Patent Application
20140299546
1. Technical Field
The present invention pertains to nanofiltration processes and systems for recovering brine and for removing sulfate impurity from a brine stream in the industrial processing of chemicals. In particular, it pertains to nanofiltration of brine streams in brine electrolysis processing.
2. Description of the Related Art
Pressure driven membrane separation processes are known wherein organic molecules or inorganic ionic solutes in aqueous solutions are concentrated or separated to various degrees by the application of a positive osmotic pressure to one side of a filtration membrane. Examples of such processes are reverse osmosis (RO), ultrafiltration (UF) and nanofiltration (NF). These pressure driven membrane processes employ a cross-flow mode of operation wherein only a portion of a feed stream solution is collected as a permeate solution and the rest is collected as a pass solution. Thus, in a nanofiltration module, the exiting process stream which has not passed through the nanofiltration membrane is referred to as the “pass stream” and the exiting process stream which has passed through the membrane is referred to as the “permeate” stream.
NF membranes are structurally similar to RO membranes in that chemically they typically are crosslinked aromatic polyamides, which are cast as a thin “skin layer” on top of a microporous polymer sheet support to form a composite membrane structure. The separation properties of the membrane are controlled by the pore size and electrical charge of the “skin layer”. Such a membrane structure is usually referred to as a thin film composite (TFC). However, unlike RO membranes, the NF membranes are characterized in having a larger pore size in its “skin layer” and a net negative electrical charge inside the individual pores. This negative charge is responsible for rejection of anionic species, according to the anion surface charge density. Accordingly, divalent anions, such as SO42-, are more strongly rejected than monovalent ones, such as Cl-. And therefore, nanofiltration can be particularly suitable for processes requiring separation of divalent anions from monovalent anions.
Commercial NF membranes are available from known suppliers of RO and other pressure driven membranes. The NF membranes are, typically, packaged as membrane modules. A so-called “spiral wound” module is most popular, but other membrane module configurations, such as tubular membranes enclosed in a shell or plate-and-frame type, are also known.
During the NF process, a minimum pressure equal to the osmotic pressure difference between the feed/pass liquor on one side and the permeate liquor on the other side of the membrane must be applied since osmotic pressure is a function of the ionic strengths of the two streams. In the case of separation of a multivalent solute, such as Na2SO4, from a monovalent solute, such as NaCl, the osmotic pressure difference is moderated by the low NaCl rejection. Usually, a pressure in excess of the osmotic pressure difference is employed to achieve practical permeate flux.
Industrial brine electrolysis processing plants (e.g., chloralkali or chlorate plants) may advantageously use nanofiltration in certain of the processing steps, and particularly in the removal of sulfate from the brine streams employed. Various products are produced using brine as the starting material. For instance, sodium chlorate is generally prepared by the electrolysis of sodium chloride brine to produce chlorine, sodium hydroxide and hydrogen. The chlorine and sodium hydroxide are immediately reacted to form sodium hypochlorite, which is then converted to chlorate and chloride under controlled conditions of pH and temperature. Alternatively, chlorine and caustic soda are prepared by electrolysis of sodium chloride brine in an electrolytic cell or electrolyser, which contains a membrane to prevent chlorine and caustic soda reacting.
However, the sodium chloride salt used to prepare the brine for electrolysis generally contains impurities which, depending on the nature of the impurity and production techniques employed, can give rise to plant operational problems familiar to those skilled in the art. While the means of controlling these impurities are varied, they include purging them out of the system into alternative processes or to the drain, precipitation by conversion to insoluble salts, and/or crystallization or ion exchange treatment. Further, control of anionic impurities presents more complex problems than that of cationic impurities.
Sulfate ion (also referred to herein as sulfate) is a common impurity in commercial salt and, being an anion, is a more complex impurity to deal with. When such salt is used directly, or in the form of a brine solution, and specific steps are not taken to remove the sulfate, the sulfate enters the electrolytic system. Sulfate ion maintains its identity under the conditions in the electrolytic system and, thus, accumulates and progressively increases in concentration in the system unless removed in some manner. In chlorate plants producing a liquor product, the sulfate ion will leave with the product liquor. In plants producing only crystalline chlorate, the sulfate remains in the mother liquor after the crystallization of the chlorate, and is recycled to the cells. Over time, the concentration of sulfate ion will increase and adversely affect electrolysis and cause operational problems due to localized precipitation in the electrolytic cells. Within the chloralkali circuit, the sodium sulfate will concentrate and adversely effect the membrane, which divides the anolyte (brine) from the catholyte (caustic soda). It is industrially desirable that sodium sulfate levels in concentrated brine (e.g., 300 g/L NaCl) be reduced to at least 20 g/L in chlorate production and about 10 g/L in chloralkali production.
