ABIOLA Well wetted colum

Authors Avatar

                Mutiu Abiola Muritala 2621997  Bng Chemical Full time.

WETTED WALL COLUMN

MASS TRANSFER COEFFICIENTS

SUMMARY

The Wetted-wall column is a simple vertical tube with gas flowing either co-current or counter-current to a volatile liquid, and it is used to determine gas/liquid mass transfer coefficients, which is would be necessary in  Petroleum and chemical industry. The process of absorption of gases in observation towers is widely used in industry.  It is also described as a thin film of liquid running down the inside of a vertical pipe, with gas flowing either concurrently or counter currently.1 Wetted wall columns may be used to determine gas/liquid mass transfer coefficients, essential when calculating the design of absorption towers.2 Such coefficients form the basis of correlations used to develop packed towers. They are commonly used for the theoretical studies of mass transfer and in some cases they are used industrially for mass transfer applications which require high rates of heat transfer.

Objectives of study

  • To establish the variation of mass transfer coefficients for humidification of air with air flow rate.
  • To determine the correlation between mass transfer coefficient and Reynolds number for Laminar and Turbulent flow
  • Experimental equipments:
  • Two vertical glass tubes measuring:
  • 4.1cm internal diameter, 74cm high
  • 1.4cm internal diameter , 64cm high
  • Rota meters for measuring air flow
  • Wet and dry bulb thermometers for measuring temperatures
  • Psychometric chart , for measuring humidity
  • Stop clock

Key results:

Key results:

LAMINAR FLOW (Re<2000)

TURBULENT FLOW (Re>2000)

Slope, N= 0.41 for laminar and for turbulent it was linear graph

The above tabulated value are the key data obtained from the experiment.

Main Conclusion

From the experimental results and calculations, the following conclusions can be made:

  • In the laminar column , as the air flow rate increases, the mass transfer coefficient also increases while in  turbulent column , as flow rate increases, the mass transfer coefficient was not stable .
  • For a given air flow in a wetted wall column with a smaller diameter, more turbulent flow occur compared to a column with a bigger diameter. The smaller the diameter and surface area of the column, the greater the Reynolds number and subsequently the increase in the mass transfer coefficient.
  • The higher the air flow rate, the higher the Reynolds number (i.e. the greater the amount of turbulence) and the increase in the mass transfer coefficient.

From this experiment I have learnt a great deal about the variation of mass transfer coefficients for humidification of air with air flow rate and the effects of Reynolds number on mass transfer coefficient. I have looked deeply into the theory behind the correlation between mass transfer coefficient and Reynolds number for laminar flow and turbulent flow and how we can use the theory and knowledge to our advantage when designing a wetted wall column so that it is at its optimum performance.

INTRODUCTION

             Wetted-wall column is a simple vertical tube with gas flowing either co-current or counter-current to a volatile liquid. It is also described as a thin film of liquid running down the inside of a vertical pipe, with gas flowing either concurrently or counter currently1.  They are mostly used for the theoretical studies of mass transfer and in some cases they are used industrially for mass transfer applications which require high rates of heat transfer.

In wetted-wall column, the liquid flows down the surface of the tube in a thin film and the interfacial area between the gas phase and the thin film of liquid can be measured and kept constant. In the gas phase, the mass transfer coefficient can be calculated from the evaporation rate of the liquid into the gas stream and the rate of mass transfer depends on the conditions at the interface between the liquid and gas stream3.

Also, the flow of the liquid film is typically laminar while the flow of the gas stream could be laminar or turbulent. Therefore, the rate of mass transfer and its coefficient will depend on the gas phase flow i.e. either laminar or turbulent3.

Wetted-wall columns can be used as absorbers for hydrochloric acid, where absorption is accompanied by a very large evolution of heat. This experiment is based on the hypothesis that state, ‘the rate of mass transfer depends on the condition of the interface which are enhanced or affected by the flow rate of both fluids. Hence if there is low flow rate, there would be a corresponding low mass transfer but if there is a high flow rate more mass transfer would occur’.

Join now!

THEORY

The practical application of this mass transfer theories are deployed in the industry when high rates of heat transfers are necessary. From Reynolds number,

(Re) = 4 ρ Q / π µ d.

A similar correlation to that shown below could be used to study the relationship between the air flow rate, Re and mass transfer (coefficient). From Gilliland and Sherwood,                                              Kg.d .PBM = 0.023Re0.83Nsc0.44

                                                            ...

This is a preview of the whole essay