Designing the CDU/VDU for 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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Opportunity crudes cost less than other crudes because they are more difficult to refine into gasoline and middle distillate. Yet there is a perception that profitability will improve simply by substituting them for more expensive feedstocks. At times, certain crudes sell at huge discounts. This may be when production exceeds the industry’s ability to process them, whether it is for short periods that are caused by unplanned shutdowns or longer periods when regional logistics arise such as distribution constraints for Canadian bitumen. Refiners tend to focus on conversion units such as the coker or hydrocracker, and the crude and vacuum unit (CDU/VDU) is often an afterthought. As more heavy and extra-heavy crudes enter the market, there will be more opportunity for refiners to run them. The question is whether it will be profitable or not. Will lessons learned be incorporated into unit designs or will refiners 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 reliably is different from running common heavy crudes such as Arab Heavy. Common problems encountered when processing extra-heavy opportunity crudes include poor desalting, which often results in severe atmospheric crude overhead system corrosion, including exchangers, piping and vessels. These crudes are viscous, contain solids, can have amines, can precipitate asphaltenes when blended with other crudes, and have a high fouling tendency in heat exchangers (see Figure 1), column internals and fired heaters. Some have a very high naphthenic acid content, causing desalter problems and corrosion requiring upgraded metallurgy. Other problems include heater coking and asphaltene precipitation inside heater tubes, which leads to short run lengths. When processing some Northern Alberta bitumens, the vacuum heater outlet temperature must be operated as low as 705°F (374°C) to avoid short run lengths even with well-designed double-fired tube designs. Asphaltene precipitation in the bottom of the atmospheric column can lead to stripping tray plugging, causing run lengths of less than a year. These are just a few of the obstacles experienced by upgraders and refiners processing heavy opportunity crude oils. These are the unfortunate realities, but it is better to design the unit for them than to suffer the consequences that many refiners continue to experience. As producers of opportunity crudes such as Canadian dilbit continue to ramp up production, refiners increasingly are suffering because they either ignored the realities of processing these crudes or were unfamiliar with them.
Global opportunity crude supply
Historically, heavy and extra-heavy crude production has largely come from North and South America. Consequently, refining experience with these crudes is primarily in Canada, the US and South America, and only a few US refiners process high blend percentages. Some producing nations’ crude units have very poor reliability and performance, but these realities are hidden from the global market. As the global distribution of opportunity crudes improves and more fields come on stream, the world’s refining system will gain experience.
Today, most European refineries process light and medium crudes, while Middle East refiners run their own crudes. They process very little opportunity crudes. Refineries in China are primarily designed to process local and Middle East crudes and have little or no experience with opportunity crudes at high blend percentages. While China is planning to build refineries to process them, their approach is to rely on their own institutes or global engineering and construction companies that have little experience designing CDU/VDUs for opportunity crudes. The experiences they do have and have applied resulted in very poor reliability and a low refinery liquid volume yield. Opportunity crudes are different and the CDU/VDUs must be designed for them. One only needs to query refiners with long-term experience of processing opportunity crudes to get a true picture of the processing costs. Short run lengths, low middle and vacuum gas oil (VGO) product yields and poor product quality are common.
Refining and upgrader capacity has to match production. Currently, most Venezuelan crude is processed by US refineries, and most, if not all, Canadian crudes are upgraded or refined in Canada or the US. Far East refiners, primarily India, are processing some Venezuelan crudes, but the blend percentages are not high. Whether Canadian or Venezuelan, these crudes are a mixture of bitumen and lighter blend stocks. PetroZuata and Hamaca consist of coker products and bitumen, whereas Merey is a blend of Orinoco bitumen and lighter crudes, albeit there may be other names under which these 15 °API gravity Orinoco bitumen blends are marketed. Much of the Canadian crudes are a blend of bitumen and condensate (naphtha) or synthetic crude from the upgraders. Conventional Canadian heavy crude production, such as Bow River, is rapidly declining, with more crudes containing bitumen from Cold Lake, Athabasca or Peace River basins. Some of these bitumens are much more challenging than others and refiners should not assume they are all created equal. They are not!
In the mid to late 1980s, US Gulf Coast refiners began processing large volumes of 21-24° API gravity crudes from Mexico and Venezuela. In the mid to late 1990s, 15-18° API blends became more prevalent and this trend continues. At least two US refiners process these crudes neat through a CDU/VDU. In all cases, the designer touted a 1050°F (565°C) VGO product cutpoint, but not one actually achieved it in spite of the rhetoric. Today, few refiners process 100% heavy crudes, with most blending them with higher- gravity crudes. Some refiners claiming they process heavy crudes have blend gravities of only 27-30° API gravity. Processing heavy and extra-heavy crudes neat or in high blend percentages is another story completely. After more than 25 years of industry experience processing these crudes, the economic losses associated with poor CDU/VDU performance are still huge. Low VGO cutpoint is the norm whether refiners admit it or not. In spite of the claims, when one peels back the curtain, the picture is not so good. Short run lengths, low middle distillate and VGO yields, as well as poor product quality are the norm.
Refining and upgrading experience with extra-heavy crude
Canadian bitumen upgraders, Venezuelan refineries and upgraders, as well as a handful of US refiners, pioneered heavy and extra-heavy crude processing. Only these refiners or upgraders have long-term experience of processing them. Today, CDU/VDUs must operate for four to six years, while maximising middle distillate recovery and minimising vacuum residue (VR) yield. Poor product recovery and quality dramatically reduce the economic benefits of lower crude costs. Avoiding unscheduled outages has been a chronic problem, reducing profitability. A 10-day shutdown to replace failed exchangers or piping, or a longer-term outage to fix fire damage caused by loss of containment, dramatically reduces profitability.
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