Apparatus, use of apparatus and process for desalination of water

Abstract

A process for desalinating water is disclosed. The process comprises the steps of passing a feed stream of saline solution 2 in a first desalination step through a reverse osmosis membrane desalination plant 3 comprising at least one reverse osmosis desalination unit 4 to form a first product water stream 5 having a reduced salt concentration relative to that of the feed stream of saline solution 2 and a first byproduct stream 6 having an increased salt concentration relative to that of the feed stream of saline solution 2 characterized in that the first byproduct stream 6 is passed in a second desalination step through a falling film crystallization unit 7 to form a second product water stream 8 having a reduced salt concentration relative to that of the first byproduct stream 6 and a second byproduct stream 9 having an increased salt concentration relative to that of the first byproduct stream 6. The invention further relates to an apparatus 1 for carrying out said process. The present invention further relates also to the use of the process or apparatus 1 for the reduction of the volume of the first byproduct stream 6 of a reverse osmosis membrane desalination plant 3, preferably an in-land desalination plant 3, or in a device or plant or process for producing desalinated water, for salt production, for co-production of power and desalinated water, or for air conditioning.

Claims

1. A process for desalinating water comprising the steps of: passing a feed stream of saline solution in a first desalination step through a reverse osmosis membrane desalination plant comprising at least one reverse osmosis desalination unit to form a first product water stream having a reduced salt concentration relative to that of the feed stream of saline solution and a first byproduct stream having an increased salt concentration relative to that of the feed stream of saline solution, wherein the first byproduct stream is passed in a second desalination step through a first falling film crystallization unit to form a second product water stream having a reduced salt concentration relative to that of the first byproduct stream and a second byproduct stream having an increased salt concentration relative to that of the first byproduct stream, wherein the salt concentration of the first byproduct stream is between 3 to 7 weight %, wherein the second byproduct stream is passed in a third desalination step through a static crystallization unit to form a third product water stream having a reduced salt concentration relative to that of the second byproduct stream and a third byproduct stream having an increased salt concentration relative to that of the second byproduct stream, wherein the third product water stream is fed into the first byproduct stream, and wherein the second byproduct stream is crystallized in the static crystallization unit at a temperature between about ?4 to about ?13? C.

2. The process of claim 1, wherein the first byproduct stream is crystallized in the first falling film crystallization unit at a temperature between about ?1 to about ?4? C.

3. The process of claim 1, wherein the salt concentration of the second byproduct stream is between about 8 to about 18 weight %.

4. The process of claim 1, wherein the second desalination step in the first falling film crystallization unit and/or the third desalination step in the static crystallization unit or the first falling film crystallization unit or the second falling film crystallization unit comprises a sweating stage in which a byproduct is removed from a product by means of partial melting of a crystal within the first falling film crystallization unit or the static crystallization unit or the second falling film crystallization unit.

5. The process of claim 1, wherein the steps occur in a reverse osmosis membrane desalination plant, or in a device or plant or process for producing desalinated water, for salt production, for co-production of power and desalinated water, or for air conditioning.

6. An apparatus for carrying out a process for desalinating water, the apparatus comprising: a reverse osmosis membrane desalination plant comprising at least one reverse osmosis desalination unit having an inlet for a feed stream of saline solution, an outlet for a first product water stream, an outlet for a first byproduct stream, wherein the outlet for a first byproduct stream is in fluid communication with an inlet of a first falling film crystallization unit having an outlet for a second product water stream, and an outlet for a second byproduct stream, wherein the outlet for the second byproduct stream is in fluid communication with an inlet of a static crystallization unit having an outlet for a third product water stream and an outlet for a third byproduct stream, wherein the third product water stream is in fluid communication with the first byproduct stream.

7. The apparatus of claim 6, wherein, in use, the feed stream of saline solution in a first desalination step is passed through the reverse osmosis membrane desalination plant to form the first product water stream having a reduced salt concentration relative to that of the feed stream of saline solution and the first byproduct stream having an increased salt concentration relative to that of the feed stream of saline solution, wherein, in use, the first byproduct stream is passed in a second desalination step through the first falling film crystallization unit to form the second product water stream having a reduced salt concentration relative to that of the first byproduct stream and the second byproduct stream having an increased salt concentration relative to that of the first byproduct stream.

