DE102006033252B4 - Method and device for measuring humidity and dew point in a hydrogen peroxide-laden environment - Google Patents

Abstract

Method for determining the partial pressures (p _H2O and p _H2O2 ) of water vapor and hydrogen peroxide in a measuring gas, comprising the following steps:
(A) measuring a temperature (T) of the measurement gas,
(B) measuring a relative humidity (RH) with a moisture sensor (3) with a reducing protective cap (1), the measured relative humidity (RH) being an amount of moisture resulting from reduction of hydrogen peroxide present in the sample gas to water vapor, is increased,
(C) calculating an increased partial pressure (p _W ) of water vapor from the measured values for the temperature and relative humidity (rF), wherein the calculated increased partial pressure (p _W ) is the sum of the partial pressures (p _H2O , p _H2O2 ) of the measured gas represented contained water vapor and hydrogen peroxide.
(D) Calculation of the individual partial pressures (p _H2O , p _H2O2 ) of the water vapor contained in the sample gas and hydrogen peroxide from the increased partial pressure (p _W ) and another parameter, the ratio of the partial pressures (p _H2O , p _H2O2 ) contained in the sample gas Water vapor and hydrogen peroxide represents.

Description

The The invention relates to a method and a device for Measurement of moisture in hydrogen peroxide loaded environment with catalytically reducing protective filter and an evaluation unit to correct the measured values for the humidity and the calculation of the mixed dew point and others variables derived from moisture. The Invention is in the field of sensor technology and more specifically in the field of moisture measurement under problematic environmental conditions, by which damages the sensors and / or the measured values are corrupted can be.

Such a problematic environmental influence is, for example, the proportion of hydrogen peroxide (H ₂ O ₂ ) in a measuring gas whose moisture content is to be measured. Hydrogen peroxide is a substance that is preferably used for sterilization in the production process of the food and beverage industry as well as in pharmacy.

at these processes are transformed into a dry carrier gas stream (usually compressed air) a commercial hydrogen peroxide solution introduced by evaporation. Process-related solutions are usually with a hydrogen peroxide content of 15 to 60 mass percent used. The evaporation typically occurs on a hot metal plate. An uncoated plate will be in the evaporation process around five to ten percent of the hydrogen peroxide due to the catalytic Effect of metal plate converted to water and oxygen. at a coated plate, this proportion is lower, in the case a Teflon coating negligible.

Around To make the process optimal, it is important to have moisture and hydrogen peroxide content in the sterilization process to know and if necessary to regulate. About that In addition, usually a condensation of the material to be sterilized be avoided. For this reason, it is particularly important that Determine the dew point of the air / water / hydrogen peroxide mixture to be able to. The gas mixture of carrier gas and evaporated water / hydrogen peroxide solution is hereinafter referred to as Measuring gas called.

Measuring devices for recording or direct measurement of H ₂ O (water) and / or H ₂ O ₂ (hydrogen peroxide) under these conditions are very expensive and therefore not economically viable for process monitoring. Conventional and cheaper to implement measuring devices, usually with capacitive humidity sensors, have the disadvantage that these sensors have cross sensitivities to hydrogen peroxide or even destroyed by this.

To protect the humidity sensors from harmful environments, protective caps are known, which are permeable to the sample gas and have a chemical reservoir, wherein the reservoir may contain a catalyst or a reducing agent. With the help of the chemical contained in the reservoir, substances that can damage the sensor should be bound or rendered harmless. In the case of measuring the moisture content in gases which also contain hydrogen peroxide, the hydrogen peroxide is to be reduced by the protective cap to water and oxygen (2H ₂ O ₂ → 2H ₂ O + O ₂ ).

It turns out that the effect of this measure is not optimal and the production of such a protective cap with a chemical reservoir is expensive and thus the response time of the sensor can be increased. In addition, with such a solution, the measurement result can be falsified. In the case of the measurement of the moisture content in H ₂ O ₂ laden environment is naturally generated by the catalytic process on the cap additionally water, which increases the moisture content within the cap and the measurement result is falsified accordingly.

The pamphlets US 4,843,867 and US 6,953,549 each describe a system and method for determining the concentration of hydrogen peroxide in a gas mixture of hydrogen peroxide and water vapor.

Of the present invention is based on the object, a measuring device to disposal to ask which with as possible low financial and constructive effort a protective cap of the above type realized to unwanted substances, in particular Hydrogen peroxide, to be able to keep away from the sensor, and on the other hand in the Location is the effects caused by this cap consequences or repercussions accordingly to compensate.

