absorption in packed

8/8/2019 Absorption in Packed

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ABSTRACT

The process of liquid/gas absorption is used in many chemical industries for purifying a gas

stream. The main objective of the experiment was to determine the rate of absorption of carbon

dioxide into water. This was achieved by titrating samples of water entering and exiting theabsorption column, at different time intervals, to determine the amount of carbon dioxide they

contained. In this experiment, the water was continuously recycled through the absorption

apparatus. As such, it was found that over time, the amount of carbon dioxide in the water

increased, but the rate at which the carbon dioxide was absorbed decreased.


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OBJECTIVES

Absorption is the taking up of matter in bulk by other matter, as in the dissolving of a gas in a

liquid [1]. Liquid/gas absorption is a process used in many chemical industries. For instance,

liquid/gas absorption can be used to remove acidic gases (H 2S, CO, CO 2) from gases beingreleased into the atmosphere to limit acid rain. The removal of ammonia from the product

stream of the Haber process utilizes the gas/liquid absorption process [2].

The main objective of this experiment is to calculate the rate of absorption of carbon dioxide

into water, from analysis of liquid solutions flowing down the absorption column. The

absorption of carbon dioxide into water is a mass transfer operation. It is important for chemical

engineers to be able to determine how fast a process can operate so that an operating time is

calculated. In particular, it is important for the rate of a mass transfer operation to be determined

so that the time necessary for a specified amount of mass transfer to occur.


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APPARATUS/PROCEDURE

List of equipment and materials used to conduct the experiment:

o The ARMFEILD gas/liquid absorption column

o Measuring cylinders

o Conical flasks

o Burettes

o Funnel

o White tile

o Distilled water

o Tap water

o Carbon dioxide gas

o Phenolphthalein indicator

o 0.0277M sodium hydroxide solution


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Figure 1 is a sketch of the ARMFIELD absorption apparatus used to determine the rate of

absorption of carbon dioxide from air into water. Sampling points for collecting S 4 and S 5are

clearly indicated in figure 1. The absorption apparatus required the use of water (supplied from

sump tank), a supply of carbon dioxide gas, and a source of air (air was taken in through the

compressor from the surroundings).

Figure 2 is a picture of the ARMFEILD gas/liquid absorption column equipment [3].


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Figure 3 shows the set up of apparatus for carrying out titrations on the samples collected from

the sump tank and the sampling point at the base of the column.

The procedure followed in conducting the experiment:

1. The sump tank was filled to its volume, V T = 37.5L, with water.

2. With the air flow and carbon dioxide flow valves closed, the pump was started and the

water flow rate was set to 6 L min -1.

3. The air flow valve was opened to allow an air flow rate of 20 L min -1 through the

absorption column.

4. The carbon dioxide valve was opened for carbon dioxide to flow up through the

column, with the air, at half the air flow rate- i.e. 10 L min -1.


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5. After 15 minutes of the mixture of carbon dioxide and air, counter-flowing water

through the packed column, a sample (S 5) of water from the sump tank and a sample

(S4) from the sampling point at the base of the column (sampling point is indicated in

figure 1), was taken.

6. Five more sets of samples (S 4 and S 5) were taken at 10-minute intervals.

7. 100ml of each sample collected, was measured using a measuring cylinder, and titrated

with 0.0277M sodium hydroxide solution. Eight drops of phenolphthalein were added

to the 100ml of sample, in a conical flask, before titrating. The end-point of the titration

was indicated by an onset of pink coloration of the solution in the conical flask.

8. The volume of sodium hydroxide used in each titration (V B) was recorded.


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THEORY

Mass transfer operations involve the changes in composition of solutions (or phases) [4] and it

occurs because of concentration differences. The solute (the substance being transferred between

phases) moves from a region of high concentration to one of low concentration. The rate of

diffusion within a phase is dependent on the concentration gradient in the phase [4]; and

diffusion occurs until the system comes into a state of equilibrium. In the experiment conducted,

the solute is carbon dioxide, which transfers from the gas phase (air and carbon dioxide mixture)

to the liquid phase (water).

