Butene separation processes

In the physical separation process, a molecular sieve adsorbent is used as in the Union Carbide Olefins Siv process (88—90). Linear butenes are selectively adsorbed, and the isobutylene effluent is distilled to obtain a polymer-grade product. The adsorbent is a synthetic 2eohte, Type 5A in the calcium cation exchanged form (91). UOP also offers an adsorption process, the Sorbutene process (92). The UOP process utilizes ahquid B—B stream, and uses a proprietary rotary valve containing multiple ports, which direct the flow of Hquid to various sections of the adsorber (93,94). The cis- and trans-isomers are alkylated and used in the gasoline blending pool. 

A synthesis problem on purification of butenes is discussed (Hurme and Heikkila, 1998). The aim is to synthesize the separation process by maximizing the inherent safety. The inherent safety is measured as the Inherent Safety Index. 

In Figure 6—1, if you look closely, you 11 see that the difference between butene-1 and the butene-2 is the location of the double bond. Butene-1 has it at the end position/ butene-2 at the middle. The methyl groups in trans-butene-2 are across from each other, on opposite sides of the fence in the CIS form they are next to each other or on the same side of the fence. The difference is more than cosmetic. It determines the way the molecule behaves, physically and chemically. Check the boiling points, for instance, in Figure 6—1. They differ, and that helps in the separation process. In a few paragraphs, the different applications for butylenes that derive from their chemical behavior differences will be covered. 

A novel homogeneous process for the catalytic rearrangement of 3,4-epoxy-l-butene to 2,5-dihydrofuran has been successfully developed and scaled-up to production scale. A tri(n-alkyl)tin iodide and tetra-(n-alkyl)phosphonium iodide co-catalyst system was developed which met the many requirements for process operation. The production of a minor, non-volatile side product (oligomer) was the dominating factor in the design of catalysts. Liquid-liquid extraction provided the needed catalyst-oligomer separation process. 

Although not a separate process, isomerization plays an important role in pretreatment of the alkene feed in isoalkane-alkene alkylation to improve performance and alkylate quality.269-273 The FCC C4 alkene cut (used in alkylation with isobutane) is usually hydrogenated to transform 1,3-butadiene to butylenes since it causes increased acid consumption. An additional benefit is brought about by concurrent 1-butene to 2-butene hydroisomerization. Since 2-butenes are the ideal feedstock in HF alkylation, an optimum isomerization conversion of 70-80% is recommended.273... 

A somewhat different application of SCFs in separation is the industrial process for the production of butan-2-ol from sc-Butene. This process, operated by the Idemitsu Petroleum Company has been described in some detail elsewhere [1]. Briefly, the reaction between butene and H2O takes place in an aqueous phase. sc-Butene is only partially miscible with H2O and an equilibrium is set up between butene and butan-2-ol in the aqueous phase but, under these conditions, butan2-ol is more soluble in sc-Butene than in H20. 

Another isomer-separation process which has been announced Is called OlefinSlv (13) This process separates iso-butene from normal butenes in a manner similar to IsoSiv. 

In view of the interest in this field, an experimental investigation was undertaken to determine the applicability of supercritical phenomena to the separation of butadiene from C4 mixtures. In particular, the separation of 1-butene from 1,3-butadiene is a key factor in the separation process. Results of these studies are considered in light of predictions obtained from a representative equation of state in the retrograde region of the SC solvent-solute VLE envelope. 

Depending on the method used to separate isobutene or butene-1, and in accordance with the deared application for the remainiag components the Q cuts initially available, supplonentary treatments may exhibit a range of complexity. Hence it may or may not be possible, both for technical and ecopomic reasons, to incorporate them in the extraction facilities that ser e as a basis for the upgrading schemes selected. In certain cases, in fact, one of the primary separation processes already examined can be used as an auxiliary for another technology, that is also designed as an initial step. 

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. 

Olefin Separation. Olefin-containing streams are separated either by the OlefinSiv process (Union Carbide Corp.) separating / -butenes from isobutenes in the vapor phase, or the Olex process (Universal Oil Product) a Hquid-phase process. 

