Selective acetylene hydrogenation production

Since both complete hydrogenation of acetylene or any hydrogenation of the ethylene results in the production of a less valuable product such as ethane, conditions must be chosen carefiiUy and a catalyst must be used that is both sufficiently active for acetylene hydrogenation and extremely selective to avoid ethylene hydrogenation. Since hydrogenation of acetylenic bonds proceeds stepwise and since acetylene is more strongly adsorbed on the catalytic... 

Acetylene hydrogenation. Selective hydrogenation of acetylene to ethylene is performed at 200°C over sulfided nickel catalysts or carbon-monoxide-poisoned palladium on alumina catalyst. Without the correct amount of poisoning, ethane would be the product. Continuous feed of sulfur or carbon monoxide must occur or too much hydrogen is chemisorbed on the catalyst surface. Complex control systems analyze the amount of acetylene in an ethylene cracker effluent and automatically adjust the poisoning level to prepare the catalyst surface for removing various quantities of acetylene with maximum selectivity. 

The high concentration of oxygen in the feed for the 20% oxygen system causes an over oxidation of the carbon species to CO, reducing the selectivity towards acetylene. This product flexibility could be useful in that it would allow for hydrogen to be produced... 

Data on the formation of carbonaceous materials from diene and acetylene over 373 K and selectivity of n-butane in butadiene hydrogenation (H2/BD=2.2, 1.33 kPa BD, 284 K) are collected in Table 3. The hydrogen content of the deposits was calculated from the amount of the self-hydrogenated products in the gas phase. Reaction of 1,3-butadiene was accompanied with formation of n-butenes whereas in reaction of acetylene 1,3-butadiene was the principal product. Hydrogen content of the deposits was also measured by TPD technique. Desorption of H2 is presented in Figure 3 measured on Cat. C before (1) and after BD poisoning at 489 K for 18 hr (2). 

Most olefinic gas streams obtained from refinery off-gases or from the cracking of saturated hydrocarbons contain acetylenic compounds in concentrations ranging from a few tenths of 1% to about 2%. These compounds must be removed if the olefins are to be used for the production of certain petrochemicals. Although liquid purification processes employing. selective. solvents have been developed, selective catalytic hydrogenation is usually the pre-... 

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. 

Oxychlorination reactor feed purity can also contribute to by-product formation, although the problem usually is only with low levels of acetylene which are normally present in HCl from the EDC cracking process. Since any acetylene fed to the oxychlorination reactor will be converted to highly chlorinated C2 by-products, selective hydrogenation of this acetylene to ethylene and ethane is widely used as a preventive measure (78,98—102). 

By-products from EDC pyrolysis typically include acetjiene, ethylene, methyl chloride, ethyl chloride, 1,3-butadiene, vinylacetylene, benzene, chloroprene, vinyUdene chloride, 1,1-dichloroethane, chloroform, carbon tetrachloride, 1,1,1-trichloroethane [71-55-6] and other chlorinated hydrocarbons (78). Most of these impurities remain with the unconverted EDC, and are subsequendy removed in EDC purification as light and heavy ends. The lightest compounds, ethylene and acetylene, are taken off with the HCl and end up in the oxychlorination reactor feed. The acetylene can be selectively hydrogenated to ethylene. The compounds that have boiling points near that of vinyl chloride, ie, methyl chloride and 1,3-butadiene, will codistiU with the vinyl chloride product. Chlorine or carbon tetrachloride addition to the pyrolysis reactor feed has been used to suppress methyl chloride formation, whereas 1,3-butadiene, which interferes with PVC polymerization, can be removed by treatment with chlorine or HCl, or by selective hydrogenation. 

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. 

Tlie cooled gaseous products are dried using an adsorbent such as molecular sieves and compressed to about 500 psig by a multistage compressor. The compressed gas is dien sent to an acetylene converter where acetylene is selectively hydrogenated to ediane. The gaseous mixture dien flows to die purification section of the plant where each component of die gas is recovered by means of cryogenic disdlladon. 

In the petrochemical field, hydrogen is used to hydrogenate benzene to cyclohexane and benzoic acid to cyclohexane carboxylic acid. These compounds are precursors for nylon production (Chapter 10). It is also used to selectively hydrogenate acetylene from C4 olefin mixture. 

The increase of selectivity in consecutive reactions in favor of the intermediate product may be sometimes extraordinarily high. Thus, for example, in the already cited hydrogenation of acetylene on a platinum and a palladium catalyst (45, 46) or in the hydrogenation or deuteration of 2-butynes on a palladium catalyst (57, 58), high selectivities in favor of reaction intermediates (alkenes) are obtained, even though their hydrogenation is in itself faster than the hydrogenation of alkynes. 

The selectivity is 100% in this simple example, but do not believe it. Many things happen at 625°C, and the actual effluent contains substantial amounts of carbon dioxide, benzene, toluene, methane, and ethylene in addition to styrene, ethylbenzene, and hydrogen. It contains small but troublesome amounts of diethyl benzene, divinyl benzene, and phenyl acetylene. The actual selectivity is about 90%. A good kinetic model would account for aU the important by-products and would even reflect the age of the catalyst. A good reactor model would, at a minimum, include the temperature change due to reaction. 

Subsequently the intercalates - without prior ejqposure to air - were reduced either by hydrogen gas or using potassium naphthalide in THF. (3U) Clearly the choice of intercalation and reduction temperature will control the nature of the final product. These materials were then e qposed to air. Some of the results obtained are given in Table IV. Clearly catalyst B in its hi conversion and selectivity to acetylene demonstrates mique properties. 

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