INTRODUCTION
Ethylene is one of the principal building blocks in the petrochemical industry, and world-wide production and consumption have been steadily increasing. Production of ethylene is accomplished primarily by the pyrolytic stripping of hydrogen from ethane or a higher molecular weight hydrocarbon. This cracking process, sometimes referred to as steam cracking, is currently accomplished in metallic tubes using high temperature furnaces and has a conversion efficiency for ethane of 60-65%. Operation at significantly higher temperatures could increase the efficiency as much as 20%, but materials with better high temperature strength would be required. To help identify suitable materials, tests have been conducted to determine the behavior of selected ceramic materials in environments similar to those anticipated for a high-efficiency, advanced steam cracking system. The effects of exposure on weight change, mechanical strength, and microstructure have been determined in a series of 100 hour tests. In addition, 500 hour tests have been conducted to determine the effect of time on material behavior. From these tests, several strong candidates have been identified.
The Department of Energy-Office of Industrial Technologies (OIT), in cooperation with Stone & Webster Engineering Corporation, is developing a high pressure heat exchanger system (HiPHES) for ethylene production. Conventional production of ethylene is by a process known as pyrolysis or steam cracking. During normal cracking operation, hydrocarbon feedstock is initially mixed with steam and heated to temperatures of approximately 815-900 ºC (1500- 1650ºF) and pressures of 1.7-2.4 atm (25-35 psia) while flowing through metal alloy tubes within a direct-fired furnace, During this cracking operation, an undesirable product, coke, is also produced and it accumulates on the walls of the metal tubes. Decoking of the tubes is required and is accomplished through a process in which steam and air are passed through the coil at about 870 ºC (1600 ºF) to burn the deposited coke. Steam cracking technology has evolved to permit utilization of reactors at increasingly higher temperatures and shorter residence times so as to enhance the overall yield of ethylene and other light olefins. The present technology is constrained by operating temperature limits of common commercial alloys. The tube material temperatures typically reach 1149 °C (2100 ºF) at the end of normal operation with a coked tube. Use of advanced ceramics, such as silicon carbide or a multiphase ceramic containing acme silicon carbide, would allow higher operating temperatures.
Commercial success of this project depends directly upon development of a heat exchange system that utilizes monolithic ceramics ardor ceramic composite materials, The concepts of applying advanced materials to steam cracking are being studied in this project. Stone & Webster is evaluating the benefits of elevated operating temperatures on ethylene yield at their Bench Scale Unit in Houston, Texas. The effect of exposure to simulated steam cracking environments on corrosion behavior, mechanical strength, and microstructure of candidate ceramic materials is being determined in stiles being conducted at Oak Ridge National Laboratory, Oak Ridge, Tennessee. This paper will summarize the work done at Oak Ridge.
