Development of a new gas sensor for binary mixtures based on the permselectivity of polymeric membranes. Application to oxygen/nitrogen mixture

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Abstract

A new oxygen/nitrogen gas concentration sensor is described in this paper. This new sensor is based on the permselectivity of a membrane element. It is especially suited for the very low price market to determine the concentration of binary or pseudo-binary gas mixtures in the 0–100% range.

The sensor is made of a permselective membrane module whose permeate stream is linked to a needle valve. The feed composition is related to the permeate build-up pressure, measured using a pressure transducer. Poly(dimethylsiloxane) (PDMS) and poly(etherimide) (PEI) hollow fiber membranes were tested. The response curves for both membranes at different temperatures display a quasi-linear behavior. The PDMS membrane-based sensor enables continuous and rapid oxygen analysis (t0−95% = 50 s) with a reproducible and long-term stable signal (k¯(O2) = 2.474 × 10−6 dm3 (STP) s−1 m−2 Pa−1, s = 0.045 × 10−6, N = 10; k¯(N2) = 1.173 × 10−6 dm3 (STP) s−1 m−2 Pa−1, s = 0.010 × 10−6, N = 11) over 5.4 × 106 s. The absolute sensitivity of the PDMS-based sensor depends on the oxygen feed concentration ranging from 0.0329 to 0.450 MPa.

A simple analytical model was developed and is presented here. Good agreement was obtained with the experimental results for both membranes.

Introduction

Membrane-based processes are currently used in a wide range of different industrial applications, such as desalination of seawater, hemodialysis, the treatment of industrial effluents (sometimes with the recovery of components of economic interest), in food technology, etc. [1]. In the field of chemical sensor applications, membranes have attracted increasing attention. Polymeric membranes have been widely used in different types of sensors, such as optical [2], [3], [4], [5], [6], [7] and electrochemical [8], [9], [10], [11], [12], [13], [14], in order to increase their selectivity. Also, zeolite membranes have been proposed to increase the selectivity of semiconductor and optical chemical sensors [15], [16], [17], [18]. Kaneyasu et al. [19] reported the use of zeolite as a filter material in order to minimize the effect of interfering gases on the electromotive force of a carbon dioxide gas sensor.

This paper describes a new oxygen/nitrogen gas sensor based on the permselectivity of polymeric membranes for binary gas mixtures. The sensor is made of a permselective membrane, a pressure transducer for measuring the permeate pressure and a non-selective barrier (e.g. needle valve) [20]. The non-selective barrier is used to control the permeate outlet to the atmosphere. This new sensor is based on the fact that different gases have different permeabilities on a selective membrane. When the feed pressure is kept constant, the permeate flow is proportional to the gas mixture concentration. However, gas flow meters are expensive and using this principle to determine concentration is not a promising solution. However, if a non-selective barrier, such as a needle valve, is placed on the permeate outlet, the permeate pressure is then proportional to the permeate flow rate and, therefore, to the feed concentration. The permeate pressure can be measured with a cheap pressure transducer. Based on a similar principle, it has been reported that the concentration or partial pressure of gases, especially oxygen, in fluids could be measured with a plastic material that is permeable to only specific gases and is in contact with the fluid [21]. Also, devices to sense dissolved carbon dioxide in beverages using a membrane to interface with the liquid phase have been developed [22]. The molecular mass distribution of a polymer can be inferred from the observed osmotic pressure across a membrane of a solution of this polymer [23].

In the development of an oxygen/nitrogen sensor, it is critical to consider the potential applications, since the design can be adapted/optimized to the purpose. There is a need for a very cheap and reliable oxygen/nitrogen sensor of low/medium precision (±2.5%) for medical oxygen concentrators. These concentrators separate oxygen from air by means of an adsorption process known as pressure swing adsorption (PSA). The sensor described here is especially applicable to these oxygen concentrators where the world market is around 400,000 units/year [24]. The oxygen concentration delivered by these units should never be below 85%, thus a sensor with an incorporated alarm could be provided. It is possible to directly adapt the new sensor to these oxygen concentrators because the product stream pressure is maintained relatively constant at about 0.25 MPa, absolute pressure. To the best of our knowledge, the only oxygen sensor that can share the same market as the permselective sensor is one based on the ultrasound speed dependence on a binary gas mixture composition and marketed under the name DigiFLO [25]. The newly proposed oxygen/nitrogen sensor could be further developed into a portable device with a satisfactory sensitivity.

In addition to the application for oxygen monitoring of the outlet stream of medical oxygen concentrators, the new sensor can also be used for determining the composition of other bi-component mixtures in the 0–100% range, such as carbon dioxide/methane, carbon dioxide/helium, hydrogen/nitrogen and hydrogen/methane.

This paper describes the new sensor and reports all tests performed. A simple mathematical model describing its behavior is also reported. Two different polymeric membranes are used: poly(dimethylsiloxane) (PDMS) and poly(etherimide) (PEI) hollow fibers. Results are reported for oxygen/nitrogen gas mixtures. For both membranes, response curves were obtained at three different temperatures. The reproducibility, sensitivity, response time and reversibility, time stability and temperature dependence of the sensor for these two polymers are discussed.

Section snippets

Mathematical model

The sketch of the binary gas mixture sensor is shown in Fig. 1. The gas mixture to be analyzed is supplied at a constant pressure that must be higher than the outlet permeate pressure. In our system, the feed pressure (PF) was set to 0.3 MPa, while the permeate pressure after the needle valve (V) was the ambient pressure (Pamb).

A simple mathematical model of this sensor was developed based on the following main assumptions: isothermal operation, ideal gas behavior, complete mixing flow pattern

Experimental

The sketch of the experimental set-up developed for testing the oxygen/nitrogen sensor is shown in Fig. 2.

The oxygen and nitrogen gases used were from Air Liquide, 99.995% purity. Different oxygen/nitrogen compositions in the range 0–100% (v/v) were produced by controlling the flow rates of oxygen and nitrogen fed to the membrane module using needle valves (V1 and V2). The feed line is made of a 3.9 ID/mm poly(urethane) tube (Festo, PU-4). The feed gas mixture was introduced at the desired flow

Results and discussion

The main objective of this work is to study the performance of the membrane-based sensors. Thus, the following items were evaluated for the oxygen/nitrogen gas sensor: response curves, reproducibility, response time and reversibility and long-term stability. The influence of operating conditions, such as temperature, was also analyzed.

Membranes were also characterized for pure and mixture mass transfer coefficients.

The experimental results presented below were obtained using the PDMS membrane

Conclusions and further research

A new oxygen/nitrogen gas sensor is described, tested and modeled. The most important application devised for this sensor is for monitoring the oxygen content in the outlet stream generated by medical oxygen concentration units. Other applications are possible. This sensor is characterized by simple instrumentation with no sample pre-treatment and a short response time, and can offer new possibilities for conventional oxygen/nitrogen concentration sensors.

The concentration sensor is suitable

Acknowledgements

This work has been supported by Agência de Inovação, s.a. (project P0046/ICPME/S - Gassense). The authors thank Prof. Fernão Magalhães (FEUP) for the careful revision of this manuscript. Rosa Rego thanks Prof. Luís Carvalho (UTAD) for helpful discussions.

References (28)

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