In place gas cleaning of diffusion elements

In a process of treating a liquid medium by passing treating gas through multi-pore diffusion elements submerged in the medium, wherein foulants in the medium or treating gas tend to form deposits in the elements or at their surfaces causing a progressive increase in the dynamic wet pressure and/or the mean bubble release pressure of the diffusion elements relative to a previous base condition of said elements, and in which the elements are cleaned in place by intermittently passing a cleaning gas through them, alone or in admixture with treating gas, said cleaning gas having a different composition than said treating gas and being capable of reducing the dynamic wet pressure and/or the mean bubble release pressure of fouled diffusion elements, the improvement which comprises:

• initiating cleaning with said cleaning gas when the dynamic wet pressure level of the elements has increased to a level above their base condition by an amount which is equal or equivalent to about 25 inches of water gauge or less at 2 SCFM per square foot of active gas discharge surface, or when the mean bubble release pressure of the diffusion elements has increased to a level above their base condition by about 25 inches of water gauge or less;
• simultaneously applying said cleaning gas to one or more groups of at least about 10 diffusion elements per group by feeding the cleaning gas to the elements through a gas distribution network having a submerged portion including a plurality of flow regulating means, a plurality of plenums and a plurality of diffusion elements, said flow regulating means being connected with said plenums to deliver gas to said plenums, said flow regulating means being sized or adjusted to deliver said gas at a substantially similar rate to each of said plenums, each said plenums being connected directly or indirectly with one or more of said diffusion elements whereby said diffusion element or elements connected to one of said plenums can be supplied with gas at a substantially similar rate as compared to the rate at which gas is supplied to the diffusion elements connected to the [outer] other plenum or plenums;
• and applying said cleaning gas in one or more periods of application and in a sufficient amount [of] for reducing said increase in dynamic wet pressure by at least about 0.3 times said increase in dynamic wet pressure or for reducing said increase in mean bubble release pressure by at least about 0.5 times said increase in means bubble release pressure.

In a process of treating a liquid medium by passing oxygen-containing gas through multi-pore diffusion elements submerged in the medium wherein foulants in the medium or oxygen-containing gas have a tendency to form deposits in the elements or at their surfaces and to cause a progressive increase in the dynamic wet pressure and/or means bubble release pressure of the diffusion elements relative to a previous base condition of said elements, a method of cleaning said elements in place comprising;

• positioning diffusion elements for submergence in said liquid medium having a bubble release pressure in the range of about 2 to about 20 inches of water gauge, at least about 90 percent of said diffusion elements when new or thoroughly cleaned being able to transmit gas at a flow rate within about 15 percent of the average flow rate of all such elements when tested in a dry condition at a pressure of 2 inches of water gauge, said elements having a coefficient of variation of bubble release pressure of the gas discharge surface not greater than about 0.25 based on values of bubble release pressure measurements for at least 5 equally spaced points along each of two perpendicular reference lines across the gas discharge surface and through the center thereof;
• initiating cleaning when the dynamic wet pressure level or mean bubble release pressure of the diffusion elements has increased above said base condition by from 0.7 to about 25 inches of water gauge by simultaneously applying a mixture of HCl and said oxygen-containing gas to the diffusion elements, said HCl being present in a mole fraction of at least about 7.5.times.10.sup.-5 and sufficiently high to effectuate cleaning of said diffusion elements, said mixture being applied through a gas distribution network having a submerged portion including a plurality of flow regulating means, a plurality of plenums, and a plurality of diffusion elements, said flow regulating means being connected to said plenums and adapted to deliver gas at a substantially similar rate to each of said plenums, each of said plenums being connected directly or indirectly with one or more of said diffusion elements whereby said diffusion element or elements connected to one of said plenums can be supplied with gas at a rate substantially similar to the rate of gas supplied to the diffusion elements connected to the other plenum or plenums;
• and
• applying said mixture of HCl and oxygen-containing gas in one or more periods in an amount sufficient for reducing said increase in dynamic wet pressure by at least about 0.3 times said increase in dynamic wet pressure or for reducing said increase in means bubble release pressure by at least about 0.5 times said increase in mean bubble release pressure.