Abstract:
A system and method for removing impurities to reconstitute a NaCl stream to a saturated solution salt solution and remove any impurities such as sodium bisulfate (NaHSO 3 ), sodium chlorate (NaClO 3 ) and sodium iodide (NaI) to improve brine quality from an electrolytic cell is disclosed, including an evaporation system connected to the electrolytic cell, a brine treatment system connected to the evaporation system and the electrolytic cell. A waste treatment system is connected to the evaporation system. The evaporation system includes a set of evaporators that concentrates the brine. Sodium chloride is precipitated from the set of evaporators to the brine treatment system. Impurities are precipitated from the set of evaporators. The brine treatment system includes a hydrocyclone and a centrifuge that separates sodium chloride from water. The sodium chloride is mixed with water to create a concentrated and purified brine.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 62/088,152, filed Dec. 5, 2014. The patent application identified above is incorporated herein by reference in its entirety to provide continuity of disclosure. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to the treatment of waste streams. In particular, the present invention relates to treating depleted brine from a chlor-alkali processing system. 
       BACKGROUND OF THE INVENTION 
       [0003]    Chlor-alkali systems and processes produce chlorine, sodium hydroxide (caustic soda) and other caustic alkali products. Typically, the process is conducted in an electrolytic membrane cell using a brine, which is an aqueous solution of sodium chloride. The brine is fed into the electrolytic cell, which includes an anode side and a cathode side separated by a membrane. A current is passed through the electrolytic cell. As a result, the sodium chloride brine splits into its constituent parts. The membrane allows sodium ions to pass through it to the cathode side, where it forms sodium hydroxide in a solution. The membrane allows only positive sodium ions to pass through to prevent the chlorine from mixing with the sodium hydroxide. The chloride ions are oxidized to chlorine gas at the anode. Hydrogen gas and hydroxide ions are formed at the cathode. After this process, the brine is depleted and cannot be used in the electrolytic cell. Therefore, the depleted brine must be treated or replaced with fresh brine in order for the membrane electrolytic cell to properly operate. Further, the depleted brine must be purified to remove impurities that may cause fouling of the membrane. 
         [0004]    The prior art has attempted to address the need for purified brine with limited success. For example, U.S. Pat. No. 4,169,773 to Lai, et al. discloses a system and method for the electrolytic production of alkali metal hydroxide and halide with acidification of part of a recirculating anolyte stream to remove halite. The system in Lai diverts a brine stream from a membrane cell to a reaction vessel for treatment. In the reaction vessel, concentrated hydrochloric acid (HCl) is added to the brine stream to minimize chlorine dioxide production. The treated stream is then irradiated with ultra violet light. The irradiated stream is passed through a scrubber before being reintroduced to the membrane cell. However, the system in Lai requires the use of hydrochloric acid and an irradiation step that increases costs and inefficiencies. 
         [0005]    U.S. Pat. No. 6,309,530 to Rutherford, et al. discloses a system and method for the concentration of depleted brine exiting a chlor-alkali membrane cell plant. The depleted brine flows from a membrane cell into a dechlorinator where chemicals, such as sodium carbonate and sodium hydroxide are added. The dechlorinated brine is fed into a concentrator system where water vapor is removed. The reconcentrated brine is then ready for use. However, like Lai, the system in Rutherford requires the addition of chemicals, thereby leading increased costs. 
         [0006]    Therefore, there is a need in art for a system and method for purifying brine that does not add chemicals to the brine and does not require costly compression and/or condensation steps. Thus, there is a need for a system and method of purifying depleted brine with minimal costs and steps to treat the depleted brine. 
       SUMMARY 
       [0007]    In a preferred embodiment, a system and method for concentrating and purifying depleted brine from an electrolytic cell is disclosed. The system includes a reconstitution/evaporation system connected to the electrolytic cell, a brine treatment system connected to the reconstitution/evaporation system and the electrolytic cell. A waste treatment system is connected to the reconstitution/evaporation system. 
