EA040404B1 - METHOD AND DEVICE FOR PLASMA-CHEMICAL GAS/GAS MIXTURE CONVERSION - Google Patents

Description

The invention relates to the field of chemistry, namely to the plasma-chemical conversion of a gas or gas mixture using a pulsed electric discharge. It can be used to process natural gas or associated petroleum gas in the petrochemical industry, can be used in environmentally friendly technologies for capturing and processing carbon dioxide, as well as in other areas of chemical production.

State of the art

Plasma is a powerful tool for high activation energy chemical reactions such as syngas production, CO2 and H2S conversion, etc. Plasma technologies are widely known using barrier and corona electric discharges, an electric arc or a microwave electric discharge for conducting plasma-chemical reactions in non-equilibrium and equilibrium plasma. The name non-equilibrium plasma means that the gas molecules remain cold (their temperature does not increase significantly), and the electrons have a high energy sufficient for the dissociation of molecules and their ionization.

Optimization of plasma parameters for carrying out a plasma-chemical reaction consists in minimizing energy costs while maximizing the yield of desired products. In order to stimulate direct chemical reactions, the plasma causes the reactant molecules to dissociate or excite, producing radicals or other active species that can react to produce the desired products.

There are two ways to ensure the occurrence of such reactions.

The first way is the dissociation of the initial molecules directly by impacts of electrons having sufficient energy. In this case, the key characteristic of the plasma is the electric field strength, or more precisely, the ratio of the electric field strength to the gas concentration, which determines whether the energy that an electron gains in an electric field between two collisions with gas molecules is sufficient for the desired process of formation of radicals or active particles.

This path is typical for all types of non-equilibrium plasma, such as barrier discharge, including intermittent barrier discharge, described in the article DBD in burst mode: 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.

The same applies to the nanosecond corona discharge described, for example, in the article 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).

Both works can be considered as analogs of the claimed invention, having an obvious drawback - the low efficiency of the conversion process.

The general problem of a nonequilibrium plasma is that all types of electron energy losses (due to elastic collisions, vibrational excitation of molecules, etc.) used to heat the gas are useless and irreversible in this case. Unfortunately, these losses are usually many times greater than the dissociation energy of molecules and even more so than the thermal effect (enthalpy) of the reaction. Therefore, the energy efficiency of a nonequilibrium plasma (the fraction of the reaction enthalpy of the total energy spent on the process) is usually very low, on the order of 10–20%.

The second way is to heat the gas molecules in the reaction chamber to a temperature sufficient to overcome the activation barrier of the reaction. In this case, heating is a useful process, and all processes leading to heating are not losses.

But when heating the reaction chamber, there is another problem: all molecules are heated, and energy is spent not only on heating and dissociation of the reagents we need, but also on heating and dissociation of the final reaction products. In this case, the main problem is reverse reactions, which reduce the conversion and energy efficiency of the process.

The solution to this problem is the removal of reaction products from the hot zone as quickly as possible. Due to this method of suppressing reverse reactions, the yield of desired products and the energy efficiency of plasma-chemical processes can be significantly increased. This approach is called quenching of the products of plasma-chemical reactions.

Known technology for conducting plasma-chemical reactions, described in US patent 7867457 B2, publ. 01/11/2011. It uses a plasma-chemical reactor built on the basis of a sliding arc moving in a gas flow organized in the form of a reverse vortex. This approach makes it possible to partially solve the problem of hardening of products due to the movement of the plasma channel, but this solution also has significant drawbacks due to the fact that the velocity of the plasma channel relative to the gas (slip velocity) is low, about 1 m/s. Thus, part of the useful reaction products has time to undergo secondary processing, which leads to a significant contribution of reverse reactions and a decrease in the conversion and energy efficiency of the process.

Ensuring optimal conditions for gas conversion in the hot zone of the discharge with efficient quenching of the reaction products makes it possible to achieve maximum conversion and energy efficiency of the conversion process.

- 1 040404

The essence of the invention

The technical result of the claimed solution is to increase the efficiency of the process of converting a gas / gas mixture into the desired products by stimulating direct reactions and minimizing the occurrence of reverse reactions.

