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Designing the CDU/VDU or  opportunity crudes New units for processing heavy crudes should not be designed using conventional practices or run length will be short, and product yields and quality poor  Scott Golden, Tony Barletta and Steve White Process Consulting Services

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pportunity crudes cost less than other encountered when processing extra-heavy opporoppor crudes because they are more difcult to tunity crudes include poor desalting, which often rene into gasoline and middle distillate. results in severe atmospheric crude overhead  Yet there is a perception that protability will system corrosion, including exchangers, piping improve simply by substituting them for more and vessels. These crudes are viscous, contain expensive feedstocks. At times, certain crudes solids, can have amines, can precipitate sell at huge discounts. This may be when producproduc - asphaltenes when blended with other crudes, and tion exceeds the industry’s ability to process have a high fouling tendency in heat exchangers them, whether it is for short periods that are (see Figure 1), column internals and red heaters. caused by unplanned shutdowns or longer peri - Some have a very high naphthenic acid content, ods when regional logistics arise such as causing desalter problems and corrosion requirrequir distribution constraints for Canadian bitumen. ing upgraded metallurgy. Other problems include Reners tend to focus on conversion units such heater coking and asphaltene precipitation inside as the coker or hydrocracker, and the crude and heater tubes, which leads to short run lengths.  vacuum unit (CDU/VDU) is often an after-  When processing processing some Northern Northern Alberta Alberta bitubituthought. As more heavy and extra-heavy crudes mens, the vacuum heater outlet temperature must enter the market, there will be more opportunity for reners to run them. The question is  whether it will be protable or not. Will lessons learned be incorporated into unit designs or will reners learn only  through the school of hard knocks? CDU/VDUs must be designed or revamped for opportunity crudes or there will  be unscheduled shutdowns, poor product yield and quality, and maintenance costs much higher than anticipated. Processing opportunity crudes in high blend percentages reliarelia  bly is different different from running running common heavy crudes such as  Arab Heavy. Heavy. Common problems problems Figure 1 Fouled exchanger — asphaltene precipitation

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 be operated as low as 705°F (374°C) to avoid Rening and upgrader capacity has to match short run lengths even with well-designed double- production. Currently, most Venezuelan crude is red tube designs. Asphaltene precipitation in the processed by US reneries, and most, if not all,  bottom of the atmospheric column can lead to Canadian crudes are upgraded or rened in stripping tray plugging, causing run lengths of  Canada or the US. Far East reners, primarily  less than a year. These are just a few of the obsta- India, are processing some Venezuelan crudes, cles experienced by upgraders and reners  but the blend percentages are not high. Whether processing heavy opportunity crude oils. These Canadian or Venezuelan, these crudes are a are the unfortunate realities, but it is better to mixture of bitumen and lighter blend stocks. design the unit for them than to suffer the conse- PetroZuata and Hamaca consist of coker products quences that many reners continue to experience. and bitumen, whereas Merey is a blend of   As producers of opportunity crudes such as Oronoco bitumen and lighter crudes, albeit there Canadian dilbit continue to ramp up production, may be other names under which these 15 °API reners increasingly are suffering because they  gravity Oronoco bitumen blends are marketed. either ignored the realities of processing these Much of the Canadian crudes are a blend of bitu crudes or were unfamiliar with them. men and condensate (naphtha) or synthetic crude from the upgraders. Conventional Canadian heavy  crude production, such as Bow River, is rapidly  Global opportunity crude supply  Historically, heavy and extra-heavy crude declining, with more crudes containing bitumen production has largely come from North and from Cold Lake, Athabasca or Peace River basins. South America. Consequently, rening experi- Some of these bitumens are much more challengence with these crudes is primarily in Canada, ing than others and reners should not assume the US and South America, and only a few US they are all created equal. They are not! reners process high blend percentages. Some In the mid- to late 1980s, US Gulf Coast renproducing nations’ crude units have very poor ers began processing large volumes of 21-24° reliability and performance, but these realities  API gravity crudes from Mexico and Venezuela. are hidden from the global market. As the global In the mid- to late 1990s, 15-18° API blends distribution of opportunity crudes improves and  became more prevalent and this trend continues. more elds come on stream, the world’s rening  At least two US reners process these crudes system will gain experience. neat through a CDU/VDU. In all cases, the Today, most European reneries process light designer touted a 1050°F (565°C) VGO product and medium crudes, while Middle East reners cutpoint, but not one actually achieved it in spite run their own crudes. They process very little of the rhetoric. Today, few reners process 100% opportunity crudes. Reneries in China are heavy crudes, with most blending them with primarily designed to process local and Middle higher- gravity crudes. Some reners claiming East crudes and have little or no experience with they process heavy crudes have blend gravities of  opportunity crudes at high blend percentages. only 27-30° API gravity. Processing heavy and  While China is planning to build reneries to extra-heavy crudes neat or in high blend percent process them, their approach is to rely on their ages is another story completely. After more than own institutes or global engineering and 25 years of industry experience processing these construction companies that have little experi- crudes, the economic losses associated with poor ence designing CDU/VDUs for opportunity  CDU/VDU performance are still huge. Low VGO crudes. The experiences they do have and have cutpoint is the norm whether reners admit it or applied resulted in very poor reliability and a low  not. In spite of the claims, when one peels back  renery liquid volume yield. Opportunity crudes the curtain, the picture is not so good. Short run are different and the CDU/VDUs must be lengths, low middle distillate and VGO yields, as designed for them. One only needs to query   well as poor product quality are the norm. reners with long-term experience of processing opportunity crudes to get a true picture of the Refning and upgrading experience with processing costs. Short run lengths, low middle extra-heavy crude and vacuum gas oil (VGO) product yields and Canadian bitumen upgraders, Venezuelan renpoor product quality are common. eries and upgraders, as well as a handful of US

