Basic flowsheet structures

Again at this stage, it cannot be determined exactly how large an excess of decane would be required in order to make Figure 13.7d feasible. This would have to be established from experimental data, but the size of the excess does not change the basic flowsheet structure. 

The basic flowsheet structure is given by the reactor and separation systems. Alternatives can be developed by applying process-synthesis methods. Use computer simulation to get physical insights into different conceptual issues and to evaluate the performance of different alternatives. 

Note that the evolution of design between the levels 4 and 5 can generate a number of alternatives, but these should not affect the basic flowsheet structure defined at the reactor/separations/recycle level. In addition, employing complex units and process-intensification techniques can produce more compact flowsheet and cheaper hardware. 

Level 7 Process-Control System. The key issues of process dynamics and control, namely fresh feed policy and stability in operation of the reaction/separation/ recycle system, are solved at Level 3. Consequently, the implementation of a process-control system may be realized without affecting the basic flowsheet structure, but taking into account fundamental process control principles, as proposed in the methodology developed by Luyben and Tyreus [20]. 

The key result of the Hierarchical Approach is the development of the basic flowsheet structure, formed by Reactor-Separations-Recycles. This structure defines the material balance envelope. In this respect of highest importance is the behaviour of the reaction system, which should deliver a realistic image of the reaction mixture. Other constraints regarding the reactor operation, as molar ratio of reactants, or safety requirements, are determinant for the structure of recycles. Optimal conversion represents a complex optimisation problem between the valorisation of raw materials and the cost of reactor, separators and recycles. 

Process plants can be represented by means of subsystems, called basic flowsheet structures (BPS). These can interact through material and energy streams. This representation reveals two steps for integrating conceptual design and plantwide control design controllable BFS s, and couple the BFS s in such a way that a controllable system is obtained. 

Design basic flowsheet structure with good controllability properties. This is possible for unit operations, where a lot of industrial experience exists. However, it is an open field of research more complex sub-systems. 

First, several candidates for plantwide controlled variables may be identified. These are not assigned to a particular basic flowsheet structure, and therefore they have a true plantwide character. Examples are production rate, recycle composition, ratio between reactants at reactor inlet, etc. 

Sensitivity analysis can show which plantwide manipulated inputs (setpoints of BFS control) are necessary. Variability of the streams connecting BFS indicates which disturbances must be rejected. In this way, the designer can identify control objectives of the basic flowsheet structures, and set them as explicit targets for design. 

Flexibility for changes, additions or upgrade of process equipment, flowsheets, instruments, etc., thus the basic canyon structure can be operated indefinitely. 

Rossiter and Douglas (1986) state that the first step in process design is to generate a basic structure for the flowsheet i.e. the choice of unit operations and interconnections which can be analysed, refined and costed, and then compared to alternatives. Thus, the generation of an industrial crystallization flowsheet gives rise to a number of optimization problems for which a systematic hierarchical decision process for particulate systems was proposed ... 

There appears to be three fundamental approaches to the synthesis of chemical process flowsheets. The first, systematic generation, builds the flowsheet from smaller, more basic components strung together in such a way that raw materials eventually become transformed into the desired product. The second, evolutionary modification, starts with an existing flowsheet for the same or a similar product and then makes modifications as necessary to adopt the design to meet the objectives of the specific case at hand. The third, superstructure optimization, views synthesis as a mathematical optimization over structure this approach starts with a larger superflowsheet that contains embedded within it many redundant alternatives and interconnections and then systematically strips the less desirable parts of the superstructure away. 

It may be observed that the two inner layers, Reactor and Separations, define the material balance envelope. Moreover, these define the basic structure of the flowsheet also, which is the object of a design activity named Process Synthesis. The outer layers of Heat Recovery and Utility systems deal with the heat balance envelope. Both are objects of a design activity that was called Process Integration. 

The use of control structures is an advanced flowsheeting feature. Flowsheet controllers are particularly useful in plant operation. An important application is the simulation of the steady state behaviour of SISO controllers. With respect to control action we can distinguish between two basic types, feedback and feedforward control. 

Figure 2 shows an existing industrial flowsheet for the manufacture of the product and figure 3 shows the optimal flowsheet as generated by the synthesis software. The basic structure of the two flowsheets is the same but Jacaranda suggested the cell concentration step uses microfiltration instead of a centrifuge which is certainly a feasible option. Results for an intracellular product also produced similar flowsheets to those used in practice (Steffens et al 2000a). 

Perhaps the first decision to be made in process development is the difficult decision of whether the enzymes to be used should be used in an integrated format. Such a question does not arise with conventional single biocatalytic steps but is highly important in multienzyme processes. One of the key criteria here is whether the enzymes can be operated together without compromise to any of the individual enzyme s activity or stability. An interaction matrix (see Section 10.6) can be used to assist such decision making. In cases where the cost of one or more of the enzyme(s) is not critical, it will be possible to combine in a one-pot operation. In other cases, where the cost of an individual enzyme becomes critical, then it may be necessary to separate the catalysts, such that each can operate under optimal conditions. Likewise, selection of the biocatalyst format (immobilized enzyme, whole cell, cell-free extract, soluble enzyme, or combinations thereof) in combination with the basic reactor type (packed bed, stirred tank, or combinations thereof) and biocatalyst recovery (mesh, microfiltration, ultrafiltration, or combinations thereof) will determine the structure of the process flowsheet and therefore is an early consideration in the development of any bioprocess. The criterion for selection of the final type of biocatalyst and reactor combination is primarily economic and may best be evaluated by the four metrics in common use to assess the economic feasibility of biocatalytic processes [29] ... 

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