Method for measuring the concentration of a dissolved gas in a fluid

The invention pertains to a method for measuring the concentration of dissolved gases in a liquid [(39)], especially of CO.sub.2 in beverages, in which the liquid [(39)] is passed across the retentate side [(140)] of a membrane [(14)] that is at least partially permeable to the dissolved gas, and [in which] the volumetric flow of the permeated gas on permeate side [(16)] of the membrane [(14)] is determined, the temperature of the liquid [(39)] is measured, and the concentration of the dissolved gas in the liquid [(39)] is calculated from these values. In this, the thickness of the membrane [(14)] can be pre-selected as a function of the flow rate of the liquid [(39)] flowing along the retentate side [(140)].

DESCRIPTION
 The invention pertains to a method for measuring the concentration of
 dissolved gases in a liquid, especially of CO.sub.2 in beverages, in which
 the liquid is passed across the retentate side of a membrane that is at
 least partially permeable to the dissolved gas, and [wherein] the
 volumetric flow of the permeated gas on permeate side of the membrane is
 determined, the temperature of the liquid is measured, and the
 concentration of the dissolved gas in the liquid is calculated from these
 values.
 A method of this kind is known (DE-OS [Offenlegungsschrift=disclosure] 44
 39 715) deriving from the same applicants. This known method has proven to
 be extraordinarily effective and economical as well as being easily used
 in practice, especially in connection with concentration measurement of
 dissolved carbon dioxide (CO.sub.2) in water or beverages.
 For certain applications, especially in the area of analysis, a still
 higher accuracy is frequently needed in determination of the dissolved gas
 concentration in a liquid.
 It is generally known that some beverages that are bottled in cans or
 bottles, for example, cola, soft drinks, or mineral water, frequently
 contain carbon dioxide in dissolved form. In manufacture, one proceeds so
 that prior to filling into bottles, cans, or the like, a specific amount
 of gaseous carbon dioxide is added to the beverage so that a part of the
 carbon dioxide is taken up by the beverage. In this, attention must be
 paid that the content of carbon dioxide is maintained within certain
 limits. On the one hand, the beverage should still contain an adequate
 amount of carbon dioxide upon consumption, and on the other, the beverage
 must not foam upon opening of the container, which can be traced to too
 high an amount of carbon dioxide.
 During the filling operation, the concentration must be monitored and
 possibly adjusted. Methods and apparatuses for measuring the carbon
 dioxide content or carbon dioxide concentration are known with which the
 carbon dioxide concentration can be determined discontinuously. For this
 purpose, samples are generally taken from the ongoing production process
 and these are studied with regard to the carbon dioxide concentration. The
 dosing of carbon dioxide is influenced based on the results. This
 discontinuous method of measurement has the disadvantage that, for
 example, short-term variations cannot be detected or lead to erroneous
 analyses so that even with more frequent measurements, rejects cannot be
 avoided. Especially with too-high a carbon dioxide concentration in the
 liquid, there is the danger that the container for the end user will
 burst, for example, at higher temperatures and/or more vigorous movement
 wherein the danger of injuring the user cannot be excluded.
 The known method is based on the recognition that with given membrane
 characteristics, namely the gas permeability and the membrane area, the
 volumetric flow of gas through the membrane can be used as a measure of
 the concentration of the gas in the liquid on the retentate side of the
 membrane. The volumetric flow of the permeate, i.e., the volumetric flow
 of the dissolved gas, depends on the permeability, the membrane area, and
 the driving pressure difference across the membrane as well as the
 concentration of the gas dissolved in the liquid on the retentate side of
 the membrane. The concentration of gas dissolved in the liquid on the
 retentate side that is required for creating the measured volumetric flow
 of the gas on the permeate side depends on the pressure on the retentate
 side in a manner that depends on the temperature of the liquid and which
 is unique for the respective substance mixture, namely via the Henry's Law
 coefficient. Specifically, this means that the measured volumetric flow at
 a particular temperature is a measure of the concentration of the
 dissolved gas on the retentate side of the membrane.
