Sieder et. al. Chapter 9 and 13 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 Separation Method Energy Separation Agent (ESA) Phase condition of feed Separation Factor Cost of Energy 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+H2B+CH4 2B Biphenyl+H2 Reactor Effluent T=1,350F P = 500 psia
Reactor Effluent Reaction Conditions T=1,350F P = 500 psia
After Flash to 100F @ 500 psia 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 Recycled Reactants
Direct Distillation Sequence B/TD, T/D Direct Distillation Sequence Biphenyl
Indirect Sequence BT/D, B/T Benzene Toluene Biphenyl Which is the Cheapest Direct or Indirect?
Column Sequences No. of Columns No. of Possible Column Sequences Nc=P-1 P= No. of Products No. of Possible Column Sequences Ns=[2(P-1)]!/[P!(P-1)!] 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 P=10, N.c=9, Ns=4,862 No. of Possible Column Sequences Blows up!
P = 4, Nc=3, Ns=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 Rmin V=D (R+1) V= Vapor Flow Rate D= Distillate Flow Rate R=Recycle Ratio
After Flash to 100F @ 500 psia Recycled Reactants
Simplified Marginal Vapor Flow Analysis R assumed to be similar for all columns and R>1, You should not make this assumption! Because separations have different degrees of difficulty.
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 Biphenyl Byproduct Benzene Toluene & Biphenyl
Vapor Pressure vs Temp Benzene BP=80.1ºC 80.1°C P(48.8ºC)=101.4 kPa =25 kPa ~0.25 atm 80.1°C
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 Bernoulli’s Equation Vacuum
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