AU2012203109C1 - Monitoring scale formation - Google Patents

Abstract

Abstract Deposition of precipitated inorganic salts on surfaces (scale formation) represents an important problem in many industrial applications, such as: seawater desalination by evaporation and reverse osmosis, boilers, cooling towers, oil recovery utilising water flooding techniques, and food processing. Surface deposits of scale forming minerals such as CaCO 3, CaSO 4, BaSO 4, CaC 2 0 4 and Mg(OH) 2 can encourage corrosion attack and growth of microorganisms and degrade conditions for heat exchange, thus lessening the efficiency of industrial processes. The present invention provides a method of monitoring scale formation, the method comprising: a) providing a scale forming solution within a cell at a predetermined temperature, the cell including an intrinsic exposed-core optical fibre sensor disposed so as to pass through the scale forming solution; b) acquiring a signal representing an intensity of light transmitted through the optical fibre sensor while maintaining the scale forming solution within the cell at the predetermined temperature; and c) processing the acquired signal so as to determine a parameter characterising the formation of scale in the scale forming solution.

Description

S&F Ref: P030976 AUSTRALIA PATENTS ACT 1990 PROVISIONAL SPECIFICATION FOR THE INVENTION ENTITLED: Monitoring scale formation Name and Address of Applicant: Ali Abdrabalrasoul Mohamed Al Hamzah, Saudi Arabian Citizen, of Sihat, Wasel 3657-32654, Eastern Provence, PO Box 6896, Saudi Arabia Name of Inventor: Ali Abdrabalrasoul Mohamed Al Hamzah This invention is best described in the following statement: 5805c(6221198_1) 1 MONITORING SCALE FORMATION TECHNICAL FIELD [0001] The present invention relates generally to scale deposition and, in particular, to monitoring scale formation in scale forming solutions. BACKGROUND [0002] Deposition of precipitated inorganic salts on surfaces (scale formation) represents an important problem in many industrial applications, such as: seawater desalination by evaporation and reverse osmosis, boilers, cooling towers, oil recovery utilising water flooding techniques, and food processing. Surface deposits of scale forming minerals such as CaCO 3 , CaSO 4 , BaSO 4 , CaC 2 0 4 and Mg(OH) 2 can encourage corrosion attack and growth of microorganisms and degrade conditions for heat exchange, thus lessening the efficiency of industrial processes. [0003] Scale deposition is usually addressed by adding small quantities (in the parts per million range) of scale inhibitors, which may be organic or inorganic compounds, either of low molar mass or polymers, to the solutions containing the scale forming ions. In the presence of scale inhibitors, the scale forming ions can exist in solution at concentrations higher than the critical concentration of precipitation. The efficiency of scale inhibitors depends on their molecular weight, polydispersity, composition, structure and concentration, and as well as the temperature and pH of the solutions in which they are used. The attractive features of certain scale inhibitors, such as ease of handling, relatively low cost, low dose rate, and minimal corrosion, have made their use widespread. For example, soluble polyelectrolytes such as poly(acrylic acid) (PAA), poly(maleic acid), poly(phosphonic acid) and co-polymers of these and similar monomers have been used to control CaCO 3 scale in desalination applications. Two phosphonate additives (methylenephosphonic acid and N,N,N',N'-ethlenediaminetetramethylenephosphonic acid) have been shown to have a significant reduction in crystal growth of Mg(OH) 2 . [0004] Currently, the experimental evaluation of the efficiency of scale inhibitors depends on one of two approaches. First, monitoring the conductivity and turbidity (light scattering) of a solution containing scale forming ions, such as calcium ions and sulphate or carbonate ions, under supersaturation conditions. Second, monitoring the changing ion concentrations over time, either by using a calcium ion selective electrode with Automatic Titrator, or by using spectrometric analytical methods such as atomic absorption and inductively coupled plasma. However, there are some difficulties with using both approaches at high temperatures and high 6218846_1 2 concentrations. For instance, turbidity measurements at high temperatures are interfered with by the formation of air bubbles. In addition, such measurements of bulk solution properties do not directly characterise the formation of scale deposits on surfaces contacting the solution. Ion selective electrodes need heavy and expensive equipment, are time-consuming, and suffer from interference from magnesium and hydrogen ions. [0005] A need therefore exists for methods and apparatus for monitoring scale formation, and determining the inhibiting effect of scale inhibitors, that are more robust, cheaper, more portable, and/or more amenable to laboratory conditions than present approaches. SUMMARY [0006] It is an object of the present invention to substantially overcome, or at least ameliorate, one or more disadvantages of existing arrangements. It is a further object to at least partially satisfy the above need. [0007] Disclosed are apparatus and methods for monitoring scale formation. The apparatus includes an optical fibre sensor. Measurements of attenuation in light transmitted through the optical fibre sensor are used to determine parameters that characterise the formation of scale in scale forming solutions. Light passing through the optical fibre sensor responds specifically to heterogeneous surface crystallisation, as occurs in scale deposition, rather than homogenous crystallisation throughout the scale forming solution, which affects the measurements of previous sensors. In addition, an optical fibre sensor is less expensive and can be used at higher temperatures and ion concentrations than previous sensors. Finally, the optical fibre sensor can be easily removed for microscopic analysis such as by scanning electron microscope and X-ray diffraction to study the morphologies of scale crystals deposited thereon. [0008] According to a first aspect of the present disclosure, there is provided a method of monitoring scale formation, the method comprising: a) providing a scale forming solution within a cell at a predetermined temperature, the cell including an intrinsic exposed-core optical fibre sensor disposed so as to pass through the scale forming solution; b) acquiring a signal representing an intensity of light transmitted through the optical fibre sensor while maintaining the scale forming solution within the cell at the predetermined temperature; and 6218846_1 3 c) processing the acquired signal so as to determine a parameter characterising the formation of scale in the scale forming solution. [0009] The following options may be used in conjunction with the first option, either individually or in any suitable combination. [00101 Step a) may comprise: al) bringing an aqueous liquid within the cell to the predetermined temperature, and a2) injecting scale forming ions into the cell to form the scale forming solution. This may comprise injecting a salt comprising the scale forming ions, optionally a solution thereof. [0011] Step a) may comprise: al) injecting the aqueous liquid at the predetermined temperature into the cell; and a2) injecting scale forming ions into the cell to form the scale forming solution. [0012] Step c) may comprise: cI) computing an attenuation of the transmitted light at a current time instant; c2) computing a rate of change of the computed attenuation with time; c3) determining whether the attenuation has reached a steady state value, using the computed rate of change; and c4) - computing, based on the determination, the parameter characterising the formation of scale in the scale forming solution. [0013] Step cl) may comprise computing the ratio of the initial transmitted intensity to the transmitted intensity at the current time instant. [0014] The parameter may be the rate of change of attenuation. It may be the steady state value. It may be the time taken to reach the steady state value. [0015] The solution may include a scale inhibitor. In this case the parameter may be the inhibition efficiency of the scale inhibitor. [00161 Step (c4) may comprise computing the complement of the ratio of the steady state value to a steady state attenuation value computed in the absence of the scale inhibitor. [0017] Step c) may further comprise, after step (c2) and before step (c3): 6218846_1 4 c2a) determining whether the computed rate of change of attenuation exceeds a predetermined threshold; c2b) computing the attenuation of the transmitted light at a current time instant; and c2c) computing a rate of change of the computed attenuation with time. In this case the parameter may be the time taken for the rate of change of the computed attenuation to exceed the predetermined threshold. [00181 The method may additionally comprise the steps of removing the optical fibre sensor from the cell and submitting the sensor to analysis. The analysis may comprise scanning electron microscopy and/or X-ray diffraction so as to study the morphologies of scale crystals deposited thereon. [0019] In an embodiment, there is provided a method of monitoring scale formation, the method comprising: a]) bringing an aqueous liquid within a cell to a predetermined temperature, the cell including an intrinsic exposed-core optical fibre sensor disposed so as to pass through the aqueous liquid, and a2) injecting scale forming ions into the cell to form a scale forming solution b) acquiring a signal representing an intensity of light transmitted through the optical fibre sensor while maintaining the scale forming solution within the cell at the predetermined temperature; and cl) computing an attenuation of the transmitted light at a current time instant; c2) computing a rate of change of the computed attenuation with time; c3) determining whether the attenuation has reached a steady state value, using the computed rate of change; and c4) computing, based on the determination, the parameter characterising the formation of scale in the scale forming solution. [001 9a] In another embodiment there is provided a method of monitoring scale formation, the method comprising: a) providing a scale forming solution within a cell at a predetermined temperature, the cell comprising a pressure vessel capable of withstanding an internal pressure of up to about 1.5 atmospheres and including an intrinsic exposed-core optical fibre sensor disposed so as to pass through the scale forming solution; b) acquiring a signal representing an intensity of light transmitted through the optical fibre sensor while maintaining the scale forming solution within the cell at the predetermined temperature; and 4a c) processing the acquired signal so as to determine a parameter characterising the formation of scale in the scale forming solution. [0020] In a second aspect of the invention there is provided an apparatus for monitoring scale formation, the apparatus comprising: a cell comprising: 5 a vessel adapted to contain a scale forming solution and maintain said scale forming solution at a predetermined temperature; an intrinsic exposed-core optical fibre passing through the vessel; and a fluid injector configured to inject a fluid into the vessel; a light source connected to one end of the fibre; a photometric detector connected to the other end of the fibre, configured to generate signals representing the intensity of light transmitted from the light source through the fibre sensor; and a computing device configured to process the generated intensity signals from the photometric detector. [0021] The following options may be used in conjunction with the second aspect, either individually or in any suitable combination. [00221 The cell may further comprises a housing, optionally a thermally conductive housing, that houses the vessel. It may also comprise a temperature controller configured to control the temperature of the housing. [0023] The light source may be monochromatic. It may be a laser light source. [0024] The apparatus may further comprise a stirring element for stirring the scale forming solution in the cell. [0025] The computing device may be further configured to control the fluid injector and / or the temperature controller. [0026] The optical fibre may be removable from the cell for surface analysis. [0027] The apparatus may comprise a pH detector disposed in the vessel so as to detect a pH of the scale forming solution therein. It may also comprise an injector disposed so as to inject a pH controlling solution into the vessel. This may serve to adjust the pH of the scale forming solution therein. The injector may be adapted to receive a control signal generated as a result of the pH detected by the pH detector. [0028] In an embodiment there is provided an apparatus for monitoring scale formation, the apparatus comprising: a cell comprising: 6218846_1 6 a vessel adapted to contain a scale forming solution and maintain said scale forming solution at a predetermined temperature; an intrinsic exposed-core optical fibre passing through the vessel; and a fluid injector configured to inject a Iluid, commonly a liquid, into the vessel; a monochromatic laser light source connected to one end of the fibre; a stirring element for stirring the scale forming solution in the cell; a photometric detector connected to the other end of the fibre, configured to generate signals representing the intensity of light transmitted from the light source through the fibre sensor; and a computing device configured to process the generated intensity signals from the photometric detector. [0028a] In an embodiment there is provided an apparatus for monitoring scale formation, the apparatus comprising: a cell comprising: a pressure vessel adapted to contain a scale forming solution and maintain said scale forming solution at a predetermined temperature and capable of withstanding an internal pressure of up to about 1.5 atmospheres; an intrinsic exposed-core optical fibre passing through the vessel; and a fluid injector configured to inject fluid into the vessel; a light source connected to one end of the fibre; a photometric detector connected to the other end of the fibre, configured to generate signals representing the intensity of light transmitted from the light source through the fibre sensor; and a computing device configured to process the generated intensity signals from the photometric detector. [0029] The method of the first aspect may use the apparatus of the second aspect. The invention therefore also encompasses the use of an apparatus according to the second aspect for monitoring scale production in an aqueous liquid. The monitoring may comprise determining at least one parameter characterising the formation of scale in the aqueous liquid. The invention also encompasses the use of an intrinsic exposed-core optical fibre for monitoring scale production in an aqueous liquid.

