Rapid Measurement of Gas Solubility in Liquids Using a Membrane

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Rapid Measurement of Gas Solubility in Liquids Using a Membrane Dispersion Microcontactor Jing Tan, Jianhong Xu, Kai Wang, and Guangsheng Luo* The State Key Lab of Chemical Engineering, Department of Chemical Engineering, Tsinghua UniVersity, Beijing 100084, China

In this study, we present a simple and fast method for the measurement of gas solubility in different liquids. Gas solubility measurements were carried out in a microcontactor using microdispersions to enhance mass transfer. The gas-liquid microcontactor, with a 5 µm pore size microfiltration membrane as the dispersion medium, was used to measure the gas solubility of six typical gas-liquid systems. The gas-liquid systems reached vapor-liquid equilibrium within 2 s, much less time than the 24 h required by the widely used static method of measurement. The obtained gas solubility data agree well with those obtained using the static method. The results produced using this measurement method were also highly reproducible, and relative measurement errors were less than 10% at the optimized operating conditions. The present method can potentially be very useful for the accurate and rapid measurement of gas solubility in many areas including gas-liquid reactions and solvent-based CO2 capture. 1. Introduction The solubility of gas in different solvents is one of the most important physicochemical properties necessary for understanding and controlling industrial chemical reaction processes. Several technologies have already been developed for the measurement of gas solubility in liquids including the static method,1-4 the headspace gas chromatographic method,5 and the total pressure measurement method6 and have been thoroughly reviewed and compared in the literature.7-9 Among the established methods, the static method1 and those similar to the static method2-4 have become some of the most widely used types of methods in industry because of their use of simple equipment, ease of operation, and accuracy. The static method is based on the precise analysis of the composition of both the vapor phase and the liquid phase in a sealed cell after the vapor-liquid equilibrium state has been realized. The accuracy of the results, reported between 0.6% and 1% in the literature, depends on the analysis of the vapor and liquid phases.10 The most significant disadvantage of this method is the long amounts of time required: 24 h in general for aqueous solutions and even longer periods of time when the solution mixtures are viscous.11 However, the application of new gas-liquid reaction and separation systems require large amounts of gas solubility data for different liquids and a more rapid, accurate measurement method is crucial for the quick adoption of these new systems by industry. For chemical absorption processes with rapid gas-liquid reactions, the limiting step is the gas dissolution process: mass transfer from vapor phase to liquid phase. Low gas-liquid mass transfer rates are the main reason behind long gas solubility measurement times. Therefore, enhancing the gas-liquid mass transfer rate will result in a reduction in the time it takes to reach vapor-liquid equilibrium, allowing for more rapid measurement of gas solubility. In the past several decades, microdispersion technology has been widely used for many diverse applications including onchip separations,12-14 kinetic analysis,15,16 and protein crystallization,17,18 due to its high mass transfer efficiency, safety, * To whom correspondence should be addressed. Tel.: +86-1062783870. Fax: +86-10-62783870. E-mail: [email protected].

repeatability, and controllability.19 The high mass transfer coefficients found in gas-liquid microdispersion systems, due to their very high interfacial areas and very short mass transfer diffusion distances,20,21 make them ideal for the development of a rapid, accurate gas solubility measurement method. In this work, we developed a microcontactor with a microfiltration membrane as the dispersion medium for the rapid and accurate measurement of gas solubility in liquids. Six typical gas-liquid systems containing physical and chemical adsorption processes were used to measure the gas solubilities of different solvents. The results are compared with data from the literature. Measurement error was analyzed and appropriate operating conditions are suggested for determining the solubility at different orders of magnitude. 2. Experimental Section 2.1. Experimental Setup. The experimental setup is shown in Figure 1. The membrane dispersion microcontactor was made with stainless steel. A stainless steel microfiltration membrane (Figure 2) with 5 µm average pore size and 0.3 mm thickness was used as the dispersion medium. The active membrane area was 30 mm2. The size of the flow channel was 1.5 cm × 2 mm × 3 mm, and the size of the gas buffer reservoir was 1.5 cm × 5 mm × 1 cm. In order to maintain the two-phase dispersion and allow for the adjustment of the residence time, a long stainless steel capillary with an inner diameter of 2 mm was connected directly downstream from the microcontactor. Capillary pipes, 40, 90, 190, 290, 390, 490, and 790 cm in length respectively, were used accordingly to adjust the residence time. A phase separator with an inner diameter of 8 cm and a height of 20 cm was connected to the capillary pipe. The liquid phase was pumped into the top side of the membrane (the flow channel) while the gas phase was pumped into the gas chamber. The pressure difference caused the gas phase to disperse as microsized bubbles as it passed through the membrane into the liquid phase, forming a microdispersion. The microdispersion flowed along the capillary pipe allowing for a sufficient residence time for mass transfer to occur before it flowed into the gas-liquid phase separator. Mass transfer from the gas phase to the liquid phase took place during both the

