Enhanced Olefin Reduction.pdf
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1、1 Enhanced Olefin Reduction CDTECH Introduction Simple hydrogenation of C5 olefins produces a mixture of isopentane and normal pentane which is high in vapor pressure and low in octane, hence not very attractive for gasoline blending. Conversion of C5 olefins to TAME is a good way to convert a probl
2、em into an asset. The TAME product has high octane and low vapor pressure as well as contained oxygen. The other option is to alkylate the C5 olefins. This route produces greater volume but with higher vapor pressure and lower octane value. The TAME route also achieves lower capital and operating co
3、st than alkylation. Production of TAME from C5 Olefins Reactive Isoamylenes can be converted to TAME by etherification with methanol. The reactive isoamylenes include 2- methyl butene-1 and 2-methyl butene-2. These reactions are shown in Figure 1. The other C5 olefins can be converted to reactive is
4、oamylenes through skeletal isomerization. The reaction of pentene- 1, cis/trans pentene-2 and 3-methyl butene-1 to reactive isoamylenes are shown in Figure 2. Figure 1 CH2=C-CH-CH3 CH3 CH3-C=CH-CH3 CH3 CH3-C-CH-CH3 CH3 O-CH3 or + CH3-OH 2MB1 2MB2 TAME CDTame Isoamylene Etherification Figure 2 CH2=C-
5、CH-CH3 CH3 2MB1 CH3-C=CH-CH3 CH3 2MB2 CH3-CH=CH-CH2-CH3 Pentene-2 ISOMPLUS Olefin Skeletal Isomerization CH2=CH-CH2-CH2-CH3 Pentene-1 CH2=CH-CH-CH3 CH3 3MB1 2 A simplified flow diagram for the production of TAME from FCC gasoline is presented in Figure 3. The FCC gasoline (light catalytic naphtha) i
6、s fractionated in a distillation column to separate the C5 fraction from the C6+ fraction. The C5 stream usually contains over 1% of diolefins. The diolefins are very reactive and readily form oligomers. These oligomers cause problems in TAME manufacture by fouling the etherification catalyst and by
7、 producing yellowish foul smelling TAME product. It is necessary to reduce the diolefin concentration to less than 100 ppm to minimize the problems. Therefore, selective hydrogenation of the diolefins in the C5 stream is required. Conventional practice is to hydrogenate the diolefins over palladium
8、catalyst in a separate fixed bed reactor. The C5 olefin stream usually contains enough mercaptans to keep the palladium catalyst from performing at its capacity. This is because the sulfur compounds are more strongly adsorbed on the catalyst than the other components. Therefore, the mercaptans are n
9、ormally removed upstream of the selective hydrogenation step. The conventional method of removing the mercaptans involves contact with a caustic stream and subsequent contact with oxygen. The refinery must deal with the fresh caustic make-up as well as dispose of the spent caustic waste stream. In a
10、ddition the residual oxygen dissolved in the treated hydrocarbon stream must be removed since it will degrade the polymeric substrate in the TAME etherification catalyst thus decreasing the catalyst life. Figure 3 Conventional FCC C5 Treating Selective Hydrogenation FCC C5+ GASOLINE FRESH CAUSTIC TR
11、EATED C5s HYDROGEN C6+ GASOLINE Mercaptan Removal Hydrogen Compression SPENT CAUSTIC ISOMPLUS CDTame CDTame METHANOL TAME TAME C5 RAFFINATE Refinery TAME Production 3 After mercaptan and diolefin removal, the next step would be etherification of the existing reactive isoamylenes with methanol. This
12、step has been commercialized using the CDTame technology from CDTECH (Figure 4). Conversions of reactive isoamylenes exceeding 90% have been achieved. The remaining C5 olefins represent additional potential olefins via skeletal isomerization. The ISOMPLUS technology is available from CDTECH/Lyondell
13、 for this purpose (Figure 5). Conversion of normal pentenes exceeding 65% at greater than 95% selectivity to reactive isoamylenes have been demonstrated. The zeolytic catalyst developed for this process provides a very simple process scheme with low capital and operating costs. No diluents or cataly
14、st activation agents are required. The process is characterized by long catalyst operating cycle length and low byproduct yield. Fresh Methanol HC Raffinate Water Recycle Methanol HC Feed Wash Water Water Feed Wash Boiling Point Reactor Catalytic Distillation Column Methanol Extractor Methanol Colum
15、n CDTame Ether Product Figure 4 Figure 5 ISOMPLUSISOMPLUS C5 Skeletal Isomerization TAME Raffinate Reactor Heater Isomerate 4 Etherification of the reactive isoamylenes resulting from skeletal isomerization is best achieved by use of a second TAME unit. This unit will be smaller and have fewer equip
16、ment items than the first TAME unit. This approach actually saves capital and operating cost relative to a single TAME unit with recycle from the skeletal isomerization step. The combination of skeletal isomerization with a second TAME unit can increase TAME by as much as 70% over that from a TAME u
17、nit only. The estimated capital costs for each of the units to process 10,000 BPD of C5s are: Unit $(million) Depentanizer3.9 Mercaptan Removal1.3 Selective Hydrogenation3.9 Hydrogen Compressor0.3 TAME/ISOM/TAME21.0 Total30.4 CDHydro CDHydro technology combines distillation and selective hydrogenati
18、on and was developed for the removal of diolefins from C3, C4 and C5 streams. This technology utilizes palladium catalyst which has been placed in the rectification section of a distillation column (Figure 6). The catalyst is contained in a proprietary structured packing (CDModulesm) which has fract
19、ionation capabilities similar to conventional trays. Hydrogen and hydrocarbon feed containing diolefins are introduced in the distillation column below the catalyst section. Incorporating catalyst in the fractionation column provides several advantages in the areas of mercaptan effect, catalyst foul
20、ing resistance, selective hydrogenation performance and capital/operating cost. Figure 6 FCC C5+ GASOLINE C5s C6+ HYDROGEN CDHydro CDModulessm 5 Mercaptan Effect When the mercaptans are adsorbed onto the catalyst at the bottom of the CDModule bed a reaction occurs as shown in Figure 7. The mercaptan
21、s such as ethyl mercaptan react with diolefins to produce sulfides, which have much higher boiling points than the surrounding hydrocarbons. As a result the sulfides are quickly fractionated to the bottom of the distillation column and exit with the C6+ product. Since the C6+ fraction already contai
22、ns similar sulfide compounds a new problem is not created. The mercaptan- diolefin reaction goes to completion in a very small portion at the bottom of the catalyst zone. The selective hydrogenation of diolefins occurs above this zone and proceeds to very high conversion at high selectivity. Overhea
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