EP0651726A4 - HOT WATER BOILER OPERATION WITH REDUCED NITROGEN OXIDE AND WASTE WATER EMISSION. - Google Patents

HOT WATER BOILER OPERATION WITH REDUCED NITROGEN OXIDE AND WASTE WATER EMISSION.

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Publication number
EP0651726A4
EP0651726A4 EP93918356A EP93918356A EP0651726A4 EP 0651726 A4 EP0651726 A4 EP 0651726A4 EP 93918356 A EP93918356 A EP 93918356A EP 93918356 A EP93918356 A EP 93918356A EP 0651726 A4 EP0651726 A4 EP 0651726A4
Authority
EP
European Patent Office
Prior art keywords
hardness
reducing agent
water
process according
acid
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
EP93918356A
Other languages
German (de)
English (en)
French (fr)
Other versions
EP0651726A1 (en
Inventor
Leonard Dubin
Vincent M Albanese
Roy A Johnson
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nalco Fuel Tech
Original Assignee
Nalco Fuel Tech
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nalco Fuel Tech filed Critical Nalco Fuel Tech
Publication of EP0651726A1 publication Critical patent/EP0651726A1/en
Publication of EP0651726A4 publication Critical patent/EP0651726A4/en
Ceased legal-status Critical Current

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Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F22—STEAM GENERATION
    • F22D—PREHEATING, OR ACCUMULATING PREHEATED, FEED-WATER FOR STEAM GENERATION; FEED-WATER SUPPLY FOR STEAM GENERATION; CONTROLLING WATER LEVEL FOR STEAM GENERATION; AUXILIARY DEVICES FOR PROMOTING WATER CIRCULATION WITHIN STEAM BOILERS
    • F22D1/00—Feed-water heaters, i.e. economisers or like preheaters
    • F22D1/02—Feed-water heaters, i.e. economisers or like preheaters with water tubes arranged in the boiler furnace, fire tubes or flue ways
    • F22D1/12—Control devices, e.g. for regulating steam temperature
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F9/00—Multistage treatment of water, waste water or sewage
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/46—Removing components of defined structure
    • B01D53/54—Nitrogen compounds
    • B01D53/56—Nitrogen oxides
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F1/00—Treatment of water, waste water, or sewage
    • C02F1/70—Treatment of water, waste water, or sewage by reduction
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F5/00—Softening water; Preventing scale; Adding scale preventatives or scale removers to water, e.g. adding sequestering agents
    • C02F5/08—Treatment of water with complexing chemicals or other solubilising agents for softening, scale prevention or scale removal, e.g. adding sequestering agents
    • C02F5/10—Treatment of water with complexing chemicals or other solubilising agents for softening, scale prevention or scale removal, e.g. adding sequestering agents using organic substances
    • C02F5/14—Treatment of water with complexing chemicals or other solubilising agents for softening, scale prevention or scale removal, e.g. adding sequestering agents using organic substances containing phosphorus
    • C—CHEMISTRY; METALLURGY
    • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
    • C02F2101/00—Nature of the contaminant
    • C02F2101/10—Inorganic compounds
    • C02F2101/16—Nitrogen compounds, e.g. ammonia

Definitions

  • the invention relates to improving the environmental impact of industrial plants which generate waste water and produce nitrogen oxides (NO ⁇ ) .
  • the invention has particular advantage in installations where waste waters having high concentrations of dissolved or suspended solids are produced and injection of aqueous solutions of NH- containing NO ⁇ -reducing agents is feasible. Processes and compositions are provided.
  • Aqueous solutions of NH-containing NO ⁇ -reducing agents such as urea, its hydrolysis products and related amidozine-generating materials are useful in reducing the environmental damage caused by NO ⁇ from large-scale combustion.
  • Carbonaceous materials including the typical hydrocarbon fuels such as coal, oil and gas, as well as refuse, are burned in increasing amounts each year. This combustion is a primary source of steam for power genera ⁇ tion and produces a variety of pollutants which must be removed from the effluents or adverse consequences to the environment will be suffered.
  • waste water is produced when spent steam is condensed for recycling or purified by reverse osmosis, or other industrial process waters are cooled in recirculating cooling towers. These waste waters and others produced by industrial processes contain considerable quantities of dissolved and dispersed solids. There are economic, environmental and regulatory restrictions on the discharge of waste waters.
  • the process for cooling steam typically entails indirect heat exchange (such as in a tube and shell heat exchanger) with cooling water.
  • the cooling water can then be cooled in a cooling tower by contact with air and re ⁇ sulting evaporation.
  • the water lost by evaporation can be replaced by the addition of fresh "make-up" water, but the solids content of the water lost by evaporation is left behind and, in time, builds up. It has been the practice to treat cooling water, boiler water and other process water to prevent scale build up and to periodically "blow down" a portion, treat it to reduce the solids content, and then to return purified water to the process. Solids removal is expensive but is required in most areas because of legal limits on the discharge of waste water. Indeed, some states restrict water dis ⁇ charge to foster conservation.
  • SCR Selective catalytic reduction
  • Consistency in N0 ⁇ reduction is made even more difficult by the fact that the temperature across any plane varies significantly at any given time and shifts with changes in rate of combustion (i.e., load) which is common for boilers used in power generation and other combustors.
  • rate of combustion i.e., load
  • the art has developed to the state where chemicals can be injected in stages (U.S. Patent 4,777,024 to Epperly et al ) , with variation in location of injection and chemical formu- lation as is necessary to meet the temperature and com ⁇ positional variations in the gas stream being treated (U.S. Patent 4,780,289 to Epperly et al ) . All piping, pumps, nozzles and associated equipment must be kept clean and clear for the objectives to be met. Frequent draining, flushing and washing are not possible without severe consequences.
  • Chelation is the binding between an inhibitor and a metal ion (e.g., Ca, Mg, Fe ions) at two or more sites.
