Isobutane from catalytic cracking

Butane Isomerization—C4 isomerization will be limited to smaller refineries which do not contain hydrocracking facilities. Since the i-C4/ n-C4 ratio for hydrocracking is about 2/1 to 3/1 sufficient isobutane should be available from this source to alkylate all of the available (C3-, C4-, and C5-) olefins from catalytic cracking. 

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. 

Isobutene is present in refinery streams. Especially C4 fractions from catalytic cracking are used. Such streams consist mainly of n-butenes, isobutene and butadiene, and generally the butadiene is first removed by extraction. For the purpose of MTBE manufacture the amount of C4 (and C3) olefins in catalytic cracking can be enhanced by adding a few percent of the shape-selective, medium-pore zeolite ZSM-5 to the FCC catalyst (see Fig. 2.23), which is based on zeolite Y (large pore). Two routes lead from n-butane to isobutene (see Fig. 2.24) the isomerization/dehydrogenation pathway (upper route) is industrially practised. Finally, isobutene is also industrially obtained by dehydration of f-butyl alcohol, formed in the Halcon process (isobutane/propene to f-butyl alcohol/ propene oxide). The latter process has been mentioned as an alternative for the SMPO process (see Section 2.7). 

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. 

The alkylation unit in a petroleum refinery is situated downstream of the fluid catalytic cracking (FCC) units. The C4 cut from the FCC unit contains linear butenes, isobutylene, n-butane, and isobutane. In some refineries, isobutylene is converted with methanol into MTBE. A typical modern refinery flow scheme showing the position of the alkylation together with an acid regeneration unit is displayed in Fig. 1. 

Besides ethylene and propylene, the steam cracking of naphtha and catalytic cracking in the refinery produce appreciable amounts of C4 compounds. This C4 stream includes butane, isobutane, 1-butene (butylene), cis- and trans-2-hutene, isobutene (isobutylene), and butadiene. The C4 hydrocarbons can be used to alkylate gasoline. Of these, only butadiene and isobutylene appear in the top 50 chemicals as separate pure chemicals. The other C4 hydrocarbons have specific uses but are not as important as butadiene and isobutylene. A typical composition of a C4 stream from steam cracking of naphtha is given in Table 8.3. 

The original source of feed stock for the production of aviation gasoline was the butylene-isobutane portion of products from thermal cracking. Later, the thermal cracking units were replaced by catalytic cracking units, and most of the feed stocks are now derived from this source. To a... 

As can be seen from Table I, only the Airlift catalytic cracking unit has sufficient isobutane to react with all of the butylenes. The isobutane requirement for alkylation is about 1.15 vol. for every volume of butylene, as indicated in Table II. Besides this quantity required for the reaction, there is also an additional quantity of isobutane which is lost from the alkylation fractionation section. The latter can be from 2 to 25% of the normal butane leaving the unit, depending upon the separation efficiency in the deisobutanizer. 

The olefin and isobutane feed streams used in the mixer alkylation comparison were obtained from Exxon Congiany USA s Baton Rouge Refinery. The olefin stream was catalytically cracked butenes while the isobutane stream was obtained as the overhead from a deisobutanizer tower. The composition of these streams is shown below ... 

The continuous increase in world consumption of MTBE has created a strong incentive to increase the production of isobutylene. Isobutylene can be produced by catalytic dehydrogenation of isobutane. However, the largest production of C4 olefins comes from the thermal cracking processes for the manufacture of ethylene which generate as by-products C4 mixtures containing C4 olefins and C4 alkanes plus butadiene. Isobutylene is also a product of fluid bed catalytic cracking units. 

The chief sources of olefins are cracking operations, especially catalytic cracking. However, olefins can be produced by the dehydrogenation of paraffins butanes are dehydrogenated commercially to provide feeds to alkylation. Isobutane is obtained from crude oils, cracking operations, catalytic reformers, and natural gas. To supplement these sources, n-butane is sometimes isomer-ized. Only small concentrations of diolefins are permissible in feeds to alkylation, particularly for sulfuric add catalyst. Diolefins increase the consumption of acid. 

We have seen that by 1973 catalytic cracking will only satisfy 2 to 4 billion lbs/year of a projected 11 billion lb/year propylene demand. Most of the balance will be produced as a by-product of ethylene manufacture. Shifting from ethane and propane to heavier stocks such as n-butane and gas oil will satisfy propylene needs. Some propylene will also be produced from isobutane steam crackers as an isobutylene co-product. 

