Gaseous Diffusion Coefficient
UNIVERSITI TUNKU ABDUL RAHMAN
Faculty
:
Engineering & Science
Unit Code
:
UEME3213
Course
:
Unit Title
:
Heat and Mass Transfer
Year/ Semester
:
Year 3/ Semester 2
Lecturer
:
Session
:
Experiment 2: Gaseous Diffusion Coefficient
Objective
To determine gas diffusion coefficient of acetone
Introduction The knowledge of physical and chemical properties of certain materials is important because very often process engineering deal with the transformation and distribution of these materials in bulk. One such property is diffusivity.
Theory
When a concentration gradient exists within a fluid consisting of two or more components, there is a tendency for each constituent to flow in such a direction as to reduce the concentration gradient. This is called mass transfer.
Description
The experiment to determine the diffusivity of gaseous is based on the Winkelmann’s method. In this method, the volatile liquid is allowed to evaporate in a vertical glass tube over the top of which a stream of vapour-free gas is passed. A water bath is provided for maintaining a steady temperature so that there is no eddy current in the vertical tube and mass transfer takes place from the surface by molecular diffusion alone. By monitoring the evaporation rate, which is the rate of fall of liquid surface, and with the knowledge of concentration gradient, the diffusivity can be calculated
The rate of mass transfer is given by:
(1)
Where, D = diffusivity [m2/s] CA = saturation concentration at interface [kmol/m3] L = effective distance of mass transfer [m] CT = total molar concentration [kmol/m3] CBm = logarithmic mean value of CB [kmol/m3]
Considering the evaporation of the liquid:
(2)
Where, ρL = density of liquid [kg/m3] M = molecular weight [kg/kmol]
From Equation (1) and (2), the diffusivity of acetone, D can be calculated as:
Where, s is the slope of the plot against
Procedure
Faculty
:
Engineering & Science
Unit Code
:
UEME3213
Course
:
Unit Title
:
Heat and Mass Transfer
Year/ Semester
:
Year 3/ Semester 2
Lecturer
:
Session
:
Experiment 2: Gaseous Diffusion Coefficient
Objective
To determine gas diffusion coefficient of acetone
Introduction The knowledge of physical and chemical properties of certain materials is important because very often process engineering deal with the transformation and distribution of these materials in bulk. One such property is diffusivity.
Theory
When a concentration gradient exists within a fluid consisting of two or more components, there is a tendency for each constituent to flow in such a direction as to reduce the concentration gradient. This is called mass transfer.
Description
The experiment to determine the diffusivity of gaseous is based on the Winkelmann’s method. In this method, the volatile liquid is allowed to evaporate in a vertical glass tube over the top of which a stream of vapour-free gas is passed. A water bath is provided for maintaining a steady temperature so that there is no eddy current in the vertical tube and mass transfer takes place from the surface by molecular diffusion alone. By monitoring the evaporation rate, which is the rate of fall of liquid surface, and with the knowledge of concentration gradient, the diffusivity can be calculated
The rate of mass transfer is given by:
(1)
Where, D = diffusivity [m2/s] CA = saturation concentration at interface [kmol/m3] L = effective distance of mass transfer [m] CT = total molar concentration [kmol/m3] CBm = logarithmic mean value of CB [kmol/m3]
Considering the evaporation of the liquid:
(2)
Where, ρL = density of liquid [kg/m3] M = molecular weight [kg/kmol]
From Equation (1) and (2), the diffusivity of acetone, D can be calculated as:
Where, s is the slope of the plot against
Procedure