         [0008]    The reconstitution/evaporation system further includes a set of effect evaporators that evaporates water vapor from the depleted brine to concentrate the brine. Sodium chloride is precipitated from the set of effect evaporators to the brine treatment system. Impurities such as sodium bisulfite (NaHSO 3 ), sodium chlorate (NaClO 3 ) and sodium iodide (NaI) are precipitated from the set of effect evaporators, to improve brine quality. 
         [0009]    The brine treatment system includes a hydrocyclone and a centrifuge that separates sodium chloride from water. The sodium chloride is mixed with water to create a concentrated and purified brine. The brine is stored for later use. 
         [0010]    The waste treatment system includes a waste treatment tank that receives waste water from the set of effect evaporators. The waste water includes the removed impurities such as sodium bisulfite (NaHSO 3 ), sodium chlorate (NaClO 3 ) and sodium iodide (NaI). Each of the pH level and the chlorine level of the waste water is adjusted to predetermined levels. The waste water is passed through a carbon filter to complete the treatment process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    In the detailed description presented below, reference is made to the accompanying drawings. 
           [0012]      FIG. 1  is a general schematic drawing of a system for treating depleted brine of a preferred embodiment. 
           [0013]      FIG. 2  is a schematic drawing of a system for treating depleted brine of a preferred embodiment. 
           [0014]      FIG. 3  is a schematic drawing of a reconstitution/evaporation system of a preferred embodiment. 
           [0015]      FIG. 4  is a schematic drawing of a brine treatment system of a preferred embodiment. 
           [0016]      FIG. 5  is a schematic drawing of a waste water treatment system of a preferred embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    Referring to  FIG. 1 , a general schematic of system  100  for treating depleted brine will be described. System  100  is connected electrolytic cell  101 . System  100  includes reconstitution/evaporation system  102  connected to electrolytic cell  101 , brine treatment system  103  connected to reconstitution/evaporation system  102  and to electrolytic cell  101 . Raw brine supply  106  is connected to brine treatment system  103 . Waste treatment system  104  is connected to reconstitution/evaporation system  102  for the output of waste  105 . 
         [0018]    Referring to  FIG. 2 , system  200  is connected to electrolytic cell  201  and forms a crystallizer which purifies depleted brine for reuse. Electrolytic cell  201  includes an anode side and a cathode side separated by a membrane. Each of water stream  202  and brine stream  203  is connected to the anode side and flows into electrolytic cell  201 . An electric current is supplied to electrolytic cell  201 . Each of catholyte stream  207  and hydrogen stream  205  is connected to and flows from the cathode side of electrolytic cell  201 . Each of lean brine stream  206  and chlorine stream  204  is connected to and flows from the anode side of electrolytic cell  201 . Lean brine stream  206  flows into brine evaporator feed tank  208  at a rate from approximately 2,800 gallons per minute (gpm) to approximately 3,200 gpm. 
         [0019]    System  200  includes brine evaporator feed tank  208 , which is connected to a set of effect evaporators. Brine evaporator feed tank  208  is connected first effect evaporator  210  with depleted brine line  209 . Depleted brine line  252  is connected to depleted brine line  209  and to salt dissolving tank  224 . Lean brine line  253  is connected to lean brine stream  206  and to salt dissolving tank  224 . First effect evaporator  210  is connected to steam supply  211  and to cooled steam line  243 . Second effect evaporator  212  is connected to first effect evaporator  210  with vapor line  228  and liquid line  229 . Second effect evaporator  212  is further connected to cooled vapor line  244 . Second effect evaporator  212  is connected to third effect evaporator  213  with vapor line  230  and liquid line  231 . Third effect evaporator  213  is connected to fourth effect evaporator  214  with vapor line  232  and to fifth effect evaporator  215  with liquid line  233 . Third effect evaporator  213  is further connected to cooled vapor line  245 . Fourth effect evaporator  214  is further connected to fifth effect evaporator  215  with vapor line  234  and liquid line  235 . Fourth effect evaporator is further connected to precipitates outlet  216 , cooled vapor line  247 , and to waste water treatment tank  217  with waste line  236 . Waste water treatment tank  217  is connected to treatment inlet  218  and to water outlet  219 . Waste water treatment tank  217  includes carbon filter  227  adjacent to water outlet  219 . 
