Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
2001-06-29
2004-03-09
Cecil, Terry K. (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C210S652000
Reexamination Certificate
active
06702944
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a multi-stage nanofiltration or reverse osmosis membrane module, to processes for using such a module to filter water or to remove hardness, to processes for cleaning or maintaining the permeability of such a module, and to a small-scale system particularly for use in private homes and small commercial buildings.
BACKGROUND OF THE INVENTION
Hollow fibre semi-permeable membranes are useful for filtering solids rich fluids. Membranes in the nanofiltration and reverse osmosis ranges may also be useful for separating salts. For example, U.S. Pat. No. 5,152,901 describes a nanofiltration membrane material capable of filtering out suspended solids and large organic molecules and generally rejecting calcium salts while generally permeating sodium salts. U.S. Pat. Nos. 4,812,270 and 5,658,460 also describe membranes useful for rejecting salts. Membranes with similar characteristics, such as Stork Friesland's NR 015-500, are available on the market.
Membranes as described above may be used in the form of hollow fibres operated in an inside-out flow mode. The hollow fibres are suspended between a pair of opposed tube sheets or headers. The headers maintain a separation between the lumens of the membranes and their outer surfaces. Thus, pressurized feed water can be supplied to the lumens of one end of the membranes, permeate can be collected as it leaves the outer surface of the membranes, and a concentrate or retentate can be extracted from the lumens at the other end of the membranes.
Various characteristics of hollow fibre membranes, however, make them difficult to use in such an inside-out flow mode. For example, the inner diameter of the hollow fibre is small which results in significant pressure and flux reductions towards the outlet end of long hollow fibres. The problem is most significant when the feed pressure is low.
U.S. Pat. No. 5,013,437 describes one method of attempting to reduce the problem of pressure and flux loss in long fibres. In an embodiment of that patent, an inside-out hollow fibre filtration module is split into two stages. The retentate from the first stage becomes the feed for the second stage. The ratio of the surface areas of the first to the second stages is preferably about 1.5:1 to 2.25:1. This helps to increase the pressure and velocity of the retentate from the first stage as it becomes the feed to the second stage such that both stages have more nearly equal pressure drops. The stages are arranged concentrically, however, and permeate, particularly from the second stage, must flow along the outside of the fibres to reach an outlet port. With a reasonable packing density of hollow fibre membranes, the head loss in the permeate flow would be substantial if used to filter liquids. Thus the transmembrane pressure differential across the membranes of the second stage is reduced. It is also difficult to pot fibres in an annular ring as required in the '437 module.
A similar principle has also been used in large scale systems using spiral wound membranes. A large number of membrane modules are arranged in stages. Each successive stage has fewer modules than the preceding stage and the retentate from preceding stages becomes the feed of the succeeding stages. Such a system is both large and complex and not suited to residential or small commercial systems.
Makers of small scale nanofiltration or reverse osmosis membrane filtration systems typically try to address the problems discussed above by using a single stage filtration module, and recirculating the retentate to the feed inlet to increase the velocity of the feed water and the transmembrane pressure. In such systems, the minimum velocity of the feed/retentate is between about 3-10 ft/s. This technique requires a high rejection membrane, and is operated at a very low per pass recovery. This leads to rapid fouling and either frequent cleaning or replacement of the membranes. Energy costs and pressure required are also high.
Another characteristic of semi-permeable membranes is that their pores become fouled over time particularly including, in the case of membranes used for water softening, because of carbonate scaling. In large scale systems, carbonate scaling may be addressed by partially softening the feed water using resin exchange beds or by adding an anti-scalant to the feed water. Such techniques are generally too complex to be practicable in small scale systems, particularly in private homes.
SUMMARY OF THE INVENTION
It is an object of the invention to improve on the prior art. It is another object of the invention to provide a membrane filtration module, particularly one that is useful for small scale filtration or water softening. It is another object of the invention to provide a process to clean or reduce scaling of a membrane module, particularly one used for water softening. It is another object of the invention to provide a small scale filtration or water softening system. These objects are met by the combination of features, steps or both found in the claims. The following summary may not describe all necessary features of the invention which may reside in a sub-combination of the following features or in a combination with features described in other parts of this document.
In various aspects, the invention provides a filtration module having a plurality of hollow fibre nanofiltration or reverse osmosis membranes suspended between a pair of opposed headers. The outer surfaces of the membranes are sealed to the headers while their lumens are open at the distal faces of the headers.
Within the module, the hollow fibre membranes are grouped into a plurality of preceding or succeeding stages (some stages being both preceding and succeeding). The lumens of the hollow fibre membranes are open at first and second ends of the stages. Flow between stages occurs across the distal faces of the headers. A module feed inlet is connected in fluid communication with the first end of a first stage. The remaining stages are connected in series behind the first stage with fluid connections between the second end of each preceding stages and the first end of each directly succeeding stage. A module outlet is connected in fluid communication with the second end of a last stage. A permeate collection plenum surrounds the stages and is in fluid communication with each stage. The surface area of the membranes of each preceding stage is between 1 and 2.5 times the surface area of the membranes of a directly succeeding stage and the surface area of the stages decreases from the first stage to the last stage.
To construct the connections between the stages, a first cap covers the distal face of one header and a second cap covers the distal face of the other header. The permeate plenum includes the space between the proximal faces of the headers and an outer shell. Dividers within one or both of the caps collect groups of the membranes into the stages while leaving open fluid connections between the second end of each preceding stage and the first end of each directly succeeding stage. The module inlet and module retentate outlet, typically provided in the caps, are in fluid communication with the first end of the first stage and the second end of the last stage respectively. Thus feed water enters the first end of the first stage and the portion not permeated exits the second end of the first stage. From there, the second end cap directs the feed/retentate to the first end of the second stage. The water not permeated in the second stage arrives at the first cap. In a two stage device, the water not permeated then leaves the module. In a module with more stages, the first cap redirects the feed/retentate to the first end of another stage and the water not permeated flows to the second cap and so on until the second end of the last stage is reached.
The stages are arranged so that each is adjacent the perimeter of the module and interstage flows are generally parallel to the periphery of the module. For example, the stages may be configured as sectors of a
Behmann Henry
Cote Pierre
Husain Hidayat
Murphy Ella
Pottinger Ian
Bereskin & Parr
Cecil Terry K.
Zenon Environmental Inc.
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