  • a metal ion e.g., Ca, Mg, Fe ions
  • chelants are ethylene diamine tetracetic acid (EDTA) , nitrilotriacetic acid (NTA) , N-hydrcxy ethyl ethylene diamine tetracetic acid, hydroxyethylene diamine triacetic acid (HEDTA) , citric acid, diethylenetriamme pentacetic acid, gluconic acid, tartaric acid, glucoheptonic acid, and the water-soluble salts of these.
  • the complexation of a metal ion with a chelant scale inhibitor results in dissolution of the metal ion.
  • Chelation requires a 1:1 mole ratio of chelating agent to metal ion, and is therefore stoichio- metric.
  • Threshold inhibition refers to the phenomenon where inhibitors prevent precipitation of mineral salts when added in amounts which are less than the amount of the scaling ion. Threshold inhibitors are typically viewed as acting through particle dispersion by steric stabil ⁇ ization and electrostatic repulsion, and retarding crystal growth by adsorption onto and blocking active growth sites. The ratio of threshold inhibitor to scaling ions is generally much smaller and is sub- stoichiometric. Typically, threshold inhibitors
  • ppm active inhibitor e.g., polymers, phosphonates
  • ppm total hardness or particulate matters such as silt, clay or precipitate.
  • compositions for treating cooling water are: Canadian Patent 1,117,395, to L. Dubin and J.A. Baumbach which teaches the use of phosphonocarboxylic acids and certain acrylic acid polymers; U.S. Patent 3,663,448 to P.H. Ralston which teaches the use of combinations of amino phos- phonate compounds and polyacrylic acid derivatives; U.S. Patent 3,890,228 to CM. Hwa, D.G. Cuisia and J.S. Gray, which teaches the use of combinations of polyacrylates and polymethacrylates with organo-phosphonic acids; U.S. Patents 4,801,388 and 4,919,821, D.W. Fong, J.E.
  • Hoots and J. Kneller which disclose various hydrocarbon polymers containing amido functionalities prepared by postpolymerization derivatization; and U.S. Patents 4,752,443 and 4,923,634 to J.E. Hoots, D.A. Johnson, D. . Fong and J.F. Kneller which teach a composition com ⁇ prising a water-soluble inorganic phosphate capable of inhibiting corrosion in an aqueous alkaline environment and a hydrocarbon polymer containing an N-substituted acrylamide polymer with a specific structure.
  • Waste waters such as blowdown water from cooling tower reservoirs treated with chemicals of the above and other types, often have solids concentrations which stress the effectiveness of scale control additives employed therein. Reducing the solids concentration in these waste waters requires either chemical or physical separation, both of which add costs. It would be desir ⁇ able to enable replacement of at least a portion of the waste waters with fresh make-up water without either discharging the waste water or incurring costs required for treatment to reuse the water.
  • the invention provides an improvement in a process for controlling emissions, conserving water usage and reducing waste water discharge, the process being of the type wherein fuel is burned with air to produce combustion gases containing NO ⁇ , and an aqueous solution of NO ⁇ -reducing agent is introduced into the combustion gases under conditions effective to reduce the NO ⁇ concentration thereof, the improvement comprising: mixing a NO ⁇ -reducing agent with waste water and scale inhibiting composition in sufficient quantities to provide an aqueous solution of the NO ⁇ -reducing agent effective for reducing the N0 ⁇ concentration of the combustion gases while moderating the tendency for precipitation of hardness factors.
  • the invention provides an improvement in a process for producing and utilizing steam while controlling emissions, the process being of the type wherein fuel is burned with air to produce hot combustion gases containing N0 ⁇ , heat is transferred from the combustion gases to boiler water to produce steam, steam is passed to a device which utilizes at least a portion of the energy of the steam and produces spent steam, the spent steam is cooled by indirect heat transfer to produce liquid boiler water, and an aqueous solution of NO ⁇ -reducing agent is introduced into the combustion gases under conditions effective to reduce the NO ⁇ concentration thereof, the improvement comprising: separating a portion of the boiler water from the remainder as boiler blowdown water, subjecting the blow down water to purification to prepare purified water for recycle to the boiler and reject water and mixing reject water with NO ⁇ -reducing agent in sufficient quantity to provide an aqueous solution of the NO ⁇ -reducing agent effective for reducing the NO ⁇ concentration of the combustion gases.
  • the invention provides an improvement in a process for producing and utilizing steam while controlling emissions, the process being of the type wherein fuel is burned with air to produce hot combustion gases containing NO ⁇ , heat is transferred from the combustion gases to water to produce steam, steam is passed to a device which utilizes at least a portion of the energy of the steam and produces spent steam, the spent steam is cooled by indirect heat trans- fer with cooling water, the cooling water is cooled by direct contact with air causing evaporation, and an aqueous solution of NO ⁇ -reducing agent is introduced into the combustion gases under conditions effective to reduce the NO ⁇ concentration thereof, the improvement comprising: separating a portion of the cooling water from the remainder and mixing it with NO ⁇ -reducing agent in sufficient quantity to provide an aqueous solu ⁇ tion of the NO ⁇ -reducing agent effective for reducing the NO ⁇ concentration of the combustion gases.
  • a portion of the cooling water blowdown is preferably separated, treated to reduce the hardness of that portion and is then recombined with the remainder of the cooling water.
  • the por ⁇ tion of the cooling water which is treated is contacted with an additive to reduce the hardness of that portion.
  • solids are precipitated and removed.
  • the portion of cooling water is treated by reverse osmosis.
  • the process includes the further step of combining the resulting treated water and make-up with the remainder of the cooling water.
  • the total hardness in the cooling water is maintained at from 400 to 2500 ppm
  • the silicate is maintained at less than 200 ppm
  • the calcium to magnesium ratio is maintained at greater than 2:1.
