Process synthesis sequencing separation processes

Conceptual process synthesis using heuristics, process simulation, heat integration and economic analysis are applied in sequence for feasible techno-economics of the integrated system. A process flowsheet can be developed logically starting from the core reaction-separation-heat recovery processes to fully developed heat integrated flowsheet. 

The economics of the various methods that are employed to sequence multicomponent columns have been studied. For example, the separation of three-, four-, and five-component mixtures has been considered (44) where the heuristics (rules of thumb) developed by earlier investigators were examined and an economic analysis of various methods of sequencing the columns was made. The study of sequencing of multicomponent columns is part of a broader field, process synthesis, which attempts to formalize and develop strategies for the optimum overall process (45) (see Separation systems synthesis). 

In most chemical processes reactors are sequenced by systems that separate the desired products out of their outlet reactor streams and recycle the unconverted reactants back to the reactor system. Despite the fact that process synthesis has been developed into a very active research area, very few systematic procedures have been proposed for the synthesis of reactor/separator/recycle systems. The proposed evolutionary approaches are always based upon a large number of heuristic rules to eliminate the wide variety of choices. Many of these heuristics are actually extensions of results obtained by separately studying the synthesis problem of reactor networks or separator systems, and therefore the potential trade-offs resulting from the coupling of the reactors with the separators have not been investigated. 

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... 

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. 

The first step is the split of the initial mixture in essentially monophase submixtures, as gas, liquid and solid. This operation, called the first separation step, can employ simple flash or a sequence of flashes, adsorption/desorption and reboiled stripping, or the combination of the above techniques. Next, the process synthesis activity can be further handled by specialized managers, namely gas split manager (GSM), liquid split manager (LSM) and solid split manager (SSM). 

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. 

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. 

In order to avoid a pronounced shift of the phase behaviour with the progress of the reaction one reactant can be fed to the reactor in a semi-batch mode. With this concept shift in phase behaviour may be compensated and the optimal state of the system is maintained during the whole reaction time. After phase separation at the most suitable temperature conventional product isolation follows. This sequence of process steps is also possible in a continuously operating process, as illustrated in Fig. 5.17 for the synthesis of 1-phenoxyoctane in a microemulsion stabilised by Triton X-100 [28],... 

Process synthesis heuristics fall into several categories. Many of these were proposed origiually for multicomponent distillation but have since been applied 10 other types of separation. Some heuristics have bean verified through calculation and experimentation while others are described best as common senes, in any case, they serve as effective guidelines in the selection and sequencing of separation processes. 

There is no doubt that, in many instances, relatively smtple approaches are still anfficient to accomplish particular control objectives. However, for example, in the case of separation sequences that involve complex flow arrangements and thermal onapling, more advanced process control strategies will be necessary. As process synthesis methodologies improve, control strategy evaluation and process design and... 

In die perspective of process synthesis, process control should be viewed am as a separate element in process desiga and optimization bas rather as a component of a coordinated approach. Therefore, the design and sequencing of separation processes me si consider the relationship that process control will have to the final process structure. This can be done only through process modeling and simulation,... 

Very little process synthesis literature has dealt successfully with the problem of selection between alternative methods of separation. Indeed, most process synthesis literature has dealt with the selection of the flowsheet sequences Tor a single sepamtion method with only simple, sharp separations between components of ndjacent selectivity, without recycle. Furthermore, most of this literature has used only distillation in illustrative epplications. In practice, much use is made of recycle, nonsharp eplits. and complex columns (in distillation) and, of coorse. distillation is not always the separation merited of choice. 

While process sym basis research has not yet produced the so In lion to the problem of generation of die best flowsheet, a number of heuristic rules have been stated by various process synthesis researchers which can be of considerable aid in guiding the selection of the near optimum or base case and Tor systematically choosing those alternate sequences to be investigated. Some of the heuristic roles developed will be compering and in some cases even contradictory, but nevertheless they do serve to reduce the enormous amount of work (hat would be involvnd in evaluating all possible separation sequences. 

---