This invention relates generally to a tray for holding contaminated adsorbent materials for desorption thereof and more particularly concerns a ventilated tray which is particularly useful in microwave desorption.
In industry, process streams carrying contaminants or other components are often purified by passing the stream in contact with an adsorbent. The contaminants or other components are adsorbed by the adsorbent, thereby removing them from the process stream. Materials commonly used as adsorbents include activated carbon, activated charcoal, zinc oxide, activated alumina and molecular sieves. Adsorption is most effective when the adsorbent is maintained at ambient temperatures or cooler. The adsorbed materials are referred to as adsorbates or simply sorbates. Thus, a sorbated adsorbent refers to an adsorbent having adsorbed materials therein. In the course of cleansing process streams, the adsorbent will eventually become saturated with sorbates and be unable to adsorb further materials. Rather than simply being disposed of, a saturated adsorbent can be recycled through a process which desorbs or strips the sorbates from the adsorbent. Once the sorbates have been desorbed, the adsorbent is again capable of being used to cleanse process streams.
Many organic contaminants can be desorbed by heating the adsorbent to relatively low temperatures (e.g., in the range of 100.degree.-300.degree. C. for activated carbon). This low temperature process is referred to as regeneration. However, some contaminants cannot be desorbed at regeneration temperatures. These remnant contaminants, which might be high boiling point materials or result from polymerization on the adsorbent, are referred to as the "heel." After many (hundreds or even thousands) regenerations, the heel buildup diminishes the sorbent capacity of the adsorbent to the extent that the adsorbent is no longer useful. At this point, the adsorbent must not only be treated at higher temperatures (e.g., about 900.degree.-1000.degree. C. for activated carbon) but must also be exposed to reactants (such as steam or carbon dioxide) which can gasify some of the heel and the adsorbent to create new surface area. This process is called reactivation and is usually performed in large, off site furnaces. As used herein, the terms "desorption" or "desorbing process" refer to both regeneration and reactivation.
Traditionally, a saturated adsorbent is regenerated by heating the adsorbent with a flow of hot gas such as steam, nitrogen or flue gases to a sufficiently high temperature at which the sorbate is desorbed. The high temperature causes the sorbated matter to vaporize and pass from the adsorbent. The flow of the hot gas also purges the vaporized or desorbed materials from the system. Some disadvantages of this method include long regeneration times, use of large amounts of purge gas, and non-uniform heating of the adsorbent material. The gas heating method also requires heating not only the adsorbent material but also the entire structure containing the adsorbent, which is necessarily several times heavier than the adsorbent. Thus, the design of the containment structure is controlled by the temperature and corrosion limits prescribed by the regeneration process. In addition, this type of gas heating usually can achieve temperatures only in the range of about 100.degree.-150.degree. C. and is thus insufficient for reactivation.
Traditional reactivation processes are conducted in rotary kilns or Herreschoff multi-hearth furnaces in which the adsorbent is heated to the high reactivation temperatures while being exposed to the gasification reactants in continuous counter flows. The incidental stirring and tumbling motion of the adsorbent in such kilns or furnaces assures thorough contact of the adsorbent with the gasification reactants, thus providing complete reactivation. However, the stirring and tumbling tends to cause a high degree of relative movement between individual granules of the adsorbent. This relative movement tends to grind some of the adsorbent into smaller, less useful particles, thus producing costly attrition losses.
Another desorption approach is to use microwave energy to heat the adsorbent material. Microwave heating is quick and uniform and can produce relatively high temperatures so as to be applicable to both regeneration and reactivation. Microwave heating has a further advantage in that the adsorbent material can be heated without directly heating the containment structure. Thus, the energy required for microwave heating is less than heating with hot gas. The cost of the containment structure can also be reduced-since the structure itself is subjected to lower temperature ranges.
A simple approach to microwave desorption is to transfer the adsorbent from the adsorber vessel to a bulk container and expose the container to microwave energy in order to heat the adsorbent to the desorption temperature. The adsorbent is thus heated while at rest and without the stirring and tumbling motions described above. The lack of agitation minimizes attrition of the adsorbent. However, the lack of adsorbent agitation during reactivation severely limits contact between the adsorbent and the gasification reactants, thereby hampering the reactions. Furthermore, complete removal of desorbed materials from a stationary bed is difficult without a flow of purge gas through the bed. Thus, the capability to force gas flow through a stationary adsorbent bed, for the removal of desorption products and to induce successful gasification reactions, is crucial for the effective desorption of the bed.
Accordingly, there is a need for a container that supports a bed of adsorbent material for desorption while permitting thorough gas flow through the adsorbent bed without agitation thereof.