Electrochemical descaling by pulsed signal reversal

Abstract

The invention provides a heater arrangement and method for heating a liquid, wherein the heater comprises a heating element, wherein the method comprises (i) heating the liquid in the heater wherein the heating element is in contact with the liquid, and (ii) applying a potential difference between the heating element and a counter electrode, wherein the potential difference has an AC component whereby the potential difference varies with an AC frequency in the range of 0.01-100 Hz and wherein the potential difference is applied with a cycle time, wherein the potential difference has a sign during a first part of the cycle time that is opposite of the sign of the potential difference during a second part of the cycle time, and wherein during one or more of the first part of the cycle time and the second part of the cycle time, the potential difference temporarily changes sign.

Claims

1. A method for heating a liquid in a heater, wherein the heater comprises a heating element, the method comprising acts of: heating the liquid in the heater wherein the heating element is in contact with the liquid, applying a potential difference between the heating element and a counter electrode, wherein the potential difference has an AC component whereby the potential difference varies with an AC frequency (f) in the range of 0.01-100 Hz and wherein the potential difference is applied with a cycle time and a duty cycle, wherein the potential difference has a sign during a first part of the cycle time that is opposite of the sign of the potential difference during a second part of the cycle time, and modulating the AC component with a pulse that reverses the signal at a pulse frequency (fp) such that during one or more of the first part of the cycle time and the second part of the cycle time, the potential difference temporarily changes sign with a frequency in the range of 50-2500 Hz, wherein the pulse frequency (fp) and the frequency (f) of the AC component has a relation fp/f>2.

2. The method according to claim 1, wherein during one or more of the first part of the cycle time and the second part of the cycle time, the potential difference temporarily changes sign and absolute value.

3. The method according to claim 1, wherein fp/f>5, wherein the pulse modulation is chosen to modulate the amplitude of the AC signal with (i) a modulation factor (mf) in the range of 2.5mf<0 and with (ii) a duty cycle in the range of 5%D95%.

4. The method according to claim 1, wherein during one or more of the first part of the cycle time and the second part of the cycle time the duty cycle is in the range of 35-65%.

5. The method according to claim 1, wherein the AC frequency (f) is the range of 0.1-50 Hz and wherein the potential difference is in the range of 1-5 V.

6. The method according to claim 1, comprising an act of heating the heating element to a temperature in the range of 120-250 C.

7. The method according to claim 1, comprising acts of flowing the liquid along the heating element with a flow speed in the range of 1.5-10 ml/s and controlling one or more of the potential difference (V), AC frequency (f), and duty cycle as function of one or more of (i) a current between the heating element and the counter electrode, and (ii) an electrical conductivity of the liquid.

8. The method according to claim 1, wherein the heater is configured to flow the liquid between the heating element and the counter electrode.

9. The method according to claim 1, wherein the heater comprises a flow through heater, wherein the heating element encloses the counter electrode.

10. The method according to claim 8, wherein the heating element and the counter electrode have a mutual distance (d2) in the range of 0.5-5 mm.

11. A heater arrangement comprising a heater, arranged for heating a liquid, comprising a heating element to heat the liquid in the heater, the heating element arranged to be in contact with the liquid, and an electrical power supply, wherein the heater arrangement is configured to apply a potential difference between the heating element and a counter electrode, wherein the potential difference has an AC component whereby the potential difference varies with an AC frequency (f) in the range of 0.01-100 Hz and wherein the potential difference is applied with a cycle time and a duty cycle, wherein the potential difference has a sign during a first part of the cycle time that is opposite of the sign of the potential difference during a second part of the cycle time, wherein the heater arrangement is further configured to apply a modulation to the AC component with a pulse that reverses the signal at a pulse frequency (fp) such that during one or more of the first part of the cycle time and the second part of the cycle time, the potential difference temporarily changes sign with a frequency in the range of 50-2500 Hz, and wherein the pulse frequency (fp) and the frequency (f) of the AC component has a relation fp/f>2.

