To understand what is commonly referred to as a “water-softening” process, one need only understand the etymology of the classic definition of water “hardness.” Traditionally, “hard” water was water that featured high levels of certain common impurities such as calcium (Ca) and magnesium (Mg). Water purification processes which facilitated the removal of these offensive “hard” cations were therefore quickly referred to as “water-softening” processes, a term that has prevailed even as purification processes have advanced and expanded in scope.
Various approaches have been adopted in the search for an industrially robust, high-efficiency water-softening process that could address a broad range of impure waters. Many of these approaches feature important shortcomings.
For example, most of the conventional water-softening processes are designed for relatively low levels of hardness (and, specifically, hardness of a sort consisting mostly of Ca and Mg). The novel water-softening process disclosed herein is effective for a broad range and level of contaminants. Specifically, it is particularly well-suited for removing a broad (but, unfortunately, common) array of contaminates (namely calcium, magnesium, barium, strontium, copper, zinc, iron, manganese, aluminum, silica, TOC [total organic carbon], oil, grease, TDS [total dissolved solids] and TSS [total suspended solids]).
Unlike many conventional water-softening processes that use lime (CaO) and soda ash (Na2Co3) as the primary chemical agents to deliver bicarbonate and carbonate alkalinities, the novel softening process can utilize carbon dioxide and/or carbon monoxide in alkaline solution, thus creating bicarbonate or carbonate ions by chemical reaction.
Unlike conventional cold lime softening, hot lime softening or lime-soda ash softening, reverse osmosis membrane, or electro-dialysis reversal processes, all of which are typically conducted at a pH level below 10.5, the novel water-softening process disclosed herein works at elevated pH levels, and not uncommonly at pH levels from 10.5 to 14.0.
Unlike conventional cold lime softening, hot lime softening, or lime-soda ash softening processes that rely on the use of lime and soda ash as the primary softening agents, the novel water-softening process disclosed herein can work with soda-ash, potassium hydroxide, or sodium hydroxide as the chemical agents.
Unlike hot-lime softening processes, which must be conducted at elevated temperatures in order to be effective, the novel water-softening process disclosed herein can be carried out at ambient temperatures, although the rate of reaction will be faster at elevated temperatures.
In the conventional lime softening process, hot lime softening process, or lime-soda ash process, it is sometimes difficult to ensure that the lime (CaO) or hydrated lime (Ca(OH)2) goes effectively into solution. The novel water-softening process disclosed herein does not feature this particular problem, as soda ash can readily go into solution and sodium hydroxide is soluble in all concentrations.
In conventional lime softening processes, hot lime softening processes, lime-soda ash processes, processes that use softening membranes, and/or processes that use electro-dialysis membranes, the treated water will nearly always contain some levels of calcium impurities, as well as magnesium, Ba, Sr, and other metals. The novel water-softening process disclosed herein works extremely efficiently in terms of removing these impurities to negligible levels.
Some prior art approaches, such as the approach described in U.S. Pat. No. 5,152,904, utilize a process frequently referred to as a seeded slurry process; however, the novel water-softening process disclosed herein does not feature or require such an approach. Similarly, some seeded slurry processes are critically dependent upon the size of the crystal; once again, the novel water-softening process disclosed herein has no such crystal size dependency.
Unlike energy-driven processes such as reverse osmosis, electro-dialysis, or electro-deionization, the novel water-softening process disclosed herein utilizes very little energy; in fact, in most cases, the energy consumption comes from mixing devices and transfer pumps. Furthermore, the water-novel softening process can be carried out under atmospheric pressure or at elevated pressures.
Unlike most competitive processes, such as reverse osmosis, the novel water-softening process disclosed herein does not require expensive materials of construction such as high-quality alloys. In most cases, inexpensive materials, such as polyvinyl chloride (hereinafter “PVC”), fiberglass, carbon steel, or stainless steel, can be used. In some applications that contain extremely high levels of chlorides, it may be advantageous to use super-stainless steel or duplex stainless steel materials.
In the conventional processes that utilize reverse osmosis membrane technologies, electro-dialysis membrane technologies, or electro-deionization membranes, it is extremely critical to remove sparingly soluble species (such as calcium, magnesium, silica, barium and strontium) in the pretreatment process; otherwise, the calcium, magnesium, silica, barium or strontium deposits could form a devastating scale on the process components. The novel water-softening process disclosed herein has no limits with respect to the levels of these scaling agents and can be effective as a pretreatment process.
Unlike the membrane based softening processes that generally get fouled in the presence of excess amounts of certain coagulation aid chemicals (such as alum or ferric salts or polymers), the novel water-softening process disclosed herein can be carried out in presence of excess amounts of coagulation aid chemicals.
