Material-Balance Envelope

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. 

It is clear that the reaction and separation systems are interrelated and, in principle, their design should be examined simultaneously. In practice, this approach turns to be very difficult It is possible to apply, however, a simplified approach. Indeed, from the systemic viewpoint the functions and the connections of units have priority on their detailed design. Therefore, a functional analysis based on the RSR structure only is valuable. This analysis should primarily show that the flowsheet architecture is feasible and appropriate for stable operation. On this basis design targets for the subsystems may be assigned. 

In the RSR approach the chemical reactor is the key unit, designed and simulated in terms of productivity, stability and flexibility. From the systemic viewpoint the key issue is the quality and dynamics of flows entering the reactor and less how they have been produced. Obviously, these flows include fresh reactants and recycle streams. The dynamics of flows must respect the overall material balance at steady state, as well as the process constraints. For this reason, the chemical-reactor analysis should be based on a kinetic model. 

A sensible assumption is that the separation units are well controlled. They may be viewed as black-boxes supplying constant purity recycle flows. However, the flows rates of recycles may exhibit large variations, some with periodical character, 

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The constant-molar-overflow assumption represents several prior assumptions. The most important one is equal molar heats of vaporization for the two components. The other assumptions are adiabatic operation (no heat leaks) and no heat of mixing or sensible heat effects. These assumptions are most closely approximated for close-boiling isomers. The result of these assumptions on the calculation method can be illustrated with Fig. 13-28, vdiich shows two material-balance envelopes cutting through the top section (above the top feed stream or sidestream) of the column. If L + i is assumed to be identical to L 1 in rate, then 9 and the component material balance... 

FIG. 13-28 Two material-balance envelopes in the top section of a distillation column,... 

FIG. 13-29 Material-balance envelope which contains two external streams D and S, where S represents a sidestream product withdrawn above the feed plate,... 

FIG. 13-30 Material-balance envelope around the bottom end of the column. The partial rehoiler is equilibrium stage 1,... 

Figure 2.14 illustrates the overall approach by pinch-point analysis. The first step is extraction of stream data from the process synthesis. This step involves the simulation of the material-balance envelope by using appropriate models for the accurate computation of enthalpy. On this basis composite curves are obtained by plotting the temperature T against the cumulative enthalpy H of streams selected for analysis, hot and cold, respectively. Two aspects should be taken into account ... 

Figure 5.17 Preliminary simulation of the material-balance envelope.

Material balances are easier, more meaningful, and more accurate when they are done for individual production units, operations, or production processes. For this reason, it is important to define the material balance envelope or boundary limit accurately, in addition to the tie compound. Ideally, a more accurate balance should be established for the unit operation that is more critical from the waste-generation and reduction point of view, and a less accurate balance could be established for other processes. 

Choose the material balance envelope in such a way that the number of streams entering and leaving the process is the smallest possible. 

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 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. 

Pinch Point Analysis starts with the input of data. The first step is the extraction of stream data from a flowsheet simulation, which describes typically the material balance envelope (Reactors and Separators). Proper selection and treatment of streams by segmentation is a key factor for efficient heat integration. The next step is the selection of utilities. Additional information regards the partial heat transfer coefficients of the different streams and segments of streams, and of utilities, as well as the cost of utilities and the cost laws for heat exchangers. 

Generate flowsheet alternatives by applying Process Synthesis methods. The basic flowsheet structure is given by the Reactor and Separation systems. These form the material balance envelope that is the basis for further development. Use flowsheeting to get insights into different conceptual issues, as well to evaluate the performance of different alternatives. 

Fig. 5.2-4 Flow sheet of a rectification column with several material balance envelopes...

The conceptual design of this column can be explained by Figure 3.49. With the inner material balance envelop as shown in Figure 3.48, the material balance lines of adding FI and OR to produce the split of VI at column top and pure IPA at column bottoms are illustrated in Figure 3.49. In the same figure, the outer material balance lines of adding FI and pure CyH to produce the split AO (aqueous outlet) and pure IPA can also be illustrated. 

The recommended operating specification to use for the first time is the bottom flowrate. The reasonable bottom flowrate, and also the makeup flowrate, can roughly be estimated by utilizing the total material and IPA component balances of the outer material balance envelop. Additional ways to estimate the stream information will be discussed in Chapter 8. 

Assuming that the composition of the top vapor stream reaches the ternary azeotrope (with the minimum temperature of the whole system), the compositions of both the AO and the OR can be estimated. With the known fresh-feed flowrate and the lever rule, the flowrate of the OR and the makeup flowrate can be estimated by the inner and outer material balance envelopes, respectively. Second, from the tie-line information and the lever rule, the flowrate of the AO stream can also be estimated. Thirdly, by using the outer material balance envelop and the lever mle, the bottom flowrate can further be estimated. Finally by using the inner material balance envelop and the lever mle, the top vapor flowrate can be estimated. 

Another factor that favors isobutyl acetate to be a suitable entrainer is the aqueous phase composition. The aqueous phase should contain as little entrainer as possible. The aqueous phase stream will be drawn out of the system, so any entrainer loss must be compensated for by the entrainer makeup stream as shown in Figure 13.19. This will contribute to an entrainer cost in the TAC calculations. The makeup flowrate can actually be estimated using the outer material balance envelope shown in Figure 13.19. 

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