Scrubbing of CO2 Greenhouse Gases, Accompanied by Precipitation

Jul 23, 2008 - A continuous bubble-column scrubber that absorbs carbon dioxide (CO2) gas using an alkaline solution under a pH-stat condition was used...
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Ind. Eng. Chem. Res. 2008, 47, 6336–6343

Scrubbing of CO2 Greenhouse Gases, Accompanied by Precipitation in a Continuous Bubble-Column Scrubber Pao-Chi Chen,* Welly Shi, Ruby Du, and Vanessa Chen Department of Chemical and Materials Engineering, Lunghwa UniVersity of Science and Technology, 300, Wan-Shou Rd., Sec. 1, Kueishan, Taoyuang, Taiwan 333, Republic of China

A continuous bubble-column scrubber that absorbs carbon dioxide (CO2) gas using an alkaline solution under a pH-stat condition was used to explore the effects of the pH of the solution, gas concentration, gas-superficial velocity, and liquid-flow rate on the absorption rate and volumetric mass-transfer coefficient of CO2. When the scrubbing factor was evaluated, this scrubber outperformed other scrubbers. Correlations of local volumetric mass-transfer coefficients with process variables are presented here and the gas-liquid interfacial area and individual mass-transfer coefficient are discussed to provide insight into the mass-transfer mechanism. Investigation of the liquid-side mass transfer coefficient revealed that the kL/[OH-]0.5 value was a constant, thus demonstrating that the system obeyed second-order kinetics. Evidence also shows that liquid-side resistances are within the range of 27%-99%, indicating that gas-side mass-transfer resistance cannot be neglected in some cases. This implies that the absorption mechanism is flexible and can be shifted by adjusting the process parameters. 1. Introduction Recently, because of environmental greenhouse effects caused by exhaust carbon dioxide (CO2) gas, the importance of removing CO2 from exhaust emissions has been recognized around the world. For the removal of exhaust CO2 gas, several methods have been proposed, such as chemical absorption, physical absorption, membrane separation, biochemical methods, and the catalytic conversion method. In these processes, chemical absorption reactions are widely used in the chemical and petroleum industries, especially for the absorption of CO2 gas by NaOH, ammonia, and amine solutions.1–8 In addition to these methods, the absorption of CO2 in an alkaline solution with crystallization has also been adopted to explore the removal of CO2 from waste gas.9–13 This approach, with the production of carbonate by means of reactive crystallization, has been found to be effective. However, some of these operations could only be performed in batch mode.11–13 Moreover, a continuous bubble-column scrubber was more effective in the removal of CO2, compared to other scrubbers.2,4,14 In addition, bubble-column reactors are widely used in the chemical, petrochemical, biochemical, and metallurgical industries,15 because they have simple construction, higher heat- and mass-transfer coefficients, higher removal efficiency, and effective control of the liquid residence time. In addition, a bubble-column scrubber could be operated in a pH-stat continuous mode with the production of carbonates9 on one hand, and the pH value could be adjusted to a desired value for the enhancement of absorption rate on the other hand. Therefore, a pH-stat continuous bubble-column scrubber has an advantage in the removal of CO2, compared to other processes. In a bubble-flow regime, the gas flow controls the fluid dynamics of the individual phases of these systems. This, in turn, controls liquid mixing and interphase mass transfer, which subsequently influences conversion and selectivity.16 In the gas-liquid reaction, the absorption mechanism is * To whom correspondence should be addressed. Tel.: 886-282093211, ext.5205. Fax: 886-2-82093211 ext, 5209. E-mail address: [email protected].

