METHOD AND APPARATUS FOR DETERMINING THE CONTENT OF A FOREIGN GAS IN A PROCESS LIQUID

Abstract

A method and an apparatus determine a content of a foreign gas in a process liquid in which a measurement gas, especially CO.sub.2, has been dissolved. A concentration of the measurement gas is ascertained and a concentration of the gas mixture formed by the measurement gas and the foreign gas is ascertained, especially via a manometric measurement method. The measurement values are supplied to an evaluation unit. A concentration of the foreign gas is determined on the basis of the ascertained concentration of the measurement gas and the ascertained concentration of the gas mixture.

Claims

1. A method for determining a content of a foreign gas in a process liquid in which a measurement gas has been dissolved, which comprises the steps of: ascertaining a concentration of the measurement gas; ascertaining a concentration of a gas mixture formed by the measurement gas and the foreign gas; supplying measurement values to an evaluation unit; and determining a concentration of the foreign gas on a basis of the concentration of the measurement gas ascertained and the concentration of the gas mixture ascertained.

2. The method according to claim 1, which further comprises: ascertaining a total pressure and a temperature of the gas mixture formed by the measurement gas and the foreign gas; determining a partial pressure of the measurement gas on a basis of the concentration of the measurement gas, the total pressure and the temperature; ascertaining a partial pressure of the foreign gas on a basis of the partial pressure of the measurement gas and on a basis of the pressure and the temperature; and determining a concentration and/or a content of the foreign gas based on the partial pressure of the foreign gas.

3. The method according to claim 1, wherein the foreign gas is added to the process liquid after an ascertainment of the concentration of the measurement gas.

4. The method according to claim 2, which further comprises ascertaining the concentration of the measurement gas by means of a manometric sensor via a manometric measurement method.

5. The method according to claim 2, which further comprises ascertaining the concentration of the measurement gas by means of a first sensor configured as an optical sensor, wherein an optical absorption measurement of the measurement gas is performed by means of the first sensor.

6. The method according to claim 5, which further comprises ascertaining the total pressure and the temperature of the gas mixture formed by the measurement gas and the foreign gas by means of a second sensor configured as a manometric sensor and the total pressure is ascertained via a manometric measurement method.

7. The method according to claim 6, which further comprises ascertaining the partial pressure of the foreign gas (P.sub.F) with an aid of equation P.sub.F=(p′.sub.Total−p.sub.MG/(1 +v/L.sub.MG))*(1+v/L.sub.F).

8. The method according to claim 1, wherein the measurement gas is carbon dioxide and the foreign gas is nitrogen or nitrous oxide.

9. The method according to claim 1, which further comprises taking into account a solubility and/or a compressibility of the measurement gas and/or the foreign gas in the process liquid in a determination of the concentration of the measurement gas and/or the foreign gas.

10. The method according to claim 6, wherein a difference between measurement values of the first and second sensors is calibrated using samples of known concentration of the measurement gas and the foreign gas and calibration curves or tables are stored in the evaluation unit.

11. The method according to claim 1, which further comprises: determining a relationship between the concentration of the measurement gas without foreign gas influence and the concentration of the gas mixture consisting of the measurement gas and the foreign gas before a start of measurement on a basis of measurements on known samples; making available values for the concentration of the foreign-gas in the evaluation unit of measurement devices as calculation curves and/or table values as a concentration difference of the measurement gas, the concentration difference being temperature-dependent; and ascertaining the concentration of the foreign-gas on a basis of the values during the measurement.

12. The method according to claim 2, wherein a determination of the concentration of the measurement gas and the measurement of the total pressure and the temperature of the gas mixture take place in a pipe system of the process liquid or in a vessel.

13. The method according to claim 1, wherein the process liquid is a beverage containing dissolved gases.

14. The method according to claim 1, wherein a content of ingredients such as extracts and alcohol that are contained in the process liquid is taken into account in a determination of the content of the measurement gas.