10.1021/ie1011504  2010 American Chemical Society Published on Web 09/14/2010

Ind. Eng. Chem. Res., Vol. 49, No. 20, 2010

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Figure 1. Experimental setup.

Figure 2. SEM picture of the microfiltration membrane surface.

dispersion and flow time. The microdispersion flowed into the phase separator and immediately separated into the corresponding gas and liquid phases. The separated gas and liquid phases flowed out from the top and bottom portions of the separator, respectively. To achieve rapid online measurement of gas solubility in liquid, the composition of gas phase is measured with a fast measurement method. The gas phase flows from the phase separator immediately into an online gas chromatograph, applying a simple and accurate sampling process. Adding an inert gas into dissolved gas as an internal standard component is a simple and effective method to determinate the solubility of gas in liquid. In this case, once the compositions of both gas samples before and after absorption are measured, gas solubility in liquid could be determined by material balance calculation. N2 is chosen as an inert gas for its low solubility in water. This is because the solubility of N2 in water is much lower than those of O2 and CO2.22-24 On the other hand, the presaturation via

sparging with pure N2 into water before measurement can also decrease the dissolved amount of N2 during measurement. In this case, N2 can be regarded as a suitable inert component to realize rapid and accurate online measurement of gas solubility in liquids. An advection pump (0-100 mL/min, with a measurement accuracy of (1.0%) was used to pump the liquid phase into the microcontactor. Two mass flow meters (0-500 SCCM of N2, with a measurement accuracy of (1.0%) were used to deliver a gas mixture at stable pressures and flow rates. A water bath (room temperature to 100 °C, with a resolution of (0.1 K) was used to maintain the temperature of all the feed pipes, the microcontactor, the capillary pipes, and the phase separator. A back-pressure valve (0-0.3 MPa range) was used to maintain the outlet pressure of the phase separator at the required pressure. A pressure sensor (0-0.3 MPa, with an accuracy of (0.5%) was set before the back-pressure valve to determine the total pressure of the gas phase. After absorption, a gas chromatograph (GC) (Tianmei, 7890Π) using a thermal conductivity detector (TCD) was applied to measure the compositions of gas samples online. Gas chromatography measurements were carried out with a carrier gas flow rate of 35 mL/min, a column temperature of 120 °C, and a detector temperature of 150 °C. The accuracy of the measurements in the experiments was (0.5%. 2.2. Materials. Six gas-liquid systems were used to measure gas solubility (Table 1). The viscosities of the liquids were measured with an Ubbelohde viscometer at 40 °C. Carbon dioxide (CO2), oxygen (O2), and nitrogen (N2) were used for the gas phases. All gases, with a purity of 99.995 mol %, were purchased from Beijing Huayuan Gas Chemical Industry Co., Ltd. Deionized water was used as an absorbent alone or as a solvent combined with other liquids. When used as an absorbent, the deionized water was pretreated by sparging pure N2 for 2 h.

Table 1. Components of the Model Systems Used in the Experiments

system 1 system 2 system 3 system 4 system 5 system 6

gas phase

liquid phase

viscosity of liquid phase, mPa · s (40 °C)

solubility data

O2, N2 CO2, N2 CO2, N2 CO2, N2 CO2, N2 CO2, N2

water water 15.3 wt % MEA, 84.7 wt % H2O 30.0 wt % MEA, 70.0 wt % H2O 15.3 wt % MEA, 42.3 wt % EG, 42.4 wt % H2O 15.3 wt % MEA, 42.3 wt % PEG 400, 42.4 wt % H2O

0.66 0.66 1.08 1.61 4.26 11.00

Rettich et al.23 Park et al.24 Lee et al.25 Jou et al.26 Song et al.27 Song et al.27

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Figure 3. Images of bubbles observed in different microdispersions. (a) System 2: QG ) 368 mL/min, QL ) 60 mL/min. (b) System 3: QG ) 591 mL/min, QL ) 11.5 mL/min. (c) System 6: QG ) 579 mL/min, QL ) 16.5 mL/min.

