WO2014071610A1

WO2014071610A1 - Electrodialysis based brine treatment

Info

Publication number: WO2014071610A1
Application number: PCT/CN2012/084383
Authority: WO
Inventors: John H. Barber; Neil Edwin Moe; Rihua Xiong
Original assignee: General Electric Company
Priority date: 2012-11-09
Filing date: 2012-11-09
Publication date: 2014-05-15

Abstract

Brine is treated with an electrical separation unit, for example an electrodialysis reversal unit. Antiscalant is added to the concentrate side of the electrical separation unit. Some of the concentrate flows through a precipitation unit. Antiscalant is deactivated in the concentrate flowing through the precipitation unit. A solid-liquid separation unit and an anti-scalant deactivation unit may be located in a side stream loop off of a concentrate recirculation loop of the electrical separation unit. A desalination unit upstream of the electrical separation unit may produce brine for the electrical separation unit.

Description

ELECTRODIALYSIS BASED BRINE TREATMENT

FIELD

[0001] This specification relates generally to methods and systems for treating water using electrodialysis.

BACKGROUND

[0002] Obtaining high water recovery from an electrodialyis unit, particularly when desalinating seawater or hard brackish water, requires operating the concentrate side of the electrodialysis unit at high concentrations of calcium carbonate and calcium sulfate. The solubility of calcium carbonate increases at low pH and acids may be used to prevent calcium carbonate scaling. Calcium sulfate solubility, however, is mainly independent of pH.

[0003] In one experiment, reported in Elyanow, D., et. al., (1980) Parametric tests of an electrodialysis reversal system with aliphatic anion membranes, an electrodialysis reversal (EDR) unit was operated at a calcium sulfate saturation index of 6.9 using sodium hexametaphosphate antiscalant and acid addition.

[0004] International Publication Number WO 201 1/106151 A1 describes a water treatment device having a pressure driven membrane desalination unit, an electrical separation unit, and a precipitation unit. The electrical separation unit removes salinity from the membrane reject. The precipitation unit removes calcium sulfate from the concentrate side of the electrical separation unit. The precipitation unit is seeded to increase the rate of calcium sulfate precipitation and reduces the calcium sulfate saturation index on the concentrate side of the electrical separation unit to 1.5 or less. In an example, an acid injection unit is used to reduce alkalinity in a synthetic nanofiltration membrane reject. The reject is sent to an electrodialysis reversal (EDR) unit. A gypsum seeded precipitation unit is placed in a concentrate recycle loop of the EDR unit. A cartridge filter is used to filter clarified water from the precipitation unit before returning the clarified water to the EDR unit.

INTRODUCTION

[0005] A system is described in this specification having an electrical separation unit and a solid-liquid separation unit. The system also includes an antiscalant injection system and an antiscalant de-activation system. Optionally, the solid-liquid separation unit and antiscalant deactivation unit are provided in a side stream loop to a concentrate recirculation loop of the electrical separation unit. Optionally, the system may further include a desalination unit upstream of the electrical separation unit, wherein the electrical separation unit receives brine from the desalination unit.

[0006] A process is described in this specification having a step of treating brine by electrical separation to produce a concentrate. An antiscalant is added to the concentrate. Solid salts are formed and removed from at least a portion of the concentrate. The antiscalant is de-activated in the concentrate subject to the salt removal step. The process may include an upstream step of producing the brine. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 is a schematic process flow diagram for a water treatment system.

DETAILED DESCRIPTION

[0008] Figure 1 shows a water treatment system 10 having a desalination unit 12, an electrical separation 14, and a precipitation unit 16. A feed stream 18 of water to be treated flows into the desalination unit 12 and is separated into a brine stream 20 and a product stream 22. The product stream 22 may require further treatment before it is used, but it has a reduced concentration of one or more ions relative to the feed stream 18. The

concentration of one or more ions may be measured, for example, as hardness, total dissolved solids (TDS), salinity or the specific concentration of one or more selected salts or ions.

