Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
2001-05-15
2004-02-10
Funk, Stephen R. (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
C324S690000, C324S663000, C324S676000, C324S664000, C324S687000, C073S061440, C073S061610, C073S118040
Reexamination Certificate
active
06690180
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process and apparatus for determining the ratio of components in a fluid mixture, particularly a sensor for use in determining the ratio of water to methanol in a reforming feed for producing hydrogen for supply to a fuel cell system.
DESCRIPTION OF THE PRIOR ART
The use of fuel cells in electrically powered vehicles has become important for the purpose of reducing air pollution normally resulting from internal combustion engines. As a way of producing the hydrogen gas to be supplied to the fuel cell in combination with oxygen gas, usually supplied as the oxygen present in atmospheric air, it is known to supply a mixture of methanol and water to a catalytic reforming reactor to convert said mixture to carbon dioxide and hydrogen according to the endothermic reaction:
CH
3
OH+H
2
O→CO
2
+3H
2
.
Such a system is described in the U.S. Department of Energy Report (DOE/CH/10435-02) of January 1996, entitled Research and Development of Proton-Exchange-Membrane (PEM) Fuel Cell System for Transportation Applications.
In order to operate such a reforming system it is important that the ratio of methanol to water in the reforming feed be continuously monitored and correspondingly controlled so that the optimum ratio of these reactants for the reforming reaction is achieved. The measurement of the ratio of methanol to water can be carried out by chromatography methods. However, such techniques used in the laboratory are not practical for industrial or commercial applications, such as for fuel cell systems in electrically powered vehicles or in electric power sources suitable for other purposes, such as in satellite communication devices and in stationary “home” power systems. In general it is desirable to have a sensor suitable for continuously measuring the ratio of components in fluids, both gases and liquids. Although it is useful to have a sensor measure the ratio of the methanol and water in the gas phase fed to the above reforming system, it is practical to measure the ratio of the methanol to water in the liquid phase in equilibrium with the gas phase mixture fed to the reformer.
It is known to use oscillometry for determining the water content of a liquid mixture since water has a dielectric constant fifteen to twenty times that of other substances (see PERRY, CHEMICAL ENGINEERS' HANDBOOK, FIFTH EDITION, 1969, PAGE 22-52). LANGE'S HANDBOOK, THIRTEENTH EDITION, 1985, cited below, discloses dielectric constants for a number of substances, including water and methanol.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a process for determining the ratio of methanol to water in a fluid mixture containing same.
Another object of the invention is to provide a sensor for determining the ratio of two components in a fluid mixture, particularly a liquid mixture, mixtures with different ratios of said components having respectively different dielectric constants at given conditions of frequency of an imposed alternating voltage and of temperature of said fluid mixture.
It is a further object of the invention to provide a sensor for determining the ratio of two gas components in a gas mixture, mixtures with different ratios of said components having respectively different dielectric constants at given conditions of frequency of an imposed alternating voltage and of temperature and pressure when said mixture is placed between opposed electrodes imposing said alternating voltage.
In accordance with one aspect of the present invention, there is provided a sensor for determining the ratio of two components in a test fluid mixture, mixtures with different ratios of said components having respectively different dielectric constants at a given frequency of an imposed alternating voltage and at a given temperature of said test fluid mixture when said test fluid mixture is subjected to said alternating voltage, said sensor comprising:
a first pair of opposed electrodes establishing there between a test cell, the opposed electrodes of said test cell being in contact with said test fluid mixture being tested, said test cell having a test cell impedance;
a reference cell established by a second pair of opposed electrodes having equivalent operating characteristics to those of the first pair of opposed electrodes, said reference cell containing a reference fluid mixture of the two components in a known ratio and being isolated from said test fluid mixture, said reference cell having a reference cell impedance;
an electrical circuit wherein one of the opposed electrodes of the first pair of electrodes is connected to one pole of a source of alternating voltage and one of the electrodes of the second pair of electrodes is connected to the other pole of the source of alternating voltage, and wherein the other of the opposed electrodes of the first and second pair of electrodes are connected together for serial flow between said pairs of electrodes of the test cell and reference cell, respectively; and
a voltage measuring device connected to said circuit for measuring the voltage drop across the test cell so as to determine the voltage drop across the test cell in relation to the voltage drop across the reference cell;
said relative voltage drops being used to determine said test cell impedance relative to said reference cell impedance and thus to determine the dielectric constant of the test fluid mixture relative to that of the reference fluid mixture at a given temperature of said test and reference fluid mixtures and at,a given voltage of the imposed alternating voltage, whereby the ratio of said components of the test fluid mixture to the ratio of the components of the reference fluid mixture is correlated with the respective dielectric constants of said test and reference fluid mixtures.
In a modification of the sensor of the invention, representing a preferred embodiment, the reference cell is replaced by a reference element including a capacitor connected to the same source of alternating voltage as the first pair of electrodes, said reference element having a known impedance corresponding to that of such a (hypothetical) reference cell containing the reference fluid mixture of two components of a known ratio and at a specific temperature. In this embodiment, there is provided a temperature measuring device for measuring the temperature of the mixture in the test cell and a voltage measuring device (connected to the electrical circuit in which the reference element replaces the reference cell) for measuring the voltage drop across the test cell electrodes and across the reference element of known impedance so as to compare the respective impedances and determine the ratio of the two fluid components of the mixture in the test cell on the basis of the impedance of the fluid mixture in the test cell and the temperature of said fluid mixture.
In accordance with another aspect of the present invention there is provided a process for testing a fluid mixture containing methanol and water in a given ratio so as to determine said ratio, which comprises:
establishing a test cell between opposed, electrodes contacting said fluid mixture, said fluid mixture being tested at a given temperature and said test cell being in open flow communication with said fluid mixture being tested;
imposing an alternating voltage of a given frequency across said electrodes;
measuring the dielectric response of the fluid mixture in said test cell at said temperature and frequency, and
determining the ratio of methanol to water in said mixture on the basis of the dielectric response thus measured.
In the usual practice of this above process the dielectric response of the test cell is measured in terms of the dielectric constant of the fluid mixture in said cell, the dielectric response of the fluid mixture being tested varying in an essentially linear and monotonic relation to the ratio of methanol to water in said mixture at a given temperature of said fluid.
The sensor provided by the present invention measures the die
Braatz Paul O.
Kirby Kevin W.
Schwartz Robert N.
Funk Stephen R.
Hamdan Wasseem H.
HRL Laboratories LLC
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