DE10158077C1 - Determining density of measuring gas, especially fuel gas, involves passing measuring gas and reference gas through apertures and mixing together - Google Patents

Abstract

Process for determining the density of a measuring gas, especially a fuel gas, involves passing the measuring gas and a reference gas of a known volumetric reference component separately through two or more apertures (4, 5, 4', 5') each with prescribed opening cross-sections (AM, AE) and mixing together; investigating the mixed gas with respect to the dilution of the reference component; and determining the density of the measuring gas. An Independent claim is also included for a device for determining the density of a measuring gas, especially a fuel gas, comprising a mixing chamber (1); a measuring gas feed line (2) and a reference gas feed line (3); and a measuring gas nozzle and a reference gas nozzle formed as the apertures. Preferred Features: The reference component is oxygen, hydrogen, carbon dioxide or hydrogen sulfide.

Description

The invention relates to a method and a device for determining the density of a measuring gas, in particular Fuel gas.

The density determination for gases is u. a. then by Bedeu tion if the gas or sample gas is a fuel gas is a gaseous medium, which, for example how to generate heat, generate electricity, etc. is burned. For this purpose, it is necessary Calorific value or the so-called density-corrected calorific value worth knowing (the Wobbe number) (cf. EP 1 067 383 B1 or DE 297 15 633 U1).

It should be noted that the fuel gas, which for example a refinery, a chemical plant, one Coking plant etc. comes from, usually different distillers contains bare components, depending on the origin process fluctuate greatly in composition. Around now a burner with a regulated supply of energy to supply, the Wobbe index must be known and in For example, natural gas can be added to the Power of the burner in a given given Keep window. However, this requires that the wobble Index of the mixed gas generated in this way is determined. However, this calorific value determination can only be made after  Mixing take place, so that a total reverse regulation is carried out.

However, since the sample gas or fuel gas changes in a short time intervals regarding the composition of each  Components can change greatly is such an approach wise disadvantageous or a measurement before the Mixing device, already for consumption and need keep agile use of natural gas as low as possible.

Because the Wobbe index is nonlinear from the relative density of the respective fuel gas (based on dry air) depends, it becomes clear that the Wobbe index also Mixed gas no linear combination of the individual wobble Indexes of its components can be. So it is required Lich, in addition to the Wobbe index of the sample gas or fuel gas also to determine its density to the following ver to optimize the supply of a burner or an energy and inexpensive mixture with, for example, natural gas to be able to ask.

In the past, it had different starting points given the density of a sample gas, fuel gas or to determine any gas. For natural gases are mostly chromatographic methods are used, but they are not sometimes waits for occurring components in the (fuel) gas do not record.

In addition, it is known to have an operational density transducer, which contains a vibrating body, with the help to calibrate a calibration gas. Then a Density value of the gas to be examined during operation conditions with the help of the operating density sensor measured. Furthermore, the speed of sound and the Viscosity of both the gas and the calibration gas  determined in each case under operating conditions (cf. DE 198 23 193 A1).

Similar approaches provide for the gas density via the measurement to obtain the speed of sound (see WO 99/02954) or the natural frequency of one through which the sample gas flows to determine the cylindrical cavity.

All known procedures for density determination of a sample gas are more or less serious Disadvantages. This usually includes the high one Acquisition price of underlying devices as well as one restrictions in the range of detectable gases. - Here the invention seeks to remedy the situation as a whole.

The invention is based on the technical problem Method for determining the density of a sample gas to further develop that on inexpensive and simple A reliable density determination taking into account changing relationships settling of the sample gas. In addition, one should go through implementation of the method particularly suitable device be created.

To solve this technical problem is counter the invention was a method for determining the density a measurement gas, in particular fuel gas, after which the measurement gas and a reference gas with volumetrically known Reference component essentially laminar and separated by two or more apertures, each with a specified one Flow opening cross section and mixed together  and what is the mixed gas thus formed visually examined the dilution of the reference component and deduced the density of the sample gas from this becomes.

So the two or more apertures essentially become laminar flow, d. H. both the sample gas and that In this context, reference gas represents a friction free and compressible fluid, which the laminar flow described performed. Therefore let the sample gas and the reference gas in the associated Mixing device using the law of Bernoulli describe how it is explained in detail below.

