Patent Application
20210086158
The present invention relates to the improvement of chemical reactions that include the use of a catalyst. More particularly, the present invention relates to systems and methods using microwaves to enhance such chemical reactions.
The industrial use of microwave energy is now well established for over 50 years, with new applications continuing to be developed besides the historical operations of bulk heating. These include the development of microwave plasma techniques as well as the use of microwave energy to replace, stimulate or enhance the operation of conventional catalytic materials either in combination with plasma operations or in a non-plasma mode.
Plasma technology, although relatively recent in terms of some applications, has rapidly grown in popularity owing to the unique and impressive properties of plasmas, particularly in the promotion of some chemical processes either as the self-catalyst or in conjunction with other catalytic materials. In particular, the class of microwave plasmas is unique in that certain reactions occur only in the case of microwave plasmas (as opposed to other types of plasmas) and also because only microwave plasmas have the inherent capability to be scaled up to industrial levels.
The use of catalysts for the promotion of chemical processes is well established, so much so that an entire industry has developed worldwide for the development, sale and use of catalysts in virtually every area of chemical processing.
The potential use of microwave energy in combination with catalysts has been known for over 30 years, however published information deals only with low-power laboratory-scale reactor systems, citing the hurdles of achieving temperature uniformity and other technical issues at larger scale. There remains a practical need for systems capable of operating and controlling these processes at larger scales, compatible with commercial operations.
Microwave reactors, or applicators, have been in use for several decades in a wide variety of applications. In the most general terms, the reactor is the device in which the microwave energy is applied to the material(s) to be processed. These processes may be thermal or non-thermal, since microwave energy is capable of inducing heating (thermal) effects in most materials and it is also capable of electronically coupling to many molecular structures by means of direct electron (non-thermal) excitation.
Nearly all the microwave reactors introduced to date are small in scale compared to many conventional industrial processes. The obstacle of increasing the scale of operation of these reactors has usually been met by simply increasing the number of reactors, thereby increasing the size of the system. In many cases (e.g. drying or cooking materials such as food) this enlarged “linearization” of the system is entirely acceptable and fits well with other associated parts of the process. Nevertheless, there remains a challenge of increasing the unit capacity of the reactor without necessarily simply making the system bigger or adding more parts.
The geometrical size of a microwave reactor is limited by several factors, depending upon how the electromagnetic field is managed within it.
Cavity reactors cannot be smaller than the minimum size required to sustain the lowest order resonant mode at the microwave frequency being used, and they cannot generally exceed a certain size related to the penetration depth of the microwaves in the material being processed; this latter restriction may be greatly modified by arranging the material to move within the reactor such that substantially all of the material is sufficiently exposed to the microwave energy.
Travelling wave reactors may be sub-resonant, however there arises the need to quickly move the material being processed through the reactor electromagnetic field while still allowing sufficient time for the process to be completed to the desired degree. These systems are usually conveyorized or pneumatically driven.
Finally, the unit capacity of a microwave reactor is related to the maximum power of the microwave source that can be employed; depending on the frequency of operation within the industrial microwave frequency bands, this power limit may range from a few tens of watts up to approximately 100 kW.
By means of the present disclosure, reactor designs are introduced that are capable of greatly increased material throughput, higher conversion efficiency and higher unit power capacity, hence significantly advancing the use of these systems for large-scale industrial applications.
One aspect of the present invention relates to a means of greatly increasing the unit capacity of microwave reactors that are of particular use in the processing of gases by means of plasma. The unit power capacity may be further significantly increased by providing a means of connecting more than one microwave generator to a single reactor.
A second aspect of the present invention relates to the combination of catalysts and microwave energy for the purpose of performing one or more chemical processes at a minimal energy cost. In the one instance, the combination is in the form of microwave plasma and catalysts, and in the other instance the combination is in the form of microwave energy (no plasma) and catalysts. This invention discloses beneficial and unique advantages of the synergy between catalysts and these (microwave energy and plasma) energy forms.
