Abstract: This invention relates to a process for treating an engine exhaust gas stream. The process involves first flowing the engine exhaust stream, which is relatively cool, over an adsorbent zone which comprises an adsorbent bed, i.e., a molecular sieve bed, capable of preferentially adsorbing pollutants such as hydrocarbons. This provides a first exhaust stream which is flowed over a primary castalyst which converts the pollutants to innocuous compounds and then discharging the resultant treated exhaust stream to the atmosphere. When the adsorbent bed reaches a temperature of about 150.degree. C., the entire engine exhaust stream is completely diverted over the primary catalyst. When the inlet temperature to the primary catalyst has reached about 350.degree. C., a minor portion of the engine exhaust stream is diverted over the adsorbent bed to desorb the pollutants adsorbed on the bed and carry them over the primary catalyst where they are converted to innocuous components.

Abstract: In the prevention of atmospheric pollution by sulfur dioxide emissions from acid absorbers in contact process sulfuric acid plants in which the unconverted SO.sub.2 is accumulated by adsorption in a zeolite adsorbent bed and desorbed back into the acid production system, it is found that ambient moist air, after partial dehydration in an acid scrubber is suitably used both to cool down and hot purge desorb the zeolite beds. In addition the pure water derived from the ambient air is advantageously used in the acid-making system.