Advances in ion transport membrane technology for oxygen and syngas production
Introduction
Air Products and its partner Ceramatec are developing ion transport membranes (ITM) for oxygen and syngas production. ITMs are ceramic, mixed conducting membranes that conduct oxygen ions at elevated temperatures. They have significant potential in the industrial gas and energy industries. The focus of the development program at Air Products and Ceramatec is ITM membranes for the separation of a pure oxygen product from air, ITM Oxygen, and ITM membranes for the conversion of methane to synthesis gas, a mixture of carbon monoxide and hydrogen, ITM Syngas. Many different organizations have been involved in their own developments of similar membranes and those developments have been reviewed by other authors [1], [2], [3], [4], [5]. The purpose of this paper is to review the state of the art of the technology being jointly developed by Air Products and Ceramatec, in particular the scale-up of the technology to large scale process demonstration facilities.
Air Products and Ceramatec have been developing ITM membranes together since the mid 1990's [6]. Significant progress has been made since then. The ITM team currently holds over 90 US patents in the ITM field, with equivalent patents issued in jurisdictions around the world. The patents cover membrane materials [7], [8], [9], [10], membrane designs [11], [12], [13], [14], [15], ceramic-to-metal seals [16], [17], [18], [19], and processes using ITM membranes [20], [21], [22], [23], [24], [25], [26], [27]. Progress in both technologies has been aided by cooperative agreements with the US Department of Energy and alliances with industrial partners.
The planar membrane configuration offers a high degree of flexibility in designing membrane modules for optimum flux performance and reliability. The modular membrane systems developed by Air Products and Ceramatec provide a high packing density of membrane for a given volume of reaction vessel and a low ratio of ceramic-to-metal seals to membrane surface area. It is well known that planar compact heat exchangers can achieve higher convective heat transfer coefficients than shell and tube heat exchangers since the heat transfer coefficients are inversely proportional to the hydraulic diameter. Planar compact heat changers can have smaller hydraulic diameters than tubular heat exchangers [28]. Analogously, planar membranes should achieve improved convective heat and mass transfer coefficients relative to tubular membranes resulting from the planar membranes' smaller hydraulic diameters due to the internal micro-channels and close external membrane spacing. Furthermore, the microchannel architecture reduces stresses due to external pressure in the membrane layers of components and, hence, improves reliability.
ITM Oxygen membranes operate under an imposed gradient of oxygen partial pressure to produce a pure oxygen product. Hot, high-pressure air is supplied to one side of a membrane and a pure oxygen product is recovered from the permeate side of the membrane. Favorable economics can be obtained when ITM Oxygen is integrated with other high-temperature processes such as Integrated Gasification Combined Cycle [29], [30]. Recovery of the energy contained in the hot, high-pressure vitiated air stream after oxygen has been extracted by the ITM membranes permits the co-production of power and steam [31]. This has been one of the drivers for continued development of ITM Oxygen technology by various research groups around the world [32].
The driving force for oxygen flux through ITM Syngas membranes is produced by supplying heated air and a hydrocarbon such as methane to opposite sides of the membrane [33]. Oxygen permeating the membrane oxidizes the methane to produce synthesis gas (syngas), a valuable feedstock for many industrial processes, including the production of ultraclean liquid fuels, hydrogen, and other chemicals. Previously published studies indicated that ITM Syngas could improve the economics of syngas production for gas-to-liquids production [34], [35]. Research on ITM Syngas disk membrane designs has been reported previously [36]. The disks described in that study could not support a significant pressure differential across the disk. Also, the reactor systems in which the disks were evaluated were not rated for pressurized operation. To evaluate the performance of new membrane configurations and compositions at elevated pressure, both the reactor design and the disk membrane module had to be modified. The benefits of introducing porous layers and components were described in the previous study, for a disk membrane system operated at near-atmospheric pressure on both sides of the membrane, and whose porous layer composition is identical to that of the dense membrane. This previous work has been extended in two directions: demonstration of operation with an elevated pressure on the reducing side of the membrane, and further flux enhancement by the introduction of catalytic components into the porous layers.
Section snippets
ITM membrane fabrication
Membrane components for ITM Oxygen and ITM Syngas applications are manufactured by similar methods. As shown in the flowchart presented in Fig. 1, the first step of manufacturing is to synthesize powders with the desired ITM material composition using solid state synthesis methods. Subsequently, powder is milled to the appropriate particle size for forming and sintering. Milled powder is combined with solvents, dispersants, binders, and plasticizers to make a suspension with a well-defined
ITM Oxygen process demonstration
From the earliest development of ITM Oxygen technology, the vehicles used to demonstrate the concept and implementation have undergone continuous scale-up. Experimentation continues at all scales, in support of the various development goals, but the focus in recent years has shifted to installations to demonstrate multi-wafer membrane stacks, a step up from apparatuses to test small disk membranes and single-wafer membrane modules. Membrane stacks containing up to 12 wafers can be tested in a
ITM Oxygen
Several recent milestone achievements have furthered the development of commercial ITM Oxygen technology. In the TDU, a 12-wafer membrane stack was operated for over 15,000 hours at full process conditions. During this time, it underwent several planned and unplanned temperature and pressure cycles, as well as variations in pressure and temperature to evaluate membrane performance at a range of conditions. Performance versus time at a single temperature and pressure condition is shown in Fig. 9.
Conclusions
Oxygen production using ceramic, mixed conducting ion transport membranes has now been demonstrated at a significant scale, producing tonnage quantities of oxygen for the first time. Very long term testing has shown that the ITM membranes can operate for the times required for commercial applications while maintaining high purity oxygen production. The advantages of using enhanced surface area and catalysts separately or in combination on the oxygen flux for oxidation reactions have also been
Acknowledgments
This technology development has been supported in part by the U.S. Department of Energy under Contract No. FC26-98FT40343. The U.S. Government reserves for itself and others acting on its behalf a royalty-free, nonexclusive, irrevocable, worldwide license for Governmental purposes to publish, distribute, translate, duplicate, exhibit and perform this copyrighted paper. Neither Air Products and Chemicals, Inc. nor any of its contractors or subcontractors nor the United States Department of
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