《化学工程设计技术》课程教学资源（参考资料，英文版）16 Design 3 Recycle-Structure

DESIGN METHODOLOGY III RECYCLE STRUCTURE OF THE PROCESS GOAL --Determine alternate recycle structures for the process based on reactor operating requirements. 11/29/99 Recycle structure

11/29/99 Recycle Structure 1 DESIGN METHODOLOGY III: RECYCLE STRUCTURE OF THE PROCESS GOAL --Determine alternate recycle structures for the process based on reactor operating requirements

RECYCLE STRUCTURE FOR SINGLE REACTION SYSTEM Vapor phase recycle Purge Products fresh Reaction Separation eel System System B products Liquid phase recycle 11/29/99 Recycle structure

11/29/99 Recycle Structure 2 Reaction System Separation System Purge Products Byproducts Liquid Phase Recycle Vapor Phase Recycle Fresh Feed RECYCLE STRUCTURE FOR SINGLE REACTION SYSTEM

REASONS FOR RECYCLE Recycle unreacted reactants E Conversion limitations E Excess reactant to shift equilibrium ◆ Recycle diluent E Reactor heat management E Flammability constraints ◆ Recycle" to extinction” E Recycle byproduct to equilibrium- no net production 11/29/99 Recycle structure

11/29/99 Recycle Structure 3 REASONS FOR RECYCLE u Recycle unreacted reactants ð Conversion limitations ð Excess reactant to shift equilibrium u Recycle diluent ð Reactor heat management ð Flammability constraints u Recycle “to extinction” ð Recycle byproduct to equilibrium - no net production

EXAMPLE #1. EFFECT OF FEED COMPOSTION ON EQUILIBRIUM CONVERISION 07 06 0.5 03 02 0.1 0 0 3 fAO/fB0 A+B今C Delta=extent of reaction for Ky=Yc/rarb=2.0 fA0=feed rate of a, fB0 =feed rate of B=1.0 fC0=0. 11/29/99 Recycle structure

11/29/99 Recycle Structure 4 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 1 2 3 4 5 fA0/fB0 Delta EXAMPLE #1. EFFECT OF FEED COMPOSTION ON EQUILIBRIUM CONVERISION A + B Û C Delta = extent of reaction for Ky = Yc/YaYb = 2.0 fA0 = feed rate of A, fB0 = feed rate of B = 1.0 fC0 = 0

REACTOR CONSIDERATIONS ◆ Heat management E What is the adiabatic temperature change? E excessive(more than 25 C), must modify design Diluent >Heating(endothermic) or cooling(exothermic ◆ Equilibriun E Modify operating conditions Change temperature or pressure Use excess of one reactant in feed 11/29/99 Recycle structure 5

11/29/99 Recycle Structure 5 REACTOR CONSIDERATIONS u Heat management ð What is the adiabatic temperature change? ð If excessive (more than 25 C), must modify design ÿDiluent ÿHeating (endothermic) or cooling (exothermic) u Equilibrium ð Modify operating conditions ÿ Change temperature or pressure ÿ Use excess of one reactant in feed

EXAMPLE #2: HEAT MANAGEMENT USING A DILUENT Reaction :A->B. AHr= 50.000 BTU/b-mol 25 BTU1b-moL-F Make 100 mohrofB: @r=5.0 MM BTU/r Let Fa=feed rate ofa Fd=feed rate of diluent Conversion Fa Fd ATadiabatic(F) 0.25000 400 0.l10000 200 0.0250000 0.110004000 40 0.25004500 40 Use diluent if cheaper and if there is a purge loss. If flow rate is too high, go to cooling 11/29/99 Recycle structure

11/29/99 Recycle Structure 6 EXAMPLE #2: HEAT MANAGEMENT USING A DILUENT Reaction: A --> B, DHr = 50,000 BTU/lb-mol Cp = 25 BTU/lb-mol-F Make 100 mol/hr of B: Qr = 5.0 MM BTU/hr Let Fa = feed rate of A Fd = feed rate of diluent Conversion Fa Fd DTadiabatic (F) 0.2 500 0 400 0.1 1000 0 200 0.02 5000 0 40 0.1 1000 4000 40 0.2 500 4500 40 Use diluent if cheaper and if there is a purge loss. If flow rate is too high, go to cooling

EXAMPLE #3: RECYCLE TO EXTINCTION Reaction system:A→B BeC where b is the desired product C is in equilibrium with B, so that Cc/cb=Ke=0. 1 Goal: recycle enough c so that the amount fed to the reactor is the equilibrium amount, hence no net ∫ formation Specs: make 100 mohr of B at a conversion ofa of% if no make of C, must feed 100 mowhr of a and recycle 100 mo/hr of A. Reactor effluent is 100 moWhr A, 100 mowhr of B and 10 mo/hr of c at equilibrium So, we must recycle 10 mor of c along with 100 mowhrofA 11/29/99 Recycle structure

11/29/99 Recycle Structure 7 EXAMPLE #3: RECYCLE TO EXTINCTION Reaction System: A Þ B B Û C where B is the desired product C is in equilibrium with B, so that Cc/ Cb = Ke = 0.1 Goal: recycle enough C so that the amount fed to the reactor is the equilibrium amount, hence no net formation. Specs: make 100 mol/hr of B at a conversion of A of 50%. If no make of C, must feed 100 mol/hr of A and recycle 100 mol/hr of A. Reactor effluent is 100 mol/hr A, 100 mol/hr of B and 10 mol/hr of C at equilibrium. So, we must recycle 10 mol/hr of C along with 100 mol/hr of A