Determining the Concentration of Water Treatment Chemicals

The present invention relates to a method of monitoring the concentration of a target water treatment chemical in water containing at least one additional water treatment chemical in an industrial process, especially an industrial water circuit. In particular the method preferably involves a method in which the monitoring generates a signal which can be used to control the dose of the target water treatment chemical. The method is especially applicable where the target water treatment chemical is a polymer. The concentration of the target water treatment chemical can be determined singly or repeatedly, for example in monitoring the concentration.

Various methods have been proposed for monitoring and controlling doses of water treatment chemical for use in industrial water circuits. Generally these methods tend to employ indirect means for monitoring the water treatment chemical.

One system employs a turbidimethc method in which a reagent, such as benzethonium chloride, is added to a sample of cooling water thereby causing turbidity if any of the water treatment chemical is present. A turbidity measurement is used to estimate the concentration of water treatment chemical. This may be performed by extraction of a sample or alternatively in line.

Another method employs a highly detectable companion chemical which is added with a polymeric water treatment chemical which is to be monitored. The companion chemical is measured spectrophotomethcally, for instance by fluorescence detection. The companion chemical and water treatment chemical are added at a known ratio. By assuming that the ratio of water treatment chemical to companion chemical remains constant, detection of the companion chemical provides an indirect measurement of water treatment chemical.

However, this assumption is based upon the expectation that the water treatment chemical and companion chemical will be consumed at the same rate.

A further system employs a water treatment chemical that is modified with a tagged group that is more easily detected, for instance by incorporating a fluorescent group or other group that causes an immunoassay response.

US 4514504 relates to a quantitative method useful for field monitoring low concentrations of water-soluble, polyacrylic acids in aqueous systems containing polyacrylic acids and other ionic materials. The method involves selective adsoption and concentration of the polyacrylic acids on other suitable adsorbent. The polyacrylic acids are then desorbed from the adsorbent and the concentration of polyacrylic acid in the aqueous system is determined by a conventional technique.

GB 2424876 defines a method for analysing and monitoring scale inhibitor content in the presence of interfering ions. The method employs treating the aqueous system with a cationic substrate of free cation so as to separate scale inhibitors from the system and allow analysis of the scale inhibitor is in the system. The treatment of the aqueous system allows the anionic scale inhibitor to become attached to the protonated cationic group of the substrate thus allowing the scale inhibitor to be adsorbed onto the substrate enabling it to be separated from the dissolved interfering ions. The scale inhibitor is then eluted by passing a basic solution through the cationic substrate containing the adsorbed scale inhibitor to deprotonate the cationic group thus allowing the anionic scale inhibitor to be released.

GB 2213933 describes a method for monitoring polyacrylic scale inhibitor in the presence of interfering polyvalent cations by treatment with a cationic exchange resin prior to conventional analysis for the polyacrylic acid.

EP 614079 relates to a method for monitoring the concentration of biocides used to treat aqueous systems. The biocides are said to have a detectable absobance or emission spectrum in the ultraviolet, visible and/or near infrared wavelengths. The spectrum of the aqueous system containing biocides is determined and chemo metric algorithms are applied to the spectrum to determine the concentration of biocides. The method is said to avoid separation techniques and so allow online monitoring.

US 5705394 concerns a method of determining the concentration of a water- soluble all-American treating agent added to a waste water treatment system. The method is said to involve several steps including dosing the body of water with a predetermined concentration of a treating agent having a fluorescent tag covalently bonded to the treating agent. A sample of the water containing the tagged treating agent is removed and by analysing the emission of the sample, the concentration of the treating agent in the sample is determined.

US 6153110 describes a method for controlling treatment chemicals in aqueous systems employing a special quad polymer as the active component. The polymers incorporate functional groups, for example aromatic groups, that exhibit chromophore or spectroscopic activity. The polymers are said to permit direct measurement of polymer levels in the water system but avoiding the expense and uncertainty associated with fluorescent tags, proteins tags, or other inert or inactive tracer species. Although this method will provide direct measurement of the treatment chemical it employs a polymer containing special functional groups to enhance the ability of detection. However, this special polymer may not necessarily provide the optimum specific treatment properties.