6a [0030] Other aspects of the invention are also disclosed. BRIEF DESCRIPTION OF THE DRAWINGS [0031] At least one embodiment of the present invention will now be described with reference to the drawings, in which: [0032] Fig. I is a diagram of an optical fibre sensor according to one embodiment; [0033] Fig. 2 is a diagram of an apparatus for monitoring scale formation according to one embodiment; [0034] Figs. 3A and 3B collectively form a schematic block diagram of a general purpose computer system which may be used as the digital computing device in the apparatus of Fig. 2; [0035] Fig. 4 is a flow diagram illustrating a method of monitoring scale formation using the apparatus of Fig. 2 according to one embodiment; 7 [00361 Fig. 5 is a plot of the attenuation for an exemplary scale forming solution without scale inhibitors, as produced by the apparatus of Fig. 2 using the method of Fig. 4; [0037] Fig. 6 is a plot of the attenuation measured under the conditions of Fig. 5 with several PAA-based scale inhibitors with different end groups; and [0038] Fig. 7 is a flow chart illustrating a method of processing the transmitted intensity signal, as used in the method of Fig. 4. DESCRIPTION OF EMBODIMENTS [0039] The apparatus and method described herein were originally developed for use in assessing the efficiency of certain scale inhibitors, in particular those based on polyacrylic acid. These are described in detail in an application filed concurrently with and having the same inventor as the present application, and entitled "Descaling polymers". The entire contents of this application are incorporated herein by cross-reference. It will be understood that the present invention, whilst suitable for this application, is not limited thereto. [0040] An important aspect of the apparatus described herein is an optical fibre sensor, which passes through the scale forming solution. As noted earlier, light passing through the optical fibre sensor responds specifically to heterogeneous surface crystallisation on the surface of the sensor. The optical fibre sensor comprises a length of optical fibre without the normal cladding, i.e. a bare core. This may be generated as such or may be obtained by removing the cladding from an optical fibre comprising a core surrounded by a cladding. The sensor passes through the vessel so that in use it will pass through the solution to be monitored. It may pass from one side or end of the vessel to the other. It may pass in a straight line or it may pass in a curve. The latter option may serve to increase the length of fibre in the solution to be monitored and therefore increase the sensitivity of the measurement. It is preferred that the sensor not contact the inner wall of the vessel other than at the points where it enters and exits the vessel. The sensor may have no cladding for the entire length that passes through the vessel, or for at least about 50% thereof, or at least about 60, 70, 80 or 90% thereof. Portions of the optic fibre that do not pass through the vessel (e.g. the portion which connects the vessel portion to the light source and/or the portion that connects the vessel portion to the photometric detector) may have cladding. This may serve to reduce attenuation of the light passing through the fibre except where it is used to monitor scale formation (i.e. except where it passes through the vessel). The fibre may be straight within the vessel or may be bent, curved, e.g. spiralled within the vessel. It may have a length within the vessel, or a length without cladding, or a measurement length, of about 10 to 6218846_1 8 about 50cm, or about 20 to 50, 30 to 50, 10 to 30, 10 to 20 or 20 to 30cm, e.g. about 10, 15, 20, 25, 30, 35, 40, 45 or 50. A suitable length is 21-23cm. The fibre may have a diameter of about 0.1 to Imm, or about 0.1 to 0.5, 0.5 to 1 or 0.3 to 0.8mm, e.g. about 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1mm. The fibre may have a smooth surface so as to optimise its optical properties. The optical fibre sensor may be removable from the vessel so as to enable microscopic analysis. This enables study of the morphologies of scale crystals deposited thereon. [0041] The sensor may be removable by virtue of the vessel being openable. For example the vessel may comprise two parts which are joinable so as to form the vessel and to allow the sensor to pass therethrough but when separate allow removal of the sensor for subsequent investigation. The vessel may comprise suitable seals so as to avoid leakage of liquid therefrom in operation. The vessel may comprise a drain to allow draining of a liquid therein prior to opening to remove the sensor. [0042] The vessel may have a volume of about 10 to 1000ml, or about 10 to 100, 10 to 50, 50 to 100, 100 to 1000, 100 to 500, 100 to 200, 200 to 1000, 500 to 1000 or 300 to 800ml, e.g. about 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 200, 300, 400, 500, 600, 700, 800, 900 or 1000ml. In some instances it may be larger or smaller than this. It may be elongated. This may be of benefit in maximising the fibre length within the solution whilst not unduly increasing the amount of solution in the vessel. It may have a length of about 10 to about 50cm, or about 10 to 30, 10 to 20, 20 to 50 or 20 to 40cm, e.g. about 10, 15, 20, 25, 30, 35, 40, 45 or 50cm. It may have a width and height, independently, of about 2 to about 10cm, or 2 to 5, 5 to 10 or 3 to 8cm, e.g. about 2, 3, 4, 5, 6, 7, 8, 9 or 10cm. It may have a length to width ratio of at least about 2, 3, 4, 5, 6, 7, 8, 9 or 10 or more than 10. It may have a square cross-section or a circular cross section or a semicircular cross-section or a rectangular cross-section or some other shaped cross section. Commonly the sensor will pass the entire length of the vessel. Alternatively it may pass the entire width or height thereof. In some particular cases it may pass more than once, e.g. 2, 3, 4, 5, 6, 7, 8, 9 or 10 times through the vessel, either lengthwise, widthwise or heightwise. The vessel may be made of any suitable material capable of withstanding the temperatures used in the experiment and the materials introduced therein. In some cases it may be a pressure vessel. This is particularly useful when the temperature of the experiment conducted using the cell is above 100*C. It may therefore be capable of withstanding an internal pressure of up to about 1.5, 2, 3, 4, 5, 10, 15, 20 , 30, 50 or 50 atmospheres. It may in some instances be transparent, or have a transparent window, so as to enable observation of the sensor and/or of the scale forming solution during the experiment. 6218846_1 9 [0043] The method of the present invention involves monitoring a scale forming solution within a cell. The solution should be maintained throughout the monitoring process at a constant temperature, preferably a predetermined temperature. In order to do this, the scale forming solution must be created at that temperature and maintained at that temperature thereafter. This can be achieved by bringing an aqueous liquid which is a precursor to the scale forming solution and then, once the aqueous liquid is in the cell and at the desired temperature, adding a a second solution containing ions which, in combination with ions in the aqueous liquid, can produce scale. Thus the addition of the second solution to the aqueous liquid generates the scale forming solution. Commonly the addition of the second solution will be attended by stirring, shaking or otherwise agitating the aqueous liquid so as to promote rapid mixing. [0044] In some embodiments, the aqueous liquid contains ions and the second solution comprises counterions which, when they encounter the ions in the aqueous liquid, can potentially form scale. In other embodiments a third solution may be added either sequentially or simultaneously with the second solution, whereby the second and third solutions separately contain ions and counterions which in combination potentially form scale. For the sake of simplicity, in the specification we refer to adding a second solution to the aqueous liquid, however it should be understood that this may equally refer to adding second and third solutions as described above to the aqueous liquid. For example, the aqueous liquid may comprise calcium ions in solution (e.g. a solution of calcium chloride) and the second solution may comprise carbonate ions in solution (e.g. a solution of sodium carbonate), so that when the second solution is added to the aqueous liquid in the vessel, a scale of calcium carbonate may be formed (depending on concentration, presence or absence of scale inhibitor and other factors). Alternatively, the aqueous liquid may be water (e.g. distilled or deionised water) and a second solution comprising calcium ions in solution (e.g. a solution of calcium chloride) and a third solution comprising carbonate ions in solution (e.g. a solution of sodium carbonate) may be added either concurrently or sequentially to the water in the vessel, so as to potentially form calcium carbonate scale. [0045] Commonly the liquid in the vessel will be continuously agitated/stirred during the experiment, during formation of the scale forming solution and during the step (if conducted) of bringing a liquid in the vessel to the desired temperature. This assists in promoting even distribution of ions throughout the liquid in the vessel and also even temperature throught the liquid. The agitation/stirring should be relatively gentle. It should be sufficiently gentle as to 6218846_1 10 avoid disturbing scale which deposits on the sensor. The stirrer should be disposed so as not to contact the sensor during conduct of the experiment, i.e. during monitoring of scale formation. [0046] The scale forming solution may, in operation of the method, completely fill the vessel, or it may partially fill the vessel. It may fill the vessel to a level of at least about 50%, or at least about 60, 70, 80, 90, 95 or 99%. In order that the aqueous liquid stay at the desired predetermined temperature, it is preferred that either the second solution is at the predetermined temperature when it is added, or else a very small amount of the second solution relative to the amount of aqueous liquid is added, or both. In one option the aqueous liquid is brought to the