Hierarchy of chemical process design

To develop a representation of the work domain of chemical process design the introduced abstraction-decomposition space has been developed [117-119]. The announced abstraction hierarchy or functional means-ends dimension supports knowledge-based reasoning and decision making in terms of functional relationships among the information objects. Each level in the hierarchy represents the goals or ends for the functions of the level below and potential resources or means for the level above. In other words, the AH spans the gap between purpose and material form. 

There are a variety of process safety risks one needs to assess with chemical processes. In general, these risks will lead to an evaluation of the potential for the process to have precipitous changes in temperature and or pressure that lead to secondary events such as detonations, explosions, over pressurizations, fires, and so forth. The most cost-effective way of avoiding these sorts of risks is through the adoption of inherent safety principles. Inherent safety principles are very similar to and complementary to pollution prevention principles, where one attempts to use a hierarchy of approaches to avoid and/or reduce the risk of an adverse event. The reader is referred elsewhere to a more complete treatment of this important area of process design. ... 

Alternative designs may also result when more than one property difference exists between a stream and its goal and these differences are eliminated in different orders. However, a natural hierarchy among property differences seems to exist in the chemical process domain. The hierarchy is the same as the order in which the differences were previously mentioned identity first, then amount, then concentration, then phase, then temperature and pressure, then form. The hierarchy arises because properties lower in the hierarchy are often more readily manipulated in order to satisfy the preconditions for the application of difference-elimination operators for properties higher in the hierarchy. Alternative solutions are still possible when multiple differences at the same level in the hierarchy exist for example, when both temperature and pressure need to be changed or when a mixture of more than two components is to be separated and sent to different destinations. 

Formal decision analysis methods, in particular Multi-Criteria Decision Analysis MCDA, see [960[ for an overview) such as Utility Analysis and Analytic Hierarchy Process AHP), seek to formally assess the importance of several criteria and the grade to which the criteria are respected by different alternatives, to detect inconsistencies in the assessments, and finally to recommend the best fitting alternative. Several applications in the domain of chemical engineering are reported in the literature, including the design of urban wastewater treatment plants [672] and separation systems for hydrocarbons [621]. 

The goal of plantwide control structure synthesis is to develop feasible control structures that address the objectives of the entire chemical plant and account for the interactions associated with complex recycle and heat integration schemes, and the expected multivariate nature of the plant. Many strategies have been proposed for accomplishing this task, and the majority of them have been demonstrated using dynamic process simulations. However, none have been accepted as the universal approach, in a manner similar to the steady-state process design synthesis hierarchy of Douglas [1]. 

In the manufacture of a chemical product that is actually a chemical, batch operations are often employed. This is because specialty chemical products are usually produced in small batches. In the Douglas hierarchy discussed in Chapter 2, the first decision to be made in designing a chemical process is batch versus continuous. For production of a commodity chemical in the quantities reflected in the exartples on the CD-ROM accompanying this book, the choice will always be a continuous process. Similarly, for production of a specialty chemical, the choice will almost always be a batch process. 

---