Flowsheets

Such sketches develop into flow sheets, which are more elaborate diagrammatic representations of the equipment, the sequence of operations, and the expected performance of a proposed plant or the actual performance of an already operating one. For clarity and to meet the needs of the various persons engaged in design, cost estimating, purchasing, fabrication, operation, maintenance, and management, several different kinds of flowsheets are necessary. Four of the main kinds will be described and illustrated. 

Such sketches develop into flow sheets, which are more 

Mi plant design is made up of words, numbers, and MW pictures. An engineer thinks naturally in terms of the sketches and drawings which are his pictures. m I Thus, to solve a material balance problem, he will start with a block to represent the equipment and then will show entering and leaving streams with their amounts and properties. Or ask him to describe a process and he will begin to sketch the equipment, show how iris interconnected, and what the flows and operating conditions are. 

Beaedek (Ed.), Steady Stale Flowsheeting of Chemical Plants, Elsevier, New York, 1980. 

Bodman, The Industrial Practice of Chemical Process Engineering, MIT Press, Cambridge, MA, 1%8. 

Braoan, Process Engineers Pocket Book, Gulf, Houston, 1976, 1983, 2 vols. 

Burklin, The Process Plant Designers Pocket Handbook of Codes and Standards, Gulf, Houston, 1979 also. Design codes standards and recommended practices, Encycl. Chem. Process. Des. 14, 416-431, Dekker, New York, 1982. 

Cremer and Watkins, Chemical Engineering Practice, Butterworths, London, 1956-1965, 12 vols. 

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Once the flowsheet structure has been defined, a simulation of the process can be carried out. A simulation is a mathematical model of the process which attempts to predict how the process would behave if it was constructed (see Fig. 1.1b). Having created a model of the process, we assume the flow rates, compositions, temperatures, and pressures of the feeds. The simulation model then predicts the flow rates, compositions, temperatures, and pressures of the products. It also allows the individual items of equipment in the process to be sized and predicts how much raw material is being used, how much energy is being consumed, etc. The performance of the design can then be evaluated. 

Figure 1.46 shows a flowsheet without any heat integration for the different reactor and separation system. As before, this is probably too inefficient in the use of energy, and heat integration schemes can be explored. Figure 1.5 shows two of the many possible flowsheets. 

Grossmann, I. E., Mixed Integer Programming Approach for the Synthesis of Integrated Process Flowsheets, Camp. Chem. Eng., 9 463, 1985. 

Kocis, G. R., and Grossmann, I. E., A Modeling/Decomposition Strategy for MINLP Optimization of Process Flowsheets, paper no. 76a, AIChE Meeting, Washingtonj D.C., 1988. 

The secondary reactions are parallel with respect to ethylene oxide but series with respect to monoethanolamine. Monoethanolamine is more valuable than both the di- and triethanolamine. As a first step in the flowsheet synthesis, make an initial choice of reactor which will maximize the production of monoethanolamine relative to di- and triethanolamine. 

An initial guess for the reactor conversion is very difficult to make. A high conversion increases the concentration of monoethanolamine and increases the rates of the secondary reactions. As we shall see later, a low conversion has the effect of decreasing the reactor capital cost but increasing the capital cost of many other items of equipment in the flowsheet. Thus an initial value of 50 percent conversion is probably as good as a guess as can be made at this stage. 

The decisions made in the reactor design are often the most important in the whole flowsheet. The design of the reactor usually interacts strongly with the rest of the flowsheet. Hence a return to the decisions made for the reactor must be made when the process design has progressed further and we have fully understood the consequences of those decisions. For the detailed sizing of the reactor, the reader is referred to the many excellent texts on reactor design. 

However, factors such as this should not he allowed to dictate design options at the early stages of flowsheet design because preheating the cold feed hy heat integration with the rest of the process might be possible. 

The introduction of an extraneous component as a heat carrier aflfects the recycle structure of the flowsheet. Figure 4.6a presents an example of the recycle structure for just such a process. 

Where possible, introducing extraneous materials into the process should be avoided, and a material already present in the process should be used. Figure 4.6h illustrates use of the product as the heat carrier. This simplifies the recycle structure of the flowsheet and removes the need for one of the separators (see Fig. 4.66). Use of the product as a heat carrier is obviously restricted to situations where the product does not undergo secondary reactions to unwanted byproducts. Note that the unconverted feed which is recycled also acts as a heat carrier itself. Thus, rather than relying on recycled product to limit the temperature rise (or fall), simply opt for a low conversion, a high recycle of feed, and a resulting small temperature change. 

Although the flowsheet shown in Fig. 4.7a is very attractive, it is not practical. This would require careful control of the stoichiometric ratio of decane to chlorine, taking into account both the requirements of the primary and byproduct reactions. Even if it was possible to balance out the... 

