Separation Trains S, S&L Chapt. 7

Simple Separation Unit Operations Flash Quench Liquid-liquid decantation Liquid-liquid Flash Crystallization Sublimation Filtration

Use of Separation Units

HW2 Separation Reaction Hydrodealkylation of Toluene T+H 2  B+CH 4 Reactor Effluent T=1,350F P = 500 psia

Reactor Effluent Reaction Conditions T=1,350F P = 500 psia

After Flash to 500 psia Recycled Components

Further Separation What separation units should be used? Liquid Separation –Toluene, BP=110.6ºC –Benzene, BP=80.1ºC –What happens to the Methane(-161.5ºC) and Biphenyl (BP=255.9ºC) impurities? Gas Separation –Hydrogen –Methane –What happens to the Toluene and Benzene impurities?

Direct Distillation Sequence

Criteria for the Selection of a Separation Method Energy Separation Agent (ESA) –Phase condition of feed –Separation Factor –Cost Mass Separation Agent (MSA) –Phase condition of feed –Choice of MSA Additive –Separation Factor –Regeneration of MSA –Cost Phases I and II, Components 1 and 2

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=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!

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 Duty (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

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.

How To Determine the 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 psia Compute the Bubble Pt. Temp for an estimate of the Bottoms Composition at Distillate Pressure –Give Bottoms Temperature Not Near Critical Point for mixture

Distillation Problems Multi-component Distillation –Selection of Column Sequences Azeotropy –Overcoming it to get pure products Heat Integration –Decreasing the cost of separations