Water is removed from air and other gases to avoid undesirable effects caused by liquid and ice formation, corrosion, and catalyst contamination. Water removal steps appear in a large number of applications, two examples of which are air conditioning and industrial gas drying.XR101 series Activated alumina figures prominently among the ways to remove water from air or gas streams; it is particularly suited to drying compressed air and to drying gases having high relative humidities. Among the reasons for the widespread acceptance of alumina in these applications are its low cost, chemical and physical resistance, regenerability and high capacity for retaining water. Commercial drying conditions are more demanding than ever, notably for air service where high gas velocities, pressures and regeneration frequencies are often specified by customers. The formulations of high quality aluminas offered by XR to satisfy these needs are based on a half-century of knowhow and experience in alumina development and manufacture.
XR's activated aluminas have very high resistance to attrition and crushing. This is a result of the production process and the spherical shape of the beads. The attrition and crushing values for spheres are well adapted to their applications, the spherical form making possible homogeneous and compact loading with very little subsequent settling. Higher flow rates are enabled and taller drying vessels can be used.
The optimized high porosity of activated alumina gives a high specific surface and a large adsorption capacity per unit weight. Even when saturated with water, activated alumina looks and feels dry. Moreover, activated alumina is chemically inert toward most liquids and gases. Its excellent resistance to fouling by olefin polymerization permits its wide use in the treatment of petrochemicals. The porosity of XR activated alumina is distributed under carefully controlled procedures during manufacture to enable straightforward and mild operating conditions for both drying and regeneration.
A high internal surface area through the presence of pores (capillaries) is necessary to create adsorption sites. Alumina is the most widely used adsorbent owing to the chemical properties of its surface and its ability to be formulated with well-defined pores defined as follows:
• Macropores (> 500 Å) to enhance diffusion into the pore system.
• Mesopores (50 to 500 Å) to accommodate medium size molecules.
• Micropores (< 50 Å) to accommodate small molecules like water.
XR has the capability of optimizing the pore size distribution for a specific adsorbent duty, while retaining high mechanical strength. Chemical species are sometimes used to alter acidity or basicity.
Drying liquidsMost fluids, including liquids, can be dried by XR activated alumina. Liquids are generally dried in the up-flow mode to maximize distribution over the alumina bed.
Regeneration can be performed either with a vaporized dry product or with a dry gas (for instance, methane). In practice, contact times specified for drying liquids are much greater than those for drying gases because the moisture content is higher per unit of fluid volume. For some liquids, very efficient drying can be achieved with a contact time of one minute or less. Regeneration The most common configurations consist of two adsorbers, one operating in the adsorption mode while the other operates in the regeneration mode. At some point, as the MTZ nears the bed outlet, most of the bed is saturated and the original adsorption capacity needs to be restored by desorption. So, once the adsorber columns have been sized, the next step is to determine how regenerate the desiccant. For less-stringent applications like small dewpoint reductions, pressure reduction may be sufficient to regenerate the bed.