METHOD AND SYSTEM FOR DETERMINING THE FRACTIONS OF A STREAMING GASEOUS MEDIUM

Abstract

The invention relates to a method and a system for determining the fractions of a flowing gaseous medium that comprises a known plurality N of known components. The method comprises the steps for determining at least N−1 parameters of a flowing gaseous medium. The N−1 parameters are chosen from a group of quantities comprising mass flow, density, viscosity, and heat capacity. At least N−1 reference values are provided for each of the known N components relating to each of the determined N−1 quantities. The fraction of each of the known components of the supplied gaseous medium is determined through solving of at least N equations. The N equations comprise N−1 equations which describe each determined parameter as a function of the fraction and the reference values, plus an equation that sets the sum of the fractions so as to be equal to 100%.

Claims

1. A method of determining the fractions of a flowing gaseous medium which consists at least substantially of a known plurality N of known components, which method comprises the following steps: providing the flowing gaseous medium of which the composition is to be determined, determining at least N−1 parameters of the gaseous medium provided, for each of the N known components, providing at least N−1 reference values for each of the determined N−1 quantities, determining the fraction of each of the known components of the provided gaseous medium through solving of at least N equations, which equations comprise: at least N−1 equations which describe each determined parameter as a function of the fraction of each of the known parameters of the medium and as a function of the provided reference values for each of the known components of the gaseous medium, and at least one equation which sets the sum of the fractions of each of the known components at least substantially so as to be equal to 100%.

2. A method according to claim 1, wherein the method comprises a step of substantially continuously providing the flowing gaseous medium and of substantially continuously determining the at least N−1 parameters.

3. A method according to claim 1, wherein the known plurality N of known components is equal to at least three or equal to at least four.

4. A method according to claim 1, wherein the method comprises a direct supply of the flowing gaseous medium without any pre-treatment.

5. A method according to claim 1, wherein the steps of determining the parameters and of determining the fractions of the components are repeated at least once.

6. A method according to claim 5, wherein a time interval between two consecutive determinations of fractions lies in a range selected from between 0 and 60 seconds, between 0 and 15 seconds, and between 0 and 5 seconds.

7. A method according to claim 1, wherein at least one of the parameters is chosen from mass flow, density, viscosity, and heat capacity.

8. A method according to claim 7, wherein the density and the heat capacity of the gaseous medium are determined by means of signals from a thermal flow sensor and a flow sensor of the Coriolis type.

9. A method according to claim 1, wherein the equations are solved by a method of least squares.

10. A method according to claim 1, wherein a measure for the calorific value of the flowing gaseous medium is additionally derived from the determined fractions.

11. A method according to claim 10, wherein the Wobbe index of the gaseous medium is additionally derived from the calorific value.

12. A method according to claim 1, comprising a step of controlling the mass flow of the flowing gaseous medium in dependence on the determined fractions thereof.

13. A system for the method according to claim 1 comprising a flow tube having an inlet and an outlet for supplying and discharging, respectively, the flowing gaseous medium, in particular in a continuous manner, of which medium the composition is to be determined, sensor means for determining the at least N−1 parameters of the supplied gaseous medium, a processing unit which is connected to the sensor means, in which the at least N−1 reference values are stored, and which is designed for determining the fraction of each of the known components of the supplied gaseous medium by solving the at least N equations.

14. A system according to claim 13, wherein the sensor means and the processing unit are designed for determining the N−1 parameters and the fractions in a repetitive manner and/or continuously.

15. A system according to claim 14, wherein the system is designed for repeatedly determining the fractions at time intervals that lie in a range selected from between 0 and 60 seconds, between 0 and 15 seconds, and between 0 and 5 seconds.

16. A system according to claim 13, wherein the sensor means comprise at least one of the following: a density sensor, a flow sensor of the Coriolis type, a thermal flow sensor, and/or a pressure sensor.

17. A system according to claim 13, wherein the sensor means comprise at least a thermal flow sensor and a flow sensor of the Coriolis type, and wherein the processing unit is designed for determining the specific heat capacity of the medium on the basis of signals from both the thermal flow sensor and the flow sensor of the Coriolis type.