Some years ago, it was found that NF membranes showed unexpected ion membrane selectivity at relatively high salt concentrations and this offered attractive applications in the treatment of brine electrolysis liquors having sodium sulfate levels unacceptable in recycle systems. U.S. Pat. No. 5,587,083 and U.S. Pat. No. 5,858,240 disclosed use of nanofiltration systems in the application of sulfate removal from spent electrolysis brine. When using these nanofiltration processes, because there was no buildup in concentration of sodium chloride in the pass liquor stream over its original level in the feed stream, it was possible to increase the content of sodium sulfate in the pass liquor to a higher level than would have been possible if the NaCl level of the pass liquor has increased. It was now possible to realize a desirable high % recovery and, in the case of electrolysis brine, to minimize the volume of brine purge, and/or the size of a reactor and the amount of chemicals for an, optional, subsequent sulfate precipitation step.
For certain reasons, various modifications have been proposed. For instance, US2008/0056981 discloses a method for at least partially removing soluble divalent anions from an aqueous divalent anion-containing brine solution comprising a crystal growth inhibitor (CGI) for the divalent anion. The method comprises the process steps: obtaining a sodium chloride concentration between 100 g/L and saturation in the presence or absence of a CGI for sodium chloride or a sodium chloride concentration above saturation in the presence of a CGI for sodium chloride, and acidifying the solution to a pH below 11.5; subjecting the solution to a membrane filtration step thereby separating the brine solution into a brine stream being supersaturated for the divalent anion (concentrate), and a brine stream being undersaturated for the divalent anion (permeate); subjecting the supersaturated brine stream comprising the crystal growth inhibitor for the divalent anion to a crystallization process, removing crystallized divalent anion; and optionally, recycling the overflow of the crystallizer to the brine solution for subjecting it again to the membrane filtration step.
Nanofiltration techniques have also been suggested for use in completely different industrial processes. For instance, U.S. Pat. No. 7,314,606 discloses a process for recovering sodium thiocyanate and separating impurities from industrial process solutions comprising sodium thiocyanate using nanofiltration techniques.
There still remains, however, a need for ever greater efficiency in the sulfate removal process and for recovery of useful brine from brine streams in industrial chemical processing settings, and particularly from spent brine streams in brine electrolysis processing.
The present invention provides for desirable sulfate removal from process brine streams while recovering more of the brine salt for reprocessing. In brine electrolysis processing, this can result in substantial savings of valuable raw material and reduction in waste. Further, in some brine electrolysis plants, pure water is itself a valuable raw material that must be provided as an input. In such plants, the present invention can also provide for greater recovery of water which meets the purity requirements for the process. And further still, the brine streams in typical brine electrolysis plants often contain sodium chlorate and/or bromate which are also therefore present to some extent in the effluent sulfate stream. However, there are increasingly restrictive environmental limits on chlorate and bromate in effluent streams. Advantageously, the present invention also results in a reduction of these species in the effluent sulfate stream.
Specifically, a process and system are provided for recovering brine and for removing sulfate impurity from a brine stream in a nanofiltration system. The brine stream here comprises an aqueous solution of NaCl, and the system comprises a nanofiltration module. The nanofiltration module comprises a nanofiltration membrane for rejecting sulfate, an inlet for a feed stream, an outlet for a permeate stream which has permeated through the membrane, and an outlet for a pass stream which has not permeated through the membrane. The nanofiltration membrane may be any of those conventional membranes suitable for rejecting sulfate. A dilution stream is introduced upstream of the feed stream inlet of the module, and thereby dilutes the feed stream at the feed inlet of the module and increases the amount of NaCl and water in the permeate stream at the permeate outlet of the module without substantially diluting the concentration of sulfate in the pass stream at the pass outlet of the module. And so, albeit more diluted, there is more brine salt present in total in the permeate stream which can be recovered. Of further advantage is that more water of sufficient purity for use in the brine electrolysis process can also be recovered. The rejected pass stream on the other hand has a much reduced concentration of brine salt while roughly maintaining the same concentration of sulfate.
The brine stream may comprise additional species such as NaClO3 and/or NaBrO3, which can also be present in the effluent rejected pass stream. Introducing a dilution stream in accordance with the invention also reduces the concentration of chlorate and/or bromate in the effluent pass stream and thus offers environmental advantages as well. Instead, an increased amount of chlorate and/or bromate is returned in the volume of permeate.
