Acetylene Hydrogenation Catalyst Operation

TABLE 3.23. Operation of Some Early Acetylene Hydrogenation Catalysts. 

The depropanizer overhead, Cj and lighter feed is compressed to about 300 psi and then passed over a fixed bed of acetylene removal catalyst, generally palladium on alumina. Because of the very large amount of hydrogen contained in this stream, the operating conditions are critical to selectively hydrogenate the acetylene without degrading the valuable ethylene to ethane. 

A new process for the manufacture of acetylene has been proposed. The process will involve the dehydrogenation of ethane over a suitable catalyst (yet to be found). Pure ethane will be fed to a reactor and a mixture of acetylene, hydrogen, and unreacted ethane will be withdrawn. The reactor will operate at 101.3 kPa total pressure and at some as yet unspecified temperature T. 

In less-coordinating solvents such as dichloromethane or benzene, most of the cationic rhodium catalysts [Rh(nbd)(PR3)n]+A (19) are less effective as alkyne hydrogenation catalysts [21, 27]. However, in such solvents, a few related cationic and neutral rhodium complexes can efficiently hydrogenate 1-alkynes to the corresponding alkene [27-29]. A kinetic study revealed that a different mechanism operates in dichloromethane, since the rate law for the hydrogenation of phenyl acetylene by [Rh(nbd)(PPh3)2]+BF4 is given by r=k[catalyst][alkyne][pH2]2 [29]. 

The commercial catalyst studied in this work was palladium (0.03w%) supported in a-alumina with less than 10 m /g of surface. Spent samples of catalyst were taken from industrial reactors after a 7 years run. The industrial unit correspond to a front end acetylene hydrogenation scheme comprising three reactors in series. The design of the unit is such that in normal operation the first reactor decrease acetylene concentration from 2700 ppm to 270 ppm, the second one from 270 ppm to 27 ppm and the last one from 27 to less than 3 ppm. 

It is important to remove all oxygen, dienes and acetylenes from the feed to the metathesis reactor. Furthermore, the C4 stream needs to contain the minimum practical level of butene-1 and isobutene to minimize the metathesis reaction of the C4 hydrocarbons, which results in the formation of C5 and Ce olefins. The higher olefins lead to polymer formation and catalyst deactivation. An excess of ethylene normally suppresses the C4 reactions. A typical steam cracker C4 stream, which has been subjected to selective hydrogenation to remove impurities and fractionation to provide a suitable butene-2 rich raffinate-2, or the butene-2 rich efflnent from an MTBE unit, can provide a suitable feed for the metathesis reactor. The catalyst operating cycle with a rhenium catalyst is usually fairly long. Regular catalyst regeneration may be necessary and the catalyst can last for several years. 

At one time, the only commercial route to 2-chloro-1,3-butadiene (chloroprene), the monomer for neoprene, was from acetylene (see Elastomers, synthetic). In the United States, Du Pont operated two plants in which acetylene was dimeri2ed to vinylacetylene with a cuprous chloride catalyst and the vinyl-acetylene reacted with hydrogen chloride to give 2-chloro-1,3-butadiene. This process was replaced in 1970 with a butadiene-based process in which butadiene is chlorinated and dehydrochlorinated to yield the desired product (see Chlorocarbonsandchlorohydrocarbons). 

The general theoretical approach to the selectivity observed in the hydrogenation of acetylene has been discussed in Sect. 2.3, where it was noted that the observed selectivity may be dependent upon both thermodynamic and mechanistic factors. A possible explanation of the operation of a mechanistic factor has been discussed in Sect. 4.3. The selectivity values, defined as S = Pc2h4/(Pc2h4 + Pc2h6) observed for various metal catalysts are shown in Table 15. Selectivities have been observed to... 

KLP [Dow K Catalyst Liquid Phase] A selective hydrogenation process for removing acetylenes from crude C4 hydrocarbons from ethylene cracking, with no loss of butadiene. The catalyst is based on either copper metal or alumina. Developed by Dow Chemical Company and first commercialized at its plant in Temeuzen, the Netherlands. Eight units were operating in 2005. The KLP licensing business was sold to UOP in 1991. 

In commercial polyethylene operations, poisons may enter the process as trace (ppm) contaminants in ethylene, comonomer, hydrogen (CTA), nitrogen (used as inert gas), solvents and other raw materials. These poisons reduce catalyst activity. Most damaging are oxygen and water. However, CO, CO, alcohols, acetylenics, dienes, sulfur-containing compounds and other protic and polar contaminants can also lower catalyst performance. With the exception of CO, aluminum alkyls react with contaminants converting them to alkylaluminum derivatives that are less harmful to catalyst performance. Illustrative reactions of contaminants with triethylaluminum are provided in eq 4.9-4.11 ... 

We shall consider first the simplest reaction so far reported (56, 94), which is the hydrogenation of 2-butyne in a flow system at room temperature and a little above, catalyzed by alumina-supported palladium (0.03%). This reaction proceeds very selectively, only a trace of butane being formed in the presence of 2-hutyne, as long as the catalyst is not completely fresh. Moreover, the reaction is highly stereoselective for the formation of cis-2-butene and only traces of mw -2-butene and 1-butene were observed. After the removal of 2-hutyne the cts-olefin both isomerized and hydrogenated, showing that a powerful thermodynamic factor is again operative (as was observed for 1-butyne, propyne and acetylene) when alkyne is present. 

The acetylene derivatives of the C, cut are eliminated by selective hydrogenation. The hydrogen employed is obtained from the demethanizer, so that some methane is reintroduced into the C, cut. which is therefore usually sent to a secondary demethanizer after hydrogenation. The palladium (or nickel) based catalysts are placed in one ot two reactors, sometimes featuring several beds with intermediate cooling. The temperature risevfrom 40 to 80°C between the iniet and outlet of a bed. and the operating presses ts about 3.10 Pa. 

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. 

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