Measuring and testing – Gas content of a liquid or a solid – By pressure of the gas
Reexamination Certificate
1998-11-20
2001-02-27
Larkin, Daniel S. (Department: 2856)
Measuring and testing
Gas content of a liquid or a solid
By pressure of the gas
C073S019010, C073S019040, C073S019050, C073S019100, C261S104000
Reexamination Certificate
active
06192737
ABSTRACT:
DESCRIPTION
The invention pertains to a method for measuring the concentration of dissolved gases in a liquid, especially of CO
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
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 fo
Cegla Dieter
Hasler Carsten
Ohlrogge Klaus
Steffens Franz Josef
Wind Jan
Larkin Daniel S.
Rosemount Analytical Inc.
Westman Champlin & Kelly P.A.
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