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Analysis of the effect of flow rate, packing size and materials on the overall performance of the absorption process in a packed absorption column. 1

Ameerul Afiq, 2Arina Hazirah, 3Mohd Asraf, and 4Yuweta 1,2,3,4

Chemical Engineering Section

Universiti Kuala Lumpur Malaysian Institute of Chemical and Bioengineering Technology 1988 Bandar Vendor, Taboh Naning, 78000 Alor Gajah, Melaka, MALAYSIA. 1

3

E-mail: [email protected] 2E-mail: [email protected]

E-mail: [email protected] 4E-mail: [email protected]

Abstract Packed columns are commonly used in chemical industry to remove volatile substances from a liquid or to absorb a gas from a mixture of gases. Usually, the columns use countercurrent flow, in which gas flows upward, and liquid flow downward. Packings are filled inside the column to provide large contact area for mass transfer between gas and liquid. There is a need to find the optimum condition for the operation of an absorption column in order to increase the absorption efficiency of the process, while at the same time reduce the energy requirement and ensure optimized mass transfer. Several factors need to be taken into consideration before this optimization can be done. In this open-ended experiment, we were tasked to run an absorption process using a Packed Absorption column and to analyze the effect of the different flow rate that can affect the overall performance of the absorption process. Not only that, we were also tasked to investigate the effects of different types and sizes of the packing materials on the efficiency of CO2 removal. This analysis is done using direct titration between HCL and mixture of NaOH and CO2 mixture from the column to find the unreacted NaOH. Keywords: absorption column, packing material, packing size, percentage removal, CO2 removal Highlights • •

•

Lower flow rate will lead to a better percentage removal of CO 2. Big sized packing material will lead to a better percentage removal of CO2 as it will have a larger contact area for carbon dioxide and water, hence facilitating the absorption process. Ceramic material will have a better CO2 removal percentage compared to glass material.

1. Introduction/Theory This study aims to analyze the effect of flow rate, packing size and packing material towards the percentage removal of CO2 and towards the overall performance of the absorption process. According to IUPAC (Absorption, 2019), absorption is defined as the process of a single material, absorbate, retained by another material, absorbent. Absorption process can be either physical or chemical, depending if any chemical reaction occurs between the absorbate and absorbent, in which atoms, molecules or ions penetrate the bulk phase of either liquid or solid material and are distributed throughout the whole volume of absorbent. If no significant chemical reaction occurs between the absorbate and absorbent, the process is referred to as physical absorption. In the experiment, gas mixtures are pumped from the bottom while water are pumped from the top, in a counter-current manner. This process is known as gas absorption or scrubbing, due to the reason that the contact between gas mixture and liquid for the purpose of dissolving components of the gas into the liquid.

Packing materials in packed absorption column plays a huge part in the absorption process as they facilitate the process of absorption through their large contact area which enables the two phases to be in contact. These packing materials are usually made from either plastics, ceramics or metals. According to Sinott, Richardson & Coulson (2013), good packing materials should provide a large contact area between the gas and liquid, have low resistance to gas flow, promote uniform liquid distribution on the packing surface, and promote uniform

vapour gas flow across the column crosssection.

Packing can be generally divided into two types, which are regular geometry packings and random packings. Regular geometry packings have regular geometry such as stacked grids and structured packings. On the other hand, random packings consist of rings, saddles and proprietary shapes, which are dumped into the column and assumed a random arrangement. Random packings are more commonly used in the process industry (Sinott, Richardson & Coulson, 2013). This experiment will also utilize random packings in the separation process.

There is a need to know the maximum gas flow rate that can be used. This is because higher gas flow rate will lead to a greater resistance felt by the liquid and a higher pressure drop across the packing materials. When the gas flow rate becomes too much, a condition called flooding occurs where the liquid will fill the entire column and will lead to difficulties in carrying out the operation. The high pressure then will damage the packings in the column. To prevent this, the gas flow rate should be ½ of the flooding velocity. Otherwise, a specified pressure drop condition can be set which will be well below the pressure drop in which flooding would occur (“Column Diameter and Pressure Drop”, 2019).

Generally, the packing size depends on the size of the column itself. However, the general rule is that the packing size should not exceed 50 mm as smaller packing cost more compared to larger packing. Besides that, the usage of large packing in a small

column will cause poor liquid distribution (Sinott, Richardson & Coulson, 2013). The recommended packing sizes according to the column diameter is shown in Table 1.