8. The apparatus of claim 6, wherein the reverse osmosis membrane desalination plant is an in-land desalination plant.

9. The apparatus of claim 8, where the plant produces desalinated water, salt power, conditioned air, or any combination thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained in more detail hereinafter with reference to various embodiments of the invention as well as to the drawings. A single quotation mark () after a reference number is used to indicate those features of the prior art. The schematic drawings show:

(2) FIG. 1 shows a schematic view of an embodiment of the process for desalinating water according to the invention having a first and second desalination step.

(3) FIG. 2 shows a schematic view of a preferred embodiment of a process for desalinating water according to the invention, in which a third desalination step takes place in a static crystallization unit.

(4) FIG. 3 shows a schematic view of a preferred embodiment of a process for desalinating water according to the invention, in which a third desalination step takes place in the same or a second falling film crystallization unit.

(5) FIG. 4 shows a schematic view of an embodiment of an apparatus according to the invention for carrying out a process for desalinating water according to the invention having a first and second desalination step.

(6) FIG. 5 shows a schematic view of a preferred embodiment of an apparatus for carrying out a process for desalinating water according to the invention, in which a third desalination step takes place in a static crystallization unit.

(7) FIG. 6 shows a schematic view of a preferred embodiment of an apparatus for carrying out a process for desalinating water according to the invention, in which a third desalination step takes place in a second falling film unit.

(8) FIG. 7 shows a summary of results obtained in the Examples.

DETAILED DESCRIPTION OF THE INVENTION

(9) FIG. 1 shows a schematic view of a process according to the invention which as a whole is labeled with reference number 100. The process 100 comprises a first desalination step, which is labeled as 110; and a second desalination step, which is labeled as 120. In the first desalination step 110, a feed stream of saline solution 2 is passed through a reverse osmosis membrane desalination plant 3, wherein the plant 3 comprises at least one reverse osmosis desalination unit 4, to form a first product water stream 5 having a reduced salt concentration relative to that of the feed stream of saline solution 2 and a first byproduct stream 6 having an increased salt concentration relative to that of the feed stream of saline solution 2. In the second desalination step 120 the first byproduct stream 6 is passed through a falling film crystallization unit 7 to form a second product water stream 8 having a reduced salt concentration relative to that of the first byproduct stream 6 and a second byproduct stream 9 having an increased salt concentration relative to that of the first byproduct stream 6.

(10) As shown in FIG. 1, as well as in FIGS. 2-6, the first byproduct stream 6 from the reverse osmosis desalination unit 4 is fed directly to the falling film crystallization unit 7 without passing through any intermediate concentrator or evaporator devices, such as a mechanical vapor compression unit or a thermal or a steam driven evaporator. Therefore the salt concentration of the first byproduct stream 6 remains relatively unchanged after it exits the reverse osmosis desalination unit 4 until it is fed directly to the falling film crystallization unit 7, as shown in these figures. The lack of such intermediate concentrator or evaporator devices has benefits in minimizing the complexity, investment and maintenance costs, and footprint of the apparatus and process for desalinating water.

(11) In the specification and claims of this application, the salt concentration of saline solution is expressed as a weight % (weight salt/weight saline solution?100%). There are several well-known methods in the art to measure the salt concentrations of water, and they include Total Dissolved Solids (TDS) usually together with a mass salt balance, gravimetric determination of weight left upon evaporation of the water, melting point (or freezing point) determination, optical refractometry, UV/Vis spectrophotometry to detect the major components of ionic components in solution (e.g. using a HACH LANGE DR 5000 UV-Vis Spectrometer), full chemical analysis taking into account the charge balance for ions, and electrical conductivity measurements.

