CA2893345C

CA2893345C - Hybrid electrochemical softening desalination system and method

Info

Publication number: CA2893345C
Application number: CA2893345A
Authority: CA
Inventors: Benjamin Stuart Sparrow
Original assignee: Saltworks Technologies Inc
Current assignee: Saltworks Technologies Inc
Priority date: 2013-07-29
Filing date: 2014-04-24
Publication date: 2017-01-24
Anticipated expiration: 2034-04-24
Also published as: CA2893345A1; WO2015013809A1

Abstract

A hybrid electrochemical water softening desalination system softens water by both electrodialytical and non-electrodialytical means before desalinating the softened water in a primary desalination subsystem. The concentrate produced as byproduct from the desalination is provided to a concentrate channel of the electrodialysis device employed, and the electrodialysis device extracts multivalent ions from the water to the concentrate channel. In some embodiments the electrodialysis device provides softened water directly to the primary desalination subsystem. In other embodiments the electrodialysis device provides electrodialytically softened water to a non-electrodialytical softening subsystem for further softening, before the softened water is supplied to the primary desalination subsystem. The arrangement integrates the concentrate disposal line of the primary desalination subsystem with the functioning of the electrodialysis device.

Description

HYBRID ELECTROCHEMICAL SOFTENING DESALINATION SYSTEM AND
METHOD
TECHNICAL FIELD
[0001] The present invention relates to the desalination of primary industrial water. More specifically, the invention is directed to a method and apparatus for controlling the hardness of saltwater input to a desalination system by integrating an electrodialysis system with more traditional chemical water softening systems.
BACKGROUND

[0002] Desalination of inland brackish waters and industrial wastewater is has become an important aspect of mining, the oil and gas industry, and manufacturing in general. The most commonly practiced desalination processes are reverse osmosis ("RO"), thermal desalination and electrochemical desalination.

[0003] RO is presently the most widely practiced seawater desalination process. In this technology water is forced through an osmotic membrane that rejects salts and allows water flux under pressures in excess of the osmotic pressure.

[0004] In thermal desalination water is evaporated and condensed, sometimes in multiple stages, in order to recycle the latent heat of condensation. While this category of process can operate at high brine levels, the energy input requirements tend to be large and expensive and the containment equipment is similarly expensive due the use of materials such as alloy steels and titanium.

[0005] The Reverse Osmosis and Thermal Evaporation-Condensation systems, so widely used in industry for desalination, are often limited in their efficacy by the presence of low solubility species such as calcium sulfate, barium sulfate, and magnesium carbonate. The presence of these species is popularly referred to as rendering the water "hard", requiring the water to be treated for these species. This treatment is popularly referred to as "softening" the water. This softening is done upstream of the Reverse Osmosis or Thermal Desalination system.

[0006] The two most widely used softening systems are Lime Softening and Ion Exchange, both being chemical softening processes. In Lime Softening, lime (calcium hydroxide) is added to the "hard" water, sometimes followed by soda ash (sodium carbonate) in order to precipitate calcium and/or magnesium as carbonate slats, thereby rendering the water "soft". These carbonate salts, often referred to as "lime sludge", are then physically removed.
This process requires addition of chemicals, which increases costs and introduces health and safety concerns. In addition, Lime Softening requires removal of the sludge, thereby yet further increasing costs.

[0007] In Ion Exchange (IX), ion exchange resins packed in resin beds exchange, for example, sodium ions with calcium or magnesium. The lower solubility species that create the "hardness" are thereby effectively replaced with sodium, to create more highly soluble salts. Ion exchange is a semi-batch process in which resins are depleted over time and require regeneration. This technique requires considerable maintenance, such as the frequent addition of sodium chloride and hydrochloric acid to regenerate the resins. During the regeneration step an acid wash is often performed to remove the calcium or magnesium from the resin and replace it with protons (H+), followed by a sodium chloride wash to replace the protons with sodium. By this method the resin is regenerated and can be re-used. The addition of the hydrochloric acid and sodium chloride adds costs and introduces health and safety concerns.
Moreover, an acid waste stream is generated that must be disposed of, adding yet further costs.

