Reactor-Separation Integration

This expression does not look very simple or instructive. However, let us check the expression if mass transfer is very fast compared to reaction, (A km/y ) k. In this case the expression becomes 

These equations give expressions for the exit concentrations of reactant in a two-phase reactor for a first-order irreversible reaction in terms of the two residence times r and r, the two feed concentrations C and Cfo, and the reaction rate coefficient, the mass transfer coefficient /tm and the area between the two phases 

We have in fact had to make many approximations to obtain even these complicated expressions, and it is evident that numerical solutions to most multiphase chemical reactor problems can only be obtained after considerable computations and approximations. 

We typically think of the chemical reactor as the unit in which reactions are run and that the reactor is preceded and followed by separation units such as distillation, extraction, centrifugation, etc., to remove reactants fi om products and to prepare products with required purities. In fact, a crucial feature of many chemical reactors is that they involve the integration of chemical reaction and separation in a single chemical reactor. 

Integrated reaction-separation units can achieve several desirable goals  

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Fig. 13.1 Two basic types of integrated reactor—separator systems. In A the membrane is used as an add-on to the reactor, whereas in B the membrane is fully integrated.

In addition to recycling reactants back into the reactor, there are several other tricks to keep reactants in the reactor and thus attain higher conversions and yields. Aunong these are formation of vapor-phase products from liquid or solid feeds and the use of membranes that pass products but retain reactants. We will discuss the integration of reactor-separation systems in Chapter 12. 

If product inhibition occurs, either a stirred-tank reactor in batch or a plug-flow reactor should be used. In these two reactors, the product concentration increases with time. Alternatively a reactor with integrated product separation (membrane, solvent, etc.) is preferable. 

Novel processing methods, such as integration of reaction and one or more unit operations in so-called multifunctional reactors and integration of two or more separation techniques in hybrid separations Use of alternative forms and sources of energy for chemical processing Novel methods of process/plant development and operation... 

The above constraints can be fulfilled only by building up an appropriate structure of recycles. At the industrial level this implies the integration of design and control of the units implied in the plantwide material balance. Hence, we may speak about the reactor/separation/recyde (RSR) as the major architectural structure defining a chemical process. 

We emphasize that traditional procedures tackle plantwide control and heat integration toward the end of the design. The newly introduced reactor/separation/recycle level (Chapters 2 and 4) allows an early solution to these problems, with the result of avoiding unnecessary loops in the design process. Rigorous design and closed-loop dynamic simulation prove the effectiveness of the approach (Section 9.6). 

Because the reactor is periectly mixed, C in this equation is the concentration of the traeer either in the effluent or within the reactor. Separating the variables and integrating with C = Q at / = 0 yields... 

The case study of vinyl acetate synthesis emphasises the benefits of an integrated process design and plantwide control strategy based on the analysis of the Reactor / Separation / Recycles structure. The core is the chemical reactor, whose behaviour in recycle depends on the kinetics and selectivity of the catalyst, as well as on safety and technological constraints. Moreover, the recycle policy depends on the reaction mechanism of the catalytic reaction. 

Figure 3 Selected schemes for the production of A (F - feed, S - solvent, SR - solvent removed), a) Reactor-separator-recycle system w/ four zones and solvent removal, b) Partially integrated process w/ three zones and side reactors, c) Fully integrated process w/ four zones, distributed reaction and solvent removal, d) Fully integrated scheme w/ three zones and distributed reaction.

Concerning the level of integration, classical flowsheet-integrated processes as reactor-separator-recycle systems (Fig. 3a) and the use of side reactors ( Hashimoto process . 

Jprgensen V (2007) Numerical investigation of integrated reactor/separator designs for precombustion with carbondioxide capture. MSc Thesis, the Norwegian University of Science and Technology, Trondheim Jones DRM, Davidson JF (1965) The Plow of Particles from a Fluidized Bed through an Orifice. Rheol Acta 4(3) 180-192... 

This method of writing the mass balance, like Eq. (3-10) for batch operation of a tank reactor, separates the extensive variables F and F and relates them to an integral dependent on the intensive conditions in the reaction mixture. It is worthwhile to note the similarity between Eq. (3-13) and the more familiar design equation for heat-transfer equipment based on an energy balance. This may be written... 

Hence, at the level Reactor-Separators-Recycles the material balance can be brought in a narrow optimal region. On this basis can be started the process integration steps regarding the optimal management of energy, mass separation agents, process water, waste minimisation, etc. 

Figure 9.17 A Pd-based membrane separator integrated with a Pd-based membrane reactor.

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