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GAS ABSORPTION

Introduction:

Packed towers are used extensively in the Chemical Industry for Mass Transfer

operations using gas absorption, whereby a solute is transferred between a gas and a liquid

phase. The liquid and gas are contacted. Based on the solubility of the gas, components of it (if it

is a multi-component gas) can be absorbed into the liquid. In order to do this, the two streams

can be sent through several different media. Successful design demands satisfactory performance

with regards to both fluid dynamic and mass transfer considerations.

Figure 1: Flue gas desulphurization plant [courtesy of Steuler]

Figure 1 shows a SO2 scrubber from Steuler [6]. The hot waste gas with a temperature of

up to 220°C reaches first the lower part of the scrubber which is provided as a quencher zone and

there it is cooled down to the cooling limiting temperature. In the subsequent absorption zone,

the waste gas is charged in the counterflow with a washing solution containing Ca. The cleaned

waste gas saturated with water steam leaves the scrubber after having passed a mist collector and

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is then released into the atmosphere via a stack. Lime stone (CaCO3) is metered from a silo into

the scrubbing solution in dependency of the pH-value. During the absorption, the SO2 reacts with

the lime stone to form gypsum which is then present in the scrubbing solution as solid matter. A

partial flow of this gypsum suspension is given on the vacuum belt filter and is dehydrated. The

filtrate returns into the scrubber receiver, so that the plant works free of waste water.

This experiment uses a SO2-air mixture with the objective being to strip off as much

sulfur dioxide gas from the air as possible. This situation is commonly found in industrial

situations such as oil refineries where sulfur compounds are removed from fuels. To prevent this

harmful gas from reaching the atmosphere, the gas mixture is sent through a scrubber or can be

contacted with a solvent (water, in this case). Another alternative is to add caustic to the water

flow, which will remove acidic traces from the gas stream and subsequently increase the

efficiency of the SO2 removal. By simulating this unit operation in the lab, one can determine

the best flow setup to remove the SO2 from the stream.

Results can be expressed in terms of the height of a Transfer Unit or an Overall Mass

Transfer Coefficient. In either case they can be compared with values predicted using published

empirical equations.

Theory:

This experiment involves the absorption of SO2 gas from air by contacting the mixture

with water thorough a packed column. The gas mixture used in the trials contains approximately

1-3% SO2, so it will be treated as a dilute mixture.

The measure of the efficiency of the absorption process can be expressed in terms of the

overall height of gas-phase transfer units, HOG

(m). The smaller the HOG

, the more efficient the

absorption process will be. The HOG

is defined as:

′ (1)

Where ≡ overall mass transfer coefficient

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NOG

is a function only of compositions and depends on the operating conditions, e.g., G, L, T, P.

It is defined as

(2)

where is a log mean (average) driving force for absorption.

The overall height of gas-phase transfer units is then calculated from the NOG

: and known z:

(3)

Where z ≡ height of column (m)

The experimental results will be compared to theoretical values of HOG

by using the following

equation:

(4)

Where HOG

≡ theoretical overall height of gas-phase transfer unit (m)

HG ≡ height of gas-phase transfer unit (m)

m ≡ slope of equilibrium line

G, L ≡ gas and liquid flow rates, respectively (kmol.m2s)

HL ≡ liquid phase transfer unit (m)

The height of gas phase and liquid phase transfer units are both functions of the gas and

liquid flow rates into the column. A number of correlations to predict HG are available from

references 2-5 listed at the end of this manual.

In the second part of the experiment, caustic soda (NaOH) is injected into the water

stream. This makes the liquid alkaline and assists the absorption process. Chemical reaction in a

liquid phase reduces the equilibrium partial pressure of a solute over the solution, which greatly

increases the driving force for mass transfer. The limiting case involves assumption of an

instantaneous, irreversible chemical reaction. This case corresponds to the maximum driving

force, due to the reduction of the equilibrium partial pressure to zero (the reaction plane

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coincides with the interface). For dilute systems NOG

reduces to:

(5)

Apparatus:

The apparatus consists of a packed tower, nominally 6 inches in diameter and packed to a

height of 8 feet with dumped 1/4-inch rashig rings. An orifice meter is provided to allow

calculation of the gas flow rate. The SO2 and water flow rates can be determined using the

appropriate manometers, which are also provided.

The orifice itself is ½” in diameter inserted in a nominal 2" diameter schedule 40 steel

pipe. An expression relating pressure drop across the orifice must be developed to predict the

gas flow rate.

Scope of work:

Memo Report:

1. Calculate %SO2 removal for each operating condition

2. Discuss effects of each parameter, i.e., water flow rate, air flow rate, SO2 flow rate and the use of

NaOH on SO2 removal

3. Compare experimentally-determined heights of mass transfer unit with values obtained from

theoretical analysis

Formal Report should contain:

1. Perform mass balance on SO2 inlet and outlet

2. Calculate %SO2 removal for each operating condition and discuss on effects of water, air and SO2

flow rates

3. Calculate overall mass transfer coefficients

4. Compare experimentally-determined heights and number of mass transfer unit with values obtained

from theoretical analysis

5. Estimate pressure drop in the column using Eckert’s chart and compare with experimental data

6. Discuss an effect of chemical absorption using NaOH

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Experimental procedures:

Before lab

• Check beakers for buffer solutions

• Prepare goggles or safety glasses, gloves, lab coat

• Turn on infrared (IR) analyzer (at least 2h of warming up)

