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Abstract

The objective of this project is to analyze the removal of H2S from a gas stream by a NaOH solution using a pilot-scale gas absorption column separating CO2 from air using a 0.05N aqueous NaOH solution. The pilot-scale gas absorption column is packed with ½-inch Raschig rings to a height of 4 feet with a 5.5 inch diameter. The effects of inlet gas and liquid flow rates on the gas absorption was investigated to find the parameters that help scrub the highest amount of H2S from a landfill gas stream. NaOH is used for the solution, as it is a strong base that will react with CO2 and H2S irreversibly to produce a solid. CO2 was used in place of H2S since they are similar in size, and they react with NaOH in a similar manner, in addition to CO2 being much safer. At a constant gas flow rate, Kya increased with the increase of liquid flow rate. In addition, at a constant liquid flow rate, Kya also increased with the increase of gas flow rate. As the flow rate is increased, the trend of Kya reaches equilibrium. Similarly, using H2O to absorb CO2 had the same trend as the NaOH solution. However, the removed CO2 concentration from H2O was less than NaOH solution. The NaOH solution removed twice as much CO2 than H2O.

Background:

                                                          (Eq. 1)

G is superficial molar velocity of gas, NCO2 is mass transfer flux of CO2 – amount of CO2 transferred per area and time, Z is a packed bed height, and a is the specific surface area of the packing - the surface area of the packing per volume of packing.

From their research, they found out the liquid flow rate has no effect on the concentration of CO2 at the outlet gas stream. However, the gas flow rate affected the concentration of CO2. As the gas flow rate increased the concentration of CO2 in the outlet gas stream also increased.

In another experiment by Yaminah Jackson, recording data for a COMSOL simulation of CO2

removal from air stream using water, using (Eq. 2) as a basis for results.[footnoteRef:3] [3: Jackson, Yaminah Z. Modeling Gas Absorption. Worcester Polytechnic Institute, 24 Apr. 2008. Web. 24 Jan. 2017.]

(Eq. 2)

is the overall mass transfer coefficient based on the gas phase driving force (mol/m3·hr), V is the average vapor molar flow rate through the column, S is the cross-sectional area of the column, and HOG is the height of a transfer unit.

According to the article by Yadolla and Hosseini, they used an NaOH solution in order to remove the CO2 from air.[footnoteRef:4] [4: Tavan, Yadollah, and Seyyed Hossein Hosseini. "A novel rate of the reaction between NaOH with CO2 at low temperature in spray dryer." Petroleum (2016): n. pag. Web. 3 Feb. 2017.]

CO2 (l) + 2NaOH (l) Na2CO3(s) + H2O (l) (Eq. 3)

From the equation 3, when CO2 reacts with NaOH, it makes Na2CO3, which is a salt, and H2O. As it produces a salt in the reaction, the y* value is 0. Furthermore, increasing the NaOH concentration leads to a high CO2 removal efficiency.

Materials and Methods:

A 180 L tank was filled with a 0.05N aqueous NaOH solution. The NaOH solution was created by combining 360 g of NaOH in two liters of water as a concentration. The solution was transferred to the tank where it was mixed thoroughly while adding 178 L of water. The solution was pumped through the packed bed column at a variety gas flow rates with constant liquid flow rate and vice versa.

First, a gas (air and CO2) flow rate was chosen and was kept constant while the liquid flow rate was varied. Measurements of the entering and leaving gas concentrations were taken in CO2 ppm using LabQuest probes. The concentration measurements were taken constantly and recorded when steady state was reached. Steady state was considered reached when the concentration had not changed in 20 seconds. Secondly, a liquid flow rate was chosen and was kept constant, while varying the gas flow rates. As before, CO2 concentration measurements were taken using the probes when steady state was reached. Steady state was reached in around 200 s. After measurements were taken using the NaOH solution, the solution was drained from the tank, and the column by flowing water through the system. Measurements were taken using water as the liquid in the column.

Results and Discussion:

The overall mass transfer coefficient, Kya was calculated using equation (7) after rearranging equations (4-6). The detailed calculations are in Appendix IV.

(Eq. 4)

(Eq. 5)

for very dilute systems (Eq. 6)

(Eq. 7)

where is the height of the column, HOG is the height of a transfer unit, NOG is the number of transfer units, is the average molar flow rate of the gas through the column, and S is the cross-sectional area of the column.

