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1、Increasing diesel production from the FCCU S ulphur concentration in the gasoline pool is being reduced in many parts of the world. In Europe, Euro V regulations, implemented in 2009, require less than 10 ppm sulphur in gasoline. Since 2008, the gasoline sulphur specification in the US and Canada ha
2、s been less than 30 ppm on average, and a further reduction to 10 ppm is under consideration in the US. In Europe and North America, diesel with a sulphur content in this range is referred to as ultra-low sulphur diesel (ULSD). In Russia, the gasoline sulphur content will be reduced to 10 ppm by 201
3、5. It is likely the sulphur specification in the rest of the world will also become more stringent over time. Gasoline is a complex product, with many important parameters determining its quality, aside from sulphur concentration; these include vapour pressure, benzene concentr- ation, boiling range
4、 and octane rating. In some regions, such as the EU and India, demand for diesel is higher than for gasoline. In others, including the US, demand for diesel is projected to grow while gasoline demand declines, as shown in Figure 1, which also illustrates the future projected change in the global gas
5、oline-to-distillate ratio. To reduce gasoline product and increase diesel product, some refiners produce lower end-point gasoline, routing the heavier cut of the full-range gasoline to the diesel pool. This results in higher capacity requirements for the diesel hydro- treaters, which are often alrea
6、dy fully utilised. To remove sulphur from FCC gasoline, CDTECH offers the FCC naphtha desulphurisation technology produces low-sulphur heavy catalytic naphtha as a separate product for blending into the diesel pool MauriCe KorpelShoeK, Gary podrebaraC, Kerry roCK and rajeSh SaMarth CDTeCh commercial
7、ly proven LCN (light catalytic naphtha) CDHydro and CDHDS processes. These processes, which employ the principle of catalytic distillation, conduct selective hydro-desulphurisation in a distillation environment. The full- range gasoline is split into three different cuts, giving the refiner blending
8、 flexibility. It is possible to use the CDHDS process to tailor the sulphur content of the distillate and bottom product, making it feasible to produce 10 ppm low end- point gasoline and a separate heavier cut that meets the less than 10 ppm sulphur specification for blending into the diesel pool. T
9、his flexibility can provide significant value by debottlenecking the diesel hydrotreater. properties of FCC gasoline Figure 2 shows a plot of the concentrations of total sulphur, mercaptan sulphur (RSH) and olefins (measured via bromine number) as a function of the boiling point of the FCC gasoline.
10、 At the light end of the gasoline, the olefin concentration is high and the total sulphur concentration is relatively low. Nearly all the total sulphur is in the form of RSH. As the boiling point increases, the sulphur concentration begins to increase quite significantly, while the RSH concentration
11、 actually declines. At the heaviest end, most of the sulphur is contained in compounds such as benzothiophene and methyl benzothiophene. Conversely, the olefin concentration profile follows the opposite trend: the lighter end of gasoline is olefin-rich, while the heavier end contains very few olefin
12、s. The data plotted in Figure 2 illustrate the challenge involved in treating the light end of the PTQ Q1 2010 75 Global gasoline-to- distillate ratio North American gasoline demand, Mt/a ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Figure 1 Decline in gasol
13、ine demand loss. As Figure 3 shows, the first step is to treat the lightest fraction of the gasoline in a LCN CDHydro unit, where the RSH is non- destructively removed. The LCN CDHydro unit is not a hydro- desulphurisation step. It operates at very mild conditions, resulting in no measurable olefin
14、loss. Part of the rectification section of the LCN CDHydro column contains catalyst packed in a distillation structure. The rest of the column contains conventional distillation trays. The LCN CDHydro unit works by performing an additional reaction between the RSH and the contained diolefins over th
15、e catalyst to form a heavier sulphide (RSR). The heavy sulphide goes to the bottom of the LCN CDHydro column and exits with the bottom product. The light product from the LCN CDHydro column has a very low RSH content, very high olefin concentration, a high octane rating and a relatively high Reid va
16、pour pressure (RVP). Some refiners choose to isolate this fraction in order to increase flexibility in the blending operation. The LCN CDHydro column bottoms go to the CDHDS column. This is the hydrodesulphurisation step that converts sulphur in the gasoline to H2S. The CDHDS column also contains a