 It is irrelevant for the functioning of the membrane whether the liquid
 with the dissolved gas is supersaturated, saturated, or less than
 saturated. Also, the actual pressure on the retentate side does not
 necessarily need to be determined. Rather, in the method of the invention,
 use is made of the fact that a certain concentration of substance on the
 retentate side of the membrane at a given temperature causes a certain
 volumetric flow of the dissolved gas through the membrane.
 With the known method, it is possible to perform continuous measurement of
 the dissolved gas in a liquid. In mixing beverages with carbon dioxide, it
 is therefore possible, for example, that a computing unit used for
 evaluation of the measured values (temperature, volumetric flow) be
 connected to a control unit by which the dosing device for carbon dioxide
 is controlled. In this manner it is possible to make available an
 essentially closed regulatory circuit for dosing of carbon dioxide in the
 preparation of beverages.
 It is the goal of the invention to create a method with which such a highly
 accurate continuous measurement of the concentration of a gas dissolved in
 a liquid is possible so that it can be used in the field of analysis,
 wherein the method itself should be able to be done by simple means and
 thus inexpensively, and that apparatus for doing the process is used that
 is essentially commercially available and thus likewise contributes to the
 inexpensive execution of the process.
 This goal is attained by the invention in that the thickness of the
 membrane can be preselected as a function of the velocity of the liquid
 flowing past the retentate side.
 This procedure has the advantage that, besides the values to be measured,
 namely the volumetric flow on the one hand and the temperature on the
 other which can be continuously determined, there is also an adjustment of
 the membrane thickness, and therefore of the permeation rate of the
 dissolved gases through the membrane as a function of the boundary layer
 formation at the membrane surface, to the velocity of the liquid and it is
 included in the calculation, so that for example, by using the computing
 unit, the concentration of the gas content [sic] in the liquid can be
 indicated continuously with sufficient accuracy so that the method is also
 suitable for application in the field of analysis as is strived for.
 According to an advantageous embodiment of the method, the flow rate of the
 liquid is determined, wherein a calculation of the dissolved gas in the
 liquid is interrupted, as a pre-set minimum flow rate in the liquid is
 detected. Below a minimum [flow] rate of a liquid, a boundary layer in the
 liquid can no longer be formed at the retentate side of the membrane,
 i.e., at the surface there, to the extent desired so that measurements
 below a minimum flow rate falsify the calculated measurement results and
 possibly make them completely unusable.
 In a further advantageous embodiment of the method, the level of the
 minimum flow rate can be set, i.e., the threshold at which the measured
 parameters are determined or a calculation is or is not done, so that an
 immediate adjustment to the measurement goal as such, to the liquid and
 the solubility of the gas in the liquid, is thereby possible.
 In order to further increase the accuracy of the concentration
 determination, it is advantageous to use the flow rate of the liquid as
 such for calculation of the concentration of the dissolved gas in the
 liquid in addition to the determined volumetric flow of the permeating gas
 and the temperature of the liquid so that, as already described above, the
 effect of the flow rate of the liquid on boundary layer formation on the
 retentate side of the membrane can come into the calculation.
 A further improvement in the accuracy of the concentration determination is
 advantageously attained in that besides the determined volumetric flow of
 the permeating gas and the temperature of the liquid, the ambient pressure
 is also used for calculating the concentration of the dissolved gas in the
 liquid, wherein due to the additional determination of the ambient air
 pressure, a correction of the value for permeation of the gas through the
 membrane is possible since the ambient air pressure acts as atmospheric
 counter-pressure to the permeation.
 In a still further advantageous embodiment of the method for determining
 the concentration of a dissolved gas in a liquid, the solubility
 coefficient of the gas or of different gases, which is different in
 different liquids, is used and likewise is included as a determining
 parameter in the overall calculation, whereby a still more accurate
 concentration determination is possible.