NaOH reacts with CO2 irreversibly to produce Na2CO3, thus y* is 0, which means CO2 is diluted.

For a constant gas flow rate, Kya increased as the NaOH solution flow rate increased (Figure 1). At constant gas flow rate of 6.2 mL/s, and a 26.3 mL/s liquid flow rate, which are the smallest flow rates tested, Kya was found to be 23 mol/m3s. At 60 mL/s liquid flow rate, which is the highest flow rate tested, Kya was found to be 25 mol/m3s. The slope between 26.3 mL/s and 44 mL/s is 0.072 mol/m3-mL, and the slope between 44 mL/s and 60 mL/s is 0.042 mol/m3-mL. The slope decreases with increasing liquid flow rate. If there is more liquid flow rate, there is more NaOH, thus there is more chance to transfer CO2 concentration from gas to liquid in the column. If there is more CO2 coming from the gas, then Kya increases more. However, when there is more CO2 than NaOH can hold, then Kya will reach equilibrium.

Figure 1: Kya of CO2 removal from air using a 0.05N aqueous NaOH solution at different flow rates with constant gas flow rate at 6.2mL/s.

For a constant liquid flow rate, Kya increased as the gas flow rate increased (Figure 2). At 26.3 mL/s liquid flow rate, Kya increased by 19 mol/m3s when the gas flow rate was increased from 1.3 mL/s to 6.2 mL/s. When the liquid flow rate was constant at 60 mL/s, Kya increased by 19 mol/m3s when the gas flow rate was increased from 1.3 mL/s to 6.2 mL/s. The increased value of Kya is similar, which is 19 mol/m3s even though constant liquid was increased from 26.3 mL/s to 60 mL/s. When there is more gas flow rate, there is more chance to transfer CO2 concentration from gas to liquid. However, if there are no more spaces for the CO2 concentration transfer, then Kya will reach equilibrium.

Figure 2: Kya of CO2 removal from air using a 0.05N aqueous NaOH solution when holding the liquid flow rate constant while varying the gas flow rate.

At the smaller gas flow rates, there is less Kya change (Figure 3). At 1.3mL/s of constant gas flow rate, the difference of Kya with varied liquid flow rate was 1.27 x10-2 mol/m3s. However, at 6.2 mL/s of constant gas flow rate, the difference was 2.08 mol/m3s. Therefore, the gas flow rate is more effective to remove CO2 compared to liquid flow rates.

Figure 3: Kya of CO2 removal from air using an NaOH solution at different liquid flow rates with constant gas flow rate.

The H2O solution had a similar trend as the NaOH solution at constant gas flow rates (Figure 4, Appendix II). At constant gas flow rate, Kya increased with the liquid flow rate. The trend of Kya was same at constant liquid flow rates (Figure 5, Appendix II). However, the value of Kya was less than the NaOH solution value. At the same liquid flow rate of 60 mL/s and a gas flow rate of 6.2 mL/s, the NaOH solution had a Kya value of 26 mol/m3s, while H2O had 17.8 mol/m3s. This means that there was less transfer of concentration CO2 in H2O liquid compared to NaOH solution. In addition, the NaOH solution removed twice as much CO2 from air than the H2O solution (Figure 6, Appendix II).

When 10000 ppm of CO2 gas inlet was used, Kya almost doubled in comparison to 5000 ppm inlet (Figure 7, Appendix II)

To calculate the flooding velocity, Eq (8) was used.

From this equation, the flooding velocity was found to be 0.0184 m/s. The detailed sample calculation is in Appendix V.

Appendix I.

PFD

Appendix II.

Figure 4: Kya of CO2 removal from air using water at different liquid flow rates with constant gas flow rates.

Figure 5: Kya of CO2 removal from air using water at different liquid flow rates with a constant gas flow rate of 1.3 mL/s

Figure 6: Comparison of the CO2 concentration at the gas outlet for the NaOH solution and water

Figure 7: Kya of CO2 removal using a concentration of 10000 ppm as desired for H2S removal concentration, compared to a concentration of 5000 ppm.

Appendix III.

Table 1 Raw data for the gas flow rate held constant (at two different rates) with varying NaOH solution flow rates. Includes the resulting CO2 concentrations in ppm, height of the column and cross-sectional area of column.

