GB2166426A - A process and apparatus for acid-free descaling - Google Patents

Abstract

Water mixed with pressurised carbon dioxide is passed through the object and then the pressure is reduced while the water containing removed scale is passed into a tank separated from the object. The scale is contained in the discharged water in the form of a slurry which is separated from the water in the tank and is intermittently discharged. In this way the equilibrium between calcium bicarbonate and calcium carbonate is controlled by a programmed variation of the partial pressure of carbon dioxide.

Description

SPECIFICATION A process and apparatus for acid-free descaling The invention concerns a process and apparatus for the removal of scale without the use of mineral acids, e.g. from scaled-up metallic surfaces.

It is known that the deposition of scaling (furring) causes significant problems primarily for heat transfer surfaces, e.g. boilers, cooking and boiling devices as well as heat exchangers because it causes a reduction of the heat transfer output, increases the flow resistance of the apparatus and other disturbing factors which may in given cases lead to shutdown of operations. Hence, in practice, numerous technical solutions are used as protection against the harmful effects of scale deposition.

These solutions may essentially be classified into two groups: one group of technical measures serves for the prevention of scaling while another group serves for the removal of the scale which has already deposited.

The existing and known measures for scale prevention consist mainly in the so-called softening of the water. This can be achieved by a chemical precipitation of the calcium and magnesium salts in the water and with a filtration of the precipitate, by means of which the alkaline earth metal ions are changed to sodium cations, but softening of the water may also be achieved by the use of ion exchanging electrolytes. The use of water softening processes is restricted not only by their high expense but also by the fact that the electrolyte content and concentration of the water is in certain circumstances controlled by official regulations in dependence on the intended purpose.Thus, for instance, most authorities prohibit the removal of electrolytes from drinking water and from domestic hot water for health reasons even where the chemical composition of the drinking water is often far from optimal from a physiological point of view.

There have been proposals for prevention of scaling and indeed for loosening of already existing scaling or furring by a magnetic treatment of the water, such treatment consisting of passing the water through a magnetic force field before feeding it into the object or artifact to be protected from scaling. Relevant publications describe many positive and negative experiences regarding the efficiency of this method, see for instance Szakáll- Nyilassy: Hidrológiai K6zldny 1965, (1), pages 477480 (Hidrol6giai Kdzlony = Hydrological Gazette).

We have not yet found a convincing hypothesis, let alone proof, for the mechanism of the effect, but from a practical point of view an even greater difficulty is that the reproducibility of this solution is problematic, both according to the literature information and practical experiences.

For the post-formation removal of scale, mostly performed intermittently, both mechanical and chemical methods are known. In the mechanical processes various polishing and wearing devices are utilized, while the chemical solutions involve dissolving the scale, usually in hydrochloric acid or other mineral acid. Both methods are laborious and are extremely limited as regards utilisability. In addition, descaling by mineral acid treatment does not reassuringly solve the problem of protecting metallic structures even when inhibitors are used and to this should be added the problems of protecting the working personnel and their health as well as environmental problems.

With due regard to all these factors it was considered necessary to develop the process and apparatus according to the invention which is based on the discovery that the known process for establishing the equilibrium between calcium carbonate and calcium hydrocarbonate can be accelerated by a programmed variation of the pressure of carbon dioxide.Both from our theoretical knowledge and practical experience it can be shown and proved that the formation of scale and its breakdown are respectively the result of a reversable chemical process which is as follows: Ca(HCO3)2 = CaCO3 + CO, + H20 The upper arrow indicates the formation of scale while the lower arrow indicates the breakdown of the scale since calcium carbonate is a hardly soluble compound while the bicarbonate is readily soluble. The reaction is illustrated by way of the calcium compounds because even where the reaction of scale formation takes place in the presence of magnesium ions the precipitation of calcium carbonate is the dominant process.The reaction equation illustrates that an increase in the partial pressure of the carbon dioxide in the presence of water favours the breakdown of scale while its reduction favours the formation of scale. The fluctuation at constant temperature of the pressure of carbon dioxide arising in practice explains the experience frequently obtained in use, namely that in the same object the scale forms and then breaks down while an increase in temperature for the same quality or hardness of water results in dissociation of the carbon dioxide, i.e. formation of scale. This is why scale always deposits first on a heat transfer surface.

