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Gas analyzer for continuous monitoring of carbon dioxide in gas streams
Sensors and Actuators B 145 (2010) 398–404
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Gas analyzer for continuous monitoring of carbon dioxide in gas streams Sayed A.M. Marzouk a,∗ , Mohamed H. Al-Marzouqi b a b
Department of Chemistry, UAE University, P.O. Box 17551, Al Ain, United Arab Emirates Department of Chemical and Petroleum Engineering, UAE University, P.O. Box 17555, Al Ain, United Arab Emirates
a r t i c l e
i n f o
Article history: Received 29 October 2009 Received in revised form 7 December 2009 Accepted 14 December 2009 Available online 24 December 2009 Keywords: CO2 gas analyzer In-line gas analysis Hollow fiber membrane contactors pH detector
a b s t r a c t The construction and characterization of a simple and inexpensive gas analyzer setup for direct measurement of CO2 concentration in gas streams are presented. The analyzer was based on a hollow fiber membrane (HFM) contactor which served as a mean for continuous gas sampling and preconcentration of CO2 from the gas stream into a dilute buffer carrier solution. The resultant pH changes of the carrier solution were measured downstream by a pH flow-through detector. A Nernstian response of 55.5 mV/log[CO2 ] was obtained over a wide dynamic range of 0.1–100% CO2 in nitrogen. Moreover, the presented analyzer offered several favorable performance characteristics such as fast response (45 s) and recovery times (1–2 min), excellent signal stability and reproducibility (RSD = 1.2%) and intrinsic high selectivity in the presence of most common neutral gases (e.g., CH4 , N2 , O2 , CO, etc.). H2 S and SO2 which are potentially interfering acidic gases were efficiently removed by a simple prior in-line treatment using acidified potassium permanganate bubbler. The analyzer was applied successfully to monitor the treatment process of CO2 removal from CO2 –CH4 gas mixtures. The merits of the developed analyzer suggest its reliable use in various applications requiring continuous carbon dioxide monitoring in gas streams for several hours. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Carbon dioxide is an important industrial gas with several uses that include production of chemicals (e.g., urea), inert agent for food packaging (to extend the shelf-life of meat, cheese, etc.), refrigeration systems, beverages, welding systems, fire extinguishers, water treatment processes, precipitated calcium carbonate for the paper industry and many other smaller scale applications [1,2]. Moreover, in greenhouses, the growth rate of several crops can be improved by controlling the concentration of carbon dioxide [3]. Measuring and controlling carbon dioxide are important as well in natural gas purification [4] and in several experimental research setups [5]. Several methods have been described in literature for the determination of carbon dioxide based on potentiometric [6–11], conductometric [12], amperometric [13,14], coulometric [15], spectrometric [16–21], impedance [22], and capacitance [23] techniques. However, most of these methods targeted the dissolved CO2 in solutions [12,18,23]. In addition, there are some carbon dioxide gas sensors and analyzers commercially available in the market for direct measurements in gas streams. The method of infrared technology is most commonly used in the commercial analyzers. Systems using the infrared technology offer the advantage of
∗ Corresponding author. Tel.: +971 3 7134939; fax: +971 3 7671291. E-mail address: [email protected] (S.A.M. Marzouk). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.12.045
standalone operation, i.e., do not require consumption of reagents. However, they are usually large and available at relatively high cost and suffer from some limitations that may affect the reliability of the carbon dioxide measurements. Furthermore, controlling the humidity of the sample gas is important for the accuracy of the infrared measurements. Alternatively, continuous gas monitoring in a given gas streams can be achieved by employing a prior step in which the analyte gas is selectively stripped in an appropriate carrier solution either by direct gas–liquid mixing [24] or gas–liquid membrane contactor (diffusion scrubber) [12]. The concomitant changes in the solution as a result of gas absorption can be measured in a subsequent step to produce the analytical signal. An interesting feature of diffusion scrubbers is that there is no interaction between the particulate matter present like those encountered in conventional filtration or sorbent sampling [25]. The aim of the present work was to develop analyzer for continuous CO2 determination in gas streams based on diffusion scrubber gas sampling and flow-through pH detector. Several advantages of such configuration can be recognized such as low cost, simple construction and operation, versatile detection schemes, tunable sensitivity. Although shares some similarities, the presented analyzer setup overcomes the limitations associated with the common Severinghaus gas sensor configuration [6] including the long recovery times and the incompatibility with continuous monitoring in dry gas streams. Moreover, Severinghaus gas sensing configuration
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mostly involves reversible physical dissolution of the analyte gas in the internal receiving solution and hence a number of possible selective detection schemes based on irreversible gas reaction/absorption are not possible with Severinghaus configuration with such fixed internal solution. The suggested continuous receiving solution provides the necessary flexibility in developing analyzers for other important gases [26]. The intrinsic physical and chemical properties of CO2 such as its appreciable solubility in aqueous solutions and the acidity of the produced carbonic acid suggested using the glass electrode as a convenient and adequately selective detector. Optimization, characterization and application of the presented CO2 analyzer are described. 