EP-A-564948 describes ultraviolet spectrographic monitoring of water soluble corrosion inhibitors which is based on measurement of absorbance of component of corrosion inhibitor formulation, for on-site determinations of

residuals. The corrosion inhibitors employed are imidazoline or pyridine derivatives. Such species are known to provide strong UV signals.

EP-A-614085 describes a method for directly monitoring the concentrations of water treatment compositions in steam generating systems. The method involves determining an absorbance or emission spectrum of the system water in the wavelength range from 200 to 2500, and applying chemometrics algorithms. The process relies on the concentration of treatment chemical in the steam being proportional to the concentration of water treatment chemical in the water.

EP-A-622630 describes a system for optimising the dose of the deposit inhibitor in a water circuit. This disclosure describes employing feedback from a heating element and thermistor in a sample or bypass line which generates a control signal in order to increase the pumping rate of the deposit inhibitor.

US 6740231 discloses a self contained treatment unit that can be controllably adapted to bleed or remove water from the aqueous system and to feed or deliver water treatment chemicals to the aqueous system. The treatment unit provides an alternative to extensive re-piping of a plant that would otherwise be required.

Japanese published application JP-A-11319885 describes an on stream concentration monitoring apparatus of process agent containing anionic polyelectrolyte in water base. The apparatus sucks sample water by moving a piston, which is then subsequently ejected to a colorimetry cell.

The objective of the present invention is to provide a method for directly determining the concentration of a water treatment chemical in an industrial process especially an industrial water circuit in which there are other water treatment chemicals. In particular it would be desirable to develop a system

which accurately determines the concentration of the water treatment chemical in the water present in the circuit. It would also be desirable for such a system to monitor conventional water treatment chemicals without the need for special tagging groups or companion chemicals.

In accordance with the present invention we provide a method of determining the concentration of a target water treatment chemical in water, in an industrial process in which the water contains at least one additional water treatment chemical(s), comprising the steps of, a) introducing a predetermined quantity of said water into a separating medium, b) separating the target water treatment chemical from the at least one additional water treatment chemical(s), c) employing a detector to determine the concentration of the target water treatment chemical, in which in step (b) the target water treatment chemical and at least one additional water treatment chemical(s) are separated in the separating medium by the target water treatment chemical taking a different period of time to pass through separating medium than the at least one additional water treatment chemical(s).

By target water treatment chemical we mean that it is the water treatment chemical for which the concentration is to be determined. Preferably the target water treatment chemical is a polymeric water treatment chemical. However, the target water treatment chemical may be non-polymeric, for instance a biocide.

Any other water treatment chemical present in the water with the target water treatment chemical may be regarded as the at least one additional water treatment chemical. The at least one additional water treatment chemical may be one or more of any conventional water treatment chemical or chemicals for

instance conventionally used in conjunction with the target water treatment chemical in the industrial process.

The industrial process may be a water treatment process involving clarification and/or filtration employing a polymeric coagulant and high molecular weight polymeric flocculant, for instance for providing potable water. Alternatively the industrial process may be the clarification or other solids liquid separation process for waste water employing a polymeric coagulant in conjunction with a high molecular weight anionic or cationic polymeric flocculant. The industrial process may also be a treatment process for mining run off water employing a polymeric coagulant and high molecular weight flocculant. A further example of suitable industrial process includes sedimentation, filtration or centrifugation of industrial effluent, paper effluent, mineral processing slurries employing a polymeric coagulant with a high molecular weight anionic or cationic polymeric flocculant. The concentration level of coagulant may be measured in the overflow, clarified run off water, filtrate or centrate. The monitoring may be to effect appropriate dose control of the polymeric coagulant or alternatively the polymeric flocculant. Alternatively this monitoring may be to ensure that residual coagulant or flocculant in the clarified water is maintained below a specified level. Whichever of the coagulant or flocculant is monitored is regarded as the target water treatment chemical in accordance with the present invention.

Preferably the water is water in an industrial water circuit, especially cooling water circuits. Typically such industrial water circuits will contain a variety of water treatment chemicals, including biocides, pH regulators, antiscalants, dispersants and corrosion inhibitors etc. any of which that is monitored in accordance with the method of the present invention would be regarded as the target water treatment chemical, whilst the other water treatment chemicals would be regarded as the at least one additional water treatment chemical. Typically the target water treatment chemical used in an industrial water circuit

will be any of polymeric antiscalants, polymeric dispersants, polymeric corrosion inhibitors or biocides. It is also possible to use a method of the invention to monitor more than one of the water treatment chemicals, each of which may then be regarded as target water treatment chemicals.