(12) Electrical conductivity measurements are based on the measurement of the amount of electrical current conducted across a centimeter length of water column of unit cross-sectional area. Water containing a higher amount of dissolved salts has a higher conductivity. As used herein, the salt concentration in weight % is obtained by dividing the salt concentration in parts per million by 10000. The salt concentration (C) in units of parts per million (ppm) as used herein is determined by an electrical conductivity measurement, where the electrical conductivity (U) is expressed in units of milli-siemens per cm (mS/cm). The relationship between C in ppm and U in mS/cm is provided by the following equation:
C=((0.0154009*(U.sup.3))?(2.67657*(U.sup.2))+(922.071*(U))?(744.133))

(13) FIG. 2 shows a schematic view of a preferred embodiment of the invention which is a process 100 comprising a first desalination step 110, a second desalination step 120, and a third desalination step, which takes place in a static crystallization unit 10, which is labeled as 130. In the third desalination step 130, a third product water stream 12 having a reduced salt concentration relative to that of the second byproduct stream 9 and a third byproduct stream 13 having an increased salt concentration relative to that of the second byproduct stream 9 are formed. As discussed earlier, this additional step beneficially reduces the volume of the first byproduct stream 6 and increases the productivity, and the third product water stream 12 may be advantageously recycled in one embodiment by feeding it into the first byproduct stream 6.

(14) FIG. 3 shows a schematic view of a preferred embodiment of the invention which is a process 100 comprising a first desalination step 110, a second desalination step 120, and a third desalination step, which takes place in the same 7 or a second falling film crystallization unit 11, which is labeled as 135. In the third desalination step 135, a third product water stream 12 having a reduced salt concentration relative to that of the second byproduct stream 9 and a third byproduct stream 13 having an increased salt concentration relative to that of the second byproduct stream 9 are formed. This preferred embodiment has similar advantages to that of the one shown in FIG. 2, and the third product water stream 12 may be advantageously recycled in one embodiment by feeding it into the first byproduct stream 6.

(15) The first desalination step 110 and its feed stream of saline solution 2, reverse osmosis membrane desalination plant 3, reverse osmosis desalination unit 4, first product water stream 5, and first byproduct stream 6 are all conventional and well-known in the art, for example, as disclosed in U.S. Pat. No. 4,125,463.

(16) Preferred process conditions for the second desalination step 120 are the same for the process of the invention as shown in FIG. 1 and its preferred embodiments shown in FIG. 2 and FIG. 3 unless otherwise specifically indicated. Falling film crystallization units 7 and their operation are well-known in the art, for example, as disclosed in U.S. Re. 32,241 or U.S. Pat. No. 3,621,664. Unless indicated otherwise, conventional falling film crystallization units 7 and 11 may be used and operated as known in the art for the second desalination step 120 and third desalination step 130. Static crystallization units 10 and their operation are well-known in the art, for example, as disclosed in Sulzer Technical Review February 1999 pp. 8-11, Sulzer Technical Review January 2006 pp. 4-6, or U.S. Pat. No. 6,145,340. Unless indicated otherwise, a conventional static crystallization unit 10 may be used and operated as known in the art for the third desalination step 130.

(17) In principle, freezing-desalination technology has many advantages over other conventional desalination processes for treating highly concentrated saline solutions, especially its lower energy requirement and reduced scaling, fouling, and corrosion problems. Due to the low operating temperatures of freezing-desalination technology, no special materials of construction (e.g. corrosion resistant) are required.

(18) In preferred embodiments of the apparatus 1, the various crystallization units 7 and/or 10 and/or 11 are equipped with appropriate buffer vessel(s) and fluid connections so that additional further desalination steps may be carried out within the same unit.

(19) The various product and byproduct streams from the crystallization units may be conveniently controlled by means of the mass balance in collecting vessels directly connected to the crystallization unit. The heating and cooling requirements of the crystallization units are varying as a function of time, and energy buffer systems may be advantageously used in order to minimize the fluctuations in the demands for steam and refrigeration. In the case of a batch operation, streams may be stored in buffer vessels prior to their passing through a specific crystallization unit. The crystallization units may be conveniently controlled by a computer system using level and temperature measurement instruments, as well as on/off or control valves.

