Superfractionator process control
Hurowitz, Scott Edward
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An in-depth study is conducted regarding product composition control of superfractionators with an emphasis on control configuration selection. A propylenepropane (C3) splitter is chosen as a representative column by which to investigate superfractionator process control issues. An ethylene-ethane (C2) splitter is also investigated for comparative purposes. Detailed steady state and dynamic simulations of a C3 and C2 splitter are developed and benchmarked against industrial C3 and C2 splitter process data. These simulations are used to investigate single-ended and dual Proportional-Integral (PI) composition control. For C3 splitter single-ended PI composition control, the (L, V) configuration provides the best control performance. For C3 splitter dual PI composition control, the (L, B) and (L, V/B)configurations provide the best control performance. The (L, V) and (L, V/B) configurations are determined as optimal for dual PI composition control of the C2 splitter. The control benefits provided by the use of decoupling techniques and feedforward compensation for dual PI composition control are also investigated. An evaluation of the control benefits realized by feedforward compensation indicate that, when a material balance (product) stream is used to control composition, feedforward compensation will provide a significant improvement in composition control performance. Dynamic Matrix Control (DMC), a model-based control algorithm, is applied to the C3 and C2 splitters, and its performance is compared to that obtained by PI control. Dynamic Matrix Control generally provides control performance that is equal to or better than that obtained by PI control for unconstrained, 2x2 distillation composition control, provided that the process is adequately modeled by the DMC controller. A technique is developed for predicting closed-loop product variabilities based on a signal processing analysis of feed composition data, from which usefiil information can be extracted and used to predict closed-loop product variabilities. This technique is applied to a C3 splitter for demonstrative purposes and is shown to accurately predict the product variabilities that result from feed composition disturbances.