Method and apparatus for plasma chemical gas / gas mixture conversion

Abstract

The present disclosure relates to the field of chemistry and, in particular, proposes a method for plasma-chemical conversion of a gas or gas mixture using a pulsed electric discharge. The technical effect of the present disclosure is to increase the efficiency of the process for converting a gas / gas mixture to a desired product by inducing a forward reaction and minimizing a reverse reaction. This is achieved by converting the gas / gas mixture using a pulsed electric discharge in the form of a high-temperature plasma channel formed between electrodes in a moving stream of the gas / gas mixture. The ratio of flow rate to average discharge current is within the following range: 250J / (m 3 *A 2 )<ρ*V 2 / I 2 <4,000J / (m 3 *A 2 ), ρ is the density of the gas / gas mixture in the reaction chamber (kg / m), V is the flow velocity of the gas / gas mixture in the reaction chamber (m / s), and I is the average current of the pulsed electric discharge (A). A method for gas / gas mixture conversion and an apparatus for carrying out the method are proposed. 2 independent claims, 9 dependent claims, 3 figures.

Description

Technical Field

[0001] The present disclosure relates to the field of chemistry, and specifically proposes a method for plasma chemical conversion using pulsed electrical discharges of gases or gas mixtures. It can be used to process natural gas or associated petroleum gas in the petrochemical industry, is an environmentally friendly technology for binding and treating carbon dioxide, and can also be used in other types of petrochemical processes.

Background Art

[0002] Plasma can be regarded as a very powerful tool for promoting chemical reactions with high activation energies, for example, in the production of synthesis gas, the conversion of CO2 and H2S, etc. There are well-known plasma-based technologies that utilize DBD and pulsed corona discharges, electric arcs, or microwave discharges to produce plasma-chemical reactions induced by high-temperature or non-equilibrium plasmas. The name non-equilibrium plasma results from the fact that electrons have very high energies above what is sufficient to dissociate and ionize molecules even though the gas molecules remain at relatively low temperatures (their temperatures do not increase).

[0003] The optimization of plasma parameters for producing plasma-chemical reactions is to minimize the energy cost while maximizing the yield of the desired product. To induce a forward chemical reaction, the plasma dissociates or excites the reagent molecules to generate radicals or other reactive particles. The radicals or other reactive particles can react with each other to obtain the desired product.

[0004] There are two ways to generate such reactions.

[0005] One is by dissociating through means of directly colliding a source molecule with an electron having sufficient energy. In this case, an important feature of the plasma is the voltage of the electric field, or more precisely, the ratio of the electric field voltage to the gas concentration. This ratio of the electric field voltage to the gas concentration determines whether the energy acquired by the electrons between two collisions with gas molecules in the electric field is sufficient for the desired process of forming radicals or active particles.

[0006] This method is typical of all types of non-equilibrium plasmas, such as dielectric barrier discharge (DBD), including the pulsed barrier discharge described in the burst-mode paper DBD: A solution for more efficient CO2 conversion? A. Oskan et al (see Plasma Sources Science and Technology, IOP Publishing, 2016, 25 (5), p. 055005), published at https: / / hal.sorbonne-universite.fr / hal-01367345.

[0007] The same also applies to nanosecond pulsed discharges. Nanosecond pulsed electrical discharges are described, for example, in the paper Nanosecond-Pulsed Discharge Plasma Splitting of Carbon Dioxide, Moon Soo Bak et al, (see IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 43, NO. 4, APRIL 2015, pp. 1002 - 1007).

[0008] Both studies can be regarded as prior art of the present disclosure and have one major drawback of very low conversion process efficiency.

[0009] One of the main problems with non-equilibrium plasmas is all types of energy losses of electrons (including elastic collisions, vibrational excitation of molecules, etc.). Energy losses lead to irreversible gas heating, and most importantly, this is not useful in this case. Unfortunately, these types of losses are usually much larger than the molecular dissociation energy and heat (enthalpy) of the reaction. For this reason, the energy efficiency of non-equilibrium plasmas (the rate of reaction enthalpy and energy cost) is usually very low, about 10% - 20%.

