Binary distillation profiles

Equimolar Counterdiffusion in Binary Cases. If the flux of A is balanced by an equal flux of B in the opposite direction (frequently encountered in binary distillation columns), there is no net flow through the film and like is directly given by Fick s law. In an ideal gas, where the diffusivity can be shown to be independent of concentration, integration of Fick s law leads to a linear concentration profile through the film and to the following expression where (P/RT)y is substituted for... 

Continuous binary distillation is illustrated by the simulation example CON-STILL. Here the dynamic simulation example is seen as a valuable adjunct to steady state design calculations, since with MADONNA the most important column design parameters (total column plate number, feed plate location and reflux ratio) come under the direct control of the simulator as facilitated by the use of sliders. Provided that sufficient simulation time is allowed for the column conditions to reach steady state, the resultant steady state profiles of composition versus plate number are easily obtained. In this way, the effects of changes in reflux ratio or choice of the optimum plate location on the resultant steady state profiles become almost immediately apparent. 

Figure 6.4. Optimal Reflux Ratio Profiles for Binary Distillation. [Mujtaba and Macchietto, 1993]e...

Alternative 2 has also three columns, as depicted in Fig. 9.18. From oo/ooanalysis we expect the presence of a small amount of acetone in chloroform, because of the constraint set by the separatrix. Fortunately, chloroform purity is better at finite reflux. In fact, the distillation border can be crossed in the first column, as demonstrated by the concentration profile in Fig. 9.21. The bottom product is free of acetone, high purity chloroform being obtained by a simple binary distillation. 

Sensitivity. Figure 18.1 shows typical temperature and composition profiles for binary distillation. Temperature is insensitive to composition below point A or above point B. Therefore, all trays above tray 8 and below tray 36 are unsuitable for temperature control from a sensitivity standpoint. Of the trays between trays 8 and 36, some are more sensitive to composition than others. 

In Fig. 5.2-29 the internal concentration profiles of preferred separation, low-boiler separation, and high-boiler separation are qualitatively shown. Limiting for the energy demand is the pinch (i.e., point of intersection of operating and equihbrium lines). At preferred separations a double pinch exists whose concentration is equal to the feed concentration (like in binary distillation). 

What do the flow, tenperature, and conposition profiles look like Our intuition would tell us that these profiles will be similar to the ones for binary distillation. As we will see, this is true for the total flow rates and tenperature, but not for the conposition profiles. 

The differences in the composition profiles for multicomponent and binary distillation for relatively ideal VLE with no azeotropes can be summarized as follows ... 

Especially the second column is practically a binary distillation column. As the exergy loss profiles in Figures 4.8 and 4.9 show that both the columns operate with rather large exergy losses. ... 

Minimum Boiling Azeotropes. AH extractive distillations of binary minimum boiling azeotropic mixtures are represented by the residue curve map and column sequence shown in Figure 6b. Typical tray-by-tray composition profiles are shown in Figure 7. 

The corresponding wave patterns of the transformed concentration variable X for a reactive distillation column are shown in Fig. 5.8. Here, a single feed with pure reactant A is introduced in the middle of the column. As in the nonreactive binary case, the composition profiles consist of a single front in each column section,... 

For single separation duty, Mujtaba and Macchietto (1993) proposed a method, based on extensions of the techniques of Mujtaba (1989) and Mujtaba and Macchietto (1988, 1989, 1991, 1992), to determine the optimal multiperiod operation policies for binary and general multicomponent batch distillation of a given feed mixture, with several main-cuts and off-cuts. A two level dynamic optimisation formulation was presented so as to maximise a general profit function for the multiperiod operation, subject to general constraints. The solution of this problem determines the optimal amount of each main and off cut, the optimal duration of each distillation task and the optimal reflux ratio profiles during each production period. The outer level optimisation maximises the profit function by... 

