Wetted Wall Column
Content:
Introduction
Experimental Description/Apparatus
Theory
Safety
Experimental Procedure
Wetted Wall Column Proforma
Calculated Results
Graphs
Physical Data of Acetone/Air
Discussion
Conclusion
References
Introduction: Convective mass transfer is an energy transfer between a surface and fluid moving over the surface. In this analysis of convection, the soluble vapour is absorbed by means of a liquid in which the solute gas is more or less soluble from its mixture. Therefore, a suitable data can be used to distinguish the calculations of vapour phase mass transfer coefficients and this useful method can also predict the influence of vapour flowrate on the vapour phase mass transfer coefficients and also compare the experimental results with suggested correlations of model mass transfer. In terms of absorption processes, however the feed is a gas introduced at the bottom of the column and the solvent is fed to the top, as a liquid. The absorbed gas and solvent leave at the bottom and the unabsorbed components leave as gas from the top. The essential difference between distillation and absorption is that the vapour has to be produced in each stage by partial vaporisation of the liquid which is therefore at its boiling point, whereas in absorption the liquid is well below its boiling point. In general, the ratio of the liquid to the gas flowrate flowrate is considerably greater in absorption than in distillation.
Experimental Description: The diagram of the wetted wall column is shown below;
Diagram 1:
AIR IN
Diagram 2:
It consists of a liquid film running down the inside of a long glass tube with gas flowing counter-current, up through the middle of the tube. Mass transfer occurs at the interface between the flowing vapour and the liquid phases. In contrast to a packed column, the interfacial area between the vapour and liquid phases is easily measured. However, the contracting area per unit volume of column is much lower in the wetted wall column. This makes the wetted wall column suitable for mass transfer experiments but unsuitable for practical applications. In this experiment, the wetted wall column is used to evaporate liquid acetone. In which the solute (acetone) is transferred from the solvent liquid to the gas phase and this operation is called ‘’stripping’’. The air is used as the stripping gas to lower the partial pressure of the acetone in the gas phase.
Theory: At any point, the force driving mass transfer is the difference in gas phase mole fraction between the evaporating acetone ac etone at the liquid surface and that in the bulk of the airstream. The direction of transfer of material across the interface is not dependent solely on the concentration difference, but also on the equilibrium relationship.
The mass transfer rate over the whole column can be calculated by integrating the rate equation over its length. This leads to an average driving force difference given by the log mean of the mole fraction differences at column entry and exit.
Mass transfer takes place by diffusion of acetone through a stagnant layer of air. The flux, using gas phase mole fraction terms, will be equal to the evaporation rate of acetone given by;
̅ The rate of evaporation, for a dilute solution, will be given by the difference in acetone concentration of the bulk airstream and its flowrate, or by the rate at which acetone liquid is lost.
As the liquid phase is pure acetone, the interface compositions can be calculated from the pure component vapour pressure. As shown below; vapo vapour ur pressu pressure re for acet acetone( one(mm mmH Hg) 1 5 10 20 40 60 100 200 400 760
temper temperat atur ure(C) e(C) -59.4 -49.5 -31.1 -20.8 -9.4 -2 7.7 22.7 39.5 56.5
The vapour pressure of acetone in mmHg can be plotted against the mole fraction of acetone and this gives a straight line which indicates that as the mole fraction of acetone increases the vapour pressure of acetone also increases.
800
) 750 g H700 m650 m ( e 600 n550 o t e 500 c a 450 f o400 e r 350 u300 s s e r 250 p200 r u150 o p100 a v 50
0 0
0.2
0.4
0.6
0.8
1
1.2
mole fraction of acetone
y = 669.53x - 5.923
Convective mass transfer can be affected by both fluid flow and the permeability of the medium, a correlation should include the relevant Reynolds and Schmidt groups. The Gilliland and Sherwood correlation as shown sh own below is one of the useful parameters in wetted wall columns only if the Reynolds number exceeds 2100 and Schmidt number lies between 0.5 and 3.0:
The Sherwood number is dimensionless group that includes convective mass transfer term as well as a critical length and the diffusivity of acetone in air. As the coefficient used here has been the appropriate conversion must be used.
The log mean mole fraction of the stagnant component, air can be deduced using Dalton’s law as shown below;
⏞
Safety: Care should be taken when handling small quantities of acetone used since it is a flammable material. Acetone can be disposed when it’s used with the appropriate waste solvent bottle.
Hazards: It’s highly flammable and irritating to eyes. Repeated exposure may cause skin
dryness or cracking. Vapours may cause drowsiness and dizziness.
Fire fighting measures :
The container should be removed in case of fire.
Accurate measurements are needed to retain water used for extinguishing.
Dispose of contaminated water.