These The object is achieved by a method according to claim 1 and a device solved according to claim 5, advantageous embodiments and developments are the subject the dependent claims.

The inventive device sees, among other things, a protective cap surrounding a humidity sensor with finely distributed through openings in front, on the surfaces of a Active substance (catalyst) is arranged. The openings are big enough so that the sample gas to be analyzed (for example, air) in sufficient Dimensions The protective cap can pass and reach the sensor. The However, hydrogen peroxide contained in the sample gas is so strong with the catalyst in contact, creating a chemical reaction takes place, through which the hydrogen peroxide in water and oxygen is converted. By this catalytic reduction, the hydrogen peroxide practically completely eliminated when passing through the cap.

Furthermore, the device according to the invention provides an evaluation unit which is designed to calculate the dew point, the moisture content and the H ₂ O ₂ content of the measurement gas outside the protective cap from the moisture value measured inside the protective cap. Within the protective cap, the total moisture is greater by that amount than outside the protective cap created by catalysis as the fluid passes through the protective cap. The total moisture inside the protective cap is therefore a measure of the sum of the water present outside the protective cap and the hydrogen peroxide present outside the protective cap. For the above calculation, a measured value for the temperature of the measuring gas at the measuring point and the temperature of the humidity sensor is additionally necessary. For this purpose, a temperature sensor is arranged in the vicinity of the humidity sensor. Normally, the measuring point, ie the point at which the moisture is to be determined, and the humidity sensor the same temperature and it is sufficient a temperature sensor, the z. B. may also be disposed within the cap in the immediate vicinity of the moisture sensor. However, if the measuring point is too hot for the humidity sensor, ie from temperatures above 180 ° C, the sample gas must be extracted from the measuring point and cooled down on the way to the sensor. In this case, the temperature of the hot measuring point and the temperature of the cooler sensor must be measured separately.

Around on the proportionate composition of water and hydrogen peroxide outside Close the protective cap to be able to is additional yet another parameter necessary. This parameter can either metrologically determined from several the real states of the Measuring gas calculated descriptive parameters, or, if it is off another source is manually entered into the evaluation unit become. This further parameter essentially corresponds to the relationship the partial pressures of water vapor and hydrogen peroxide at the measuring point or one directly derived therefrom.

Possible sizes which can influence this ratio of the partial pressures is, for example, the moisture content of the carrier gas stream. In the simplest case, the said further parameter is determined by the mass fraction of hydrogen peroxide in the total mass of water and hydrogen peroxide. This mass fraction can, for. B. from the mass fraction of hydrogen peroxide in the vaporized water / hydrogen peroxide solution can be calculated. By weighing a certain volume of the H ₂ O ₂ solution to be evaporated, the standard density of the solution can be determined taking into account the temperature. The standard density is then directly attributable to the mixing ratio of water and hydrogen peroxide. Depending on the type of application considered, consideration of the moisture in the carrier gas stream is also necessary since it also increases the proportion of water vapor in the sample gas.

The already mentioned Evaluation unit is designed to be within the protective cap determined measured value for Total humidity and temperature and the other parameters mentioned Proportions of water and hydrogen peroxide outside the protective cap as partial pressures, Calculate volume fractions or other derivable quantities.

With Knowledge of the tabulated vapor pressure curves for the substance mixture water / hydrogen peroxide or air / water / hydrogen peroxide, for example, from the partial pressures of Dew point of the mixture can be determined. Alternative to the table values The dew point can also be determined with the aid of model equations become. A possibility this is, for example, the NRTL model, which in the field of technical chemistry is widely used and the thermodynamic Balance of multiphase systems with the help of few parameters can describe efficiently. Interpolate in Tables is for the usual device hardware often too memory intensive, the use of the NRTL model often too computationally intensive, so that the inventive method with an approximate solution a simple polynomial sentence used. It is first of the model a mixture of two ideal gases and in a last Step the ideal gas behavior on the real gas behavior with help a polynomial approximation approach displayed.

