Preliminary Process Synthesis

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 safety evaluation can be seen as a part of the process synthesis which includes economical, environmental and safety aspects. Different process alternatives have to be compared based on all of these criterias. Problems of the preliminary safety evaluations arise from the lack of information. To solve this problem a dedicated methodology for preliminary inherent safety evaluations is required. 

Conception is the most critical step in the development of a new process. Though still done largely on the basis of experience and intuition, it may be implemented with process synthesis. Computerized algorithms may provide for a large number of possible routes to a product. This method, combined with the analysis of raw material costs and DS-51 ASTM tests for process hazardousness, are the best options to speed up the bench-scale development of new chemical processes. Thus, one or a reduced number of routes to the desired product can be identified for preliminary process development. 

Although rigorous computer methods are available for solving multicomponent separation problems, approximate methods continue to be used in practice for various purposes, including preliminary design, parametric studies to establish optimum design conditions, and process synthesis studies to determine optimal separation sequences (Seader and Henley, 2006). A widely used approximate method is commonly referred to as the Fenske-Underwood-Gilliland (FUG) method. 

Clearly, as data are obtained in the laboratory, they are tabulated and usually regressed, to allow addition to the preliminary database for use by the design team in preliminary process synthesis, the subject of the next section. 

As discussed earlier, preliminary process synthesis occurs after an alternative processing concept has been created. Having defined the concept and assembled the preliminary database, usually with some experimentation, the design team sets out to synthesize a flowsheet of process operations to convert the raw materials to the desired products. First, it decides on the state of the raw materials, products, and byproducts, before assembling different configurations of the process operations. 

In this example, only the production of the monomer is considered, with a focus on the specific problems identified by the design team as alternatives 2 and 3 in Section 1.2. The objective is to create several promising flowsheets, as candidate solutions to these specific problems, to be inserted later into Figure 3.3. In addition to data from the chemistry laboratory, two patents (Benedict, 1960 B.F. Goodrich Co., 1963) play a key role in process synthesis. These were located by the design team during their literature search and entered into the preliminary database. When appropriate, they will be referred to in connection with the synthesis steps that follow. 

Finally, before leaving this section on preliminary process synthesis, the limitations of the heuristic approaches should not be overlooked. Many algorithmic methods are very effective for the synthesis of alternative flowsheets, their analysis, and optimization. These methods are usually used by design teams in parallel with their work on the development of the base-case design, which is the subject of the next section. The algorithmic methods are easily implemented and are illustrated with many examples in Part Two of this text (Chapters 6-12). 

As mentioned throughout the discussion of preliminary process synthesis and the creation of the process flow diagram, the process simulator usually plays an important role, even if a simulation model is not prepared for the entire flowsheet. When parts of a simulation model exist, it is common for the design team to assemble a more comprehensive model, one that enables the team to examine the effect of parametric changes on the entire process. In other cases, when the process simulators have not been used for design, a simulation model is often created for comparison with the pilot-plant data and for parametric studies. 

Be able to create a preliminary database for use in preliminary process synthesis. 

Understand the steps in preliminary process synthesis and be able to use them to develop other flowsheets for the manufacture of vinyl chloride and tPA (corresponding in the other branches of the synthesis trees in Figures 3.9 and 3.17), as well as for the manufacture of other chemicals. 

This chapter returns to the steps of preliminary process synthesis in Section 3.4, in which a strategy is recommended that involves assembling the process operations in a specific order, as follows ... 

An approximate measure of the profitability of a potential process that does take into account the size of the project is venture profit. It is used often for preliminary estimates when comparing alternative flowsheets during the process synthesis stage of process design. VP is the annual net earnings in excess of a minimum acceptable return on investment, Thus,... 

A measure of economic goodness, which does not involve sales revenues for products and is also used for preliminary estimates when comparing alternative flowsheets during process synthesis, is the annualized cost. It is the sum of the production cost and a reasonable return on the original capital investment where, again, the reasonable return on investment, i i , is taken here as 0.2. Thus,... 

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