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reners, pioneered heavy and  with incremental VGO recovery  extra-heavy crude processing. are huge. When crudes cost Only these reners or upgraders $100/bbl, the prot loss of  have long-term experience of  turning 1000 b/d of VGO into processing them. Today, CDU/ coke is more than $35 million/  VDUs must operate for four to  y. Proper vacuum unit design is six years, while maximising essential to maximise VGO middle distillate recovery and and diesel product yield (see minimising vacuum residue Figure 2). (VR) yield. Poor product recov Poor diesel recovery and high ery and quality dramatically  diesel boiling range material in reduce the economic benets of  the VGO are another conselower crude costs. Avoiding quence of poor CDU/VDU unscheduled outages has been a design. Maximum diesel recovery  chronic problem, reducing prof requires production from the itability. A 10-day shutdown to  vacuum unit, yet most US renreplace failed exchangers or ers have not designed their units piping, or a longer-term outage for diesel production. Many large to x re damage caused by loss engineering and construction of containment, dramatically  companies continue to design reduces protability. CDU/VDUs without it, hence the There are only a handful of  atmospheric gas oil (AGO) and long-term producers in Canada Figure 2 Deep-cut VGO vacuum unit  VGO product contain large  with experience of processing producing diesel rom the top draw  volumes of diesel. It is not 100% Canadian bitumen over uncommon for the FCC or hydromore than a decade. Over the years, they have cracker feed to contain 25-30 vol% diesel boiling made many changes to the process and equip- range material. ment designs to run reliably. Only recently have US reneries started to process these crudes in Heavy crude oil properties high blend percentages and they are experienc- Heavy crude oil’s properties make it difcult to ing problems similar to those encountered by the reliably process through a conventional CDU/ reners processing Venezuelan crudes in the  VDU. Essential CDU/VDU design properties are mid-1980s to early 1990s. TBP distillation and API gravity curves, contamiReners make money producing gasoline and nants distribution (MCR, asphaltenes and middle distillates; coke is a by-product often metals), viscosity, salt content, total acid number having a negative value. A common CDU/VDUs (TAN), solids content, and tramp amine and design basis is a 1050°F (565°C) VGO product other trace components. These properties dictate cutpoint; however, the reality is that most the design, and if they are ignored the unit will achieve only 950°F (510°C) or lower. This not work as expected. For example, many of  represents an almost 10 vol% higher VR yield these crudes contain amines due to either on whole crude and means VGO is fed to the production water treatment or an H 2S scavenger coker instead of the FCC unit or hydrocracker. used. If the desalted crude has a high salt content Coking the heavy portion of the VGO converts and the amines are not removed they will cause 15-20% into coke, with the remainder down - severe plugging and corrosion in the top of the graded to lower-value product than the virgin crude column, requiring replacement of the  VGO. Even though VGO produced from extra-  vessel’s shell with exotic metallurgy. This has heavy crude has higher contaminants and happened in several instances. consumes hydrogen to make ULSD and high As crude blend distillations get heavier, the VR  quality FCC feed, the quality of the coked VGO content increases and they become difcult to products is even lower. Heavy and extra-heavy   vapourise in the atmospheric and vacuum unit. crudes contain large amounts of VGO compared Units designed with a high crude column AGO  with other crudes, and the economics associated cutpoint or a vacuum preash column make the

