《Benzene Reduction.pdf》由会员分享，可在线阅读，更多相关《Benzene Reduction.pdf（11页珍藏版）》请在三一文库上搜索。
1、1 Benzene Saturation in Gasoline CDTECH Background Fuel Specification Changes Refinery products used for fuels are receiving increasing levels of attention. Product specifications are being scrutinized by governmental agencies, whose interests are decreased emissions from mobile and stationary sourc
2、es, and by the industries that produce the engines and vehicles that utilize these fuels (Table 1). The European Union, followed closely by the United States, is implementing significant changes in gasoline specifications. CARB imposed dramatic changes in the early 1990s, Automakers worldwide have p
3、roposed a set of limitations for gasoline and diesel to allow them to manufacture vehicles which will produce significantly lower emissions over their lifetime. This paper addresses processes that offer the refiner cost effective options for reducing benzene levels in the gasoline pool. Table 1 - US
4、 and EU Gasoline Proposed Specifications SpecificationCARBEUAUTOMAKERS Max Level2005 Sulfur, ppm wt405010 Aromatics, vol%253535 Olefins, vol%618 - AO-II10 Benzene, vol%11.01 Summer RVP, kpa48(7)60(8.7) - AO-II60 Lead000 Oxygen, wt%2.22.7 - AO-II2.7 E100C, min vol%50210F46 AO-II50 E150C, min vol%9031
5、0F75 AO-II E180C, min vol%90 EP, Deg C,max195(383F) DI570 Figure 1- Benzene in Gasoline Light Straight Run Reformate FCC Isomerate Coker 2 Source of Benzene In the typical refinery benzene comes from several sources (Figure 1) however, the benzene from the reformer usually represents 50-80% of the t
6、otal. FCC gasoline is only a minor contributor to benzene in the gasoline pool. As a result, reformate is the natural place to focus benzene reduction. Chemistry of Benzene Saturation Benzene Hydrogenation The primary reaction is the saturation of Benzene. The primary product is cyclohexane (CH). Th
7、ere are small amounts of cyclohexadiene and cyclohexene under some conditions, but these are not significant for fuel products. Benzene saturation represents a considerable loss of octane. The pure component octane of CH more than 20 numbers less than benzene. Cyclohexane Isomerization A beneficial
8、side reaction of benzene saturation is the isomerization of cyclohexane to methylcyclohexane (MCP). This occurs at elevated temperatures with an acidic catalyst. The pure component octane of MCP is 8 numbers higher than CH. Straight Chain Paraffin Isomerization The isomerization of normal pentane (n
9、C5) and normal hexane (nC6) to isopentane (iC5) and isohexane (iC6) increases the octane of the stream. This occurs at elevated temperatures with an acidic catalyst. The pure component octane of iC5 is 35 numbers higher than nC5, while iC6 exceeds nC6 by more than 50 numbers. Benzene Mitigation Exam
10、ples The following examples are based on 50,000 bpd of straight run naphtha containing 1.0% benzene. The base case assumes that the feed is deisohexanized with 0.5% benzene in the distillate and the reformer operates with 80% gasoline yield, 1000 scf/bbl hydrogen yield and 3.0% benzene in the reform
11、ate. If the light straight run (LSR) naphtha is sent to an isomerization unit, there are additional considerations relative to its benzene content. Any benzene in the isom unit feed is saturated in 3 the isom reactor. The heat of reaction increases the temperature which shifts the reaction equilibri
12、um, thus limiting the amount of isomerization. The resulting isomerate has a lower octane number than that from LSR with low benzene content. Table 2 - Naphtha Comparisons Straight Run Naphtha Coker NaphthaHeavy Hydrocrackate Light Hydrocrackate Hydrocracker Main Frac Tops Aromaticslowhigh15% 15% 15
13、% Olefinsnilhighvery lowvery lowvery low Benzene0.5 1% 2 4% nil2 4%2 4% Sulfur500 2000 ppm typ. 1% 5ppm 15ppm 15ppm Nitrogenlow high nilnilnil ProcessingDebut DIH NHT Reformer Debut DIH NHT Reformer NHT if high S Reformer DIH Reformer Debut DIH Reformer Benzene from Reformer 2 5% full 5 10% light 0.
