CN1310827C

CN1310827C - Chemical processing using non-thermal discharge plasma

Info

Publication number: CN1310827C
Application number: CNB028181026A
Authority: CN
Inventors: 帕斯卡尔·理卡托; 爱德华·豪斯顿; 理查德·克罗
Original assignee: Plasmasol Corp
Current assignee: Plasmasol Corp
Priority date: 2001-08-02
Filing date: 2002-08-02
Publication date: 2007-04-18
Anticipated expiration: 2022-08-02
Also published as: WO2003040027A9; CA2456202A1; WO2003040027A1; CN1555340A; JP2005519729A; EP1472179A2; EP1472179A4; KR20040029388A

Abstract

A method for activating chemical reactions using non-thermal capillary discharge plasma (NT-CDP) unit or a non-thermal slot discharge plasma (NT-SPD) unit (collectively referred to as 'NT-CDP/SDP'). The NT-CDP/SDP unit includes a first electrode disposed between two dielectric layers (8 and 9), wherein the first electrode and dielectric layers having at least one opening (e.g., capillary or a slot) defined therethrough. A dielectric sleeve (3) inserted into the opening, and at least one second electrode (2) (e.g., in the shape of a pin, ring, metal wire, or tapered metal blade) is disposed in fluid communication with an associated opening. A non-thermal plasma discharge is emitted from the opening when a voltage differential is applied between the first and a second electrodes. Chemical feedstock to be treated is then exposed to the non-thermal plasma. This processing is suited for the following exemplary chemical reactions as (i) partial oxidation of hydrocarbon feedstock to produce fictionalized organic compounds; (ii) chemical stabilization of a polymer fiber (e.g., PAN fiber precursor in carbon fiber production; (iii) pre-reforming of higher chain length petroleum hydrocarbons to generate a feedstock suitable for reforming; (iv) natural gas reforming in a chemically reducingatmosphere (e.g., ammonia or urea) to produce carbon monoxide andhydrogen gas; or (v) plasma enhanced water gas shifting.

Description

Chemical treatment using non-thermal discharge plasma

Technical Field

The present invention relates to plasma-assisted chemical processing, and in particular to the use of non-thermal capillary discharge plasma (NT-CDP) or non-thermal slot discharge plasma (NT-SDP) (collectively "NT-CDP/SDP") to energize specific chemical species more uniformly than conventional discharge techniques (e.g., arc, dielectric barrier discharge, and corona discharge) in order to achieve higher throughput and more energy efficient chemical transformations. Unlike conventional discharge techniques which tend to result in temporally and spatially non-uniform filament discharges, non-thermal capillary or slot discharge (NT-CDP/SDP) devices produce a stable diffuse plasma which helps ensure the most cost-effective processing within the same volume. The apparatus can also be specifically tailored to selectively initiate certain rates that determine the chemical reactions that can be readily cascaded to obtain the desired product. Energizing the system in this particular manner enables chemical reactions that would normally only occur at higher temperatures and pressures to occur at ambient conditions. The adjustment is achieved by changing the following: power; the composition and concentration of the reactants; composition and flow rate of carrier gas; (ii) temperature; pressure and/or reactor physical dimensions.

Background

The industrial importance of using electrical discharges to cause chemical reactions has long been well known and used for a long time. The oldest and most effective example of a chemical reaction occurring under electrical discharge is the generation of ozone. The generated ozone can be reacted with unsaturated hydrocarbons to synthesize ozonides, aldehydes and ketones. As published by henry de boe en jazz, london chapman and hall press in 1960 as arc discharge; as said in power regulation, typical early gas discharge devices operate by exposing different reactive gases to an arc (thermal plasma).

A recent development is the use of both thermal and non-thermal plasmas in chemical processing. U.S. patent No. 6,372,192 to Paulauskas et al describes one method of using plasma in the manufacture of carbon fibers. In this patent process, the first step in treating the carbon fibers is to convert Polyacrylonitrile (PAN) fibers to carbon graphite fibers using a 10 mhz frequency plasma in a low pressure oxygen-free atmosphere. However, this patent does not disclose or suggest the use of an oxygen-rich plasma to stabilize polyacrylonitrile fibers in the initial steps of the process, and teaches people to use a non-thermal plasma?

