This work addresses the systematic tuning of proportional−integral (PI) controllers for dividing wall distillation columns. By following an approach of stable pole assignment to a linear dynamics that approximately describes the control system convergence, a technique results that forces the gains of controllers to be dependent on well-known parameters for each control loop: a static gain and a time constant that characterize the open-loop response of the output with respect to the control input, and a damping factor and a response velocity that outline the path of the closed-loop response. Then, it becomes possible to tune all the controllers in a simultaneous way, substantially reducing trial-and-error activities on tuning the entire control system. Via simulation, the control system performance is illustrated for disturbance rejection and set-point tracking in a representative dividing wall distillation column, showing that this tuning technique is an effective choice.
The problem of controlling an energy-efficient dividing-wall distillation column (DWDC) with a discrete key variable measurement is addressed. Usually, controllers are tuned in a systematic and simultaneous way, based on a certain control system configuration and conventional discrete proportional-integral controllers. The same stable pole-assignment approach to linear dynamics is followed here that approximately describes the convergence of each control loop. Tuning relationships in terms of sampling time and predecessor parameters of physical insight are obtained. The control system performance is simulated for disturbance rejection in an energy-efficient DWDC for separating a ternary mixture of benzene, toluene, and xylene. This tuning technique can be effective with a sampling time as long as that corresponding to current measurement instruments.
Experiments and simulations were carried out in a bench scale absorption unit; the runs were made using solutions of monoethanolamine (MEA) at 25 wt%. The gas used was a mixture of air and CO 2. Runs were conducted using three different gas flow rate, in a range between 40 to 190 NL/min, and the flow rate of MEA solution was varied to have different ratios of CO 2 :MEA, in a range between 1:2 to 1:5, for each flow rate. The results are compared with those obtained with simulations using Aspen Hysys software with a constant Murphree efficiency value of 24%, and varying it for those values calculated from experiments. In this work it was found that bench scale absorption unit has a percentage of absorption up to 99% for a CO 2 :MEA ratio of 1:4.3.
One of the main criteria for the selection of a suitable solvent is the CO2 solubility capacity or CO2 loading. The objective of this work is to provide students and early-career scientists a detailed description of a titration-based experiment to measure the CO2 loading using simple and inexpensive volumetric and gravimetric lab apparatus. The performance of the method is corroborated by comparing the experimental uncertainty obtained during the determination of the CO2 concentration in test samples (in an absorption unit at lab scale) with reference values obtained by mass balance based on a certified gas analyser. The results indicate that CO2 loading values between the experimental method and the reference range from ±3 to 13%, which is in good agreement with other similar methods.
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