N2 Mixed-Gas

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RESEARCH NOTES A New Testing System To Determine the O2/N2 Mixed-Gas Permeation through Hollow-Fiber Membranes with an Oxygen Analyzer Yi Li, Lan Ying Jiang, and Tai-Shung Chung* Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore (NUS), 10 Kent Ridge Crescent, Singapore 119260

A new testing system to determine the O2/N2 mixed-gas separation through hollow fiber membranes has been set up in this study. This system consists of an oxygen analyzer and two mass flow meters, with a total cost of less than S$10 000 (approximately US$6 000). Not only can it reduce the expense, but it also may eliminate some problems encountered when using gas chromatography (GC) in the mixed-gas separation. To validate the new system, two types of hollow-fiber membranes were tested. Experimental results show that the separation performance of the mixed gas agrees well with that of pure gas. It is concluded that the newly developed system may provide an economical alternative in determining the permeance and selectivity of hollow-fiber membranes specifically for mixed O2 and N2 separation. 1. Introduction Pure gas permeation measurements have often been used to estimate the intrinsic separation properties of polymeric membranes for mixed-gas pairs. However, some differences between pure-gas and mixed-gas permeation results may exist. Possible causes are (i) the competition in sorption among the penetrants, (ii) the plasticization induced by CO2 and hydrocarbon gases, (iii) the concentration polarization, and (iv) the nonideal gas behavior.1-8 Therefore, the conduction of mixed-gas measurements is highly recommended for polymeric membranes to obtain the true membrane performance in industrial applications. During the last two decades, the techniques based on gas chromatography (GC) to measure the mixed-gas permeation through polymer membranes have been well-developed. Initially, researchers determined the actual permeability of each species in a gas mixture for flat membranes, using a sweep gas at the downstream side of the membrane to carry the permeate gas to a GC system for composition analysis.9,10 However, the undesirable back-diffusion of sweep gas across the membrane to the feed side might not be negligible. O’Brien et al.11 devised modified manometric (i.e., constant volume) permeation cells for the measurement of multicomponent gas transport through polymer membranes. In combination with GC, their approach significantly simplified the measurement process and could directly estimate the true permeability and selectivity over a wide range of mixed-gas feed pressures and compositions. However, their technique was designed for flat membranes. With the advances in membrane fabrication technology, asymmetric hollow fiber membranes have become a favorable configuration in the membrane-based systems for gas separation.12-15 To investigate their separation performance for mixed gases, Jones and Koros may be the pioneers on mixed-gas tests for hollow fibers,16 where they revised a flat permeation cell for hollow fiber carbon membranes with a modified manometric * To whom correspondence should be addressed. Fax: (65)-67791936. E-mail: [email protected].

permeation cell. Other approaches have also been proposed, using bubble flow meters to determine the retentate and permeate flow rates of a hollow-fiber module and then backcalculated the individual permeance, depending on the flow pattern.17-19 However, no matter which type of mixed gases was separated and which method was applied to test the gas flow rate, the compositions of the permeate, retentate, and feed gases were all determined by a GC system in these reports. A modern GC system is fairly expensive. In addition, it is not trivial to identify proper operation conditions for GC and to accurately calculate the individual fluxes when overlap peaks occur or the permeance is very small. This is especially true for O2/N2 mixed gas separation. Based on our experience, it is more difficult to measure the separation performance of O2/N2 than that of CO2/CH4 mixed gas when using the same GC-based system, because of the partial overlapping of peak areas of O2 and N2. Therefore, the purpose of this short note is to introduce a relatively economical and easier testing system for the characterization of hollow-fiber membranes for mixed O2 and N2 separation based on an oxygen analyzer. The detailed design, testing procedure, and data validation will be elucidated. 2. System Design and Measurement Procedure A schematic diagram of the apparatus using an oxygen analyzer for O2/N2 mixed-gas permeation tests through a hollowfiber module is shown in Figure 1. Swagelok 316 and 316L stainless steel components such as tubing, union tees, and connectors were used in the construction of testing system. A hollow-fiber module was installed in a temperature-controlled chamber and tested in the shell-side feed method. This module was made by assembling several pieces of fibers into a bundle. The open end of the bundle was sealed with a 5-min rapid epoxy resin (Araldite, Switzerland), while the other end was glued onto the Swagelok VCR union tee using a regular epoxy resin (Eposet). Eight hours were required to fully cure the Eposet resin. Two hollow-fiber modules that were composed of

