Modeling ethane cracker with application to flowsheet simulation of an ethylene plant

Pyrolysis is a principal technique for production of olefins in the petrochemical industry and ethylene has been the most basic and important building block in the industry today. The main source of ethylene in Taiwan is derived from naphtha cracking; however, for those countries where natural gas is abundant, ethane has been the predominant feedstock for production of ethylene. Although the manufacturing process of naphtha cracking is very complicated, its principle is virtually identical with that of ethane cracking. In this work, a complete ethane cracking plant is designed with an ethylene throughput of 50,000 metric tons per year. The major process of the plant consists essentially of cracking, quenching, compression, dehydration, refrigeration and separation. We begin with a discussion of proposing a mathematical model describing the ethane reactor-the core of the process. The mathematical model incorporates three portions: kinetics, fire box, and thermophysics. In the model, ethane reaction kinetics, material/energy balance of cracked gas, heat transfer mechanisms such as radiation in the cracking furnace, heat conduction and convection of tube, pressure drop across the tubular reactor were all considered. Furthermore, the model also permits thermophysical properties such as viscosity, heat conductivity, and heat capacity of cracked gas to be altered as the reactor temperature varies. The downstream units after the reactor include quencher, staged compressor, dryers, refrigerator and distillation towers. Their process synthesis and design were carried out by commercial software DESIGN II. The yield of the ethane cracker simulated by the proposed mathematical model is in good agreement with plant data. In addition, the design results of the downstream unit operations by DESIGN II appear to be satisfactory.