17、hydrodesulphur- isation catalyst packed in a distillation structure. Since the process is based on distillation, the catalyst structure below the feed point operates at much higher temperatures than the catalyst structure above the feed point. This has the effect of increasing reaction severity for
18、the heavy portion of the gasoline, where the requirement for sulphur conversion is the highest and the olefin content is the lowest. Simultaneously, the medium catalytic naphtha (MCN), which contains higher olefin levels and fewer refractory sulphur com- pounds, goes up the column, where conditions
19、are less severe than they are at the bottom. Thus, the reaction conditions in the CDHDS unit are ideally suited to the goals of preserving olefins and minimising octane loss. gasoline through to the heavy end. To meet a 10 ppm sulphur specification, the light cut requires only 90% conversion of sulp
20、hur; the middle cut (near 135C) requires 99% conversion, while the heaviest cut requires 99.8% sulphur conversion. The preservation of olefins is vital to reducing hydrogen consumption and minimising octane loss. Olefin saturation, although inevitable in a hydrodesulphurisation process, is minimised
21、 when sulphur conversion is kept to a minimum. With this fact in mind, an ideal desulphurisation process would provide an environment where the highest severity is applied only to the heavy fraction of the gasoline, which has high sulphur and low olefin concentrations. The olefins in the heavy fract
22、ion also have a lower octane content than the olefins in the light fraction. Reaction severity would be decreased for the lighter fractions, which have lower sulphur and higher olefin concentra- tions. Treating the lighter fraction at lower severity limits the saturation of valuable olefins in this
23、olefin-rich region. process overview CDTECH has developed a selective treatment method for full-range FCC gasoline, which optimises the severity of treatment for different cuts of gasoline to maximise sulphur removal while minimising olefin 76 PTQ Q1 2010 Bromine No Total sulphur Mercaptans ? ? ? ?
24、 ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Figure 2 Concentration profiles of total sulphur, mercaptans and olefins (as bromine number) in FCC gasoline LCN CDHydro CDHDS ? ? ? ? ? ? ? ? ? Figure 3 A proposed flow scheme to maximise blend flexibility Since the CDHDS unit performs distillation, there is an oppo
25、rtunity to isolate two additional FCC gasoline fractions to increase blending flexibility and control the end point of the product. The CDHDS column distillate product (MCN) makes an ideal low end- point blendstock for the gasoline pool. end-point flexibility The CDHDS column bottom product (heavy c
26、atalytic naphtha, or HCN) can also be isolated and is a high end-point product; however, it is important to realise that this product has a very low sulphur concentr- ation. The HCN can be desulphur- ised to sulphur concentrations that are lower than those required for the gasoline pool without sacr
27、ificing olefins and octane in the MCN bound for the gasoline pool. This increases its number of potential uses, while maintaining good economics for the treatment of the MCN. The HCN can be blended into the gasoline pool if desired. If low end-point requirements become constraining, the HCN can be s
28、ent into a low-sulphur or an ULSD pool without requiring any further sulphur removal, making it ideal for blending into the diesel pool. Thus, the HCN will not occupy capacity in the refinerys diesel hydrotreater (DHT), which otherwise could potentially cause a DHT bottleneck. The ability of the CDH
29、DS column to produce a higher sulphur distillate and lower sulphur bottom product is unique and cannot be replicated by installing a gasoline splitter downstream of a fixed-bed FCC gasoline hydrodesulphurisation (HDS) unit. The product from a fixed-bed FCC gasoline HDS unit contains heavy sulphur co
30、mponents such as benzothiophenes. When the gasoline produced from a fixed-bed unit is distilled, the bottom product will contain more sulphur than the distillate product. To meet the sulphur specifications required to send this bottom product into the diesel pool, two options exist: Send it for furt
31、her treatment in the DHT, which may restrict diesel production in the refinery Treat the entire gasoline stream to reach a sulphur level well below the desired diesel specification, which will result in a very large octane loss in the gasoline pool, but will not restrict diesel production. The refin
32、er may opt to undercut the gasoline and send only a low end-point feed through the fixed- bed FCC gasoline HDS unit, but this still requires running the heavy gasoline-range material through the DHT. The CDTECH flow scheme does not take up capacity in the DHT and therefore gives refiners an opportun
