The Effect of Liquid Flow Rate and Gas Flow Rate on a Packed Absorption Tower and Repair of Equipment

The Effect of Liquid Flow Rate and Gas Flow Rate on a Packed Absorption Tower and Repair of Equipment

Chapter One

SCOPE/OBJECTIVE OF THE PROJECT

The project deal first and foremost with the repairs and restoration of the packed absorption tower to a good working condition and the performance of a laboratory experiment using it.

The experiment want to measure the relationship between the liquid flow rate and gas flow rate with pressure in the packed absorption tower.

CHAPTER TWO

LITERATURE REVIEW

The most useful concept of the process of absorption is given by the two film theory due to WHITMAN. According to this theory, material is transferred in the bulk of the phases by convection currents and concentration difference are regarded as negligible except in the vicinity of the interface between the phases. One either side of this interface it is supposed that the current die out and that there exists a thin film of fluid of through which the transfer is solely molecular diffusion. This film will be slightly thicker than the laminar sub- layer, because it offers a resistance equivalent to that of he whole boundary layer. According to Fick’s law, the rate of transfer by diffusion is proportional to the concentration gradient and to the area of interface over which the diffusion is occurring. Fick’s law is limited to cases where the concentration of the absorbed component is low. At high concentrations, bulk flow occurs and the mass transfer rate, which is increased by a factor G/CB is governed by Stefan’s low. Under these arcumstance,t the concentration gradient is no longer constant throughout the film and the lines AB and DE are curred.

The direction of transfer of material across the interface is not dependent solely on the concentration difference, but also on the equilibrium relationship

There is therefore a very big concentration gradient across the interface but this is not the controlling factor in the mass transfer, as it is generally assumed that there is no resistance at the interface itself, where equilibrium conditions will exist.

An alternative theory is one given by HIGBIE (1)and later extended by DANCWERTS(2)and DANCWERTS and KENNEDY (3)in which the liquid surface is considered to be composed of a large number of small elements each of which is exposed to the gas phase for an internal of time, after which they are replaced by fresh elements arising from the bulk of the liquid.

The application of the penetration theory to the interpretation of the experimental results obtained in wetted- wall columns has been studies by LVNN,STRAATEMETER, and KRAMERS (4) they absorbed pure sulphur dioxide in water and various aqueous solution of salts and found that in the presence of a trace of Tecpol which suppressed ripple formation, the rate was overstated because of the formation of a region in which the surface was stagnant over the studies were extended to columns containing spheres and again the penetration theory has found to hold, there being little mixing of the surface layers with the bulk of the fluid as it flowed from one layer of spheres to the next.

Absorption experiments in columns packed with spheres (37.8mm diameter) were also carried out by DAVIDSON otal (5) who absorbed pure carbon dioxide in water. When a small amount of surface active agent was present in the water no appreciable mixing was found between the layers of spheres. With pure water, however, the liquid was almost completely mixed in this region.

Absorption of gases and rapour by drop has been studies by GARNER and KENDRICK (6) and GARNER and LANE (70 who developed a rerticaltennel in which drop could be suspended for considerable period of time in the rising gas stream

PRESSURE DROP

It is important to be able to predict the drop in pressure for the flow of the two fluid streams through a packed tower.

In the majority of cases the gas flow is turbulent and the general from of the relation between the drop pressure- DP and the volumetric gas flow rate per unit area of tower shown on curve A of fig 2.

-DP is then proportional to about Ua 1.8 in agreement with curve A at high Reynolds numbers. If in addition to the gas flow, liquid is not significantly affected at low liquid rates and the pressure drop line is similar to line A but for given value of U_G the value of –DP is some what increased. When the gas then rises very much move quickly and is proportional to U_G^2.5, as shown by the section XY on curve C. over this section the liquid flow is interfering with the gas flow and the hold-up of liquid is progressively increasing. The free space in the packing is therefore being consciously taken up by the liquid, and this the resistance to flow rises quickly. At gas flows beyond Y, -DP rises very steeply and the liquid held up in the column. The point X is known is known as the loading point and point Y as the flooding point for the given liquid flow. If the flow rate of liquid is increased, a similar plot D is obtained in which the loading point is achieved at a lower gas rate but at a similar value of –DP. Whilst it is advantageous to have a reasonable hold-up in the column as this promotes inter phase contact, it is not practicable to operate under flooding conditions and columns are best operated over the section XY. Since this is a section with a relatively short range in gas flow, the safe practice is to design for operation at the loading point X. it is of interest to note that, if a column is flooded and then allowed to drain, the value of –DP for a given gas flow is increased over that for an entirely dry packing as shown by curve B. ROSE and YOUNG(8) who correlated their experimental pressure drop data for Rasching by the rings by the following equation

-DP_W = DP_a (1+3.30) –(1)

where –DP_W is the pressure drop across the wet drained column

-DP_d is the pressure drop across the dry column d_nis the nominal size of the Rasching rings in mm. There are several ways to calculate pressure drop across a packed column when gas and liquid are flowing simultaneously and the column is operating below the loading point.

