Ceramic membranes membrane elements

In the second half of the 1980s, an increasing number of companies entered the field of inorganic membranes, the most significant ones being ceramic companies such as NGK of Japan which also developed a multichannel membrane element (19 channels, 3 mm diameter), Nippon Cement and Toto also from Japan and very recently Coming who also developed a multichannel membrane structure. 

The porosity at both ends of a tubular or monolithic honeycomb membrane element can be a potential source of leakage. These extremities need to be made impervious to both permeates and retentates so that the two streams do not remix. Typically the end surfaces and the outer surfaces near the ends of a commercial membrane element are coated with some impervious enamel or ceramic materials. 

A membrane system consists of many membrane modules which, in turn, are made of several membrane elements. Both ends of a membrane element are sealed with such materials as enamels or ceramic materials. The connections between elements and between elements and the housing or pipings are typically made from plastics or elastomers for liquid phase applications. 

These ceramic membranes are relatively easy to operate for filtering particulates. The pressure drop across the thickness of a membrane element and the gas flow rate follows a linear or nearly linear relationship. It has been found, however, that not all inorganic membranes are suitable for clean room air filtration. Glass membranes, for example, suffer from the problem of particle shedding under mechanical shock conditions [Jensen and Goldsmith, 1987]. Sometimes high moisture content in the filtered air can be a problem. Some chemical treatments to ceramic membranes prior to their utilization as... 

A recent development of a ceramic membrane appears to be promising for selectively removing oxygen from air. Multi-channel membrane elements have been fabricated for that purpose [Anonymous, 1995]. The membrane has the potential for reducing the cost of converting natural gas to synthesis gas. 

Other ceramic sealing materials containing calcined colloidal particles are being pursued by the membrane manufacturers as the issue of sealing extremities of a membrane element continues to be one of the important materials selection and engineering challenges for inorganic membranes. 

A simpler (one-step) and more economical joining process is direct brazing in a furnace under a vacuum or inert gas atmosphere through the use of active filler metals [Mizuhara Cl al., 1989]. An active element such as the commonly used Ti in the filler metal forms a true alloy with the base metal. The difference in the thermal expansion coefficients between the ceramic membrane and the metal housing can lead to high stress at the... 

Figure 11.2 Schematic diagram of a honeycomb ceramic membrane element with rectangular channels [Dolccek, 1995]...

Figure 12.1 General trend of ceramic membrane element or system cost as a function of the volumetric permeation rate [Lahiere and Goodboy, 1993]...

Many methods have been proposed to address this issue (see Chapter 9). Beside thermal and chemical resistances of the sealing materials other issues need to be considered as well. One such important issue is the mismatch of the thermal expansion coefficients between the membrane element and the sealing material or joining material. While similar material design and engineering problems exist in ceramic, metal and ceramic-metal joining developmental work in this area is much needed to scale up gas separation units ot membrane reactors for production. The efforts are primarily p ormed by the industry and some national laboratories. 

Although some ceramic membrane elements are proposed with a flat geometry, most of them exhibit a cylindrical shape for a multichannel element (Figure 6.1). The reason for that is the much better mechanical properties obtained for cylindrical-shaped ceramics and the easier sealing of the elements compared to flat shapes. 

Since their early development, the geometrical and structural characteristics of ceramic membrane elements have readily changed (Figure 6.2). Originally, they were prepared as single tubes with an inside diameter ranging from 6 to 15 mm and a wall thickness of about 2 mm. These ceramic tubes are still available from some suppliers, but the main handicaps with such... 

FIGURE 6.1 Schematic representation of the multichannel structure of a ceramic membrane element. 

Originally, multichannel ceramic membranes have been produced at the industrial scale by SCT-Exekia and Orehs in France with commercial elements registered, respectively, as Membralox and Kerasep. The membrane filtration area in this case can reach 0.35 m /element depending on channel diameter and the number of channels per element. FILTANIUM elements, representative of flower-like geometries, have been produced more recently by TAMI industries in France (Figure 6.3). These elements with a cross-section diameter of either 10 or 25 mm exhibit a number of channels that vary from 3 to 39 and a membrane filtration area of 0.5 m for the largest elements. The increase in membrane surface compared to equivalent cylindrical-shaped channels can be estimated at 30%. 

FIGURE 6.2 Evolution of the geometiy of ceramic membrane elements, (a) Conventional cyhndrical-shaped channels (b) flower-like designed channels and (c) honeycomb-t5fpe stmcture. 

FIGURE 6.3 Ceramic membrane elements with a flower-like geometry, from TAMI industries. 

FIGURE 6.5 Details of the structure of a monolithic ceramic membrane element with filtrate conduits, from CeraMem. 

Compared to modules based on cylindrical elements, flat ceramic membrane modules are not developed in a large extent and are limited to date to small liquid volume treatment [27]. Flat ceramic membranes are generally implemented as disks in laboratory scaled cells, offering a limited filtration surface area. Indeed a diameter of 90 mm that is one of the largest available dimensions for these membrane disks results in a filtration surface of -56 cm. Anopore alumina membranes supplied by Whatman or ATZ ceramic membrane disks with zirconia or titania top-layers from Sterlitech are typical examples of these commercially available flat ceramic membranes. Sterlitech ATZ ceramic membrane disks and the corresponding membrane holder are shown in Figure 6.16. 

FIGURE 6.1S Examples of ceramic membrane modules, (a) Multi-elements module from Orelis and (b) single-element module from CeraMem. 

FIGURE 6.27 General working principle of a pervaporation or vapor permeation module equipped with tubular ceramic membrane elements. 

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