RU2593297C2 - Method of producing gas mixture containing nitrogen oxide - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to plasma chemistry, particularly to production of nitrogen oxide (NO) from initial gas containing at least nitrogen and oxygen, using electric discharge and can be used in scientific research, industry, agriculture and medicine. Method of producing gas mixture containing nitrogen oxide, includes following steps: providing annular gas gap between two electrodes with tangential input and output of initial gas containing at least oxygen and nitrogen, through gas gap, feeding to electrodes high-voltage pulses with duration 1-10 mcs with controlled repetition frequency within range from 250 Hz to 8.5 kHz, providing rate of gas passage and repetition frequency of high-voltage pulses, where in direction of passage of gas a sequence of spark discharges is formed, displaced relative to each other at distance not shorter than 0.15 mm.

EFFECT: longer service life of electrode system and wider range of control of concentration of nitrogen oxide.

1 cl, 3 dwg

Description

The invention relates to plasma chemistry, in particular, to a technology for producing nitric oxide (NO) from a source gas containing at least nitrogen and oxygen by means of an electric discharge and can be used in scientific research, industry, agriculture and medicine.

The process of nitrogen oxidation in an electric discharge plasma is widely used in a number of industrial and medical technologies. The advantages of the plasma method over the chemical one are readily available reserves of raw materials (atmospheric nitrogen and oxygen), simplicity of equipment, and the possibility of placing a plasma chemical plant in the immediate vicinity of the experiment site, medical procedure, etc.

A spark discharge at a gas pressure of about atmospheric is attractive for producing an NO-containing mixture, primarily from a technological point of view. In this discharge, it is possible to put significant energy (1-2 J / cm ³ ) into the gas necessary to ensure effective synthesis of NO in a nonequilibrium plasma, while the temperature of the gas mixture can not exceed ~ 500 ° C. For comparison: in an equilibrium arc discharge plasma, effective ozone synthesis requires heating the gas to a temperature of ~ 4000 ° C, which at least reduces the service life of the electrode system.

Known patent authors RW Treharne, CK McKibben US No. 4010897, class. IPC ⁷ B05B 7/30; B05B 17/04, C05C 11/00, A01C 23/04, published 03/08/1977, according to which the oxidation of nitrogen is carried out in a spark plasma. The oxidation process includes the following steps: provide a gas gap between the two electrodes, ensure the passage of the source gas containing at least oxygen and nitrogen through the gas gap, bring the high voltage voltage to the above electrodes, while providing a voltage amplitude sufficient to form a spark discharge (channel) in the gas gap, the synthesis of nitric oxide under the action of a spark discharge and the formation of an NO-containing mixture.

In the known patent, the air flow is passed through a gas gap separating the cylindrical external electrode and the rod inner electrode mounted on its axis (coaxially). In this case, a high-voltage voltage with an amplitude of about 5 kV of an alternating current of industrial frequency 60 Hz is applied to the electrodes. When the amplitude voltage value (twice per period) reaches the breakdown voltage of the air gap, a spark discharge is formed between the electrodes with a characteristic plasma channel diameter of ~ 0.2 mm. An active current flows in the channels, sufficient energy is invested in the gas to activate nitric oxide synthesis reactions. As a result, the NO-containing mixture is removed from the gas gap.

In the known patent, a spark discharge is formed at a slowly increasing voltage with a front duration of about 4.15 ms. The long time of transferring energy from the power source to the discharge unnecessarily increases the current in the spark channel and the temperature in the spot on the surface, which is a drawback, since it increases erosion and reduces the service life of the electrodes.

In the known patent, the spark gas processed by the source gas crosses the cylindrical gas gap in the longitudinal direction, passing along long helical paths. In this case, a significant part of the NO molecules manages to enter the oxidation reaction with the formation of nitrogen dioxide NO ₂ . As a result, the average concentration of NO in the output mixture decreases. This is a disadvantage because it reduces the range of regulation of the concentration of NO.