The dilution stream can desirably be water or any suitable, compatible liquid, such as a very dilute brine. (Depending on the processing and systems involved, very dilute brine streams may possibly be derived from elsewhere in a specific plant.) An additional advantage of diluting the feed brine stream is that a more neutral, desirable stream pH (i.e., pH from about 5 to 9) can be obtained.
A preferred nanofiltration system may be a multi-stage system comprising at least a first nanofiltration module and a second nanofiltration module in series. A greater number of nanofiltration modules in series may be contemplated depending on the specific circumstances. Each module in the system may comprise a nanofiltration membrane for rejecting sulfate, a feed stream inlet, a permeate stream outlet, and a pass stream outlet, wherein the pass stream outlet of the first module is connected to the feed stream inlet of the second module.
Introducing the dilution stream between the pass stream outlet of the first module and the feed stream inlet of the second module in such a series arrangement allows for more efficient use of the volume of the dilution stream. Desirable results can be obtained for instance for a volumetric flow rate ratio of the total dilution streams to that of the brine stream of less than or about 12:70. And with a sufficient number of module and dilution stream stages in series, the brine and other content in the process pass stream may be reduced enough to purify the sulfate in the pass stream to a commercial grade and thus become a valuable by-product as opposed to a waste.
As demonstrated in the following Examples, an upstream brine stream comprising greater than or about 200 g/L NaCl and less than about 10 g/L Na2SO4 can be subjected to the nanofiltration process to produce a pass stream comprising less than or about 50 g/L NaCl and greater than about 50 g/L Na2SO4.
The nanofiltration system may be particularly employed to remove sulfate impurity and recover substantial brine from the spent brine stream or product liquor coming from the electrolysers used in industrial brine electrolysis chemical processing. A related brine electrolysis plant, such as a chloralkali or chlorate plant, can thus comprise an electrolyser and the nanofiltration system in which the spent brine outlet of the electrolyser is connected to the feed stream inlet of the nanofiltration module.
In a brine electrolysis chemical processing plant in which an initial NaCl brine is prepared in a saturator from a concentrated supply of salt (e.g., crystalline salt), a spent brine stream from an electrolyser may be fed to the nanofiltration system of the invention and the permeate stream recycled directly to the saturator. In a brine electrolysis chemical processing plant in which crystalline salt is not a salt source (e.g., well brine as the salt source), the spent brine stream may first need to be concentrated in an evaporator (which concentrates both the NaCl brine salt and the sulfate in the stream). Sulfate impurity may then be removed by feeding a portion of the now-concentrated spent brine stream to the nanofiltration system, recycling the permeate stream to the evaporator, and rejecting the sulfate-containing pass stream. In a further embodiment, the nanofiltration system of the invention may be employed upstream of an electrolyser in a brine electrolysis chemical processing plant, that is wherein the brine inlet of the electrolyser is connected to the permeate stream outlet of the nanofiltration module.
In certain embodiments, the dilution stream can be introduced at a temperature substantially lower than that of the upstream brine stream. If the dilution stream is introduced in a sufficient amount at a sufficiently low temperature, a heat exchanger for cooling the upstream brine stream may not be required. The system may thus be absent the heat exchanger which may be employed for cooling the upstream brine stream in conventional systems.
Unless the context requires otherwise, throughout this specification and claims, the words “comprise”, “comprising” and the like are to be construed in an open, inclusive sense. The words “a”, “an”, and the like are to be considered as meaning at least one and not limited to just one. Further, in a numerical context, the word “about” is to be construed as meaning plus or minus 10%.