 Depending on apparatus-related conditions that have an effect on the
 procedure of doing the method, strong changes in the determined parameters
 like [sic; as well as] the gas concentration that is calculated from them
 have an effect on the rapidity of the measurement of the parameters and
 the concentration level calculated from them. The larger the changes in
 the measured values from one measuring time to another and thus from one
 time of calculation to another, the larger will be the average time for
 evaluation, that is, the time required for and between calculation
 processes, so that it is extraordinarily advantageous to determine the
 final measured value by calculation or estimation from the slope of a
 parameter in order to shorten the equilibration time when there are
 changes in the measured values and in this manner obtain an intelligent
 set-up for attaining an overall rapid determination of the concentration
 of the gas in the liquid with quasicontinuous measurements even with large
 changes in the measured values.
 With wear, down time, or other effects impairing the functioning of the
 apparatus for performing the method of the invention, after appropriate
 repair or exchange of parts, especially also of the membrane, a new
 calibration of the apparatus for doing the method is required. Calibration
 of a computing or evaluating unit used in performing the method is
 advantageously done by using a pure gas whose parameters are known,
 wherein this pure gas is preferably CO.sub.2.

With respect to their basic construction, the devices 10 and 30 in FIGS. 1
 and 2 essentially correspond to the devices 10 as they are known from the
 known, related DE-OS 44 39 715. The device 10 for measuring the
 concentration of a gas in a liquid has a measuring vessel 11 that is
 formed as a measuring sensor which is equipped with an essentially tubular
 measuring channel 12. There is a membrane 14 in the wall 13 surrounding
 the measuring channel 12, [which membrane] is connected to the measuring
 vessel 11 via an appropriate membrane holding device 15. In this, the
 surface of the membrane 14 is oriented so that its retentate side 140 is
 essentially parallel to the direction of flow of the liquid 39 in the
 measuring channel 12.
 The permeate side 16 of membrane 14 that is away from the liquid 39 is
 connected to a cavity 17 whose outlet 18 is connected to a flowmeter 19.
 Heretofore, it was possible with this arrangement to measure the
 volumetric flow of a gas passing through the membrane, for example carbon
 dioxide.
 Additionally, in wall 13 of the measuring channel 12 in the region of the
 membrane 14, there is a measuring device 20 for determining the
 temperature of the liquid 39. Moreover, in measuring channel 12, there is
 a flow rate indicator 40 for determining the flow rate of the liquid 39.
 The signal output of the flowmeter 19 is connected to a computing unit 23
 via a signal line 21, as is the signal output of the temperature measuring
 device 20 via signal line 22. The signal output of the flow rate indicator
 40 is likewise connected with the computing unit 23 via a signal line 41.
 The concentration of dissolved gas in the liquid is calculated in this
 computing unit 23 based on the determined values, namely the volumetric
 flow of the permeated gas stream on one side and the temperature of the
 liquid 39 on the other side, the [flow] rate of the liquid 39, and the
 ambient air pressure.
 Finally, there is an air pressure gauge 42 that likewise via a signal line
 43 supplies the appropriate air pressure values at predetermined time
 intervals quasicontinuously to the computing unit 23.
 For example, the concentration of dissolved CO.sub.2 in water in this
 measuring device is obtained from:
 ##EQU1##
 wherein
 V is the volumetric flow in m.sup.3 /h
 L is the permeability of CO.sub.2, m.sup.3
 N/m.sup.2.multidot.h.multidot.bar
 A is the membrane area, m.sup.2
 P.sub.p is the permeate pressure, bar
 T is the temperature, .degree.C
 .sub.N is the density of CO.sub.2 under normal conditions, (1.97
 kg/M.sup.3)
 C.sub.CO2 is the carbon dioxide concentration, g CO.sub.2 per 1 of water
 H is the Henry's Law coefficient, m.sup.3.sub.CO2
 /m.sup.3.sub.w.multidot.bar
 In this relation, the H(T) function (Henry's Law coefficient) depends on
 the temperature and, for a CO.sub.2 -water mixture, is: [In equations,
 commas in numbers represent decimal points.]
EQU H=1,6431-0,059017.multidot.T+0,0012226.multidot.T.sup.2 -1,36E-05T.sup.3
 +6,17E-08T.sup.4
EQU 0.degree.&lt;T&lt;60.degree.
