Modeling of industrial naphtha thermal cracking furnaces

Abstract

Ethylene plant is one of the largest chemical plants to handle large amount of materials and utilities. Thermal cracking furnace is a key unit in the ethylene plant. Since a slight improvement of production yield or energy saving in the cracking furnace can make large benefit, the analysis and the optimization of operation have been important issues in the ethylene industry under the rapidly changing market situation. The optimization of operation needs a first principles model that can embody the effects of various operating variables. However, the modeling of naphtha cracker is known to be extremely difficult. First, naphtha feedstock consists of a lot of hydrocarbon components that range from C5 to C10 broadly. Numerous thermal cracking reactions with complex mechanism also occur in the reactor tube with transient carbon deposition. Thus a rigorous reactor model that can cope with all these mechanisms is essential for the modeling of a cracking furnace. In addition, overall performance of the cracking furnace is mainly determined by the complex heat transfer phenomena between the reactor tube and the radiant box which provides the combustion heat to the cracking reactors by fuel-firing. Therefore, a rigorous integrating model is very important for the detail analysis of cracking furnaces. In this study, a rigorous cracking furnace model has been developed. The model is not only as flexible to any reactor configuration in the radiant box but also as rigorous to calculate the detail profiles of product compositions and temperature along the reactor length. The radiant box has been modeled by the single gas zone method. The carbon deposition model has used the relative rates of coke production for several precursors selected by isotope experiments. The reactor has been modeled as a general plug flow reactor. The developed model can be tuned easily with plant operating data to incorporate the detail characteristics of process equipment and coke deposition. The...