predetermined temperature and then introduced into the vessel (which is also set at the predetermined temperature so as to maintain the aqueous liquid at that temperature) or else the aqueous liquid is introduced into the vessel and then brought to the desired predetermined temperature. In either case, the second solution is added once the aqueous liquid has been brought to the desired temperature. The predetermined temperature may be between about 15 and about 150*C, or about 20 to 150, 25 to 150, 50 to 150, 100 to 150, 15 to 100, 15 to 50, 15 to 25, 20 to 100, 20 to 50 or 50 to 100*C, e.g. about 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140or150*C. [0047] The cell may comprise a thermostat disposed so as to measure the temperature of the scale forming solution, or the aqueous liquid. It may be coupled to a temperature controller, so as to bring a liquid in the cell to the predetermined temperature and/or to maintain it at that temperature. The temperature controller may be for example a heater within a liquid heating bath in which the vessel is at least partially immersed, or it may be an electrical heating element, or it may be a heat exchanger or some other form of temperature controller. [0048] The apparatus of the invention includes a light source disposed so as to pass light through the sensor to a photometric detector. The light source may be a laser light source. It may be a monochromatic light source. It may have a visible wavelength, or a UV (e.g. near UV) wavelength or an IR (e.g. near IR) wavelength. It may be a polychromatic light source. The wavelength, or each wavelength in the event of a polychromatic source, may be between about 300 and about 800nm, or about 300 to 400, 400 to 500, 500 to 600, 600 to 700, 700 to 800, 400 to 700, 400 to 600 or 500 to 700, e.g. about 300, 350, 400, 450, 500, 550, 600, 650, 700, 750 or 800nm. The wavelength may be selected so as to optimise the refractive index difference between the detector and the scale. The wavelength and the sensor should be selected in combination so that the sensor has a low as possible, preferably zero, absorbance for the selected wavelength. A suitable wavelength is 632.9nm. The light source may have a power of about 1 to 6218846_1 11 about 100mW, or about I to 50, 1 to 20, 1 to 10, 1 to 5, 5 to 100, 10 to 100, 50 to 100, 5 to 20, 5 to 10 or 3 to 8mW, e.g. about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 40, 50, 60, 70, 80, 90 or 100mW. [0049] The photometric detector is disposed so as to detect an intensity of light that has passed through the sensor from the light source. This provides a signal which may be used to determine a parameter characterising the scale formation on the sensor (or optionally the absence of such formation). Commonly in a scale forming solution, scale formation proceeds until the amount of scale produced reaches a steady state. In some cases, particularly in the presence of scale inhibitors, there may be a delay period or induction period before production of appreciable amounts of scale. Suitable parameters that may be measured, therefore, include the length of the delay period, the rate of increase during the formation period, the steady state value and the time taken to reach the steady state value. [0050] The detector may detect the light intensity continuously or it may detect it intermittently or, in an extreme case, it may detect it at time zero (as the scale forming solution is being formed) and at one further time. Commonly measurements will be taken at regular intervals. These may be intervals of from 1 to 60 seconds or more, or may be even more frequent. The intervals may be about I to 30, 1 to 20, 1 to 10, 10 to 60, 20 to 60, 30 to 60, 10 to 40, 10 to 20 or 20 to 30 second intervals, e.g. about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 or 60 second intervals. The acquisition may be continued (either continuously or at regular intervals) until the system is determined to be at a steady state. It may be continued for about 1 to about 60 minutes, or about 1 to 30, 1 to 20, i to 10, 10 to 60, 20 to 60, 30 to 60, 10 to 40, 10 to 20 or 20 to 30 minutes, e.g. about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 or 60 minutes. [0051] Measurement of the delay or induction period may involve determining the time until the difference between successive measurements exceeds a predetermined value. Measurement of the rate of increase may involve determining the difference between successive measurements once that difference exceeds a predetermined minimum and determining the difference once that difference is constant, or sufficiently constant, for successive pairs of measurements. Measurement of the steady state value or of the time taken to reach the steady state value may involve determining when the difference between two successive measurements returns below a predetermined value. These calculations may be conducted by a computer or similar device which receives signals from the photometric detector. 6218846_1 12 [0052] In a particular application of the method, it may be used to determine the inhibition efficiency of a scale inhibitor. This may be valuable in developing new improved scale inhibitors. In this application, a parameter (as described above) is obtained in successive experiments, one in which no scale inhibitor is present in the scale forming solution and another in which scale inhibitor is present, the experiments being otherwise as identical as possible. By comparison of the parameter (or of more than one parameter) in the two experiments, the inhibition efficiency of the scale inhibitor may be determined. If a series of experiments is conducted in which different concentrations of the scale inhibitor are present in the scale forming solution, a concentration dependence of scale inhibition efficiency may be determined, allowing an experimenter to optimise dosages for efficient scale inhibition. [0053] As discussed above, the optical fibre sensor may be removed from the cell following deposition of scale thereon. This enables any scale which has formed on the surface of the sensor to be analysed and/or or imaged e.g. by scanning electron microscopy and/or X-ray diffraction. This enables an experimenter to study the morphologies of scale crystals deposited on the sensor. The cell may also comprise electrodes disposed so as to be capable of measuring a conductivity of the liquid within the vessel. These may be for example platinum electrodes. They may be disposed on either side of the sensor, or on opposing inner walls of the vessel or in some other location. By measuring the conductivity of the scale forming solution and following that conductivity over time (either intermittently or continuously), additional information concerning the formation of scale within the vessel may be obtained. This may complement the information from other sensors (in particular the optical fibre sensor/photometric detector and/or pH sensor) in order to provide more complete information regarding scale formation within the vessel. [0054] The apparatus of the invention may comprise a stirring element for stirring liquid in the cell. This enables rapid and efficient mixing of added components, e.g. the second solution when forming the scale forming solution, a solution of a scale inhibitor, etc. Suitable stirring elements include a magnetic stirrer (i.e. a magnetic stirrer bar within the vessel and a stirrer base for stirring the bar external to the vessel), a mechanical stirrer (commonly with a motor outside the vessel, a blade inside the vessel and a shaft coupling these and passing through a wall of the vessel), a sonicator, a shaker or some other means for agitating or stirring a solution within the vessel. [0055] The apparatus may comprise a pH detector. This may be useful in adjusting the aqueous liquid in the vessel to a desired pH. Thus the pH detector may provide a signal which causes a control signal to be sent to an injector disposed so as to inject a pH-controlling solution into the 6218846_1 13 vessel. In some cases there may be more than one pH-controlling solution (and correspondingly more than one injector), one being acidic and another alkaline. This enables the system (or a user manually) to adjust pH either up or down as required. Suitable pH-controlling solutions should not have scale forming ions such as magnesium, calcium, carbonate etc. Suitable such solutions include sodium, ammonium or potassium hydroxides, organic ammonium hydroxides (e.g. methylammonium hydroxide), strong acids such as hydrochloric acid, hydrobromic acid, trifluoroacetic acid etc. These may be in relatively dilute solution so as to enable relatively fine pH adjustment. They may for example be about 0.01 to IN, or about 0.01 to 0.1, 0.1 to I or 0.05 to 0.5N, e.g. about 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or IN, although in some cases they may be more concentrated than this. The vessel may have one or more separate injector ports for introducing these to the vessel, or they may be introduced via the single port used for introducing the aqueous liquid, second solution and/or scale forming solution. The pH to which the liquid in the vessel is controlled may depend on the nature of the scale which is to be formed. The pH may be between about 5 and about 9, or about 5 to 7, 7 to 9 or 6 to 8, e.g. about 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5 or 9, although in some cases it may be more than 9 or less than 5. [0056] The apparatus may have a system controller, which may be a programmable logic controller (PLC), computer or similar. This may control one or more functions of the apparatus, for example temperature control, addition of the second solution to form the scale forming solution, illumination of the light source, introduction of the pH-controlling solutions into the vessel for pH control etc. The controller may also have a data acquisition and/or processing function for acquiring data from the photometric detector and other detectors if present and/or processing that data so as to generate the parameter or other output data. Alternatively a separate data acquisition and/or processing device may be present for that purpose. [0057] Where reference is made in any one or more of the accompanying drawings