[0010] An alternative method is to heat the gas molecules in the reaction chamber to a temperature sufficient for them to overcome the activation barrier of the reaction. In this case, heating is a useful process, and also, any process that results in more heat generation is not a loss.

[0011] However, there is another problem when heating the reaction chamber: all molecules are heated, and the energy is spent not only on heating and dissociating the reagents required by the practitioner, but also on heating and dissociating the final products of the reaction. In this case, the main problem is the reverse reaction. The reverse reaction reduces the conversion rate and energy efficiency of the process.

[0012] The solution is to remove the reaction products from the high-temperature region as quickly as possible. This method of suppressing the reverse reaction can substantially increase the yield of the desired product and the energy efficiency of the plasma-chemical process. This approach is sometimes called quenching of the products of plasma-chemical reactions.

[0013] As described in Patent US 7867457 B2, published on January 11, 2011, a technique for performing plasma-chemical reactions is known. This technique involves a special plasma-chemical reactor that uses a gliding arc moving through a gas flow constructed as a counter-rotation. This partially solves the problem of product quenching by moving the product through the plasma channel. However, this solution also has several major drawbacks resulting from the fact that the speed of the plasma channel with respect to the gas (slip speed) is relatively low at about 1 meter per second. Therefore, at least some of the reaction products may be subject to secondary treatment. The secondary treatment results in a significant contribution to the reverse reaction, reducing the conversion rate and energy efficiency of the process.

[0014] By ensuring optimal gas conversion conditions in the high-temperature zone of the reaction product and ensuring effective quenching, it becomes possible to achieve maximum conversion as well as the energy efficiency of the conversion process.

Summary of the Invention

[0015] The technical effect of the present disclosure is to increase the efficiency of the process of converting a gas / gas mixture into a desired product by inducing the forward reaction and minimizing the reverse reaction.

[0016] To achieve this effect, it is proposed to use a new method for the plasma-chemical gas / gas mixture conversion process. This new process includes generating a pulsed electric discharge in a flow of a gas / gas mixture moving through a reaction chamber at a predetermined speed. This process generates a short-lived plasma channel connecting electrodes located inside the reaction chamber.

[0017] The proposed method solves the problem of quenching the reaction products generated in the high-temperature plasma channel. The flow of the gas / gas mixture moving at a predetermined speed within the reaction chamber supplies a new portion of the reagent for conversion while promoting the rapid extinction of the newly formed plasma channel, thereby limiting its duration. The optimal plasma-chemical conversion process is achieved when the ratio of the flow rate of the gas / gas mixture in the reaction chamber to the average current during the discharge is as follows: 250 J / (m 3 *А 2 ) < ρ * V 2 / I 2 <4,000 J / (m 3 *А 2 ) where ρ is the density of the gas / gas mixture in the reaction chamber (kg / m 3 ), V is the flow rate of the gas / gas mixture in the reaction chamber (m / s), and I is the average current of the said discharge (A).

[0018] This method is implemented by creating a special device for the plasma-chemical conversion of a gas / gas mixture, comprising a reactor consisting of a reaction chamber and an input / output module, a high-voltage power supply unit connected to the electrodes within the reaction chamber section, and a gas flow rate controller. The high-voltage power supply unit generates a pulsed electric discharge between the electrodes in the form of a high-temperature plasma channel having a duration of (10 - 500) ns and a frequency of (20 - 300) kHz.

Brief Description of the Drawings

[0019]

Figure 1

Figure 2

Figure 3

Embodiments for Carrying Out the Invention

[0020] The plasma conversion of a gas or gas mixture into the desired product involves the dissociation or excitation of the molecules of the initial reagent. The dissociation or excitation of the molecules of the initial reagent does not enter into a chemical reaction without an external action, prevents affecting the reaction product, and is formed to avoid reverse reactions. For example, when acetylene is produced from natural gas (methane), it is necessary to subject methane molecules, rather than acetylene molecules, to plasma treatment. And when CO and hydrogen are produced from a mixture of CO2 and methane, it is necessary to subject CO2 and CH4 initial reagents, rather than CO and H2 reaction products, to the treatment.