The l,4-dichloro-2-butene can also be separated and hydroly2ed with aqueous NaOH to form 1,4-butenediol, which is hydrogenated with Ni catalyst to produce 1,4-butanediol. In 1971 this process was commerciali2ed in Japan (55). The plant is now shut down because of unfavorable economics. 

Isomerization. Isomerization of any of the butylene isomers to increase supply of another isomer is not practiced commercially. However, their isomerization has been studied extensively because formation and isomerization accompany many refinery processes maximization of 2-butene content maximizes octane number when isobutane is alkylated with butene streams using HF as catalyst and isomerization of high concentrations of 1-butene to 2-butene in mixtures with isobutylene could simplify subsequent separations (22). One plant (Phillips) is now being operated for this latter purpose (23,24). The general topic of isomerization has been covered in detail (25—27). Isomer distribution at thermodynamic equiUbrium in the range 300—1000 Kis summarized in Table 4 (25). 

There are currentiy three important processes for the production of isobutylene (/) the extraction process using an acid to separate isobutylene (2) the dehydration of tert-huty alcohol, formed in the Arco s Oxirane process and (3) the cracking of MTBE. The expected demand for MTBE wHl preclude the third route for isobutylene production. Since MTBE is likely to replace tert-huty alcohol as a gasoline additive, the second route could become an important source for isobutylene. Nevertheless, its avaHabHity wHl be limited by the demand for propylene oxide, since it is only a coproduct. An alternative process is emerging that consists of catalyticaHy hydroisomerizing 1-butene to 2-butenes (82). In this process, trace quantities of butadienes are also hydrogenated to yield feedstocks rich in isobutylene which can then be easHy separated from 2-butenes by simple distHlation. 

Refining and Isomerization. Whatever chlorination process is used, the cmde product is separated by distillation. In successive steps, residual butadiene is stripped for recycle, impurities boiling between butadiene (—5° C) and 3,4-dichloto-l-butene [760-23-6] (123°C) are separated and discarded, the 3,4 isomer is produced, and 1,4 isomers (140—150°C) are separated from higher boiling by-products. Distillation is typically carried out continuously at reduced pressure in corrosion-resistant columns. Ferrous materials are avoided because of catalytic effects of dissolved metal as well as unacceptable corrosion rates. Nickel is satisfactory as long as the process streams are kept extremely dry. 

The acid extract phase is separated, diluted with water, and heated to regenerate isobutylene. The isobutylene is then caustic and water washed to remove traces of acid, distillation dried, and rerun. The unreacted C4 stream, containing normal butenes, is also caustic washed before further processing. 

The principal components of the cut are butene-1, butene-2, isobutylene and butadiene-1,3. Methyl, ethyl, and vinyl acetylenes, butane and butadiene-1,2 are present in small quantities. Butadiene is recovered from the C4 fraction by extraction with cuprous ammonium acetate (CAA) solution, or by extractive distillation with aqueous acetonitrile (ACN). The former process is a liquid-liquid separation, and the latter a vapor-liquid separation. Both take advantage of differences in structure and reactivity of the various C4 components to bring about the desired separation. 

The ionic liquid process has a number of advantages over traditional cationic polymerization processes such as the Cosden process, which employs a liquid-phase aluminium(III) chloride catalyst to polymerize butene feedstocks [30]. The separation and removal of the product from the ionic liquid phase as the reaction proceeds allows the polymer to be obtained simply and in a highly pure state. Indeed, the polymer contains so little of the ionic liquid that an aqueous wash step can be dispensed with. This separation also means that further reaction (e.g., isomerization) of the polymer s unsaturated ot-terminus is minimized. In addition to the ease of isolation of the desired product, the ionic liquid is not destroyed by any aqueous washing procedure and so can be reused in subsequent polymerization reactions, resulting in a reduction of operating costs. The ionic liquid technology does not require massive capital investment and is reported to be easily retrofitted to existing Cosden process plants. 

Figure 2-1 shows the two processes for the separation of n-butenes from isobutene. ... 

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