Dryer

CHAPTER TWO

CLASSIFICATION AND SELECTION OF INDUSTRIAL DRYERS
Arun S. Mujumdar

1. INTRODUCTION
Dryer selection has long been practiced as an art rather than science depending more on prior experience and vendors’ recommendations. As drying technologies have evolved and become more diverse and complex, this has become an increasingly difficult and demanding task for the non-expert not conversant with the numerous types of equipment, their pros and cons, etc. Further, the task is exasperated by the need to meet stricter quality specifications, higher production rates, higher energy costs and stringent environmental regulations. In the absence of in-house experts in drying, there have been some attempts, albeit not fully successful, to develop expert systems for a non-expert to use. It is therefore necessary for an engineer responsible for selection of a dryer or, more appropriately, a drying system to be aware of what is available in the market, what the key criteria are in the selection process and thus arrive at alternative possibilities before going to vendors of such equipment for comparative quotes. It is time and effort well spent since the cost of incorrect selection can be very high. This chapter is intended to give an introduction to this subject; the reader is referred to Mujumdar (1995) for further details. Note that over 80 percent of major chemical companies in Europe – each using over 1000 dryers in their production facilities – made errors in selecting dryers in the past year alone. What is optimal choice in one location at one point in time may be a wrong choice for another geographic location some years later. Prior use is a definite help but not the only criterion to be used in selecting drying systems.

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As an example, concentrated nickel ore is dried in different parts of the world at very high production rates (20-75 t/h) using flash dryers, fluid bed dryers, rotary dryers as well as spray dryers. It is thus not a simple task to select a dryer for such applications based on what is done elsewhere. Over 400 dryer types have been cited in the technical literature although only about 50 types are commonly found in practice. In this chapter, we will examine the key classification criteria for industrial dryers and then proceed to selection criteria with the explicit understanding that the latter is a complex process, which is not entirely scientific but also involves subjective judgment as well as considerable empiricism. It should also be noted that the pre-drying as well as post-drying stages have important bearing on the selection of appropriate dryer types for a given application. Indeed, for an optimal selection of process, one must examine the overall flowsheet as well as the “drying system.” This chapter will be confined, however, only to the classification and selection of dryers. Another important point to note is that several dryer types (or drying systems) may be equally suited (technically and economically) for a given application. A careful evaluation of as many of the possible factors affecting the selection will help reduce the number of options. For a new application (new product or new process), it is important to follow a careful procedure leading to the choice of the dryers. Characteristics of different dryer types should be recognized when selecting dryers. Changes in operating conditions of the same dryer can affect the quality of the product. So, aside from the dryer type, it is also important to choose the right operating conditions for optimal quality and cost of thermal dehydration. According to a very recent survey conducted by SPIN (Solids Processing Industrial Network, UK, founded by 14 large chemical companies based in Europe) selection of dryers is a key problem faced by all companies (Slangen, 2000). Over ninety percent of the companies had made errors in selection of their new dryers. Sometimes the selection is easy but when a new product is involved or the production capacity required for exceeds current practice, it is not always an easy task. New requirements on safety and environmental aspects can also make the selection more difficult. The SPIN report recommends development of user-friendly expert systems and better standardization to assist with this complex selection process. It should be noted that the selection process is further complicated by the fact that each category of dryers (e.g., fluid bed, flash, spray, rotary) has a wide assortment of sub-classes and, furthermore, each must be operated at optimal conditions to benefit from appropriate selection. Baker (1997) has presented a “structural approach” for dryer selection, which is iterative. It includes the following steps: • • • • • • List all key process specifications Carry out preliminary selection Carry out bench scale tests including quality tests Make economic evaluation of alternatives Conduct pilot-scale trials Select most appropriate dryer types

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Often, for same materials, a specific dryer type is indicated from the outset. If selection is based exclusively on past experience, it has some limitations: • If the original selection is not optimal (although it works satisfactorily), the new choice will be less-than-optimal • No new drying technologies are considered by default • It is implicitly assumed the “old” choice was arrived at logically, which is often not the case

2. CLASSIFICATION OF DRYERS
There are numerous schemes used to classify dryers (Mujumdar, 1995; van't Land, 1991). Table 1 lists the criteria and typical dryer types. Types marked with an asterisk (*) are among the most common in practice.

Table 1 Classification of dryers Criterion Mode of operation
Heat input-type

State of material in dryer Operating pressure Drying medium (convection)

Drying temperature

Relative motion between drying medium and drying solids Number of stages

Types • Batch • Continuous* • Convection*, conduction, radiation, electromagnetic fields, combination of heat transfer modes • Intermittent or continuous* • Adiabatic or non-adiabatic • Stationary • Moving, agitated, dispersed • Vacuum* • Atmospheric • Air* • Superheated steam • Flue gases • Below boiling temperature* • Above boiling temperature • Below freezing point • Co-current • Counter-current • Mixed flow • Single* • Multi-stage

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Residence time

• • •

Short (< 1 minute) Medium (1 – 60 minutes) Long (> 60 minutes)

* Most common in practice The above classification is rather coarse. Just the fluidized bed dryer can be subclassified into over thirty types depending on additional criteria. Each type of dryer has specific characteristics, which make it suited or unsuitable for specific applications. Details can be found in Mujumdar (1995). Certain types are inherently expensive (e.g., freeze dryers) while others are inherently more efficient (e.g., indirect or conductive dryers). Thus, it is necessary to be aware of the wide variety of dryers available in the market as well as their special advantages and limitations. It should be noted that the aforementioned classification does not include most of the novel drying technologies, which are applicable for very specific applications. The reader is referred to Kudra and Mujumdar (1995) for details on novel drying technologies. Following is a general scheme proposed by Baker (1997) for classification of batch and continuous dryers. Note that there is a more limited choice of batch dryers – only a few types can be operated in both batch and continuous modes.

Batch Dryers: Classification (Baker, 1997) (Particulate Solids) Major Classes: Layer (packed bed); Dispersion type
1. Layer type: a. Contact (conductive or indirect type), e.g., vacuum tray, agitated bed, rotary batch b. Convective (atmospheric tray) c. Special types (e.g., microwave, freeze, solar) 2. Dispersion type: a. Fluidized bed/spouted bed b. Vibrated bed dryer

Continuous Dryers: Classification Major Classes: Layer; Dispersion type
1. Layer type: a. Conduction, e.g., drum, plate, vacuum, agitated bed, indirect rotary b. Convective, e.g., tunnel, spin-flash, throughflow, conveyor c. Special, e.g., microwave, RF, freeze, solar 2. Dispersion type: a. Fluid bed, vibrated bed, direct rotary, ring, spray, jet-zone

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Classification of dryers on the basis of the mode of thermal energy input is perhaps the most useful since it allows one to identify some key features of each class of dryers.

Direct dryers – also known as convective dryers – are by far the most common. About 85 percent of industrial dryers are estimated to be of this type despite their relatively low thermal efficiency caused by the difficulty in recovering the latent heat of vaporization contained in the dryer exhaust in a cost-effective manner. Hot air produced by indirect heating or direct firing is the most common drying medium although for some special applications superheated steam has recently been shown to yield higher efficiency and often higher product quality. Flue gases may be used when the product is not heatsensitive or affected by the presence of products of combustion. In direct dryers, the drying medium contacts the material to be dried directly and supplies the heat required for drying by convection; the evaporated moisture is carried away by the same drying medium. Drying gas temperatures may range from 50º C to 400º C depending on the material. Dehumidified air may be needed when drying highly heat-sensitive materials. An inert gas such as Nitrogen may be needed when drying explosive or flammable solids or when an organic solvent is to be removed. Solvents must be recovered from the exhaust by condensation so that the inert (with some solvent vapor) can be reheated and returned to the dryer. Because of the need to handle large volumes of gas, gas cleaning and product recovery (for particulate solids) becomes a major part of the drying plant. Higher gas temperatures yield better thermal efficiencies subject to product quality constraints. Indirect dryers – involve supplying of heat to the drying material without direct contact with the heat transfer medium, i.e., heat is transferred from the heat transfer medium (steam, hot gas, thermal fluids, etc.) to the wet solid by conduction. Since no gas flow is presented on the wet solid side it is necessary to either apply vacuum or use gentle gas flow to remove the evaporated moisture so that the dryer chamber is not saturated with vapor. Heat transfer surfaces may range in temperature from -40º C (as in freeze drying) to about 300º C in the case of indirect dryers heated by direct combustion products such as waste sludges. In vacuum operation, there is no danger of fire or explosion. Vacuum operation also eases recovery of solvents by direct condensation thus alleviating serious environmental problem. Dust recovery is obviously simpler so that such dryers are especially suited for drying of toxic, dusty products, which must not be entrained in gases. Furthermore, vacuum operation lowers the boiling point of the liquid being removed; this allows drying of heat-sensitive solids at relatively fast rates. Heat may also be supplied by radiation (using electric or natural gas-fired radiators) or volumetrically by placing the wet solid in dielectric fields in the microwave or radio frequency range. Since radiant heat flux can be adjusted locally over a wide range it is possible to obtain high drying rates for surface-wet materials. Convection (gas flow) or vacuum operation is needed to remove the evaporated moisture. Radiant dryers have found important applications in some niche markets, e.g., drying of coated papers or

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printed sheets. However, the most popular applications involve use of combined convection and radiation. It is often useful to boost the drying capacity of an existing convective dryer for sheets such as paper. Microwave dryers are expensive both in terms of the capital and operating (energy) costs. Only about 50 percent of line power is converted into the electromagnetic field and only a part of it is actually absorbed by the drying solid. They have found limited applications to date. However, they do seem to have special advantages in terms of product quality when handling heat-sensitive materials. They are worth considering as devices to speed up drying in the tail end of the falling rate period. Similarly, RF dryers have limited industrial applicability. They have found some niche markets, e.g., drying of thick lumber and coated papers. Both microwave and RF dryers must be used in conjunction with convection or under vacuum to remove the evaporated moisture. Standalone dielectric dryers are unlikely to be cost-effective except for high value products in the next decade. See Schiffmann (1995) for detailed discussion of dielectric dryers. It is possible, indeed desirable in some cases, to use combined heat transfer modes, e.g., convection and conduction, convection and radiation, convection and dielectric fields, to reduce the need for increased gas flow which results in lower thermal efficiencies. Use of such combinations increases the capital costs but these may be offset by reduced energy costs and enhanced product quality. No generalization can be made a priori without careful tests and economic evaluation. Finally, the heat input may be steady (continuous) or time varying. Also, different heat transfer modes may be deployed simultaneously or consecutively depending on individual application. In view of the significant increase in the number of design and operational parameters it is desirable to select the optimal operating conditions via a mathematical model. In batch drying intermittent energy input has great potential for reducing energy consumption and for improving quality of heat-sensitive products.

