labreport gas diffusion.docx

7/28/2019 LabReport gas diffusion.docx

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1.0 ABSTRACT

Regarding to the experiment objectives which is to determine the diffusivity of the vapour of

acetone and to study the effect of temperature on the diffusivity, this experiment is based on

the mass transfer theory. The instrument used is the Gas Dispersion Apparatus that consists

of an acrylic assembly which is sub-divided into two compartments. One compartment is

constructed from clear acrylic and is used as a constant temperature water bath. The other

compartment is incorporates an air pump and the necessary electrical controls for the

equipment. The experiment is run by using two difference temperatures in order to study the

effect of temperature on the diffusivity of the vapour of acetone. At temperature of40C, the

diffusivity of acetone that obtained is 6.17 x m2/s. Meanwhile, at temperature of 50C,

the diffusivity of acetone that obtained is 2.252 x m2/s. This experiment showed that

gas diffusivity decreased with increasing temperature. However, supposedly the temperature

increase with the diffusivity of the vapour of acetone increases as well.


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2.0 INTRODUCTION

Figure 1: Gaseous Diffusion Apparatus

Gaseous diffusivity or gas dispersion apparatus which involves diffusion with bulk flow is

one of the items of laboratory equipment that have been designed to allow measurement of

molecular diffusivities and also to make the students become more familiar with the basic

notions 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 comprise an air pump, a travelling microscope with accurate focus

adjustment and mounted for vertical axis movement against a Vernier scale and a

thermostatically controlled water bath, in which to place the capillary tube, capable

of accurate temperature control.


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The experimental capabilities of this apparatus are direct measurement of mass transfer rates

in the absence convective effects, use of a gas laws to calculate concentrations differences in

terms of partial pressures, use of Ficks Law 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.

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

Winklemanns 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, D, is a kinetic parameter

associated with static and dynamic conditions of a process. All the complexity and

unwieldiness of many calculations is, indeed, connected with the determination of this

quantity.


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3.0 OBJECTIVES

The objective of this experiment is:

3.1 To determine the diffusivity of the vapour of acetone.

3.2 To study the effect of temperature on the diffusivity.


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4.0 THEORY

The diffusion of vapour A from a volatile liquid into another gas B can be conveniently

studied by confining a small sample of the liquid in a narrow vertical tube and observing its

rate of evaporation into a stream of gas B passed across the top of the tube. Normally, for

simple instructional purposes, gas B is air and vapour A is an organic solvent such as acetone

or methyl alcohol.

The apparatus consist essentially of a glass capillary tube placed in a transparent-sided

temperature controlled water bath. A horizontal glass tube is fixed to the upper end of the

capillary tube and air is blown through this by a small air pump included within the unit. This

arrangement allows the maintenance of a partial pressure difference within the capillary

tube between the evaporating liquid surfaces and the flowing air stream. A travelling

microscope, with sliding vernier scale, is mounted on a rigid stand alongside the thermostatic

bath and is used to measure the rate of fall of the solvent or air meniscus within the capillary.

The relation between the measured molar mass transfer rate (N Aper unit area), the partial

pressure gradient and the diffusion coefficient, D is deduced based on the following;

... Equation [1]

Where D = Diffusivity (m2/s)

CA = Saturation concentration at interface (kmol/ m3)

L = Effective distance of mass transfer (mm)

CBm = Logarithmic mean molecular concentration of vapour (kmol/ m3)

CT = Total molar concentration = CA + CBm (kmol/ m3)

N A = D {CA/L}{CT/CBM}


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Considering the evaporation of the liquid:

. Equation [2]

Where is the density of liquid

Thus,

. Equation [3]

Integrating and putting L - Lo at t = 0

Equation [4]

Lo and L cannot be measured accurately but L-Lo can be measured accurately using

thevernier on the microscope

. Equation [5]

Or

where:

M = molecular weight (kg/mol)

t = time(s)

where

s are the slopes of a graph t/(L-L0) against L - Lo then:

NA = {L/M}{dL/dt}

{L/M}{dL/dt} = D {CA/L}{CT/CBM}

L2L20 = {2DM/L}{(CACT)/CBm}t

(LL0)(L-L0+2L0) = {2DM/L}{(CACT)/CBm}t

t/(L-L0) = {L/2MD}{ CBm/( CACT)}(L-L0) + {(L CBm)/( CACTMD)}L0


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. Equation [6]

.... Equation [7]

s = (L CBm)/( CACT2MD)

D = (L CBm)/( CACT2sM)


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5.0 PROCEDURES

1. The capillary tube is partially filled with acetone to a depth of approximately 35mm.the top nut was removed from metal fitting.

