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 is considerably greater in absorption than in distillation.

Experimental Description:

The diagram of the wetted wall column is shown below;

Diagram 1:

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 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.

∆ y LM=¿¿

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;

N A=N A A=k y A ∆ y LM

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.

k y=nair

A( youtlet− y inlet )

∆ yLM

As the liquid phase is pure acetone, the interface compositions can be calculated from the pure component vapour pressure. As shown below;

vapour pressure for acetone(mmHg) temperature(C)1 -59.45 -49.5

10 -31.120 -20.840 -9.460 -2

100 7.7200 22.7400 39.5760 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.

-0.2

-1.66533453693773E-16 0.2 0.4 0.6 0.8 1 1.2

0100200300400500600700800

f(x) = 669.527620434678 x − 5.92301605935458

mole fraction of acetone

vapo

ur p

ress

ure

of a

ceto

ne(m

mHg

)

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 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:

Sh=0.023 ℜ0.83 Sc0.33

The Sherwood number is dimensionless group that includes convective mass transfer term k ' c as well as a critical length and the diffusivity of acetone in air. As the coefficient used here has been k y ,the appropriate conversion must be used.

k ' cPtotal

RT= yBM k y

Sh=k ' cD

DAB

=k y

DRT yBM

D AB Ptotal

The log mean mole fraction of the stagnant component, air can be deduced using Dalton’s law as shown below;

yBM ¿⏞∆

y B1− yB 2

ln ( y¿¿B 1/ y B2)=(P¿¿ B1 /Ptotal)−(P¿¿B 2 /Ptotal)

ln (P¿¿B 1¿¿ PB2)=1

P total

PB1−PB2

ln (P¿¿ B1 /PB2)=PBM

Ptotal

¿¿¿

¿¿¿

∴Sh=k y

DRT PBM

DAB P2total

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 1.04 1.05 1.05 1.07 1.09Inlet.Flow(ml/min) 150 150 150 150 150outlet.Flow(ml/min) 144.23 142.9 142.9 140.2 137.61

Experimental data collected

Run1 Run2 Run3 Run4 Run5

Inlet air litres/min 10.8 19.3 30.0 39.8 35Inlet liq.flow ml/min 150 150 150 150 150Outlet liq.flow ml/min 144.23 142.9 142.9 140.2 137.61Inlet liq.temp ¿) 18.4 16.0 17 14.4 14.1Outlet liq.temp¿) 16.8 14.0 10.8 11.14 10.5Ambient pressure air (mbar)

1026 1026 1026 1026 1026

Ambient temp. T(air) (℃)

23 23 23 23 23

Interpreted Data Run1 Run2 Run3 Run4 Run5Acetone evaporation Rate(kg/s)

0.0000762 0.0000937 0.0000937 0.00013 0.000164

Mass flowrate of air(kg/s) 0.000215 0.000384 0.000597 0.000792 0.000697Average vapour temp. in column (K )

290.75 288.15 287.05 286.05 285.45

dimensions

Internal Diameter of column, d(m) 0.025

Length of column, L (m) 1.5

Interfacial area between liquid/vapour A(m¿¿2)¿

0.118

Cross-sectional area of column, S(m¿¿2)¿ 0.00049

Calculations of interpreted data (Run 1):

Acetone evaporation rate = (liqinlet−liqoutlet)

1× 106 ×1

60 (Run1)

=5.77

1× 106×

160

= 9.6167×10−8 m3 /s.

The actual rate of acetone = 9.6167×10−8 × 792 = 7.62×10−5 kg/s .

Mass flow rate of air = (inlet air)

1000 × 1

60 (Run1)

= 10.81000

× 160

= 1.8×10−4 m3/s

Ambient air pressure at 23℃ is 1.194 kg/m3.

The actual mass flow rate = 1.194× 1.8×10−4 = 2.1492× 10−4kg/s .

Average temperature = (T ¿¿ inlet+T outlet)

2¿ (Run1)

= (18.4+16.8)2

= 290.15 K.

Calculated Results (run 1):

Molar mass of air m = 29kg/kmol

Experimental molar flow of air (Run 1) = 2.1492× 10−4 kg/s

nair=molarflowair

m = 2.149× 10−4

29 = 7.41103×10−6 kmol /s.

Antoine parametersof acetone

A B C

7.31414 1315.67 240.479

Run1:

log10 P=A− BC+T bot

(Bottom)

log10 P=7.31414− 1315.67240.479+16.8

= 102.200353=158.62 mmHg .

log10 P=A− BC+T top

(Top)

log10 P=7.31414− 1315.67240.479+18.4

= 102.231959

¿170.6mmHg .

Mole fraction of acetone inlet = 0

Mole fraction of acetone outlet =nacet

nacet+nair

nacet = 7.62× 10−5

58.08 = 1.312×10−6 kmol/s

yace .out= nacet

nacet+nair

= 1.312×10−6

1.312× 10−6+7.410×10−6 = 0.15034

Interfacial mole fraction yibot=158.62

770 = 0.206

Interfacial mole fraction yitop=170.6770

=0.222

Driving force at the top of the column ( y i− y ¿=0.222−0=0.222

Driving force at the bottom of the column ( y i− y ¿

¿0.206−0.15034

¿0.056

Logarithmic mean driving force, ( y i− y ¿LM=¿¿

=0.222−0.056

ln (0.2220.056

)=0.120

Bulk log mean pressure of air in column PBM=

PB1−PB2

ln (PB1

PB2

)

PB 1=PAir−PBot=770−158.62=611.4mmHg .