Figure 4. Mole fraction of dissolved gas, xG, versus residence time. The operating conditions for (a)-(f) are listed in Table 2.

Monoethanolamine (MEA), ethylene glycol (EG), and poly(ethylene glycol) with a molecular weight of 400 (PEG 400) were all chemically pure and were purchased from Beijing Chemical Plant.

be calculated from the analysis of the gas sample and total pressure of gas phase using eqs 1 and 2:

3. Results and Discussion

where ∆QD is the amount of gas dissolved in the liquid, mol/ min; QD0 is the initial flow rate of the dissolved gas, mol/min; QI is the flow rate of inert gas (N2), mol/min; and xI is the mole fraction of inert gas, mol/mol.

3.1. Determination of Solubility Data. The small volume of the microcontactor allowed for steady flow to be reached in a short amount of time (no more than 2 min). At steady state flow, the amount of gas dissolved in the liquid and the partial pressure of the dissolved gas at a certain residence time could

∆QD ) QD0 + QI - QI /xI

PD ) (P - PH2O)(1 - xI)

(1)

(2)

Ind. Eng. Chem. Res., Vol. 49, No. 20, 2010 Table 2. Operating Conditions and Residence Times in Figure 4

Figure 4a

system 1

Figure 4b

system 2

Figure 4c

system 3

Figure 4d

system 4

Figure 4e

system 5

Figure 4f

system 6

Aa B Aa B A Ba Aa B Aa B A Ba

QD, mmol/ min

QI, mmol/ min

QL, mL/ min

P, kPa

t0,b s

3.58 3.58 9.87 9.87 16.12 16.12 16.12 16.12 16.12 16.12 16.12 16.12

0.89 0.89 4.46 4.46 6.88 12.43 17.60 11.22 11.62 2.15 6.42 6.87

100 100 60 60 11.5 5 4.5 2 10 4 16.5 8

101.3 301.3 101.3 301.3 101.3 101.3 101.3 351.3 101.3 101.3 101.3 101.3

0.79 1.23 0.40 0.93 0.44 0.36 0.30 0.84 0.37 0.56 0.44 0.44

a The operating conditions with maximum total flow rate (QD + QI + QL) were used for each system, and accordingly the residence times shown are the shortest times for the same flow channel length. b The minimum residence time, t0, to ensure equilibrium was reached for different operating conditions and working systems.

Table 3. Comparison of the Gas Solubilities Obtained for Systems 1-4 Using Different Measurement Methods

system system system system a

1 2 3 4

range of QG/QL

ADP,a %

0.38-1.15 4-7 37-147 52-237

7.1 6.1 4.7 4.9

Figure Figure Figure Figure

5a 5b 6a 6b

n |[(PCO2)iexp - (PCO2)icalc]/(PCO2)iexp|. APD ) ∑i)1

where PD is the partial pressure of the dissolved gas, kPa; P is the total pressure of gas, measured by the pressure sensor, kPa; and PH2O is the partial pressure of water, 7.38 kPa at 40 °C. The amount of gas dissolved in the absorbent solution can be calculated using eq 3 or 4 depending on the type of system. For systems 1 and 2 (H2O as absorbent alone) c)

∆QD QH2O

(3)

where c is concentration of dissolved gas in liquid phase, mol/ L, and QH2O is the flow rate of water, L/min. For systems 3-6 (absorption of CO2 with MEA solutions) R)

∆QDMMEA QLφMEA

(4)

where R is the CO2 loading of MEA, mol of CO2/mol of MEA; QL is the flow rate of liquid phase, g/min; φMEA is the mass