controlled by both the mass-transfer and chemical reaction steps, depending on the operating conditions. To describe the concentration distributions, several models of the absorption reaction have been proposed in the literature.17–21 In these reports, the chemical reaction kinetics in gas-liquid reactions was first order or second order, depending on the pH value. However, in some systems, the mass action in solution has been found to be very complicated, while in others, it has been demonstrated that the pH has an effect on the species.22 In a word, the absorption of CO2 gas in an alkaline solution is strongly affected by the pH of the solution. Thus, it is clear that controlling the pH of the solution is important for controlling the absorption rates of gases. This is due to the ionic strength (and, hence, Henry’s constant, H) being affected by the pH of the solution. In this study, a continuous bubble column scrubber under a constant pH value was adopted to study the effects of the gas flow rate, the concentration of gas, the pH of the solution, the liquid flow rate, and the concentration of absorbent on the absorption rate of CO2 and the mass-transfer coefficients with precipitation and without precipitation. To understand the mass-transfer mechanism in an alkaline solution with and without precipitation, a two-film model was utilized to obtain individual mass-transfer coefficients. The results are also compared here with those reported in the literature. 2. Determination of Absorption Rate In a scrubber, a simulated gas mixture containing A (carbon dioxide) and B (nitrogen), and flowing into a bubble column at the bottom, comes into continuous contact with a barium chloride solution, under alkaline conditions, flowing into the column from the top. If we assume that nitrogen gas is essentially insoluble in the liquid phase and that the liquid does not vaporize to the gas phase, the gas phase is a binary A-B system. To determine the absorption rate, we assume that liquid holdup and gas holdup are maintained constant through the column. Therefore, the absorption rate of carbon dioxide can be determined using a material balance under steady-state

10.1021/ie070324x CCC: $40.75  2008 American Chemical Society Published on Web 07/23/2008

Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 6337 19

operation. Under fast chemical reaction kinetics and precipitation, from the material balance for carbon dioxide at steady state, the average absorption rate (RA) is equal to the rate of consumption of carbon dioxide through the bubble column. This value can be determined by measuring the concentration of carbon dioxide and the gas-flow rate: FA1 - FA2 FA1 - FA2 ) RA ) VL εLVb

[ ( )( )]

FA1 1 - y1 y2 1VL y1 1 - y2

(1)

(2)

Therefore, the absorption rate RA can be determined with measurable quantities. The result was the same as that obtained with the multiple-tube plug-flow model reported in the literature.14

(3)

where CA is the concentration of CO2 gas in the gas phase and CLA is the concentration of CO2 gas in the liquid phase. If we assume plug flow for the gas phase and well-mixed flow for the liquid phase,16 the material balance equation with z, at steady state, is (4)

where u is the gas-superficial velocity, based on the column cross-sectional area. Take the limit for eq 4, dCA + rAεL ) 0 u dz

(5)

Substituting eq 3 into eq 5 yields dCA + (KGa)loc(CA - HCAL)εL ) 0 u dz

In eq 6, CAL is almost less than 4 × 10-8 M when the pH value is >12.22 In addition, H ≈ 1.5 and CA > 1 mM (see Table S-1 in the Supporting Information). Therefore, CA . HCLA, eq 6 could be written as u

dCA + (KGa)locCAεL ) 0 dz

(7)

We assume that εL is kept constant through the column, if the slurry is a homogeneous suspension. Integrating eq 7, we have u

∫

CA2

CA1

dCA + εL CA

∫

L

0

(KGa)loc dz ) 0

∫

1 L (K a) dz L 0 G loc Multiplying A by eq 9, the equation becomes KGa )

( )

Qg CA1 ln VL CA2

(8)

(10)

(11)

Using this equation, the average overall mass-transfer coefficient can be evaluated in terms of measurable quantities. Furthermore, the overall mass-transfer coefficient is correlated with the individual mass-transfer coefficients: 1 H 1 ) + KGa kGa kLa

(12)

Here, 1/KGa represents the overall resistance, 1/(kGa) is the gasside resistance, and H/(kLa) denotes the liquid-side resistance. For a second-order reaction, by introducing the enhancement factor12 into eq 12, we have (13)

where k2 is the second-order reaction constant and DA is the diffusion coefficient for carbon dioxide. Henry’s constant is a function of the ionic strength and temperature, which can be adjusted with pH values and temperature; therefore, a plot of eq 13 to obtain kGa and kLa becomes possible. 4. Determination of Henry’s Constant At a temperature of 30 °C, Henry’s constant can be expressed as23 pKH ) 1.53 + 0.1039I - 0.0148I2