[0009] The feed stream 18 may be, for example, seawater, groundwater, surface water, industrial waste water, waste irrigation water, or produced water. The first ion separating unit 12 may be, for example, a membrane separation unit or a distillation unit. A membrane separation unit may comprise one or more reverse osmosis (RO) or nanofiltration (NF) membrane modules. A distillation unit may comprise, for example, one or more evaporators such as mechanical vapor compression (MVC or MVR) evaporators.

[0010] The electrical separation unit 14 receives the brine stream 20 and produces a de-salted brine stream 24. Optionally, acid may be added to the brine stream 20 to lower its pH to about 5.5. The de-salted brine stream 24 may be suitable for use as product water, either blended with the product stream 22 or separately. However, where the electrical separation unit 14 is treating an already concentrated feed stream, it is more likely that the de-salted brine stream 24 still contains considerable ion concentrations. The de-salted brine stream 24 is optionally recycled to the feed stream 18 for further treatment. Regardless of the potential capacity of an electrical separation unit 14, it may be more economical to treat the brine stream 20 such that the de-salted brine stream 24 has one or more ion

concentrations, for example TDS or salinity, within about 50% of the corresponding concentration in the feed stream 18, and to recycle the de-salted brine stream 24 to the feed stream 18.

[0011] In some cases, for example when treating waste water or ground water that already has one or more salts near their saturation limits, the first ion separating unit 12 may be omitted and the feed stream 18 may be fed directly to the electrical separation unit 14.

[0012] Electrical separation unit 14 uses an electrical current to draw ions from the brine stream 20 across a membrane 26 separating a dilute side 28 of the electrical separation unit 14 from a concentrate side 30 of the electrical separation unit 14. The type of electrical separation unit 14 is optional and may be, for example, a supercapacitator desalination (SCD) device or an electrodialysis (ED) device. However, an electrodialysis reversal (EDR) device is preferred at higher feed water concentrations as it has a higher capacity for deslination. An EDR unit can be operated under conditions that tend to foul other electrical separation units, and it is more tolerant to scaling species.

[0013] In an EDR unit, the polarity of the applied current is periodically reversed, and valves are used to switch the locations of the dilute side 28 and concentrate side 30. When water electrolysis occurs at the electrodes in the EDR unit, the polarity reversal causes periodic changes in the pH of the electrode chambers within the EDR unit which inhibits scale formation in these regions. This operation is known in the art and not shown in detail in Figure 1 to simplify the figure. This operation is described, for example, in International Publication Number WO 201 1/106151 A1. International Publication Number WO

2011/106151 A1 is incorporated by reference. Inhibiting scale formation is a useful property in the system 10 because the electrical separation unit 14 is being fed an already

concentrated brine stream 20 to the dilute side 28 and will also be operated at an even higher salt concentration in the concentrate side 30.

[0014] Most or all of the brine stream 20 flows into the dilute side 28 of the electrical separation unit 14. The de-salted brine stream 24 flows out from the opposite end of the dilute side 28 of the electrical separation unit 14. A portion of the brine stream 20 may also flow to the concentrate side 30 of the electrical separation unit 14 to provide make up water to a concentrate recycle loop 32 around the concentrate side 30 of the electrical separation unit 14. Optionally, make up water may be provided from another source, for example feed water 18. Flow in the recycle loop 32 is driven by a pump 46.

[0015] Precipitated salts and some liquid are removed from the concentrate recycle loop 32 through the precipitator unit 16, which is located in a side loop 34 of the concentrate recycle loop 32. The EDR recirculation rate, relative to the flow rate through the dilute side 28 of the electrical separation unit 14, may be about 1 :1 . In the example of Figure 1 , the side loop 34 discharges into a pump 42 that also circulates water through the concentrate recycle loop 32. An antiscalant injector unit 36 mixes an antiscalant with water entering the concentrate side 30 of the electrical separation unit 14.

