Four-component distillation

What happens for a four-component distillation if there are LKs and HKs and LNKs and HNKs present Since there is an LNK, we would expect the LK curve to show maxima and since there is an HNK, we would expect maxima in the HK concentration profile. This is the case shown in Figure 5-6 for the distillation of a benzene-toluene-xylene-cumene mixture. Note that in this figure the secondary maxima near the feed stage are drastically repressed, but the primary maxima are readily evident. 

Roat et al. (1986) analyzed the choice of manipulated variables for a complex, four-component distillation column. The four components were propane, isobutane, n-butane, and isopentane. There were six possible manipulated variables, and ratios of these variables were also permissible. Table 18.2 shows the condition numbers for six control configurations that were evaluated for the column. Note that the last three strategies have approximately the same low CN. Subsequently, these... 

FIG. 13-99 Distillate composition profile for a batch distillation of a four-component mixture. 

A distillation column separating 150 kmol-h 1 of a four-component mixture is estimated to have a minimum reflux... 

Table 11.9 shows the composition of a four-component mixture to be separated by distillation. The K-values for each component at the bubble point temperature of this mixture are given. The liquid- and vapor-phase mixtures of these... 

Develop a mathematical model for the three-column train of distillation columns sketched below. The feed to the first column is 400 kg mol/h and contains four components (1, 2, 3, and 4), each at 25 mol %. Most of the lightest component is removed in the distillate of the first column, most of the next lightest in the second column distillate and the final column separates the final two heavy components. Assume constant relative volatilities throughout the system ai, CI2, and a3. The condensers are total condensers and the reboilers are partial. Trays, column bases, and reflux drums are perfectly mixed. Distillate flow rates are set by reflux drum... 

In (i), a column separates its feed into products without overlap in the components. An example is the separation of a stream consisting of four components A,B,C, and D via a distillation column, into one product consisting of only A and another product featuring B, C, and D. If all columns are sharp, then the separation sequence is termed as sharp sequence. 

We begin here with a very simple isomerization process that is similar to some of the simplified processes studied in Chap. 2. The process consists of a reactor, two distillation columns, and a liquid recycle stream. There are four components to consider. In subsequent chapters we look at processes with many more units, many more components, and multiple recycle streams. 

The components involved in this example are proprietary, but the results are general (Siirola, 1981). During the species allocation stage of the process synthesis procedure, it was determined that each species of a particular four-component stream was required to be relatively pure at four different destinations. The components are liquids at ambient temperatures, have about equal relative volatility differences, and form no azeotropes. Distillative separation methods were selected to resolve all composition property differences. The feed stream composition was dominated (about 70%) by the heaviest component (D). 

From exhaustive application of alternative simple distillation operators to all possible separations for this four-component system, or from application of ranked-list-based separations synthesis methods, it is easily shown that there are five different separation train structures for this four-component problem. Each can be generated systematically or since this pattern of solutions is already well known, each can be written down immediately or design heuristics can be used to generate one or more of the structures expected to be most suitable. After each structure is synthesized, its performance can be analyzed and evaluated with a flowsheet simulator. 

A depropanizer is a distillation operation encountered in almost all oil refineries. Our task here is to design a column to separate 1000 mol/s of a four component mixture containing 300 mol/s n-propane and 500 mol/s -butane so that there is no more than 3.5 mol/s of n-propane present in the bottom product and no more than 3.5 mol/s of n-butane is... 

Gorak (1991) conducted a number of distillation experiments with the four component system acetone(l)-methanol(2)-2-propanol(3)-water(4). The data for one of Gorak s experiments were used as the basis for Example 12.3.3. Here we ask you to estimate the mass transfer coefficients and the numbers of transfer units for the conditions existing in a different experiment ... 

Based on material balance considerations only, which of the following sets of specifications uniquely define the separation of a four-component stream into a distillate and a bottoms product ... 

It is proposed to design a distillation column for coarse separation of a four-component mixture between components 2 and 3. (The components are numbered by decreasing volatility). The combined mole fraction of components 1 and 2 in the distillate should be 0.65, and the ratio of the mole fraction of components 3 and 4 in the bottoms should be... 

Two columns are needed in either of these sequences the first to recover one of the components in the ternary and the second to separate the remaining two. For a four-component mixture, five altemative sequences could be used, each requiring three distillation columns ... 