2. Carefully, the capillary tube was inserted through the rubber ring, inside the metal nutuntil the top of the tube rests on the top of the nut.

3. Gently the assembly is screwed onto the top plate, with the T piece normal to themicroscope. The flexible air tube was connected to one end of the T piece. The

object lens is adjusted to within 20-30 mm from the tank.

4. The vertical height of the microscope was adjusted until the capillary tube is visible.When the meniscus has been determined, the vernier scale should be aligned with asuitable graduation on the fixed scale.

5. Then, the air pump was switched on.6. The level inside the capillary tube was recorded.7. The temperature controlled water bath was switched on and a steady temperature was

obtained.

8. The reading was taken every 5 minutes for 10 times. The experiment was done attemperature 45C and 50C.


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6.0 APPARATUS AND MATERIALS

6.1 Apparatus

Figure 2: Gaseous Diffusion Apparatus

1) TR 14 Membrane Test Unit apparatus.2) 500 mL beakers.3) Electronic balance.4) Gloves5) Ruler

6.2 Materials

1) Acetone2) Sodium Chloride3) Water


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7.0 RESULTS

Intial Length, Lo = 10.3

Temperature, T = 45C

Time, t (kilosecond, ks) (LoL) (mm)

(ks/mm)

0.3 0.3 1.00

0.6 0.6 1.00

0.9 1.0 0.90

1.2 1.3 0.92

1.5 1.8 0.83

1.8 2.1 0.86

2.1 2.4 0.88

2.4 2.7 0.89

2.7 3.0 0.90

3.0 3.4 0.88

Table 1

Intial Length, Lo = 7.30

Temperature, T = 50C

Time, t (kilosecond, ks) (LoL) (mm)

(ks/mm)

0.3 0.3 1.00

0.6 1.0 0.60

0.9 1.6 0.56

1.2 0.8 1.50

1.5 1.3 1.15

1.8 2.8 0.64

2.1 2.8 0.75

2.4 3.3 0.73

2.7 3.8 0.71

3.0 4.3 0.69

Table 2


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8.0 CALCULATIONS

8.1 Sample calculations

8.1.1 Time from commencement of experiment:

x

1000 = 0.3ks

Liquid level (LLo) : = 0.3

=

= 1.0 ks/mm

8.1.2 Calculation on diffusivity, D (T = 40 C )

Figure 3: Graph of

against LoL

Density of acetone, = 790 kg/ Gas constant, R =

Molecular weight of acetone =

Vapor pressure, Pv = 56 kN/m3 Slope, s = 0.036 = 3.6 x


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Assume standard conditions (P = 101.32 kN/, V=22.4 , T = 273 K) Tempereature, Ta = 40 C = 313 K

CT = (1/V )(T / Ta ).. Equation [8]

= (1/22.4)(273/313)

= 0.0389 kmol/m3

CB1 = CT = 0.0389 kmol/m3

To find CB2 :

CB2 = (PaPv / Pa)CT... Equation [9]

= (101.32-56/ 101.32) 0.0389

= 0.0174 kmol/m3

To find CBM :

CBM = (CB1-CB2)/ln (CB1/CB2).... Equation [10]

= (0.03890.0174)/ln (0.0389/0.0174)

= 0.0267

To find Ca :

Ca = (Pv/Pa)CT.. Equation [11]

= (56/101.32) 0.0389

= 0.0215 kmol/m3


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To find diffusivity,D :

......... Equation [7]

D = (790)(0.0267/3.6 x (2x58.08x0.0215x0.0389)

D = 6.17 x m2/s

Calculation steps is repeated to find diffusivity, D at T = 50 C

D = (L CBm)/( CACT2sM)


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8.1.3 Calculation on diffusivity, D (T = 50 C )

