Separation Trains
S, S&L Chapt. 8
T &S Chapter 16 and 17
Use of Separation Units
Simple Separation Unit
Operations
• Flash
• Quench
• Liquid-liquid decantation
– Liquid-liquid Flash
• Crystallization
• Sublimation
• Filtration
MSA = Mass Separating Agent
ESA = Energy Separating Agent
Criteria for the Selection of a
Separation Method
• Energy Separation
Agent (ESA)
– Phase condition of feed
– Separation Factor
– Cost of Energy
I
1
C
SF 
C
I
2
C
II
2
II
1
C
• Mass Separation
Agent (MSA)
– Phase condition of feed
– Choice of MSA
Additive
– Separation Factor
– Regeneration of MSA
– Cost of MSA
Phases I and II,
Components 1 and 2 (light key and heavy key)
Separation
Reaction
Hydrodealkylation of
Toluene
T+H2B+CH4
2B Diphenyl+H2
Reactor Effluent
T=1,350F
P = 500 psia
Reactor Effluent
Reaction Conditions
T=1,350F
P = 500 psia
Component
Hydrogen
Methane
Benzene
Toluene
Biphenyl
Total
kmole/hr
1292
1167
280
117
3
2859
After Flash to 100F @ 500 psia
Effluent
Vapor
Liquid
Component kmole/hr kmole/hr kmole/hr
Hydrogen
1292
1290
2
Methane
1167
1149
18
Benzene
280
16
264
Toluene
117
2
115
Biphenyl
3
0
3
Total
2859
2457
402
Recycled Reactants
Further Separation
What separation units should be used?
• Liquid Separation
– Toluene, BP=110.6ºC
– Benzene, BP=80.1ºC
– What happens to the Methane (BP= -161.5ºC) and
Biphenyl (BP=255.9ºC) impurities?
• Gas Separation
– Hydrogen
– Methane
– What happens to the Toluene and Benzene impurities?
Direct Distillation Sequence
Column Sequences
• No. of Columns
– Nc=P-1
• P= No. of Products
• No. of Possible Column Sequences
– Ns=[2(P-1)]!/[P!(P-1)!]
• P= No. of Products
–
–
–
–
–
P=3, Nc=2, Ns=2
P=4, Nc=3, Ns=5
P=5, Nc=4, Ns=14
P=6, Nc=5, Ns=42
P=7, Nc=6, Ns=132
No. of Possible
Column Sequences
Blows up!
How do I evaluate which is best
sequence?
Marginal Vapor Rate
• Marginal Annualized Cost~ Marginal Vapor Rate
• Marginal Annualized Cost proportional to
–
–
–
–
–
Reboiler Duty (Operating Cost)
Reboiler Area (Capital Cost)
Condenser Duty (Operating Cost)
Condenser Area (Capital Cost)
Diameter of Column (Capital Cost)
• Vapor Rate is proportional to all of the above
Selecting Multiple Column
Separation Trains
• Minimum Cost for Separation Train will
occur when you have a
– Minimum of Total Vapor Flow Rate for all
columns
– R= 1.2 Rmin
– V=D (R+1)
• V= Vapor Flow Rate
• D= Distillate Flow Rate
• R=Recycle Ratio
After Flash to 100F @ 500 psia
Effluent
Vapor
Liquid
Component kmole/hr kmole/hr kmole/hr
Hydrogen
1292
1290
2
Methane
1167
1149
18
Benzene
280
16
264
Toluene
117
2
115
Biphenyl
3
0
3
Total
2859
2457
402
Recycled Reactants
Simplified Marginal Vapor Flow
Analysis
Liquid
kmole/hr
Hydrogen
2
Methane
18
Benzene
264
Toluene
115
Biphenyl
3
Total
402
Sequence Total
Direct Sequence
Distillate Flow Distillate Flow
Column 1
Column 2
x
x
x
x
D=
284
399
115
Indirect Sequence
Distillate Flow
Distillate Flow
Column 1
Column 2
x
x
x
x
x
x
x
399
683
284
R assumed to be similar for all columns and R>1
Separation Train Heuristics
• 1. Remove thermally unstable, corrosive, or chemically
reactive components early in the sequence.
• 2. Remove final products one by one as distillates (the
direct sequence).
• 3. Sequence separation points to remove, early in the
sequence, those components of greatest molar percentage
in the feed.
• 4. Sequence separation points in the order of decreasing
relative volatility so that the most difficult splits are made
in the absence of the other components.
• 5. Sequence separation points to leave last those
separations that give the highest-purity products.
• 6. Sequence separation points that favor near equimolar
amounts of distillate and bottoms in each column.
Vapor Pressure vs Temp
Benzene
BP=80.1ºC
120F
A Word About Column Pressure
• Cooling Water Available at 90ºF
• Distillate Can be cooled to 120ºF min.
• Calculate the Bubble Pt. Pressure of Distillate
Composition at 120ºF
– equals Distillate Pressure
– Bottoms Pressure = Distillate Pressure +10 psi delta P
• Distillate P > Atm, Pressure generated by system.
• For Vacuum, how is it that generated?
Steam Ejector Generates the
Vacuum.
High Pressure
High Velocity
Steam
Velocity > Mach 1
Vacuum
Bernoulli’s Equation
Steam Ejectors
Distillation Problems
• Multi-component Distillation
– Selection of Column Sequences
• Azeotropy
– Overcoming it to get pure products
• Heat Integration
– Decreasing the cost of separations