  • an aqueous solution of NO ⁇ -reducing agent is prepared by admixing cooling water, an effective NH- containing composition, and a hardness-suppressing com ⁇ position comprising at least one member selected from the group consisting of polymers, phosphonates, chelants, phosphates and mixtures of these, in an amount effective to moderate the tendency for precipitation of hardness factors.
  • concentration of the hardness-suppressing composition will preferably be sufficient to enable protection against calcium scale formation at calcium concentrations of from 400 to 2500 parts per million.
  • the NH-containing composition comprise a member selected from the group consisting of ammonia, urea, urea precursors, urea hydrolysis products, car- bamates, ammonium carbonate, ammonium bicarbonate, cyanurates, ammonium salts of organic acids, other ammonium salts, and other a idozine-generating composi- tions and mixtures of these.
  • the pH of these aqueous solutions is preferably within the range of from 7 to 11.
  • Figure 1 is a schematic diagram of one preferred process flow scheme of the invention wherein cooling tower blowdown water is used to prepare an aqueous solution of NO ⁇ -reducing agent for effluent treatment;
  • FIG. 2 is a schematic diagram of another preferred process scheme wherein boiler water blowdown is employed to prepare an aqueous solution of NO ⁇ -reducing agent for effluent treatment.
  • the present invention recognizes that industrial plants, in addition to creating NO ⁇ and other air-borne pollutants, produce waste water in related processes which can be an economic and environmental burden. Discharge of the waste water is usually restricted because of the dissolved and suspended solids content. In some areas, water conservation is the purpose for restricting the discharge of water as waste. The inven ⁇ tion enables removal of the water from these waste waters by integrating waste water disposal with NO ⁇ reduction.
  • the known processes for NO ⁇ reduction involve preparing an aqueous solution of NO ⁇ -reducing agent, transporting the solution to the gas stream through conduits effective for this purpose, and introducing the solution into the gas stream at a concentration and at a rate effective to reduce NO ⁇ under the conditions prevailing in the gas stream.
  • This type of process can be practiced with or without a catalyst, by selective gas-phase reactions. Without a catalyst the process is selective, non- catalytic reduction (SNCR) , and with a catalyst it is selective catalytic reduction (SCR) .
  • SNCR non- catalytic reduction
  • SCR selective catalytic reduction
  • Hybrid processes can also be practiced.
  • compositions are effective if scale or precipitation is reduced by any mechanism, but chelation and threshold inhibition are the typical mechanisms. While “scale” is considered by some as hard, adherent, heat-transfer hindering and nozzle-orifice- blocking solids, no differentiation is made here between types of scale or between scale and other forms of “precipitates”.
  • Hardness factors of particular concern include Ca 2+ , Mg 2+ , S0 4 2" , Cl “ , Fe 2+ , Fe 3+ , HC0 3 ⁇ , silica, Mn 2+ , Mn 4+ , Cu + , Cu 2+ , Zn 2+ , P0 4 3" and C0 3 2”” , and can include various particulates and other impurities in the water and other solution ingredients.
  • concentrations of hardness factors e.g., "H", Ca, Mg
  • problems normally associated with these hardness factors are exacerbated by the presence of amidozine-generating NO ⁇ -reducing agents.
  • Figure 1 shows a representative steam generation and utilization system which is improved according to the invention.
  • Figure l shows a large industrial boiler 10 which burns a fuel with air in combustion zone 12. The hot combustion gases have heating value for the production of steam and con ⁇ tain NO ⁇ and other pollutants which must be reduced prior to discharge from the stack 14. While the description which follows will focus on the reduction of NO ⁇ levels in this exemplary effluent situation, it will be recognized by those skilled in the art that SO ⁇ particulates and other effluent components are also subject to control. Effluents from other types of combustion will also need treatment for NO ⁇ removal. Among these are effluents from large utility boilers, circulating fluidized bed boilers, and gas turbines.
  • Hot combustion gases from combustion zone 12 heat water and produce steam which is utilized for its heat value and produces spent steam which is cooled and recycled.
  • the boiler 10 is shown to include a steam drum 15, a mud drum 16, riser 17 and downcomer 19. In operation, the steam is produced in steam drum 15 for circulation to external steam utilizing equipment via line 18. Cooler water flows from steam drum 15 to mud drum 16 by convection through downcomer 19 as hotter water flows in the opposite direction through riser 17.
  • the steam can be utilized to provide heat as such or to produce mechanical energy, such as in turbine 20, which can in turn be used to produce electrical energy via a generator 22.
  • a preferred utility of the invention is in a plant designed for the cogeneration of heat and electricity.
  • the spent steam is then cooled by indirect heat exchange with cooling water, such as in a tube and shell heat exchanger represented as 24 in Figure 1. Once cooled, the resulting condensate from the spent steam can be recirculated to the boiler 10.
  • the cooling water is passed from heat exchanger 24 via line 26 to cooling tower 28 where it is cooled by direct heat exchange with air.
  • Figure 1 shows the cooling water being sprayed at 30 and descending through air which can be renewed by natural or forced convection. The contact of air and water transfers sensible heat from the water to the air and cools both by evaporation of a portion of the water.
  • the cooled cooling water is collected in reservoir 32.
  • Evaporation necessitates supplying additional make-up water to maintain a desired water level.
  • make-up water contains solids, and solids are scrubbed from the air used for cooling. Accordingly, dissolved and suspended solids tend to build up in the cooling water. It is typical to separate a portion of this warer for discharge or treatment. This portion, such as is shown taken off via line 34 in Figure 1, is called a blowdown stream.
  • the blowdown water can be treated, mechanically or chemically, to reduce its hardness and then recycled to the remainder of the cooling water in reservoir 32, such as via lines 36, 38 and 40.
  • the treatment can be by addition of chemical softening agents which typically precipitate out selected species such as in vessel 42.
  • lime can be added to precipitate out salts such as magnesium silicate to reach the target levels for these materials as is explained further below.
