Abstract

The experiment is based on pressure drop, the air flow rate , the water flow rate and also

the packed column. The pressure drop is increased when the water flow rate and air flow

rate is increased. This experiment is to examine the air pressure drop across the column as

a function of air flow rate for different water flow rates through the column. The graph of

log pressure drop against log of air flow rate is plotted. The graph of generalized

theoretical pressure drop correlation chart for random packing is also plotted. Both of the

graph have same principle where high flow rate parameter is meant for high liquid flowand

high pressure drop while low flow rate parameter is meant for low liquid flow and low

pressure drop.

Introduction

Absorption is a mass transfer process in which a vapor solute A in a gas mixture is

absorbed by means of a liquid in which the solute more or less soluble. The gas mixture

consists mainly of an inert gas and the soluble. The liquid also is primarily in the gas

phase; that is, its vaporization into the gas phase is relatively slight. A typical example is

absorption of the solute ammonia from an air-ammonia mixture by water. Subsequently,

the solute is recovered from the solution by distillation. In the reverse process desorption

or stripping, the same principle and equations hold.

Gas absorption is the unit operation in which one or more soluble components of a gas

mixture are dissolved in a liquid. The gas phase or gas mixture is inert gas while the

liquid phase is immiscible in the gas phase. Therefore, the liquid phase will vapourize

very slightly in gas phase. Gas absorption (also known as scrubbing) is an operation in

which a gas mixture is contacted with a liquid for the purpose of preferentially dissolving

one or more components of the gas mixture and to provide a solution of them in the

liquid. We can see that there is a mass transfer of the component of the gas from the gas

phase to the liquid phase. The solute so transferred is said to be absorbed by the liquid.

There are 2 types of absorption processes which are physical absorption and chemical

absorption, depending on whether there is any chemical reaction between the solute and

the solvent (absorbent).

When water and hydrocarbon oils are used as absorbents, no significant chemical

reactions occur between the absorbent and the solute, and the process is commonly

referred to as physical absorption.

When aqueous sodium hydroxide (a strong base) is used as the absorbent to dissolve an

acid gas, absorption is accompanied by a rapid and irreversible neutralization reaction in

the liquid phase and the process is referred to as chemical absorption or reactive

absorption.

More complex examples of chemical absorption are processes for absorbing CO2 and H2S

with aqueous solution of monoethanolamine (MEA), diethanolamine (DEA),

diethyleneglycol (DEG) or triethyleneglycol (TEG), where a reversible chemical reaction

takes place in the liquid phase. Chemical reactions can increase the rate of absorption,

increase the absorption capacity of the solvent, increase selectivity to preferentially

dissolve only certain components of the gas, and convert a hazardous chemical to a safe

compound.

Aim

1. To determine the pressure drop across the dry column as a function of air flowrate.

2. To examine the air pressure drop across the column as a function of air flow rate for

different water flow rates through the column.

Theory

Packed columns are used for efficient gas-liquid contact processes ‘contact interface’ into

the bulk of the liquid. The driving force for absorption involves a concentration gradient

across the gas-liquid interface. Figure 1 provides a visual of the gas-liquid ‘contact

interface’. Packed columns are used for efficient gas-liquid contact processes .It is used

in processes like gas absorption, desorption(stripping) , distillation etc. It mainly consists

of a cylindrical column filled with packings , liquid inlet and distributor at the top, gas

inlet at the bottom, liquid and gas outlets at the bottom and top respectively. Column

packings can be of two types mainly: dumped and structured . A distributor consists of

several perforated pipes used for spreading the liquid uniformly throughout the cross-

section of column.

Figure 1: gas liquid interface

Loading:

Amount of liquid accumulate in side packed column that generate pressure drop.

Figurer 2 : loading in side packed column

Flooding :

Amount of the liquid flood in the top of column with increasing pressure drop due to

accumulation of liquid in side packed column

Figure 3 :flooding in packed column

Pressure drop is the result of fluid friction between liquid flow and the packings.

loading

flooding

The graph above shows the relationship between pressure drop and gas flow rate and

fordry column, a straight line is plotted and wet column three curvy lines are plotted. The

three curves are parallel to the straight line. The point where liquid holdup starts to

increase is the point where the slope starts to change. This point is known as the

loading point. When the gas flow rate is further increased, pressure drop rises

tremendously untilthe lines plotted are almost vertical and at this point, liquid is of

continuous phase.