(14)

where I is the ionic strength and KH is defined as KH )

[CO2] PCO2

(15)

The relation between KH and H is H)

(6)

(9)

where

( )

According to a two-film model, the local absorption rate, expressed as the overall mass-transfer coefficients, based on both gas and liquid sides at a local point, can be written as

(uCA|z - (uCA)|z+∆z)A - rAεLA ) 0

CA2 + KGaLεL ) 0 CA1

1 1 1 H ) + KGa kGa (D k )1⁄2a C 1⁄2 A 2 B0

3. Determination of Mass-Transfer Coefficients

rA ) (KGa)loc(CA - HCLA)

u ln

KGa )

where VL is the volume of the liquid phase, Vb the volume of the bubble column, and εL is the holdup of the liquid phase in the entire column. Because the molar flow rate of inert gas is equal to FA1(1 - y1)/y1, the molar flow rate of carbon dioxide at the outlet (FA2) is FA1[(1 - y1)/y1][y2/(1 - y2)]. Thus, eq 1 can be rewritten as RA )

and

1 KHRT

(16)

To match up with eq 13, here, KH and H are based on the bulk-liquid phase, because we assume that the liquid phase is well-mixed. Therefore, the evaluated values are according to the bulk-liquid phase at steady-state values. If the ionic strength is obtained, the Henry’s constant, H can be determined using eqs 14 and 16. Using No. 1 as an example, the calculated value of H is 1.55 if we substitute the I value (0.59 M, as shown in Table S-1 of the Supporting Information) into eqs 14 and 16. For the CO2-H2O-BaCl2 system, the mass action equations and total mass balance equations are shown in Table 1: this table gives eqs 17–26. From the given pH value and the total mass balance equations, mass action equations, and activity coefficient equations, the concentrations of chemical species can be obtained using an ionic strength approximation method.9 Given this information, the ionic strength can be

6338 Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 Table 1. Mass Action Equations in the BaCl2-CO2-H2O Crystallization System reaction

reaction number

5. Experimental Section

Mass Action Equations H2O ) OH- + H+

(17)

CO2(g) + H2O ) H2CO3 *

(18)

H2CO3 * ) HCO3- + H+

(19)

HCO3- ) CO32- + H+

(20)

BaCO3° ) Ba2+ + CO32-

(21)

BaCO3(s) ) Ba2+ + CO32-

(22)

Total Mass Balance Equations TCO ) [H2CO3 * ] + [HCO3-] + [CO32-] + [BaCO3°]

(23)

TBa ) [Ba2+] + [BaCO3°]

(24)

TCl ) [Cl-]

(25)

TNa ) [Na+]

(26)

determined. The activity coefficients of electrolytes can be estimated from the Bromley correlation equation:24

( )

(0.06 + 0.6B1)I B1I √I 1 log γi ) -0.511 + + 2 2 zi (1 + 1.5I ⁄ zi)2 zi 1 + √I

tively, CL represents the feed concentration of the barium chloride solution, and C2 denotes the feed concentration of the sodium hydroxide solution.

The experimental device is shown in Figure 1. It consisted of a bubble column, a pH control system, a gas-bubbling system, and a feeding system. The inside diameter of the column was 5 cm, and the gas distributor was a perforated plate designed with four holes per square centimeter, with each hole being 1 mm in diameter. The location of the BaCl2 solution feeding point was fixed at a position 25 cm below the top cover. Initially, a known amount of distilled water was introduced into the column, in which a pH electrode also was inserted. The solution was adjusted to the desired pH value by adding a known concentration of NaOH solution. The experiment started when the mixed N2-CO2 gas began bubbling through the column. Simultaneously, barium chloride solution from a storage tank was continuously fed into the bubble column. During the operation, the pH of the solution decreased, because of absorption and crystallization in the scrubber. To maintain the pH value of the solution at a desired value during operation, NaOH solution was introduced into the column through the action of a pH controller. A digital gauge pressure meter measured the inlet gas pressure. The overflow slurry solution flowed out to the reservoir. A Gas Data PCO2 meter was used to measure the concentration of CO2 gas at the outlet of the bubble column. When the concentration of CO2 gas reached a steady state, the slurry solutions were withdrawn using a syringe and divided into two parts: one part was used to measure the total dissolved CO2 and the barium ion concentration in the solution, by means of the potential titration method (Oring, model PH/ISE290APlus), and the other was used to measure crystal size distribution, using a laser particle size analyzer (Galai, model CIS-1). The experiments were conducted at 30 °C in most of the runs, and the operational pH values were kept within the range of 12-13. During the operation, the pH, gas flow rate, feed rate of the