3. SELECTION OF DRYERS
In view of the enormous choices of dryer types one could possibly deploy for most products, selection of the best type is a challenging task that should not be taken lightly nor should it be left entirely to dryer vendors who typically specialize in only a few types of dryers. The user must take a proactive role and employ vendors' experience and benchscale or pilot-scale facilities to obtain data, which can be assessed for a comparative evaluation of several options. A wrong dryer for a given application is still a poor dryer, regardless of how well it is designed. Note that minor changes in composition or physical properties of a given product can influence its drying characteristics, handling properties, etc., leading to a different product and in some cases severe blockages in the dryer itself. Tests should be carried out with the “real” feed material and not a “simulated” one where feasible. Although here we will focus only on the selection of the dryer, it is very important to note that in practice one must select and specify a drying system which includes predrying stages (e.g., mechanical dewatering, evaporation, pre-conditioning of feed by

CLASSIFICATION AND SELECTION OF DRYERS 29 ___________________________________________

solids backmixing, dilution or pelletization and feeding) as well as the post-drying stages of exhaust gas cleaning, product collection, partial recirculation of exhausts, cooling of product, coating of product, agglomeration, etc. The optimal cost-effective choice of dryer will depend, in some cases significantly, on these stages. For example, a hard pasty feedstock can be diluted to a pumpable slurry, atomized and dried in a spray dryer to produce a powder, or it may be pelletized and dried in a fluid bed or in a through circulation dryer, or dried as is in a rotary or fluid bed unit. Also, in some cases, it may be necessary to examine the entire flowsheet to see if the drying problem can be simplified or even eliminated. Typically, non-thermal dewatering is an order-of-magnitude less expensive than evaporation which, in turn, is many-fold energy efficient than thermal drying. Demands on product quality may not always permit one to select the least expensive option based solely on heat and mass transfer considerations, however. Often, product quality requirements have over-riding influence on the selection process (see Section 4). As a minimum, the following quantitative information is necessary to arrive at a suitable dryer: • • • • • • • • • • • • • • Dryer throughput; mode of feedstock production (batch/continuous) Physical, chemical and biochemical properties of the wet feed as well as desired product specifications; expected variability in feed characteristics Upstream and downstream processing operations Moisture content of the feed and product Drying kinetics; moist solid sorption isotherms Quality parameters (physical, chemical, biochemical) Safety aspects, e.g., fire hazard and explosion hazards, toxicity Value of the product Need for automatic control Toxicological properties of the product Turndown ratio, flexibility in capacity requirements Type and cost of fuel, cost of electricity Environmental regulations Space in plant

For high value products like pharmaceuticals, certain foods and advanced materials, quality considerations override other considerations since the cost of drying is unimportant. Throughputs of such products are also relatively low, in general. In some cases, the feed may be conditioned (e.g., size reduction, flaking, pelletizing, extrusion, back-mixing with dry product) prior to drying which affects the choice of dryers. As a rule, in the interest of energy savings and reduction of dryer size, it is desirable to reduce the feed liquid content by less expensive operations such as filtration, centrifugation and evaporation. It is also desirable to avoid over-drying, which increases the energy consumption as well as drying time.

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Drying of food and biotechnological products require adherence to GMP (Good Manufacturing Practice) and hygienic equipment design and operation. Such materials are subject to thermal as well as microbiological degradation during drying as well as in storage. If the feed rate is low (< 100 kg/h), a batch-type dryer may be suited. Note that there is a limited choice of dryers that can operate in the batch mode. In less than one percent of cases the liquid to be removed is a non-aqueous (organic) solvent or a mixture of water with a solvent. This is not uncommon in drying of pharmaceutical products, however. Special care is needed to recover the solvent and to avoid potential danger of fire and explosion. Table 2 presents a typical checklist most dryer vendors use to select and quote an industrial dryer.

Table 2 Typical checklist for selection of industrial dryers
Physical form of feed • • • • • • • • • • • • • • • • • • Granular, particulate, sludge, crystalline, liquid, pasty, suspension, solution, continuous sheets, planks, odd-shapes (small/large) Sticky, lumpy kg/h (dry/wet); continuous kg per batch (dry/wet) Oil Gas Electricity Mean particle size Size distribution Particle density Bulk density Rehydration properties Dry basis Wet basis Melting point Glass transition temperature Drying curves Effect of process variables

Average throughput Expected variation in throughput (turndown ratio) Fuel choice

Pre- and post-drying operations (if any) For particulate feed products

Inlet/outlet moisture content Chemical / biochemical / microbiological activity Heat sensitivity Sorption isotherms (equilibrium moisture content) Drying time

CLASSIFICATION AND SELECTION OF DRYERS 31 ___________________________________________

Special requirements

Foot print of drying system

• • • • • • • •

Material of construction Corrosion Toxicity Non-aqueous solution Flammability limits Fire hazard Color/texture/aroma requirements (if any) Space availability for dryer and ancillaries

Drying kinetics play a significant role in the selection of dryers. Aside from simply deciding the residence time required, it limits the types of suitable dryers. Location of the moisture (whether near surface or distributed in the material), nature of moisture (free or strongly bound to solid), mechanisms of moisture transfer (rate limiting step), physical size of product, conditions of drying medium (e.g., temperature, humidity, flow rate of hot air for a convective dryer), pressure in dryer (low for heat-sensitive products), etc., have a bearing on the type of suitable dryer as well as the operating conditions. Most often, not more than one dryer type will likely meet the specified selection criteria. We will not focus on novel or special drying techniques here for lack of space. However, it is worth mentioning that many of the new techniques use superheated steam as the drying medium or are simply intelligent combinations of traditional drying techniques, e.g., combination of heat transfer modes, multi-staging of different dryer types. Superheated steam as the convective drying medium offers several advantages, e.g., higher drying rates under certain conditions, better quality for certain products, lower net energy consumption if the excess steam produced in the dryer is used elsewhere in the process, elimination of fire and explosion hazard. Vacuum steam drying of timber, for example, can reduce drying times by a factor of up to four while enhancing wood quality and reducing net fuel and electricity consumption by up to 70 percent. The overall economics are also highly favorable.

4. SELECTION OF A DRYER BASED ON QUALITY
As the product quality requirements become increasingly stringent and as the environmental legislation becomes more and more demanding it is often found that we need to switch from one drying technology to the others. The rising cost of energy as well as the differences in the cost of fossil fuels versus electrical energy can also affect the choice of a dryer. Since up to 70 percent of the life cycle cost of a convective dryer is due to energy it is important to choose an energy-efficient dryer where possible even at a higher initial cost. Note that energy costs will continue to rise in the future so this will become increasingly important. Fortunately, improved efficiency also translates into better environmental implications in terms of reduced emissions of the greenhouse gas (CO2) as well as NOx resulting from combustion.

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Following is an example of how selection of the dryer is affected by quality of the dried product that may be used as raw material to produce different consumer products. Shah and Arora (1996) have surveyed the various possible dryers used for crystallization/drying of polyester chips from an initial moisture content of about 0.30.5% (w.b.) to under 50 ppm. Aside from low average moisture content it is also necessary to ensure uniform distribution of moisture, especially for some certain products, e.g., production of thin films. The uniformity constraint is less severe if the chips are to be used to make PET bottles. Figure 1 shows schematics of the crystallization/drying steps involved. Generally, it is a two-step process. The material is heat-sensitive. The initial crystallization/drying is faster than the drying step at low moisture levels. A two-stage dryer is indicated and is commonly used. It is possible to use different dryer types for each stage as shown in Figure 2. A single dryer type (e.g., column or packed bed dryer with the chips moving downward slowly under gravity) is cheaper and hence recommended for the lower quality grade but a more expensive fluid bed followed by another fluid bed or column dryer may be needed for the higher quality grade. Note that numerous alternatives are possible in each case. It is also important to operate the dryers at the correct conditions of gas flow rate, temperature and humidity. Dehumidified air is needed to achieve low final moisture contents in accordance with the equilibrium moisture isotherms of the product.

Wet chips

Crystallizer/ dryer

500-1000 ppm moisture

Final Dryer

< 50 ppm moisture

Batch
Vacuum tumbler

Continuous
DIRECT INDIRECT
Paddle dryer

Fluid bed Vibro-fluid bed Pulsed fluid bed Vortex (spouted) bed Column dryer (with mixer)

Batch
Vacuum tumbler

Continuous
DIRECT INDIRECT
Paddle dryer

Column dryer with internal tube Multi-stage fluid bed

Figure 1 Schematic diagram of crystallization/drying steps in the production of polyester chips

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Polymers Chips: Quality Parameter HIGH
e.g. for magnetic tape

MEDIUM
e.g. for speciality fibre, film A. Crystallizer: fluid bed or pulse fluid bed or paddle crystallizer B. Finish dryer: Column dryer with a central tube for smooth downward flow of chips

AVERAGE
e.g. for PET bottles, staple fibre, etc. Single column crystallizer/dryer with a mixer in the top crystallizer section to avoid agglomeration Low capital/operating cost, smaller space requirements

A. Crystallizer: fluid bed B. Finish dryer: Multi-stage fluid bed with dehumidified air

Figure 2 Possible dryer types for drying of polyester chips
Another example of dryer selection is related to the choice of a suitable atomizer for a spray dryer. A spray dryer is indicated when a pumpable slurry, solution or suspension is to be reduced to a free-flowing powder. With proper choice of atomizer, spray chamber design, gas temperature and flow rate it is possible to “engineer” powders of desired particle size and size distribution. Table 3 shows how the choice of the atomizer affects chamber design, size, as well as energy consumption for atomization and particle size distribution. The newly developed two-fluid sonic nozzles appear to be especially attractive choices when nearly monodisperse powders need to be produced from relatively moderate viscosity feeds (e.g., under 250 cp) at capacities up to 80 t/h by using multiple nozzles. More examples may be found in Masters (1985).

Table 3 Spray drying of emulsion-PVC. Effect of selection of atomizer on spray dryer performance: A Comparison between different atomizers
Parameter Dryer geometry Evaporation capacity (water) Chamber (D × H) Number of nozzles Power for atomizer Capital cost Operating cost Rotary disk Conical/cylindrical H/D ≈ 1.2-1.5 1600 kg/h 6.5 m × 8 m 1, 175-mm disk 15,000 rpm 25 W/kg slurry High Medium Two-fluid (sonic) Tall-form Cylindrical H/D ≈ 4 1600 kg/h 3.5 m × 15 m 16 nozzles 4 bar pressure 20 W/kg slurry Medium Low Two-fluid (standard) Tall-form Cylindrical H/D ≈ 5 1600 kg/h 3 m × 18 m 18 nozzles 4 bar pressure 80 W/kg slurry Medium High

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New dryers are being developed continuously as a result of industrial demands. Over 250 US patents are granted each year related to dryers (equipment) and drying (process); in the European Community about 80 patents are issued annually on dryers. Kudra and Mujumdar (2000) have discussed a wide assortment of novel drying technologies, which are beyond the scope of this chapter. Suffice it to note that many of the new technologies (e.g., superheated steam, pulse combustion – newer gas-particle contactors as dryers) will eventually replace conventional dryers in the next decade or two. New technologies are inherently more risky and more difficult-to-scale-up. Hence there is natural reluctance to their adoption. Readers are encouraged to review the new developments in order to be sure their selection is the most appropriate one for the application at hand. Some conventional and more recent drying techniques are listed in the Table 4.