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fraction of MEA in absorbent solution; and MMEA is the molecular weight of MEA, 61.08 g/mol. 3.2. Determination of Operating Conditions for Measurement. As mentioned before, measurement is based on the fast mass transfer performance of the gas-liquid microdispersion system. In the microdispersion process, the size of the dispersed bubbles and the residence time are the most important factors influencing mass transfer performance, and they are controlled by adjusting the two-phase flow rate when the size and geometry of the microcontactor are fixed. Therefore optimizing the two-phase flow rate is crucial for obtaining a high mass transfer rate and shorter amount of time to reach equilibrium. Figure 3 shows images of bubbles observed at the outlet of the microcontactor when the gas to liquid phase ratio ranged between 6 and 60. The bubble sizes ranged from 200 to 400 µm for system 2 and from 10 to 50 µm for systems 3 and 6 at the operating conditions specified for measurement. As reported in the literature,20,28 the micro size and high gas to liquid phase ratio can significantly enhance the mass transfer performance. The gas-liquid microdispersion flows out of the microcontactor and into the subsequent capillary tube. The capillary pipe length was used to control the residence time, ensuring that the microdispersion remained in the system long enough for equilibrium to be reached. Figure 4 shows plots of the mole fraction of dissolved gas, xG, versus residence time under different operating conditions. For all six working systems, xG remained nearly constant at residence times greater than 0.58 s and gas-liquid equilibrium was reached at residence times of 1.2 s for systems 1 and 2 and 0.8 s for systems 3-6. Here, the values for xG could be considered as the solubility of the dissolved gas in the solvent. The operating conditions used and residence times, t0, to ensure equilibrium was reached are listed in Table 2. Here, t0 was less than 1.2 s for systems 1 and 2 and less than 0.8 s for systems 3-6 to ensure equilibrium was reached in each of the six different gas-liquid microdispersion systems. It is wellknown that the mass transfer rate decreases as the viscosity increases; however, the minimum residence time for systems with higher viscosity (systems 5 and 6) does not show a significant increase compared with that for systems 3 and 4. Thus, within the range of operating conditions identified in here, sufficient mass transfer rate will be provided and vapor-liquid equilibrium will be achieved. The error bars in Figure 4 show the variances of the xG value at different operating times, indicating that the measurement of xG provides good repeatability. In the measurement, the capillary pipe with the length

Figure 5. Solubility data for systems 1 and 2. The dashed lines were calculated using Henry’s law, PG ) HcG, in which PG is the partial pressure of dissolved gas in kPa, H is the dissolved gas constant in kPa · m3/kmol, and cG is the concentration of dissolved gas in the liquid in kmol/m3. (a) At 40 °C, HO2 ) 9.49 × 104 kPa · m3/kmol, from Rettich et al.23 (b) At 40 °C, HCO2 ) 4.07 × 103 kPa · m3/kmol, from Park et al.24

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Figure 6. Solubility data for systems 3 and 4. The dashed lines were calculated using the model from Park et al.24 The reaction equilibrium constants used in the model were calculated using data from different referenced results: (a) ∆, Lee et al.;25 (b) ∆, Jou et al.26

Figure 7. Solubility data for systems 5 and 6: ∆, Song et al.27

of 490 cm was used, providing sufficient residence time for the gas-liquid system to reach equilibrium. The phase separation time, t1, was studied for different systems. For systems 1-4, the large difference in density and low viscosity of the liquid phase caused the gas/liquid mixtures to separate almost immediately upon flowing into the phase separator. Systems 5 and 6 required longer phase separation times (∼20 and 40 s, respectively) because of higher liquid phase viscosity. In the measurement, the volume of the separator could offer a residence time of at least 2 min for phase separation. Therefore, gas solubility for different working systems can be measured with good repeatability when sufficient residence and phase separation times are provided. The precision and accuracy of the measurements are discussed in section 3.3. 3.3. Measurement of Solubility. The six different systems used for gas solubility measurements were divided into three groups, according to their solubilities and liquid phase viscosities: (1) systems with low gas solubility and low liquid phase viscosity (systems 1 and 2); (2) systems with high solubility and low liquid phase viscosity (systems 3 and 4); (3) systems with high solubility and high liquid phase viscosity (systems 5 and 6). When the solubility was measured, changes in the operating conditions were followed by a 2 min waiting period to allow the system to reach steady state, while changes in the working system were followed by at least a 15 min waiting period. Each cycle, which consisted of measuring the gas sample three times to obtain an average value for the solubility, took less than 10 min. Table 3, Figure 5, and Figure 6 show a comparison of the solubility values measured with this device for systems 1-4 and those obtained using the static method as reported in the literature.23-26 The variances in the solubility value are more significant for system 2 and especially for system 1. Thus an error analysis was necessary to help understand the measurement limits and determine the appropriate operating conditions.