[0008] The present invention addresses the need for reducing the costs presented by Lime Softening and Ion Exchange bulk water softening systems and ameliorating the associated health and safety concerns.
SUMMARY

[0009] In a first aspect a desalination system is provided comprising an electrodialysis device configured for preferentially extracting multivalent ions from input saltwater to produce electrodialytically softened water; a non-electrodialysis water softening subsystem configured for softening water to produce softened water; and a primary desalination subsystem configured for desalinating the softened water. The non-electrodialysis water softening subsystem is arranged to supply softened water to the primary desalination subsystem. The electrodialysis device comprises a concentrate channel and the primary desalination subsystem may be in fluid communication with the concentrate channel of the electrodialysis device. The primary desalination subsystem may be configured for supplying to the concentrate channel of the electrodialysis device concentrate generated in the primary desalination subsystem. The desalination system may further comprise a water pre-treatment system configured for removing suspended solids and organic species.

[0010] In some embodiments, the electrodialysis device may be arranged to supply electrodialytically softened water to the non-electrodialysis water softening subsystem. In other embodiments, the electrodialysis device may be arranged to supply electrodialytically softened water to the primary desalination subsystem and the non-electrodialysis water softening subsystem arranged to soften the input saltwater and to provide non-electrodialytically softened water to the primary desalination subsystem.

[0011] The non-electrodialysis water softening subsystem may be an ion exchange system comprising an ion exchange resin bed, and the desalination system further comprisesa storage tank for collecting a concentrate from the primary desalination subsystem and a conduit for directing the concentrate from the storage tank to the ion exchange resin bed.

[0012] In a further aspect a desalination system is provided comprising an electrodialysis device comprising a diluent channel, a membrane system and a concentrate channel; a non-electrodialysis water softening subsystem configured for producing softened water from input saltwater; and a primary desalination subsystem configured for desalinating the softened water.
The primary desalination subsystem is further configured for generating concentrate from the softened water and arranged to supply the concentrate to the concentrate channel of the electrodialysis device. The electrodialysis device is arranged for receiving input saltwater on the diluent channel and configured for extracting multivalent ions from the diluent channel to the concentrate channel through the membrane system to produce electrodialytically softened water in the diluent channel.

[0013] In some embodiments, the electrodialysis device may be arranged for supplying the electrodialytically softened water to the primary desalination subsystem.
In other embodiments the electrodialysis device may be arranged for supplying the electrodialytically softened water as input saltwater to the non-electrodialysis water softening subsystem.

[0014] In another aspect a method is provided for desalinating input saltwater, the method comprising: electrodialytically softening the input saltwater to produce electrodialytically softened water; non-electrodialytically softening the input saltwater to produce non-electrodialytically softened water; and desalinating the electrodialytically softened water and the non-electrodialytically softened water softened water in the same desalination system to produce desalinated water and concentrate. The electrodialytically softening the input saltwater may comprise electrodialytically extracting multivalent ions from the input saltwater to the concentrate. The method may further comprise pretreating the input saltwater to remove suspended solids and organic species. The non-electrodialytically softening may be ion exchange softening and the method may further comprise suspending ion-exchange softening and regenerating an ion exchange resin bed by treating the resin bed with the concentrate.

[0015] In yet another aspect a method is provided for desalinating input saltwater, the method comprising: electrodialytically softening the input saltwater to produce electrodialytically softened water; non-electrodialytically softening the electrodialytically softened water to produce softened water; and desalinating the softened water to produce desalinated water and concentrate. The electrodialytically softening the input saltwater may comprise electrodialytically extracting multivalent ions from the input saltwater to the concentrate. The method may further comprise pretreating the input saltwater to remove suspended solids and organic species. The non-electrodialytically softening may be ion exchange softening and the method may further comprise suspending ion-exchange softening and regenerating an ion exchange resin bed by treating the resin bed with the concentrate.

[0016] This summary does not necessarily describe the entire scope of all aspects. Other aspects, features and advantages will be apparent to those of ordinary skill in the art upon review of the following description of specific embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In the accompanying drawings, which illustrate exemplary embodiments:

[0018] Figure 1 is a schematic view of a first embodiment of a hybrid electrochemical softening desalination plant.