Table 4 Conventional versus innovative drying techniques
New techniques* • Fluid/spouted beds of inert particles • Spray/fluid bed combination • Vacuum belt dryer • Pulse combustion dryers Paste/sludge • Spray • Spouted bed of inerts • Drum • Fluid bed (with solid backmixing) • Paddle • Superheated steam dryers Particles • Superheated steam FBD • Rotary • Flash • Vibrated bed • Fluidized bed (hot air • Ring dryer or combustion gas) • Pulsated fluid bed • Jet-zone dryer • Yamato rotary dryer Continuous sheets • Multi-cylinder • Combined impingement/radiation (coated paper, contact dryers dryers paper, textiles) • Impingement (air) • Combined impingement and through dryers (textiles, low basis weight paper) • Impingement and MW or RF *New dryers do not necessarily offer better techno-economic performance for all products Feed type Liquid Suspension Dryer type • Drum • Spray

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CLOSING REMARKS
It is difficult to generate rules for both classification and selection of dryers because exceptions occur rather frequently. Often, minor changes in feed or product characteristics result in different dryer types being the appropriate choices. It is not uncommon to find different dryer types being used to dry apparently the same material. The choice is dependent on production throughput, flexibility requirements, cost of fuel as well as on the subjective judgment of the individual who specified the equipment. We have not considered novel dryers in this chapter. Kudra and Mujumdar (2000) have discussed in detail most of the non-conventional and novel drying technologies reported in the literature. Most of them have yet to mature; a few have been commercialized successfully for certain products. It is useful to be aware of such advances so that the user can make intelligent decisions about dryer selection. Since dryer life is typically 25-40 years that effect of a poor “prescription” can have a long-term impact on the economic health of the plant. It is typically not a desirable option to depend exclusively on prior experience, reports in the literature or vendors’ recommendations. Each drying problem deserves its own independent evaluation and solution.

REFERENCES
Baker, C.G.J., 1997, Dryer Selection, pp. 242-271, in C.G.J. Baker (Ed.) Industrial Drying of Foods, Blackie Academic & Professional, London. Kudra, T., Mujumdar, A.S., 2000, Advanced Drying Technologies, Marcel Dekker, New York. Kudra, T., Mujumdar, A.S., 1995, Special Drying Techniques and Novel Dryers, pp. 1087-1149, in A.S. Mujumdar (Ed.) Handbook of Industrial Drying, 2nd Edition, Marcel Dekker, New York. Masters, K., 1985, Spray Drying Handbook, Halsted Press, New York. Mujumdar, A.S. (Ed.), 1995, Handbook of Industrial Drying, 2nd Edition, Marcel Dekker, New York. Schiffmann, R.F., 1995, Microwave and Dielectric Drying, pp. 345-372, in A.S. Mujumdar (Ed.) Handbook of Industrial Drying, 2nd Edition, Marcel Dekker, New York. Shah, R.M., Arora, P.K., 1996, Two Fluid Nozzles and their Application in Spray Drying of E-PVC, pp. 1361-1366, in C. Strumillo, Z. Pakowski, A.S. Mujumdar (Eds.) Drying’96: Proceedings of the Tenth International Drying Symposium, Lodz, Poland.

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Slangen, H.J.M., The Need for Fundamental Research on Drying as Perceived by the European Chemical Industry, to appear in Drying Technology – An International Journal, 18(6), 2000. van't Land, C.M., 1991, Industrial Drying Equipment: Selection and Application, Marcel Dekker, New York.

THREE

CHAPTER

DRYERS FOR PARTICULATE SOLIDS, SLURRIES AND SHEET-FORM MATERIALS
Arun S. Mujumdar

1. INTRODUCTION
Wet materials come in different physical forms and are required to be dried to different desired specifications. Over 400 different dryer types have been proposed in the technical literature although only about 50 types are commonly used and readily available from various vendors. No two dryers are identical even when used for drying nominally the same material. Even minor changes in feed condition and/or product specification may make the two dryers different in design or in operation or both. Mujumdar (1995), among many others, have provided detailed classification schemes and selection criteria of dryers; major topics of study by themselves. In this chapter the focus is on providing a brief overview of the more common drying equipment (which naturally excludes the novel drying techniques, many of which have come on stream only recently and are not yet readily available on the market). It is also not extensive enough to cover all types and sub-types of dryers. However, it will allow the reader to obtain a quick understanding of the key features, main advantages and limitations of the various dryer types and their modifications. For ease of presentation, the chapter is categorized according to the physical form of the feedstock to be dried since the first qualification of the selected dryer is the ability to physically handle the feedstock and the dried product themselves. For an in-depth discussion of various dryer types the reader is referred to Mujumdar (1995).

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2. DRYERS FOR PARTICULATES AND GRANULAR SOLIDS 2.1 Tray Dryers
By far the most common dryer for small tonnage products, a batch tray dryer (Figure 1) consists of a stack of trays or several stacks of trays placed in a large insulated chamber in which hot air is circulated with appropriately designed fans and guide vanes. Often, a part of the exhausted air is recirculated with a fan located within or outside the drying chamber. These dryers require large amount of labor to load and unload the product. Typically, the drying times are long (10-60 hours). The key to successful operation is the uniform air flow distribution over the trays as the slowest drying tray decides the residence time required and hence dryer capacity. Warpage of trays can also cause poor distribution of drying air and hence poor dryer performance.

Figure 1 A batch tray dryer
It is possible to convert the batch tray dryer into a continuous unit. Figure 2 shows the so-called Turbo dryer, which consists of a stack of coaxial circular trays mounted on a single vertical shaft. The product layer fed onto the first shelf is leveled by a set of stationary blades, which scratch a series of grooves into the layer surface. The blades are staggered to ensure mixing of the material. After one rotation, the material is wiped off the shelf by the last blade and falls onto the next lower shelf. Up to 30 trays or more can be accommodated. Hot air is supplied to the drying chamber by turbine fans. In the design shown, the air is heated indirectly by passage over internal heaters. The wet granular material is fed at the top and it falls under gravity to the next tray through radial slots in each circular shelf. A rotating rake mixes the solids and thus improves the drying performance. Such dryers can be operated under vacuum for heat-sensitive materials or when solvents must be recovered from the vapor. In a modified design, it is possible to heat the trays by conduction and apply vacuum to remove the moisture evaporated.

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Figure 2 A Turbo dryer 2.2 Rotary Dryers
The cascading rotary dryer is a continuously operated direct contact dryer consisting of a slowly revolving cylindrical shell that is typically inclined to the horizontal a few degrees to aid the transportation of the wet feedstock which is introduced into the drum at the upper end and the dried product withdrawn at the lower end (Figure 3). To increase the retention time of very fine and light materials in the dryer (e.g., cheese granules), in rare cases, it may be advantageous to incline the cylinder with the product end at a higher elevation.

Figure 3 A cascading rotary dryer
The drying medium (hot air, combustion gases, flue gases, etc.) flows axially through the drum either concurrently with the feedstock or countercurrently. The latter mode is preferred when the material is not heat-sensitive and needs to be dried to very low moisture content levels. The concurrent mode is preferred for heat-sensitive materials and for higher drying rates in general.

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In this type of dryer, a wide assortment of granular products of diverse shapes, sizes and size distributions can be processed by proper design of the internal flights and lifters. Special internals are needed for materials that tend to form large lumps that must be broken to avoid major problems in the later stages of drying. The lifters lift the material to the top of the drum where it showers down in the form of cascades. The major heat and mass transfer processes are accomplished during the flight of the particles from the top to the bottom of the drum by gravity. The drying medium is in cross-flow with respect to the cascading particles. Clearly, particles with terminal velocities below the cross-flow gas velocity will be entrained and collected in the gas cleaning equipment. The cascading action may cause severe attrition of fragile materials, especially when the drum diameter is large. Although numerous attempts have been reported which permit calculation of particle residence times in rotary dryers, the design of commercial units is still based on pilot tests and empirical rules (often proprietary) based on prior experience with similar material and similar design of rotary dryer hardware. The drying process is essentially intermittent. It is intense during the cascading motion under gravity when the particles contact the cross-flowing hot gas stream. When the particles settle on the drum wall as a bed and carried upward by the revolving shell, there is a “soaking” or “tempering” period when the temperature and moisture content fields in the particles tend to equalize before the particles are exposed to the convective drying condition again. Rotary dryers can be designed for drying time from 10 to 60 minutes. If large retention time is needed for removing the internal moisture in the falling rate period, it is possible to use a smaller shell diameter at the wet end for surface moisture removal with low holdup of material in the drum and then increase the shell diameter at the dry end to allow longer retention time with larger holdup. In some designs, it is possible to use a pneumatic conveyor to carry the product out of the dryer. Thermal efficiencies of rotary dryers vary widely in the range of 30-60%. For good efficiency, the product holdup (typically 10-15 percent of volume) should be such as to cover the flights or lifters fully. The lifters should be carefully designed to ensure good cascading action, avoiding large clusters of material falling from the flights. Length-todiameter ratios of 4 to 10 are common in industrial practice. Rotary dryers can be operated at very high temperatures to accomplish various reactions in addition to or instead of simple drying; these units are referred to as kilns. It is necessary to line the shell of rotary kilns with suitable refractory materials. In order to enhance the drying rates in the rotary dryer without raising the gas temperature or gas flowrate excessively, it is possible to introduce steam-heated tubes or coils within the shell. Aside from providing additional energy for drying, such internals can also help with redistribution or delumping of the material. Of course, it is possible to use internal heaters only if the material does not stick to the walls of the internals. A new variant of the classical rotary dryer uses a central axial header for the drying gas that is injected at discreet intervals along the length of the rotating shell directly into the “kilning” bed of particles. This type of flow distribution is more effective for heat and mass transfer and results in volumetric heat and mass transfer coefficients up to two times larger than those in the cascading dryer. However, this design is not suited for all types of materials.

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Rotary dryers are very flexible, very versatile and are especially suited for high production rate demands. On the negative side, they are typically less efficient, demand high capital costs and significant maintenance costs depending on the material being dried. They are not recommended for fragile materials and for low production rates. Finally, it is useful to note that while most of the continuous rotary dryers are operated under near atmospheric pressure, the term vacuum rotary dryer refers to an entirely different class of dryers. It is, in fact, an indirect type batch dryer because of the difficulty of maintaining vacuum under continuous feeding and discharge conditions. Here, the horizontal cylindrical shell is stationary while a set of variously designed agitator blades revolves on a central shaft to agitate the material contained in the dryer shell. Heat is supplied by heating the shell jacket using condensing steam or a thermal fluid. In larger units, the central agitator shaft and the blades may also be heated. The agitator may be a single- or double-spiral. The outer blades are set close to the wall and may have a scraper attached to keep the material from building up on the walls and deteriorating the thermal performance of the unit. This type of dryer is useful for handling heat-sensitive materials, which dry at lower temperatures because of the vacuum conditions.

