Butenes hydroisomerization

Although not a separate process, isomerization plays an important role in pretreatment of the alkene feed in isoalkane-alkene alkylation to improve performance and alkylate quality.269-273 The FCC C4 alkene cut (used in alkylation with isobutane) is usually hydrogenated to transform 1,3-butadiene to butylenes since it causes increased acid consumption. An additional benefit is brought about by concurrent 1-butene to 2-butene hydroisomerization. Since 2-butenes are the ideal feedstock in HF alkylation, an optimum isomerization conversion of 70-80% is recommended.273... 

Description Crude C4 streams are converted into propylene and an isobutylene-rich stream in three IFP process steps (1) butadiene and C4 acetylenes selective hydrogenation and butenes hydroisomerization, (2) isobutylene removal via distillation or MTBE production and (3) metathesis (Meta-4). 

There are currentiy three important processes for the production of isobutylene (/) the extraction process using an acid to separate isobutylene (2) the dehydration of tert-huty alcohol, formed in the Arco s Oxirane process and (3) the cracking of MTBE. The expected demand for MTBE wHl preclude the third route for isobutylene production. Since MTBE is likely to replace tert-huty alcohol as a gasoline additive, the second route could become an important source for isobutylene. Nevertheless, its avaHabHity wHl be limited by the demand for propylene oxide, since it is only a coproduct. An alternative process is emerging that consists of catalyticaHy hydroisomerizing 1-butene to 2-butenes (82). In this process, trace quantities of butadienes are also hydrogenated to yield feedstocks rich in isobutylene which can then be easHy separated from 2-butenes by simple distHlation. 

Isopol A hydroisomerization process for converting 1-butene to 2-butene. Developed by the Institut Frangais du Petrole. 

It is advantageous to pretreat butene feeds before alkylation.294-298 1,3-Butadiene is usually hydrogenated (to butenes or butane) since it causes increased acid consumption. The additional benefit of this process is that under hydrogenation conditions alkene isomerization (hydroisomerization) takes place, too. Isomerization, or the transformation of 1-butene to 2-butenes, is really attractive for HF alkylation since 2-butenes give better alkylate (higher octane number) in HF-cata-lyzed alkylation. Excessive 1,3-butadiene conversion, therefore, ensuring 70-80% isomerization, is carried out for HF alkylation. In contrast, approximately 20% isomerization is required at lower butadiene conversion for alkylation with H2SO4. 

In the hydrogenation of olefins such as 3-3-dimethyl-1-butene and cyclohexene on Ni/kieselguhr, the addition of sulfur caused hydroisomerization to 2-3-dimethylbutane and methylcyclopentane, respectively (247). 

The hydroisomerization step features complete C4 acetylenes and butadiene conversion to butenes, maximum 2-butenes production, flexibility to process different feeds, polymer-free product and no residual hydrogen. The second step separates isobutylene either by conventional distillation, or by reacting the isobutylene with methanol to produce MTBE. 

Application Increase the value of steam cracker C4 cuts via low-temperature selective hydrogenation and hydroisomerization catalysis. Several options exist removal of ethyl and vinyl acetylenes to facilitate butadiene extraction processing downstream conversion of 1, 3 butadiene to maximize 1-butene or 2-butene production production of high-purity isobutylene from crude C4 cuts total C4 cut hydrogenation and total hydrogenation of combined C3/C4 and C4C5 cuts for recycle to cracking furnaces or LPG production. 

CDTECH Isobutylene Raffinate 1 Selective hydrogenation of butadiene and hydroisomerization of butene-1 to butene-2 via catalytic distilation to recover isobutylene 1 1994... 

The latest industrial application of metathesis was developed by Phillips who started up a plant in late 1985 at Cbannelview, Texas, on the L ondell Petrochemical Complex with a production capacity of 135,000 t/year of propylene from ethylene. This facility carries out the disproportionation of ethylene and 2-butenes, in the vapor phase, around 300 to 350°C, at about 0.5.10 Pa absolute, with a VHSV of 50 to 200 and a once-througb conversion of about 15 per cent 2-butenes are themselves obtained by the dimerization of ethylene in a homogeneous phase, which may be followed by a hydroisomerization step to convert the 1-butene formed (see Sections 13.3.2. A and B). IFP is also developing a liquid phase process in this area. 

Furthermore, since the C4 cuts employed exhibit a 1-butcne percentage of about 10 to 15 per cent weight, this graph also shows that, to achieve effective conversion, it is necessary to operate at less than ISO C. In practice, the operation is conducted at about lOCPC, which maintains a residual 1-butene content of about 5 per cent weight in the effluenL la these conditions, however, the reaction rate becomes slow. Catalysts are used to accelerate it, usually based on precious metals deposited on inert alumina (palladium, rhodium etc.), whose operation is considerably improved in a hydrogen atmosphere. This also permits the selective hydrogenation of the residual butadiene, and explains why this conversion is called hydroisomerization. 

Fig. 32. The values of the ratio

When highly pure propene is not available commercially, it can be prepared by the reverse metathesis reaction of ethene and 2-butene [Eq. (1)]. The process is performed either at high temperatures (150-350°C) in the gas phase, over molybdenum or tungsten catalysts (Phillips triolefin process) [4], or at low temperatures (50°C) in the liquid phase, in the presence of rhenium-based catalysts (IFP-CPC process) [13], The raw material may be either ethene and the C4 fraction available from the hydroisomerization unit (previously submitted to an isomerization step to maximize its 2-butene content) or ethene alone, which, before admission to the metathesis unit, is partly dimerized to 1-butene, then isomerized to 2-butene in separate units. The process is useful in the event of a high demand for propene, since the C4 fraction is readily available from a cracking unit. 

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