to steps and/or features, which have the same reference numerals, those steps and/or features have for the purposes of this description the same function(s) or operation(s), unless the contrary intention appears. [0058] Optical fibres are dielectric waveguides of circular cross-section, comprising a solid core of high refractive index material surrounded by a cladding of lower refractive index material. Optical fibres are capable of guiding light through the core via the optical mechanism of "total internal reflection" and are often employed as extrinsic and intrinsic components of chemical and physical sensors. An "intrinsic exposed-core optical fibre sensor" (IECOFS) 6218846_1 14 comprises an exposed section of optical fibre core, with the cladding not present, for example stripped away. When the IECOFS is inserted in a medium of lower refractive index than the core, such as water, the fibre core remains capable of guiding light via total internal reflection. However, when inserted in a supersaturated solution (still of lower refractive index), the IECOFS, while remaining capable of guiding light, provides a surface for heterogeneous surface crystallization of the dissolved salts. The crystals formed on the IECOFS surface typically have a refractive index higher than the IECOFS itself, resulting in light incident on the core/crystal interface refracting out of the IECOFS. This causes a measurable attenuation in light transmitted through the IECOFS that can be used to determine parameters that characterise the formation of scale in scale forming solutions. [0059] Fig. 1 is a diagram of a cell 100 for monitoring scale formation according to one embodiment of the invention. The cell 100 comprises a vessel 120, such as a glass cylinder, adapted to contain and maintain a scale forming solution 150 at a predetermined temperature. In one embodiment, a housing 110 of thermally conductive material, such as aluminium, houses the vessel 120. The housing 110 is heatable electrically or otherwise, and its thermally conductive properties allow the scale forming solution 150 to be maintained at the predetermined temperature. [0060] Along the longitudinal axis of the vessel 120 is disposed an IECOFS 130 of fused silica (of refractive index 1.457). The vessel 120 has small central holes at either end through which passes the IECOFS 130. The housing 110 also has holes at either longitudinal end through which the IECOFS 130 passes. The housing 110 and the vessel 120 also have holes through which a hollow fluid injector 140 is passed. Scale forming solution 150 may be injected into the vessel 120 via the injector 140. Typical dimensions of the vessel 120 are 15 cm in length, 2.5 cm in diameter (approximately 70 ml in volume); however other dimensions may be chosen. Typical dimensions of the IECOFS 130 are 19 to 21 cm in length and 0.1 to 1 mm in diameter. [0061] The vessel may also function as heat transfer tube. Thus the thermocouple which may be present in the vessel may be used for determining heat transfer efficiency. Since scale formation can affect heat transfer, heat transfer efficiency may be correlated with scale formation. This may supplement information obtained from the optical fibre sensor/photometric detector, in order, for example to provide more complete information regarding the efficiency of particular scale inhibitors. 6218846_1 15 [0062] Fig. 2 is a diagram of an apparatus 200 for monitoring scale formation according to one embodiment. The apparatus 200 comprises a cell 210 such as the cell 100 illustrated in Fig. 1. The IECOFS 215 of the cell 210 is connected at one end to a light source 220. In one implementation, the light source is a monochromatic laser, for example a 5 mW He-Ne laser of wavelength 632.9 nm. The other end of the IECOFS 215 is connected to a photometric detector 230, which is configured to generate a signal representing the intensity of the light transmitted by the IECOFS 215, which corresponds to the IECOFS 130 in Fig. 1. If the intensity signal is an analog signal, the apparatus 200 includes an analog / digital converter (ADC) 240 configured to convert the analog intensity signal to a digital signal at predetermined sampling intervals, typically 30 seconds. The digital signal, representing a series of discrete measurements of the intensity of the light transmitted through the IECOFS 215 at respective time instants, is suitable for processing by a digital computing device 250. In one implementation, the ADC 240 is incorporated within the digital computing device 250. If the intensity signal generated by the photometric detector 230 is a digital signal, the apparatus 200 does not include an ADC 240, and the intensity signal is passed directly to the digital computing device 250. [0063] Liquid may be injected into the cell 210 via the injector 260, which corresponds to the injector 140 in Fig. 1, so as to provide scale forming solution 205 within the cell 210. A temperature controller 270 is connected to the thermally conductive housing (e.g. the housing 110 in Fig. 1) that houses the cell 210, so as to control the temperature of the scale forming solution 205 within the cell 210. The apparatus 200 also includes a temperature sensor (for example a type J (Iron vs. Copper-Nickel) thermocouple that provides a signal representing the temperature of the scale forming solution 205 within the cell 210 to the temperature controller 270. The apparatus 200 also includes a stirring element 280 within the cell 210, adapted to effect stirring of the scale forming solution 205. In the implementation illustrated in Fig. 2,, the stirring element 280 is a magnetic stirring element that is controlled by a stirring controller 285 positioned outside the cell 210. In some implementations, all of the injector 260, the temperature controller 270, and the stirring controller 285 are under the control of the computing device 250. In other implementations, at least one of those elements is controlled manually. The range of temperature operation is from room temperature (about 200 C) to about 150* C. [0064] Figs. 3A and 3B collectively form a schematic block diagram of a general purpose computer system 300, which may be used as the digital computing device 250 in the apparatus 200 of Fig. 2 for the processing of the digital signal representing the- intensity of the light transmitted through the IECOFS 215. As seen in Fig. 3A, the computer system 300 is formed by 6218846_1 16 a computer module 301, input devices such as a keyboard 302, a mouse pointer device 303, and a microphone 380, and output devices including a printer 315, a display device 314 and loudspeakers 317. [0065] The computer module 301 typically includes at least one processor unit 305, and a memory unit 306 for example formed from semiconductor random access memory (RAM) and semiconductor read only memory (ROM). The module 301 also includes an number of input/output (I/O) interfaces including an audio-video interface 307 that couples to the video display 314, loudspeakers 317 and microphone 380, an I/O interface 313 for the keyboard 302 and mouse 303, and an interface 308 for the printer 315 and for any external data sources 316, such as the ADC 240 in the apparatus 200 of Fig. 2. The computer module 301 also has a local network interface 311 which, via a connection 323, permits coupling of the computer system 300 to a local computer network 322, for example a Local Area Network (LAN). The interface 311 may be formed by an EthernetTM circuit card, a BluetoothTM wireless arrangement or an IEEE 802.11 wireless arrangement. [0066] The interfaces 308 and 313 may afford either or both of serial and parallel connectivity, the former typically being implemented according to the Universal Serial Bus (USB) standards and having corresponding USB connectors (not illustrated). Storage devices 309 are provided and typically include a hard disk drive (HDD) 310. Other storage devices such as a floppy disk drive and a magnetic tape drive (not illustrated) may also be used. A reader 312 is typically provided to interface with an external non-volatile source of data. A portable computer readable storage device 325, such as optical disks (e.g. CD-ROM, DVD), USB-RAM, and floppy disks for example may then be used as appropriate sources of data to the system 300. [0067] The components 305 to 313 of the computer module 301 typically communicate via an interconnected bus 304 and in a manner which results in a conventional mode of operation of the computer system 300 known to those in the relevant art. Examples of computers on which the described arrangements can be practised include IBM-PC's and compatibles, Sun SparcstationsTM, Apple MacTM or computer systems evolved therefrom. [0068] The methods described hereinafter may be implemented as one or more software programs 333 executable within the computer system 300. In particular, with reference to Fig. 3B, the steps of the methods are effected by instructions 331 in the software 333 that are carried out within the computer system 300. The software instructions 331 may be formed as one or more code modules, each for performing one or more particular tasks. The software 333 may 6218846_1 17 also be divided into two separate parts, in which a first part and the corresponding code modules performs the methods and a second part and the corresponding code modules manage a user interface between the first part and the user. [0069] The software 333 is generally loaded into the computer system 300 from a computer readable medium, and is then typically stored in the HDD 310, as illustrated in Fig. 3A, or the memory 306, after which the software 333 can be executed by the computer system 300. In some instances, the application programs 333 may be supplied to the user encoded on one or more storage media 325 and read via the corresponding reader 312 prior to storage in the memory 310 or 306. Computer readable storage media refers to any non-transitory tangible storage medium that participates in providing instructions and/or data to the computer system 300 for execution and/or processing. Examples of such storage media include floppy disks, magnetic tape, CD-ROM, DVD, a hard disk drive, a ROM or integrated circuit, USB memory, a magneto-optical disk, semiconductor memory, or a