15. The method according to claim 1, which further comprises taking into account a vapor pressure of the process liquid in a determination of the concentration of the measurement gas and the foreign gas.

16. The method according to claim 5, wherein the process liquid, the measurement gas and the foreign gas are present in a vessel in a two-phase state, wherein the process liquid containing dissolved portions of the measurement gas and the foreign gas forms a first phase and an undissolved portion of the measurement gas and the foreign gas forms a second phase as the gas mixture, wherein the first sensor and the second are disposed in a region of the vessel in which the measurement gas and the foreign gas are presently dissolved in the process liquid.

17. An apparatus for determining a content of a foreign gas in a process liquid in which a measurement gas has been dissolved, the apparatus comprising: a pipe system in which the process liquid can flow and/or a vessel in which the process liquid is collectable; a first sensor by which a content of the measurement gas is ascertained; and a second sensor by which a total pressure and a temperature of the measurement gas and the foreign gas in the process liquid are ascertained; an evaluation unit configured to determine the content of the foreign gas in the process liquid in which the measurement gas having been dissolved, said evaluation unit configured to: ascertain a concentration of the measurement gas; ascertain a concentration of a gas mixture formed by the measurement gas and the foreign gas; receive measurement values; and determine a concentration of the foreign gas on a basis of the concentration of the measurement gas ascertained and the concentration of the gas mixture ascertained.

18. The apparatus according to claim 17, wherein at least one of said first and second sensors is an optical sensor.

19. The apparatus according to claim 17, wherein at least one of said first and second sensors is a manometric sensor.

20. The apparatus according to claim 17, wherein said first sensor and said second sensor are disposed at a short distance from one another in said pipe system.

21. The apparatus according to claim 17, wherein said first sensor and said second sensor are disposed opposite one another in said vessel.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0058] FIG. 1 is a graph showing portions of partial pressures of CO.sub.2 and foreign gases nitrogen and laughing gas as a function of a volume increase ΔV in a measurement by the expansion method;

[0059] FIGS. 2 and 3 are graphs showing measurement results of an experimental test for determination of a nitrogen content and a laughing-gas content in a process liquid with an aid of the method according to the invention; and

[0060] FIG. 4 is an illustration showing a pipe system.

DETAILED DESCRIPTION OF THE INVENTION

[0061] In the following, the apparatus according to the invention and the method according to the invention are described in more detail in FIG. 4. Thus, in a first exemplary embodiment, the apparatus can contain a pipe system 1 which a process liquid flows. Dissolved in the process liquid, for example beer, is a measurement gas, for example carbon dioxide, in this case in low concentration. Integrated in the pipe system 1 is a first sensor 2, by means of which the content of the measurement gas is ascertained as accurately as possible, for example by an optical measurement. Furthermore, the apparatus contains a second sensor 3 for carbon dioxide measurement, configured as a manometric sensor for example, by means of which the total pressure and the temperature of the measurement gas and the foreign gas, i.e., the gas mixture formed from these two gases, in the process liquid is then ascertained. The measurement data of the first sensor 2 and the second sensor 3 are then supplied to an evaluation unit 4, which determines the content of the foreign gas from said measurement data or from the measurement values obtained by the sensors.

[0062] For example, according to the invention, the CO.sub.2 concentration is determined optically by the first sensor 2 by an ATR method without foreign gas influence and the so-called apparent CO.sub.2 content is then ascertained using the second sensor, for example via pressure and temperature measurement, and the apparent CO.sub.2 content is then compared with the exactly measured CO.sub.2 content. From the twin measurement of CO.sub.2 content, the partial pressure and thus the concentration of the foreign gas is then ascertained via the partial pressure or the concentrations of the measured CO.sub.2 values by the method according to the invention, for example by means of Henry and Dalton's equation.

[0063] The method according to the invention utilizes a measurement value not influenced by the foreign gas for the determination of the exact CO.sub.2 concentration, which measurement value is captured using an optical sensor, for example an ATR sensor, and compares the measurement value with the “error-containing” CO.sub.2 concentration value from the manometric method in order to infer the partial pressure and the concentration of the foreign gas.