Table 1: Packing size according to column diameter. (Source: Sinott, Richardson & Coulson, 2013) Column diameter

Packing size to be used

< 0.3 m (1 ft)

< 25 mm (1 in.)

0.3 to 0.9 m (1 to 3 ft)

25 to 38 mm (1 to 1.5 in.)

> 0.9 m

50 to 75 mm (2 to 3 in.)

There are many parameters that need to be considered before designing a packed absorption column to obtain maximum efficiency of gas absorption and to prevent flooding problem from occurring. When carbon dioxide is absorbed into the liquid water, it reacts with the liquid water to form dihydrogen carbonate (carbonic acid), which dissociates further to for aqueous hydrogen and aqueous hydrogen carbonate ions.

𝐶𝑂2(𝑔)+ 𝐻2𝑂 (𝑙)→ 𝐻2𝐶𝑂3 (𝑎𝑞) 𝐻2𝐶𝑂3 (𝑎𝑞)→ 𝐻+ + 𝐻𝐶𝑂−3

(1) (2)

In order to determine the amount of carbon dioxide being absorbed into the water, neutralization of carbonic acid is done since the amount of carbonic acid formed id proportional to the amount of carbon dioxide absorbed by the water. Thus, sodium hydroxide is used, which are readily dissociates in water into sodium ion and hydroxide ion. The carbonic acid then reacts

with sodium hydroxide to for sodium hydrogen carbonate, which are neutral, thus completing the reaction.

𝑁𝑎𝑂𝐻 (𝑎𝑞) → 𝑁𝑎+ + 𝑂𝐻− (3) 𝐻2𝐶𝑂3 (𝑎𝑞) + 𝑁𝑎𝑂𝐻 (𝑎𝑞) → 𝑁𝑎𝐻𝐶𝑂3 (𝑎𝑞) + 𝐻2𝑂(𝑎𝑞) (4)

When determining the efficiency of water in removing carbon dioxide content in air by using a packed bed reactor, four factors influencing the rate of absorption have to be taken into consideration; flowrate of water, flowrate of gas, size of packing material and material of construction of the packing material.

As for the effect of water flowrate and gas flowrate to the absorption of carbon dioxide into water, it should be fairly straightforward. Since physical absorption involve an absorbate to be absorbed by absorbent, an increase in water (absorbent) flowrate and decrease in gas (absorbate) flowrate should result in increase of absorption of carbon dioxide into water, proven by the findings of Tan, Shariff, Lau, & Bustam (2012).

Figure 1: Effect of gas flowrate to CO 2 concentration (Tan, Shariff, Lau, & Bustam, 2012).

According to Arachchige & Melaaen (2012), the effect of packing material and packing material size on carbon dioxide absorption are influenced by factors including specific surface area, surface area spread uniformity, void space per unit column volume and friction. In general, maximum specific surface area in uniform manner with maximum void space per unit column volume and minimal friction will result in the greatest removal of carbon dioxide in gas mixture. This is due to the result of improved vaporliquid contact from increased surface area and uniformity of packing material. By maximizing the void space per unit column volume and minimizing friction, the gas faces minimal resistance when going up the flow.

2. Procedure 2.1 Start up Firstly, recirculation vessel was filled up with water for about half of the vessel. Valves FCV1, FCV2, FCV3, FCV4, V7, V12 and V13 were ensured to be closed. The column to be operated was chosen and the valves were adjusted according to the table below. The manometer reading was set to zero, and the selector valve switch, SV1 must be connected to the chosen column. Table 2: Valve opening & column relation

V1 V2 V3 V4 V5 V6 V7 V8 V9 V10 SV1

Column 1 O X X O X X O O X X O

Column 2 X O X X O X O X O X O

Column 3 X X O X X O O X X O O

O – Open X – Closed

2.2 Comparison of Material Size: Column 1 vs Column 2 For Column 1, the power switch of recirculation pump is turned on. All valves were ensured to be closed except for V4, V7, V8, and V13. Next, FCV4 was slowly opened and was adjusted to obtain the flowrate of 5 L/min as indicated by FI3. After that, FCV1,

