METHOD FOR CHARACTERIZING THE GAS LOAD OF A MEDIUM

Description

[0001] The present invention relates to a method for characterizing the gas loading of a medium comprising a gas-laden liquid.

[0002] DE 10 2018 112 002 A1 discloses a method for correcting the influence of frequency fluctuations when determining the density or viscosity of a medium using a vibronic measuring device. DE 10 2016 114 974 A1 and DE 10 2016 114 972 A1 disclose methods for determining the gas volume fraction of a gas-laden liquid medium using a vibration-type measuring sensor based on the natural frequency of two vibration modes, taking into account the resonator effect, and using the Sorokin equation. It is also known from published patent application DE 10 2017 131 267 A1 to determine the gas loading of a liquid based on the resonator effect and the Sorokin equation. However, the gas loading determined in this way leaves much to be desired in that suspended and free bubbles occur in liquids, which have different effects on the measurement of parameters of the medium, such asits density or flow rate. It is therefore an object of the present invention to provide a method that enables a more differentiated characterization of the gas loading of a medium.

[0003] The object is achieved according to the invention by the method according to independent patent claim 1.

[0004] The method according to the invention serves to characterise the gas loading of a medium which comprises a gas-laden liquid by means of a measuring sensor which guides the medium in at least one vibrating measuring tube, the method comprising: Determining a sound velocity value and a resonator density value of the medium based on the natural frequencies of at least two different vibration modes of the measuring tube; Determining a pressure measurement value for the medium conveyed in the measuring tube and the natural frequencies of at least two different vibration modes of the measuring tube; Determining a sound velocity value and a resonator density value of the medium based on the natural frequencies of the at least two different vibration modes of the at least one measuring tube;Determining a value for the gas volume fraction of free bubbles as a function of the resonator density value of the medium and of the gas volume fraction of suspended bubbles, wherein the value for the gas volume fraction of free bubbles is proportional to a difference between an expected value of the media density based on a reference density value of the liquid and the gas volume fraction of the suspended bubbles on the one hand and the resonator density value on the other hand, wherein to determine the value for the gas volume fraction of free bubbles said difference is divided by a product of the expected value of the media density based on the reference density value of the liquid and the gas volume fraction of the suspended bubbles on the one hand and a correction factor on the other hand, wherein the correction factor is not less than 1 and not greater than 4, in particular not greater than 3. ;

[0005] In a further development of the invention, the correction factor depends on the expected mobility and / or the Stokes number of the free bubbles in the liquid.

[0006] In a further development of the invention, the correction factor has the value 2.

[0007] In a further development of the invention, the gas volume fraction of the suspended bubbles is determined on the basis of the Sorokin equation.

[0008] In a further development of the invention, the resonator density value is determined on the basis of the natural frequencies of the F1 bending vibration mode and the F2 bending vibration mode or the F3 bending vibration mode.

[0009] In a further development of the invention, the measuring sensor has at least two different measuring tubes in which similar bending vibration modes have different natural frequencies, wherein the determination of the resonator density value is carried out on the basis of the different natural frequencies of two similar bending vibration modes, in particular the two F1 bending vibration modes of the different measuring tubes.

[0010] In a further development of the invention, a mixed phase density value for the medium is determined as a function of the gas volume fraction of suspended bubbles and the gas volume fraction of free bubbles.

[0011] In a further development of the invention, the method is carried out at least partially iteratively, wherein in a second iteration the determined mixed phase density value is used instead of the resonator density value to determine the gas volume fraction of suspended bubbles.

[0012] In a further development of the invention, at least one of the following values ​​is provided as measured values: the mixed phase density value, the gas volume fraction of free bubbles, the gas volume fraction of suspended bubbles, and the sum of the aforementioned gas volume fractions.

[0013] An example of a density meter to perform the procedure includes: a measuring sensor with at least one measuring tube that can be excited to oscillate for guiding a flowable medium; an exciter for exciting the oscillations; at least one oscillation sensor for detecting oscillation-dependent signals; and a measuring and operating circuit configured to drive the exciter, to detect the oscillation-dependent signals, and to carry out the method according to the invention.

[0014] In a further development, the at least one measuring tube is a measuring tube of a pair of essentially identical measuring tubes which can be excited to oscillate against each other.

[0015] The invention will now be explained in more detail with reference to the exemplary embodiment illustrated in the drawings. It shows: Fig. 1 : a flowchart of an embodiment of the method according to the invention; Fig. 2a : measurement data on gas volume fractions obtained using the method according to the invention; and Fig. 2b : a corresponding density determination according to the state of the art and according to the method according to the invention.

[0016] The Fig. 1The illustrated embodiment of a method 100 according to the invention for characterizing a gas-laden liquid begins in a step 110 with the determination of the natural frequencies of the f 1 bending vibration mode and the f 3 bending vibration mode of a measuring tube pair of a Coriolis mass flow meter, which here serves in particular as a density meter. For this purpose, the f 1 bending vibration mode and the f 3 bending vibration mode can be excited, in particular simultaneously. By maximizing the ratio of the vibration amplitude to the mode-specific excitation power by varying the excitation frequencies, the desired natural frequencies can be determined. Furthermore, a pressure measurement value p valid at the time of the frequency measurement is recorded.

