Ion Transport Membranes (ITM)

The air separation process (oxygen plant) is an integrated part of a gasification plant. Current commercial air separation units (ASUs) are based on cryogenic distillation technology that is energy-intensive and costly. The ASU can account for 15% of the total gasification plant capital cost. Ion Transport Membrane (ITM) technology, currently under development, uses a radically different approach to produce high-purity oxygen (O₂). It offers tremendous opportunities to improve the efficiency and cost for air separation, and thus, on the overall gasification economics.

Technology Description
ITMs are nonporous ceramic membranes that are permeable only to oxygen ions and are therefore 100% selective. At temperatures of 800-900°C (1,500-1,650°F), oxygen from the air feed adsorbs and dissociates on the membrane to form oxygen ions by electron transfer. The oxygen anions migrate through the ceramic lattice counter-currently with electrons, and are driven toward the permeate side by the oxygen partial pressure differential. Flux through the membrane can be increased by introducing a sweep gas on the permeate side to maintain a low oxygen partial pressure. To minimize the mechanical load on the membrane, the feed stream is typically pressurized to 100-300 psia, while the permeate side is kept sub-atmospheric.

The energy of the hot, pressurized, non-permeate stream is recovered by a gas turbine for power generation and an optional steam turbine for low-level heat recovery.

APCI/DOE Project
Air Products and Chemicals, Inc., (APCI) and industrial partners GE Energy, Penn State University, Eltron Research, Ceramatec, Siemens and others, are pursuing a project with an objective to develop, scale-up, and demonstrate the ITM concept for large-scale gasification applications. The project is co-sponsored by the Department of Energy (DOE). It is a phased program with Phase I focusing on the materials and processing R&D, and the design, construction and operation of an approximately 0.1 ton/day (tpd) Technology Development Unit (TDU). During Phase I, APCI successfully built a 0.5 tpd commercial-scale ITM TDU unit and demonstrated its ability to produce high-purity (>99%) O₂ at a high flux rate.

The success of Phase I led to Phases II and III, which continued the development of high-flux materials, and the design and construction of a 5 tpd Subscale Engineering Prototype (SEP) pilot plant to verify the performance of commercial-scale modules. Currently, APCI is operating a 5 tpd SEP unit at its Sparrows Point gas plant, and has demonstrated that the unit can be operated under full driving-force conditions, meet/exceed wafer performance for flux and purity, and that it can be cycled from idle to operating conditions without loss of performance. Design and construction of a 150-tpd SEP unit has begun.

Subscale Engineering Prototype (SEP) ITM Test unit at APCI's Sparrows Point gas plant.

Since ITMs are thermally activated, the basic process cycle must include heating the pressurized air feed to high operating temperatures, either by indirect heat exchange or direct firing. APCI has performed studies to integrate the ITM module into a gas turbine power cycle to produce oxygen, power and/or steam, and concluded that integration is practically feasible to achieve acceptable cycle efficiency. The energy associated with the hot, pressurized, non-permeate stream can be recovered.

Partial or minimum integration is also feasible to design a “stand-alone” ITM system with reduced power co-production. It potentially offers good early entry prospects for ITM technology.

Preliminary Economics
Air Products and Chemicals, Inc. compared the economics of ITM against a state-of-the-art cryogenic ASU and projected that ITM would decrease the installed capital cost of air separation equipment by 35%. This translates to a 7% decrease in the installed capital cost of an IGCC plant and a 1% increase in efficiency.

Future Work
APCI and DOE are planning to expand the Phase III program to include a 100 tpd SEP unit, with a 1,500 to 2,000 tpd commercial scale unit planned for Phase IV’s program, to be completed by 2015.

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