Incidentally, in nature the changes in the partial pressure of carbon dioxide results in the formation of stalactites. In the internal cracks and pores of limestone and dolomite strata the pressure exceeds the ambient pressure when the calcium compounds, namely limestone, pass into an aqueous solution enriched with carbon dioxide, and subsequently the calcium bicarbonate solution seeps to the surface of the rock and there, due to a decrease in pressure, the carbon dioxide breaks down while simultaneously the water evaporates and accordingly the limestone precipitates in the form of stalactites or stalagmites.

An important task in the development of the process and apparatus according to the invention was not only the elimination of the use of acids to obviate the disadvantages mentioned above but also a significant acceleration of the known process of stalactite - stalagmite formation and thus the reduction to industrial practicability of these processes. Accordingly, an aim of the invention was the provision of a readily performable process for descaling by means of utilising carbon dioxide at variable pressures within a range of pressures which conform to safety prescriptions as well as to the properties of the given apparatus.

In an advantageous preferred mode of carrying out the invention the closed or closable object affected by scaling is filled with water of any hardness, then carbon dioxide is forced into the object either from an external source or by partially dissolving the scale in a non-mineral acid and the pressure of the gas is maintained for a constant time, namely the reaction time; subsequently the water subjected to the given pressure is permitted to discharge towards a space or reaction chamber of lower pressure, whereby the water flows to the space (e.g. the ambient) or reaction space. Thereafter the water is maintained in circulation, expediently turbulent flow circulation, between the two spaces while the supply of carbon dioxide is interrupted, whereby the pressure of the carbon dioxide dissolved in the water flowing between the two spaces will in practice be the same everywhere.

The circulation of water is then interrupted, the object affected by scale is sealed again, the pressure of carbon dioxide therein is increased once more to the value maintained in the previous reaction time and the process is continued as described above. In this way, the above-described process steps are essentially periodically repeated and each cycle consists of two periods, which are not necessarily of identical length, namely the reaction time and the circulation (flushing) time.

During the reaction time the equilibrium between the carbonate and bicarbonate is displaced towards the bicarbonate under the effect of the increased gas pressure. Since the gas pressure is maintained during the reaction time, the gas will flow into the object from its source, e.g. a gas bottle, even after the desired pressure has been attained, but at a rate determined by the scale breakdown reaction. In the circulation or flushing period, and especially in its initial part, the sudden reduction in pressure causes carbon dioxide to be removed from the liquid and the equilibrium is displaced towards the formation of the carbonate, whereby the carbonate, i.e. scale, dissolved in the previous period is precipitated once more, but in accordance with the high speed of the reaction it precipitates not as a compact coating or deposit but as a finely divided precipitate, 'mud' or slurry.

The use of turbulent water circulation is advantageous for the reason that such flow is more effective in sweeping the precipitate out of the object into the space, e.g. vessel or tank, maintained at all times under low pressure. A filter disposed between the tank and the circulating pump serves to prevent the re-entry of the precipitate from the tank into the object. Thus in the above-described manner the scale is literally carried out from the object into a tank which is expediently emptiable, and moreover the scale is not in the form of an environment-polluting acidic solution but in the form of a slurry which has no harmful effect on the environment and precisely because of this, may be periodically discharged from the tank and directly passed into the ground or soil anywhere at all.

Since no mineral acid is employed in the process, the method may also be used without any special protective measures directly in objects used in the food industry and the descaling process does not cause any additional corrosive damage in the object concerned. The above-described process not only removes scale by chemical means but also by mechanical means since the turbulent water flow entrains scale particles which have been loosened but not reacted by the chemical reaction and such particles are also entrained and carried off into the tank. The same may be said of the 'attendant' components embedded in the scale, e.g. haematite particles, siderite particles, silicate compounds, formed as products of corrosion.All these are discharged during the process from the scale-affected object and are concentrated in the tank, but not as a coherent and compact coating but as a slurry which at any desired time may be readily removed.

If the physical properties of the apparatus permit this, the possibility opens up of collecting the carbon dioxide that is liberated in the flushing period; moreover, it may be possible to promote the dissociation of carbon dioxide by placing the whole system under vacuum and compressing the carbon dioxide which is quasi-regenerated in this way and recycling it into the process. In such a case the carbon dioxide mediates the continuous descaling process in the same way in a closed cycle as the water. However, where difficulties arising from adequate sealing of the whole process are in given cases disproportionate to the costs of carbon dioxide make-up, then another practical possibility is to permit the carbon dioxide liberated on the re-precipitation of the carbonate compounds to escape and to cover the carbon dioxide requirements for descaling from a portable gas bottle.Another preferred embodiment of the apparatus according to the invention is a 'portable variant' because in this case descaling can be performed as a service activity and the apparatus according to the invention may be continuously and well exploited.