[0021] The idea underlying the proposed gas conversion process is to generate a gas region during a pulsed electric discharge in the form of a high-temperature plasma channel connecting the electrodes. The temperature inside the channel reaches thousands of degrees Celsius. The high temperature causes the dissociation or excitation of the molecules in the gas within the plasma channel. After the disappearance of the plasma channel, a chemical reaction involving radical or excited gas particles occurs, resulting in the generation of the desired components. Since the temperature of the environment is lower than (100 - 150)°C, the desired components remain stable thereafter.

[0022] The efficiency of this gas conversion process, assuming its continuous use, directly depends on the form of the pulsed electric discharge generated. The maximum efficiency is achieved when the form of the pulsed electric discharge is a high-temperature plasma channel that intermittently appears and reappears. The discharge in the form of an intermittent plasma channel is the optimal form of discharge for the treatment.

[0023] The parameters for generating the optimal discharge form in a gas flow moving at a predetermined speed are different from the parameters for generating the same type of discharge in a stationary gas. During the experiment, a correlation was established between the speed of the gas flow and the average current of the pulsed electric discharge. The pulsed electric discharge is necessary to create the optimal discharge form in a given gas composition. The following range of values was determined for the ratio of the speed of the gas flow to the average current of the discharge necessary to maintain the optimal discharge form: 250J / (m 3*A 2 ) <ρ * V 2 / I 2 <4,000 J / (m 3 *A 2 ) Here, ρ is the density of the gas / gas mixture in the reaction chamber (kg / m 3 ) and V is the flow rate of the gas / gas mixture in the reaction chamber (m / s), and I is the average current of the discharge (A).

[0024] The velocity V of the gas / gas mixture flow used in the above formula is calculated as the ratio of the volume of the gas / gas mixture entering the reaction chamber per unit time to the cross-section of the working zone of the reaction chamber. This velocity can be directly measured by means of the Doppler effect.

[0025] By using intermittent pulsed electric discharges in a moving gas flow, it becomes possible to solve several problems simultaneously: 1) A new portion of the reagent required for the conversion is always supplied, enabling more reagent to be processed in less time.

[0026] 2) The final product of the reaction is rapidly removed from the active zone of the reactor, thus minimizing the possibility of the reverse reaction and enhancing the efficiency of the conversion process.

[0027] 3) The high-temperature plasma channel is blown away with the gas flow, resulting in better promotion of its extinction and control of its duration.

[0028] The duration of the plasma channel affects the efficiency of gas conversion. If it is too short, dissociation within the channel does not occur in all gas molecules. Consequently, the number of particles that can participate in the reaction decreases, thereby also reducing the number of forward reactions that generate the desired product. If the plasma channel persists for too long, the high temperature affects the obtained molecules of not only the reagent but also the desired product, dissociating them and causing a reverse reaction.

[0029] The optimal duration for the plasma channel to have a sufficiently high temperature and thus to obtain a sufficiently high degree of molecular ionization to ensure an electrical resistance of the channel below 10,000 ohms is (10 - 500) ns. If the duration of the plasma channel is less than 10 ns, the energy efficiency of the forward reaction decreases. Also, if it exceeds 500 ns, the influence of the reverse reaction increases and the quenching effect decreases.

[0030] Also, through experiments, the optimal frequency of the plasma channel for plasma-chemical conversion, which is (20 - 300) kHz, could be determined. At low frequencies, the productivity of the reactor decreases, and at high frequencies, there are technical difficulties in stabilizing the optimal form of the discharge.

[0031] The principle of plasma-chemical gas / gas mixture conversion will be explained below using the diagram of the apparatus shown in FIG. 1.

[0032] The plasma-chemical gas / gas mixture conversion apparatus comprises a reactor 1 consisting of a reaction chamber 2 and an input module 3 and an output module 4, and a high-voltage power supply unit 5 connected to electrodes 6 and 7 disposed inside the reaction chamber 2.