(27)

where zi is the charge number, I is the ionic strength, and B1 is composed of ionic contributions. This equation can be used to estimate activity coefficients in solutions of high ionic strength (up to ∼6 M). The ionic strength of a solution can be calculated with the following equation: I)

1 2

∑z

1 Ci ) ([H+] + 4[Ba2+] + [Na+] + [OH-] + 2 [HCO3-] + 4[CO32-] + [Cl-])

2

i

(28)

However, the total chloride ion concentration (TCl) and the total sodium ion concentration (TNa) can be determined from material balances, because Cl- and Na+ ions are both nonreacting components. Under steady-state conditions, the TCl and TNa values can be evaluated as follows: TCl )

2QLCL QL + Q2

(29)

TNa )

Q2C2 QL + Q2

(30)

and

where QL and Q2 are the volumetric flow rates for the barium chloride solution and the sodium hydroxide solution, respec-

Figure 1. Schematic diagram of the experimental device used in these experiments. Legend: 1, pH controller; 2, pressure gauge; 3, speed controller; 4, PCO2 meter; 5, pH electrode; 6, bubble column; 7, motor; 8, heater; 9, feed tank for BaCl2 solution; 10, feed tank for NaOH solution; 11, motor; 12, digital temperature meter; 13, mixing bottle; 14, gas flow meters; 15, gas tanks; 16, outlet line; 17, storage tank; and 18, pump.

Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 6339 Table 2. Operating Conditions Maintained in This Work property With Precipitation pH working temperature gas superficial velocity liquid flow rate concentration of gas concentration of NaOH solution concentration of barium chloride solution

value 12-13 30 °C 1.70-9.10 cm/s 50-320 mL/min 10%-30% 2.0 M 0.01-0.2 M

Without Precipitation pH working temperature gas superficial velocity liquid-flow rate concentration of gas concentration of NaOH solution concentration of barium carbonate slurry

12-13 30 °C 4.07-12.3 cm/s 50 mL/min 20% 2.0 M 16 g/L

liquid, and inlet concentrations of gas and liquid were held constant. In this manner, the mass-transfer coefficients could be determined by a two-film model by adjusting the pH of the solutions while keeping the other process variables constant, according to eq 12. To explore the effects of process variables on the absorption of carbon dioxide, changes in the gas feed concentration, gas flow rate, liquid flow rate, liquid feed concentration, and pH of the solution were required. The operating time was 30 min in most of the runs. At the end of the operations, the slurry was filtered and the volume of filtrate (VL) was measured, while the solids were dried in an oven at 100 °C for at least 1 h. For comparison, an absorption-withoutprecipitation procedure was also investigated in this study, using a BaCO3 slurry as well as a NaOH solution as an absorbent. Table 2 shows the operating conditions for both of the procedures. Experimental data obtained for both procedures were listed in the Supporting Information (see Tables S-1 and S-2). 6. Results and Discussion 6.1. Steady-State Absorption Rate. Evidence also showed that the outlet concentrations for all runs remained almost constant when the operating time was >10 min, indicating that a steady-state operation was achieved. When the CO2 gas outlet concentration at steady state for each run was determined, the absorption rate (RA) could be obtained with known values of y2 and y1, as shown in eq 2. Using No. 1 as an example, the inlet and outlet CO2 concentrations were 20% and 9.6%. In addition, the gas flow rate (FA1) and the liquid volume (VL) were 1.26 × 10-4 mol/s and 0.68 L, respectively. Substituting