  • reverse osmosis can be employed, where pressure is employed to force water through a semi-permeable membrane to leave behind a brine bearing the greatest concentration of dissolved solids.
  • Further available techniques include electrodialysis reversal, other bipolar membrane technology, and vapor recompression techniques.
  • Fresh make-up water can be added, such as via line 44, in amounts required.
  • the invention has the advantage of not only reducing waste water discharge problems to stay within regulation, but also the advantage of reducing costs of operation, and, for new installations, of capital equipment.
  • Effluent temperatures will typically be greater than about 500°F and can be as high as about 1900°F, even 2100°F and higher.
  • the tem- perature of the effluent at the point of introduction of the NO ⁇ -reducing agent is typically within the range of 1500° to 2000°F for SNCR systems. For SCR operations it will typically be within the range of from 200° to 1000°F.
  • An aqueous solution of NO ⁇ -reducing agent is shown being introduced into the effluent by injection at 50.
  • an injector is employed and generally comprises a conduit, sometimes fitted at the tip with a nozzle, extending into the effluent.
  • Apparatus of varying degrees of sophistication are known for introducing NO ⁇ -reducing compositions into a high temperature envi ⁇ ronment. Some comprise coaxial, multi-tubular struc ⁇ tures, such as those disclosed by Burton in U.S. Patent 4,842,834, and by DeVita in U.S. Patent 4,985,218, the disclosures of each of which are incorporated herein by reference.
  • a portion of effluent or other gas is often employed to help atomize and disperse the solution of NO ⁇ - reducing agent.
  • the spray or injection pattern is defined with precision, often with the aid of computer to assure good distribution and reaction.
  • the incorporation of a hardness-suppressing composition as described below in greater detail and in copending U.S. Patent Applica- tion Serial No. 07/770,857, helps assure operation according to the defined pattern by minimizing injector fouling in this severe environment where the NO ⁇ - reducing agents tend to pronounce the effects of hard ⁇ ness.
  • hardness- suppressing compositions in amounts effective to increase the stability of hardness factors, and thereby moderate the tendency for precipitation of hardness factors such as in the conduits, nozzles, or storage vessels.
  • the control of SNCR and SCR systems is difficult. These processes require individual molecules of NO ⁇ -reducing agents (or, more precisely, free radical components of them, e. g. , the amidozine radical) to react with individual NO ⁇ molecules — both the reducing agent and the NO ⁇ being present at very low (e.g., ppm) concentrations.
  • Figure 1 shows a controller 52 which receives a first signal from means 54 for sensing the NO ⁇ concentration of the effluent, and includes means for generating a control signal responsive to that first signal for controlling the supply of aqueous solution of NO ⁇ -reducing agent to the effluent to injector 50 by regulating a valve or other flow-controlling means 56.
  • the solution of NO ⁇ -reducing agent is prepared by diluting a suitable NO ⁇ -reducing agent and hardness- suppressing composition with water and holding the resul ⁇ ting aqueous solution in a vessel such as 58. Additional water or solution can be added as necessary such as through line 59.
  • the preferred NO ⁇ -reducing agents will be NH- containing compositions and will comprise at least one member selected from the group consisting of ammonia, urea, urea precursors, urea hydrolysis products, car- bamates, ammonium carbonate, ammonium bicarbonate, cyanurates, ammonium salts of organic acids, other ammonium salts (e.g., inorganic ammonium salts), other amidozine-generating compositions and mixtures of these.
  • the NO ⁇ -reducing agents based on carbamates and urea hydrolysates such as those described in U.S. Patent 4,997,631 to Hof ann et al , and U.S. Patent Applica ⁇ tion Serial No. 07/561,154, filed August 1, 1990 in the names of von Harpe et al, particularly stress the ability of the solution to retain hardness factors in soluble form.
  • This patent and the application are incor ⁇ porated herein by reference. Accordingly, the present invention is of particular advantage when dealing with materials of this type or its components such as ammonium or calcium carbonates, bicarbonates and carbamates.
  • the NO ⁇ -reducing compositions employed by the invention will typically be prepared and shipped as concentrates which are diluted for use.
  • these concentrates will contain 25 to 65% urea and 0.05 to 1.0% of an effective hardness-suppressing composition, more preferably from 40 to 55% urea, e.g., 50%, and from 0.1 to 0.75%, e.g., 0.5%, of the hardness- suppressing composition.
  • This concentrate is diluted as required to achieve a urea concentration effective under the conditions.
  • dilution to concentrations of from 5 to 25% urea are effective.
  • Lower concentrations e.g., l to 5%
  • the concentration of certain hardness factors in the blow-down water used for dilution will be kept within prescribed ranges to assure satisfactory operation.
  • Figure 1 also illustrates a simple system for controlling blowdown of cooling water, its treatment to maintain tolerances, its utilization as dilution water for NO ⁇ -reducing agent and the supply of make-up water to the cooling tower 28.
  • a controller 61 receives a signal indicative of the chemical composition of the cooling water in reservoir 32 via line 62. This signal is processed and according to a program generates a control signal to either open valve 63 to discharge the blow-down water for use as dilution water and/or to open valve 64 to enable treatment of the water at 42.
  • a surge tank (not shown) would be employed to hold discharged blowdown water until called for in the preparation of the aqueous solution of NO ⁇ -reducing agent.
  • the drawing illustrates a chemical separation of hardness factors as a precipitate, but a physical separation by reverse osmosis could be employed if desired.
  • the controller will preferably also generate signals to open and close valve 65 to control return of treated blow-down water to the reservoir and valve 66 to control supply of cooling water make-up.
  • the aqueous solution of NO ⁇ -reducing agent is prepared by admixing cooling water, an effective NH-containing composition, and a hardness-suppressing composition comprising at least one member selected from the group consisting of polymers, phosphonates, chelants, phosphates and mixtures of these, in an amount effective to moderate the tendency for precipitation of hardness factors. It is preferred that the concentration of the hardness-suppressing composition is sufficient to enable protection against calcium scale formation at calcium concentrations of from 400 to 2500 parts per million.
