Monash University Chemical Engineering Faculty

Gas Absorption CHE3165 Separations Processes Laboratory Report

Nasser Cura 21493863, David De La Cruz 21537399, Gabriella Gunawan 20780249, Jimmy Nguyen 21498849 4/16/2012

Table of Contents

Introduction ................................................................................................................................................. 3

Aims .............................................................................................................................................................. 4

Experimental Work .................................................................................................................................... 4

Procedure Safety ..................................................................................................................................... 4

Apparatus ................................................................................................................................................. 6

Experimental Method ............................................................................................................................ 7

Experiment A ...................................................................................................................................... 7

Experiment B ...................................................................................................................................... 8

Experiment C ...................................................................................................................................... 8

Experiment D ..................................................................................................................................... 9

Introduction

Separation of a component from a mixture is instrumental in many industrial processes. The

method used may be determined by the phases of solute/solvent, direction of solute travel or

specific chemical properties of the solvent/solute involved, to name a few.

In a process where only the solute of interest has considerable solubility in a liquid solvent and

none of the other gas components have low solubility in the liquid solvent, gas absorption may

be used to separate the solute of interest from a gas phase into a liquid phase. Quoting from

Perrys Chemical Engineering Handbook, gas absorption is a unit operation in which soluble components

of a gas mixture are dissolved in liquid. The liquid solvent chosen must also possess low solubility in

the gas phase to ensure mass transfer only occurs for the solute of interest.

Gas absorption processes are carried out in vertical columns. These columns are packed with

materials that increase surface area, hence contact, between the gas and liquid phases and operate

counter-current as gas and liquid inherently flow in opposite directions.

In his book, Geankoplis states that gas absorption is a combination of limiting factors of

molecular diffusion and turbulent diffusion processes.

The above is Ficks law of molecular diffusion which gives us a mathematical representation of

the process of molecular diffusion. JAZ is molar flux in the z-direction, DAB is the diffusion

coefficient for component A being transferred to component B, cA is the concentration of A in

the bulk phase and z is the diffusion distance (Geankoplis).

From Equation 1, turbulent diffusion can be altered through the diffusion coefficient to account

for eddy diffusivity, where there is backward fluid flow from areas of high to low concentration

in the column.

Combining eddy diffusivity and molecular diffusion coefficient into one term gives us an

equation much like equation 1. As such, molar flux of component A is a function of a diffusivity

constant, the driving force (concentration gradient) and resistance (z). If we assume that the

diffusion coefficient and driving force is unaffected by area, then added packing increases overall

contact area but has no effect on molar flux but increases overall mass transfer when compared

to no packing present.

The experiment uses 75mm diameter borosilicate glass column with an effective length of 1.4m.

The column is packed with glass Raschig rings, 10mm x 10mm. The column is set ip with a

specific set of valves and Hempl apparatus, allowing the gas composition at top, middle and

bottom of column to be analysed.

Carbon dioxide is chosen as solute of interest, with the solvents being pure water or NaOH

solution for a comparison of neutral and basic medium effects on transfer rate of solute. The

solvent found to absorb the most carbon dioxide will be deemed the better of the 2 mediums

considered when removing CO2 from a gas phase mixture.

Carbon dioxide has been chosen as solute of interest for its importance in industry. Keeping

global warming in mind, CO2 is a known greenhouse gas and regulatory bodies are setting limits

on CO2 emissions. Carbon dioxide is a by-product of most plants, formed during combustion of

organic matter, most notably methane or coal in a furnace, which is also used not only in

manufacturing plants but electricity generating plants that consume fossil fuels.

Aims

This experiment compares the absorption of CO2 into 2 different mediums: water and a caustic

solution. The variable rate of absorption will be compared by way of comparing which medium

is more effective for removing CO2 from a stream.

To achieve the above-stated aim, the experiment is broken down into 4 parts: Part A aims to

quantify the amount of CO2 absorbed into water, part B aims to measure the rate of absorption

of CO2 into water, part C measures the rate of absorption of CO2 into sodium hydroxide (a

caustic solution), and part D aims to verify that the CO2 absorbed during part C was indeed due

to absorption from the gas stream.

Experimental Work

Procedure Safety

For safety, this experimental procedure requires safety glasses, closed toes, flat heeled shoes and

full pants to be worn at all times. There will be no food and drink allowed in the experimental

area at any time. Smoking is not allowed in the corridors or within the laboratory.

All wastes produced by the experiment must be disposed of in the provided containers as

labelled and NOT poured into the sink.

The following hazards table identifies sources of hazard during the course of the experiment.

