Methodology Development of a Flexible and Operable Energy Integrated Distillation Columns Mohd Faris Mustafa1,a, Noor Asma Fazli Abdul Samad2,b, Mohd Kamaruddin Abd. Hamid1,c* 1
Process Systems Engineering Centre, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia. 2 Faculty of Chemical Engineering and Natural Resources, Universiti Malaysia Pahang, Lebuhraya Tun Razak, 26300 Gambang, Pahang, Malaysia a
[email protected],
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Keywords: Energy integrated distillation columns, Integration of process design and control, driving force, pinch technology
Abstract. This paper presents the development of a new methodology that will enable to design flexible and operable energy integrated distillation columns (EIDCs). Distillation is the primary separation process used in the industrial chemical processing. Although it has many advantages, the drawback is its large energy requirement, which can significantly influence overall plant profitability. The large energy requirement of these processes can be reduced by using energy integration. Therefore, a new methodology that will enable to design flexible and operable of EIDCs has been proposed in this study. This can be successfully obtained by implementing the integration of process design and control (IPDC) methodology, which has been drawn great attention in the past decades. The design of EIDCs can be further improved to ensure that the design is more cost efficient, flexible, controllable, and operable. This can be achieved by developing a new model-based IPDC method, which includes cost optimality and controllability at the early design stage, which is also the main objective of this study. It is expected that this new methodology will help engineers to solve EIDCs design problem in a systematic and efficient manner. Introduction
The demand for energy has been continuously increasing for years and operation units with large energy demand have become more difficult to be supplied. Reconsideration and rationalization of industrial plants is recommended. On the other hand, reducing energy requirements of distillation systems leads to lower CO2 emission. This is the reason why the plant designer must take the different energy solutions into account and choose the adequate distillation system for the specific separation task [1]. Reducing product cost in chemical industry has been very effectively used in energy saving method for distillation columns by heat integration columns system. Heat integration by two columns is based on an idea to match utilizing overhead vapour of one column in order to provide heat content for boiling up a second column [2]. Heat integration column is the process of hot streams was heat exchanged with cold streams. In this process the rectifying and stripping sections was designed by internally coupled through heat exchanger. These designs have proven an enormous improvement by reducing the reboiler and condenser duties will lead to energy saving efficiency. Methodology Development In most of EIDCs design activities, the sole consideration in solution derivation is about energy saving. Process operational issues especially controllability, and flexibility, are frequently a concern in the process design. The designed EIDCs may face controllability and flexibility challenges because of the tight-fixed design and may lead to dynamic constraint violations, and may not guarantee robust performance. If disturbances propagate severely through the process, the process may be extremely difficult to operate or even uncontrollable regardless of advanced of control techniques. Industrial
practice has made it clear that process controllability should be considered during process synthesis. This realization has led to the introduction of the integration of process design and control (IPDC), which has been drawn great attention in the past decades. Hamid formulated the IPDC problem as a mathematical programming (optimization with constraints) problem [3]. The EIDCs design can be further improved to ensure that the design is more cost efficient, controllable, and flexible. This can be achieved by developing a new model-based IPDC method for EIDCs design, which includes sensitivity, controllability, and flexibility aspects at the early design stage, which is also the main objective of this study. This paper proposes a development of a new methodology that will enable to design flexible and operable EIDCs. Another aspect that needs to be taking into consideration is the optimal EIDCs controller structure (pairing of the controlled-manipulated variables). In this case, the best pairing must be selected based on the one where the effect of a given set of disturbances can be accommodated internally without requiring too much external “help” from the utilities. However, finding the optimal solution for a complex optimization problem is not an easy task. While other optimization methods may or may not be able to find the optimal solution, depending on the performance of their search algorithms and computational demands, the proposed methodology will use the so-called reverse approach by decomposing the complex optimization problem into several sequential hierarchical sub-problems. Energy Integrated Distillation Columns Design The design method for a new methodology of distillation column can be summarized into four types which are: 1) conventional type distillation column with multicomponent separation, 2) conventional type distillation column with driving force, 3) conventional type distillation column with pinch technology and 4) conventional type distillation column with driving force combining with pinch technology. Moreover, each of the distillation columns produces a top product with the present of condenser and bottom products with the present of reboiler. The McCabe-Thiele graphical technique has been used as a basic and simple technique to determine the design values of distillation column [4]. In this case, McCabe-Thiele graphical technique will be applied in the conventional type of distillation column to determine design variables such as number of stages, feed location, reflux and reboiled ratio. In this study, two graphical methods are used to determine the optimal design for EIDCs, which are driving force and pinch technology. Driving force method usually use in the earliest stage of designing distillation column in order to successfully achieve in desired separation. In distillation column, driving force is the difference in composition between vapour phase and liquid phase which occurs when the difference of properties such as boiling point and vapour pressure [5]. The ideal designs for distillation column is based on the driving force approach to maximum, will lead to energy necessary in maintaining the two phase system in minimum or highly energy efficient design. Therefore, in the second type of distillation column design, the driving force method will be used as an additional step in similar conventional distillation column with the present of driving force diagram. Pinch technology represent as a simple thermodynamically method that produce minimum energy consumption by using the first key of pinch analysis (setting energy target) as a key part for energy monitoring [6]. Pinch technology method helps to optimize the heat transfer equipment during temperature crossover between hot streams and cold streams according to the first and second Law of thermodynamics. Then, the third type of distillation design method is achieved by using pinch technology in the conventional type of distillation column in order to reduce energy consumption in the process which mean more energy saving can be obtained. Lastly, the design of energy efficient distillation column can be created by combining driving force method with pinch technology in the conventional type of distillation column. Theoretically, by combining these two methods with the conventional type of distillation columns can produce more energy efficient EIDCs.