CHAPTER THREE

EXPERIMENTAL PRODURE

We opened the liquid control needle value slightly.
We started the pump
We carefully opened the liquid control needle valve until desired rate of flow between 1.5 gallon per hour (1.8945×10⁶m³/s) and 15 gallon by the liquid flow meter.
We opened the gas external connection to the gas supply.
We carefully opened the external connection to gas supply
We carefully opened the gas control valve until the desired rate of flow of gas was indicated by the gas flow meter.
We adjusted the liquid control needle valve and the gas needle valve until desired absorption situation had been obtained.
As we then shut down we closed the gas control needle valve
We closed the external connection to the gas supply
We closed the liquid control needle valve
We stopped the pump
We opened the solution exit drain cock to allow the tower to drain

GLASS BLOWING PROCESS AND PROCEDURE

Glass according to Encydopedia of chemistry, glass is not readily a solid but a supper cooled liquid. This is because, when heated it does not suddenly pass from the solid state to the liquid state at a definite temperature but softens slowly as the temperature rises and gradually become liquid.

Glass is produced by heating together dioxosilicate (S_i 0₂) in from of sand with sodium trioxo carbonate IV (Na₂C0₃) or sodium tetraoxosulphate IV (Na₂S0₄) and some broken glasses and a little coke added as catalyst.

Equation of reaction

CaCO₃ + S_iO₂ High temp CaCO₃ +CO₂(g)

1440-1600⁰c

GENERAL PHYSICAL PROPERTIES OF GLASS

MECHANICAL STRENGTH: The surface of glass probably contains numerious extremely small cracks and then a tensile stress is applied there is a concentrate of stress at the ends of these cracks which causes then to grow further into the glass, until at some crack breakage occurs and propagated through the glass. Glass usually breaks in a direction at right angles to the direction of maximum tensile stress.

A newly drawn glass fibre is free form these surface cracks and actually strengthened by removing the surface layer with hydrofluoric acid even through the cross- section is reduced. Basically, glass is a bad heat conductor.

CHAPTER FOUR

RESULTS AND DISCUSSION

RAW DATA FROM EXPERIMENT

CHAPTER FIVE

CONCLUSION

Since the graph obtained was linear, we include that the liquid flow rate and gas flow rates have effect on the pressure drop. And from the graph increase in gas flow rates causes increase in the pressure drop in the tower.

It therefore means that large pumping cost is required to achieve larger pressure and so larger absorption.

Old age and fouling due to longtime non- use affected the reading making for time-kg and response delay. We suggest overall maintenance or replacement with digital response equipment.

RECOMMENDATION

The equipment has better model with quicker response to change. We therefore recommend a better model be installed for the department. Model having digital output as compared with the analog is also recommended to yield a more accurate reading.

We also suggest a glass blowing workshop with up to data equipment be establish in the campus and close or in the chemical engineering department. It will save time for repair and fabrication of glass wares or the packed absorption tower and the like.

More studies should be carried on the packed absorption tower bearing in mind what its technology can do in any nation.

Since the tower can be used in chemical scrubbing for neutralizing environmental pollutants from chemical synthesis
Since it can be used for waste water treatment, removal of prickling corrosive dust and odours form chemical
Since it can be used for gas purification, for example carbon IV oxide gas which is of high demand in both lining of beverages.
Since every industrial nation cannot do without a proper fire prevention programme, fire extinguishers gas content are deansed of their inflammable constituent using the tower.
One direct application is in production of rated capacity of carbon IV oxide, it is important to maintain the proper ratio of air/ fuel. The correct ratio is determined by oil tank via filter and flow meter to the burner to suit that of the blown air. These adjustments produce maximum of carbon IV oxide and the bye-product is nitrogen (H₂) which is rented to the atmosphere

REFERENCES

HIGHER, R: Trans am inst chem. eng 31 (1935)365 The rate of absorption of pure gas into a still liquid during short period of exposure
DANCKWERTS, P.V. Ind. Eng. Chem. 43 (1951) 1460. significance of liquid-film coefficients in gas absorption
DANCKWERTS, P.V. And KENNEDY, A. M.: Trans inst chem. eng 32 (1954) 549. kinetics of liquid- film process in gas absorption.
LYNN, STRAATEMEISER,J.R, And KRAMERS,H. Chem. Eng. Sci. 4 (1955)49,58,63
DAVIDSON, J.F. Trans inst. Chem. eng. 37(1959)122. the hold-up and liquid film coefficient of packed towers.
GARNER, F.H. and KENDRICK, P. Trans inst. Chem. eng (37) (1959) 155. Mass transfer to drops of liquid suspended in a gas stream
GARNER, F.H. and LANE,J.J. Trans inst chem. Eng 37 (1959) 162. Mass Transfer to drop of liquid suspended in a gas stream.
ROSE,H.E. and YOUNG, PH. Proc. Inst. Mech. Eng. 1B(1952)114 hydraulic characteristics of packed towers operating under countercurrent flow conditions

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