18. A system according to claim 13, wherein the sensor means comprise at least a flow sensor of the Coriolis type and a pressure sensor, and wherein the processing unit is designed for determining the viscosity of the medium on the basis of signals from both the flow sensor of the Coriolis type and the pressure sensor.

19. A system according to claim 13, wherein the sensor means comprise at least a pressure sensor and a thermal flow sensor, and wherein the processing unit is designed for determining the differential pressure across the thermal flow sensor.

20. A system according to claim 13, further comprising signalling means connected to the processing unit for providing a signal when one of the determined fractions deviates from a standard value.

Description

[0049] The invention will be explained in more detail below with reference to the appended figures, in which:

[0050] FIG. 1 diagrammatically shows a system according to the present invention;

[0051] FIG. 2 diagrammatically shows a system having a plurality of sensors according to the present invention;

[0052] FIG. 3 shows the dependence of the Wobbe index of a biogas as a function of CO.sub.2 and N.sub.2;

[0053] FIG. 4 is a graph showing the Wobbe index WI as a function of the viscosity η; and

[0054] FIGS. 5 to 8 show results of measurements carried out by a system according to the present invention.

[0055] FIG. 1 diagrammatically shows a system 100 according to the present invention with which fractions of a flowing gaseous medium, which comprises at least substantially a known plurality N of known components, can be determined. The system 100 comprises a flow tube 2 for the medium of which the fractions are to be determined. The system comprises sensor means 30 which are connected to the flow tube 2 or which form part thereof. The sensor means 30 are designed for determining at least N−1 parameters of the medium. Said parameters are chosen from a group comprising density, viscosity, and specific heat capacity, indicated with the respective symbols ρ, η and c.sub.p in FIG. 1. The system is further provided with a processing unit 40 which is connected to the sensor means 30 and which is designed for determining the fraction of each of the components on the basis of the measured and/or determined parameters. The processing unit 40 in the embodiment shown is provided with a reference table 60 or database 60, shown schematically in FIG. 1, in which reference values for the measured and/or determined parameters of the known components are stored.

[0056] The operation of the system 100 will be explained below. The gaseous medium with the known components is conducted through the flow tube 2. The sensor means 30 are used for determining the at least N−1 parameters, either in that direct measurements are carried out, or in that the relevant parameters are determined on the basis of signals from the sensor means 30. It is alternatively possible that the signals are directly fed to the processing unit 40, where the parameters are determined. The processing unit 40 of FIG. 1 is designed for utilizing the data from the reference table 60 for determining the composition of the medium 2, for example by comparing the at least two parameters of the medium 2 with data from the reference table 60. The reference table 60 preferably also comprises information on the dependencies between the at least two parameters and the respective fractions pi of the components, for example in the form of formulae or functions.

[0057] The processing unit 40 of FIG. 1 comprises equations wherein are present on the one hand the at least two parameters of the medium and on the other hand the fractions of the components and the associated data from the reference table 60, such as the equations (1), (2), (3), and (4) described above. In other words: each of the at least N−1 parameters, for example ρ, η, and/or c.sub.p, of the medium is a function of the fraction φ.sub.i of the respective component and the associated data in the reference table 60. The processing unit 40 of FIG. 1 is capable of solving this set of equations for the N−1 parameters.

[0058] In an embodiment, the processing unit is designed for determining the fractions of the components in real time, i.e. substantially instantaneously. To achieve this, the set of equations may be arranged in the form of a matrix equation such as (3) or (4) for a simple and fast solution thereof by the processing unit 40.

[0059] In an embodiment, the processing unit 40 is designed also to determine a calorific value of the medium. It is possible in particular to determine the Wobbe index WI of the medium. The Wobbe index can be calculated from the fractions of the medium in combination with data from the reference table 60 by means of the equation mentioned above.

[0060] FIG. 2 diagrammatically shows a system 100 according to the present invention with sensor means 30 comprising sensors 5, 6, 7, and 8 which are provided on or adjacent to the flow tube 2. Said sensor means 30 in particular comprise a thermal flow sensor 5, a flow sensor of the Coriolis type 6, a density sensor 7, and a pressure sensor 8.

