Device for converting a liquid into vapour and associated method for regulating a heating power

Abstract

A device for converting a liquid into vapor, having an evaporation surface, a liquid inlet, a heater for heating the evaporation surface, a flow controller, a control unit configured to control a liquid flow rate injected into the liquid inlet, a chamber containing the evaporation surface, and a temperature sensor arranged on the evaporation surface. The control unit is configured to control a heating power of the heater according to a flow rate and to a temperature measured by the temperature sensor according to a predetermined control law. The predetermined control law varies, for each flow rate, non-linearly and inversely proportionally to the difference between a reference temperature of the chamber and the measured temperature.

Claims

1. A device for converting a liquid into vapor for a high-temperature vapor electrolyzer, this device comprising: an evaporation surface, a liquid inlet connected to the evaporation surface, a heater for heating the evaporation surface, a flow controller arranged at a level of the liquid inlet, and a control unit configured to control a liquid flow rate (D.sub.L) injected into the liquid inlet by the flow controller, wherein the device also comprises: a chamber containing a helix forming the evaporation surface, said chamber having an opening connected to the liquid inlet and a vapor outlet, said device being configured to input the liquid at a top of the helix and to allow the liquid to descend down the helix to form said evaporation surface; and a temperature sensor arranged on the evaporation surface, and wherein the control unit is configured to control a heating power (P) of the heater according to a flow rate (D.sub.L) and to a temperature (T) measured by the temperature sensor according to a predetermined control law, said predetermined control law varying, for each flow rate (D.sub.L), non-linearly and inversely proportionally to a difference between a reference temperature (T.sub.ref) of the chamber and a temperature (T) measured by the temperature sensor.

2. The device for converting a liquid into vapor of claim 1, wherein the temperature sensor is positioned at a level of an end of the evaporation surface opposite to an end connected to the liquid inlet.

3. The device for converting a liquid into vapor of claim 1, wherein the device also comprises a gas inlet emerging into the chamber and a second flow controller, arranged at the level of the gas inlet, the control unit being configured to control a heating power (P) of the heater according to a flow rate (D.sub.L) of the first flow controller, to a flow rate (D.sub.g) of the second flow controller, and to a temperature (T) measured by the temperature sensor according to a predetermined control law.

4. A method of regulating a heating power (P) of the device for converting a liquid into vapor of claim 1, wherein the method comprises the steps of: if the flow rate set point (D.sub.L) is zero, regulating the heating power (P) of the heater to obtain a temperature (T) of the chamber substantially equal to the reference temperature (T.sub.ref), if the flow rate set point (D.sub.L) is non-zero, calculating a theoretical heating power (P.sub.T) corresponding to the flow rate (D.sub.L) divided by a coefficient (R.sub.L), correcting the theoretical heating power (P.sub.T) according to the temperature (T) measured by the temperature sensor, and applying the corrected theoretical heating power to the heater.

5. The heating power regulation method of claim 4, wherein when the device comprises a gas inlet emerging into the chamber and a second flow controller arranged at the level of the gas inlet, the calculation of the theoretical heating power (P.sub.T) corresponds to a maximum power (P.sub.MAX) of the heater as a percentage multiplied by the flow rate (D.sub.L) divided by a coefficient (R.sub.L) added to the flow rate (D.sub.G) of the second flow controller divided by a second coefficient (R.sub.G).

6. The heating power regulation method of claim 4, wherein the coefficient (R.sub.L) is determined according to the latent heat of vaporization of the considered liquid and to the specific heat capacity of the vapor.

7. The heating power regulation method of claim 4, wherein the reference temperature (T.sub.ref) corresponds to an average temperature selected for operation of the device for converting a liquid into vapor, the reference temperature (T.sub.ref) being higher than a vaporization temperature (T.sub.vap) of the liquid.