Plasmas consist of ions, electrons and charged molecular particles; plasma streams are highly chemically active due to their energetic species composition and are often self-catalytic, i.e. they can promote certain chemical operations at lower energy input than similar operations using non-plasma techniques.
Plasmas may be categorized as being either thermal or non-thermal, the distinguishing feature being the relative temperature of the gas with respect to the energy (equivalent temperature) of the electrons. Thermal plasmas are characteristically “hot”, meaning that the gas temperature is approximately equal to the electron temperature. Non-thermal plasmas (also known as non-equilibrium plasmas) are characterized as having gas temperature significantly less than the electron temperature.
In order to achieve minimum plasma energy, there are several operating conditions which must be simultaneously met:
Although it may not be practical to achieve all of the above conditions in every case, the maximum possible number of these conditions should be met in order to minimize reaction energy requirements.
In order to be commercially viable, plasma systems in this application must be capable of handling process gas volumes in the range of several hundred to several thousand Standard Liters per minute (SLPM). Furthermore, the plasma must operate in the non-thermal mode in order to achieve optimum energy efficiency. These constraints limit the plasma to essentially the microwave plasma type. Microwave plasma torches are capable of satisfying some of these constraints, however the presence of an electrode in this configuration leads to metal erosion, poor heat distribution and resultant added maintenance cost. The most satisfactory configuration is therefore one which has no electrode(s) and which generates a spatial plasma of sufficient volume and intensity to achieve commercial process flow rates.
Non-thermal plasmas produce ions, radicals and electronically excited species with internal energies that are often high enough to enhance plasma volume reactions. This can be attributed to the high threshold energies required to generate these species through electron collision processes. For ions and radicals, threshold energies of 5-20 eV are typically required and for electronically excited species, threshold energies are in the range of 1-10 eV. Vibrationally excited species are produced with the lowest threshold energies of 0.1-1 eV, hence the internal energies are too low to facilitate plasma volume reactions by themselves.
The importance of emphasizing the vibrationally excited species is that, of the three modes of molecular excitation (vibrational, rotational, translational), only the vibrational mode is non-thermal, i.e. no energy is consumed in the generation of heat, and hence it is the most energy efficient mode of excitation.
The simple configuration of a test tube mounted transversely (between the broad walls) in a rectangular waveguide, in which a gas is passed through the tube and is ionized (forming plasma) within the tube-waveguide intersection, is commonly used in laboratory situations. Such a configuration, however, is severely limited in size and gas throughput, being limited to typical gas flows of the order of a few liters per minute using microwave power levels of a few kilowatts. Such systems, although useful in laboratory operation, cannot practically be scaled up to industrial capacity.
A common improvement to the simple waveguide system referred above is the addition of a secondary, non-reagent gas flow upstream of the plasma (ionization) region, the purpose of said secondary gas flow being to form a vortex sheath which simultaneously constricts (stabilizes) the plasma gas to a narrow axial filament along the center of the reactor tube (where the microwave intensity is greatest) while forming a cooling sheath between the (hot) plasma and the tube wall, thereby protecting the tube from damaging thermal effects. This vortex arrangement, while providing plasma stability and thermal protection, often degrades the plasma process by introducing large quantities of non-reagent gas (such as Nitrogen, Argon, etc.) which must be subsequently removed from the product gas stream. Reactors such as described above can achieve complete gas conversion, however only within a narrow range of gas flow rate and power level; decreasing the power or increasing the gas flow leads to a rapid reduction ion gas conversion and, ultimately, to plasma extinction. For these reactor systems, the energy levels are in the range of 4-5 eV/molecule, indicating that these plasmas are not operating in the non-equilibrium mode (vibrational electron excitation) but rather in the thermal (plasma torch) mode.