PB 1=PAir−PTop=770−170.6=599.4mmHg .

PBM=611.4−599.4

ln (611.4599.4

) = 605.4 mmHg.

Correction factor for unimolecular diffusion:

yBM =PBM

Ptotal

=605.4770

=0.786

Rate of loss liquid acetone (kmol/s) =nair × ( ybottom− y top)

= 7.41103×10−6× ¿0.15034−0¿

= 1.114 ×10−6 kmol /s.

Predicted acetone evaporation = nair × ( yibottom− yitop)

= 7.41103×10−6×(0.222−0.206)

= 1.186×10−7 kmol/ s

Mean evaporation rate = rate of lossacet−predictedacet

2

= 1.114× 10−6+1.186×10−7

2

= 6.163×10−7 kmol/ s.

k y=nair

A( youtlet− y inlet )

∆ yLM

k y=7.41103 ×10−6

0.118(0.15034−0)

0.120

¿7.9×10−5 kmol

m2 s.molfr .

Sh=k ' cD

DAB

=k y

DRT yBM

D AB Ptotal

Sh=7.9×10−5 (0.025× 8314× 290.75× 0.786)(0.0000115 ×770)

Sh=423.77

Vapour phase superficial velocity:

u=Qair

A

u=1.8×10−4

0.00049=0.367 m/ s

Vapour phase Reynolds number:

ℜ= pudμ

ℜ=(1.194× 0.367×0.025)

(1.827× 10−5)=599.61

Sherwood number for Re>2100:

Sh=0.023 ℜ0.83 Sc0.33

Sh=0.023×599.610.83×1.3310.33

Sh=5.109

Sc= 1.827×10−5

(1.194× 0.0000115)=1.331

Heat removed by evaporation of acetone:

Q 'C=mCp∆T

Q 'C=¿ 2.1492× 10−4× 121.3×(17.6−16.8)

= 0.0209 Watts

Cooling effect estimate by evaporation of acetone:

Q 'C=7.62×10−5× 121.3(T c−T B)

0.0209=7.62× 10−5 ×121.3(T c−17.6)

0.0209=7.62× 10−5 ×121.3(T c−17.6)

T c=19.9℃

Theoretical estimate of outlet liquid temperature; T liq. out℃

T liq. out℃=19.9−T out

T liq. out℃=19.9−16.8=3.06℃

Graphs:

Figure1:

400 600 800 1000 1200 1400 1600 1800 2000 2200 24000

0.00005

0.0001

0.00015

0.0002

0.00025

f(x) = 5.51063960107593E-08 x + 4.10623327207957E-05R² = 0.450220484950812

Reynolds number

ky e

xper

imen

tal

Figure2:

400 600 800 1000 1200 1400 1600 1800 2000 2200 24000

2

4

6

8

10

12

14

16

f(x) = 0.00618893624377524 x + 1.51806493301576R² = 0.999208705907013

Reynolds number

Sh co

rrel

ation

Physical data of Acetone/Air:

Physical properties of air and acetoneUniversal gas constant (J/kmol.K) 8314.5Molar mass of air, (kg/kmol) 29Inlet air density to column (kg/m3) 1.194Molar mass of acetone (kg/kmol) 58.08Density of acetone at ambient T (kg/m3) 792Air viscosity at column temperature(Ns/m2) 0.00001827Diffusivity of acetone in column air (m2/s) 0.0000115Latent heat of vaporisation of acetone DHv(J/kmol)Heat capacity of liquid acetone @ 283K, Cp(J/kg.K) 121.3Antoine parameter A of acetone 7.31414Antoine parameter B of acetone 1315.67Antoine parameter C of acetone 240.479Acetone Schmidtz number (dimensionless) 1.33030369Ambient air pressure mbar 1026Ambient air pressure(mmHg) 770Boiling point of acetone @56.1(deg.C) in mmHg 755.07

͵ ൈ�ͳͲ

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)

Air flow rate=50 ml/min= 0.0008333 m3/s pair= 1.194 kg/m3

mair=¿1.194×0.0008333=0.000995 kg/sQc=mair Cp ∆ T=¿ 0.000995×121.3× (17.6-16.8) =0.09655 W

Air flow rate=10 ml/min= 0.0001667 m3/s pair= 1.194 kg/m3

mair=¿1.194×0.0001667 = 0.00019904 kg/sQc=mair Cp ∆ T=¿ 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 air from ml/min to m3/s then this was multiplied with density of air at 1.194 kg/m3 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:

T c=Qc

Cp × macet

+T average

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. 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. 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. 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, 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 by Sherwood Pigford and Wilke, McGraw Hill Publication.http://www.mycheme.com/calculation-methods/bubble-a-dew-point.html.http://www.chem.tamu.edu/class/majors/tutorialnotefiles/percentcomp.htm.http://edibon.com/products/catalogues/ru/units/chemicalengineering/chemicalengineeringgeneral/CAPC.pdf.http://www.nt.ntnu.no/users/skoge/prost/proceedings/distillation02/dokument/6-12.pdf.Coulson and Richardson's Chemical Engineering Volume 2.Yaws' Handbook of Antoine Coefficients for Vapor Pressure (2nd Electronic Edition).