By passing a predetermined amount of the water from the process stream (for instance water in the circuit or other industrial process) into a separating medium the target water treatment chemical is separated from other chemical compounds present in the water. Typically the water treatment chemicals will be separated in the separating medium and each take different periods of time to pass through separating medium.

The separation of the target water treatment chemical from the one or more additional water treatment chemicals tends to occur by a size exclusion mechanism. By size exclusion mechanism we mean that molecules of different sizes pass through the separating medium at different rates with the consequence that compounds of molecules of different sizes become separated. Larger bulkier molecules tend to pass through the separating medium more quickly than relatively smaller molecules.

The mechanism of separating the target water treatment chemical from the at least one additional water treatment chemical(s) tends to avoid any significant adsorption onto the separating medium. Preferably the separating medium is selected and/or adapted to prevent any significant adsorption of any of the water treatment chemicals. This may be achieved by careful selection of the separating medium.

Preferably, however, in addition to a careful choice of separating medium, the method involves contacting the separating medium a treatment compound that prevents or significantly reduces any adsorption of the water treatment chemicals onto the separating medium. More preferably this involves passing

an aqueous mobile phase containing the treatment compound such as dissolved inorganic salts through the separating medium before or simultaneous with the introduction of the predetermined quantity of water from the industrial process into the separating medium. More preferably still the predetermined quantity of water should also contain the treatment compound, such as dissolved inorganic salts.

Typically the treatment compound used to prevent significant absorption of the water treatment chemicals will include alkali metal salts of mineral acids or organic acids. Suitable salts will include sodium chloride, disodium sulphate, sodium hydrogen sulphate, sodium acetate, sodium nitrate, thsodium phosphate, disodium hydrogen phosphate, sodium dihydrogen phosphate, potassium chloride, dipotassium sulphate, potassium hydrogen sulphate, potassium acetate, potassium nitrate, tripotassium phosphate, dipotassium hydrogen phosphate and potassium dihydrogen phosphate. Preferred salts include sodium chloride, sodium acetate, dipotassium hydrogen phosphate and mixtures thereof. The concentrations of salt dissolved in the aqueous phase may be between 0.001 and 1 mole, preferably between 0.01 and 0.5 mole.

The residence time required to separate the target water treatment chemical in a particular separating medium will depend on the specific chemical and separating medium. This can be determined by routine experimentation. In general the time required will be at least 30 seconds and can be up to 15 minutes or longer. Normally the residence time will be below 30 minutes. Preferably the residence time will be up to 10 minutes and typically within the range of from 1 minute to six or seven minutes.

Where the method is being used to establish the concentration of more than one target water treatment chemical, the resolution time can be determined for each of the target water treatment chemicals and then appropriate detector or detectors may be used to measure the separate signals issued by each of the

target water treatment chemicals. This can be achieved using a single sampling point and single separating medium and appropriate detector or detectors. Alternatively this can be achieved using a multiplicity of sampling points, separating media and detectors for each target water treatment chemical.

Typically the separating medium can be held in a column, cartridge or other housing. Preferably the separating medium is a chromatography packing capable of separating compounds of different molecular weight. Such a separating medium can facilitate the separation of the target water treatment chemical, which is of a specific molecular weight or molecular weight range, from other chemical species within the water of process stream. Typically the target water treatment chemical will be polymeric and have a weight average molecular weight at least 500 and usually greater. In general the polymeric target water treatment chemical can be separated by this method from the low molecular weight species, such as simple electrolytes, surfactants and other low molecular weight water treatment chemicals. When the target water treatment chemical is polymeric it may also be separated from additional polymeric water treatment chemicals of different molecular weight. For instance the polymeric target water treatment chemical for which the concentration is to be determined may be separated from additional polymeric water treatment chemicals of lower molecular weight and/or additional polymeric water treatment chemicals of higher molecular weight.

