Process synthesis flowsheets

Dynamic simulation, process control, real-time optimization Process synthesis, flowsheet convergence, simultaneous modular vs. equation-oriented... 

Process. synthesis. Concept development, construction of flowsheets for new processes, and redesign or modification of existing process flowsheets. 

Chemical Flowsheet Basic unit operation selection with flow rates, conversion factors, temperatures, pressures, solvents and catalyst selection Process synthesis route Laboratory and pilot scale trials Knowledge of existing processes... 

The result of process synthesis is a flowsheet which represents the configuration of the various pieces of equipment and their interconnection. Next, it is necessary to analyze the performance of this flowsheet. 

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. 

FIGURE 8.2 Example of a flowsheet generated by computer-aided process synthesis, as would be seen on the screen of a computer terminal. The column on the left side of the figure shows options available in the particular design program being run. Courtesy, Peter Piela, Carnegie-Mel Ion University. 

The primary objective of process synthesis is to develop systematically process flowsheet(s) that transform the available raw materials into the desired products and which meet the specified performance criteria of... 

The next section will reflect on common conceptual process design practices for overall flowsheet development. The remainder of this chapter reviews recent developments in process synthesis methods for reaction and separation systems that systematically guide the design decision-making process toward novel and improved designs. 

Synthesis is the step in design where one conjectures the building blocks and their interconnection to create a structure which can meet stated design requirements. This review paper first defines chemical process synthesis and indicates the nature of the research problems—to find representations, evaluation functions and search strategies for a potentially nearly infinite problem. It then discusses synthesis research and the most significant results in each of six areas—heat exchanger networks, separation systems, separation systems with heat integration, reaction paths, total flowsheets and control systems. 

Stephanopoulos, G. and Westerberg, A.W., Studies in Process Synthesis—II. Evolutionary Synthesis of Optimal Process Flowsheets, Chemical Engineering Science, Vol. 31, pp 195-204, 1976. 

The case studies cover key applications in chemical process industries, from petrochemistry to polymers and biofuels. The selection of processes was confronted with the problem of availability of sufficient design and technology data. The development of the flowsheet and its integration is based on employing a systems viewpoint and systematic process synthesis techniques, amply explained over three chapters. In consequence, the solution contains elements of originality, but in each case this is compared to schemes and economic indices reported in the literature. 

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. 

Process synthesis by superstructure optimization consists of the identification of the best flowsheet from a superstructure that considers many possible alternatives, including the optimal one. A substantial advantage is that integration and design features may be considered simultaneously. At today s level of software technology the superstructure optimization is still an emerging technique. However, notable success has been achieved in numerous applications. The reference in this field is the book of Biegler et al. [6]. 

We should mention that the modern process-synthesis methods can ensure intrinsically the optimization of subsystems, such as reactions, separations and heat exchange. Therefore, by searching in the first stage the structural optimization of the flowsheet and by performing later the optimization of units should lead quite soon to the true overall optimum in a large number of situations. 

Formally, the hierarchical design procedure can be divided in two steps process synthesis and process integration. The first deals with the synthesis of the process flowsheet and basic material balance. The second step handles the problem of efficient use of energy and helping material resources (water, solvents, hydrogen). Clearly, the two steps are interrelated. 

Have mass integration possibilities been evaluated Are mass exchange networks (MEN) in place in the flowsheet MEN synthesis has been addressed in the book by El-Halwagi,9 and application of this method to water management has been addressed in a paper by Liu et al.10 These aspects should be looked at carefully during the process synthesis stage, as they offer considerable scope for solvent or MSA reduction. 

Process synthesis tries to find the flowsheet and equipment for specified feed and product streams. We define process synthesis as the activity allowing one to assume which process units should be used, how those units will be interconnected and what temperatures, pressures and flow rates will be required [2.15, 2.16[. 

Process flowsheet generation is an important part of process synthesis. The following tasks have been established for process flowsheet generation [2.17] (i) the generation of alternative processing routes, ii) the identification of the necessary unit operations, (iii) the sequencing of unit operations into an optimal flowsheet. 

Most of the applications discussed here were based on algorithmic systematic generation approaches to process synthesis. There also exist a number of heuristic rule-based or expert-systems-based approaches including PIP for total flowsheet synthesis, SPLIT for separation schemes (Wahnschafft et ai, 1991),... 

Advances in problem formulation and in computational hardware and software capability offer the promise of a new generation of practical process synthesis techniques based directly on structural optimization. Soon the goal of synthesizing provably unbeatable conceptual process flowsheets may be at hand. 

Discrete variables are also sometimes used in process design, for example, the number of trays or the feed tray of a distillation column, and in process synthesis, to allow selection between flowsheet options, as described later. 

In the hierarchical decomposition method, or, process synthesis one only looks at the input-output structure of the process at the first level. In subsequent levels more detail is added, finally ending with the entire flowsheet. Design decisions are made by using heuristics and models. The amount of detail in the models depend on the level at which the models are applied. 

The integration activities have been mainly related to heat and mass exchangers. However, there are many more popular applications related to integration of flowsheet creation with simultaneous consideration of safety aspects. The design of control system is also beginning to be integrated with process synthesis and... 

The most characteristic task of CAPE and historically the most popular is process synthesis. The development of process synthesis is confirmed by the use of a very broad spectrum of methods for building a process flowsheet. 

A different approach to process synthesis is offered by means-ends method. It is based on the observation that the purpose of material processing is to apply various operations in such a sequence that the differences in properties between the raw materials and the products are systematically eliminated. As a result, the raw materials are transformed into the desired products. The means-ends method starts with an initial state and successively applies transformation operators to produce intermediate states with fewer differences until the goal state is reached. The hierarchy for the reduction of property differences is as follows identity, amount, concentration, temperature, pressure, and finally form. This property changing method has its limitations, as it ignores the influences and the impacts on other properties. Moreover, the search method takes an opportrmistic approach, which cannot guarantee the generation of a feasible flowsheet. The means-ends analysis approach has been used as a systematic process synthesis method for overall process flowsheet synthesis, as well as for the more detailed case of a separation system to resolve the concentration differences in nonideal systems that include azeotropes. 

The phenomena-driven method for the process synthesis analyzes not the processing units, the so-called building blocks, but the phenomena that occur in those blocks. This method is based on opportunistic task identification and integration. It was applied to separation process synthesis, based on thermodynamic phenomena. It explored the relationship between the physicochemical properties, separation techniques, and conditions of operation. The number of alternatives for each separation task is reduced by systematically analyzing these relationships. Then, possible flowsheets are produced with a list of alternatives for the separation tasks. 

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