Post date: Jan 31, 2013 9:37:48 PM

Moges Retta

Abstract

This paper describes a model of the absorption process in a falling film POE oil/CO2 absorber. Absorption of CO2 into a laminar falling film on the surface of a vertical plate is considered. Necessary boundary conditions and simultaneous mass and heat transfer model equation are solved. The coolant side is assumed water at specified inlet and outlet temperature. To solve this, finite element software has been used. The concentration, temperature and velocity profiles are obtained. The mass and heat fluxes along plate length are also depicted.

Introduction

In this study, the flow of the liquid film is assumed to be laminar, Newtonian, one-dimensional and fully developed. No shear force is exerted on the liquid by the vapor. The flow is schematically depicted as shown below.

Modeling equations

See the attached doc

The velocity field is given by

Results

The resulting concentration, temperature and velocity profile are shown in the figures below. Also, the mass and heat flux along the plate length are drawn.

Concentration profile along the length of plate

Temperature profile along film thickness

Temperature profile along the length

Velocity profile

Heat flux

Diffusive flux

Discussions

The concentration profile figure 1 shows that the concentration drops at the interface where absorption of CO2 takes place. It is highest at the wall since the CO₂ absorbed at the interface diffuses through the oil film to the wall. Figure 2 shows the concentration distribution along the wall. The concentration drops rapidly at the inlet region but decreases slowly at x increases. Figure 3 shows the temperature profile along the film thickness. It is highest at the interface and decreases to the lowest at the wall. This is because the heat of absorption released at the interface is transported through the liquid film to the plate wall. Figure 4 shows the profile of temperature along the plate. It decreases rapidly at the inlet region but slowly as x increases. This is because the temperature difference between the film and the wall decreases as x increases. Figure 5 shows the velocity reaches maximum at the interface as expected from parabolic velocity profile. Figure 7 shows the diffusive flux along the plate length. In a thermodynamic equilibrium state at the interface, the vapor pressure driving force is zero. The liquid film starts to be cooled by cooling water which decreases the saturation pressure. The driving force increases and the mass flux increases while the concentration drops. The vapor pressure driving force decreases whereas the cooling water causes the it to increase. Thus, the flux increases and reaches its maximum value and after this it continues to decreases rapidly. The vapor pressure driving force decreases slowly and thus the absorption mass flux and interface concentration decreases slowly.

References

1. Yoon et al. (2005) Numerical study on heat and mass transfer characteristics of plate absorber. Applied thermal engineering,25 p.2219-2235

2. Bo et al. (2010) Numerical simulation on the falling film absorption process in a counter flow absorber. Chemical engineering Journal. 156,p.607-612

3. Yigit et al. (1999)A numerical study of heat and mass transfer in falling film absorber. Int.Comm.Heat mass transfer,26 p.269-278