Feed isobutane alkylation

The butanes are used as gasoline-blending components. Normal butane is sometimes an olefins plant feed. Isobutane is used in refinery alkylation plants with propylene or butylene to make alkylate, a high-octane gasolineblending component. 

The hydrogen lost during their formation apparently goes into chain termination, i.e., the formation of isobutane most probably and possibly some propane when propylene is present In alkylation feed. HF alkylation has found no benefit from having acid-soluble oils present in the catalyst. When they are present in amounts greater than about one weight percent, they have a detrimental effect on alkylate quality and yield. 

Even with propylene feed, a high isobutane-to-olefin ratio influences the product toward predominantly Cg hydrocarbons which have the highest octane number and also Improves yields. Thus, both alkylate quality and yield are found to improve with increasing ratio and olefin dilution. In Table IX, detailed propylene-isobutane alkylate composition data are shown, where the volume ratio was increased from 4.6 to 126. For quick reference, composition data are summarized in Table IV. 

The same effects were seen with mixed olefin feeds. The refinery stream described in Table III was blended with C.P. Grade Isobutane to obtain a 9.0-to-l.O Isobutane-to-olefln molar ratio. This feed was alkylated at temperatures ranging from to 45°C. The contact time was held constant at 1.0 minutes. The results are shown in Table IV. The alkylate compositions include pentanes derived from the feed, but only... 

Two to four distillation columns are usually required to separate the liquid hydrocarbon product stream that contains unreacted isobutane, alkylate mixture, n-butane, and propane. The major column is designated as the deisobutanizer (DIB) column. Often this column separates the isobutane as the overhead stream, the alkylate as the bottom stream, and a n-butane rich sidestream. In many plants, the feed isobutane is also fed to the DIB to remove most of the n-butane. A second column is generally needed to remove propane from the isobutane. Sometimes a third column is provided to purify further the n-butane sidestream and to recover more isobutane. In an alternate arrangement, the bottom stream of the DIB column is a mixture of alkylate and n-butane. This mixture is then separated in another column. 

The chief sources of olefins are cracking operations, especially catalytic cracking. However, olefins can be produced by the dehydrogenation of paraffins butanes are dehydrogenated commercially to provide feeds to alkylation. Isobutane is obtained from crude oils, cracking operations, catalytic reformers, and natural gas. To supplement these sources, n-butane is sometimes isomer-ized. Only small concentrations of diolefins are permissible in feeds to alkylation, particularly for sulfuric add catalyst. Diolefins increase the consumption of acid. 

A simplified process flow diagram of the alkylation process is shown in Figure 1.4. The process has a reactor with olefin feed, isobutane makeup and isobutane recycle as the inlet streams. Fresh acid is added to catalyze the reaction and spent acid is withdrawn. The exothermic... 

Mechanism 1 is basically the reaction of 1 mole of isobutane (or isopentane) with 1 mole of olefin to give a C7, Cg, or C9 isoparaffin (or Cg, C9, or Cio isoparaffin) depending on the carbon number of the olefin. As already indicated, the olefin reacts significantly faster than the isobutane (or isopentane) and there are many more reaction steps than equation (2) and (3). The approximate octane numbers of isoparaffins produced by mechanism 1 vary greatly depending on the olefin and isoparaffin feed (isobutane or isopentane) (18). The best quality alkylates are produced when 2-butenes and isobutylene are reacted to form TMPs, which have research octane number (RON) averaging about 102-102.5. Much lower quality alkylates are produced, however, when dimethyUiexanes are the major isoparaffins produced RON values in such cases average less than 70. 

To run a refinery alkylation unit isobutane and light olefins are required as feedstock. However, the composition of the olefin stream varies significantly with the local refinery situation and this requires careful adjustment of the process conditions. The most commonly used olefins are butenes and propene but sometimes the use of pentenes is also considered. New gasoline specifications and the Chan Air Act (a United States federal law) amendments make it necessary to remove pentenes from the gasoline pool, because of their potential for atmospheric pollution. The main sources of olefins are catalytic cracking and coking processes. The isobutane feed for alkylation units is mainly obtained from hydrocrackers, catalytic crackers, and catalytic reformers. Additional amounts of isobutane are directly available from crude distillation and natural gas processing. Moreover, n-butane can be... 

The volnmes of isobutane, propylene, and butylenes was also increased to provide feed for alkylation units. 

Alkylation is the reaction of an olefin with an isoparaffin (usually isobutane) in the presence of a catalyst (either 98 percent sulfuric acid or 75 to 90 percent hydrofluoric acid) under controlled temperatures and pressures to produce high octane compounds known as alkylate. These products are separated in a settler where the acid is returned to the reactor and the al late is farther processed. This hydrocarbon stream is scrubbed with caustic soda to remove acid and organically combined sulfur before passing to the fractionation section. Isobutane is recirculated to the reactor feed, the alkylate is drawn off from the bottom of the debutanizer, and the normal butane and propane are removed from the process. 

Figure 11.4-2 shows process flows for an HF alkylation unit. The three sections are 1) reaction, 2). settling and 3) fractionation. In the reaction section isobutane feed is mixed with the olefin feed (usually propylene and butylene) in approximately a 10 or 15 to 1 ratio. In the presence of the HF acid catalyst the olefins react to form alkylate for gasoline blending. The exothermic reaction requires water cooling. The hydrocarbon/HF mixture goes to the settling... 