Figure 4: Graph of

against LoL

Assume standard conditions (P = 101.32 kN/, V=22.4 , T = 273 K) Tempereature, Ta = 50 C = 323 K

CT = (1/V )(T / Ta ).. Equation [1]

= (1/22.4)(273/323)

= 0.0377 kmol/m3

Density of acetone, = 790 kg/ Gas constant, R =

Molecular weight of acetone =

Vapor pressure, Pv = 56 kN/m3 Slope, s = 0.1 = 1.0 x


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CB1 = CT = 0.0377 kmol/m3 .. Equation [2]

To find CB2 :

CB2 = (PaPv / Pa)CT..Equation [3]

= (101.32-56/ 101.32) 0.0377

= 0.0169 kmol/m3

To find CBM :

CBM = (CB1-CB2)/ln (CB1/CB2) Equation [4]

= (0.0377 - 0.0169)/ln (0.0377/0.0169)

= 0.02597

To find Ca :

Ca = (Pv/Pa)CT.. Equation [5]

= (56/101.32) 0.0377

= 0.0208 kmol/m3

To find diffusivity,D :

D = CBM /s(2MCa CT)... Equation [6]

D = (790)(0.02597/1.0 x (2x58.08x0.0208x0.0377)

D = 2.252 x m2/s


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9.0 DISCUSSIONS

In this gas diffusion experiment, equipment used was the Gaseous Diffusion Apparatus

[Model: CER-A (ARMFIEL)]. This equipment has been designed to study the application of

mass transfer focussing on the gas diffusion.

This experiment was conducted to realize the main objectives which are to study the gas

diffusion coefficient and its relationship with the change in temperature. The manipulating

variable involved is the temperature and the responding variable was the level inside the

capillary tube, L which was taken every 0.3 ks until 3.0 ks was reached. This experiment was

carried out for two different temperatures which are at 40 C and 50 C with respect to gas

diffusivity. In particular, gas diffusivity of vapour acetone is calculated.

Utilizing the data obtained, graph of

against LoL was plottedas shown in Figure 3 and 4.

From both of these graph, the gradient or also known as the slope,s was calculated. However,

due to the present of errors, data collected has shown to deviate variedly. A linear graph was

not able to be obtained and thus to overcome this problem the slope,s was calculated taking

from two points relative to the line obtained.


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The calculations were proceeds with the aim to find the diffusivity of vapour acetone at

different temperature. It was based on the data collected for T = 40 C, D equivalent to 6.17 x

m2/s while at T = 50 C, D was found to be 2.252 x m2/s. This result has proven

that as the temperature increase, the diffusivity decreases which opposes the principle of gas

diffusion from this experiment. Supposedly, the diffusivity of vapor acetone increase with

increasing temperature. Diffusion is the movement of molecules from area of high

concentration to area of lower concentration and this is increased with increasing

temperature which means when the temperature increase the diffusion will speeds up. This

explained when temperature is higher, then the rate of diffusion would probably increase

caused by increasing kinetic activity of the solution. However, due to this

inaccuracy, values for gas diffusivity calculated using Equation [7] may not explain correctly

the definition of gas diffusion.

It has then clearly proved that errors arose during this experiment. Firstly, error mentioned

may be classified as a parallax error. This occurence is especially when data taken for the

level inside the capillary tube was performed. Besides that, inaccurate time relay may causes

human error due to lack in focus in conducting the experiment. Stabilizing of temperature

was also unable to achieve during the experimnent therefore deviating the data obtained.


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10.0 CONCLUSION

As a conclusion, the result for diffusivity coefficient at 40 C is -2504.53 m 2/s and the

diffusivity coefficient at 45 C is -3650.55 m

2

/s. There are some errors when the experiment

was done. Some of recommendations step should be taken otherwise the result for diffusivity

would not be accurate. To sum up, the experiment had been done with quite accurate results

although some error happened.

11.0 RECOMMENDATIONS

Some recommendations should be implements in this experiment. One of them is insulating

the glass container. It will maintain the temperature through the experiment and the heat will

not loss to the surrounding. Next, the reading meter should be stabilized with the person who

read the meter. The person who read it should be at the same level to the meter. By doing

this, parallax and reading error can be minimized.