[0016] An antiscalant de-activation system 38 in a side loop 34 at least partially deactivates the antiscalant upstream of, or in, the precipitator unit 16. A slurry stream 40 is drawn out of the precipitator unit 16. The precipitator unit 16 may be, for example, a tank sized to provide a retention time of about 25-50 minutes. The area of the precipitator unit 16 may provide a clarification rise rate of 0.25 to 0.75 gpm/ft2 or a velocity of 0.001 to 0.002 ft/second. The precipitator unit 16 may be a gypsum seed precipitator as described in International Publication Number WO 201 1/106151 A1. However, seeding is not required in the system 10 and a cartridge filter, though optional, may also not be required.

[0017] Under steady state operating conditions, the rate of ion removal from the concentrate recycle loop 32 through the slurry stream 40 equals the rate of ion addition to the concentrate recycle loop 32. Ions may enter the concentrate recycle loop 32 by passing through membranes 26 or by being added with make up water. The flow rate of the slurry stream 40 is also equal to the flow rate of the feed brine stream 20 to the concentrate side 30 of the electrical separation unit 14 plus any fluid crossing the membranes by electro-osmosis.

[0018] The precipitator unit 16 is preferably located in a side loop 34 that receives water from, and returns water to, the concentrate recycle loop 32. The rate of flow though the side loop 34 is, for example, 25% or less, or 15% or less, of the rate of flow in the concentrate recycle loop 32. The volume and footprint of the precipitator unit 16 is reduced relative to a precipitator unit 16 located in the concentrate recycle loop 32 having the same hydraulic retention time (HRT) and velocity. A high rate of solids removal in slurry stream 40 is maintained despite the reduced flow in the side loop 34 by operating the system 10 with a high concentration of ions in the concentrate recycle loop 32. Operation at a high concentration of ions in the concentrate recycle loop 32 is facilitated by the use of antiscalant from the antiscalant injector unit 36.

[0019] Optionally, the precipitation unit 16 may be replaced with or include another solid-liquid separation unit, for example a membrane or other filter, a centrifuge or a hydrocyclone. In order to inhibit uncontrolled scaling as the antiscalant is deactivated, nucleation crystals may be added to the concentrate before or after the antisclant is deactivated, but before passing the concentrate through the solid-liquid separation unit.

[0020] De-activating some or all of the antiscalant by way of an antiscalant deactivation system 38 increases the rate of precipitation in the precipitator unit 16.

Accordingly, the precipitator unit 16 may have a reduced HRT or increased velocity relative to a precipitator unit 16 operated without antiscalant deactivation. The reduced flow in side loop 34 relative to concentrate recycle loop 32 reduces the amount of energy or chemicals required by the antiscalant deactivation system 38. An optional antiscalant activator may also be provided in the side loop 34. The reduced flow in side loop 34 also reduces the effect of antiscalant deactivation on antiscalant in the concentrate recycle loop 32 if there is no antiscalant activator 42, or if the antiscalant activator 42 is only partially effective.

[0021] While many salts can cause scaling, calcium sulfate (CaS0₄) scaling may be the most important form of scaling to be controlled in an electrical separation unit 14, particularly when desalinating seawater or sulfate rich ground, waste or surface waters. Calcium carbonate (CaC0₃) scaling may also be a concern, but can often be controlled by adding an acid to water flowing in the electrical separation unit 14. The system 10, however, is preferably operated with a high degree of supersatu ration of calcium sulfate in order to allow a reduction in the size of the precipitator unit 16, and a reduction in the cost of operating the precipitator unit 16. In particular, water exiting the concentrate side 30 of the electrical separation unit 14 into the concentrate recycle loop 32 may be supersaturated to a saturation index of 6 or more, or 7.5 or more, or 9 or more..

