C4 hydrocarbons, separation

A notable feature of high-pressure distillation is the high efficiency that is usually obtained on trays. Figures close to 100% are not uncommon. However, the efficiency of trayed columns has been shown to increase only from atmospheric pressure up to a pressure of 11.5 bar. At higher operating pressures, the efficiency of the trays decreases with increasing pressure. There is an entrainment of vapour in the liquid phase which is carried back down the column. For example, for a C4-hydrocarbon separation the tray efficiency will be reduced by 16% as the pressure is raised from 11.5 bar to 27.6 bar. 

Extractive distillation is commercially used for separating mixtures of butanes, butenes, butadienes, and various acetylenes with four carbon atoms (13). Separating these multicomponent mixtures by fractional distillation is very difficult because the natural volatilities pf the various components, paraffinic as well as olefinic, overlap considerably. For instance, n-butane is less volatile than 1-butene but more volatile than cis-and trans-2-butenes. Thus, separation of butanes from butenes is more difficult by fractional distillation than by extractive distillation where the solvent increases the volatilities of all the butanes to make them greater than the butene volatilities. For 1,3-butadiene recovery extractive distillation is also more attractive than ordinary distillation because the large polarizability of the conjugated double bonds interacts strongly with the polar solvent. Also, in C4 hydrocarbon separations the solvent often only enhances and does not reverse the natural relative volatility for many of the components however, even for those components for which the rela-... 

BUTENEX A process for separating several C4 components from C4 hydrocarbon streams by extractive distillation using Butenex, a proprietary extraction agent. Piloted by Krupp Koppers in 1987. Several plants were being engineered in 1994. 

Distex A family of extractive distillation processes used in the petroleum industry from 1940. In one such process, furfural is used as the extracting agent for separating butadiene from other C4 hydrocarbons. 

Al-Thamir, W. K., Laub, R. J. and Purnell, J. H. J. Chromatog. 142 (1977) 3. Gas chromatographic separation of ail C1-C4 hydrocarbons by multi-substrate gas-sohd-hquid chromatography. 

Neuzil, R.W. and Fergin, R.L (1979) Desorbent for separation of butene-1 from a C4 hydrocarbon mixture using zeolite X. U.S. Patent 4,119,678. 

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. 

Uses, Because of its good solvency and relatively low boiling point, acetonitrile is used widely as a recoverable reaction medium, particularly for the preparation of pharmaceuticals. Its largest use is for the separation of butadiene from C4 hydrocarbons by extractive distillation. 

Mixed Solvents Effect. Using mixed solvents can improve selectivity. For example, adding small amounts of water has improved the selectivity of furfural in separating C4 hydrocarbons (24). Baumgarten and Gerster (25) have studied how various solvents affect the selectivity of furfural for the pentane-pentene pair. They concluded that for only a few solvents some improvement was observed. The resulting selectivity lies between the selectivity of the pure solvents (see Table III). To avoid immiscibility at high solvent concentrations, a second solvent is sometimes added (25). 

A cracker C4 stream contains all of the possible C4 hydrocarbons which are listed in Table 5.2. Of these commercial interest focuses on butenes, isobutene, 1,3-butadiene and butanes. Efficient separation is impossible by distillation alone and complete separation is by a combination of distillation, selective hydrogenation and selective absorption. If butadiene is not required this ean be hydrogenated and the butenes and butane separated by distillation. 

The SSF membranes, which are produced by carbonization of PVDC, contain nanopores that allow all of the molecules of a feed gas mixture to enter the pore structure. However, the larger and more polar molecules are selectively adsorbed on the carbon pore walls at the high pressure side, and then th dif se selectively to the low pressure side. The smaller molecules are enriched at the high pressure side. These membranes can be used to enrich H2 from mixtures with C1-C4 hydrocarbons or from mixtures with CO2 and CH4. They can also be used to separate CH4-H2S and H2S-H2 mixtures. Table 5 compares performances of SSF carbon and polymeric PTMSP membranes for H2 enrichment from FCC off gas [15]. Clearly, the SSF membrane is much superior for this application. 

McCartney, R. F., Plate Efficiencies in the Separation of C4 Hydrocarbons by Extractive Distillation, M.S. Thesis, University of Delware, Delaware, 1948. 

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