2.3 Freeze Dryers
Highly heat-sensitive solids, such as some certain biotechnological materials, pharmaceuticals and foods with high flavor content, may be freeze dried at a cost that is at least one order-of-magnitude higher than that of spray drying – itself not an inexpensive drying operation. Here, drying occurs below the triple point of the liquid by sublimation of the frozen moisture into vapor, which is then removed from the drying chamber by mechanical vacuum pumps or steam jet ejectors. Generally, freeze drying yields the highest quality product of any dehydration techniques. A porous, non-shrunken structure of the product allows rapid rehydration. Flavor retention is also high due to the low temperature operation (-40o C). Living cells, e.g., bacteria, yeast's and viruses can be freeze dried and the viability on reconstitution can still be high. Mammalian cells, however, cannot be preserved by freeze drying. Because of its inherently high cost nature, freeze drying is not common in the chemical industry. Most freeze dryers are batch-type with rather low capacities although some continuous freeze drying units are in operation. Industrial freeze dryers can be of several types; simple tray freeze dryers are by far the most common. Heat for sublimation is supplied by conduction through the tray bottom. Vacuum pressure is typically under 25 Pa and the condenser operates at around –40o C. The heaters start at a higher temperature (say, 120o C) but the temperature drops with time, according to schedules determined empirically, to lower values, say 40o C over 8-10 hour runs. To minimize drying times, freeze dryers are program-controlled. Multi-batch freeze dryers are used to permit nearly equal load on all systems throughout the drying cycle. A number of batch cabinets are programmed to operate with staggered, overlapping drying cycles. Tunnel freeze dryer (Figure 4) is basically a large vacuum cabinet into which traycarrying trolleys are loaded at intervals through a vacuum lock at one end of the tunnel. Dried products are unloaded through a vacuum lock at the other end of the drying

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chamber. Low pressure steam is used to heat the plates on which the trays sit. Liapis and Bruttini (1995) have provided a detailed analysis of the drying characteristics, costs and details on various freeze dried products.

Figure 4 A tunnel freeze dryer 2.4 Vacuum Dryers

Figure 5 A paddle-type vacuum dryer
For drying of granular solids or slurries, vacuum dryers of various mechanical designs are available commercially. They are more expensive than atmospheric pressure dryers but are suited for heat-sensitive materials or when solvent recovery is required or if

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there are risks of fire and/or explosion. Single-cone and double-cone mixers can be adapted to drying by heating the vessel jackets and applying vacuum to remove moisture. Figures 5 and 6 show two vacuum dryers available commercially. The paddle dryer is suited for sludge-like materials while the vacuum band dryer is good for thin pastes or slurries. The material forms a film over the heated band; it may boil and form a highly foamy, porous structure of very low bulk density.

Figure 6 A band-type vacuum dryer 3. DRYERS FOR SLURRIES AND SUSPENSIONS 3.1 Spray Dryers
Over 20,000 spray dryers are presently in use commercially to dry products from agro-chemicals, biotechnologicals, fine and heavy chemicals, dairy products, dyestuffs, mineral concentrates to pharmaceuticals in capacities ranging from a few kg per h to 50 tons per h evaporation capacity. Liquid feedstocks, such as solutions, suspensions or emulsions can be converted into powder, granular or agglomerate form in one step operation in spray dryer. Figure 7 gives a process schematic for a spray dryer plant. Atomized feedstock in the form of a spray is contacted with hot gas in a suitably designed drying chamber. Proper selection and design of the atomizer is vital to the operation of the spray dryer as it is affected by the type of feed (viscosity), abrasive property of the feed, feed rate, desired particle size and size distribution as well as the design of the chamber geometries and mode of flow, e.g., concurrent, countercurrent or mixed flow (see Figure 8).

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Figure 7 A process schematic of a spray dryer plant

Figure 8 Concurrent, countercurrent and mixed flow spray dryer chambers
It is beyond the scope of this chapter to cover all the important aspects of spray dryers in detail. The reader is referred to Masters (1991) and Filkova and Mujumdar (1995) for further information. Here, we will summarize the key aspects of spray dryers in a tabular form. It must be noted that design of spray dryers depends heavily on pilot scale testing. It is impossible to scale-up quality criteria for spray dryers. Fortunately, in most

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cases, it is found that the larger scale dryer provides better quality product than the one obtained in smaller scale pilot tests. Aside from drying rate and quality tests, it is also important to check potential of deposits in the drying chamber as this may lead to fire and explosion hazards. Essentially, three major types of atomizers are used in practice. They are: (a) Rotary wheel (or disk) atomizers, (b) Pressure nozzle and (c) Two-fluid nozzle. Figure 9 shows some typical atomizer designs. Ultrasonic and electrostatic atomizers can also be used for special applications to produce monodisperse sprays but they are very expensive and low capacity units. Most spray dryers operate at slight negative pressure. New designs may use low pressure chambers to enhance drying rates at lower temperatures to dry highly heat-sensitive products.

Figure 9 Typical spray dryer atomizer designs
The design of the spray drying chamber depends on the needed residence time (see Table 1) as well as the type of atomizers used (see Table 2). The mode of flow, i.e., concurrent, counter-current, mixed flow, depends on the desired characteristics of the product as summarized in Table 3. Finally, Table 4 gives suggested spray dryer system

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layout depending on the feedstock characteristics, e.g., presence of organic solvents, danger of fire or explosion. Collection of the dried powder from the spray dryer is also an important issue. Table 5 lists general recommendations for the selection of the dried powder collection system.

Table 1 Residence time requirements for spray drying of various products
Residence time in chamber Short (10-20 s) Medium (20-35 s) Long (> 35 s) Recommended for Fine, non-heat sensitive products; surface moisture removal, non-hygroscopic Fine-to-coarse sprays (dmean = 180 m); drying to low final moisture Large powder (200-300 m); low final moisture, low temperature operation for heat-sensitive products

Table 2 Atomizer selection criteria
Rotary wheel Pressure nozzle

Type of chamber * Concurrent X X X * Counter- current * Mixed (fountain) X Feed type Solution/slurries * X * Low viscosity X High viscosity X * Slurries * Non-abrasive X X * Slightly abrasive X X * Highly abrasive X Feed rate * < 3 m3 per h X X 3 * > 3 m per h X X* Droplet size * 30-120 m X * 120-150 m X X: Applicable; -: Not applicable; X*: Multi-nozzle assembly

Two-fluid nozzle
X X X X X X X X X* -

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Table 3 Selection of mode of flow in spray drying chamber based on desired powder characteristics Dryer design – flow type Concurrent Mixed flow with integrated fluidized beds Mixed flow (fountain type) Characteristics Low product temperature To produce agglomerated powder

For coarse sprays in small chambers; product no heat-sensitive Counter-current flow Products which withstand high temperatures; coarse particles; high bulk density powders Table 4 Spray dryer system layout System layout Open cycle Closed cycle Semi-closed; selfinertizing Characteristics General; all aqueous feeds Recovery of solvents; prevention of vapor emissions; elimination of explosion or fire hazards Prevent powder explosion (keep O2 content low) yet use higher inlet temperature

Table 5 Selection of dry powder collection system Requirement Low cost, efficient, easy to clean Medium cost, very efficient, high running cost Large air volumes Product recovery; fines Recommended system Cyclones Bag filter
Electro-static precipitator Cyclone + wet scrubber

Since the choice of the atomizer is very crucial it is important to note the key advantages and limitations of the wheel and pressure nozzles, which are most common in practice. Although both types may be used for the same feedstocks, the product properties (bulk density, porosity, size, etc.) will be different.

a. Rotary wheels (or disk) atomizers Advantages:
• • • • • Handle large feed rates with single wheel Suited for abrasive feeds with proper design Negligible clogging tendency Change of rpm controls particle size More flexible capacity

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Limitations: • • • Higher energy consumption compared to pressure nozzles More expensive Broad radical spray requires large drying chamber (cylindrical-conical type)

b. Pressure nozzles Advantages:
• • • Simple, compact, cheap No moving parts Low energy consumption

Limitations: • • • Low capacity (flow rates) High tendency to clog Erosion can change spray characteristics

Figure 10 Spray dryer schematics. (a) Wheel atomizer; (b) Single or two-fluid nozzle

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Figure 10 shows schematics of two spray dryers, one fitted with a wheel atomizer (cylindrical-conical) and the others with a nozzle atomizer (single or two-fluid), which is a cylindrical vessel. These figures also show other components of the system, i.e., feed tank, filter, pump, air heater, fan cyclone, exhaust fan. Figure 11 shows the layout of a spray dryer system, which is self-inertizing and used to handle materials with high risk of fire and explosion. Here, excess air entering the system passes through the burner flame and used as combustion air, thus inactivating it.

Figure 11 Self-inertizing spray dryer system

Figure 12 A two-stage spray dryer followed by a fluidized bed agglomerator
When the product coming out of the spray dryer is too fine it does not wet readily and so is harder to reconstitute. To make the product instantly soluble it is agglomerated in a small fluidized or vibrated fluidized bed, as shown in Figure 12. This two-stage arrangement is used in the production of instant coffee, milk powder, cocoa, etc. An extension of this basic concept is the so-called “Spray-Fluidizer” which dries the material in two stages. The surface moisture from droplets is removed fully, along with some internal moisture, which takes longer time to come out, in the first stage (spray dryer).

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The final moisture content is achieved in a fluidized bed located at the bottom of the spray chamber as an integral part of it. This two-stage arrangement makes the drying process very efficient and economic. The fluidized bed drying unit can be replaced with a through circulation band dryer at the bottom of the chamber; this concept is the basis of the so-called Filtermat dryer used for sticky and sugar-rich materials which are hard to dry. The spray chamber in this case is much wider at the bottom, unlike the SprayFluidizer.

3.2 Drum Dryers
In drum dryers, slurries or pasty feedstocks are dried on the surface of a slowly rotating steam-heated drum. A thin film of the paste is applied on the surface in various ways. The dried film is doctored off once it is dry and collected as flakes (rather than powder). Figure 13 shows four types of commonly used drum dryer arrangements, which are self-explanatory. The design of applicator rolls is important since the drying performance depends on the thickness and evenness of the film applied. The paste must stick to the surface of the drum for such a drop to be applicable.

Figure 13 Four types of drum dryers in common use
Four key variables influence the drum dryer performance. They are: (a) steam pressure or heating medium temperature, (b) Speed of rotation, (c) Thickness of film and (d) Feed properties, e.g., solids concentration, rheology and temperature. Because it allows good control of the drying temperature, drum dryers may be used to produce a precise hydrate of a chemical compound rather than a mixture of hydrates. Vacuum operation of both single- and double-drum dryers are done commercially to enhance drying rates for heat-sensitive materials, such as pharmaceutical antibiotics. They are also used when a porous structure of product is desired. When recovery of

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solvents is an issue, once again, vacuum operation is recommended. When recovering high boiling point solvents such as ethylene glycol, lowering the pressure depresses the boiling point. For a detailed description and discussion of the various types of drum dryers, the reader is referred to Moore (1995).

