Portable dual-channel gas analyzer for continuous monitoring of carbon dioxide in gas streams Muna S. Bufaroosha a , Mohamed A.R.A. Alnaqbi a , Mohamed H. Al-Marzouqi b , Sayed A.M. Marzouk a, c, ⁎ a Department of Chemistry, UAE University, Al Ain, P.O. Box 17555, United Arab Emirates b Department of Chemical and Petroleum Engineering, UAE University, P.O. Box 17555, Al Ain, United Arab Emirates c Department of Chemistry, Faculty of Science, Ain Shams University, Cairo, Egypt abstract article info Article history: Received 26 February 2013 Received in revised form 15 March 2013 Accepted 16 March 2013 Available online 26 March 2013 Keywords: Carbon dioxide determination Gravity-driven flow Portable gas analyzer Dual-channel monitoring Diffusion scrubbers pH detector Development, characterization and application of a portable dual-channel analyzer based on gravity driven flow for continuous monitoring of CO 2 in gas streams are presented. The analyzer is constructed from (i) a carrier/stripping solution reservoir, (ii) a diffusion scrubber (DS) in the form of a hollow fiber membrane (HFM) module integrated with a custom-designed flow-through pH-detector cell, and (iii) a waste solution reservoir. The analyzer is designed in such a way that the dilute sodium bicarbonate carrier/stripping solution flows downward by gravity into the lumen side of the hollow fibers in the HFM module which allows efficient contact with the gas stream. Absorption of CO 2 into the carrier solution produces pH changes that constituted the analytical signal. Flat-bottom glass electrode and solid-state IrO 2 electrodes were used as pH detectors in two types of custom-built flow cells, respectively. Under the optimized conditions, the measured pH signal showed Nernstian response to the CO 2 concentration (59.6 and 62.0 mV/log[CO 2 ] and r 2 = 0.9992 and 0.9984 for the glass and IrO 2 pH detectors, respectively) in the gas stream over a wide dynamic range (0.1-100 % CO 2 in balance of nitrogen). Moreover, the presented analyzer based on either detector offered several favorable performance characteristics such as reasonably short response and recovery times; excel- lent signal stability and reproducibility and high selectivity in the presence of non-ionogenic gases, e.g., CH 4 ,N 2 ,O 2 , CO, etc. The suggested analyzer was applied successfully in the dual-channel monitoring of CO 2 absorption from CO 2 -CH 4 binary feed gas mixtures. © 2013 Elsevier B.V. All rights reserved. 1. Introduction Due to the essential role of carbon dioxide in our life, in several in- dustries and the greenhouse effect as well as its importance in several experimental setups [1–6], development of analytical devices for the observation of carbon dioxide has been an active area of research. The operating principles of carbon dioxide sensors and their manifold applications in environmental control, biology, biotechnology, medi- cine and food production and control were reviewed recently [7]. Among various analytical methods, portable analyzers offer the advantage of convenient mobility. However, there are several re- quirements that should be considered in developing portable ana- lyzers. These include: ease-of-use, light weight (≤10 kg) [8], low power consumption, rugged, no sample preparation, and real time monitoring capability. Several types of gas analyzers have been commercialized for monitoring of important gases such as CO 2 , CO, NO x ,H 2 S, SO 2 ,O 2 ,O 3 , THC and VOCs. Commercial gas analyzers based on different detection principles, e.g., non-dispersive infra- red, Fourier-transform infrared, Chemiluminescence, UV absorption, UV fluorescence, electrochemical detection, flame ionization, para- magnetic cell, and mass spectrometry have been used successfully in several applications [9–19]. Some of the commercial analyzers are also available as portable devices. Although such commercial and portable gas analyzers pro- vide several attractive features such as high sensitivity, standalone operations, i.e., they do not require consumable reagents for their op- erations. The portable gas analyzers are still of considerable cost and sometimes of limited linear range. In addition to the commercialized gas analyzers, considerable efforts have been reported in literature to describe portable gas analyzers based on several detection schemes. Gas analyzers based on colorimet- ric detection were developed for the determination of several gases such as acetylene [20], ammonia [21,22], nitrogen dioxide [23], and carbon dioxide [24]. Portable analyzers for H 2 S [25] and NO [26] employed fluorimetric detection were reported. Electrochemical detec- tion methods [27–29] were also commonly used in developing ana- lyzers for several important gases such as hydrogen fluoride [30], hydrogen chloride [31], CO 2 [32–35], SO 2 [36,37] and volatile alcohols [38]. Other detection principles such as thermometric [39], gravimetric Microchemical Journal 110 (2013) 185–191 ⁎ Corresponding author at: Department of Chemistry, UAE University, Al Ain, P.O. Box 17555, United Arab Emirates. Tel.: +971 3 7136149; fax: +971 3 7671291. E-mail address: sayedm@uaeu.ac.ae (S.A.M. Marzouk). 0026-265X/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.microc.2013.03.013 Contents lists available at SciVerse ScienceDirect Microchemical Journal journal homepage: www.elsevier.com/locate/microc