Less time for the removal of the NO-containing mixture from the gas gap is provided in the well-known patent of the authors HG Wyse, MS Smith US No. 4141715, class. IPC ⁷ B01K 1/00; C05C 5/00, published on 02.27.1979, according to which the oxidation of nitrogen is carried out in a spark plasma, which includes the following steps: provide a gas gap between the two electrodes, ensure the passage of the source gas containing at least oxygen and nitrogen through the gas gap high voltage is supplied to the above electrodes, while providing a voltage amplitude sufficient to form a spark discharge (channel) in the gas gap, the synthesis of nitric oxide under the influence of a spark discharge and the formation of an NO-containing mixture.

In the known patent, in the gas gap between the coaxial external disk electrode with the central circular hole and the internal wire electrode, the source gas (air) is given a swirl movement in the cross section of the gas gap (aperture). A high-voltage voltage with an amplitude of 7 kV AC of an industrial frequency of 60 Hz is supplied to the electrodes and a radially directed spark discharge is formed. Within the aperture, a compact region for the formation of NO molecules with an axisymmetric flow outlet is provided, which reduces the time for the removal of the NO-containing mixture from the gas gap.

However, in the known patent, a spark discharge excited by a slowly rising voltage has too long a duration. This leads to local overheating in the near-electrode region of the discharge and thermal erosion of the surface, which is a disadvantage because it reduces the life of the electrodes. In addition, the surface of the wire electrode, due to its small diameter, is almost continuously subjected to erosion by a spark discharge. This is a disadvantage, as it reduces its service life.

In the known patent, in the cross section of the gas gap, the gas mixture forms a vortex, the angular velocity of which is small at the periphery and large at the axis in the outlet region. Part of the kinetic energy of the inner layers is used to increase the temperature of the gas, which intensifies the mixing of the mixture components and the oxidation of NO. As a result, the average concentration of NO in the output mixture is significantly reduced. This is a disadvantage because it reduces the range of regulation of the concentration of NO.

With almost no stirring, the NO-containing gas is removed in the method for producing a gas mixture containing nitric oxide by WM Zapol as described in US Pat. No. 5,339,682, cl. IPC ⁷ A61M 11/00, published on March 14, 1995, which is the closest in technical solution to the claimed invention.

The known method includes the following steps: provide a gas gap between the two electrodes, ensure the passage of the source gas containing at least oxygen and nitrogen through the gas gap, bring the high voltage voltage to the above electrodes, while providing a voltage amplitude sufficient to form a spark discharge (channel) in the gas gap, the synthesis of nitric oxide under the action of a spark discharge and the formation of an NO-containing mixture.

In the known method, the source gas (air) is passed transversely to the discharge gap separating two coaxially arranged rounded rod electrodes. Under the action of a high voltage voltage U (~ 20 kV) of alternating current of industrial frequency (50/60 Hz), a spark discharge is formed between the electrodes, initiating chemical reactions of the synthesis of nitric oxide. Since the gas flows smoothly around the electrodes without vortices, the NO-containing stream is removed without mixing along the shortest path, which reduces the loss of nitric oxide.

However, in the gap between the indicated electrodes, the electric field has a sharply inhomogeneous distribution with a maximum value of tension in the areas adjacent to the vertices of the electrodes. Here, in each pulse, the processes of gas ionization develop in a faster manner and a spark channel is formed. As a result, there is a "binding" of the spark channels to the tops of the electrodes. This is a disadvantage because it accelerates thermal erosion and reduces the life of the electrodes.

The yield of chemical reactions in a spark discharge is determined by the specific energy input parameter W = P · F / V, where F is the frequency of repetition of the high voltage voltage U, P is the average energy deposited in the discharge under the influence of U.

In the known method, the concentration of NO is controlled by the flow rate V of air or voltage U (F = const). In this case, the edges of the spark discharge remain “tied” to the vertices of the electrodes, and its central part bends in the direction of flow. In the first case, the higher the flow rate V, the greater the deflection of the plasma channel. Moreover, the electric field strength in the channel and the energy P deposited in the discharge deviate significantly from the initial values. The presence of the P (V) bond is a drawback, since even with small changes in V, a nonlinear dependence of the NO concentration on the flow rate arises, which reduces the control range for the NO concentration. In the second case, the drawback is the narrow limits of the change in the value U-between the breakdown voltage of the gas gap and the allowable voltage on the elements of the electric circuit, since this reduces the depth of regulation of the NO concentration.