Theoretically, even the direct alkylation of carbenium ions with isobutane is feasible. The reaction of isobutane with a r-butyl cation would lead to 2,2,3,3-tetramethylbutane as the primary product. With liquid superacids under controlled conditions, this has been observed (52), but under typical alkylation conditions 2,2,3,3-TMB is not produced. Kazansky et al. (26,27) proposed the direct alkylation of isopentane with propene in a two-step alkylation process. In this process, the alkene first forms the ester, which in the second step reacts with the isoalkane. Isopentane was found to add directly to the isopropyl ester via intermediate formation of (non-classical) carbonium ions. In this way, the carbenium ions are freed as the corresponding alkanes without hydride transfer (see Section II.D). This conclusion was inferred from the virtual absence of propane in the product mixture. Whether this reaction path is of significance in conventional alkylation processes is unclear at present. HF produces substantial amounts of propane in isobutane/propene alkylation. The lack of 2,2,4-TMP in the product, which is formed in almost all alkylates regardless of the feed (55), implies that the mechanism in the two-step alkylation process is different from that of conventional alkylation. 

With propene, n-butene, and n-pentene, the alkanes formed are propane, n-butane, and n-pentane (plus isopentane), respectively. The production of considerable amounts of light -alkanes is a disadvantage of this reaction route. Furthermore, the yield of the desired alkylate is reduced relative to isobutane and alkene consumption (8). For example, propene alkylation with HF can give more than 15 vol% yield of propane (21). Aluminum chloride-ether complexes also catalyze self-alkylation. However, when acidity is moderated with metal chlorides, the self-alkylation activity is drastically reduced. Intuitively, the formation of isobutylene via proton transfer from an isobutyl cation should be more pronounced at a weaker acidity, but the opposite has been found (92). Other properties besides acidity may contribute to the self-alkylation activity. Earlier publications concerned with zeolites claimed this mechanism to be a source of hydrogen for saturating cracking products or dimerization products (69,93). However, as shown in reaction (10), only the feed alkene will be saturated, and dehydrogenation does not take place. 

A variety of solid acids besides zeolites have been tested as alkylation catalysts. Sulfated zirconia and related materials have drawn considerable attention because of what was initially thought to be their superacidic nature and their well-demonstrated ability to isomerize short linear alkanes at temperatures below 423 K. Corma et al. (188) compared sulfated zirconia and zeolite BEA at reaction temperatures of 273 and 323 K in isobutane/2-butene alkylation. While BEA catalyzed mainly dimerization at 273 K, the sulfated zirconia exhibited a high selectivity to TMPs. At 323 K, on the other hand, zeolite BEA produced more TMPs than sulfated zirconia, which under these conditions produced mainly cracked products with 65 wt% selectivity. The TMP/DMH ratio was always higher for the sulfated zirconia sample. These distinctive differences in the product distribution were attributed to the much stronger acid sites in sulfated zirconia than in zeolite BEA, but today one would question this suggestion because of evidence that the sulfated zirconia catalyst is not strongly acidic, being active for alkane isomerization because of a combination of acidic character and redox properties that help initiate hydrocarbon conversions (189). The time-on-stream behavior was more favorable for BEA, which deactivated at a lower rate than sulfated zirconia. Whether differences in the adsorption of the feed and product molecules influenced the performance was not discussed. 

The catalyst is reported to be a true solid acid without halogen ion addition. In the patent describing the process (239), a Pt/USY zeolite with an alumina binder is employed. It was claimed that the catalyst is rather insensitive to feed impurities and feedstock composition, so that feed pretreatment can be less stringent than in conventional liquid acid-catalyzed processes. The process is operated at temperatures of 323-363 K, so that the cooling requirements are less than those of lower temperature processes. The molar isobutane/alkene feed ratio is kept between 8 and 10. Alkene space velocities are not reported. Akzo claims that the alkylate quality is identical to or higher than that attained with the liquid acid-catalyzed processes. 

To form the process model, regression analysis was carried out. The alkylate yield x4 was a function of the olefin feed xx and the external isobutane-to-olefin ratio jc8. The relationship determined by nonlinear regression holding the reactor temperatures between 80-90°F and the reactor acid strength by weight percent at 85-93 was... 

The isobutane makeup x5 was determined by a volumetric reactor balance. The alkylate yield x4 equals the olefin feed xx plus the isobutane makeup x5 less shrinkage. The volumetric shrinkage can be expressed as 0.22 volume per volume of alkylate yield so that... 

Butane isomerization is usually carried out to have a source of isobutane which is often reacted with C3-C5 olefins to produce alkylate, a high octane blending gasoline [13]. An additional use for isobutane was to feed dehydrogenation units to make isobutene for methyl tert-butyl ether (MTBE) production, but since the phaseout of MTBE as an oxygenate additive for gasoline, this process has decHned in importance. Zeolitic catalysts have not yet been used industriaUy for this transformation though they have been heavily studied (Table 12.1). 

The use of acidic chloroaluminates as alternative liquid acid catalysts for the alkylation of light olefins with isobutane, for the production of high octane number gasoline blending components, is also a challenge. This reaction has been performed in a continuous flow pilot plant operation at IFP [44] in a reactor vessel similar to that used for dimerization. The feed, a mixture of olefin and isobutane, is pumped continuously into the well stirred reactor containing the ionic liquid catalyst. In the case of ethene, which is less reactive than butene, [pyridinium]Cl/AlCl3 (1 2 molar ratio) ionic liquid proved to be the best candidate (Table 5.3-4). 

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