  • the pH of the aqueous solution will be above 5, prefer ⁇ ably within the range of from 7 to 11.
  • aqueous solu ⁇ tions comprise both a water-soluble polymer and a phos- phonic acid or salt as hardness-suppressing composi ⁇ tions.
  • hardness-suppressing compositions comprising a member selected from the group consisting of polyacrylic acid and 2-acrylamido-2-methylpropane sul ⁇ fonic acid.
  • preferred compositions comprise a phosphonate selected from the group consisting of: 1-hydroxy ethylidene-l,l-diphosphonic acid and 2-phosphono-l,2,4-tricarboxy butane. It is also useful to further employ a surfactant.
  • the dilution water can be a waste water stream from a cooling system blowdown stream and have unusually high hardness levels. This facilitates the non-polluting disposal of these streams.
  • a stream 60 is shown coming from a reservoir of cooling water 32 in cooling tower 28 via line 34.
  • a hardness-suppressing composition can contain the above combinations but chelants are per se not as important, and phosphates (including esters) may be employed for corrosion control.
  • Cooling water blowdown typically has very high hardness levels although they are preferably kept to less than about 2500 ppm total hardness.
  • the weight ratio of calcium to magnesium should be less than 20:1, preferably less than 15:1, and the silicate level is preferably maintained at less than 150 ppm. These levels can be achieved by diluting or otherwise treating the blowdown water prior to use for dilution, or simply setting these as targets for operation of the cooling tower.
  • NO ⁇ -reducing agent e.g., less than 10% and prefer ⁇ ably at 5% or below in the case of the highest levels with urea or urea hydrolysate.
  • a hardness-suppressing additive package comprising (on a solids basis) 3.25% polyacrylic acid (PAA) , 12.0% 2-phosphono-l,2,4-tricarboxybutane (PBTC) , 44.4% of a 37% actives acid form terpolymer of acrylamide/acrylic acid/sulfomethyl acrylamide (pH-5) (AMS) , and 0.75% l-hydroxyethylidene-l,l-diphosphonic acid (HEDP) , is effective at 2.5% urea in a solution containing 2250 ppm and below total hardness with magnesium, calcium and silicate within the above limits.
  • This additive package is effective at a level of about 500 ppm.
  • the above additive package can be employed at effective levels, e.g., about 300 to 700 ppm.
  • the dispersant level can be decreased even at urea concentrations up to 30%.
  • the AMS and PBTC can be dispensed with in an additive package at levels of 4000 to 6000 ppm of product containing only the polyacrylic acid and HEDP.
  • Threshold mechanisms are the primary factor and are the preferred method because of cost and efficiency. Chelation is typically effective in softer waters such as from high pressure steam boilers. Threshold inhibition refers to the phenomenon where inhibitors prevent precipitation of mineral salts when added in amounts which are less than the amount of the scaling ion. Threshold inhibitors are typically viewed as acting by particle dispersion through steric stabili ⁇ zation and electrostatic repulsion, and retarding crystal growth by adsorbing onto and blocking active growth sites. Chelation is the binding between the inhibitor and the metal ion (e.g., Ca, Mg, Fe ions) at two or more sites.
  • the metal ion e.g., Ca, Mg, Fe ions
  • Chela ⁇ tion requires a 1:1 mole ratio of chelating agent (e.g., citric acid, EDTA, etc. ) to metal ion, and and is therefore stoichiometric.
  • chelating agent e.g., citric acid, EDTA, etc.
  • the ratio of threshold inhibitor to scaling ions is generally much smaller and is substoichiometric.
  • threshold inhibitors e.g., polymers, phosphonates
  • threshold inhibitors are applied at a dosage ratio of 1:10,000 to 1:2 ppm active inhibitor/ppm total hardness or par- ticulate matters such as silt, clay or precipitate.
  • Figure 2 also illustrates an industrial plant which generates steam and utilizes the steam to operate a turbine, but in this case employs at least a portion of the boiler blowdown water as dilution water for the aqueous solution of NO ⁇ -reducing agent.
  • the process represented in Figure 2 is the same in many regards with that of Figure 1, and like parts are given like reference numbers.
  • the drawing shows line 76 drawing boiler blowdown water from mud drum 16 for use as dilution water for the NO ⁇ -reducing agent and further shows make-up boiler water being introduced via line 72.
  • the use of boiler water blowdown presents a less severe environment than that of cooling water blowdown; but as with the cooling water situation described above, the actual operating requirements of the boiler will 5 determine the exact treatment necessary.
  • some low-pressure steam operations operate at pressures of less than 100 psig and can tolerate up to 5000 ppm total hardness.
  • Blowdown water from these boilers may require substantial dispersant levels, e.g., several 0 thousand ppm, and may require treatment to reduce solids prior to use in the solution of NO ⁇ - reducing agent.
  • High-pressure boilers on the other hand, cannot tolerate hardness, and blowdown water from these boilers can be treated to separate a high solids fraction portion from a 5 purified portion which is recycled to the steam drum.
  • Figure 2 shows a reverse osmosis unit 74 which receives blowdown from the mud drum 16 via lines 76 and 78 and separates it into a solids rich fraction which is passed via lines 80 and 82 to vessel 58, and a solids 0 poor fraction which is recycled back to steam drum 15 via lines 84 and 25.
  • reverse osmosis can be employed to reduce solids in the poriton fed to line 82.
  • line 84 would go to line 5 82
  • line 80 would go to discharge, such as a sedimentation pond.
  • the hardness-suppressing composition can contain: che- lant(s) ; polymer(s) ; a combination of chelant(s) and
  • n polymer(s) a combination of phosphonate(s) and polymer(s) ; a combination of chelant(s) , phosphonate(s) and polymer(s) ; and combinations of any of these with a phosphate such as tripolyphosphate which is employed for scale control.