IDENTIFY the hazard DETERMINE the risk CONTROL the risk

Ethanol (high potential to do harm)

Mild irritant Flammable (low probability that harm results)

Wear appropriate PPE Wash spills promptly Ensure work area is free of ignition sources

Sodium hydroxide (NaOH, caustic soda) (high potential to do harm)

Strong base: corrosive (skin contact) Heated dissolution in water (high probability that harm results)

Wear appropriate PPE (including gloves) Use appropriate mixing method (excess water required) Wash any skin exposure thoroughly with water, remove exposed clothing

Hydrochloric acid (HCl) (high potential to do harm)

Strong acid Irritant Burns on skin contact (high probability that harm results)

Wear appropriate PPE (including gloves) Wipe up spills quickly Wash any skin exposure thoroughly with water, remove exposed clothing

Barium chloride (BaCl2)

Harmful if swallowed Toxic

Wear appropriate PPE (including gloves)

(moderate potential to do harm)

Absorbed transdermal (moderate probability that harm results)

Wipe up spills quickly Wash hands after use

Phenolphthalein indicator (moderate potential to do harm)

Harmful if swallowed Carcinogen Irritant (low probability that harm results)

Wear appropriate PPE Do not ingest Wipe up spills quickly

Methyl orange indicator (moderate potential to do harm)

Harmful if swallowed Toxic (low probability that harm results)

Wear appropriate PPE Avoid skin contact Use under fume hood

Sodium bicarbonate solution (low potential to do harm)

Skin irritant (low probability that harm results)

Wear appropriate PPE Avoid skin contact Wipe up spills quickly

Glassware (low potential to do harm)

Broken glassware is a sharps hazard (moderate probability that harm results)

Wear appropriate PPE Only use glassware how it has been designed Ensure any breaks are reported and cleaned up quickly

Electrical equipment (low potential to do harm)

Main electricity supply can cause fatal shocks (low probability that harm results)

Check all electrical equipment wires for integrity Use equipment in moisture-free environment

Heating equipment (moderate potential to do harm)

Hot equipment may cause burns (high probability that harm results)

Make it clear that hot equipment is in use Communication and effective placement of CAUTION HOT signs

In an emergency, the following emergency procedures must be followed:

1. Clear the immediate area

2. Turn power off from main power point

3. Close CO2 gas cylinder

4. Keep clear of area until area is declared safe by authorised personnel

5. Contact Chief Technical Officer ASAP

Apparatus

Figure : Diagrammatic representation of the gas absorption column (2012, Manual)

The apparatus has a sump tank (ST), a large 50L rectangular tank located underneath the base of

the absorption column. It holds the liquid that is used in the experiments. Valves on the

underside of it allow it to be drained for cleaning and replacing of liquid absorbent.

The absorption column (AC1) is a packed absorption column made of two 75 mm diameter

borosilicate glass sections joined end to end to give a total column length of 1.4 m. It is arranged

vertically and filled with 10 mm x 10 mm glass Raschig rings.

The Hempl Apparatus (HA) consists of two glass bulbs filled with liquid, a level reader and a

sampling syringe (SS1). This device determines the concentration of CO2 in a gas stream.

The system has 2 pumps: P1 is a rotary compressor pump that feeds air into the system and P2 is

a centrifugal pump that delivers liquid to the top of the column.

Flow is measured by three different flow meters: F1 measures the flow rate of the liquid being

delivered to the top of the column, F2 measures the flow rate of air being delivered to the

bottom of the column and F3 measures the flow rate of CO2 being delivered to the bottom of

the column. Flow is regulated by control valves C1 which adjust the flow of liquid being

delivered to the top of the column, C2 adjusts the flow rate of air, C3 adjust the flow rate of

CO2 and C4 adjust the flow rate of liquid returning the sump tank (usually fully open).

Valves are used to direct gas in the apparatus. V1 directs gas stream from middle of the column

to either the Hempl apparatus or manometers. The V2 valve directs gas stream from bottom of

the column to either Hempl Apparatus or M2. V3 directs gas stream from top of the column to

either Hempl Apparatus or manometer 1. V4 isolates Hempl Apparatus from the column and

V5 allows samples to enter/exit syringe or allows syringe to vent to atmosphere. The isolation

valves in the system are IV1 which isolates the middle of the column; IV2 isolates the bottom of

the column and IV3 isolates the top of the column.

Manometers are used in the system to indicate pressure drops. There are 2 manometers used: M1

is a U-tube manometer that measures the pressure drop between the top and middle of the

column and M2 is another U-tube manometer that measures the pressure drop between the

middle and bottom of the column

Experimental Method

Experiment A

Set up the column as follows:

1. 40g NaOH was dissolved in 1L of water to make 1.0 M NaOH solution

2. Globes of Hempl apparatus were filled with NaOH solution, approximately 300mL was

needed

3. Globes were zerod off by using drain pipe

4. Water supply for the sump tank was turned on

5. A drain pipe was added to divert away liquid return to sump tank

6. Valves 1-3 were closed off

7. P2 (liquid pump) was turned on and flow adjusted using C1 to 4L/min as shown by F1

8. P1 (air pump) was turned on and flow adjusted using C2 to 25L/min as shown by F2

9. CO2 flow rate was adjusted using C3 to approx.. 12.5L/min as shown by F3

10. After 5 minutes the apparatus is ready, as steady state was assumed to be achieved.

Analysis was performed by

1. Sample line was flushed by correctly positioning V3, V4 and V5 and exposing sample

syringe to top of column. The syringe was withdrawn to fill and then V5 was adjusted to