[0019] Figure 2 is a schematic view of a second embodiment of a hybrid electrochemical softening desalination plant.

[0020] Figure 3 is a flow chart of a method for softening water in a desalination plant

[0021] Figure 4 is a flow chart describing in more detail the desalination step of the flow chart of Figure 3.

[0022] Figure 5 is a flow chart describing in detail a first embodiment of the water-softening step of the flow chart of Figure 3.

[0023] Figure 6 is a flow chart describing in detail a second embodiment of the water-softening step of the flow chart of Figure 3.
DETAILED DESCRIPTION

[0024] Electrodialysis has been presented as a promising method for the desalination of saltwater. This process transfers salt ions across ion exchange membranes under the action of a galvanic potential. The galvanic potential is supplied as a voltage generated at an anode and cathode. Ion exchange membranes offer the advantage that they do not require regeneration, thereby reducing the need for chemical inputs as required by IX processes.
Membrane inorganic scaling can be managed through polarity reversal (electrodialysis reversal ¨
EDR) and fouling managed through periodic flushes or dilute acid washes. Unlike reverse osmosis, the output product water salt concentration from electrodialysis can be adjusted by adjusting the voltage applied to the stack. This technology provides an output water product that is desalinated to a degree that retains usefulness in industrial applications, including but not limited to enhanced oil recovery, while the energy requirements are more nominal. As will be described below, electrodialysis is also efficacious in the softening of saltwater input to traditional desalination systems. It may be employed to reduce the frequency of resin regeneration and the chemical consumption in traditional chemical softening systems and thereby reduce the associated costs and health and safety concerns.

[0025]
Embodiments described herein are directed to the use of an electrochemical softening (ECS) process in combination with a chemical water softening process, with the ECS
process removing "hardness" from the incoming feed of water to be softened, thereby reducing the salt load placed on the more traditional chemical softening step.

[0026]
Electrodialysis stacks consist of at least two sets of chambers ¨ a first set containing a diluent, and a second set containing a concentrate separated from the first by a set of membranes. Salt ions are transferred from the diluent to the concentrate under the action of a DC
electric field applied at the electrodes. The concentration factor across any single membrane separating a diluent chamber from a concentrate chamber, which is expressed as the ratio of concentrate to diluent salt mass, has practical limits. A practical concentration factor of five to ten is common; that is, transferring ions from a diluent with a concentration of 2000 ppm to a concentrate with a concentration of between 10,000 ppm and 20,000 ppm.

[0027]
Salts are transferred from the input water to saline water through ion exchange membranes in an ED stack. With suitable membrane selection, only strongly ionized and low molecular weight species such as sodium, chloride, calcium, magnesium, sulfates and the like are transferred from the input water through the ion exchange membrane to the saline water. Non-ionic species such as hydrocarbons and larger weakly ionized molecules, such as organics present in the input water, do not cross the membrane into the saline water.
The salinity of the input water is beneficially reduced and the desalinated input water may be re-used in the industrial process or discharged to the environment, provided it meets regulatory specifications.

[0028]
Electrochemical desalination systems exhibit strong cross-membrane transport for multivalent ions as compared with the generally lower transference monovalent ions due to their lower charge. Typical examples of multivalent ions strongly transported, and therefore preferentially removed, are the Alkaline Earth Metals hardening the water, such as magnesium, calcium, barium, as well as elements of profound concern in the Environmental and Health fields such as selenium and arsenic,. The low multivalent ion concentration partially desalinated water from an electrochemical desalination system may therefore be used as source of water for Reverse Osmosis or Thermal Desalination systems. More particularly, electrochemical desalination systems may be combined with more traditional Lime Softening or Ion Exchange . , . , softening systems to provide softened water as input to the Reverse Osmosis or Thermal Desalination system. This reduces the load on and operating costs of new or existing chemical softening plants.