To achieve this result, a method for plasma-chemical conversion of a gas/gas mixture is proposed, which consists in creating in a gas/gas mixture flow moving in a reaction chamber at a certain speed, a pulsed electric discharge, which is a sequentially emerging and extinguishing hot plasma channels connecting the electrodes located inside reaction chamber.

The proposed method provides a solution to the problem of quenching the reaction products generated in a hot plasma channel. The flow of gas/gas mixture moving at a certain speed in the reaction chamber, in addition to providing new portions of reagents for conversion, contributes to the rapid quenching of the formed plasma channel, thereby limiting its lifetime. The optimal process of plasma-chemical conversion is carried out when the ratio of the gas/gas mixture flow rate in the reaction chamber to the average current in the electric discharge is in the following range:

250 J / (m ³ * A ² ) < p * V ² / I ² < 4000 J / (m ³ * A ² ), where ρ is the density of the gas / gas mixture in the reaction chamber (kg / m ³ ), V - gas/gas mixture flow rate in the reaction chamber (m/s), I - average discharge current (A).

The lifetime of the channel, during which it has a temperature that provides a high degree of ionization of the reagents and the electrical resistance of the channel is less than 10 kOhm, is 10-500 ns. At a shorter lifetime, the channel temperature turned out to be low and the energy efficiency of direct reactions decreased, while at times longer than 500 ns, the contribution of reverse reactions increased and the quenching efficiency decreased.

The optimal repetition frequency of plasma channels is 20-300 kHz. At a lower frequency, the productivity of the reactor decreases, and at a higher frequency, technical difficulties in stabilizing the optimal form of an electric discharge sharply increase.

To implement the claimed method, a device is proposed for conducting plasma-chemical conversion of a gas / gas mixture, containing a reactor consisting of a reaction chamber and input and output modules, a gas flow regulator and a high-voltage power supply connected to the electrodes located inside the reaction chamber, while the high-voltage power supply creates between the electrodes pulsed electrical discharges, which are hot plasma channels with a duration of 10-500 ns with a repetition rate of 20-300 kHz.

Brief description of the drawings

In FIG. 1 shows a diagram of the proposed device for conducting plasma-chemical conversion of a gas/gas mixture.

In FIG. 2 shows a diagram of an exemplary embodiment of a device for conducting plasma-chemical gas/gas mixture shifting with gas/gas mixture recirculation.

In FIG. 3 shows the electrical circuit of a high-voltage power supply for generating pulsed electrical discharges.

Detailed description of the invention

Plasma conversion of a gas or gas mixture into the desired product involves the dissociation or excitation of the molecules of the initial reagents that do not enter into a chemical reaction without external influence, while preventing this effect on the formed reaction products to exclude reverse reactions. For example, in the production of acetylene from natural gas (methane), methane molecules, and not molecules of acetylene, should be subjected to plasma treatment, and in the production of CO and hydrogen from a gas mixture of CO2 and methane, the initial reagents CO2 and CH ₄ should be treated, and not the reaction products CO and H2.

The essence of the proposed process of gas conversion is to create a pulsed electric discharge in the gas region, which is a periodically arising and extinguishing hot plasma channels connecting the electrodes. The temperature inside the channel reaches several thousand degrees Celsius. Under the influence of high temperature, dissociation or excitation of molecules occurs in the entire volume of gas that has entered the channel. After the plasma channel is extinguished, the necessary chemical reaction occurs with the participation of radicals or excited gas particles, as a result of which the desired components are formed, which remain stable, since the ambient temperature no longer exceeds 100-150°C.

The efficiency of such a process of converting gas into the desired product during its continuous use directly depends on the form of the generated pulsed electrical discharges. The greatest efficiency is achieved when the forms of pulsed electric discharges are sequentially emerging and extinguishing hot plasma channels. An electric discharge, which is a rapidly emerging and extinguishing hot plasma channel, is the optimal

- 2 040404 discharge form.