The last of the four flowsheet options generated above, which features excess decane in the reactor, is therefore preferred (see Fig. 4.7[Pg.104]

Ideally, the K value for the light key component in the phase separation should be greater than 10, and at the same time, the K value for the heavy key should be less than 0.1. Having such circumstances leads to a good separation in a single stage. However, use of phase separators might still be effective in the flowsheet if the K values for the key components are not so extreme. Under such circumstances a more crude separation must be accepted. 

Absorption. If possible, a component which already exists in the flowsheet should be used as a solvent. Introducing an extraneous component into the flowsheet introduces additional complexity and the possibility of increased environmental and safety problems later in the design. 

Figure 4.8 A flowsheet for the production of benzene uses a purge to remove the methane, which enters as a feed impurity and also is formed as a byproduct.

Solution Having synthesized the continuous flowsheet shown in Fig. 4.136, let us now convert this into batch operation. 

Clearly, the time chart shown in Fig. 4.14 indicates that individual items of equipment have a poor utilization i.e., they are in use for only a small fraction of the batch cycle time. To improve the equipment utilization, overlap batches as shown in the time-event chart in Fig. 4.15. Here, more than one batch, at difierent processing stages, resides in the process at any given time. Clearly, it is not possible to recycle directly from the separators to the reactor, since the reactor is fed at a time different from that at which the separation is carried out. A storage tank is needed to hold the recycle material. This material is then used to provide part of the feed for the next batch. The final flowsheet for batch operation is shown in Fig. 4.16. Equipment utilization might be improved further by various methods which are considered in Chap. 8 when economic tradeoffs are discussed. 

Figure 4.16 Final flowsheet for the production of butadiene sulfone in a batch process.

The use of excess reactants, diluents, or heat carriers in the reactor design has a significant effect on the flowsheet recycle structure. Sometimes the recycling of unwanted byproduct to the reactor can inhibit its formation at the source. If this can be achieved, it improves the overall use of raw materials and eliminates effluent disposal problems. Of course, the recycling does in itself reuse some of the other costs. The general tradeoffs are discussed in Chap. 8. 

Consider the simple flowsheet shown in Fig. 6.2. Flow rates, temperatures, and heat duties for each stream are shown. Two of the streams in Fig. 6.2 are sources of heat (hot streams) and two are sinks for heat (cold streams). Assuming that heat capacities are constant, the hot and cold streams can be extracted as given in Table 6.2. Note that the heat capacities CP are total heat capacities and... 

Figure 6.2 A simple flowsheet with two hot streams and two cold streams.

TABLE 6.2 Heat Exchange Stream Data for the Flowsheet in Fig. 6.2... 

Example 6.1 The flowsheet for a low-temperature distillation process is shown in Fig. 6.19. Calculate the minimum hot and cold utility requirements and the location of the pinch assuming AT, m = 5°C. 

Solution First, extract the stream data from the flowsheet. This is given in Table 6.4. 

Consider again the simple process shown in Fig. 4.4d in which FEED is reacted to PRODUCT. If the process usbs a distillation column as separator, there is a tradeofi" between refiux ratio and the number of plates if the feed and products to the distillation column are fixed, as discussed in Chap. 3 (Fig. 3.7). This, of course, assumes that the reboiler and/or condenser are not heat integrated. If the reboiler and/or condenser are heat integrated, the, tradeoff is quite different from that shown in Fig. 3.7, but we shall return to this point later in Chap. 14. The important thing to note for now is that if the reboiler and condenser are using external utilities, then the tradeoff between reflux ratio and the number of plates does not affect other operations in the flowsheet. It is a local tradeoff. 

Plots of economic potential versus reactor conversion allow the optimal reactor conversion for a given flowsheet to be identified (Fig. 8.2). Although this approach allows the location of the optimum to be found, it does not give any indication of why the optimum occurs where it does. 

Also, if there are two separators, the order of separation can change. The tradeoffs for these two alternative flowsheets will be different. The choice between different separation sequences can be made using the methods described in Chap. 5. However, we should be on guard to the fact that as the reactor conversion changes, the most appropriate sequence also can change. In other words, different separation system structures become appropriate for different reactor conversions. 

In Sec. 4.4 the possibility of using batch rather than continuous operations in the flowsheet was discussed. At that time, our only interest was the recycle structure of the flowsheet. There the approach was first to synthesize a flowsheet based on continuous... 

The best way to deal with a hazard in a flowsheet is to remove it completely. The provision of safety systems to control the hazard is much less satisfactory. One of the principal approaches to making a process inherently safe is to limit the inventory of hazardous material, called intensification of hazardous material. The inventories we wish to avoid most of all are flashing flammable liquids or flashing toxic liquids. 

Once the process route has been chosen, it may be possible to synthesize flowsheets that do not require large inventories of materials in the process. The design of the reaction and separation system is particularly important in this respect, but heat transfer, storage, and pressure relief systems are also important. 

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