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Batch Distillation

Batch Distillation. History. Alembic still for distillation of brandy. Early distillation of alcohol. http:// essentialspirits.com/history.htm. Major types of batch distillation. Simple batch distillation. Multistage batch distillation. Reasons to u se batch distillation.

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Batch Distillation

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  1. Batch Distillation

  2. History Alembic still for distillation of brandy Early distillation of alcohol http://essentialspirits.com/history.htm

  3. Major types of batch distillation Simple batch distillation Multistage batch distillation

  4. Reasons to use batch distillation • Small capacity (e.g., specialty chemicals) • Intermittent need • Test run for a new product • Up-stream operations are batch (e.g., alcoholic spirits) • Feed inappropriate for continuous distillation (suspended solids) • Feed varies widely in composition

  5. Simple batch distillationno rectification ( = no column) Characteristics: • no column; a single equilibrium stage (= the still pot) • single charge (F) to still pot at time = 0 • vapor is withdrawn continuously • composition of liquid in still pot (xW) changes continuously • composition of liquid distillate (xD) changes continuously D, xD V, y time t: W, xW time 0: F, xF still pot with heater

  6. Rayleigh equation TMB: F = Wfinal + Dtotal CMB: FxF = WfinalxW,final + DtotalxD,avg D, xD Specify F, xF and xW,final or xD,avg Leaves 3 unknowns: Wfinal, Dtotaland xW,final or xD,avg Need one more equation: V, y time 0: F, xF still pot, with heater time t: W, xW y = xD dCMB: - xDdW = - d(WxW) (vapor withdrawn)=(change in stillpot composition) chain rule: - xDdW = - WdxW - xWdW WAIT! K is not constant; K= K(T) where xD = f(xW) Rayleigh equation

  7. Integration of the Rayleigh equation Numerical integration: Constant relative volatility: • • 1/(xD – xD) xW,final xF x = xW Specify F, xF, and either Wfinal or xW,final. Simpson’s rule:

  8. Solvent switching using simple batch distillation Goal: replace one solvent by another, in order to facilitate crystallization of a non-volatile product, or for a subsequent reaction step. The Hard Way: • Boil off most of original solvent in a batch still. • Add second solvent. • Perform second batch distillation to remove residual original solvent. The Easy Way: Constant-level batch distillation Add second solvent continuously as first solvent vaporizes, keeping W constant; more energy efficient and uses less solvent. dTMB: dV = dS (vapor withdrawn)=(new solvent added) dCMB: 0 - ydV = - xDdS = WdxW (W constant)

  9. Batch steam distillation D, xD = 1 Used for thermally fragile organics (e.g., essential oils in perfume industry), and for slurries/sludges containing organics. A single charge (F) added to still pot at time = 0. Steam is added continuously. H2O(l) (to waste) V, y still pot, no heater If W, D are immiscible with water, we have a heterogeneous azeotrope. H2O(l) D.o.F. = 2 components – 3 phases + 2 = 1 W, xW steam Fix Ptotal, then T cannot vary! constant T < Tbp(H2O) How much steam is required? Both H2O and organic vaporize well below their single-component boiling points. Also, constant vapor composition. Raoult’sLaw: Ptotal = P*WxW + P*H2O Steam also needed to heat and vaporize the material in the still pot.

  10. Batch distillation with rectification TMB: Vj+1 = Lj + D CMB: Vj+1yj+1 = Ljxj + DxD • both are time-dependent • either D or xD (or both) change over the course of the distillation D, xD L0, x0 stage 1 V1, y1 y1 ≠ K / xW stage j CMO: Vj+1 = Vj and Lj = Lj-1 operating line equation: y = (L/V) x + (1 - (L/V)) xD y = x = xD slope = L/V • actually a family of operating lines, since L/V or xD(or both) changeover the course of the distillation • therefore the operating line moves on the M-T diagram y1 = x0 = xD Vj+1, yj+1 Lj, xj stage N VN+1, xN+1 LN, xN time 0: F, xF still pot, with heater stage N+1 time t: W, xW

  11. Choice of operating methods Constant reflux ratio (variable xD) Constant distillate composition (variable R) •xD VLE VLE •xD • time • time • total reflux • y=x y=x distillation must end when (or before) xD,avg = xF distillation must end when (or before) R = ∞ (L/V = 1) Harder to monitor and control (need to detect xD on-stream and adjust R accordingly). Easy to monitor and control. Can solve graphically, if we assume no liquid holdup on the column.

  12. Multistage batch distillation with constant R Given F, xF, xW,final, R and N, find Dtotal, xD,avg VLE For N = 2 (incl. reboiler) 1 • • • • •xD,1 1. For an arbitrary set of xD values, draw a series of parallel operating lines, each with slope R/(R+1) 2 • • • • 1 •xD,2 2 2. Step off N stages on each operating line to find its corresponding xW 1 •xD,3 2 3. Perform numerical integration: plot 1/(xD-xW) vsxW limits: xF, xW,final 1 • xD,4 2 4. Calculate Wfinalusing Rayleigh equation y=x xW,1 xW,4 xW,2 xW,3 5. Solve mass balances for Dtotal and xD,avg If xD,avg is specified instead of xW,final: guess xW,final, calculate xD,avg, iterate.

  13. Operating time at constant R (D) shut down, cleaning and recharging still pot, restart depends on vapor flow rate (V), which depends on boilup rate • if the boilup rate is constant, then V is constant, and D will be constant • V = Vmax when vapor velocity u = uflood • uflood depends on column diameter • typically, operate at D = 0.75 Dmax condenser TMB:

  14. Calculating column diameter We want to use the smallest diameter that will not cause the column to flood. • where σ is surface tension, ρL and ρV are liquid and vapor densities, respectively. Csb,flood is the capacity factor, depends on flow parameter FP and tray spacing; obtain from graphical correlation. • where η is the fraction of the column cross-sectional area available for vapor flow (i.e., column cross-sectional area minus downcomer area).

  15. Multistage batch distillation with constant xD Given F, xF, xD (maybe) xw,final and N, find Rinitial, Rmin, xW,min VLE • N = ∞, R = Rmin •xD (L/V)min 1. Draw trial op. lines and step off N stages to end at xF This is trial-and-error, except for N = 2, or N = ∞ (Rmin) • • (L/V)initial 3. Find xW,min using (L/V) = 1. Rayleigh equation not needed! Verify xW,final > xW,min. y=x 4. Solve mass balance for Wfinal and Dtotal. xW.min N = 2 (incl. reboiler) xF

  16. Operating time with constant xD Numerical integration: mass balance: • • xW,final xF x = xW 1. Draw a series of arbitrary operating lines, each with a different slope L/V 2. Step off N stages on each operating line to find its corresponding xW 3. Perform numerical integration (plot graph, use Simpson’s rule) 4. Calculate toperating

  17. Optimal control • use optimal, time-dependent reflux ratio (not constant R, not constant xD) • more energy-efficient • useful for difficult separations • usually, we can assume vapor holdup is negligible • liquid holdup causes xw to be lower than it would be in the absence of holdup • causes the degree of separation to decrease To assess the effect on batch distillation: • measure the amount of holdup at total reflux • perform computational simulation Effect of liquid holdup on the column

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