8. The heating power regulation method of claim 4, wherein the step of correcting the theoretical heating power (P.sub.T) comprises the steps of: if the measured temperature (T) is lower than a first temperature threshold (T.sub.1), correcting the power (P) by an addition of a maximum power (P.sub.MAX) of the heater as a percentage multiplied by a first coefficient (A.sub.1), the first temperature threshold (T.sub.1) being selected between a vaporization temperature (T.sub.vap) of the liquid and the reference temperature (T.sub.ref), if the measured temperature (T) is lower than a second temperature threshold (T.sub.2), correcting the power (P) by a subtraction of the maximum power (P.sub.MAX) of the heater as a percentage multiplied by the temperature difference between the measured temperature (T) and the second temperature threshold (T.sub.2) and by a second coefficient (A.sub.2), the second temperature threshold (T.sub.2) being selected to be higher than the reference temperature (T.sub.ref), if the measured temperature (T) is higher than the second temperature threshold (T.sub.2), correcting the power (P) by a subtraction of the maximum power (P.sub.MAX) of the heater as a percentage multiplied by the temperature difference between the measured temperature (T) and the second temperature threshold (T.sub.2) and by a third coefficient (A.sub.3), and if the measured temperature (T) is higher than a third temperature threshold (T.sub.3), correcting the power (P) by a subtraction of the maximum power (P.sub.MAX) of the heater as a percentage multiplied by a fourth coefficient (A.sub.4), the third temperature threshold (T.sub.3) being selected to be higher than the second temperature threshold (T.sub.2).

9. The heating power regulation method of claim 8, wherein the step of correcting the theoretical heating power (P.sub.T) comprises the step of: if the measured temperature (T) is higher than a fourth temperature threshold (T.sub.4), correcting the power (P) by a subtraction of the maximum power (P.sub.MAX) of the heater as a percentage multiplied by a fifth coefficient (A.sub.5), the fourth temperature threshold (T.sub.4) being selected to be higher than the third temperature threshold (T.sub.3).

10. The heating power regulation method of claim 8, wherein the step of correcting the theoretical heating power (P.sub.T) comprises the steps of: if the value of the corrected power (P) is smaller than zero, setting the power (P) to zero, and if the value of the corrected power (P) is greater than a maximum heating power, setting the power (P) to the maximum heating power.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be better understood on reading of the following description provided as an example only in relation with the accompanying drawings, where the same reference numerals designate the same or similar elements, among which:

(2) FIG. 1 is a simplified representation of a device for converting a liquid into vapor according to an embodiment of the invention;

(3) FIG. 2 is a law of control of the power of a heating surface according to the temperature of the device of FIG. 1 according to a first embodiment of the invention;

(4) FIG. 3 is a flowchart showing the steps of regulation of the power of a heating surface of the device of FIG. 1 according to an embodiment of the invention; and

(5) FIG. 4 is a representation of four laws of control of the power of a heating surface according to the temperature of the device of FIG. 1 according to second embodiment of the invention.

DETAILED DESCRIPTION

(6) FIG. 1 illustrates a device for converting a liquid into vapor comprising a chamber 6 provided with a vapor outlet 4 and with a liquid inlet 2, coupled to a first conventional flow controller 9, itself supplied from a liquid inlet 1. First flow controller 9 may be a commercial controller, for example, a thermal mass flow or Coriolis controller. Chamber 6 contains different elements ensuring the conversion of liquid into vapor, particularly, a helix 3 having a wire electric heating resistor, for example, having a round cross-section, used a heating surface 7 and as an evaporation surface 17, inserted therein. A temperature sensor 8 is installed in the lower portion of helix 3 in contact with the electric resistor. A thermally-insulating jacket 5 is arranged on the wall of the vapor generator to avoid cold spots. Preferably, jacket 5 is maintained at a temperature much higher than the boiling temperature of the liquid.

(7) In the non-limiting example of FIG. 1, a gas flow controller 11, powered by a gas line 10, is connected to the vapor generator by an inlet 12 to enable the forming of a gaseous mixture.

(8) A control unit, not shown, is connected to first controller 9, to second controller 11, to temperature sensor 8, and to the electric resistor, to control a flow rate D.sub.L in first controller 9, a flow rate D.sub.g in the second controller, and a heating power P of the electric resistor. Heating power P of the electric resistor is controlled according to flow rates D.sub.L and D.sub.g and to temperature T measured by temperature sensor 8 according to a control law.