12. The heater arrangement according to claim 11, wherein fp/f>5, wherein the pulse modulation is chosen to modulate the amplitude of the AC signal with (i) a modulation factor (mf) in the range of 2.5mf<0 and with (ii) a duty cycle in the range of 5%D95%.

13. The water heating arrangement according to claim 11, wherein the heater is configured to flow the liquid between the heating element and the counter electrode, and wherein the heater comprises a flow through heater, wherein the heating element encloses the counter electrode.

14. An electronic device for providing a beverage comprising a liquid at elevated temperature, the electronic device comprising a heater arrangement according to claim 11, wherein the electronic device is arranged to produce heated water and/or steam for the beverage.

15. A method for heating a liquid in a heater, wherein the heater comprises a heating element, the method comprising acts of: heating the liquid in the heater wherein the heating element is in contact with the liquid, applying a potential difference between the heating element and a counter electrode, wherein the potential difference has an AC component whereby the potential difference varies with an AC frequency (f) in the range of 0.01-100 Hz and wherein the potential difference is applied with a cycle time and a duty cycle, wherein the potential difference has a sign during a first part of the cycle time that is opposite of the sign of the potential difference during a second part of the cycle time, wherein during one or more of the first part of the cycle time and the second part of the cycle time, the potential difference temporarily changes sign with a frequency in the range of 50-2500 Hz, and wherein during one or more of the first part of the cycle time and the second part of the cycle time the duty cycle is in the range of 35-65%.

16. A method for heating a liquid in a heater, wherein the heater comprises a heating element, the method comprising acts of: heating the liquid in the heater wherein the heating element is in contact with the liquid, applying a potential difference between the heating element and a counter electrode, wherein the potential difference has an AC component whereby the potential difference varies with an AC frequency (f) in the range of 0.01-100 Hz and wherein the potential difference is applied with a cycle time and a duty cycle, wherein the potential difference has a sign during a first part of the cycle time that is opposite of the sign of the potential difference during a second part of the cycle time, wherein during one or more of the first part of the cycle time and the second part of the cycle time, the potential difference temporarily changes sign with a frequency in the range of 50-2500 Hz, wherein the heater is configured to flow the liquid between the heating element and the counter electrode and wherein the heating element and the counter electrode have a mutual distance (d2) in the range of 0.5-5 mm.

17. A heater arrangement comprising a heater, arranged for heating a liquid, comprising a heating element to heat the liquid in the heater, the heating element arranged to be in contact with the liquid, and an electrical power supply, wherein the heater arrangement is configured to apply a potential difference between the heating element and a counter electrode, wherein the potential difference has an AC component whereby the potential difference varies with an AC frequency (f) in the range of 0.01-100 Hz and wherein the potential difference is applied with a cycle time and a duty cycle, wherein the potential difference has a sign during a first part of the cycle time that is opposite of the sign of the potential difference during a second part of the cycle time, and wherein during one or more of the first part of the cycle time and the second part of the cycle time, the potential difference temporarily changes sign with a frequency in the range of 50-2500 Hz, and wherein during one or more of the first part of the cycle time and the second part of the cycle time the duty cycle is in the range of 35-65%.

18. A heater arrangement comprising a heater, arranged for heating a liquid, comprising a heating element to heat the liquid in the heater, the heating element arranged to be in contact with the liquid, and an electrical power supply, wherein the heater arrangement is configured to apply a potential difference between the heating element and a counter electrode, wherein the potential difference has an AC component whereby the potential difference varies with an AC frequency (f) in the range of 0.01-100 Hz and wherein the potential difference is applied with a cycle time and a duty cycle, wherein the potential difference has a sign during a first part of the cycle time that is opposite of the sign of the potential difference during a second part of the cycle time, and wherein during one or more of the first part of the cycle time and the second part of the cycle time, the potential difference temporarily changes sign with a frequency in the range of 50-2500 Hz, wherein the heater is configured to flow the liquid between the heating element and the counter electrode and wherein the heating element and the counter electrode have a mutual distance (d2) in the range of 0.5-5 mm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

