(237f) A Method for Pressure Drop Targeting of Heat Exchanger Networks
Heat recovery and, therefore, energy savings in the process industries require heat exchanger networks (HENs). A HEN is an interconnected arrangement of heat exchangers, mixers and splitters, in which cold and hot process streams interchange internal process heat as complete as possible for decreasing energy consumption. The objective of the HEN design problem for a given set of process streams, utility streams and cost data is to find the optimal network structure along with the optimal size of their heat transfer units. Typically, total annualized cost has been used as an objective function for obtaining an optimal solution for the HEN design problem. Total annualized cost includes the annualized investment of the heat exchangers and also the annual operating costs of auxiliary utilities.
The two most common approaches that have been developed for optimal synthesis of a HEN are pinch technology and mathematical programming methods. Pinch technology is based on heuristics and thermodynamic principles (Linnhoff and Hindmarsh, 1983; Linnhoff and Ahmad, 1989), while mathematical programming approaches (Floudas et al., 1986; Yee and Grossmann, 1991) formulate and solve the HEN synthesis problem as a constrained optimization problem. Thorough reviews of the principles, advances and applications of both these approaches have been presented in the literature by Gundersen and Naess (1988), Jezowski (1994a, 1994b), Shenoy (1995), and Furman and Sahinides (2002). Although the mathematical programming methods have a more rigorous approach of obtaining an optimal HEN design while satisfying stipulated constraints, the pinch technology is currently more used because it allows the design engineer to incorporate real plant situations easily for industrial scale problems.
Pinch technology decomposes the HEN design problem into a sequential procedure. The first stage is to generate economic trade-offs between capital and operating costs ahead of design in order to select the optimal value for the minimum allowed approach temperature, ÄTmin. Then, the pinch design method (Linnhoff and Hidmarsch, 1983) is applied to develop a network based on the targets generated in the first stage. Targeting and synthesis stages constitute the conceptual design phase of pinch technology. The conceptual design is followed by the detailed design. In this stage, the detailed heat-exchanger designs based on the stream pressure drops are performed. Until recently, however, targeting and synthesis stages had been applied assuming constant film heat-transfer coefficients for all streams, which can lead to nonoptimal or infeasible networks due to inconsistencies with detailed designs of the heat exchangers at the last stage.
To overcome this limitation of pinch technology, Polley and Panjeh Shahi (1991) developed the first systematic procedure for HEN targeting considering pressure drop effects. The original area targeting algorithm was modified to include an expression that relates the stream pressure drop to the heat exchanger area and the stream heat transfer coefficient. Values of the allowable pressure drops of streams, fluid properties, volumetric flowrate and hydraulic diameters of the exchangers are specified to calculate the film heat-transfer coefficients of streams and network area. The calculated heat transfer coefficients are then used in synthesis stages. This work improves the consistency between conceptual design and detailed design phases for given allowable pressure drops of streams. However, for grassroots designs, the pressure drops of streams are usually unknown and can be treated as part of the degrees of freedom of the HEN design problem. Therefore, it is convenient to consider the optimization of pressure drops instead of specifying fixed allowable pressure drops in the targeting stage.
As a step forward to the optimal design of HENs based on pinch technology, a method that treats as optimization variables the stream pressure drops at the targeting stage is proposed in this work. For that purpose, an optimization model for stream contact areas, total network area, utility consumption and stream pressure drops is developed in terms of unknown film heat-transfer coefficients. The Kern method (Kern, 1956) is used to relate pressure drops, film heat-transfer coefficients and contact areas. For each energy recovery level, the stream contact areas are obtained using the UA-values of the individual matches from the spaghetti design. The model is formulated so as to minimize the total annualized cost, which includes the capital costs of heat exchangers and pumping devices, the utilities costs and the electricity costs to run the pumping equipment. By this method, the optimal pressure drops and film heat-transfer coefficients can be calculated, and the targeting stage can remain simple. Thus, no assumed heat-transfer coefficients or fixed allowable pressure drops need to be used, as is the case with previous grassroots design methods based on pinch technology.
Three case studies from the literature that include the detailed design for each heat exchanger of the networks are used to test the method developed. The results confirm that this method provides the consistency of heat exchanger design information through conceptual design and detailed design phases.
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