Heuristics equipment design

A conparison of the actual equipment design and the predictions of the heuristic methods are given below. 

The initial aim of the procedure is to generate a reasonable base case design that can be used for preliminary economic evaluation of the process. This can subsequently be optimized and/or compared with any process alternatives that are identified. The complete process is always considered at each decision level, but additional fine detail is added to the structure of the flowsheet at any stage. Established heuristics and equipment selection procedures are used together with new process synthesis insights to guide each flowsheet decision. 

The key attribute of flows in micro devices is their laminar character, which stands in contrast to the mostly turbulent flows in macroscopic process equipment. Owing to this feature, micro flows are a priori much more accessible to a model description than macro flows and can be described by first-principle approaches without any further assumptions. In contrast, for the simulation of turbulent flows usually a number of semi-heuristic models are applied, and in many situations it is not clear which description is most adequate for the problem under investigation. As a result, it stands to reason to assume that a rational design of micro reactors... 

This approach to creation of the design involves making a series of best local decisions. This might be based on the use of heuristics or rules of thumb developed from experience4 on a more systematic approach. Equipment is added only if it can be justified economically on the basis of the information available, albeit an incomplete picture. This keeps the structure irreducible, and features that are technically or economically redundant are not included. 

The requirements for expert systems for process control have inspired new designs based on real-time knowledge base inferencing. Object oriented representation of plant equipment, knowledge representation of the interactions of processes and models of process behavior - heuristic as well as analytical --are incorporated into a real-time expert system for process control. The application of inference in real-time requires using metaknowledge to focus the inferencing resources of the expert system. Finally truth maintenance requires a temporal model of the time dependence of the truth of data and inferred results. 

Over the years some heuristics have been developed for estimating liquid holdups in most systems. Holdup times (based on total flow in and out of the surge volume) of about 5 to 10 minutes work well. If a distillation column has a fired reboiler, the base holdup should be made larger. If a downstream unit is particularly sensitive to rate or composition changes, then holdup volumes in upstream equipment should be increased. Such matters should be considered when the vessels are designed. 

In creation of the detailed database, it is common to add transport and kinetics data, as well as data concerning the feasibility of the separations, the identity of any forbidden matches in heat exchange, heuristic parameters, and data for sizing the equipment. Each process requires somewhat different data, and hence it is inappropriate to generalize. However, it is instructive to examine the mix of data needed by a design team in connection with the vinyl chloride process in Figure 3.19. 

Table 16.32 is accompanied by the following brief equipment descriptions, which include, in some cases, heuristics for estimating the size factor when design data are not readily available. More equipment descriptions and detailed methods for determining size for most of the types of equipment described below may be found in Perry s Chemical Engineers Handbook (1997). 

Once the issues have been defined, available best practice, in the form of design heuristics and technologies to satisfy requirements, will be reviewed. Cormnent will be made upon the efficacy of their application within extremities PPE alongside the theatre in which they could be used. Future technologies that may be available in the short, medium and longer term will be discussed in a later section of the chapter. Generic requirements for extremities protection equipment products are ... 

Next, the size of the major process lines must be determined. In order to estimate these pipe sizes, it is necessary to make use of some heuristics. A heuristic is a sinple algorithm or hint that allows an approximate answer to be calculated. The preliminary design of a piece of equipment might well use many such heuristics, and some of these might conflict with each other. Like any simplifying procedure, the result from a heuristic must be reviewed carefully. For preliminary purposes, the heuristics from Chapter 11 can be used to estimate approximate pipe sizes. Example 1.12 illustrates the heuristic for calculating pipe size. 

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