         [0020]    Third effect evaporator  213  is further connected to hydrocyclone  220  with precipitate line  237 . Hydrocyclone  220  is connected to waste water outlet  221  to waste water treatment tank  217 . 
         [0021]    In one embodiment, hydrocyclone  220  is connected to centrifuge  222  with centrifuge line  238 . In this embodiment, centrifuge  222  is connected to waste water outlet  223  to waste water treatment tank  217  and to salt dissolving tank  224  with solids line  239 . 
         [0022]    In another embodiment without the centrifuge, hydrocyclone  220  is connected to salt dissolving tank  224 . 
         [0023]    Fifth effect evaporator  215  is further connected to salt dissolving tank  224  with solids line  240 , water vapor outlet  226 , and to cooled vapor line  246 . Salt dissolving tank  224  is further connected to raw brine tank  225  with brine line  241 . Feed line  249  is connected to raw brine tank  225 . Raw brine tank  225  is connected to brine treatment  251  with brine supply line  242 . Brine treatment  251  is connected to electrolytic cell  201  with brine stream  203 . 
         [0024]    In a preferred embodiment, each of first effect evaporator  210 , second effect evaporator  212 , third effect evaporator  213 , fourth effect evaporator  214 , and fifth effect evaporator  215  is a falling film effect evaporator. Other suitable evaporators known in the art may be employed. 
         [0025]    In one embodiment, any of first effect evaporator  210 , second effect evaporator  212 , third effect evaporator  213 , fourth effect evaporator  214 , and fifth effect evaporator  215  includes a recirculation line. 
         [0026]    In a preferred embodiment, hydrocyclone  220  is a stainless steel hydrocyclone manufactured by ChemIndustrial Systems, Inc. of Cedarburg, Wis. Other suitable hydrocyclone separators known in the art may be employed. 
         [0027]    In a preferred embodiment, centrifuge  222  is a disc-stack centrifuge. Other suitable centrifuges known in the art may be employed. 
         [0028]    In one embodiment, steam supply  211  is a multiple effect evaporator (MEE) steam driver. Any steam driver known in the art may be employed. 
         [0029]    In another embodiment, steam supply  211  is a mechanical vapor recompressor (MVR) power driver. Any power driver known in the art may be employed. 
         [0030]    In a preferred embodiment, brine treatment  251  is a clarifier to remove any sludge in the brine prior to introduction into electrolytic cell  201 . Other types of solids separation known in the art may be employed. 
         [0031]    It will be appreciated by those skilled in the art that any type of suitable piping means may be employed to connect the previously described vessels including the effect evaporators, tanks, hydrocyclone, centrifuge, and brine treatment. It will be further appreciated that any suitable directing devices including pumps, valves, sensors, controllers, supervisory control and data acquisition (“SCADA”) system and software may be employed to direct steam, a liquid stream, a vapor stream and/or solids into and/or out of any of the previously described vessels. 
         [0032]    In other embodiments, any number of holding tanks may be connected adjacent to any of effect evaporators  210 ,  212 ,  213 ,  214 , and  215 . 
         [0033]    In other embodiments, any number of heat exchangers may be connected any of effect evaporators  210 ,  212 ,  213 ,  214 , and  215  to provide additional heating or cooling to the effect evaporators. 