Table 2 Kya results from the measured data of liquid flow rate, CO2 flow rate, air flow rate and molar fraction change for CO2 in aqueous NaOH solution.

Table 3 Raw data for the gas flow rate held constant (at two different rates) with varying water flow rates. Includes the resulting CO2 concentrations in ppm, height of the column and cross-sectional area of column.

Table 4 Kya results from the measured data of liquid flow rate, CO2 flow rate, air flow rate and molar fraction change for CO2 in water.

Appendix IV.

Converting ppm to mole fractions:

NOG was found from the change in mole fraction between the top and bottom of the column:

Finding the molar flow rates of the vapor, CO2 + air:

For the incoming stream:

Calculating overall mass transfer coefficient:

Appendix V.

Flooding velocity

The superficial gas velocity at flood is correlated by

= superficial gas velocity at flood, m/s

= total surface area packing, bed

= fractional voids in dry packing

g = gravitational constant, 9.8067

= liquid and has densities,

L/ G = liquid and gas flow ratio

= liquid viscosity, mPa*s (or cP)

= 1.225 kg/m3, = 1.98 kg/m3.There is 0.05% of CO2 is in the mixed air, so the density can be assumed as air. Therefore, the density of gas is 1.225kg/m3.

The density of liquid can be assumed as water density because in aqueous NaOH solution, there is 0.2% of NaOH and 99.8% of water. Therefore, the density of liquid is 1 kg/m3.

In trial 1, the liquid volumetric flow rate was 26.3mL/s and the gas volumetric flow rate was 103.2 mL/s.

From the generalized correlation of flood points, packed columns graph[footnoteRef:5], the x-axis is 0.282, therefore the y-axis is 0.06. [5: Generalized correlation of flood points, packed columns, Sherwood et al., Ind. Eng. Chem. 30, 768 (1938).]

From the characteristics of dumped tower packing chart, = 370, = 0.64. The density of has is calculated in Eq. 6. = 1.225kg/m3, g = 9.8067m/s, = 1kg/m3, and =1cP.

= 3.403 x 10-4 m2/s2

= 0.0184 m/s

H2O flowrate (mL/s)

Kya (mol/m3·s)

Trial 1 26.3 44.0 60.0 3.7955119583101 3.970535838768209 4.077797265394341 Trial 2 26.3 44.0 60.0 3.73339181737684 3.940898760780611 4.07356315980745

Liquid flowrate (mL/s)

Kya (mol/m3·s)

Concentration of CO2 in outlet (ppm)

Liquid flowrate (mL/s)

10000 ppm model 30.0 50.0 70.0 0.0281211215530425 0.028829813515346 0.0290998429631448 5000 ppm model 30.0 50.0 70.0 0.0114808947401888 0.0122233681595977 0.0124596548178303

NaOH flow rate (mL/s)

Kya (mol/m3·s)

Trial 1 26.3 44.0 60.0 22.93607092378319 24.41758796794011 24.88902487041074 Trial 2 26.3 37.4 44.0 60.0 23.00340794132821 23.54212007899421 24.215550249345 24.88902487041074 Trial 3 30.0 50.0 70.0 0.00704679667033133 0.00704679667033133 0.00703407630039255 Trial 4 30.0 50.0 70.0 0.0246215717661307 0.0259282738709048 0.0266082305416003 Trial 5 30.0 50.0 70.0 0.0245276646452005 0.0261431807186952 0.0263202599197678

Liquid flow rate (mL/s)

Kya (mol/m3 · s)

Liquid 60 mL/s 1.3 6.2 6.2 7.03407630039255 26.6082305416003 26.3202599197678 Liquid 26.3 mL/s 1.3 6.2 6.2 7.04679667033133 24.6215717661307 24.5276646452005 Liquid 44 mL/s 6.2 1.3 6.2 25.92827387090472 7.04679667033133 26.1431807186952

Gas flowrate (mL/s)

Kya (mol/m3 ·s)

Gas 1.3mL/s 60.0 26.3 44.0 7.03407630039255 7.04679667033133 7.04679667033133 Gas 6.2mL/s 60.0 60.0 26.3 26.3 44.0 44.0 26.6082305416003 26.3202599197678 24.6215717661307 24.5276646452005 25.92827387090472 26.1431807186952

Liquid flowrate (mL/s)

Kya (mol/m3 ·s)