It is known that in techniques intended to assure a mixing or putting different phases in contact, for example, gas-liquid-solid phases, numerous difficulties are encountered, particularly on the hydraulic level and in particular when an effort is made to create several zones of different activities or effects in the liquid mass or liquid-solid mixture.
For example, in treating aqueous media where air injections are made in large-surface filters having granular materials with co-current circulation of the fluids, the phenomena are kept under control by using two well known processes: either the "air cushion" where the air and water penetrate between the false bottom (or support) and floor (or base) of the filter and then are sent into the aqueous medium to be treated, charged with filtering material, by means of a series of pump strainers having calibrated orifices; or "air rackets" in which the air is channeled in branched pipes placed on the surface of the floor to calibrated orifices provided on each branching at right angles to tail-less pump strainers, the air that escapes from the calibrated orifices then going with the wash water into the pump strainers before going into the filtering material.
However, difficulties begin when the air is supposed to be sent uniformly over the entire surface of a granular mass countercurrent to a liquid also going through the filtering mass. Such difficulties become very serious when, in addition, zones of different activities are to be maintained in the bed, which is or is not fluidized, for example in the case of biological filtering where the air for oxidizing the polluted materials of the water must be blown in, in an ascending manner, at an intermediate level of the submerged, fixed bed of granular filtering material (see, for example, French Pat. Nos. 76.21246 and 78.30282).
Actually, for there to be a possibility of air injection, it is necessary that the air pressure be greater than that of the column of water on the air injection points; the air can rise as bubbles in the liquid mass only if the force linked to the speed of the water that is filtering in the opposite direction (descending) is less than the buoyancy exerted under the air bubbles; now, this force increases with the clogging of the filter and the travel of the air quickly becomes preferential in the form of "air pockets" which obviously impair the filtering or purification efficiency. It is possible to send the air through perforated pipes placed above the false bottom of the filter which supports the pump strainers for washing the clogged material with an air and water mixture to inject air in the lower half or active zone of the biological filter. Installing these pipes, superposed on standard filter elements, makes the reactor or filter larger and entails additional installation and maintenance costs.
Both for simplification and in an effort to obtain minimum pressure drops, the applicant undertook a series of studies and tests aimed at improving particularly the pump strainers, for example, by developing "double" pump strainers comprising an internal air injection, separated from the free volume of filtering or washing air-water entrainment; and by working on the number of pump strainers per square meter of filtering surface or the diameters and lengths of the air injection pipes. The results were disappointing; in particular, the height of the air cushion under the false bottom of the filter (or reactor) was excessive and impossible to maintain at a constant rate during increase of the air delivery, because the variation in the pressure drop is proportional to the square of the delivery of gas injection.