Dynamic Analysis and Control for Divided Wall Column

Transcription

1 Dynamic Analysis and Control for Divided Wall Column Claudia J. G. Vasconcelos and Maria Regina Wolf-Maciel State University of Campinas, School of Chemical Engineering, Campinas/SP, Brazil Abstract Process intensification is an increasing area turned over using smaller equipments. The divided wall column is a way of process intensification, where one column is used to represent two columns, through a wall dividing the shell. In this work, two case studies are investigated: the separation of a ternary mixture containing butanol, pentanol and hexanol and an extractive distillation process used for recovering aromatics from pyrolysis gasoline. Optimization and control aspects were studied. Fractional Factorial Design and Response Surface Methodology were used for optimization. Keywords: Process Intensification, Distillation, Divided Wall Column, Process Control 1. Introduction The integration of two unit operations into one apparatus (process intensification) implies in cost reductions, but it, generally, causes strong interactions. Integrated process must be carefully designed. Examples of process intensification include novel reactors, intense mixing devices, heat and mass-transfer designs that provide high surface area per unit of volume, equipment that performs one or more unit operations, and alternative ways for delivering energy in processing equipments. Clearly, it is desired from economic perspectives - high productivity from a small volume of material. The integration of several unit operations into one common apparatus has the potential to substantially improve the economics of chemical processes, if applied adequately. Such a plant also reduces the magnitude of potential accidents. On the other hand, the integration can cause strong interactions and requires a more robust control scheme. In this work, divided wall column (DWC) processes are studied. The steady state process is simulated, dynamic behavior is evaluated and an efficient control strategy is proposed. Adrian et al. (2003) compared the performance of PI and Model Predictive controller applied to divided wall columns. Kolbe and Wenzel (2003) studied the Morphylane process. Serra et al (2001) analysed control possibilities using feedback diagonal and dynamic matrix control and Serra et al (2003) compared the efficiency for different distillation column arrangements for a ternary mixture separation.

2 2. Results Case study 1 Butanol/n-Pentanol/n-Hexanol system Divided wall column (DWC) is a complex distillation arrangement which consists of a prefractionator connected to a main column at both ends. It can be considered as thermodynamically equivalent to a Petyluk Column, however, the physical set-up is quite different. DWC is built up in only one shell and a vertical wall divides its core in two parts, which work as the prefractionator and the main column. Thermal coupling makes the energy consumption lower than that observed in conventional arrangements for ternary separations. In this work, analyses of degrees of freedom of the process and simulation aspects are discussed. The number of trays for each section, feed position and reflux flow rate were optimized in order to minimize the energy consumption. Factorial design was used as optimization tool. Simulations were performed using HYSYS.Plant process simulator (Hyprotech, inc.). Figure 1 (a) shows the scheme for divided wall column arrangement applied to the separation of a butanol/pentanol/hexanol mixture and Figure 1 (b) shows the simulation flowsheet. n-butanol Feed n-pentanol n-hexanol Figure 1(a) DWC Conceptual design. Figure 1(b) Simulation flowsheet. Optimization Two level full factorial design and Response Surface Methodology were used for the process optimization. The independent variables used were: number of stages above the wall (N1), number of stages divided by the wall (N2), number of stages below the wall (N3) and liquid and vapor splits (Split). The responses analysed were: butanol mole fraction at the top (x1) and hexanol mole fraction at the bottom (x3). Reflux ratio was specified in order to obtain pentanol mole fraction in the side-draw (x2) equal to Figure 2 shows the energy consumption behavior when N2 and Split are changed. It can be observed that increasing N2, the energy consumption decreases. When the Split effect is considered, the energy consumption decreases at the ends.