(9) FIG. 2 illustrates such a control law of power P for two specific flow rates D.sub.L and D.sub.g according to temperature T. The control law is centered on a reference temperature T.sub.ref, and varies non-linearly and inversely proportionally to the difference between reference temperature T.sub.ref and the measured temperature T. Power P is shown between zero Watt and the maximum power P.sub.MAX of heating surface 7. Theoretical power P.sub.T is positioned on portion 20 of the control law when flow rates D.sub.L and D.sub.g are low, which enables to anticipate an increase in the flow rate since the system operates at a temperature higher than T.sub.ref. Conversely, portion 21 shows theoretical power P.sub.T when flow rates D.sub.L and D.sub.g are significant, while a decrease in these flow rates is anticipated since the system operates at a temperature lower than T.sub.ref.

(10) FIG. 3 illustrates a flowchart of the method of regulating power P implemented with very simple functions, such as comparisons, additions, subtractions, or multiplications. The flowchart shows an automated loop of tests and calculations, which results in a value of power P which is applied to heating surface 7.

(11) At a first step 30, when the generator vaporizes no liquid, that is, when flow controller 9 has a zero set point, then power P is adapted to maintain the heater cable at reference temperature T.sub.ref. The regulation may be performed by a conventional regulation, of threshold or PID type.

(12) At a step 31, when the generator vaporizes liquid, that is, when liquid flow controller 9 has a set point greater than zero, then a theoretical power P.sub.T is calculated according to the flow rate of liquid to be evaporated D.sub.L and to a coefficient R.sub.L taking into account the latent heat of vaporization of the considered liquid and the specific heat capacity of vapor, and the contribution of the flow rate of gas to be heated D.sub.G divided by a coefficient R.sub.G taking into account the specific heat capacity of the considered gas, according to the following formula:

(13) $P_T=\frac{P_{\mathrm{MAX}}}{100}.Math.\left(\frac{D_L}{R_L}+\frac{D_G}{R_G}\right)$

(14) Power P to be applied to the heater cable is thus initialized with this theoretical value P.sub.T and will be adapted in the next steps according to temperature T.
P=P.sub.T

(15) At a step 32, the measured temperature T is compared with a first temperature threshold T.sub.1 selected between the vaporization temperature T.sub.vap of the liquid and reference temperature T.sub.ref. If the measured temperature T is lower than first temperature threshold T.sub.1, then power P is increased by a positive coefficient A.sub.1 greater than 1 according to the following formula:

(16) $T<T_1.fwdarw.P=P+A_1.Math.\frac{P_{\mathrm{MAX}}}{100}$

(17) At a step 33, the measured temperature T is compared with a second temperature threshold T.sub.2 greater than reference temperature T.sub.ref. If the measured temperature T is lower than second temperature threshold T.sub.2, then power P is increased by a positive coefficient A.sub.2 smaller than 1 according to the following formula:

(18) $T<T_2.fwdarw.P=P-A_2.Math.\left(T-T_2\right).Math.\frac{P_{\mathrm{MAX}}}{100}$

(19) At a step 34, the measured temperature T is compared with the second temperature threshold T.sub.2. If the measured temperature T is higher than second temperature threshold T.sub.2, then power P is linearly decreased by a positive coefficient A.sub.3 smaller than 1 according to the following formula:

(20) $T>T_2.fwdarw.P=P-A_3.Math.\left(T-T_2\right).Math.\frac{P_{\mathrm{MAX}}}{100}$

(21) At a step 35, the measured temperature T is compared with a third temperature threshold T.sub.3 higher than second temperature threshold T.sub.2. If the measured temperature T is higher than third temperature threshold T.sub.3, then power P is decreased by a positive coefficient A.sub.4 greater than 1 according to the following formula:

(22) $T>T_3.fwdarw.P=P-A_4.Math.\frac{P_{\mathrm{MAX}}}{100}$

(23) At a step 36, the measured temperature T is compared with a fourth temperature threshold T.sub.4, higher than third temperature threshold T.sub.3. If the measured temperature T is higher than fourth temperature threshold T.sub.4, then power P is decreased by a positive coefficient A.sub.5 greater than 1 according to the following formula:

(24) $T>T_4.fwdarw.P=P-A_5.Math.\frac{P_{\mathrm{MAX}}}{100}$

(25) Steps 37 and 38 enable to limit the values of power P. If the result of the above calculations provides a value P smaller than zero, then P is set to zero. If the result of the above calculations provides a value P greater than P.sub.MAX, then P is set to P.sub.MAX. Eventually, at step 39, power set point P is applied to heating surface 7, after which the test loop starts again from the beginning.