It is understood that one (or more) apertures are one be used for the flow of the sample gas, so a sample gas orifice (or several sample gas orifices) is provided is (are) and on the other hand the same for the reference gas applies. In other words, the reference gas flows through a (or more) of the sample gas orifice (s) ver different reference gas orifice plate (s).

It is possible that the reference component not only is contained in the reference gas but also in the sample gas. Then the invention proposes the proportion of the reference component in the sample gas before the mixture of reference gas and Sample gas to determine the subsequent dilution Correct the measurement of the reference component accordingly can.  

It has proven to be advantageous if both the measurement gas and the reference gas are essentially laminar the respective orifice or mixing nozzle flows, d. H. Reynolds numbers around 2000 or below can be observed. With in other words, the conditions are roughly like this to deliver, as in EP 1 067 383 B1 with reference to the there openings 4, 5 is described.

In the same way, it has proven to be beneficial if the pressure and temperature of the sample gas and the Reference gas within specified pressure limits and Temperature limits set the same and in particular be kept constant. It can be done easily Real pressure differences between sample gas and reference gas sieren in the range of +/- 0.1 mbar or also clearly are below (<0.01 mbar). As temperature differences The invention recommends between sample gas and reference gas Values less than 1 mK.

As already indicated, the sample gas and reference gas advantageously mixed in a mixing chamber to the mixed gas, from where from the mixed gas into a connected Wobbe index Measuring device or a comparable device for Determination of the calorific value (or other gas properties) arrives. In this case, the mixing chamber takes over one Dual function. On the one hand, it serves to determine the density of the sample gas and on the other hand as a preparation room for Determination of the calorific value of the mixed gas.

Oxygen (O ₂ ), hydrogen (H ₂ ), carbon dioxide (CO ₂ ), hydrogen sulfide (H ₂ S) or comparable gases and associated gas mixtures have proven themselves as reference components. These can be easily evaluated with regard to their dilution in the mixture of sample gas and reference gas. For example, a simply constructed and inexpensive oxygen sensor of known design is sufficient for measuring the concentration of oxygen in the resulting mixed gas.

The invention also relates to a device for Determination of the density of a measuring gas, e.g. B. fuel gas, which are particularly suitable for carrying out the described Process is suitable. Advantageous embodiments of this before direction in the related to the relevant claim 7 described subsequent claims.

The invention starts out from the known fact that the respective pressure drop ΔP at the associated orifices, taking into account a gas density ρ and a gas velocity ν, is measured as follows within the framework of the Bernoulli law (ideal flow):

If one now also takes into account that the flow ϕ of the sample gas or reference gas through the respective orifice with opening cross section A is described as follows:

ϕ = A.ν, 1.2)

taking into account relationship 1.1) this results in the following dependency:

If one now determines the flow of the measuring gas ϕ _M and the flow of the reference gas ϕ _E according to the aforementioned regulation 1.3) and puts them in relation to each other, the mixture ratio is formed

The following dependencies result:

where ΔP _M = ΔP _E (pressure and temperature of the sample gas and the reference gas are largely set within predetermined pressure and temperature limits and in particular kept constant) and d _{rel represents} the relative density of the sample gas based on the reference gas, i.e.

applies.

From equation 1.4) it becomes clear that the mixing ratio between the sample gas and the reference gas, i.e. the quotient

in addition to the (constant) geometric parameters (opening cross-sections of the orifices A _M , A _E ) only depends on the relative density d _{rel of} the sample gas in relation to the reference gas. This assumption assumes that both A _M and A _{E are} constant, which is regularly the case. The invention also includes variants with adjustable and possibly varying opening cross sections A _M , A _E.

The dependency of the mixing ratio is namely

mathematically designed in the same way as that of the Wobbe index WI on the relative density in question:

with H the heat of combustion and d _rel in turn the density of the sample gas or combustion gas in relation to air under standard conditions or the reference gas (which in some cases is air).

The previously worked out relative density d _{rel of} the sample gas is now determined on the basis of the total gas flow ϕ _total , which is measured as follows:

with ϕ O2 | E the flow of the reference component (O ₂ ) in the reference gas and ϕ Rest | E the flow of the remaining components of the reference gas (i.e. ϕ _E = ϕ O2 | E + ϕ Rest | E).

