Batch Distillation Uses o Relatively small amounts of

Batch Distillation Uses o Relatively small amounts of product. o Non continuous operation (batch). o Different distillations are to be done using the same equipment. Lecture 20 1

Batch Distillation o o o Unit may be a single pot or multi staged. There is no continuous feed – the pot is charged with liquid and then drained at the end of the run. Distillate (usually the desired product) may be withdrawn continuously or collected in an accumulator. Lecture 20 2

Batch Distillation o o o One is more interested in the amounts of bottoms and distillate collected rather than the rates. The amount and compositions in the pot (bottoms) change as the more volatile component(s) decrease(s) with time and the less volatile component(s) increase(s) with time. Because the bottoms amount and concentrations change with time, the distillate amount, D, and concentration, x. D, in general, change with time. Lecture 20 3

Batch Distillation Mass Balances over Total Operation Time Lecture 20 4

Multistage Batch Distillation Lecture 20 5

Multistage Batch vs. Continuous o o Since there is no feed, there is only one operating line. The batch system can be operated with a constant L/D, which means that x. D will change with time, or it can be operated with a constant x. D, which means that the L/D must be continuously changed. Lecture 20 6

Batch Trade Off o o o For a constant L/D, the distillate concentration fed to the accumulator decreases with time. The composition of the desired component in the distillate is at a maximum at the beginning of the batch run and decreases with time as it is distilled from the bottoms pot. The concentration of the more volatile component in the accumulator also decreases with time – the trade off is a lower concentration with more distillate accumulated. Lecture 20 7

Multistage Batch Distillation Operating Line o For a multistage batch column, if one can assume CMO, then the operating line for the column is essentially the same as that previously used for continuous distillation: Lecture 20 8

Mc. Cabe-Thiele -- Constant x. D Lecture 20 9

Mc. Cabe-Thiele -- Constant L/D Lecture 20 10

Constant L/D vs. x. D o o Operating at constant L/D is easy – one sets it and runs while the x. D continuously changes. Operating at constant x. D is more difficult since one needs to continuously monitor the distillate concentrations and control the L/D. Lecture 20 11

Multistage Batch Distillation o o o o We have only one operating line since there is no feed. We can plot this operating line on a Mc. Cabe Thiele plot along with our equilibrium curve. We can step down the operating line from x. D to x. W to determine the number of stages. Note that the bottoms concentration, x. W, keeps changing with time as the liquid is boiled off. Also note that x. D changes with time for a constant reflux ratio, L/V or L/D. How do we determine the composition at the bottoms in the pot? We need to relate x. D and x. W with respect to time… Lecture 20 12

Differential Change Lecture 20 13

Integrating Lecture 20 14

Rayleigh Equation Lecture 20 15

Rayleigh Equation Notes Lecture 20 16

x. D = f(x. W) Relationship Lecture 20 17

Algorithm o o o For a given L/D and number of column stages, assume x. D’s, perform a Mc. Cabe Thiele Analysis at each x. D stepping down to determine the corresponding x. W’s. Plot 1/(x. D – x. W) vs. x. W. Graphically integrate or do a polynomial curve fit between x. W = x. F and x. W = x. W, final and integrate. Lecture 20 18

Total Reflux o o Note that a multistage batch still, operated such that all of the distillate is returned to the top of the column, is essentially the same as a multi stage distillation column operated under total reflux. For a binary separation, given a column containing an N number of equilibrium stages, one can measure x. D at the top of the column and x. B at the bottom of the column and perform a Mc. Cabe Thiele analysis to determine theoretical Nmin. Lecture 20 19

Column Overall Efficiency o For a column containing an Nactual number of stages, the overall efficiency can be determined from Eo = Nequil /Nactual where Nequil is theoretical N obtained from the Mc. Cabe Thieleanalysis. Lecture 20 20