[0064] In the following embodiment of the method according to the invention, the first sensor 2, a Carbo 520 from Anton Paar (https://www.anton-paar.com/at-de/produkte/details/carbo-520-optical/), measures the CO.sub.2 concentration optically by the ATR method without foreign gas influence. A second sensor 3 for pressure and temperature measurement, Carbo 510 from Anton Paar (https://www.anton-paar.com/corp-de/produkte/details/carbo-510-smart-sensor/), ascertains the apparent or error-containing CO.sub.2 content from the measured total pressure. If no second gas is present, the apparent CO.sub.2 content corresponds to the actual CO.sub.2 content and it is possible, for example, to check whether a foreign gas is contained in the process liquid in the first place.

[0065] If the CO.sub.2 contents ascertained by the first sensor 2 and the second sensor 2 differ, the partial pressure and the concentration of the foreign gas, for example nitrogen or nitrous oxide, can then be ascertained from the concentration and/or the determined partial pressure of the CO.sub.2 on the basis of the following calculation.

[0066] In the case of the presence of a foreign gas, the total pressure measured using the manometric method is yielded by the sum totals of the partial pressures.

[0067] Dalton's law: p.sub.Total=Σp.sub.i

[0068] The following therefore applies to the CO.sub.2 and foreign gas: p.sub.Total=p.sub.CO2+p.sub.FG.

[0069] The concentration can then be determined using Henry and Dalton's law: ci=ξi(T)*pi [0070] where: [0071] ci is in g/L (concentration of a gas), and [0072] ξi is in g/L/bar (solubility of a gas).

[0073] The CO.sub.2 and N.sub.2O concentration can also be specified in the unit “volume” (vol) instead of “g/L”. 1 vol=1 liter of CO.sub.2 or N.sub.2O under standard pressure (1 bar) per liter of beverage. (The units can be converted via the gas law).

[0074] The CO.sub.2 concentration is then calculated by: c.sub.cO2V=p.sub.co2*L.sub.CO2 [0075] where: [0076] c.sub.co2V . . . CO.sub.2 concentration in unit of volume, [0077] L.sub.CO2 . . . solubility in volume/bar=ξ/(M/(R*T0*Z0))=ξ/1.951, [0078] T0=273.2 K, Z0=0.993, M.sub.CO2=44.01 g/mol, and [0079] R . . . gas constant=8.3145*10−2[l*bar/(K*mol)].

[0080] From the CO.sub.2 concentration value determined using the foreign gas-independent optical sensor, it is possible to infer or back-calculate the true partial pressure of the CO.sub.2 p.sub.CO2:

[0081] p.sub.CO2=c.sub.CO2OPT/ξ.sub.CO2.

[0082] From the manometric total-pressure measurement value for CO.sub.2 plus foreign gas and the actual partial pressure of the CO.sub.2, it is then possible to infer the partial pressure of the foreign gas and thus also the concentration of any foreign gas such as, for example, nitrogen or laughing gas.

[0083] FIG. 1 shows the differences in the pressure profile of CO.sub.2 and the foreign gas nitrogen in the expansion of different volumes. The specific solubility of N.sub.2 in beverages such as water or beer is much lower than that of CO.sub.2. At a pressure of 1 bar, what can be dissolved in 1 liter of water is 1 liter of CO.sub.2 under normal pressure, but only 0.17 liter of nitrogen at the same pressure. This means that the greatest portion of the nitrogen is already in the gas phase in the measurement using the expansion method with, for example, 10% expansion; the influence due to the nitrogen thus rises with small expansion volumes. The nitrogen and CO.sub.2 concentration in the liquid falls. The partial pressure of the nitrogen drops rapidly in the expanded chamber compared to the CO.sub.2 pressure, meaning that the majority of the foreign gas nitrogen has so to speak diffused out of the solution. The partial pressure then changes only slightly.

