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Ind. Eng. Chem. Res. 1997, 36, 2514-2516
CORRESPONDENCE Comments on “Kinetic Analysis of Isobutane/Butene Alkylations over Ultrastable H-Y Zeolite” Lyle F. Albright School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907
Sir: Interesting and valuable information was reported in this paper. Unfortunately, the kinetic analyses claimed by the investigators are flawed due to several reasons reported here. Choice of Catalyst As mentioned by Simpson et al. (1996), numerous investigators have tested solid catalysts. The major problem reported has been the rapid deactivation of the catalyst. In the past, solid catalyst systems have been reported which apparently can be used for several hours before reactivation is needed. It is surprising that the present investigators chose the ultrastable Y-type faujasite as a catalyst since serious deactivation occurred within several seconds (see Results and Discussion). To obtain what the authors claim to be is reliable kinetic data, the investigators extrapolated their results back to zero time. Whether such an extrapolation provides reliable results is open to question. For example, each product sample was collected over a time period, and the composition of the product probably varied, perhaps appreciably, during each period. Yields Based on Olefins Reacted The investigators either incorrectly calculated yields or had inconsistent data; see, for example, Table 1. They state on p 3863 (first column) that “from the reaction stoichiometry that the total number of moles of various alkanes should add up to the total moles of butenes reacted provided no alkenes or carbonaceous deposit is formed.” That is incorrect. Instead, the total moles of alkanes formed should add up to the total moles of isobutane reacted. Such a conclusion is based on material balances of the carbon and hydrogen atoms. Several past investigators have discussed such material balances including Albright, Kranz, and Masters (1993). Using the analytical results of Table 1, as an example, the molar yields of the three examples were calculated as 0.89, 0.93, and 0.92 instead of 0.75, 1.1, and 1.0, respectively. In making these new calculations of yields, it was assumed the “others” were C12 isoparaffins, which is probably an overly conservative estimate. Based on the recalculated values, yields change little in the 0.11.6 min period. These new values negate the discussion on p 3863 in the paper on why yields increase so much with time. The investigators are, of course, correct to indicate that part of the olefin reacts to form byproducts including the “carbonaceous deposit”, which is possibly retained in the pores. Based on the recalculations provided above, about 8-11% of the butene probably reacted to form the carbonaceous deposit (which will be discussed later). S0888-5885(96)00818-4 CCC: $14.00
Analytical Results for Alkylate Products The reliability and completeness of analytical results of the alkylates produced in this investigation are highly questionable. To minimize catalyst deactivation, ratios of isobutane/olefin in the feedstream were extremely high, in the 500-3500 range. Product streams, as a result, contained about 99.6-99.94 wt % of isobutane and only 0.4-0.06 wt % of alkylate (C5 to C16 isoparaffins). Commercial alkylates produced using HF or H2SO4 have been found to contain at least 140-160 chromatographic peaks. In the present investigation, some chromatographic peaks were obviously very small. It is recognized that relatively large samples were injected into the unit, but splitters are often required to get satisfactory operation when capillary columns are used. In any case, the results presented here suggest that the peaks of numerous isoparaffins were so small that one of the following occurred: first, the peak could not be detected or was not reported or, second, the relative accuracy of the value reported was poor. Such problems apparently lead to all of the following: (1) Problems in calculating yields, as already discussed. (2) Failure to report several isoparaffins found in other alkylates, including 2,2,3-trimethylpentane, 2-methylpentane, 2,3-dimethylbutane, 2-methylhexane, and 3-methylhexane. (3) Low values of others (presumably C10 and heavier isoparaffins). Alkylates produced commercially using either H2SO4 or HF frequently contain 12-15 wt % of heavy isoparaffins and contain at least 100-120 different isoparaffins. The present investigators postulate that at the extremely high ratios of isobutene/olefin reduced amounts of heavy isoparaffins are produced or such heavy materials diffuse to only a small extent out of the catalyst pores. As will be discussed later in the deactivation section, considerable oligomerization appears to have occurred, which suggests that high concentrations of heavy isoparaffins actually occurred. There is clearly a need for further investigations of heavy end production. It seems highly probable, however, that the results for “others” as reported in this paper are quite inaccurate and incomplete. Chemistry of Alkylation Table 4 entitled “Mechanism for Isobutane/Butene Alkylation over USHY” is misleading and incomplete in key details. The table and the discussion in the paper indicate, or at least imply, that the alkylation mechanisms when H2SO4 and HF are used are identical; that is incorrect in key details. The authors did not provide any experimental answers to the chemical steps (or © 1997 American Chemical Society
