The use of a two-phase liquid/gas stream to clean pipelines is disclosed in European Patent Application 0 490 117 A1 to Kuebler that was published on Jun. 17, 1992 as an alternative to conventional clean-in-place techniques in order to reduce the amount of chemicals used. Kuebler describes cleaning pipelines using a two-phase liquid/gas stream and a reduction in throughput of the cleaning liquid by several orders of magnitude relative to conventional clean-in-place techniques. According to Kuebler:                If two phases, namely liquid and gas, flow through a pipeline, then extremely different flow patterns are possible which are dependant, in particular, on the proportion of the gas passing through and also on whether these phases are passing through horizontal, vertical, or inclined pipes. The gas and liquid generally flow in the same direction. In vertical or markedly inclined pipes, however, counter flow of the gas relative to the liquid is also possible. The transitions between the individual existence regions of the flow patterns can be fluid; depending on the pair of phases, considerable differences can also arise from the boundaries of the flow patterns.        The cleaning process is also conceivable by means of plug flow or froth flow within the framework of the cleaning process. However, the use of two-phase gas/liquid flow is preferable, whereby such flow is in the form of annular flow through the pipeline system. The term annular flow is to be understood to mean flow in which the liquid forms a film, which is usually thin, along the wall of the pipe, and the gas flows in the center of the pipe. The two phases are thereby separated from one another by a more or less well-defined boundary layer that corresponds approximately to the interior wall of the pipeline. A comprehensive presentation of two-phase annular flow phenomena is given in Hewitt, Hall Taylor: “Annular Two-Phase Flow” (Pergamon Press, 1970), the contents of which are incorporated herein by reference.        The form in which the liquid is fed into the pipeline is not especially important for the formation of annular flow. Thus, the liquid could be fed into the stream of gas by one or more central nozzles, or even in an annular manner via a porous pipe component or an annular line. If liquid is fed centrally, then disperse annular flow will generally be produced in which finely divided liquid droplets are swept along in the stream of gas.        However, the pressure loss is reduced by a tangential inward flow direction of the liquid and/or an inward flow direction of the liquid that is inclined at an angle to the pipe axis; this is because the stream of gas has to muster less energy in order to accelerate the liquid annulus, and less liquid will be swept along by the gas in the center of the pipe.        Annular feeding at the pipe's periphery is to be provided, rather, for mild cleaning processes for vertical or slightly inclined pipes in which case the liquid flows in the form of a thin film along the interior wall of the pipe and the gas is capable of flowing either in the direction of the stream of liquid or even counter thereto.        A quite well-defined operating region is found to be especially advantageous for the formation of annular flow with high cleaning action and minimal consumption of liquid, whereby this is independent of the pipe's being orientated horizontally or vertically. The two-phase liquid/gas flow ratio being 1 m3:3000 to 7500 m3 or, respectively, 1 kg:2.0 to 6.0 kg. Thus, for example, the consumption of approximately 30 L of liquid is required for a cleaning time of approximately ten minutes in the case of a pipe line system that has a pipe diameter of 65 mm, whereby this is independent of it's length, whereas approximately 80,000 L of liquid would be necessary in the case of complete filling of the pipeline with the cleaning liquid.        Various flow patterns are possible for a two-phase flow. The following list indicates the various flow patterns: bubble flow, which contains gas bubbles in the uppermost part of the pipe; plug flow, which contains bullet-shaped gas plugs moving along in the uppermost part of the pipe; stratified flow, where the two-phases are separated by a smooth interface; pulse flow, where the phase boundary is formed in an undulating manner; surge flow, which results after pulse flow with increasing speed of the gas flow; and annular flow, which occurs after surge flow with yet a further increase in gas speed flow. Annular flow is certainly to be preferred for economical though effective cleaning. However, plug flow or surge flow would be usable for pipe cleaning with more economical consumption of the cleaning liquid than the conventional pipe filling with cleaning liquid.        There are various ways that liquid can be fed into the pipe. The liquid can be introduced via an annular inlet. In addition, a more favorable approach, the liquid is fed, under pressure, into the pipe via an inlet nozzle. The nozzle can be inclined at an angle, preferably 45°, to the axis of the pipe. The inlet nozzles can stop at the inner surface of the pipe or protrude essentially into the center of the pipe.        
