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.
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
11/29/99 Recycle Structure 3
REASONS FOR RECYCLE
u Recycle unreacted reactants
e Conversion limitations
e Excess reactant to shift equilibrium
u Recycle diluent
e Reactor heat management
e Flammability constraints
u Recycle,to extinction”
e Recycle byproduct to equilibrium - no net production
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 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.
11/29/99 Recycle Structure 5
REACTOR CONSIDERATIONS
u Heat management
e What is the adiabatic temperature change?
e If excessive (more than 25 C),must modify design
Diluent
Heating (endothermic) or cooling (exothermic)
u Equilibrium
e Modify operating conditions
Change temperature or pressure
Use excess of one reactant in feed
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.
11/29/99 Recycle Structure 7
EXAMPLE #3,RECYCLE TO EXTINCTION
Reaction System,A B
B 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.