(20) Additional information on crystallizers and their operation is disclosed in Handbook of Industrial Crystallization, 2.sup.nd Edition, by Allan S. Myerson, published Jan. 9, 2002 by Butterworth-Heinemann, Woburn, Mass. ISBN: 978-0750670128 and Crystallization Technology Handbook, 2.sup.nd Edition, edited by A. Mersmann, published 2001 by Marcel Dekker, Basel, ISBN: 0-8247-0528-9.

(21) In one embodiment, the salt concentration of the first byproduct stream 6 is between about 3 to about 7 weight %, preferably between about 4 to about 7, more preferably between about 5 to about 7, most preferably between about 6 to about 7. As discussed earlier, feeding the falling film crystallization unit 7 with a stream 6 having such salt concentrations ensures that the unit 7 produces a second product water stream 8 of sufficiently good quality, while simultaneously minimizing the volume of the first byproduct stream 6.

(22) In another embodiment, the first byproduct stream 6 is crystallized in the falling film crystallization unit 7 at a temperature between about ?1 to about ?4? C., preferably about ?1.5 to about ?4, more preferably about ?2 to about ?4, most preferably about ?3 to about ?4. As discussed earlier, such temperature ranges offer a process optimum in terms of the quality and volume of the second product water stream 8.

(23) The temperature of the crystallization in a specific crystallization unit as referred to in the specification of this application is expressed in degrees Celsius (? C.), and it is measured by measuring the melting point of the product water stream removed from the specific crystallization unit.

(24) In yet another embodiment of the process of the invention having a third desalination step 130 or 135, the second byproduct stream 9 is crystallized in the static crystallization unit 10 or the same 7 or a second falling film crystallization unit 11 at a temperature between about ?4 to about ?13? C., preferably about ?6 to about ?10, more preferably about ?9 to about ?10.

(25) In still yet another embodiment, the salt concentration of the second byproduct stream 9 is between about 8 to about 18 weight %, preferably between about 10 to about 15, more preferably between about 14 to about 15. As discussed earlier, these ranges act to optimize the water recovery of the process.

(26) It will be understood by one skilled in the art that the use of very low crystallization temperatures in order to achieve even higher salt concentrations of the byproduct stream will be limited in practice to a maximum salt concentration of about 23 weight % due to the eutectic point of saline solution.

(27) In embodiments of the process of the invention in which a second desalination step 120 takes place in a falling film crystallization unit 7 or in which a third desalination step 135 takes place in a static crystallization unit 10, for example, such as that shown in FIG. 2, or a second falling film crystallization unit 11, for example, such as that shown in FIG. 3, in some specific further embodiments the second desalination step 110 and/or the third desalination step 130 or 135 comprises a sweating stage in which a byproduct 18 is removed from a product 19 by means of partial melting of a crystal 20 within the falling film crystallization unit 7 or the static crystallization unit 10 or the second falling film crystallization unit 11.

(28) A sweating stage is a temperature-induced purification based on a partial melting of crystals or crystal layers by heating the cooled surface to a temperature close to the melting point of the pure substance (water). As a result, impurities or inclusions are in near equilibrium with the solid phase. An increase in temperature will cause material in the adjacent solid phase to dissolve into such inclusions until phase equilibrium is re-established. The primary result of this phenomenon is to increase the porosity of the layer, which allows inclusions to drain out. A secondary effect is a reduction in viscosity, which also assists the draining process. As discussed earlier, these specific further embodiments have benefits in significantly improving the purity of the product stream obtained and with only a relatively small loss of water.

(29) FIG. 4 shows a schematic view of an apparatus according to the invention which as a whole is labeled with reference number 1. The apparatus 1 comprises a reverse osmosis membrane desalination plant 3 comprising at least one reverse osmosis desalination unit 4 having an inlet 21 for a feed stream of saline solution 2, an outlet 51 for a first product water stream 5, an outlet 61 for a first byproduct stream 6, wherein the outlet 61 is in fluid connection with an inlet 62 of a falling film crystallization unit 7 having an outlet 81 for a second product water stream 8, and an outlet 91 for a second byproduct stream 9.