computer readable card such as a PCMCIA card and the like, whether or not such devices are internal or external to the computer module 301. A computer readable storage medium having such software or computer program recorded on it is a computer program product. The use of such a computer program product in the computer module 301 effects an apparatus for monitoring scale formation. [0070] Alternatively the software 333 may be read by the computer system 300 from the network 322 or loaded into the computer system 300 from other computer readable media. Examples of transitory or non-tangible computer readable transmission media that may also participate in the provision of software, application programs, instructions and/or data to the computer module 301 include radio or infra-red transmission channels as well as a network connection to another computer or networked device, and the Internet or Intranets including e mail transmissions and information recorded on Websites and the like. [0071] The second part of the application programs 333 and the corresponding code modules mentioned above may be executed to implement one or more graphical user interfaces (GUIs) to be rendered or otherwise represented upon the display 314. Through manipulation of typically the keyboard 302 and the mouse 303, a user of the computer system 300 and the application may manipulate the interface in a functionally adaptable manner to provide controlling commands and/or input to the applications associated with the GUI(s). Other forms of functionally adaptable user interfaces may also be implemented, such as an audio interface utilizing speech prompts output via the loudspeakers 317 and user voice commands input via the microphone 380. 6218846_1 18 [0072] Fig. 3B is a detailed schematic block diagram of the processor 305 and a "memory" 334. The memory 334 represents a logical aggregation of all the memory devices (including the HDD 310 and semiconductor memory 306) that can be accessed by the computer module 301 in Fig. 3A. [0073] When the computer module 301 is initially powered up, a power-on self-test (POST) program 350 executes. The POST program 350 is typically stored in a ROM 349 of the semiconductor memory 306. A program permanently stored in a hardware device such as the ROM 349 is sometimes referred to as firmware. The POST program 350 examines hardware within the computer module 301 to ensure proper functioning, and typically checks the processor 305, the memory (309, 306), and a basic input-output systems software (BIOS) module 351, also typically stored in the ROM 349, for correct operation. Once the POST program 350 has run successfully, the BIOS 351 activates the hard disk drive 310. Activation of the hard disk drive 310 causes a bootstrap loader program 352 that is resident on the hard disk drive 310 to execute via the processor 305. This loads an operating system 353 into the RAM memory 306 upon which the operating system 353 commences operation. The operating system 353 is a system level application, executable by the processor 305, to fulfil various high level functions, including processor management, memory management, device management, storage management, software application interface, and generic user interface. [0074] The operating system 353 manages the memory (309, 306) in order to ensure that each process or application running on the computer module 301 has sufficient memory in which to execute without colliding with memory allocated to another process. Furthermore, the different types of memory available in the system 300 must be used properly so that each process can run effectively. Accordingly, the aggregated memory 334 is not intended to illustrate how particular segments of memory are allocated (unless otherwise stated), but rather to provide a general view of the memory accessible by the computer system 300 and how such is used. [0075] The processor 305 includes a number of functional modules including a control unit 339, an arithmetic logic unit (ALU) 340, and a local or internal memory 348, sometimes called a cache memory. The cache memory 348 typically includes a number of storage registers 344 346 in a register section. One or more internal buses 341 functionally interconnect these functional modules. The processor 305 typically also has one or more interfaces 342 for communicating with external devices via the system bus 304, using a connection 318. 6218846_1 19 [0076] The application program 333 includes a sequence of instructions 331 that may include conditional branch and loop instructions. The program 333 may also include data 332 which is used in execution of the program 333. The instructions 331 and the data 332 are stored in memory locations 328-330 and 335-337 respectively. Depending upon the relative size of the instructions 331 and the memory locations 328-330, a particular instruction may be stored in a single memory location as depicted by the instruction shown in the memory location 330. Alternately, an instruction may be segmented into a number of parts each of which is stored in a separate memory location, as depicted by the instruction segments shown in the memory locations 328-329. [0077] In general, the processor 305 is given a set of instructions which are executed therein. The processor 305 then waits for a subsequent input, to which it reacts to by executing another set of instructions. Each input may be provided from one or more of a number of sources, including data generated by one or more of the input devices 302, 303, data received from an external source across the network 322, data retrieved from one of the storage devices 306, 309 or data retrieved from a storage medium 325 inserted into the corresponding reader 312. The execution of a set of the instructions may in some cases result in output of data. Execution may also involve storing data or variables to the memory 334. [0078] The described methods use input variables 354, that are stored in the memory 334 in corresponding memory locations 355-358. The methods produce output variables 361, that are stored in the memory 334 in corresponding memory locations 362-365. Intermediate variables may be stored in memory locations 359, 360, 366 and 367. [0079] The register section 344-346, the arithmetic logic unit (ALU) 340, and the control unit 339 of the processor 305 work together to perform sequences of micro-operations needed to perform "fetch, decode, and execute" cycles for every instruction in the instruction set making up the program 333. Each fetch, decode, and execute cycle comprises: (a) a fetch operation, which fetches or reads an instruction 331 from a memory location 328; (b) a decode operation in which the control unit 339 determines which instruction has been fetched; and (c) an execute operation in which the control unit 339 and/or the ALU 340 execute the instruction. 6218846_1 20 [0080] Thereafter, a further fetch, decode, and execute cycle for the next instruction may be executed. Similarly, a store cycle may be performed by which the control unit 339 stores or writes a value to a memory location 332. [0081] Each step or sub-process in the methods described below is associated with one or more segments of the program 333, and is performed by the register section 344-347, the ALU 340, and the control unit 339 in the processor 305 working together to perform the fetch, decode, and execute cycles for every instruction in the instruction set for the noted segments of the program 333. [0082] The methods described below may alternatively be implemented in dedicated hardware such as one or more integrated circuits performing the functions or sub functions of the methods. Such dedicated hardware may include graphic processors, digital signal processors, or one or more microprocessors and associated memories. [0083] Fig. 4 is a flow diagram illustrating a method 400 of monitoring scale formation using the apparatus 200 of Fig. 2, according to one embodiment. At least one step of the method 400 is carried out by the processor of the computing device 250, for example the processor 305 of the computer system 300 of Fig. 3, in the manner described above. [0084] The method 400 begins at step 410, where a predetermined volume of scale forming solution 205 is provided in the cell 210 at a predetermined temperature. [0085] In one implementation of step 410, the predetermined volume of aqueous liquid is first injected into the cell 210 through the injector 260 as the basis of the scale forming solution 205. The pH of the aqueous liquid may optionally be set to a predetermined value by injecting a predetermined quantity of one or more non-scale-forming acid or alkaline solutions into the cell 210 via the injector 260, or, optionally, through a separate injector port (not shown in the figures) dedicated to injection of pH controlling solutions. The cell may comprise a pH sensor so as to detect the pH of the aqueous liquid. This may generate a signal to control injection of the pH controlling solution, so as to control the aqueous liquid at a desired pH. Alternatively, the pH sensor may send a signal to a controller (e.g. a computer or PLC), which generates a signal to control the injection of the pH controlling solution. The temperature controller 270 is then set to heat the thermally-conducting housing of the cell 210 so as to bring the aqueous liquid to the predetermined temperature. When the temperature sensor 275 indicates that aqueous liquid has reached the desired temperature, predetermined quantities of the scale forming ions in solution(s), and any scale inhibitors, are then injected into the cell 210 via the injector 260 to 6218846_1 21 form the scale forming solution 205. This instant represents the time origin ( = 0) for a given experiment. [0086] In a further implementation of step 410, the predetermined volume of aqueous liquid at the predetermined temperature is first injected into the cell 210 through the injector 260 as the basis of the scale forming solution 205. The pH of the aqueous liquid may optionally be set to the predetermined value by injecting predetermined quantities of non-scale-forming acid or alkaline ions into the cell 210 via the injector 260. Predetermined quantities of the scale forming ions in solution(s), and any scale inhibitors, are