4. DRYERS FOR SHEET-FORM MATERIALS 4.1 Drying of Boards and Sheets
Although not common in the chemical industry, drying of sheet-form materials or materials in the form of large and small pieces is a major question in paper, textile, coated or impregnated fabrics, wood processing, food processing, polymer and photographic film production as well as in the graphic arts industries. Here, we will only review, in a general way, types of dryers to choose from when the feedstock is other than particulate or liquid-like. Figure 14 gives a coarse classification of possible dryers – the list is far from complete, however. In the following paragraphs, we will review very briefly the main dryer types suited for such materials.

Drying of Continuous Sheets
Conductive/contact dryers
Multi-cylinder dryer for paper

Convective
Impinging jets (air or superheated steam) Through dryers (for porous sheets)

Radiant (with mild convection)

Combined modes
Impinging jets + MW Impinging jets + IR Impinging jets + throughflow

MW/RF (usually as "assists")

Figure 14 A coarse classification of dryers for continuous sheets

4.2 Dryers for Continuous Sheets
Convection, conduction as well as infrared dryers can be used for such materials although combined mode dryers are often more efficient. Paper, coated webs or textiles are dried on steam-heated cylinders (conduction heating) or jets of hot air may be impinged on the sheet for convective heating as well. In some cases, it may be desirable to use infrared heating to augment the drying rate if the material is not very heat-sensitive.

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For drying of thin permeable sheets, it is possible to draw drying air through the sheet for highly enhanced drying rates. Combined through and impingement drying is a particularly attractive option for drying of tissue or newsprint, for example. Furthermore, it is possible to use superheated steam as the drying medium in place of air or combustion gases. For thin sheets, the total drying time may be in the order of seconds (e.g., tissue paper) to several minutes (e.g., textiles).

4.3 Dryers for Piece-Form Sheets
Materials like plywood or chipboard require long residence times to dry. Impinging jets may be used initially to remove surface moisture; internal moisture comes out at much slower rate and this can be achieved in a tunnel dryer with modest parallel flow of drying medium.

4.4 Dryers for Very Thick Sheets (or Odd Shapes)
Here, the drying times may range from days to months. Wood, for example, is dried in hot air kilns from weeks to months depending on the size of the pieces to be dried and the type of wood species. Superheated steam drying under vacuum conditions has been shown to enhance drying rates as well as product quality. Only batch dryers are suited for these long drying time requirements.

4.5 Drying of Materials in the Form of Thin Wafers
Such problems are encountered in the wood (e.g., wafer board) and food (e.g., potato chips) industries. Here, one may use a continuous conveyor or through-circulation dryer or even the so-called impinging jet-fluidized bed dryer. In the latter, hot air jets impinge on a thin layer of wet chips, which are conveyed mechanically; the high velocity jets “pseudo-fluidize” the material to accomplish drying. Wood chips, for example, may be dried in a rotary or a conveyor dryer.

5. SELECTED DRYERS AND DRYING SYSTEMS
In this section, we will consider the key features and applications of some specialized dryer types that are used in the chemical and ancillary industries but perhaps less commonly than that the spray, rotary and fluidized bed types discussed earlier. The following types of dryers will be discussed briefly: two-stage dryers, flash or pneumatic dryers, spin-flash dryers, Roto-Louvre dryer, tunnel dryers, band dryers, infrared, microwave and radio frequency dryers. Note that most of the dryers mentioned here have several variants that make them more efficient or otherwise desirable for a given application; here we will cover only the most basic dryer concepts, however.

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5.1 Two-Stage Dryers
When both surface and internal moistures must be removed from large quantities of feedstock, it is desirable to look into a two-stage operation, where the two stages may be the same dryer types (e.g., fluidized bed) or may be different. The fundamental advantage of such a system is that one can remove the surface moisture rapidly using dryers or conditions suitable for rapid removal of surface moisture (e.g., using higher gas temperatures or velocities), and use a dryer allowing longer residence time or gentler drying conditions as the second stage. A plug flow continuous fluidized bed dryer can be zoned along its length by lowering the gas temperature from inlet to outlet, for example. Figure 15 shows a two-stage arrangement, where the top first stage is a wellmixed fluidized bed dryer for a filter cake which is difficult to fluidize unless it is mixed with a fluidized bed of lower moisture content. In this figure, the first stage also uses internal heating panels to increase the drying rate since this stage receives drying air, which is the exit air from the lower second stage. The lower stage, which receives the output of the first stage by gravity through a centrally located discharge tube, is a spiral plug flow fluidized bed dryer, which controls the particle residence time to yield a uniform product moisture content. Figure 16 is another example of a commercial two-stage dryer for crystallization/drying of polyester chips. A small fluidized bed, as the first stage, removes the readily removable liquid while the tall “column” dryer allows a very long residence time during which the material crystallizes and dries very slowly. There are numerous examples of two-stage dryers used commercially; perhaps they should be considered as viable options more often than they are today to reduce drying costs and even enhance product quality. Many more examples can be found in Mujumdar (1995).

Figure 15 A two-stage fluidized bed dryer

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Figure 16 A commercial two-stage dryer for crystallization/ drying of polyester chips 5.2 Flash Dryers
Figure 17 shows a schematic of the simple flash (pneumatic) dryer system. Here, the wet feed is dispersed mechanically into a hot gas stream (commonly air, combustion gases) and conveyed for long enough time to allow drying of the particulates in the size range of 10-500 microns during their transport. Clearly, only surface moisture of small particles can be removed economically in such a system of reasonable length of the insulated conveying tube. Most dryers are thus adiabatic and use a flash tube of circular and uniform cross-section. In some cases, the tube may diverge and converge, may have sudden expansions and contractions. The tube may be heated through the wall to keep up the temperature driving force as the gas loses its energy to the particles in the forms of heat of vaporization and sensible heat. Noncircular cross-section (e.g., rectangular with rounded corners) and tubes of non-rectilinear configurations (e.g., in the form of a ring) are also employed for special applications. For details, see Mujumdar (1995). Flash dryers may be used to dry heat-sensitive solids in view of the short exposure time to the drying medium. They have low capital cost although, in some cases, the ancillary equipment (e.g., disperser, blender – if solids backmixing is needed prior to dispersion, heat exchangers, product collection devices) may cost much more than the basic flash dryer tube itself. There is a risk of fire and explosion so care must be taken to avoid flammability limits in the dryer. The dryer must be designed with suitable rupture disks to minimize damage in the event of an explosion. The dryer has small “foot print” (e.g., small floor area) since the flash tube generally rises vertically so the flow of

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particulates against gravity increases the residence time in the tube of a given length. When it is feasible, it is a good idea to consider a flash dryer. It does cause attrition, however. It can be used as the first stage of a two-stage dryer system to remove only the surface moisture fast and cheaply while a higher residence time dryer (e.g., fluidized bed) may be deployed as the second stage. Removal of the surface moisture also helps fluidize the material well aside from reducing the size of the fluidized bed unit.

Figure 17 Schematic of a simple flash dryer
Design of the feeding system is crucial in flash dryer design. For free-flowing powdery solids, a screw feeder or a rotary valve may be used effectively. Pasty or sticky materials need to be pre-conditioned by blending them with dried product using a single or twin-shaft paddle blender and then dispersed mechanically using a kicker mill or one of several other designs of rotating disperser. The product may be collected in cyclones, baghouses and the very fine material removed prior to exhaust in wet scrubbers. Flash dryers utilizing superheated steam as the drying medium have some unique quality and energy advantages over air drying systems. More recently, flash dryers consisting of inert media have been employed at pilot scales to dry slurries and suspensions, which are sprayed onto them. The particles are coated thinly by the slurry and dried rapidly as a thin film. Attrition due to inter-particle collisions and shrinkageinduced breakage of the dried film allows entrainment of the dry powder into the drying

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gas for collection in a cyclone or baghouse. This process is yet to be commercialized, however.

5.3 Spin-Flash Dryers
This dryer is basically a mechanically agitated fluidized bed designed for very short residence times so it is suited for removal of only the surface moisture. It is suited for drying sludges, pulps, pastes, filter cakes, high viscosity liquids, without the use of an atomizer. As shown in Figure 18, a rotor placed at the bottom of the chamber serves to disperse the feed, which falls by gravity onto it. Hot drying air enters the chamber tangentially and spirals upwards carrying and drying the dispersed particles. The exhaust containing the dried powder is cleaned and the powder recovered. Heavier wet particles remain within the chamber for a longer time and are broken up by the rotor – only the dried fine powder escapes to the gas cleaning system. This type of dryer can be a replacement for the more expensive spray dryer (which needs more thermal energy because the feed is wetter due to the pumpability requirements and also expensive because of the need for an atomizer). Such dryers are recommended for some special applications only although numerous materials have been dried successfully in such units at capacities up to 10 tons per h. They are more expensive than the conventional flash or fluidized bed dryers. Care must be taken to ensure in pilot tests that there is no danger of product accumulation on the walls due to stickiness.

Figure 18 A spin-flash dryer 5.4 Roto-Louvre Dryer
This dryer type is a modification of the conventional rotary dryer, in which the drying gas contacts the wet particles rather inefficiently as the particles shower down from the flights and get exposed to the axial cross-flow of the gas. In a Roto-Louvre design, (Figure 19) the slowly rotating (2-3 rpm) horizontal drum is fitted with longitudinal louvres which make a tapered drum within the external drum. Diameters up to 3.5 m and lengths up to 12 m have been built commercially. The particles form a

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gently rolling fluidized bed at the bottom of the inner drum as the drying gas is introduced. The resulting heat and mass transfer rates are much greater than those achieved in a conventional rotary dryer. This may reduce the size of the dryer by up to 50 percent. However, the added complexity of the equipment increases the initial cost. Product handling is gentler and hence results in less attrition.

Figure 19 A Roto-Lourve rotary dryer 5.5 Tunnel Dryers
In this simple dryer concept, cabinets, trucks or trolleys containing the material to be dried are transported at an appropriate speed through a long insulated chamber (or tunnel) while hot drying gas is made to flow in concurrent, countercurrent, cross-flow or mixed flow fashion (Figure 20). In the concurrent mode, the hottest and driest air meets the wetted material and hence results in high initial drying rates but with relatively low product temperature (wet-bulb temperature if surface moisture is present). Higher gas temperatures can be used in concurrent arrangements while in counter-current dryers the inlet drying gas must be at a lower temperature if the product is heat-sensitive. If the material to be dried is not heat-sensitive and low residual moisture content is a requirement, one may employ higher gas temperatures in the countercurrent arrangement as well. Combination flow or cross-flow arrangements are used less commonly. The latter offer high drying rates but the tunnels must be designed to fit the trolleys snugly so the drying gas flows through the material much like a through-circulation packed bed dryer. Total drying times that can be handled range from 30 minutes to 6 hours.