(26) FIG. 4 illustrates the specific case of a device for converting water into vapor, integrating gas supplies with hydrogen and nitrogen. The maximum water flow rate is 6,400 g/h, that of hydrogen is 500 l/h, and that of nitrogen is 1,000 l/h. The maximum power P.sub.Max of the heating resistor is 6,000 W. Preferably, reference temperature T.sub.ref is in the range from 150 C. to 250 C. In the example of FIG. 4, reference temperature T.sub.ref is set to 200 C., that is, 100 C. above the boiling point of water.

(27) At first step 30, when the generator vaporizes no liquid, that is, when flow controller 9 has a zero set point, then power P is adapted to maintain the heater cable at reference temperature T.sub.ref, that is, 200 C.

(28) At step 31, when the generator vaporizes liquid, that is, when liquid flow controller 9 has a set point greater than zero, then a theoretical power P.sub.T is calculated according to the water flow rate to be evaporated (D.sub.H2O in g/h), to the hydrogen flow rate (D.sub.H2 in l/h), and to the nitrogen flow rate (D.sub.N2 in l/h) according to the following formula:

(29) $P_T=\frac{P_{\mathrm{MAX}}}{100}.Math.\left(\frac{D_{H2O}}{80}+\frac{D_{H2}}{500}+\frac{D_{N2}}{500}\right)$

(30) Numerical values 80 and 500 have been determined by calculation and then verified and refined by experimental tests: Knowing that flow rate D.sub.H2O is in g/h, ratio D.sub.H2O/80 corresponds to the percentage of the total power P.sub.Max necessary to evaporate 1 g/h of water, that is, the electric power P.sub.1 necessary to heat a flow rate D.sub.0=1 g/h=2.78.Math.10.sup.7 kg/s of water from 20 to 100 C. (1), P.sub.2 for the evaporation (2), and P.sub.3 to heat the generated vapor from 100 to 200 C. (3).
P.sub.1=D.sub.0.Math.Cp.sub.water.Math.(20100)=0.093 W with Cp.sub.water=4,195 J/(kg.Math.K)(1)
P.sub.2=D.sub.0.Math.C.sub.Lvapor=0.627 W with C.sub.Lvapor=2.26.Math.10.sup.6 J/kg(2)
P.sub.3=D.sub.0.Math.Cp.sub.vapor.Math.(100100)=0.056 W with Cp.sub.vapor=2,030 J/(kg.Math.K)(3) The theoretical ratio is thus P.sub.Max/100/(P.sub.1+P.sub.2+P.sub.3)=77.2 This theoretical ratio does not take into account thermal losses and has to be experimentally verified, which has led to selecting value 80 in the regulation algorithm. Knowing that flow rate D.sub.H2 is in l/h, ratio D.sub.H2/500 corresponds to the percentage of the total power P.sub.Max necessary to heat 1 l/h of hydrogen, that is, the electric power P.sub.H2 necessary to heat a flow rate D.sub.H2=1 l/h=2.78.Math.10.sup.4 l/s of hydrogen from 20 to 200 C.
P.sub.H2=D.sub.H2.Math.Cp.sub.H2.Math.(20200)=0.035 W with Cp.sub.H2=1.27 J/(l.Math.K) The theoretical ratio thus is P.sub.Max/100/P.sub.H2=1,695 This theoretical ratio does not take into account thermal losses and has to be experimentally verified, which has led to selecting value 500 in the regulation algorithm. The same type of calculation is performed for the N.sub.2 gas. Knowing that flow rate D.sub.N2 is in l/h, ratio D.sub.N2/500 corresponds to the percentage of the total power P.sub.Max necessary to heat 1 l/h of nitrogen, that is, the electric power P.sub.N2 necessary to heat a flow D.sub.N21 l/h=2.78.10.sup.4l/s of nitrogen from 20 to 200 C.
P.sub.H2=D.sub.N2.Math.Cp.sub.N2.Math.(20200)=0.036 W with Cp.sub.N2=1.28 J/(l.Math.K) The theoretical ratio thus is P.sub.Max/100/P.sub.N21,685 This theoretical ratio does not take into account thermal losses and has to be experimentally verified, which has led to selecting value 500 in the regulation algorithm.

