Membrane dehumidification process using defect-free hollow fiber membrane

https://doi.org/10.1016/j.ijhydene.2017.08.018Get rights and content

Highlights

  • Defect-free hollow fiber membranes were prepared for H2/water vapor separation.

  • Water vapor removal efficiency increased from 54% to 90% (at 120 °C).

  • The process with combined H2 separation consumes less energy.

Abstract

Water vapor removal by the polymeric membrane to reduce the energy cost during the water–gas shift reaction in a catalytic membrane reactor was investigated. In this study, polyamideimide (PAI) defect-free hollow fiber membranes were produced by a dry/wet phase inversion method. The purpose of this study was to investigate the water vapor removal efficiency under high pressure and high temperature. The morphologies of the hollow fiber membranes were characterized by SEM. The water vapor and hydrogen mixed gas separation properties were used to verify the performance of a defect-free membrane. The water vapor removal efficiency increased from 54% to 90% (at 120 °C) as a function of the operating conditions because of the enhanced water vapor flux. However, the H2 retention ratio was negatively related to the water removal efficiency.

Introduction

The water–gas shift (WGS) reaction (CO + H2O ↔ CO2 + H2, ΔH = −41.1 kJ/mol) is an important process for an integrated gasification cycle with pre-combustion CO2 capture and storage (CCS), as well as for hydrogen generation [1]. The utilization of membranes for dehumidification processes has huge applications in a broad assortment of industries. Recently, a membrane reactor (MR) was designed by Jokar et al., to produce hydrogen from waste gas (flare gas + flue gas) of a domestic gas refinery [2]. Palladium (Pd)-based membrane reactors have long been identified as a hydrogen separation membranes for generating CO2-rich streams and hydrogen [3]. In particular, Pd and Pd alloy membranes with dense/porous hybrid structure exhibit high permeability and H2 selectivity, and their practical utilization at high temperatures (300–500 °C) and pressures (20–60 bar) has been demonstrated [4], [5], [6]. Also, Pd and Pd alloy membranes have good durability and stability in high temperature and pressure. However, the water vapor interrupted the H2/CO2 separation. Therefore H2/water vapor separation defect-free membrane can be used for pretreatment system of Pd composite membrane.

Many researchers used various types of membrane to developed dehumidification process such as moisture dehumidification and its application to a 3 kW proton exchange membrane, dehumidification process for desalination brackish water [7], [8] along with desalination system based on air humidification-dehumidification is investigated theoretically and experimentally [9]. In the water gas shift reaction and sorption enhanced water gas shift reaction numerous efforts have been made to simplify the conventional water gas shift process. However, researchers are trying to combine the reaction and separation step in one unit with the recent expansion of membrane reactors.

A defect-free membrane with an ultra-thin, dense, and selective layer provides intrinsic selectivity with a low resistance to gas permeance, whereas the porous substrate, which has no significant macrovoids, provides mechanical strength to the membrane for use in high-pressure applications [10]. In addition, post-treatment such as a silicone rubber coating can be eliminated, and hence the cost and time associated with manufacturing the membrane can be reduced [11]. A defect-free hollow fiber membrane from PAI withstood extremely high pressures up to 50–100 bar, and its glass transition temperature was 273 °C [12], [13]. The membrane process has some advantages over the previous methods such as low power consumption, low cost, simplicity of operation, compactness, and ease of handling. It does not need additives for separation, and it easily combines with other processes [14], [15]. Therefore, a defect-free membrane dehumidification process could reduce the energy consumption for the operating conditions as compared to the condenser.

Water vapor separation membranes have been widely researched in industry for the drying of natural gas and compressed air and for use as protective apparel, packing material, roofing covers, and humidity control in confined spaces. Air conditioning in buildings, aviation and space flight, and steam recovery are in this category [16]. Sijibesma et al. [17] studied flue gas dehydration using a PEBAX® 1074 polymer membrane. They demonstrated the technical viability through a pilot-scale polymer membrane process. The consequent research works in the gas humidification process using proton exchange membranes are mainly focused on optimization of the humidification parameters, together with temperature, relative humidity (RH), and gas flow rate in the inlet [18], [19], while no literature concerning the effect of the defect-free hollow fiber membrane on the performance of water vapor removal efficiency under high pressure and high temperature has been reported. Recent studies revealed that PAI is a promising polymer for gas separation because of its excellent intrinsic properties for O2/N2 and CO2/CH4 separation [20]. In addition, dehydration using hydrophilic polymers such as poly(vinyl alcohol) and polycarbonate membranes has been widely researched to recover organic species from gas streams [21], [22]. Recently, Ingole et al. [23], [24] studied the interfacial polymerization (IP) coating technique for separating water vapor from a gas mixture [25]. The advantage of IP is the formation of thin and hydrophilic selective layers. However, both the mechanical and chemical coating methods are difficult to apply in a high-temperature, high-pressure membrane process such as membrane reactor.

In this study, PAI hollow fiber membranes with a defect-free, ultra-thin, and dense selective layer were fabricated for pre-treatment in a membrane reactor (MR) process. PAI hollow fiber membranes are interesting materials for removing water vapor from gas streams because they possess a high permeance for water vapor. Water and inert gas are transported simultaneously through highly permeable polymers. Especially, PAI defect-free membranes suppress the plasticization effect because of their ability to form intra- and inter-chain hydrogen bonds [26]. The manufactured PAI defect-free hollow fiber membrane was characterized by SEM and pure gas permeation experiments. After that, water vapor and hydrogen mixed gas separation experiments were performed as a function of various operating conditions such as effect of the temperature, and operating pressure on the performance of PAI defect-free hollow fiber membrane is systematically investigated. The block diagram of WGS membrane reactor is shown in Fig. 1.

Section snippets

Dope formulation and spinning process

Defect-free hollow fiber membranes were produced by a dry/wet phase inversion method [27], [28]. Hollow fiber membranes were spun using the setup schematically illustrated in Fig. 2. PAI (Torlon® T-LV, Solvay, USA) was dried at 80 °C over 3 days to remove moisture. The dried PAI was dissolved in a N-methylpyrrolidone (NMP, Merck, Germany) and tetrahydrofuran (THF, Sigma-Aldrich, USA) mixture at 70 °C under continuous mechanical stirring for 2 days at 160 rpm to obtain a homogeneous solution. To

Characteristics of defect-free membrane

In this study, the dry/wet phase inversion method was used to fabricate hollow fiber membranes. After the dope solution passed through the spinneret at the set air gap, the volatile and non-volatile solvents evaporated [31]. The polymer thus became highly concentrated near the outer surface and a dense membrane top layer formed. After passing through the air gap, the nascent fibers entered a coagulation bath where phase inversion occurred and the membrane structure was arrested, resulting in

Conclusion

In this research study, a PAI defect-free hollow fiber membrane was fabricated by the dry/wet phase inversion method. The morphologies of the membranes were characterized by SEM. A dense selective layer was formed on the outside of the hollow fiber membrane. The mixed gas separation experiment was conducted as a function of the operating pressure and temperature. The water vapor removal efficiency increased with increasing operating pressure and temperature because of the enhanced solubility

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