gas liquid absorption

8/13/2019 Gas liquid absorption

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MT-304

Gas-Liquid Absorption

Batch-A2 Rig 2.

09002058 Rohit Geesa

09011045 Shivi Jain

09D02032 Vivek Pandey

Date of Experiment: 14-02-2011

Date of Submission: 17-02-2011


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AIM:

To study the reaction of carbon dioxide with the hydroxide solution in the Absorption Column.

MOTIVATION:

The absorption of a gas into a liquid is one of the fundamental processes of industrial chemistry.

It makes it possible to separate one or more components of a gaseous mixture and to produce a

liquid containing a desired quantity of a gas. Absorption is a powerful tool for the gas separationand purification. The chemical reaction brings two benefits for absorption. One is the increase of

carrying capacity for gas components. Another is the reduction of mass transfer resistance or

increase of mass transfer coefficient. Both factors contribute to the increase of absorption rate.

THEORY:

The following reaction takes place in the set up:

Gas containing and air, is passed through the packed column from the bottom. The packed

column is packed with Raschig rings. A solution of known concentration (1.43N) of NaOH is

passed from the top. As it trickles down the column, it comes in contact with the rising gas

mixture, and is absorbed into the liquid with the above reaction. NaOH is passed in excess

of the theoretical requirement for the column. The gas is then passed through a column of

standing liquid in the bubbling pot. As it bubbles through the liquid, more

is absorbed into

the liquid. When samples are titrated with 0.1N HCl solution prepared, two end points are

obtained, one with Phenolphthalein, and another with Methyl Orange. Phenolphthalein gives the

first end point, with the following reactions taking place:

Methyl Orange gives the second end point which is due to an additional reaction:

Using reverse titration technique, amount of dissolved in the solution can be calculated and

a comparison can be made between the packed column and the Bubbling Column Yields.


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Experimental procedure :

Add 1500gm of solid NaOH to 25 Litres of water and

prepare a 1.5N solution of NaOH.

Flow rate of NaOH solution is set to 10lph in theadsorption tower. Fill bubbling pot with the samesolution of NaOH.

Set flow rate of air to 30 lph and 2 to 70lph for firstset of readings and then 90lph for the second set ofreadings.

NaOH solution prepared is first titrated with the HClsolution to check its normality.

Samples required for the two endpoint titrations arewithdrawn from the bubbling pot outlet and the packedcolumn outlet. This is done at regular intervals of 10,20and 30 minutes for both the flow rates of 2.


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Observations :

normality of NaoH from titration = 1.48 N

HCl conc. = 0.1 N

liquid flow rate (NaoH) = 10 l/hr

air flow rate = 30 l/hr

CO2 flow rate = 70 l/hr

Absorption Column

Time (min.) 10 20 30

NaOH 1st end point (ml) 14.3 13.7 14.4

NaOH 2nd end point (ml) 1.1 1.7 1.6

Deviation (ml) -13.2 -12 -12.8

Bubble Column

Time (min.) 10 20 30



Deviation (ml) -13.7 -14.2 -13.6

Table 1 : Table shows the calculated values for the Absorption column and the Bubble Column for 70

L/hr flow rate

Absorption Column

Time (min.)

N2

(mol/l) NaOH Reacted (mol/hr) CO2 fed (mol/hr) CO2 reacted (mol/hr) Yield of CO2 (%

10 1.32 1.8 3.125 0.9 25

20 1.2 3 3.125 1.5 4430 1.28 2.2 3.125 1.1 32

Bubble Column

Time (min.)

N2

(mol/l) NaOH Reacted (mol) CO2 fed (mol/hr) CO2 reacted (mol/hr) Yield of CO2 (%

10 1.37 0.39 2.225 1.17 42

20 1.42 0.24 1.625 0.72 31

30 1.36 0.42 2.025 1.26 50

Table 2.

.In this table, the amount of CO2 which reacted was calculated based on the titrations done, and the

amount of CO2 present instantaneously and over a cumulative timeframe was calculated. Shown below

is a table which highlights the values of instantaneous and cumulative yield for the absorption column.

For the bubble column, a similar analysis was carried out. For cumulative calculations, an average value

between the two data points was used to give us a more realistic picture of the true nature of the yield.

The table below shows the data obtained for the bubble column for the given flow rate.


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Co2 flow rate = 90 lph

Absorption Column

Time (min.) 10 20 30


NaOH 2nd end point (ml) 1.5 1.6 1.5Deviation (ml) -13 -13 -13.2

Bubble Column

Time (min.) 10 20 30



Deviation (ml) -12.8 12.6 13.7

Table 3 : Table shows the calculated values for the Absorption column and the Bubble Column for 90

L/hr flow rate.

Absorption ColumnTime (min.) 10 20 30


NaOH 2nd end point (ml) 16 16.2 16.3

Deviation (ml) 1.5 1.6 1.6

Bubble Column

Time (min.) 10 20 30



Deviation (ml) 0.4 0.6 0.8

Absorption Column

Time

(min.)