The target water treatment chemical can be added to the industrial process at the point or points of addition normally conventional for the particular industrial process. Likewise the at least one additional water treatment chemical can be added to the industrial process at the point or points of addition normally conventional for the particular industrial process.

Typically the separating medium can include but is not limited to any of the materials selected from the group consisting of silica, modified silica,

polyacrylamide, polysaccharide (including agarose or dextran), polymers of sulphonated styrene with divinylbenzene, polymers of,allyl dextran, polyvinyl alcohol), polyvinyl acetate), hydroxyethyl cellulose, hydroxylated poly(ethers), methacrylate polymers or copolymers and polymers designed with increased levels of hydroxyl functionality. Generally these would be in the form of commercially available chromatography column packing materials. Preferably, the separating medium would be a polysaccharide. The more preferred specific separating medium is a commercially available GPC packing material Sephadex ® (Amersham Biosciences Ltd) or similar or commercially available columns such as TSK (Toso Haas Corporation), PL aquagel (Varian) et cetera.

Suitably samples from the process stream can be injected directly into a separating medium by means of an injection valve or loop followed by passing it to a detection device.

Preferably, the detector is a spectrophotometer, preferably employing a signal that is selected from the group consisting of infrared light, ultraviolet light, visible light. Alternatively the detector is an evaporative light scattering detector or a device for measuring electrical conductivity or pH.

Preferably the detector produces a signal which is used to control the dose of target water treatment chemical. More preferably the detector signal is monitored and provides either manual or automated control of a dosing pump to increase or decrease the flow of target water treatment chemical. Typically the control of the dosing pump will depend upon the determined concentration of the target water treatment chemical by comparison to the required concentration. In effect if the determined concentration falls below the required concentration, the dosing pump will be controlled to increase the flow of target water treatment chemical. On the other hand is the determined concentration rises above the required concentration, the dosing pump will be controlled to decrease the flow of target water treatment chemical.

In one preferred form of the invention the three steps a) introducing the predetermined quantity of water into the separating medium; b) separating the target water treatment chemical from the at least one additional water treatment chemical(s); and employing a detector to determine the concentration of the target water treatment chemical are desirably contiguous. By this we mean that the steps a, b and c occur sequentially and each subsequent step b and c occurs substantially immediately on completion of the respective previous step a and b. In such a preferred form of the invention additional steps between the steps a, b and c would be unnecessary.

The present invention is particularly suitable as an in line system.

Therefore method according to the present invention may include an in line analyser which comprises a pump (allowing a mobile phase to pass through the analyzer), injection device, separating medium and a detector. The detector may be a spectrophotometer as described earlier or alternatively it may be a device, such as a probe, for measuring electrical conductivity or pH. When the device is a spectrophotometer it could contain a flow cell facilitating measurement at the sample flows through a flow cell. Desirably the output signal of the detector is monitored and variations in the signal strength used to actuate changes to the flow rate of the dosing pump. This can be achieved by manual input or preferably by automated control.

In a particularly preferred embodiment, a sample is taken from the process stream, which desirably is a cooling water circuit, and is gravity fed or pumped into a sample loop, tube or vessel of fixed volume. In this form the target water treatment chemical is preferably a target polymeric water treatment chemical. Suitably a switching valve is used to redirect the contents of the sample loop, tube or vessel on to a column containing the separation medium. This switching may be effected manually as required or preferably it is done automatically, for

instance at preset time intervals. Preferably the detector is a UV detector. The detector output is monitored at the elution time of the target water treatment chemical, which is preferably a polymer. If the signal is less than the predetermined signal value corresponding to the optimal dose a signal is sent to an actuator to increase the flow rate of the dosing pump. Conversely if the signal is greater than the predetermined signal value a signal is sent to decrease the flow rate of the dosing pump. It is possible that the target water treatment chemical can be added anywhere in the circuit. However, typically the target water treatment chemical is added into the makeup water which is fed into a cold water tank. The sample is typically taken from the flow line between the cold water tank and heat source. Nevertheless, other dosing and sampling points could be used appropriate for the particular configuration of the water circuit.

In another particularly preferred embodiment, where the target water treatment chemical in the sample does not have an active UV chromophore the preferred detector is a differential refractive index detector.