(2) FIGS. 1a-1d schematically depict some possible configurations of a heater arrangement where the heating element is immersed in the water or where the heating element is a wall of a (water) heater;

(3) FIGS. 2a-2e schematically depict some examples of the electrochemistry and AC voltages;

(4) FIGS. 3a-3c schematically depicts an embodiment of an AC voltage comprising two components; and

(5) FIGS. 4-4b schematically depict some examples of electronic devices.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(6) FIG. 1a schematically depicts a (water) heater arrangement (heater arrangement) 1 comprising a (water) heater (heater) 100, arranged for heating an (aqueous) liquid 20. The (aqueous) liquid 20, especially water, is contained in the heater 100. The heater 100 comprises a metal heating element 110 to heat the (aqueous) liquid 20 in the heater 100. The heater arrangement 1 further comprises an electrical power 200 supply, arranged to apply a AC voltage between the heating element 110 and a counter electrode 120. In addition to the AC voltage a second, lower frequency AC voltage may be added and/or a DC voltage. When a DC voltage is applied, the heating element 110 is chosen as positive electrode. By application of the voltage(s), the metal heating element 110 is protected against scaling and/or scaling formed may be removed. FIG. 1a schematically depicts an embodiment wherein the heating element 110 is a submerging/immersion heating element, indicated with reference 111. The wall (or at least part of it) of the heater 100 is used as counter electrode in this embodiment. The wall may for instance be of steel as conductive material. Typically stainless steel is used with Cr, Ni as alloying elements (e.g. 304) or with additional Mo added (e.g. 316).

(7) FIG. 1b schematically depicts a heater arrangement 1 wherein the heating element 110 is a curled immersed heating element, and wherein the counter electrode 120 is partly surrounded by at least part of the submerging heating element 111. In this case the counter electrode can be of stainless steel or Inconel or other oxidation resistant electrode material like titanium, platinum, mixed metal oxide coated titanium, platinum coated titanium or carbon based.

(8) FIG. 1c schematically depicts an embodiment of a flow through heater (FTH), wherein the heater 100 is a heater through which the (aqueous) liquid 20 flows, while being heated. In the schematic embodiment of FIG. 1c, a heat generation device 115 is connected to the wall of the heater 100, and a rod within the heater is used as counter electrode 120. The wall is connected to an generation device 115 to heat the wall and is preferably of (stainless) steel; the wall is in contact with the (aqueous) liquid (not depicted) and is thus used as heating element 110. The counter electrode 120 may comprise a material as indicated in the description above for FIG. 1b. The heating element 110 and the counter electrode 120 have a (mutual) distance d2, which may e.g. be in the range of 0.5-5 mm.

(9) Optionally, the circumferential heating element and the counter electrode may be arranged the other way around, with the counter electrode enclosing the heating element (this embodiment is not depicted).

(10) FIG. 1d schematically depicts substantially the same embodiment as schematically depicted in FIG. 1c, but now in cross-sectional view. Elements 115 heat wall of the heater 100. Therefore, the wall is indicated as heating element 110. Over this wall, i.e. heating element 110, and the counter electrode 120, a voltage is applied with electrical power supply 200. Here, the wall is used as heating element 110, and is preferably of (stainless) steel. The counter electrode 120 may comprise a material as indicated in the description above for FIG. 1b.

(11) A basic idea behind the AC signal is that corrosion is suppressed by preventing metal ions to move from the electrode into solution. When the signal is positive, metal ions tend to leave the electrode into the water (corrosion). When the signal is sufficiently fast reversed the metal ions are pulled back to the electrode. When the reversal is fast enough ions cannot escape the boundary layer at the metal surface and corrosion is prevented. FIG. 2a schematically depicts two electrodes, like the heating element and the counter electrode. The electrodes are indicated with EL. When an AC voltage is applied, there will be surface charge SC formed, which changes in sign continuously because of the AC character. In the vicinity of the electrodes, there will be an electrical double layer, indicated with DL. The dashed line indicates the slipping plane SP that separates mobile fluid from fluid that remains attached to the surface.