N2

(mol/l) NaOH Reacted (mol/hr) CO2 fed (mol/hr) CO2 reacted (mol/hr) Yield of CO2 (%)

10 1.3 2.00 4.02 1 22.5

20 1.3 2.00 4.02 1 22.5

30 1.31 1.90 4.02 0.95 20.0

Bubble ColumnTime

(min.)

N2

(mol/l) NaOH Reacted (mol) CO2 fed (mol/hr) CO2 reacted (mol/hr) Yield of CO2 (%)

10 1.28 0.66 3.02 1.98 56.25

20 1.26 0.72 3.02 1.08 76.15

30 1.37 0.39 3.07 0.39 33


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In order to better understand the trends observed both with respect to the behaviour of the two

columns with time, and the behaviour of the two columns with respect to increasing flow rate, a graph

was plotted and the graph plotted is shown below.

A) Absorption ColumnA graph showing Instantaneous yield for both flow rate.

A graph showing cumulative yield for both flow rate from table 3 [appendix]

B) Bubble ColumnA graph showing Instantaneous yield for both flow rate.

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10min 20 min 30 min

absorption column 90 lit

absorption column 70lit

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10min 20 min 30 min




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A graph of instantaneous yield versus time was also plotted and the variations seen are shown in the

figure given above.

A graph showing cumulative yield for both flow rate from table 3 [appendix]

.

A graph of cumulative yield versus time was plotted and the variations seen are shown in the figure

given above.

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70.00

10min 20 min 30 min

bubble column 90 lit

bubble column 70lit

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10min 20 min 30 min

bubble column 90 lit

bubble column 70lit


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SAMPLE CALCULATIONS

Flow rate of Carbon Dioxide = 70 L /Hr :

For sample collected from absorption column after 10 mins:

Volume of HCl (V1) = Burette Reading of phenolphthaleinDeviation = 14.31.1 = 13.2 ml

NaOH concentration used in titration (N2)= N1 * V1/V2*10 =0.1*13.2*10/10 = 1.32 mol/lit

Balance on hydroxide solution

Moles of NaOH reacted = (nNaOH InletnNaOH outlet) x flow rate of NaOH

= (1.5-1.32)*10 = 1.6 mol/hr

As per the reaction one mole of CO2reacts with two mole of NaOH.

So, Moles of CO2absorbed in the column = nNaOH/2 = 0.8 mol/hrMolar flow rate of CO2= 70Lit/hr/22.4 = 3.125 mol/hr

Yield of reaction:

Yield = moles of CO2 absorbed / moles of CO2 fed*100

= (0.80/3.125)*100 = 25.60 %

For sample collected from bubbling pot After 10 mins:

Volume of HCl (V1) = Burette Reading of phenolphthaleinDeviation = 14.60.9 = 13.7 ml

NaOH concentration used in titration=N1 * V1/V2 = 0.1*13.7*10/10 = 1.37 mol/lit

Balance on hydroxide solution

(Moles of NaOH t=0moles of NaOH t) x volume = nNaOH reacted = (1.48-1.37)*3 = 0.33 moles

Moles of CO2 reacted = (moles of NaOH reacted*60/10)/2 = 0.33/2 = 0.99 moles/HR

Final balance on CO2

Moles of CO2 fed to bubbling pot = moles of CO2 in column outlet.

= moles of CO2in column inletmoles of CO2reacted in column

= 3.1250.8 = 2.325 mol/hr

Yield of reaction:Yield = moles of CO2 absorbed / moles of CO2fed

= (0.99/2.325)*100

= 42.58 %


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CONCLUSION AND DISCUSSIONS :

1. We observe that the amount of CO2absorbed at equilibrium is higher in the packed towerthan in the bubble column. This is explained as the packing slows the easy flow of the

carbon dioxide and NaOH solutions. Thus contact time between the hydroxide and CO2

increases. Raschig rings also help increase the surface area for absorption

2. By comparing the amount of CO2absorbed in corresponding samples taken at differentair flow rates, we see that a higher flow rate of air leads to higher absorption.

SOURCES OF ERROR:

1) Error in preparing 0.1N HCl can lead to erroneous results because all calculations are donewith concentration of HCl as 0.1N. In our experiment, the normality of HCl was found out to

be less than 0.1N, so we adjusted it taking concentration of NaOH to be 1.48N rather than1.5N.

2) Least count of the beakers used for making the titre solutions were 10 ml, therebyintroducing significant errors in making samples of the appropriate concentration.

3) Flow rate of the gas was constantly fluctuating, leading to irregular absorption4) NaOH is measured using a simple analytical balance and there is a source of error in the

measurement in its weight during preparation of 1.5N NaOH solution.

5) We may have stopped the titration at different colour density occurred during the titration.

gas liquid absorption

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