Fig. 5.1. Chromatograms of products of catalytic cracking (A) without reactor and (B) with reactor. Sorbent, 11% quinoline on refractory brick temperature, 25 C column length, 10.5 m. Peaks 1 = propane 2 = propylene 3 = isobutane 4 = n-butane 5 = isobutene 6 = butene-1 7 = rmns-butene-2 8 = cis-butene-2 9 = isopentane 10 = 3-methylbutene-l 11 = n-pentane 12 = pentene-1 13 = 2,2-dimethylbutene 14 = 2-methylbutene-l 15 = tnms-pentene-2 16 = cfsi)entene-2 17 = 2-methyl-butene-2 18 = 2,3-dimethylbutane 19 = 2-methylpentane 20 = 3-methylpentane 21 = 3-methylpen-tene-1 22 = 4-methylpentene-l 23 = c -4-methylpentene-2 24 = cyclopentane 25 = 2,3-dimethyl-butene-1 26 = fmns-4-methylpentene-2 27 = w-hexane 28 = cyclopentene 29 = 2-methylpentene-l 30 = hexene-1 31 = 2,4-dimethylpentane 32 = cis-hexene-3 33 = tnms-hexene-3 34 = 2-ethylbu-tene-1 35 = trans-hexene-2 36 = methylcyclopentane 37 = cis-methylpentene-2 38 = 2-methylpen-tene-2 39 = pisns-3-methylpentene-2 40 = methylcyclopentene-4 41 = 4-methylcyclopentene 42 = cw-3-methylpentene-2 43 = 2,3-dimethylpentane 44 = 2-methylheptane 45 = 2,3-dimethylbutene-2 46 = methylheptane 47 = cyclohexane 48 = C, olefin. Reprinted with permission from ref. 1.

To run a refinery alkylation unit isobutane and light olefins are required as feedstock. However, the composition of the olefin stream varies significantly with the local refinery situation and this requires careful adjustment of the process conditions. The most commonly used olefins are butenes and propene but sometimes the use of pentenes is also considered. New gasoline specifications and the Chan Air Act (a United States federal law) amendments make it necessary to remove pentenes from the gasoline pool, because of their potential for atmospheric pollution. The main sources of olefins are catalytic cracking and coking processes. The isobutane feed for alkylation units is mainly obtained from hydrocrackers, catalytic crackers, and catalytic reformers. Additional amounts of isobutane are directly available from crude distillation and natural gas processing. Moreover, n-butane can be... 

MTBE is produced by reacting methanol with isobutene. Isobutene is contained in the C4 stream from steam crackers and from fluid catalytic cracking m the crude oil-refining process. However, isobutene has been in short supply in many locations. The use of raw materials other than isobutene for MTBE production has been actively sought. Figure 2 describes the reaction network for MTBE production. Isobutene can be made by dehydration of i-butyl alcohol, isomerization of -butenes [73], and isomerization and dehydrogenation of n-butane [74, 75]. t-Butanol can also react with methanol to form MTBE over acid alumina, silica, clay, or zeolite in one step [7678]. t-Butanol is readily available by oxidation of isobutane or, in the future, from syngas. The C4 fraction from the methanol-to-olefins process may be used for MTBE production, and the C5 fraction may be used to make TAME. It is also conceivable that these... 

Poisoning studies carried out by several groups have shown that the equivalents of poison needed to quench the catalytic activity of the dealuminated Y-type zeolites are much less than the number of Alf atoms. Beyerlein et al. (9) reported that residual sodium cations extensively decreased the isobutane cracking activity of steam-dealuminated Y-type zeolites. From their results it was concluded that only one-third of the Alf atoms were associated with strong acidity throughout the Si/Al > 5 composition domain. 

The cracking pattern on Pt-Ce is similar to the one observed on Platinum catalysts on Pt-Co catalysts, the demethylation reaction is more important than on Pt catalyst. When Ni is added, repetitive cracking reactions occur the ratio of isobutane over n-butane is lower than 1 on these latter catalysts which means that multiple processes occur on the catalytic surface because n-butane cannot be obtained from 2-methylpentane with only one carbon-carbon bond breaking. 

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