The purpose of a packed column is to provide sufficient contact time between phases to allow

mass to transfer between the phases [4]. The mass transfer of carbon dioxide from air to water is

a situation of the transfer of a substance to a non-transferring substance. That is, there is no

counter-transfer, and the situation can be compared to that of diffusion of a substance through a

stagnant liquid. For such circumstances and steady state operation, the molar rate of diffusion [4]

of the substance is given by:

NA = k y (yA,g y A,i) = k x (xA,i x A,l)

Where; A_ the substance being transferred,

k y, k x _ mass transfer coefficient in gas phase and liquid phase, respectively,

yA,g , yA,i _ mole fraction of A in the gas, and at the gas-liquid interface, respectively, and

(yA,g y A,i) _ the driving force in the gas phase,

xA,i , xA,l _ mole fraction of A at the liquid-gas interface, and in the liquid, respectively,

and (x A,i x A,l) _ the driving force in the liquid phase.

This equation shows that the rate of mass transfer of A (carbon dioxide), depends on the

concentration gradients of A within each phase. The greater the concentration gradient, the

greater the rate of absorption of carbon dioxide into the water.


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In this experiment, not enough measurements was taken for the rate of absorption to be

determined from the previous equation. Also, as the water is recycled in this experiment this

makes the use of the previous equation more difficult. Instead, the absorption rate for carbon

dioxide across the column at any given time is given by:

Absorption rate = (inlet flow of dissolved CO 2) (outlet flow of dissolved CO 2)

= (F C D(5,i) ) (F C D(4,i) ).

The average rate of absorption over a given period, t was given by:

Average rate = [(amount of CO 2 absorbed over a period t) (V T) / t], where t is in seconds.


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RESULTS/DISCUSSION

Table 1 shows flow rates of water, air, and carbon dioxide through the absorption column; and

the volume of water in the sump tank, (V T).

VT , L 37.5

Water Flow Rate, L min -1 6

Air Flow rate, L min -1 20

Carbon dioxide flow rate, L min -1 10

Table 2 shows titration results for the sump water samples (S 5), and samples collected at the

sampling point at the base of the column (S 4). Where V B,4 and V B,5 are the volumes of sodium

hydroxide used to neutralize S 4 and S 5 samples respectively.

Time Elapsed (mins) S 5 Sample V B,5 (ml) S 4 Sample V B,4 (ml)

15 S5,1 12.8 S 4,1 15.4

25 S5,2 12.5 S 4,2 21.1

35 S5,3 17.2 S 4,3 26.25

45 S5,4 15.1 S 4,4 32.45

55 S5,5 -- S 4,5 20.7

65 S5,6 18.9 S 4,6 18.25

In conducting the experiment an experimental error occurred in that, indicator, unfortunately,

was not added to the sump sample that was taken after an elapsed time of 55 minutes, before

being titrated. This error was noticed only after too much alkali was added to the sump sample,

to determine the samples end-point. Hence an estimation of volume of 0.0277M sodium

hydroxide required to neutralise sample S 5,5 is needed.


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However, from table 2, it is difficult to notice a trend in the V B,5 results for the sump samples.

Taking more than one sump and sampling point samples per time interval would give greater

accuracy; since mean volumes of sodium hydroxide needed for neutralization, can then be

calculated for a S 4 or S 5 at different time intervals.

Now considering what is happening in the absorption apparatus, within each time interval, a

fraction of sump water is passed through the packed column. This fraction of sump water

ALWAYS absorbs some carbon dioxide; hence, the amount of carbon dioxide in the sump water

should always increase with time.

A continuous increase of carbon dioxide concentration in the sump water is not apparent from

the experiment results. This is due to experimental errors on behalf of the experimenter.

Phenolphthalein indicator is colourless in acidic solutions, but pink in alkaline solutions. The

end-point of the titration was indicated by the contents in the conical flask obtaining a light pink

colouration. It was difficult to determine when a solution had just changed colour; and as such,

some solutions, at the end of the titration, would have a more distinct pink colour than others

would. This means than some results for V B are more accurate than others were.

Phenolphthalein has a pH range of 8.3 to 10 [5] so an indicator with a similar pH range can be

used to titrate the strong alkali (0.0277M NaOH) with a weak acid (water with dissolved CO 2). A

suitable replacement for phenolphthalein is thymol-blue indicator with a pH range of 8 to 9.6.

This indicator is yellow in acidic solutions and blue in alkaline solutions. Hence, a very

distinctive green colouration would indicate the end-point.

To determine if the increase for carbon dioxide in the sump water is linear or otherwise, the

results were compared with another groups results group L results (shown in table 3 below).

Table 3 shows group L titration results for samples collected.