The most significant improvement in microwave plasma operation came as the result of incorporating supersonic gas flow in combination with the simple waveguide plasma system described above. The characteristics of gas flow through a supersonic divergent nozzle are well understood and include a transfer of the gas rotational and translational energies into vibrational energy with a large increase in velocity. The energy transfer into the vibrational mode at the nozzle throat is accompanied by an extremely rapid cooling and drop in pressure sufficient to meet the necessary quench rate and pressure conditions described above for optimum (minimum energy) plasma operation.
The incorporation of the supersonic expansion nozzle and the waveguide plasma system results in a plasma apparatus wherein microwave energy contained in a waveguide conduit excites a plasma in a transversely-oriented second conduit, said second conduit within the boundary of the waveguide structure being essentially transparent to the microwave frequency being used such that microwave energy passes substantially unrestricted into the gas contained within the second conduit. The plasma so formed extends beyond the boundary of the waveguide, while being fully contained in the second conduit. Gas enters the reactor by means of an axial feed as well as by means of one or more tangential feeds. The purpose of the tangential feed(s) is to induce a vortex flow within the reactor plasma zone. Located immediately at the downstream end of the reactor is a supersonic nozzle of the type described earlier whose purpose is to quickly quench the plasma reactions. Thermal re-generation is controlled by adjusting the diverging nozzle angle to ensure critical heat transfer, i.e. plasma heat is transferred to the nozzle walls rather than being allowed to increase the gas temperature, thus reducing the gas flow to sub-sonic velocity. After leaving the nozzle the gas stream is further expanded at sub-sonic speed in a discharge vessel.
The arrangement in which the supersonic nozzle is located at the downstream end of the reactor as described above is preferred when the gas flow through the reactor is by means of presenting a vacuum pump or similar device at the exhaust end of the system, the significance being that the pressure in the plasma zone need not exceed atmospheric pressure; high pressure in the plasma zone acts to deter the formation of the plasma and may lead to erratic plasma operation.
Notwithstanding the improvements introduced by the above combination of supersonic expansion and microwave plasma, there remain the important restrictions of (i) introduction of a secondary vortex gas which requires subsequent removal, and (ii) restriction to low-pressure operation to avoid high gas pressure in the plasma zone.
In an attempt to overcome these further limitations, the supersonic nozzle may be moved upstream from the plasma zone; in this case the gas flow is maintained by applying high pressure at the nozzle inlet. Although the gas pressure is relatively high in the pre-supersonic, pre-plasma region, the action of the supersonic nozzle is to greatly reduce the pressure in the plasma zone, a desirable condition for plasma ignition and stability. The high velocity of the gas through the plasma region helps to meet the short-duration residence time in the plasma.
In order to confine and stabilize the plasma in the post-nozzle plasma region, a secondary gas is introduced to form a vortex sheath, however this secondary gas introduces the same limitation described earlier and hence represents a limitation to practical operation. Extensive laboratory tests using this system have confirmed the ability to operate in the non-equilibrium mode (with energy levels in the range of 0.6-0.7 eV/molecule) as shown in the following examples, however the typical small size of these systems, together with the limited power supplies in use, are a serious impediment to their commercial, large-scale use.
By means of the following examples the non-equilibrium plasma characteristics have been confirmed using the supersonic fixture with the nozzle mounted at the input end of the reactor. Also presented is the case of the same reactor operated without the supersonic nozzle shown in
All the above embodiments have been disclosed at various times in the open literature, all with the noted limitations involving the use of a secondary (vortex) gas. Furthermore, even in the case where the secondary gas is a reagent gas, because of the relatively high gas flows required to maintain an effective vortex effect, the large volume and location of the secondary gas around the reaction (plasma) zone leads to reduced gas conversion since much of the gas bypasses the active plasma region, a condition known as “gas slip”, in which case the gas conversion is typically not greater than 60% at the highest (rated) gas flows.
There remains, therefore, a need to be able to couple the largest microwave power sources (hundreds of kW) with sufficiently large reactors to achieve industrial-scale capacity while preserving the energy advantages of non-equilibrium operation and simultaneously achieving complete or nearly complete gas conversion.
