Liquid isobutane alkylation

Such reactions can take place predominantly in either the continuous or disperse phase or in both phases or mainly at the interface. Mutual solubilities, distribution coefficients, and the amount of interfadal surface are factors that determine the overall rate of conversion. Stirred tanks with power inputs of 5-10 HP/1000 gal or extraction-type equipment of various kinds are used to enhance mass transfer. Horizontal TFRs usually are impractical unless sufficiently stable emulsions can be formed, but mixing baffles at intervals are helpful if there are strong reasons for using such equipment. Multistage stirred chambers in a single shell are used for example in butene-isobutane alkylation with sulfuric acid catalyst. Other liquid-liquid processes listed in Table 17.1 are numbers 8, 27, 45, 78, and 90. 

Olefin Dimerization. Other investigators (Schaad, 1955, Sparks et al., 1939) have reported that the catalytic polymerization of isobutene produced a liquid polymer consisting mainly of isooctenes. Hydrogenation of these isooctenes gave isooctanes consisting of 70 to 90 percent 2,2,4-trimethylpentone the remainder was reported as mainly 2,3,4-trimethyl-pentane. That earlier work used analytical techniques inferior to those now available. Typically, It Is now found that the C0 fraction of Isobutene-isobutane alkylate catalyzed by HF contains about 62.6 percent 2,2,4-trimethylpentane, 13.4 percent 2,3,4-trimethylpentane,... 

The emulsion leaving the reactor enters a settler. Residence times there often average up to 60 min to permit separation of the two liquid phases. Most of the acid phase is recycled to the reactor, being injected near the eye of the impeller. The hydrocarbon phase collects at the top of the decanter it contains unreacted isobutane, alkylate mixture, often some light n-paraffins, plus small amounts of di-isoalkyl sulfates. The sulfates must be removed to prevent corrosion problems in the distillation columns. Caustic washes are often employed to separate the sulfates they result in destruction of the sulfates. Acid washes have the advantage that most of the sulfates eventually react to reform sulfuric acid, which is reused, and to produce additional alkylate product. 

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. 

Note, however, that liquid acids are still largely used in refinery and petrochemical processes. For example, HF alkylation (for isobutane alkylation with light olefins) is still among the top-ten refining processes licensed by UOP, with over 100 units installed worldwide. However, UOP introduced from 2002 the Alkylene process, which uses a liquid phase riser reactor with a solid acid catalyst for the isobutane alkylation. However, HF alkylation remains the best economic choice [223], notwithstanding environmental and corrosion problems. Also in this case, the conventional process has been improved, for example by HF aerosol vapor suppression. Other aspects of isobutane alkylation have been reviewed by Hommeltoft [224]. 

The catalytic behavior of an Al-ITQ-7 zeolite, with a three-dimensional system of large pore channels, has been evaluated for the liquid phase alkylation of isobutane with 2-butene, and compared to that of a Beta zeolite. In absence of deactivation (TOS=l min), zeolite ITQ-7 gives a higher proportion of C5-C7/C5+, obtained by cracking of Cs and specially of the bulky C9+. However, the main differences are observed in the distribution of the trimethylpentane (TMP) isomers. Although zeolite ITQ-7 is more selective to TMP in the C8 fraction than Beta, the most abundant isomers are 2,3,3- and 2,3,4-TMP instead of the primary 2,2,3-TMP or the thermodynamically favored 2,2,4-TMP. This is a clear shape selectivity effect, due to the smaller pore size of ITQ-7 as compared to Beta, and the fact that 2,3,3- and 2,3,4-TMP are the isomers with less restricted transition states and smaller diffusion problems. 

Four processes have been employed for isobutane alkylation units in the last 20-30 years. Prior to that, one unit was built to alkylate isobutane with ethylene a liquid AICI3 complex was then employed as the catalyst. The processes that are currently important are discussed next. 

Alkylation units take two small molecules of isobutane and olefin (propylene, butylenes, or pentylenes) and combine them into one large molecule of high-octane liquid called alkylate. This alkylation combining process (Figure 11-8) takes place inside a reactor filled with an acid catalyst. Alkylate is a superior antiknock product that is used in blending unleaded gasoline. 

To obtain light ends conversion, alkylation and polymerization are used to increase the relative amounts of liquid fuel products manufactured. Alkylation converts olefins, (propylene, butylenes, amylenes, etc.), into high octane gasoline by reacting them with isobutane. Polymerization involves reaction of propylene and/or butylenes to produce an unsamrated hydrocarbon mixture in the motor gasoline boiling range. 

Figure 4-13. Liquid-liquid heterogeneous tubular flow reaotor (e.g., alkylation of olefins and Isobutane). (Source J. M. Smith, Chemloal Engineering KInetlos, 3rd ed., McGraw-Hill, Inc., 1981.)...

Flowever, information concerning the characteristics of these systems under the conditions of a continuous process is still very limited. From a practical point of view, the concept of ionic liquid multiphasic catalysis can be applicable only if the resultant catalytic lifetimes and the elution losses of catalytic components into the organic or extractant layer containing products are within commercially acceptable ranges. To illustrate these points, two examples of applications mn on continuous pilot operation are described (i) biphasic dimerization of olefins catalyzed by nickel complexes in chloroaluminates, and (ii) biphasic alkylation of aromatic hydrocarbons with olefins and light olefin alkylation with isobutane, catalyzed by acidic chloroaluminates. 

Refinery alkylation Liquid alkanes (e.g., isobutane) Gaseous alkenes (e.g., 1-butene) HF or H2SO4... 

The co-refining synergy of natural gas liquids and Fe-HTFT was exploited for alkylate production. The natural gas liquids serve as a source of butane that can be hydroisomerized to yield isobutane that is alkylated (HF process) to produce a... 

Chemistry and Technology of Isobutane/Alkene Alkylation Catalyzed by Liquid and Solid Acids... 

This contribution is an in-depth review of chemical and technological aspects of the alkylation of isobutane with lightalkenes, focused on the mechanisms operative with both liquid and solid acid catalysts. The differences in importance of the individual mechanistic steps are discussed in terms of the physical-chemical properties of specific catalysts. The impact of important process parameters on alkylation performance is deduced from the mechanism. The established industrial processes based on the application of liquid acids and recent process developments involving solid acid catalysts are described briefly. 2004 Elsevier Inc. 

Table I gives the compositions of alkylates produced with various acidic catalysts. The product distribution is similar for a variety of acidic catalysts, both solid and liquid, and over a wide range of process conditions. Typically, alkylate is a mixture of methyl-branched alkanes with a high content of isooctanes. Almost all the compounds have tertiary carbon atoms only very few have quaternary carbon atoms or are non-branched. Alkylate contains not only the primary products, trimethylpentanes, but also dimethylhexanes, sometimes methylheptanes, and a considerable amount of isopentane, isohexanes, isoheptanes and hydrocarbons with nine or more carbon atoms. The complexity of the product illustrates that no simple and straightforward single-step mechanism is operative rather, the reaction involves a set of parallel and consecutive reaction steps, with the importance of the individual steps differing markedly from one catalyst to another. To arrive at this complex product distribution from two simple molecules such as isobutane and butene, reaction steps such as isomerization, oligomerization, (3-scission, and hydride transfer have to be involved.

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