Considered

The most important compounds considered to fall within the scope of this chapter are listed in the following subsections a-g, but reactivity data are as yet by no means available for all of them. 

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For typical conditions in the chemical industry, the effect of pressure on liquid-liquid equilibria is negligible and therefore in this monograph pressure is not considered as a variable in Equation (2). 

The calculation of single-stage equilibrium separations in multicomponent systems is implemented by a series of FORTRAN IV subroutines described in Chapter 7. These treat bubble and dewpoint calculations, isothermal and adiabatic equilibrium flash vaporizations, and liquid-liquid equilibrium "flash" separations. The treatment of multistage separation operations, which involves many additional considerations, is not considered in this monograph. 

The boundary between condensable and noncondensable components is somewhat arbitrary, especially because it depends on the range of temperatures where calculations are made. In this monograph we consider only common volatile gases (e.g. N2,... 

We can now consider the most convenient form for writing the liquid-phase fugacity of component i. First we consider a condensable component and write... 

In the calculation of vapor-liquid equilibria, it is necessary to calculate separately the fugacity of each component in each of the two phases. The liquid and vapor phases require different techniques in this chapter we consider calculations for the vapor phase. 

An adequate prediction of multicomponent vapor-liquid equilibria requires an accurate description of the phase equilibria for the binary systems. We have reduced a large body of binary data including a variety of systems containing, for example, alcohols, ethers, ketones, organic acids, water, and hydrocarbons with the UNIQUAC equation. Experience has shown it to do as well as any of the other common models. V7hen all types of mixtures are considered, including partially miscible systems, the... 

The critical temperature of methane is 191°K. At 25°C, therefore, the reduced temperature is 1.56. If the dividing line is taken at T/T = 1.8, methane should be considered condensable at temperatures below (about) 70°C and noncondensable at higher temperatures. However, in process design calculations, it is often inconvenient to switch from one method of normalization to the other. In this monograph, since we consider only equilibria at low or moderate pressures in the region 200-600°K, we elect to consider methane as a noncondensable component. 

In ternary systems, we distinguish between two common types. In type II, two binaries are partially miscible and the third binary is completely miscible in type I, only one binary is partially miscible. (A third type, where all three binaries are only partially miscible, is relatively rare and not considered here.)... 

We consider three types of m-component liquid-liquid systems. Each system requires slightly different data reduction and different quantities of ternary data. Figure 20 shows quarternary examples of each type. 

For many liquid mixtures. Equation (19) can be used to provide a crude estimate of excess enthalpy. A much better estimate is obtained if the UNIQUAC parameters are considered temperature-dependent. For example, suppose Equations (4-9) and (4-10) are modified to = + k /t... 

If the m Equations (7-1) are considered independent, additional restrictions must be imposed as... 

A much preferable form of Equation (7-10) was described by Rach-ford and Rice (1952), who considered Equations (7-6) and (7-7) in the form... 

The equation systems representing equilibrium separation calculations can be considered multidimensional, nonlinear objective functions... 

The vapor-liquid equilibrium separation calculations considered here are for two cases, isothermal and adiabatic, both at fixed pressure. 

Consider the process illustrated in Fig. 1.2. The process requires a reactor to transform the FEED into PRODUCT (Fig. 1.2a). Unfortunately, not all the FEED reacts. Also, part of the FEED reacts to form BYPRODUCT instead of the desired PRODUCT. A... 

On the other hand, this approach has a number of advantages. Many different design options can be considered at the same time. Also, the entire design procedure can be accommodated in a computer program capable of producing designs quickly and efficiently. 

However, there are many other factors to be considered in the choice of reaction path. Some are commercial, such as uncertainties regarding future prices of raw materials and b3q)roducts. Others are technical, such as safety and energy consumption. 

The lack of suitable catalysts is the most common reason preventing the exploitation of novel reaction paths. At the first stage of design, it is impossible to look ahead and see all the consequences of choosing one reaction path or another, but some things are clear even at this stage. Consider the following example. 

The market values and molecular weights of the materials involved are given in Table 2.1. Oxygen is considered to be free at this stage, coming from the... 

Single reactions. Most reaction systems involve multiple reactions. In practice, the secondary reactions can sometimes be neglected, leaving a single primary reaction to consider. Single reactions are of the type... 

