Electrodialysis method and apparatus for passivating scaling species
View Patent ↗In one embodiment, this invention relates to an electrodialysis device comprising an inlet for directing a feed stream into a plurality of first feed paths and a plurality of second feed paths; the feed stream is comprised of a first anionic scaling species and a first cationic scaling species; the first cationic scaling species is transferred from the second feed paths to the first feed paths through a first membrane group, the first anionic scaling species is transferred from the first feed paths to the second feed paths through the first membrane group. In another embodiment, this invention relates to a method for passivating scaling species in an electrodialysis device.
1. A method of reducing scale imparting ion content of a desalination unit (DSU) feed stream comprising:
a) passing a direct current through a pair of electrodes in an electrolysis device (ED) so as to energize said pair of electrodes as a cathode and an anode respectively;
b) feeding an aqueous stream to said ED, said aqueous stream having an anionic scale imparting species (ASIS), an anionic non-scale imparting species (ANSIS), a cationic scale imparting species (CSIS), and a cationic non scale imparting species (CNSIS) therein, said ASIS being present in a first ASIS concentration and said CSIS being present in a first CSIS concentration;
c) contacting said aqueous stream in said ED along a first feed path and along a second feed path separated by a first membrane group, said second feed path also being in fluid communication with a second membrane, and said first feed path also being in fluid communication with a third membrane group, said first membrane group permitting transfer of ASIS from the first feed path to the second feed path and permitting transfer of CSIS from the second feed path to the first feed path, said second membrane permitting passage of ANSIS therethrough and out of said second feed path, said third membrane permitting passage of CNSIS therethrough and out of said first feed path, whereby after passage of said aqueous stream through said first feed path a first effluent is provided and after passage of said aqueous stream through said second feed path a second effluent is provided, wherein said first effluent has a reduced ASIS concentration compared to said first ASIS concentration and said second effluent has a reduced CSIS concentration compared to said first CSIS concentration; and
d) feeding either or both of said first and second effluents to said DSU as said DSU feed stream, with the proviso that when both said first and second effluents are fed to said DSU, they are fed separately and are not mixed.
2. The method of claim 1 , wherein said CSIS is selected from the group consisting of Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ , and said ASIS is selected from the group consisting of SO 4 2− , HCO 3 − , CO 3 2− , OH − , F − , PO 4 3− , HPO 4 2− , and H 2 PO 4 − .
3. The method of claim 2 wherein said CSIS is Ca 2+ and wherein said ASIS is HCO 3 − .
4. The method of claim 2 wherein said CSIS is Ca 2+ and wherein said ASIS is SO 4 2− .
5. The method of claim 2 wherein said CSIS is Ca 2+ and wherein said ASIS comprises SO 4 2− and HCO 3 − .
6. The method as recited in claim 3 wherein said anionic selective exchange membrane is a monovalent HCO 3 − selective membrane.
7. The method as recited in claim 3 wherein said cation selective exchange membrane is divalent Ca 2+ selective membrane.
8. The method as recited in claim 4 wherein said anionic selective exchange membrane is a monovalent HCO 3 − selective membrane.
9. The method as recited in claim 4 wherein said cationic selective exchange membrane is divalent Ca 2+ selective membrane.
10. The method of claim 5 wherein said cation selective exchange membrane is divalent Ca 2+ selective membrane.
11. The method as recited in claim 5 wherein said anion selective exchange membrane comprises a first membrane component defined by a monovalent HCO 3 − membrane and a second membrane component defined by a divalent SO 4 2− membrane.
12. The method as recited in claim 1 wherein said first effluent comprises a reduced ASIS concentration of less than about 90% of said first ASIS concentration and said second effluent comprises a reduced CSIS concentration of less than about 90% of said first CSIS concentration.
13. The method as recited in claim 1 wherein said first effluent comprises a reduced ASIS concentration of less than about 50% of said first ASIS concentration and said second effluent comprises a reduced CSIS concentration of less than about 50% of said first CSIS concentration.
14. The method as recited in claim 1 wherein said first effluent comprises a reduced ASIS concentration of less than about 20% of said first ASIS concentration and said second effluent comprises a reduced CSIS concentration of less than about 20% of said CSIS concentration.