102811batch-distill

102811batch-distill - Batch Dis*lla*on Wankat ch 9 Alembic s*lls for dis*lla*on of brandy Simple batch s*ll Mul*stage

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Unformatted text preview: Batch Dis*lla*on Wankat ch. 9 Alembic s*lls for dis*lla*on of brandy Simple batch s*ll Mul*stage batch dis*lla*on Simple batch dis*lla*on Objec*ves for this lecture •  Describe uses for batch dis*lla*on •  Diagram simple batch dis*lla*on (no column) •  Obtain differen*al mass balance (Rayleigh equa*on) •  Solve for two special cases: (1) constant K; (2) constant α Recap: •  In batch dis*lla*on, the dis*llate composi*on xD is *me- dependent, requiring a differen*al mass balance. •  The design equa*on for batch dis*lla*on is called the Rayleigh equa*on: Wfinal ln = F xW ,final ∫x F dxw xD − xw •  Before we integrate, we have to find a rela*onship between xD and xW. •  For € simple batch dis*lla*on (no column), this rela*onship is VLE: yD xD K= = xW xW WAIT! K is not constant; K = K(T) € Objec*ves for this lecture •  Evaluate the Rayleigh equa*on for (a) constant rela*ve vola*lity; (b) general case •  Uses of simple batch dis*lla*on: (a)  Solvent switching (b)  Batch steam dis*lla*on • Adding rec*fica*on to batch dis*lla*on Homework this week: 9D4 and 9D6a (due Weds); 9D21 (due Fri) Friday Reading: Ch 9 Recap: •  There is no need for rec*fica*on in batch dis*lla*on for solvent switching, or with batch steam dis*lla*on. •  The Rayleigh equa*on has an analy*cal solu*on for simple batch dis*lla*on if rela*ve vola*lity is assumed constant. •  If α is not constant, but we have (x,y) VLE data, we can integrate numerically, using Simpson’s rule. •  In mul*stage batch dis*lla*on, R, xD, or both, must vary. Objec*ves for this lecture •  Use McCabe- Thiele analysis for mul*stage batch dis*lla*on with constant R •  Determine batch opera*ng *me •  Use McCabe- Thiele analysis for mul*stage batch dis*lla*on with constant xD •  Effect of liquid holdup •  Introduc*on to con*nuous countercurrent extrac*on Mul*stage batch dis*lla*on Constant reflux ra*o (variable xD) Constant dis*llate composi*on (variable R) 1 1 •xD 0.9 • 0.8 0.7 • 0.6 y(MeOH) 0.6 y(MeOH) 0.8 • 0.7 • 0.5 0.4 0.5 0.4 0.3 0.3 0.2 dis*lla*on must end when xD,avg = xF 0.1 0 •xD 0.9 0 0.1 0.2 0.3 0.4 0.5 x(MeOH) 0.6 0.7 0.8 0.9 dis*lla*on must end when R = ∞ (L/V = 1) 0.2 0.1 1 0 • 0 0.1 0.2 0.3 0.4 0.5 x(MeOH) 0.6 0.7 0.8 0.9 1 Mul*stage batch dis*lla*on with constant R Given F, xF, xW,final, and N, find Dtotal, xD,avg 1 4. Use Simpson’s rule to integrate Rayleigh equa*on, then calculate Wfinal 5. Solve mass balance for Dtotal and xD,avg y(MeOH) 1. For an arbitrary set of xD values, draw a series of parallel opera*ng lines, each with slope R/(R+1) 2. Step off N stages on each opera*ng line to find its corresponding xW 3. Plot y=1/(xD- xW) vs x=xW; read y- values at x = xF, xW,final and (xF+xW,final)/2 Assume N = 2 (incl. reboiler) 0.9 0.8 0.7 1 • 2 1 • • 0.6 0.5 2 • 1 • 0.4 0.3 1 • 2• •xD,1 •xD,2 •xD,3 •xD,4 2• 0.2 0.1 xW,4 xW,3 xW,2 0 0 0.1 0.2 0.3 xW,1 0.4 0.5 x(MeOH) 0.6 If xD,avg is specified instead of xW,final: guess xW,final, calculate xD,avg, iterate. 0.7 0.8 0.9 1 Opera*ng *me at constant R (D) tbatch = toperating + tdown depends on vapor flow rate (V), which depends on boilup rate € shut down, cleaning and recharging s*ll pot, restart • if the boilup rate is constant, then V is constant, so D is constant toperating = Dtotal F − Wfinal F − Wfinal = = D V −L V (1 − L / V ) 1 1− L / V = R +1 € toperating = € D= € V R +1 € Dmax = Vmax R +1 (R + 1) F − Wfinal V ( ) • V = Vmax when vapor velocity u = uflood • uflood depends on column diameter • typically operate at D = 0.75 Dmax Calcula*ng column diameter We want to use the smallest diameter that will not flood. uflood ⎛ྎ σ ⎞ྏ0.2 ρ − ρ L V = Csb,flood ⎜ྎ ⎟ྏ ρV ⎝ྎ 20 ⎠ྏ where σ is surface tension, ρL and ρV and liquid and vapor densi*es, respec*vely. € Csb,flood is the capacity factor, depends on flow parameter FP and tray spacing; obtain from graphical correla*on (e.g., Figure 10- 16 in Wankat 3rd Ed) diameter (feet ) = ( 4V MWv ) πηρVuop (3600) where η is the frac*on of the column cross- sec*onal area available for vapor flow (i.e., column cross- sec*onal area minus downcomer area). € Mul*stage batch dis*lla*on with constant xD Given F, xF, xD (maybe) xw,final and N, find Rini*al, Rmin, xW,min 1. Draw trial op. lines and step off N stages to end at xF 3.Find xW,min using (L/V) = 1. Verify xW,final > xW,min N = ∞, R = Rmin 0.9 0.8 • 0.7 • /V) min (L •xD • 0.6 y(MeOH) This is trial- and- error except for N = 2 and N = ∞ (Rmin) 1 0.5 0.4 0.3 4. Solve mass balance for Wfinal and Dtotal. 0.2 0.1 0 N = 2 (incl. reboiler) xW.min xF 0 0.1 0.2 0.3 0.4 0.5 x(MeOH) 0.6 0.7 0.8 0.9 1 Opera*ng *me with constant xD mass balance: W =F xD − xF xD − xW dW dD − = = V − L = V (1 − L / V ) dt dt dW xD − xF dxw =F dt € x − x 2 dt D w ( ) toperating = F € xD − xF € V xF ∫x W ,final dxW ( (1 − L / V ) xD − xw ) 2 1. Draw a series of arbitrary opera*ng lines, each with a different slope L/V 2. Step off N stages on each opera*ng line to find its corresponding xW € 3. Plot y=1/[(1- L/V)(xD- xW)2] vs x=xW; read y- values at x = xF, xW,final and (xF+xW,final)/2 4. Use Simpson’s rule to evaluate integral, then calculate topera*ng Effect 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 separa*on to decrease To assess the effect on dis*lla*on • measure the amount of holdup at total reflux • perform computa*onal simula*on ...
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This note was uploaded on 12/29/2011 for the course CHE 128 taught by Professor Scott,s during the Fall '08 term at UCSB.

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