RU2548614C1 - Method of determining coefficient of combustion gas diffusion in nitrogen - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: method consists in passing of an electric current through an electro-chemical cell, the value of which is changed until a limit current, passing through the border of the phase separation, is obtained, as well as the calculation of a diffusion coefficient. Into a gas flow with a known content of a combustion gas, mixed with nitrogen, placed is the electro-chemical cell with a cavity, formed by hermetically connected to each other two disks from an oxygen-conducting solid electrolyte, with a pair of electrodes being placed on opposite surfaces of one of the disks, and a capillary, connecting the cavity with the gas flow. Then the direct current voltage in the range of 300÷500 mW with the supply of the positive pole to the electrode, located inside the cell, is supplied to the electrodes, and the value of the limit current, produced in this way, is used to calculate a coefficient of the combustion gas diffusion in nitrogen.

EFFECT: possibility of measuring the combustion gas diffusion in nitrogen in a wide temperature range by the well-studied oxygen-conducting solid electrolyte, accuracy increase.

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Description

The invention relates to an analytical technique and can be used to measure the diffusion coefficients of combustible gases, such as hydrogen, carbon monoxide, methane and ethane, in nitrogen and other inert gases.

The study of the properties of substances is an integral part of the study of the structure of matter. Knowledge of the properties of substances is also necessary for the calculation of various technological devices and installations, in particular, the problems of chemical kinetics, the calculation of mass transfer in the boundary layer, and many others. To date, certain experimental material has been accumulated and kinetic theories of diffusion in discharged gases have been created, which allow one to calculate diffusion coefficients. For moderately dense and dense gas systems, only very limited, often conflicting, experimental data exist. There is no reliable method and corresponding theory for calculating the coefficient of mutual diffusion in gases, although this coefficient is widely used for calculating the combustion process, in chemical kinetics, it enters into many dimensionless criteria for heat and mass transfer. To determine the diffusion coefficients of gases, the Loshmidt method, the two-bulb method, the capillary method, the magnetic resonance method, the dynamic method, and several others have been developed and are used. Most of them are characterized by the complexity of the hardware design, the high complexity, the high qualifications of the personnel and the significant measurement error (Reed R., Prausnits J., Sherwood T. Properties of gases and liquids. JL: Chemistry, LO, 1982-592 p. [1] ; Sherwood T., Pigford R., Wilkie C. Mass transfer. M: Chemistry, 1982-696 p. [2]; Rezibua P., DeLener M. Classical kinetic theory of liquids and gases. M: Mir, 1980-423 with [3]).

A known method for determining the diffusion coefficient of a gaseous substance (SU No. 480012, publ. 08/05/1975) [4], diffusing into the capillary outlet from the hollow tube in which the test substance is located. At the same time, a carrier gas stream is passed through the tube into which the test substance is pulsed; at the outlet of the capillary outlet, the substance with the carrier gas is directed to an additional carrier gas stream, which has the same velocity as the main stream, and is recorded at the outlet of the main and additional flows, the concentration of the test substance and the obtained data determine the diffusion coefficient.

This method relates to time-consuming, requires special chromatographic equipment, complicated in execution and allows you to determine the diffusion coefficient of a substance only at room temperature.

Closest to the proposed invention is a method for determining the diffusion coefficient of a substance (SU 1141311, publ. 23.02.1985) [5]. This method consists in passing an electric current, the density of which exponentially changes in time from a given initial value, through the electrolyte-electrode interface, measuring the dependence of the polarization of the electrode on time and calculating the diffusion coefficient, while the current density is reduced in time, the polarization is measured electrode at the time of reaching the extreme value, and the diffusion coefficient is calculated by the equation:

where: D is the diffusion coefficient of the studied reagent, cm ² / s;

In - initial current density, A / cm ²

Z is the number of electrons per one diffusion particle;

F - Faraday constant, C / g · equiv .;

Cv is the volume concentration of the studied reagent, mol / cm ³ ;

R is the universal gas constant, J / mol · K;

T is the absolute temperature, K;

τ is the characteristic time of current decrease, s;

r is the extreme value of polarization, V.