Figure 2. Absorption rate (RA) versus the gas flow rate (u) at different pH values.

these values into eq 2, the calculated absorption rate was determined to be 1.06 × 10-4 mol s-1 L-1. Absorption rates obtained in this study are listed in the Supporting Information (see Tables S-1 and S-2). Figure 2 exhibits the absorption rate versus the gas flow rate with precipitation at various pH values. The result shows that the absorption rate increased as the gas flow rate as well as the pH increased. In addition, the absorption rate also increased obviously with an increase in gas concentration, y1, but was only slightly affected by the liquid flow rate. 6.2. Overall Volumetric Mass-Transfer Coefficient and Scrubbing Factor. After the gas outlet concentration and VL were obtained, the overall volumetric mass-transfer coefficient could be evaluated by eq 11. Measured values with precipitation were within the range of 0.0277-0.2115 s-1, whereas the without-precipitation ranges were 0.0651-0.3396 s-1. To better understand the performance of various scrubbers,4,5 measured KGa values obtained in the literature (given in units of kmol m-3 h-1 kPa-1) were converted to units of s-1, which are listed in Table 3. In addition, KGa values for the rotating packed bed (RPB)2 are also listed in Table 3. The result shows that packed beds with structured packing have the highest KGa values, while bubble column scrubbers have moderate KGa values, in comparison to other scrubbers. Alternatively, from the scrubbing factor (φ), which is defined as φ ) FgE/(FNaOHVb), which means the number of moles of CO2 gas to be removed per mole of absorbent and per unit volume of the scrubber; estimated values for various scrubbers were also calculated and listed in Table 3. In these systems, FNaOH is defined as the NaOH molar flow rate. The result showed that φ values in this study, both for systems with precipitation and for systems without precipitation, are much greater than those in packed-bed systems, indicating that bubblecolumn scrubbers are indeed more effective than packed-bed scrubbers. Comparisons in the gas flow range of u ) 4.07-9.10 cm/s also revealed that bubble-column scrubbers with precipitation (φ ) 0.180-0.637) exhibited more effectiveness than bubble-column scrubbers without precipitation (φ ) 0.178-0.515), indicating that precipitation more or less has an enhancement effect on the absorption of CO2 gas. 6.3. Individual Volumetric Mass-Transfer Coefficients. 6.3.1. Liquid-Side Mass-Transfer Coefficient. According eq 13, a linear plot of 1/(KGa) vs H/(CB0)1/2 was conducted in this work. The gas-side mass-transfer coefficients obtained under different conditions were within the range of 0.0863-1.1991 s-1. On the other hand, the slope, 1/[(DAk2)1/2a], multiplied by H/CB01/2 was H/(kLa), where the liquid-side mass transfer kLa could be obtained. The liquid-side mass-transfer coefficients obtained with precipitation were within the range of 0.0418-0.7414 s-1, while the values obtained without precipitation were within the range of 0.1186-1.5511 s-1. All the data for individual mass-transfer coefficients was also collected in the Supporting Information (see Tables S-1 and S-2). Figure 3 is a plot of the liquid-side mass-transfer coefficient versus the superficial velocity of the gas at various pH values for both procedures in this study. The liquid-side mass-transfer coefficient increased as the superficial velocity increased at various pH values. Both procedures show that the data are close together, larger data points for precipitation and smaller data points for no precipitation. The values obtained in this study were higher than those reported by Fan,19 as shown in the parallelogram, and were close to the values reported by Sada et al.12 On the other hand, the trend in our data, extrapolated into smaller gas-superficial velocity, was higher than that reported in Sanchez et al.16 However, at zero velocity, Cournil

