Thermodynamic Analysis in the Design of Process Networks

This thesis discusses the use of thermodynamic Second Law analysis in the context of chemical process network design. It is divided into two parts. Part I is based on the study of entire processes while Part II concentrates on the problem of heat exchanger network design. This division into two parts facilitates a clear presentation of the results obtained. Second Law analyses are frequently referred to in the academic literature as giving a more valid account of inefficiencies in engineering systems than simple heat balances, a view that would seem to be well founded in thermodynamic theory. On the other hand, process design engineers in industry do not seem to make much use of this type of analysis. They usually comment that the results obtained either state the obvious (e.g., "... do not degrade heat...", etc.) or lead to recommendations that are not practical (e.g., "... use fuel cells instead of thermal reactors...", etc.) Thus, there seems to be a conflict between theoretical claims and practical experience. The present thesis attempts to clarify this situation by giving a balanced view of both the potential value of Second Law analysis as well as its shortcomings. In Part I, it is shown that Second Law analyses are both difficult to produce and difficult to interpret in the context of chemical process design. Consequently, an approach is developed to overcome these difficulties. However, the approach somewhat transforms the meaning of the words "thermodynamic analysis". Namely, it is no longer a strict application of Second Law textbook theory that is implied, but a rather more broad minded approach involving the use of carefully considered thermodynamic concepts. In other words, a somewhat "slackened" form of thermodynamic analysis is recommended. This slackened form is less well defined than the classical one but easier to produce and more meaningful to interpret. (A more detailed explanation of the concepts involved is given in the "Extended Abstract of Part I" on page iii) . When applied to the case studies, the approach leads quickly to attractive design suggestions. In Part II, the approach is applied to the rather specialised and recently well researched problem of heat exchanger network design. This leads to the development of several explicit design methods which consistently identify better solutions for identical problems than methods described previously in the literature. Also, an understanding of the subject matter is achieved that is quite unprecedented (see "Extended Abstract of Part II" on page v). These results seem to support the conclusions arrived at in Part I. An interesting observation made in Part I and Part II is that some of the designs put forward would not only be cheap to run but also cheap to build. Thus, the thermodynamic approach developed here appears to be capable not only of identifying energy savings but capital savings, too. This gives rise to a very ambitious claim (compare pp. 270-271) : namely, that the approach may lead to designs that are "driving force conscious" in general. In other words, it may help the engineer to develop a better feel for the natural driving forces in his problem and may therefore stimulate him to create, in a quite general sense, more appropriate, simpler, and more elegant processes.