Synthesis of multipass heat exchanger networks based on pinch technology

Abstract

The multipass heat exchanger is the most common type of heat transfer equipment used in heat exchanger networks (HENs) by the chemical process industries. There are many methods that have been proposed for the synthesis of HENs with multipass heat exchangers, which are mostly derived from the FT design method. In this paper, an alternative new method to synthesis multipass HENs is presented based on the classical pinch technology. In the multipass heat exchanger, both countercurrent and co-current flow are involved. For the co-current flow, composite curves and problem tables are modified, and compared with that of the countercurrent flow. A proper minimum temperature difference is also selected considering the energy-capital cost trade-offs, and then a multipass HEN is synthesized. Results of the case study demonstrate that the new approach meets operating requirements and minimizes the total cost successfully.

Introduction

In industrial practice, multipass heat exchangers are commonly used because of advantages such as longer flow-paths for a given exchanger length, allowance for thermal expansion, easy mechanical cleaning, as well as good heat transfer coefficients on the tube side due to higher velocities (Ponce-Ortega, Serna-Gonzalez, Salcedo-Estrada, & Jimenez-Gutierrez, 2006).

Research efforts on the synthesis of heat exchanger networks (HENs) have been significant in the last few decades (Gundersen, 2000). However, most of the methods published for HENs synthesis problem consider the use of single pass exchangers (Colbert, 1989, Linnhoff and Flower, 1978, Zhu et al., 1995). In these methods, the pinch technology-based techniques have found applications in a wide range of designs for the countercurrent exchanger networks. Pinch technology is a sequential graphical HENs method based on the first law of thermodynamics and some constrains originated from the second law of thermodynamics (Predrag & Sreten, 2009). The first law is concerned with the energy balance (i.e. the conservation of energy constraint), and the second law ensures the positive temperature difference between the hot stream and the cold stream, in case they exchange the heat. Based on the concepts of pinch technology, it is feasible to design multipass HENs with the help of the first and second laws of thermodynamics.

The most common approach for the synthesis of multipass HENs is based on the F_T correction factor (Galli and Cerda, 2000, Jose et al., 2008, Kern, 1950). During the design process, the rule of keeping F_T > 0.75 is used to calculate the number of shells. The size of the exchanger is then found from the basic design equation. However, the evaluation of the F_T correction factor depends on trial iterations and may be difficult to compute (Ponce-Ortega, Serna-Gonzalez, & Jimenez-Gutierrez, 2008). Fakheri (2003) has presented some explicit expressions that avoid the difficulties associated with the use of the F_T charts. Ahmad and Smith (1989) introduced a new parameter, X_p, and derived simple equations to calculate the number of shells in series explicitly. Reddy, Rao, and Davies (1998) also proposed a set of rules for designing multipass HEN with a smaller number of 1–2 shells without the consideration of the trade-offs among the energy consumption, the number of units, and the area. Ponce-Ortega et al. (2006) presented an optimization method for the design of multipass heat exchangers by using the F_T correction factor. In short, all of these studies are mainly based on the F_T correction factor to optimize the multipass HENs.

To account effects of co-current flows in heat exchangers, the synthesis of the HEN with only co-current heat exchangers is firstly developed in this paper. By taking this as a starting point, and using classical pinch technology (Linnhoff et al., 1982), an alternative new method to synthesis the multipass HEN is proposed with the consideration of energy-capital cost trade-offs.