FCV 2, and V1 were opened to allow the air and CO2 to enter the column and gave the required air flowrate as indicated by FI1 and FI2, and V11 will be opened. The absorption process was let to stable for 10 minutes to attain steady state. After 10 minutes, samples will be collected at valve V12 for half an hour to analyze the removal of CO2. For Column 2, the power switch of recirculation pump is turned on. All valves were ensured to be closed except for V5, V7, V9, and V13. Next, FCV4 was slowly opened and was adjusted to obtain the flowrate of 5 L/min as indicated by FI3. After that, FCV1, FCV 2, and V2 were opened to allow the air and CO2 to enter the column and gave the required air flowrate as indicated by FI1 and FI2, and V11 will be opened. The absorption process was let to stable for 10 minutes to attain steady state. After 10 minutes, samples will be collected at valve V12 for half an hour to analyze the removal of CO2.

2.3 Comparison of Material Type: Column 2 vs Column 3 For Column 2, the same data was taken from previous step at column 2. For Column 3, the power switch of recirculation pump is turned on. All valves were ensured to be closed except for V6, V7, V10, and V13. Next, FCV4 was slowly opened and was adjusted to obtain the flowrate of 5 L/min as indicated by FI3. After that, FCV1, FCV 2, and V3 were opened to allow the air and CO 2 to enter the column and gave the required air flowrate as indicated by FI1 and FI2, and V11 will be opened. The absorption process was let to stable for 10 minutes to attain steady state. After 10 minutes, samples will be collected at valve V12 for half an hour to analyze the removal of CO2.

2.4 Analysis of Carbon Dioxide in Water Sample Every 10 minutes, 10mL sample from the outlet valve was taken. After that, 30mL of 0.01 M NaOH solution was added to the sample. The volume of NaOH was in excess to allow all CO2 had reacted with NaOH in the mixture. Next, a few drops of indicator (phenolphthalein) was dropped to the solution. The mixture was then titrated with 0.01 M HCl solution to obtain the amount of unreacted NaOH. The titration process was repeated every 10 minutes with fresh samples until 30 minutes. The results were

then tabulated, and the experiment was repeated with samples from different columns.

2.5 Shut Down The CO2 supply and the compressed air supply at valve SG & CA is slowly closed. Next, the power switch of recirculation pump is turned off. After that, the air supply and CO2 supply were also turned off. Valve V11 will be opened to allow fresh water to enter the column for a few minutes to drain off the water.

3. Result Volume of NaOH = 30mL Volume of CO2 in sample = 10mL

Table 3: Volume of HCL at air flowrate 20 L/min and 40 L/min at Colum 1 (small glass) Air flowrate (L/min)

20

Time (min)

Water flowrate (L/min)

Volume of HCL (mL)

10

1

21.90

20

3

22.50

30

5

23.40

10

1

17.90

20

3

18.80

30

5

21.15

40

Table 4: Volume of HCL at air flowrate of 40 L/min at Column 2 Time, t (min) 10 20 30

Water flowrate (L/min) 1 3 5

Volume of HCL (mL) 24.00 22.50 22.10

Table 5: Volume of HCL at air flowrate of 40 L/min at Column 3 Time, t (min) 10 20 30

Water flowrate (L/min) 1 3 5

Volume of HCL (mL) 24.35 23.25 22.30

Table 6: Concentration of CO2 in inlet and outlet and percentage removal of CO2 at air flow rate at 20 L/min at column 1 Time, t (min)

Concentration of CO2 inlet (mol/L)

Concentration of CO2 outlet (mol/L)

Concentration of CO2 in water (mol/L)

Percentage removal of CO2 (%)

10

0.045

4.05 ×10-3

0.04095

91.00

20

0.045

3.75×10-3

0.04125

91.67

30

0.045

3.25×10-3

0.04175

92.78

Table 7: Concentration of CO2 in inlet and outlet and percentage removal of CO2 at air flow rate at 40 L/min at column 1 Concentration Concentration Concentration Percentage Time, t (min) of CO2 inlet of CO2 outlet of CO2 in water removal of (mol/L) (mol/L) (mol/L) CO2 (%) 10