The disadvantage of this method is that the magnitude of the polarization of the electrode used in it depends on many factors, including the manufacturing technology of the electrodes, the material of the electrodes, the method of measuring polarization, and it is poorly reproducible, which determines the low accuracy of the measurement.

The objective of the present invention is to create a methodically and technically simple accurate and reliable method for determining the diffusion coefficient of a combustible gas in a mixture with nitrogen or other inert gas over a wide temperature range.

To solve this problem, a method for determining the diffusion coefficient of combustible gases in nitrogen involves passing an electric current through an electrochemical cell, the voltage of which is changed to obtain the limiting current flowing through the phase boundary, as well as calculating the diffusion coefficient, while into a gas stream with a known fuel content of a gas in a mixture with nitrogen, an electrochemical cell is placed with a cavity formed by hermetically interconnected two oxygen-wire disks solid electrolyte, on the opposite surfaces of one of the disks there is a pair of electrodes, and a capillary connecting the cavity to the gas stream, a direct current voltage of 300–500 mV is applied to the electrodes with a positive pole supplied to the electrode inside the cell and the magnitude of the while the current limit is calculated diffusion coefficient of a combustible gas in nitrogen in accordance with the equation:

where: D is the diffusion coefficient of a combustible gas in nitrogen, cm ² / s;

I is the value of the limiting current, A;

R is the universal gas constant, J / mol · K;

T is the absolute temperature, K;

L is the length of the capillary, mm;

F is the Faraday constant, C / g · equiv;

S is the cross-sectional area of the capillary, mm ² ;

P is the total pressure of the gas mixture, atm;

X _{(flammable gas)} is the mole fraction of flammable gas mixed with nitrogen.

The essence of the claimed method is as follows. A gas mixture containing a known amount of combustible gas in nitrogen washes an electrochemical cell with a cavity formed by disks of an oxygen-conducting solid electrolyte with electrodes and a capillary, one end of which is located in the cell cavity and the other in the gas mixture washing the electrochemical cell. The electrochemical cell is heated to a predetermined temperature in the range from 400 to 700 ° C. The gas mixture is washed from the outside of the electrochemical cell and through the capillary enters the internal cavity. Under the action of the voltage applied to the electrodes, oxygen is pumped from the gas mixture washing the cell through the solid oxygen-conducting electrolyte into the cell cavity. In the cavity of the cell, the pumped oxygen interacts with the combustible gas that has been received therein in a mixture with nitrogen or other inert gas via a capillary from the gas mixture washing the cell. Combustible gas interacts with oxygen in accordance with reactions (1-4):

A current is generated in the circuit between two electrodes deposited on a solid electrolyte disk. And when the DC voltage reaches in the range from 300 to 500 mV, the current value stabilizes and stops growing with increasing applied voltage. The resulting current is the limiting current, and its value is due to gas exchange between the gaseous medium washing the cell and the gas in the cell cavity. In this case, the capillary is a diffusion barrier that limits this gas exchange flow, which will also correspond to the cell current. The magnitude of the limiting current, limited by the diffusion capillary barrier, obeys the equation from the source (Reed R., Prausnits J., Sherwood T. Properties of gases and liquids. JL: Chemistry, LO, 1982-592 p.) [1].

where: D is the diffusion coefficient of a combustible gas in nitrogen, cm ² / s;

X _{(combustible gas)} - molar fraction of combustible gas in a mixture with nitrogen;

S is the cross-sectional area of the capillary, mm ² ;

P is the total pressure of the gas mixture, atm;

T is the analysis temperature, K;

L is the length of the capillary, mm

In accordance with equation (6), it is possible to calculate the diffusion coefficient of a given combustible gas in nitrogen or in another inert gas at a given temperature from the measured value of the limiting current (I _{L (combustible gas-nitrogen)} ):

A new technical result achieved by the claimed method lies in the possibility of measuring the diffusion coefficients of combustible gases in nitrogen in a wide temperature range by means of a well-studied oxygen-conducting solid electrolyte, simply and with high accuracy.

