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.
N-Nitrosamines are believed to act as carcinogens by alkylating DNA in their ultimate carcinogenic forms which are produced by metabolic activation. Alkylation at certain oxygen sites in DNA, described as "promutagenic," appears to be of particular significance for mutagenesis and cancer, as indicated by experimental findings. This theoretical study deals with two factors involved in the alkylation of these promutagenic oxygen sites by N-nitrosamine ultimate carcinogens. The first is the competition between alkylation at the promutagenic 06-guanine and 04-thymine sites and that at the nonmutagenic N7-guanine site, which is here related to the degree of participation of cationic ultimate carcinogens as compared with neutral ultimate carcinogens. Parent dialkylnitrosamines are classified structurally according to their degree of cationic ultimate carcinogen participation and preference for promutagenic alkylation. The second factor is the thermodynamic facileness of alkylation at the 06-guanine and 04-thymine sites. Heats of alkylation by candidate ultimate carcinogens are calculated here for numerous parent dialkylnitrosamines. Finally, these two factors are jointly considered in an attempt to correlate them with experimental carcinogenic potency of the parent nitrosamines. Out of the patterns of correlation observed, light is shed on mechanistic factors likely to be involved in the modulation of parent carcinogenic potency.
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