(15b) Simulation and Optimization of Depropanizer, C3-Splitter and Debutanizer

The ethylene production system consists of highly complex sub-systems. The simulation and/or optimization of the whole system could generate severe problems. It is imperative to divide the process into the sub-systems for efficacious optimization purposes. On the basis of a sensitivity analysis, the most sensitive thermodynamic model parameters from each critical separation task are tuned in accordance with the industrial data and the product separation requirements. The most difficult, expensive, and energy-intensive separations are demethanizer as well as ethylene/ethane and propylene/propane splitters. The separation of multicomponent mixtures by distillation can be difficult when the Vapour-Liquid Equilibrium (VLE) system is non-ideal and azeotropes are present. The products obtained from various distillation towers could be estimated via simulation of the separation systems.

The purity of components and the composition of the output depend on the thermodynamic models. In this work, the effect of SRK, PSRK, and Peng-Robinson (PR) models on the depropanizer, C3-splitter and debutanizer was studied. Thermodynamic modeling and simulation efforts were performed for an industrial ethylene production plant with a yearly ethylene throughput of 500,000 tons. Depending on the composition of the feedstock and the operating conditions (feedstock temperature, column top pressure, column pressure drop, number of stages, feed stage position) different K-Value and enthalpy model combinations were suggested for each distillation unit.

Optimization tool was implemented on selected distillation towers to increase the ethane, ethylene, propane, propylene yields as well as to decrease the condenser and/or reboiler duties. The feed to the fractionation train is composed of ethane, propane, methane, hydrogen, acetylene, ethylene, propylene, n-butane, i-butene, 1-butene, 1,3-butadiene, propyne, benzene, toluene, and heavy gasoline.