Development of Anticoking Additives for Thermal Cracking of Naphtha

Abstract

Thermal cracking of naphtha, and light hydrocarbons such as ethane and propane and their mixtures is a major process for production of ethylene, propylene, butadiene and aromatics. One of the main problems in the process is deposition of coke on inner surfaces of the cracker reactor coil and on the inner surfaces of the transfer line exchangers which are used to recover most of the heat from the hot product stream. Yet methods to minimize or eliminate the deposition of coke on the coil walls have been investigated only to a limited extent. Cracking experiments were conducted in a bench scale naphtha cracker unit having a reactor coil made of Incoloy 800. Extensive experiments were carried out to optimize operating conditions that would simulate commercial plant performance with respect to yields and to obtain pressure increase up to 3 kg/cm2g in 100 hours. The optimized conditions are: temperature 852 ºC, steam dilution ratio 0.15; residence time 1.5 seconds. Decoking experiments were performed for 24 hours at 850-880 ºC using steam - air mixture. A water soluble additive comprising of acetate of potassium and calcium metals and phosphorous containing compound and or sulfur containing compound has been tested in cracker unit in amounts effective to reduce the coke deposition on coil surface.The addition of the additive during cracking resulted in increase in ethylene and propylene yields, increase in run length and reduction in coke deposition.