Polymerization reactions. There are two broad types of polymerization reactions, those which involve a termination step and those which do not. An example that involves a termination step is free-radical polymerization of an alkene molecule. The polymerization requires a free radical from an initiator compound such as a peroxide. The initiator breaks down to form a free radical (e.g., CH3 or OH), which attaches to a molecule of alkene and in so doing generates another free radical. Consider the polymerization of vinyl chloride from a free-radical initiator R. An initiation step first occurs ... 

Consider the system of parallel reactions from Eq. (2.4) with the corresponding rate equations. " ... 

Consider now which of the idealized models is preferred for the five categories of reaction systems introduced in Sec. 2.2. 

Multiple reactions in parallel producing byproducts. Consider again the system of parallel reactions from Eqs. (2.16) and (2.17). A batch or plug-flow reactor maintains higher average concentrations of feed (Cfeed) than a continuous well-mixed reactor, in which the incoming feed is instantly diluted by the PRODUCT and... 

Mixed parallel and series reactions producing byproducts. Consider the mixed parallel and series reaction system from Eq. (2.10) with the corresponding kinetic equations ... 

In the preceding section, the choice of reactor type was made on the basis of which gave the most appropriate concentration profile as the reaction progressed in order to minimize volume for single reactions or maximize selectivity for multiple reactions for a given conversion. However, after making the decision to choose one type of reactor or another, there are still important concentration effects to be considered. 

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

The equilibrium conversion can be increased by employing one reactant in excess (or removing the water formed, or both). b. Inerts concentration. Sometimes, an inert material is present in the reactor. This might be a solvent in a liquid-phase reaction or an inert gas in a gas-phase reaction. Consider the reaction system... 

Product removal during reaction. Sometimes the equilibrium conversion can be increased by removing the product (or one of the products) continuously from the reactor as the reaction progresses, e.g., by allowing it to vaporize from a liquid-phase reactor. Another way is to carry out the reaction in stages with intermediate separation of the products. As an example of intermediate separation, consider the production of sulfuric acid as illustrated in Fig. 2.4. Sulfur dioxide is oxidized to sulfur trioxide ... 

Multiple reactions in parallel producing byproducts. Once the reactor type is chosen to maximize selectivity, we are in a position to alter selectivity further in parallel reaction systems. Consider the parallel reaction system from Eq. (2.20). To maximize selectivity for this system, we minimize the ratio given by Eq. (2.21) ... 

If the reaction involves more than one feed, it is not necessary to operate with the same low conversion on all the feeds. Using an excess of one of the feeds enables operation with a relatively high conversion of other feed material, and still inhibits series reactions. Consider again the series reaction system from Example 2.3 ... 

If the byproduct reaction is reversible and inerts are present, then changing the concentration of inerts if there is a change in the number of moles should be considered, as discussed above. Whether or not there is a change in the number of moles, recycling byproducts can suppress their formation if the bj iroduct-forming reaction is reversible. An example is in the production of ethylbenzene from benzene and ethylene ... 

Temperature control. Let us now consider temperature control of the reactor. In the first instance, adiabatic operation of the reactor should be considered, since this leads to the simplest and cheapest reactor design. If adiabatic operation produces an unacceptable rise in temperature for exothermic reactions or an unacceptable fall in temperature for endothermic reactions, this can be dealt with in a number of ways ... 

Having considered reactor temperature and pressure, we are now in a position to judge whether the reactor phase will be gas, liquid, or multiphase. Given a free choice between gas- and liquid-phase reactions, operation in the liquid phase is usually preferred. Consider the single reaction system from Eq. (2.19) ... 

Further consideration of the reaction system reveals that the ammonia feed takes part only in the primary reaction and in neither of the secondary reactions. Consider the rate equation for the primary reaction ... 

Another possibility to improve selectivity is to reduce the concentration of monoethanolamine in the reactor by using more than one reactor with intermediate separation of the monoethanolamine. Considering the boiling points of the components given in Table 2.3, then separation by distillation is apparently possible. Unfortunately, repeated distillation operations are likely to be very expensive. Also, there is a market to sell both di- and triethanolamine, even though their value is lower than that of monoethanolamine. Thus, in this case, repeated reaction and separation are probably not justified, and the choice is a single plug-flow reactor. 

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