《化学工程设计技术》课程教学资源（参考资料，英文版）5 Equilibrium Staged Separations

EQUILIBRIUM STAGED SEPARATIONS o Single Stage Flash/Flash Drum ◆ Distillation E Simple distillation E Complex distillation E Non-deal distillation o Gas Absorption and Stripping Liquid-Liquid Extraction ◆ Multi-Effect evaporation 0/6/99 Equilibrium Staged separations

0/6/99 Equilibrium Staged Separations 1 EQUILIBRIUM STAGED SEPARATIONS u Single Stage Flash [Flash Drum] u Distillation ð Simple Distillation ð Complex Distillation ð Non-Ideal Distillation u Gas Absorption and Stripping u Liquid-Liquid Extraction u Multi-Effect Evaporation

PHASE EQUILIBRIUM Q Conditions for thermodynamic equilibrium between two phases f=,i=1,…,ne TO= TO) and P()=P(2) For an ideal system fA(=yi P and L=x; Pi(T 0/6/99 Equilibrium Staged separations

0/6/99 Equilibrium Staged Separations 2 PHASE EQUILIBRIUM uConditions for thermodynamic equilibrium between two phases f i *(1) = fi *(2) , i = 1,…,nc T(1) = T(2) and P(1) = P(2) u For an ideal system f i *(V) = yi P and fi *(L) = xi Pi S (T)

PHASE EQUILIBRIUM DEFINITIONS K-value Ki=y/A For an ideal system K=Pi()/P o Relative Volatility ak of component with respect to component k a n=K Kk 0/6/99 Equilibrium Staged separations

0/6/99 Equilibrium Staged Separations 3 PHASE EQUILIBRIUM DEFINITIONS u K-value Ki Ki = yi /xi For an ideal system Ki = Pi S (T)/ P u Relative Volatility ajk of component j with respect to component k ajk = Kj / Kk

FLASH DRUM VAPOR Yi EED Zi iting Or Cooli g OUD 0/6/99 Equilibrium Staged separations

0/6/99 Equilibrium Staged Separations 4 VAPOR V Yi LIQUID L Xi Heating Q or Cooling FEED F Zi FLASH DRUM

FLASH DRUM PERFORMANCE ◆ Separation Factor SF ∥ Detween components I dk SF ik= SR, SRk But SR VY/lX and sR k VY/LX So sFik =/vY; /LX /Yk/lx=K /k=aik ◆ Conclusion The maximum separation is limited by the relative volatility aik which is set by nature Fractional Recovery of component FR=/F;=两y+Lx/=/SS+1 where S V.L 0/6/99 Equilibrium Staged separations 5

0/6/99 Equilibrium Staged Separations 5 FLASH DRUM PERFORMANCE u Separation Factor SFjk between components j and k SF jk = SRj / SRk But SRj = V Yj / L Xj and SRk = V Yk / L Xk So SF jk = [V Yj / L Xj ] / [V Yk / L Xk ] = Kj / Kk = ajk u Conclusion The maximum separation is limited by the relative volatility ajk which is set by nature. u Fractional Recovery of component j FRj = Vyj / F zj = Vyj / [Vyj + L xj ] = [ Sj / Sj + 1] where Sj = VKj / L

TWO-STAGE FLASH F Zi X2 LI XIi 0/6/99 Equilibrium Staged separations

0/6/99 Equilibrium Staged Separations 6 F Zi L1 X1i V1 Y1i V2 Y2i L2 X2i TWO-STAGE FLASH

TWO-STAGE FLASH PERFORMANCE o How can chemical engineers improve on the single stage flash? Answer: Try two stages. Overall Separation Factor sf =sFa= a So sF OveraLL = a. 2 Since ai>l, this is an improvement in the separation factor Overall Fractional Recovery of component lboth stages are operated at the same s, then FR OVERALL FR. FR.= FR2< FR So the overall recovery factor is lower for two stages than for one. 0/6/99 Equilibrium Staged separations

0/6/99 Equilibrium Staged Separations 7 TWO-STAGE FLASH PERFORMANCE u How can chemical engineers improve on the single - stage flash? Answer: Try two stages. u Overall Separation Factor SF1jk = SF2jk = a jk So SFOVERALLij = a ik 2 Since a ij > 1, this is an improvement in the separation factor. u Overall Fractional Recovery of component j If both stages are operated at the same Sj , then FROVERALLj = FR1j FR2j = FRj 2 < FRj So the overall recovery factor is lower for two stages than for one

THE SOLUTION COUNTERCURRENT MULTESTAGING OF FLASH UNITS 0/6/99 Equilibrium Staged separations

0/6/99 Equilibrium Staged Separations 8 THE SOLUTION COUNTERCURRENT MULTI-STAGING OF FLASH UNITS

COUNTERCURRENT MULTI-STAGE FLASH SYSTEM 0/6/99 Equilibrium Staged separations

0/6/99 Equilibrium Staged Separations 9 F COUNTERCURRENT MULTI-STAGE FLASH SYSTEM V L F

SIMPLE COMPLEX DISTILLATION DISTILLATION B 0/6/99 Equilibrium Staged separations

0/6/99 Equilibrium Staged Separations 10 D F B F1 F2 D S1 S2 B SIMPLE DISTILLATION COMPLEX DISTILLATION