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Dr Saad Al-ShahraniChE 334: Separation Processes Binary Multistage Distillation Condenser At the top of column, three type of condenser: a)Total condenser: takes the overhead vapor from the column and liquefies it completely and split it into two portions: 1.Distillate which is called overhead product. 2.Reflux which return back to the top plate. accumulator D V Condenser L Reflux Overhead vapour

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Dr Saad Al-ShahraniChE 334: Separation Processes b)Partial condenser: takes the over head vapor from the column and liquefies a portion of it to return to the top plate as reflux, while the other portion is vapor product or distillate. c)Mixed condenser Partial condenser V Condenser L Vapor overhead product accumulator Reflux Overhead vapor Vapor distillate V Condenser L liquid distillate accumulator Mixed condenser Reflux Overhead vapor Binary Multistage Distillation

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Dr Saad Al-ShahraniChE 334: Separation Processes Re-boilers a)Kettle type re-boiler At the bottom of column which heat the bottom liquid to produce vapor partially and return it back to column, while the rest of liquid withdraw as a bottom product. Binary Multistage Distillation bottoms

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Dr Saad Al-ShahraniChE 334: Separation Processes b)Vertical thermosyphone-type re-boiler. In this type of re-boilers, re-boiler liquid with drown from the bottom sump. Binary Multistage Distillation

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Dr Saad Al-ShahraniChE 334: Separation Processes c)Vertical thermosyphone-type re-boiler. In this type of re-boilers, re-boiler liquid with drown from the bottom- tray down comer. Binary Multistage Distillation

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Dr Saad Al-ShahraniChE 334: Separation Processes Reflux It is apart of condensed vapor returned back to stage below. There are two types of reflux: 1. Operating reflux ratio (External Reflux Ratio) External Reflux Ratio= L a /D Binary Multistage Distillation DxDDxD accumulator VayaVaya Condenser LaxaLaxa Top plate Reflux

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Dr Saad Al-ShahraniChE 334: Separation Processes 2. Operating reflux ratio (Internal Reflux Ratio) Internal Reflux Ratio or Plate n+2 Plate n+1 Plate n V n+2 L n+3 V n+1 L n+2 VnVn L n+1 V n-1 LnLn Binary Multistage Distillation

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Dr Saad Al-ShahraniChE 334: Separation Processes Feed McCabe Thiele Graphical Equilibrium-Stage Method for trayed tower MacCape-Thiele Diagram DxDDxD accumulator VayaVaya water Stripping Section Rectifying Section BxBBxB Condenser Re-boiler VbybVbyb L b, x b Bottom plate Top plate LaxaLaxa steam Feed plate Equations for the analysis of continuous flow binary distillation were developed by sorel (1893), but the simplest and most convenient method for analyzing binary distillation column is graphical method by [McCape and Thiele] consider the equilibrium-stage distillation column shown in this figure. Feed is a mixture of two components (A,B) y a mole fraction of vapor x a mole fraction of liquid

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Dr Saad Al-ShahraniChE 334: Separation Processes McCabe Thiele Graphical Equilibrium-Stage We can group values of y and x into 2 groups: Group 1: Vapor and liquid leaving each plate Group 2: Vapor and liquid passing each other between two plates n-1 n V n-1 L n-2 VnVn L n-1 V n+1 LnLn n n-1 VnynVnyn L n-1 x n-1 e.g. y n, x n e.g. y n, x n-1

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Dr Saad Al-ShahraniChE 334: Separation Processes McCabe Thiele Graphical Equilibrium-Stage By defining of an ideal plate, the vapor and liquid leaving plate n are in equilibrium, so y n and x n represent equilibrium concentration. n-1 n VnynVnyn L n-1 x n-1 V n+1 y n+1 LnxnLnxn V n+2 y n+2 L n+1 x n+1 V n-1 y n-1 L n-2 x n-2 n+1 Subcooled liquid 0 1.0 Superheated Vapor y Temperature y n+1 x xnxn ynyn y n-1 x n+1 Saturated liquid line Saturated vapor line x n-1 operating x,y

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Dr Saad Al-ShahraniChE 334: Separation Processes McCabe Thiele Graphical Equilibrium-Stage Since the concentration in both phases (vapor, liquid) increases with the hight of the column, x n-1 > x n > x n+1 y n > y n+1 > y n+2 In spite of x n, y n (streams leaving plate n) are in equilibrium, x n-1, y n+1 ( streams entering plate n) are not in equilibrium (see the last figure)

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Dr Saad Al-ShahraniChE 334: Separation Processes McCabe Thiele Graphical Equilibrium-Stage V n+1 y n+1 LnxnLnxn V m+1 y m+1 LmxmLmxm Feed DxDDxD accumulator VayaVaya Feed plate n+1 m Stripping Section Rectifying Section BxBBxB Condenser Re-boiler VbybVbyb L b, x b Bottom plate Top plate LaxaLaxa I II Overall material balances for Two components systems Total material balance Component A balance Eliminating B Eliminating D F

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Dr Saad Al-ShahraniChE 334: Separation Processes Equation (3), (4) are the true for all values of flows of vapor and liquid within the column Net flow rates By making a material balance around the condenser and accumulator The difference between the flow rates of vapor and liquid anywhere in the upper section of the column (Rectifying section) = D DxDDxD accumulator VayaVaya Condenser LaxaLaxa Top plate McCabe Thiele Graphical Equilibrium-Stage