(30) FIG. 5 shows a schematic view of a preferred embodiment of the invention which is an apparatus 1 as shown in FIG. 4, but wherein the apparatus 1 additionally comprises a static crystallization unit 10 having an inlet 92 in fluid communication with the outlet 91 of the falling film crystallization unit 7 and an outlet 121 for a third product water stream 12 and an outlet 131 for a third byproduct stream 13.

(31) FIG. 6 shows a schematic view of another preferred embodiment of the invention which is an apparatus 1 as shown in FIG. 4, but wherein the apparatus 1 additionally comprises a second falling film crystallization unit 11 having an inlet 92 in fluid communication with the outlet 91 of the falling film crystallization unit 7 and an outlet 121 for a third product water stream 12 and an outlet 131 for a third byproduct stream 13.

(32) Preferred apparatus configurations for the falling film crystallization unit 7 are the same for the apparatus of the invention as shown in FIG. 4 and its preferred embodiments shown in FIG. 5 and FIG. 6 unless otherwise specifically indicated. As discussed earlier, the reverse osmosis membrane desalination plant 3, reverse osmosis desalination unit 4, inlet 21, outlet 51, and outlet 61 are all conventional and well-known in the art, for example, as disclosed in U.S. Pat. No. 4,125,463. Falling film crystallization units 7 and 11 are also well-known, for example, as disclosed in U.S. Re. 32,241 or U.S. Pat. No. 3,621,664, and they may be conventionally used and operated in the invention as described in the art unless specifically indicated otherwise. Static crystallization units 11 are also well-known, for example, as disclosed in Sulzer Technical Review February 1999 pp. 8-11, Sulzer Technical Review January 2006 pp. 4-6, or U.S. Pat. No. 6,145,340, and they too may be conventionally used and operated in the invention as described in the art unless specifically indicated otherwise.

(33) In some specific preferred embodiments of the apparatus 1 comprising a static crystallization unit 10 or a second falling film crystallization unit 11 having an inlet 92 in fluid communication with the outlet 91 of the falling film crystallization unit 7 and an outlet 121 for a third product water stream 12 and an outlet 131 for a third byproduct stream 13, such as the embodiments shown in FIG. 5 or FIG. 6, the outlet 121 for a third product water stream 12 is in fluid communication with the first byproduct stream 6 in order to advantageously recycle the stream 12 in specific embodiments.

(34) In preferred embodiments of the apparatus 1, the various crystallization units 7 and/or 10 and/or 11 are equipped with appropriate buffer vessel(s) and fluid connections so that additional further desalination steps may be carried out within the same unit.

(35) Yet another aspect of the invention is the use of the process or the apparatus 1 for the reduction of the volume of the first byproduct stream 6 of a reverse osmosis (RO) membrane desalination plant 3, preferably an in-land desalination plant 3, or in a device or plant or process for producing desalinated water, for salt production, for co-production of power and desalinated water, or for air conditioning. Incorporation of the invention into a RO plant allows for a reduction of the required capacity of the RO plant and thus investment for a particular application. For example, productivity is increased by the use of the invention in that less saline solution feed is required because some of the water that would otherwise be lost is recovered. In addition these various uses typically benefit in a reduction of maintenance costs due to the use of proven crystallization units and technologies. Furthermore heat pump crystallizers may be used to reduce energy costs and minimize the equipment.

EXAMPLES

(36) The following examples are set forth to provide those of ordinary skill in the art with a detailed description of how the processes, apparatuses and uses claimed herein are evaluated, and they are not intended to limit the scope of what the inventors regard as their invention. Unless indicated otherwise, parts are by weight, and temperature is in degrees Celsius (? C.).

