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

Binary Multistage Distillation 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. Mixed condenser Partial condenser V Condenser L Vapor overhead product accumulator Reflux Overhead vapor Condenser Vapor distillate Overhead vapor V accumulator liquid distillate L Reflux Mixed condenser ChE 334: Separation Processes Dr Saad Al-Shahrani

Binary Multistage Distillation Re-boilers 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. bottoms ChE 334: Separation Processes Dr Saad Al-Shahrani

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

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

Binary Multistage Distillation 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= La/D Condenser Va ya accumulator La xa Top plate Reflux D xD ChE 334: Separation Processes Dr Saad Al-Shahrani

Binary Multistage Distillation 2. Operating reflux ratio (Internal Reflux Ratio) Internal Reflux Ratio Vn+2 Ln+3 Plate n+2 or Vn+1 Ln+2 Plate n+1 or Vn Ln+1 or Plate n Vn-1 Ln ChE 334: Separation Processes Dr Saad Al-Shahrani

McCabe Thiele Graphical Equilibrium-Stage Method for trayed tower D xD accumulator Va ya water Stripping Section Rectifying Section B xB Condenser Re-boiler Vb yb Lb, xb Bottom plate Top La xa steam Feed plate MacCape-Thiele Diagram 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] Feed consider the equilibrium-stage distillation column shown in this figure. Feed is a mixture of two components (A,B) ya mole fraction of vapor xa mole fraction of liquid ChE 334: Separation Processes Dr Saad Al-Shahrani

McCabe Thiele Graphical Equilibrium-Stage n-1 n Vn-1 Ln-2 Vn Ln-1 Vn+1 Ln We can group values of y and x into 2 groups: Group 1: Vapor and liquid leaving each plate e.g. yn, xn Group 2: Vapor and liquid passing each other between two plates n n-1 Vn yn Ln-1 xn-1 e.g. yn, xn-1 ChE 334: Separation Processes Dr Saad Al-Shahrani

McCabe Thiele Graphical Equilibrium-Stage By defining of an ideal plate, the vapor and liquid leaving plate n are in equilibrium, so yn and xn represent equilibrium concentration. Subcooled liquid 1.0 Superheated Vapor y Temperature yn+1 x xn yn yn-1 xn+1 Saturated liquid line Saturated vapor line xn-1 operating x,y n-1 n Vn yn Ln-1 xn-1 Vn+1 yn+1 Ln xn Vn+2 yn+2 Ln+1 xn+1 Vn-1 yn-1 Ln-2 xn-2 n+1 ChE 334: Separation Processes Dr Saad Al-Shahrani

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

McCabe Thiele Graphical Equilibrium-Stage Condenser Va ya accumulator La xa I Overall material balances for Two components systems Top plate D xD Total material balance Rectifying Section Ln xn Vn+1 yn+1 F Component A balance Feed n+1 Feed plate Eliminating B m Stripping Section Lm xm Vm+1 ym+1 Eliminating D II Bottom plate Vb yb Re-boiler B xB ChE 334: Separation Processes Dr Saad Al-Shahrani Lb, xb

McCabe Thiele Graphical Equilibrium-Stage 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 D xD accumulator Va ya Condenser La xa Top plate The difference between the flow rates of vapor and liquid anywhere in the upper section of the column (Rectifying section) = D ChE 334: Separation Processes Dr Saad Al-Shahrani

McCabe Thiele Graphical Equilibrium-Stage Total balance around the control surface (I) Vn+1 yn+1 Ln xn D xD accumulator Va ya Condenser Top plate La xa 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 ChE 334: Separation Processes Dr Saad Al-Shahrani

McCabe Thiele Graphical Equilibrium-Stage By the same manner in the stripping section Total balance around control surface (II) Vm+1 ym+1 Lm xm B xB Re-boiler Vb yb Lb, xb Bottom plate II m is to designate a general plate in the stripping section B= net flow in the bottom (stripping section) ChE 334: Separation Processes Dr Saad Al-Shahrani

