“Chemical Engineering Equilibrium Separations” CBE 417 “Chemical Engineering Equilibrium Separations” Lecture: 8 24 Sep 2012

Overview Multicomponent Flash Flash Unit Operation (AspenPlus) Staged systems McCabe-Thiele

Distillation Column

Distillation Column Design (binary) Simulation (binary) Specify Za, XD, XB (more volatile) Reflux Ratio (Lo/D) Optimum feed stage location P column (condenser) F (feed flowrate) Feed condition Find N (number of stages) Nfeed (feed stage) D, and B (flowrates) Heat duties Diameter, height XD Za Simulation (binary) Use existing column Simulate to see performance Any needed modifications? XB

Distillation Column Overall Column Balances (SS) QC Distillation Column P Overall Column Balances (SS) XD hD Material Balance (MB): Za hF Energy Balance (EB): heat added is (+) heat removed (-) adiabatic (well insulated) QR XB hB

McCabe-Thiele Graphical Method (binary) Used to simplify analysis of binary distillation (ease of understanding) Assumptions: Pure components a, b have equal latent heats of vaporization / mole ( ) and they stay constant. are much larger than Sensible heat changes Heats of mixing Column is adiabatic (well – insulated) Constant pressure (P) throughout the column (i.e. no P in the column) Called Constant Molal Overflow (CMO) Assumes for every 1 mole of light material vaporized that 1 mole of heavy material condenses from the vapor phase Net result: Total molar flowrates (i.e. L and V) remain constant within that column section (rectifying or stripping, or other) Do not need a stage by stage energy balance McCabe-Thiele is done with MB and thermodynamic information.

MB on Rectifying Section At Steady State (SS) for light material (LK) MB: Moles In = Moles Out For CMO General Operating Line

General Operating Line: Rectifying Section Equilibrium Stage xN-1 yN+1 yN xN (xD, y1) Equilibrium Line: Relates composition of liquid leaving stage N (i.e. xN) to the composition of vapor leaving stage N (yN) Operating Line: Relates composition of liquid leaving stage N (i.e. xN) to the composition of vapor entering stage N (yN+1)

Tie Together Equilibrium & Operating Lines y1 x1 xD y2 y1 V1 LO y3 x3 x2 y4 (xD, y1) (x1, y1) D (x2, y2) (x1, y2) (x3, y3) (x2, y3) 1 2 3

MB on Stripping Section At SS and CMO still assumed, so: Operating Line Stripping Section (xN, yN) (xN-1, yN) (xB, yB) (xN, yB) (xB)

Feed Stage F Depending on Feed “condition” will get changes to vapor and liquid flowrates… ZF Define q = Moles of liquid flow in Stripping section that result from one mole of feed. Suppose: q = 1 q = 0

Feed Stage Operating Line ZF rectifying section stripping section Feed stage & overall column MBs: feed line eqn.

Plot Feed MB Line Lets put all three lines together: subcooled liq. Feed Condition q Slope sat’d liq. q>1 sat’d liquid = 1  partial V&L q = 1 sat’d vapor = 0 0 0<q<1 mixed V & L 0 < q < 1 neg. (-) subcooled L > 1 pos. (+) q = 0 sat’d vap. superheated V < 0 pos. (+) zF Superhtd vapor q<0 Lets put all three lines together: rectifying section stripping section feed line

Operating Lines (McCabe-Thiele) q is constant R incr. Rectifying & stripping lines must intersect at the same point on the feed line. Consider limits: R =  R where rectifying line intersects the equil. curve zF

Operating Lines (McCabe-Thiele) R is constant Rectifying & stripping lines must intersect at the same point on the feed line. q incr. zF

McCabe-Thiele Graphical Method Binary Distillation zF “step off” equilibrium stages on the XY diagram. Feed stage location: point where switch from rectifying operating line to the stripping operating line.

McCabe-Thiele Graphical Method Binary Distillation zF “step off” equilibrium stages on the XY diagram. Feed stage location: point where switch from rectifying operating line to the stripping operating line. Optimum feed stage location: switching point to obtain smallest number of stages. Switch when intersection of 3 operating lines is first crossed.

McCabe-Thiele Graphical Method Binary Distillation zF Minimum Number of Stages Total reflux; so D = ? and R = ??

McCabe-Thiele Graphical Method Binary Distillation zF Minimum Reflux Ratio One or both operating lines intersect the equilibrium line. Result: infinite number of stages.

McCabe-Thiele Graphical Method Binary Distillation zF Minimum Reflux Ratio One or both operating lines intersect the equilibrium line. Result: infinite number of stages.

McCabe-Thiele Graphical Method (binary) Used to simplify analysis of binary distillation (ease of understanding) Assumptions: Pure components a, b have equal latent heats of vaporization / mole ( ) and they stay constant. are much larger than Sensible heat changes Heats of mixing Column is adiabatic (well – insulated) Constant pressure (P) throughout the column (i.e. no P in the column) Called Constant Molal Overflow (CMO) Assumes for every 1 mole of light material vaporized that 1 mole of heavy material condenses from the vapor phase Net result: Total molar flowrates (i.e. L and V) remain constant within that column section (rectifying or stripping, or other) Do not need a stage by stage energy balance McCabe-Thiele is done with MB and thermodynamic information.

Problem Solving Exercise

Questions?