[0084] The partial pressures reached as a result of a volume increase v can be calculated using a modified Henry and Boyle's formula as described below. In order to simplify the formula, calculations are made with the solubility in “volume/bar” instead of “g/l/bar”.

[0085] p′.sub.CO2=p.sub.CO2/(1+v/L.sub.CO2)

[0086] p′.sub.FG=p.sub.FG/(1+v/L.sub.FG)

[0087] p′.sub.x partial pressure of the gas after the measurement-chamber volume increase

[0088] p.sub.x saturation pressure of the gas in the liquid

[0089] Volume increase v=0.1 (=10%)

[0090] L.sub.x solubility of the gas in the sample liquid in “volume/bar”

[0091] The following apply:

[0092] p′.sub.Total=p.sub.CO2/(1+v/L.sub.CO2)+p.sub.FG/(1+v/L.sub.FG)

[0093] p.sub.FG=(p′.sub.Total−p.sub.CO2/(1+v/L.sub.CO2))*(1+v/L.sub.FG))

[0094] For the specific solubility, literature values for the fluids and gases to be tested can be used.

[0095] c.sub.FG=ξ.sub.FG(T)*p.sub.FG

[0096] In the following, a method according to the invention is described exemplarily on the basis of an exemplary embodiment of CO.sub.2 as measurement gas and nitrogen as foreign gas.

[0097] The first sensor 2 configured as an optical sensor, Carbo 520 from Anton Paar (https://www.anton-paar.com/at-de/produkte/details/carbo-520-optical/), measures the CO.sub.2 concentration without foreign gas influence. A second sensor 3 which is configured as the manometric sensor Carbo 510 (https://www.anton-paar.com/corp-de/produkte/details/carbo-510-smart-sensor/) and functions according to the expansion method is used for foreign gas determination, i.e., for nitrogen determination.

[0098] The volume increase of the second sensor 3 is set at around 10% in order to minimize measurement errors. A reduction would have the advantage that an error of the optical measurement value has less influence, but that the influence of the volume increase or pressure dependency rises. In this exemplary embodiment, the expansion volume for the manometric sensor is thus advantageously optimally chosen on the basis of the actual specific solubility of the foreign gas to be tested. Alternatively for different gases, the expansion volume can also be adapted thereto and also be chosen differently in the exemplary embodiment described.

[0099] From the CO.sub.2 concentration ascertained by the first sensor 2 without foreign gas influence, it is possible to calculate the CO.sub.2 saturation pressure:

a) p.sub.CO2=c.sub.(CO2)OPT/L.sub.CO2

[0100] Besides the apparent CO.sub.2 value, the second sensor also provides the total pressure according to volume increase: p′.sub.Total. From both values, it is possible to ascertain the partial pressure of the foreign gas, in this case nitrogen by way of example.

[0101] p.sub.N2=(p′.sub.Total−p.sub.CO2/(1+v/L.sub.CO2))*(1+v/L.sub.N2).

[0102] For the specific solubility, literature values for the process liquids to be tested can be used.

[0103] ci=ξi(T)*pi

[0104] The following apply:

[0105] Solubilities L at a defined measurement temperature, for example 10° C.:

[0106] L.sub.CO2=1.058 bar−1, ξ.sub.CO2=2.06 g/L/bar

[0107] L.sub.N2=0.017 bar−1, ξ.sub.N2=20 mg/L/bar

[0108] Saturation pressures in the gas phase: p.sub.CO2=2. 50 bar, p.sub.N2=2. 00 bar

[0109] For nitrogen as foreign gas, rearrangement and insertion therefore yields:

[0110] p′.sub.Total=p.sub.CO2/(1+v/L.sub.CO2)+p.sub.N2/(1+v/L.sub.FG)=2.284+0.291=2.574 bar

[0111] p.sub.N2=(p′.sub.Total−p.sub.CO2/(1+v/L.sub.CO2))*(1+v/L.sub.N2)=0.290*6.882=2 bar