(12) FIG. 2b shows a solution by way of reference, wherein a slow AC is modulated with a fast AC. Although the addition of a high frequency signal can suppress the corrosion of a single low frequency AC signal there is a drawback. The second AC signal needs to have certain amplitude to be effective. That means that when for example the amplitudes of both AC signals are equal that at the peak of the low frequency signal the amplitude is modulated by the second frequency bringing it down to 0V in the minimum but doubles it at the maximum (FIG. 2b). The doubling (in this example) of the amplitude can then result again in corrosion in spite of the second AC signal being present that could have prevented this corrosion due to the peak amplitude exceeding corrosion threshold.

(13) Hence, another solution is sought, of which some aspects are described in FIGS. 2c-2e, and 3a-3c.

(14) FIG. 2c is showing a pulse modulated AC signal with duty cycle of 50% at both at positive and negative part of the SC signal. Hence, 50% of the time the signal does not follow the normal sine, but deviates therefrom (due to a pulse thatin this embodimentswaps the signal). FIG. 2d shows a pulse modulated signal where the positive part is modulated at 90% and the negative part at 75%. The duty cycle in the first part is 90% and the duty cycle in the second part is 75%. Hence, in the first part 10% of the time the signal does not follow the normal sine (only evaluating the first part of the cycle), but deviates therefrom (due to a pulse thatin this embodimentswaps the signal); likewise in the second part 25% of the time the signal does not follow the normal sine (only evaluating the second part of the cycle), but deviates therefrom (due to a pulse thatin this embodimentalso swaps the signal). Here, a mean duty cycle may be 82.5%. FIG. 2e shows a pulse modulated signal where the reversal is not 100%

(15) FIG. 3a schematically depicts a sine wave as example of an AC voltage. This can be a component of the AC potential difference. The frequency (f) is 0.5 Hz, the cycle time is 2 s, and the amplitude is 1 (by way of example 1 V). FIG. 3b schematically depicts an embodiment of a pulse width modulation (signal). Here, as frequency (fp) 15 Hz is chosen. The modulation factor (mf) is 1. This does not exclude that in principle part of the signal may also be modulate with a modulation factor mf in the range of 0mf1 (or even larger). However, as will be clear to a person skilled in the art, when mf=1, there is no modulation. Further, by way of example the modulation is chosen such that the effective time in the positive part of the signal, here the first part is 80%; in the second part is 60%. When such modulation of FIG. 3b is applied to the sinusoidal AC voltage of FIG. 3a, the AC potential difference of FIG. 3c is obtained. This AC potential difference has an AC component, as well as temporary changes in signal. Note that here the modification factor is 1 (and +1), however, also a value different to 1 (or a value different to +1), such as 0.75 might be chosen.

(16) FIG. 4a schematically depicts an electronic device 2. FIG. 3 schematically depicts an electric kettle as example of the electronic device 2. The electronic device 2 comprises the heater arrangement 1. Here, electronics 300 may be arranged to control the heating of heating element 120 and provide power to the electric power supply 200 for imposing the AC and the temporary sign (and optional value) change of the AC to the heating element 120 and counter electrode 110. FIG. 4b schematically depicts an example of a vending machine as example of an electronic device 2. Reference 300 indicates a beverage.

(17) The heater arrangement 1 may further comprise a sensor (not depicted) to sense parameters like conductivity of the (aqueous) liquid, the temperature of the (aqueous) liquid, etc. Further, the heater arrangement 1 may further comprise a controller, to control the one or more features of the AC and the temporary sign change of the potential difference. The controller may control those one or more features in dependence of the one or more parameters and one or more predefined relations between the one or more parameters and the one or more features.

