2 History Early distillation of alcohol Alembic still for distillation of brandy
3 Major types of batch distillation Simple batch distillationMultistage batch distillation
4 Reasons to use batch distillation Small capacity (e.g., specialty chemicals)Intermittent needTest run for a new productUp-stream operations are batch (e.g., alcoholic spirits)Feed inappropriate for continuous distillation (suspended solids)Feed varies widely in composition
5 Simple batch distillation no rectification ( = no column) Characteristics:no column; a single equilibrium stage (= the still pot)single charge (F) to still pot at time = 0vapor is withdrawn continuouslycomposition of liquid in still pot (xW) changes continuouslycomposition of liquid distillate (xD) changes continuouslytime t: W, xWV, yD, xDtime 0: F, xFstill pot with heater
6 Rayleigh equation TMB: F = Wfinal + Dtotal time t: W, xWy = xDV, yD, xDTMB: F = Wfinal + DtotalCMB: FxF = WfinalxW,final + DtotalxD,avgSpecify F, xF and xW,final or xD,avgLeaves 3 unknowns:Wfinal, Dtotal and xW,final or xD,avgNeed one more equation:time 0: F, xFstill pot, with heaterdCMB: - xDdW = - d(WxW)(vapor withdrawn) = (change in still pot composition)chain rule: - xDdW = - WdxW - xWdWWAIT! K is not constant;K = K(T)Rayleigh equationwhere xD = f(xW)
7 Integration of the Rayleigh equation 1/(xD – xD)x = xWNumerical integration:Constant relative volatility:xW,final••xFSpecify 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 distillationAdd 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as first solvent vaporizes, keeping W constant; more energy efficient and uses less solvent.dTMB: dV = dS(vapor withdrawn) = (new solvent added)dCMB: ydV = - xDdS = WdxW(W constant)
9 Batch steam distillation Used for thermally fragile organics (e.g., essential oils in perfume industry), and for slurries/sludges containing organics.W, xWV, yD, xD = 1A single charge (F) added to still pot at time = 0. Steam is added continuously.H2O(l)(to waste)still pot,no heatersteamIf W, D are immiscible with water, we have a heterogeneous azeotrope.H2O(l)D.o.F. = 2 components – 3 phases + 2 = 1Fix 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’s Law: Ptotal = P*WxW + P*H2OSteam 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 distillationtime t: W, xWy1 = x0 = xDV1, y1D, xDL0, x0stage 1y1 ≠ K / xWstage jCMO: Vj+1 = Vj and Lj = Lj-1operating line equation:y = (L/V) x + (1 - (L/V)) xDy = x = xD slope = L/V• actually a family of operating lines, since L/V or xD (or both) change over the course of the distillation• therefore the operating line moves on the M-T diagramVj+1, yj+1Lj, xjstage NVN+1, xN+1LN, xNtime 0: F, xFstill pot, with heaterstage N+1
11 Choice of operating methods Constant reflux ratio (variable xD)y=xConstant distillate composition (variable R)VLEy=x•xD•xDtotal reflux•timeVLEtime•••distillation must endwhen (or before) xD,avg = xFdistillation must endwhen (or before) R = ∞ (L/V = 1)Easy to monitor and control.Harder to monitor and control (need to detect xD on-stream and adjust R accordingly).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,avgy=xVLE12For N = 2 (incl. reboiler)•xD,4•xD,1•xD,2•xD,3••1. For an arbitrary set of xD values, draw a series of parallel operating lines, each with slope R/(R+1)•12xW,2•12xW,32. Step off N stages on each operating line to find its corresponding xW3. Perform numerical integration:plot 1/(xD-xW) vs xWlimits: xF, xW,final•12xW,44. Calculate Wfinal using Rayleigh equationxW,15. Solve mass balances for Dtotal and xD,avgIf xD,avg is specified instead of xW,final: guess xW,final, calculate xD,avg, iterate.
13 Operating time at constant R (D) depends on vapor flow rate (V), which depends on boilup rateshut down, cleaning and recharging still pot, restart• if the boilup rate is constant, then V is constant, and D will be constantcondenser TMB:• V = Vmax when vapor velocity u = uflood• uflood depends on column diameter• typically, operate at D = 0.75 Dmax
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,miny=xVLEN = ∞, R = Rmin••xD(L/V)initial(L/V)min1. Draw trial op. lines and step off N stages to end at xFThis is trial-and-error, except for N = 2, or N = ∞ (Rmin)xF••xW.min3. Find xW,min using (L/V) = 1.Rayleigh equation not needed!Verify xW,final > xW,min.Is xWfinal required?4. Solve mass balance for Wfinal and Dtotal.N = 2 (incl. reboiler)
16 Operating time with constant xD mass balance:x = xWNumerical integration:xW,final•xF1. Draw a series of arbitrary operating lines, each with a different slope L/V2. Step off N stages on each operating line to find its corresponding xW3. Perform numerical integration (plot graph, use Simpson’s rule)4. Calculate toperating
17 Effect of liquid holdup on the column Optimal control• use optimal, time-dependent reflux ratio (not constant R, not constant xD)• more energy-efficient• useful for difficult separationsEffect of liquid holdup on the column• 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 simulationNext year: add heater and condenser duties
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