[0033] The reaction chamber 2 is made of a heat-resistant dielectric material such as ceramic or quartz glass and can usually be in a cylindrical shape.

[0034] The electrodes 6 and 7 each consist of an anode and a cathode having their own power supply, or a pair of a plurality of anodes and cathodes, and the electrodes can take several shapes. For example, they can be shaped as a cylinder with flat ends, a cylinder with one end having a sharp edge, a cylinder with a rod or a needle at one end, or a cone having a sharp end and radial holes. Any combination of design and shape is possible. The following materials can be used for the electrodes and their components: steel, stainless steel, copper, brass, bronze, titanium, tungsten, molybdenum, hafnium, zirconium, or any combination thereof.

[0035] The power supply units 5 are each connected to the electrodes 6 and 7 via the high-voltage terminals of the anode 8 and the cathode 9. The number of high-voltage inputs 8, 9 for each of the electrodes 6 and 7 may vary; those numbers are selected based on the required inductance in the discharge circuit in order to achieve the required plasma channel duration.

[0036] One or several initial gaseous reagents, or the flow rate thereof, which are set by one or several gas flow controllers, are fed into the reactor 1. In the reactor 1, the gaseous reagents are flowed through the holes of the input module 3 towards the reaction chamber 2. Once inside the reaction chamber, they start to move at a predetermined speed. The high-voltage power supply unit 5 ensures that an intermittent high-temperature plasma channel with optimal duration and frequency parameters is generated between the electrodes 6 and 7 in order to achieve plasma-chemical conversion. The sufficient flow rate of the reagents in the reaction chamber 2 and the shape of the electrodes 6 and 7 rapidly extinguish the plasma channel. The extinction of the plasma channel minimizes the possibility of reverse reactions. The components generated as a result of the conversion are discharged from the chamber 2 through the holes of the output module 4, just like the remaining reagents, helping to maintain the required gas velocity in the reaction chamber.

[0037] FIG. 2 shows a type of plasma-chemical conversion apparatus capable of flowing the unreacted reagents back towards the reaction chamber 2 again.

[0038] After being mixed with the reagents and the product is discharged from the output module 4, it passes through the heat-transfer type heat exchanger 10, where it is cooled to a temperature suitable for the gas separation unit. Then, the mixture is fed into the gas separation unit 11, where the product is separated from the reagents. Thereafter, the remaining reagents are flowed towards the initial reagent supply channel and towards the reactor 1. If the rate at which the initial reagent is fed from the gas flow controller 13 is insufficient, the apparatus can also be further provided with a recirculation blower 12 in order to generate the required gas flow rate in the reaction chamber 2.

[0039] Products 1 and 2 can be, for example, CO and oxygen when CO2 is the raw material used in the process. This system can also produce a single product, for example, acetylene from methane. For example, when the purpose is to produce synthesis gas (a mixture of H2 and CO) from a mixture of CO2 and methane, it is not necessary to separate the products from each other.

[0040] As one option for the design of the gas separation module 11, it can be a two-stage system. This system is similar to that used for the production of acetylene from methane. And the gas separation module 11 comprises a unit for dissolving acetylene in a solvent, or a unit for extracting acetylene by pressure swing absorption (PSA), and a PSA unit for separating hydrogen from the methane residue.

[0041] The input and output modules are parts made of metal, heat-resistant plastic, or ceramic. Their role is to generate a localized gas flow within the region adjacent to the electrodes. This gas flow is such that the gas enters the discharge chamber and then exits through the holes of these modules. During this process, a velocity field localized within the region adjacent to the electrodes is generated (which is different from the gas / gas mixture velocity V). If necessary, the gas can be sprayed onto the electrodes, and by moving the gas spirally, a reverse vortex can be formed within the discharge chamber.

