Design and Analysis of Divided Wall Column

Transcription

1 Proceedings of the 6th International Conference on Process Systems Engineering (PSE ASIA) June 2013, Kuala Lumpur. Design and Analysis of Divided Wall Column M. Shamsuzzoha, a* Hiroya Seki b, Moonyong Lee c a Department of Chemical Engineering, King Fahd University of Petroleum and Minerals, Daharan, Saudi Arabia, mshams@kfupm.edu.sa ( ) b Chemical Resources Laboratory, Tokyo Institute of Technology, Yokohama, Japan c School of Chemical Engineering and Technology, Yeungnam University, Korea Abstract In this study, an efficient design method has been proposed for determining the optimal design structure of the dividing wall column. Preliminary design parameters are fixed on the basis of well-known shortcut method (Fenske-Underwood-Gilliland). The rigorous optimization is used to obtain the optimal design parameters of the divided wall column. Divided wall column has total eight design variables, which affect the energy consumption. The optimal deign could save significant energy consumption in compare to the nominal design based on the shortcut method. The study has been also carried out to investigate the effect of energy supply on product compositions. It is the usual case in the real practice when the column is operated with the energy lower than the optimal energy required during operation. Keywords: divided wall column, Petlyuk column, structure design, shortcut method 1. Introduction It is widely accepted that the distillation is most used separation process and will continue to be an important process for the future because there is currently no other industrially viable alternative. Dividing Wall Columns (DWC) for distillation is currently receiving a lot more attention from industry because of their energy saving capability and capital cost reduction. These reductions occur due to the fact that only one column, reboiler and condenser are needed, as compared to two complete conventional columns when a middle-cut is required. Despite the high potential of the DWC economic benefits, a lack of reliable design methods has prevented their commercial application. In DWCs, the entire separation task occurs in one thermally coupled column shell, which makes much more difficult design structure compared to conventional distillation columns. Accordingly, several studies [1-6] have been conducted to address the DWC design structure. Triantafyllou and Smith [1] proposed a fully thermally couple distillation column (FTCDC) design using a three-column model. The method provides a good basis for investigating the degrees of freedom and the number of trays in an easy manner; however, it requires trial-error steps for matching the compositions of the interlinking streams. Amminudin et al. [3] developed a semi-rigorous method for the initial design of an FTCDC based on the concept of equilibrium stage composition. In their study, the FTCDC was divided into two separate columns to eliminate interlinking and obtain an optimal initial design that could be confirmed through rigorous simulation. Premkumar and Rangaiah [4]

2 398 M. Shamsuzzoha et al. utilized a three-column configuration for the initial design structure of the DWC in their study for the retrofit of a conventional column system to a DWC. The initial structure obtained by a shortcut method was then optimized in a rigorous simulation steps. Recently, Shamsuzzoha and Lee [5,6] proposed an efficient design method for determining the optimal design structure of a dividing wall column. The internal section of the DWC is divided into four separate sections and it matched to the sloppy arrangement with three conventional simple columns. The light and heavy key component mole-fractions are used as the design variables in each column. They found that the structure that gives superior energy efficiency in the shortcut sloppy case also brings superior energy efficiency in the DWC, while the optimal internal flow distribution of the DWC is different from that obtained from the sloppy configuration. It is clear from open literature that there is need of effective design and optimization method of dived wall column. Therefore, the main focus of the present study is to fill this gap and develop a simple procedure to determine the optimal DWC structure. This contains the location of the feed tray in prefractionator, the side-stream product draw tray, vapor and liquid feed stage location to main column, total number of trays in both main and pre-fractionation section, liquid and vapor draw rate. This study is also extended to investigate the effect of energy supply on product compositions. It is the usual case in the real practice when the column is operated with the energy lower than the optimal energy required for operating the column. 2. Divided Wall Column Structure The Petlyuk configuration in Figure 1 represents an arrangement that can separate three or more components using a single reboiler and a single condenser. This configuration has more thermal coupling than the pre-fractionator which increases efficiency. It has greater internal flows with no hold-ups due to not having an intermediate reboiler or condenser. The exchange of vapor and liquid between the columns in Petlyuk configuration poses strict pressure and operability constraints. Figure 2 represents the dividing wall column configuration which is the most compact and allows for both considerable energy and capital saving. There is a partition between the feed and sidedraw sections of the column which provides greater capacity and increased separation efficiency yet still externally resembles a normal side-draw column. This column is thermodynamically identical to the Petlyuk column provided that there is negligible heat transfer across the dividing wall of the column. 3. Design of Divided Wall Column Structure The preliminary design parameters have been fixed on the basis of well-known shortcut Fenske-Underwood-Gilliland method N-N m R-R m = (1) N+1 R+1 where the minimum number of theoretical stages at total reflux (N m ) was estimated by the Fenske equation. Underwood equation has been utilized which gives the minimum reflux for an infinite number of theoretical equilibrium stages (R m ). In this study, the actual reflux ratio (R) was chosen as 1.2R m. The feed tray location was determined by assuming that the relative feed location was constant as the reflux ratio changed from total reflux to a finite value:

3 Design and Analysis of Divided Wall Column 399 N F,m N F = (2) N m where N F,m denotes the feed stage at the total reflux estimated by the Fenske equation as: XD, LK XF, LK ln X X N F,m = ln D, HK F, HK (3) LK HK Figure 1: The Petlyuk Configuration Figure 2: The Dividing Wall Column (DWC) Once the preliminary design parameters are fixed like total number of stages in both prefractionator and main column, feed location and side stage draw location etc. The simulation has been initiated to get the nominal divided wall column parameters. After getting nominal design parameters the rigorous optimization is used to obtain the optimal design parameters of the divided wall column. In these study total six key design variables is utilized to obtain the optimal DWC structure. 4. Simulation Studies In this study, UniSim Design Suite simulation program is used with the Fenske- Underwood equation for the design of column structure. Three different feed mixtures of Benzene, Toluene and p-xylene were considered to check the feasibility of the proposed study and that are: a mixture with low amounts of the intermediate component (F1); an equimolar mixture (F2); and a mixture with high amounts of the intermediate component (F3). The feed condition of ternary mixture is shown in Table 1. It is assumed that the operating pressure is 1050 KPa and the top, side draw and bottom product purity are required as 99.5 mol%, 91 mol% and 92 mol%, respectively. Table 1: Feed Condition of Ternary Mixture Types of Feed F1 F2 F3 Feed A: Benzene A : 0.40 A : 0.33 A : 0.15 Mixture B: Toluene C: p-xylene B : 0.20 C : 0.40 B : 0.33 C : 0.34 B : 0.70 C : 0.15 Feed rate: 100 kmol/h, Pressure: 1050 KPa, Saturated liquid

4 400 M. Shamsuzzoha et al. As discussed earlier shortcut design method is utilized for the preliminary column configuration. The simulation result of the F2 feed mixture is given in Table 2. From the result of the shortcut column in Table 2, the number of tray for the prefractionator column is set as 20 stages and the number of tray for the main column is 63. Table 2: Main Structure of the Equivalent Conventional Column Configuration (F2 Mixture) 1st column 2nd column 3rd column Light Key Benzene: Benzene: Toluene: Heavy Key p-xylene: Toluene: p-xylene: Ext. Reflux Ratio Total Stage No Feed Stage No Rectify Vapor (kmol/h) Rectify Liquid (kmol/h) Stripping Vapor (kmol/h) Stripping Liquid(kmol/h) Reflux ratio (R)= 1.2R m The Petlyuk column configuration was used for the simulation of the DWC because both the column is thermodynamically equivalent. Once the column structure is fixed, the distribution of the internal liquid and vapor flows to the prefractionator and main section is the most significant factor which affects the energy consumption. Furthermore, the internal flow distribution is other important factor to determine the easiness for realization of the DWC structure. The temperature and composition profile of the main column of the DWC is shown in the Figure 3. Although the temperature and composition of the shortcut rigorous 2 nd and 3 rd column profile is not shown here, the similarity in the temperature and composition profiles between the two configurations has been seen. This also implies the validity of the structural similarity between the sloppy and DWC configurations. The effect of internal flow distribution on the energy consumption in DWC has been studied extensively. For getting the optimal energy consumption in divided wall column, the MATLAB program is connected with UniSim for the optimization purpose. The MATLAB optimization tools have been used to find the optimal vapour and liquid split ratio. Figure 3: Temperature and Composition vs. Tray Position from Top for main column