[0112] c.sub.N2=ξ.sub.N2*p.sub.N2=20*2=40 mg/L

[0113] An alternative calculation of the content of the foreign gas can also be done with the aid of an empirically ascertained polynomial composed of CO.sub.2, apparent CO.sub.2 and temperature. The CO.sub.2 contents ascertained by the first sensor 2 and the second sensor 3 or the partial pressures calculated therefrom are used for this purpose. The difference between the measured pressure values is studied by measurement of known samples having known contents of CO.sub.2 and a defined foreign gas. The different measurement values are measured using standard measurement instruments, for example the CarboQC from the applicant or other highly accurate laboratory devices.

[0114] The difference between the thus ascertained CO2 concentrations is ascertained from the difference of

[0115] c.sub.DEV=c.sub.(CO2)Manc.sub.(CO2)OPT.

[0116] From this difference, the foreign gas content, for example nitrogen content or laughing gas, is then ascertained and a table containing foreign gas values in relation to respective c.sub.DEV at various temperatures is stored in the evaluation unit 4. In the process line, the measured differences are then read from a corresponding lookup table.

[0117] The measurement values can optionally also be fitted with a higher order polynomial and be stored in the evaluation unit 3 as calculation curves. The corresponding solubilities, temperatures and pressure differences are evaluated and taken into account:

[0118] c.sub.N2=f(c.sub.DEV, c.sub.(CO2)OPT, t).

[0119] Thus, the relationship between measurement gas concentration without foreign gas influence and measurement gas concentration with foreign gas influence can alternatively be determined in advance at the factory on the basis of measurements on known samples. The values for the foreign-gas concentration are then made available in the evaluation unit of the measurement devices or stored therein as calculation curves and/or table values as the concentration difference of the measurement gas, which concentration difference is especially temperature-dependent. When the concentrations and the pressure and the temperature are measured, the foreign-gas concentration is then ascertained on the basis of these values during the measurement.

[0120] The applicable formula is then not universally valid, but only for one process liquid in each case, for example beer, and is, depending on the process liquid to be measured, adapted for, for example, soft drinks and a corresponding formula is developed and stored in the evaluation unit 4. The user can thus select the tested process liquid, for example beer, cola, diet cola, etc., and the foreign gas in the evaluation unit 4 and thus consult the correct evaluation curve.

[0121] The evaluation unit can be respectively present in each sensor itself, i.e., the measurement values can be processed directly in an evaluation unit assigned to the sensor or arranged therein or can be supplied to a central evaluation unit in which the measurement values of the sensors are then processed and the determination of the concentration of the foreign gas takes place.

[0122] Optionally, accuracy can be improved even in the case of a low CO.sub.2 content in the liquid. If the process liquid is expanded in the case of low CO.sub.2 concentrations in the process liquid, the pressure which is reached arises not only from the CO.sub.2 gas from the process liquid, but also from the water vapor of the aqueous solution, particularly in the case of a large expansion. In order to then increase accuracy, the vapor pressure of the water is also taken into account in the determination of the concentration of the foreign gas; the pressure value ascertained in the measurement of the process gas, for example the measurement of CO.sub.2, is therefore corrected:

p.sub.Cor=p.sub.Gas−p.sub.Vapor.

[0123] This is done using a formula, the Magnus formula, which can be gathered from the literature and takes into account the dependency of the vapor pressure on the measurement temperature.

[0124] (Magnus formula for the saturation vapor pressure above level water surfaces:

p.sub.Vapor=0.006112*Exp(17.62*t/(t+243.12) (for −45° C.<T<60° C.).

[0125] Furthermore, the accuracy of the method according to the invention can be increased by using the real gas factor or the compressibility factor. The real gas factor or compressibility factor describes the deviation of a real gas from ideal behavior. Gases sometimes considerably deviate from ideal behavior in the case of finite volume and higher pressures.

