ION TRANSPORT MEMBRANE TECHNOLOGY

 

ION TRANSPORT MEMBRANE TECHNOLOGY(ITM)

Ion transport membrane (ITM) technology is a key perspective for efficient oxygen separation. At the present time, semi-industrial modules based on ceramic ITMs produce oxygen of 98.9%–99.9% purity. In order to improve the oxygen purity, using newly developed liquid-oxide ITMs along with the highest oxygen selectivity, these membranes exhibit competitive oxygen permeability and could be successfully used for ultrahigh purity oxygen separation. Oxygen is the second-largest volume industrial gas that has numerous applications in aerospace, metallurgy, power engineering, environmental, medicine, etc. Ultrahigh purity oxygen (>99.999% purity) is in demand in the solar, semiconductor, chemical, and pharmaceutical industries. Recently, renewable sources of energy (sun, wind, biomass, etc.) are rapidly gaining in popularity. Clean energy, especially photovoltaics (PV), is a field of major growth and investments.

 Oxygen is one of the ultrahigh-purity process gases needed by the PV cell manufacturing. At the present time, ultrahigh purity oxygen is produced by water electrolysis or distillation method. However, these method-based oxygen production technologies are energy-intensive. In recent decades, energy-efficient ion transport membrane (ITM) technology is developing to produce pure oxygen. Conventional ITMs are the ceramic membranes with high oxygen ion conduction at elevated temperatures. There are two ITM processes. In the first, a mixed ionic electronic conducting (MIEC) membrane operates with a difference of the oxygen partial pressures, as illustrated in Fig. 1a. Under the oxygen electrochemical potential gradient, ambipolar conductivity of ions and electrons provides a sufficient oxygen permeation flux through the MIEC membrane. The MIEC membrane-based separation process is referred to as ITM oxygen. In the second process, an ion-conducting membrane (or an electrolyte) operates with a voltage.

In contrast to the first process, electron transfer occurs in the outer circuit, as illustrated in Fig. 1b. Devices utilizing electrolytes are referred to as oxygen generators. The envisioned ITM applications vary from the generation of pure oxygen and partial oxidation of methane (membrane reactors for syngas production) to the capture of CO2 in oxy-fuel power plants. Oxygen permeation flux through MIEC membrane is limited by diffusion. Therefore, the minimization of membrane thickness is necessary to achieve a high oxygen permeation flux. However, the brittle thin-film ceramic membrane material has a very low mechanical strength. As a rule, a thin membrane film is deposited on a porous support (it is so-called asymmetric membrane). To provide the thermochemical compatibility between membrane and support, they are usually made of the same material. The surface exchange reaction rates can limit the oxygen permeation flux through a thin MIEC membrane. An appropriate catalyst is deposited on the membrane to increase the rate of surface exchange reactions. Gas transport in a porous support can also control the oxygen permeation flux through asymmetric membranes. In order to ensure the sustainable production of oxygen, numerous asymmetric membranes with high oxygen permeability have been developed. Currently, asymmetric membrane-based semi-industrial modules produce oxygen of 98.9%–99.9% purity.

Three types of liquid-oxide ITM materials have been developed: (i) mixed ionic electronic conducting (MIEC), (ii) bilayer mixed ionic electronic conducting—redox (MIEC-Redox) and (iii) ionic conducting (electrolytes). In contrast to the MIEC and ionic conducting membrane materials (Figs. 1a and 1b), the bilayer MIEC-Redox membrane materials have a combined diffusion bubbling oxygen mass transfer. The chemical diffusion of oxygen takes place in the external layer of the membrane material, while in the internal layer, the redox reactions and nucleation, growth and transport of oxygen gas bubbles occur, as illustrated in Fig. 1c. The concept of highly selective liquid-oxide ITMs opens up ample opportunities for oxygen separation technology. However, in order to realize the potential of these membranes and successfully commercialize them, many scientific and technical challenges remain to be solved.

The rare earth stabilized bismuth oxide and rare-earth doped ceria are usually used as the oxygen generator electrolytes. To achieve a sufficient ionic conductivity of these electrolytes, oxygen generators operate in the temperature range of 700 °C–800 °C. The purity of oxygen produced by the ceramic electrolyte depends on the electrolyte density. At the present time, the product grades of 98%–99.99% oxygen are available.