Selective acetylene hydrogenation operation

If the reaction is not selective and the required product specification is not achieved, performance can be improved by the addition of a few ppm of carbon monoxide to the hydrogen in a similar manner to acetylene hydrogenation. Operating conditions are summarized in Table 3.24. 

Acetylene hydrogenation is widely practiced and efficient. However, a green-oil which comprises vinyl acetylene oligomers is also produced and in some instances can foul the unit. In large cracking operations, the acetylene may be recovered by absorption processes based on copper salts, which selectively absorb the acetylene. 

The removal of acetylenes and dienes from steam-cracked olefins is a critical step in purification. Selective hydrogenation processes and catalysts have become more important as worldwide olefin production has increased in 1999 to more than 90 million tormes of ethylene and almost 50 million tonnes of propylene. Demand for better catalysts with improved selectivity and longer operating cycles has grown as larger plants are built. Tighter product specifications have also been imposed now that more of the olefins produced are being converted to polyolefins. 

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. 

Significant quantities of Cj and C, acetylenes are produced in cracking. They can be converted to olefins and paraffins. For the production of high purity ethylene and propylene, the contained Cj and C3 acetylenes and dienes are catalytically hydrogenated leaving only parts per million of acetylenes in the products. Careful operation is required to selectively hydrogenate the small concentrations of acetylenes only, and not downgrade too much of the wanted olefin products to saturates. 

With other acetylenes steric factors may be operative which render the selective reduction somewhat difficult. In the aldosterone intermediates (53) and (54), for instance, selective hydrogenation is obtained only with the 14 -acetylenic ether " (hydroxyl group effect). 

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. 

Thus, batadiaie is first recovered from steam cradced C4 cuts by solvent extraction, an operation that is sometimes facilitated by preliminary selective hydrogenation of the acetylenic compounds. In a number of applications, the raffinate itsdf must undergo similar treatment to rid it of residual dioiefins. The initial cut, after being debutadienized by hydrogenation, can also serve the same purpose. This also applies to catalytic cracker eflluents that are very often directly upgradable, but whose albeit low butadiene content may justify hydrogenation pretreatment for certain uses. 

The product gases are cooled and compressed (4) to facilitate separation of products and by-products. The suction-side of the compressor ensures that upstream units operate at a low pressure. The product gases are first dried (5) and the cooled product passed to a cryogenic separator (6) which removes hydrogen from the system. Some is recycled with the other portion passed-on for other uses. A selective hydrogenation unit (7) removes dienes and acetylenes. A final distillation train removes light hydrocarbon (C2-), propylene product, propane, which is recycled, and a C4 by-product. 

For downstream cracking operations, the main catalytic process of interest is the selective hydrogenation of acetylene and related compounds. The process is considered to be selective and to only form ethylene, but this could be improved because there is some evidence (including the formation of green oil) that the process is not as selective as generally claimed. It is not clear that the small amount of acetylene present is in fact reduced to ethane rather than ethylene. It is clear there is some room for improvement. 

The effect of trace contaminants on the reaction has been investigated carefully. All uncondensed effiuent gases were recycled to the reactor, except for the amounts present in the streams taken off for analysis or flashed upon depressuring of the organic phase. Aqueous phase from the separator containing the water soluble by-products has been used as the water feed to the reactor. Hydrogen chloride containing chlorinated hydrocarbons and acetylene was used in all operations. In addition, certain possible impurities were tested for their effect on the kinetics and selectivity of the process. Paraffins, carbon monoxide, sulfide, carbon dioxide, alkali, and alkaline earth metals were found to be chemically inert. Olefins, diolefins and acetylenic compounds are chlorinated to the expected products. No deleterious effects of by-product recycle were observed even when some of the main by-products were added extraneously. 

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. 

As stated above, gas chromatographs used in process control are the analysers most commonly used in the industrial field. This is particularly true of oil refineries and petrochemical industries as a result of the versatility, selectivity and suitability with respect to the products to be analysed, i.e. gases or volatile liquids. These instruments have been used for a variety of applications control of ambient air in industry (determination of vinyi chloride), of reagent purity (determination of traces of water in xylenes) and of processes (determination of the products obtained in the hydrogenation of acetylenes). These instruments are widely used in the fractionation of LPGs (Fig. 16.4) in controlling fractionators to ensure their optimum and economic operation with acceptable impurity levels. 

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