Howe

A classic in its field, giving a splendid survey of solution physical chemistry from a chemist s point of view. While seriously out of date, it nevertheless provides physical insight into how molecules "behave" in mixtures. 

In Equation (24), a is the estimated standard deviation for each of the measured variables, i.e. pressure, temperature, and liquid-phase and vapor-phase compositions. The values assigned to a determine the relative weighting between the tieline data and the vapor-liquid equilibrium data this weighting determines how well the ternary system is represented. This weighting depends first, on the estimated accuracy of the ternary data, relative to that of the binary vapor-liquid data and second, on how remote the temperature of the binary data is from that of the ternary data and finally, on how important in a design the liquid-liquid equilibria are relative to the vapor-liquid equilibria. Typical values which we use in data reduction are Op = 1 mm Hg, = 0.05°C, = 0.001, and = 0.003... 

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. 

Hgure 1.1 Synthesis is the creation of a process to transform feed streams into product streams. Simulation predicts how it would behave if it was constructed. 

We might think that we can find all the structural options by inspection, at least all of the significant ones. The fact that even long-established processes are still being improved bears evidence to just how difficult this is. 

Before we can explore how reactor conditions can be chosen, we require some measure of reactor performance. For polymerization reactors, the most important measure of performance is the distribution of molecular weights in the polymer product. The distribution of molecular weights dictates the mechanical properties of the polymer. For other types of reactors, three important parameters are used to describe their performance ... 

We shall see later how temperature and pressure affect equilibrium conversion. For now, let us consider how concentration affects equilibrium conversion. 

However, the laboratory data seem to indicate that a constant concentration in the reactor to maintain 63 percent sulfuric acid would be beneficial. Careful temperature control is also important. These two factors would suggest that a continuous well-mixed reactor is appropriate. There is a conflict. How can a well-defined residence time be maintained and simultaneously a constant concentration of sulfuric acid be maintained ... 

Having made an initial specification for the reactor, attention is turned to separation of the reactor effluent. In addition, it might be necessary to carry out separation before the reactor to purify the feed. Whether before or after the reactor, the overall separation task normally must be broken down into a number of intermediate separation tasks. The first consideration is the choice of separator for the intermediate separation tasks. Later we shall consider how these separation tasks should be connected to the reactor. As with reactors, we shall concentrate on the choice of separator and not its detailed sizing. 

At this stage, how great the excess of chlorine should be for Fig. 4.7c to be feasible cannot be specified. Experimental work on the reaction chemistry would be required in order to establish this. However, the size of the excess does not change the basic structure. 

There is a tradeoff between energy and capital cost i.e., there is an economic degree of energy recovery. Chapter 7 explains how this tradeoff can be carried out using energy and capital cost targets. 

Details of how this design was developed in Fig. 6.9 are included in Chap. 16. For now, simply take note that the targets set by the composite curves are achievable in design, providing that the pinch is recognized, there is no transfer of heat ac ss it, and no inappropriate use of utilities occurs. However, insight into the pinch is needed to analyze some of the important decisions still to be made before network design is addressed. 

The question now is, given that there are often constraints to deal with, how do we evaluate the effect of these constraints on the system performance The problem table algorithm cannot be used directly if constraints are imposed. However, often the effect of constraints on... 

The process requires (Qup + Qlp) to satisfy its enthalpy imbalance above the pinch. If there were no losses from the boiler, then fuel W would be converted to shaftwork W at 100 percent efficiency. However, the boiler losses Qloss reduce this to below 100 percent conversion. In practice, in addition to the boiler losses, there also can be significant losses from the steam distribution system. Figure 6.336 shows how the grand composite curve can be used to size steam turbine cycles. ... 

As with heat pumping, the grand composite curve is used to assess how much heat from the process needs to be extracted into the refrigeration system and where, if appropriate, the process can... 

It was also noted in Sec. 4.4 that the hatch nature of a process can lead to less than full utilization of the equipment. Let us consider how utilization of equipment can be improved. 

It is easy to say that operation of a reactor at higher temperature might lead to a safer plant if the inventory can be reduced as a result, but how do we assess such changes quantitatively Lowering the inventory makes the plant safer, but raising the temperature makes it less safe. Which effect is more significant ... 

Let us take each of these in turn and consider how reactor waste can... 

Product removal during reaction. Separation of the product before completion of the reaction can force a higher conversion, as discussed in Chap. 2. Figure 2.4 showed how this is done in sulfuric acid processes. Sometimes the product (or one of the products) can be removed continuously from the reactor as the reaction progresses, e.g., by allowing it to vaporize from a liquid phase reactor. 

Perhaps the most extreme situation is encountered with purge streams. Purges are used to deal with both feed impurities and byproducts of reaction. In the preceding section we considered how the size of purges can be reduced in the case of feed impurities by purifying the feed. However, if it is impractical or uneconomical to reduce the purge by feed purification, or the purge is required to remove a byproduct of reaction, then the additional separation can be considered. 

Once the life-cycle inventory has been quantified, we can attempt to characterize and assess the eflfects of the environmental emissions in a life-cycle impact analysis. While the life-cycle inventory can, in principle at least, be readily assessed, the resulting impact is far from straightforward to assess. Environmental impacts are usually not directly comparable. For example, how do we compare the production of a kilogram of heavy metal sludge waste with the production of a ton of contaminated aqueous waste A comparision of two life cycles is required to pick the preferred life cycle. 

Flue gas recirculation. Recirculation of part of the flue gas as shown in Fig. 11.4 lowers the peak flame temperature, thus reducing formation. There is clearly a limit to how much flue gas can be recirculated without affecting the stability of the flame. 

In Chap. 12 it was discussed how the pinch takes on fundamental significance in improving heat integration. Let us now explore the consequences of placing reactors in different locations relative to the pinch. 

The reactor is highly exothermic, and the data have been extracted as the molten salt being a hot stream. The basis of this is that it is assumed that the molten salt circuit is an essential feature of the reactor design. Thereafter, there is freedom within reason to choose how the molten salt is cooled. 

Having decided that some essential matches need to be made around the pinch, the next question is how big should the matches be ... 

In Sec. 6.3 it was mentioned that some problems, known as threshold problems, do not have a pinch. They need either hot utility or cold utility but not both. How should the approach be modified to deal with the design of threshold problems ... 

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