Process synthesis distillation

Whereas process simulation includes quantitative analysis of a design given the stmcture of the design, process synthesis involves determining the stmcture that will meet the requirements of the design as well as finding the best stmcture for the requirements. For example, if components A, B, C, and D whose relative volatOities were in the order D, C, B, and A were to be separated by distillation for which each column produced a top and a bottom fraction, five schemes of three columns arise as possible stmctures (53) (Fig. 8). 

The task of process synthesis is to evaluate these schemes and select the best. The creative possibilities of design do not stop here. For example, these separations can be carried out by heat integration, by multiple-draw columns, or by use of processes other than distillation many other variations are possible. 

Inhibitors and retarders are used to stabilize monomers during storage or during processing (e.g, synthesis, distillation). They are often used to quench polymerization when a desired conversion has been achieved. They may also be used to regulate or control the kinetics of a polymerization process. 

Westerberg, A. W. and O. Wahnschafft. Synthesis of Distillation-Based Separation Processes. In Advances in Chemical Engineering, Vol. 23, Process Synthesis, J. L. Anderson, ed. Academic Press, New York (1996), pp. 63-170. 

Uses Solvent standardized hydrocarbon manufacturing paraffin products biodegradable detergents Jet fuel research rubber industry paper processing industry distillation chaser component of gasoline and similar fuels organic synthesis. 

Part 3 of this book presents a number of major developments and applications of MINLP approaches in the area of Process Synthesis. The illustrative examples for MINLP applications, presented next in this section, will focus on different aspects than those described in Part 3. In particular, we will consider the binary distillation design of a single column, the retrofit design of multiproduct batch plants, and the multicommodity facility location/allocation problem. 

Process synthesis and design of these non-conventional distillation processes proceed in two steps. The first step—process synthesis—is the selection of one or more candidate entrainers along with the computation of thermodynamic properties like residue curve maps that help assess many column features such as the adequate column configuration and the corresponding product cuts sequence. The second step—process design—involves the search for optimal values of batch distillation parameters such as the entrainer amount, reflux ratio, boiler duty and number of stages. The complexity of the second step depends on the solutions obtained at the previous level, because efficiency in azeotropic and extractive distillation is largely determined by the mixture thermodynamic properties that are closely linked to the nature of the entrainer. Hence, we have established a complete set of rules for the selection of feasible entrainers for the separation of non ideal mixtures... 

Sargent RWH. Functional approach to process synthesis and its application to distillation systems. Comp Chem Eng 1998 22 31. 

Early work in process synthesis focused on the solution of specific problems, such as the best sequence of distillation columns to perform separation of components in feedstreams into product streams. Another early problem was the synthesis of heat-exchanger networks. 

Azeotropic distillation deals with the separation of mixtures involving one or several azeotropes. This problem, which in the past was tackled by means of experience and intuition, is today approached by means of systematic methods based on a deeper thermodynamic analysis. Here, we review the indispensable aspects for process synthesis. More details can be found in recent specialized books [8, 14]. 

Westerberg, A.W., O.M. Wahnshaft, Synthesis of distillation based separation processes, in Adv. Chem. Eng., Vol. 23, Process Synthesis, Academic Press,... 

This case study deals with the design and simulation of a medium size plant of lOOkton cumene per year. The goal is performing the design by two essentially different methods. The first one is a classical approach, which handles the process synthesis and energy saving with distinct reaction and separation sections. In the second alternative a more innovative technology is applied based on reactive distillation. 

Fischer-Tropsch. The process most frequently considered for indirect coal liquefaction is the Fischer-Tropsch (F-T) synthesis, developed in 1925 by German chemists Franz Fischer and Hans Tropsch. In the F-T process, synthesis gas is reacted over a catalyst, typically iron or cobalt based, at 1-30 atm and 200-350°C to produce a wide range of mainly aliphatic hydrocarbons, including gas, LPG, gasoline, jet fuel, diesel oil, middle distillates, heavy oil, and waxes. Germany used this technology during World War II to produce nearly 15,000 barrels/day of military fuels. 

Much has been written about this reactive distillation scheme, including works by Bessling et al. [Chem. Eng. Tech., 21, 393 (1998)], Song et ah, [Ind. Eng. Chem. Res., 37,1917 (1998)], Huss et al. [Corn-put. Chem. Eng., 27,1855 (2003)], Siirola ("An Industrial Perspective on Process Synthesis, pp. 222-233 in Biegler and Doherty, eds.. Foundations of Computer-Aided Process Design, AIChE, New York, 1995), and Krishna (chap. 7 in Sundmacher and Kienle, eds.. Reactive Distillation, Wiley-VCH, 2003). 

The components involved in this example are proprietary, but the results are general (Siirola, 1981). During the species allocation stage of the process synthesis procedure, it was determined that each species of a particular four-component stream was required to be relatively pure at four different destinations. The components are liquids at ambient temperatures, have about equal relative volatility differences, and form no azeotropes. Distillative separation methods were selected to resolve all composition property differences. The feed stream composition was dominated (about 70%) by the heaviest component (D). 

Sargent, R. W. S. H. A Functional Approach to Process Synthesis and its Application to Distillation Systems, Tech. Rep. Centre for Process Systems Engineering, Imperial College, London, 1994. 

The first chapter, by Siirola, reviews the impact of process synthesis in industry and shows how process synthesis fits into the innovation process within industrial manufacturing and research. It also highlights a number of industrial successes leading to substantial energy savings and overall cost reductions. Most of these savings are in the areas of distillation sequences, and examples include heat-integrated separation sequences and separation of azeotropic systems. 

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. 

While process synthesis gives qualitative reference points, for industrial implementation we need quantitative results. Therefore, tools for rigorous process simulation including all effects are needed. In practice, the application of staged models with increasing complexity can be reconunended but tools with this complexity are not yet on the market, so that in many cases reactive distillation cannot be simulated so far that a process design is possible without experiments. 

---