EXAMPLES

(18) Water Preparation

(19) Stock solutions of CaCl.sub.2.2H.sub.2O (65.6 gr/ltr), MgSO.sub.4.7H.sub.2O (38 gr/ltr) and NaHCO.sub.3 (76.2 gr/ltr) were made. Standard hard water was made by mixing 50 gram of each stock solution into 9 liter of de-ionized water and adding up to 10 liter. The resulting water had a total hardness of 16.8 .sup.0DH and a temporary hardness of 11.2 .sup.0DH. Total hardness is defined as 2.82[mmol/ltr Ca+mmol Mg/ltr]. Temporary hardness is defined as 2.8[mmol HCO.sub.3.sup./ltr]

(20) Corrosion Experiments (Low Temperature)

(21) In the following examples the effect of various parameters is shown especially on the corrosion of the electrodes when the amplitude is increased. In a typical experiment two stainless steel electrodes (316 grade) of 2.5 and 6 mm diameter were immersed in a beaker filled with standard hard water. Water was heated to 75 C. and an electrical signal was applied across the electrodes. Current was run for 30 minutes and electrodes inspected visually.

Experiment 1

(22) A sinusoidal AC signal of 3V and 0.5 Hz was applied across the two electrodes. Severe corrosion occurred at both electrodes.

Experiment 2

(23) An AC signal of 3V and 0.5Hz was modulated with 100 Hz at 80% duty cycle and applied on the two electrodes as in previous experiment. After 30 min the electrodes were visually inspected. The small (2.5 mm) electrode showed a weak yellow cast while the 6 mm electrode was colorless.

Experiment 3

(24) An AC signal of 3V and 0.5 Hz was modulated with 100 Hz at 85% duty cycle and applied on the two electrodes as in previous experiment. After 30 min the electrodes were visually inspected. The small (2.5 mm) electrode showed a weak yellow cast while the 6 mm electrode showed a small discoloration. Coloration of the 85% duty cycle was comparable to the 80% duty cycle of experiment 2

Experiment 4

(25) An AC signal of 3V and 0.5 Hz was modulated with 100 Hz at 90% duty cycle and applied on the two electrodes as in previous experiment. After 30 min the electrodes were visually inspected. The small (2.5 mm) electrode had corroded and showed a deep yellow color. Also the larger 6 mm electrode was clearly yellow colored.

(26) Experiments of above show clearly the effect of reversing the basis signal at a certain frequency in order to prevent corrosion and the effect of the duty cycle of the pulses.

Experiment 5

(27) An AC signal of 3V and 0.5 Hz was modulated with 50 Hz instead of 100 Hz at 80% duty cycle and applied on the two electrodes as in previous experiment. After 30 min the electrodes were visually inspected. The small (2.5 mm) electrode was slightly yellow as was the 6 mm electrode. The discoloration was stronger than with the 100 Hz pulse.

Experiment 6

(28) An AC signal of 3V and 0.5 Hz was modulated with 500 Hz at 80% duty cycle and applied on the two electrodes as in previous experiment. After 30 min the electrodes were visually inspected. Both electrodes now were clean and showed no sign of corrosion.

Experiment 7

(29) An identical experiment with the duty cycle increased to 90% showed again a strong yellowing of the electrodes.

Experiment 8

(30) An AC signal of 3V and 0.5 Hz was modulated with 100 Hz at 80% duty cycle and applied on the two electrodes. The signal was not 100% reversed but only to 0V. After 30 min the electrodes were visually inspected. The small (2.5 mm) electrode had severely corroded while the 6 mm electrode was yellow in color.

(31) In experiments above, the reversing of the signal was more than 50% meaning that overall the signal was still resembling the original base signal. When the duty cycle is increased more than 50% the signal is in fact merely swopped.