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Figure 20 A tunnel dryer 5.6 Band Dryers
For relatively free-flowing granules and extrudates that may undergo mechanical damage if they are dispersed, band dryers are a good option. It is essentially a conveyor dryer wherein the band is a perforated band over which the bed of drying solids rests. Drying air at rather low velocities flows upwards through the band to accomplish drying. Clearly, this type of dryer is not a good choice for very wet or very fine solids. If the bed depth is large (over 10-15 cm) there may be a significant moisture profile in the bed with the solids resting on the band over dried and overheated. One option to alleviate this problem is to reverse the gas flow direction alternately over the length of the dryer. This evens out the moisture profile while increasing the drying rate as well. Another option is to cause mixing of the bed at appropriate interval of space. In some commercial designs, so-called multi-pass dryers, several bands are stacked one above the other and the material is made to drop under gravity from the higher to the next lower band which causes some random mixing of the material before it undergoes further throughcirculation drying. It is possible to use a temperature profile along the length of the conveyor so that the drier product can be exposed to lower gas temperatures if that is desired. Also, the final section may be a simple cooler so the product is ready for packaging or storage. Residence times from 10 minutes to 60 minutes are economically feasible. These dryers are quite versatile and can handle relatively large and arbitraryshaped particles that may be heat-sensitive and fragile at the same time. Gas cleaning requirements are minimal as low gas velocities are used. Also, power requirements for air handling are low due to the low pressure drops needed. In commercial designs of very large band dryers, it is important to ensure uniform distribution of the product on the band and also uniform distribution of the air flow within the chamber of the dryer to ensure uniform product moisture content. A schematic of a single-pass band dryer is shown in Figure 21.

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Figure 21 A single-pass band dryer 5.7 Infrared Dryers
Infrared (IR) dryers may be gas-fired ceramic radiators or electrically heated panels. The IR wavelength range is from 0.1 µm to 100 µm, which generates heat in the exposed physical body. The wavelength ranges 0.75 - 3.0 µm; 3.0 - 25 µm and 25 - 100 µm are referred to as near IR, middle IR and far IR ranges, respectively. Industrial radiators are of two types: (1) Light radiators (near IR), e.g., quartz glass with peak radiation intensity at 1.2 µm and (2) Dark radiators, e.g., ceramic (3.1 µm) or metal radiators (2.7 - 4.3 µm). While convection can yield heat fluxes of the order of 1-2 kW m-2, radiation can yield much higher levels of heat flux, i.e., 4-12 kW m-2 (light radiators) or 4-25 kW m-2 (dark radiators). In many drying operations, the evaporation rates feasible are not high enough to require IR radiators, however. There are some niche applications for IR dryers in some certain industries, e.g., drying of coated paper, booster drying of paper in paper machines. They offer the advantages of compactness, simplicity, ease of local control and low equipment costs. Also, in combination with convection, IR dryers offer the potential for significant energy savings and enhancement in drying rates with better product quality. On the negative side, the high heat flux may scorch product and enhance fire and explosion hazards. Clearly, IR must be used in conjunction with convection or vacuum. Good control is essential for the safe operation, i.e., IR power source must be cut off if there is upset in the process which may lead to overheating of the product.

5.8 Microwave (MW) and Radio Frequency (RF) Drying
Unlike conduction, convection or radiation, dielectric heating heats a material containing a polar compound volumetrically, i.e., thermal energy supplied at the surface does not have to be conducted into the interior, as limited by Fourier's law of heat conduction. This type of heating provides the following advantages: • Enhanced diffusion of heat and mass • Development of internal pressure gradients which enhance drying rates • Increased drying rates without increasing surface temperatures • Better product quality

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When an alternating electromagnetic field is applied to a “lossy” dielectric material, heat is generated due to friction of the excited molecules with asymmetric charges, e.g., water. This is a result of ionic conduction or dipole oscillations (Strumillo and Kudra, 1986). The radio frequency range extends from 1-300 MHz while the microwave range is from 300 to 3000 MHz. However, only specific frequency ranges are permitted for industrial heating applications, i.e., ranges 13.56, 27.12 and 40 MHz for RF and 915 (896 in Europe) and 2450 MHz for MW. Bound and free waters have different loss factor. Since loss factors increase with temperature there is a danger of runaway, i.e., an accelerated heating rate causing a thermal damage to the material. Table 6 summarizes the basic characteristics of MW and RF techniques. The main limitation of MW and RF drying is that the technique is highly capital-intensive. It also consumes high-grade energy, i.e., electricity, and the conversion efficiency to dielectric field is only in the order of 50%. Thus, these techniques are suited only for special applications involving very high value products, extremely long drying time to remove traces of moisture or to obtain products of special characteristics not obtained otherwise. It is therefore not surprising that MW/RF drying is used only in special niche applications. Further, these techniques are used mainly to boost drying capacity (to remove free water rapidly without generation of large thermal gradients in the material) or to remove the last few percent of water which comes out very slowly. Generally, dielectric heating is combined with convection or vacuum to reduce the energy consumption. Microwave vacuum drying and microwave freeze drying are among the commercial drying technologies that have so far found some applications. Microwave freeze drying is typically carried out at temperatures well below the triple point of water. Typical conditions are: pressure in the range of 500 Pa and temperature of –40o C. Use of excessive power as well as maldistribution of power due to nonhomogeneities in the frozen solids can cause problem in MW drying. The main hurdle to commercialization of MW freeze drying is the high cost. Numerous laboratory and pilot scale studies have been reported on MW drying at atmospheric as well as vacuum conditions. It is also possible to “pipe” microwave energy in various dryer configurations, e.g., fluidized bed, spouted bed, vibrated bed or tray dryers, to enhance convective drying rates. Unfortunately, while all these techniques do provide significant enhancement of the drying time required, the initial and operating costs are such that the enhancement obtained does not offset the added cost. Drying of treated grapes in combined microwave and convection dryer has been shown to be very rapid and energy-efficient. However, the costs ate prohibitively high.

CLOSING REMARKS
The reader probably now has a sense of the bewildering array of dryer designs that one could possibly use for a given application. As noted in Chapter 2, selection of dryers and drying systems is not a task to be taken lightly since it can lead to major costs and even catastrophic failures. Since the dryer will typically last 25-40 years in operation, the incremental cost (in terms of capital costs, operating costs, loss of productivity, loss of product quality, etc.) persists over a long period. One must devote the necessary time,

DRYERS FOR PARTICULATE SOLIDS, SLURRIES… 61 __________________________________________

effort and even budget to the selection phase to avoid paying for it over the lifetime of the dryer. Retrofitting an existing malfunctioning dryer can be expensive with long payback times. It is strongly suggested that the newly emerging technologies of drying should be evaluated closely before selecting the time-honored drying schemes. Finally, the user is encourages to make the preliminary selection of one or more possible systems before enlisting the assistance of a vendors who typically specialize is a narrow range of drying equipment for obvious reasons.

REFERENCES
Filkova, I., Mujumdar, A.S, 1995, Industrial Spray Drying Systems, pp. 263-307, in A.S. Mujumdar (Ed.) Handbook of Industrial Drying, 2nd Edition, Marcel Dekker, New York. Strumillo, C., Kudra, T., 1986, Drying: Principles, Applications and Design, Gordon and Breach, New York. Liapis, A.I., Bruttini, R., 1995, Freeze Drying, pp. 309-343, in A.S. Mujumdar (Ed.) Handbook of Industrial Drying, 2nd Edition, Marcel Dekker, New York. Masters, K., 1991, Spray Drying Handbook, Longman Scientific & Technical, Burnt Mill, Essex. Moore, J.G., 1995, Drum Dryers, pp. 249-262, in A.S. Mujumdar (Ed.) Handbook of Industrial Drying, 2nd Edition, Marcel Dekker, New York. Mujumdar, A.S. (Ed.), 1995, Handbook of Industrial Drying, 2nd Edition, Marcel Dekker, New York.

CHAPTER

FOUR

INNOVATION IN DRYING TECHNIQUES AND FUTURE TRENDS
Arun S. Mujumdar

1. INTRODUCTION
As an operation of pre-historic origin, one would normally not associate drying with innovation. Since whatever products need to be dried currently are being dried with existing technology (some literally centuries old) it is often hard to justify the need for innovation and concomitant need for R&D in drying and dewatering to the layman. This is reflected in the relatively low level of R&D resources drying is able to attract as opposed to some of the exotic bio-separation processes, which on an economic scale may be an order-of-magnitude less significant. It is interesting to note, however, that some 250 patents – the titles of which contain words dryer, drier or drying in them – are issued by the US Patent Office each year. Only about ten percent or less of this number is being issued in some of the other unit operations, e.g., membrane separations, crystallization, adsorption, distillation. There appears to exist a negative correlation between a current level of industrial interest and academic research activity, at least as measured by the number of publications in the archival literature. It is instructive to start our discussion here with a definition of innovation, types of innovation and then identifying the need for innovation in drying as well as the features common to some of the novel drying technologies. Finally, we close by briefly mentioning some of the novel technologies developed in the past decade or two. At the outset, it is important to recognize that novelty per se is not an adequate justification for embracing new

64 INNOVATION IN DRYING TECHNIQUES ___________________________________________

technology; it must be technically feasible and cost-effective compared to the current technology.

2. INNOVATION: TYPES AND COMMON FEATURES
It is interesting to begin with Webster's Dictionary’s meaning of innovation, which is innovation, n.: • • The introduction of something new A new idea, method, or device

Notice that it does not use adjectives like better, superior, improved, more costeffective, higher quality, etc., to qualify an innovation. In our vocabulary, however, we are not interested in innovation for the sake of novelty or even originality of concept but for the sake of some other positive economic attributes. I prefer instead the following definition given by Howard and Guile (1992): “A process that begins with an invention, precedes with development of the invention, and results in the introduction of new product, process or service in the marketplace” To make it into a free marketplace, the innovation must be cost-effective. What are the motivating factors for innovation? For drying technologies, I offer the following list; one or more of the following attributes may call for an innovative replacement of existing products, operation or process. • • • • • • • New product or process not made or invented heretofore Higher capacities than current technology permits Better quality and quality control than currently feasible Reduced environmental impact Safer operation Better efficiency (resulting in lower cost) Lower cost (overall)

Innovation is crucial for their very survival of industries with short time scales (or life cycles) of products/processes, i.e., a short half-life (less than one year, as in the case of some electronic and computer products). For longer half-lives (e.g., 10-20 years – typical of drying technologies) innovations come slowly and less readily accepted. The management of innovation depends on the “stage” it is at. Thus, • • Initially, value comes from rapid commercialization Later, value comes from enhancing product, process or service

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•

At maturity, value may come from discontinuing and embracing newer technology.