then injected into the cell 210 via the injector 260 to form the scale forming solution 205. This instant represents the time origin (t = 0) for a given experiment. [0087] In either implementation of step 410, magnetic stirring of the scale forming solution 205 may be commenced using the stirring element 280. [0088] Step 410 is carried out by the processor 305 if the injector 260, the temperature controller 270, and (if present) the stirring controller 285, are controlled by the computing device 250. Otherwise, step 410 is carried out manually. [0089] At the next step 420, a digital signal representing measurements at predetermined intervals of the intensity of light transmitted through the IECOFS 215 is acquired by the computing device 250 from the ADC 240 while maintaining the scale forming solution 205 within the cell at the predetermined temperature by appropriate control of the cell 210 via the temperature controller 270. Step 420 is wholly carried out by the processor 305 if the temperature controller 270 is controlled by the computing device 250. [0090] At the next step 430, the acquired digital signal is processed by the computing device 250 so as to determine a parameter characterising the formation of scale in the scale forming solution 205. One implementation of the processing step 430 is described below with reference to Fig. 7. The method 400 then concludes. [0091] The intensity of light transmitted through the IECOFS 215 of the cell 210 as measured by the photometric detector 230 and digitised by the ADC 240 at a given time t is denoted as P,. The transmitted intensity Po at time t = 0, when no scale has yet formed, is the base value for the computation of the amount of attenuation A, at time t. The amount of attenuation A, at time t is computed as the ratio of the initial transmitted intensity Po to the transmitted intensity P, at time t, expressed in decibels: 6218846_1 22 A, = -10 log j (1) [0092] At t = 0, P, = Po so A, is zero. As t increases, scale begins to be deposited on the IECOFS 215, hence P, decreases, so the attenuation A, increases at a rate that is approximately proportional to the rate of heterogeneous crystallisation. Eventually, when the heterogenous surface crystallisation of the scale forming ions reaches equilibrium, the rate of change of the attenuation A, typically decreases to near zero, and the attenuation reaches a steady state value. The steady state attenuation for a particular scale inhibitor is written herein as A., with the subscript x indicating the inhibitor used. For example, the "blank" steady state attenuation, achieved in the absence of scale inhibitors, is written as A blank. [0093] Fig. 5 is a plot 500 of the attenuation A, over 1000 seconds at 30 second intervals for an exemplary scale forming solution 205, as produced by the apparatus 200 using the method 400. The exemplary scale forming solution 205 has pH 9.2, temperature 1000 C, and contains 100 ppm of C03 ions and 66 ppm of Ca ions, but no scale inhibitors (i.e. "blank" conditions). [0094] Fig. 5 shows that under blank conditions there is an initial increase in attenuation at a rate of approximately 0.0048 dB/sec for the first 30 seconds, indicating the depletion of reactants in solution. Thereafter the attenuation increases steadily at a rate of approximately 0.0026 dB/sec. A steady state attenuation value of around 1.5 dB is reached after about six minutes. [0095] Fig. 6 is a plot 600 of the attenuation A, measured under the conditions of Fig. 5 with several PAA-based scale inhibitors with different end groups added to the scale forming solution 205 at a concentration of 6.70 ppm, alongside the "blank" attenuation from Fig. 5. The scale inhibitors are labelled EIB (short length hydrophobic end group, 6 carbon atoms); CIB (middle length hydrophobic end group, 10 carbon atoms), and CMM (hydrophilic end group, 4 carbon atoms). [0096] Fig. 6 shows that the attenuation for each scale inhibitor follows a curve that can be characterised by at least three stages: the initial linear increase stage, the maximum rate stage, and the steady state stage. The maximum rate of increase for the CMM, EIB, and CIB scale inhibitors may be computed from the attenuation curves in Fig. 6 as .0059 dB / sec, .0021 dB / sec, and .0023 dB / sec respectively. The steady state attenuation value for each inhibitor, Acmm, AEIB, and Ac/B, is less than that achieved in blank conditions, Ablank, showing that the formation of scale has been inhibited. With some scale inhibitors, there is a fourth stage before the linear increase stage commences, during which the rate of increase of attenuation is insignificant. 6218846_1 23 During this fourth stage, there is a delay in the onset of scale formation caused by the scale inhibitor. For example, in Fig. 6 for the CIB scale inhibitor this "induction" stage occupies approximately the first sixty seconds of the experiment. [0097] Fig. 7 is a flow chart illustrating a method 700 of processing the transmitted intensity signal, as used in step 430 of the method 400 of Fig. 4. The method 700 is carried out by the processor of the computing device 250, for example the processor 305 of the computer system 300 of Fig. 3, in the manner described above. The method 700 is adapted for the characterisation of scale inhibitors that have the four stages of attenuation over time mentioned above in relation to Fig. 6. [0098] The method 700 starts at step 710, at which the processor 305 uses equation (1) to compute the attenuation A, from the transmitted power P, at the current time t. At the following step 720, the processor 305 computes the rate of change of attenuation with time, A',, using the current value A, and one or more previously measured values of attenuation. Step 730 follows, at which the processor 305 determines, based on the rate A', computed at step 720, whether the attenuation A, has begun to increase. In one implementation, step 730 tests whether the rate A', exceeds a predetermined threshold. If not, the method 700 returns to step 710 to await the next transmitted power value P,. If so, the method 700 proceeds to step 740, at which the processor 305 uses equation (1) to compute the attenuation A, from the transmitted power P, at the current time t. At the following step 750, the processor 305 computes the rate of change of attenuation with time, A',, using the current value A, and one or more previously measured values of attenuation. Step 760 follows, at which the processor 305 determines, based on the rate A', computed at step 720, whether the attenuation A, has reached a steady state value. In one implementation, step 750 tests whether the rate A', has fallen below a second predetermined threshold. If not, the method 700 returns to step 740 to await the next transmitted power value P,. If so, the method 700 proceeds to step 770, at which the processor 305 computes the parameter characterising the formation of scale in the scale forming solution 205 from the values computed in the preceding steps 710 to 760. Suitable parameters include the rate of formation of scale, the induction period before scale is produced, the steady state amount of scale formed and the time taken to achieve the steady state amount. The method 700 then concludes. [0099] In an alternative implementation of the method 700, steps 730, 740, and 750 are omitted, i.e. step 720 proceeds to step 760, which returns to step 710 in the event of a negative determination. The alternative implementation of the method 700 is adapted for the 6218846_1 24 characterisation of scale inhibitors that have no induction stage of the kind mentioned above with reference to Fig. 6. [00100] In one implementation of the step 770, the parameter characterising the formation of scale in the scale forming solution 205 is the steady state value A. In this implementation, step 770 simply returns the most recent value of the attenuation A, computed at step 750 as the steady state value A. In an alternative implementation of the step 770, the parameter characterising the formation of scale in the scale forming solution 205 is returned as one of the values of the rate of change of attenuation with time, A',, computed at step 750 before the steady state was reached. [00101] In yet another implementation of the step 770, the parameter characterising the formation of scale in the scale forming solution 205 is the time taken to reach the step 740, i.e. the length of the induction stage before the rate of increase of attenuation becomes significant. In yet another implementation of the step 770, the parameter characterising the formation of scale in the scale forming solution 205 is the time taken to reach the step 770, i.e. the time taken to reach the steady state. [00102] A scale inhibitor, by definition, gives a steady state attenuation value A, that is less than the "blank" steady state attenuation value Ablank, so the ratio A, / Ablank is less than one. The inhibition efficiency (IEx) of a given scale inhibitor x may be computed as the complement of the ratio of the steady state attenuation A, achieved using that scale inhibitor to the "blank" steady state attenuation A blank, expressed as a percentage: IE,% =[l A ]x 100 (2) [00103] According to equation (2), the inhibition efficiency with no scale inhibitor is 0%, while a "perfect" scale inhibitor S which completely prevents heterogeneous crystallisation gives As = 0 and hence IEs = 100%. [00104] In yet another implementation of step 770, the parameter characterising the formation of scale in the scale forming solution 205 is the inhibition efficiency IE/o, computed from the steady state attenuation value A. using equation (2). [00105] Using the results illustrated in Fig. 6, the inhibition efficiencies IE/ may be computed from the steady state attenuation values of the CMM, EIB, and CIB PAA scale inhibitors as 56%, 64%, and 68% respectively. 6218846_1 25 [00106] The arrangements described are applicable to the desalination and food processing industries, to name just two examples. [00107] The foregoing describes only some embodiments of the present invention, and modifications and/or changes can be made thereto without departing from the scope and spirit of the invention, the embodiments being illustrative and not restrictive. 6218846_1