[0029] Figure 1 shows an embodiment of a hybrid electrochemical softening desalination plant 100. Plant input saltwater feed line 102 supplies water to be softened to pre-treatment subsystem 104 in which suspended solids and organic species are removed.
Pretreated input water leaves pre-treatment subsystem 104 on pretreated input water supply conduit 106 to be directed along conduit 108 to water softening subsystem 110 and along conduit 120 to electrodialysis subsystem 122. This may be done with a suitable arrangement of pumps and valves, which are omitted for the sake of clarity. Suitable pumps and valve arrangements are well known in the art and will not be dwelt upon here. Water softening subsystem 110 may be a Lime Softening or Ion Exchange Softening system, or any other non-dialysis water softening system.

[0030] Electrodialysis subsystem 122 may be a simple Electrodialysis device (ED), an Electrodialysis Reversal device (EDR), a multi-compartment Electrodialysis reversal device (MC-EDR), a multivalent splitter device (MVS), or other suitable electrodialysis device. Suitable devices of these types are described in considerable detail in United States Patent US8,137,552 titled "Method, for desalinating saltwater using concentration difference energy"; United States Patent US8,236,158 titled "Method, apparatus and plant for desalinating saltwater using concentration difference energy"; United States Patent US8,317,992 titled "Method, apparatus and plant for desalinating saltwater using concentration difference energy";
United States Provisional Patent Application 61/764,076 titled "Adaptive current limiting in electrodialysis systems"; United States Provisional Patent Application 61/774,530 titled "Multivalent Ion Separating Desalination System"; United States Provisional Patent Application 61/814,317 titled "Hybrid Electrodialysis Desalination System"; Canadian Patent Application CA2792516A1 titled "Apparatus for compression of a stack and for a water treatment system"; World Intellectual Property Organization Application Number W02013/037047A1 titled "Method, Apparatus and System for Desalinating Saltwater"; and United States Patent Number US8317992B2 titled "Modular apparatus for a saltwater desalinating system, and method for using same".

,

[0031] Conduit 120 supplies pretreated input water to a set of diluent chambers 124 of electrodialysis subsystem 122. In operation, electrodialysis subsystem 122 preferentially transfers multivalent ions from the diluent chambers 124 across a set of suitable membranes 126 to a set of concentrate chambers 128 under the action of an applied electric field. For the sake of clarity in Figure 1, electrodialysis subsystem 122 is schematically shown as a single diluent chamber 124, a single concentrate chamber 128 and a single membrane 126. In practice, electrodialysis subsystem 122 comprises a plurality of such chambers and membranes, as described in patents and provisional patent applications US8,137,552, US8,236,158, US8,317,992, US8317992B2, 61/764,076, 61/774,530, 61/814,317, CA2792516A1, and W02013/037047A1. The plurality of diluent chambers 124 of electrodialysis subsystem 122 is referred to in the present specification as "diluent channel" 124. The plurality of concentrate chambers 128 of electrodialysis subsystem 122 is referred to in the present specification as "concentrate channel" 128.

[0032] We refer in the present specification to the water output from the diluent channel 124 of as "electrodialytically softened water" in order to distinguish it from water softened by other means. Water softened by non-electrodialysis means is referred to in the present specification as "non-dialytically softened water". Water softened first by electrodialysis and then by non-electrodialysis means is referred to in the present specification as "dual softened water". When water softened by electrodialysis and water softened by non- electrodialysis means is mixed, we refer to it in the present specification as "mixed softened water".

[0033] The diluent in the diluent channel 124 is output as electrodialytically softened water on diluent output conduit 130 and via conduit 114 to primary desalination system 116, which may be by way of non-limiting example a Reverse Osmosis system or a Thermal Desalination system, or any other non-dialysis system. Non-dialytically softened water from softening subsystem 110 is directed via conduits 112 and 114 to primary desalination system 116.
Desalinated water from desalination system 116 is directed out of the plant 100 via desalinated water conduit 118.
Concentrate from desalination system 116 is directed along concentrate conduit 132 to the concentrate chambers 128 of electrodialysis subsystem 122. Concentrate from concentrate chambers 128 of electrodialysis subsystem 122 is disposed of via concentrate output conduit 134.

[0034] In this embodiment electrodialysis subsystem 122 may be described as having its diluent channel disposed in parallel with softening subsystem 110 and its concentrate channel downstream in series with the concentrate channel of primary desalination system 116.
Electrodialysis subsystem 122 is therefore in direct fluid communication with both the input and the output side of primary desalination system 116.

