Propadiene, selective hydrogenation

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... 

C4 raw cuts of stream crackers typically contain butanes (4-6%), butenes (40-65%) and 1,3-butadiene (30-50%), as well as some vinylacetylene, 1-butyne, propadiene and methylacetylene. First, acetylenes are selectively hydrogenated and the 1,3-butadiene is extracted resulting in butene cut (or raffinate I). Isobutylene is next removed to produce raffinate II which contains linear butenes and some residual 1,3-butadiene. The latter needs to be removed to achieve maximum butene yields. The methods and catalysts for this process are chosen according to the final use of butenes. The demand for polymer-grade... 

BRICI/ABB Lummus Propylene C3s Selective hydrogenation of methyl acetylene and propadiene to propylene 12 1998... 

At the bottom of the deethanizer, a C3+ cut is collected, which is fractionated in more conventional columns. The C3 cut collected at the top of the depropanizer is selectively hydrogenated to remove methylacetykne and propadiene. Since its propylene content may be as high as 95 per cent weight, depending on the severity, this cut is often... 

The C3+-fraction of the C2 /C3+ splitter enters the C3/C4+ splitter that separates propane, propene, propadiene, and propyne from all heavier products. The C3 stream undergoes a selective hydrogenation step in a fixed bed reactor that converts propyne and propadiene mainly into propene. Propene and propane are separated in a very similar way as ethane/ethene. Again, distillation columns with more than 100 trays are applied, making these separation units very costly in investment and energy consumption. The bottom fraction of the C3/C4+ splitter is transferred to C4/C5+ splitter. The C4 fraction leaving this column at the top contains mainly butadiene, isobutene, 1-butene, 2-butene, and butane. The further use of this crack-C4 mixture is described in detail in Section 5.3. 

The a-selectivity for carbon radical addition to propadiene (la) is retained on substituting chlorine or fluorine for hydrogen in radicals of the type CX3 (X=F, Cl), no matter whether the reaction is conducted in the liquid or in the gas phase (Table 11.4) [14, 49-51]. /3-Selective addition to allenes becomes progressively more important for the CC13 radical with an increase in number of methyl substituents [14, 47]. For example, treatment of optically active (P)-(+)-2,4-dimethylpenta-2,3-diene [(P)-(lc)] with BrCCl3 affords a 59 41 mixture of a- and /3-monoadducts [47]. The a-addition product consists of a 20 80 mixture of E- and Z-stereoisomers, whereas the product of /3-addition exclusively exhibits the Z-configuration. The fraction of 2,4-dimethylpenta-2,3-diene (P)-(lc) that was recovered from this reaction mixture had completely retained its optical activity. These results indicate that the a-and the /3-CCl3 addition proceed under kinetic control. If one of the addition steps were reversible, at least partial racemization would inevitably have taken place. 

Tetrahydrofuran (12) adds to propadiene when heated in the presence of DTBP to 160°C to afford a minor fraction of diadduct in addition to 71% of a mixture of monoaddition products 13 and 14 (Scheme 11.8). The reaction proceeds via the nucleophilic 2-tetrahydrofuryl radical (not shown) that adds with a low a-selectivity to propadiene (la), thus leading after hydrogen atom trapping to a 66 34 ratio of functionalized heterocydes 13 and 14 [59]. 

Selectivities, S, Observed in Propadiene Hydrogenation Catalyzed by Some Alumina- and Pumice-Supported... 

Development of the fiont-end catalyst revealed the most important factor in achieving selectivity carbon monoxide is adsorbed by the catalyst surface so that acetylene, metltyl acetylene, and, less strongly, propadiene are adsorbed in preference to ethylene. Consequently, only small amounts of ethylene are hydrogenated at the bottom of the final bed when most of the acetylenes have been removed. 

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