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Simulation and analysis of extractive distillation process in a valve tray column using the rate based model

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Abstract

Valve trays are becoming popular in the chemical process industries owing to their flexibility to handle a wide range of vapor throughputs. Using the rigorous rate based model, the importance of the non-equilibrium approach is demonstrated for a typical extractive distillation process in a Glitsch V-1 valve tray column. Simulation results based on an in-house developed code indicated that the rate based model predictions for a valve tray column operation showed significant differences relative to the equilibrium model. Even small errors in product purities translated into nonoptimal feed stage locations and inaccurate number of stages required. The counter-intuitive effect of high reflux ratio on separation is explained.

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Abbreviations

a:

interfacial area per unit active area

d:

characteristic length, taken as 1 m

D:

diffusion coefficient [m2·s-1]

E MV :

Murphree tray efficiency

f Lij :

liquid feed flow rate of species i to stage j [mol/s]

f Vij :

vapor feed flow rate of species i to stage j [mol/s]

F(suj):

total vapor feed flow rate to stage j [mol/s]

F Lj :

total liquid feed flow rate to stage j [mol/s]

FV Lj :

total vapor feed flow rate to stage j [mol/s]

G:

superficial gas mass velocity [kg·m-2·s-1]

H Lj :

enthalpy of liquid stream leaving stage j [J/mol]

H LFj :

enthalpy of liquid feed stream entering to stage j [J/mol]

H Vj :

enthalpy of vapor stream leaving stage j [J/mol]

H VFj :

enthalpy of vapor feed stream entering to stage j [J/mol]

k x a:

liquid-side mass-transfer coefficient based on mole fraction driving force [kg-mol·m-2·s-1]

k y a:

gas-side mass-transfer coefficient based on mole fraction driving force [kg-mol·m-2·s-1]

L:

superficial liquid mass velocity [kg·m-2·s-1]

L j :

inter-stage total liquid flow rate from stage j [mol/s]

M:

molecular weight

N ij :

inter-phase mass transfer rate of species ‘i’ in stage ‘j’ [mol/s]

P:

total pressure [kPa]

Qj :

external heat transfer to or from the stage j [J/s (Watt)]

Q Lj :

external heat transfer to or from the stage j in liquid phase [J/s (Watt)]

Q Vj :

external heat transfer to or from the stage j in vapour phase [J/s (Watt)]

r Lj :

fraction of the liquid exiting the stage j withdrawn as liquid side stream [Uj/Lj]

r Vj :

fraction of the vapor exiting the stage j withdrawn as vapor side stream [Wj/Vj]

Re G :

gas-phase Reynolds number=Gd/µ G

Re L :

gas-phase Reynolds number=Ld/Μ L

Sc G :

Schmidt number in gas-phase=v G/D G

Sc L :

Schmidt number in gas-phase=v L /D L

Sh G :

gas-side Sherwood number=k y ad/ρ G D G

Sh L :

gas-side Sherwood number=k x ad/ρ L D L

T:

temperature [K]

U j :

liquid side stream flow rate from stage j [mol/s]

V i :

molar volume of the pure liquid of species [kmol/m3]

V j :

interstage total vapor flow rate from stage j [mol/s]

W:

weir height [m]

W′ :

dimensionless weir height=W/d

W j :

liquid side stream flow rate from stage j [mol/s]

x i :

mole fraction, liquid

y i :

mole fraction, gas

µ:

viscosity in Re [kg·m-1·s-1]

p :

bulk-phase molar density [kg-mol·m-3]

φi :

fugacity coefficient of species i

γi :

activity coefficient of species i

δ ij :

Kronecker delta, 1 if i=j, 0 if i≠j

V:

vapor phase

L:

liquid phase

I:

interface

V:

vapor phase

L:

liquid phase

I:

interface

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Authors and Affiliations

  1. Department of Chemical Engineering, Indian Institute of Technology Madras, 600 036, Chennai, India

    Sasmita Pradhan & Aravamudan Kannan

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  1. Sasmita Pradhan
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  2. Aravamudan Kannan
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Correspondence to Aravamudan Kannan.

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Pradhan, S., Kannan, A. Simulation and analysis of extractive distillation process in a valve tray column using the rate based model. Korean J. Chem. Eng. 22, 441–451 (2005). https://doi.org/10.1007/BF02719424

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  • DOI: https://doi.org/10.1007/BF02719424

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