A preferred embodiment of the invention is described, purely by way of example, with reference to the accompanying diagrammatic drawings, wherein :- Figure 1 is a view of the apparatus illustrating the apparatus and process according to the invention, and Figure 2 illustrates an automatically operating switching/valve mechanism.

The object 1 to be descaled may be a structural component, a tank, a material, or a surface thereof, will hereafter for brevity be referred to as 'the object' is connected to ducting. Upstream of the object is a valve 2 which may be manually actuated and which may also simultaneously function as a throttle, while downstream of the object 1 the ducting has an electromagnetic valve 3. At the commencement of the reaction period a time switch 4 shown in Figure 2 closes the electromagnetic valve 3 and opens an electromagnetic valve 12 (Figure 2). The valve 2 is permanently open dur ing operation.A buffer container 6 is arranged to have low pressure at the commencement of the reaction period and a circulation pump 5, actuated in a preceding cycle carries on operating until the system pressure attains the value set by means of a pressure-regulating device 7, which value is of course higher than the gas pressure set by reductors 14 connected in the path of carbon dioxide flowing from a bottle 13 and non-return valve 15 as well as an electromagnetic valve 12 into the system.

The circulating pump 5 feeds water from a slurry-collecting vessel 9 via a filter 10 and a nonreturn valve 11 to the buffer tank 6 and from there to the object 1. On opening the electromagnetic valve 12 the path is freed for the carbon dioxide stored in the bottle 13 to flow via reductors 14 and the non-return valve 15. The gas penetrates into the buffer tank 6 and from there to the object 1.

The tank 6 is provided with a pressure gauge 8 enabling the monitoring of the pressure increase while during the chemical reaction phase gas continues to flow into the object 1 even after the pressure attains the value predetermined by the pressure-regulating device 7.

The reaction, that is to say the descaling, lasts in each cycle until the elapse of the time pre-set by the time switch 4, e.g. five minutes. Then the time switch 4 opens the magnetic valve 3 and closes the valve 12. Consequently the pressure prevailing in the buffer tank 6 suddenly forces the liquid in the object 1 out into a vessel 9, whereby the pressure in the buffer tank 6 drops to the value set by regulating device 7, e.g. to the value of the ambient pressure. Then the regulating device 7 starts up the motor of pump 5 and the liquid is circulated for a time pre-set by time switch 4, e.g. for 30 seconds, between the object 1 and the slurry-collecting tank 9, whereby the hardness of the water in the object is reduced to the level of the water in the tank 9.At the end of the circulating-flushing period the electromagnetic valve 3 is closed again and the electromagnetic valve 12 is opened again whereby the above-described cycle may be repeated.

In the circulating-flushing period the water entering the tank 9 brings the scale removed from the object 1 partly in a chemically bound form, i.e. as Ca(HCO3)2, and partly as suspended or floating contamination, accompanied by the other components embedded in the scale which are liberated when the descaling takes place, e.g. corrosion products and sediment. The sudden pressure drop promotes the dissociation of the calcium bicarbonate, i.e. the re-formation of calcium carbonate (slurry), while the pressure drop is accelerated by the dispersion of the water. This purpose is served by the baffle plate 16 which simultaneously also impedes the slurry and sediment accumulating at the bottom of the tank 9 from becoming disturbed and passing into suspension.A pipe 17 mounted in the tank 9 protects the pump 5 and unloads the filter 10 by permitting water from the zone above the level of slurry to pass out into the circulation system. The carbon dioxide that dissociates in the slurry collecting tank 9 exits via drip catcher 18 which at the same time allows water to trickle back into the tank, while a venting valve 19 serves for the periodic discharge of the slurry.

The duration and length ratio of the reaction and flushing periods are chosen with due regard to the volume of the object to be descaled. The periods of five minutes and half a minute mentioned by way of example evidently justified when the volume of the object to be descaled is relatively small, e.g. not very large diameter pipe section or heat exchanger of the tube-bundle type etc., because the hardness of the water dwelling in the object rises rapidly in the reaction period and correspondingly the rate of descaling reduces. However, when the volume of the object to be descaled is large, e.g. steam-generating boiler, industrial distillation plant, the reaction period may last even one to two hours. However, in such a case the flushing period is also extended since even a single full circulation of the whole mass of water requires a longer time.

