SEAWATER DESALINATION PLANT AND
PRODUCTION OF HIGH PURITY SALT
BACKGROUND OF THE INVENTION
Field of the Invention
[0001] This invention relates generally to desalination, salt production, and water
production. In particular, it relates to a process for converting a saltwater desalination
reject stream into pure salt, where this desalination process can be via thermal process
or via membrane process.
Description of Related A t
[0002] As is stated in U.S. Patent Application 12/345,856 filed on December 30,
2008, herein incorporated by reference, for centuries, common salt has been produced
by evaporative concentration of seawater or of another naturally occurring brine,
typically by using open-air evaporation lagoons or thermal concentration equipment and
processes. A number of modern industrial processes require salt of substantially high
purity, such as a sodium chloride salt substantially free of undesirable chemical or taste
components. Such high purity salt may be mined from some natural geological
formations, and may also be obtained from other saline waters by concentration and
treatment steps that remove the principal unwanted impurities present in a starting
solution.
[0003] Potable, high-quality or pure water has also historically been produced, when
fresh water is not available, from natural saline or brackish waters, originally by
thermal processes such as freezing or distillation, and more recently by membrane
processes such as reverse osmosis or membrane vapor permeation, and/or by hybrid
membrane/thermal processes. When starting with a saline feed, all of these water
production processes recover or purify only a fraction of the water present in the feed,
and generally produce waste brine that is substantially more concentrated than the
original feed stream.
[0004] One problem is that seawater and other natural saline waters contain many
solutes and impurities, so the salt-enriched side streams of a pure water production
process, i.e., the concentrated reject of a reverse osmosis water treatment, or the
residue of a distillation process, include other solids that both limit flux or treatment
rate and/or recovery of the water side and must be removed on the brine side if a high
quality salt is desired. These dissolved solids can be corrosive and scale forming in the
salt plant evaporators and crystallizers. Currently, chemicals are introduced on the
brine side to prevent or reduce the scale formation. These chemicals are costly and
reduce the salt purity.
[0005] Accordingly, a need exists for a non-chemical purification solution to reduce
or prevent scale formation in the salt plant.
SUMMARY OF THE INVENTION
[0006] The present invention concerns a saltwater desalination and salt plant. The
desalination plant has a feed stream and a reject stream. The desalination plant reject
stream is used as the salt plant feed stream. A filter selectively removes scaling species
from said salt plant feed stream. Additional equipment processes the filtered salt plant
feed stream into a high purity moist salt or slurry.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] These and other aspects of the invention will be understood from the
description and claims herein, taken together with the drawings showing details of
construction and illustrative embodiments, wherein:
[0008] Figure 1 schematically illustrates a system for the integrated production of
salt and water outputs in accordance with one embodiment of the present invention; and
[0009] Figure 2 is a water quality table showing representative concentrations of
components in a salt plant feed and the corresponding filter permeate streams calculated
for one representative plant.
DETAILED DESCRIPTION OF THE INVENTION
[0010] Approximating language, as used herein throughout the specification and
claims, may be applied to modify any quantitative representation that could permissibly
vary without resulting in a change in the basic function to which it is related.
Accordingly, a value modified by a term or terms, such as "about", is not limited to the
precise value specified. In at least some instances, the approximating language may
correspond to the precision of an instrument for measuring the value. Range limitations
may be combined and/or interchanged, and such ranges are identified and include all
the sub-ranges stated herein unless context or language indicates otherwise. Other than
in the operating examples or where otherwise indicated, all numbers or expressions
referring to quantities of ingredients, reaction conditions and the like, used in the
specification and the claims, are to be understood as modified in all instances by the
term "about".
[0011] "Optional" or "optionally" means that the subsequently described event or
circumstance may or may not occur, or that the subsequently identified material may or
may not be present, and that the description includes instances where the event or
circumstance occurs or where the material is present, and instances where the event or
circumstance does not occur or the material is not present.
[0012] As used herein, the terms "comprises", "comprising", "includes",
"including", "has", "having", or any other variation thereof, are intended to cover a
non-exclusive inclusion. For example, a process, method, article or apparatus that
comprises a list of elements is not necessarily limited to only those elements, but may
include other elements not expressly listed or inherent to such process, method, article,
or apparatus.
[0013] The singular forms "a", "an", and "the" include plural referents unless the
context clearly dictates otherwise.