2. Experimental 2.1. Materials and reagents Carbon dioxide (99.99%), nitrogen (99.99%), methane (99.99%), sulfur dioxide standard (1.0% in N2 ) and hydrogen sulfide (2.0% in N2 ) were received from Air products (UAE). Microporous polypropylene (PP) loose fibers (OD 300 m and ID 220 m), and polypropylene Mini-module (Model G543, Lumen side volume 16 mL, 2300 fibers per module) were received from Membrana (USA). Mini-modules based on Silicone Rubber (SR) hollow fibers (Model M-300, 300 fibers per module) were received from Nagayanagi Co. (Japan). All chemicals used were of the highest available purity. All solutions were prepared using deionized water. 2.2. Instrumentation A 4-channel computer controlled gas mixer, Model MFC-4, Sable Systems (USA) was used to control four Mass flow controllers, Sierra Instruments, Inc. (USA) to prepare variable concentrations of CO2 (and/or H2 S and SO2 ) in a balance of N2 or CH4 for calibration and characterization purposes. The MFC-4 utility software (Sable Systems) was used to run a given preset program of concentration steps. The potentiometric pH detector was based on a commercial acrylic flow cell, Sensorex, model FC47C (USA), with 50 L internal volume and equipped with a flat-bottom combination glass-reference electrode (Sensorex, model S450C). A custom made high-input impedance (1015 ) differential amplifier was used for pH measurements. The output of the amplifier was measured using a 16 bit-analog to digital converter (ADC) interface, Pico Technology, model ADC 16 (UK) connected to a PC installed with PicoLog software (Pico Tech.) for data display and storage. The obtained potential resolution was 0.01 mV. All measurements were carried out in an air-condition lab at 23 ± 1 ◦ C. 2.3. Construction of the CO2 analyzer The proposed CO2 analyzer in this work was designed for continuous monitoring of CO2 in gas streams. The analyzer consisted of two main components, i.e., the gas sampling unit and the detector as shown in Fig. 1. The gas sampling unit was based on a diffusion scrubber in the form of HFM module. The HFM modules were either the commercially available or custom fabricated. The latter were fabricated by potting a given number of loose PP fibers (30 cm in length) using epoxy at both ends in transparent acrylic tube (10 mm ID) to provide an active fibers length of 20 cm. Stainless steel tubes (3 mm OD, 3 cm in length) were used as inlet and outlet for both gas stream and the liquid carrier. The details of the custom fabricated HFM modules were shown in the insert in Fig. 1. The commercially available SR module was equipped with
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300 fibers and the active length of the fibers was 6.5 cm. The commercial polypropylene Mini-module (Model G543) was modified to reduce the number of the active fibers and hence the lumen volume (from 16 mL to about 6 mL). The 1/4” plastic female NPT thread cavity of the lumen inlet and outlet were cut approximately 3 mm away from the fibers ends to expose the cross-section of the circular bundle of the PP fibers (2300 fibers) at both ends. Then Araldite epoxy was used to block the central fibers at the inlet side in such away the active fibers were only those located away from the module axis as shown in Fig. 1s (supporting material). An appropriately selected carrier solution was pumped through the tube side (lumen) of the module using a peristaltic pump (Masterflex, Model 7519-20) and the gas stream was allowed to flow in a counter flow configuration through the shell side. 3. Results and discussion 3.1. Principle of operation of the proposed gas analyzer The overall performance of the present gas analyzer was determined by several individual factors including: (i) HFM type; (ii) composition of the carrier solution; (iii) flow rate of the carrier solution; (iv) module size; and (v) response properties of the pH electrode and the flow cell geometry. The membrane type should be hydrophobic and reasonably permeable to the analyte gas, CO2 in the present work. The liquid carrier should be able to capture, to some extent, the analyte gas from the gas stream and the gas–liquid carrier interaction should selectively produce some changes in at least one of the conveniently measurable properties of the liquid carrier. For CO2 , significant pH changes in the carrier solution can be produced given that the buffer capacity of the carrier was kept at sufficiently low value. The use of potentiometric pH detectors to construct Severinghaus-type electrodes for gaseous species such as CO2 [6], NH3 [27] NOx [28], hydrazoic acid [29] has been well documented in literature. However, several advantages offered by the proposed CO2 analyzer compared to Severinghaus-type configuration can be rationalized. These include: (i) the response time can be controlled by adjusting the receiving solution flow rate and/or the total internal volume; (ii) the proposed analyzer was more suitable than Severinghaus gas probes for continuous monitoring of the analyte gas in gas streams; (iii) the gas sampling step (which takes place in the HFM module) was spatially separated from the detection step which takes place downstream. Such remote placement of the detector could be of exceptional importance if the gas stream conditions (e.g., high temperature) do not suit the Severinghaus-type gas probes based on glass electrode; and (iv) schemes based on physical dissolution of the analyte in the receiving solution as well as chemical reaction can be utilized. Whereas Severinghaus-type sensors were limited to the physical dissolution to keep the properties of the internal solution and hence the sensor response fixed to a reasonable extent. The electromotive force (EMF) of the pH electrode cell can be shown to relate linearly to the logarithmic CO2 concentration [29] as shown in Eq. (1), given that the bicarbonate ion exit in the carrier solution at relatively high concentration: EMF = constant + S log[CO2 ]
(1)
where S is the slope of the calibration graph. 3.2. Optimization of the experimental parameters for CO2 analyzer The experimental variables of significance to the described setup were (i) nature of the hollow fiber membranes; (ii) buffer capacity of the carrier solution; and finally (iii) the ratio (R) between
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Fig. 1. The experimental setup and construction of CO2 gas analyzer based on pH flow-through detector and HFM module.