When creating the claimed invention, the problem was solved of creating a method for producing a gas mixture containing nitric oxide in a wide range of concentrations.

The technical result in solving this problem was to increase the service life of the electrode system and increase the range of regulation of the concentration of nitric oxide.

The specified technical result is achieved in that, in comparison with the known method for producing a gas mixture containing nitric oxide, comprising the following steps: provide a gas gap between two electrodes, ensure the passage of the source gas containing at least oxygen and nitrogen through the gas gap, the high voltage voltage is supplied to the above electrodes, while providing a voltage amplitude sufficient to form a spark discharge in the gas gap, the synthesis of nitric oxide under The effect of spark discharge and the formation of an NO-containing mixture is new in that they provide an annular gas gap between the indicated electrodes with tangential gas inlet and outlet, high voltage voltage pulses of 1-10 μs duration with an adjustable repetition rate are supplied to the electrodes, while providing a gas transmission rate and the repetition rate of high-voltage pulses, at which a sequence of spark discharges displaced relative to each other by p Normal distance no smaller than the characteristic diameter of the spark discharge.

и NO^-, между моментами подачи высоковольтных импульсов, смещаясь в направлении газового потока, остаются в области разрядного промежутка. Причем молекулярные ионы, образованные в малом объеме искрового канала, перемещаются в кольцевом промежутке в виде компактной группы (облако на фиг. 1). Во время очередного высоковольтного импульса нарастающее электрическое поле вызывает ударное отлипание электронов от указанных молекулярных ионов, чем создает характеристическую область с высокой концентрацией свободных электронов. Именно в этой характеристической области, смещенной на расстояние L, электронные лавины формируют следующий искровой разряд.In the inventive method, the source gas stream is introduced tangentially into the annular gap, passed along the gap, is treated with a spark discharge, and also tangentially removed from the gap. The small turbulences that arise in the flow attenuate quickly and do not mix the gas layers. As a result, products of electrosynthesis, including electronegative ions

and NO ^- , between the moments of the supply of high-voltage pulses, shifting in the direction of the gas flow, remain in the region of the discharge gap. Moreover, molecular ions formed in a small volume of the spark channel move in the annular gap in the form of a compact group (cloud in Fig. 1). During the next high-voltage pulse, an increasing electric field causes shock detachment of electrons from these molecular ions, which creates a characteristic region with a high concentration of free electrons. It is in this characteristic region, displaced by a distance L, that the electron avalanches form the next spark discharge.

Thus, the first gas breakdown initiates a sequential, in step with the frequency F of high voltage pulses, movement of the spark discharge in the annular gap. Since the ion cloud during the time T = 1 / F _max (F _max is the upper limit of the control range) is displaced by the gas stream V by a distance L no less than the characteristic diameter of the spark channel (~ 0.15 mm), the heat load is distributed over the length of the electrodes more evenly. This reduces erosion and accordingly increases their service life.

поскольку основная энергия быстро заселяемых колебательных уровней N₂ направляется на реакцию синтеза окиси азота, а не на разогрев газа. В результате увеличиваются концентрация NO и соответственно диапазон ее регулирования.The electric field in the longitudinal direction of the annular gap is uniform, which increases the stability of the parameters that determine the electrosynthesis of NO. In particular, in a discharge excited by high-voltage pulses of duration T _imp = 1-10 μs, the average electron energy lies at the maximum of the cross section for vibrational excitation of nitrogen molecules N ₂ and the specific energy input exceeds the threshold value W = 1 W / cm ³ . This increases the yield of the nitric oxide formation reaction.

since the main energy of rapidly populated vibrational levels of N _{2 is} directed to the reaction of synthesis of nitric oxide, and not to the heating of the gas. As a result, the concentration of NO and, accordingly, the range of its regulation increase.