The method of the present invention is particularly useful for monitoring the concentration of target polymeric water treatment chemicals or biocides as target water treatment chemicals in for instance cooling water circuits. Additionally, the method is also useful for monitoring target polymeric water treatment chemicals in clarification of drinking water, overflows, filtrates, centrates, run-off waters reverse osmosis circuits or in boilers. The target polymeric water treatment chemical may be a dispersant, antiscalant, corrosion inhibitor, coagulant, flocculant or other target polymeric water treatment chemical commonly used in solid liquid separation processes or in industrial water circuits, especially cooling water circuits.

In accordance with the present invention it is generally not necessary to include any additional tracer compounds with the target water treatment chemical.

In general the target water treatment chemical is frequently an anionic polymer, a non-ionic polymer, or a cationic polymer. When the polymer is used as a dispersant, antiscalant or corrosion inhibitor it will usually have a weight average molecular weight of at least 500 but below 500,000. Often this is within the range of 1 ,000 and 100,000. Suitable target polymeric water treatment chemicals for industrial water circuits are described in the prior art. When the target polymeric water treatment chemical is used as a coagulant it will usually have a weight average molecular weight of at least 1000 but below 1 ,000,000. Often this is within the range of 10,000 and 500,000. When the target polymeric water treatment chemical is used as a flocculant it will usually have a weight average molecular weight of at least 500,000 and generally above one million. Typically the flocculant will be a polymer that has a molecular weight of several million, for instance between 5 million and 20 million.

Particularly suitable target water treatment chemicals used as dispersants, antiscalants or corrosion inhibitors include polymers formed from a monomer or a monomer blend consisting of, i) at least one ethylically unsaturated carboxylic acid (or salt thereof), and/or ii) at least one ethylically unsaturated sulphonic acid (or salt thereof), and iii) optionally (meth) acrylamide.

The target water treatment chemicals used as coagulants tend to be either cationic or anionic having a relatively high ionic charge density, for instance at least 4 milliequivalents per gram.

Particularly preferred coagulant target water treatment chemicals tend to be cationic. They can be formed from a monomer or monomer blend comprising i) at least one ethylically unsaturated cationic monomer and ii) optionally (meth) acrylamide.

Usually the cationic monomer is selected from dialkyl amino alkyl -(meth) acrylate or -(meth) acrylamide quaternised or salified to render them cationic or diallyl dialkyl ammonium salts.

Alternatively preferred coagulant target water treatment chemicals can be poly amines, amine epichlorohydrin polymeric adducts or dicyandiamide polymeric adducts.

The flocculant target water treatment chemicals can be non-ionic, anionic, cationic or amphoteric polymers. Typically they can be made from a monomer or monomer blend formed from one or more of the following: i) at least one ethylically unsaturated carboxylic acid (or salt thereof), and/or ii) at least one ethylically unsaturated sulphonic acid (or salt thereof), and /or iii) (meth) acrylamide, and/or iv) at least one ethylically unsaturated cationic monomer selected from dialkyl amino alkyl -(meth) acrylate or -(meth) acrylamide quaternised or salified to render them cationic or diallyl dialkyl ammonium salts.

An especially preferred target water treatment chemical used as dispersants, antiscalants or corrosion inhibitor is the homopolymer of acrylic acid or salts thereof, for instance the homopolymer of sodium, ammonium or potassium acrylate.

An especially preferred coagulant target water treatment chemical is the homopolymer of diallyldimethyl ammonium chloride.

Yet another especially preferred coagulant target water treatment chemical is a condensation reaction product of epichlorohydrin with a polyamine.

Especially preferred flocculant target water treatment chemicals include polymers of sodium acrylate optionally with acrylamide, polyacrylamide,

polymers of dimethyl amino ethyl acrylate quaternised with methyl chloride with acrylamide.

The method offers the advantage that any appropriate water treatment chemical, especially polymeric water treatment chemicals, can be used as the target water treatment chemical and is not limited specifically to specially designed water treatment chemicals, for instance containing tagged groups. Furthermore, conventional polymeric water treatment chemicals can be used without the need of tracer companion chemicals. The method also has the advantage that the analysis can occur in line and measurements may be made as often as required without in the need for an operator. By employing a separation stage only target water treatment chemical, for instance the target polymeric water treatment chemical, in solution passes through the separating medium and therefore the final measurement is made on the target water treatment chemical remaining in the water. We have also found that the measurement tends to be less prone to interference and therefore provides a more accurate indication of the treatment chemical concentration.