the total internal volume of the system and the carrier flow rate. The ratio R is directly proportional to the carrier residence time within the module and hence the extent of the gas preconcentration in the carrier solution. Hollow fibers fabricated from different polymeric materials were commercially available in different dimensions (i.e., ID, OD and wall thickness). The fibers were either microporous (e.g., polypropylene) or non-porous (e.g., silicone rubber). Gas permeation in the latter type was based on dissolution–evaporation mechanism [30]. Microporous fibers offer the advantage of higher gas flux but the non-porous fibers could provide some selective permeation for different gases. The interfacial membrane area and hence the efficiency of gas absorption and detection sensitivity should be enhanced with smaller fiber dimensions. Hydrophobic and microporous polypropylene (PP) fibers and SR were selected in the present work due to their high CO2 permeability, a requirement necessary for high sensitivity. Buffer capacity of the carrier solution should be a trade off between baseline stability (enhanced with increased buffer capacity) and signal sensitivity which should be enhanced at lower buffer capacity. Strongly buffered carrier solution would suppress the pH changes upon CO2 absorption. The resultant sensitivity and linearity of the analyzer could be a direct combination of the CO2 gas permeability in the HFM module, carrier buffer capacity and the ratio (R) which determines the extent of the gas preconcentration and, in turn, the observed magnitude of the pH change. The ratio (R) also participated to the overall response and recovery times of the analyzer. Therefore, the selected modules were among the smallest commercially available modules and the custom made ones were constructed from even smaller number of fibers (140–250 fibers) and the dead volume in the lumen side, through which the carrier solution flows, was kept to the minimum. Given the above consideration, the response of the analyzer equipped with SR module was initially obtained using NaHCO3 carrier solutions with different concentrations. The obtained results were presented in Fig. 2. The enhanced linearity at lower concentration was attributed to the smaller buffer capacity and hence smaller signal attenuation. A linear response with almost Nerns-
tian slope (57.5 mV per CO2 concentration decade) was obtained at 1 mM NaHCO3 carrier solution. The same trend was obtained with custom fabricated HFM modules based on microporous polypropylene fibers (250 fibers per module, data not shown). Therefore, 1 mM NaHCO3 carrier solution was used throughout the remaining work. The effect of carrier solution flow rate was studied at different values and the results obtained for 0.6, 1.5 and 3 mL min−1 were presented in Fig. 3. The observed enhanced linearity and sensitivity at lower flow rates were attributed to the increased residence time, and hence higher CO2 preconcentration in the carrier solution. A small hysteresis between the analyzer response and recovery was observed when the SR module was used (∼3.5 mV at 10% CO2 ). This
Fig. 2. Effect of NaHCO3 carrier concentration on the analyzer response to CO2 . Silicone rubber HFM module; carrier flow rate 1.5 mL min−1 ; gas flow rate 200 mL min−1 .
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Fig. 3. Effect of carrier solution flow rate on the analyzer response to CO2 concentrations. Silicone rubber HFM module; 10−3 mol L−1 NaHCO3 carrier solution.
behavior was shown in Fig. 4 as indicated by the dotted lines connecting some corresponding CO2 levels in the rising and decreasing ramps, respectively. Such relatively unfavorable recovery was significantly improved when the SR module was replaced with the custom fabricated modules based on microporous polypropylene fibers. Then, a modified commercial module (Membrana® , Model G543, constructed from the same type of PP fibers) as described above was used throughout the remaining work. The module modification was conducted to allow smaller carrier solution flow rates and to minimize the solution consumption. The response obtained with the modified PP module was shown in Fig. 5. A linear response was obtained with a near Nernstian slope (55.5 mV per CO2 concentration decade) as shown by the multi-step calibration curve (Fig. 5A) and the hysteresis was almost eliminated as indicated by the horizontal dotted lines connecting two
Fig. 4. Trace of the analyzer real-time response to CO2 showing a noticeable hysteresis between the signal readings obtained with SR HFM module. 10−3 mol L−1 NaHCO3 carrier solution; carrier flow rate 4 mL min−1 ; and gas flow rate 200 mL min−1 .
similar CO2 concentrations in both ramps as shown in Fig. 5B. The obtained sensitivity and the wide linearity (0.1–100% CO2 ) were attributed to the optimized analyzer construction and the intrinsic wide dynamic response range of the glass electrode detector. More sensitive detection than the LOD obtained under the optimized conditions (i.e., 0.05% CO2 ) could be realized if needed in any particular application by decreasing the carrier flow rate to increase the extent of CO2 preconcentration in the carrier solution. However, this will be on the expense of longer response and recovery times. Full analyzer characterization was carried out using such modified commercial polypropylene module and described in the following section.
Fig. 5. (A) Time response curves and the corresponding potentiometric calibration plots obtained with the modified PP commercial HFM module. 10−3 mol L−1 NaHCO3 carrier solution, carrier solution flow rate 6 mL min−1 ; and gas flow rate 200 ml min−1 . (B) Trace of the analyzer real-time response to two cycles of CO2 level change over a wide range indicating excellent baseline and signal stability and complete recovery.
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Fig. 6. The analyzer response to series of step changes in CO2 level between 1.0% and 10.0% CO2 in N2 . The horizontal doubly head arrow near the baseline of the first step represents the recovery time of the presented step change. The experimental conditions were similar to those mentioned in Fig. 5.
3.3. Characterization of the CO2 analyzer The repeatability of the signal response was investigated by series of step changes in CO2 level between two values as shown in Fig. 6. The time response obtained for six cycles between 1% and 10% CO2 , respectively, clearly revealed fast response time (t95% = 45 s). The analyzer response showed high accuracy for CO2 measurements, i.e., 98.2% and 99.0%, and excellent repeatability (RSD = 2.0% and 1.2%) at 1.0% and 10.0% CO2 concentration levels, respectively. Within day repeatability and between-day reproducibility of the analyzer response to CO2 were 1.8% and 3.5%, respectively. These are important favorable features in continuous monitoring of CO2 for prolonged time (several hours) which eliminate the need for frequent calibration. The ability of the analyzer to detected small changes at relatively high CO2 concentration was also assessed by series of step changes in CO2 level between 10.0% and 9.9% (i.e., 1% of the initial value). The obtained response was smoothed with adjacent averaging and presented in Fig. 7. Two well distinguished and stable signal levels with S/N > 5 were obtained indicating the ability of the developed analyzer to reliably detect small changes in the CO2 level.