3 Figure 2 N2 and Split effect on X3 Figure 3 shows the hexanol mole fraction at the bottom when N2 and Split are changed. It can be observed that there are two high purity regions: 1. high N2 and low Split and 2. low N2 and high Split. Figure 3 N2 and Split effects on x3

4 Control strategy Because the DWC complexity, more possibilities of control variables exist in relation to the conventional distillation arrangements. The control objectives are: maintain the product specifications under disturbances on feed flow rate and on composition and smooth variation on the manipulated variables. The control strategy used here consists of: - Condenser level control manipulating the butanol draw - Condenser pressure manipulating the condenser cooling - Top temperature control manipulating the reflux flow rate - Bottom temperature control manipulating the reboiler heat duty - Temperature control on tray 15 manipulating the pentanol flow rate - Reboiler level control manipulating the hexanol flow rate Figure 4 (a) Hexanol composition control at 50% feed flow rate disturbance PID Figure 4 (b) Hexanol composition control at 50% feed flow rate disturbance MPC controller.

5 Figure 4 (a) shows the performance of the control strategy using only PID controllers. It was able to maintain the desired specification of hexanol composition when the feed flow rate increases 50%. Figure 4 (b) shows the control strategy performance when a model predictive controller is used to control the bottom temperature. It was used a first order transfer function in a DMC controller. The same disturbance was applied. Case study 2 Extractive Distillation: BTX Process The second case study is an extractive distillation process used in refineries to recover high-purity benzene, toluene and xylenes from pyrolysis gasoline. Several solvents can be used to perform the separation, for example: sulfolane, N-methyl pyrolidone, N- formyl morpholine and glycol blends. The process using N-formyl morpholine (Morphylane process) was presented in Kolbe and Wenzel (2003). In this work, the selected solvent was the N-methyl pyrolidone. The conventional extractive distillation process is shown on Figure 5. The first column is the extractive column, where the solvent extracts the aromatics fraction and the non-aromatics are obtained as the top product. The second column is used to recover the solvent. The feed to be processed is barrel/day: non-aromatics wt%, benzene wt %, toluene wt% and xylenes 1.57 wt%. The number of stages for each section is: 20 at the top, 20 at the wall region and 20 at the bottom. Figure 5 Conventional extractive distillation for recovering aromatics from pyrolysis gasoline. An important difference when comparing with the first case study is that the side-draw (Aromatics) must be in vapor phase (in the previous case, the Pentanol side-draw was as liquid phase). The flow rate split on the divided section is an optimization task, so that, for each case study a different situation is found out. Table 1 shows the comparison between the conventional and DWC processes. It can be observed that the reboiler heat duty is smaller for DWC arrangement. Table 1 Comparison between de conventional and DWC processes. Reboiler heat duty (Kcal/h) Conventional process x 10 7 DWC x 10 7 Figure 6 illustrates the temperature profile along the column (60 equilibrium stages plus a reboiler, counted from the top of the column). Observing the profile near stage 60, it can be seen an abrupt increase in the temperature.

6 Temperature (C) Stage Figure 6 Temperature profile for DWC arrangement 3. Conclusions Two case studies were proposed to study divided wall column arrangements: 1. a ternary mixture of butanol, pentanol and hexanol in order to obtain high purity of all components; 2. Morphylane process - an extractive distillation process used for recovering aromatics from pyrolysis gasoline. Decentralized control schemes using PID controllers was used for the composition control for the first case study. It was found that this simple structure was able to maintain column operation, although strong interaction between control loops is present, making very important the tuning of controller parameters. Furthermore, in this kind of column arrangement, it is very important to the success of the process, to evaluate the side draw phase (vapor or liquid). We have shown these both situations in this work. 4. References Adrian, T., Schoenmakers, H., Boll, M. Model predictive control of integrated unit operations: Control of a divided wall column Chemical Engineering and Processing (2003) in press. Kolbe, B., Wenzel, S. Novel distillation concepts using one-shell columns Chemical Engineering and Processing (2003) in press. Serra, M., Perrier, M., Espuna, A., Puigjaner, L. Study of the divided wall column controllability: influence of design and operation Computers and Chemical Engineering 25 (2001) 859 Serra, M., Espuña, A., Puigjaner, L. Controllability of Different Multicomponent Distillation Arrangements Ind. Eng. Chem. Res. 2003, 42,