Some prior art efforts in this area, such as, for example, U.S. Pat. No. 3,976,569, utilize cross flow filtration membranes; however, the instant novel water-softening process does not require the use of such membranes.
In addition, unlike the Green and Behrman process disclosed in U.S. Pat. No. 1,653,272, which is mostly intended for hardness based upon Ca and Mg impurities, and which mostly uses a lime and soda-type process (again, used for mostly low-hardness surface waters), the novel water-softening process is intended for a broad range of contaminations. It uses a high-pH mode of operation by using chemicals such as soda ash and/or sodium hydroxide, potassium carbonate, or potassium hydroxide and is intended to treat surface waters, seawater, produced waters from oil and gas drilling operations and wastewaters from municipal as well as industrial applications.
The novel water-softening process disclosed herein is not an ion exchange process.
Ion exchange processes are mostly batch processes; they generally achieve water softening by exchanging ions on an ion exchange resin. Once the ion exchange resin is fully exhausted (i.e., it has no further capability for exchanging hardness for, e.g., the sodium ion or the hydrogen ion), it has to be regenerated, typically, by either a sodium chloride solution (NaCl), a hydrochloric acid solution (HCl), or a sulfuric acid solution (H2SO4).
Conventional ion exchange processes are very inefficient in terms of chemicals usage for the removal of specific impurities. Also, note that ion exchange systems simply do not work for highly contaminated streams, because the throughput capacities become very small (i.e., the ion exchange systems in such applications tend to require almost constant regeneration). Furthermore, spent regeneration chemicals have to be disposed of which presents a further managerial/technical problem.
The novel water-softening process disclosed herein is extremely efficient in terms of producing high-quality effluent while simultaneously generating a minimum volume disposal stream. In fact, a typical sludge/waste stream from the novel softening process can be reprocessed to recover the water stream, thus making the novel softening process an important part of any zero liquid discharge (hereinafter “ZLD”) process.
Conventional ion exchange water softeners require use of a sodium chloride (NaCl) solution for regeneration. These processes are mostly effective in exchanging Ca and Mg species, and low levels of to Ba and Sr. Furthermore, any presence of iron, manganese, oil, grease, and/or organic matter tends to create serious fouling of the ion exchange resin. The novel water-softening process disclosed herein does not have such process limitations.
Finally, the conventional ion exchange water softeners of the prior art commonly require removal of suspended solids; from the treated stream otherwise, once again, the ion exchange resin can get plugged up and/or foul. The novel water-softening process disclosed herein does not have such limitations.
Zero liquid discharge (hereinafter “ZLD”) technologies utilize a combination of pretreatment processes such as those described in the sections hereinabove.
In the conventional ZLD systems that utilize either lime, lime/soda ash, or hot lime, the resultant process stream is typically further treated with acid or scale inhibitors to lower scale-forming tendencies and/or to prevent further precipitation or scaling due to silica, calcium, magnesium, barium and/or strontium salts. The novel water-softening process disclosed herein does not have such requirements.
In some ZLD systems, the effluent pH from the pretreatment is lowered (to less than about 5) to reduce the scaling potential due to the presence of calcium, magnesium, strontium and/or barium. The lower pH effluent is typically highly aggressive on conventional metals such as carbon steel or lower grades of stainless steel, and requires the use of exotic (and expensive) metallurgy such as titanium alloys, Hastalloy C, and/or Alloy 20.
The novel water-softening process effluent, with pH values typically exceeding 10.5, do not require lowering of pH; thus, the metallurgy of the equipment downstream of the novel water-softening process can be fabricated from lower-cost alloys such as carbon steel, grade three zero four stainless steel (hereinafter “304 SS”), three hundred sixteen stainless steel (hereinafter “316 SS”), or Duplex stainless steel or Super duplex stainless steel or SMO 254. In certain situations, it is possible to fabricate the equipment downstream of the novel softening process from non-metallic materials such as polyvinyl chloride (hereinafter “PVC”), chlorinated PVC (hereinafter “CPVC”), polypropylene (hereinafter “PPL”), Teflon (hereinafter “PTFE”), or fiberglass reinforced plastic (hereinafter “FRP”).
If the process stream is known to contain high levels of sodium, chlorides, sulfates, or carbonates and bicarbonates, the treated effluent from the novel water-softening process can be further treated by concentration processes or ZLD processes such as reverse osmosis, electro-dialysis, evaporators, or crystallizers. Concentrated streams from these processes can be highly pure, sterile, and could be recycled or reused for further industrial or non-industrial uses (such as, for example, dry salt or chemicals manufacturing processes). Concentrated streams from these processes can also be recycled or reused to “kill” gas wells after completion of gas extraction process.
In view of the continuing need for an improved process to efficiently and effectively remove contaminates from water streams, an improved high-efficiency water-process for removing contaminates has been developed.