By way of the particle sizes, preferably their partial pressures, of water or hydrogen peroxide, all other relevant parameters can be calculated. These sizes are different depending on the application For example, it is the volume or mass fractions of water vapor in ppm or percent, the absolute humidity in g / m ³ carrier gas (eg moist air), the degree of moisture in g / kg dry air, the dew point, the Dew point distance, the enthalpy of the sample gas, the degree of saturation or the saturation deficit and the corresponding quantities of hydrogen peroxide. In principle, all the values derived therefrom can be calculated from the measured values for the temperature and the relative humidity within the measuring cap and the further parameter (for example the partial pressure ratio of water and hydrogen peroxide).

The Invention will be described below with reference to the figures embodiments explained in more detail. It demonstrate:

1 : a protective body according to the invention in the form of a protective cap in perspective view,

2 : a protective cap in a longitudinal section and a part of the sensor, on which the protective cap can be screwed.

3 : the entire measuring system with capacitive humidity sensor including protective cap and evaluation unit.

1 shows schematically a cylindrical hollow protective cap 1 at her closed end 2 has a rounded edge and which is placed on a humidity sensor, such as a capacitive polymer moisture sensor for protection against aggressive environments. For this purpose, the protective cap, for example, on a pedestal 4 on which the sensor 3 attached is screwed on. The protective cap 1 consists of a porous material, which has been brought into a mold by sintering polytetrafluoroethylene granules, the granules having been mixed with manganese oxide prior to pressing. The wall thickness of the protective cap 1 , The degree of compression or the packing density of the granular particles is chosen so that a fluid to be measured sufficiently penetrate through the protective cap and can reach the sensor. On the way through the openings, however, the parts of hydrogen peroxide contained in the sample gas come so intensely into contact with the brownstone that a catalytic reduction to water and oxygen makes the hydrogen peroxide virtually completely harmless. The manganese oxide represents in the case illustrated three percent of the total mass of the protective body and is evenly distributed on the polytetrafluoroethylene granules.

For others Applications can as active ingredients alternatively other catalysts are used, such as for example precious metals (for example platinum, gold and silver) for the conversion of nitrogen oxides and hydrogen peroxide, also metal oxides and also enzymes (for example catalase). Additionally or alternatively, indicators as active substances in the protective body which are used to display the ph value and other chemical quantities, for example, sensor impairments like damage or Indicate consumption of catalysts or the expiration of the useful life or filter functionality to signal.

2 shows except a longitudinal section through a protective cap 1 also a sensor 3 with a shaft 4 , on the thread of the protective cap can be screwed. Basically, instead of the screw but also any other connection possible, such. B. an adhesive or a solder joint.

In 3 is the entire measuring system including capacitive humidity sensor 3 with protective cap 1 and evaluation unit 10 shown. By that in the evaluation unit 10 implemented method is made from within the protective cap 1 measured value rF for the relative humidity of the proportion of water and the proportion of hydrogen peroxide in the sample gas outside the protective cap 1 determines from which in turn the mixture dew point TP _{Gem of} the sample gas can be calculated.

• (1) Einlesen eines Messwertes für die Temperatur T des Messgases
• (2) Einlesen eines unkorrigierten Messwertes für die relative Feuchte rF innerhalb der Schutzkappe.
• (3) Ermitteln eines weiteren Parameters V. Dieser ist a priori bekannt und repräsentiert das Verhältnis der Partialdrücke von Wasserdampf und Wasserstoffperoxid im Messgas außerhalb der Schutzkappe 1. Vereinfachend kann für den weiteren Parameter V auch das Volumenverhältnis von Wasserdampf und Wasserstoffperoxid des verdampften Wasser-H₂O₂-Gemisches verwendet werden. Bei bekannten Massenanteilen in Prozent für Wasser rM_H2O und Wasserstoffperoxid rM_H2O2 und bekannter Dichte von Wasserdampf ρ_H2O und Wasserstoffperoxid ρ_H2O2 erfolgt die Berechnung nach der Gleichung
  
• (4) Berechnen des Partialdruckes p_W für Wasserdampf innerhalb der Schutzkappe aus den Messwerten für die Temperatur T und die relative Feuchte rF. Dazu gibt es verschiedene bekannte Möglichkeiten, z. B. die Berechnung mit Hilfe der Magnus-Formel
  