[0042] The rotation of the gas in the discharge chamber is generated to stabilize the region where the plasma channel is generated at the center of the discharge chamber and to prevent the plasma channel from "sticking" to the reactor wall. The adhesion of this plasma channel to the reactor wall is an undesirable and dangerous phenomenon that can lead to damage to the wall of the reaction chamber due to overheating. The sticking effect occurs as a result of non-uniform heating of the wall of the discharge chamber, and therefore, it can be promoted and prevented by directing the gas vortex flowing through the chamber through the tangential holes of the input and output modules. This serves to equalize the temperature inside the reactor and causes the formation of a plasma channel following an orbit close to the closest distance between the cathode and the anode.

[0043] The high-voltage power supply unit 5 maintains a voltage sufficient to cause the destruction and formation of the plasma channel on the electrodes between the electrodes 6 and 7. After the channel is heated and its resistance drops, the discharge capacitance loses its charge, and the voltage in the discharge gap drops to zero at a certain point and similarly drops. Then, the inductance in the circuit causes a negative pressure. At this point, the energy generated in the plasma channel is close to zero. It takes some time for the discharge capacitance to acquire a voltage sufficient for the next breakdown. This time is determined by the parameters of the power supply unit 5. The unit 5 ensures the necessary pause during the pulsed electric discharge. The next plasma channel is formed in a different location, does not affect the reaction products, and ensures that the reaction products reach an ideal tempering state. The described image is similar to pulsed-periodic spark discharge, but in this case, the lifetime of the plasma channels and their repetition frequency are twice as low, providing the quality required for the described device for plasma-chemical conversion.

[0044] For an example of the electrical circuit diagram of the high-voltage power supply unit 5 used in the device for generating pulsed electric discharge, refer to Figure 3.

[0045] The high-voltage power supply unit includes a frequency converter 14, a high-voltage high-frequency transformer 15, a voltage-doubling circuit 16, and an output pulse capacitor 17. The voltage-doubling circuit 16 includes a high-voltage diode 18 and a high-voltage capacitor 19.

[0046] The high-voltage transformer 15 and the voltage-doubling circuit 16 based on the diode 18 and the capacitor 19 operate as a limiter for the charging current of the output pulse capacitor 17. The charging current can be controlled by changing the frequency of the high voltage fed into the unit or the frequency of the impulse generated by the power frequency converter 14. Once the output pulse capacitor is charged to a voltage sufficient for the breakdown of the electrode gap in the plasma-chemical reactor 1, the output pulse capacitor 17 discharges through the plasma channel formed in the process. The capacitance of the capacitor 17 should ideally be no greater than I * 100 mkF. Here, I is the average discharge current.

[0047] When the plasma channel disappears, the process itself is repeated. A high repetition frequency ensures the required conversion degree of the reagent and the productivity of the reactor.

[0048] In the experiment, a 100 pF capacitor was used as the capacitor 19 in the voltage-doubling circuit 16. The frequency converter 14 generated frequencies of 60 kHz or 120 kHz. The output pulse capacitor 16 had a capacitance of 300 pF. Thus, a stable regime of ignition and extinction of the plasma channel was obtained, with frequencies of 30 kHz or 60 kHz respectively. The effective inductance of the circuit of the output pulse capacitor 16 through the plasma channel in the plasma-chemical reactor 1 was 0.5 μH, 0.125 μH, or 0.03 μH. On the other hand, the duration of the plasma channel was 180 ns, 80 ns, and 30 ns respectively.

[0049] An example of the results obtained during the experiment is shown below.

[0050] 〔Example 1.〕 Production of acetylene from a 50 / 50 mixture of methane and hydrogen at atmospheric pressure. The density was 0.4 kg / m 3 It was. The average discharge current was set to 0.4 A. The average gas flow velocity in the discharge chamber was 11.5 m / s. Therefore, the ratio of the flow velocity to the average discharge current was: ρ*V 2 / I 2 = 330.6 J / (m 3 *А 2 ) and it fell within the target range.

[0051] During the experiment, a discharge in the pure form as described above was obtained, and the energy cost for acetylene molecule formation was 8 eV per molecule.

