Process Selection — Considerations

FOLLOWING THE feasibility study, the next stage in the design project is the evaluation and comparison of the alternative process routes for manufacture of the chemical. The selection of an appropriate process is an important decision, all the subsequent work depends upon this choice. Although the selection can be changed or modified at a later stage, at least before the plant is built, such a decision results in a serious waste of time and money. However, probably not such a waste as building an uneconomic or unsafe plant  

The reactor is the heart of most processes, developments in process technology often centre around improvements in the design and operation of the reactor. This is often the basis of new and improved processes. Subsequent stages in the chemical process are usually concerned with the separation of various chemicals from the desired product, followed by the final purification stages. 

Previously in the feasibility study (Chapter 2) the appropriate purity of the final product should have been established. The product purity must be dictated by the customer (market) requirements. In some 

Processes often differ in terms of the process conditions, e.g. high-pressure and low-pressure processes, or the type of reactor that is used, e.g. gas-phase catalytic reactor or liquid-phase CSTR. These differences should be clearly marked on both the simple block diagrams for each process and the detailed process flowsheets, they often determine which process route is ultimately selected. Many older, established processes were operated under conditions of high pressure or high temperature (mainly in the reactor), whereas the newer processes have often been improved by operation under less severe conditions. 

Many older processes (developed before the 1970s oil price crisis) were less energy efficient than those developed more recently. This should be apparent by consideration of the energy conservation features included in new plants, and the subsequent increase in complexity of the associated P ID. Energy conservation is discussed in Section 8.2.1. The selection of a process route for production of a chemical will depend upon the following factors/considerations  

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Additional process selection considerations. Material limitations will affect the process selection because none of the processes will accept all of the plastics. Therefore, the optimum process may not accept the desired material. When this problem occurs, one of the two will need to be changed. Table 8.6 lists the acceptable materials for the principal processes. Bear in mind that both resin and equipment manufacturers are continuously working to enhance their products and a list such as this can be made obsolete at any time. If the desired process does not indicate that the preferred polymer can be used, it may be worthwhile to investigate further. 

Filter Selection. A variety of product- and process-related factors govern filter selection. Considerations include the characteristics of the fluid to be filtered, ie, its chemical composition and compatibiHty with the filtration system (inclusive of the membrane, filter hardware, piping, etc), the level of bioburden present, specifications on effluent quaHty, the volume of product to be filtered, flow rate, and temperature. 

Table 12.1 Some considerations in reaction process selection and design...

For aromatics production, similar considerations apply. Maximum yields of xylenes and other heavy aromatics can be obtained in cyclic units, but, again, at somewhat higher investments. The process selection, thus, again requires the balancing of process credits versus debits for the specific application. For light aromatics (benzene-toluene) production, however, the situation tends to favor a... 

Often the decision to select a batch or continuous processing mode involves a determination of the relative contributions of capital and operating expenses to total process costs for the proposed level of capacity. As Denbigh (1) points out, what is best for a highly industrialized country with high labor costs is not necessarily best for a lesser developed country. In many cases selectivity considerations determine the processing mode, particularly when the reaction under study is accompanied by undesirable side reactions. The yield of the desired product may differ considerably between batch and continuous operation and between the two primary types of continuous processes. When the yield is lower for a continuous process, this factor may be so important in the... 

Early attempts to utilize the high acid activity of faujasite zeolite catalysts for direct xylene isomerization suffered from low selectivity. Considerable improvement was obtained first by using a large pore zeolite (7) catalyst and subsequently in several process modifications that use ZSM-5 as catalyst (2). In the following we will show how these selectivity differences can be related to structural differences of the various zeolites. 

Syngas (typically a mixture of CO, H, and CO ) reacts over the active catalyst (Cu/Zn/AljOj) dispersed in an inert oil medium. This process offers considerable advantages over the conventional vapor phase synthesis of methanol in the areas of heat transfer, exothermicity, and selectivity toward methanol. However, this process suffers from the drawback that the methanol synthesis reaction is a thermodynamically governed equilibrium reaction. 

Decision-making process involving considerations of political, social, economic, and technical factors with relevant risk assessment information relating to a hazard so as to develop, analyze, and compare regulatory and non-regulatory options and to select and implement appropriate regulatory response to that hazard. 

Other considerations for process selection include fabrication facilities and suitability of the explosive for its intended application (Ref 13)... 

There are no simple rules of thumb in defining the cost of reinforced plastic components. Their successful use has resulted from proper design, utilizing the benefits these materials offer, process selection, tooling cost advantages that fit the production needs, and consideration of life cycle economics. Each existing application illustrates the cost-performance advantage of reinforced plastic over the traditional material that is displaced. 

Royalty cost also is a factor in process selection. It varies with the individual plant under consideration hence, its economic significance cannot be evaluated in general terms. 

SELECTIVITY CONSIDERATIONS IN MODELING THE TREATMENT OF PERCHLORATE USING ION-EXCHANGE PROCESSES Anthony R. Tripp and Dennis A. Clifford... 

Another more arbitrary but equally reasonable technique is to choose a single reaction as a model typical of the processes under consideration. The results from study of this reference reaction then could be employed to define the parameters for the substituent groups in other similar reactions. Thus, Hammett (1940) selected the ionization equilibria for substituted benzoic acids in water as a reference reaction and Taft (1956) employed the ionization equilibria of 4-substituted bicyclo[2.2.2]octane-l-carboxylic acids (Roberts and Moreland, 1953) for evaluating the inductive e-constants. The constants so defined are invariant. The validity of this scheme is measured by the agreement between the predictions of the reference reaction and the actual systems under study. 

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