  Die Magnus-Koeffizienten für Wasser C_1W, C_2W und C_3W können je nach Wertebereich angepasst werden. Beispielsweise ist C_1W = 6,1078 hPA, C_2W = 17,08085 und C_3W = 234,175°C. Eine äquivalente Möglichkeit wäre die Verwendung der Antoine-Gleichung.
• (5) Berechnen des Partialdrucks von Wasserdampf p_H2O und des Partialdrucks von Wasserstoffperoxid p_H2O2 im Messgas außerhalb der Schutzkappe 1 aus dem Partialdruck von Wasserdampf p_W innerhalb der Schutzkappe und dem weiteren Parameter V. Der Partialdruck für Wasserdampf p_H2O und für für Wasserstoffperoxid p_H2O2 ergibt sich aus den folgenden Gleichungen
  
• (6) Berechnen von aus den Partialdrücken (p_H2O, p_H2O2) von Wasserdampf und Wasserstoffperoxid abgeleiteten Größen wie beispielsweise den für reale Gase korrigierten Gemischtaupunkt TP_Korr.

The evaluation or correction procedure comprises the following steps:

• (1) Reading a measured value for the temperature T of the measuring gas
• (2) Reading in an uncorrected measured value for the relative humidity RH within the protective cap.
• (3) Determining another parameter V. This is known a priori and represents the ratio of the partial pressures of water vapor and hydrogen peroxide in the sample gas outside the protective cap 1 , For simplicity, the volume ratio of water vapor and hydrogen peroxide of the vaporized water-H ₂ O ₂ mixture can also be used for the further parameter V. For known mass fractions in percent for water rM _H2O and hydrogen peroxide rM _H2O2 and known density of water vapor ρ _H2O and hydrogen peroxide ρ _H2O2 is calculated according to the equation
  
• (4) Calculate the partial pressure p _W for water vapor inside the protective cap from the measured values for the temperature T and the relative humidity rF. There are several known ways, such. For example, the calculation using the Magnus formula
  
  The Magnus coefficients for water C _1W , C _2W and C _3W can be adjusted depending on the value range. For example _1W C = 6.1078 hPa, C _{2W 3W} = 17.08085 and C = 234.175 ° C. An equivalent possibility would be the use of the Antoine equation.
• (5) Calculation of the partial pressure of water vapor p _H2O and the partial pressure of hydrogen peroxide p _H2O2 in the sample gas outside the protective cap 1 from the partial pressure of water vapor p _W within the protective cap and the further parameter V. The partial pressure for water vapor p _H2O and for hydrogen peroxide p _H2O2 is given by the following equations
  
• (6) Calculating quantities derived from the partial pressures (p _H2O , p _H2O2 ) of water vapor and hydrogen peroxide, such as the mixed _dewpoint TP _corr . Corrected for real gases.

• (1) Berechnen des Wasserstoffperoxid-Taupunktes TP_H2O2 aus dem Partialdruck von Wasserstoffperoxid p_H2O2. Mit Hilfe der Magnus-Formel erhält man für den Wasserstoffperoxid-Taupunktes TP_H2O2
  
  Die Konstanten C_1WP, C_2WP und C_3WP sind dabei die Magnus-Koeffizienten für Wasserstoffperoxid.
• (2) Berechnen eines äquivalenten Sättigungsdampfdruckes p_Wäqui als Summe aus dem Sättigungsdampfdruck für Wasserdampf bei der Wasserstoffperoxid-Taupunkttemperatur TP_H2O2 und dem ermittelten Wasserdampf-Partialdruck p_H2O entsprechend der Magnus-Formel:
  
• (3) Berechnen des Gemisch-Taupunktes TP_Gem für ideale Gase aus dem äquivalenten Sättigungsdampfdruck p_Wäqui für Wasserdampf nach folgender Gleichung:
  
• (4) Korrektur des für die Annahme idealer Gase ermittelten Wertes für den Wasserdampf-Taupunktes TP_Gem über einen Polynomansatz in Abhängigkeit des Wertes selbst und dem weiteren Parameter, so dass ein für reale Gase korrigierter Gemisch-Taupunkt TP_Korr berechnet werden kann.

