Hydrothermally stable alumina

Abstract

The hydrothermal stability of transition aluminas used as adsorbents and catalyst carriers is improved through their treatment with a soluble silicon inorganic compound such as sodium silicate wherein the silicon compound is mixed with the alumina powder at the production stage of forming particulates by liquid addition. The silicon containing particulates are activated by heating at a temperature lower than 500° C. and treated, before or after the thermal activation, by a colloidal silica solution to produce a hydrothermally stable, low dust alumina. The total silica content of the final product is typically less than 5 mass-%.

Description

CROSS-REFERENCE TO RELATED APPLICATION[0001]This application is a Division of copending application Ser. No. 11 / 439,547 filed May 24, 2006, the contents of which are hereby incorporated by reference in its entirety.FIELD OF THE INVENTION[0002]The present invention relates to a hydrothermally stable alumina, its process of manufacture and its use as a desiccant. More specifically, the present invention relates to a process of treating transition aluminas with a soluble silicon inorganic compound.[0003]The industrial activated alumina adsorbents are produced exclusively by the rapid (flash) calcination of the Bayer process derived aluminum hydroxide (Gibbsite, ATH) powder followed by wet agglomeration and thermal activation. These adsorbents exhibit X-ray diffraction patterns of transition alumina phases. They typically have high BET surface area and good adsorption properties for moisture and other contaminants. This makes them suitable for treatment of various industrial streams.[00...

AI Technical Summary

Benefits of technology

[0017]The present invention greatly improves the hydrothermal stability of alumina desiccants and simultaneously reduces the dust formation with activated aluminas. The modified adsorbent maintains low reactivity and is still suitable for application in reactive streams.

[0018]The existing processes for manufacturing activated alumina can easily accommodate the production of the hydrothermally stable alumina described in the present invention. The additives used are inexpensive and no adverse environmental effects are expected. No heat treatment is needed as is the case in the prior art methods to prepare a thermally stable alumina carrier.

[0019]The hydrothermally stable alumina desiccants of the present invention will prolong the lifetime and improve the performance of all processes employing thermal regeneration of the adsorbent. Severe regeneration applications such as natural gas drying will especially benefit from this invention.

[0020]The transition alumina phases formed by rapid calcinations of aluminum hydroxide have high BET surface area and are very reactive toward water. While this feature is generally useful since it helps forming beads by agglomeration and allow for the fast pick up of moisture during adsorption, in long term, especially at severe conditions of thermal regeneration of the adsorbent, it causes irreversible re-hydration effects, which speed up the aging process of alumina.

[0022]Activation at higher temperature increases somewhat the hydrothermal stability of alumina since it produces alumina phases, which are more stable toward re-hydration. Unfortunately, the BET surface area and the adsorption capacity decline after high temperature calcinations. On the other hand, this approach achieves only a moderate improvement of the hydrothermal stability of alumina.

Problems solved by technology

In the course of regeneration, the adsorbent experiences the simultaneous effect of elevated temperature, pressure and high moisture content, with hot liquid water percolating through the adsorbent bed, causing hydrothermal aging and loss of adsorption performance.

While the loss of performance over regeneration cycles is small in some desiccant applications and the adsorbent can last thousand of cycles, there are some severe applications resulting in much faster deterioration of performance, which are challenging even for the most stable alumina adsorbents.

However, the short lifetime caused by hydrothermal aging led to replacement of activated alumina by molecular sieves in most of the units.

Alumina quickly loses its drying performance when used as protective layer.

In spite of the fact that the need of improvement in the hydrothermal stability of activated alumina has been acknowledged (see the article of R. Dale Woosley “Activated Alumina Desiccants” in ALUMINA CHEMICALS—SCIENCE AND TECHNOLOGY HANDBOOK edited by L. D. Hart, American Ceramic Society, 1990, page 241-250), there remains a lack in reported success in preparing hydrothermally stable aluminas.