This point is known as the flooding point and happens when liquid accumulates due to hi

ghgas flow rate and this accumulation continues until the packed column is completely

filled with liquid.

Material and apparatus

1. Solteq-QVF absorption column (model: BP 751-B)

Procedure

1. All valves is ensured are closed except the ventilation valve V13.

2. All the gas connection is checked properly fitted.

3. The power for control panel is on.

4. The receiving vessel B2 is filled through the charge port with 50 L of water by

opening valve V3 and V5.

5. Valve V3 is closed.

6. Valve V10 and V9 is opened slightly. The flow of water from vessel B1 through

pump P1 is observed.

7. Pump P1 is switch on, valve V11 is open and adjusted slowly to give a water flow

rate of around 1L/min. The water is allowed to enter the top of the column K1, flow

down the column and accumulate at the bottom until it overflows back into vessel B1.

8. Valve V11 is adjusted and open to give a water flow rate of 1L/min into the column

K1.

9. Valve V1 is open and adjusted to give an air flow rate of 20L/min into column K1.

10. The pressure drop is recorded .

11. Step 9 is repeated with the different values of air flow rate, each time increasing by

20 L/min while maintaining the same flow rate.

12. Step 8 to 9 is repeated with different values of water flow rate, each time increasing

by 1L/min by adjusting valve V11.

Result

Flow rate

(L/min)

Pressure drop (mm H20)

Air

water

20 40 60 80 100 120 140 160 180

1 0 1 2 4 8 14 28 51 -

2 0 1 3 5 10 17 73 - -

3 0 4 8 16 48 58 - - -

Table 1: Flow rate and pressure drop

Flow rate Pressure drop (mm H20)

(L/min)

Air

water

1.3 1.6 1.8 1.9 2.0 2.1 2.15 2.2 2.3

1 0 0 0.3 0.6 0.9 1.1 1.4 1.7 -

2 0 0 0.5 0.7 1 1.2 1.9 - -

3 0 0.6 0.9 1.2 1.7 1.8 - - -

Table 2: Log air flow rate and Log pressure drop

Calculation

Data:

Density of air = 1.175 kg/m3

density of water = 996 kg/m3

Column diameter =80 mm

Area of packed column diameter = 0.005027m2

Packing Factor = 900 m-1

0

0.5

1

1.5

2

1.3 1.6 1.8 1.9 2 2.1 2.15 2.2

Log

Pre

ssu

re D

rop

, m

mH

20

Log Gas Flow Rate, Gy

Graph of Log Pressure Drop against Log Gas Flow Rate

1LPM

2LPM

3LPM

Water viscosity = 0.001Ns/m2

Theoretical flooding point:

GG, gas flow rate (kg/m2s)

GG = 𝐺𝑦𝑋𝜌

𝐴

=

20𝐿

𝑚𝑖𝑛x

1𝑚𝑖𝑛

60𝑠𝑒𝑐𝑥

1.175𝑘𝑔

𝑚3𝑥

1𝑚3

1000𝐿

0.005027

=0.0779kg/m2s

Capacity parameter, y-axis

y-axis =

13.1 (𝐺𝐺 )2𝐹𝑝(𝜇𝐿𝜌𝐿

)0.1

𝑃𝐺 (𝑃𝐿 −𝑃𝐺 )

= 13.1 (0.0779) 2900

(0.001996

)0.1

1.175 (996−1.175)

= 0.0154

GL, liquid flow rate per unit column cross-sectional area

GL = 𝐺𝐿 𝑋𝜌

𝐴

=

1𝐿

𝑚𝑖𝑛x

1𝑚𝑖𝑛

60𝑠𝑒𝑐𝑥

1.175𝑘𝑔

𝑚3𝑥

1𝑚3

1000𝐿

0.005027

=3.3002 kg/m2s

Flow parameter, x-axis

x-axis = 𝐺𝐿

𝐺𝐺(√

𝜌𝐺

𝜌𝐿)

= 3.3002

0.0779(√

1.175

996)

= 1.4551

Water flow rate

(L/min)

GL (kg/m2s)