[0035] In another embodiment shown in Figure 2, a hybrid electrochemical softening desalination plant 200 comprises the same pre-treatment subsystem 104, electrodialysis subsystem 122, softening subsystem 110, and primary desalination system 116, but these components are disposed in a different arrangement. Here also plant saltwater feed line 102 supplies saltwater to be softened to pre-treatment subsystem 104 in which suspended solids and organic species are removed. In this embodiment, however, pretreated input water from pre-treatment subsystem 104 is directed along conduit 202 to diluent channel 124.
As in the embodiment of Figure 1, multivalent ions are preferentially transferred from diluent channel 124 across a set of suitable membranes 126 in electrodialysis subsystem 122 to a set of concentrate chambers 128 of electrodialysis subsystem 122 under the action of an applied electric field.
Diluent from diluent channel 124, now much reduced in "hardening" multivalent ions and environmentally troublesome species, is directed as electrodialytically softened water along conduit 204 to the input of water softening subsystem 110. The softened water output from softening subsystem 110 is directed along conduit 206 to the input of primary desalination system 116.

[0036] The remainder of the plant 200 functions exactly as the corresponding segment of plant 100 of Figure 1. Desalinated water from desalination system 116 is directed out of the plant via desalinated water conduit 118. Concentrate from desalination system 116 is directed along concentrate conduit 132 to the concentrate chambers 128 of electrodialysis subsystem 122.
Concentrate from concentrate channel 128 is disposed of via plant concentrate output conduit 134. As with plant 100, the water may be directed with a suitable arrangement of pumps and valves, which are omitted from Figure 2 for the sake of clarity. Suitable pumps and valve arrangements are well known in the art and will not be dwelt upon here.

[0037] In this embodiment electrodialysis subsystem 122 may be described as having its diluent channel disposed upstream in series with softening subsystem 110 and its concentrate channel downstream in series with the concentrate side of primary desalination system 116.
Electrodialysis subsystem 122 is therefore in direct fluid communication with the output channel of primary desalination system 116, but in this alternative embodiment the diluent channel of electrodialysis subsystem 122 is not in direct fluid communication with primary desalination system 116.

[0038] In another embodiment, being a variant of both embodiments detailed above, the non-electrodialytical softening subsystem 110 is an IX water softening system and the concentrate from concentrate channel 128 is directed to a storage tank, not shown, where, in this embodiment, it is collected for directing to the IX water softening system 110 along a conduit (not shown) for use in the regeneration of the ion exchange resin bed of IX
water softening system 110 after cleaning the resin bed with a suitable acid wash.

[0039] Figure 3 is a flow chart providing a method for desalinating saltwater in a hybrid electrochemical softening desalination plant. The method comprises pretreating [310] the input saltwater to remove suspended solids and organic species from the input saltwater, resulting in pretreated input saltwater; softening [320] the pretreated input saltwater to produce softened water; and desalinating [360] the softened water. The softening [320] the pretreated input saltwater may be by different combinations of electrodialysis and non-eletrodialytic means, as explained inmore detail below.

[0040] Figure 4 is a flow chart showing detail of the method for desalinating [360] the softened water in more detail. The desalinating [360] the softened water comprises providing [362] the softened water to a desalination system 116 of Figures 1 and 2;
delivering [364]
desalinated water from the desalination system 116 on a first fluid conduit 118; supplying [366]
saltwater concentrate from the desalination system 116 on a second fluid conduit 132 to concentrate channel 128; extracting multivalent ions [368] from the pretreated input saltwater in diluent channel 124 through a membrane system 126 of the electrodialysis device 122 to concentrate channel; disposing [370] of the concentrate in the concentrate channel 128 via a third fluid conduit 134. The desalinating process described here is the same for both the hybrid electrochemical softening desalination plant of Figure 1 and the hybrid electrochemical softening desalination plant of Figure 2.