14、3 1% heavy 2 5% full 0.3 1%13 18%5 10% 4 6% if highly paraffinic 4 Reformer Feed Dehexanizer Removing the benzene precursors from the reformer feed (Figure 2) nearly eliminates the benzene in the reformate (minimum is generally 0.3 %wt and ranges up to 1.0% depending on conditions). There is however
15、 a sharp increase in the concentration of benzene in the light gasoline (former DIH tops) from 0.5% to 5.0%. The higher benzene content of some crudes can result in dehexanizer overheads with as much as 8 to 10% benzene. There is also a shift in yields. The dehexanizer tops rate increases by 1/3 (fr
16、om 15% to 20% of naphtha) with a corresponding decrease in reformer feed of 6%. There is a loss of hydrogen yield of up to 10 %, as the entire hydrogen loss cannot be made up with increasing reformer severity. This does, however, reduce the combined benzene in these two streams by 40%. Reformer Feed
17、 Dehexanizer with Side Column This option (Figure 3) still removes the benzene precursors from the reformer feed (Figure 2) and nearly eliminates the benzene in the reformate (as described above). The tops composition is unchanged. The side draw contains most of the benzene and precursors. The benze
18、ne is saturated in the CD portion of the side column. The combined distillate from the dehexanizer and side column is essentially the same as the previous DIH tops. The bottoms of the dehexanizer is the same in this case as Figure 2 - Reformer Dehexanizer Feed C5-iC6 )De-isohexanizer Stablizer H2 Ve
19、nt LPG Ref Reformer Figure 3 - Reformer Dehexanizer with CDHydro Side Column Feed C5-iC6 DehexanizerStablizer H2 Vent LPG Ref Reformer cC6 Stripper 5 Figure 5 - Reformate Splitter with Saturation Feed C5-iC6 DeisohexanizerStablizer H2 Vent LPG HRef Reformer Splitter Hydro LRef above. The new C6 prod
20、uct contains less than 0.5% benzene. This reduces the combined benzene in these three streams by 80% compared to the original two streams. There is a loss of hydrogen yield by chemical consumption of 38 scf/bbl of Straight Run Naphtha in addition to the above yield loss. This option also minimizes t
21、he benzene content of the LSR. If the resulting LSR is isomerized, the low benzene content will maximize the octane produced in the isom unit. Reformate Fractionation The reformate can be fractionated to produce a benzene concentrate (Figure 4) . For a single, three-product splitter column, the conc
22、entrate is approximately 10- 15% of the full range reformate and recovers ca 90% of the benzene, more or less depending on the column design. This concentrate can be further processed in a benzene extraction or recovery unit. Feed C5-iC6 DeisohexanizerStablizer H2 Vent LPG HRef Reformer Figure 4 - R
23、eformate Fractionation Bz Conc Dehexanizer 6 Reformate Splitter with Saturation The reformate can be split into two fractions with the bottoms containing, say, 0.2% benzene and the tops free of toluene. Conventionally, the distillate is processed in a high pressure, fixed bed benzene saturation unit
24、. (Figure 5) CD Hydro technology saturates benzene within the splitter column itself (Figure 6). Both the fixed bed and CD versions of the technology convert the benzene to cyclohexane. An alternative configuration is a low pressure existing reformate splitter with a side column incorporating CD tec
25、hnology (Figure 7). Advantages of this configuration include: less CD packing and a higher pressure vent In all cases, the benzene reduction can be controlled in a wide range, but is assumed to be near 90% in all cases. The loss of hydrogen yield by chemical consumption is 69 scf/bbl of Straight Run
26、 Naphtha. Vent losses vary with local conditions. A further refinement of the above splitter scheme is to modify the reformate stabilizer upstream to enable a benzene/toluene heartcut, to be drawn. The heartcut is then routed to a reformate splitter; the splitter in this case is smaller and less cos
27、tly because of the reduced hydrocarbon throughputs. Comparison of Alternatives Reformer Feed Dehexanizers The biggest impact of feed dehexanizing is the yield of gasoline and hydrogen. The actual yields depend on how the severity and the reformer feed is adjusted. If there is no change, the combined
28、 gasoline production from the DIH and the reformer increases by 1% but the hydrogen yield decreases by 10% and there is a slight reduction in the octane value. The only real cost of this alternative is the loss of hydrogen production from the reformer. There is little or no capital cost required for
29、 changing a deisohexanizer into a dehexanizer. 7 The addition of a side column for benzene saturation doubles the benzene reduction. The major operating costs are hydrogen, octane loss and catalyst cost. There is potentially a net energy credit from the high temperature condenser. Reformate Fraction
30、ation Distillation of the Reformate to produce a benzene concentrate followed by benzene extraction requires the largest capital expenditure and highest energy cost of all alternatives. The value of benzene produced does not cover the operating costs. Reformate Splitter with Saturation There are thr
31、ee variations of benzene saturation processes. All can achieve the same benzene removal. All three have the same total cost if compared on a grass- roots basis with variations in capital, energy and catalyst costs. Hydrogen cost and octane loss dominate the operating cost. Table 3 - Comparison of Op