In recent years, research and development in the field of plasma-assisted fuel modification and fuel conversion has also made significant progress, primarily due to the interest in hydrogen fuel cells. For example, U.S. patent No. 6,322,757 to cohn et al and the references cited therein disclose plasma fuel converters such as reforming hydrocarbons with a plasma tube to produce a hydrogen-rich gas. U.S. patent No. 6,395,197 to Detering et al discloses a high temperature apparatus and process for the thermal conversion of light hydrocarbons (natural gas) into desired end products, particularly diatomic hydrogen and elemental carbon. Another patented invention is U.S. patent No. 6,375,832 to ilayasen et al, which teaches a method of chemically converting hydrogen rich gas (e.g., methane) and carbon rich gas (e.g., carbon dioxide) into a general liquid fuel. This plasma-assisted fischer-tropsch synthesis is carried out using a process in which an insulating barrier discharge is combined with a solid fluorite catalyst.

Another area of interest in plasma treatment is the plasma-excited surface treatment of polymeric materials to improve wet performance and/or surface adhesion, as described, for example, in the electrical characteristics of corona discharge surface treatment, published by L.A.Rosenthal and D.A.These, proceedings of the institute of Electrical and electronics Engineers, 1A-11, third volume 328-335 (1975, 6), S.Han, Y.Li, H.gold, G.gold, J.Li, J.Yoon, G.gold, surface and surface treatment layer technology, 93 volume 262-264 (1997), in the modification of polymer surfaces by plasma ion implantation, and in U.S. Pat. No. 6,399,159 to Grace et al.

Thus, there is a need to optimize chemical treatment with a non-thermal capillary discharge plasma or a non-thermal slot discharge plasma as described in U.S. patent No. 09/738,923, filed 12/15/2000 and U.S. patent No. 60/358,340, filed 2/9/2002. Both of the above patents are incorporated by reference herein.

Disclosure of Invention

The present invention relates to a method of enhancing chemical processing. In particular, the invention relates to a method of exciting a chemical reaction with a non-thermal-capillary discharge plasma/slot discharge plasma device. The non-thermal-capillary discharge plasma/slot discharge plasma device according to the present invention comprises a first electrode disposed between two insulating layers, wherein the first electrode and the two insulating layers have at least one opening (e.g., capillary or slot) therethrough. At least one second electrode (e.g., in the shape of a pin, ring, metal pin, wire, or tapered metal blade) is also provided in fluid communication with the opening. A non-thermal plasma discharge is radiated from the opening when a voltage difference is applied between the first and second electrodes. The chemical feed to be treated is then exposed to the non-thermal plasma. This treatment is suitable for the chemical reactions listed below: (i) partially oxidizing a hydrocarbon feed to produce functional organic compounds; (ii) chemically stabilizing polymer fibers (e.g., polyacrylonitrile fiber precursors in carbon fiber production); (iii) pre-reforming longer chain length petroleum hydrocarbons to produce a feed suitable for reforming; (iv) natural gas reforming in a chemically reducing atmosphere (e.g., ammonia or urea) to produce carbon monoxide and hydrogen; or (v) plasma enhanced water gas shift.

Drawings

The above features and other features of the present invention will become more readily apparent from the following detailed description and drawings illustrating embodiments of the invention. Like elements in the above-described figures are labeled with like reference numerals, and wherein:

FIG. 1a is a side perspective view of an embodiment of a circular non-thermal capillary plasma treatment device for chemically stabilizing (oxidizing) polyacrylonitrile fibers in accordance with the present invention;

FIG. 1b is a side cross-sectional view in a direction perpendicular to the longitudinal axis of the device of FIG. 1 a;

FIG. 1c is an enlarged longitudinal cross-sectional view of one capillary ring electrode of the device of FIG. 1 b;