(37) The pilot plant used in the examples consisted of a single, standard falling film crystallization element (length of 12 m, diameter of 70 mm, equipped with the same type of crystallizer elements as are used for industrial crystallizers), a collecting vessel with observation window and a product circulation pump. The product section was constructed of DIN 1.4404 (ANSI 316 L).

(38) An energy section provided the process with heat transfer medium (HTM) of the required temperature. This section consisted of an electrical heater for heating and a heat exchanger for cooling by water, which in turn was cooled by a refrigeration plant. Specific temperatures during crystallization, partial melting and total melting were maintained by a by-pass mixing system.

(39) The pilot plant was equipped with a computer system to control, maintain and record the temperature gradients.

(40) The pilot plant allowed the determination of crystallization behavior under conditions identical to those in a full scale falling film crystallization plant. As a result, the results obtained from this unit match those which could be expected in a full scale plant with a number of crystallization elements in parallel.

(41) General Test Description

(42) Pilot testing on the falling film crystallizer proceeded by the same steps or phases found in a full scale plant:

(43) Phase 1

(44) During phase 1 a predetermined weight of molten residue was fed to the collecting vessel and was pumped to the top of the crystallizer tube by the product circulation pump. The saline solution concentration and composition was selected to be representative of the first 6 byproduct stream coming from a typical reverse osmosis (RO) desalination unit. At the top of the tube, the product was distributed onto the inner wall of the crystallizer tube, and it flowed as a falling film back to the collecting vessel.

(45) The HTM flowed as a co-current falling film on the outside of the crystallization tube. Over the course of the crystallization, the HTM temperature was gradually lowered. When the temperature of the product passed the solidification point, a crystal layer begins to grow on the inner tube wall. The product circulation and cooling was continued until the mother liquor reached a predetermined level in the collecting vessel.

(46) At the end of the first phase the remaining liquid residue was drained from the collecting vessel and stored in a container.

(47) Phase 2

(48) In the second phase (referred to here as a sweating or partial melting phase) the temperature was increased gradually to just below the melting point of the crystal layer. Further purification of the crystal mass was carried out in this step, since trapped impurities melt and migrate to the inner surface. This partial melting results from local solid-liquid equilibrium around impurity pockets. The partially remolten product rinsed the crystal layer, thereby removing adhering residue and flushing off impurities. The partial melt fraction was collected and drained into a container and stored for reuse. The sweating phase provided a very effective additional purification of the crystal layer.

(49) Phase 3

(50) In the third phase the HTM temperature was set above the melting point of the crystal layer. Once enough molten product had accumulated in the collecting vessel, the circulation pump was switched on to circulate the melt and increase the speed of melting. At the end of the total melting, all product was in liquid form and could be removed from the crystallizer.

(51) If further purification was desired, the product liquid was recrystallized in a higher stage in either the same falling film crystallization 7 or a static crystallization 10.

(52) In the case of further purification in a static crystallizer, the pilot plant used in these examples consisted of a standard static crystallization unit of a 70 liter crystallizer, equipped with the same type of crystallizer elements as used for industrial crystallizers. It therefore avoids any risk in the design for final capacity as no scale up needs to be done. The operating phases of the static crystallization are analogous to those of the falling film process, only the product melt remains stagnant during crystallization and total melting

(53) The weights of all streams removed from the crystallizer were measured using a digital balance, and freezing point measurements were carried out where appropriate (at lower purities, where deviations from the pure product freezing point are significant enough to be detected). Samples were taken during the test runs and analyzed [residue+partial melt fractions (in certain examples (e.g. 7 or 8))+stage product].

(54) The salt concentration (C) was determined by an electrical conductivity measurement, where the electrical conductivity (U) is expressed in units of milli-siemens per cm (mS/cm). The relationship between C in ppm and U in mS/cm is provided by the following equation:
C=((0.0154009*(U.sup.3))?(2.67657*(U.sup.2))+(922.071*(U))?(744.133))

(55) The water recovery (WR) in percent (%) in the examples was determined by the following equation:
WR=(w.sub.p/w.sub.f)*100%
wherein w.sub.p=weight of the product and w.sub.f=weight of the feed.