[0126] m.sub.G=p*V.sub.H*M/(R*T*Z) Mass of the gas in the headspace

[0127] p . . . Partial pressure of CO.sub.2

[0128] m.sub.G . . . Mass of CO.sub.2

[0129] V.sub.H . . . Headspace volume

[0130] M . . . Molar mass

[0131] R . . . Gas constant

[0132] T . . . Temperature in K

[0133] Z . . . Compressibility

[0134] Z=1.0005+p.sub.Cor*(−0.007+T*0.0000674)

[0135] Z is the compressibility factor of CO.sub.2; it is temperature—and pressure-dependent and can in principle be gathered from the literature. When applied, the factor lies in the order of magnitude of 0.98 and plays a greater role, the larger the volume of expansion.

[0136] FIG. 2 shows the results of an example of nitrogen measurement using the combination of a first optical sensor Carbo520 and a second manometric sensor Carbo510.

[0137] The system was started with air-saturated water, which is cooled to 20° C. via a heat exchanger with thermostat.

[0138] At two time points, t1 13:25 (1:25 PM) and t2 14:25 (2:25 PM), nitrogen was injected in each case into the fluid tested: the nitrogen concentration determined correlates with the saturation pressure.

[0139] FIG. 3 shows the results of an example of nitrous oxide measurement (laughing gas) using the combination of a first optical sensor Carbo520 and a second manometric sensor Carbo510.

[0140] The system was started with water as process liquid, which is cooled to 20° C. via a heat exchanger with thermostat.

[0141] At two time points, t1 14:38 (2:38 PM) and t2 15:05 (3:05 PM), nitrous oxide was injected in each case into the process liquid tested. The nitrous oxide concentration determined correlates with the saturation pressure.

[0142] As an alternative to the first exemplary embodiment, it is, for example, also possible for the first and/or the second sensor 3 to be arranged within a vessel in which the process liquid is collected. The process liquid, for example beer, is collected in such vessels, for example beverage tanks, and what is formed above the process liquid is a gas phase, the so-called headspace, in which the measurement gas in the undissolved portion is present. Thus, the process liquid, the measurement gas and the foreign gas are present in the vessel in a two-phase state, wherein the process liquid containing dissolved portions of the measurement gas and the foreign gas form the first liquid phase and, thereabove in the headspace, an undissolved portion of the measurement gas and the foreign gas in a gas mixture form the second phase. Here, according to the invention, the first sensor 2 and the second sensor 3 can be arranged in the region of the vessel in which the measurement gas and the foreign gas are present dissolved in the process liquid. The method according to the invention can then be performed according to the invention, as explained in relation to the first exemplary embodiment.

[0143] Alternatively, the foreign gas can likewise be added only in the course of the production of the process liquid, for example beer. Thus, alternatively, the content of the measurement gas, for example carbon dioxide, can first be ascertained by means of the first CO.sub.2 sensor, for example via the ATR method or an ATR sensor, in order to determine the exact CO.sub.2 content within the process liquid. In a further step, the foreign gas, for example nitrogen, is then added and measurement is subsequently performed using the second sensor, for example a manometric sensor, and the content of the foreign gas is then ascertained via the method according to the invention via the measured pressures or determined partial pressures of the measurement gas and the foreign gas.

[0144] In a further alternative embodiment of the apparatus according to the invention and the method according to the invention, only one sensor is provided inside the process line or the vessel, and the measurement gas is first determined before addition of a foreign gas and then the process liquid admixed with the foreign gas is guided past the sensor again and a second measurement is performed using said sensor, and, in this way, the added portion of the foreign gas is determined on the basis of the different measurement values.

[0145] In the case of the embodiment of the apparatus with two sensors and the performance of the method according to the invention with two sensors, it is particularly preferred that said two sensors, or the measurement points of exact determination of CO.sub.2 and of determination of total pressure and temperature, are effected at a short distance from one another or the two sensors are arranged opposite one another in the pipe system 1.