Sustainable Integrated Process Design and Control for a Continuous-Stirred Tank Reactor System Siti Aminah Zakariaa, Mohd Jufri Zakariab, Mohd Kamaruddin Abd Hamidc* Process Systems Engineering Centre (PROSPECT), Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia. a
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Keywords: Sustainability, process design, process control, CSTR
Abstract. The objective of this paper is to highlight the use of a two-dimensional (2D) sustainability index in performing a sustainable integrated process design and control (Sustain-IPDC) for a continuous-stirred tank reactor (CSTR) system. Sustain-IPDC for a CSTR system is formulated as a mathematical programming problem and solved by decomposing it into six sequential hierarchical sub-problems: (i) pre-analysis, (ii) design analysis, (iii) controller design analysis, (iv) sustainability analysis, (v) detailed economic analysis, and (vi) final selection and verification. The proposed methodology is applied to the production of cyclohexanone using a CSTR. The results show that the proposed methodology is capable in finding an optimal solution for a CSTR design problem that satisfy design, control, sustainability and economic criteria in an easy and systematic manner. Introduction Sustain-IPDC is a new alternative method used to solve the multi-objective problems related to process design, process control, sustainability and economics, which is upgraded from the previous Integrated Process Design and Control (IPDC) [1]. Sustain-IPDC method includes the optimization of design, control as well as sustainability criteria with respect to economic, social and environmental issues. This paper highlights the use of a 2D sustainability index which includes economic-environment, socio-economic, and socio-environmental indicators. A 2D index is used in this paper because of its simplicity in measuring sustainability assessment instead of using a 1D index which consists of too many indicators. Methodology The proposed methodology as shown in Figure 1 consists of six hierarchical stages which are (i) preanalysis, (ii) design analysis, (iii) controller design analysis, (iv) sustainability analysis, (v) detailed economic analysis, and (vi) final selection and verification. The detailed explanation of stage 1, 2, 3 and 6 can be referred to Hamid’s work [2]. While the detailed explanation for stage 5 can be referred to Sinnot [3] and Li et al. [4] works. A 2D sustainability index was used in a Stage 4 in the proposed methodology. This is due to the simplicity of the equation which simultaneously includes two out of three aspects of sustainability. In addition, since there are too many indicators need to be measured in order to do the sustainability analysis for a 1D index, thus, a 2D indicators are selected, which are mass intensity index (Mindex), water intensity index (Windex) and also energy intensity index (Eindex) [5].
Figure 1: Sustainable Integrated Process Design and Control methodology for CSTR systems. Application of the Methodology for a CSTR System The production of cyclohexanone was used as a case study in order to verify the performance of the proposed methodology. Details step-by-step results are shown below. Problem Formulation. The problem formulation can be referred to previous paper [1]. For this case, each component used is presented as a, b, and c which respect to cyclohexanol, cyclohexanone and high boiler, respectively.
Stage 1: Pre-analysis. The objective of this stage is to define the operational window and set the targets for the design-controller solution by using attainable region diagram [1]. Based on the plotted region, the maximum point at the diagram (Point A) is chosen as the design target together with the other two points (alternative design for verification purposes) (refer to Figure 2).