         [0034]    Referring to  FIG. 3 , a preferred use of reconstitution/evaporation system  300  will now be described. In a preferred embodiment, approximately 45% of the depleted brine stored in brine evaporator feed tank  301  is sent to first effect evaporator  303  as depleted brine streams  302  and  330  at a rate of approximately 2,800 gpm to approximately 3,200 gpm. Each of depleted brine streams  302  and  330  has a concentration level of approximately 2% NaCl and an iodine level of 0.15 parts per million (ppm). Each of the concentration levels of the liquid streams as used in this application is defined as a percentage of the total weight. Steam  304  is introduced into first effect evaporator  303  at a rate of approximately 21,000 pounds per hour (lbm/hr) to heat depleted brine stream  302  to an operating temperature of approximately 237° F. for a time of approximately 15 minutes. First effect evaporator  303  has an operating pressure of approximately 7 psig. Cooled steam  325  is produced by steam  304  heating depleted brine stream  302 . Cooled steam  325  is directed out of first effect evaporator  303  at a rate of approximately 21,000 lbm/hr. Water vapor  305  is evaporated from depleted brine stream  302  in first effect evaporator  303  to create liquid stream  306 . Water vapor  305  is directed from first effect evaporator  303  into second effect evaporator  307  at a rate from approximately 20,000 lbm/hr to approximately 22,000 lbm/hr. Water vapor  305  has a temperature in a range from approximately 240° F. to approximately 245° F. Liquid stream  306  is directed from first effect evaporator  303  into second effect evaporator  307  at a rate from approximately 2,800 gpm to approximately 3,200 gpm. Liquid stream  306  has a concentration level of approximately 12% NaCl and an iodine level of 0.22 ppm. 
         [0035]    In a preferred embodiment, the levels and concentrations are measured with a spectrophotometer, such as an Agilent Cary-60 UV-Vis spectrophotometer manufactured by Agilent Technologies. The iodine levels are measured in accordance with ASTM D3869-09 (Standard Test Methods for Iodide and Bromide Ions in Brackish Water, Seawater, and Brines by the International Association for Testing and Materials (ASTM International)) wherein a 25 milliliter (ml) brine sample undergoes buffering to a slightly acidic state, along with subsequent treatment with bromine solution, sodium formate, potassium iodide, and a starch indicator. The treated sample is then read on the spectrophotometer, at a wavelength of 570 nanometers (nm), to determine concentration. 
         [0036]    In one embodiment, cooled steam  325  is reheated and recirculated as steam  304 . Any heating and recirculating means known in the art may be employed. 
         [0037]    Water vapor  305  heats liquid stream  306  in second effect evaporator  307  to an operating temperature of approximately 212° F. for a time of approximately 15 minutes. Cooled water vapor  326  is produced from water vapor  305  heating liquid stream  306 . Cooled water vapor  326  is directed out of second effect evaporator  307  at a rate from approximately 20,000 lbm/hr to approximately 22,000 lbm/hr. Second effect evaporator  307  has an operating pressure of approximately 0 psig. Water vapor  309  is evaporated from liquid stream  306  in second effect evaporator  307  to create liquid stream  308 . Water vapor  309  is directed from second effect evaporator  307  into third effect evaporator  310  at a rate from approximately 20,000 lbm/hr to approximately 22,000 lbm/hr. Water vapor  309  has a temperature in a range from approximately 240° F. to 245° F. Liquid stream  308  is directed from second effect evaporator  307  into third effect evaporator  310  at a rate from approximately 2,800 gpm to approximately 3,200 gpm. Liquid stream  308  has a concentration level of approximately 17% NaCl. 