If one now puts the proportion O2 | E of the flow of the reference gas with the reference component (in the present case oxygen O ₂ ) in relation to the total gas flow ϕ _total , the concentration of the reference component O ₂ in the mixed gas C ^O2 is obtained taking into account the relationship 1.8) as follows :

This relationship can be transformed as follows:

If one now takes into account the relation 1.4), the final dependency results:

The factor C O2 | E expresses the concentration of the reference component (O ₂ ) in the reference gas (i.e.

which is either determined beforehand or (where Use of air) is known (approx. 20.9 vol.% Acid substance in air).

Using the relationship 1.11) it becomes clear that, for example, an oxygen concentration measurement in the mixed gas (i.e. determination of C ^O2 taking into account the known values for the opening cross sections of the orifices (A _E , A _M ) and knowledge of the concentration of the reference component (O ₂ ) in the reference gas C O2 | E (in the present case approx. 20.9 vol.% O ₂ in air) the relative density d _{rel of} the measuring gas can be inferred based on the reference gas (air).

The corresponding and calculated value can be then immediately when determining the Combustion heat H using the (also use the measured Wobbe index WI (see Equation 1.7). All of this is accomplished with the help of a reliable, easy built sensor (oxygen sensor), which also cost works cheaply.

Of course, the method described can still be modified if, in addition to the reference gas, the measurement gas or the flow ϕ _{M associated} therewith also contains the reference component (O ₂ ). This only requires a modification of equation 1.11). In this case, however, it is necessary to examine the sample gas before mixing with regard to the concentration of this reference component.

The invention also includes embodiments such that the concentration of the reference component (O ₂ ) in the reference gas is initially unknown, which requires a prior determination.

Either way, these individual concentrations carry out measurements easily, quickly and cost-effectively,  resulting in an extremely precise density determination of the sample gas results, and at low Expenditure. It actually plays within the scope of the invention no matter whether the reference component is only in the sample gas, only occurs in the reference gas or in both. It matters rather, whether the differences in the concentrations of this Reference component in the sample gas and possibly in the Reference gas large enough for a metrological Are recorded using the presented method.

The fact that the Sample gas and the reference gas same temperature and same Pressure (within specified pressure limits and temperature borders). In addition, pressure and Temperature of the sample gas and the reference gas within given limits are kept constant. The constancy these values are of less importance than the Setting the same temperature and pressure. In other words, the variation in the constancy of pressure and temperature of the measurement gas and the reference gas much be greater than that of temperature and equality Pressure for the sample gas and for the reference gas.

In the following, the invention is based on only one Embodiment showing drawing he closer purifies; show it:

Fig. 1, the device according to the invention in a schematic representation;

Fig. 2 shows the concentration C ^O2 in vol .-% against the root relative density, that is, for ver different cross-sectional ratios of the opening cross sections A _E to A _M and

Fig. 3 shows a calibration curve of the Mischvor direction or measuring device based on the measurement of several methane-nitrogen mixtures with known methane concentrations and consequently known values for the relative density of these gas mixtures.

The mixing device to be seen in the drawing has a basic structure of a mixing chamber 1 , at least one measuring gas supply 2 leading into the mixing chamber 1 and a reference gas supply 3 . Orifices 4 , 5 are also found in the mixing chamber 1 . An orifice 4 serves to reduce the pressure of the measurement gas, while the other orifice 5 ensures that the reference gas is reduced. Consequently, a sample gas orifice 4 and a reference gas orifice 5 are realized.

According to the exemplary embodiment, the measuring gas orifice 4 and the reference gas orifice 5 are circular openings in a bottom end plate 6 of the mixing chamber 1 , which in turn has a truncated cone-shaped outer jacket and is equipped with an upper connection chamber 7 . In this connection chamber 7 there is a mixing nozzle 8 designed as a flame arrester. The flame barrier or mixing nozzle 8 ensures that an occasional flashback of the downstream burns in the mixing chamber 1 is avoided. - Mixing chamber 1 and connection chamber 7 are made of stainless steel.