Experiment 9

(32) An AC signal of 3V and 0.1 Hz was modulated with 100 Hz at 80% duty cycle when in positive amplitude and with 100 Hz 20% when in negative amplitude. (The 2.5 mm electrode was connected to the positive terminal). The 2.5 mm electrode showed yellowing while the 6 mm electrode was clean. On top of that the 6 mm showed scale formation. The latter indicates that clearly water was decomposed as due to the duty cycles the 6 mm electrode was facing continuously a high pH giving rise to scale formation.

Experiment 10

(33) An AC signal of 3V and 10 Hz was modulated with 100 Hz at 80% duty cycle when in positive amplitude and with 100 Hz 20% when in negative amplitude. (The 2.5 mm electrode was connected to the positive terminal). Now both electrodes showed no corrosion while there was still scale formation onto the 6 mm electrode.

(34) Increasing base frequency diminished the corrosion while maintaining the water decomposition as indicated by the scale formation.

(35) In previous experiments the base signal was sinusoidal. Similar behavior can be observed when the base signal is in the form of a block signal or a triangle.

(36) Test Setup for a Flow Through Heater

(37) The heater is schematically shown in the FIGS. 1c and 1d. The outer tube, first heating element 110, is a stainless steel tube with two heat generation devices 115 at both sides. The outer tube has a 15 cm length and a 13 mm inner diameter Together, they in fact form one heating element, surrounded by aluminum, with which the liquid is in contact when the liquid flows through the tube. The inner tube may also be of stainless steel, and had a larger diameter than schematically depicted in FIG. 1c/1d: as counter electrode 120 a stainless steel tube of 8 mm diameter was inserted in the tube. Water was passed through the area between the two stainless steel tubes at a speed of 2.25 ml/sec; the pressure in the heater system was 6-10 bar. Both the outer tube and the inner tube of the water heating system were connected electrically. A typical test cycle consisted of heating water for 70 seconds and cooling down for 50 seconds. This cycle was repeated for 500 times.

Comparative Experiment

(38) Said (water) heater was used for testing the calcification by the two water types without any electrical signal applied across the two electrodes. In both cases especially the wall was fully calcified and a dense layer of scale was formed that was strongly adhering to the wall.

Experiment 11

(39) A similar experiment as in the comparative experiment was run but now with a sinusoidal signal applied of 0.5 Hz and 1.6 V amplitude. The signal was reversed with 500 Hz; the duty cycle during the positive half of the sine is 80%, the duty cycle during the negative half 70%.

(40) As a result of the experiment, the counter-electrode was clean of calc but slightly colored while the wall of the outside tube showed slight scale formation and pitting corrosion. The adhesion of the scale to the wall was weak and far less than in the comparative experiment.

Experiment 12

(41) In a second experiment the signal reversal frequency was increased from 500 Hz to 1000 Hz. Slight scale formation was observed, comparable to experiment 11. Corrosion was less as in experiment 11 but still present.

Experiment 13

(42) In a third experiment the signal reversal frequency is increased from 1000 Hz to 2000 Hz. No corrosion is observed, but a dense layer of calc remains, comparable to the amount of calc in the comparative experiment. Clearly the 2000 Hz frequency limits both oxidation of water and oxidation of metal.

(43) In order to effectively prevent corrosion in the operating conditions as described above (high temperature), the duty cycle was lowered further.

Experiment 14

(44) A test was performed with a 0.5 Hz sine, reversed with 1000 Hz with a duty cycle of 70% and 60% for the positive and negative half of the sine respectively. Still corrosion is observed.

Experiment 15

(45) A test is performed with a 0.5 Hz 1.6V amplitude sine, reversed with 1000 Hz with a duty cycle of 50% and 50% for the positive and negative half of the sine respectively. Corrosion is effectively prevented. The amplitude can be increased up to 2.8V to enhance scale prevention while still avoiding corrosion.

(46) From these experiments it can be concluded that at high heater wall temperatures the mobility of the (metal) ions is very high. In order to prevent metals ions from moving into the bulk of the water, a 50% duty cycle was necessary. It can also be concluded that at 1000 Hz the descaling action is still in operation; 2500 Hz may be too high for the electrochemistry to follow.