The analyzer recovery times upon step changing the CO2 from 100% level to various lower values were evaluated as shown in Fig. 2s (supporting material). Similar plots for changes from 10% and 1% levels to lower values were also obtained, respectively (data not shown). Zero in the time axis marked the onset of the concentration change. The obtained data revealed fast recovery times which tend to be longer for larger step changes and for lower concentrations levels. For example, the recovery times (t95% ) upon step change from 100% to 10%, and from 10% to 1% and from 1% to 0% were 65, 100, and 120 s, respectively. The effect of gas flow rate on the analyzer response was investigated by changing the gas flow rate at a fixed CO2 concentration level. The analyzer response to 10% CO2 at substantially different gas flow rates (i.e., from 1000 to 50 mL min−1 ) showed impressive insensitivity to wide variations in the gas flow rate as shown in Fig. 3s (supporting materials). This was an important feature in gas analyzers which otherwise would require strict flow rate control or calibration at different gas flow rates. Two gases, i.e., hydrogen sulfide and sulfur dioxide were identified as potential interfering species in the determination of CO2 because they are both acidic gases and also commonly co-exist with CO2 in a number of important applications such natural gas industry and CO2 sequestration. Both gases exerted significant interference on CO2 measurements in the order of SO2 > H2 S. To address such interference problem, two simple approaches were suggested. First, for low concentration of hydrogen sulfide (ppm levels) a (20cm long, 1 cm ID) acrylic column filled with acid washed metallic copper particles proved effective to eliminate hydrogen sulfide from the gas stream without affecting the CO2 level. Second, a bubbler containing 250-mL of 0.25 M potassium permanganate in 2 M sulfuric acid proved highly efficient in eliminating completely and selectively both H2 S and SO2 by oxidizing them to elemental sulfur and sulfate ions, respectively. H2 S and SO2 at 20,000 ppm and 10,000 ppm tested level, respectively were completely removed from gas streams (200 mL min−1 ) whereas an almost 100% CO2 recovery was obtained after passing through the same bubbler. This conclusion was further confirmed by independent analysis of the treated gas streams with commercial CO2 -infrared analyzer (Model 602, California Analytical Instruments, Inc.) and H2 S-UV analyzer (Model AMO 300, Applied Analytics Inc.), respectively. Other acidic gases which are less likely to co-exist with CO2 such as NOx and acetic acid vapor are expected to exert some interfering effect if present in appreciable levels in the gas stream. In such cases, further investigation will be required to estimate the effect of any particular gas and a suitable prior treatment can be similarly suggested. Alternatively, a carbonate selective ISE can be used instead of the glass electrode in combination with appropriate stripping carrier if interference from acidic gases presented serious interference problem in any particular application [31]. 3.4. Monitoring of CO2 removal from CO2 –CH4 gas mixture using the proposed analyzer
Fig. 7. The analyzer response to series of step changes in CO2 level between 9.9% and 10.0% CO2 . The conditions were similar to those mentioned in Fig. 5.
Removal of acid gases from various contaminated gas streams using hollow fiber membrane modules in combination with various stripping solvents has been an active area of research [5,32]. There is a considerable reliance on use of commercial gas analyzers or GC to monitor contaminated gas streams (e.g., flue gas, natural gas, etc.) both before and after the treatment step. Therefore the analyzers should provide a wide linear dynamic range (although with good sensitivity) to suit these important applications. In the present work, the described CO2 analyzer was successfully applied in monitoring continuously CO2 level in the treated gas stream after passing through the absorption module (Membrana® , Model G542 based on microporos polypropylene hollow fibers) in which CO2 was preferentially removed from methane by contacting
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Fig. 8. Experimental setup for CO2 removal from CO2 –CH4 mixture using HFM module and NaOH as stripping solvent. The treated gas was continuously monitored with the proposed CO2 gas analyzer.
presented analyzer for other specific application requirements such as longer continuous monitoring and/or at higher temperatures. The basic analyzer components, i.e., the peristaltic pump head, mini HFM module and flow-through pH detector can also be integrated into a small portable device for in-line real-time CO2 monitoring. Developing analyzers for other important gases using different detection schemes and the same analyzer setup is in progress. Acknowledgements The authors would like to thank JCCP (Japan Corporation Center, Petroleum) for the financial support. They would also like to acknowledge the support of Research Affairs at the UAE University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.snb.2009.12.045. References Fig. 9. Effect of sodium hydroxide concentration on the CO2 level in the treated gas stream as monitored by the presented CO2 gas analyzer. The first two steps correspond to the analyzer response to 1.0% and 10.0% CO2 without applying NaOH in the treatment module.
with and dissolution in NaOH solution. The experimental setup was shown in Fig. 8. The effect of NaOH concentration on the CO2 level in the treated gas was shown in Fig. 9. Complete removal of CO2 from the gas stream was achieved by using a flow of 1.00 mol L−1 NaOH solution at 50 mL/min. 4. Conclusions The suggested low-cost and simple CO2 analyzer proved successful in continuous monitoring of CO2 in gas streams in laboratory setups and offered several attractive characteristics. More characterization may be required to evaluate the performance of the
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Biographies Sayed A.M. Marzouk received his PhD in Analytical Chemistry from Ain Shams University in 1997. In 2005 he is appointed as associate professor of Analytical Chemistry at the United Arab Emirates University. His research interests are in the field of chemical sensors and biosensors, gas analysis and gas separation. Mohamed H. Al-Marzouqi received his PhD in Chemical Engineering from Oregon State University in 1994. In 2006 he is appointed as the Chair of Chemical and Petroleum Engineering Department at the United Arab Emirates University. His research interests are in the field of gas separation using membrane contactors.