Acetone forms explosive mixtures with air and extremely flammable. It may explode in a fire. Vapour may travel considerable distance to source of ignition and flash back.
Handling and storage:
Wear safety glasses at all times.
Wear lab coat that is fastened.
Experimental Procedure: In the first run, syringe was used to fill the pump inlet hose with acetone. A measuring cylinder was used also to collect 150ml/min of acetone before the pump was switched on and this was repeated for every minute for each run. Then, the pump was switched on with speed adjusted to 2. While, the acetone was draining into collection vessel steadily a beaker and a stopwatch was used to collect exactly one minute’s worth of liquid.
The measuring vessel, however was used to measure the volume pumped and this information was obtained accurately. The flowrate outlet was between 100ml and 145ml and these figures were noted for every minute. The air pressure was set roughly to 0.8 bar and the flowmeter was adjusted to one of the following values: 10.8, 19.3, 30, 39.8 and 35 litres/min.
After 4 minutes period, the temperatures of both the feed and the collection vessel were measured substantially and the pump was switched off when all the runs were calculated.
Wetted wall column proforma: Time/min Inlet.Flow(ml/min) outlet.Flow(ml/min)
1.04 150 144.23
Experimental data collected Inlet air litres/min Inlet liq.flow ml/min Outlet liq.flow ml/min Inlet liq.temp ) Outlet liq.temp ) Ambient pressure air (mbar) Ambient temp. T(air) ()
Interpreted Data Acetone evaporation Rate(kg/s) Mass flowrate of air(kg/s) Average vapour temp. in column
1.05 150 142.9
1.05 150 142.9
1.07 150 140.2
Run1
Run2
Run3
Run4
Run5
10.8 150 144.23 18.4 16.8 1026
19.3 150 142.9 16.0 14.0 1026
30.0 150 142.9 17 10.8 1026
39.8 150 140.2 14.4 11.14 1026
35 150 137.61 14.1 10.5 1026
23
23
23
23
23
Run1 Run2 Run3 Run4 0.0000762 0.0000937 0.0000937 0.00013 0.000215 290.75
0.000384 288.15
0.000597 287.05
dimensions Internal Diameter of column, d(m)
0.025
Length of column, L (m)
1.5
Interfacial area between liquid/vapour A
0.118
Cross-sectional area of column, S
0.00049
1.09 150 137.61
Run5 0.000164
0.000792 0.000697 286.05 285.45
Calculations of interpreted data (Run 1): Acetone evaporation rate = =
(Run1)
= 9.6167 . The actual rate of acetone = 9.6167 792 = 7.62 kg/s. Mass flow rate of air = =
(Run1)
= 1.8 Ambient air pressure at 23 is 1.194 kg/ . The actual mass flow rate = 1.194 1.8 = 2.149 kg/s. Average temperature =
=
(Run1)
= 290.15 K.
Calculated Results (run 1): Molar mass of air = 29kg/kmol Experimental molar flow of air (Run 1) = 2.149
kg/s
= = 7.41103 .
Antoine parameters of acetone
A
B
C
7.31414
1315.67
240.479
Run1:
(Bottom)
=
(Top)
=
Mole fraction of acetone inlet = 0 Mole fraction of acetone outlet =
=
= 1.312 kmol/s
= 0.15034 = =
Interfacial mole fraction
= 0.206
Interfacial mole fraction
Driving force at the top of the column (
Driving force at the bottom of the column (
Logarithmic mean driving force, (
=
Bulk log mean pressure of air in column
= 605.4 mmHg.
Correction factor for unimolecular diffusion:
Rate of loss liquid acetone (kmol/s) = ( ) = 7.41103 0.15034 = . Predicted acetone evaporation =
( )
= 7.41103 =
Mean evaporation rate =
=
= .
