REVERSE OSMOSIS WITH RECTIFICATION

Abstract

Two or more reverse osmosis filtration units are assembled together such that rectification of the retentate and permeate streams is provided.

Claims

1. A process for the desalination of salt water to produce freshwater using first and second osmotic filtration units, each of which comprises a membrane dividing the unit into permeate and retentate sides, wherein the process comprises the steps of: a. feeding the retentate from the second unit to the permeate side of the first unit; b. feeding the permeate from the first unit to the retentate side of the second unit; c. feeding the saltwater to the retentate side of the first unit; and d. discarding brine from the retentate side of the first unit whereby the permeate from the second unit is the freshwater product.

2. A process for the desalination of salt water by reverse osmosis using two or more filtration units each of which comprises a membrane dividing the unit into retentate and permeate sides wherein the process uses countercurrent streams according to the steps of: a. feeding the retentate from unit n to the permeate side of the unit n1; b. feeding the permeate from unit n to the retentate side of unit n+1; and c. feeding saltwater to the retentate side of unit n1 while brine is discarded from said retentate side whereby the permeate from unit n+1 is the desalinated water product.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 shows a filter unit for reverse osmosis as in the current practice;

[0013] FIG. 2 is a representation of the present invention with two filter units;

[0014] FIG. 3 shows an arrangement for multiple filter units using rectification as specified by the present invention; and

[0015] FIG. 4 shows the results of calculations for an example in which three filter units are used.

BRIEF DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0016] In order to appreciate the workings of the present invention, some knowledge of the relevant physics is necessary.

[0017] When a salt solution is separated from pure water by a semipermeable membrane, water will seep from the purse water through the membrane into the salt water. If the volume of the salt water is constrained, the salt water will exert a pressure known as the osmotic pressure.

[0018] The osmotic pressure of a solution will depend on several factors including the nature of the dissolved salt, its concentration, and the temperature. The osmotic pressure can be quantified by the following expression known as the Van't Hoff equation.

=kCT

In the above equation, represents the osmotic pressure, k is a constant, C is the salt concentration and T is the absolute temperature. When sodium chloride is the solute, k will vary slightly with concentration, but this deviation is low for the most practical purposes.

[0019] Reverse osmosis is achieved by applying sufficient pressure to a salt solution to overcome the osmotic pressure. Now water will pass through the membrane in the reverse direction so that it flows from salt water into fresh water. In practice, an excess of pressure is used in order to achieve acceptable flow rates.

[0020] This description of reverse osmosis is a good example of how the technology is practiced today. The state of the art is illustrated in FIG. 1. Salt water is fed by high pressure pump A to the retentate side of membrane type filtration unit B. Brine is discarded from the retentate side after losing water that passes through membrane C. Fresh water is recovered from the permeate side D of unit B.

[0021] In contrast with existing RO technology, the present invention provides for the rectification of countercurrent streams that connect two or more filtration units. Such an arrangement is shown in FIG. 2 for two RO units. In this layout, the permeate from unit 1 is pumped to unit 2 and the retentate from unit 2 is fed to unit 1.

[0022] The advantage of combining filtration with rectification can be seen from a case study. Again referring to FIG. 2, the saltwater feed as a concentration of 3.5 percent and the fresh water product has close to zero salinity. As shown by a material balance, the concentration of salt in the permeate of unit 1 may be 1.75 percent. Because the concentration gradient is only half of that encountered in conventional RO units, the pressure of the saltwater feed can be reduced to 500 psi from the usual 1000 psi.

[0023] The rectification format shown in FIG. 2 can be extended to any number of filtration units as illustrated in FIG. 3. This particular arrangement has five RO units 8, 10, 12, 14, and 16. For further explanation purposes, unit 12 is denominated n, unit 10 is n1 and unit 14 is n+1. Referring to FIG. 3, the retentate from unit n becomes the feed to the permeate side of unit n1, and permeate from unit n becomes the feed to the retentate side of unit n+1. With the use of five filtration units, the pressure drop across each unit is reduced to 200 psi.

[0024] For maximum energy efficiency, the pressure of each retentate exit stream can be recovered. In this manner, the pressure of the permeate of each RO unit can be raised when discarded. Regenerative pumps are suitable for this procedure.

[0025] The modest pressures required by the present invention have numerous benefits. Equipment costs can be reduced significantly. The membrane requirements are more flexible. This result should be a great help in designing new and more advanced materials for porous membranes.

[0026] No breakthroughs in technology are required to facilitate the adoption of the present invention. The risks involved with this process are minimal. The potential rewards, on the other hand, are substantial and justify a major effort in this direction.

[0027] Aside from its use in the desalination of salt water, membrane filtration can be used for separating solutes from solvents in non-aqueous solutions. This technology has general applicability in chemical synthesis where solvents, reactants, products and catalysts need to be recovered. In these applications, the use of membrane filtration can be cost-effective.

Example

[0028] FIG. 4 illustrates the results for a reverse osmosis installation with rectification employing three filter units 18, 20, and 22. These results are quite dramatic. As can be noted, the differential salt concentration across each filter unit is 1.17 pound of salt per hundred pounds of solution or one-third the concentration drop if only one unit is used.