Two binary mixtures are being processed in a batch distillation column with 15 plates and vapour boilup rate of 250 moles/hr following the operation sequence given in Figure 7.7. The amount of distillate, batch time and profit of the operation are shown in Table 7.6 (base case). The optimal reflux ratio profiles are shown in Figure 7.8. It is desired to simultaneously optimise the design (number of plates) and operation (reflux ratio and batch time) for this multiple separation duties. The column operates with the same boil up rate as the base case and the sales values of different products are given in Table 7.6. 

The one level optimal control formulation proposed by Mujtaba (1989) is found to be much faster than the classical two-level formulation to obtain optimal recycle policies in binary batch distillation. In addition, the one level formulation is also much more robust. The reason for the robustness is that for every function evaluation of the outer loop problem, the two-level method requires to reinitialise the reflux ratio profile for each new value of (Rl, xRI). This was done automatically in Mujtaba (1989) using the reflux ratio profile calculated at the previous function evaluation in the outer loop so that the inner loop problems (specially problem P2) could be solved in a small number of iterations. However, experience has shown that even after this re-initialisation of the reflux profile sometimes no solutions (even sub-optimal) were obtained. This is due to failure to converge within a maximum limit of function evaluations for the inner loop problems. On the other hand the one level formulation does not require such re-initialisation. The reflux profile was set only at the beginning and a solution was always found within the prescribed number of function evaluations. 

Figure 10.1 shows typical distillate composition profiles for close boiling mixtures (binary) and implications of using CBD column for such mixtures. 

Vogelpohl (193) and Medina et al. (203) applied the diffusional interaction method for predicting ternary distillation composition profiles using binary data. They achieved this by eliminating the first two steps and assuming that all the mass transfer resistance is in the vapor. This procedure was shown to give excellent agreement with experimental data for dissimilar components. Biddulph and Kalbassi (194), however, report some discrepancies between prediction and experiment due to this assumption. 

Many industrial columns use temperatures for composition control because direct composition analyzers can be expensive and unreliable. Although temperature is uniquely related to composition only in a binary system (at known pressure), it is still often possible to use the temperatures on various trays up and down the column to maintain approximate composition control, even in multicomponent systems. Probably 75 percent of all distillation columns use temperature control of some tray to hold the composition profile in the column. This prevents the light-key (LK) impurities from dropping out the bottom and the heavy-key (HK) impurities from going overhead. 

Let s consider the separation of a binary minimum azeotrope AB with a medium boiling entrainer C (Fig. 9.12). Note that the AB azeotrope and the component B are nodes, while both A and C are saddles. The separation regions for the first split are delimited by direct and indirect sequences, respectively. If the boiling points of A and AB azeotrope are not too close, A can be obtained as distillate, even if it is a saddle Rooks et al. (1998) has given recently a consistent explanation the split is feasible when the concentration profiles of both rectification and stripping zones points to the same common saddle, in this case the component C. 

The concentration profile of the second column shows that this works completely on the right side of the distillation border, even if the feed seems to be on the left side. The concentration profile follows closely the residue curves. The third column is a binary separation, without much interest. Hence, the simulation indicates that the separation sequence designed in a RCM by oo/oo analysis is feasible and leads to high purity products. Sizing and optimisation of columns can be done by standard procedures. 

The ethanol column (C-1) has practically only stripping zone. Fig. 9.31-left shows composition profile both for liquid and vapour phase. The examination of the composition profiles highlights the role of the entrainer. In the zone close to the top the benzene extracts the ethanol in the liquid phase, and as a result increases the volatility of water, so that on lower stages the water is completely removed. In the lower part practically only the binary ethanol/benzene remains. The distillation trajectory starts from the ternary azeotrope, goes along the ethanol/ benzene saddle and terminates in the ethanol vertex. Because the boiling point of the azeotrope ethanol-water is close to the pure ethanol, the profile could easily jump to the ethanol/water azeotrope. Consequently, the design and operation of the column (C-1) is very sensitive. 

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