[0022] As noted above, an antiscalant is added to water flowing into the concentrate side 30 of the electrical separating unit 14. The term "antiscalant" refers to a compound or change in one or more operational parameters which reduces or inhibits scale formation. For example, an antiscalant may be: (a) a chemical additive; (b) a change in the temperature which increases the solubility of a scaling salt; or (c) a combination thereof. An antiscalant may work, for example, according to one or more mechanism such as crystal growth inhibition, sequestration or chelation. [0023] Sufficient antiscalant is used such that precipitation of one or more dissolved salts, for example calcium sulfate, is prevented or inhibited thereby reducing formation of scale deposits in the electrical separation unit 14 and associated piping. The antiscalant deactivator system 38 deactivates at least some of the antiscalant effect so that one or more salts may be precipitated in the precipitator system 16. The one or more salts are thereby encouraged to precipitate preferentially in the precipitator unit 16 rather than in the

separation unit 14. The antiscalant may be, for example, pH sensitive, temperature sensitive, chemical sensitive, or UV sensitive. The anti-scalant may be reversibly or irreversibly deactivated.

[0024] In the case of calcium sulfate, which has an inverted solubility curve, the temperature of water in the concentrate recycle loop 32 may be increased by the antiscalant deactivation system 38 before it enters the precipitator unit 16 to help form a precipitate, alone or in combination with chemical de-activation of an antiscalant. Water leaving the precipitator unit 16 may be cooled through a heat exchanger to provide some of the energy required to heat water flowing into the precipitator unit 16.

[0025] Phosphonate antiscalants are often effective at preventing calcium sulfate fouling. Phospohantes can be deactivated by adding an acid to reduce the pH.

Phosphonates can also be deactivated by oxidizing them, for example with ozone or hydrogen peroxide.

[0026] Polyacrylate antiscalants may also be useful for preventing calcium sulfate scaling. Polyacrylates can be deactivated, for example, by adding a ferric iron salt.

[0027] A dispersant also inhibits scale formation and may be used in combination with an anitscalant. In particular, while the electrical separation unit 14 may be able to operate at a calcium sulfate saturation index of 1 1.5 or more, preventing the formation of scale in pipes of the system may require an additional dispersant. The dispersant may be, for example, a polycarboxilic dispersant. One suitable dispersant is Depositrol PY5203 available from GE Water and Process Technologies.

[0028] If an antiscalant is used that is deactivated by lower pH, it may be necessary to avoid a transfer of acid from the electrode chambers, not shown in Figure 1 , of the electrical separation unit 14. For example, a heavy anion-exchange membrane may be used to preclude H+ migration from the electrode chambers to the dilute and concentrating chambers, or a capacitive carbon electrode may be used in place of faradaic electrodes which produce H+ and OH- during water electrolysis reactions when polarized in the positive and negative polarities respectively. An alternative electrode configuration which circulates a redox couple between the anode and cathode can also be used to avoid H+ and OH- production, for example the Fe2+/Fe3+ redox couple.

[0029] Some antiscalant is removed in the slurry 40. Accordingly, the antiscalant injector unit 36 is provided, even when using a reversibly de-activated antiscalant, to replace active antiscalant leaving the system. When using an irreversibly deactivated antiscalant, additional antiscalant is added to generally replace the deactivated antiscalant.

[0030] One particularly useful antiscalant is Hypersperse™ MDC706, available from GE Water and Process Technologies. This antiscalant is ordinarily used in reverse osmosis processes. Hypersperse™ MDC706 is an anionic polymer with phosphonate or carboxylate functional groups, or both. This antiscalant is sold in a liquid form and is preferably added through a mixer, for example a static mixer, upstream of the electrical separation unit 14. Typical dosage is between 3 and 6 ppm. The dosage should not exceed 10 ppm when adding MDC706 to the feed stream of a reverse osmosis unit used to produce potable water.

[0031] In tests, Hypersperse™ MDC706 was able to inhibit formation of calcium sulfate scale in reverse osmosis filters at a saturation index (SI) of up to 8 at a dosage of about 40 ppm. SI in this document means the ratio of ion product (concentration of Ca ions times concentration of S04 ions) to Ksp, where Ksp is a function of the ionic strength of the solution. Ksp as a function of ionic strength may be determined, for example, from Figure 2.6 of the online version of the Dow FILMTEC Membranes Technical Manual which reproduces a graph from Marshall, W.L. and Slusher, R., "Solubility to 200 C of Sulfate and its Hydrates in Sea Water and Saline Water Concentrates and Temperature, Concentration Limits," Journal of Chemical and Engineering Data, 13(1 ), 83 (1968) SI was determined in the tests using Argo software that applies this definition.

