Isobutane thermal alkylation

The principal use of the alkylation process is the production of high octane aviation and motor gasoline blending stocks by the chemical addition of C2, C3, C4, or C5 olefins or mixtures of these olefins to an iso-paraffin, usually isobutane. Alkylation of benzene with olefins to produce styrene, cumene, and detergent alkylate are petrochemical processes. The alkylation reaction can be promoted by concentrated sulfuric acid, hydrofluoric acid, aluminum chloride, or boron fluoride at low temperatures. Thermal alkylation is possible at high temperatures and very high pressures. 

The main products formed by the catalytic alkylation of isobutane with ethylene (HC1—AICI3, 25-35°C) are 2,3-dimethylbutane and 2-methylpentane with smaller amounts of ethane and trimethylpentanes.13 Alkylation of isobutane with propylene (HC1—AICI3, — 30°C) yields 2,3- and 2,4-dimethylpentane as the main products and propane and trimethylpentanes as byproducts.14 This is in sharp contrast with product distributions of thermal alkylation that gives mainly 2,2-dimethylbutane (alkylation with ethylene)15 and 2,2-dimethylpentane (alkylation with propylene).16... 

Thermal alkylation was never a totally successful commercial process because of the severe operating conditions required. The reaction was carried out in a heater coil with temperatures of 900°-975°F, pressures in the range of 3000-5000 psig, and contact times of 2-7 seconds (16). Polymerization of the olefins occurred readily under these conditions, and low olefin concentrations had to be used to minimize undesirable side reactions. Ethylene could be alkylated more readily than the higher molecular weight olefins, and either normal butane or isobutane could react with the olefin. In general, the yields and quality of the product were not equal to those obtained with catalytic alkylation. 

The thermal alkylation of ethylene-isobutane mixtures at high pressures in the gas phase has been studied in the presence and absence of HCl, and it has been found that HCl can (a) dramatically increase the total yield of alkylate, (b) increase the fraction of the alkylate which is C6 rather than C8> and (c) both increase and decrease the ratio of 2-methyl pentane to 2J2-dimethylbutane in the C6 fraction of the alkylate, this latter depending on the amount of HCl used. All of these effects can be explained readily in terms of the generally accepted free radical mechanism of thermal alkylation, provided one assumes that HCl acts as a catalyst for those reaction steps that involve transfer of a hydrogen atom between a free radical and a hydrocarbon. 

Thus, it would appear that catalyzing the thermal alkylation with HC1 might achieve the same result as operation at high isobutane-to-ethylene ratio. 

To study the relative ease with which various hydrocarbons undergo thermal alkylation with ethylene, a mixture of propane, normal butane, isobutane, and neopentane (9.2 mol % each), 4.6 mol % C2H4, and 17 mol % HC1 were reacted at 399°C and at an initial pressure of 177 atm for 1 hr. The yields of the alkylation products (nC5Hi0 and iC5H10 from C3H6 nC6Hi4 and 3-methyl pentane from nC4Hi0 2-methyl pentane and 2,2-dimethyl butane from isobutane and 2,2-dimethyl pentane from neopentane) were measured and are shown in Table I on a relative basis. 

Thermal alkylation occurs readily with ethylene, less readily with propene and n-butylenes, and with difficulty with isobutylene. (As wiU be shown, the reverse is true in catalytic alkylation.) The reaction of propane with propene at 505° and 6300 p.s.i. yielded a liquid product containing 18.0% 2,3-dimethylbutane, 17.7% 2-methylpentane, and 5.4% n-hexane. An even higher pressure, 8000 p.s.i. was required for the reaction of isobutylene with isobutane at 486° the liquid product, whose yield was only 10-20% of that obtained when ethylene w as used, contained 34.0% octanes but also 32.7% octenes. 

The acid alkylation process works most successfully on the higher molecular weight olefins (such as the butenes), whereas thermal alkylation attacks ethene most readily. Acid alkylation is limited to the isoparaffin hydrocarbons (isobutane and isopentane), but the therma.1 process handles either the iso or the normal compounds. 

Reactions other than those of the nucleophilic reactivity of alkyl sulfates iavolve reactions with hydrocarbons, thermal degradation, sulfonation, halogenation of the alkyl groups, and reduction of the sulfate groups. Aromatic hydrocarbons, eg, benzene and naphthalene, react with alkyl sulfates when cataly2ed by aluminum chloride to give Fhedel-Crafts-type alkylation product mixtures (59). Isobutane is readily alkylated by a dipropyl sulfate mixture from the reaction of propylene ia propane with sulfuric acid (60). 