10.1021/ie051152b CCC: $33.50 © 2006 American Chemical Society Published on Web 12/17/2005

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Figure 1. Schematic diagram of the apparatus using an oxygen analyzer for O2/N2 mixed-gas permeation tests through a hollow-fiber module. Table 1. Module Specifications and Experimental Conditions (at 24 °C) parameter

hollow fiber 1

hollow fiber 2

membrane material outer diameter of fibers inner diameter of fibers length of fibers number of fibers compositions of mixed gas feed gas pressure permeate gas pressure retentate flow rate

composite membranes based on polyethersulfone 950 µm 520 µm 5.5 cm 11 21 mol % ( 1% O2 in air or CO2/CH4 (50/50 mol %) 200 psig (for air separation) or 190 psig (for CO2/CH4) ∼0 psig 0.50 mL/min (for air separation); 1.38 mL/min (for CO2/CH4) 0.0219 mL/min (for air separation); 0.0819 mL/min (for CO2/CH4)

composite membranes based on polysulfone 1000 µm 550 µm 4.5 cm 4 21 mol % ( 1% O2 in air or CO2/CH4 (50/50 mol %) 200 psig (for air separation) or 200 psig (for CO2/CH4) ∼0 psig 0.57 mL/min (for air separation); 2.70 mL/min (for CO2/CH4) 0.028 mL/min (for air separation); 0.135 mL/min (for CO2/CH4)

permeate flow rate

different polymers were tested. Table 1 summarizes their specifications and experimental conditions. Purified air containing 21 mol % ( 1 mol % O2 was feed to the shell side of hollow-fiber membranes from a compressed gas cylinder (shown in Figure 1). A very small part of the feed gas went through the wall of the hollow-fiber membranes and entered the permeate side, while most of the feed gas entered the retentate side from the mid-arm of the union tee. The feed gas pressure was controlled by needle valve 1 and displayed by a digital pressure gauge. For O2/N2 mixed-gas permeation tests, the feed gas pressure was ∼200 psig, the permeate side was open to atmosphere, and the system is at ambient temperature. However, they can be also conducted at different temperatures and pressures. To obtain accurate mixed-gas permeation results, the testing system is comprised of the following four steps. The first step is to decide the retentate and permeate flow rates, using a mass flow controller and bubble flow meter, respectively. The model of mass flow controller used in this work is GFC 17 (Aalborg), which costs S$1740 (approximately US$1000), including calibration. Its measurement range is 0-10 mL/min (under standard temperature and pressure). The accuracy of the mass flow controller is approximately (1.5% of the full scale. For different gases, the actual flow rate is the product of the set point of the mass flow controller times an adjustable coefficient, because

this equipment was calibrated with N2 as received. In our case, the adjustable coefficient is ∼1 for air, because the adjustable coefficient is 1 for N2 and 0.996 for O2. The following briefly describes our procedure to control the stage cut approximately at or below 5% to eliminate the effect of concentration polarization.11,18 First, the set point of mass flow controller (1) (see Figure 1) was randomly set at a certain value (for example, 2 mL/min). Second, the permeate flow rate was measured by the bubble flow meter shown in Figure 1. If the ratio of permeate flow rate to retentate flow rate is ∼0.05 or lower (i.e., the stage cut was at or below 0.05), the first step (i.e., measurement of the flow rates) was finished. Otherwise, the set point of mass flow controller (1) would be readjusted to a new value until this requirement was achieved. In this work, the ultimate retentate and permeate flow rates in the mixed O2 and N2 separation were 0.50 and 0.0219 mL/min for hollow fiber 1, respectively; and were 0.57 and 0.0283 mL/min for hollow fiber 2, respectively, as shown in Table 1. The second step is to measure the composition of retentate gas using the oxygen analyzer. An Advanced Micro Instrument (AMI) oxygen analyzer (model 201) was applied in this study. The analyzer costs S$5500 (approximately US$3300), inclusive of the installation fee. Its measurement range is 0%-100% O2 concentration. This O2 analyzer is calibrated on air, and its reading is adjusted to 20.9 in the calibration at moderate

Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006 873 Table 2. Comparison of Separation Performance of Hollow-Fiber Membranes between the O2/N2 Mixed Gas and Pure Gas Measurementsa Mixed Gas (21 mol % ( 1% O2 in air) at 24 °C Permeance (GPU)

Pure Gas at 35 °C Permeance (GPU)

membrane name

total feed pressure (psig)

O2

N2

selectivity (O2/N2)

O2

N2

ideal selectivity (O2/N2)

Hollow Fiber 1 Hollow Fiber 2

200 200

0.0631 0.271

0.00918 0.0423

6.9 6.4

0.0741 0.306

0.0114 0.0493

6.5 6.2

a

Pure O2 and N2 gases were tested at 132 psig.

Table 3. Comparison of Separation Performance of Hollow-Fiber Membranes between the CO2/CH4 Mixed-Gas and Pure-Gas Measurementsa Mixed Gas (50/50 mol % CO2/CH4) at 24 °C Permeance (GPU)

Pure Gas at 35 °C Permeance (GPU)

membrane name

total feed pressure (psig)

CO2

CH4

selectivity (CO2/CH4)

CO2

CH4

ideal selectivity (CO2/CH4)

Hollow Fiber 1 Hollow Fiber 2

190 200

0.164 0.807

0.00485 0.0249

33.8 32.4

0.187 0.835

0.00641 0.0267

29.2 31.2

a

Pure CO2 and CH4 gases were tested at 100 psig.

temperature and humidity. Both the sensitivity and accuracy of the AMI model 201 oxygen analyzer used in this work are 0.5% of full scale. Therefore, the measurement error from the oxygen analyzer is much smaller, compared to that from the mass flow controller, and the oxygen analyzer is sensitive and accurate enough to measure the concentration changes in the feed and retentate gases. Measurement of the O2 concentration is performed using an electrochemical oxygen sensor, the basic functioning of which is similar to a battery. Oxygen gas diffuses through a membrane; contacts an electrode and is reduced to a negatively charged hydroxyl ion. This ion moves through an electrolyte in the oxygen sensor to a positively charged electrode typically made of lead. The hydroxyl ion reacts with the lead and releases electrons. The electron flow is measured and can mathematically be converted to an oxygen concentration. This electrochemical oxygen sensor is sealed in the cell compartment of oxygen analyzer by an O-ring. The gas sample can only enter and exit the cell compartment through the 1/4-in. Swagelok® tube; therefore, there is no internal leak within the oxygen analyzer. Because the retentate flow rate was too small in our study, it was impossible to produce a high positive pressure from the inlet to the outlet of the oxygen analyzer, to prevent the interference of atmosphere entering into the oxygen analyzer. To overcome this problem, a special design was made here. Another mass flow controller (denoted as mass flow controller (2)) was connected to the oxygen analyzer, as shown in Figure 1. As a result, the gas can only flow from the inlet to the outlet in the mass flow controller and cannot flow inversely, because of its special inner structure; therefore, the gas in the atmosphere could not diffuse into the system. This design (i.e., the use of the second mass flow controller) is very important, because it significantly extends the application range of our testing system at various gas flow rates. After needle valves 2, 3, and 5 were opened and needle valve 4 was closed, the line from the outlet of mass flow controller (1) to the outlet of mass flow controller (2) was evacuated, to remove the residual gas of the system (i.e., hollow-fiber module and tubing) before test. The electric power of the oxygen analyzer must be shut down during this period; otherwise, this action may damage the sensor in the oxygen analyzer by boiling the electrolyte. Thereafter, needle valves 3 and 5 were closed and the electric power of the oxygen analyzer was turned on. Thereupon, the retentate gas that accumulated inside the oxygen analyzer was indicated by a gradual increase in the reading of the oxygen analyzer. After an appropriate period of time, needle valves 3 and 4 were opened and the superfluous gas that was