Note that management must be agreeable to discontinuing a currently viable technology in the interest of future of the company if the technology has reached its asymptotic limit of performance. This principle applies to all technologies. Numerous studies have appeared in the literature on the fundamental aspects of the process of innovation. One of the models of the innovation process assumes a linear progress from (a) discovery of laws of nature to (b) invention to (c) development of a marketable product or process in this order. It is well known, however, that some of the truly remarkable revolutionary technologies evolved well before the fundamental physics or chemistry responsible for their success was worked out. I believe that true innovation is nonlinear – even chaotic – trial-and-error, serendipitous process. Therefore, it is difficult to teach innovation in a logical sense although one could presumably encourage creativity or try to remove blockages in the process of creativity. What may be classified as innovations can represent different characteristics. Following is a list of the quality parameters of innovations in general (Howard and Guile, 1992): • • • • Innovation establishes an entirely new product category Innovation is the first of its type in a product category already in existence Innovation represents significant improvement in existing technology Innovation is a modest improvement in existing product/process

Innovations trigger technological changes, which may be revolutionary or evolutionary. From our experience, we know that the latter are more common. They are often based on adaptive designs, have shorter gestation periods, shorter times for market acceptance and are typically a result of “market-pull” – something the marketplace demands, i.e., a need exists currently for the product or process. These usually result from a linear model of the innovation process (an intelligent modification of the dominant design is an example). Revolutionary innovations, on the other hand, are few and far between, have longer gestation periods, may have larger market resistance and are often a result of “technology-push”, where development of a new technology elsewhere prompts design of a new product or process for which market demand may have to be created. They are riskier and often require larger R&D expenditures as well as sustained marketing efforts. The time from concept to market can be very long for some new technologies. It is well known that the concept of a helicopter appeared some 500 years before the first helicopter took to the air. The idea of using superheated steam as the drying medium was well publicized over one hundred years ago, yet its real commercial potential was first realized only about fifty years ago and that too not fully. In fact it is not fully understood even today! Most recent example of this long gestation period is the Condebelt drying process for high basis weight (thick grades) paperboard proposed and developed by late Dr. Jukka Lehtinen for Valmet Oy of Finland. It took a full twenty years of patient and high quality R&D before the process was first deployed successfully.

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The vision required by the management teams of such organizations must be truly farsighted! It is natural to inquire if it is possible to predict or even estimate the best time when the marketplace requires an innovative technology or the mature technology of the day is ripe for replacement. Foster's well-known “S” curve (Figure 1; Foster, 1986), which gives a sigmoid relationship between product or process performance indicators and resources devoted to develop the corresponding technology, is a valuable tool for such tasks. When the technology matures (or is “saturated” in some sense), no amount of further infusion of R&D resources can enhance the performance level of that technology. When this happens (or even sooner), time is right to look for alternate technologies which should not be incremental improvements on the dominant design but truly new concepts, which once developed to their full potential, will yield a performance level well above that of the current one. As proven by Foster with the help of real world examples, the performance versus effort (resources) curve occurs in pairs when one technology is replaced by another. They represent discontinuity when one technology replaces another and industry moves from one S-curve to the next.

Resources

Performance

Figure 1 Foster's S-curve
Table 1 lists examples of some new drying technologies that were developed via technology-push versus market-pull. In some cases, a sharp distribution or grouping in just two types is not possible since a “market-pulled” development may require a “technology-push” to succeed.

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Table 1 Examples of new drying technologies developed through technology-push and market-pull
Technology-push* Microwave/RF/induction/ultrasonic drying Market-pull** Superheated steam dryers – enhanced energy efficiency, better quality product, reduced environmental impact, safety, etc. Impulse drying/Condebelt drying of paper (also need technology-push)

Pulse combustion drying – PC developed for propulsion and later for combustion applications Vibrating bed dryers – originally deveCombined spray-fluid bed dryers – to loped for solids conveying improve economics of spray drying Impinging streams (opposing jets) – origi- Intermittent drying – enhance efficiency nally developed for mixing, combustion applications *Technology originally developed for other applications applied to drying **Developed to meet current or further market demand

3. SOME EXAMPLES OF INNOVATIVE DRYING TECHNOLOGIES
Since one must think in terms of drying systems (including pre-drying operations, such as dewatering), we will include in our listing some innovative dewatering technologies as well. For lack of space, it is impossible to include schematics of all the dryers mentioned in this chapter. Details are available in the chapter on Special Drying Technologies and Novel Dryers by Kudra and Mujumdar (2000). Only the innovative aspects will be mentioned here. It should be noted that the focus here is on innovation and not on dryers per se.

3.1 Mechanical Dewatering
To reduce the thermal load on dryers, it is important to minimize the water content of the wet feed material. Conventionally, this is done using vacuum or pressure filters, decanters, centrifuges, etc. With colloidal materials, e.g., waste streams from tertiary treatment of paper mills, food processing wastes, tailings from coal mines or oil sands, dewatering is difficult due to the small particle sizes (< 5 m) involved. In recent years, the following novel processes have evolved successfully, partly as a result of technologypush and partly as a result of market-pull. • Electro-osmotic dewatering (EOD) – application of a DC field to a bed of colloidal suspension

68 INNOVATION IN DRYING TECHNIQUES ___________________________________________

• • • •

Interrupted electro-osmotic dewatering – periodic interruption of power by shorting the electrodes. This process is more effective than the continuous one for fundamental reasons Combination of vacuum filtration with EOD – both steady and intermittent applications Combined field dewatering – EOD coupled with ultrasonic field Vibration-assisted micro-filtration – superior to cross-flow filtration

While some of the above innovative concepts have been commercialized successfully, there is still a potential to improve and exploit them further. Some of these processes could be coupled with a batch drying operation similar to that of a conventional Nutsch filter or combined filter-dryer. Filter-dryers are batch units that avoid transfer of the contents from one unit to another and thus avoid potential of contamination – a particularly attractive feature for the pharmaceutical industry. Novel dewatering techniques could be coupled with drying giving a synergistic benefit overall.

3.2 Fluidized Bed Dryers (FBDs)
FBDs have become very popular over the past three decades owing to their numerous favorable features for drying of particulates that can be fluidized. Table 2 of Chapter 5 summarizes the enormous number of possible variants of FBDs that are now used to dry not only particulates (which was the original idea) but also slurries, pastes, continuous webs and sheet-form materials. Large pieces that cannot be fluidized by themselves can be immersed in a fluidized bed of smaller fluidizable inert particles and dried. Most of the variants shown are used in industrial drying applications to varying extent. Many users seem to be unaware of some of these innovative modifications of the FBDs. Interestingly, by fluidizing only parts of the particulate bed at a time, it is possible to effect a major saving in energy costs, e.g., so-called pulsed fluid beds (Gawrzynski and Glaser, 1996). In batch fluidized bed drying, a control strategy that keeps the bed temperature constant by adjusting heat input saves energy (and time) while enhancing the quality of heat-sensitive products (Devahastin and Mujumdar, 1999). Such a dryer based on a fuzzy logic control is already on the market.

3.3 Spouted Bed Dryers (SBDs)
These are essentially modified fluidized beds of larger particles belonging to Geldart's D group (e.g., grains, beans) that have a characteristic internal recirculatory motion and a spout (or fountain) at the top free surface. The particle motion is regular rather than chaotic (or random) as in the fluidized bed. Table 7 of Chapter 5 shows some of the variants of the SBDs, which could be used to dry not only larger particulates but also slurries and pastes. Using internal draft tubes, two-dimensional design and/or a mechanical spouting action, it is possible to eliminate or alleviate several of the weaknesses of the conventional axisymmetric SBDs. These are very simple devices

INNOVATION IN DRYING TECHNIQUES 69 __________________________________________

which have not been exploited fully yet. It is noteworthy that for particles with primarily internal resistance to heat and mass transfer (e.g., grains) use of intermittent heating of the spouting air as well as intermittent spouting can lead to a substantial saving in energy costs with possible improvement in quality. This is achieved in the rotating jet spouted bed (Jumah et al., 1996).

3.4 Impinging Jet Dryers (IJDs)
Impinging jets (IJs) provide the best configuration for convective heat/mass transfer to a surface. For optimal design, it is important to choose the right geometry for the nozzles as well as the right operating conditions. IJs are used in paper, photographic film, textile, coatings, veneer, etc., industries extensively; sometimes in conjunction with infrared heat sources between modules of IJs. In some cases (e.g., textiles, double-sided coated papers, pulp sheets), the web may be supported by jets impinging on both sides of the web for contactless drying. IJs can be used also to dry particles or chips by pseudofluidizing a bed supported on a conveyor belt. In order to improve the drying rates even further, it is important to find ways of enhancing the conventional impinging jet heat transfer rates. One way to do this is to attach a collar that causes oscillations and vortex shedding in the jet exit flow to a tubular nozzle. The so-called SOJIN (Self-Oscillating Jet Impinging Nozzle) has been shown to enhance the heat transfer rate significantly (Chinnock and Page, 1994). No application in drying has been reported so far although the concept is a truly innovative result of technology-push. Use of a gas-particle impinging jet has also been found to yield significant enhancement in impingement heat transfer.

3.5 Drying of Paper
Table 2 summarizes the various drying technologies for paper, both conventional and alternates are being considered (de Beer et al., 1998). While a need exists for an improved drying technology for paper to replace the century-old multi-cylinder dryers, none is on the horizon yet. The Condebelt dryer developed by Tampella-Valmet, Finland for linerboard has already been successfully commercialized in Finland and South Korea. The superheated steam dryer concept for paper first proposed and initially demonstrated by the author in 1981 has yet to be validated at mill scale. Paper dried in superheated steam using impinging jets and/or through drying has been shown to yield better strength properties especially when the pulp is mechanical pulp, i.e., has low lignin content. Mechanical pulp is so-called high-yield pulp since one can obtain a higher yield of mechanical pulp per ton of wood used. Chemical pulp is low yield and highly polluting, i.e., environmentally unfriendly. Thus, by using steam drying one can use less amount of chemical pulp and yet produce newsprint of good mechanical strength. Therefore, steam drying of paper saves energy as well as resources. However, the dryer becomes mechanically complex to design and operate.

Table 2 Comparison of several paper drying technologies
Measure Possible saving on Fuel Dry sheet 100% Electricity Increase because of higher pressing demands; decrease because of less pumping power and elimination of drying section Increase because of higher pressing demands and heating press (IR, induction); decrease because of shorter drying section. Probably net increase Increase because of higher pressure demands; decrease because of shorter drying section and less refining Increase because of pressing demands and extra vacuum suction boxes; decrease because of elimination of conventional drying section Increase because of power for compressor; decrease because of elimination of drying section Increase due to power for compressor; decrease due to smaller drying section

Number and type of groups working on development If any, paper manufacturers

State of the art

Bottlenecks for further development Fundamental; no technology exists for broadening field of application Fundamental/ technical: sheet delamination, paper quality

Motivation for development High-speed machinery, energy conservation,

Expected costs of new paper mill Low: elimination of drying section

Commercial for bulky grades of paper; no research for other grades Several pilot plants; experiments with IR and induction heating; speed approx. 240-400 m/min. Large number of pilot scale experiments Pilot plant is being built with a length of 18 m and a web width of 0.7 m Not tested yet for paper drying Basic R&D; lab/pilot scale tests

Impulse drying

50-75%, depending on the in- and out-going consistency and the type of heating

Machinery manufacturers and large research institutes Machinery manufacturers and large research institutes One machinery manufacturer

Paper quality improvement, highspeed machinery, energy conservation, size of machinery Paper quality improvement, energy conservation, shorter machine possible High-speed operation, paper quality improvement, energy conservation, size of machinery Energy conservation, cost reduction Quality, energy conservation

Medium: reduction of drying section. New equipment

Press drying

50-75%, depending on the in- and out-going consistency and the type of heating Approx. 20%

Fundamental: delamination, operational speed is limited Standard: increasing machine speed

Medium: reduction of drying section. New equipment Medium: reduction of drying section. New equipment

Condensing belt drying

Airless drying

Almost 100%

One small research institute (one to three persons) McGill Univ., VTT (Finland)

Superheated steam drying (impinging jets, through drying)

60-75%

Technical: air tightless with continuous drying Air infiltration at high machine speeds

Medium: reduction of drying section. New equipment Medium: reduction of drying section. New equipment

Because of the enormously capital-intensive nature of the papermachine it is difficult to introduce a totally new drying technology in a large scale. Most likely, the initial mill-scale testing will take place in smaller machines producing specialty papers and not commodities like newsprint, or tissue, where the potential benefits of successful deployment are enormous. The dilemma in introducing innovative technologies is that no one wants to be the first in the field due to the higher risk levels involved.