[0014] The present invention discloses an integrated plant for the production of both
pure water and a salt or slurry product, operable to effectively provide salt of high
purity while reducing or preventing scale formation in evaporators and crystallizers in
the salt plant without the use of chemical purification.
[0015] Disclosed in Fig. 1 is a combination desalination and salt plant 10 comprised
of a desalination plant 20 and a salt plant 40. In this embodiment, desalination plant 20
is comprised of a seawater or brackish water feed 22, pure water permeate steam Al,
reject stream Bl, and an optional waste stream CI. The desalination plant is further
comprised of a pretreatment section 24, an optional nanofiltration (NF) section 26, and
a reverse osmosis (RO) section 28.
[0016] In operation, feed 22 passes through the pretreatment section 24 before
progressing to the optional NF section 26, The NF section 26 produces waste stream
CI and directs feed stream 22 into the RO section 28, RO section 28 produces a pure
water permeate stream Al and a reject stream Bl. Reject stream B becomes salt plant
feed stream 41. In some embodiments in which the NF section 26 is not present, feed
22 passes from the pretreatment section 24 into the RO section 28.
[0017] Pretreatment section 24 is of a known type (e.g., coarse screen, media filter,
ftoccu!ation and clarification, ultrafiltration, dual media, and/or other pretreatment
processes) and removes suspended solids and a substantial portion of organic matter.
[0018] Desalination plant 20 can be any standard saltwater reverse osmosis (SWRO)
desalination plant or any standard thermal Multiple Effect Distillation (MED)
desalination plant or any standard thermal Multi-Stage Flash (MSF) desalination plant
having a reject stream Bl, which desalinates water taken from a saltwater source, such
as an ocean, sea, or brackish body of water.
[0019] The salt plant 40 is comprised of a feed stream 41, a distillate stream A2, a
reject stream C2 and moist salt or slurry S. The salt plant 40 is further comprised of a
NF section 42, an optional concentrator section 43, an evaporator section 44, and a
crystallizer section 48. In some embodiments, optional concentrator section 43 is an
RO section.
[0020] In operation, feed 4 1 passes through NF section 42 and exits as permeate
before progressing to the optional concentrator section 43. The NF section 42 also
produces waste stream C2 and optional concentrator section 43 produces permeate
stream A3. After exiting optional concentrator section 43 as concentrate, feed 4 1
proceeds to evaporator section 44 and exits as concentrate before entering crystallizer
section 48. Evaporator section 44 also produces a distillate stream A2. Feed 4 exits
crystallizer section 48 as a moist salt or slurry S. In some embodiments in which
concentrator section 43 is not present, feed 4 1 passes from NF section 42 directly into
evaporator section 44.
[0021] In Fig. 1, NF section 42 selectively removes scaling species from feed 41.
Such scaling species can include hardness ions (e.g., calcium and magnesium) and
sulfates (e.g., native di- and polyvalent sulfur ions). NF section 42 effects a substantial
reduction in sulfates, removes bivalent ions while at least somewhat selectively passing
monovalents. NF section 42 operates at a relatively low feed pressure, and preferably
includes several stages so that NF section 42 permeate passed along feed 4 1 represents
from about 70% to about 80% or more of the feed volume, achieving high water
recovery. This NF section 42 permeate forms an intermediate permeate stream that
comprises feed 4 1 that is substantially free of scaling sulfate, relatively depleted of
bivalent ions, and rich in monovalent salts, primarily NaCl, with a total dissolved solids
(TDS) that is about 2/3 that of the feed. Preferably NF section 42 is comprised of a 2-
Pass Nanofiltration system. NF section 42 is constructed and operates in a similar
fashion as NF section 26 in U.S. Patent Application 12/345,856, which is incorporated
by reference above.
[0022] In one embodiment, NF section 42 rejects anions with more than one
negative charge, rejects cations depending on shape and size, rejects organic if
molecular weight is > 200-300 Daltons. The pore sizes are on the order of about
.0009-. 0085 microns. Further, the typical operating pressure of NF section 42 is about
70-400psig with a maximum operating pressure of about oOOpsig. The maximum
pressure drop of NF section 42 is 12psi over an element and 50psi per housing. The
typical operating flux of NF section 42 is about 8-20GFD.