[0041] In a further embodiment pertaining to the case when softening subsystem 110 is an IX water softening system, the disposing [370] comprises storing the concentrate in a tank (not shown), halting the operation of the plant when the IX resin bed of the softening subsystem 110 is depleted, acid washing the IX resin bed, and treating the IX resin bed with the sodium rich concentrate, thereby regenerating the IX resin bed. The regeneration is particularly efficacious when the sodium content in the concentrate is higher 5 to 10 times higher than calcium content.
While the plant is halted, the opportunity may also be used to reverse flush the membrane system of the electrodialysis device 122 in order to improve its operation.

[0042] We now direct our attention back to the flowchart of Figure 3, and more particularly to the step of softening [320] the pretreated input saltwater.
Figure 5 is a flowchart showing this step in more detail for the arrangement of Figure 1, while Figure 6 is a flowchart showing the corresponding step in more detail for the arrangement of Figure 2.

[0043] We consider first the flowchart of Figure 5, in which the method for softening [320] the pretreated input saltwater comprises extracting multivalent ions [322] from a first portion of the pretreated saltwater in diluent chamber system 124 through a membrane system 126 of the electrodialysis device 122 to produce electrodialytically softened water; softening by a non-dialysis water softening process [324] a second portion of the pretreated saltwater in water softening subsystem 110 to produce non-dialytically softened water; and supplying [326] the first portion of electrodialytically softened water and the second portion of non-dialytically softened water to primary desalination system 116 as mixed softened water.

[0044] In the flowchart of Figure 6, the corresponding softening method comprises electrodialytically softening the pretreated saltwater from pre-treatment subsystem 104 by extracting multivalent ions [332] from the pretreated saltwater in diluent channel 124 through a membrane system 126 of the electrodialysis device 122 to produce electrodialytically softened water; supplying [334] electrodialytically softened water from the diluent chamber system 124 to water softening subsystem 110; softening [336] the electrodialytically softened water further in water softening subsystem 110 to produce softened water; and supplying [338]
the dual softened water to primary desalination system 116. In this implementation, pretreated water is first electrodialytically softened and then non-electrodialytically softened and the output stream of softened water from the non-electrodialytical softening process is supplied to primary desalination system 116.

[0045] The routing of the concentrate byproduct of the primary desalination process to the concentrate channel of the electrodialysis device usefully integrates the concentrate disposal line of the primary desalination subsystem 116 with the functioning of the electrodialysis device 122. The arrangement of the diluent channel 124 of the electrodialysis device 122 upstream in series with the water softening subsystem 110 as in Figure 1, or in parallel with it, as in Figure 2, reduces the salt load on the water softening subsystem 110 and thereby reduces the costs of operating softening subsystem 110. The arrangement has further merit in that the electrodialysis device 122 removes environmentally unacceptable species as such as arsenic and selenium.

[0046] While particular embodiments have been described in the foregoing, it is to be understood that other embodiments are possible and are intended to be included herein. It will be clear to any person skilled in the art that modification of and adjustments to the foregoing embodiments, not shown, are possible. It is contemplated that any part of any aspect or embodiment discussed in this specification can be implemented or combined with any part of any other aspect or embodiment discussed in this specification.

Claims

What is claimed is

1. A method for desalinating input saltwater, the method comprising:
a. electrodialytically removing from input saltwater multivalent ions using an electrodialysis device to produce electrodialytically softened water;
b. non-electrodialytically softening the input saltwater to produce non-electrodialytically softened water; and c. desalinating the electrodialytically softened water and the non-electrodialytically softened water in the same desalination system to produce desalinated water.

2. The method of claim 1, further comprising pretreating the input saltwater to remove suspended solids and organic species.

3. The method of claim 1, wherein the non-electrodialytically softening is selected from ion exchange softening and lime softening.

4. A method for desalinating input saltwater, the method comprising:
a. electrodialytically removing from input saltwater multivalent ions using an electrodialysis device to produce electrodialytically softened water;
b. non-electrodialytically softening the electrodialytically softened water to produce softened water; and c. desalinating the softened water to produce desalinated water.

5. The method of claim 4, further comprising pretreating the input saltwater to remove suspended solids and organic species.

6. The method of claim 4, wherein the non-electrodialytical softening is selected from ion exchange softening and lime softening.