From the connection chamber 7 , the mixed gas produced in the mixing chamber 1 passes via a gas line 9 to a Wobbe index measuring device, not expressly shown, for example a catalytic reactor as described in DE-GM 297 15 633. Alternatively, a burner can also be provided at this point, as is disclosed in principle in European documents EP 0 323 658, EP 0 405 693 and EP 0 445 861. The Wobbe number WI is determined by recording the oxygen content, as documented in the documents mentioned.

In the supplied via the investigation gas supply 2 gas, it is either the measuring gas which passes through an associated gas conduit 10 a into the mixing chamber 1 or one or more calibration gases, which conduit a gas are supplied b 10th Corresponding solenoid valves in the aforementioned lines or gas lines 10 a, 10 b ensure the desired exposure. A fuel gas of unknown and possibly changing composition is used as the measuring gas. In the example, the reference gas is air. The reference gas contains oxygen O _{2 of a} given concentration C O2 | E (20.9% by volume) as the reference component.

Following the aforementioned lines 10 a, 10 b, there is a pressure regulator 11 within the sample gas supply 2 . This pressure regulator 11 is connected via a pressure control line 12 and a differential pressure regulator 13 to a differential pressure transmitter 14 .

The mixing device shown has a two-part cylinder chamber 15 a, 15 b below the mixing chamber 1 , which carries the bottom 6 of the mixing chamber 1 on the head side. One part 15 a of the cylinder chamber 15 a, 15 b is connected to the reference gas supply 3 , while the measuring gas supply 2 acts on the other part 15 b. Both aforementioned parts 15 a, 15 b are separated by a vertical bulkhead 16 with two bulkheads. In addition, there are connecting lines 17 a, 17 b, which lead on the one hand from the cylinder chamber part 15 a to the differential pressure transmitter 14 , and on the other hand from the cylinder chamber part 15 b to the differential pressure transmitter 14 . Consequently, a pressure difference between the two aforementioned cylinder chambers 15 a, 15 b can be detected by means of the differential pressure transmitter 14 . Depending on whether there is a pressure difference, (additional) measurement gas is optionally fed via the pressure control line 12 into the cylinder chamber 15 b.

At the same time, it is possible to apply an additional pressure regulator 18 in the oxidizing gas supply 3 accordingly. All of this is done by means of a control / regulating device 19 . This not only evaluates measurement data from the differential pressure transmitter 14 and controls the differential pressure controller 13 accordingly, but also serves to actuate the pressure controllers 11 and 18 . Likewise, the solenoid valves in the lines 10 a, 10 b can be acted upon by the control device 19 . Consequently, the pressure in both cylinder chambers 15 a, 15 b on the one hand by means of the pressure regulator 18 , on the other hand by means of the differential pressure regulator 13 in connection with the Drusk control line 12 can be set sensitively and separately, depending on whether a differential pressure from the differential pressure transmitter 14th was recorded and, if so, depending on its size. This takes place by means of the control device 19 in the course of a closed control loop.

The temperature of the test gas and the oxidation gas is set the same within predetermined temperature limits. For this purpose, the mixing chamber 1 including the connection housing 7 and cylinder chambers 15 a, 15 b and corresponding add-on parts 13 , 14 are arranged in a heat insulation chamber 20 , the inside and, if appropriate, the outer wall of which consists of a heat-insulating material, for example ceramic, or is coated therewith ,

In addition, the burner (not shown) or the catalytic reactor can be located in this heat insulation chamber 20 . In this way, the same temperature of the test gas and the oxidizing gas can be set, with a relative deviation of approximately ± 1 ° C. to ± 5 ° C., preferably ± 2 ° C. The waste heat from the burner or reactor, which is not expressly shown, is used for heating, while a controllable fan 21 , which is connected to the outside air, is provided for cooling. This fan 21 can be acted upon as well as a temperature sensor 22 provided in or on the heat insulation chamber by the already described control / regulating device 19 , to which the aforementioned construction parts 21 , 22 are connected. Depending on the actual temperature values measured with the aid of the temperature sensor 22 , the controllable fan 21 is acted upon to set predetermined temperature setpoints, in the sense of a regulation.