[0023] Further, in some embodiments, NF section 42 can be operated at a reduced
pH, preferably at a pH of less than 7.0, to enhance calcium and magnesium removal.
Additionally, in some embodiments, NF section 42 rejects less than about 30% of
chloride, greater than about 90% of sulfate, greater than about 68% of calcium, and
greater than about 77% of magnesium average per pass.
[0024] Accordingly, in one aspect of the invention, NF section 42 eliminates the
requirement for a separate chemical purification stage to prevent scaling in equipment
downstream of NF section 42, such as optional membrane-based concentrator 43,
evaporator 44, and crystallizer 48. Further, NF section 42 allows salt to be
continuously recovered by crystallization at purity above 99% that meets an intended
purity standard (e.g., NaCl purity level and absence of critical contaminants) for chloralkali,
soda ash production or other user applications.
[0025] Further NF section 42 allows optional concentrator section 43 to operate on
the NF section 42 permeate at high recovery without scaling and with no antiscalant to
produce a pure water output and a substantially concentrated reject stream.
[0026] The optional RO section 43 can be a seawater RO, brackish water RO, or
other RO system. NF section 42 allows the RO section 43 to operate on NF section 42
permeate at high recovery without scaling and with no need for antiscalant, to produce
a pure water output and a substantially concentrated reject stream. By way of example,
a 2-pass NF section 42 may operate at from about 70% to about 80% recovery, and the
RO section 43 may include a third stage high pressure brine recovery stage to operate at
from about 70% to about 80% or more recovery on this NF permeate, giving an overall
recovery of from about 50% to about 70% or more in the salt plant 40.
[0027] Further, concentration of the RO section 43 reject stream may be by a
thermal process or other evaporator section 44. Evaporator section 44 can also include
a concentrator such as an evaporative brine concentrator, preferably a unit such as a
falling film evaporator, and may operate with a vapor recompressor unit for enhanced
energy efficiency and augmented water recovery. A mechanical vapor compression
unit may be used to enhance evaporative efficiency while recovering additional water in
this section. In one embodiment, evaporator section 44 is a mechanical vapor
compressor that produces additional pure water or distillate A2 while further
concentrating feed 41. By way of example, from about 70% to about 90% or more of
the water present in the RO section reject stream B that passes to the concentration/salt
production stage may be recovered as additional water.
[0028] In crystallizer section 48, the high purity salt product is crystallized and
additional pure water of distillate A4 is produced. The high purity salt may be
extracted as a moist salt or as a salt slurry from an evaporator/centrifuge loop in which
the stream temperate may be easily controlled, e.g., with mechanical vapor
recompression, to provide supersaturated salt solution and optimize sodium chloride
crystallization. Crystallizer section 48 may be driven by crystal seeding, allowing
efficient and continuous take-off of the salt output from a precipitation and
centrifiigation loop, and both the crystallization and purity of the product may be
enhanced by allowing a small periodic blowdown from the loop to keep remaining
unwanted species, such as potassium, below saturation in the crystal!izer section 48,
and below a level that might impair crystallization or product quality. For this, a purge
under about 1% of the initial brine feed volume or 3% of the crystallizer volume
suffices, resulting in a near zero-liquid discharge (ZLD) process from producing a
highly purified NaCl product.
[0029] Stated alternatively, in desalination plant 20, feed 22 passes downstream to
pretreatment section 24, which removes suspended solids and a substantia! portion of
organic matter from feed 22. Feed 22 then travels downstream into optional NF section
26. Feed 22 exits NF section 26 as permeate and concentrate exits NF section 26 as
waste stream CI. RO section 28 is located downstream of NF section 26 and receives
feed 22 from NF section 26 located upstream. Permeate exits RO section 28 as
permeate stream Al and concentrate exits RO section 28 as reject stream Bl.
Downstream, reject stream Bl exits desalination plant 20, becomes salt plant feed 41,
and enters salt plant 40.
[0030] After entering salt plant 40, feed 4 1 travels downstream into NF section 42.