6340 Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 Table 3. Comparisons of Overall Volumetric Mass-Transfer Coefficient and φ Factor for the CO2-NaOH Absorption System in Various Scrubbers scrubber

reference/source

operating conditions

packed bed

Tontiwachwuthiku et al.5

Qg ) 0.87-1.47 L/min QL ) 1230-1760 mL/min y1 ) 11.5%-19.5% [NaOH] ) 1.2-2.0 M

packed bed

Aroonwilas and Tontiwachwuthiku4 Aroonwilas and Tontiwachwuthiku4

packed bed with structured packing

rotating packed bed (RPB)

Lin et al.2

bubble column without precipitation

this work

bubble column with precipitation

this work

KGa (s-1)

φa (mol mol-1 L-1) (1.33-6.56) × 10-3

0.08-0.27b Qg ) 4.9-10.4 L/min QL ) 23-70 mL/min y1 ) 1%-15% [NaOH] ) 1.0-2.0 M Qg ) 4.4-13.1 L/min QL ) 42 mL/min y1 ) 1%-10% [NaOH] ) 2.0 M Qg ) 4.7-14.5 L/min QL ) 50 mL/min y1 ) 20% [NaOH] ) 2.0 M Qg ) 2.0-10.27 L/min QL)50-320 mL/min CL ) 0.01-0.2M y1 ) 10%-30% [NaOH] ) 2.0 M

0.68-2.74b

0.0176-0.173

0.41-0.53c 0.0651-0.3396

0.0508-0.151d 0.178-0.872

0.0227-0.2115

0.0422-0.637

a φ ) FgE/(FNaOHVb). b Parameter a is based on the packing volume. c Parameter a is based on the dispersion phase. d The value of E is assumed to be 0.8.

and Herri20 found that the kLa values were within the range of 0.0001-0.005 s-1, in which parameter a is much smaller than that found here. If u ) 0.1 cm/s, we can substitute that value into eq 32 or use Figure 4; the value of a was determined to be 0.04978 cm-1. If we use a value of kLa ) 0.0025 s-1, which is a middle value reported by Cournil and Herri,20 the value of kL is 0.0502 cm/s, which is close to our values (0.04-0.1267 cm/ s). Therefore, the kL values were similar to those reported here. The discussion of the kLa values demonstrates that the masstransfer model used here was reasonable. For comparison, a plot of kLa/[OH-]0.5 vs u is shown in Figure 4 for absorption, with and without precipitation. It was found that both sets of data are close together; indicating that absorption with precipitation could not obviously affect the mass-transfer coefficient. Excluding other effects, a linear regression for kLa/[OH-]0.5 and u was determined to give kLa ) 0.2449u1.09[OH-]0.5

(31)

The coefficient of determination becomes 0.9670. The resulting factor was u ) 1.09, which was greater than the value of u )

Figure 3. Plot of kLa vs u at various pH values for various systems; the operating conditions for both procedures in this study are y1 ) 0.20 and QL ) 50 L/min.

0.58 that was determined by Juvekar and Sharma13 and u ) 0.86, which was obtained by Sada et al.12 However, the value of u was close to 1.0, as reported in the literature.19 In addition, the exponent was lower than the values of 1.52,16 as also shown in this figure, for the purpose of comparison. In this manner, our data here fall in the range of u ) 0.58-1.52 that has been reported in the literature. Moreover, the parameters kL and a can be discussed separately. Data for parameter a can be estimated from the slope, 1/[(DAk2)1/2a], as shown in eq 13, if the second-order rate constant (k2) and the diffusion coefficient of carbon dioxide (DA) are available. In this study, the values of k2 and DA were determined to be 8000 L s-1 mol-1 and 2.5 × 10-5 cm2/s,17 respectively. Values of a that have been obtained in this work were in the range of 1.04-5.85 cm-1, which were comparable to those reported in the literature.12,13 According to eq 31, kLa was proportional to [OH-]0.5. Therefore, the term kL/[OH- ]0.5 could be estimated when the value of a was determined. In this work, all the estimated kL/[OH-]0.5 values were ∼0.40. Therefore, adjusting the pH value could give a desired kL value. A

Figure 4. Plot of kLa/[OH-]0.5 and a vs u, showing the effect of superficial velocity.

Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 6341

Figure 5. Plot of kGa vs u for various systems.

plot of a vs u was also shown in Figure 4. Therefore, the following correlation for a and u was obtained: (32) a ) 0.6125u1.09 The coefficient of determination was determined to be 0.9673. The results show that the gas-liquid interfacial area was proportional to u1.09. The exponent order was the same as those obtained in eq 31. From eqs 31 and 32, the results also show why the value of kL/[OH-]0.5 was constant (i.e.,kL ) 0.3998[OH-]0.5). A similar result was also found for absorption without precipitation. From the data of Sada et al.12 and Fan,19 the effect of u on a can be found, i.e., a ) 0.3995u0.87; the order of 0.87 is slightly less than that reported here. For other gas-liquid-solid systems, Quicker et al.25 found that the orders

were 0.81 and 0.65 for activated carbon and Al2O3 solids, respectively. 6.3.2. Gas-Side Mass-Transfer Coefficient. The effect of u on a for both procedures in this study is shown in Figure 4. Therefore, here, we only discuss the effect of process variables on kG. The effects of process variables on kGa were also observed as shown in the Supporting Information (Tables S-1 and S-2). Table S-1 shows that, except for CL the effects of u, QL, and y1 on the term kGa were obvious. Data showed that the change of CL was 20-fold, whereas the change in kG was approximately 2-fold. In contrast, the values of QL and y1 changed by a factor of 6.4 and 3, respectively; the kG values changed by factors of 14 and 21, respectively. The results showed that kG decreased as u increased, while kG increased as QL and y1 increased. The result also showed that the gasside mass-transfer resistance (1/(kGa)) increased as u increased. Figure 5 is a plot of kG vs u. It was found that kG decreased significantly as u increased in the range of 1.7 cm/s to 4 cm/s, and the effect was attenuated when u increased further. For the sake of comparison, data obtained in the literature12,19 are also shown in this figure, and the values are much higher than those in this study. On the other hand, the decreasing trend observed in this work was similar to that reported in the literature,12,19 except the first point at very low gas superficial velocity. Therefore, the gas-side mass-transfer resistance can be adjusted using process parameters, such as u, QL, and y1. In this study, the volumetric gas-side mass-transfer resistance could not be neglected in some cases, because the resistance was determined to be within the range of 1%-73%, depending on the operating

Figure 6. Effects of process parameters on the mass-transfer resistance: (a) effect of u, (b) effect of CL, (c) effect of QL, and (d) effect of y1.

6342 Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008

conditions. Therefore, a discussion of the effects of the process variables on the mass-transfer resistance is required. 6.4. The Role of Process Parameters in Mass-Transfer Resistances. The liquid-side mass-transfer resistance, Rx, can be defined as follows: Rx (%) )

H ⁄ kLa × 100 [1 ⁄ (kGa)] + [H ⁄ (kLa)]

(33)

The effects of process parameters on Rx are shown in Figures 6a-d. In these figures, we found that the higher the pH value, the lower the Rx value. In Figures 6a, it was determined that Rx decreased as the superficial velocity of the gas increased, while the effect was attenuated when the superficial velocity of the gas increased further. Figure 6b shows that Rx slightly decreases with CL, whereas the effect becomes significant at CL ) 0.2 M. Figure 6c shows that Rx increases as QL increases; however, at QL ) 320 mL/min, Rx is ∼100%. On the other hand, the effect of y1 on Rx is significant, as shown in Figure 6d. The Rx value is ∼100% when the y1 value is >30%, indicating that the absorption process is shifted to liquid-side mass-transfer control. From Figures 6a-d, we determined that the liquidside mass-transfer resistances could be kept at higher values; most of them were >80%, if the process parameters were controlled at a lower pH value, a higher y1 value, and a higher QL value. In other words, the absorption process could be shifted to liquid-side mass-transfer control as desired. 7. Conclusion The absorption (with precipitation) of carbon dioxide (CO2) gas by a barium chloride alkaline solution in a continuous pHstat bubble-column scrubber was possible in this process. Overall mass-transfer rates could be adjusted to significantly higher values by adjusting the pH or superficial velocity of the gas. The scrubbing factor showed that the bubble-column scrubber was more effective than the packed-bed and rotating packedbed (RPB) scrubbers. Using a two-film model with second-order reaction kinetics, the individual volumetric mass-transfer coefficient could be determined and the results were reasonable, compared to the literature. Mass-transfer coefficients obtained from both procedures were similar, showing the significance of pH values on the absorption of CO2 gas. However, a separate analysis for the interfacial surface area and liquid-side mass transfer coefficient demonstrated that the liquid-side mass coefficient was proportional only to the square root of the concentration of hydroxide ion, showing the strong effect of pH value. A separate analysis found that the mass-transfer resistance could be adjusted by process parameters, such as pH value, superficial velocity of the gas, liquid-flow rate, and gas concentration. In addition, the process could be used to remove CO2 through the production of other carbonates.