Time Elapsed (mins) S 5 Sample V B,5 (ml) S 4 Sample V B,4 (ml)


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15 S5,1 22.4 S 4,1 24

25 S5,2 29.0 S 4,2 31.9

35 S5,3 29.2 S 4,3 36.1

45 S5,4 33.9 S 4,4 36.0

55 S5,5 34.1 S 4,5 34.4

65 S5,6 35.0 S 4,6 34.5

Graph 1 showing group L titration results.

The most suitable trend found for group Ls V B,5 results with respect to time is a linear one.

Hence a linear relationship of V B,5 results with respect to time will also be found (for GROUP F

experiment results); and used to determine the amount of 0.0277M sodium hydroxide required to

neutralise the S 5,5 sample.


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Graph 2 shows titration trends for the experiment conducted (by group F); specifying a linear

relation of the volume of 0.0277M NaOH used to neutralise the sump water samples with time.

Now, both graphs 1 and 2 show a rapid increase in V B,4 values with time, followed by a

decrease. The reason for the decrease in the trend of V B,4 values is that, with time, the inlet water,

to the absorption column, contains more carbon dioxide. Accordingly, the driving force for the

mass transfer of carbon dioxide, the concentration gradient with respect to the liquid phase, is

therefore increasingly smaller with time (the concentration gradient in the gas phase is relatively

constant). This is in agreement with the theory.

Now the initial increase in the trend of V B,4 values can be explained by considering the flow

rates of gases and water. The carbon dioxide at a flow of 10 Lmin -1 was mixed with an air flow

rate of 20 Lmin -1. This means that the stream of gas flow through the absorption apparatus is not

very concentrated with carbon dioxide. Carbon dioxide, when compared with air, is more soluble

in water. However, carbon dioxide is still only sparingly soluble in water. So with a water flow

rate of 6 Lmin -1 only small amounts of carbon dioxide is absorbed with time. This small increase

in carbon dioxide in the liquid phase is too insignificant to decrease the driving force, with

respect to the liquid phase, for a noticeable decrease in the absorption rate. However, the initial

small increase is significant enough to add to the carbon dioxide that was absorbed in the water,


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to increase the volume of sodium hydroxide needed for neutralisation. Hence the initial increase

in V B,4 values.

CALCULATIONS/DISCUSSION

Determining a value for V B,5 corresponding to sample S 5,5.

Using the relation shown in graph 2, when the time elapsed is 55 minutes, that is x = 55, then

y = 0.1243x +10.7

y = 0.1243(55) + 10.7

y = 17.5

Hence, the volume of sodium hydroxide used to neutralise the sump sample collected after 55

minutes is 17.5 ml.

Calculating the moles of carbon dioxide per litre of sample:

The stoichiometric equation of the reaction of carbon dioxide in the water with the sodium

hydroxide is CO 2 + 2NaOH Na 2CO 3 + H 2O.

Moles of sodium hydroxide used in titration = V B (NaOH molarity).

Mole ratio of carbon dioxide to sodium hydroxide is 1:2.

Moles of carbon dioxide in sample = [V B (NaOH molarity)].

Therefore, mole/litre of free CO 2, C D = [V B (ml) 0.0277(mol L -1)] / 100 (ml).

Sample calculation:

For sample S 5,1, V B = 12.8 ml.

CD(5,1) = [12.8 (ml) 0.0277(mol L -1)] / 100 (ml).


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= 1.773 10 -3 mol L -1.

Table 3 shows experiment results along with the amount of carbon dioxide per mole of solution

is contained in samples collected

Time Elapsed

(mins)S5 Sample V B,5 (ml)

C D(5)(mol/L)

S4 Sample V B,4 (ml)C D(4)

(mol/L)

15 S5,1 12.8 0.001773 S 4,1 15.4 0.002133

25 S5,2 12.5 0.001731 S 4,2 21.1 0.002922

35 S5,3 17.2 0.002382 S 4,3 26.25 0.003636

45 S5,4 15.1 0.002091 S 4,4 32.45 0.004494

55 S5,5 17.5 0.002424 S 4,5 20.7 0.002867

65 S5,6 18.9 0.002618 S 4,6 18.25 0.002528

Calculating the amount of carbon dioxide dissolved over different time intervals:

Sample calculation:

Considering the S 5 samples, for a time interval of 20 minutes;

Average rate = [(C D(5,3) C D(5,1) ) (V T)] / (20 60)

= [(0.002382 0.001773) (37.5)] / (20 60)

= 19.03 10 -6 mol s -1.