Claims (26)

1. A method of monitoring scale formation, the method comprising: a) providing a scale forming solution within a cell at a predetermined temperature, the cell comprising a pressure vessel capable of withstanding an internal pressure of up to about 1.5 atmospheres and including an intrinsic exposed-core optical fibre sensor disposed so as to pass through the scale forming solution; b) acquiring a signal representing an intensity of light transmitted through the optical fibre sensor while maintaining the scale forming solution within the cell at the predetermined temperature; and c) processing the acquired signal so as to determine a parameter characterising the formation of scale in the scale forming solution.

2. A method according to claim 1, wherein step a) comprises: al) bringing an aqueous liquid within the cell to the predetermined temperature, and a2) injecting scale forming ions into the cell to form the scale forming solution.

3. A method according to claim 1, wherein step a) comprises: al) injecting the aqueous liquid at the predetermined temperature into the cell; and a2) injecting scale forming ions into the cell to form the scale forming solution.

4. A method according to any one of claims 1 to 3, wherein step c) comprises: cl) computing an attenuation of the transmitted light at a current time instant; c2) computing a rate of change of the computed attenuation with time; c3) determining whether the attenuation has reached a steady state value, using the computed rate of change; and c4) computing, based on the determination, the parameter characterising the formation of scale in the scale forming solution.

