Inside-Out Algorithm - Usual specifications
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Distillation columns in CHEMCAD Short Cut Tower Tower Plus SCDS - Fenske-Underwood-Gilliland - Limited design, Rating - Rigorous - Inside-Out Algorithm - Usual specifications - Simple columns - Rigorous - Inside-Out Algorithm - Extended specifications - Complex columns - Rigorous - Simultaneous corrections method - Usual specifications Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net SHOR column Short Cut Method (SHOR) • Fenske-Underwood-Gilliland method • One feed and two products • Design mode – Calculates reboiler and condenser duties, determines number of theoretical stages, finds optimum feed tray location (Fenske, Kirkbride) • Rating mode – Calculates reboiler and condenser duties, computes fractional recovery of key components • Generally not useful beyond initial column estimates: constant molar underflow requirement is too unrealistic Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net SCDS Column Simultaneous Correction Method: • Large number of stages • Stable with non-linear (non-ideal) thermodynamics • Run time = B * (NCOMP)2 * NSTAGES • Mass transfer and tray efficiencies • Dynamics Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net TOWR column Inside-Out Method: • Fast with EOS thermodynamics • Large number of components, small number of stages • Run time = A * NCOMP * (NSTAGES)2 • No mass transfer or stage efficiencies • Complex columns • Dynamics Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net General Recommended Selection EOS thermodynamics Tower Non-ideal thermodynamics SCDS Three phase distillation SCDS Reactive distillation SCDS Mass transfer SCDS Complex column Tower Plus Trays > 100 SCDS Components > 20 Tower Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net Distillations Models… Degrees of Freedom.. Simple absorber 0 Reboiler 1 Condenser 1 Side exchanger 1 Pumparound 2 Side stripper 1 Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net Distillations Models… Typical Specifications Relatively “Safe”: One material balance specification (D, B) One heat balance specification (R/D, V/B) May require estimates: Compositions Temperatures Recoveries Heat loads Poor Choices: Two material balance specifications Pure stream temperature Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net Distillations Models… Column Estimate Guidelines • No estimate better than poor estimate • Avoid pure component temperature • Avoid azeotrope temperature • Tray 2, N-1 temperatures for unusual problems • Use thermodynamic (phase) plots for guidance • Use profile options for difficult problems • Use preliminary material balances Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net Distillations Models… Column Convergence – Problem Recognition SCDS: Profile oscillations Increasing errors Many NR steps, decreasing size Inside-Out: Increasing errors Many Jacobian Inversions Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net Distillations Models… Column Convergence – Problem Management • Review thermo • Poor specifications • Specification conflicts? • Poor estimates? • High pressure? • Close boiling? Dry trays? (adjust T profiles) • Create restart profiles • Use damping factors • Converged with poor mass balance? (tighten tolerances) • All else fails? (reduce tray efficiencies) Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net Dehydration of ethanol using normal pentane as the entrainer Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net Note: To find multicomponent azeotropes you can use CC-Batch Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net Benzene/Ethanol/Water Residue Curve Map 1 0.9 0.8 liquid boiling (at fixed pressure) binodal plotenvelope (fixed temperature) Ethanol mole fraction 0.7 binodal plot (at fixed temperature) liquid boiling envelope (fixed P) 0.6 0.5 0.4 0.3 0.2 0.1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Benzene mole fraction Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net material balance line excluding condenser 1. Fix bottoms composition at 99.99% EtOH. 2. Pick tie line that crosses relevant boundaries. Fixes composition of decanter liquid phases. 3. Determine distillate flowrate by pegging water flowrate. 4. Pick y(n) on tie line so that it is in equilibrium with homogeneous liquid. 5. Determine organic phase flowrate using lever rule. tie line Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net Simulation results Liquid compositions on trays Composition of top tray liquid Composition of combined feed y(n) Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net Design considerations in Azeotropic Distillation • Entrainer requirements • Column sequence • Reflux ratio • Number of stages • Effect of trace impurities Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net distillation boundary Distillation boundaries can be crossed but it is almost always very difficult to take advantage of it. Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net Distillations Models… VOC Stripping… H(TCE,25C) = 650 atm. T = 85 F P = 14.7 psia H2O = 696,000 lb/hr TCE = 69 lb/hr Air H(TCE in H2O) = 650 atm. TCE = 1 ppb Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net Distillations Models… Debutanizer – Split Iso and N butanes Feed: C3 83.06 lbmoles/hr iC4 4329.755 nC4 725.09 iC5 62.12 nC7 1286.98 Chemstations, Inc – Houston, TX – 800.243.6223 – http://www.chemstations.net