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Gas analyzer for continuous monitoring of carbon dioxide in gas streams Sayed A.M. Marzouk a,∗ , Mohamed H. Al-Marzouqi b a b
Department of Chemistry, UAE University, P.O. Box 17551, Al Ain, United Arab Emirates Department of Chemical and Petroleum Engineering, UAE University, P.O. Box 17555, Al Ain, United Arab Emirates
a r t i c l e
i n f o
Article history: Received 29 October 2009 Received in revised form 7 December 2009 Accepted 14 December 2009 Available online 24 December 2009 Keywords: CO2 gas analyzer In-line gas analysis Hollow fiber membrane contactors pH detector
a b s t r a c t The construction and characterization of a simple and inexpensive gas analyzer setup for direct measurement of CO2 concentration in gas streams are presented. The analyzer was based on a hollow fiber membrane (HFM) contactor which served as a mean for continuous gas sampling and preconcentration of CO2 from the gas stream into a dilute buffer carrier solution. The resultant pH changes of the carrier solution were measured downstream by a pH flow-through detector. A Nernstian response of 55.5 mV/log[CO2 ] was obtained over a wide dynamic range of 0.1–100% CO2 in nitrogen. Moreover, the presented analyzer offered several favorable performance characteristics such as fast response (45 s) and recovery times (1–2 min), excellent signal stability and reproducibility (RSD = 1.2%) and intrinsic high selectivity in the presence of most common neutral gases (e.g., CH4 , N2 , O2 , CO, etc.). H2 S and SO2 which are potentially interfering acidic gases were efficiently removed by a simple prior in-line treatment using acidified potassium permanganate bubbler. The analyzer was applied successfully to monitor the treatment process of CO2 removal from CO2 –CH4 gas mixtures. The merits of the developed analyzer suggest its reliable use in various applications requiring continuous carbon dioxide monitoring in gas streams for several hours. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Carbon dioxide is an important industrial gas with several uses that include production of chemicals (e.g., urea), inert agent for food packaging (to extend the shelf-life of meat, cheese, etc.), refrigeration systems, beverages, welding systems, fire extinguishers, water treatment processes, precipitated calcium carbonate for the paper industry and many other smaller scale applications [1,2]. Moreover, in greenhouses, the growth rate of several crops can be improved by controlling the concentration of carbon dioxide [3]. Measuring and controlling carbon dioxide are important as well in natural gas purification [4] and in several experimental research setups [5]. Several methods have been described in literature for the determination of carbon dioxide based on potentiometric [6–11], conductometric [12], amperometric [13,14], coulometric [15], spectrometric [16–21], impedance [22], and capacitance [23] techniques. However, most of these methods targeted the dissolved CO2 in solutions [12,18,23]. In addition, there are some carbon dioxide gas sensors and analyzers commercially available in the market for direct measurements in gas streams. The method of infrared technology is most commonly used in the commercial analyzers. Systems using the infrared technology offer the advantage of
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standalone operation, i.e., do not require consumption of reagents. However, they are usually large and available at relatively high cost and suffer from some limitations that may affect the reliability of the carbon dioxide measurements. Furthermore, controlling the humidity of the sample gas is important for the accuracy of the infrared measurements. Alternatively, continuous gas monitoring in a given gas streams can be achieved by employing a prior step in which the analyte gas is selectively stripped in an appropriate carrier solution either by direct gas–liquid mixing [24] or gas–liquid membrane contactor (diffusion scrubber) [12]. The concomitant changes in the solution as a result of gas absorption can be measured in a subsequent step to produce the analytical signal. An interesting feature of diffusion scrubbers is that there is no interaction between the particulate matter present like those encountered in conventional filtration or sorbent sampling [25]. The aim of the present work was to develop analyzer for continuous CO2 determination in gas streams based on diffusion scrubber gas sampling and flow-through pH detector. Several advantages of such configuration can be recognized such as low cost, simple construction and operation, versatile detection schemes, tunable sensitivity. Although shares some similarities, the presented analyzer setup overcomes the limitations associated with the common Severinghaus gas sensor configuration [6] including the long recovery times and the incompatibility with continuous monitoring in dry gas streams. Moreover, Severinghaus gas sensing configuration
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mostly involves reversible physical dissolution of the analyte gas in the internal receiving solution and hence a number of possible selective detection schemes based on irreversible gas reaction/absorption are not possible with Severinghaus configuration with such fixed internal solution. The suggested continuous receiving solution provides the necessary flexibility in developing analyzers for other important gases [26]. The intrinsic physical and chemical properties of CO2 such as its appreciable solubility in aqueous solutions and the acidity of the produced carbonic acid suggested using the glass electrode as a convenient and adequately selective detector. Optimization, characterization and application of the presented CO2 analyzer are described. 