Retroﬁtting Heat Exchanger Networks Based on Simple Pinch Analysis
Department of Chemical Engineering, Dalian Nationalities UniVersity, Dalian 116600, China
Department of Chemical Engineering, National Cheng Kung UniVersity, Tainan 70101, Taiwan, ROC
It has been well-established that the energy and capitalcosts of a heat exchanger network are both dependent upon the minimum allowable temperature approach ∆Tmin. As a result of the rapidly growing oil prices in recent years, there appears to be an urgent need to retroﬁt the existing “optimal” networks so as to reduce the current utility consumption rates with smaller ∆Tmin values. A simple pinch-based approach is proposed here to accomplish this taskwhile keeping additional capital investment to a reasonable level. In particular, every cross-pinch match is removed, and its heat loads on the hot and cold streams are both divided into two according to the pinch temperatures. At either side of the pinch, the divided heat loads on each stream are combined and then matched according to a systematic procedure derived from simple pinch analysis. Twoexamples are provided to illustrate this procedure.
1. Introduction The heat exchanger network (HEN) design method is a matured technology for energy integration in the process industries,1-4 which has already been applied successfully in numerous grass-root and revamp projects for over two decades. With the rapidly growing energy costs in recent years, there is a renewed interest in retroﬁttingthe existing “optimal” HENs which were designed under the presently outdated cost structure. A number of good reviews on this issue can be found in the work of Yee and Grossmann,5 Asante and Zhu,6 and PonceOrtega et al.7 Thus, a full literature survey is omitted here for the sake of brevity. Generally speaking, the existing HEN retroﬁt methods can be considered as either pinch- or model-based.The former approach is adopted in this study due to the fact that, in practical applications, it is easier to implement the manual design steps and to exercise engineering judgment. Tjoe and Linnhoff8 proposed a calculation procedure to determine the appropriate minimum temperature approach ∆Tmin after retroﬁt by considering the energy savings, investment cost, and payback period. On the basis of aset of general design guidelines, the existing cross-pinch exchangers were then eliminated, shifted, or rematched strictly above or below the new pinch temperature. Additional exchangers could also be placed if necessary. Finally, the resulting network was evolved manually by shifting loads around the heat-load loops and along the heat-load paths so as to yield a retroﬁt design which is closelycompatible with the existing one.5 Despite the fact that satisfactory results were reported, there is still a lack of systematic and speciﬁc procedure to produce the modiﬁed HEN designs. An improved pinch-based retroﬁt procedure is developed in this work to lower the utility consumption levels of any given HEN at the cost of minimal capital investment. For illustration clarity, the remainder of thispaper is structured as follows: The identiﬁcation and partition methods of the cross-pinch heat loads are ﬁrst presented in the next section. The speciﬁc steps to modify a given HEN are then listed in section 3. Two examples
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are provided next in thesubsequent sections to demonstrate the effectiveness of the proposed approach. Conclusions are given at the end of this paper. 2. Identiﬁcation of Cross-Pinch Heat Loads It is assumed in this work that the updated ∆Tmin after retroﬁt can be determined in advance by considering the payback period, investment cost, and energy savings.8 Consequently, the corresponding pinch temperatures can also be...