Combined Heat and Mass Transfer Model in Amines System

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Combined Heat and Mass Transfer Model in Amines System

Xiao Luo and Hallvard F. SvendsenNorwegian University of Science and Technology (NTNU)

Trondheim, NORWAY

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Contents

• Literature work– Motivation– Multi-component system– Influence of convection– Combined heat and mass transfer

• Work plan• Expectation

XIAO LUO, Combined Heat and Mass Transfer Model in Amines System

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Motivation

• In CO2 absorption processes large mass and heat transfer

driving forces occur several places, absorber bottom, water

wash section and in the desorber.

• Good fundamental models for the combined heat and mass

transfer including chemical reaction and convective

mechanisms are still lacking.

• A proper description of the transfer processes is vital for

improved absorption process simulation and optimization.


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General description

Vapor bulk Liquid bulk

NCO2

NH2O

NAmines

Interface

QL QS

Water

Amines

CO2

CO2

Other Gases

H2OVap

AminesVap


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Multi-components system

• Conventional approaches to mass transfer in multi-component

mixtures are based on an assumption that the transfer flux of

each component is proportional to its own driving force.

• Ross Taylor & R. Krishna, (1993)


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Fick’s Law

• Diffusion in binary mixtures:

• Multi-component Fickian Diffusion Coefficients– Obtained from composition profiles measured in a diffusion apparatus.

– Wilke (1950), Dunlop, 1972; Cussler, (1976);

– Tyrrell and Harris, (1984);

– Schiesser, (1991)

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1

n

i t ik kk

J c D x


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Maxwell-Stefan Equation

• Derivation of Maxwell-Stefan equation– Sum of driving force = Sum of friction force– Theoretic significance

• Driving force including– Gravity driving force (usually ignored)– Chemical potential driving force; Frank, (1994); – Pressure driving force– Centrifugal driving force– Electrical potential driving force; Schneider, R. (1999)

2

1

ln ni j j ii i

i i ij t ij

x N x Nd x dP dRT V M r FZ RT

dz dz dz c D


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Mass transfer theories• Penetration Theory (Higbie, 1935)

– Small fluid eddy; Retention time τ

– Samanta, A. (2007); Tobiesen, F. A. (2007);

– van Elk, E. P. (2007); Zhang, X. (2004);

– Mandal, B. P. (2001); Bhat, R. D. Vas, (1998);


• Surface renewal theory (Danckwerts, 1951)– Based on Penetration Model; Statistical age distribution function Φ(θ)

– Li, Zhengxing (2007); Shen, Shuhua (2007);

– Rahimpour, M. R. (2004); Pacheco, M. A. (2000);

• Film theory (Lewis & Whitman, 1924)– Steady state; Thickness of film δ;

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Combined heat and mass transfer

• Heat transfer including

– Sensible heat by difference in temperature between two phases

– Latent heat by evaporation or condensation

– Heat produced by solvation and chemical reactions

– Y. T. Kang & Y. Fujita (2001); I. Alatiqi (1993)


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Modeling separately

• Model in liquid phase

– Similar property of amines

– Water > 90 mol%

– Seen as binary mixture

– Using Fick’s Law


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Modeling separately

• Model in gas phase

– More complicated

– Differences between each components

– Using Maxwell-Stefan Equation


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Combined model

• Assumptions– Equilibrium at the interface

– Driving force is caused by chemical potential (Liquid)

– Consider fugacity driving forces (Gas)

– Convective contributions

– ……

• Combine such models above by using Henry’s Law at

the interface


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Experimental validation

• Falling film apparatus

– Mass transfer at interface

– Heat transfer at interface

– Considering the influence

of momentum transfer near

the wall

WallG-L Interface

LiquidGas


NH2O

NCO2

NAmines

TG TLHeat transfer

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Expectation

• More accurate

– Reduce the number of assumptions of existing models

• More scientific

– Modify the parameters which only depend on experimental data or

empirical equations.


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THANK YOU FOR YOUR ATTENTION


Combined Heat and Mass Transfer Model in Amines System

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