Constant reflux ratio time requirement

Hence the reflux ratio, the amount of distillate, and the bottoms composition can be related to the fractional distillation time. This is done in Example 13.4, which studies batch distillations at constant overhead composition and also finds the suitable constant reflux ratio that enables meeting required overhead and residue specifications. Although the variable reflux operation is slightly more difficult to control, this example shows that it is substantially more efficient thermally—the average reflux ratio is much lower—than the other type of operation. 

The time t required for batch distillation at constant reflux ratio and negligible holdup in the column and condenser can be computed by a total material balance based on constant boil-up rate V to give the following equation (Seader and Henley, 2006) ... 

A batch distillation column with three theoretical stages (the first stage is the still pot) is charged with 100 kmol of a 20 mol% n-hexane in n-octane mixture. At a constant reflux ratio R - 1.0, how many moles of the charge must be distilled if an average product composition of 70 mol% n-hexane is required If the boilup ratio is 10 kmol/h, calculate the distillation time. The equilibrium distribution curve at column pressure is given in Figure 6.27. 

The resulting savings are considerable. In comparison to operation with constant reflux ratio, a reduction in the distillation time and energy requirement of about 30-60% can be expected. 

Reflux ratio. This is defined as the ratio between the number of moles of vapour returned as refluxed liquid to the fractionating column and the number of moles of final product (collected as distillate), both per unit time. The reflux ratio should be varied according to the difficulty of fractionation, rather than be maintained constant a high efficiency of separation requires a liigh reflux ratio. ... 

Alternatively, by careful control of the reflux ratio, it is possible to hold the composition of the distillate constant for a time until the required reflux ratio becomes intolerably large, as illustrated in Figure 14.9. A mass balance gives ... 

This method is one of the most important concepts in chemical engineering and is an invaluable tool for the solution of distillation problems. The assumption of constant molar overflow is not limiting since in very few systems do the molar heats of vaporisation differ by more than 10 per cent. The method does have limitations, however, and should not be employed when the relative volatility is less than 1.3 or greater than 5, when the reflux ratio is less than 1.1 times the minimum, or when more than twenty-five theoretical trays are required(13). In these circumstances, the Ponchon-Savarit method described in Section 11.5 should be used. 

X/1. If after a certain interval of time the composition of the top product starts to fall, then, if the reflux ratio is increased to a new value R2, it will be possible to obtain the same composition at the top as before, although the composition in the still is weakened to xS2. This method of operating a batch still requires a continuous increase in the reflux ratio to maintain a constant quality of the top product. 

Constant reflux, varying overhead composition. Reflux is set at a predetermined value at which it is maintained for the run. Since pot liquid composition is changing, instantaneous composition of the distillate also changes. The progress of a binary separation is illustrated in Fig. 13-98. Variation with time of instantaneous distillate composition for a typical multicomponent batch distillation is shown in Fig. 13-99. The shapes of the curves are functions of volatility, reflux ratio, and number of theoretical plates. Distillation is continued until the average distillate composition is at the desired value. In the case of a binary, the overhead is then diverted to another receiver, and an intermediate cut is withdrawn until the remaining pot hquor meets the required specification. The intermediate cut is usually added to... 

Counting steps, about 27 ideal stages are required for this separation, compared to 21 based on the assumption of constant molal overflow. The difference would be smaller if a higher reflux ratio were used. The calculations were based on 1.2 times the nominal value of but this really corresponds to about 1.1 times the true minimum reflux, as can be seen from Fig, 18.25. 

In the case of a batch distillation it is therefore not sufficient to determine the conditions of distillation at the start it is necessary to consider what the final composition of the contents of the still pot is to be, or what is the highest reflux ratio that one is prepared to use, considering the total time required. Suppose that we adopt r = 25 as the limiting reflux ratio, that xb = 80 mol% and that we keep xb and the number of theoretical stages constant at 98% and 10 respectively. The data found for a reduction of the stiU pot concentration from 80 to 5 mol% are then as shown in Table 14. 

In Figure 8-13 the A product will be the ethanol product and the B product (distillate from column 2) will be water. We want the A product to be 0.9975 mole frac ethanol (this exceeds requirements for ethanol used in gasoline). Use an external reflux ratio in column 1 L7D = 1.0. (Normally these would be optimized, but to save time leave it constant.) The reflux is returned as a saturated liquid. This set of conditions should remove sufficient ethylene ycol from the distillate to produce an ethanol of suitable purity (check to make sure that this happens). The bottoms product from column 1 should have less than 0.00009 mole frac ethanol. This number is low to increase the recovery of ethanol. 

These lines are drawn on Pig. 7-21, and the corresponding steps are shown. A still plus approximately 12 theoretical plates are required. This is in close agreement with the result of Part 2, and this is generally the case. Thus, if a tower is calculated on the basis of a certain factor times the minimum reflux ratio, the theoretical plates required are usually approximately the same on either the (y x) or enthalpy-composition basis, provided the method chosen is used consistently throughout the calculation. However, the heat and cooling requirements may be seriously in error when calculated on the constant overflow basis. 

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