Table 4 shows average rate of absorption of carbon dioxide over different time intervals for

sump (S 5) samples and sampling point (S 4) samples.

Time Interval (min) Average Rate for S 5 (mol/s) Average Rate for S 4 (mol/s)

10 -2.59688 10 -6 4.93406 10 -5

20 1.90438 10 -5 4.69602 10 -5

30 6.63646 10 -6 4.91964 10 -5


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40 1.01711 10 -5 1.14695 10 -5

50 1.05606 10 -5 4.93406 10 -6

Graph 3 shows the absorption rate of carbon dioxide over different time intervals with respect to

the sump water (S 5) samples and sampling point (S 4) samples.

Graph 3 simply reflects graph 2 and the experiment results. The average rate for S 4 values

relation shows a decrease in the rate of absorption, of carbon dioxide, with time, as the driving

force in the liquid phase decreases. This was previously explained in examining graph 2. The

average rate for S 5 values relation reflects a slowing increase in carbon dioxide absorption with

time. This is because while the amount of carbon dioxide in the sump water increases, less

carbon dioxide is absorbed with time.

Calculating the absorption rate of carbon dioxide in the column at any time:

Absorption rate = (inlet flow of dissolved CO 2) (outlet flow of dissolved CO 2)

= (F C D(5,i) ) (F C D(4,i) )

= F (C D(5,i) C D(4,i) )


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Sample calculation:

For samples taken after an elapsed time of 35 minutes;

Absorption rate = F (C D(5,3) C D(4,3) )

= (6/60) L s -1 (0.002382 - 0.003636) mol L -1

= - 125.4 10 -6 mol s -1.

Table 5 shows the rate of absorption of carbon dioxide across the column at any time.

Time Interval (min) Absorption Rate (mol/s)

15 -0.00003601

25 -0.00011911

35 -0.000125343

45 -0.000240298

55 -0.00004432

65 9.0025 10 -6


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Graph 4 shows the rate at which carbon dioxide is absorbed across the column at any time.

The negative absorption rate is due to the values for V B,5 being smaller than the values for V B,4

except the V B,5 corresponding to sample S 5,6. Now, the water in the sump tank at any particular

time during the operation of the absorption apparatus has a greater amount of carbon dioxide

than the volume of water in the packed column at that time. However, because of the greater

amount of water in the sump tank, the carbon dioxide from a sump water sample would be of a

small concentration. The increasing negative portion of graph 4 corresponds with the increasing

portion of the V B,4 relation with time intervals, in graph 2. This suggests that initially the

concentration of carbon dioxide in the S 4 samples increases greatly when compared with the

increase in the S 5 (sump water) samples. The decreasing negative portion of graph 4 is due to the

decreasing ratio of concentration of carbon dioxide in S 4 to that in S 5, as rate of absorption of

carbon dioxide decreases with time.


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CONCLUSIONS/RECOMMENDATIONS

The rate of absorption of carbon dioxide into water was determined by titrating samples of water

entering and exiting the absorption column, at different time intervals, to determine the amount

of carbon dioxide they contained. As the water used was recycled, it was found that the total

amount of carbon dioxide in the water, increased with time. The rate of absorption of the carbon

dioxide into the water depended on the concentration of carbon dioxide already in the water. It

was shown that the rate of the absorption of the carbon dioxide, into the water, decreased with

time.

Possible improvements to increase the accuracy in the results for this experiment is to take more

than one sump sample and more than one sampling point sample for each time; and to use

another indicator thymol blue.


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REFERENCES

[1] Cited 27 th September 2009. Available from the internet:



[4] Treybal, R.E. (1980). Mass Transfer Operations (3 rd edition). McGraw-Hill, New York.

[5 ] Cited 27 th September 2009. Available from the internet:
http://encyclopedia2.thefreedictionary.com/Absorptionhttp://www.uic.edu/depts/chme/UnitOps/CO2.pdfhttp://www.armfield.co.uk/uop7_datasheet.htmlhttp://www.ausetute.com.au/indicata.htmlhttp://encyclopedia2.thefreedictionary.com/Absorptionhttp://www.uic.edu/depts/chme/UnitOps/CO2.pdfhttp://www.armfield.co.uk/uop7_datasheet.htmlhttp://www.ausetute.com.au/indicata.html


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APPENDIX

absorption in packed

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