The following examples illustrate the invention.

Example 1

A slurry of the packing material is packed into the chromatography column. The column is connected in series to an autosampler and a detector. The detector is connected to a PC via a data manager.

column: Pharmacia Fast Desalting column packing: Sephadex ^® G-25 superfine packing flow rate: 1.0 ml/min mobile phase: 0.1 M sodium chloride + 0.01 M di-potassium hydrogen phosphate in an aqueous solution detector: Shimadzu SPD-10Avp

PC software: TriSEC GPC software, version 3, Viscotek Corporation autosampler: Kontron HPLC 360 autosampler

5, 10, 15, 20 and 25 ppm active solutions of Antiprex A (sodium polyacrylate solution polymer) are prepared in the mobile phase and injected onto the column using the autosampler. Peak heights and peak areas are measured for each peak and plots are constructed of peak height versus polymer concentration and peak area versus concentration as shown in figures 1 to 3.

Example 2

A slurry of the packing material is packed into the chromatography column. The column is connected in series to an autosampler and a detector. The detector is connected to a PC via a data manager.

column: Pharmacia Fast Desalting column packing: Sephadex ^® G-25 superfine packing flow rate: 1.0 ml/min

mobile phase: 0.1 M sodium chloride + 0.01 M di-potassium hydrogen phosphate in an aqueous solution detector: Shimadzu SPD-10Avp PC software: TriSEC GPC software, version 3, Viscotek Corporation autosampler: Kontron HPLC 360 autosampler

1 , 3 ,5, 7.5 and 10 ppm active solutions of Antiprex A (sodium polyacrylate solution polymer) are prepared in the mobile phase and injected onto the column using the autosampler. Peak heights and peak areas are measured for each peak and plots are constructed of peak height versus polymer concentration and peak area versus concentration as shown in figures 4 to 6.

Example 3

A size exclusion chromatography column is connected in series to an autosampler and a detector. The detector is connected to a PC via a data manager.

column: Toso Haas TSK PWXL G6000 flow rate: 1.0 ml/min mobile phase: 42.5g sodium acetate trihydrate + 18 mis glacial acetic acid in one litre of purified water. detector: Showdex RI-71 differential refractive index detector PC software: Polymer Laboratories Cirrus GPC software autosampler: Agilent 1100 series

A 50ppm active dilution of a poly(diallyl dimethyl ammonium chloride) coagulant is prepared in deionised water. A 50ppm active dilution of a cationic flocculant (80% polyacrylamide, 20% poly(dimethylamino ethyl acrylate) quaternised with methyl chloride) is prepared in deionised water.

Both dilutions are injected into the chromatography system separately to determine the elution characteristics, followed by a blend of the two to demonstrate the resolution between the two components. (Figure 7 and 8).

Example 4

A slurry of the packing material is packed into the chromatography column. The column is connected in series to an autosampler and a detector. The detector is connected to a PC via a data manager.

column: Pharmacia Fast Desalting column packing: Sephadex ^® G-25 superfine packing flow rate: 1.0 ml/min mobile phase: 0.06 molar sodium chloride solution in purified water detector: Shimadzu SPD-10Avp

PC software: TriSEC GPC software, version 3, Viscotek Corporation autosampler: Kontron HPLC 360 autosampler

A IOOppm dilution of active antiscalant is prepared in the mobile phase. A IOOppm dilution of active biocide is prepared in mobile phase. Both solutions are further diluted to a concentration of 10ppm. The following combination of samples and detector conditions are applied to the chromatographic system,

a) 10ppm biocide with wavelength set at 200nm b) 10ppm biocide with wavelength set at 270nm c) 10ppm biocide + 10ppm antiscalant with wavelength set at 200nm d) 10ppm biocide + 10ppm antiscalant with wavelength set at 270 nm e) 10ppm biocide (dilution made with cooling water field sample) with wavelength set at 200nm f) 10ppm biocide (dilution made with cooling water field sample) with wavelength set at 270nm

10ppm biocide (dilution made with cooling water field sample) with wavelength changed from 200nm after elution of antiscalant to 270nm.