[0061] The sensor means 30 of FIG. 2 comprise a sensor processing unit 10. The latter is provided with a number of calculation models 15, 16, 17, 18 with which a plurality of parameters, comprising the specific heat capacity c.sub.p, the mass flow rate m, the density ρ, and the viscosity η of the medium, can be determined on the basis of the signals of the sensors 5, 6, 7, 8. Applicant's NL 2 012 126 cited above describes in great detail how the plurality of parameters can be determined by means of the sensors 5, 6, 7, and 8 mentioned above, as does Lötters, J. C. et al., 2014, Integrated multi-parameter flow measurement system, in 2014 IEEE 27th International Conference on Micro Electro Mechanical Systems (MEMS) [DOI: 10.1109/MEMSYS.2014.6765806]. A short clarification will be given below for the sake of brevity.

[0062] The output signal of the thermal flow sensor 5 is a measure for the flow rate and the heat capacity of the gas mixture. The pressure drop across the thermal flow sensor 5 is measured by the pressure sensor 8, which in particular is a differential pressure sensor 8. The output signal of the flow sensor of the Coriolis type 6 provides the mass flow rate, and the density is obtained from the density meter 7.

[0063] By comparing the output signals of the flow sensor of the Coriolis type 6 and the pressure sensor 8, taking into account the density, it is possible to calculate the viscosity.

[0064] Comparing the output signals of the thermal flow sensor 5 and the flow sensor of the Coriolis type 6 renders it possible to calculate the heat capacity of the gaseous medium.

[0065] The one or more parameters 20 thus obtained are fed to the equations 45 stored in the processing unit 40. The fractions (pi of the components, and preferably also the Wobbe index WI, can be determined in that the set of equations 45 is solved.

[0066] The processing unit 40 is designed, for example, for drawing up a matrix equation 45 such as described with reference to the equations (3) and (4). The processing unit completes the vector for the values of the parameters of the medium with the values determined by the assembly of sensors 1 and transmitted to the processing unit 40 via the parameter output 20. The quantities of the components, with the exception of the fractions are derived from a reference table 60 by the processing unit 40 and entered in the equations 45. The processing unit 40 subsequently solves the set of equations 45, as a result of which the fractions of the components of the medium are determined.

[0067] It is conceivable for a flow measuring system according to the cited Dutch Patent Application NL 2 012 126 to be connected to a processing unit according to the present invention so as to form a system according to the present invention. In an embodiment, the sensor signal processing unit 10 is integral with the processing unit 40.

[0068] FIG. 3 shows the dependence of the Wobbe index of a gas on the CO.sub.2 and N.sub.2 fractions. Such a gas may be, for example, a natural gas that is supplied to the gas grid. FIG. 3 shows the dependence of the Wobbe index on the nitrogen and carbon dioxide contents of the gas. Given such a strong variation in the composition of the gas mixture, an accurate and quick determination of that composition is desirable.

[0069] FIG. 4 is a graph showing the Wobbe index WI on the vertical axis as a function of the viscosity on the horizontal axis. It was found that there is a strong correlation between the viscosity and the Wobbe index if CO.sub.2 is the only inert gas in the mixture. If there is also N.sub.2 present, however, the correlation becomes less strong owing to the higher viscosity. This leads to a comparatively wide range within which the actual Wobbe index is situated. This range within which the Wobbe index may be situated is indicated by a lower index limit a and an upper index limit d in FIG. 4.

[0070] The method and the system according to the present invention render it possible to distinguish between CO.sub.2 and N.sub.2 by taking into account the density of the gas mixture, so that the range within which the actual value of the Wobbe index may lie can be narrowed so as to lie between a corrected lower index limit b and a corrected upper index limit c. According to the present invention, the determination of the Wobbe index becomes more accurate in that more than one parameter of the medium are determined, and the composition and thus the Wobbe index are determined on the basis thereof.

[0071] The FIGS. 5 to 8 show further results of measurements with a system according to the present invention. Methane, propane, carbon dioxide and nitrogen were added in quantities of the order of approximately 500 ml.sub.n/min to the system at a pressure of the order of 1.5 bar (absolute pressure). The output signals of the density sensor, pressure sensor, thermal sensor and the flow sensor of the Coriolis type were recorded during the measurements and processed by the method according to the present invention.