By means of the present invention, this limitation in gas conversion has been overcome while maintaining all the advantages of supersonic gas expansion, and without the introduction of any secondary (non-reagent) gas. This improvement is based on the introduction of a reverse vortex gas flow in the post-nozzle plasma zone. The gas used to form the reverse vortex is the same composition as the plasma gas, hence eliminating the need for subsequent gas removal. The reverse vortex serves the purpose of providing a thermal barrier between the plasma and the reactor vessel wall, providing a pressure “containment” barrier for the plasma and ensuring that all the gas in the system is constrained to pass directly through the high-energy central region along the reactor axis as illustrated in
With reference to
The other fittings shown in
It will be immediately recognized by one who is knowledgeable in microwave science and engineering that the reactor system described generically in
Recognizing the potential limitations of reactor systems generically similar to that illustrated in
With reference to
With reference to
With reference to
The synergy between plasmas and catalysts is based on the fundamental principles of operation of each of these components. Catalyst operation requires the preparation of specific activation sites on the surface of the catalyst material; at these sites, the work function for a particular chemical operation is reduced, thus allowing the chemical operation to proceed with a reduced input energy or an equivalent reduction in operating temperature. The preparation of the catalyst activation sites may be carried out by several means, often involving the deposition of specific metallic molecular groups which are “tuned” to target molecules or ions in the process stream. Since catalyst activity is a surface phenomenon, catalyst effectiveness increases with the surface area exposed to the process stream and is negatively affected by any operation which occludes, blocks, abrades or otherwise neutralizes the catalyst material coating.
Plasma streams are highly chemically active due to their energetic species composition and are often self-catalytic, i.e. they can promote certain chemical operations at lower energy input than similar operations using non-plasma techniques. The synergy between plasmas and catalytic materials is at least partially intuitive since both are fundamentally defined by an energetic, charged, chemically active material composition.
Non-thermal plasmas produce ions, radicals and electronically excited species with internal energies that are often higher than the activation energies for thermal catalysis; these species can enhance plasma volume reactions. This can be attributed to the high threshold energies required to generate these species through electron collision processes. For ions and radicals, threshold energies of 5-20 eV are typically required and for electronically excited species, threshold energies are in the range of 1-10 eV. Vibrationally excited species are produced with the lowest threshold energies of 0.1-1 eV, hence the internal energies are too low to facilitate plasma volume reactions. However, activation energies for reactions involving vibrational species can be lowered when adsorbed to a catalyst surface; consequently, the vibrational state can be a significant contributor to the acceleration of catalysis. In addition, the energy required for surface adsorption of radical species may be lower than for adsorption of ground state gas molecules.
Several studies have revealed the synergistic effects of the plasma-catalyst combination, however these studies have focused on the use of very small-scale applicators (reactors), usually involving dielectric barrier discharge (DBD) plasmas which, for several reasons, cannot achieve economic large-scale operation. Although not capable of commercial scale of operation, these reactors have successfully demonstrated plasma-catalyst synergistic effects in dry reforming of methane and carbon dioxide over Cu—Ni/γ-Al2O3 where the result for the plasma-catalytic reaction was greater than the sum of the catalyst only and plasma only results. Hydrogen and carbon monoxide selectivities were also enhanced by the use of plasma catalysis. Synergistic effects have also been observed for other reactions including steam methane reforming of biogas over Cu—Ni/γ-Al2O3 catalysts, hydrogenation of carbon dioxide and destruction of toluene, benzene and hydrofluorocarbons.
By means of the present invention, these and other process reactions may be carried out at industrial scale and at minimum energy cost. The realization of this operation is made possible, as herein disclosed, by the introduction of a large-scale microwave plasma source in combination with an inhomogeneous catalyst structure.
Within the cavity (25) the chemical reaction is completed to the desired extent and the product gas exits via an exhaust duct (27).