McCabe Thiele Graphical Equilibrium-Stage Material balance around the re-boiler B xB Vb yb Lb, xb Bottom plate Re-boiler ChE 334: Separation Processes Dr Saad Al-Shahrani

McCabe Thiele Graphical Equilibrium-Stage Vn+1 yn+1 Ln xn D xD accumulator Va ya Condenser Top plate La xa I Operating lines Because there are two sections in the column, there are two operating lines 1. For rectifying section (I) For control surface (I) ( ) Vn+1 yn+1 Ln xn Operation relationship n n+1 ChE 334: Separation Processes Dr Saad Al-Shahrani

McCabe Thiele Graphical Equilibrium-Stage Vm+1 ym+1 Lm xm B xB Re-boiler Vb yb Lb, xb Bottom plate II 2. For stripping section (II) For control surface (II) ( ) Vm+1 ym+1 Lm xm Operation relationship m m+1 Vm ym Lm-1 xm-1 m-1 ChE 334: Separation Processes Dr Saad Al-Shahrani

McCabe Thiele Graphical Equilibrium-Stage Number of ideal plates, McCape Thiele Method When the operating lines represented by: For rectifying section For stripping 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 ChE 334: Separation Processes Dr Saad Al-Shahrani

McCabe Thiele Graphical Equilibrium-Stage 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 D xD V Top plate L Reflux to overhead product Reflux to the vapor ChE 334: Separation Processes Dr Saad Al-Shahrani

McCabe Thiele Graphical Equilibrium-Stage 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 xD/(RD+1) ChE 334: Separation Processes Dr Saad Al-Shahrani

McCabe Thiele Graphical Equilibrium-Stage xD is set by the conditions of design RD , 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. Vn+1 yn+1 Put xn= xD xn=xD L Ln Reflux n+1 Top plate yn+1= xD So the operating line for rectifying section intersect the diagonal at point (xD,xD). This is true for both partial and total condenser xD yn+1 ChE 334: Separation Processes Dr Saad Al-Shahrani

McCabe Thiele Graphical Equilibrium-Stage Condenser and top plate Vn+2 yn+2 Ln+1 Xn+1 n+1 Vn+1 yn+1 Top plate Ln=L =Reflux Xa=xD Top plate xn Concentration of vapor from top plate =yn+1 Concentration of reflux to the top plate xn= xD =yn+1 ChE 334: Separation Processes Dr Saad Al-Shahrani

McCabe Thiele Graphical Equilibrium-Stage Total Condenser Top plate y’ yn+1 n+1 Vn+1 yn+1 yn+2 Equilibrium line n+2 Top plate yn+1 yn+3 Ln xn n+3 xn=xD D, xD n+1 xn+1 yn+2 Reflux xn+1 n+2 xn+2 yn+3 xn+2 Operating line (rectifying section) n+3 composition Equilibrium with Operating with xn=xD y’ yn+1 xn+1 yn+2 xn+2 yn+3 ChE 334: Separation Processes Dr Saad Al-Shahrani

McCabe Thiele Graphical Equilibrium-Stage yn+1 Top plate Ln yn+1 xn=xD n+1 n+1 xn+1 yn+2 yn+2 For total condenser n+2 n+2 xn+2 yn+3 yn+3 n+3 n+3 xn=xD xn+1 xn+2 xn+3 Operating line equation for the rectifying section: 45 o line x = y ChE 334: Separation Processes Dr Saad Al-Shahrani

McCabe Thiele Graphical Equilibrium-Stage Partial condenser Partial Condenser yD=xD D xD Top plate yn+1 V yD n+1 Vn+1 yn+1 yD=xD yn+2 Equilibrium line n+2 Top plate yn+1 yn+3 xn Ln xn n+3 n+1 xn+1 yn+2 Reflux xn+1 n+2 xn+2 yn+3 xn+2 Operating line (rectifying section) n+3 composition Equilibrium with Operating with xn xD=yD yn+1 xn+1 yn+2 xn+2 yn+3 ChE 334: Separation Processes Dr Saad Al-Shahrani