Front-End Acetylene Hydrogenation

The catalyst is normally divided into several beds, with interbed cooling, to limit the temperatnre rise in each bed to less than 15-25°C and to control the selectivity of the reaction. Temperatnre increases by 40°C and 75°C for every 1.0% of acetylene converted to ethylene or ethane, respectively, and the increase for converting 1.0% MAPD to propylene is 40°C. The number of catalyst beds needed in a reactor can easily be estimated. Reaction is readily controlled by variation of the bed inlet temperature. 

For economic reasons the palladium catalyst has occasionally been used to hydrogenate all of the acetylene in a single bed. Performance was satisfactory although, with large-diameter reactors and shallow catalyst beds, there was occasionally a small ethylene loss. Table 3.25 gives an example of front-end catalyst operatioa 

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. 

The need for carbon monoxide inhibition can be seen as the catalyst is commissioned. A hot spot may develop when cold gas enters the reactor and the overall temperature is much lower than normal operating levels. However, the catalyst surface in the vicinity of the hotspot is rapidly deactivated with carbon 

Note With increased acetylene content the number of beds increases. If the earbon monoxide content changes the inlet temperature must be adjusted. Catalyst volume required about 6 m for 100,000 tonnes of ethylene produced per year. 

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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. 

Figure 3.2. Reactor in front-end acetylene hydrogenation unit. Reprinted with permission from Linde AG.

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