  • a phosphate such as tripolyphosphate which is employed for scale control.
  • cationic and anionic polymeric dispersants can be important.
  • waste or impure waters from other sources can also be employed.
  • These other sources include: river water; lake water; various industrial pond waters, including sedimentation ponds, cooling ponds, evaporation ponds, ash ponds, and sumps; brines from various sources such as sea water and concentrates from reverse osmosis; waste water streams from various cleaning processes, including air heater wash water; and the like.
  • An additive of this type is preferably dispersed in the waste water prior to addition to the concentrate or into the NH-containing concentrate prior to dilution.
  • Any water-soluble polymer which is effective to sup ⁇ press hardness can be employed.
  • a wide variety of polymers is commercially available.
  • water-soluble acrylic polymers Exemplary of these are polymers and copolymers (including those which are substituted or derivatized) of acrylic acid, methacrylic acid, ethyl acrylic acid, acrylamide, esters of acrylic monomers, itaconic acid, and maleic acid or its anhydride as described, for example in U.S. Patents 3,890,228, 4,680,124, 4,744,949, 4,752,443, 4,756,881, 4,818,506, 4,834,955, 4,904,413, 4,919,821, 4,923,634, and 4,959,156 and Canadian patent 1,117,395.
  • the dis- closures of each of these and the references cited therein are incorporated herein by reference in their entireties.
  • the preferred water-soluble polymers will have molecular weights within the range of from 500 to 300,000, typically from 1,000 to 50,000, e.g., 2,000 to 25,000, as measured by gel permeation chromatography in water.
  • acrylates those including recurring groups represented by the following general formula:
  • each R independently is hydrogen or lower alkyl (e.g., l to 4 carbons) and each Y indepen ⁇ dently is hydroxyl (OH) , oxymetalic (OM) , oxyorgano (OR) , oxyammoniu (0NH 4 ) , amino (NR 2 ) , amino carbosulfonic (NHRSO ⁇ M) , amino carbosulfonic ester (NHRSO- j R) and the like.
  • M is H or a metal, particularly one selected from the group consisting of alkali metals (e.g., Na f K, Li), alkaline earth metals (e.g., Ca Mg) , transition metals (e.g., Zn, Cu, Ni) and mixtures of these; and each R independently is hydrogen or an aliphatic, aromatic, or carbocyclic group and can be saturated or unsat rate ⁇ , and either su__st ⁇ t te ⁇ cr unsubst tut ⁇ with aikoxy, : ⁇ eto, carboxyamide, polycarboxy, -oi v - nydrcxy, cara ⁇ xylate ester, am o, phosphonic acid, hospnate ester, sulfonic acid, sulfonate salt or ester cr other compatible subsistent.
  • alkali metals e.g., Na f K, Li
  • alkaline earth metals e.g., Ca Mg
  • acrylic polymer and "poiy- acrylate” as used herein include homopolymers, and copolymers, including terpolymers- which comprise one or more of the monome ⁇ c residues defined by formula I as recurring units, other monomers including maleic or taconic acids and their derivatives and precur ⁇ sors, vinyl acetate (which can be hydrolized after polymerization to polyv yl alcohol) , lower alkyl vinyl ethers, styrene, styrene anhydride, other vinyl monomers (e.g., 3-allyloxy-2-hydro__ypropane sul onic acid) , derxvatized starches, and the like can also be employed.
  • residues of maleic acid or itaconic acid or their anhydrides can be employed to prepare effective homopolymers cr copolymers including the residues of other monomers. They can be copolymers of two, three or more different monomers. Block copolymers as well as random copolymers can be employed. These homo or copolymers will typically include one or more of the following as recurring units:
  • each Y independently is as defined a b ove and each z independently is: hydrogen, lower alkyl ( e.g., i to 4 carbons), hydroxyl (OH), oxymetalic ( OM ) , oxyorgano (OR) , alkylsulfonic (RSO3M ) , alkylsulfonic ester (RSO ⁇ ) and the like an d M and R being as defined above.
  • oxyorgano (OR) substituents are mono an d polyhydric alcohol residues, particularly those having from 1 to 4 carbons and up to three hy d roxyls. Prominent among these are the residues of methanol, ethanol. ethylene glycol, 1,3-propane diol and 1,3- b utane diol, and isomeric forms of these.
  • acrylamide monomers and polymers results in acrylamide monomers and polymers.
  • these are the following N-, ,N- and mixe d acryla ides: methyl, ethyl, propyl, i-propyl, butyl, i-butyl, t-butyl, pentyl, hexyl, carboxy pentyl, methoxy propyl, tris(hydroxy methyl) methyl, ( 1,2-dicarboxy) ethyl, (1,2-dihydroxy) ethyl, ( 2,3-dihydroxy) propyl, (2-methyl- ⁇ ,2-dih ⁇ droxy) propyl, 2-(2 ,4-trimethyl pentyl) , 2-(2- ethyl-
  • amino carbosulfonic groups are represented by the following: 4 -amino b enzene sulfonic acid, aminomethane sulfonic acid, 2-aminoethane sulfonic acid, ⁇ -aminobenzene sulfonic acid, 2-amino-2-methyl propyl sulfonic acid (N(H) 2 C(CH 3 ) 2 CH 2 SO.,H) (AMPS), l-amino-2-hydroxy-3-propane sulfonic acid, and 2,3-hydroxy propylamine.
  • NHRSO ⁇ R amino carbosulfonic groups
  • amino carbosulfonic ester groups (NHRS 3 R) of the formula are represented by the alkyl, aralkyi, aryl, and carbocyclic esters of the above amino carbosulfonic groups.
