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Ind. Eng. Chem. Res. 2002, 41, 230-250
Influence of Impurities on the Control of Heterogeneous Azeotropic Distillation Columns Jan Ulrich and Manfred Morari* Automatic Control Laboratory, ETH-Z, Swiss Federal Institute of Technology, CH-8092 Zu¨ rich, Switzerland
A concept is presented for the design of heterogeneous distillation columns under aspects of operability. The column behavior is analyzed using residue curve maps and a theoretical finite reflux/infinite length column. In particular, the influence of impurities on the operation of the column is discussed. Depending on the impurities, different control schemes with different designs of the process have to be used to guarantee robust performance. Here, robustness is not against modeling errors but against typical nonlinear phenomena such as the disappearance of the phase split in the decanter. The theoretical findings are illustrated by steady-state and dynamic simulations of an industrial column where a heavy-boiling organic substance is dewatered using methyl tert-butyl ether as a light entrainer. 1. Introduction Focus. In this paper, we provide a concept for the design of heterogeneous distillation columns under aspects of operability. The influence of additional components in the feed (impurities) on the design and operation of the process is discussed. This study was motivated by an actual industrial process where operational problems were observed. In that process, methylisobutinol (MBI) is dewatered using methyl tert-butyl ether (MTBE) as a light entrainer. In the first part of the paper, a general design procedure for heterogeneous columns is proposed, which is based on residue curve map information and a theoretical finite reflux/infinite length column only. In the second part, the specific industrial process is used as an example to illustrate the implementation of the design and its dynamic behavior. Review. Simulation studies on heterogeneous azeotropic distillation appeared in the literature in the late 1970s. Detailed simulation studies about the industrial operation of these columns were reported about 10 years later. Bozenhardt1 studied the dewatering of an unspecified alcohol in the presence of its complementary ether as the entrainer. He described temperature and online composition control in an industrial plant, which resulted in improved economic operation. Rovaglio et al.2 proposed control strategies for the ethanol dewatering column3,4 where the entrainer benzene is recovered in a separate column. To maintain the desired product purity, the position of a temperature front in the stripping section of the dewatering column was fixed by controlling the average of two temperatures via the reboiler duty. By manipulation of the entrainer flow, a second temperature front was fixed in the rectifying section to ensure the liquid-liquid-phase split in the decanter. In a succeeding paper, Rovaglio et al.5 reported the importance of the entrainer inventory for good controller performance. In particular, the entrainer makeup flow was adjusted to the water content in the feed by a feed-forward controller. * Author to whom correspondence should be addressed. Phone: +41 1 632 7626. Fax: +41 1 632 1211. E-mail:
[email protected].
Correˆa and Jørgensen6 studied the same column with two different control schemes: manipulation of the decanter bypass and reboiler duty and manipulation of the decanter bypass and entrainer makeup flow. Their analysis showed that the first scheme is better conditioned for control than the second one. Tonelli et al.7 analyzed the control strategy of a whole column train that dewaters 2-propanol using cyclohexane as the entrainer. For the heterogeneous column, they found that controlling a temperature close to the top via manipulation of the reboiler duty and a temperature close to the bottom via manipulation of the reflux (inverse pairing8) was less interactive than the opposite (conventional8) pairing, which is normally used. They provided no clear statement about the adjustment of the entrainer makeup flow. We suspect that the entrainer makeup flow was manipulated by controlling the decanter level, as is done in most industrial applications.9 This level control can either be continuous or discontinuous. In the latter case, the decanter is refilled with the entrainer when its level drops below a specified value as the result of entrainer loss. Chien et al.8 presented simulation results for dewatering 2-propanol using cyclohexane as the entrainer. Temperature fronts were fixed by manipulating the reflux flow and reboiler duty. The entrainer makeup flow was manipulated to control the level of the buffer tank. Like Tonelli et al.,7 they found that the inverse pairing is less interactive than the conventional one. The experimental validation of the simulation results was published recently.10 All of the above-mentioned heterogeneous distillation columns have one point in common: At the nominal operation point, the top composition of the column is close to the “unstable node” (the global temperature minimum in the three-component mixtures), which is a heterogeneous azeotrope. The aqueous phase of the overhead is separated in a decanter and leaves the process as a distillate. In the aforementioned control studies, the heterogeneous columns are operated with two-point control. The entrainer flow rate is manipulated directly as a continuous variable2 or indirectly.1,6-8 In the latter case, the reflux is the continuously manipulated variable and the process is operated pseudocontinuously. The process is not in
10.1021/ie001004f CCC: $22.00 © 2002 American Chemical Society Published on Web 12/28/2001
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Figure 1. Setup of the industrial dewatering column.
steady state because the entrainer loss causes the decanter level to decrease. The decanter is refilled with entrainer when its level drops below a specified value. The importance of manipulating the entrainer flow rate will be studied in detail. 2. Industrial Plant for the Production of MBI MBI is an intermediate product for isoprene and vitamin A production. MBI is produced by the reaction of acetylene with acetone in the presence of ammonia as the solvent and aqueous KOH as the catalyst.11,12 Hence, the product stream contains MBI, acetylene, ammonia, acetone, water, and some byproducts, which are mainly heavy boilers. In the plant, ammonia and acetylene are recovered first and recycled to the reactor. In a second step, acetone is separated from the product stream and recycled. After the removal of some very heavy boilers by simple flashing, the product stream contains mainly MBI (approximately 0.95), about 0.02 water, traces of acetone (