• Fill the ice in the blue bucket at the back of the column for bubbler

pH Meter calibration

Read and follow instructions;

• The calibration is needed to be done before any run

• Use three buffers on the table (sometimes the pH 2.00 buffer is adequate)

• Take the pH probe off the column

• Immerse it in the beaker

• Check the temperature of the buffer and set it on the pH meter

• Adjust to buffer value

• Repeat the steps for each buffer solution

• Put the electrode system back into the column

Procedure for Physical experiment

1. Standardize the pH meter

2. Turn on the sample pump (above bucket) to SO2 analyser

3. Turn on water valves

First on the front side

On the platform (try the side valve again if there is no flow)

4. Direct water to column (30-40% reading on rotameter)

5. Raise the water level control (at the bottom of the column) to set steady water level to below the air

inlet to prevent bubbles from the pH probe

6. Turn on the blower on the wall (right-hand switch)

7. Open the air valve on the back of the panel (red valve)

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Pay close attention to the incline manometer

Stop when it reads 80mm (The maximum reading is 140mm)

8. Turn on the SO2 cylinder valve

9. Start the flow into the column by adjusting SO2 rotameter

10. Wait approximately 10-15 min for the system to reach its steady state

11. Work on the mass balance to check the accuracy of results

12. Measure all parameters:

Water flow at inlet and outlet

Air flow at inlet

SO2 flow at inlet

SO2 content at inlet and outlet

Temperature

Pressure drop

pH at the bottom of the column

13. Start the next run by changing one parameter (in a good range) when keeping other parameters

constant

14. Follow the same procedure for each run

15. After the experiments, leave the water on for at least one hour to prevent SO2 smell in the lab

Procedure for Chemical experiment

1. Prepare 20L of 0.1M NaOH solution in the tank under the control board

(calculate NaOH weight)

2. Plug the outlet tube to the metal tube going to the pump

3. Turn off SO2 valve approximately for 20 min to let only air flows through the column

4. Loosen the screw on the tube going from the pump to the column

5. Turn on the pump (to left-hand-side) to high flow rate at the beginning for 10-15 min to purge the

NaOH into the system then reduce the flow rate for the run

6. Select 3 to 4 runs from the physical experiment and repeat them with adding NaOH

7. Check the pH at the outlet of the column (it takes 2 to 3 min for NaOH to flow down)

8. Turn on the valve for SO2

9. Record all parameters

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Calculations:

Attachment

• 2 graphs of SO2 content for Range 1, Range 3 of the IR analyzer

• 2 graphs of SO2 content and pH

• 1 graph for SO2 content and flow rate

Example of Recorded Data sheet

Run #

Tempe-rature

Pressure drop

Water Flow inlet

Air Flow inlet

SO2 flow inlet

SO2 content

inlet (ySO2)

SO2 content outlet

pH

1 2 3

… Some data treatment has to be performed to compare the values and to get the mass balance.

Mass balance

In order to check the accuracy of the experiment and if the steady state is reached, a mass balance on

SO2 is to be performed.

The general form is the following:

Moles of SO2 at inlet = Moles of SO2 at outlet

From this we can calculate everything.

Moles of SO2 at inlet = Moles of SO2 in air + SO2 flow at inlet water.

Moles of SO2 at outlet = Moles of SO2 in the water + Moles of SO2 in the air outlet

Example of a mass balance:

Readings on the apparatus:

Run Qair Qwater QSO2 SO2in SO2out pHin pHout T ΔP

# mm

H2O

% mm % % °C mm

H2O

1 110 25 75 75 10 8.39 1.93 15.7 94

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Now we need to work on these values to get the good units for the mass balance.

Water flow: 100% is equal to 2 USGal/min

min/89.11

78.3)min

(225.0 LusGal

LusGalQwater =××=

Air flow: must calculate the air flow through a pipe.

ρPACdQair

Δ=

2..

Where Cd=0.6 (discharge coefficient), A=πd²/4 is the surface of opening (m²), ΔP=ρwater.g.h is the pressure

difference (Pa) and ρ is the air density (kg/m3).

min/667.172/1088.22.1

110.081.9100024

012.06.0 332

LsmQair =×=×××

××= −π

SO2 flow: read directly from the graph

75mm = 1800 cc/min = 1.8 L/min

Volume fraction of SO2 at the inlet: based on flow ratio

%03.1667.1728.1

8.1

2

2

2vol

QQQ

yairSO

SOSO =

+=

+=

References:

[1] Eic, M. Mass Transfer Lab Notes – ChE 4404. Department of Chemical Engineering, University of New Brunswick. Fredericton, NB. [2] Perry, Robert H. Perry’s Chemical Engineer’s Handbook. McGraw-Hill.

[3] Geankoplis, C. Transport Processes and Separation Process Principles, Prentice Hall, 2003.

[4] Treybal, Robert E. Mass-Transfer Operations: 3

rd Edition. McGraw-Hill. Toronto, ON. 1987

[5] Seader, J. D., and Ernest J. Henley. Separation Process Principles. John Wiley & Sons, Inc. Toronto, ON. 1998 [6] http://www.steuler.de/anlb/absorp/hist/rgentsa/e_rgent.html

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Attachment 1of 5: SO2 content for Range 1 of the IR analyzer

%Vol SO2

% READING

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Attachment 2 of 5: SO2 content for Range 3 of the IR analyzer

%Vol SO2

% READING

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