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Batch Distillation Uses

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sandra_john
20 diapositives · 1.2K Views · Dec 07, 2011

Batch Distillation Uses. Relatively small amounts of product. Non-continuous operation (batch). Different distillations are to be done using the same equipment. Batch Distillation. Unit may be a single pot or multi-staged.

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

  2. Batch Distillation • 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 • Distillate (usually the desired product) may be withdrawn continuously or collected in an accumulator.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

  3. Batch Distillation • 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, xD, in general, change with time. Lecture 20

  4. Batch Distillation Mass Balances over Total Operation Time Lecture 20

  5. Multistage Batch Distillation Lecture 20

  6. Multistage Batch vs. Continuous • 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 xD will change with time, or it can be operated with a constant xD, which means that the L/D must be continuously changed. Lecture 20

  7. Batch Trade-Off • 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

  8. Multistage Batch Distillation Operating Line • 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

  9. McCabe-Thiele -- Constant xD Lecture 20

  10. McCabe-Thiele -- Constant L/D Lecture 20

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

  12. Multistage Batch Distillation • We have only one operating line since there is no feed. • We can plot this operating line on a McCabe-Thiele plot along with our equilibrium curve. • We can step down the operating line from xD to xW to determine the number of stages. • Note that the bottoms concentration, xW, keeps changing with time as the liquid is boiled off. • Also note that xD 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 xD and xW with respect to time… Lecture 20

  13. Differential Change Lecture 20

  14. Integrating Lecture 20

  15. Rayleigh Equation Lecture 20

  16. Rayleigh Equation Notes Lecture 20

  17. xD = f(xW) Relationship Lecture 20

  18. Algorithm • For a given L/D and number of column stages, assume xD’s, perform a McCabe-Thiele Analysis at each xD stepping down to determine the corresponding xW’s. • Plot 1/(xD – xW) vs. xW. • Graphically integrate or do a polynomial curve fit between xW = xF and xW = xW,final and integrate. Lecture 20

  19. Total Reflux • 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 xD at the top of the column and xB at the bottom of the column and perform a McCabe-Thiele analysis to determine the theoretical Nmin. Lecture 20

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