Separation Trains Sieder et. al. Chapter 9 and 13 Terry A Ring Chemical Engineering University of Utah
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 Energy Separation Agent (ESA) SF Separation Method Phase condition of feed Separation Factor Cost of Energy = C C I 1 II 1 C C I 2 II 2 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+H 2 B+CH 4 2B Biphenyl+H 2 Reactor Effluent T=1,350F P = 500 psia
Reaction Conditions T=1,350F P = 500 psia Reactor Effluent Component kmole/hr Hydrogen 1292 Methane 1167 Benzene 280 Toluene 117 Biphenyl 3 Total 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?
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
Direct Distillation Sequence B/TD, T/D Biphenyl
Indirect Sequence BT/D, B/T Benzene Biphenyl Toluene Which is the Cheapest Direct or Indirect?
Column Sequences No. of Columns N c =P-1 P= No. of Products No. of Possible Column Sequences N s =[2(P-1)]!/[P!(P-1)!] P= No. of Products P=3, N c =2, N s =2 P=4, N c =3, N s =5 P=5, N c =4, N s =14 P=6, N c =5, N s =42 P=7, N c =6, N s =132 No. of Possible Column Sequences Blows up!
P = 4, N c =3, N s =5 Example A/BCD, B/CD, C/D A/BCD, BC/D, B/C AB/CD, A/B, C/D ABC/D, A/BC, B/C ABC/D, AB/C, A/B
How do I evaluate which is best sequence?
Marginal Vapor Rate Marginal Annualized Cos t~ 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 R min 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
R assumed to be similar for all columns and R>1, You should not make this assumption! Because separations have different degrees of difficulty. Simplified Marginal Vapor Flow Analysis Direct Sequence Indirect Sequence Distillate Flow Distillate Flow Distillate Flow Distillate Flow Liquid Column 1 Column 2 Column 1 Column 2 kmole/hr Hydrogen 2 x x x Methane 18 x x x Benzene 264 x x x Toluene 115 x x Biphenyl 3 Total 402 D= 284 115 399 284 Sequence Total 399 683
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 to 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.
Distillation Sequences for Separation Direct Biphenyl Byproduct Biphenyl Benzene Toluene & Biphenyl
80.1 C Vapor Pressure vs Temp Benzene BP=80.1ºC P(48.8ºC)=10 1.4 kpa =25 kpa ~0.25 atm
A Word About Column Pressure Cooling Water Available at 90ºF (32.2ºC) Distillate Can be cooled to 120ºF (48.8ºC) min. Calculate the Bubble Pt. Pressure of Distillate Composition at 120ºF (48.8ºC) 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
Pressure Vessel with Vacuum Withstand atmospheric pressure exerted on it. They always leak a little! What is the leak rate? 1 Bar = E is elastic Modulus of Steel, t is the thickness, R is the cylinder radius.
Distillation Problems Multi-component Distillation Selection of Column Sequences Azeotropy Overcoming it to get pure products Heat Integration Decreasing the cost of separations