[0017] Based on the determined natural frequencies fi, 120 preliminary density values ​​are calculated in one step p 1 and p 3 determined as: ρ i = c 0 i + c 1 i 1 f i 2 + c 2 i 1 f i 4 , where c 0i , c 1i , and c 2i , are mode-dependent coefficients.

[0018] In a step 130, which is explained in more detail below, the speed of sound of the gas-laden liquid and a mode-specific resonator correction term K res-i for the density measurement are determined.

[0019] Subsequently, in a step 140, the speed of sound c res a resonator density value p res for the gas-laden liquid.

[0020] Using the current pressure measurement value p for the gas-laden liquid, whose speed of sound c res and the resonator density value p res In a step 150, a gas volume fraction of suspended bubbles α susp is determined.

[0021] Based on the gas volume fraction of suspended bubbles α susp and knowing the density of the pure liquid phase p lin a next step 160 a mixed phase density intermediate value p l-susp to determine the density of the gas-laden liquid. ρ l − susp = ρ l ∗ 1 − α susp

[0022] Based on a difference between the mixed phase density intermediate value p l-susp and the Resonator density value p res can for p res < p l-susp In the next step 170 the gas volume fraction α free of free bubbles is calculated according to: α free = ρ l − susp − ρ res k gas ∗ ρ l − susp

[0023] Here, k gas is a correction factor with a value between 1 and 3, which depends on the Stokes number of the gas bubbles, and is well estimated with 2 for most cases. Für ρ res > ρ l − susp gilt : α free = 0 .

[0024] For a true mixed phase density value p mix In a subsequent step you get 180: ρ mix = ρ l ∗ 1 − α susp − α free

[0025] In addition to this mixed-phase density value, the individual gas volume fractions α susp and α free, as well as their sum, can be output as the total gas volume fraction α total. Based on these gas volume fractions and a flow variable, the flow regime in a flowing medium can be characterized, for example.

[0026] Details of the process steps are explained below: To determine the resonator correction term K res-i to calculate a resonator density value p res First, the ratio V of the preliminary density values ​​is calculated, for example, the division of the preliminary density values p 1 and p 3 to V : = ρ 1 / ρ 3 .

[0027] Subsequently, a value of the speed of sound c res is determined, which, together with the measured natural frequencies f 1 and f 3 of the bending vibration modes in the following equation, leads to the observed ratio V of the preliminary density values: 1 + r g ⋅ c res f 1 2 − b 1 + r g ⋅ c res f 3 2 − b = V where r is approximately 0.84, b=1, and g is a measuring tube-dependent proportionality factor between the speed of sound c res and the resonance frequency, which can, for example, assume a value of 10 / m. The value of the speed of sound c res that satisfies the above equation is the desired value for the speed of sound of the gas-laden liquid.

[0028] Based on the determined sound speed value c res , a mode-specific correction term K res-i for the resonator effect can then be calculated according to: K res − i : = 1 + r g ⋅ c res f i 2 − 1

[0029] The resonator density value p res can be calculated in the next step 140 as: ρ res ⋅ = ρ i K res − i

[0030] Furthermore, according to Sorokin, the following relationship exists between the speed of sound c mix of a gas-laden liquid and other parameters: c S − mix = α s c g 2 + 1 − α s 2 c l 2 + α s 1 − α s ⋅ ρ l γ ⋅ p − 1 2

[0031] Here, α s is a gas volume fraction (or the Gas Void Fraction GVF), cg is the speed of sound of the pure gas, cl is the speed of sound of the pure liquid, γ is the adiabatic coefficient for the gas, p is the current pressure of the gas-laden liquid and ρ l is the density of the pure liquid.

[0032] The mixture density value according to Sorokin is ρ S-mix is ​​related to the density of the liquid phase ρ l and the gas density via the gas volume fraction α by: ρ S − mix = ρ l 1 − α s + ρ g α s

[0033] Since the liquid density is significantly greater than the gas density and since the gas volume fraction is usually in the single-digit percentage range, the following approximation applies: ρ S − mix ≈ ρ l 1 − α s

[0034] Thus, equation (viii) can be rewritten as: c S − mix = α s c g 2 + 1 − α s 2 c l 2 + α s ρ S − mix γp − 1 2

[0035] Neglecting quadratic terms in α we obtain: c S − mix = α c g 2 + 1 − 2 α s c l 2 + α s ρ S − mix γp − 1 2

[0036] By solving equation xii for α s, an expression for calculating the gas volume fraction according to Sorokin is found: α s <menclose notation="box"> = 1 c S − mix 2 − 1 c l 2 c g 2 − 2 c l 2 + ρ S − mix γP < / menclose>

[0037] Neglecting the terms with (1 / cl ) 2< and (1 / cg ) 2< , which is justified for pressure values ​​up to a few bar, one obtains a value for the gas volume fraction α with a relative accuracy in the lower single-digit percentage range: α s <menclose notation="box"> = γp c S − mix 2 ρ S − mix < / menclose>

[0038] If in equation xiii or xiv the sound speed c res found with equation v is substituted for the mixing sound speed c S-mix and the resonator density value determined with equation vii p res for the mixture density value ρ S-mix according to Sorokin, the gas volume fraction α s according to Sorokin corresponds to the gas volume fraction α susp of suspended bubbles, which is used in equations ii and iv.

