Hydrogenation propadiene

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

The overhead of the depropanizer is sent to the propylene fractionator. The methylacetylene (MA) and propadiene (PD) are usually hydrogenated before entering the tower. An MAPD converter is similar to an acetylene converter, but operates at a lower temperature and in the Hquid phase. Due to recent advances in catalysis, the hydrogenation is performed at low temperatures (50—90°C) in trickle bed reactors (69). Ordy rarely are methylacetylene and propadiene recovered. 

Figure 14.7 Typical clnomatogram obtained by using the refinery analyser system shown in Figure 14.6. Peak identification is as follows 1, hydrogen 2, Cg+, 3, propane 4, acetylene 5, propene 6, hydrogen sulfide 6, iso-butane 8, propadiene 9, n-butane, 10. iso-butene 11, 1-butene 12, traw-2-butene 13, cw-2-butene 14, 1,3-butadiene 15, iso-pentane 16, w-pen-tane 17, 1-pentene 18, tro 5-2-pentene 19, cw-2-pentene 20, 2-inethyl-2-butene 21, carbon dioxide 22, ethene 23, ethane 24, oxygen + argon, 25, niti Ogen, 26, carbon monoxide.

Figure 3-13. The influence of conversion severity on the theoretical product yield for the cracking of propane. Acetylene, methyl acetylene, and propadiene are hydrogenated and both ethane and propane are recycled to extinction (wt%)." ...

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]. 

C3 Hydrorefining. The aim of C3 hydrorefining is to hydrogenate methylacetylene and propadiene present in the cut. Efficient liquid-phase processes were developed by Bayer314-316 (cold hydrogenation process carried out at 10-20°C) and IFP,317 but hydrogenation in the gas phase is also practiced. 

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... 

Propadiene polymerisation during hydrogenation has been observed to occur, particularly with iron, cobalt and nickel. Over these metals, up to approximately 25% of the propadiene has been observed to polymerise [204], although the chemical identity of the polymeric products was not established. 

In the platinum-catalysed reaction, it has been observed [155] that the effect of increasing the hydrogen/propadiene reactant ratio was to increase the yield of propane without affecting the yield of propene. This has been interpreted as showing that propane may be formed directly from adsorbed propadiene by a mechanism which does not involve adsorbed propene as an intermediate. However, no conclusions were drawn regarding the nature of the surface intermediates involved in such an interconversion. 

Propadiene adds hydrogen chloride to yield 2-chloropropene. However, the possibility exists that initial attack of a proton might lead to the 2-propenyl cation (Section 6-6), which then would react with chloride ion to form 3-chloropropene. Using the rules for application of the resonance method (Section 6-5B) and the atomic-orbital model for 1,2-propadiene (Figure 13-4), rationalize why a 2-propenyl cation might not be formed easily by addition of a proton to 1,2-propadiene and why 2-chloropropene is the observed product. 

Propadiene is represented in Figure 13-4 as if it were two isolated Huckel ring systems. This molecule also may be represented as a stable Mobius system of Att electrons. Draw an orbital diagram of 1,2-propadiene to indicate this relationship. If 1,2-propadiene twisted so that the hydrogens on the ends all were in the same plane, 57, would it be a Huckel or a Mobius polyene, or neither ... 

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... 

Various polar and chemical compounds reportedly are capable of poisoning or deactivating disproportionation catalysts if present in the feed or allowed to contact the catalyst after activation. For example, propylene conversion over cobalt-molybdate catalyst was reduced when 300—2000 ppm of oxygen, water, carbon dioxide, hydrogen sulfide, ethyl sulfide, acetylene, or propadiene... 

Similar photolysis of 20 in the presence of 1-trimethylsilylpropyne produces a silacyclopropene in 33% yield. In this case, small amounts of two other compounds, l-trimethylsilyl-l-(r-phenyl-2, 2, 2 -trimethyldi-silanyl)propadiene (2% yield) arising from a 1,3-hydrogen shift of the initially formed silacyclopropene and l-bis(trimethylsilyl)phenylsilylpro-pyne (2% yield), are also obtained. Formation of the latter compound can be best explained in terms of another type of photochemical migration in-... 

The C3 from the deethanizer bottoms (10) is depropanized (15), hydrogenated (16) to remove methyl acetylene and propadiene (16) and fractionated to recover polymer grade propylene. C4 components are separated from heavier components in the debutanizer (18) to recover a C4 product and a C5 stream. The C5, together with the... 

C3s are combined and hydrogenated to remove methyl acetylene and propadiene (10). Polymer or chemical-grade propylene is then produced overhead from the C3 superfractionator (11). 

The methyl-acetylene and propadiene in the C3 cut are hydrogenated to propylene in a liquid-phase reactor. Polymer-grade propylene is separated from propane in a C3 splitter (16). The residual propane is either recycled for further cracking, or exported. C4s and light gasoline are separated in a debutanizer (17). 

---