5 Design and Analysis of Divided Wall Column 401 Although it is not shown, it is clear from three different case studies that there is significant difference in the energy consumption in both the nominal and optimal design. It is important to note that minimum energy requirement for specified product is strongly dependents on the vapour and liquid split in the divided wall column. Figure 4 shows the variation in energy consumption as the vapour and liquid flow rate change in the DWC. The significant amount of energy can be saved using proper design approach of the DWC. Reb q Vapor101 out Liquid101 out F2: Feed Mixture: (0.33:0.33:0.34) vapor_flow = kgmole/h liquid_flow = kgmole/h reboiler_duty = 1.511e+003 kw Optimal value vapor_flow = kgmole/h liquid_flow = kgmole/h reboiler_duty = e+003 kw Figure 4: Effect of internal flow distribution on the energy consumption in DWC for F2 5. Operation of the Divided Wall Column This section emphasised on analysis of the effect on product compositions in case of column is operated with the energy lower than the optimal energy required for operating the DWC. The optimal energy required for running the column can be calculated from above procedure as mentioned earlier. The effect of the lowering the energy has been investigated on the side product in the main column and shown in Figure 5. Figure 5 illustrate the composition profile for F2 case when available energy is 20% lower energy than original optimal value. Figure 5: composition profile of the main column of the DWC (for 20% lower energy) The investigation has been carried out to check the purity of the side product of the divided wall column for variation in the wide range of the reboiler duty. Figure 6 clearly shows that there is direct impact of energy on the purity of the side product while top and bottom product purity is almost fixed.

6 Mole Fraction of Toluene as a side product(purity in fraction) 402 M. Shamsuzzoha et al. Conclusions This study emphasizes the design of DWC structure on the basis of shortcut method using the Fenske-Underwood equation. The method utilizes the three conventional column configuration equivalents to the DWC to find the proper structure in a simple manner. The rigorous optimization is used to obtain the optimal design parameters of the divided wall column in an effective way. Extensive simulation studies illustrate that the proposed method is suitable to DWC structure design. The study has been also carried out to investigate the effect of available energy on the side product compositions. It shows that there is direct impact of energy consumption on the purity of the side product while top and bottom product purity is almost constant purity = (Reboiler duty) orignal value of energy % less than orignal value of energy Reboiler duty kw Figure 6: purity verses reboiler duty of the side products in the DWC for F2 case. Acknowledgement The authors would like to acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project number 11-ENE as part of the National Science Technology and Innovation Plan. References 1. C. Triantafyllou, and R. Smith, The Design and Optimization of Fully Thermally Coupled Distillation. Trans IChemE, 70 (Part A), , I. J. Halvorsen and S. Skogestad, Shortcut Analysis of Optimal Operation of Petlyuk Distillation, Ind. Eng. Chem. Res, pp , K. A. Amminudin, R. Smith, D. Y. C. Thong, and G. P. Towler, Design and Optimization of Fully Thermally Coupled Distillation Columns: Part 1: Preliminary Design and Optimization Methodology. Trans IChemE, 79 (Part A), , R. Premkumar and G. P. Rangaiah, Retrofitting Conventional Column Systems to Dividing Wall Columns, Chem. Eng. Res. Des., 87, 47, S. H. Lee, M. Shamsuzzoha, M. Han, Y. H. Kim, and M. Y. Lee, Study of Structural Characteristics of a Divided Wall Column Using the Sloppy Distillation Arrangement Korean Journal of Chemical Engineering, 28, , M. Shamsuzzoha, Hiroya Seki and Moonyong Lee, Structural Design by Shortcut Method of the Divided Wall Column, Petrotech-20012, 10 th International Oil & Gas Conference and Exhibition, October 2012, New Delhi, India.