Process Systems Engineering 2003 B. Chen and A.W. Westerberg (editors) 9 2003 Published by Elsevier Science B.V.
Design and Control of a Complete Heterogeneous Azeotropic Distillation Column System I-Lung Chien, Huan-Yi Chao, and Yao-Pin Teng Department of Chemical Engineering, National Taiwan University of Science and Technology 43, Keelung Road, Sec. 4, Taipei, 106-72, Taiwan
Abstract In this work, design and control of a complete heterogeneous azeotropic distillation column system of isopropyl alcohol (IPA) + water (H20) with cyclohexane (CyH) as an entrainer is investigated. There are two feasible approaches to design the complete flowsheet for the separation of IPA and H20. One approach is to use total of three columns, another simpler design is to only use two columns. From TAC (Total Annual Cost) analysis, the simpler approach is more economical. The suitable control strategy for this complete heterogeneous azeotropic distillation column system is also studied in this paper. From process analysis and dynamic simulation, the overall control strategy is developed which requires two temperature loops in the first column and one temperature loop in the second column. The overall control strategy is tested with composition and flow rate variations in the fresh feed stream. The specifications of the IPA product and waste water composition are all met despite of the various disturbances. Keywords
heterogeneous azeotropic distillation, column design, distillation control
1. INTRODUCTION Separation of isopropyl alcohol (IPA) and water ( H 2 0 ) is an important process in industry. Previous studies ([ 1]-) of this system by using cyclohexane (CyH) as an entrainer showed that the optimum operating point should be located at a critical reflux, a transition point at which the distillation path switches from a route that passes through IPA+H20 azeotrope to one that passes through IPA+CyH azeotrope. At this critical reflux, a very high purity IPA product can be obtained with minimum energy consumption and maximum product recovery. However, from bifurcation analysis, this critical reflux point is right at the edge of top stable branch and the middle unstable branch of steady-state operating conditions, thus this steady-state is extremely sensitive to any small perturbations. An inverse double loop conventional control strategy was recommended to maintain a steady column temperature profile. This control strategy was verified via dynamic simulation and experimental study. In previous studies, only a single heterogeneous distillation column was focused. Although the bottom product composition can reach high purity specification of over 99.9 mol% IPA, the top aqueous product from decanter contents large amount of IPA. This represents loss of IPA product through the top stream. This loss can not be reduced because
761 the lowest temperature (in the top of the column) in the system is the three-component (IPA+H20+CyH) azeotrope. After the two liquid phase separation in the decanter, the organic stream which is rich in CyH will be refluxed back to the column, while the aqueous stream will always contain large amount of IPA. In order to minimize the IPA product loss while purify the waste water to high water purity, additional separation sequence will be needed. The organization of this paper is as follows. Section 2 will explore different steady-state designs of this complete IPA and H20 separation. The goal of the overall complete system is to have only two exit streams with one stream having high-purity IPA product and the other stream with high-purity waste water for discharge. Section 3 will discuss the suitable control strategy of this complete heterogeneous azeotropic distillation column system. Sensitivity analysis will be performed to determine the proper temperature control points. Several different control strategies will be compared using Aspen DynamicsTM simulation package. Fresh feed composition and also flow rate changes will be used to test the closed-loop performance of the different control strategies. The goal is to keep two product compositions at their specifications under various disturbances through proper temperature control loops. Some conclusions will be drawn in Section 4. 2. STEADY-STATE DESIGN In order to purify the aqueous outlet stream from decanter, two feasible approaches will be discussed in this section. One approach is to use two additional columns with the first additional column to separate into three-component azeotrope at the top of the first column and IPA+H20 mixture at the bottom of the first column. The top product of this column can be recycled back to decanter while the bottom product feeds to the next additional column. The next column is designed to produce high-purity water at the bottom with IPA+H20 azeotrope at the top of the column. This IPA+H20 azeotrope can be recycled back to combine with the fresh feed into the heterogeneous column. The flowsheet of this three-column system can be seen in Fig. 1 and the conceptual design of this three-column system can be seen in Fig. 2.
Figure 1. Flowsheet of the three-column system.
762 IPA 1 IN.. ~ ~ ~ .
v FeedComposi'don o OrganicReflux Composition 9 AqueousFlowComposition 9 Columnl&Colurnn2 Top Vapor Composition 9 Column1Bottom Composition 9 Column2Bottom Composition 4, Column3Top Vapor Corn.position n
Figure 2. Conceptual design of the three-column system. The column design of the two additional columns can be carried out using shortcut design first to determine the total stage number and the feed location. The total stage number is determined initially in the shortcut design to be twice of the minimum number of stages. The product specification is set to give 99.9 mol% of water at the bottom of the last column. The rigorous simulation is developed next so that Total Annual Cost (TAC) can be calculated from the steady-state simulation result. By varying the total number of stages in the rigorous simulation, optimum stage number for the additional columns can be obtained. The resulting optimum stage number of the first additional column is 5 and the optimum stage number of the last additional column is 15. Another simpler design with only one additional column is to directly obtain high-purity water at the bottom of this additional column with the top product containing IPA and H20 with small amount of CyH. This top product will be recycled back to combine with the fresh feed into the heterogeneous column. The flowsheet of this simpler system can be seen in Fig. 3. The conceptual design of this simpler two-column system can be seen in Fig. 4.
Figure 3. Flowsheet of the two-column system.