1.0 3.3002

2.0 6.6004

3.0 9.9006

Air flow rate

(L/min)

GG

(kg/m2s)

Capacity

parameter

(y-axis)

Flow parameter (x-axis)

1 LPM 2 LPM 3 LPM

20 0.0779 0.0154 1.4551 2.9102 4.3653

40 0.1557 0.0614 0.7280 1.4560 2.1840

60 0.2336 0.1383 0.4852 0.9705 1.4557

80 0.3115 0.2459 0.3639 0.7278 1.0917

100 0.3893 0.3841 0.2912 0.5823 0.8735

120 0.4672 0.5532 0.2428 0.4852 0.7279

140 0.6229 0.7531 0.2079 0.4159 0.6238

160 0.6229 0.9832 0.1820 0.3639 0.5459

Discussion

In this experiment, we are going to determine the pressure drop across the dry column as

a function of air flow rate and the air pressure drop across the column as a function of air

flow rate for different flow rates through the column. These two objectives are achieved

by using the same apparatus but different method. The experiment is bases on the flow

rate of liquid and gas in the packed.

Firstly, the water flow rate is kept constant to 1L/min and air flow rate reading is

recorded when achieved 1 minute. The air flow rate is kept rising constantly by 20 L/min

by each minute. All reading of pressure drop is recorded until flooding point is reached.

The pressure drop for flow rate of air are 1, 2, 4, 8 , 14, 28 and 51 mm H20 consequently

to 20, 40, 60, 80, 100, 120, 140, and 160 L/min. It cannot reach 180 of air flow rate,

which the water will sprayed out from the column due to the high flow rate.

The the flow rate is adjusted to 2L/min, by using the same step as recording the 1L/min.

the data recorded are 0, 1, 3, 5, 10, 17 and 73 mm H20 consequently to 20, 40, 60, 80,

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0.0154 0.0614 0.1383 0.2459 0.3841 0.5532 0.7531 0.9832

Cap

acit

y P

aram

ete

r

Flowrate parameter

Generalised Thepretical Pressure Drop Correlation Chart for Random

Packings

1LPM

2LPM

3LPM

100, 120, and 140 L/min. Whereby, for flow rate of water 3L/min is 0, 4, 8, 16, 48 and 58

consequently to 20, 40, 60, 80, 100, and 120.

With these data record, the graph of log of pressure drop against the log of air flow rate is

plotted. From the graph plotted, all the log pressure drop is directly proportional log gas

flow rate. We can conclude that the higher the log of gas flow rate the higher the log of

pressure drop. But the higher the water flow rate, the lower the log gas flow rate.

For correlated value of the pressure drop is calculated and the graph of capacity

parameter against flow rate parameter is plotted. The capacity parameter is indirectly

proportional to flow rate parameter. Two of the graph is different as one is directly

proportional while the other one is indirectly proportional.

Conclusion

In conclusion, the aim of experiment which is to determine the pressure drop across the

column as a function of air flow rate for different water flow rate through the column is

achieved. They are some errors made when the experiment is being conducted resulting

in a slight in accuracy of the experimental chart plotted.

Recommendation

When conducting this experiment, there are several recommendations that will produce

better observation which will not differ much from the theoretical observations.

Firstly, safety is very important when doing experiment. Thus, we need to wear

laboratory coat, helmet and fully cover shoes to avoid any danger for safety precaution.

Titration must take place in fume chamber and must be stop when the solution turns to

light pink. Next, when taking the reading of volume of sodium hydroxide solution, make

sure that eyes is directly perpendicular with the level of sodium hydroxide solution inside

the burrette to avoid any parallax error. Before conducting the experiment, we must

ensure that all the apparatus are in good condition and follow all the procedures in order

to get more accurate result.

Reference

I. http://en.wikipedia.org/wiki/Liquid%E2%80%93liquid_extraction

II. http://www.academia.edu/3641270/Liquid-Liquid_Extraction_Basic_Principles

III. http://www.chem.ualberta.ca/~orglabs/Interactive%20Tutorials/separation/Theory

/theory1_1.htm

IV. http://courses.chem.psu.edu/chem36/Experiments/PDF's_for_techniques/Liquid_

Liquid.pdf

V. Lab Manual