Although the waveguide configuration illustrated in
With reference to
With reference to
For gas volume rates of the order of 100 SLPM, the required aperture diameter may not exceed 2 mm in order to ensure supersonic operation. Higher gas flow rates will allow a larger-diameter aperture to be used.
In another embodiment shown in
The plasma so formed within the cavity (40) exits via an exhaust conduit (45) and enters a second cavity (46) containing catalyst materials (47) in the form, for example, of powder, pellets or short, hollow cylinders but not limiting the catalyst structure to these forms. The second cavity (46) is disposed to operate as a fluidized bed (specifically a bubbling fluidized bed) in which the catalyst materials (47) are suspended and continuously intermixed in an expanded bed supported by the plasma gas flow from the reactor cavity (40). The design of fluidized beds is well known such that the size of the second cavity (46), the size and shape of the catalyst “particles”, the depth of the fluidized bed (47) and the gas characteristics can be used to produce the desired fluidized bed operating characteristics. The dimensions of the fluidized bed cavity (46) are further constrained to ensure that the cavity is below the cutoff of the microwave frequency being used, thus ensuring that the microwave energy is fully contained within the reactor cavity (40). The gas stream thereafter exits from the fluidized bed chamber (46) through an exhaust conduit (53) and may be further processed, cleaned or otherwise disposed.
The characteristics of the fluidized bed (47) are such that the individual catalyst “particles” are continuously circulated throughout the bed, with the fluidizing gas passing through the spaces between the “particles” such that the processing occurring in the bed, being between the process gas components and the catalyst materials, achieves an overall steady-state condition, and although the bed itself may not have completely uniform characteristics (such as temperature), the gas product passing through it, by virtue of the many possible random paths through the bed, will achieve a steady state condition. In order to assist in minimizing heat loss from the fluidized bed, an external insulation (48) may be added to the vessel.
Furthermore, by the nature of the disclosed system including the fluidized bed, the catalyst material may be periodically exchanged by opening a discharge pipe (49) through a gas interlock valve (50), and similarly adding new catalyst material through an inlet pipe (51) through a gas interlock valve (52).
The plasma so formed within the cavity (40) exits via a small-diameter aperture (45a) such that the gas velocity becomes supersonic; the gas is thereafter expanded in a nozzle (45b) before entering a second cavity (46) containing catalyst materials (47) in the form of powder, pellets or short, hollow cylinders. The second cavity (46) is closely affixed to the reactor cavity (40) such that the transit time of gas (plasma) exiting the supersonic nozzle (45b) is minimized, preferable to less than 1 millisecond. For example, at approximately Mach 2, the gas travels about 0.5 m in 1 millisecond, meaning that the second reactor must be located within 0.5 m from the first reactor (40). Once in the second reactor (46), the gas velocity rapidly decreases. The benefits of the supersonic nozzle include prevention of unwanted reverse reactions and isolation of pressure fluctuations in the second cavity (46) from the plasma environment in the first cavity (40). The second cavity (46) is disposed to operate as a fluidized bed (specifically a bubbling fluidized bed) in which the catalyst materials (47) are suspended and continuously intermixed in an expanded bed supported by the plasma gas flow from the reactor cavity (40). The design of fluidized beds is well known such that the size of the second cavity (46), the size and shape of the catalyst “particles”, the depth of the fluidized bed (47) and the gas characteristics can be used to produce the desired fluidized bed operating characteristics. The dimensions of the fluidized bed cavity (46) are further constrained to ensure that the cavity is below the cutoff of the microwave frequency being used, thus ensuring that the microwave energy is fully contained within the reactor cavity (40). The gas stream thereafter exits from the fluidized bed chamber (46) through an exhaust conduit (53).