3.6 Rotary Dryer
Rotary dryers have been the workhorses of many industries that produce high tonnage products. They are generally capital-intensive, less efficient but very flexible. Use of steam tubes immersed within the rotating shell makes the cascading rotary dryer thermally more efficient. However, there has not been much true innovation in this technology for some time. Recently, Yamato Sankyo Mfg. Co. of Tokyo, Japan has patented a simpler design of the rotary dryer wherein the drying air is injected into the bed of material being carried in a rotating cylindrical shell through a multiplicity of pipes branching off from a central pipe. The heat and mass transfer rates are almost doubled with all the resulting advantages of smaller size, simplicity and lower cost (Yamato, 1996). Such a dryer is not suited for all types of materials normally handled by a cascading rotary dryer, however. When feasible, the Yamato design can reduce dryer volume by a factor of two for similar operating conditions. This is a major advantage of this truly innovative idea in rotary drying.

3.7 Impinging Streams Dryers (ISDs)
The impingement zone created by the head-on collision of two confined turbulent streams of gas or gas-particle is particularly favorable for high heat/mass transfer rates. It also is a zone wherein de-agglomeration, atomization or dispersion of particles can occur. Larger particles have a longer residence time in the confined opposing jet flow field due to their higher inertia. Impinging streams are thus ideal for flash drying of particulates, pastes or slurries. Several stages of impingement zones can be generated to reach the desired final moisture content. Kudra and Mujmdar (1995) have classified the wide assortment of ISDs although only a few have been studied so far. Most recently, Hosseinalipour and Mujumdar (1997) examined, via computational fluid dynamic modeling and Monte Carlo simulations, a novel two-dimensional ISD using superheated steam as the carrier medium. Effects of the degree of superheat, operating pressure and jets Reynolds number were examined numerically assuming a power law model for the falling rate drying kinetics. New criteria are formulated to characterize performance of dispersion dryers. Experimental validation is required although the computed results do appear physically plausible. The model predicts the number distribution function for particle moisture content and residence time. Particle Biot numbers were assumed to be small and the maximum number of particles tracked in the Eulerian-Lagrangian simulation was limited to 2000 owing to the enormous computer time required.

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While much of the laboratory and pilot scale work in this area was done in the former USSR, no commercial ISD suppliers exist in the rest of the world. ISDs have strong potential to replace conventional flash dryers in some applications once the problems of scale-up are resolved. See Tamir (1992) for additional information on ISDs.

3.8 Remaflam Drying of Textiles
This by far is the most exotic and innovative drying process. By mixing the fuel with the liquid (water) to be evaporated and combusting it in a controlled manner, where the energy is needed, the drying is both efficient and rapid. The dryer is effectively a combustion chamber (at 600o C) wherein the fabric residence time is just equal to the time required for complete drying. A 34% methanol solution in water gives an ideal fuelwater mixture to accomplish combustion to meet the drying needs. It is unfortunate that this idea is not generally applicable to many other products. Details are available in von der Eltz and Schon (1984). Use of alcohol as the fuel has the additional advantage of environmentally friendly, pollution-free combustion. Among its other advantages noted by the manufacturer are: • Smaller space (~ 1 m long) • Easily cleaned and maintained • No energy wastage due to over drying • Energy consumption unaffected by width of fabric • Fully automatic control, safe operation Among the limitations are cost variability of methanol, extra care needed in transportation, handling, and storage of methanol, non-suitability for drying knitted goods, limitation of fabric speed and possible “singing” or melting of edges. The idea of producing heat exactly where it is needed to evaporate the moisture does not have wide applicability, however.

3.9 Spray Drying
Today, over 20,000 spray dryers are in operation around the world. It is often considered a mature technology. However, there is still ample scope for improvement, particularly in the following: • Higher production rates (using multi-stage designs) • More uniform (ideally monodisperse) particle size distribution – determined by atomizer design • Containment of powder – preferably within dryer chamber • Reduction or elimination of deposits on walls which lead to fire hazard, large down time and high maintenance costs • Better designs using modern CFD (Computational Fluid Dynamics) Table 3 compares the performance of a 1600 kg/h capacity spray dryer for production of emulsion PVC using rotating disk, conventional two-fluid nozzles and

newer two-fluid sonic nozzles (Shah and Arora, 1996). It is easy to see the advantages of the sonic nozzle as it yields a smaller drying chamber, lower power consumption for atomization and also a better monodisperse powder. More recently, ultrasonic nozzles are being tested for high value, low production rate product applications (e.g., pharmaceutical, biotech products) using a spray chamber at low pressures. For high production rates two-stage or three-stage spray dryers are most cost-effective. The final stage (fluid bed or vibrated bed) can also granulate or agglomerate the product for easy handling, better rehydration characteristics, etc.

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Table 3 Spray drying of emulsion-PVC. Effect of selection of atomizer on spray dryer performance: A Comparison between different atomizers
Parameter Dryer geometry Evaporation capacity (water) Chamber (D × H) Number of nozzles Power for atomizer Capital cost Operating cost Rotary disk Conical/cylindrical H/D ≈ 1.2-1.5 1600 kg/h 6.5 m × 8 m 1, 175-mm disk 15,000 rpm 25 W/kg slurry High Medium Two-fluid (sonic) Tall-form Cylindrical H/D ≈ 4 1600 kg/h 3.5 m × 15 m 16 nozzles 4 bar pressure 20 W/kg slurry Medium Low Two-fluid (standard) Tall-form Cylindrical H/D ≈ 5 1600 kg/h 3 m × 18 m 18 nozzles 4 bar pressure 80 W/kg slurry Medium High

To reduce the footprint of the drying system and fully contain the powder within the spray chamber, Niro A/S have introduced integrated particulate filters within the drying chamber near the roof of the dryer so that external cyclones are not needed (Masters, 1999). Figure 2 shows a spray dryer with a second stage dryer and a filter assembly at the top. Figure 3 compares the plant layouts for the conventional system versus the new integrated filter system. Design of the new atomizers which consume less power (e.g., low rpm disk atomizers) or produce a more uniform spray (e.g., sonic or ultrasonic nozzles) are central to advances in spray drying technology. Operation under vacuum or using superheated steam may also find niche applications as a result of some unique properties such processing may impart to the powders produced. Finally, multistage operations involving spray drying as the first stage to remove the surface moisture and hence the surface stickiness of the particles as well as to “engineer” the product size and geometry, followed by less-expensive drying technologies such as fluid bed, vibrated bed, through circulation conveyor drying, will no doubt become increasingly common in the future.

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3.10 Batch Dryers

Batch dryers are commonly used to dry small quantities or when the drying times are very long (of the order of several hours, days or even months as in the case of some certain wood drying applications). They are common in the pharmaceutical industry for the former reason while they are used in the wood industry and for freeze drying of ultraheat-sensitive products in the pharmaceutical and biotechnological industries for the latter reason. Some dryers can operate in both batch and continuous modes, e.g., fluid beds. On the other hand, most continuous dryers cannot operate in the batch mode, e.g., spray, rotary, flash dryers. There is a limited choice of dryers for batch drying.

Figure 2 Spray drying chamber with integrated fluid bed and particulate filters

Figure 3 Plant layouts. (A) Conventional layout (B) Powder containment layout

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Among some of the recent developments in batch drying one may cite the following: • Combined filtration and drying in a single unit, e.g., Nutsch dryers used in the pharmaceutical industry to minimize chances of contamination during transfer of a wet cake from a filter to a dryer. • Use of intermittent and/or time-dependent drying, e.g., varying the drying air temperature or velocity with time to match the requirements of the instantaneous drying kinetics. The drying air temperature can be varied from a higher initial value in the constant rate drying regime when the surface moisture is being removed to a lower value when the critical moisture is reached. For heat-sensitive materials reducing the heat input as the moisture content decreases ensures that the product temperature will not exceed a prespecified permissible value during drying. Numerous studies have been reported in the literature on intermittent drying using variable air temperature, air velocity, pressure of drying chamber as well as intermittent supply of other forms of energy, e.g., microwave or infrared radiation. The possibilities are immense and limited only by the imagination of the designer.

CLOSING REMARKS
A summary is provided on a selection of novel drying processes as well as trends in drying technologies. No pretense is made that the list is all-inclusive; it is only illustrative. Some of the common features of innovations are identified. There is need for further R&D and evaluation of new concepts. A collaborative interaction between academia and industry is most likely to lead to improved drying and dewatering technologies of the future. Most innovations are likely to be evolutionary and driven by market-pull. There will be fewer revolutionary innovations driven by technology-push because the market penetrability of such innovations is difficult and unpredictable. Since many of the conventional drying technologies have reached or are close to maturity further influx of R&D is unlikely to yield major dividends in several of these areas. R&D investment is needed in the development of new technologies and understanding of the existing ones so that they may be scaled-up and optimized confidently. While no mention was made here of the role of basic research to enhance the fundamental understanding of drying and dryers it is indeed a key factor in identifying and testing new drying concepts and dryer designs. If mathematical models of drying could be developed that considered not only the transport phenomena but also product quality predictions they could be a valuable engineering design tool for development of novel dryers. In the meantime, it is necessary to test and validate new concepts of drying in the laboratory and if successful then on a pilot scale. The willingness of industry to accept a certain amount of risk will be central to the development of new drying technologies in the coming decade.

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REFERENCES
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Tamir, A., 1992, Impinging Streams and Their Applications in Drying, pp. 209-223, in A.S. Mujumdar (Ed.) Drying' 92, Vol. 1, Elsevier, Amsterdam. von der Eltz, H-U., Schon, F., 1984, The Remaflam Process, pp. 350-364, in A.S. Mujumdar (Ed.) Drying' 84, Hemisphere, Washington. Yamato, Y., 1996, A Novel Rotary Dryer with Through Air Combination, pp. 624-630, in C. Strumillo, Z. Pakowski, A.S. Mujumdar (Eds.) Drying’96: Proceedings of the Tenth International Drying Symposium, Lodz, Poland.

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