Figure 2: Attainable region diagram for Ca with respect to Cb at 394.26 K. Stage 2: Design analysis. In this stage, all the targets selected in the previous stage then undergo the validation process for finding its design values by calculating its residence time and the design variables such as volume and height of the CSTR. Stage 3: Controller design analysis. At this stage, the controllability performance of each of the feasible candidates is evaluated and validated for the selection of controller structure. There are two criteria that need to be analyzed which are sensitivity of controlled variable y with respect to disturbance d, dy/dd and sensitivity of controlled variable y with respect to the manipulated variable u, dy/du. The details procedure for controller design analysis can be referred in Hamid’s work [2]. The results clearly show that design A has the best controller design compared to designs B and C. Stage 4: Sustainability Analysis. The purpose of this stage is to analyze the optimal design of reactor systems in terms of sustainability. The sustainability of each CSTR designs is assessed by using a 2D metrics, which are based on the economic-environmental indicators [5]. In order to fulfill the sustainability criteria, it must have low impacts to the economic losses as well as environmental impacts. Material and energy consumption will be measured to satisfy sustainability objective functions. The values of sustainable metric are tabulated in Table 1. It can be seen that CSTR design at Point A has the lowest index compared with other points. Thus, it proves that the CSTR design at the maximum point of attainable region will have the best objective in term of sustainability. Table 1: Mass, water and energy intensity index at different CSTR design candidates. Candidates Mindex Windex Eindex A B C
0.4121 0.4812 0.4636
0 0 0
0 0 0
Stage 5: Detailed Economic Analysis. The analysis is continued by identifying the profit and total cost of each design candidates. Based on the analysis, design at Point B is the best in economic analysis because of its high profit. This is due to the large design volume of CSTR at this point which leads to high production of the products. Then, it follows by design and Points A and C, respectively. The CSTR designs will be analyzed by using the profit function in order to analyze the design that will provide the maximum profit. There are four criteria used to calculate the profit function, which are profit of product, cost of material, depreciation cost and operating cost. The economic analysis is calculated by assuming
that the CSTR is operating in 5 years and 340 days per year. From the results, design at Point B has the highest profit compared to designs at Points A and C. Stage 6: Final selection and verification. The objective of this stage is to select the best candidate by analyzing the value of the multi-objective function. The multi-objective function is calculated by summing up the multi-objective function value. From the tabulated results in Table 2, it is clearly seen that the multi-objective function, J for CSTR design A is higher compared to other designs. Therefore, it is verified that the CSTR design at the maximum point of attainable region is an optimal solution for Sustain-IPDC of a cyclohexanone production in CSTR which satisfies the design, control, sustainability and economic criteria. Table 2: Final result of multi-objective function. The best candidate is highlighted in bold. P1,1 P2,1 P2,2 P3,1 P3,2 P4,1 0.2180 0.0065 0.0720 0.9600 0.730 23,347,971 A 0.2098 0.2205 0.0680 0.9700 0.750 23,287,304 B 0.2082 0.1558 0.0690 1.0400 0.820 23,120,306 C P1,1s P2,1s P2,2s P3,1s P3,1s P4,1s J 1.00 0.03 1.00 0.92 0.89 1.00 A 38.54 0.96 1.00 0.99 0.93 0.91 0.99 6.11 B 0.96 0.71 0.98 1.00 1.00 0.99 6.34 C Conclusion In this work, a systematic model-based methodology has been developed for sustainable integrated process design and control (Sustain-IPDC) for a continuous-stirred tank reactor system (CSTR) by using a 2D sustainability index. This methodology has been applied and verified for a single CSTR system. The main achievement that has been obtained from this work is that, the methodology applied a stepby-step procedure which allows a systematic analysis being done at each stage. Every step of the design methodology is clear with respect to calculations/analysis and generic in terms of application range which makes the application of the methodology is quite easy. References [1] Hamid, M. K. A., Sin, G., and Gani, R., Integration of Process Design and Controller Design for Chemical Processes using Model-based Methodology, Computers and Chemical Engineering. 34 (2010) 683–699. [2] Hamid, M. K. A., Model-based Integrated Process Design and Controller Design of Chemical Processes, Ph. D. Thesis, Technical University of Denmark, 2011. [3] Li, C., Zhang, X., Zhang, S., and Suzuki, K., Environmentally Conscious Design of Chemical Processes and Products: Multi-optimization Method, Chemical Engineering Research and Design, IChemE. (2009) 233-243. [4] Sinnott, R. K., Chemical Engineering Design, Chemical Engineering, 4th Edition. Oxford, Elsevier Butterworth-Heinemenn, United Kingdom, 2005. [5] Uhlman, B. W., and Saling, P., Measuring and communicating sustainability through eco-efficiency analysis, Chemical Engineering Progress. (2010) 17-26.