(56) The salt rejection (SR) in percent (%) in the examples was determined by the following equation:
SR=(1?x.sub.p/x.sub.f)*100%
wherein x.sub.p=salt concentration in the product and x.sub.f=salt concentration in the feed.

(57) The yield (Y) in percent (%) in the examples was determined by the following equation:
Y=[x.sub.p*(x.sub.f?x.sub.r)]/[x.sub.f*(x.sub.p?x.sub.r)]*100%
wherein x.sub.p and x.sub.f are as defined above and x.sub.r=salt concentration in the residue.

(58) The HTM temperature was varied between 15 and ?24? C. The initial feed inlet temperature was 15? C., and the temperature was cooled down during the examples. The final temperature for crystallization was different for the different examples in order to observe the effect on the water recovery, the salt rejection, and the yield. The final HTM temperature during crystallization was between ?6 and ?24? C.

Examples 1 to 4

(59) In these examples, a first byproduct stream 6 having a salt concentration of from 2.64 to 6.11 weight % was treated in a second desalination step 120 by crystallization in a single stage of the falling film crystallization unit 7 at temperatures of from ?0.76 to ?2.46? C. and yielded the results shown in Tables 1 to 4. It is noted that Examples 3 and 4 both used a sweating stage.

(60) FIG. 7 summarizes the results obtained for the water recovery and salt rejection and product salt concentration obtained in a second desalination step 120 in a falling film crystallization unit 7 at a variety of operating temperatures (temperature of the heat transfer medium). It can be seen that in reducing the operating temperature an optimum is achieved at about ?16? C. An operating temperature of about ?16? C. results in a residue (third byproduct stream 13) having a melting point of about ?9 to about ?10? C., which corresponds to about 14 to about 15 weight % salt concentration, The product salt concentration is about 4 to about 5 weight %, which is quite favorable for recycling into the feed stream of saline solution 2 fed to the RO desalination plant 3. Further reduction of the operating temperature does not result in a significant increase in the water recovery, and the salt rejection and product salt concentration properties actually decrease as the temperature is further reduced. Of course, lower operating temperatures also result in higher energy costs. Therefore an operating temperature (heat transfer medium temperature) of about ?16? C. will provide an optimum trade-off for many situations.

(61) These results demonstrate that the falling film crystallization is particularly effective in providing quite useful water recovery, salt rejection and yield results over the breadth of the preferred ranges for the salt concentration and temperature disclosed in this application. In addition, the desalination of a first byproduct stream 6 of an RO plant is particularly effective for salt concentrations between about 6 to about 7 weight % when crystallized at a temperature between about ?2 to about ?4? C. It is generally preferred to have the salt concentration of the first byproduct stream 6 higher in order to have a more economical use of the RO desalination unit 4. Nonetheless one skilled in the art will understand how to appropriate trade-offs between the water recovery, salt rejection, and yield obtained for optimizing the process based on a specific situation and requirements.

Examples 5 to 6

(62) In these examples, a second byproduct stream 9 similar to that obtained as a residue in Example 1 was further purified (third desalination step 135 or 130) by crystallization in the same falling film crystallization pilot plant (Example 5) or in a static crystallization pilot plant (Example 6). The third desalination step 135 in the falling film crystallization unit 7 was carried out with a sweating stage.

(63) These results demonstrate that either a falling film or a static crystallization may be used in treating the second byproduct stream from a falling film crystallization unit used in a second desalination step 120. It will generally be preferred to have the salt concentration of the second byproduct stream 9 from the falling film crystallization unit 7 in the range of about 14 to about 15 weight % so that the second desalination step 120 operates most economically. However higher salt concentrations of the second byproduct stream 9 will preferably use lower crystallization temperatures in the third desalination step (130 or 135), and thus the energy costs will be increased. It is noted that the product water obtained in these examples (third product water stream 12) has a quality sufficient for it to be fed into the feed stream of the falling film crystallization unit 7 used in the second desalination step 120.