2. Experimental 2.1. Materials and reagents Carbon dioxide (99.99%), nitrogen (99.99%), methane (99.99%), sulfur dioxide standard (1.0% in N2 ) and hydrogen sulfide (2.0% in N2 ) were received from Air products (UAE). Microporous polypropylene (PP) loose fibers (OD 300 m and ID 220 m), and polypropylene Mini-module (Model G543, Lumen side volume 16 mL, 2300 fibers per module) were received from Membrana (USA). Mini-modules based on Silicone Rubber (SR) hollow fibers (Model M-300, 300 fibers per module) were received from Nagayanagi Co. (Japan). All chemicals used were of the highest available purity. All solutions were prepared using deionized water. 2.2. Instrumentation A 4-channel computer controlled gas mixer, Model MFC-4, Sable Systems (USA) was used to control four Mass flow controllers, Sierra Instruments, Inc. (USA) to prepare variable concentrations of CO2 (and/or H2 S and SO2 ) in a balance of N2 or CH4 for calibration and characterization purposes. The MFC-4 utility software (Sable Systems) was used to run a given preset program of concentration steps. The potentiometric pH detector was based on a commercial acrylic flow cell, Sensorex, model FC47C (USA), with 50 L internal volume and equipped with a flat-bottom combination glass-reference electrode (Sensorex, model S450C). A custom made high-input impedance (1015 ) differential amplifier was used for pH measurements. The output of the amplifier was measured using a 16 bit-analog to digital converter (ADC) interface, Pico Technology, model ADC 16 (UK) connected to a PC installed with PicoLog software (Pico Tech.) for data display and storage. The obtained potential resolution was 0.01 mV. All measurements were carried out in an air-condition lab at 23 ± 1 ◦ C. 2.3. Construction of the CO2 analyzer The proposed CO2 analyzer in this work was designed for continuous monitoring of CO2 in gas streams. The analyzer consisted of two main components, i.e., the gas sampling unit and the detector as shown in Fig. 1. The gas sampling unit was based on a diffusion scrubber in the form of HFM module. The HFM modules were either the commercially available or custom fabricated. The latter were fabricated by potting a given number of loose PP fibers (30 cm in length) using epoxy at both ends in transparent acrylic tube (10 mm ID) to provide an active fibers length of 20 cm. Stainless steel tubes (3 mm OD, 3 cm in length) were used as inlet and outlet for both gas stream and the liquid carrier. The details of the custom fabricated HFM modules were shown in the insert in Fig. 1. The commercially available SR module was equipped with
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300 fibers and the active length of the fibers was 6.5 cm. The commercial polypropylene Mini-module (Model G543) was modified to reduce the number of the active fibers and hence the lumen volume (from 16 mL to about 6 mL). The 1/4” plastic female NPT thread cavity of the lumen inlet and outlet were cut approximately 3 mm away from the fibers ends to expose the cross-section of the circular bundle of the PP fibers (2300 fibers) at both ends. Then Araldite epoxy was used to block the central fibers at the inlet side in such away the active fibers were only those located away from the module axis as shown in Fig. 1s (supporting material). An appropriately selected carrier solution was pumped through the tube side (lumen) of the module using a peristaltic pump (Masterflex, Model 7519-20) and the gas stream was allowed to flow in a counter flow configuration through the shell side. 3. Results and discussion 3.1. Principle of operation of the proposed gas analyzer The overall performance of the present gas analyzer was determined by several individual factors including: (i) HFM type; (ii) composition of the carrier solution; (iii) flow rate of the carrier solution; (iv) module size; and (v) response properties of the pH electrode and the flow cell geometry. The membrane type should be hydrophobic and reasonably permeable to the analyte gas, CO2 in the present work. The liquid carrier should be able to capture, to some extent, the analyte gas from the gas stream and the gas–liquid carrier interaction should selectively produce some changes in at least one of the conveniently measurable properties of the liquid carrier. For CO2 , significant pH changes in the carrier solution can be produced given that the buffer capacity of the carrier was kept at sufficiently low value. The use of potentiometric pH detectors to construct Severinghaus-type electrodes for gaseous species such as CO2 [6], NH3 [27] NOx [28], hydrazoic acid [29] has been well documented in literature. However, several advantages offered by the proposed CO2 analyzer compared to Severinghaus-type configuration can be rationalized. These include: (i) the response time can be controlled by adjusting the receiving solution flow rate and/or the total internal volume; (ii) the proposed analyzer was more suitable than Severinghaus gas probes for continuous monitoring of the analyte gas in gas streams; (iii) the gas sampling step (which takes place in the HFM module) was spatially separated from the detection step which takes place downstream. Such remote placement of the detector could be of exceptional importance if the gas stream conditions (e.g., high temperature) do not suit the Severinghaus-type gas probes based on glass electrode; and (iv) schemes based on physical dissolution of the analyte in the receiving solution as well as chemical reaction can be utilized. Whereas Severinghaus-type sensors were limited to the physical dissolution to keep the properties of the internal solution and hence the sensor response fixed to a reasonable extent. The electromotive force (EMF) of the pH electrode cell can be shown to relate linearly to the logarithmic CO2 concentration [29] as shown in Eq. (1), given that the bicarbonate ion exit in the carrier solution at relatively high concentration: EMF = constant + S log[CO2 ]
(1)
where S is the slope of the calibration graph. 3.2. Optimization of the experimental parameters for CO2 analyzer The experimental variables of significance to the described setup were (i) nature of the hollow fiber membranes; (ii) buffer capacity of the carrier solution; and finally (iii) the ratio (R) between
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Fig. 1. The experimental setup and construction of CO2 gas analyzer based on pH flow-through detector and HFM module.