  • polyacrylate-based polymers include but are not limited to acrylic acid/acrylamide, acrylic acid/vinyl acetate, acrylic acid/acrylate ester, acrylic acid/maleic acid/acrylamidomethylpropane sulfonic acid, and acrylic acid/acrylamido methyl- propane sulfonic acid polymers, and combinations thereof.
  • Belciene 200 polymaleate from Ciba Geigy Belciene 233 polymaleic anhydride terpolymer 5 with ethyl acrylate and vinyl- acetate, MW 1,500, from Ciba Geigy
  • Betz 2020 acrylic acid, yroxypropyl- acrylate copolymer, from Betz Other Carboxylate ⁇ Polymers
  • polymers may be prepared using conventional polymerization techniques. Many, as indicated above, are commercially available. Others can be prepared following the techniques described in the above- identified references.
  • phosphonic acids including all poiyphosphonic acids and salts and esters of these acids, which are water soluble and effective to suppress hardness in solutions of the type disclosed herein.
  • polymers there is a wide variety of phosphonates which are commercially available and will be effective for the purposes of the present invention. See U.S. Patents 4,303,568 and 4,923,634 which list a number of representative phosphonates. The disclosures of these patents are incorporated herein by reference.
  • the organophosphonic acid compounds are those having a carbon to phosphorous bond, i.e. ,
  • These compounds can be organized into several groups including the acid, ester and salt forms of the followm ⁇ : ⁇ r ⁇ ano monoohosohonates, ⁇ r ⁇ ano diphospnonates, ammo monophospnonates, and amino poiyphosphonates.
  • R is lower alkyl having from about l to 6 carbon atoms, (e.g., methyl, ethyl, butyl, propyl, isopropyl, pentyl, isopentyl and hexyl) ; substituted lower alkyl of from 1 to 6 carbon atoms (e.g., hydroxyl and amino-substituted alkyls) ; a mononuclear aromatic (aryl) radical (e.g., phenyl, benzyl, etc.
  • a substituted mononuclear aromatic com ⁇ pound e.g., hydroxyl, carboxy, amino, lower alkyl substituted aromatic such as benzyl phosphonic acid
  • M is a water-soluble cation, e.g. , sodium potassium, ammonium, lithium, etc. or hydrogen.
  • each R iV independently is hydrogen, a carboxyl (C0 2 H) group or a phosphono (P0 3 H 2 ) group.
  • this group is 2-phosphono-l,2,4- tricarboxybutane (PBTC) .
  • m is an integer' (e.g., from 1 to 12) ; each R v independently is hydrogen or an alkyl group (e.g., having 1 to 6 carbons) ; and each R v ⁇ independently is hydrogen, hydroxyl or an alkyl group (e.g. , from 1 to 6 carbons) .
  • the composition c this inven t ion are hydroxy alkylidene diphosphonic acids correspon d ⁇ i ng t o formula (VI) above such as those disclose d in U.S.
  • an alkylene diphosp h onic acid corresponding to formula (VI) such as those disclosed in U.S. Patent 2,303,129, the entire disclosure of which is also incorporated herein b y reference.
  • R V11 is a lower alkylene having from about one to about four carbon atoms, or an amine or hydroxy substituted lower alkylene
  • R v ⁇ jm is [R v"ii -P0 3 M 2 ] H, OH, amino, substituted amino, an alkyl having from 1 to 6 carbon atoms, a substituted alkyl of from 1 to 6 carbon atoms (e.g., OH, NH 2 substituted) , a mononuclear aromatic radical or a substituted mononuclear aromatic radical (e.g. , OH, NH 2 substituted)
  • R jjJJC is R vlii or the group represented by the formula
  • R and R X1 are each hydrogen, lower alkyl of from about 1 to 6 carbon atoms, a substituted lower alkyl (e.g., OH, NH 2 substituted), hydro ⁇ gen, hydroxyl, amino group, substituted amino group, a mononuclear aromatic radical, or a substituted mono ⁇ nuclear aromatic radical (e.g. , OH and amine substituted);
  • R*-"- is R x , R x , or the group iVli -P0 3 M 2 ⁇ R V1Z is as defined above)
  • n is an integer of from 1 througn about 15;
  • y is an integer ⁇ f from about 1 through about 14; and is as earlier defined.
  • R*"- 1 3 hydrogen, hydroxyl, or an alkyl (e.g., l to 10 carbons) group, aryl (e.g., phenyl) group or aralkyi (e.g. , benzyl) group.
  • alkyl e.g., l to 10 carbons
  • aryl e.g., phenyl
  • aralkyi e.g. , benzyl
  • R xlv is hydrogen, alkyl, aryl, aralkyi, hydroxyl, hydroxy alkyl ((CH 2 ) n 0H or a phosphono alkyl ( (C__ 2 ) n P0 3 H 2 ) group; and n is an integer, typically f om l to .
  • R ;v is an alkyl group having l to 10 carbons, (e.g., 2 to 6) ; and n is an integer, (e.g., from 0 to 4 carbons, typically 0 to 1) .
  • Prominent among these compounds are:
  • HMDTP hexamethylene diamine tetra(methylene phosphonic acid)
  • EDTP ethylene diamine tetra(methylene phosphonic acid
  • dialkylene triamme penta (alkylene phosphonic acid)
  • the water-soluble salts of these acids include the alkali metal, ammonium, amine, lower alkanolamine salts, and like salts.
  • suitable esters are the lower alkyl (e.g., methyl and ethyl) esters. Mixtures of the organophosphonic acid compounds described above are also contemplated.
  • Phosphates can be employed alone or with other hardness-suppressing compositions in amounts effective to suppress hardness.
  • suitable inorganic phosphates are the acid forms of inorganic phosphate and any of their metal, ammonium or amine salts.
  • Representative of the inorganic phosphates (ortho and condensed) are those chosen from the group: orthophosphates, pyrophosphates, tripolyphosphates, hexametaph ⁇ sphates, and higher molecular weight polyphosphate oligomers.