[0072] FIG. 5 shows the determination of the composition of a gas mixture with CH.sub.4, CO.sub.2 and N.sub.2. Known quantities were supplied to the system. The known values of the added fractions (the so-termed applied fractions) are plotted against time tin FIG. 5: the applied CH.sub.4 fraction CH.sub.4 (a), the applied CO.sub.2 fraction CO.sub.2 (a) and the applied N.sub.2 fraction N.sub.2 (a). The applied fractions “(a)” are set, for example by means of a flowmeter, and vary in time step by step, as can be seen in the square waveforms of the applied fractions CH.sub.4 (a), CO.sub.2 (a), and N.sub.2 (a). The values measured by a system according to the present invention are denoted “(m)”. The measured CH.sub.4 fraction CH.sub.4 (m), the measured CO.sub.2 fraction CO.sub.2 (m), and the measured N.sub.2 fraction N.sub.2 (m) are plotted against time t in FIG. 5. It is apparent from FIG. 5 that the values of the fractions CH.sub.4 (m), CO.sub.2 (m), and N.sub.2 (m) as determined by a system according to the present invention follow the applied, i.e. actual fractions CH.sub.4 (a), CO.sub.2 (a), and N.sub.2 (a) in real time. The values of the measured fractions CH.sub.4 (m), CO.sub.2 (m), and N.sub.2 (m) lie within 5 percent of the applied values CH.sub.4 (a), CO.sub.2 (a), and N.sub.2 (a). The system according to the present invention is thus not only fast, but also accurate.

[0073] FIG. 6 shows the determination of the Wobbe index of the gas mixture of FIG. 5. Since the composition of the gas mixture is known, as is its density, the Wobbe index can be calculated. The applied Wobbe index of the gas mixture is denoted WI (a). It is apparent from FIG. 6 that the Wobbe index varies stepwise in time. A system according to the present invention thus determines the Wobbe index of the gas mixture, the relevant values of which are denoted WI (m). FIG. 6 shows that the curve of the determined Wobbe index follows the curve of th e applied, i.e. actual Wobbe index WI (a). A change in the applied value of the Wobbe index is followed substantially instantaneously by an adaptation of the determined Wobbe index WI (m). The deviation e is plotted in the lower part of FIG. 6. The determined values WI (m) lie within a deviation range of five percent with respect to the applied values WI (a). The system according to the present invention is accordingly designed for an instantaneous and accurate determination of the Wobbe index values WI (a).

[0074] FIG. 7 shows the determination of the composition of a gas mixture comprising CH.sub.4, C.sub.3H.sub.8, and N.sub.2. The values of the fractions measured by a system according to the present invention CH.sub.4 (m), C.sub.3H.sub.8 (m), and N.sub.2 (m) as well as the applied, i.e. actual values CH.sub.4 (a), C.sub.3H.sub.8 (a), and N.sub.2 (a) of the fractions are plotted on the vertical axis against time t, which is plotted on the horizontal axis. Again, the measured values CH.sub.4 (m), C.sub.3H.sub.8 (m), and N.sub.2 (m) follow the actual values CH.sub.4 (a), C.sub.3H.sub.8 (a), and N.sub.2 (a) quickly and accurately. The deviation between the measured values “(m)” and the applied values “(a)” is below five percent.

[0075] FIG. 8 shows a further determination of the Wobbe index of a gas mixture comprising CH.sub.4, C.sub.3H.sub.8, and N.sub.2. This measurement corresponds to the measurement of FIG. 6, but with the difference that in FIG. 8 the applied Wobbe index WI (a) is given a flatter waveform than in FIG. 6. Again, the determined values WI (m) lie within a five percent deviation with respect to the applied values WI (a).

[0076] It will be clear to those skilled in the art that the invention was described above with reference to a few possible embodiments which are regarded as preferable. The invention, however, is by no means limited to these embodiments. Many modifications are possible within the scope of the invention. The protection applied for is defined by the appended claims.