David W. Pennington

Abstract
The identification and comparison of design modifications is not typically practiced consistently or routinely, particularly in the context of pollution prevention.  A structured heuristic (rule of thumb) approach is demonstrated in this chapter that can help designers systematically identify pollution prevention opportunities.  The heuristics were derived from basic chemical engineering principles and encoded into a prototype expert system, P2TCP.  The reaction and separation systems of a process are first analyzed independently and then potential interactions are considered.  Prior specification of a process flow diagram is not required, facilitating the identification of alternatives during conceptual as well as retrofit design.  The key concepts of the approach are illustrated using a hypothetical example based on the production of allyl chloride.
 

Introduction
The synthesis or analysis of a process flow diagram is a highly skilled and creative task, typically involving expert knowledge to identify options and mathematical analysis techniques for subsequent evaluation.  Once the design problem has been specified, the next step is the generation of alternatives.  There are many different methods for generating chemical process design alternatives.¹ As time constraints exist, tools that help generate and evaluate such alternatives are extremely beneficial.  In this chapter we focus on the generation of process design alternatives for continuous chemical processes (Figure 1) using a structured approach based on heuristics derived from basic chemical engineering principles.

Figure 1: Potential Streams Reaction and Separation Systems

As the task synthesis or analysis is knowledge intensive (heavily dependent on experience and qualitative information that is often uncertain, imprecise and incomplete) this makes the use of knowledge-based expert systems that rely heavily on heuristics attractive aids for designers.  A heuristic is a rule of thumb, simplification or educated guess that reduces or limits the search for solutions in domains that are difficult and poorly understood.  Unlike algorithms, heuristics do not guarantee optimal, or even feasible, solutions and are commonly used with no theoretical guarantee.

A chemical process design can be divided into a number of generic sections to provide a structured basis for the identification of pollution prevention opportunities.  For example, detailed design theory can help to determine pollution prevention opportunities at different levels of the "onion model" in Figure 2, as described by Smith. ^2-4  Nevertheless the use of this theory and available simulation tools can involve a significant number of skilled man-hours and there is no guarantee that a systematic approach will be adopted within each ring of the onion or that interactions between the rings will be properly addressed in a structured manner.  Using software tools can help reduce these limitations.

Figure 2: The Onion Model ²

Douglas^5-7 proposed the hierarchical procedure (Table 1) and associated synthesis heuristics that can help identify pollution prevention opportunities in a structured manner.  In addition to its use in conceptual design, several retrofit studies have been performed using this approach.^{e.g. 8-10}

Table 1: Hierarchical Design Decision Levels⁶

Using the hierarchical procedure and the economic-orientated heuristics presented by Douglas^5-7 a base case Process Flow Diagram (PFD) can be rapidly synthesized.  The minimum design knowledge required is embodied in these heuristics and hence the base case is not intended to be a final or detailed design.  Importantly, a number of decisions will be made in the hierarchy that can be re-considered to identify alternative process options.  The scope of these alternatives relates to the depth of design knowledge considered and the degree to which iterations of the hierarchical procedure are performed.

Limitations of the heuristics presented by Douglas⁵ may include their economic orientation (although maximizing economic performance and minimizing pollution are often covariant) and the minimalist level of design knowledge represented.  As the recommendations can be fairly general, the use of these heuristics and the hierarchical procedure to identify P2 opportunities can be considered similar to using guideword or brainstorming techniques, albeit more structured.  A more in-depth analysis might be achieved by using detailed design procedures in conjunction with such a structured hierarchy, analogous to a combination of the work of Smith et al.^2-4 and Douglas^5-7.  Advantages over the use of "simplistic" design heuristics can include:

• representation of reaction kinetics and thermodynamics to help identify effects of changes in operating conditions, heat mode and reactor type
• consideration of other separation unit operations in addition to phase splitting and distillation
• trade-offs between energy consumption and waste generation can be identified

In the hierarchical procedure, process systems or the rings of the onion are sequentially considered.  For example, the reaction system is commonly specified before the consideration of separation techniques.^8,12 Process conditions are often frozen to optimize selectivity and conversion in the reaction system, hence to minimize process development time.¹³  As the systems interact, this traditional iterative approach may not result in the development of an "optimum" in terms of overall process waste generation and energy consumption.  Therefore it may not be appropriate to select among alternatives until all layers of the onion have first been considered.^8,12,14

Process flow diagram alternatives can first be identified independently for the reaction and separation systems using basic chemical engineering principles.^15,16  Interactions between the reaction and separation systems can then be evaluated to identify a shorter list of potential process flow diagram options.  This adaptation of the hierarchical procedure and the use of detailed design theory to help determine pollution prevention opportunities is demonstrated in this chapter.  The approach is illustrated here for a hypothetical example based on the production of allyl chloride and was represented in a computer-based prototype expert system, P2TCP.¹¹

 More detailed design theory, for example the consideration of mixture properties to refine the identification of potential separation options, and the use of mathematics orientated tools (e.g. design simulation, heat integration, mathematical programming approaches to design synthesis) can help to further screen preliminary flow diagram options initially identified using heuristics.  Similarly, taking into account different chemical reaction paths, alternative separation and reaction agents, the waste treatment infrastructure, life-cycle analysis, metrics to environmentally compare alternatives (the topic of another chapter in this book) and fugitive emissions can all augment the process of identifying pollution prevention opportunities.¹  The consideration and illustration of all these valuable tools is obviously beyond the scope of this chapter.
 

Summary
The prototype of a structured heuristics based approach is presented in this chapter that can help designers of continuous chemical processes systematically identify process flow diagram alternatives to inherently reduce waste generation and energy consumption.  Advantages of using such an approach include:

• the analysis performed is not resource intensive and does not require the prior specification or simulation of a process flow diagram; it can therefore help identify pollution prevention opportunities during conceptual as well as retrofit design.
• the heuristics can be derived systematically using fundamental chemical engineering principles; this approach can therefore provide a greater depth of analysis and more structure than key-word approaches or other similar brainstorming techniques.
• each system (reaction and separation) of a process can be analyzed independently, not sequentially, and then potential interactions are considered to help ensure all options are identified within the search space and scope of the heuristics used

As with any tools, the results of expert systems should not be accepted blindly and there is room for considerable further development.  As well as efforts to validate and extend the current core of knowledge used, focus on the development of heuristics to identify opportunities for the use of "advanced" technologies, for example combined reaction-separation techniques, is considered a high priority.