In dependence on the structure and properties of the object to be descaled, the maximum gas pressure is expediently 10-15 bar, while the minimum pressure may be atmospheric pressure. The temperature of the water utilised in the process is expediently ambient temperature, i.e. it is unnecessary to heat the water because on increasing its temperature the solubility of carbon dioxide associated with the same pressure decreases. This prescription is valid independently of the dimensions and pressure resistance of the object to be descaled, while the duration of the reaction and flushing periods and the pressure relations are determined with due regard to the mechanical properties of the object.

Either the whole of the slurry-collecting tank 9 or at least one side of it is expediently made from a methyl methacrylate polymer, in which case the progress of slurry formation (descaling) can be followed by visual monitoring or survey and the time for discharging the slurry can be reliably selected.

The degree of descaling of the object may be ascertained in a variety of ways, expediently by noting when slurry formation in the tank 9 ceases.

The parameters of the process and apparatus shown in Figures 1 and 2 by way of example may be varied to match the concrete properties of the object in question, thus for instance the reaction time is increased when the dimensions of the object to be descaled, especially its volume, are increased but it may also be possible to descale the object in a single cycle. At the same time, for large-volume apparatus, e.g. a boiler, the flushing period will be totally obviated because the maximum attainable water velocity does not make it possible for the slurry to be swept out or discharged. Instead, this task can be done in a separate process, expediently by the use of separating flushing or rinsing water.

The advantages of the preferred process according the invention may be summarised by saying that the scale from an object to be descaled will be periodically removed when the object is inoperative, with a frequency selected so that the amount of scale or its thickness should remain below a tolerable threshold value, the removal of the scale obviates the need of acids which damage the apparatus and are dangerous to human health and as a result the environment is also undamaged by the deposition of the by-products and residue of descalling.

Claims (13)

1. A process for descaling without the use of mineral acid treatment, comprising bringing the scale into contact with a liquid, expediently water, mixed with pressurised carbon dioxide, passing the liquid to a space separated from the object affected by scale while simultaneously reducing the pressure, separating in said space the slurry from the liquid, then, if required, again bringing the liquid which is re-saturated with carbon dioxide under pressure, into contact with the scale, and continuously repeating the process.

2. A process according to claim 1, in which the feeding of carbon dioxide into the liquid is repeatedly intermittently interrupted and during the interruption periods the liquid is caused to flow between the object containing the scale and the said separated space.

3. A process according to claim 1 or 2, in which the liquid is repeatedly fed into the object containing the scale and is repeatedly removed therefrom, and the hardness of the liquid fed in is maintained at all times at a lower level than that of the liquid that is discharged.

4. A process according to any preceding claim, in which the partial pressure of the carbon dioxide in the liquid used for removing the scale is periodically changed.

5. A process according to any preceding claim, in which by means of sedimentation andlor filtration the slurry as well as suspended contaminants and sediment are removed from the liquid after its separation from the object containing the scale and then the liquid is brought into contact again with the scale in the object.

6. Descaling apparatus comprising means for feeding CO2-rich water into the object affected by sealing, means for passing the water from said object to a space separated from said object and maintained at a relatively low pressure, means for separating slurry entrained in the water from the water in said space, and means for repeating the cycle as often as required.

7. Apparatus according to claim 6, wherein the said object is connected with means for periodi cally feeding carbon dioxide at a regulated pressure.

8. Apparatus according to claim 6 or 7, wherein non-return valves are provided for preventing flow of water towards the source of carbon dioxide and the flow of carbon dioxide in a direction opposite to that of the water and in the direction towards the water discharging from the said object.

9. Apparatus according to any of claims 6 to 8, wherein valves for periodically regulating the flow are connected into the flow paths of the carbon dioxide and of the water.

10. Apparatus according to any of claims 6 to 9, wherein an overflow and/or filter is/are connected into the path of the water to prevent the slurry swept up by the water from the said object into a recirculation water duct.

11. Apparatus according to any of claims 6 to 10, wherein a pressure gauge serves as a signal source for controlling a pump that circulates the water.

12. A process according to claim 1, substantially as herein described with reference to and as shown in the accompanying drawings.

13. Apparatus according to claim 6, substantially as herein described with reference to and as shown in the accompanying drawings.