The parameters for the formation of the optimal form of a discharge in a gas flow moving at a certain speed differ from the parameters of its formation in a stationary gaseous medium. In the course of the experiments, a relationship was established between the gas flow rate and the average current of a pulsed electric discharge for the appearance of an optimal discharge form in a certain gaseous medium. The following range of values for the ratio of the flow rate to the average discharge current was determined, at which the optimal discharge shape is maintained:

250 J / (m ³ * A ² ) < p * V ² / I ² < 4000 J / (m ³ * A ² ), where ρ is the density of the gas / gas mixture in the reaction chamber (kg / m ³ ), V - gas/gas mixture flow rate in the reaction chamber (m/s), I - average discharge current (A).

The gas/gas mixture flow rate V used in the formula above is calculated as the ratio of the volume of gas/gas mixture entering the reaction chamber per unit time to the cross-sectional area of the working area of the reaction chamber. Also, this speed can be measured directly, for example, by the Doppler method. The rate V of the gas/gas mixture flow in the reaction chamber is controlled by the feed rate of the feedstock, for example, by a gas flow controller.

The use of periodic pulsed electrical discharges in a moving stream provides a solution to several problems at once:

1) there is a continuous flow of new portions of reagents for conversion, which allows you to process a larger amount of reagents in less time;

2) a quick withdrawal of finished products from the working zone of the reactor is carried out, thereby reducing the possibility of formation of reverse reactions, which increases the efficiency of the conversion process; And

3) the hot plasma channel is blown, which contributes to its rapid quenching and allows you to control its duration of existence.

The lifetime of the plasma channel has an impact on the gas conversion efficiency. With its short duration, not all gas molecules in the channel have time to dissociate, as a result of which the number of particles capable of participating in the reaction decreases, thereby reducing the number of direct reactions that form the necessary product. And with a long duration of the existence of the plasma channel, the effect of temperature will also occur on the newly formed molecules of the product, causing their dissociation, which will lead to the appearance of reverse reactions.

The optimal value of the lifetime of the plasma channel, during which the channel has a sufficiently high temperature and, accordingly, a sufficiently high degree of ionization of molecules, providing an electrical resistance of the channel of less than 10,000 Ω, is 10–500 ns. At a time less than 10 ns, the energy efficiency of the direct reaction decreases, and at a time greater than 500 ns, the contribution of reverse reactions increases and the quenching efficiency decreases.

Also during the experiments, the optimal frequency of repetition of electrical discharges for carrying out plasma-chemical conversion was determined. It is 20-300 kHz: at a lower frequency, the productivity of the reactor fell, and at a higher frequency, technical difficulties in stabilizing the optimal discharge form sharply increased.

The principle of conducting plasma-chemical gas/gas mixture shifting will be described below using the scheme of the device shown in Fig. 1.

The device for carrying out plasma-chemical conversion contains a reactor 1, consisting of a reaction chamber 2 and input modules 3 and output 4, and a high-voltage power supply 5 connected to electrodes 6 and 7 located inside the reaction chamber 2.

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

Electrodes 6 and 7 represent an anode and a cathode or several pairs of anodes and cathodes with separate power supply and can be made in various versions in the form of, for example, a cylinder with a flat end, a cylinder with one end made with a sharpened edge, a cylinder with pins or needles at one end or a cone with a sharp end and radial holes. In this case, the combination of execution can be any. As materials for the electrodes or parts thereof, steel, stainless steel, copper, brass, bronze, titanium, tungsten, molybdenum, hafnium, zirconium, or combinations thereof can be used.

The power supply 5 is connected to the electrodes 6, 7 through the high-voltage inputs of the anode 8 and cathode 9, respectively. The number of high-voltage inputs 8, 9 for each electrode 6 and 7 can be different, their number is selected based on the required inductance of the discharge circuit to achieve the required development time of the plasma channel.

The initial gaseous reagent or several initial gaseous reagents, the flow rate of which is set by one or more gas flow regulators, enter the reactor 1, where they are directed through the openings of the input module 3 to the reaction chamber 2. The high-voltage power supply 5 provides periodic generation between electrodes 6 and 7 of short hot plasma channels with

- 3 040404 optimal parameters of duration and frequency for the implementation of plasma-chemical conversion.

A sufficient flow rate of the reagents in the reaction chamber 2 and the geometry of the electrodes 6, 7 lead to rapid quenching of the plasma channel, thereby minimizing the possibility of back reactions. The resulting components and the remaining unreacted reagents leave chamber 2 through the holes of the output module 4, which additionally form the required velocity field in chamber 2.