In addition, an additional air supply 23 is provided for calibrating the mixing chamber 1 and / or the burner or catalytic reactor. This additional air supply 23 opens into its own calibration chamber 24 with a calibration opening 25 in the bottom 6 of the mixing chamber 1st

With the aid of this additional air supply 23 , at a specific and measured Wobbe index WI, the supply of additional air or an oxidizing gas (predetermined and defined amount) can dilute the mixed gas processed in the mixing chamber 1 and consequently reduce the Wobbe number in a targeted manner to reach. This reduction or dilution is determined because the calibration chamber 24 and the calibration nozzle 25 only allow the supply of a certain additional gas flow. For this purpose, a solenoid valve 26 is also provided in the additional Luftzu guide 23 , which is also acted upon by the control device 19 .

In order to set the temperature of the measuring gas and the reference gas as equal as possible, the measuring gas supply 2 and the reference gas supply 3 are arranged next to one another or as concentric tubes at least in the region of the heat insulation chamber 20 . Here, a heat exchanger 27 is realized, which ensures reliable heat transport between the two aforementioned gases.

With the help of the differential pressure regulator 13 in conjunction with the differential pressure transmitter 14 and the control device 19 , the differential pressure line 12 and finally the pressure regulator 18 , the pressure in the cylinder chamber 15 a can be practically the same as compared to the cylinder chamber 15 b to adjust. In any case, regulated differential pressures between the test gas and oxidizing gas of up to approx. ± 0.1 mbar are possible. In this context, the pressure of the supplied reference gas and the sample gas must be kept the same and constant. Pressure fluctuations of ± 100 mbar have proven to be harmless in practice, provided that the reference gas and the sample gas are subject to the same fluctuations and the differential pressure between the two aforementioned gases is reduced to the aforementioned value of approx. ± 0.1 mbar regulated.

In the area of the sample gas orifice 4 and, if appropriate, the reference gas orifice 5 , the invention ensures that laminar flow conditions are present here. The like is basically possible because the cylinder chambers 15 a, 15 b act practically like pressure reservoirs, from which the respective gas relaxes in the mixing chamber 1 . In other words, the gases to be mixed enter the mixing chamber 1 practically without internal friction (Reynolds number Re <2000). In addition to this first mixing chamber 1 (Wobbe index mixing chamber 1 ) there is another mixing chamber 1 '(density mixing chamber 1 ' ) realized, but this is not mandatory. This second mixing chamber 1 'has essentially the same structure, with the corresponding reference numerals (except for the gas line 9 ) corresponding and identified by single quotes.

The second mixing chamber 1 'is supplied with both the measurement gas and the reference gas via a gas branch line 28 . Because of the same pressure conditions in the two cylinder chamber parts 15 a, 15 b of the first mixing chamber 1 , this also applies to the chamber parts 15 a ', 15 b' of the second mixing chamber 1 ', because the corresponding chambers 15 a, 15 a' and 15 b, 15 b 'are connected to each other. The temperatures are also the same for the reasons already mentioned.

The first mixing chamber 1 serves to determine the Wobbe index WI in accordance with equation 1.7). In contrast, in the context of the second mixing chamber 1 ', the relative densities d _{rel are recorded in} accordance with the relationship 1.11 ) using an indicated sensor 29 as a concentration measuring device 29 , in the present case an oxygen sensor 29 (measurement of C ^O2 ) and using the control / Control device 19 or a computer integrated therein calculated according to equation 1.11).

The heat of combustion H can now be deduced from the relative density d _rel and the Wobbe index WI, taking into account equation 1.7). In this way, the fact can be mastered that the Wobbe index of the mixed gas as a whole or of the individual mixing components shows no linear relationship with the relative density d _rel .

There have been for natural gases or measuring gases or fuel gases, the original or by admixing such natural gases such. B. contain methane in the first mixing chamber 1 ratios of the opening cross sections A _M , A _{E of} the panels 4 , 5 of A _E / A _M ≈ 10 proved to be advantageous. On the other hand, the ratio of the opening cross sections A _E / A _M in the second mixing chamber 1 'is lower and set such that the largest possible differences occur at the associated sensor 29 at this point. This means that work is usually carried out at this point in the steep area of the ratios A _E / A _M shown in FIG. 2.