         [0038]    Water vapor  309  heats liquid stream  308  in third effect evaporator  310  to an operating temperature of approximately 212° F. for a time of approximately 15 minutes. Cooled water vapor  327  is produced from water vapor  309  heating liquid stream  308 . Cooled water vapor  327  is directed out of third effect evaporator  310  at a rate from approximately 20,000 lbm/hr to approximately 22,000 lbm/hr. Third effect evaporator  310  has an operating pressure of approximately 0 psig. Water vapor  319  is evaporated from liquid stream  308  in third effect evaporator  310  to create liquid stream  312 . Liquid stream  312  has a concentration level of NaCl above saturation. Sodium chloride  311  is precipitated from third effect evaporator  310  at a rate from approximately 8,000 lbm/hr to approximately 9,000 lbm/hr into brine treatment system  323 , as will be further described below. The concentration level of liquid stream  312  is reduced to approximately 21% NaCl. 
         [0039]    Water vapor  319  is directed from third effect evaporator  310  into fourth effect evaporator  314  at a rate from approximately 20,000 lbm/hr to approximately 22,000 lbm/hr. Water vapor  319  has a temperature in a range from approximately 240° F. to approximately 245° F. Liquid stream  312  is directed from third effect evaporator  310  into fifth effect evaporator  313  at a rate from approximately 2,800 gpm to approximately 3,200 gpm. 
         [0040]    Water vapor  315  from fourth effect evaporator  314  is directed into fifth effect evaporator  313  at a rate from approximately 20,000 lbm/hr to approximately 22,000 lbm/hr to heat liquid stream  312  in fifth effect evaporator  313  to an operating temperature of approximately 110° F. for a time of approximately 15 minutes. Cooled water vapor  328  is produced from water vapor  315  heating liquid stream  312 . Cooled water vapor  328  is directed out of fifth effect evaporator  313  at a rate from approximately 20,000 lbm/hr to approximately 22,000 lbm/hr. Fifth effect evaporator  313  has an operating pressure of approximately −14 psig. Water vapor  317  is evaporated from liquid stream  312  in fifth effect evaporator  313  to create liquid stream  316 . Liquid stream  316  has a concentration level of NaCl above saturation. Sodium chloride  318  is precipitated from fifth effect evaporator  313  at a rate from approximately 8,000 lbm/hr to approximately 9,000 lbm/hr into brine treatment system  323 , as will be further described below. The concentration level of liquid stream  316  is reduced to approximately 21% NaCl. 
         [0041]    Water vapor  317  is directed out of fifth effect evaporator  313  at a rate from approximately 20,000 lbm/hr to approximately 22,000 lbm/hr. Liquid stream  316  is directed from fifth effect evaporator  313  into fourth effect evaporator  314  at a rate from approximately 2,800 gpm to approximately 3,200 gpm. 
         [0042]    Water vapor  319  from third effect evaporator  310  is directed into fourth effect evaporator  314  at a rate from approximately 20,000 lbm/hr to approximately 22,000 lbm/hr and heats liquid stream  316  in fourth effect evaporator  314  to an operating temperature of approximately 212° F. for a time of approximately 15 minutes. Cooled water vapor  329  is produced from water vapor  319  heating liquid stream  316 . Cooled water vapor  329  is directed out of fourth effect evaporator  314  at rate from approximately 20,000 lbm/hr to approximately 22,000 lbm/hr. Fourth effect evaporator  314  has an operating pressure of approximately 0 psig. Water vapor  315  is evaporated from liquid stream  316  in fourth effect evaporator  314  to form waste water  322 . Waste water  322  has a concentration level of NaCl above saturation. Sodium sulfate (NaSO 4 )  320  and sodium chloride  321  precipitate from waste stream  322  in fourth effect evaporator  314  at a rate of approximately 8,000 lbm/hr. Waste water  322  then has a concentration level of approximately 19% NaCl. Waste water  322  is directed from fourth effect evaporator  314  at a rate of approximately 3,000 gpm into waste treatment system  324 , as will be further described below. 
         [0043]    In one embodiment, cooled water vapors  326 ,  327 ,  328 , and  329  are combined with water vapor  317  and condensed for use as a water supply. Any type of condensation means known in the art may be employed. 