[0032] In further tests with an EDR unit, the Hypersperse™ MDC706 antiscalant was able to prevent scale deposits in water flowing from the concentrate side of an EDR unit with calcium sulfate saturation indexes of 9.5 to 1 1.1 (up to 11.9 in shorter runs) using MDC706 dosages of 48 ppm to 55 ppm. Attempted operation at a calcium sulfate saturation index of 12.5 caused scaling in piping but the membranes in the EDR unit had not scaled when the run was stopped. The feed water had a conductivity of 6,360 to 6,670 uS/cm; temperature of 22 to 25 degrees C; and pH of 7.1 to 8.0. The concentrate in this example was at a saturation index of 9.5 was estimated to have about 3,000 ppm calcium and 15,000 ppm sulfate, LSI of about 0.5, and calcium hardness of about 7,500 ppm.

[0033] Since the antiscalant remains on the concentrate side of the EDR units, and ions rather than water pass through the membrane, high dosages of antiscalant are acceptable even for potable water applications.

[0034] In the example of Figure 1 , the antiscalant injection unit 36 injects a phosphonate antiscalant such as MDC706. This antiscalant can be de-activated by decreasing the pH of the concentrate. The system 10 has an antiscalant deactivator system 38 that injects an acid into the water in the side loop 34 and, optionally, also heats the water. Optionally, the system 10 may have an antiscalant activator 42 located downstream of the precipitator unit 16 in the side loop 34 or the concentrate recycle loop 32. The antiscalant activator 42 adds a base to the water and/or cools the water.

[0035] When using MDC-706, or another antiscalant that is deactivated by lowering the pH, the use of acid to control calcium carbonate scaling should be considered. For some brine streams 20, calcium carbonate scaling will not occur even if water flowing to the concentrate side 30 of the electrical separation unit 14 is kept at a pH that maintains MDC- 706 activity. In other cases, an optional carbonate removal unit 44 may be added to treat the portion of the brine stream 20 that is used as make up water for the concentrate side 30 of the electrical separation unit 14. The carbonate removal unit 44 may be, for example, a membrane based or a forced draft decarbonator that uses an acid to convert alkalinity to carbon dioxide and then removes the carbon dioxide.

[0036] The system of Figure 1 operates according to a method including steps of feeding a brine to an electrical separation unit, adding an antiscalant to the concentrate side of the electrical separation unit, diverting a portion of a concentrate recycle loop to a side stream loop, deactivating the antiscalant in the side stream loop and precipitating and removing solids from the side stream loop.

[0037] In a comparative design example based on Figure 2 of International

Publication Number WO 201 1/106151 A1 , a feed flow of 151.5 gpm is divided into 150 gpm sent to the dilute side of an EDR unit and 1.5 gpm sent to the concentrate side of the EDR unit. 150 gpm of product water is produced, assuming that there is no electro-osmosis. The concentrate recycle loop 32 has a flow of 120 to 150 gpm. Concentrate leaves the concentrate side of the EDR unit at a saturation index of about 1.5. A seeded precipitator in the concentrate recycle loop is sized to provide a retention time of about 25-50 minutes, a clarification rise rate of 0.25 to 0.75 gpm/ft2 and a velocity of 0.001 to 0.002 feet per second. This precipitator has a volume of 40,000 US gallons and a footprint of 300 square feet. 1.5 gpm of slurry leaves from the precipitation tank. Clarified water from the precipitation tank passes through a particle filter and returns to the concentrate side of the EDR.