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basic mechanism) with zeolite catalysts. When H2SO4 and HF are used as the catalyst, much better mechanistic information is now available. (1) Formation of t-C4H9+’s from 2-butenes. Selfalkylation is said to be the mechanism for forming t-C4H9+’s, and n-butane is a byproduct. Although nbutane is said to be formed on p 3862, it is later reported (p 3865) that no positive evidence for n-butane formation was found. For alkylations with HF as a catalyst, appreciable amounts of n-butane are produced; for alkylations using H2SO4 as a catalyst, indetectable amounts of n-butane are formed. The current investigators indicated that because their feedstocks contained n-butane their results relative to n-butane formation were inconclusive. To support their claim that n-butane was produced, they report that n-pentane was produced when 2-pentene was used as the olefin feed. Such evidence is not convincing, as explained next. Albright and Kranz (1992) also found that when isobutane was alkylated with n-pentenes in the presence of H2SO4, n-pentane was a byproduct. In conclusion, when H2SO4 is the catalyst, self-alkylation is of importance with n-pentenes but not with n-butenes. Table 4 fails to include an important method for producing t-C4H9+’s. A hydride ion can be transferred from isobutane to conjunct polymers (also often referred to as acid-soluble oils or red oils); see, for example, Albright et al. (1988a). Conjunct polymers are always byproducts formed during alkylations using HF or H2SO4. The quality of alkylate increases significantly when a relatively small amount of conjunct polymers is added to 99-99.5 wt % H2SO4. As will be discussed later, such conjunct polymers are almost certainly produced as byproducts during alkylations using zeolites. Hopefully, future investigators using solid catalysts will get definite answers on whether n-butane is a byproduct when 2-butenes or 1-butenes are used as olefins. (2) Disproportionation (R9 of Table 1): R9 is undoubtedly of major importance. Based on especially p 3866 and Table 5, the current investigators appear to have greatly underestimated its importance. (a) The basic chemistry was reported by Hofmann and Schriesheim (1962), who used tagged carbon atoms. Since then, additional information has been obtained. All C4-C16 isoparaffins are produced via disproportionation-type chemistry (Albright et al., 1988a,c; who refer to it as Mechanism 2). Both TMP’s and DMH’s were produced in part at least by this mechanism in all runs of this investigation. This fact is of key importance for the development of theoretical mathematical models. (b) In Table 5, both TMP’s and DMH’s were produced when 2-pentene was employed as a feed. It was suggested by the investigators that self-alkylation was the probable mechanism. If that were true, considerably more n-pentane would have been produced. A much more probable explanation is disproportionationtype reactions. (c) DMH production occurs predominantly via disproportionation-type reactions for alkylations using both C4 olefins and sulfuric acid. In such a case, R4, R5, and R7 as shown in Table 4 are of minor importance. With H2SO4, considerably fewer DMH’s are produced using 1-butene as compared to isobutylene. (d) R9 in Table 4 is partly incorrect, since neither C2’s nor C3’s are produced. The authors corrected themselves at the top of p 3866. In addition to C12+’s, at least C13+’s through C20+’s disproportionate; these heavy ions