Vapour phase superficial velocity:
Vapour phase Reynolds number:
Sherwood number for Re>2100:
Heat removed by evaporation of acetone:
2.149 = 0.0209 Watts Cooling effect estimate by evaporation of acetone:
Theoretical estimate of outlet liquid temperature;
Graphs: Figure1: 0.00025
0.0002 l a t n0.00015 e m i r e p x e 0.0001 y k
0.00005
y = 6E-08x + 4E-05 R² = 0.4502
0 0
500
1000
1500
2000
2500
Reynolds number
Figure2: 16 14 12 n10 o i t a l e 8 r r o c h S 6
4
y = 0.0062x + 1.5181 R² = 0.9992
2 0 0
500
1000
1500
Reynolds number
2000
2500
Physical data of Acetone/Air: Physical properties of air and acetone
Uni ve rsal gas constant ( J/kmol .K) Mol ar mass of ai r, ( kg/kmol ) Inl e t ai r de nsi ty to col umn ( kg/m3) Mol ar mass of ace tone ( kg/kmol ) De nsi ty of ace tone at ambi e nt T ( kg/m3) A ir visc iscosity ity at colum lumn temp empe rature(N e(Ns/m2) Di f f usi vi ty of ace tone i n col umn ai r ( m2/s) Latent Latent heat of vaporisation vaporisation of acetone acetone DHv(J/kmol) He at capacity ity of liq liquid aceto etone @ 283K, Cp( J/kg.K) A ntoi ne parame te r A of ace tone A ntoi ne parame te r B of ace tone A ntoi ne parame te r C of ace tone A ce tone Sc Schmi dtz numbe r ( di me nsi onl e ss) A mbi e nt ai r pre ssure mbar A mbi e nt ai r pre ssure ( mmHg) Boi l i ng poi nt of ace tone @56.1( de g.C) i n mmHg
8314.5 29 1.194 58.08 792 0.00001827 0.0000115
121.3 7.31414 1315.67 240.479 1.33030369 1026 770 755.07
Discussion: The vapour phase mass transfer coefficient (ky) versus the vapour phase Reynolds number as shown above in figure 1 doesn’t show a straight line graph due the fact
that there is not a significant resistance to mass transfer at its interface. In order to obtain proper results it is essential to operate with a system of more simple geometry. The rate of diffusion in liquids is much slower than in gases, and mixtures of liquids may take a long time to reach equilibrium unless agitated. In engineering, the mass transfer coefficient is a diffusion rate constant that relates the mass transfer rate, mass transfer area, and concentration gradient as driving force. This process involves simultaneous, mass and heat transfer, however there are other processes which also involve simultaneous mass and heat transfer there are named as follows: Distillation, evaporation and drying process.
During the experiment, the liquid and hence the apparatus at the base of the column becomes cooled due to the evaporation of acetone into the air-stream which mainly takes place in this section of the column. The cooling effect of air at the highest (50ml/min) 3
Air flow rate=50 ml/min= 0.0008333 m /s
= 1.194 kg/m3
1.194×0.0008333=0.000995 kg/s 0.000995×121.3× (17.6 -16.8) =0.09655 W 3
Air flow rate=10 ml/min= 0.0001667 m /s
= 1.194 kg/m 3
1.194×0.0001667 = 0.00019904 kg/s 0.00019904×121.3× (17.6 -16.8) = 0.0193 W To calculate the cooling effect, the air flowrate was converted to the mass flowrate of 3 3 air from ml/min to m /s then this was multiplied with density of air at 1.194 kg/m and then to get the cooling effect from mass flowrate of air this variable was also multiplied with the heat capacity of acetone (121.3 J/kg.K) and finally multiplied with cooling temperature as shown above: The cooled liquid's temperature is given by:
The steady state temperature at the base of the column can be determined by the pumps working conditions such as in the outlet and inlet flowrate of air. air. This is useful method because it estimates the efficiency and the amount of power supplied on the pump. The experiment could be improved if more advanced equipment was used such as using turbulent jet will give a higher absorption rate than the predicted values because of the increased velocity. velocity. Therefore, an increase in fluid velocity gives higher mass transfer coefficient. The wetting area could also be improved only if the flowrate of the column is increased so this will give more accurate data of acetone. acetone. Increasing the surface area will also give more reliable figures.
Conclusion: The experiment had low Reynolds number this is due to the influence of axial mixing. For some cases in laminar flow the absorption rate is greater than that in the turbulent flow, flow, while the mass transfer rates in turbulent case are significantly larger than in laminar flow. According to this theory, a low Reynolds number can indicate that the flow measurements were not accurate in terms of the wetted wall conditions. Therefore, to get more systematic data a low viscosity with high superficial velocity is needed so this will give higher Reynolds number.
Reference: Mass Transfer Transfer by Sherwood Pigford and Wilke, W ilke, McGraw Hill Publication. http://www.mycheme.com/calculation-methods/bubble -a-dew-point.html. http://www.chem.tamu.edu/class/majors/tutorialnot http://www.chem.tamu.edu/class/majors/tutorialnotefiles/percentc efiles/percentcomp.htm. omp.htm. http://edibon.com/products/ca http://edibon.com/products/catalogues/ru/units/chemic talogues/ru/units/chemicalengineering/chemicalengi alengineering/chemicalengi neeringgeneral/CAPC.pdf. http://www.nt.ntnu.no/users/sk http://www.nt.ntnu.no/users/skoge/prost/proceedings/dist oge/prost/proceedings/distillation02/dokument/6 illation02/dokument/6 12.pdf. Coulson and Richardson's Chemical C hemical Engineering Volume 2. Yaws' Handbook of Antoine Coefficients for Vapor Pressure (2nd Electronic Edition).