stored in the oxygen analyzer streamed into the atmosphere through mass flow controller (2). The set point of mass flow controller (2) should be regulated to a value slightly higher than that of mass flow controller (1), to eliminate the accumulation of the retentate gas in the oxygen analyzer, which may mislead the reading. With a decrease in the superfluous gas contents in the oxygen analyzer, the set point of mass flow controller (2) should be gradually adjusted until it was the same as that of mass flow controller (1) (i.e., 0.50 mL/min for hollow fiber 1 and 0.57 mL/min for hollow fiber 2). Under these conditions, the oxygen analyzer would provide a stable and reliable reading of O2 concentration in the retentate gas. The aforementioned proposed operation sequence is very critical to obtain a stable and accurate gas flow rate through the oxygen analyzer, especially at low gas flow rates, which determines the reliability of the reading of O2 concentration. To validate the O2 composition of the feed gas to be 21 mol %, the third step is to measure the composition of the feed gas, using the oxygen analyzer, and then calculate the composition of permeate gas through the mass balance. In this study, the composition of permeate gas was not measured directly using the O2 analyzer, because the permeate flow rate was so small that a much larger error may be introduced during the measurement. The tubing of the feed gas was connected directly to the inlet of mass flow controller (1), as illustrated in Figure 1, and the composition of the feed gas was measured similarly to test the retentate gas. The last step was that the apparent permeance of each species in the O2/N2 mixed gas was calculated based on one set of differential equations developed by Wang et al.,18 in which the nonideal gas behavior and pressure drop inside the hollow fiber have been considered. In the case of negligible permeate gas pressure, the selectivity of hollow fiber membranes for a mixed gas was equal to the ideal selectivity; that is, it can be calculated from the ratio of multicomponent permeances measured at the partial pressure.1,11,20 3. Results and Discussion The performance of this new testing system is evaluated by measuring the permeance of two types of hollow-fiber membranes in the mixed gas. Their pure-gas permeance was first measured using the similar technique described in the paper by Jones and Koros.16 The O2/N2 separation results of mixed gas and pure gas are listed in Table 2. It can be found that both permeance and selectivity of mixed gas are very close to those of pure gas when considering the effect of different testing

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temperatures. The results demonstrate that the new testing system devised in this work can effectively determine the permeance and selectivity of hollow-fiber membranes in the separation of O2/N2 mixed gas. The total price of the apparatus applied in this new testing system, including one oxygen analyzer, two mass flow controllers, and one bubble flow meter, is no more than S$10 000 (approximately US$6000) which is much lower than that of a GC system. To further prove the authenticity of the O2/N2 mixed-gas permeation results measured by the oxygen analyzer, the same hollow fiber samples were used to separate the CO2/CH4 (50/50 (mol %)) mixture in this study. The testing procedure was similar to that applied in the mixed O2/N2 separation, and the only difference is that the composition of the CO2/CH4 mixed gas was determined via GC instead of using the oxygen analyzer. Both the oxygen analyzerbased testing system and the GC-based testing system use the mass flow controller or bubble flow meter to measure the gas flow rate; therefore, the errors of these two testing systems should be comparable and are mainly determined by the accuracy of the mass flow controllers. Experimental conditions and parameters can be observed in Table 1. CO2/CH4 separation results of the mixed gas and the pure gas are given in Table 3. These data show that the difference between mixed gas and pure gas is almost negligible if considering the effect of different testing temperatures, which, again, justify that this new testing system with the much lower cost can fulfill the purpose of measuring O2/N2 mixed gas separation performance through the hollow-fiber membranes. 4. Conclusions A new economical testing system that uses an oxygen analyzer has been designed to determine the permeance and selectivity of hollow-fiber membranes in the O2/N2 mixed-gas separation. Two types of hollow fiber membranes are used to evaluate the performance of this new testing system. Experimental results indicate the difference of both permeance and selectivity between mixed gas and pure gas is almost neglectable, which demonstrates that this new testing system with the much lower cost can accurately measure the permeance of each species in the O2/N2 mixed-gas separation. Acknowledgment The authors would like to thank NUS for funding this research (under Grant No. R-279-000-184-112).

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ReceiVed for reView October 16, 2005 ReVised manuscript receiVed December 8, 2005 Accepted December 9, 2005 IE051152B