ChEN 5253 Design II Chapter 13 Terry A. Ring University of Utah Distillation ChEN 5253 Design II Chapter 13 Terry A. Ring University of Utah

Distillation Add extra Tanks and control system for liquid level

Plate Types Bubble Cap Tray Sieve Tray

Packed Towers Random Packing Structured Packing Note: Importance of Distributor plate

Distillation α=KL/KH Relative Volatility Equilibrium Line

Distillation Rectifying Section Stripping Section Feed Line R= reflux ratio V=vapor flow rate Stripping Section VB= Boil-up ratio Feed Line

Minimum Reflux Ratio

McCabe-Thiele

Step Off Equilibrium Trays

Marginal Vapor Rate Marginal Annualized Cost~ Marginal Vapor Rate Marginal Annualized Cost proportional to Reboiler Duty (Operating Cost) Condenser Duty (Operating Cost) Reboiler Area (Capital Cost) Condenser Area (Capital Cost) Column Diameter (Capital Cost) Vapor Rate is proportional to all of the above

Short cut to Selecting a Column Design Minimum Cost for Distillation Column will occur when you have a Minimum of Total Vapor Flow Rate for column Occurs at R= 1.2 Rmin @ N/Nmin=2, Nmin=log[(dLK/bLK)(bHK/dHK)]/log[αLK,HK] Rmin~(F/D)/(α-1) V=D (R+1) V= Vapor Flow Rate D= Distillate Flow Rate (=Production Rate) R=Reflux Ratio

How To Determine the Column Pressure given coolant 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 delta P Compute the Bubble Pt. Temp for an estimate of the Bottoms Composition at Distillate Pressure Give Bottoms Temperature Not Near Critical Point for mixture

Design Issues Packing vs Trays Column Diameter from flooding consideration Trays, DT=[(4G)/((f Uflood π(1-Adown/AT)ρG)]1/2 eq. 13.11 Uflood= f(dimensionless density difference), f = 0.75-0.85 Packed, DT =[(4G)/((f Uflood πρG)]1/2 eq. 13.14 Uflood= f(flow ratio), f = 0.75-0.85 Column Height Nmin=log[(dLK/bLK)(bHK/dHK)]/log[αLK,HK] eq. 13.1 N=Nmin/ε (or 2 Nmin/ ε) Column Height = N*Htray Tray Height = typically 1 ft (or larger) Packed Height = Neq*HETP (or 2 Neq*HETP) HETP(height equivalent of theoretical plate) HETPrandom = 1.5 ft/in*Dp Rule of thumb p. 392 Tray Efficiency, ε = f(viscosityliquid * αLK,HK) Fig 13.3 Pressure Drop Tray, ΔP=ρLg hL-wier N Packed, ΔP=Packed bed (weeping)

Tray Efficiency μL * αLK,HK

Costing

Column Costs Column – Material of Construction gives ρmetal Pressure Vessel Cp= FMCv(W)+CPlatform Height may include the reboiler accumulator tank Tray Cost = N*Ctray(DT) Packing Cost = VpackingCpacking + Cdistributors Reboiler CB α AreaHX Condenser CB α AreaHX Pumping Costs – feed, reflux, reboiler Work = Q*ΔP Tanks Surge tank before column, reboiler accumulator, condensate accumulator

CPI

Problem Methanol-Water Distillation Feed Desired to get 10 gal/min 50/50 (mole) mixture Desired to get High Purity MeOH in D Pure Water in B

Simulator Methods - Aspen Start with simple distillation method DSDTW or Distil Then go to more complicated one for sizing purposes RadFrac Sizing in RadFrac Rate Based Calculation – Efficiency is calculated for you Costing

Simulation Methods- ProMax Start with 10 trays (you may need up to 100 for some difficult separations) set ΔP on column, reboiler, condenser and separator set ΔT on condenser Create a component recovery for HK in bottom with large ± Set Reflux ratio = 0.1 (increase to get simulation to run w/o errors) Or R~1.2*(F/D)/(α-1) May need pump around loop estimate. Determine αLK,HK* viscosity (use Plots Tab to determine extra trays) determine Nmin and feed tray Use Fig. 13.1 to determine Rmin from R, N from Nmin Redo calc with tray efficiency defined see Figure 19.3 correlation. Recommendations for final design Use N/Nmin=2 (above and below feed tray) @ R/Rmin=1.2

Figure 13.1

Tray Efficiency μL * αLK,HK

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