The method is illustrated by the drawing of the device for its implementation. The device consists of two disks 1 of an oxygen-conducting solid electrolyte, interconnected by a gas tight sealant 2 with the formation of an internal cavity 3. On the opposite surfaces of the disk 1 there are two electrodes 4. A capillary 5 is located between the disks. Voltage is supplied to the electrodes 4 from a DC voltage source (IN). The current generated in the circuit is measured with an ammeter (A). The cell is placed in a gas stream containing a mixture of combustible gas of known concentration with nitrogen or other inert gas, which washes its outer surface and through the capillary 5 enters the inner cavity of cell 3.

Under the action of a voltage of 300 ÷ 500 mV, applied from the source (ID) to the electrodes 4, and a plus is applied to the internal electrode, oxygen is pumped from the gas mixture through the solid oxygen-conducting electrolyte, washing the cell into the internal cavity 3 of the device. In cavity 3, the incoming oxygen interacts with a combustible gas. The resulting interaction products in accordance with equations (1-4) are exchanged through capillary 5 with a gas mixture washing the cell. In this case, capillary 5 is a diffusion barrier that limits this gas exchange flow. The maximum current of the cell will correspond to this exchange flow. When the applied voltage reaches 0.3 ÷ 0.5 V, the gas exchange between the cell cavity and the gas mixture washing the cell is stabilized and the limiting diffusion current is established in the circuit - I _{L (combustible gas-nitrogen} ), which is measured using an ammeter (A) . Using equation (7) from the measured value - I _{L (combustible gas-nitrogen} ) and the known value of X _{(combustible gas),} it is possible to calculate the value D - diffusion coefficient of a particular combustible gas in nitrogen.

By measuring with the help of an electrochemical cell the diffusion coefficient of a combustible gas at one temperature T ₁ can be converted to another temperature T _n without additional measurements using equation (8), which follows from equation (7):

Where:

D ₁ - diffusion coefficient of a combustible gas in nitrogen, measured at a temperature T ₁ on a given electrochemical cell, cm ² / s;

D _n - diffusion coefficient of a combustible gas in nitrogen, measured at a temperature T _n on a given electrochemical cell, cm ² / s;

T ₁ - temperature of the first measurement, K;

I ₁ - current limit obtained during the first measurement, A;

T _n is the temperature of the nth measurement K;

I _n is the limiting current obtained during the nth measurement, A.

It should be noted that in equations (5-8), all values are either constant (R, F) or measured with high accuracy (S, L, P, T, X _{(combustible gas)} ), which ensures high accuracy and the method itself.

Thus, the claimed method allows to measure the diffusion coefficient of a specific combustible gas in nitrogen for a given temperature by means of an amperometric cell with an oxygen-conducting solid electrolyte.

Claims (1)

A method for determining the diffusion coefficient of combustible gases in nitrogen, including passing an electric current through an electrochemical cell, the value of which is changed to obtain the limiting current flowing through the interface, as well as calculating the diffusion coefficient, characterized in that in a gas stream with a known content of combustible gas, mixed with nitrogen, an electrochemical cell is placed with a cavity formed by hermetically interconnected two disks of an oxygen-conducting solid electrolyte , on the opposite surfaces of one of the disks there is a pair of electrodes, and a DC voltage is applied to the electrodes within the range of 300 ÷ 500 mV with a positive pole connected to the electrode inside the cell and using a capillary connecting the cavity with the gas flow current calculate the diffusion coefficient of a combustible gas in nitrogen in accordance with the equation:

where: D is the diffusion coefficient of a combustible gas in nitrogen, cm ² / s;
I is the value of the limiting current, A;
R is the universal gas constant, J / mol · K;
T is the absolute temperature, K;
L is the length of the capillary, mm;
F is the Faraday constant, C / g · equiv;
S is the cross-sectional area of the capillary, mm ² ;
P is the total pressure of the gas mixture, atm;
X _{(flammable gas)} is the mole fraction of flammable gas mixed with nitrogen.