The first mixing chamber 1 is thus designed such that the mixed gas is completely combusted in the connected burner and in this way the Wobbe index WI is reliably determined. In other words, in the first mixing chamber 1 , one is mostly with ratios of

A _E / A _M ≈ 10

(taking natural gas into account) while the density ratio is determined using the second mixing chamber 1 '

A _E / A _M ≈ 1.. .2

have proven to be advantageous. Because in this area the sensor or oxygen sensor 29 mostly has a (albeit very flat) maximum for the area of the relative density d _rel that is interesting for natural gases.

With reference to FIG. 3, like the device described above for measuring the O ₂ content can be recognized at a series of gas mixtures in the system methane-nitrogen is used. I.e. This graphic shows different measuring gases (methane / nitrogen mixture) of different compositions, which are mixed with air as the reference gas. The dilution of the oxygen (O ₂ ) in the air by the measuring gas mixed with it is detected with the help of sensor 29 and, taking into account equation 1.11), leads to the relative density d _{rel of} the measuring gas or the respective methane / nitrogen mixture.

It can be seen that when using pure methane (100 vol .-% methane, 0 vol .-% nitrogen) the density of the sample gas (in this case only methane) is approx. 0.7175 kg / m ³ . This is the so-called standard density D. This increases to 1.2505 kg / m ³ if only nitrogen (0 vol.% Methane, 100 vol.% Nitrogen) is used as the measuring gas.

Claims (9)

1. A method for determining the density of a measuring gas, in particular fuel gas, according to which the measuring gas and a reference gas with a volumetrically known reference component are separated by two or more orifices ( 4 , 5 ; 4 ', 5 '), each with a predetermined opening cross section (A _M , A _E ) flow and are mixed with one another, and then the mixed gas thus formed is examined with regard to the dilution of the reference component (O ₂ ) and from this the density (d _rel ) of the measurement gas is inferred (see equation 1.11). 2. The method according to claim 1, characterized in that the reference component (O ₂ ) is contained not only in the reference gas but also in the sample gas and its proportion in the sample gas is determined before the mixture of reference gas and sample gas. 3. The method according to claim 1 or 2, characterized in that both the measurement gas and the reference gas flow substantially laminar through the respective aperture ( 4 , 5 ; 4 ', 5 '). 4. The method according to any one of claims 1 to 3, characterized characterized that the pressure and temperature of the measurement gases and the reference gas within the specified pressure limits and temperature limits set the same and before preferably additionally within specified limits be kept constant.   5. The method according to any one of claims 1 to 4, characterized in that the measuring gas and the reference gas are mixed in a mixing chamber ( 1 , 1 '), from where the mixed gas reaches a connected Wobbe index measuring device or the like. 6. A method according to any one of claims 1 to 5, characterized in that as a reference component (O ₂₎ Sauer material (O _2), hydrogen (H _2), carbon dioxide (CO _2), hydrogen sulfide (H ₂ S) or the like used becomes. 7. Device for determining the density of a measuring gas, for. B. fuel gas, in particular for carrying out the method according to one of claims 1 to 6, with
at least one mixing chamber ( 1 , 1 '), also with
at least one sample gas supply ( 2 , 2 ') leading into the mixing chamber ( 1 , 1 ') and a reference gas supply ( 3 , 3 '), and with
at least one measuring gas nozzle ( 4 , 4 ') and one reference gas nozzle ( 5 , 5 ') as respective orifices ( 4 , 4 '; 5 , 5 ') in the mixing chamber ( 1 , 1 '),
wherein the measurement gas and the reference gas are fed into the mixing chamber ( 1 , 1 ') by means of the respective gas feeds ( 2 , 2 ', 3 ) via the associated nozzles ( 4 , 4 '; 5 , 5 '), characterized in that at least a concentration measuring device ( 29 ) is provided for determining the dilution of the reference component (O ₂ ) in the mixed gas. 8. The device according to claim 7, characterized in that two mixing chambers ( 1 , 1 '), each with measuring gas nozzle ( 4 , 4 ') and reference gas nozzle ( 5 , 5 ') are realized, which by a common measuring gas supply ( 2 , 2 ') and reference gas supply ( 3 , 3 ') are charged. 9. The device according to claim 7 or 8, characterized in that the one mixing chamber ( 1 ) as a Wobbe index mixing chamber ( 1 ) and the other mixing chamber ( 1 ') is designed as a density mixing chamber ( 1 ').