the total internal volume of the system and the carrier flow rate. The ratio R is directly proportional to the carrier residence time within the module and hence the extent of the gas preconcentration in the carrier solution. Hollow fibers fabricated from different polymeric materials were commercially available in different dimensions (i.e., ID, OD and wall thickness). The fibers were either microporous (e.g., polypropylene) or non-porous (e.g., silicone rubber). Gas permeation in the latter type was based on dissolution–evaporation mechanism [30]. Microporous fibers offer the advantage of higher gas flux but the non-porous fibers could provide some selective permeation for different gases. The interfacial membrane area and hence the efficiency of gas absorption and detection sensitivity should be enhanced with smaller fiber dimensions. Hydrophobic and microporous polypropylene (PP) fibers and SR were selected in the present work due to their high CO2 permeability, a requirement necessary for high sensitivity. Buffer capacity of the carrier solution should be a trade off between baseline stability (enhanced with increased buffer capacity) and signal sensitivity which should be enhanced at lower buffer capacity. Strongly buffered carrier solution would suppress the pH changes upon CO2 absorption. The resultant sensitivity and linearity of the analyzer could be a direct combination of the CO2 gas permeability in the HFM module, carrier buffer capacity and the ratio (R) which determines the extent of the gas preconcentration and, in turn, the observed magnitude of the pH change. The ratio (R) also participated to the overall response and recovery times of the analyzer. Therefore, the selected modules were among the smallest commercially available modules and the custom made ones were constructed from even smaller number of fibers (140–250 fibers) and the dead volume in the lumen side, through which the carrier solution flows, was kept to the minimum. Given the above consideration, the response of the analyzer equipped with SR module was initially obtained using NaHCO3 carrier solutions with different concentrations. The obtained results were presented in Fig. 2. The enhanced linearity at lower concentration was attributed to the smaller buffer capacity and hence smaller signal attenuation. A linear response with almost Nerns-
tian slope (57.5 mV per CO2 concentration decade) was obtained at 1 mM NaHCO3 carrier solution. The same trend was obtained with custom fabricated HFM modules based on microporous polypropylene fibers (250 fibers per module, data not shown). Therefore, 1 mM NaHCO3 carrier solution was used throughout the remaining work. The effect of carrier solution flow rate was studied at different values and the results obtained for 0.6, 1.5 and 3 mL min−1 were presented in Fig. 3. The observed enhanced linearity and sensitivity at lower flow rates were attributed to the increased residence time, and hence higher CO2 preconcentration in the carrier solution. A small hysteresis between the analyzer response and recovery was observed when the SR module was used (∼3.5 mV at 10% CO2 ). This
Fig. 2. Effect of NaHCO3 carrier concentration on the analyzer response to CO2 . Silicone rubber HFM module; carrier flow rate 1.5 mL min−1 ; gas flow rate 200 mL min−1 .
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Fig. 3. Effect of carrier solution flow rate on the analyzer response to CO2 concentrations. Silicone rubber HFM module; 10−3 mol L−1 NaHCO3 carrier solution.
behavior was shown in Fig. 4 as indicated by the dotted lines connecting some corresponding CO2 levels in the rising and decreasing ramps, respectively. Such relatively unfavorable recovery was significantly improved when the SR module was replaced with the custom fabricated modules based on microporous polypropylene fibers. Then, a modified commercial module (Membrana® , Model G543, constructed from the same type of PP fibers) as described above was used throughout the remaining work. The module modification was conducted to allow smaller carrier solution flow rates and to minimize the solution consumption. The response obtained with the modified PP module was shown in Fig. 5. A linear response was obtained with a near Nernstian slope (55.5 mV per CO2 concentration decade) as shown by the multi-step calibration curve (Fig. 5A) and the hysteresis was almost eliminated as indicated by the horizontal dotted lines connecting two
Fig. 4. Trace of the analyzer real-time response to CO2 showing a noticeable hysteresis between the signal readings obtained with SR HFM module. 10−3 mol L−1 NaHCO3 carrier solution; carrier flow rate 4 mL min−1 ; and gas flow rate 200 mL min−1 .
similar CO2 concentrations in both ramps as shown in Fig. 5B. The obtained sensitivity and the wide linearity (0.1–100% CO2 ) were attributed to the optimized analyzer construction and the intrinsic wide dynamic response range of the glass electrode detector. More sensitive detection than the LOD obtained under the optimized conditions (i.e., 0.05% CO2 ) could be realized if needed in any particular application by decreasing the carrier flow rate to increase the extent of CO2 preconcentration in the carrier solution. However, this will be on the expense of longer response and recovery times. Full analyzer characterization was carried out using such modified commercial polypropylene module and described in the following section.
Fig. 5. (A) Time response curves and the corresponding potentiometric calibration plots obtained with the modified PP commercial HFM module. 10−3 mol L−1 NaHCO3 carrier solution, carrier solution flow rate 6 mL min−1 ; and gas flow rate 200 ml min−1 . (B) Trace of the analyzer real-time response to two cycles of CO2 level change over a wide range indicating excellent baseline and signal stability and complete recovery.
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Fig. 6. The analyzer response to series of step changes in CO2 level between 1.0% and 10.0% CO2 in N2 . The horizontal doubly head arrow near the baseline of the first step represents the recovery time of the presented step change. The experimental conditions were similar to those mentioned in Fig. 5.
3.3. Characterization of the CO2 analyzer The repeatability of the signal response was investigated by series of step changes in CO2 level between two values as shown in Fig. 6. The time response obtained for six cycles between 1% and 10% CO2 , respectively, clearly revealed fast response time (t95% = 45 s). The analyzer response showed high accuracy for CO2 measurements, i.e., 98.2% and 99.0%, and excellent repeatability (RSD = 2.0% and 1.2%) at 1.0% and 10.0% CO2 concentration levels, respectively. Within day repeatability and between-day reproducibility of the analyzer response to CO2 were 1.8% and 3.5%, respectively. These are important favorable features in continuous monitoring of CO2 for prolonged time (several hours) which eliminate the need for frequent calibration. The ability of the analyzer to detected small changes at relatively high CO2 concentration was also assessed by series of step changes in CO2 level between 10.0% and 9.9% (i.e., 1% of the initial value). The obtained response was smoothed with adjacent averaging and presented in Fig. 7. Two well distinguished and stable signal levels with S/N > 5 were obtained indicating the ability of the developed analyzer to reliably detect small changes in the CO2 level.
