Isobutane from steam cracking

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. 

When ethylene is reacted at 573 K in the presence of water in static conditions, oligomerization and conjunct polymerization give rise to paraffinic, olefinic and aromatic products (8). Nevertheless, the distribution of the aliphatics and aromatics is quite different from that of the steam-cracking products. In the former a great variety of products is formed they include propane, n-butane, isobutane and isopentane as aliphatics, and toluene, xylenes and ethylbenzene as aromatics (Figure 6B). 

The cracking of isobutane to isobutylene is of special interest because of the high demand for isobutylene as a feed for the production of oxygenates, mainly MTBE, as octane-enhancing gasoline additives. Isobutane separated from the steam cracking C4 cut or produced by butane isomerization is cracked in the presence of steam to yield isobutylene and propylene.148,149... 

Olefins from cat-cracker operations (or even steam cracking of natural gas liquids in the U.S.A.), may be used to alkylate branched alkanes, particularly isobutane (c. H2SO4 or HF catalyst at near ambient temperature). Alternatively, propylene and butenes may be oligomerized, and the resulting Cg -h olefins hydrogenated to give polygasoline . 

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. 

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. 

Calculated concentration of radicals, molecules, and conversion along an isothermal reactor for isobutane cracking T = 775°C Po=lA atm abs 8 = 0.4 kg of steam/kg of butane. From Sundaram and Froment [1978]. 

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