An additional technical result can be considered that when providing resonant vibrational excitation of N ₂ , the excitation energy of vibrational oxygen levels does not exceed several percent, due to which the yield of ozone-forming reactions is extremely small and the amount of ozone in the NO-containing mixture is not more than 0.01 ppm. A low ozone content (not more than MPC - 0.1 ppm) in the mixture is an important requirement of a number of medical technologies, for example, when carrying out inhaled NO-therapy.

In the inventive method, the concentration of nitric oxide, which depends on the specific energy input W = P · F / V, is controlled by the repetition rate F (or period T = 1 / F) of the high voltage voltage pulses. Here P is the average energy invested in the gas in separate pulses, V is the flow rate of the source gas. The linear control region is limited both by the condition F <F _max = 1 / T _NO , where T _NO = 10 ^-4 sec is the maximum reaction time for the formation of nitric oxide, and by the condition F> F _min = 1 / T _io , where T _io = 10 ^-2 sec - the lifetime of the ion cloud.

At F> F _{max, the} growth of the NO concentration slows down due to the fact that under the action of frequently following high-voltage pulses, plasma-chemical reactions proceed under conditions with incomplete kinetics. At F <F _{min, the} stability of NO synthesis decreases, since if the pause between pulses is too long, the ion cloud disappears (Fig. 1), which ensures the initiation of a discharge in each voltage pulse.

Compared with the prototype, the frequency control has a wider linear region of almost two orders of magnitude (F _max / F _min ≈100), which increases the range of regulation of the concentration of NO.

The invention is illustrated by an example implementation, photographs of a spark discharge, graphical dependence of the concentration of nitric oxide for the case when the source gas is air under normal conditions.

In FIG. 1 shows a diagram of an annular gas gap (transverse section) according to the invention, as well as a schematic process of initiating sequential displacement of a spark discharge and its integral photograph at F = 4.8 kHz, exposure time 0.1 sec. Here T and U are the repetition period and the amplitude of the high voltage pulses, L is the step of the discharge, h is the width of the gas gap.

In FIG. 2 shows an enlarged characteristic photograph (F = 4.8 kHz, exposure time 5 · 10 ^-3 sec) of three spark discharges in the gas gap according to the invention. Here, h is the width of the gas gap, d is the diameter of the near-electrode region of the spark discharge, and L is the step of the discharge.

In FIG. 3 shows the dependence of the concentration of nitric oxide on the frequency F of the repetition of high-voltage pulses at the outlet of the gas gap according to the invention. Here the solid line is a linear approximation.

A method for producing an NO-containing gas mixture is carried out in the example shown in FIG. 1 as follows.

, NO^-. Место образования первого искрового канала носит статистически случайный характер. Следующая область формирования отстоит от предыдущей на расстоянии L=v·T≈0,55 мм, куда газовый поток за время между импульсами T=2,08 мс со скоростью v≈265 см/сек перемещает компактную группу ионов (облако на фиг. 1). Здесь в нарастающем электрическом поле ионы отдают лишние электроны, которые в результате лавинной ионизации газа образуют канал искрового разряда. В смещенном на расстояние L искровом разряде продолжается наработка NO и вновь образуется ионное облако. Далее формирование искровых разрядов осуществляется по указанному выше алгоритму.The source gas, dry air, with an oil content of not more than 0.1 mg / m ³ , is passed through an annular discharge gap 1 with a width of h = 3 mm, separating a cylindrical electrode 2 with an inner diameter of 70 mm and a disk electrode 3 with an outer diameter of 64, mm. The source gas at room temperature and atmospheric pressure through the inlet channel 4 ⌀ 2 mm is introduced into the gap 1 tangentially to its middle circle with a diameter of 67 mm. The flow rate is v≈265 cm / s (flow rate 0.5 l / min). Through the output channel 5 ⌀ 2 mm, the gas is removed tangentially from the gap 1. Then, voltage pulses of amplitude U = 15 kV with a base width T _imp = 8 μs, with a frequency of 4.8 kHz are applied to the electrodes 2, 3 and form the initial spark discharge, during which they form nitric oxide and negative ions