[0038] In a design example based on Figure 1 of this specification, a feed flow of

151.5 gpm is divided into 150 gpm sent to the dilute side of an EDR unit and 1.5 gpm sent to the concentrate side of the EDR unit. 150 gpm of product water is produced. The

concentrate recycle loop has a flow of 120 to 150 gpm but 15 gpm is diverted into a side loop of the concentrate recycle loop. Concentrate leaves the concentrate side of the EDR unit at a saturation index of 9.5. The side loop has inline acid addition and mixing upstream of a precipitation tank. The precipitation tank is sized to provide a retention time of about 25-50 minutes, a clarification rise rate of 0.25 to 0.75 gpm/ft2 and a velocity of 0.001 to 0.002 feet per second. However, the precipitation tank receives a flow of only one tenth of the flow through the precipitator in the comparative example. The precipitation tank has a volume of 4,000 US gallons and a footprint of 30 square feet, which is a significant reduction in size relative to the comparative example. 1.5 gpm of slurry leaves from the precipitation tank. 13.5 gpm of clarified water travels from the precipitation tank without passing through a particle filter to rejoin the concentrate recycle loop and return to the concentrate side of the EDR. 50 ppm of MDC706 antiscalant is added to the returning concentrate. This example ignores electro-osmosis. Electro-osmosis may occur in an actual system but would not materially change the reduction in size of the precipitation tank.

[0039] This written description uses examples to help disclose the invention and also to enable any person skilled in the art to practice the invention. Alterations, modifications and variations can be effected to the particular examples described above by those of skill in the art without departing from the scope of the invention. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art.

Claims

WHAT IS CLAIMED IS:

1. A water treatment system comprising,

a) an electrical separation unit;

b) a solid-liquid separation unit in communication with a concentrate recycle loop of the electrical separation unit; and,

c) an antiscalant injection system in communication with the concentrate recycle loop of the electrical separation unit.

2. The water treatment system of claim 1 further comprising an antiscalant de-activation system in communication with the concentrate recycle loop of the electrical separation unit.

3. The water treatment system of claim 1 or 2 wherein the precipitation unit is provided in a side stream loop to the concentrate recirculation loop.

4. The water treatment system of claim 3 further comprising an antiscalant de-activation system in communication with the side stream loop upstream of, or in, the solid-liquid separation unit.

5. The water treatment system of any of claims 2 to 4 wherein the antiscalant deactivation system comprises an acid injection system.

6. The water treatment system of any of claims 2 to 5 wherein the antiscalant deactivation system comprises a heater for water upstream of or in the solid-liquid separation unit.

7. The water treatment system of any of claims 2 to 6 further comprising an antiscalant activation system downstream of the solid-liquid separation unit.

8. The water treatment system of claim 7 wherein the antiscalant activation system comprises a base injection system.

9. The water treatment system of claim 7 or 8 wherein the antiscalant activation system comprises a cooler.

10. The water treatment system of any of claims 7 to 9 wherein the antiscalant activation system comprises an antisclant oxidation system.

1 1. The water treatment system of any preceding claim wherein the antiscalant injection system comprises a phosphonate antiscalant injection system.

12. The water treatment system of any preceding claim further comprising a desalination unit upstream of the electrical separation unit, wherein the electrical separation unit receives brine from the desalination unit.

13. The water treatment system of any preceding claim wherein the solid-liquid separation unit comprises a source of seed or nucleation crystals.

14. A process for treating water comprising the steps of,

a) treating brine by electrical separation to produce a concentrate;

b) adding an antiscalant to the concentrate;

c) deactivating the antiscalant in a portion of the concentrate;

d) removing one or more salts from the portion of the concentrate; and,

e) returning the portion of the concentrate to step a).

15. The process of claim 14 further comprising a step of concentrating a feed water to produce the brine.

16. The process of claim 14 or 15 further comprising a step of reactivating the antiscalant in the returning concentrate.

17. The process of any of claims 14 to 16 wherein step c) comprises adding an acid to the concentrate.

18. The process of any of claims 14 to 17 wherein step d) comprises adding seed or nucleation crystal to the concentrate.

19. The process of any of claims 14 to 18 wherein the concentrate produced in step a) has a calcium sulfate saturation index of 6 or more.

20. The process of any of claims 14 to 19 wherein the antiscalant comprises a phosphonate.

21. The process of any of claims 14 to 20 further comprising a step of adding a dispersant to the concentrate.