Feed 4 exits NF section 42 as permeate and concentrate exits NF section 42 as reject
stream C2. Optional concentrator section 43 is located downstream of NF section 42
and receives feed 4 1 from NF section 42. Permeate exits concentrator section 43 as
permeate stream A3, and concentrate exits as feed 4 1 and travels downstream to
evaporator section 44. Evaporator section 44 receives feed 4 1 from concentrator
section 43 located upstream. Evaporator section 44 removes water from feed 4 1 and
sends feed 4 downstream to crystallizer section 48. The water removed from feed 4 1
by evaporator section 44 forms distillate stream A2. Crystallizer section 48 receives
feed 4 1 from evaporator section 44 located upstream. Crystallizer section 48 removes
water from feed 4 and outputs a moist salt or slurry S downstream. The water
removed from feed 4 by crystallizer section 48 forms distillate stream A4. It is
anticipated that some embodiments can include a dryer located downstream from
crystallizer section 48 to receive moist salt or slurry S and produce dry salt.
[0031] Turning to Fig 2 which shows some representative operating conditions for
salt plant 40 of Fig. 1, NF section 4 effectively removes sulfate and may greatly
reduce the level of calcium, magnesium, bicarbonate, or other components of the feed
41. Fig, 2 shows the concentrations of principal dissolved species in the feed 4 and
NF section 42 permeate streams. At least about 99% of the sulfate, about 90% of the
calcium, and about 95% of the magnesium from feed 4 1 are removed by NF section
42, Preferably, NF section 42 removes at least about 99,9% of the sulfate, about 95%
of the calcium, and about 98% of the magnesium. Further, NF section 42 passes at
least about 50% of the chloride in feed 41, Preferably, NF section 42 passes at least
about 70% of the chloride in feed 41. The NF membrane may be a membrane such as,
but not limited to, the ones commonly sold for sulfate removal by The Dow Chemical
Company (Midland, Michigan) and GE Osmonics (Minnetonka, Minnesota), SWNF
membranes from the Dow Chemical Company (Midland, Michigan) Filmtec line, DK
series or SeaSoft membranes from GE Osmonics (Minnetonka, Minnesota), and
seawater NF membranes from Toray (Poway, California). GE Osmonics membranes
may have a particularly high sulfate rejection that is relatively independent of feed
concentration. This allows use of two or more stages of NF to achieve high recovery.
[0032] Advantageously, the removal of a substantial portion of the calcium and
magnesium in NF section 42 greatly reduces the quantity of chemicals required in the
conventional purification stage of salt plant. Calculations show that for a desalination
plant producing 106,000 m3 of pure water per day or 854,000 tons per year of salt, the
chemical savings are substantial . Without NF section 42, the amount of NaOH and
Na>C0 to remove bivalent ions to (a) avoid scaling in crystallizers and (b) keep purge
from crystailizer (discussed further below) to a minimum would be 329,411 tons/yr
NaOH consumption and 92,927 tons/yr Na C0 3 consumption. The corresponding
figures calculated for a stream treated with NF section 42 as described herein are 0
tons/yr NaOH consumption and 0 tons/yr of Na C0 3 consumption, so that the
incremental chemical savings are 329,411 tons/yr of NaOH and 92,927 tons/yr of
Na C0 3. At a price of $0. 10/kg for NaOH and $0.25/kg for Na C0 , this translates
into annual savings of $32.94 million for NaOH and $23.23 million for Na C0 . In
addition to the direct chemical savings, by arranging that the purification step treats a
generally lower level of bivalent impurities, the stream that passes to the crystallizer
can be dependably processed with greatly decrease scaling propensity, and operated
with smaller volume, less frequent purges while assuring that the remaining impurities
do not reach a concentration that would interfere with crystallization or impair purity of
the salt product.
[0033] While this invention has been described in conjunction with the specific
embodiments described above, it is evident that many alternatives, combinations,
modifications and variations are apparent to those skilled in the art. Accordingly, the
preferred embodiments of this invention, as set forth above are intended to be
illustrative only, and not in a limiting sense. Various changes can be made without
departing from the spirit and scope of this invention. Therefore, the technical scope of
the present invention encompasses not only those embodiments described above, but
also all that fall within the scope of the appended claims.
[0034] This written description uses examples to disclose the invention, including the
best mode, and also to enable any person skilled in the art to practice the invention,
including making and using any devices or systems and performing any incorporated
processes. The patentable scope of the invention is defined by the claims, and may
include other examples that occur to those skilled in the art. These other examples are
intended to be within the scope of the claims if they have structural elements that do not
differ from the literal language of the claims, or if they include equivalent structural
elements with insubstantial differences from the literal language of the claims.