The flow patterns in a horizontal pipe can be estimated roughly in a diagram referred to as a Baker diagram. The diagram that is obtained purely empirically is usable for liquid/gas phases that are comparable to water/air as the pair of substances. The abscissa/x-axis is represented by GL*λB*ΨB/GG and is plotted against the expression GG/λB. λ is a density/surface tension parameter and Ψ is a viscosity/surface tension parameter. GL and GG are the mass flows for the liquid and gas in lb/h*ft2 and λB and ΨB are defined by
                                          λ            B                    =                                    [                                                (                                                            ρ                      G                                                              ρ                      A                                                        )                                ⁢                                  (                                                            ρ                      L                                                              ρ                      W                                                        )                                            ]                        0.5                          ;        and                                          Ψ          B                =                                                            σ                w                            σ                        ⁡                          [                                                (                                                            μ                      L                                                              μ                      W                                                        )                                ⁢                                                      (                                                                  ρ                        W                                                                    ρ                        L                                                              )                                    2                                            ]                                            1            3                              ρ are the density values, σ the surface tension, and μ the viscosity of the liquid. The indices A and W designate the values for air and water, respectively; L and G, as already indicated above, designate those for the liquid and gas, respectively. It is important to find an operating region that insures annular flow with the flow of a stream of liquid water, which is as small as possible. This region is defined by having the expression GL*λ*ΨB/GG be between 0.1 and 0.4, and the expression GG/λB be between 2*104 and 105.                A mixture of gas and a suitable liquid, preferably including one or more cleaning agents, can also be used to create a mixed-phase flow along an interior surface of a small bore tubing, which creates shear or impact stresses or similar conditions sufficient to remove biofilm, debris and contaminants from their surfaces. The small bore tubing usually has a diameter of less than 0.8 inches. The cleaning agent is commonly a surfactant, but may also be or include an oxidizing agent, an alcohol, a non-surfactant detergent or a solid material. The method may be applied to passageway geometries of considerable complexity, including surfaces made of a porous membrane. It further includes optimally varying parameters such as the fluid mechanics regime of the mixed-phase flow, the chemistry of the cleaning liquid, temperature, and, in the case of membranes, the direction, magnitude and sequencing of pressure differences across the membrane.        
Additional publications describing mixed phased flow include, for example, U.S. Pat. No. 6,326,340 to Labib et al.; U.S. Pat. No. 6,454,871 to Labib et al.; and U.S. Pat. No. 6,027,572 to Labib et al.
Filtration membranes have a tendency to foul during processing. Fouling manifests itself as a decline in flux with time of operation. Flux decline should occur when all operating parameters, such as pressure, flow rate, temperature, and feed concentration are kept constant. In general, membrane fouling is a complicated process and is believed due to a number of factors including electrostatic attraction, hydrophobic and hydrophilic interactions, the deposition and accumulation of feed components, e.g., suspended particulates, impermeable dissolved solutes, and even normally permeable solutes, on the membrane surface and/or within the pores of the membrane. It is expected that almost all feed components will foul membranes to a certain extent. See Munir Cheryan, Ultrafiltration and Microfiltration Handbook, Technical Publication, Lancaster, Pa., 1998 (Pages 237-288). Fouling components and deposits can include inorganic salts, particulates, microbials and organics.
Filtration membranes typically require periodic cleaning to allow for successful industrial application within separation facilities such as those found in the food, dairy, and beverage industries. The filtration membranes can be cleaned by removing foreign material from the surface and body of the membrane and associated equipment. The cleaning procedure for filtration membranes can involve a CIP process or “clean-in-place” process where cleaning agents are circulated over the membrane to wet, penetrate, dissolve and/or rinse away foreign materials from the membrane. Various parameters that can be manipulated for cleaning typically include time, temperature, mechanical energy, chemical composition, chemical concentration, soil type, water type, hydraulic design, and membrane materials of construction.
Chemical energy in the form of detergents and cleaners can be used to solubilize or disperse the foulant or soil. Thermal energy in the form of heat can be used to help the action of the chemical cleaners. In general, the greater the temperature of the cleaning the solution, the more effective it is as a cleaning treatment, although most membrane materials have temperature limitations due to the material of construction. Many membranes additionally have chemical limitations. Mechanical energy in the form of high velocity flow also contributes to the successful cleaning of membrane systems. See Munir Cheryan, Ultrafiltration and Microfiltration Handbook, Technical Publication, Lancaster, Pa., 1998, pages 237-288.
In general, the frequency of cleaning and type of chemical treatment performed on the membrane has been found to affect the operating life of a membrane. It is believed that the operating life of a membrane can be decreased as a result of chemical degradation of the membrane over time. Various membranes are provided having temperature, pH, and chemical restrictions to minimize degradation of the membrane material. For example, many polyamide reverse osmosis membranes have chlorine restrictions because chlorine can have a tendency to damage the membrane. Cleaning and sanitizing filtration membranes is desirable in order to comply with laws and regulations that may require cleaning in certain applications (e.g., the food and biotechnology industries), reduce microorganisms to prevent contamination of the product streams, and optimize the process by restoring flux. See Munir Cheryan, Ultrafiltration and Microfiltration Handbook, Technical Publication, Lancaster, Pa., 1998, pages 237-288.
Other exemplary techniques for cleaning filtration membranes are disclosed by U.S. Pat. No. 4,740,308 to Fremont et al.; U.S. Pat. No. 6,387,189 to Gröschl et al.; U.S. Pat. No. 6,071,356 to Olsen; and Munir Cheryan, Ultrafiltration and Microfiltration Handbook, Technical Publication, Lancaster, Pa., 1998 (Pages 237-239).
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