Method and apparatus for high efficiency evaporation operation
A process for treatment of an aqueous stream to produce a low solute containing distillate stream and a high solute/solids containing blowdown stream utilizing a method to increase the efficiency of an evaporator while providing an essentially scale free environment for the heat transfer surface. Multi-valent ions and non-hydroxide alkalinity are removed from aqueous feed streams to very low levels and then the pH is increased preferably to about 9 or higher to increase the ionization of low ionizable constituents in the aqueous solution. In this manner, species such as silica and boron become highly ionized, and their solubility in the concentrated solution that is present in the evaporation equipment is significantly increased. The result of this is high allowable concentration factors and a corresponding increase in the recovery of high quality reusable water with essentially no scaling.
1. A process for treatment of a silica-containing aqueous feed water stream using an evaporator, said process comprising:
(a) providing a feed water stream containing soluble and insoluble inorganic and organic species therein, said species comprising:
(I) multi-valent metal cations,
(II) alkalinity,
(III) at least one molecular species which is at low ionization levels when in solution at around neutral pH, said molecular species comprises silica (SiO 2 ), metasilicic acid (H 2 SiO 3 ), or orthosilicic acid (H 4 SiO 4 );
(b) removing a portion or substantially all multi-valent metal cations from said feed water stream to a non-precipitating residual level by passing said feed water stream through membrane softening equipment, or by increasing the pH to at least 10 in said feed water stream and passing the pH adjusted stream through membrane separation equipment to filter out hardness precipitate, and thereafter passing said feed water stream through a cation ion exchange system, and
(c) processing the feed water stream by:
(I) removing substantially all non-hydroxide alkalinity from said feed water stream; and
(II) raising the pH thereof to at least 9 or higher;
(d) after removing multi-valent metal ions, removing substantially all non-hydroxide alkalinity, and raising the pH of said feed water stream to at least 9 or higher, thereafter feeding the feed water stream having a pH of at least 9 or higher into an evaporator, wherein said evaporator:
(I) contains a plurality of heat transfer surfaces,
(II) contains a circulating high solids solution, and
(III) wherein the pH of said circulating solution is maintained to at least 9, or higher; and
(e) condensing the vapor produced by the evaporator to produce a distillate stream, the evaporator also producing a high solids containing blowdown stream.
2. The process as set forth in claim 1 , wherein the step of said multi-valent metal cation removal is accomplished in a weak acid cation ion exchange system operated in a hydrogen form.
3. The process as set forth in claim 1 , wherein the step of said multi-valent metal cation removal is accomplished in a weak acid cation ion exchange system that is operated in a sodium form.
4. The process as set forth in claim 1 , wherein said feed water stream contains more multi-valent metal cations than alkalinity, and further comprising, before feeding said feed water stream to said cation ion exchange system, the step of adjusting the ratio of multi-valent metal cations to alkalinity by adding a base to said feed water stream, so as to raise the alkalinity of said feed water stream.
5. The process as set forth in claim 1 , wherein said feed water stream contains more alkalinity than multi-valent metal cations, and further comprising, before feeding said feed water stream to said cation ion exchange system, the step of addition of acid to said feed water stream, so as to remove the excess alkalinity in said feed water stream.
6. The process as set forth in claim 1 , wherein the step of multi-valent metal cation removal is accomplished by passing said feed water stream through a sodium form strong acid cation ion exchange system.
7. The process as set forth in claim 1 , wherein after removing substantially all non-hydroxide alkalinity from said feed water stream dissolved gas is removed from said feed water stream.
8. The process according to claim 1 , wherein said evaporator is falling thin film evaporation equipment operating as a single unit or in series or parallel, or forced circulation evaporation equipment which operates as a single unit or in parallel, or natural circulation evaporation equipment which operates as a single unit or in parallel, to generate said distillate stream and said high solids blowdown stream.
9. The process as set forth in claim 8 , wherein said evaporator is operated in a steam driven multiple effect mode, or a mechanical vapor recompression mode, or a thermal compression mode, or as a multiple stage flash evaporator.
10. The process as set forth in claim 1 , further comprising heating said feed water stream to enhance gas removal in a degasifier prior to entering said evaporator.
11. The process according to claim 1 , wherein the step of raising the pH is accomplished by addition of a base in aqueous solution, said base selected from the group consisting of sodium hydroxide, sodium carbonate, potassium hydroxide, and potassium carbonate.
12. The process according to claim 1 , wherein the step of raising the pH is accomplished by addition of an aqueous organic base.
13. The process according to claim 1 , wherein said feed water stream is cooling tower blowdown, scrubber blowdown, water utilized in ash transport in a coal fired steam-electric power plant, ash pond water, ash-sluicing water, effluent from sewage treatment, effluent from a food processing treatment, boiler blowdown, a concentrated stream from membrane separation equipment, effluent from oil refining operations, or effluents from hydrocarbon recovery operations.
14. The process as set forth in claim 1 , further comprising, during the step of removing alkalinity, the additional step of removing substantially all non-hydroxide alkalinity not associated with hardness.
15. The process as set forth in claim 1 , wherein after step (c) (II) and prior to step (d), the process further comprises the step of removing dissolved gas in a degasifier.