5. A method according to claim 0, wherein the step (cl) comprises computing the ratio of the initial transmitted intensity to the transmitted intensity at the current time instant.

6. A method according to any one of claims 1 to 5, wherein the parameter is the rate of change of attenuation. 27

7. A method according to any one of claims 1 to 5, wherein the parameter is the steady state value.

8. A method according to any one of claims 1 to 5, wherein the parameter is the time taken to reach the steady state value.

9. A method according to any one of claims 1 to 5, wherein the solution includes a scale inhibitor, and the parameter is the inhibition efficiency of the scale inhibitor.

10. A method according to any one of claims 4 to 0, wherein step (c4) comprises computing the complement of the ratio of the steady state value to a steady state attenuation value computed in the absence of the scale inhibitor.

11. A method according to any one of claims 4 to 10, wherein step c) further comprises, after step (c2) and before step (c3): c2a) determining whether the computed rate of change of attenuation exceeds a predetermined threshold; c2b) computing the attenuation of the transmitted light at a current time instant; and c2c) computing a rate of change of the computed attenuation with time.

12. A method according to claim 0, wherein the parameter is the time taken for the rate of change of the computed attenuation to exceed the predetermined threshold.

13. A method according to any one of claims I to 12 additionally comprising the steps of removing the optical fibre sensor from the cell and submitting the sensor to analysis.

14. The method of claim 13 wherein the analysis comprises scanning electron microscopy and/or X-ray diffraction so as to study the morphologies of scale crystals deposited thereon.

15. An apparatus for monitoring scale formation, the apparatus comprising: a cell comprising: a pressure vessel adapted to contain a scale forming solution and maintain said scale forming solution at a predetermined temperature and capable of withstanding an internal pressure of up to about 1.5 atmospheres; an intrinsic exposed-core optical fibre passing through the vessel; and 28 a fluid injector configured to inject fluid into the vessel; a light source connected to one end of the fibre; a photometric detector connected to the other end of the fibre, configured to generate signals representing the intensity of light transmitted from the light source through the fibre sensor; and a computing device configured to process the generated intensity signals from the photometric detector.

16. An apparatus according to claim 15, wherein the cell further comprises a thermally conductive housing that houses the vessel, and a temperature controller configured to control the temperature of the thermally conductive housing.

17. An apparatus according to claim 15, wherein the light source is monochromatic.

18. An apparatus according to claim 17, wherein the light source is a laser light source.

19. An apparatus according to claim 15, further comprising a stirring element.

20. An apparatus according to claim 15, wherein the computing device is further configured to control the fluid injector and / or the temperature controller.

21. An apparatus according to any one of claims 15 to 20 wherein the optical fibre is removable from the cell for surface analysis.

22. An apparatus according to any one of claims 15 to 20 comprising a pH detector disposed in the vessel so as to detect a pH of the scale forming solution therein.

23. The apparatus of claim 22 comprising an injector disposed so as to inject a pH-controlling solution into the vessel to adjust the pH of the scale forming solution therein, said injector being adapted to receive a control signal generated as a result of the pH detected by the pH detector.

24. Use of an apparatus according to any one of claims 15 to 23 for monitoring scale production in an aqueous liquid. 29

25. Use according to claim 24 wherein the monitoring comprises determining at least one parameter characterising the formation of scale in the aqueous liquid.

26. Use of an intrinsic exposed-core optical fibre for monitoring scale production in an aqueous liquid at elevated temperature and/or elevated pressure. Ali Abdrabalrasoul Mohamed Al Hamzah Patent Attorneys for the Applicant/Nominated Person SPRUSON & FERGUSON