Varying reflux ratio time requirement

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. ... 

In this case, the energy requirement is a variable as reflux ratio varies. For a differential time interval ... 

In batch distillation, as the overhead composition varies during operation, a number of main-cuts and off-cuts are made at the end of various distillation tasks or periods (see Chapter 3). Purities of the main-cuts are usually determined by the market or downstream process requirements but the amounts recovered must be selected based on the economic trade off between longer distillation times (hence productivity), reflux ratio levels (hence energy costs), product values, etc. Increasing the recovery of a particular species in a particular cut may have strong effects on the recovery of other species in subsequent cuts or, in fact, on the ability to achieve at all the required purity specifications in subsequent cuts. The profitable operation of such processes therefore requires consideration of the whole (multiperiod) operation. 

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 In the case of a binary, the overhead is then diverted to another receiver, and an intermediate cut is withdrawn until the remaining pot hquor meets the required specification.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... 

The coordinates of the initial simplex of the final search were obtained by varying those of the final simplex of the initial search by + 10 percent through the use of random numbers. This procedure promoted the expansion of the region of the search for the minimum. As in the case of conventional columns, the computer time requirement was reduced by the convergence criterion placed on 0 at each vertex. At the end of the final search, one of the simplex vertices was picked as the optimum. Then additional trials were made as required in order to obtain a solution which met the convergence criteria. If the constraints were not satisfied by this solution, minor adjustments of the variables were made such as adding one plate or increasing the reflux ratio. In the solution of the problems presented, it was seldom necessary to make these adjustments. 

A batch distillation column with six theoretical stages (the first stage is the still pot) is charged with 100 kmol of a 20 mol% ethanol in water mixture at atmospheric pressure. The boilup ratio is constant at 10 kmol/h. If the distillate composition is to be maintained constant at 80 mol% ethanol by varying the reflux ratio, calculate the distillation time required to reduce the still residue composition to 5 mol% ethanol. Calculate the fractional recovery of ethanol in the distillate. The equilibrium distribution curve at column pressure is given in Table 6.2. 

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