[0052] [Example 2.] Production of acetylene from a 50 / 50 mixture of methane and hydrogen. The density was 0.38 kg / m3. The average discharge current was set to 0.4 A. The average gas flow velocity in the discharge chamber was 3.5 m / s. Therefore, the ratio of the flow velocity to the average discharge current was: ρ*V 2 / I 2 = 29 J / (m 3 *А 2 ) and it was outside the target range.

[0053] During the experiment, the discharge took the form of continuous, non-stop plasma filaments connecting the electrodes and was a constricted glow discharge in the gas flow. The energy cost for producing acetylene molecules was 32 eV per molecule.

[0054] [Example 3.] Production of acetylene from a 50 / 50 mixture of methane and hydrogen at an absolute pressure of 1.5 atmospheres. The density was 0.57 kg / m3. The average discharge current was set to 0.4 A. The average gas flow velocity in the discharge chamber was 12 m / s. Therefore, the ratio of the flow velocity to the average discharge current was: ρ*V 2 / I 2 = 513 J / (m 3 *А 2 ) and it was within the target range.

[0055] During the experiment, the optimal discharge form as described above was obtained, and the energy cost for acetylene molecule generation was 10.5 eV per molecule.

Claims

1. A step of flowing a gas / gas mixture through a reactor comprising a discharge chamber, electrodes, an input module and an output module, wherein the gas / gas mixture is characterized by an average flow velocity in the discharge chamber; A step of generating a pulsed electric discharge at the electrodes at a frequency of 20 to 300 kHz in the discharge chamber, thereby generating a high-temperature plasma channel between the electrodes; A method for plasma-chemical conversion of a gas / gas mixture, comprising: The duration of the high-temperature plasma channel is 10 to 500 ns, The ratio of the average flow velocity to the average discharge current is within the following range: 250 J / (m ３ *A ２ ) < ρ * V ２ / I ２ < 4,000 J / (m ３ *A ２ )、 where ρ is the density of the gas / gas mixture (kg / m ３ ), V is the average flow velocity (m / s), and I is the average current of the pulsed electric discharge (A). A method for plasma-chemical conversion of a gas / gas mixture.

2. The method according to claim 1, wherein the flow of the gas / gas mixture in the discharge chamber is a vortex flow.

3. The method according to claim 2, wherein the vortex flow includes a reverse vortex.

4. A reactor comprising the discharge chamber, electrodes, an input module, an output module, a gas flow controller, A high-voltage power supply unit connected to the electrodes, wherein the high-voltage power supply unit is configured to apply a voltage to the electrodes to generate a pulsed electric discharge at the electrodes in the discharge chamber at a frequency of 20 to 300 kHz, The pulsed electric discharge is in the form of a high-temperature plasma channel, The input module is configured to generate a directed flow of the gas / gas mixture in the discharge chamber, An apparatus for performing the method for plasma-chemical conversion of a gas / gas mixture according to claim 1.

5. The electrodes are the anode and the cathode, or a pair of the anode and the cathode, or a plurality of pairs of the anode and the cathode, having a high-voltage power supply unit connected thereto via high-voltage inputs of the anode and the cathode. The apparatus according to claim 4.

6. The electrodes are designed as a cylinder having a flat bottom, a cylinder having one end with a sharp edge, a cylinder having a rod or a needle protruding from one end, and / or a cone having a sharp end and radial holes. The apparatus according to claim 5.

7. The directed flow of the gas / gas mixture is generated in the form of a vortex flow. The apparatus according to claim 4.

8. ​ The apparatus according to claim 4, further comprising a heat transfer heat exchanger and a gas separation unit for recirculating and reusing the reagent of the gas / gas mixture, wherein the gas flow controller is configured to supply the gas / gas mixture to the input module.

9. Further comprising a recirculation blower, the recirculation blower is configured to generate a required gas flow rate (V) in the discharge chamber to obtain a predetermined ratio between the current (I) of the pulsed electric discharge and the flow rate V, and the predetermined ratio is within the following range: 250 J / (m ３ *A ２ ) < ρ*V ２ / I ２ < 4,000 J / (m ３ *A ２ ) where ρ is the density of the gas / gas mixture (kg / m ３ ). The apparatus according to claim 4.

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