  • organo phosphates such as phosphate esters, especially those ⁇ f a type repr ⁇ s ⁇ nted by the f ollowing structures :
  • R (CH 2 CH 2 0) q P0 3 H 2 , and q is an integer, e.g., i to 2;
  • any of these phosphates may be used alone or in combination , orthophosphates and condensed (poly- phosphates) are preferred.
  • a combination of at least one polyphosphate and one of the o t her phosphates will be utilized.
  • Tne invention can take advantage of conventional chelants, alone or in combination with other hardness- suppressing compositions, in amounts effective to suppress hardness.
  • ethylene diamine tetracetic acid (EDTA) ethylene diamine tetracetic acid
  • NTA nitrilotriacetic acid
  • HEDTA hy d roxyethylene diamine triacetic acid
  • citric acid diethylenetriamine pentacetic acid, gluconic acid, tartaric acid, glucoheptonic acid, and the water-soluble salts of these.
  • Ca/Mg ratios were varied between 2:1 and 5:1 as a function of total hardness.
  • Mg 2+ (as CaC0 3 ) was either 150, 200 or 300 ppm, with most experiments done at 150 or 200 ppm.
  • Test solutions employed the following hardness- suppressing compositions in the amounts indicated in the tables that follow:
  • PAA Polyacrylic acid (MW approximately 2000) AMS - 37% actives acid form terpolymer of acrylamide/ acrylic acid/sulfomethyl acrylamide (pH-5) PBTC - 2-phosphono-l,2 ,4-tricarboxybutane HEDP - 1-hydroxyethylid ⁇ ne-1,1-diphosphonic acid.
  • Table 1 shows the results of experiments in simulated cooling water blowdown containing 150 ppm Si0 2 with total hardness values, "H", varying between 450-900 ppm as CaC0 3 .
  • the data describing scale control is expressed in terms of obs ⁇ rvations such as "scale/no scal ⁇ " and the four following parameters:
  • Percent Ca 2+ Inhibition (I) is based on the NACE* Method for calculating inhibition. The data is from ICP analysis of filtered solutions. With respect to just I, better performance is associated with high % Ca Inhibition.
  • Percent Dispersancy is a simple mathematical percentage, expr ⁇ ssed by the following calculation:
  • NACE National Association of Corrosion Engineers
  • D is derived from EDTA total hardness analysis of exp ⁇ rimental solutions before and after filtering.
  • P ⁇ rc ⁇ nt disp ⁇ rsancy is simply that fraction of wat ⁇ r hardn ⁇ ss which is not water soluble and not precipitated adher ⁇ nt scal ⁇ . This m ⁇ ans it is a ⁇ asure of dispers ⁇ d particl ⁇ s which can b ⁇ filt ⁇ r ⁇ d by an 0.45 micron illipor ⁇ filter. If a particle is small ⁇ r than 0.45 microns and thus not filt ⁇ r ⁇ d, it is consid ⁇ r ⁇ d solubl ⁇ hardness".
  • High % Dispersancy is associated with high % Retention of Mg + and Si0 2 and increasing total wat ⁇ r hardn ⁇ ss test conditions.
  • the achievable I decreas ⁇ s and D incr ⁇ as ⁇ s is a function of increasing l ⁇ v ⁇ ls of Ca 2+ .
  • P ⁇ rcent retained Mg + and Si0 2 are simple mathe- matical percentages derived by dividing the values obtained by inductiv ⁇ ly-coupl ⁇ d plasma spectroscopy (ICP) analysis of filter ⁇ d solutions, by th ⁇ th ⁇ or ⁇ tical conc ⁇ ntrations of each. Because of the experimental and analytical methods used, 100% ⁇ 7% probably repr ⁇ s ⁇ nts full r ⁇ cov ⁇ ry of Mg 2+ and Si0 2 for both components.
  • ICP inductiv ⁇ ly-coupl ⁇ d plasma spectroscopy
  • the recommended use concentrations for PAA and HEDP are 15 and 65 ppm respectively, even with Ca/Mg ratios increased to 5:1.
  • the reasons for decreasing the PAA and HEDP concentration are the requirements for lower urea solids, ionic strength and alkalinity as total hardness levels approach approximately 900 ppm.
  • the 900 ppm "H” data in Table 1 show the interrela ⁇ tionships that exist among Ca/Mg ratios, "H", and con- centrations of PAA and HEDP and the combination of AMS and PBTC with respect to acceptable scale control. For example, at 2:1 Ca/Mg ratio, acceptable performance cannot be achieved. At a 5:1 Ca/Mg ratio, acceptable performance can be achieved but at least 300 ppm AMS and 30 ppm PBTC are required.
  • the concept of "Dispersancy" and its importance to scale control in high-hardness, high-silica (150 ppm Si0 2 ) water is shown in Tables 1 and 2.
  • PBTC maintains dispersancy but at the cost of an appreci ⁇ able loss in I and overall performance.
  • the presence of a combination of AMS and PBTC is important; however, it is more important to control the concentration of Mg 2+ .
  • the Ca/Mg ratio must be increased to 4 to 5:1 and hence Mg 2+ to 150 ppm if Mg + and Si0 2 is to be prevented from precipitating and adding to the CaC0 3 scale. This is shown by the results in Table 1.
  • Silica can be tolerated at levels of 150 ppm Si0 2 at hardness levels up to 900 ppm providing Mg 2+ is limited to 150 ppm by controlling the Ca/Mg ratio to about 5:1.
  • At total hardness levels above 900 ppm extensive studies have not been made; however, data in Table 3 suggest that even low levels, e. g. , 30 to 60 ppm Si0 2 , may not be readily tolerated.
  • the 2,250 ppm results in Table 3 suggest that nearly all the Mg 2+ would have to be removed and Ca/Mg ratios of at least 15-20:1 will be required.

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