A variant of the device for plasma-chemical conversion with the possibility of re-entry of unreacted reagents into the reaction chamber 2 is shown in Fig. 2.

The mixture of reactants and formed products, after exiting the output module 4, passes through the heat exchanger-recuperator 10 for cooling to a temperature acceptable for the operation of the gas separation module, and then enters the gas separation module 11, where the mixture is separated into products and reagents. Next, the unreacted reagents are sent to the channel for supplying the initial reagents to the reactor 1. The device can also be additionally equipped with a recirculation compressor 12 to create the required gas flow rate in the reaction chamber 2, if the feed rate of the initial reagents from the gas flow controller 13 is not enough.

Products 1 and 2 can be, for example, CO and oxygen, if the feedstock is carbon dioxide. It is also possible that there can be only one product, such as acetylene from methane. Sometimes the products may not be separated from each other, for example, in the production of syngas (a mixture of H ₂ and CO) from a gas mixture of CO ₂ and methane.

An example of a gas separation module 11 can be a two-stage system used in the production of acetylene from methane, consisting of a stage for dissolving acetylene in acetone, followed by its extraction under pressure release, and a pressure swing adsorption unit for separating hydrogen from methane residues.

The input and output modules are parts made of metal, heat-resistant plastic or ceramics, which form local gas flows in the near-electrode region due to the fact that the gas enters the discharge chamber and leaves it through the holes located in these modules. In this case, local velocity fields are formed in the near-electrode region (not to be confused with the gas/gas mixture flow velocity V), and, if necessary, the electrodes are blown and the gas rotates with the possibility of the formation of reverse vortices in the discharge chamber.

Rotation of the gas in chamber 2 is organized to stabilize the zone of formation of plasma channels in the central region of the discharge chamber and prevent sticking of plasma channels to the reactor wall. Such sticking is an undesirable and dangerous phenomenon, which can lead to the destruction of the walls of the reaction chamber due to their overheating. Sticking caused by uneven heating of the walls of the reaction chamber can be prevented by swirling the gas in it due to tangential holes in the input and output modules, which evens out the temperature field in the reactor and stimulates the formation of plasma channels that follow trajectories close to the shortest distance between the anode and cathode .

The high-voltage power supply 5 provides a voltage on the electrodes sufficient to cause a breakdown and the formation of a plasma channel between electrodes 6 and 7. After the channel warms up and its resistance drops, the discharge capacitance is discharged and the voltage across the discharge gap drops, at a certain moment passes through zero and becomes negative due to the inductance of the circuit. At this moment, the energy release in the plasma channel is close to zero. For the next breakdown, some time is required to charge the discharge capacity to a voltage sufficient for it. This time is determined by the parameters of the power supply 5, which provides the necessary pause between electric discharges. In this case, the next plasma channel is formed already in another place, without affecting the finished reaction products and ensuring their ideal hardening. The described picture is similar to a repetitively pulsed spark discharge, but in this case, the lifetime of the plasma channel and the period of their repetition are two orders of magnitude shorter, which provides the qualities necessary for use in the described device for plasma-chemical conversion.

An example of an electrical circuit of a high-voltage power supply 5 used in a device for generating pulsed electrical discharges is shown in Fig. 3.

The high-voltage power supply contains a frequency converter 13, a high-voltage high-frequency transformer 15, a voltage doubling circuit 16, and a pulsed output capacitor 17. The voltage doubling circuit 16 consists of high-voltage diodes 18 and high-voltage capacitors 19.

The high-voltage transformer 15 and the voltage doubling circuit 16 based on diodes 18 and capacitors 19 work as current limiters for charging the output pulse capacitor 17. The charging current can be controlled by changing the frequency of the applied high voltage or the applied pulses generated by the power frequency converter 14. After charging the output pulse capacitor to a voltage sufficient for the breakdown of the interelectrode gap of the plasma-chemical reactor 1, the pulsed output capacitor 17 is discharged through the resulting plasma channel. It is desirable that the capacitance of the capacitor 17 be greater than the value of I*100 (uF), where I is the average discharge current.