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Dr Saad Al-ShahraniChE 334: Separation Processes Total balance around the control surface ( I) Then D= net flow in top (rectifying section). Regardless of changes in V and L, their difference is constant and = D Component balance A V n+1 y n+1 LnxnLnxn DxDDxD accumulator VayaVaya Condenser Top plate LaxaLaxa I McCabe Thiele Graphical Equilibrium-Stage

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Dr Saad Al-ShahraniChE 334: Separation Processes By the same manner in the stripping section Total balance around control surface ( II) m is to designate a general plate in the stripping section B= net flow in the bottom (stripping section) V m+1 y m+1 LmxmLmxm BxBBxB Re-boiler VbybVbyb L b, x b Bottom plate II McCabe Thiele Graphical Equilibrium-Stage

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Dr Saad Al-ShahraniChE 334: Separation Processes Re-boiler Material balance around the re-boiler BxBBxB VbybVbyb L b, x b Bottom plate McCabe Thiele Graphical Equilibrium-Stage

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ChE 334: Separation Processes Operating lines Because there are two sections in the column, there are two operating lines 1. For rectifying section ( I ) V n+1 y n+1 LnxnLnxn Operation relationship n n+1 For control surface ( I) ( ) McCabe Thiele Graphical Equilibrium-Stage V n+1 y n+1 LnxnLnxn DxDDxD accumulator VayaVaya Condenser Top plate LaxaLaxa I Dr Saad Al-Shahrani

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ChE 334: Separation Processes 2. For stripping section ( II ) For control surface ( II) ( ) V m+1 y m+1 LmxmLmxm Operation relationship m m+1 VmymVmym L m-1 x m-1 m-1 m McCabe Thiele Graphical Equilibrium-Stage V m+1 y m+1 LmxmLmxm BxBBxB Re-boiler VbybVbyb L b, x b Bottom plate II

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Dr Saad Al-ShahraniChE 334: Separation Processes Number of ideal plates, McCape Thiele Method When the operating lines represented by: For stripping section For rectifying section are plotted with xy equilibrium diagram, the McCabe Thiele step by step construction can be used to compute the number of ideal plates needed to accomplish a definite concentration difference in either rectifying or the stripping section McCabe Thiele Graphical Equilibrium-Stage

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Dr Saad Al-ShahraniChE 334: Separation Processes Constant Molal overflow For most distillation, the molar flow rates of vapor and liquid are nearly constant in each section of the column (rectifying and stripping) and the operating lines are almost straight. (note: V-L=D, L-V=B) Reflux Ratio Reflux to overhead product Reflux to the vapor DxDDxD V Top plate L McCabe Thiele Graphical Equilibrium-Stage

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Dr Saad Al-ShahraniChE 334: Separation Processes In both numerator and denominator of the terms on the right hand side of the last two equations are divided by D, the result is for constant molal over flow. intercept slope This equation is the operating line of the rectifying section. The y axis intercept of this line is x D /(R D +1) McCabe Thiele Graphical Equilibrium-Stage

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Dr Saad Al-ShahraniChE 334: Separation Processes x D is set by the conditions of design R D, the reflux ratio, is an operating variable that can be controlled by adjusting the split between reflux and overhead product or by changing the amount of vapor formed in the re-boiler for a given flow rate of the over head product. Put x n = x D x D y n+1 So the operating line for rectifying section intersect the diagonal at point (x D,x D ). This is true for both partial and total condenser y n+1 = x D McCabe Thiele Graphical Equilibrium-Stage x n =x D LnLn L Reflux n+1 Top plate V n+1 y n+1

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Dr Saad Al-ShahraniChE 334: Separation Processes Condenser and top plate Concentration of vapor from top plate =y n+1 Concentration of reflux to the top plate x n = x D =y n+1 McCabe Thiele Graphical Equilibrium-Stage V n+2 y n+2 L n+1 X n+1 n+1 V n+1 y n+1 Top plate L n =L =Reflux X a =x D Top plate xnxn

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Dr Saad Al-ShahraniChE 334: Separation Processes n+1 n+2 n+3 y n+1 y n+2 y n+3 x n =x D x n+1 x n+2 y Operating line (rectifying section) compositionEquilibrium withOperating with x n =x D yy n+1 x n+1 y n+1 y n+2 x n+2 y n+2 y n+3 Total Condenser n+1 D, x D V n+1 y n+1 Top plate LnxnLnxn y n+2 x n+1 y n+3 x n+2 n+2 n+3 y n+1 McCabe Thiele Graphical Equilibrium-Stage Equilibrium line Reflux Top plate

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Dr Saad Al-ShahraniChE 334: Separation Processes n+1 n+2 n+3 y n+1 y n+2 y n+3 x n =x D x n+1 x n+2 x n+3 45 o line x = y n+1 y n+2 x n+1 y n+3 x n+2 n+2 n+3 y n+1 x n =x D LnLn Top plate For total condenser Operating line equation for the rectifying section: McCabe Thiele Graphical Equilibrium-Stage

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Dr Saad Al-ShahraniChE 334: Separation Processes n+1 n+2 n+3 y n+1 y n+2 y n+3 y D =x D x n+1 x n+2 Operating line (rectifying section) compositionEquilibrium withOperating with xnxn x D =y D y n+1 x n+1 y n+1 y n+2 x n+2 y n+2 y n+3 Partial Condenser n+1 D x D V n+1 y n+1 Top plate LnxnLnxn y n+2 x n+1 y n+3 x n+2 n+2 n+3 y n+1 McCabe Thiele Graphical Equilibrium-Stage Equilibrium line VyDVyD Reflux xnxn Partial condenser y D =x D Top plate

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