2- Butenes from catalytic cracking

Butadiene is mainly obtained as a byproduct from the steam cracking of hydrocarbons and from catalytic cracking. These two sources account for over 90% of butadiene demand. The remainder comes from dehydrogenation of n-butane or n-butene streams (Chapter 3). The 1998 U.S. production of butadiene was approximately 4 billion pounds, and it was the 36th highest-volume chemical. Worldwide butadiene capacity was nearly 20 billion pounds. 

Dehydrogenation ofTertiary Amylenes. The starting material here is a CB fraction which is cut from catalytic cracking of petroleum. Two of the tertiary amylene isomers, 2-methyl-l-butene and 2-methyl-2-butene, are recovered in high purity by formation of methyl tertiary butyl ether and cracking of this to produce primarily 2-methyl-2-butene. The amylenes are mixed with steam and dehydrogenated over a catalyst. The crude isoprene can be purified by conventional or extractive distillation. 

MTBE is produced by reacting methanol and isobutylene under mild conditions in the presence of an acid catalyst. The isobutylene feed is either mixed butylenes, a butylenes stream from catalytic cracking, or a butylenes coproduct from ethylene production. The reaction conditions are mild enough to permit the n-butenes to pass through without ether formation. Figure 10.31 shows a typical process for making MTBE. 

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). 

Manufacturing processes today employ petroleum raw materials. In Europe and Japan, butadiene is obtained entirely by extraction from steam C4 cuts (see Se on 3.1.2). In the United States, it is also produced by the dehydrogenation of butane and particularly of butenes contained in C4. cuts from catalytic cracking. 

The naphtha fraction from catalytic cracking contains 30 to 40 per cent of the isoamytenes 2-methyl l-butene and 2-methyl 2-buienes (see Section Z3.1). They can be obtained in a purity of 95 to 99 per cent by tuo-step extraction (such as the ARCO technolo ) ... 

C4 cuts from catalytic cracking contain little butadiene and acetylenic compounds. Hence they can be used directly for isobutene separation processes, but require prior hydrogenation to obtain 1-butene. By contrast, steam cracked effluents must systematically undergo hydrogenation pretreatmcnL This is necessary to eliminate the compounds liable to cause highly exothermic side-polymerizations, and to form gums that disturb the operation of the catalyst systems, solvents and adsorbents used in steps designed to produce the different C4 olefins. 

IFP Process for 1-Butene from Ethylene. 1-Butene is widely used as a comonomer in the production of polyethylene, accounting for over 107,000 t in 1992 and 40% of the total comonomer used. About 60% of the 1-butene produced comes from steam cracking and fluid catalytic cracker effluents (10). This 1-butene is typically produced from by-product raffinate from methyl tert-huty ether production. The recovery of 1-butene from these streams is typically expensive and requires the use of large plants to be economical. Institut Francais du Petrole (IFP) has developed and patented the Alphabutol process which produces 1-butene by selectively dimerizing ethylene. 

Butadiene is obtained mainly as a coproduct with other light olefins from steam cracking units for ethylene production. Other sources of butadiene are the catalytic dehydrogenation of butanes and butenes, and dehydration of 1,4-butanediol. Butadiene is a colorless gas with a mild aromatic odor. Its specific gravity is 0.6211 at 20°C and its boiling temperature is -4.4°C. The U.S. production of butadiene reached 4.1 billion pounds in 1997 and it was the 36th highest-volume chemical. ... 

The three isomers constituting n-hutenes are 1-hutene, cis-2-hutene, and trans-2-hutene. This gas mixture is usually obtained from the olefinic C4 fraction of catalytic cracking and steam cracking processes after separation of isobutene (Chapter 2). The mixture of isomers may be used directly for reactions that are common for the three isomers and produce the same intermediates and hence the same products. Alternatively, the mixture may be separated into two streams, one constituted of 1-butene and the other of cis-and trans-2-butene mixture. Each stream produces specific chemicals. Approximately 70% of 1-butene is used as a comonomer with ethylene to produce linear low-density polyethylene (LLDPE). Another use of 1-butene is for the synthesis of butylene oxide. The rest is used with the 2-butenes to produce other chemicals. n-Butene could also be isomerized to isobutene. ... 

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. 

Butylenes are four-carbon monoolefins that are produced by various hydrocarbon processes, principally catalytic cracking at refineries and steam cracking at olefins plants. These processes yield isomeric mixtures of 1-butene, cis- and tra s-butene-2, and isobutylene. Derivatives of butylenes range from polygas chemicals and methyl t-butyl ether, where crude butylenes streams may be used, to polybutene-1 and LLDPE, which require high-purity 1-butene. In 1997, the estimated consumption of butylenes (in billions of pounds) was alkylation, 32.0 MTBE, 12.0 other, including polygas and fuel uses, 0.5. 

The dehydrogenation process feed can be refinery streams from the catalytic cracking processes. This mixed C4 stream typically contains less than 20 percent n-butenes. For use in dehydrogenation, however, it should be concentrated to 80-95 percent. The isobutylene generally is removed first by a selective extraction-hydration process. The n-butenes in the raffinate are then separated from the butanes by an extractive distillation. The catalytic dehydrogenation of n-butenes to 1,3-butadiene is carried out in the presence of steam at high temperature (>600°C) and... 

Separadon of butenes from C4 cuts produced by steam cracking and catalytic cracking... 

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 ... 

These olefins are present in light gasolines produced by catalytic cracking, steam cracking or resid coking, from which they can be extracted. However, they are usually produced by the dimerization or codimerization of propvlene and butenes (see Section... 

Refineries that have access to isobutylene streams from steam cracking may face the problem that the existing alkylation and possibly catalytic condensation units cannot take the normal butenes which are contained in the pyrolysis stream. 

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.

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