(31) At step 32, the measured temperature T is compared with a first temperature threshold T.sub.1 selected at 150 C. If the measured temperature T is lower than 150 C., then power P is decreased by a coefficient A.sub.1 corresponding to value 15 according to the following formula:

(32) $T<150C..fwdarw.P=P+15.Math.\frac{P_{\mathrm{MAX}}}{100}$

(33) This 15% increase has been determined experimentally during tests of qualification of the vapor generator. The same has been done for coefficients A.sub.2, A.sub.3, A.sub.4, and A.sub.5.

(34) At step 33, the measured temperature T is compared with a second temperature threshold T.sub.2 selected at 250 C. If the measured temperature T is lower than 250 C., then power P is linearly decreased by a coefficient A.sub.2 corresponding to value 0.15 according to the following formula:

(35) $T<250C..fwdarw.P=P-0.15.Math.\left(T-250\right).Math.\frac{P_{\mathrm{MAX}}}{100}$

(36) At step 34, the measured temperature T is compared with second temperature threshold T.sub.2. If the measured temperature T is higher than 250 C., then power P is linearly decreased by a coefficient A.sub.3 corresponding to value 0.3 according to the following formula:

(37) 0$T>250C..fwdarw.P=P-0.3.Math.\left(T-250\right).Math.\frac{P_{\mathrm{MAX}}}{100}$

(38) At step 35, the measured temperature T is compared with a third temperature threshold T.sub.3 selected at 300. If the measured temperature T is higher than 300, then power P is decreased by a coefficient A.sub.4 corresponding to value 5, that is, a 5% decrease, according to the following formula:

(39) $T>300C..fwdarw.P=P-5.Math.\frac{P_{\mathrm{MAX}}}{100}$

(40) At step 36, the measured temperature T is compared with a fourth temperature threshold T.sub.4 selected at 310 C. If the measured temperature T is higher than 310 C., then power P is decreased by a coefficient A.sub.5 corresponding to value 5, that is, a 5% decrease, according to the following formula:

(41) $T>310C..fwdarw.P=P-5.Math.\frac{P_{\mathrm{MAX}}}{100}$

(42) As a variation, the values of the temperature thresholds and of the coefficients may vary without changing the invention. For example, first temperature threshold T.sub.1 may be in the range from 140 C. to 175 C. Second temperature threshold T.sub.2 may be in the range from 220 C. to 280 C. Third temperature threshold T.sub.3 may be in the range from 280 C. to 350 C. Fourth temperature threshold T.sub.4 may be in the range from 300 C. to 350 C. Coefficients A.sub.1, A.sub.2 and A.sub.3 may be in the range from 1.05 to 10. Coefficients A.sub.4 and A.sub.5 may be in the range from 0.1 to 1.

(43) FIG. 4 illustrates the variation of power P according to the measured temperature T corresponding to typical production values covering the entire production range, with 100 g/h, 2,000 g/h, 4,000 g/h, and 6,000 g/h. The theoretical power P.sub.T is located above the reference temperature (200 C.) at a low flow rate (100 g/h, 2000 g/h) and below this temperature at a high flow rate (4,000 g/h and 6,000 g/h).

(44) The tests performed with the invention, such as illustrated in FIG. 4, have proven to be fully satisfactory by allowing a stable regulation over long operating time periods at a constant flow rate and allowing significant flow rate changes without creating an overpressure which would adversely affect downstream equipment using the vapor.