Control and optimization of cold recovery processes of naphtha cracking center using dynamic simulation

Abstract

In naphtha cracking center, which is one of the largest scale chemical processes, the cold recovery system separates gas mixture discharged from furnace and produces purified ethylene and propylene. A number of distillation columns included in this cold recovery system should be well controlled to satisfy the high purity specification of final products. In order to design control systems of distillation columns and tune them, dynamic simulation was performed over the system. Dynamic simulation, different from steady state simulation that was introduced a few decades ago, directly mimics the plant operation with respect to time. It gives the engineers the possibility to evaluate the process designs with respect to dynamic response using a number of control scenarios. At first, for a train of columns, which are deethanizer-I, deethanizer-II, depropanizer and debutanizer, control configurations were selected by referring to real plant control configurations and to guidelines suggested by Shinskey [7]. Open-loop test was performed to identify process model of each column. After determining the process transfer function, PID-controllers were tuned using DCLR (Desired Closed-Loop Response) tuning method. Next in order to acquire a synergism between optimization and control, which are actually two distinct features in the operation of a plant, optimization to minimize energy consumption was conducted for three different scenarios. As a result of optimization, 1.32% of energy could be saved and plant capacity could increase by 3% more than before optimization. And servo problem tests with new set points showed how well the controllers could adapt the system to the optimized condition. Finally the decentralized control system for the C2 fractionator, one of high purity distillation columns, was designed and tested. And a model-based control strategy based on a nonlinear wave theory [24] was recommended to control the quality of propylene from the C3 fractionator.