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Problems associated with opportunity crudes • Poor exchanger design — low exchanger heat transer coecients • Poor desalting caused by low temperature, small desalters, poor water system design, high pH water, too small transormers, stable rad layer ormation, periodic carry-over, etc • Failure to design or high crude water content • Crude hydraulic limits caused by poor exchanger design, resulting in rapid ouling, wrong  viscosities used, low pump eciencies and head, etc • Prefash drum oaming and carryover  • Much lower crude preheat, causing high crude heater ring  • Crude column ouling and corrosion in top section plugging trays, resulting in shutdown • Fouling in crude column top pumparound exchangers • Severe crude column top section vessel corrosion • Stripping section asphaltenes precipitation ouling trays and limiting stripping steam • Low atmospheric crude diesel product yield caused by low vapourisation • Crude heater asphaltenes precipitation and heater tube coking, requiring shutdown to pig tubes • Vacuum heater coking requiring shutdown to pig tubes • Crude and vacuum heater transer line corrosion • Vacuum column wash zone coking (see Fig ure 3) • Vacuum ejector system ouling and corrosion, causing poor vacuum and high column pressure • Fouling top pumparound section o vacuum column with amine chlorides • Low HVGO product draw temperature caused by low crude column distillate yield, resulting  in vacuum column heat removal limit • Low VGO yield • High metals VGO • VGO pumparound and product system corrosion • High chloride content light vacuum gas oil (LVGO)

CDU/VDU processing  difculties Table 1 identies some of the specic problems that reners face when processing heavy  crude oil blends. These are only a few of the difculties experienced by reners processing high percentages of these opportunity crudes.

Future heavy crude sources

Up to about 10 years ago,  Venezuelan and Mexico crudes  were the major sources of heavy  and extra-heavy crudes. The US  was the predominant market for it. Very little Canadian bitumen  was processed in the US and none outside. Today, the production of Canadian extraheavy bitumen-based crude is around 2 million b/d from the Table 1  Athabasca, Peace River and Cold Lake regions. Much of the Northern Alberta production is upgraded in the Fort McMurray  region by the long-term operators Suncor and Syncrude, with newer upgraders operating for only the last ve years. Northern  Alberta crudes are the most difcult to processes and, in the near future, increasing volumes  will reach the US market as diluted bitumen or synbit. Currently, Southern Athabasca, Cold Lake and Peace River  blends are being processed, but only a very few reners run high percentages in their blends. And none currently have long-term experience of processing 100% Figure 3 Severe wash zone coking  Canadian bitumen blends in the  vacuum column feed very heavy. A high crude 18-20 °API gravity range. heater outlet temperature lifts more diesel and Heavy Venezuelan crudes have been processed  VGO range material out of the vacuum column since the mid-1980s in high blend percentages feed, making it more difcult to vapourise in the  by only a few US reners. In nearly all cases, the  vacuum column. A heavier vacuum unit feed CDU/VDUs did not meet design capacity, prodlowers the VGO product yield for a given vacuum uct yields or reliability. In some instances, none column ash zone pressure and vacuum heater of these goals were achieved, with the units outlet temperature. requiring major revamps just to meet their

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original design basis. Today, Venezuelan crudes are increasingly going to India and the Far East  because they are seen as low-cost alternatives to Middle East and West African crudes. In most instances, the lessons learned in the US have not  been incorporated in the CDU/VDU designs. Few major engineering and construction companies have designed units for Oronoco Bitumen  blends, and the ones that have often repeat the same mistakes on future designs because they  rarely perform rigorous post start-up audits and almost never look at unit performance one to four years after start-up when reliability problems show up.

Conclusions Lessons learned by US reners processing heavy   Venezuelan crudes and Canadian upgraders processing bitumen should be applied to new  projects that will process increasing amounts of  opportunity crudes. These units should not be designed with conventional practices used for light and medium crudes; otherwise, run length  will be short, product yields and quality poor, and maintenance costs high. Opportunity crude properties are very different and design require ments will raise initial costs. These higher initial

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costs will pay out over the life of the project through an increased VGO product yield, lower maintenance costs, fewer VGO contaminants and a longer run length between decokings. Scott Golden is a Chemical Engineer with Process Consulting  Services, Inc, in Houston, Texas. He specialises in ront-end process engineering or renery unit revamps and has authored more than 100 technical papers on revamping and troubleshooting renery process units. Email: [email protected] Tony Barletta is a Chemical Engineer with Process Consulting  Services, Inc., in Houston, Texas. His primary responsibilities are conceptual process design and process design packages or  large capital revamps. Email: [email protected] Steve White is a Chemical Engineer with Process Consulting  Services, Inc, in Houston, Texas. He has more than 30 years o  process design experience or renery revamps and grassroots units. Email: [email protected]

Links More artcle from: Proce Coultg servce More artcle from the followg categore: Crude Vacuum Ut Heavy/sour Crude

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