FIG. 2a is a cross-sectional view of an embodiment of a gas phase non-thermal capillary discharge plasma assisted chemical treatment apparatus according to the present invention;

FIG. 2b is an enlarged view of one capillary of the device of FIG. 2 a;

FIG. 3a shows a reaction from NH₃In the formation of H₂An exemplary graph of the relationship between time and hydrogen detector reading;

FIG. 3b is a graph showing the relationship between time and hydrogen concentration;

FIG. 4a is a side perspective view of an embodiment of an annular non-thermal-bath discharge plasma treatment device for chemically stabilizing (oxidizing) polyacrylonitrile fibers in accordance with the present invention;

FIG. 4b is a side cross-sectional view in a direction perpendicular to the longitudinal axis of the device of FIG. 4 a;

FIG. 4c is an enlarged longitudinal cross-sectional view of one longitudinal wire electrode of the device of FIG. 4 b.

Detailed Description

The present invention relates to a method of stimulating (catalysing) a chemical reaction. The discharge in the volume exposing the chemical feed to the plasma may improve the yield and/or energy efficiency of a particular chemical transformation. Gaseous, liquid, aqueous and/or solid precursors can be processed. Some illustrative examples of chemical reactions enhanced by exposure to non-thermal-capillary discharge plasma or slot discharge plasma are provided below, where each process is accompanied by a specific example reaction.

(i) Partial oxidation of hydrocarbon feeds produces functional organic compounds such as alcohols, aldehydes, ketones, and carboxylic acids.

For example

(ii) The polyacrylonitrile precursor becomes a chemically stable "oxidation" in the carbon fiber production process.

Examples of the invention

(iii) Pretreatment ("cracking") of longer chain length petroleum hydrocarbons to produce a feed suitable for retrofitting.

Examples of the invention

(iv) Natural gas is reformed in a chemically reducing (ammonia or urea) atmosphere to produce carbon monoxide and hydrogen.

Examples of the invention

(v) The water gas shift reaction is enhanced with plasma.

Examples of the invention

In processes (i) and (ii) above, the non-thermal oxidation ("cold combustion") of the hydrocarbon feed is excited with a non-thermal-capillary discharge plasma or a non-thermal-slot discharge plasma. The plasma generates the following oxidizing species from the surrounding air into the gas stream: oxygen atom (O: (¹D) Hydroxyl (OH), ozone (O)₃) And a peroxy group (HO)₂). These highly reactive species selectively oxidize hydrocarbon molecules to produce the desired product from the reaction. In the case of example (i), the desired product is CH₃(CH₂)_nCH₂OH。

The reaction in (iii) above is preferably carried out in a chemically neutral plasma environment. The term "chemically neutral" refers to an environment with a chemically inert carrier gas such as, but not limited to, helium, and is the result of direct impact decomposition of electrons. The aboveprocesses (iv) and (v) preferentially occur in the chemically reduced plasma, i.e., the plasma that tends to increase the number of electrons of the target chemical. Ammonia or urea may be added to the gas stream (reduction as opposed to oxidation) to produce an electron rich plasma suitable for enhancing the reduction of hydrogen to hydrogen in methane (process (i)) and water (process (ii)).

Figures 1a-1c all show different views of examples of a circular non-thermal-capillary discharge plasma treatment device particularly suitable for chemically stabilizing (oxidizing) polymer fibers such as polyacrylonitrile fibers. The device comprises a second electrode arranged between two insulating layers 8,9, said two insulating layers forming a hollow tube. The second electrode 2 is selected to have a desired coefficient of expansion. Although the treatment device is shown and described as being tubular, other shapes are contemplated and are within the scope of the present invention. Disposed around the outer insulating layer 9 is a high voltage bus 5, such as a wire mesh or metal sheath.

