Conversion isobutane alkylation

Figure 13.7 Conversion of 2-butene and the selectivities to cracking products, TMP, and C9+ hydrocarbons during the isobutane alkylation at 50°C on nafion/Si02 (NS-1), sulfated zirconia (SZ), and MCM-41-supported 12-tungstophosphoric acid (HPW/MCM). Experimental conditions T = 32 C TOS = 1 min molar ratio of 15.

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. 

PFAS were obtained with 2 moles of water, for each mole of acid and they could not be dehydrated with physical methods. Hydrated acids, both as such and supported on silica using water as solvent, were not active in isobutane alkylation. Therefore the effect of different dehydrating solvent was studied, in order to remove residual water. The catalysts obtained by supporting perfluoroethanedisulphonic acid on Si02 (PFES-Si02) after dissolution in various dehydrating solvents were tested in the reaction and resulted active with high butene conversion (Table 1). 

The zeolite composition and structure, which can affect hydrogen transfer activity, are important parameters determining the activity, selectivity, and stability of the zeolite during isobutane alkylation. In the case of USY zeolites, a maximum initial 2-butene conversion was observed for a framework Si/Al ratio of about 6 (63). However, the TMP/DMH ratio, which can be taken as a measure of the alkylation/oligomerization ratio, continuously increased when decreasing the framework Si/Al ratio. On the other hand, the amount and nature of extraframework Al (EFAL) species also affected the alkylation properties of USY zeolites (64). 

Hydrogen transfer resulting in the conversion of more or less of the olefin to paraffin always accompanies the catalytic alkylation of isoparaffins with olefins. At the same time, some of the isoparaffin is converted to paraffin containing twice the number of carbon atoms thus, for example, trimethylpentanes are formed in all isobutane alkylations regardless of the olefin used. 

Fluorinated silica and alumina have also been used for isobutane alkylation with olefins in a batch reactor at 0°C. The fluorinated alumina was active only when mixed with BF3 H2O, but the fluorinated silica was active by itself The selectivity to trimethylpentanes obtained with these catalysts is much lower than that obtained with H2SO4 as a catalyst (48). At 80°C, however, F/AI2O3 catalysts are active for isobutane/2-butene alkylation, though relatively low butene conversions (27% for the most active catalyst containing 1.3% F) are obtained (49). As expected, octenes are the predominant hydrocarbons in the Cg fraction at such a low conversions. The F/AI2O3 catalysts contained both Bronsted and Lewis acid sites in a proportion that depends on the F loading. A good correlation between... 

We cite isomerization of Cs-Ce paraffinic cuts, aliphatic alkylation making isoparaffinic gasoline from C3-C5 olefins and isobutane, and etherification of C4-C5 olefins with the C1-C2 alcohols. This type of refinery can need more hydrogen than is available from naphtha reforming. Flexibility is greatly improved over the simple conventional refinery. Nonetheless some products are not eliminated, for example, the heavy fuel of marginal quality, and the conversion product qualities may not be adequate, even after severe treatment, to meet certain specifications such as the gasoline octane number, diesel cetane number, and allowable levels of certain components. 

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. 

Seasonal chances in gasoline sales and heating oil sales compel some modifications to be made in conversion level. Therefore, the conversion pattern of a given catalytic cracking unit can vary from season to season. In summer operations, for instance, higher yields of motor gasoline are desired, both from direct production of 5/430° FVT catalytic naphtha and also from conversion of butylenes and isobutane to alkylate. 

A variety of solid acids besides zeolites have been tested as alkylation catalysts. Sulfated zirconia and related materials have drawn considerable attention because of what was initially thought to be their superacidic nature and their well-demonstrated ability to isomerize short linear alkanes at temperatures below 423 K. Corma et al. (188) compared sulfated zirconia and zeolite BEA at reaction temperatures of 273 and 323 K in isobutane/2-butene alkylation. While BEA catalyzed mainly dimerization at 273 K, the sulfated zirconia exhibited a high selectivity to TMPs. At 323 K, on the other hand, zeolite BEA produced more TMPs than sulfated zirconia, which under these conditions produced mainly cracked products with 65 wt% selectivity. The TMP/DMH ratio was always higher for the sulfated zirconia sample. These distinctive differences in the product distribution were attributed to the much stronger acid sites in sulfated zirconia than in zeolite BEA, but today one would question this suggestion because of evidence that the sulfated zirconia catalyst is not strongly acidic, being active for alkane isomerization because of a combination of acidic character and redox properties that help initiate hydrocarbon conversions (189). The time-on-stream behavior was more favorable for BEA, which deactivated at a lower rate than sulfated zirconia. Whether differences in the adsorption of the feed and product molecules influenced the performance was not discussed. 