Examples 7 to 8

(64) In these examples, the beneficial effect of including a sweating stage is demonstrated in Example 7 for the case of a second desalination step 120 in a falling film crystallization unit 7 and in Example 8 for the case of a third desalination step 130 in a static crystallization unit 10. The incorporation of a sweating stage results in a higher product purity and higher salt rejection; however, the water recovery and yield are somewhat reduced and larger equipment and a longer processing time is required. One skilled in the art will understand how to make trade-offs in these particular aspects in order to obtain an optimized result for a particular situation and requirements.

(65) While various embodiments have been set forth for the purpose of illustration, the foregoing descriptions should not be deemed to be a limitation on the scope herein. Accordingly, various modifications, adaptations, and alternatives can occur to one skilled in the art without departing from the spirit and scope herein.

(66) TABLE-US-00001 TABLE 1 Example 1 1st 2nd product 2nd byproduct water byproduct Parameter Units stream 6 stream 8 stream 9 Mass Percent (%) 6.11 4.45 14.78 Melting Point (? C.) ?3.24 ?2.46 ?9.66 Water Recovery (%) 84.17 Salt Rejection (%) 27.15 Yield (%) 85.43

(67) TABLE-US-00002 TABLE 2 Example 2 1st 2nd product 2nd byproduct water byproduct Parameter Units stream 6 stream 8 stream 9 Mass Percent (%) 4.45 3.25 9.53 Melting Point (? C.) ?2.46 ?1.88 ?5.20 Water Recovery (%) 81.86 Salt Rejection (%) 27.10 Yield (%) 81.82

(68) TABLE-US-00003 TABLE 3 Example 3 1st 2nd product 2nd byproduct water byproduct Parameter Units stream 6 stream 8 stream 9 Mass Percent (%) 3.37 1.79 9.79 Melting Point (? C.) ?1.94 ?1.10 ?5.38 Water Recovery (%) 71.79 Salt Rejection (%) 46.85 Yield (%) 81.57

(69) TABLE-US-00004 TABLE 4 Example 4 1st 2nd product 2nd byproduct water byproduct Parameter Units stream 6 stream 8 stream 9 Mass Percent (%) 2.64 1.23 6.28 Melting Point (? C.) ?1.57 ?0.76 ?3.32 Water Recovery (%) 63.48 Salt Rejection (%) 53.53 Yield (%) 73.04

(70) TABLE-US-00005 TABLE 5 Example 5 3rd product 2nd water 3rd byproduct stream byproduct Parameter Units stream 9 12 stream 13 Mass Percent (%) 14.20 11.38 21.75 Melting Point (? C.) ?9.09 ?6.58 ?17.51 Water (%) 69.72 Recovery Salt Rejection (%) 19.85 Yield (%) 75.21

(71) TABLE-US-00006 TABLE 6 Example 6 3rd product 2nd water 3rd byproduct stream byproduct Parameter Units stream 9 12 stream 13 Mass % (%) 14.78 5.94 22.80 Melt. Point (? C.) ?9.66 ?3.16 ?18.82 Water (%) 41.91 Recovery Salt Rejection (%) 59.83 Yield (%) 46.67

(72) TABLE-US-00007 TABLE 7 Example 7 Fract. Fract. Fract. Fract. Parameter Units Feed Prod. Residue 1 2 3 4 Mass (%) 6.05 3.37 14.54 13.04 9.35 7.80 6.44 Percent Melting (? C.) ?3.21 ?1.94 ?9.42 ?8.00 ?5.08 ?4.12 ?3.40 Point Water (%) 69.85 Recovery Salt (%) 44.28 Rejection Yield (%) 78.19

(73) TABLE-US-00008 TABLE 8 Example 8 Before Sweating After Sweating Parameters Units Feed Product Product Residue Mass (%) 13.00 10.19 5.25 20.89 Percent Melting (? C.) ?7.97 ?5.67 ?2.83 ?16.47 Point Water (%) 77.29 49.84 Recovery Salt (%) 21.63 61.10 Rejection Yield (%) 87.13 60.59