763 IPA 1
v o 9 9 9 9 9
FeedComposition Organic Reflux Composition AqueousRow Composition Colurnnl Top Vapor Composition Column1 Bottom Composition Column2Top Vapor Composition Column2 Bottom Composition
Figure 4. Conceptual design of the two-column system. Again, the column design of this additional column can be carried out using shortcut design first to determine the total stage number and the feed location with the same product specification of 99.9 mol% H20. The rigorous simulation is developed next so that TAC can be calculated from steady-state simulation result. By varying the total number of stages in the rigorous simulation, optimum stage number for the additional columns can be obtained to be 15. The TAC for the two-column system is only about 55% of the three-column system. 3. CONTROL STRATEGY Several possible overall control strategies are proposed for this complete heterogeneous azeotropic distillation column system. Only conventional control strategy using temperature measurements of the column stages will be considered. Also for easy implementation in industry, only PID control loops and some ratio control schemes are applied to this complete system. Although this flowsheet is quite simple, many alternative control strategies can be derived. First of all, we can consider the number of temperature control loops in the first and the second columns. We will consider alternative single-point or two-point temperature control in the first column and keep the second column under single temperature control. The alternative level control strategies including in the first column using makeup flow or organic reflux flow to control the organic phase level and in the second column using distillate (recycle) flow or reflux flow to control the reflux drum level. In combination, there will be total of eight different control strategies to be considered. Through many dynamic simulation runs, we conclude that the simpler single temperature loop control strategies can not reject feed disturbances especially the feed composition changes. Due to page limitation, we will not show simulation results for the single temperature loop control strategies here. Next, we will consider two temperature loops in the first column and one temperature loop in the second column. One possible overall control strategy can be seen in Fig. 5. An alternative control strategy will be to fix the flow rate of the recycle stream instead of the reflux flow rate in Fig. 5. This alternative control strategy although commonly designed for processes having recycle stream but the dynamic behavior during feed disturbance changes is very poor. During the feed disturbance changes, the heat duty as well as the reflux flow rate for the second column may become increasing continuously until control valves saturated. This is due to the internal snowball effect in the second column.
Figure 5. Proposed overall control strategy. Another two alternative control strategies can be considered including in the first column using organic reflux flow instead of the makeup flow to control the organic phase level. Although intuitively sound via steady-state consideration because organic reflux flow is much larger, using organic reflux flow to control the organic phase level does not produce good dynamic responses. The reason is because one crucial temperature control loop has to be controlled by the extremely small makeup flow rate. The dynamic response of this temperature control loop is very poor resulting in large upset in the product compositions. The temperature control point for the first column and the second column in the proposed control strategy in Fig. 5 can be determined by locating the most sensitive points while not too nonlinear in the column temperature profile. The resulting control points can be seen in Fig. 5. Inverse control loop pairing must be used in the first column to insure stability as can be seen in [2,3]. For the second column, since tray 14 is closed to the bottom of the column with high sensitivity, heat duty instead of the organic reflux will be selected as the manipulated variable in the second column. After deciding the proposed control strategy as in Fig. 5, the next step is to do closed-loop dynamic simulation of the candidate control strategy with some load disturbances. The feed rate disturbances are easier to reject because ratio schemes are used in the control strategy in Fig. 5. The most difficult load disturbance to reject is the ummeasurable feed composition changes. Aspen DynamicsTM will be used to do the closed-loop dynamic simulation runs. Figure 6 shows the tray temperature profiles for the two columns under the proposed control strategy before the disturbance and after the +_10% IPA feed composition changes. Notice that with this inferential control scheme together with the overall control strategy, both tray temperature profiles for columns 1 and 2 can be maintained quite nicely despite disturbance changes. The bottom compositions of the two columns are all kept very close to their product specifications of over 99.9 mol% despite +_10% feed IPA composition changes. This dynamic study shows that although not directly-measured, the two important product composition specifications can be maintained by holding some temperature control points during disturbance changes. No recycling snowball effect causing by this overall control strategy is evident
Figure 6. Tray temperature profiles for the proposed overall control strategy. 4. CONCLUSIONS The design and control of a complete heterogeneous azeotropic distillation column system has been studied in this paper. From TAC analysis, a two-column system with only one additional column besides the heterogeneous column results in the most economical overall flowsheet while satisfying IPA and H20 product specifications. An overall control strategy is proposed for this complete column system. This control strategy including inverse double temperature control loops in the first column and a single temperature control loop in the second column is tested via feed flow rate and feed composition changes. The compositions of the IPA product and also the waste water stream are all kept at their specifications. ACKNOWLEDGEMENTS This work is supported by the National Science Council of the R. O. C. under grant No: NSC 89-2214-E-011-025 REFERENCES
C. J. Wang, D.S.H. Wong, I-L. Chien. R.F. Shih, W.T. Liu, and C.S. Tsai, Ind. Chem. Eng. Res., 37, 7 (1998) 2835.
 I-L. Chien, C.J. Wang, and D.S.H. Wong, Ind. Chem. Eng. Res., 38, 2 (1999) 468.  I-L. Chien, C.J. Wang, D.S.H. Wong, C.-H. Lee, S.-H. Cheng, R.F. Shih, W. T. Liu, and C.S. Tsai, J. Process Control, 10, 4 (2000) 333. 
I-L. Chien, W.-H. Chen, and T.-S. Chang, Comput. Chem. Engng., 24, 2-7 (2000) 893.