CL ) concentration of the BaCl2 solution (mol/L) DA ) diffusivity (cm2/s) E ) removal efficiency FA1 ) molar flow rate of the CO2 gas at the inlet (mol/s) FA2 ) molar flow rate of the CO2 gas at the outlet (mol/s) Fg ) molar flow rate of gas (mol/s) FNaOH ) molar flow rate of NaOH (mol/s) H ) Henry’s constant I ) ionic strength (mol/L) k2 ) second-order reaction constant (L mol-1 s-1) kLa ) individual volumetric liquid-side mass-transfer coefficient (s-1) kGa ) individual volumetric gas-side mass-transfer coefficient (s-1) KGa ) overall volumetric mass-transfer coefficient (s-1) (KGa)loc ) local volumetric mass-transfer coefficient (s-1) KH ) Henry’s constant defined in eq 15 L ) height of the bubble column (cm) PCO2 ) partial pressure of CO2 gas (atm) Q2 ) feed rate of NaOH solution (mL/min) Qg ) flow rate of gas (L/min) QL ) feed rate of BaCl2 solution (mL/min) R ) universal gas constant (atm L K-1 mol-1) RA ) average absorption rate (mol s-1 L-1) Rx ) liquid-side mass-transfer resistance (%) rA ) mass-transfer rate (mol s-1 L-1) S ) slurry concentration (g/L) T ) absolute temperature (K) TBa ) total barium ion concentration (mol/L) TCO ) total carbonate ion concentration (mol/L) TCl ) total chloride ion concentration (mol/L) TNa ) total sodium ion concentration (mol/L) u ) superficial velocity (cm/s) Vb ) volume of the bubble column (L) VL ) volume of liquid in bubble column (L) y1 ) molar fraction of CO2 gas at the inlet (%) y2 ) molar fraction of CO2 gas at the outlet (%) z ) position in the bubble column (cm) Greek Symbols γi ) activity coefficient εL ) holdup of liquid φ ) scrubbing factor (mol CO2/mol L)

ACKNOWLEDGEMENT The authors gratefully acknowledge the financial supports provided by the National Science Council of the Republic of China (under Grant No. NSC 93-2623-7-262-002-ET). Supporting Information Available: The Supporting Information includes tables for experimental data. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited

Nomenclature A ) cross-sectional area of the bubble-column scrubber (cm ) a ) gas-liquid interfacial area based on the slurry volume (cm-1) B1 ) constant in eq 27 C2 ) feed concentration of sodium hydroxide solution (mol/L) CA ) concentration of component A in the gas phase (mol/L) CAL ) concentration of component A in the liquid phase (mol/L) CA1 ) concentration of component A in gas phase at the inlet (mol/L) CA2 ) concentration of component A in gas phase at the outlet (mol/L) CB0 ) concentration of hydroxide ions (mol/L) 2

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ReceiVed for reView March 03, 2007 ReVised manuscript receiVed June 15, 2008 Accepted June 24, 2008 IE070324X