Figure 1b is a side cross-sectional view perpendicular to the longitudinal axis of the processing device of figure 1 a. As is clearly shown in fig. 1b, the plurality of capillaries 4 extend radially outwardly through the insulating layer 8, the second electrode 2 and the opposite insulating layer 9. Insulating sheaths 3, for example made of quartz, are inserted into the respective capillaries 4, and the pin electrodes 1 are embedded in the respective insulating sheaths 3, so that the pin electrodes 1 are insulated from the second electrode 2. The high voltage bus 5 connects the array of pin electrodes 1 to a common high voltage power supply (HV). In an alternative configuration, the geometry of the electrodes may be different and does not necessarily require insertion into a capillary tube, so long as the electrodes are in fluid communication with the capillary tube. Some other shapes of capillary discharge structures are shown and described in U.S. patent application No. 09/738,923. FIG. 1c is an enlarged view of one of the capillaries of FIG. 1 b.

In operation, the polyacrylonitrile fibers 6 pass through the channels within the tube and are subjected to the generated non-thermal-capillary discharge plasma. The polyacrylonitrile fiber 6 is subjected to a non-thermal-capillary discharge plasma treatment in the treatment apparatus 10 while the stabilized polyacrylonitrile fiber 7 emerges from the opposite end.

Fig. 2a and 2b are two views of a non-thermal-capillary discharge plasma gas-phase chemical treatment apparatus according to the present invention. Referring to the cross-sectional view in fig. 2a, there is a set of capillaries 20 through the insulating plate 11. The insulating sleeve 12 is inserted into each capillary 20 to form a high-insulation current-limiting capillary. Embedded in each capillary 20 is a pin or needle electrode 10. Electrically connecting a set of pin or pin electrodes to a common high voltage power supply is a high voltage bus 13 such as a wire mesh or metal sheath. An insulating plate 14 made of, for example, quartz, glass, or ceramic is used for insulating the electrode plate 15. The in and out transition ducts 16, 17 pass the gas undergoing treatment through the reactor volume 21 transversely to the array of capillary plasma nozzles. The closed manifold 18 allows gaseous chemical reactants to be injected into the process gas stream after passing through the needle electrode 10 and capillary 20 directly through the plasma nozzle. Element 19 is an auxiliary reactant gas inlet. In a preferred embodiment, the system can easily ramp up plasma power from about 500 watts to 10 kilowatts. The processing means preferably selects the radio frequency power supply. The preferred peak-to-peak voltage across the reactor gap required for the carrier gas is between about 5 kilovolts and about 50 kilovolts.

Fig. 3a and 3b are graphical illustrations of experimental results of non-thermal-capillary discharge plasma assisted generation of hydrogen gas from isooctane and ammonia vapor in a nitrogen carrier gas. The discharge was started after an equilibrium time of 300 seconds to ensure that the reduction state was stable. In particular, FIG. 3a graphically represents a slaveNH₃In the formation of H₂The relationship between the time during the test and the hydrogen detector reading (mA). The test was carried out at a power of 200W, NH₄OH concentration of 15M, N₂Is carried out at a flow rate of 11L/min. FIG. 3b shows the time-hydrogen concentration (in ppM) curve of the test results. This test was carried out at a power of 2000W, NH_3(aq)OH concentration of 15M, N₂Is carried out at a flow rate of 11L/min.

These experimental results demonstrate that the use of a non-thermal-capillary discharge plasma structure according to the present invention is due to the plasma (N) alone₂Trace) and only minimal hydrogen is formed from the iso-octane improvement within the chemically neutral plasma (iso-octane trace), the interference, if any, being very small. The trace of ammonia indicates that a large amount (-1000 ppmV) of hydrogen is formed due to autocatalytic non-uniformity of ammonia. The maximum production due to synergistic effects was found when using isooctane in the presence of ammoniaHydrogen (1500 ppmV) in an amount. Gas chromatography and mass spectrometric analysis of the product stream also showed that a number of plasma-assisted pre-modifications (fragmentation) were combined with this hydrogen formation. These results, when optimized, can provide a cost-effective method of generating hydrogen from condensed fuel.

