Recovery 1,3-butadiene

C4 Hydrorefining. The main components of typical C4 raw cuts of steam crackers are butanes (4-6%), butenes (40-65%), and 1,3-butadiene (30-50%). Additionally, they contain vinylacetylene and 1-butyne (up to 5%) and also some methylacetylene and propadiene. Selective hydrogenations are applied to transform vinylacetylene to 1,3-butadiene in the C4 raw cut or the acetylenic cut (which is a fraction recovered by solvent extraction containing 20-40% vinylacetylene), and to hydrogenate residual 1,3-butadiene in butene cuts. Hydrogenating vinylacetylene in these cracked products increases 1,3-butadiene recovery ratio and improves purity necessary for polymerization.308... 

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-... 

FIGURE 2 Butadiene recovery using jV-methyl-2-pyrollidone (NMP). 

A separation process is sought that can satisfy both our present economic and enviromental constraints. It would also provide an alternative to present practice that relies on expensive azeotropic or extractive distillation processes used in the recovery of products from low relative volatility streams. As an example, virtually all industrial butadiene recovery processes now rely on extractive distillation using acetonitrile or other equivalent agent to enhance the relative volatility of the C4 components. The use of supercritical or near critical separation of these streams may satisfy these requirements provided certain pressure, temperature and recompression criteria can be met. Such a process would also reduce the need for a complex train of distillation towers. 

Process Evaluation of Improved Solvents for Butadiene Recovery... 

Operation. Typical operating conditions for butadiene recovery plants have been described by Buell and Boatright (I). These data were used as a guide for setting the conditions used in this study. At the start of a run, the column was preheated electrically to the desired column temperature profile. The C4 feed was pumped into the column until the pressure reached 30-35 psig. Solvent (preheated to 130°F) was then circulated through the column, and the reboiler temperature increased to about 275°F. The hydrocarbon feed was pumped in at a rate which gave the desired solvent-to-hydrocarbon feed ratio. A portion of the butene stream was returned to the column to provide reflux. Temperature of the solvent feed tray was used to control the amount of reflux. Only that portion of C4 s which dissolved in the solvent could be carried down the column. The excess C4 s were revaporized to the hydrocarbon trays. 

The performance of butadiene recovery plants improves as solvent selectivity increases. The greater the solvents selectivity, the better the separation of trans-2-butene (light key) and butadiene (heavy key) in the extractive distillation column. Relative volatility of the key pair in /3-MOPN was 1.337 at 70 psig and 130 °F. This results from inversion of the volatilities. In normal distillation trans-2-butene is the heavy key (a — 0.848). Lower trans-2-butene levels in the butadiene concentrate require less reflux and enables handling of increased loads in the final purification column. 

Figures 3 and 4 show that furfural and methyl Cellosolve solvents cannot produce butadiene concentrates free of trans-2-butene at solvent-to-feed ratios where the more selective solvents acetonitrile and p-meth-oxypropionitrile (/3-MOPN) are able to reject all of this isomer from the concentrate. In fact, with furfural solvent, considerable loss of butadiene must be accepted to approach a low level of trans-2-butene in the butadiene concentrate. On the other hand, it is possible to recover 100% of the butadiene in the feed while no trans-2-butene is extracted with the /3-methoxypropionitrile or acetonitrile solvents. As much as 40% of the higher boiling cis-2-butene isomer is rejected at 100% butadiene recovery at solvent ratios as low as 12 to 1. These data indicate much lower solvent ratios could be achieved with these solvents.

Coogkr, W. W Butadiene recovery proems employs new solvent system"/ Chem. Engng, 74(16) 70-72(1967) Thomas. E- H "DMAC butadiene recovery process offers many advantages. Europ. Chem. Sews. Large Plant SuppL 62-64127 Sept. 1968). 

Klein. H, Weitz, H. M, Extract butadiene with NMP", Hydrocarbon Processing, 47 (11) 135-138 (1968) Reis, T, Compare butadiene recovery method, processes, solvents/economra",- Petro/Chem Engng, 41 (8) 12-22(1969)... 

The other isomer, 1,2-butadiene, a small by-product in 1,3-butadiene production, has no significant current commercial interests. However, there are a number of pubHcations and patents on its recovery and appHcations, particularly in the specialty polymer area (8,9) and as a gel inhibitor (10). 

Butane and heavier bottoms from the depropanizer flow to the debutanizer where the C4 stream (almost entirely olefins and diolefins) is taken overhead and sent to butadiene and isobutylene recovery facilities. 

Tower bottoms-ACN, butadiene, with some butenes and acetylenes - are fed to a recovery/stripping column. The hydrocarbons are taken overhead and then rerun to meet product specifications. The stripping column bottoms, (ACN) is then remrned near the top of the extractive distillation tower. A small slipstream goes to the ACN recovery tower, where solvent is also recovered from the water wash streams. 

Troublesome amounts of C and Q acetylenes are also produced in cracking. In the butadiene and isoprene recovery processes, the acetylenes in the feed are either hydrogenated, polymerized, or extracted and burned. Acetylene hydrogenation catalyst types include palladium on alumina, and some non-noble metals. 

Figure 3-16. Flow diagram of the Lummus process for producing butadiene (1) reactor, (2) quenching, (3) compressor, (4) cryogenic recovery, (5) stabilizer, (6) extraction.

Because of the high pyrolysis temperature, the C4-fraction contains quantities of vinyl acetylene and ethyl acetylene, the removal of which prior to the recovery of butadiene is necessary in certain cases, particularly if butadiene of low acetylene content is desired. Similar considerations apply to effractions obtained by the dehydrogenation of n-butane and n-butenes. 

This comprehensive article supplies details of a new catalytic process for the degradation of municipal waste plastics in a glass reactor. The degradation of plastics was carried out at atmospheric pressure and 410 degrees C in batch and continuous feed operation. The waste plastics and simulated mixed plastics are composed of polyethylene, polypropylene, polystyrene, polyvinyl chloride, acrylonitrile butadiene styrene, and polyethylene terephthalate. In the study, the degradation rate and yield of fuel oil recovery promoted by the use of silica alumina catalysts are compared with the non-catalytic thermal degradation. 9 refs. lAPAN... 

Acrylic textile fibers are primarily polymers of acrylonitrile. It is copolymerized with styrene and butadiene to make moldable plastics known as SA and ABS resins, respectively. Solutia and others electrolytically dimerize it to adiponitrile, a compound used to make a nylon intermediate. Reaction with water produces a chemical (acrylamide), which is an intermediate for the production of polyacrylamide used in water treatment and oil recovery. 

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