The plasma so formed within the cavity (57) exits via a connecting conduit (60) before entering a second cavity (61) containing catalyst materials (62), for example in the form of powder, pellets or short, hollow cylinders. The second cavity (61) is closely affixed to the reactor cavity (54) such that the transit time of gas (plasma) exiting the first reactor (54) is minimized. The second cavity (61) is disposed to operate as a fluidized bed of catalyst material (62) (specifically a bubbling fluidized bed) in which the catalyst materials (62) are suspended and continuously intermixed in an expanded bed supported by the plasma gas flow from the reactor cavity (54). The design of fluidized beds is well known such that the size of the second cavity (61), the size and shape of the catalyst “particles”, the depth of the fluidized bed (62) and the gas characteristics can be used to produce the desired fluidized bed operating characteristics. The dimensions of the fluidized bed cavity (61) are further constrained to ensure that the cavity is below the cutoff of the microwave frequency being used, thus ensuring that the microwave energy is fully contained within the reactor cavity (54). The gas stream thereafter exits from the fluidized bed chamber (61) through an exhaust conduit (63).
The characteristics of the fluidized bed (62) are such that the individual catalyst “particles” are continuously circulated throughout the bed, with the fluidizing gas passing through the spaces between the “particles” such that the processing occurring in the bed, being between the process gas components and the catalyst materials, achieves an overall steady-state condition, and although the bed itself may not have completely uniform characteristics (such as temperature), the gas product passing through it, by virtue of the many possible random paths through the bed, will achieve a steady state condition.
Furthermore, by the nature of the disclosed system including the fluidized bed, the catalyst material may be periodically exchanged by opening a discharge pipe (64) through a gas interlock valve (65) and similarly adding new catalyst material through an inlet pipe (66) through a gas interlock valve (67).
In order to assist in minimizing heat loss from the fluidized bed, an external insulation (68) may be added to the vessel.
The operation of catalyst materials is based on the ability to deposit energy (or energized materials) in such a way as to interact with a process stream whereby the deposited energy allows chemical reactions to occur with less input energy and/or in a preferential manner so as to favor certain chemical reactions.
Microwave energy is able to interact directly with most materials either through electronic stimulation or through thermal excitation by means of dielectric heating. It has been shown that catalyst materials, when heated directly by microwave energy, demonstrate enhanced catalytic properties for many reactions. This enhancement occurs without the formation of a plasma. Although this effect has been demonstrated only at very small-scale, by means of the present invention the effect may be realized at much larger commercial scales of operation.
One fundamental limitation in the combined use of microwave energy and catalysts is due firstly to the characteristic non-uniform microwave energy distribution throughout the dielectric catalyst medium, and secondly due to the inherent non-uniform microwave field distribution within all microwave reactor systems.
As disclosed herein, several techniques may be employed to counter the effects of these non-uniformities, with the objective of producing a relatively uniform bulk heat distribution throughout the catalyst material. An important distinction here in reference to the bulk heat distribution is the recognition that, on a micro-scale, the temperature distribution within the catalyst material may be highly non-uniform due to the interaction of microwave energy with the catalyst metallic deposition sites, resulting in relatively higher temperatures at these sites.
Methodologies designed to mitigate the effects of these non-uniformities described herein may include, without limitation, the following:
It is recognized that some chemical reactions which one may wish to carry out using this invention require elevated pressures (compared to atmospheric pressure) within the reactor; in such cases, it may be advantageous to interpose a pump or compressor at the inlet (70).
In one embodiment as shown in
It may be beneficial in some operations to introduce additional gas material(s) into the reactor vessel (69) by means of separate inlet duct(s) (70).
As an example of another embodiment (
In another embodiment (
In another embodiment (
In a further development of this geometry as shown in
In another embodiment of the present invention according to the methodology above, the catalyst materials may be arranged in a stationary manner within the microwave reactor either in the form of concentric cylindrical tubes (each having different dielectric absorption properties) or arranged as stacked “pucks” (each having different dielectric absorption properties), said pucks comprising a catalyst material which is either mixed with or coated by a separate “promoter” material designed to selectively absorb microwave energy.
The processing system shown in
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2018/026044 | 4/4/2018 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 62483138 | Apr 2017 | US | |
| 62483150 | Apr 2017 | US |