The non-thermal-capillary discharge plasma chemical treatmentmethod according to the present invention is superior to conventional thermal and/or catalytic methods because of the substantial reduction in power consumption and the reduction in catalyst consumption over time. The power consumption is reduced because it is not necessary to heat a large volume of gas in order for the transition to occur. Furthermore, non-thermal-capillary discharge plasma chemical treatment is also advantageous compared to other plasma treatments such as Dielectric Barrier Discharge (DBD) and Corona Discharge (CD). The reason is that the relatively large amount of diffuse plasma obtained with the non-thermal-capillary discharge plasma is advantageous for substantially uniform and efficient chemical treatment. The illustrated chemical treatment is for illustrative purposes only and is not meant in any way to limit the scope of the invention to other chemical treatments.

Fig. 4a-4c illustrate exemplary embodiments of a non-thermal-capillary discharge plasma vapor phase chemical processing apparatus according to the present invention. This embodiment is the same as the processing apparatus shown and described in fig. 2a-2c, except that a slot discharge structure is used instead of a capillary discharge structure. The cell discharge structure in fig. 4a-4c is particularly suitable for chemically stabilizing polymer fibers such as polyacrylonitrile fibers. Elements that function identically to those in figures 2a-2c are labeled with like numerals. The grooves 4 shown in fig. 4a are arranged substantially parallel to the longitudinal axis. Furthermore, the grooves 4 can also be arranged in a helical or substantially perpendicular direction to the longitudinal axis of the reactor. Electrodes are inserted into the respective slots. By way of example, the electrodes may be metal wires, complementary in shape to the associated slots and may be partially inserted, buried or abutted against the slots. In yet another embodiment, such electrodes may betapered blades. Other shapes for such a slot discharge structure are described in U.S. patent application No. 60/358,340, which is incorporated by reference herein in its entirety. The surface area of the plasma radiation of such a slot discharge structure is broader than that of a capillary discharge structure.

While there have been shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit and scope of the invention. For example, it is readily contemplated that various combinations of substantially the same elements and/or steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. It is also contemplated and intended that replacement of a component in one of the illustrated embodiments with a component in another is also contemplated and intended. It is also to be understood that the drawings are not necessarily drawn to scale and that the drawings are merely illustrative of certain concepts. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.

Including all references, publications and patents mentioned herein, are hereby incorporated by reference.

Claims

1. A method of stimulating a chemical reaction which is the partial oxidation of a hydrocarbon feed to produce a functional organic compound, or the chemical stabilization of polymer fibers, or the pre-reforming of increasing the chain length of petroleum hydrocarbons to produce a feed suitable for reforming, or the reforming of natural gas in a chemically reducing atmosphere to produce carbon monoxide and hydrogen, or a plasma enhanced water gas shift, using a non-thermal discharge device comprising a first electrode disposed between two insulating layers, the first electrode and both insulating layers having at least one opening therethrough and at least one second electrode disposed in fluid communication with the opening, the method comprising the steps of:

applying a voltage difference between the first and second electrodes to produce a non-thermal plasma discharge from the opening;

the chemical feed is exposed to a non-thermal plasma radiating from the opening.

2. The method of claim 1, wherein said opening is a capillary tube and said device further comprises an insulating sleeve inserted within said capillary tube.

3. The method of claim 2, wherein said opening passes radially outward through said first electrode and both insulating layers.

4. The method of claim 2, wherein said second electrode is a metal pin or ring.

5. The method of claim 1, wherein said opening is a slot.

6. A method according to claim 5, wherein the openings are provided in a longitudinal direction, in a helical direction or substantially perpendicular to the longitudinal axis.

7. A method according to claim 5, wherein said second electrode is a wire or a tapered metal blade.

8. The method of claim 1, further comprising a voltagebus connecting the second electrode to a voltage source.

9. The method of claim 8, wherein said voltage bus is a wire mesh or metal sheath.

10. The process according to claim 1, characterized in that the polymer fibers are polyacrylonitrile precursors used in the production of carbon fibers.

11. The method according to claim 1, characterized in that the chemical reducing atmosphere is ammonia or urea.