Isobutane olefin alkylation

A large number of investigators have offered theories on reaction mechanisms in HF- or H2SO -catalyzed isobutane-olefin alkylation. 

Isobutane-olefin alkylation Is accepted to proceed via carbonlutn-lon mechanism. The close similarity (Table II of the product obtained over a zeolite catalyst (Catalyst 4 of Table I) and that produced In the presence of H-SO. contributes to the point. ... 

Some specific factors should be considered when using zeolites as catalysts in isobutane/olefin alkylation. The first is the strong hydrocarbon adsorption of zeolites, especially at low temperatures, which makes the actual concentration of the reactants in the zeolite quite high. In addition, and due to the high concentration of acid sites in the zeolite cavities, zeolite would behave as a liquid-acid catalyst but with a high solubility of reactants. Taking this into account it can be easily assumed that the Si/Al ratio can determine the nature of reaction products. In this way, it has been proposed that zeolites... 

On the other hand, ethylene cannot be used in isobutane/olefin alkylation on sulphuric acid, because of the formation of stable ethylsulfates. However, when using a R,Ca-Y catalysts in combination with transition-metal cations (especially Ni) (Minachev et al. 1977) an alkylate containing octane isomers as the major product (about 80%), without the formation of hexanes has been obtained. They observed a strong influence of the chemical composition on the catalytic behaviour of R,Ca-Y zeolites, showing a maximum on a R,Ca-Y (16.6% Ca and 64.2% R) zeolite. 

The gasoline obtained by isobutane-olefins alkylation is an ideal component for the gasoline pool through its substantial contribution to the octane number and the absence of aromatics and olefins. The technologies used go back a long way and are technically well mastered, but the liquid catalysts used pose some problems human risk regarding hydrofluoric acid, large quantities of waste ( red oils ) that have to be retreated for the sulphuric acid. These problems somewhat slow down development which would otherwise be rapid. 

Several applications of the in situ decoking concept have appeared in the literature. One such application involves stabilizing the activity of solid acid catalysts such as in alkylation reactions. As reviewed elsewhere (55), numerous efforts aimed at developing solid acid alkylation catalysts and solid acid-based isobutane-olefin alkylation processes have been reported for more than three decades. However, to date, none of the solid alkylation catalysts has gained acceptance in industry for one or more of the following drawbacks rapid catalyst... 

A range of PILs was trialed in the Friedel—Craft alkylation of phenol with tert-butyl alcohol to produce tert-butyl alcohol as mentioned in section 6.1. These included a series of pyridinium PILs specifically designed for use as acid catalysts by Duan et al. and SO3H functionalized Bronsted acidic AILs. " It was shown successfully from the pyridinium PILs that a range of PILs could be easily produced that modified the acidities, catalytic activities, and, hence, selectivities and conversions for this reaction type, with the best performance by 2-methylpyridinium, with a conversion of 95%, and a selectivity toward 2,4-di- r butylphenol of 82%. In general, the SO3H functionalized AILs led to comparable selectivities and conversions for this reaction as the PILs. " Some nonstoichiometric salts containing pyridinium cations with HF anions have been used successfully as the catalyst and reaction media for the isobutane—olefin alkylation reactions. 

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. 

The ability of zeolites to catalyze paraffin-olefin alkylation has been known since 1968. Garwood and Venuto of Mobil described the use of rare-earth hydrogen X faujasite for the alkylation of isobutane with ethylene to give branched Cg-Cg... 

The proposed mechanisms may also be used to explain the formation of paraffins having both lower and higher molecular weights than would be expected from simple addition of olefin molecules to isobutane molecules. A typical example is the formation of heptanes and nonanes when isobutane is alkylated with butene. The first step consists of... 

In the alkylation of isobutane with butenes, several variables have an important bearing on the quality of the alkylate produced. The most important is the concentration of isobutane in the reactor. Although theoretically only equimolecular ratios of isobutane and butene are required for the reaction, a large excess of isobutane in the reaction zone has been found necessary to suppress undesirable side reactions which result in loss of yield and octane number. Over-all isobutane-olefin ratios of 5 to 1 or higher are necessary for the production of high quality aviation alkylate. 

The catalyst consumption for sulfuric acid alkylation is expressed in terms of pounds of fresh acid depleted per barrel of alkylate produced. When alkylating isobutane with butenes at 50° F. and maintaining an isobutane-olefin ratio of 5 to 1, the acid consumption will average 35 to 40 pounds per barrel when charging 98% acid and discarding 88% acid in a batchwise operation. 

Alkylation processes usually combine isobutane with an alkene or with mixed alkene streams (C3-C5 olefins from FCC units). The best octane ratings are attained when isobutane is alkylated with butylenes. Alkylation of higher-molecular-weight hydrocarbons (>C5) is less economic because of increased probability of side reactions. Phillips developed a technology that combines its triolefin process (metathesis of propylene to produce ethylene and 2-butenes) with alkylation since 2-butenes yield better alkylate than propylene.290 Since ethylene cannot be readily used in protic acid-catalyzed alkylations, a process employing AICI3 promoted by water was also developed.291... 

