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INTRODUCTION

Gaseous diffusivity or gas dispersion apparatus, which involves diffusion with

 bulk flow, is one of the laboratory equipment that have been designed to allow

measurement of molecular diffusivities. From this model, it will make the students

 become more familiar with the basic notions or concepts of mass transfer theory. This

apparatus is a bench mounted apparatus for the determination of diffusion coefficients of 

a vapour in air, which uses the method of measuring the rate of evaporation of a liquid

through a stagnant layer into a flowing air stream, comprising a precision bore capillary

tube, which may be filled from a syringe and the top of which means are provided to pass

air (or an inert gas) stream to remove vapour. The apparatus also consist an air pump, a

travelling microscope with accurate focus adustment and mounted for vertical a!is

movement against a "ernier scale and a thermostatically controlled water bath, in which

to place the capillary tube, capable of accurate temperature control.

Figure #$ Gas %iffusion &pparatus

The e!perimental capabilities of this apparatus are direct measurement of mass

transfer rates in the absence convective effects. Gas laws was used to calculate

concentrations differences in terms of partial pressures while Fick's aw was used to

measure diffusion coefficients in the presence of a stationary gas, measurement of the

effect of temperature on diffusion coefficients and gaining familiarity with the use of 

laboratory instruments to achieve accurate measurements of data required for industrial

 process design.

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The diffusivity of the vapour of a volatile liquid in air can be conveniently

determined by inklemann's method in which liquid is contained in a narrow diameter 

vertical tube, maintained at a constant temperature, and an air stream is passed over the

top of the tube to ensure the partial pressure of the vapour is transferred from the surface

of the liquid to the air stream by molecular diffusion. The molecular diffusivity, %, is a

kinetic parameter associated with static and dynamic conditions of a process. &ll the

comple!ity and unwieldiness of many calculations is, indeed, connected with the

determination of this quantity.