New Methodology Development for Energy Integrated Distillation Columns (EIDCs) This section will briefly present the development of a new methodology for an energy integrated distillation columns design. The flowchart of the proposed methodology is shown in Figure 1.
Figure 1: A new proposed methodology for an energy integrated distillation columns. Accordingly, the proposed methodology is dividing into four hierarchical steps. The first stage deals with the distillation column design with driving force method. In this stage the design of conventional distillation columns will be improved in terms of energy saving by using driving force methods. The second stage deals with the heat integration design using pinch technology. The design of conventional distillation columns will be improved here in terms of energy saving by using pinch technology The third stage deals with the controllability analysis for all type of distillation columns. Here, four types of distillation columns design will be verified in terms of flexibility analysis, sensitivity analysis and also controller structure selection. Lastly the fourth stage of this methodology will select the final designs of EIDCs. In this stage the best EIDCs design will be selected according to the highest value of the objective function which satisfies process design, process control and economic criteria. Then, the selected design will be verified further in terms of dynamic performances. The separation case study of a ternary mixture of ethanol, n-propanol and n-butanol is selected and studied. In Table 1, the flow rate feed and product is presented for the three components are demanded with equal distribution of the impurities in B product stream. In this case study, A, B and C denoted as light, intermediate and heavy components. Pressure both columns is 101.33 kPa.
Table 1: Case study of flow rate feed and product specification Streams Components Ethanol (A) n-propanol (B) n-butanol (C) Total
α1 4.7 2.3 1 -
Feed kmol/hr X 33.333 0.333 33.333 0.333 33.333 0.333 100 1.000
A kmol/hr 33.168 0.335 0.000 33.503
X 0.990 0.010 0.000 1.000
B kmol/hr 0.165 32.660 0.165 32.990
C X kmol/hr X 0.005 33.168 0.000 0.990 0.335 0.010 0.005 0.000 0.990 1.000 33.503 1.000
Conclusion The new methodology for flexible and operable energy integrated distillation columns has been proposed in this paper. The methodology is based on the driving force approach and pinch technology in order to select the best design of energy integrated distillation columns. With this approach, the design for energy integrated distillation columns will be based on the maximum energy saving and also easier to control with the minimum costs. The proposed methodology consists of four main stages, where each stage is integrated with each other according to the type of energy integrated distillation columns to be designed. Consequently, it is possible to make an early assumption on type of distillation columns that will give not only the best design for energy efficient, but also flexible and operable. It is expected that this new methodology will help engineers to solve energy integrated distillation columns design problem in a systematic and efficient manner. Acknowledgement The financial support from Universiti Teknologi Malaysia (RUGS Tier 1 Q.J130000.2509.07H39) and Ministry of Education of Malaysia are highly acknowledged. References [1] Gadalla, M., Olujic, Z., de Rijke, A., and Jansens, P. J. (2006). Reducing CO2 Emissions of Internally Heat-Integrated Distillation Columns for Separation of Close Boiling Mixtures. Energy, 31(13), 2409-2417. [2] Annakou, O., and Mizse, P. (1996). Rigorous Comparative Study of Energy-Integrated Distillation Schemes. Ind. Eng. Chem. Res., 35, 1877-1885 [3] Hamid, M. K. A. (2011). Model-based integrated process design and controller design of chemical processes. Technical University of Denmark (DTU) PhD Thesis. ISBN: 978-87-9248139-9. [4] Wang, J. L., and Mansoori, G. A. (1994). A Revision of the Distillation Theory (Part I). Scientia Ir., 1(3), 267-287. [5] Bek-Pedersen, E., and Gani, R. (2004). Design and Synthesis of Distillation Systems Using a Driving-Force Based Approach. Chemical Engineering and Processing, 43, 251-262. [6] Kemp, I. C. (2007). Pinch Analysis and Process Integration. A User Guide on Process Integration for the Efficient Use of Energy. Elsevier, Amsterdam, the Netherlands.