Nafion-H, a perfluorinated sulfonic acid resin, is another strongly acidic solid that has been explored as alkylation catalyst. Rprvik et al. (204) examined unsupported Nafion-H with a nominal surface area of 0.2 m2/g (surface area of a swellable polymer is difficult to define) in isobutane/2-butene alkylation at 353 K and compared it with a CeY zeolite. The zeolite gave a better alkylate and higher conversion than Nafion-H, which produced significant amounts of octenes and heavy-end products. The low surface area of the resin and questions about the accessibility of the sulfonic acid groups probably make the comparison inadequate. 

TABLE 6.1. The 1-butene conversion and product distribution after 1 h of alkylation reaction of isobutane on as-prepared JML-I50 and zeobte Beta catalysts. 

The 1-butene conversion and product distribution obtained at 25°C after 1 h of alkylation reaction of isobutane on JML-I50 and Beta catalysts are summarized in Table 6.1. The conversion (97%) with JML-I50 catalyst is higher than that (86%) with zeolite Beta. The primary products with the above catalysts are Cs compounds (59.9% with JML-I50 and 62% with Beta). The Cg products mainly consist of trimethylpentanes (TMPs), 58.7% for JML-I50 and 73% for zeolite Beta. The TMP/DMH (dimethylhexane) ratios are 13.5 for JLM-I50 and 4.1 for Beta, demonstrating that the selectivity of JML-I50 is higher than that of zeolite Beta. The yields of alkylate are 6.6 mL and 5.2 mL for JML-I50 and Beta zeolite, respectively. The weights of alkylate produced per weight of butene fed to the reactor are 1.13 and 0.95 for JML-I50 and zeolite Beta, respectively. 

FIGURE 6.7. Catalytic conversion of 1-butene in the alkylation of isobutane with 1-butene (at a mol ratio of 12 1) versus reaction time over various catalysts (1 g each) JML-I50 (A) JML-I50 regenerated five times by calcination and sulphation (B) SZ(C) SZ/Si02 (Zr/Si = 50/100, mol/mol) (D) zeolite Beta (Si02/Al203 = 40) (E) zeolite ZSM-5 (Si02/Al203 = 40) (F). 

Rorvik, T., Mostad, H.B., Karlsson, A., and Ellestad, O.H. (1997) Isobutane/ 2-butene alkylation on fresh and regenerated La-EMT-51 compared with H-EMT. The catalysts selectivity changes at high butene conversion in a slurry reactor. Appl. Catal. A, 156, 257-283. 

The propylene-butylene fraction constitutes a large part of the useful hydrocarbons produced by synthesis. It differs from similar fractions derived from petroleum refining in its high olefin (over 80%) and low isobutylene content, but this is no handicap in converting it to high octane gasoline by polymerization or by alkylation, if isobutane is available from another source. Polymerization is effected readily over a phosphoric acid on quartz catalyst with high conversion of propylene as well as butylene. The polymer... 

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. 

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... 

Silica-supported triflic acid catalysts were prepared by various methods (treatment of silica with triflic acid at 150°C or adsorption of the acid from solutions in trifluoroacetic acid or Freon-113) and tested in the isobutane-1-butene alkylation.161 All catalysts showed high and stable activity (near-complete conversion at room temperature in a continuous flow reactor at 22 bar) and high selectivity to form saturated C8 isomers (up to 99%) and isomeric trimethylpentanes (up to 86%). Selectivities to saturated C8 isomers, however, decreased considerable with time-on-stream (79% and 80% after 24 h). 

The isobutane-1 -butene alkylation was studied in dense CO2 in both fixed-bed and slurry reactors.165-167 Both Nafion SAC-13 and Nation SAC-25 exhibited steady-state conversions and selectivities for 50 h. Enhanced Cg alkylate selectivity could be achieved at near total butene conversion. The maximum value attained, however, was only about 40%. The higher effective alkylation rate constant for SAC-25 compared to SAC-13 indicates improved accessibility of the acid sites. Nafion SAC-13 and SAC-25 applied in a study to test the effect of supercritical fluids on alkylation exhibited only modest activities.168... 

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