Lecture 11. Binary Distillation (1) [Ch. 7] • Distillation - Oi&Overview & IdIndust tilrial use -Operation • McCabe-Thiele Method -Rectifying section - Stripping section - Feed-stage considerations -q-line - Optimal and nonoptimal locations of feed stage • Limiting Conditions - Minimum number of eqqguilibrium stages -Minimum reflux ratio - Perfect separation Distillation
• Distillation (fractionation) vs. absorption and stripping : the second fluid phase is usually created by thermal means (vaporization and condensation) rather than by introduction • 11th century, distillation was used in Italy to produce alcoholic b(bth)beverages (batch process) 16th century, it was known that separation could be improved by multiple vapor-liquid contacts (stages) 20th century, multistage distillation became the most widely used industrial method for separating liquid mixtures Distillation: Industrial Use
• Distillation is technically the mosttt mature separation operation • Distillation is very energy intensive, especially when the relative volatility, , is low (< 1.50) • Most significant distillation energy hbhas been consumed by petroleum refining to separate crude oil into petroleum fractions,
light hydrocarbons (C2’s to C5’s), and aromatic chemicals Distillation Operation
• Total condenser: the Total condenser overhead vapor leaving the
Overhead vapor top stage is totally Reflux drum condensed liquid distillate Distillate (D) product + liquid reflux that is Top stage 1 Reflux Light-key mole returned to the top stage fraction = xD Rectifying section • Partial reboiler: liquid from stages the bottom stage is partially Feed Light-key f Feed stage vaporized liquid bottoms mole fraction = z F SSitripp ing secti on product + vapor boilup that stages is returned to the bottom N Bottom stage Boilup stage • Multiple, countercurrent Partial contacting stages can reboiler achieve a sharp separation Bottom (B) Light-key mole unless an azeotrope is
fraction = xB formed Phase Equilibrium of Binary Mixture
• Goal of distillation: from the feed to produce a distillate, rich in the lig ht key (xD 10)1.0), an d a bo ttoms pro duc t, r ic hih in the heavy key (xB 0.0) • Relative volatility ,
1,2 KK 1/ 2 Raoult’s law
ss KPP11/and KPP 2 2 / s s 1,2 P 1/ P 2
yx11/(1) y 1 x 1 1,2 y22/(1)xx 1 y 1 x y 1,2 1 1 1( x 1) For ideal binary mixtures of components with close boiling 11,2 points, T changes are small and is almost constant McCabeMcCabe--ThieleThiele Method
Graphical equilibrium-stage methdfhod for trayed towers Rectifying Section
< 1
Assuming R=L/D constant L/V = R/(R+1) R: reflux ratio molar overflow D/V=1/(R+1) Vnn11y Lx nn Dx D LD R 1 Ln D y xx y xxD y xxD nnD1 VV RR11 VVnn11 Stripping Section
> 1 Boilup
Assuming VVBB / constant L /(VV1 )/ V BBVB: boilup ratio molar overflow BV// 1 VB LmmxV m11y mBx B L B VB 1 1 Lm B y x x y x x y xx B B mmB1 V V VB VB VVmm11 FeedFeed--StageStage Considerations
• xD and xB can be selected independently
• RandVR and VB are related by the feed phase condition Subcooled-liquid feed Bubble-point liquid feed Partially vaporized feed V V V L V V L V V L V V VF V F F F F
LF
L L L LL F V L L F V L L LF V
Dew-point vapor feed Superheated-vapor feed L VV V F L V VV F Possible feed F F conditions
LL L V L L V Relations for Reflux Ratio and Boilup Ratio • Relations covering feed conditions from a saturated liquid to a saturated vapor V L V V VF Boilup ratio Reflux ratio V F F VVV F LDV F LLL F VBL F L F V LDVF L VBLF VB R L BB DD L L LF V
For the specification of distillation operation, R or R/Rmin is used traditionally because the distillate product is often the more imppportant product
• q: ratio of the increase in molar Feed condition Value of q reflux rate across the feed Subcooled liquid >1> 1 stage to the molar feed rate Bubble-point liquid 1 Partially vaporized L /F L L V V F q q 1 Dew-point vapor 0 F F Superheated vapor < 0 qq--LineLine
• q-line: one point of which is the intersection of the rectifying and stripping operating lines
Rectifying operating line Stripping operating line L D L B y x x y x x V V D V V B
y()()VV LLxDxBx D B
DxD BxBF Fz F VLVL L L q F q z yxF qq11 Optimal and Nonoptimal Locations of Feed Stage
5 stages, The smallest number of optimal feed total stages occurs location when the transfer is made at the first opportunity after a horizontal line of the staircase passes over point P
646.4 s tages, 595.9 s tages, feed-stage feed-stage location location below above optimal optimal stage stage Minimum Number of Equilibrium Stages • Increasing reflux LV/ increases to limiting value 1 • Increasing boilup ratio LV/ decreases to limiting value 1
RtifiRectifying opera tiliting line L D y x x V V D
Stripping operating line L B y x x V V B
V Minimum number of stages
L L= V : total reflux B=D=0 : no product Minimum Reflux Ratio
• The number of equilibrium stages increases when operating line moves closer to equilibrium curve
Typical ideal or near-ideal system Typical nonideal system
Pinch point above the feed stage Pinch point at the feed stage
(/)LVmin R min /( R min 1) RLVLVmin(/) min /[1(/) min ]
Minimum boilup ratio ()VLVB min 1/[(/) max 1] Perfect Separation
• Perfect separation
xD = 1, xB = 0 • Number of stages : infinite • Reflux ratio : finite value • Slope of operating line : finite value
xB zF xD [Example] Distillation of a Binary Mixture of Benzene and Toluene A trayed tower is to be designed to continuously distill 450 lbmol/hr of a binary mixture of 60 mol% benzene and 40 mol% toluene. A liquid distillate and bottom product of 95 mol% and 5 mol% benzene are to be produced. The feed is preheated so that it enters the column with a molar percent vaporization equal to the distillate-to-feed ratio. Use the McCabe-Thiele method to compute following, assuming a uniform pressure of 1 atm throughout the column.
(a) Nmin, (b) Rmin, and (c) N for R/Rmin=1.3 and the optimal location of feed stage
• Overallmaterialbalanceonbenzene&totalbalanceOverall material balance on benzene & total balance D 275lbmol/h z F x D x B 0.60(450) 0.95(D) 0.05(B) F D B B 175lbmol/h F B D 450 B D DF/0.611 [Example] (a) Minimum Number of
Theoretical Stages, Nmin
LL 1and 1 VV x and y : benzene, more-volatile component xD =095= 0.95 xB = 0.05
⇒ Nmin = 6.7 [Example] (b) Minimum Reflux
Ratio, Rmin
VF / F D / F 0.611 LFV() q FF 0.389 F F y = 0.684 Slope of q-line x = 0.465 q 0.389 0.637 q 1 0.389 1
Slope of operating line 0950.95 00684.684 R 0.55 0.95 0.465R 1
Rmin 1.22 [Example] (c) Number of Equilibrium Stages, N
RR1.3 min 1.3(()1.22) 1.59
Slope of operating line for rectifying section R 1.59 0.614 R 11.591 N 13.2 Optimal location of feed stage: 7