33、ity to debottleneck diesel production. However, some CDTECH licensees operate their units in this manner on a seasonal basis to satisfy changing market conditions. By taking advantage of the HCN as a separate product, the refiner avoids having to treat this material in a separate hydrotreater. This
34、mode of operation commercially demon- strates the flexibility of the technology to operate on feedstocks of variable boiling range. The modifications required to maximise diesel production are negligible. A typical configuration of a CDHydro/CDHDS unit, together with the DHT, is shown in Figure 5. I
35、n this configuration, it is assumed that H2S stripping of the MCN Yes 10 ppm Fixed bed ? ? ? ? ? ? ? ? ? ? ? ? ? ? Figure 4 Comparison of sulphur content in FCC gasoline fraction PTQ Q1 2010 77 cetane number is achieved by hydrotreating the diesel fraction, which converts the aromatics into naphthe
36、nes. The resulting cetane number after hydrotreating is generally in the order of 40. Given that the amount of diesel from the CDHDS unit is small relative to the total diesel pool, the impact on the cetane number is negligible. long catalyst life Treating cracked streams in fixed- bed FCC gasoline
37、HDS units can result in catalyst coking and fouling, which in turn can cause substantial pressure drop. These problems are addressed by periodically shutting down the units to vacuum out the upper layers of catalyst. To help reduce fouling, these units require a selective hydrogenation unit (SHU) up
38、stream to significantly reduce reactive dienes from the feed. An added benefit of conducting the HDS reaction in a catalytic distillation environment is the significantly longer catalyst life. The reflux in the CDHDS column provides a sink for the heat of reaction, so catalyst hotspots are virtually
39、 eliminated. The reflux also provides a washing action, where coke precursors are removed from the surface of the catalyst before they have an opportunity to develop into larger formations that deactivate the catalyst or plug the catalyst bed. Inspections of many CDHDS units over the past 15 years h
40、ave confirmed the validity of this mechanism. CDHDS units, which do not require an upstream SHU, have been in operation for more than six years with no apparent loss of catalyst activity. The CDHDS technology is unique, in that the desulphurisation step does not limit the run length of the FCCU, eve
41、n as refiners press for FCC cycles of up to six years and eventually more. For refiners employing shorter FCC turnaround cycles, the catalyst in the CDHDS column can last for multiple cycles. hydrogen usage The CDHDS process is very selective in minimising olefin saturation and maximising octane ret
42、ention. As a result, hydrogen consumption is moderate. In addition, the CDHDS column is designed to run at modest hydrogen recirculation rates, which results in a smaller recycle compressor with less power consumption. Since the hydrogen recirculation rates are low, the CDHDS process can run with lo
43、wer purity hydrogen than a typical fixed-bed unit. Given the tight supply of hydrogen in most refineries, these can be significant operating factors. Another consideration is the refiners fallback position in the event that the hydrogen recycle compressor goes down unexpect- edly. Given the modest h
44、ydrogen requirements of the CDHDS unit, it is possible to run it on once-through hydrogen until the repair is completed. In a typical fixed-bed unit, it is impractical to attempt to match the recirculation rates on a once-through basis due to the much higher required hydrogen rates. and HCN streams
45、is performed in a single stripper column. Refiners who aim for maximum blending flexibility usually have separate MCN and HCN stripper columns, as depicted in Figure 3. In that case, the HCN stream can be directly blended to the diesel pool. If separate MCN and HCN stripper columns are not available
46、, the bottom product must be routed to the H2S stripper column in the DHT unit. Generally, the HCN stream is small relative to the diesel product, so the additional load can be handled in the H2S stripper column. Hence, the only hardware modification needed is to install piping between the CDHDS col
47、umn and the H2S stripper column in the DHT unit. Treatment of the HCN fraction in the CDHDS column has very little impact on the relevant properties of the diesel fraction. Flashpoint The MCN/HCN cutpoint is usually set by the flash point requirement, which is typically set at 55C minimum. This corr
48、esponds with a D86 10% point of approximately 180C, and is easily controlled in the CDHDS column. Cetane number The cetane number of the diesel fraction from an FCCU (light cycle oil) is rather low, generally in the order of 20. Some improvement in ? ? ? ? ? ? ? ? ? ? ? ? ? Figure 6 Boiling range of heavy catalytic naphtha (HCN) the CdhdS process is very selective in minimising olefin saturation and maximisin