The recent advancements in heat exchanger network synthesis provide efficient thermodynamic methods and programming methods for generating optimum heat exchanger networks (HENS). The articles by Gundersen and Naess (1988) and Linnhoff (1993) provide a comprehensive review of the earlier references on this topic. Most of these methods consider the use of single pass exchangers. In industrial practice, the use of a single pass exchanger is limited and the use of multipass exchangers is common. In spite of the common use, little work has been reported (Parkinson, 1982;Liu et al., 1985) on the synthesis of multipass heat exchanger networks. The approach of Liu et al. (1985) starts the design of multipass networks from the optimum singlepass networks; as shown later in this article, this may lead to nonoptimal designs for multipass networks with a greater number of shells. In this article, a systematic procedure is presented for the synthesis of multipass HENS with the objective of a minimum number of shells without violating either the minimum utility requirement or the minimum approach specifically; this procedure does not start the design of multipass networks from optimum singlepass networks. Further, for multipass exchangers, the cost function (Liu et al., 1985) includes a number of shells and the logarithmic mean temperature difference (LMTD) ("C) correction factor, which isIn the present work the values of a and b are taken as 1,456 and 0.6, respectively, except for problem TC1 for which the same are taken as 3,000 and 0.5, respectively. The cost of a multipass exchanger appears to depend mainly on the number of shells rather than the heat-transfer area, because the cost exponent b is usually less than one. Based on such a cost function, a network with a less number of shells is likely to require less capital investment than the one with a greater number of shells. Therefore, there is an attempt to develop a procedure which leads to networks with a smaller number of shells. In this article, such a procedure is outlined as a set of seven rules, and the application of the rules is illustrated with two example problems.
A digital simulation package for dynamic simulation of continuous distillation units , in the frequency domain, is presented-The package is based on a rigorous dynamic model that can account for all dynamic phenomena known to be important for control studies. Two industrial scale problem are taken up tco demonstrate the developed software: a fictitious but representative 64 tray deisobutanizer and an actual 91 tray ethane-ethylene splitter. Simulation results reveal the power of the package in computing a wide variety of frequency responses with sufficient accuracy.
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