0.045

6.05 ×10-3

0.03895

86.56

20

0.045

5.60×10-3

0.03940

87.56

30

0.045

4.43×10-3

0.04057

90.16

Table 8: Concentration of CO2 in, inlet and outlet and percentage removal of CO2 at air flow rate at 40 L/min at column 2 Concentration Concentration Concentration Percentage Time, t (min) of CO2 inlet of CO2 outlet of CO2 in water removal of (mol/L) (mol/L) (mol/L) CO2 (%) 10

0.045

3.00 ×10-3

0.04200

93.30

20

0.045

3.75×10-3

0.04125

91.67

30

0.045

3.95×10-3

0.04105

91.22

Table 9: Concentration of CO2 in, inlet and outlet and percentage removal of CO 2 at air flow rate at 40 L/min at column 3 Concentration Concentration Concentration Percentage Time, t (min) of CO2 inlet of CO2 outlet of CO2 in water removal of (mol/L) (mol/L) (mol/L) CO2 (%) 10

0.045

2.825×10-3

0.042175

93.72

20

0.045

3.37510-3

0.041625

92.50

30

0.045

3.850×10-3

0.041150

91.44

Table 10: Average percentage removal of CO2 Column 1 at 20 mL/min 1 at 40 mL/min 2 at 40 mL/min 3 at 40 mL/min

Percentage removal CO2 (%) 91.82 88.09 92.06 92.35

Percenntage of CO2 removal (%)

94 93 92 91 90

20 L/min

89

40 L/min

88 87 86 0

5

10

15

20

25

30

35

Time (min)

Figure 2: Percentage removal of CO2 (%) against time (min) for Column 1 at air flowrate of 20 L/min and 40 L/min

Percentage of CO2 removal (%)

94 93 92 91 90

Column 1

89

Column 2

88 87 86 0

5

10

15

20

25

30

35

Time (min)

Figure 3: Percentage removal of CO2 (%) against time (min) for Column 1 and Column 2 at flowrate of 40 L/min

Percentage of CO2 removal (%)

94 93.5 93 92.5

Column 2 Column 3

92 91.5 91 0

5

10

15

20

25

30

35

Time (min)

Figure 4: Percentage removal of CO2 (%) against time (min) for Column 2 and Column 3 at flowrate of 40 L/min

4. Discussion

phenolphthalein. The sample is then directly

Throughout

this experiment,

the

objective was to study how the different flowrate of gas and liquid effect the rate of absorption of carbon dioxide. Besides, this

titrated with 0.01M hydrochloric acid. The samples were titrated with hydrochloric acid until an observation of first pale pink presents.

experiment was carried out to study how the

According to Table 3, the volume of

packing material of different material and

the titrated hydrochloric acid needed to

size effect the rate of absorption of carbon

obtain

dioxide. There were three types of packed

recorded.

column

the

causes the carbon dioxide in the solution to

experiment and which were Column 1 (small

react with the sodium hydroxide to form

sized glass material), Column 2 (big sized

sodium bicarbonate. The phenolphthalein

glass material) and Column 3 (ceramic

indicator then determines the amount of

material).

the

unreacted sodium hydroxide with carbon

experiment is separated into two parts; the

dioxide in the sample mixture. In flow rate at

first part, the experiment is conducted using

20 L/min, the hydrochloric acid volume

the instrument of packed column gas

increased with the time when the flowrate of

absorption. Meanwhile, another experiment

water was increased of 1,3 and 5 L/min; until

focused on the chemical preparations which

the experiment reaches the time at 30th

used for the sample analysis from the first

minute. There were three readings taken due

experiment,

to study the differences of 2mL between the

being

used

Within

using

to

the

direct

conduct

objectives,

titration.

The

the

pale

pink

observation

are

The hydrochloric acid titration

instrument of packed column gas absorption

flowrate of water.

The highest volume of

was stable by constant water flow rate by 10

HCL for the flow rate 20 L/min was 23.4 mL

minutes. This was to ensure that the surface

at 30 minutes while the lowest volume was

of glass was wet by the water to produce gas

21.9 ml at the beginning of zero minute.

carbon dioxide. Overall, this experiment was

Likewise, to the flow rate at 40 L/min the

conducted by the changes of variable which

volume of HCL needed increase within time.

are at air flow rate 20 L/min and 40 L/min at

Based on the Figure 2, when compared

Column 1 while for Column 2 and 3 the air

between the two flowrates, was concluded

flowrate was at 40 L/min. Every 10 minutes,

that the percentage of carbon dioxide

the sample was taken for the measurement

removal at the flowrate of 20 L/min was

of the concentration of carbon dioxide. Each

higher compared to flowrate of 40 L/min

sample were added with 30 mL of 0.01 M

which was the average of carbon dioxide

sodium hydroxide and a few drops of

removed at 20 L/min is 91.82%.