32、tions Octane-bbl lost (octane * kbpd)8.617.618.618.618.6 Hydrogen Yeild loss (scf/bbl)6868 H2 required (scf/bbl)41848484 benzene concentrate3.4 % bz15% Separation energy (MM BTU/hr)3.13.19625.035.725.0 Steam Generation (MM BTU/hr)6.513.5 packing volume cf or lb338.33631,751700 New equipment count571
33、0510 Capital Cost (,000) incl 25% osbl-$-$2,000$12,50011,250$8,750$10,000 Yields Cost (Gain) $,000/yr($2,923)($2,136)($2,123)$1,630$1,630$1,630 Hydrogen cost ($,000/yr)$3,570$5,701-$4,412$4,412$4,412 Energy cost ($,000/yr)$79($3)$2,410$630$900$460 Catalyst / packing cost$237$102$350$140 Maintenance
34、(4% of Cap)$80$500$450$350$400 Total Operating Cost ($,000/yr)$725$3,879$787$7,224$7,642$7,042 Capex (x% per year)27%$533$3,332$2,999$2,332$2,666 Total Cost ($,000 per year)$725$4,412$4,119$10,223$9,974$9,708 cost per bz bbl$5$14$12$31$31$30 8 The conventional system employs a low pressure splitter
35、and a high pressure feed pump, reactor, air cooling, high pressure flash drum and recirculating pump. The estimated cost of the hydrotreater is $3,000,000 for 6,800 bpd of light reformate, in addition to the cost of a low pressure stripper. A higher pressure CD splitter (ca 75 psig) cost 15% more to
36、 build than a low pressure splitter due to the larger reboiler and condenser capacity. This assumes a fired reboiler is not required. This however eliminates the additional equipment associated with the conventional hydrotreater for capital savings of $2,000,000. This is partially offset in addition
37、al operating costs of $410,000 per year due to higher energy requirements and higher catalyst costs. The option of a CD side column for saturation has estimated capital savings of $1,000,000 compared to the conventional high pressure hydrotreater. This is expected due to the lower CD operating press
38、ure and 4 of 5 pieces of equipment being smaller than conventional. The operating costs are also lower by $170,000 per year mainly due to an energy credit on the CD column condenser (ca 350F) with all other things being equal. Economic Case Comparison Table 3 compares the Capital, operating and yiel
39、d changes for the cases discussed above. The most economic case for a refiner will depend on the level of benzene that must be removed from the reformate, if any, to reduce the blended gasoline benzene level below 1.0%. The gasoline grade which has the highest concentration of reformate will likely
40、be controlling, unless FCC gasoline is also a significant contributor to another reformate containing grade. Refiners who are extracting benzene already will likely not have to make any change. However, they may want to provide a minimal benzene manufacture option for periods when benzene extraction
41、 is not economically attractive. Some saturation may be required in that option. If a refiner requires up to about 40% reduction in his reformate benzene, a simple shift in reformer feed fractionation is the most likely option. The reduction in 9 hydrogen production would require some alternative so
42、urcing, but can likely be justified relative to higher capital alternatives. If the refiner already has a C5/C6 isom unit fed from the dehexanizer top, additional benzene will be saturated in that unit, and nothing further will likely be required although isomerate octane will be reduced. Generally
43、about 80% reduction can be achieved by adding a CDHydro side stripper reactor to a side draw from the reformer feed dehexanizer. If after all this there is still too much benzene in the reformate, reformer product must be treated. The three options discussed are comparable in cost, unless a naphtha
44、splitter is already in place. Retrofit of that column to add a CDHydro side stripper reactor provides a clear economic advantage over a fixed bed unit. Each refiner must determine what the benzene reduction requirements is. After that we can provide guidance on the most economic route to accomplish
45、that reduction. CDHydro is a low cost leader in this application. Development of the Downflow System A series of tests were conducted to establish a new range of operation for aromatics saturation. This included the effect of operating pressure, reboiler rate and location of the catalyst relative to
46、 feed. The piloted scheme is shown in Figure 8. Figure 8 -Downflow Benzene Saturation btms EDU 20 ovhd vent feed H2 10 Operating Pressure Increasing the operating pressure has three effects on the reaction kinetics: increasing the temperature, increasing the hydrogen concentration in the liquid phas
47、e and increasing the hydrogen partial pressure. All of these effects are positive and have and have an expected positive overall effect (Figure 9). This higher pressure and temperature however has a strong negative effect on the distillation. This higher operating pressure and temperature is impract
48、ical in a reformate splitter result in elevated bottoms temperature and an increase in reflux. Reboil Increasing reboil increases vapor / liquid traffic. This improves contacting which improves mass transfer. This effect is offset by a reduction in H2 concentration, vapor residence time, liquid resi
49、dence time and the concentration of aromatics. The overall effect is negative (Figure 10). This would suggest that the best results would be obtained by the minimum reboiler rate. Tests have demonstrated continuous, stable operation in the 3” CDU at over 90% conversion with the reboiler shut off. Figure 10 - Effect of Reboil 0% 20% 40% 60% 80% 100% 01,0002,0003,000 Gross Reboiler Rate (BTU/hr) Benzene Conversion Figure 11 - Effect of Feed Location 2_81sat 4082bzh2 50% 60% 70% 80% 90% Hydrogen Conversion downflowupflow Figure 9 - Effect of Pressure 0% 2