can be produced with C3-C5 olefins. It is even known that TMP+’s fragment, or disproportionate, as discussed later. The 2-pentene run (Table 5) provides information that can be used to predict the importance of disproportionation for production of both TMP’s and DMH’s. In that run, the molar ratio of C8/(C6 + C7) ) 2.1 and TMP/ DMH ) 4.8. These ratios may be somewhat different when other olefins are used, but they suggest that 2035% of TMP’s are produced by disproportionation in 2-butene runs, such as is shown in Table 1. (3) Methods to produce TMP’sand DMH’s: It is claimed on p 3866 that dimethylhexanes (DMH’s) are produced by two routes and trimethylpentanes (TMP’s) by only one. It is unclear what is meant by “routes”, but it seems highly incorrect for H2SO4, HF, and zeolite catalysts. There are at least three sequences to produce TMP’s: R3 and R6; R8, R9, and R6; and R1, R2, R3, and R6. As already discussed, Table 4 does not include all steps to produce t-C4H9+’s or to transfer hydride ions. More than two sequences of reactions are also available to produce DMH’s. It should be emphasized that at least when H2SO4 is the catalyst R3 is of minor importance for DMH+ production. (4) Intermediate formation: Table 4 fails to report that esters are produced as intermediates to a relatively large extent when either H2SO4 or HF is used as a catalyst. Esters readily form when either acid contacts normal olefins including propene, 1-butene, 2-butenes, 1-pentene, or 2-pentenes but not isoolefins. Such formation of esters explains why n-olefins often, if not always, react more rapidly than isobutane during alkylation. These esters then react later in the reaction sequence with isobutane to form alkylate. In this paper, it is reported that 2-butene adsorbed strongly on the zeolite; see p 3867. Maybe an “ester” was produced. Of interest, isoolefins also react more rapidly than isobutane. The isoolefins tend to form C12-C20+’s, discussed earlier. (5) Deactivation of zeolite catalysts: The authors suggest that the zeolite catalysts are deactivated by a deposit that was “mostly paraffinic in nature with a H/C ratio of 1.8”; see p 3867. How can a compound be mostly paraffinic when the C/H ratio is as low as 1.8? This deposit appears to be quite similar to conjunct polymers, which are byproducts formed during alkylations using HF, H2SO4, and AlCl3. They have C/H ratios of about 1.7-1.8 and contain numerous -CdC- bonds and conjugated diene groups (Miron and Lee, 1963; Hengstebeck, 1965; Carlson et al., 1966; Albright et al., 1988b). These low polymers contain C5 and possibly C6 rings, branch groups, and frequently have molecular weights of several hundred. It seems most probable that the deactivating “deposits” for zeolite catalysts are really conjunct polymers. Oligomerization of olefins seems to be involved as part of their method of formation. As discussed earlier, such evidence supports the conclusion that appreciable amounts of C10 and heavier isoparaffins were also produced. The conjunct polymers react at least to some extent to form sulfate ester groups in the presence of H2SO4 (Albright et al., 1988b). Presumably, fluoride esters form when contacted to HF. The presence of numerous tertiary C-H groups in the conjunct polymer acts as a reservoir of hydride ions. Under some conditions, the polymers are a source of hydride ions but accept these ions under other conditions. The apparent formation of conjunct polymers with solid catalysts undoubtedly has important effects. Almost certainly it would be a deactivating agent since it
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would have low rates of diffusion out of the pores. Furthermore, ester-like compounds might form on the catalysts surfaces. Some olefins, such as isobutene and isopentenes, likely promote even more rapid rates of deactivation than 2-butenes. It is not clear if the current investigators considered this approach. (6) Isomerization of 2-butenes: Some 2-butenes were found to isomerize to 1-butene (see p 3862), and R1 of Table 5 is such an isomerization step. Based on thermodynamic considerations, less than 1% of the 2-butenes isomerize to 1-butene. Hence, R5 of Table 5 is of minor importance. It is of interest that H2SO4 isomerizes n-butenes to a much higher degree during alkylation as compared to HF. (7) Kinetic modeling: Since the chemistry of alkylation is more complicated than that implied in Table 1, serious reservations result as to the theoretical value of the kinetic model developed. Concerns include ignoring the disproportionation sequence, failing to consider probable intermediates, and emphasizing R3 and R6 which is essentially a chain mechanism, as proposed by Schmerling (1964). As already discussed, chain mechanisms for alkylations with both H2SO4 and HF are at best only partly true, and both R4 and R5 are of only minor importance for alkylations with H2SO4 and probably with HF. There is no known evidence that they are of importance with zeolite catalysts. Estimates based on the analytical data obtained, such as those reported in Tables 1 and 5, indicate that slightly more than 50% of the isoparaffins are produced by disproportionation and other steps other than the R3-R7 sequence, used for modeling purposes. As already reported, R6 and R7 account for only part of the hydride transfer. In conclusion, R3-R7 likely accounts for considerably less than
