Approximate Methods Fenske-Underwood-Gilliland (FUG) Method Selection of Two Key Components

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1 Lecture 3. Approximate Multicomponent Methods () [Ch. 9] Approximate Methods Fenske-Underwood-Gilliland (FUG) Method Selection of Two Key Components Column Operating Pressure Fenske Equation for Minimum Equilibrium Stages Distribution of onkey Components at Total Reflux

2 Approximate Methods for Multicomponent Separation Use of approximate methods - Preliary design - Parametric studies : establish optimal design conditions - Process synthesis studies : detere optimal separation sequence - Obtaining i an initial i i approximation i for a rigorous method Fenske-Underwood-Gilliland (FUG) method - Approximate method for simple distillation - Manual calculation : if physical properties are independent of composition - Computer-aided calculation : included in most process design programs

3 Variable Specification for Distillation Distillation with one inlet stream, total condenser, and partial reboiler o. of specifications Feed flow rate Feed mole fractions C- Feed temperature Feed pressure Adiabatic stages (excluding reboiler) - Stage pressures (including reboiler) Split of light key component Split of heavy key component Feed stage location Reflux ratio (as multiple of R ) Reflux temperaturet Adiabatic reflux divider Pressure of total condenser Pressure at reflux divider 2+C+9

4 Algorithm for FUG Method Start (Specified feed) Repeat only if estimated and calculated splits of nonkey components differ considerably Specify splits of two key components Estimate splits of nonkey components Detere column pressure and type of condenser Flash the feed at column pressure Calculate imum theoretical stages Calculate splits of nonkey components Calculate imum reflux ratio Calculate actual theoretical stages for specified R Calculate feed stage location Calculate condenser and reboiler duties Exit Bubble/Dew point calculations Adiabatic flash procedure Fenske equation Fenske equation Underwood equations Gilliland correlation Kirkbride equation Energy-balance equations

5 Selection of Two Key Components S l ti f di till ti l Selection of distillation columns including a deisobutanizer and a debutanizer

6 Component Splits for Alkylation Reactor Effluent In practice, the deisobutanizer p, is usually placed first

7 Column Operating Pressure Factors to detere column pressure - Condition of condenser If distillate temperature is less than 20 (49 ), a refrigerant, rather than cooling water, is used Reflux drum pressure - To 25 psia (.48 MPa): total t condenser is recommended d - 25 psia-365 psia (2.52 MPa): partial condenser is appropriate - Condition i of reboiler Column bottom temperature should not exceed decomposition or near-critical conditions - Pressure drops Condenser: 0-2 psia Column: 5 psia or 0. psi/tray

8 Fenske Equation for Minimum Equilibrium Stages () Distillation column operation at total reflux For steady-state operation without heat loss (heat input to reboiler)=(heat output from condenser) From material balance V y L x i, i, Equilibrium relations for stage y K x i, i, i, x K x i, 2 i, i, y x i, i, 2 Equilibrium relations for stage 2 y K x i, 2 i, 2 i, 2 K K x i, 2 i, i,

9 Fenske Equation for Minimum Equilibrium Stages (2) Equilibrium relations for stage y K K K K x i, i, i, i, 2 i, i, y K K K K x j, j, j, j, 2 j, j, y y x i 2 x j i,, j,, x i, x j, k x i, x j, k y x i, i, y x j, j, Relative enrichments of any two components i and j over a cascade of theoretical stages

10 Fenske Equation for Minimum Equilibrium Stages (3) If an average relative volatility is used x i, x j, x, j, log i x x i, x j, x i, x j, log i, j Fenske equation More convenient form using distillate and bottoms flow rates (d and b) Geometric mean log ( d / )( / ) i d j bj bi / 2 m ( i, j) ( i, j) log m The Fenske equation is exact only if () does not vary and/or (2) the mixture forms ideal solutions

11 Distribution of onkey Components at Total Reflux Once is known, the Fenske equation can be used to calculate molar flow rates d and b for nonkey components d d i r ( ir, ) m bi br i :a nonkey component r : the reference component (or the heavy key component) f d b i i i b d i i f i r br i, r m ( d / )( ) f ( d / b )( ) i r r i, r m ( dr / br)( i, r) m

Shortcut Distillation. Agung Ari Wibowo, S.T., M.Sc Politeknik Negeri Malang Malang - Indonesia

Shortcut Distillation. Agung Ari Wibowo, S.T., M.Sc Politeknik Negeri Malang Malang - Indonesia Shortcut Distillation Agung Ari Wibowo, S.T., M.Sc Politeknik Negeri Malang Malang - Indonesia The use of separation column in HYSYS The column utilities in HYSYS can be used to model a wide variety of