[0039] In a second iteration, the true mixed phase density value found in a first iteration with equation iv can be used in equation xiii or xiv p mix instead of the resonator density value p res for the mixture density value ρ S-mix according to Sorokin. The gas volume fraction α s according to Sorokin thus found is in turn used as the gas volume fraction α susp of suspended bubbles in equations ii to obtain a new intermediate mixed phase density value p l-susp which is then used in equation iii to determine a second value for the gas volume fraction of free bubbles. The iteratively determined values ​​for the gas volume fractions are then used in equation iv to determine a second true mixed phase density value. p mix If necessary, this iteration can be continued until a convergence criterion is met. However, experience shows that one iteration is completely sufficient.

[0040] The diagram in Fig. 2ashows measurement data collected using the method according to the invention on the gas volume fractions of gum arabic into which air was introduced. The dashed line shows the gas volume fraction α s of suspended bubbles, while the solid line represents the gas volume fraction α free of free bubbles. Depending on the type of introduction, the various gas volume fractions change significantly. The method according to the invention is able to distinguish between the two types of gas loading and to generate precise measured values ​​for the respective gas volume fractions. The diagram in Fig. 2b shows the resonator density values ​​resulting from equation vii according to the state of the art p res with a dashed line and the mixed phase density values ​​according to equation iv p mixwith a solid line. The inventive determination of the mixed-phase density proves to be superior in determining density when different types of gas loading are present.

Claims

1. A method (100) for characterizing the gas content of a medium that comprises a gas-charged liquid, using a measuring sensor that conveys the medium in at least one vibrating measuring tube, wherein the method comprises: Identification of a measured pressure value (110) for the medium being conveyed in the measuring tube and of the resonant frequencies of at least two different vibrational modes of the measuring tube; Identification of an acoustic velocity value (130) and a resonator density value (140) of the medium on the basis of the resonant frequencies of the at least two different vibrational modes of the at least one measuring tube; Identification of a gas volume content (150) of suspended bubbles in the medium on the basis of the resonator density value, of the acoustic velocity value, and of the measured pressure value; Identification of a value for the gas volume content (170) of free bubbles on the basis of the resonator density value of the medium, and of the gas volume content of suspended bubbles, wherein the value for the gas volume content of free bubbles is proportional to a difference between an expected value of the media density on the basis of a reference density value of the liquid and of the gas volume content of the suspended bubbles on the one hand, and of the resonator density value on the other, wherein, in order to determine the value for the gas volume content of free bubbles, said difference is divided by a product of the expected media density value on the basis of the reference density value of the liquid and of the gas volume content of suspended bubbles on the one hand, as well as a correction factor on the other, wherein the correction factor is not smaller than 1 and not greater than 4, in particular not greater than 3.

2. A method according to claim 1, wherein the correction factor is dependent on the expected mobility and / or the Stokes number of free bubbles in the liquid.

3. A method according to claim 1 or 2, wherein the correction factor exhibits the value 2.

4. A method according to one of the preceding claims, wherein the gas volume content of suspended bubbles is determined on the basis of the Sorokin equation, according to which the following relationship is in place between the sound velocity cS-mix of a gas-charged liquid and further parameters: c S − mix = α s c g 2 + 1 − α s 2 c l 2 + α s 1 − α s ⋅ ρ l γ ⋅ p − 1 2 , wherein αs represents the gas volume content, cg the sound velocity of the pure gas, ci the sound velocity of the pure liquid, γ the adiabatic coefficient for the gas, p the current pressure of the gas-charged liquid, and p the density of the pure liquid.

5. A method according to one of the preceding claims, wherein the resonator density value is determined on the basis of the resonant frequencies of the F1 bending vibrational mode and the F2 bending vibrational mode, or the F3 bending vibrational mode.

6. A method according to one of the preceding claims, wherein the measuring sensor exhibits at least two different measuring tubes, wherein the same bending vibrational modes exhibit different resonant frequencies, wherein the resonator density value is determined on the basis of the different resonant frequencies of two identical bending vibrational modes, in particular the two F1 bending vibrational modes of the different measuring tubes.

7. A method according to one of the preceding claims, wherein a mixed phase density value is determined for the medium as a function of the volume fraction of suspended bubbles and of the gas volume content of free bubbles.

8. A method according to claim 7, wherein the method is carried out at least in part iteratively, wherein in a second iteration the determined mixed phase density value is used in place of the resonator density value in order to determine the gas volume content of suspended bubbles.

9. A method according to one of the preceding claims, wherein at least one of the following values is output and provided as a measured value: the mixed phase density value, the gas volume content of free bubbles, the gas volume content of suspended bubbles, and the total of all these gas volume contents.