Control of the important operating parameters other than temperature and the degree of mixing has also led to significant improvements in alkylation (17). For example, the effects of isobutane-olefin ratio, acid strength, acid concentration in the reaction mixture, and olefin space velocity have been recognized, and efforts to control these variables in the preferred ranges have resulted in more profitable operations. 

Considerable effort has been put into minimizing the adverse effects of these olefins. It was found that alkylating propylene and pentylenes in a mixture with butylenes promoted the desired reactions and reduced the octane and acid consumption penalties. Furthermore, by optimizing temperature, isobutane-to-olefin ratio, acid strength, and other variables, the deleterious effects of propylene and pentylenes in the feed can be minimized (4, 8, 21). The decision as to how much of these olefins to include in the alkylation unit feed depends on many different factors, such as their value relative to alkylate, butylene and isobutane avails, alkylate volume and octane requirements, acid costs, etc. 

In the alkylation process, the main reaction involves the olefin and isobutane. In contrast, the secondary reactions consist of olefin polymerization or the reaction between the olefin and C8 paraffins. For this reason, a high concentration of isobutane in the reactor is necessary. In our design the isobutane olefin ratio is 7.3 1, while typical values are in the range 5 1 to 10 1 [7]. Note that the reactants are fed to the process in a nearly stoichiometric proportion, the high excess of isobutane being accomplished by recycling. 

The effect of FSO3H on HF alkylation was very much like that of CF3SO3H. The 9-to-l blend of Isobutane with refinery olefins, alkylated earlier with HF alone (Table IV), was used to study the catalytic properties of HF-FSO3H blends. Table VIII gives the results of runs made at 4 C and 1.0 minute... 

It is noteworthy that good performance was achieved at high olefin space velocities with low external isobutane/butene ratio. For comparison purposes sulfuric acid alkylation operates at an olefin space velocity of approximately 0.2-0.4, and an external iso-butane/olefin ratio of about 5. HF alkylation, while permitting operation at higher olefin space velocities than H2SO4 alkylation, requires external isobutane/ olefin ratio of about 15. 

It is well known that olefin space velocity and external isobutane/olefin ratio has a pronounced effect on alkylate quality with both H2SO4 and HF alkylation. 

When isobutane-olefin mixtures are contacted with sulfuric acid at alkylation conditions of coimierclal Interest, the olefins often. If not always, disappear from the hydrocarbon phase at a faster rate than the Isobutane (1,2). Subsequently, the Isobutane reacts to produce alkylate tdilch Is often predominantly trimethyl pentanes (TMP s). Other Isoparaffins formed to a large extent during the Initial stages of alkylation Include dimethyl-hexanes (DMH s), C5-C7 Isoparaffins referred to as light ends (LE s), Cg and heavier Isoparaffins often called heavy ends (HE s), and acid-soluble hydrocarbons sometimes referred to as conjunct polymer or red oil. The relatively rapid disappearance of the olefins from the hydrocarbon phase Is undoubtedly caused In part by the relatively high solubility of the olefins In the acid phases. 

In this Investigation, acid was rather slowly added to the Isobutane-olefin mixture. By the time sufficient acid was added to obtain an acid/olefin (A/0) molar ratio of about 1.2, all olefins had been removed from the hydrocarbon phase. While the acid was being added, acid droplets were well dispersed because of vigorous agitation throughout the hydrocarbon phase. Acids tested were 92.5 to 98% fresh acid (water being the only Impurity) and both Amoco and Sohio used alkylation acids. When 98% fresh acid was used, part of the acid Initially froze and deposited on the walls of the reactor. Gradually, the frozen acid disappeared presumably because It reacted to form butyl sulfates. 

AAcid diluent commonly referred to as red oil or sludge which is formed primarily in the reactor (emulsion residence time in the settler is an order of magnitude less than the reactor). Although feed contaminants such as dienes or sulfur are important red oil precursors, it can also be formed from the olefin, isobutane and alkylate. 

Before the days of the sophisticated mechanisms such as those being presented in this Symposium, it was thought that probably the olefin first formed an alkyl sulfate before reacting with isobutane. In alkylation, reaction conditions were selected so as to keep the concentration of the alkyl sulfates low. It was standard procedure in batch laboratory work to have a finishing period, that is, a period at the end of a run under alkylation conditions except that no olefin was charged. 

From early work done for other purposes. It seemed certain that at least qualitatively olefins could be absorbed in recycle sulfuric acid alkylation catalyst, the resulting dialkyl sulfates extracted with isobutane and alkylated. Evidence as to what would happen to the conjunct polymer present in the recycle acid and the additional conjunct polymer formed in the absorption step when the acid phase of the absorption mixture was extracted with isobutane was missing. There was some concern that the conjunct polymer, or at least part of it, would also be extracted by the isobutane. 

Isobutane is alkylated with C3-C5 olefins to produce the highest-quality and cleanest-burning gasolines. More than 1.1 million barrels of alkylate/day are produced in the United States (P. Pyror, personal communication). The amounts of alkylate produced in the remainder of the world are considerably less, but they are increasing at a significant rate. In the United States, about 11-13% of the total gasoline pool is alkylate. This percentage depends on the location in the United States and on the season. Alkylate production will likely increase substantially in the future. 

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