open the syringe to atmosphere. Contents were expelled and syringe exposed back to

apparatus using V5

2. Step 1 was repeated approximately 3-4 times

3. A sample was taken from top of column ensuring that V4 was isolated at the same time

4. Gas was allowed to reach thermal equilibrium

5. Gas was exposed to atmosphere through V5, allowed to equilibrate to atmospheric

pressure for 10 seconds

6. The sampling cylinder was connected to absorption columns by adjusting V4 and V5. If

liquid level has changed, repeat step 5

7. Gas was injected into Hempl apparatus

8. Piston was reinserted and withdrawn until volume level of Hempl app. stopped

noticeably changing

9. The volume change of liquid was noted as V2

10. System was reset: V4 was adjusted to expose absorption globes to atmosphere and V5 set

to isolate sampling syringe from whole apparatus.

Experiment B

For this experiment we needed phenolphthalein, standardised 0.02778M NaOH and

standardised 0.01M Na2CO3.

Column was setup by:

1. Sump tank water supply was switched on. Ensure no liquid return.

2. Valves C1 to C3 were closed

3. P2 was started, liquid feed was set to 6L/min (C1, F1)

4. P1 was started, air feed was set to 10% of full scale (C2, F2)

5. CO2 flow was adjusted to half of air flow rate (C3, F3)

6. Steady state was assumed to be achieved after 5 minutes.

Sampling of liquid done by

1. Took minimum of 100mL of sample from mains inlet

2. Sample was poured into graduated cylinder (100mL) and volume noted as Vsample

3. 5-10 drops of phenolphthalein was added

4. Acid base titration was performed as outlined in titration method below

5. Sampling was taken from bottom of column and titrated as well.

Titration method:

1. NaOH solution was loaded to burette and start volume noted as Vinitial

2. Liquid solution with phenolphthalein indicator was titrated by adding drops of NaOH

while continually swirling the solution

3. When permanent color change was reached, the volume in burette was noted as Vfinal

4. Volume of NaOH consumed was then calculated

Experiment C

This experiment is prepared by filling the sump tank with 0.2M NaOH solution (30L tap water

and 7.5L caustic soda solution). Methyl orange indicator, standardised 0.2M hydrochloric acid

solution and standardised BaCl2 solution (5% wt) was also needed.

Column was setup by:

1. Sump tank water supply was switched on. Ensure no liquid return.

2. Valves C1 to C3 were closed

3. P2 was started, liquid feed was set to 3L/min (C1, F1)

4. P1 was started, air feed was set to 30L/min (C2, F2). Column was not allowed to flood

5. CO2 flow was adjusted to 3L/min (C3, F3)

6. Steady state was assumed to be achieved after 5 minutes.

Sampling was done by taking around 250mL of both sump tank and column liquid outflow each

and placing 50mL of each in 2 separate flasks (4 flasks total). The flasks were then labelled Flask

1 sump, Flask 1 outflow, Flask 2 sump, Flask 2-outflow and then tested as below.

Flask 1 analysis

1. Phenolphthalein indicator was added (1 drop)

2. Burette was loaded with HCl solution and volume noted as T1,initial

3. Titrant was added dropwise until permanent color change was observed

4. Endpoint volume was noted as T1, final

5. Methyl orange indicator was added (1 drop)

6. Burette was loaded again with HCl solution and volume noted as T2,initial

7. Titrant was added dropwise until permanent color change was observed

8. Endpoint volume was noted as T2, final and total added T2 was calculated

9. Titrations were then performed in triplicate to ensure accuracy by using the original

250mL sample

Flask 2 analysis

1. BaCl2 was added to flask 2 and shaken to precipitate all carbonates as barium carbonate.

Volume added was 10% excess of difference T2-T1

2. Two drops of phenolphthalein indicator was added

3. Burette was filled with HCl and volume noted as T3, initial

4. Titrant was added dropwise to the solution while continually swirling until permanent

color change was observed

5. Endpoint volume was noted as T3,final and T3 as difference between T3, initial and T3,final

6. Titrations were then performed in triplicate to ensure accuracy by using the original

250mL sample

Experiment D

Experiment D is carried out alongside experiment C so all the settings are the same.

The experiment is carried as follows:

1. Valves V1, V2 and V3 were used to direct gas at top of column to manometer M1 as well

as gas at middle of column. Gas from middle and bottom of column were directed to

manometer M2

2. Height difference in each manometer was noted as a pressure drop across column

3. Manometer M1 reading was recorded was Ptop

4. Manometer M2 reading was recorded was Pbottom

5. Valves were then returned to original position to measure the CO2 concentration as

performed in previous parts of the experiment

6. Experiment was repeated in 20 minute intervals

References

CHE3165 Separation Processes: Absorption Lab Manual, 2012, Monash University

Geankoplis C, Transport Processes and Separation Process Principles, 2003, Prentice Hall, 4th

International Edition

Green D W, Perry R H, Perrys Chemical Engineers Handbook, 2008, McGraw Hill, 8th E-

book edition

MSDS, Science stuff, 2010, http://www.sciencestuff.com/chemicals/S1.shtml