Equilibrium data fur multicomponent systems may be measured, but beeause of the ranges oF temperature and pressure likely to be encountered in the distillation, some sort of model must still be available for extending such experimental data. The best approach is to start with the bienry data (or all possible binary pairs—a chore not too difficult for, say, ternary or quartemary systems, but one that is quite challenging for mixtures with more than four components. A six-component mixture. For example, would require evaluation of 15 binary pairs. 

In multicomponent distillation, there are three or more components in the products, and specifying the concentrations of one component in each does not fully characterize these products. However, if the concentrations of two out of three or three out of four components are specified for the distillate and bottoms products, it is generally impossible to meet these specifications exactly. An increase in reflux ratio or number of plates would increase the sharpness of the separation, and the desired concentration of one component in each product could be achieved, but it would be a coincidence if the other concentrations exactly matched those specified beforehand. The designer generally chooses two components whose concentrations or fractional recoveries in the distillate and bottoms products are a good index of the separation achieved. After these components are identified, they are called key components. Since the keys must differ in volatility, the more volatile, identified by subscript L, is called the light key, and the less volatile, identified by subscript H, is called the heavy key. 

To find an explanation Table 7.30 present comparative results regarding the sequences 9 and 12. It can be observed that the sequence 9 handles the most difficult separation D/E as the last, while sequence 12 treated it as the first. We would expect the first solution to be more advantageous. The results shows practically the same duty, 16.15 versus 16.76, although we would expect much more energetic consumption when the four components ABCD are taken as overhead distillate instead just D. Actually, the presence of the other components has as effect a considerable reduction of the minimum reflux, 3 Instead 12 Thus, minimum reflux calculation based only on key components would give false predictions. This result reinforces the statement made before that the minimum vapour flow in sequencing should be determined by accurate methods. 

Although we firmly believe that the CPM technique is an invaluable distillation synthesis tool, we would like to point out a shortcoming of the method, namely the ability to deal with higher order systems. The vast majority of problems discussed in this book deal with ternary systems. Ternary systems allow one to visualize the problem graphically and many conclusions can be drawn from this analysis. However, when more components are introduced, the problem becomes exceedingly difficult and insights are diminished. Although a four-component system can be... 

The conventional process for the hydrolysis of methyl acetate uses a fixed-bed reactor followed by a complex arrangement of several distillation/extraction columns. The conversion is limited by unfavorable equilibrium (equilibrium constant 0.14-0.2) and a large amount of unconverted methyl acetate needs to be separated and recycled. The reaction is carried out in a fixed-bed reactor and the product stream contains all four components. It needs four additional columns to separate the methanol and acetic acid streams and recycle unconverted methyl acetate along with methanol to the reactor. 

Equation (7.9) gives five possible sequences of three columns for a four-component feed. These sequences are shown in Figure 7.11. The first, where all final products but one are distillates, is often referred to as the direct sequence, and is widely used in industry because... 

Distillation is the process of vaporizing a liquid, condensing the vapor, and collecting the condensate in another container. This technique is useful for separating a liquid mixture when the components have different boiling points or when one of the components will not distill. It is one of the principal methods of purifying a liquid. Four basic distillation methods are available to the chemist simple distillation, vacuum distillation (distillation at reduced pressure), fractional distillation, and steam distillation. This technique chapter will discuss simple distillation. Vacuum distillation will be discussed in Technique 16. Fractional distillation will be discussed in Technique 15, and steam distillation will be discussed in Technique 18. 

Figure 1.7. The examples of four-component structures (bonds and distillation regions Reg°°). Separatrix surfaces are shaded. Arrows, direction of residium curves dotty lines, separatrixes.

Let us examine the case of four-component mixture (Fig. 3.4). Let us consider the split 1 2,3,4. The distillation trajectory goes from vertex 1 = Regz> at edge 1-2, to vertex 2 and further inside face 2-3-4 = Regs by c-line to the bottom point Xb... 

Product simplex for three-component mixtures is a triangle for four-component mixtures, it is a tetrahedron for five-component mixtures, it is a pentahedron etc. Inside one distillation subregion at m > n, product simplexes cross each other (i.e., one and the same feed point can simultaneously enter several product simplexes). 

Figure 4.5. A sequence of four-component reversible distillation. C-1, C-2, C-3, C-4, C-5, C-6, columns arrows, heat input and output.

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