Joonho Shin

A nonlinear profile observer using tray temperatures for high-purity binary distillation column control

Abstract A new, concise nonlinear profile position observer using tray temperatures instead of tray compositions is proposed. Composition measurement has been one of the major difficulties associated with the composition control of distillation columns because the on-line analyzers still suffer from large measurement delays, high investment/maintenance costs and low reliability. One of common alternatives to the analyzers is to use the secondary measurement such as tray temperatures which is able to infer the tray compositions. The performance of the nonlinear control scheme which consists of this nonlinear profile observer and the model-based controller for profile position control of high-purity distillation column proposed by Han and Park (1993, A.I.Ch.E. Journal, 39(5), 787–796) has been tested and compared with that of Han (1993, Control of high-purity distillation columns using a nonlinear wave theory, Ph.D. thesis, KAIST) in simulation of high-purity distillation column. The results of this new control scheme is very similar to Han and Park (1993). The control scheme is insensitive to the noise of the temperature measurements and so simple that it can be directly applied to the real high-purity distillation columns.

CONSTRAINED OPTIMAL CONTROL OF LIQUID LEVEL LOOP USING A CONVENTIONAL PROPORTIONAL-INTEGRAL CONTROLLER

An analytical design method for a conventional proportional-integral (PI) controller is developed for the optimal control of the liquid level loop in order to explicitly handle important control specifications. The constrained optimal control problem for the liquid level loop is formulated and converted into an unconstrained optimization problem to find an optimal PI controller tuning rule. The proposed PI controller minimizes the rate of change of the outlet flow and the deviation of the level subject to two important constraints in the level loop: the maximum allowable rate of change in the outlet flow rate and the maximum allowable deviation of the level. The simulation results show that the constrained optimal control of the liquid level loop can be successfully implemented using a simple PI controller with the proposed design method.

Nonlinear static composition estimator for distillation columns using open equation-based nonlinear programming

In distillation column control, secondary measurements such as temperatures and flows are widely used to infer product composition. This paper addresses the development of nonlinear static estimators using secondary measurements for estimating product compositions of distillation columns. An open equationbased optimization problem, which minimizes the differences between the measured outputs and the estimated outputs, has been formulated and solved by using the nonlinear program (NLP) solver, MINOS5. It is shown that the proposed nonlinear estimator is robust and more powerful than the conventional PLS (Partial-Least-Squares) estimator.

DESIGN OF A COMPOSITION ESTIMATOR FOR INFERENTIAL CONTROL OF DISTILLATION COLUMNS

Abstract In distillation column control, secondary measurements such as temperatures and flows are widely used in order to infer product composition. This paper addresses the design of the linear static estimators using the secondary measurements for estimating product compositions of distillation columns. Based on the unified framework for the estimator design, the relationships among various static estimators are discussed in terms of the estimator structure. Il is shown that the projection estimator is equivalent to the regression estimators in the special cases. Since the projection estimator heavily depends on the measured inputs such as reflux flow and heat input to the reboiler due to its structural characteristic, the estimation performance is far more sensitive to measurement noise and nonlinearity of them, compared wiih the regression estimators based on the PCR or PLS method. It is also found that the use of the measured inputs leads to performance deterioration of both the projection and regression estimators because of their nonlinear effects on the product compositions especially in high-purity columns. Design guidelines for the PCR and PLS estimators are presented by analyzing the results of the simulation studies on a high-purity column example. The estimator based on the guidelines is robust to sensor noise and has a good predictive power

A cost-effective retrofit of conventional distillation sequence to dividing-wall prefractionator configuration

Abstract A dividing-wall prefractionator configuration was investigated for a safe and economic retrofit of the conventional sequence using simple distillation columns. In the proposed retrofit configuration, the first column was modified to a dividing wall column as a prefractionator to supply prefractionated multi-feeds to the subsequent column. To investigate the effectiveness of the proposed configuration, nine near-ideal feed mixtures were considered for analysis. The proposed configuration was then compared with several other alternative configurations. The proposed dividing-wall prefractionator efficiently generates prefractionated multi-feed streams avoiding feed mismatch and remixing effect with low modification cost. Moreover, because the proposed retrofit configuration allows for flexible switching between the dividing-wall prefractionator and the conventional operating mode, a safe retrofit is also ensured by reducing the operational risks. Several industrial retrofit cases were studied to validate the proposed dividing-wall prefractionator configuration.