         [0044]    Referring to  FIG. 4 , brine treatment system  400  will now be further described. Sodium chloride  401  is directed from third effect evaporator  413  into hydrocyclone  402  at a rate from approximately 8,000 lbm/hr to approximately 9,000 lbm/hr. Hydrocyclone  402  has a feed pressure of approximately 50 psi and a split ratio of 80/20, heavies (solids) to lights (liquids). Hydrocyclone  402  spins sodium chloride  401  to separate waste water  403  from sodium chloride  404 . 
         [0045]    In one embodiment, sodium chloride  404  is directed from hydrocyclone  402  into centrifuge  405  at a rate from approximately 8,000 lbm/hr to approximately 9,000 lbm/hr. Centrifuge  405  spins sodium chloride  404  to further separate sodium chloride  407  from waste water  406 . Centrifuge  405  spins at a rate of approximately 10,000 rpm. Sodium chloride  407  is now in a purified crystalized form. 
         [0046]    In another embodiment, sodium chloride  404  is directed into salt dissolving tank  408  at a rate from approximately 8,000 lbm/hr to approximately 9,000 lbm/hr. 
         [0047]    Sodium chloride  407  is directed from centrifuge  405  into salt dissolving tank  408  at a rate from approximately 8,000 lbm/hr to approximately 9,000 lbm/hr. Sodium chloride  409  is directed from fifth effect evaporator  414  into salt dissolving tank  408  at a rate from approximately 8,000 lbm/hr to approximately 9,000 lbm/hr. Lean brine  416  having a concentration level of approximately 2% NaCl is directed into salt dissolving tank  408  at a rate from approximately 2,800 gpm to approximately 3,200 gpm. Depleted brine  422  having a concentration level of approximately 2% NaCl is directed into salt dissolving tank  408  at a rate from approximately 2,800 gpm to approximately 3,200 gpm. Sodium chloride  407  is mixed with sodium chloride  409 , depleted brine  422 , and lean brine  416  in salt dissolving tank  408  for a time of approximately 15 minutes to form concentrated brine  410 . Concentrated brine  410  has a concentration level of approximately 21% NaCl. Concentrated brine  410  is sent from salt dissolving tank  408  into raw brine tank  411  for storage. Raw brine  419  having a concentration level of approximately 21% NaCl is directed into raw brine tank  411  at a rate from approximately 2,800 gpm to approximately 3,200 gpm. Untreated brine  420  having a concentration level of approximately 21% NaCl is directed from raw brine tank  411  to brine treatment  417  at a rate from approximately 2,800 gpm to approximately 3,200 gpm to remove sludge. Brine supply  412  having a concentration level of approximately 21% NaCl is directed into electrolytic cell  415 , which is connected to reconstitution/evaporation system  418 , at a rate from approximately 2,800 gpm to approximately 3,200 gpm. 
         [0048]    Referring to  FIG. 5 , waste treatment system  500  will now be further described. Waste water  502  is directed from a fourth effect evaporator into waste water treatment tank  501  at a rate of approximately 3,000 gpm. Waste water  502  includes sodium chlorate (NaClO 3 ) and sodium iodide (NaI) having concentration levels of approximately 5% and 7%, respectively. In waste water treatment tank  501 , the pH level of waste water  502  is adjusted to be in a pH range of approximately 6 to 7. Any suitable means for adjusting the pH level known in the art may be employed. A chlorine level of waste water  502  is measured. If the chlorine level exceeds 4 ppm, then sodium bisulfite (NaHSO 3 )  503  is added to waste water  502  to lower the chlorine level. Waste water  502  is passed through a carbon filter to form treated water  504 , which is sent to a plant outfall at a rate of 3,000 gpm. 
         [0049]    It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept. It is understood, therefore, that this disclosure is not limited to the particular embodiments herein, but it is intended to cover modifications within the spirit and scope of the present disclosure as defined by the appended claims.