US 4,784,672 AGranted

Regeneration of adsorbents

AssigneeAir Products and Chemicals, Inc.

InventorShivaji Sircar

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Abstract

In the recovery of methane and carbon dioxide from landfill gas (LFG) a selective adsorption system is employed having a pretreat section operated in temperature swing adsorption mode (TSA) to remove water and trace hydrocarbon contaminants, in flow communication with a pressure swing adsorption section (PSA) for separation of the methane from CO.sub.2 contained in the purified gas stream discharged from the pretreat section. The LFG is introduced into a column in the TSA section and passed consecutively through a layer of activated carbon followed by a layer of solid desiccant such as a molecular sieve zeolite. The activated carbon layer adsorbs trace contaminants such as hydrocarbons and halohydrocabons boiling above methane; the zeolite layer adsorbs water. Regeneration of the adsorbent layers is carried out by flowing hot regeneration gas first through the zeolite layer then into and through the carbon layer. When the zeolite layer has been heated to the highest regeneration temperature by the regeneration gas, the inlet temperature of the gas is lowered by 100 or more degrees (.degree.F), so that the carbon layer is regenerated at a lower temperature level to avoid detriment to the activated carbon adsorbent that might otherwise occur.

Claims10

Dependent claims

In the process for pretreatment of landfill gas by selective adsorption for removal of trace impurities therein prior to bulk separation of CO.sub.2 from CH.sub.4 contained in the pretreated gas, the improvement which comprises:

(A) passing said landfill gas at superatmospheric pressure into the feed end and through a column containing successive layers of adsorbent wherein hydrocarbons and halohydrocarbons boiling above methane are removed by selective adsorption in the first of such layers and water and sulfur compound are retained by adsorption in the succeeding layer, thereby producing a purified dry gas effluent discharge at the exit end of said column comprised of substantially purified CH.sub.4 and CO.sub.2 ;
(B) thereafter regenerating the adsorbent layers in said column by the successive steps of:
(a) depressurizing the column to near ambient pressure level by withdrawal of gas therefrom at its initial feed end;
(b) passing hot regenerating gas into the column through the initial exit end thereof whereby said gas passes first through said water-retaining adsorbent layer at the highest regeneration temperature until a portion of said layer is heated to said highest regeneration temperature thereby driving off water and impurities earlier sorbed in said layer;
(c) reducing the inlet temperature of the hot regenerating while continuing passage of the hot gas successively through said water-retaining adsorbent layer and said hydrocarbonladen layer, thereby thermally desorbing the hydrocarbon impurities from the hydrocarbon-laden layer without damage to the adsorbent in said layer;
(d) cooling the column to about ambient temperature range by flowing unheated gas through the consecutive adsorbent layers;
and
(e) then restoring the column to the starting superatmospheric pressure level by introduction therein of a stream of purified dry gas comprised of CH.sub.4 and CO.sub.2.

5 more elements

Dependencies are inferred from the claim text.