On control of high relative volatility distillation columns
Duvall, Philip Marshall
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A detailed study of single-ended and dual-ended product composition control of four high relative volatility distillation columns, depropanizers with relative volatility ranging from 1.5 to 2.0, was conducted with emphasis on control configuration selection. Product impurities in the overhead and bottoms for the four designs ranged from 2.0 mol% (low purity) to 0.1 mol% (high purity). Rigorous, tray-to-tray steady state and dynamic simulations for four multi-component depropanizers were developed. The base case design (0.5 mol% impurity overhead and bottoms) was benchmarked against data from an industrial depropanizer. The simulations were used to compare nine different control configuration pairings, [L,V], [L,B], [L,V/B1, [L/D,V], [L/D,B], [L/D,V/B1, [L,V/B], [D,V/B], and [D,B], using Proportional-Integral (PI) control. Steady State Generic Model Control (GMC), and Dynamic Matrix Control (DMC). Controllers were tuned for setpoint changes and were tested for disturbance rejection performance using unmeasured feed composition changes (step and sinusoidal). All control studies were for unconstrained process control. For single-ended composition PI control, reflux (L) and reboiler duty (V) provided optimal control of the overhead and bottom compositions, respectively. For dual-ended composition PI control, the [L/D,V/B] configurations provided superior control for feed composition disturbances when compared to other configurations. Two-way decoupling of PI control for the [L,V] configuration resulted in significant control improvement in the [L,V] configuration for all product purity designs. The addition of feedforward control to the [L/D,V/B] configuration provided marginal, if any, improvement in depropanizer control when compared to the control performance of the [L/D,V/B] configuration without feedforward. Multi-Model Decoupled GMC (MMD-GMC) was introduced as a control technique to improve traditional GMC by providing dynamic compensation of the steady state targets to account for dynamic differences in vapor/liquid traffic in the column. The use of steady state MMD-GMC with the [L/D,V/B] configuration outperformed double ratio PI control for the low to mid purity designs (2.0 mol% to 0.5 mol% impurity in overhead and/or bottoms) but showed poor performance when compared to PI control for the high purity design (0.1 mol% impurity in overhead and bottoms). The [L,V] configuration combined with [2x2] Dynamic Matrix Control (DMC) control provided superior control performance for the low to mid purity depropanizer designs and outperformed double ratio PI control. For high purity distillation, DMC control performance using the [L,V] configuration was on par with the double ratio, dual-ended PI composition controller.