The technically important case of a dew point determination in the above-mentioned step (6) takes place in the following steps:

• (1) Calculate the hydrogen peroxide dew point TP _H2O2 from the partial pressure of hydrogen peroxide p _H2O2 . With the help of the Magnus formula, TP _H2O2 is obtained for the hydrogen peroxide dew point
  
  The constants C _1WP , C _2WP and C _3WP are the Magnus coefficients for hydrogen peroxide.
• (2) calculating an equivalent saturation vapor pressure p _Wäqui as the sum of the saturation vapor pressure for water vapor at the hydrogen peroxide dew point temperature TP _H2O2 and the determined water vapor partial pressure p _H2O according to the Magnus formula:
  
• (3) Calculate the mixture dew point TP _Gem for ideal gases from the equivalent saturation vapor pressure p _Wäqui for water vapor according to the following equation:
  
• (4) Correction of the value determined for the assumption of ideal gases for the water vapor dew point TP _Gem via a polynomial theorem as a function of the value itself and the further parameter so that a mixture dew point TP _Korr corrected for real gases can be calculated.

above approach delivers a good match in the technically interesting area with the results according to the NRTL model, with deviations smaller 0.5 K absolute or 0.7% relative to the mixed dew point. Thus, the accuracy is essentially influenced by the measurement data acquisition and the correctness of the further parameter. Optimal results can be achieved through an adjustment can be achieved at the operating point.

1

protective cap

2

closed end of 1

3

sensor

4

base

10

evaluation

T

temperature

rh

reading for the relative humidity

p _W

Water vapor partial pressure within 1

rM _H2O

mass fraction percent of water

rM _H2O2

mass fraction percent of hydrogen peroxide

V

Volume ratio of water / hydrogen peroxide

p _H2O

Water vapor partial pressure outside in the sample gas

p _H2O2

Hydrogen partial pressure in the sample gas

TP _H2O2

Hydrogen peroxide dewpoint

TP _Gem

Mixture dewpoint

TP _Corr

corrected Mixture dewpoint

p _Wäquiv

Saturation vapor pressure of water vapor in the sample gas

C _1w, C3 _W, C3 _W

Magnus coefficient for water

C _1WP , C3 _WP , C3 _WP

Magnus coefficient for hydrogen peroxide

Claims (5)

Method for determining the partial pressures (p _H2O and p _H2O2 ) of water vapor and hydrogen peroxide in a measuring gas, comprising the following steps: (A) measuring a temperature (T) of the measuring gas, (B) measuring a relative humidity (RH) with a moisture sensor ( 3 ) with a reducing protective cap ( 1 ), wherein the measured relative humidity (rF) is increased by an amount of moisture resulting from reduction of the hydrogen peroxide present in the measurement gas to water vapor, (C) calculating an increased partial pressure (p _W ) of water vapor from the measured values for the temperature and the relative humidity (rF), wherein the calculated increased partial pressure (p _W ) represents the sum of the partial pressures (p _H2O , p _H2O2 ) of the water vapor contained in the measurement gas and hydrogen peroxide. (D) Calculation of the individual partial pressures (p _H2O , p _H2O2 ) of the water vapor contained in the sample gas and hydrogen peroxide from the increased partial pressure (p _W ) and another parameter, the ratio of the partial pressures (p _H2O , p _H2O2 ) contained in the sample gas Water vapor and hydrogen peroxide represents. The method of claim 1, further comprising the step of: (E) calculating the real gas corrected mixture dew point (TP _Korr ) from the partial pressure (p _H2O , p _H2O2 ) of steam and hydrogen peroxide and the temperature reading (T ). Method according to Claim 2, in which the correction step (E) comprises the following substeps: (E.1) calculating the hydrogen peroxide dew point (TP _H2O2 ) from the partial pressure of hydrogen peroxide (p _H2O2 ) and the measured value for the temperature (T) (E .2) calculation of an equivalent saturation vapor pressure (p _Wäqui ) as the sum of the saturation vapor pressure for water vapor at the hydrogen peroxide dew point temperature (TP _H2O2 ) and the determined water vapor partial pressure (p _H2O ), (E.3) Calculate a mixture dewpoint (TP _Gem ) from the equivalent saturation vapor pressure (p _Wäqui ) for water vapor assuming an ideal gas. (E.5) Calculate a mixture corrected for real gases mixture dew point (TP _Korr ) using a polynomial interpolation. The method of claim 2, wherein the in step (E) performed Calculation using the NRTL model is performed. Measuring device for measuring the moisture content of a H ₂ O ₂ -containing fluid with the following components: - a capacitive humidity sensor ( 3 ), - a temperature sensor, - a protective cap ( 1 ), which the humidity sensor ( 3 ), - evaluation unit ( 10 ), which is designed to carry out a method according to claims 1 to 4, wherein the protective cap ( 1 ) has finely distributed through openings, on whose surfaces an active substance is arranged, which almost completely eliminates the H ₂ O ₂ contained therein as it passes through the fluid.