 The computing unit 23 is equipped with an indicator 24 by means of which
 the CO.sub.2 concentration determined in the liquid can be indicated. It
 can also be arranged that the signal output of the computing unit 23 is
 connected via a signal line 25 to a controlling unit 26. The controlling
 unit 26 can be connected, for example, to a dosing device for CO.sub.2 27
 of a beverage bottling plant that is not shown. In this manner, in
 bottling of beverages, it is possible to control dosing of the CO.sub.2
 directly via the measured CO.sub.2 concentration.
 The device 30 for measuring the concentration of dissolved gases in a
 liquid shown in FIG. 2 essentially corresponds to the device 10 in FIG. 1
 so that the same elements are designated with the same reference numbers.
 In device 30 of FIG. 2, the measuring vessel 11 is essentially identical
 with the measuring vessel of device 10 in FIG. 1. Here however, a rinsing
 gas can flow across the free space 17 on the permeate side 16 of membrane
 14. Specifically, the device is constructed in such a manner that the
 membrane holding device 15 which adjoins the free space 17 on the permeate
 side 16 of the membrane 14 is equipped with an inlet 31 for rinsing gas
 and an outlet 32 for the rinsing gas/gas mixture. In the embodiment shown
 in FIG. 2, rinsing gas is passed co-current to the liquid in measuring
 channel 12. It is of course also possible that the rinsing gas be passed
 counter-current or cross-current to the liquid. As rinsing gas, nitrogen
 can preferably be used.
 The outlet 32 for the nitrogen-gas mixture is connected with a measuring
 device 33 for determining the concentration of gas in the rinsing gas
 stream. The inlet 31 is connected with a flowmeter 34 to determine the
 volumetric flow of the rinsing gas. At the other end of the inlet 31,
 there is a rinsing gas supply 35 as well as a filter 36 for the rinsing
 gas.
 The signal outputs of the temperature measuring device 20, the flowmeter
 34, and the concentration measuring device 33 are connected via signal
 lines 37 to a computing unit 38 as are the air pressure gauge 42 via
 signal line 43 and the flow rate indicator meter 40 for measuring the flow
 rate of the liquid 39 via signal line 41, which [computing unit]
 calculates the concentration of the dissolved gas in the liquid 39 from
 the determined values.
 In this embodiment, the concentration of dissolved CO.sub.2 in water can be
 determined, for example, as follows:
 ##EQU2##
 The H(T) function corresponds to the above-indicated function. In this, the
 permeate pressure P.sub.p is negligible since the amount of rinsing gas is
 advantageously adjusted in such a manner that the partial pressure of the
 carbon dioxide in the rinsing gas stream is relatively low.
 In the embodiment shown in FIG. 2, the flow measuring device 34 is situated
 in the inlet line 31 for the rinsing gas. It is of course also possible
 that the flow measuring device 34 be situated behind the outlet 32 in
 front of or behind the concentration measuring device 33.
 The computing unit 38 has an indicator 24 for the concentration of the
 dissolved gas in the liquid. Of course, here too, the signal output can be
 connected in an appropriate manner via a signal line 25 to a control unit
 26 that controls a dosing device 27, for example for addition of CO.sub.2
 in a beverage bottling plant.
 List of reference numbers
 10 Device
 11 Measuring vessel
 12 Measuring channel
 13 Wall
 14 Membrane
 140 Retentate side
 15 Membrane holding device
 16 Permeate side
 17 Free space
 18 Outlet
 19 Flowmeter
 20 Temperature measuring device
 21 Signal line
 22 Signal line
 23 Calculating unit
 24 Indicator
 25 Signal line
 26 Control unit
 27 Dosing device
 30 Device
 31 Inlet
 32 Outlet
 33 Concentration measuring device
 34 Flowmeter
 35 Rinsing gas supply
 36 Filter
 37 Signal line
 38 Calculating unit
 39 Liquid
 40 Flow rate indicator
 41 Signal line
 42 Air pressure gauge
 43 Signal line