The Column 1 and 2 were studied on

between the volume of hydrochloric acid as

how the size of the packing material affect

per Table 4 and 5 , the values were not

the absorption of carbon dioxide at flowrate

showed in big range of differences between

of 40 L/min. As shown in Table 4 and 5, at

the values were obtained from Column 2 and

the flowrate or 40 L/min, the percentage of

3. Based on Figure 4, the trendline of Column

carbon dioxide removed at Column 1

2 and 3 were individually decreased with time

increased with time, while at the Column 2

increased but according to Table 10, the

the percentage of carbon dioxide removal

percentage removal of carbon dioxide at

decreased with time. Based on Figure 3, it

Column 3 was 92.35% while Column 2 was

was shown that the trendline of Column 2

92.06% which showed that at Column 3

was being decreased with time increased

which used ceramic material absorbed more

compared to the Column 1 at the flowrate of

carbon dioxide compared to Column 2 which

40 L/min. however, based on Table 10,

was glass material. This is due to the friction

average percentage of

carbon dioxide

property of material. The ceramic material

removal was higher at Column 2 compared

has less friction compared to the glass

to Column 1.This is because the big sized

material.

packing material inside of Column 2 has larger contact area for the absorption process to occur compared to the smaller material inside Column 1, which means that Column 2 has the capacity of absorbing higher amount of carbon dioxide compared to Column 1.

There were three main factors that affected the percentage removal of CO2, which are the interfacial between gas phase and liquid phase, the resistance in the gas phase and the resistance in the liquid phase. In the process of absorption, the total gas flow rate is constantly changing due to CO2

Furthermore, the experiment was

absorption into water solution. According to

conducted on comparing the types of

Yeh & Pennline (2001), the mass transfer

material of packed Column 2 with the packed

resistance of the gas phase will decrease

Column 3 which the Column 2 was packed

with the increasing CO2 partial pressure

with glass material while Column 3 was

which is according to two-film theory.

packed with ceramic material at constant

Basically, an increase in the CO 2 partial

flowrate of 40 L/min. According to Table 4,

pressure allows more CO2 molecules to

for Column 2, the volume of hydrochloric acid

travel from gas bulk to the gas-liquid

used increased with time and likewise

interface, which would result in higher mass

happened at Column 3, as proven from

transfer performance. In case of the liquid

Table 5. When the values were compared

flow rate, it was found that an increase in the

liquid flow rate results in an increase in mass

This is because ceramic packing material

transfer coefficient value. This means there

has less friction compared to glass material

were more liquid would be spread on the

which will then improve the absorption

packing surface and this led to an increasing

process. Large packing size is preferred as it

in the interfacial area per unit volume. For the

has a larger contact area between the

case of gas flow rate, increase in the gas flow

carbon dioxide and water, thus further

rate leads to a higher mass transfer

facilitates the absorption process to occur

coefficient value especially when the carbon

compared to smaller packing materials.

dioxide concentration is high. This is due to

References

the increase of the wetted surface packing material for gas-liquid contact increased with the increasing of the liquid flow rate. Thus, increasing the efficiency of mass transfer process through the absorption column (Yeh & Pennline, 2001). From the conducted experiment, several potential errors have been detected that may obstruct the result obtained. Firstly, any valve or whatnot can be mistakenly operated and alas which could affect the product. Next, while running the experiment, it can be observed that the gas absorption column contained algae in it which it may contaminate the liquid flowing through the column. 5. Conclusion The results from this experiment shows that lower flow rate leads to a better overall performance of the absorption process. On the other hand, the usage of large packing size and ceramic packing material will increase the efficiency of CO2 removal, as evident from the result from this experiment.