The analyzer recovery times upon step changing the CO2 from 100% level to various lower values were evaluated as shown in Fig. 2s (supporting material). Similar plots for changes from 10% and 1% levels to lower values were also obtained, respectively (data not shown). Zero in the time axis marked the onset of the concentration change. The obtained data revealed fast recovery times which tend to be longer for larger step changes and for lower concentrations levels. For example, the recovery times (t95% ) upon step change from 100% to 10%, and from 10% to 1% and from 1% to 0% were 65, 100, and 120 s, respectively. The effect of gas flow rate on the analyzer response was investigated by changing the gas flow rate at a fixed CO2 concentration level. The analyzer response to 10% CO2 at substantially different gas flow rates (i.e., from 1000 to 50 mL min−1 ) showed impressive insensitivity to wide variations in the gas flow rate as shown in Fig. 3s (supporting materials). This was an important feature in gas analyzers which otherwise would require strict flow rate control or calibration at different gas flow rates. Two gases, i.e., hydrogen sulfide and sulfur dioxide were identified as potential interfering species in the determination of CO2 because they are both acidic gases and also commonly co-exist with CO2 in a number of important applications such natural gas industry and CO2 sequestration. Both gases exerted significant interference on CO2 measurements in the order of SO2 > H2 S. To address such interference problem, two simple approaches were suggested. First, for low concentration of hydrogen sulfide (ppm levels) a (20cm long, 1 cm ID) acrylic column filled with acid washed metallic copper particles proved effective to eliminate hydrogen sulfide from the gas stream without affecting the CO2 level. Second, a bubbler containing 250-mL of 0.25 M potassium permanganate in 2 M sulfuric acid proved highly efficient in eliminating completely and selectively both H2 S and SO2 by oxidizing them to elemental sulfur and sulfate ions, respectively. H2 S and SO2 at 20,000 ppm and 10,000 ppm tested level, respectively were completely removed from gas streams (200 mL min−1 ) whereas an almost 100% CO2 recovery was obtained after passing through the same bubbler. This conclusion was further confirmed by independent analysis of the treated gas streams with commercial CO2 -infrared analyzer (Model 602, California Analytical Instruments, Inc.) and H2 S-UV analyzer (Model AMO 300, Applied Analytics Inc.), respectively. Other acidic gases which are less likely to co-exist with CO2 such as NOx and acetic acid vapor are expected to exert some interfering effect if present in appreciable levels in the gas stream. In such cases, further investigation will be required to estimate the effect of any particular gas and a suitable prior treatment can be similarly suggested. Alternatively, a carbonate selective ISE can be used instead of the glass electrode in combination with appropriate stripping carrier if interference from acidic gases presented serious interference problem in any particular application [31]. 3.4. Monitoring of CO2 removal from CO2 –CH4 gas mixture using the proposed analyzer
Fig. 7. The analyzer response to series of step changes in CO2 level between 9.9% and 10.0% CO2 . The conditions were similar to those mentioned in Fig. 5.
Removal of acid gases from various contaminated gas streams using hollow fiber membrane modules in combination with various stripping solvents has been an active area of research [5,32]. There is a considerable reliance on use of commercial gas analyzers or GC to monitor contaminated gas streams (e.g., flue gas, natural gas, etc.) both before and after the treatment step. Therefore the analyzers should provide a wide linear dynamic range (although with good sensitivity) to suit these important applications. In the present work, the described CO2 analyzer was successfully applied in monitoring continuously CO2 level in the treated gas stream after passing through the absorption module (Membrana® , Model G542 based on microporos polypropylene hollow fibers) in which CO2 was preferentially removed from methane by contacting
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Fig. 8. Experimental setup for CO2 removal from CO2 –CH4 mixture using HFM module and NaOH as stripping solvent. The treated gas was continuously monitored with the proposed CO2 gas analyzer.
presented analyzer for other specific application requirements such as longer continuous monitoring and/or at higher temperatures. The basic analyzer components, i.e., the peristaltic pump head, mini HFM module and flow-through pH detector can also be integrated into a small portable device for in-line real-time CO2 monitoring. Developing analyzers for other important gases using different detection schemes and the same analyzer setup is in progress. Acknowledgements The authors would like to thank JCCP (Japan Corporation Center, Petroleum) for the financial support. They would also like to acknowledge the support of Research Affairs at the UAE University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.snb.2009.12.045. References Fig. 9. Effect of sodium hydroxide concentration on the CO2 level in the treated gas stream as monitored by the presented CO2 gas analyzer. The first two steps correspond to the analyzer response to 1.0% and 10.0% CO2 without applying NaOH in the treatment module.
with and dissolution in NaOH solution. The experimental setup was shown in Fig. 8. The effect of NaOH concentration on the CO2 level in the treated gas was shown in Fig. 9. Complete removal of CO2 from the gas stream was achieved by using a flow of 1.00 mol L−1 NaOH solution at 50 mL/min. 4. Conclusions The suggested low-cost and simple CO2 analyzer proved successful in continuous monitoring of CO2 in gas streams in laboratory setups and offered several attractive characteristics. More characterization may be required to evaluate the performance of the
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Biographies Sayed A.M. Marzouk received his PhD in Analytical Chemistry from Ain Shams University in 1997. In 2005 he is appointed as associate professor of Analytical Chemistry at the United Arab Emirates University. His research interests are in the field of chemical sensors and biosensors, gas analysis and gas separation. Mohamed H. Al-Marzouqi received his PhD in Chemical Engineering from Oregon State University in 1994. In 2006 he is appointed as the Chair of Chemical and Petroleum Engineering Department at the United Arab Emirates University. His research interests are in the field of gas separation using membrane contactors.