An exemplary industrial brine electrolysis plant is a chloralkali plant. Such plants are commonly found throughout the world. A simplified schematic for a prior art chloralkali plant 10 is shown in
As mentioned previously, sodium sulfate impurity (typically appearing in the process salt provided) undesirably increases in concentration in the recycling anolyte unless it is continually removed. In the chloralkali plant of
Series modules 21, 22, and 23 comprise nanofiltration membranes suitable for rejecting sulfate, for instance single spiral wound type nanofiltration units. Modules 21, 22, 23 comprise feed stream inlets 21a, 22a, 23a, permeate stream outlets 21b, 22b, 23b and pass stream outlets 21c, 22c, and 23c respectively. The modules are connected in series by connecting the pass stream outlet from an upstream module to the feed stream inlet of the adjacent module downstream (e.g., pass stream outlet 21c is connected to feed stream inlet 22a). Process anolyte brine 26 comprising NaCl brine salt and Na2SO4 impurity is thus concentrated in sulfate in stages in the pass streams from the nanofiltration modules while the brine salt concentration in both the pass streams and the permeate streams is only slightly reduced. The pass stream from final module 23 in the series is rejected at system pass outlet 20c. The several permeate streams from outlets 21b, 22b, and 23b may be combined into a single resultant purified brine stream which is directed back to the recycle brine line from output 20b. (Again, not shown in simplified
While nanofiltration systems such as that shown in
Modules 31, 32, 33 comprise feed stream inlets 31a, 32a, 33a, permeate stream outlets 31b, 32b, 33b and pass stream outlets 31c, 32c, and 33c respectively. Again, the modules are connected in series by connecting the pass stream outlet from an upstream module to the feed stream inlet of the adjacent module downstream (e.g., pass stream outlet 31c is connected to feed stream inlet 32a).
In the embodiment of
Although an objective of increased brine recovery can be obtained by introducing a dilution stream at any of the locations indicated, introducing the dilution stream after the first module in the series (i.e., at 35b between the pass stream outlet of the first module and the feed stream inlet of the second module, or for instance at 35c) allows for more efficient use of the volume of the dilution stream. As illustrated in the following Examples, desirable results can be obtained for example in a system comprising three nanofiltration modules in series and employing two such dilutions for a volumetric flow rate ratio of the total dilution streams to that of the brine stream of less than or about 12:70.
Preferably the volume of fluid making up the dilution stream is obtained from elsewhere in the overall chloralkali plant 10. In this way, little or no additional process fluid needs to be provided to the system overall. For instance, in the industrial chloralkali plant of
The present approach can offer other advantages along with the recovery of brine salt and efficiency. Introducing a dilution stream can be generally advantageous in that the brine stream feed to a given nanofiltration module is at a more desirable stream pH.
Further, the pass stream comprising the rejected concentrated sulfate is often considered a waste product that may be discharged to sewer. However, with a sufficient number of module and dilution stream stages in series, the brine and other content in the process pass stream may be reduced enough to purify the sulfate in the pass stream to that of commercial grade for industrial purposes and thus become a valuable by-product instead of a waste.
Still further, if water or other dilution stream volume is available in a sufficient amount at a sufficiently low temperature, the heat exchanger often employed in conventional sulfate removal systems (e.g., heat exchanger 25 in
As in prior art systems, the several permeate streams from outlets 31b, 32b, and 33b in
The preceding discusses a chloralkali chemical processing plant in which the initial process brine is prepared in a saturator and the permeate stream from the nanofiltration system is recycled directly to the saturator. However, another conventional configuration for the preceding chloralkali process (not shown) uses well brine as the initial brine supply as opposed to concentrated crystal brine. In this alternative conventional process, the spent anolyte stream leaving the electrolyser needs to be concentrated in order to be recycled since there is no crystalline salt supply available to mix therewith. In this system then, an evaporator subsystem is typically employed to concentrate the spent anolyte brine stream and this will obviously concentrate any Na2SO4 present in the stream as well. It may be desirable then to employ a nanofiltration system of the invention in the evaporator subsystem instead in order to remove sulfate from the stream.
The following Examples have been included to illustrate certain aspects of the invention but should not be construed as limiting in any way. Those skilled in the art can be expected to appreciate how to modify the nanofiltration system and process according to the specifics of a given industrial application for brine recovery and sulfate removal.
Calculated models were obtained for purposes of comparing the characteristics expected of an exemplary nanofiltration system in which two dilution streams were introduced in accordance with the invention to those expected of the same nanofiltration system but without dilution streams (i.e., a conventional system). In both cases, it was assumed that the systems were provided with a spent brine stream composition from a typical chloralkali electrolysis plant. This brine stream contained 200 g/L NaCl and 10 g/L Na2SO4 and was supplied at a flow rate of 70 m3/hr, a temperature of 75° C., and a pressure of 40 bar.
The modeled nanofiltration system 40 comprised three nanofiltration modules 41, 42, 43 in series as depicted in
The calculated flow rates and concentrations of the involved species at various locations throughout system 40 are given on
The same calculations were then performed on a system similar to that of
As is evident from the models in
All of the above U.S. patents, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification, are incorporated herein by reference in their entirety.
While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the foregoing teachings. Such modifications are to be considered within the purview and scope of the claims appended hereto.