Absorption. (2019). Gas Absorption & Desorption. Retrieved from http://www.separationprocesses.com /Absorption/GA_Chp03.htm “Absorption (chemistry)” (2019). Retrieved from https://en.wikipedia.org/wiki/Absorpti on_(chemistry) Alex Randomkat e.t al. (2016). Retrieved from https://www.quora.com/Howdoes-the-reaction-of-carbon-dioxidegas-with-sodium-hydroxide-andwater-solution-occur Andselisk e.t al. (2017). Reaction between NaOH and CO2. Retrieved from https://chemistry.stackexchange.co m/questions/57288/reactionbetween-naoh-and-co2 Arachchige, U. S., & Melaaen, M. C. (2012). Selection of Packing Material for Gas Absorption. European Journal of Scientific Research,87(1), 117-126. “Column Diameter and Pressure Drop”. (n. d.). Column Diameter and Pressure Drop. Retrieved from http://www.separationprocesses.com /Absorption/GA_Chp04a.htm

International Union. (2019). Absorption. Retrieved from https://goldbook.iupac.org/html/A/A0 0036.html Sinott, R., Richardson, J. F., Coulson, J. M. (2013). Chemical Engineering: An Introduction to Chemical Engineering Design. Amsterdam, NL: Elsevier Tan, L., Shariff, A., Lau, K., & Bustam, M. (2012). Factors affecting CO2 absorption efficiency in packed column: A review. Journal of

Industrial and Engineering Chemistry,18(6), 1874-1883. doi:10.1016/j.jiec.2012.05.013 Yeh, T. J. & Pennline, W. H. (2001, February 20). Study of CO2 Absorption and Desorption in a Packed Column. energyfuels. National Energy Technology Laboratory, U.S. Department of Energy, P.O. Box 10949, Pittsburgh, Pennsylvania 15236-0940.

Appendices Based on the reaction below: 2NaOH + CO2 → Na2CO3 + H2O Na2CO3 + HCL → CO2 + H2O+ NaCl 2 moles of NaOH results with 1 mole of CO2 Let x = moles of NaOH added = 0.01V1 V1= volume of NaOH, V2 = volume of HCL y = moles of HCl used = 0.01V2

I.

Sample of calculation at Flowrate 20 L/min with time at 10 min

𝑉1 = NaOH being added = 30 𝑚𝐿 ×

1𝐿 1000 𝑚𝐿

= 0.03 L

𝑚1 𝑉1 = 𝑥 = 𝑚𝑁𝑎𝑂𝐻 × 𝑉𝑁𝑎𝑂𝐻 𝑥 = 0.01

𝑚𝑜𝑙 𝐿

× (0.03 L) = 3×10−4 mol

𝑉2 = HCl being used = 21.90 𝑚𝐿 ×

1𝐿 1000 𝑚𝐿

= 0.0219 L

𝑚2 𝑣2 = 𝑦 = 𝑚𝐻𝐶𝑙 × 𝑣𝐻𝐶𝑙 y = 0.01

𝑚𝑜𝑙 𝐿

× (0.0219 𝐿) = 2.19×10−4 mol

Then: moles of NaOH reacted = (x-y) moles of CO2 reacted = 0.5 (x-y) Hence: Concentration CO2 outlet = 0.5 (x-y)/ (VCO2 sample) [0.5(3×10−4 mol −2.19×10−4 mol)] 0.01 𝐿

= 4.05× 10−3

𝑚𝑜𝑙 𝐿

II.

Concentration of CO2 inlet CO2 Inlet stream: 2.0

L x5 min = 10 L =0.01m3 min

Density of CO2=1.98

=

kg m3

mass volume

Mass of CO2 = 1.98

kg x0.01m 3 = 0.0198 kg m3

Mol of CO2 =mass CO2 / molar mass CO2 n = 19.8 g

conc, c =

mole = 0.45mole 44 g

n 0.45mol = = 0.045 mol L v 10 L

CO2 inlet = 0.045 mol

III.

L

Concentration of CO2 in water

Formula: concentration of CO2 inlet – concentration of CO2 outlet = 0.045

𝑚𝑜𝑙 𝐿

= 0.04095

− (4.05 × 10−3

𝑚𝑜𝑙 𝐿

𝑚𝑜𝑙 𝐿

)

IV.

Percentage 𝑪𝑶𝟐 removal.

(Concentration of 𝐶𝑂2 in water / Concentration 𝐶𝑂2 inlet) × 100% =

0.04095 0.045

= 91 %

× 100 %