According to the measurement method, the solubility can be determined by eqs 5-7: PD ) (P - PH2O)(1 - xI) ∆QD QH2O

(6)

QD0 - QI(1/xI - 1) MMEA QL φMEA

(7)

c)

R)

(5)

Set QD1 ) QI(1/xI - 1) in which QD1 is the flow rate of the dissolved gas after absorption in mol/min and QEG ) QI /xI in which QEG is the total flow rate of the gas phase after absorption in mol/min. A general error analysis was performed using eqs 8 and 9:

( ) ( )( ) ( )( ) ( ) ( )( ) ( )( ) ( )( ) ( )( ) δPD PD

( δcc )

2

)

δR R

2

2

)

P P - PH2O

)

QD0 ∆QD

2

2

δP P

δQD0 QD0

QEG ∆QD

2

2

2

+

+

δxI xI

2

xI 1 - xI

2

δxI xI

2

(8)

QD1 2 δQI 2 + ∆QD QI QL 2 δQL 2 + (9) ∆QD QL

As described in the Experimental Section, the accuracies of the different types of equipment used in the experiments were

Ind. Eng. Chem. Res., Vol. 49, No. 20, 2010 δP ) (0.5%, P

δQD0 δQI ) ) (1.0%, QD0 QI δQH2O δQL ) ) (1.0%, QH2O QL

δxI ) (0.5% xI

The appropriate operating conditions were determined by substituting these values into eqs 8 and 9: 1. To determine the solubility for a given partial pressure, the proper total pressure can be determined by setting (δPD/ PD)2 to a minimum and solving for P. 2. For systems with low solubility (systems 1 and 2), the error is inevitably large due to the large value of QEG/∆QD and the gas solubility’s high sensitivity to fluctuations in the gas composition. Increase of the absorbent’s flow rate at a fixed gas flow rate raises ∆QD and thus decreases measurement error. 3. Solubility measurement error significantly decreases as solubility increases. For systems with high gas solubility, error for the absorption system is estimated to be lower than 5%. In our experiments, the appropriate operating conditions were chosen based on this analysis, and the results showed relative measurement errors as large as 10% for systems 1 and 2 and errors less than 5% for systems 3 and 4. A comparison of the measured solubility data for systems 5 and 6 with those taken from the literature27 is shown in Figure 7. The comparison indicates that this method also provides accurate measurements for systems with viscous liquid. 4. Conclusions In this study, a simple method for rapid and accurate measurement of the gas solubility in liquids was developed by using a membrane dispersion microcontactor, to enhance mass performance. The solubilities of six different working systems were measured, and the results show that the small size of the dispersed phase bubbles in the microcontactor reduces the amount of time it takes to reach gas-liquid equilibrium to no more than 2 s. The gas solubility was measured by analyzing the composition of the vapor phase. The solubility data for the six systems using the new method agreed well with those already reported in the literature. The precision and accuracy of the measurement method were analyzed and appropriate operating conditions were selected to reduce the error to less than 10% and especially less than 5% for chemical absorption processes. This is a simple and practical method for the rapid and accurate measurement of gas solubility in liquids, physical absorption processes and chemical processes with rapid gas-liquid reactions included, and can potentially be very useful for obtaining basic solubility data in many industrial processes which involve gas-liquid reactions or gas absorption. Acknowledgment We would like to acknowledge the support of the National Natural Science Foundation of China (20876084 and 21036002) and SRFDP (20090002110070) for this work. Literature Cited (1) Battino, R.; Clever, H. L. Solubility of gases in liquids. Chem. ReV. 1966, 66, 395. (2) Benson, B. B.; Krause, D. Empirical laws for dilute aqueous-solutions of nonpolar gases. J. Chem. Phys. 1976, 64, 689. (3) Rettich, T. R.; Battino, R.; Wilhelm, E. Solubility of gases in liquids. 13. High-precison determination of henry constants for methane and ethane in liquid water at 275-K to 328-K. J. Phys. Chem. 1981, 85, 3230.

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ReceiVed for reView May 24, 2010 ReVised manuscript receiVed August 2, 2010 Accepted August 25, 2010 IE1011504