After the plasma channel is extinguished, the process is repeated again. High repetition rate provides

Claims (5)

bakes the required degree of conversion of the reagents and the performance of the reactor. Capacitors 19 of the doubling circuit 16 with a capacity of 100 pF were used in the experiment. The frequency converter 14 gave a frequency of 60 or 120 kHz. The pulse output capacitor 16 had a capacitance of 300 pF. Thus, stable modes of ignition and quenching of plasma channels with a frequency of 30 or 60 kHz, respectively, were obtained. The effective inductance of the discharge circuit of the pulsed output capacitor 16 through the plasma channel of the plasma-chemical reactor 1 was 0.5, 0.125, or 0.03 μH. In this case, the lifetime of the plasma channel was 180, 80, and 30 ns, respectively. The following are examples obtained during the experiment. Example 1 An experiment on the generation of acetylene from a mixture of methane and hydrogen in a ratio of 50/50 at atmospheric pressure. The density was 0.4 kg/m ³ . The average discharge current was set at 0.4 A. The average gas flow velocity in the discharge chamber was 11.5 m/s. Thus, the ratio of the flow rate to the average discharge current corresponds to p *V ² / t = 330.6 J/(m ³ *A ² ), which is in the specified range. During the experiment, a pure form of the electrical discharge described above was obtained, and the energy cost of producing an acetylene molecule was 8 eV/molecule. Example 2 An experiment on the generation of acetylene from a mixture of methane and hydrogen in a ratio of 50/50. The density was 0.38 kg/m ³ . The average discharge current was set at 0.4 A. The average gas velocity in the discharge chamber was 3.5 m/s. Thus, the ratio of the flow rate to the average discharge current corresponds to p *V ² / I ² = 29 J/(m ³ *A ² ), which is not in the specified range. During the experiment, the discharge was in the form of a permanent non-extinguishing plasma cord connecting the electrodes - a contracted glow discharge in a gas flow. The energy cost of producing a molecule was 32 eV/molecule for acetylene. Example 3 An experiment on the generation of acetylene from a mixture of methane and hydrogen in a ratio of 50/50 at an absolute pressure of 1.5 atm. The density was 0.57 kg/m ³ . The average discharge current was set at 0.4 A. The average gas velocity in the discharge chamber was 12 m/s. Thus, the ratio of the flow rate to the average discharge current corresponds to p *V ² /I ² = 513 J/(m ³ *A ² ), which is in the specified range. During the experiment, the optimal form of the electric discharge described above was obtained, and the energy cost of producing an acetylene molecule was 10.5 eV/molecule. CLAIM 1. The method of plasma-chemical conversion of a gas / gas mixture, characterized in that the gas / gas mixture flow is directed into a reactor consisting of a reaction chamber, electrodes and input and output modules, while the gas / gas mixture flow moves at a certain speed inside the reaction chamber; generate a pulsed electrical discharge in the reaction chamber, which is a hot plasma channels connecting the electrodes; wherein the ratio of the gas/gas mixture flow rate in the reaction chamber to the average discharge current is in the following range: 250 J / (m ³ * A ² ) < p * V ² / I ² < 4000 J / (m ³ * A ² ), where ρ is the density of the gas / gas mixture in the reaction chamber (kg / m ³ ), V - the flow rate of the gas/gas mixture in the reaction chamber (m/s), I is the average current of the pulsed electrical discharge (A). 2. The method according to claim 1, characterized in that the lifetime of hot plasma channels is 10-500 ns, and their generation is carried out at a frequency of 20-300 kHz. 3. The method according to claim 1, characterized in that the moving gas/gas mixture flow in the reaction chamber is a swirling flow. 4. The method according to claim 3, characterized in that reverse vortices are also formed in the reaction chamber. 5. A device for plasma-chemical conversion of a gas/gas mixture, containing a reactor, including a reaction chamber, electrodes and input and output modules, a gas flow regulator and a high-voltage power supply connected to the electrodes, characterized in that the high-voltage power supply is capable of generating in the reaction chamber, a pulsed electrical discharge, which is a hot plasma channels connecting the electrodes, while the ratio of the gas/gas mixture flow rate in the reaction chamber to the average discharge current is in the following range: -