What is claimed is:
CLAIMS
1. A saltwater desalination and salt plant comprising:
a desalination plant having a feed stream and a reject stream;
a salt plant comprising:
a feed stream, said desalination plant reject stream forming at
least a portion of said salt plant feed stream;
a filter to selectively remove scaling species from said salt plant
feed stream to result in a filtered salt plant feed stream; and
equipment for processing said filtered salt plant feed stream into a
high purity moist salt or slurry.
2. The saltwater desalination and salt plant of claim 1 wherein said filter removes
at least about 99% of the sulfates from said salt plant feed stream and passes at least
about 50% of the chloride in said salt plant feed stream.
3. The saltwater desalination and salt plant of clai 2 wherein said filter removes
at least about 99.9% of the sulfates from said salt plant feed stream.
4. The saltwater desalination and salt plant of claim 3 wherein said filter removes
at least about 90% of the calcium and at least about 95% of the magnesium from said
salt plant feed stream.
5 . The saltwater desalination and salt plant of claim 4 wherein said filter removes
at least about 95% of the calcium and at least about 98% of the magnesium from said
salt plant feed stream.
6. The saltwater desalination and salt plant of claim 5 wherein said filter is a
nanofiliration section.
7. The saltwater desalination and salt plant of claim 6 wherein said nanofiltration
section is a 2-PASS nanofiltration system; wherein said nanofiltration section rejects
less than about 30% of chloride, greater than about 90% of sulfate, greater than about
68% of calcium, and greater than about 77% of magnesium average per pass.
8. The saltwater desalination and salt plant of claim 7 wherein said equipment for
processing said filtered feed stream into a moist salt or slurry comprises:
an evaporator section and a crystallizer section;
said evaporator section adapted to receive permeate from said
nanofiltration section;
said crystallizer section adapted to receive concentrate from said
evaporator section.
9. The saltwater desalination and salt plant of claim 8 wherein said salt plant can
include an optional concentrator section located between said nanofiltration section and
said evaporator section.
10. The saltwater desalination and sa t plant of claim 9 wherein said concentrator
section is a reverse osmosis section.
11. The saltwater desalination and salt plant of claim 8 wherein said desalination
plant further comprises:
a pretreatment section, and a reverse osmosis section;
said desalination plant feed stream adapted to pass through said
pretreatment section and into said reverse osmosis section;
said reverse osmosis section receiving said desalination plant feed stream
and produces a permeate stream and a reject stream, said reject stream
adapted for feeding into said desalination plant reject stream.
12. The saltwater desalination and salt plant of claim 8 wherein said desalination
plant further comprises:
a pretreatment section, a nanofiltration section, and a reverse osmosis
section;
said desalination plant feed stream adapted for passage through said
pretreatment section and for direction into said nanofiltration section;
said nanofiltration section produces a permeate stream and a reject
stream, said nanofiltration section permeate stream being directed into
said reverse osmosis section;
said reverse osmosis section produces a permeate stream and a reject
stream, said reverse osmosis reject stream defining said desalination
plant reject stream.
13. A method of producing salt comprising:
directing a saltwater desalination plant reject stream into a salt plant feed
stream;
selectively filtering said salt plant feed stream to remove scaling species;
and
processing said filtered feed stream into a high purity moist salt or
slurry.
14. The method of claim 3 wherein said selective filtering removes at least about
99% of the sulfates from said salt plant feed stream.
15. The method of claim 14 wherein said selective filtering removes at least about
99.9% of the sulfates from said salt plant feed stream.
16. The method of claim 15 wherein said selective filtering removes at least about
90% of the calcium and at least about 95% of the magnesium from said salt plant feed
stream.
17. The method of claim 16 wherein said selective filtering removes at least about
95% of the calcium and at least about 98% of the magnesium from said salt plant feed
stream.
18. The method of claim 17 wherein a nanofiltration section is used to selectively
filter said plant feed stream, said nanofiltration section can be operated at a reduced pH
to enhance calcium and magnesium removal.
19. The method of claim 18 wherein said nanofiltration section is a 2-PASS
nanofiltration system.
20. The method of claim 19 wherein said filtered feed stream is processed into a
moist salt or slurry by:
an evaporator section and a crystallizer section;
said evaporator section receives said filtered feed stream from said filter;
and
said crystallizer section receives said filtered feed stream from said
evaporator section.