Alkylation in the petroleum industry, a process by which an olefin (e.g., ethylene) is combined with a branched-chain hydrocarbon (e.g., isobutane) alkylation may be accomplished as a thermal or a catalytic reaction. 

The use of thermal and catalytic cracking processes for the production of high-octane motor gasolines is accompanied by the production of quantities of light hydrocarbons such as ethylene, propylene, butene, and isobutane. These materials are satisfactory gasoline components octane-wise, but their vapor pressures are so high that only a portion of butanes can actually be blended into gasoline. Alkylation is one of several processes available for the utilization of these excess hydrocarbons. 

In broad terms, alkylation refers to any process, thermal or catalytic, whereby an alkyl radical is added to a compound. In the petroleum industry, however, the term alkylation generally refers to the catalytic process for alkylating isobutane with various light olefins to produce highly branched paraffins boiling in the gasoline range. This specific process will be discussed in this paper. 

Successful catalytic alkylation of isobutane with ethylene has been accomplished in one commercial installation using aluminum chloride catalyst (I). The chief product of the reaction is 2,3-dime thy lbutane, a hydrocarbon having very high aviation octane ratings. Ethylene has also been alkylated with isobutane in a thermal process to give 2,2-dimethylbutane as the chief product component (6). When sulfuric or hydrofluoric acid alkylation with ethylene is attempted, the ethylene forms a strong bond with the acid, and fails to react with isobutane. The net result is the formation of little or no product, accompanied by excessive catalyst deterioration. 

Catalytic reforming92-94 of naphthas occurs by way of carbocationic processes that permit skeletal rearrangement of alkanes and cycloalkanes, a conversion not possible in thermal reforming, which takes place via free radicals. Furthermore, dehydrocyclization of alkanes to aromatic hydrocarbons, the most important transformation in catalytic reforming, also involves carbocations and does not occur thermally. In addition to octane enhancement, catalytic reforming is an important source of aromatics (see BTX processing in Section 2.5.2) and hydrogen. It can also yield isobutane to be used in alkylation. 

Studies with sulfated zirconia also show similar fast catalyst deactivation in the alkylation of isobutane with butenes. It was found, however, that original activities were easily restored by thermal treatment under air without the loss of selectivity to trimethylpentanes. Promoting metals such as Fe, Mn, and Pt did not have a marked effect on the reaction.362,363 Heteropoly acids supported on various oxides have the same characteristics as sulfated zirconia.364 Wells-Dawson heteropoly acids supported on silica show high selectivity for the formation of trimethylpentanes and can be regenerated with 03 at low temperature (125°C).365... 

Feed stock for the first sulfuric acid alkylation units consisted mainly of butylenes and isobutane obtained originally from thermal cracking and later from catalytic cracking processes. Isobutane was derived from refinery sources and from natural gasoline processing. Isomerization of normal butane to make isobutane was also quite prevalent. Later the olefinic part of the feed stock was expanded to include propylene and amylenes in some cases. When ethylene was required in large quantities for the production of ethylbenzene, propane and butanes were cracked, and later naphtha and gas oils were cracked. This was especially practiced in European countries where the cracking of propane has not been economic. 

In G, the methyl disproportionation reaction observed in thermal decomposition of tetramethylammonium cations, the hydride receiver may be a surface methoxyl group, v hile the hydrogen-deficient ( oxidized ) moiety is a formaldehyde-like species, and ultimately, C=0 and H2. Ethane was detected (H) as a minor product during the alkylation of isobutane with ethylene over REHX catalyst assuming a classical... 

Derivation By the thermal or catalytic union (alkylation) of ethylene and isobutane, both recovered from refinery gases. 

Propane and higher paraffins may be alkylated thermally. On the other hand, only paraffins which contain tertiary carbon atoms readily undergo catalytic alkylation. No method for the alkylation of methane and ethane in practical yield has been reported. The alkylation of isobutane has been most extensively investigated because the reaction converts this gaseous isoparaffin to liquid gasoline of high antiknock value. 

For example, the reaction of propane with ethylene at 510° and 4500 p.B.i. pressure resulted in a liquid product containing 55.5% by weight isopentane and 16.4% ii-pentane, 7.3% hexanes and 10.1% heptanes 7.4% alkenes were also present. The formation of alkenes and other by-products indicates that thermal cracking occurred. The alkylation of isobutane with ethylene under the same conditions yielded a liquid product containing 44.3% by weight neohexane, 11.6% isohexane, 1.1% n-hexane, 4.5% heptanes, 9.6% octanes as well as minor amounts of other alkanes and alkenes. This reaction has served as a means for producing neohexane commercially. 

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