, NO ^- . The place of formation of the first spark channel is statistically random. The next region of formation is separated from the previous one at a distance L = v · T≈0.55 mm, where the gas flow during the time between pulses T = 2.08 ms with a speed of v≈265 cm / s moves the compact group of ions (the cloud in Fig. 1 ) Here, in an increasing electric field, ions give off extra electrons, which, as a result of avalanche ionization of the gas, form a spark discharge channel. In the spark discharge displaced by a distance L, the accumulation of NO continues and an ion cloud forms again. Further, the formation of spark discharges is carried out according to the above algorithm.

The input and output channels 4, 5 are placed in different half-rings of gap 1, with a symmetric offset of 0.5 mm (not shown in Fig. 1) relative to the half-cutting gap of 1 plane perpendicular to the axis of the annular gap 1. The indicated relative channel offset provides at least one a complete revolution of the gas before it is withdrawn from gap 1. In the case of sufficiently long series of high-voltage pulses, the displacement of ion clouds and, accordingly, spark discharges along the annular gas gap 1 is cyclical. Moreover, as follows from FIG. 1 photographs, spark discharges in the form of thin plasma channels, curved in the direction of the gas flow, have a clear spatial separation. Near-electrode brightly glowing areas of spark channels (see Fig. 2) with a characteristic size d = 0.15 mm, within which erosion of the electrodes is possible, are mixed relative to each other by a distance L = 0.5-0.6 mm.

The noted features of the spatio-temporal structure of the spark discharge are preserved in a wide frequency range, up to 12 kHz — the channels of the spark discharge are not “tied” to a certain point on the surface of the electrodes, each time a spark discharge is formed in a new place, and the step of moving L exceeds the diameter of the plasma channel d. The value of L has a statistical spread within ± 0.5 mm. Therefore, over a long period of time (the number of cycles), the spark channels are distributed along the annular gap evenly, which reduces the specific heat load on the electrodes. As a result, erosion is reduced and, compared with the “tied” mode, the service life of the electrodes is increased by at least l / d≈1400 times, where l is the average length of the annular gap.

The repetition rate F of high voltage pulses sets the specific energy input W and, accordingly, the concentration of NO in the stream. The results of the experiments are presented in FIG. 3 in the form of the dependence of the concentration of nitric oxide in air on the frequency F. From those shown in FIG. 3 results that the frequency regulation of the concentration of NO has a wide linear region from 250 Hz to 8.5 kHz and increases (~ 2 times compared with the prototype) the range of regulation of the concentration of nitric oxide. Lowering the frequency F below ~ 250 Hz violates the regulation due to unstable (not in every pulse) breakdown of the gas gap, and increasing the frequency significantly above 10 kHz is impractical due to the intense decomposition of nitric oxide and the onset of saturation of the NO concentration.

Thus, the application of the proposed method for producing a gas mixture containing nitric oxide, due to more than 2-fold increase in the range of regulation of the concentration of NO and 1000-fold increase in the service life of the electrodes allows us to solve the problem.

Claims (1)

A method of producing a gas mixture containing nitric oxide, comprising the following steps: providing a gas gap between two electrodes, allowing a source gas containing at least oxygen and nitrogen to pass through the gas gap, supplying a high voltage voltage to the above electrodes, while providing an amplitude voltage sufficient to form a spark discharge in the gas gap, the synthesis of nitric oxide under the action of a spark discharge and the formation of an NO-containing mixture, characterized in that provide an annular gas gap between the indicated electrodes with tangential gas inlet and outlet, high voltage voltage pulses of 1-10 μs duration with an adjustable repetition rate in the range from 250 Hz to 8.5 kHz are supplied to the electrodes, while providing a gas propagation speed and repetition rate high voltage pulses, in which a sequence of spark discharges displaced relative to each other by distances not less than 0.15 mm is formed in the direction of gas passage.

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