Alkylate quality

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. 

The amount of acid in the acid-hydrocarbon reaction mixture also has an important bearing on the alkylate quality. If the reaction mixture contains less than 40% acid by volume, an acid-in-hydrocarbon emulsion results. Above this 40% inversion point, a hydrocarbon-in-acid emulsion is formed. The latter type produces the better product and consequently an acid volume of 60 to 70% of the reaction mixture is normally maintained. 

The catalyst usually employed for sulfuric acid alkylation is 98% sulfuric acid, although concentrations as high as 100% are equally satisfactory. The use of fuming acids is not desirable since the excess sulfur trioxide reacts with isobutane and does not serve as a catalyst. It is possible to use sulfuric acid of concentrations as low as 90%, but the use of these weaker acids has an adverse effect on the alkylate quality and catalyst life. 

Because propylene is highly volatile and must be marketed as fuel gas rather than as gasoline, it is low in cost and would appear to be a desirable alkylation feed stock. Balanced against its low cost, however, are the increased catalyst consumption and decreased product quality encountered in its alkylation. Consequently, its inclusion in alkylation feed is usually limited to minor quantities by the alkylate quality required for the maximum production of aviation gasolines. 

Pentene alkylation also has the disadvantages of increased catalyst consumption and decreased alkylate quality. A further disadvantage is that pentenes are a satisfactory motor gasoline blending stock and are thus a more expensive alkylation charge stock. For these reasons, commercial alkylation of pentenes is not extensively practiced. 

Butenes can also be alkylated in the form of various polymers, such as the by-product diisobutene polymers from butadiene plants. In this operation, each octene molecule appears to react as two individual butene molecules, and the high alkylate quality and low catalyst consumption characteristic of butene alkylation are obtained. For the most part, polymers have been alkylated only as supplemental feed stocks from external sources in periods of high aviation gasoline demand. 

This correlation is based on a factor which combines those variables exerting the greatest influence on alkylate quality in any specific operating unit. The correlating factor has been called F and is defined as follows ... 

A change in any one of the three included variables which improves the quality of the product will increase the numerical value of the factor hence, the larger the factor, the better the alkylate quality. In ordinary commercial operation F will vary from about 10 to 40. Acid content of the reaction mixture and reaction time, while not directly a part of the factor, are so interrelated that if the acid content is maintained above 40%, the olefin space velocity term correlates the effects of these variables satisfactorily. While reaction temperature has an important effect on product quality, it has not been included in this factor since this variable is ordinarily held constant at an optimum value for any well-... 

The permission of the Sinclair Refining Co. for the publication of the alkylate quality correlations is gratefully acknowledged. 

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... 

Over the years H2S04 alkylation capacity has declined relative to HF alkylation.303,304 Reasons for this trend are a significantly lower acid consumption, overall lower operation costs, and less expensive regeneration plant cost and operating expenses of the HF alkylation process. Because of the mentioned environmental hazard associated with anhydrous HF and the need for further improvement of alkylate quality, improved technologies for sulfuric acid alkylation emerged, although it is improbable that there will be a major turnback to sulfuric acid alkylation. 

Albright, L., 2002, Alkylation of Isobutane with C3-C5 Olefins Feedstock Consumption, Acid Usage, and Alkylate Quality for Different Processes, Ind. Eng. Chem. Res., 41, 5627-5631... 

In preliminary experiments it was found that alkylate quality was essentially Independent of stirring speeds above 1000 rpm. Accordingly all runs were made at 1200 rpm except when stirring was being investigated. 

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. 

A comparison of Tables I and II demonstrates the importance of side reactions In determining alkylate quality. The quality of the alkylates produced from isobutene, diisobutene, cis-2-butene, and trans-2-butene was greatly improved when side reactions were reduced by lowering the reaction temperature to 4°C. At 45°C, the alkylates obtained from these olefins were remarkably similar in composition. The estimated ROM s ranged from 94.6 to 94.8. Reducing the reaction teiq>erature to 4"C Inhibited the formation of DMH s and increased the yield of IHP s, boosting octane ratings 1.5 to 4.0 RON. 

The net effect on alkylate quality was that RON Increased from... 

In Table IX alkylation was carried out at various temperatures with a catalyst blend containing 9.7 wt % FSO3H. As the reaction temperature was raised, undesirable side reactions Increased. A conq>arison of Tables IX and IV shows that above 25°C the addition of 9.7 wt % FSO3H was detrimental to alkylate quality. 

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. 

For resin/BF3 catalyst, the above process variables also affect alkylate quality. However, with the resin/ BF3 catalyst, the surface area of resin in addition to the functional group of the resin, may also play an important role in directing alkylation. Some results illustrating the effect of the resin s surface area on alkylate quality are shown in Table III. clearly, increasing the resin s surface area improves the alkylate quality both in terms of the fraction of trimethyl-pentanes in the C5+ alkylate and the clear research octane number (RON) of the C5+ alkylate. 

For the resin with a particle size of 30-40 mesh, less TMP and more Cg" " were produced in the C5+ resulting in an alkylate with a lower RON+0. With particles smaller than 100 mesh, there is almost no difference in alkylate quality, although the Cg+ content seems to increase only slightly with increasing particle size. It is concluded that particles with 100+ mesh are sufficiently small to overcome the diffusion limitation problem. 

Among the resins tested, the Amberlyst XN-IOIO/BF3 catalyst system appeared to be the most selective one, producing the best alkylate quality. This catalyst showed much less difference among different butene feedstocks than conventional HF and H2SO4 alkylation catalysts. In addition, its alkylate quality was improved as the temperature was decreased, reaching a plateau at about 0°C at which an alkylate with a R( I clear of... 

Alkylates produced In runs made at different operating conditions often were found to have relatively different amounts of the four main groups (or families) of Isoparaffins, namely TMP s, LE s, DMH s, and HE s. As a general rule. Increased RON values resulted because of Increased fractions of TMP s and decreased fractions of the other three. Variables that had a significant effect on the alkylate quality were as follows ... 

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