PARTIAL CATALYTIC HYDROGENATION OF ACETYLENE IN ETHYLENE PRODUCTION
Lee Siang Hua
Acetylene is a byproduct of modern ethylene production processes, and it acts as a poison to the catalysts used for making polyethylene out of the ethylene product. Due to the above reasons, polymer-grade ethylene product should contain not more than 5 ppm of acetylene. The trace acetylene removal is a great challenge to ethylene producer and catalyst manufacturer due to low acetylene concentration in the reactor feed and nearly 100% conversion of acetylene required in the acetylene converter. In this paper, we will first review the available technologies currently apply in most of the ethylene cracker worldwide, the challenges faced, and the new developments in the catalyst and reactor design.
Keywords: Acetylene, hydrogenation, catalyst, deactivation, regeneration
Ethylene is one of the most widely produced petrochemicals in the world, with a production history reaching back to the early 20th century. It has been recovered since 1930 as a product of coke-oven gas and in the 1940’s became an important industrial intermediate when American companies began producing it by separation from waste gas. This era also saw deliberate ethylene production from ethane and natural gas. More recently, ethylene has taken the place of acetylene in virtually all large-scale chemical syntheses. However, acetylene itself is a byproduct of modern ethylene production processes, and the removal of this contaminant will be considered.
To begin, the formation of acetylene in ethylene product streams will be examined. More than 97% of ethylene around the world is produced by pyrolysis of hydrocarbons, which is the thermal cracking of petrochemicals in the presence of steam. This process can be described as the heating of a mixture of steam and hydrocarbon to the necessary cracking temperature, which can range from 260 °C to 340 °C depending on the hydrocarbon used. This mixture is then fed to a fired reactor or furnace and heated to between 400 °C and 470 °C. As a result, the original saturated hydrocarbon “cracks” into smaller unsaturated molecules. This process is extremely endothermic, and the product must be cooled back to the original feed temperature upon leaving the reactor in order to minimize secondary reactions. Possible alkane feedstocks for pyrolysis include ethane, propane, n-butane, isobutane, naphthas, kerosene, and various gas oils. In the U.S., ethane and natural gas liquids (often a mixture of ethane and propane) are most commonly used in ethylene production. Incidentally, using an ethane feedstock produces the smallest amount of acetylene byproduct, which averages about 0.26 % by weight of the product stream. For other feeds, this quantity can become as large as 0.95 % by weight. Ethylene to be used for polymerisation processes has to be 99.90% pure. This is known as polymer-grade ethylene and the maximum allowable limit of acetylene should not be higher than 5 ppm.
The reason that acetylene removal from the ethylene stream is so vital is that acetylene acts as a poison to the catalysts used for making polyethylene out of the ethylene product. In addition, acetylene can form metal acetylides, which are explosive contaminants. It is thus imperative that acetylene in the ethylene product be reduced to an acceptable level. In fact, this paper will focus more on the catalytic hydrogenation rather than other technology available such as solvent extraction.
acethylene converter. PDF