12.0 APPLICATIONS


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All gas diffusion methods are based on interfacing the donor stream containing the analytic,

with an acceptor stream containing a reagent. The porous hydrophobic membrane serves as a

barrier that allows only the gaseous species passed through. This makes the gas diffusion

methods very selective, since the non-volatile species will not reach the detector. The

instrumental setup can be configured in two ways:

1. The continuous programmable flow method where both donor and acceptor streamare continuously pumped.

2. The stopped flow technique where both donor and acceptor stream are controlled bysyringe pumps.

The gas diffusion unit for continuous flow has diffusion path 2 cm long and 2 mm wide. The

acceptor stream is monitored by a flow cell with 10 cm long light path. The flow injection

system is operated at a programmable flow rate of 0.75mL/min per channel while the sample

passes through the diffusion unit and at a flow rate of 3.0mL/min during flush period. The

injection volume is 300 microliters of sample and sampling frequency is 40 injections per

hour.

The stop flow method uses the sandwich gas sensor, where the injected analytic is in contact

with the gas diffusion membrane during the stopped flow period. It yields higher sensitivity

and consumes far less of sample and reagents then continuous flow method.

The Sandwich Gas Diffusion Sensor uses a pair of optical fibres that monitor the acceptor

solution adjacent to the diffusion membrane. The stop flow FI technique is robust, since it


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uses syringe pumps, and therefore suitable also for continuous monitoring. Sensitivity of this

method is easily adjusted by selecting the duration of the stopped flow time, while using the

same experimental setup and reagent concentrations. The Sandwich Sensor provides response

in real time, since it monitors the kinetics of the diffusion and of the subsequent chemical

reaction and uses smaller volumes of sample than the continuous flow method.

Figure 6

Figure 5

13.0 REFFERNCES


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Journal of Physical and Chemical Reference Data. Gaseous Diffusion Coefficients.Retrieved November 15, 2012 from

http://jpcrd.aip.org/resource/1/jpcrbu/v1/i1/p3_s1?isAuthorized=no

Gases: Grahams Laws of Diffusion and Effusion. Grahams Law. RetrievedNovember 15, 2012 from

http://www.chem.tamu.edu/class/majors/tutorialnotefiles/graham.htm

Diffusion of Gases. Diffusion and Effusion. Retrieved November 16 2012 fromhttp://chem.salve.edu/chemistry/diffusion.asp

USEC. Gaseous Diffusion. RetrievedNovember 16, 2012 fromhttp://www.usec.com/gaseous-diffusion

Advancing the Chemical Sciences. Learn Chemistry: Diffusion of gases of ammoniaand hydrogen chloride. Retrieved November 16, 2012 from http://www.rsc.org/learn-

chemistry/wiki/TeacherExpt:Diffusion_of_gases_-_ammonia_and_hydrogen_chloride

14.0 APPENDIX

Refer attachment on the next page (page 22)
http://jpcrd.aip.org/resource/1/jpcrbu/v1/i1/p3_s1?isAuthorized=nohttp://www.chem.tamu.edu/class/majors/tutorialnotefiles/graham.htmhttp://chem.salve.edu/chemistry/diffusion.asphttp://www.usec.com/gaseous-diffusionhttp://www.rsc.org/learn-chemistry/wiki/TeacherExpt:Diffusion_of_gases_-_ammonia_and_hydrogen_chloridehttp://www.rsc.org/learn-chemistry/wiki/TeacherExpt:Diffusion_of_gases_-_ammonia_and_hydrogen_chloridehttp://www.rsc.org/learn-chemistry/wiki/TeacherExpt:Diffusion_of_gases_-_ammonia_and_hydrogen_chloridehttp://www.rsc.org/learn-chemistry/wiki/TeacherExpt:Diffusion_of_gases_-_ammonia_and_hydrogen_chloridehttp://www.usec.com/gaseous-diffusionhttp://chem.salve.edu/chemistry/diffusion.asphttp://www.chem.tamu.edu/class/majors/tutorialnotefiles/graham.htmhttp://jpcrd.aip.org/resource/1/jpcrbu/v1/i1/p3_s1?isAuthorized=no

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