INTERNALLY COOLED IMPEDANCE TUNER FOR MICROWAVE PYROLYSIS SYSTEMS

Abstract

An internally cooled microwave stub tuner assembly with stubs having hollow ducts for receiving circulating cooling fluid while in operation. The microwave stub tuner assembly for a pyrolysis reactor includes at least one elongated hollow body plunger projecting into a waveguide cavity. Each of the hollow body portion of the plungers has at least one internal cooling duct for receiving a circulating cooling fluid and is adapted to be cooled by the circulating cooling fluid as the circulating cooling fluid enters the plunger, courses through each the internal cooling ducts and exits the plunger. Each plunger has a position adjuster for adjusting the position of the plunger within the waveguide cavity.

Claims

1. A microwave stub tuner assembly for a pyrolysis reactor, comprising: at least one elongated hollow body plunger projecting into a waveguide cavity; each said elongated hollow body plunger having at least one internal cooling duct for receiving a circulating cooling fluid and adapted to be cooled by said circulating cooling fluid as the circulating cooling fluid enters the plunger, courses through each said internal cooling duct and exits the plunger; and each said elongated hollow body plunger having position adjusting means for adjusting the position of the plunger within the waveguide cavity.

2. A microwave stub tuner assembly as in claim 1 wherein the circulating cooling fluid is in a closed circuit.

3. A microwave stub tuner assembly as in claim 1 wherein the circulating cooling fluid is in an open circuit.

4. A microwave stub tuner assembly as in claim 1 wherein the microwave stub tuner assembly further comprises means for monitoring and controlling the temperature of the circulating cooling fluid.

5. A microwave stub tuner assembly as in claim 1 wherein the microwave stub assembly further comprises means for monitoring and controlling a flow rate of the circulating cooling fluid.

6. A microwave stub tuner assembly as in claim 1 wherein said position adjusting means for adjusting the position of the plunger within the waveguide cavity is automatically adjusted for microwave impedance matching.

7. A microwave stub tuner assembly as in claim 1 wherein the number of elongated hollow body plunger is three or more.

8. A microwave stub tuner assembly as in claim 7 wherein the at least one elongated hollow body plunger is comprises three elongated hollow body plungers.

9. A microwave stub tuner assembly as in claim 1 wherein the diameter of the cooling duct within the elongated hollow body plunger is about (λ/6) where λ is the wavelength.

10. A microwave stub tuner assembly as in claim 7 wherein the axial distance between each plunger is about (λ/3) where λ is the wavelength.

11. A microwave stub tuner assembly as in claim 1 wherein each said elongated hollow body plunger is provided with more than one of the cooling duct.

12. A microwave stub tuner assembly as in claim 1 wherein the cooling ducts are all connected to a single cooling fluid circuit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Further features and advantages of the present invention will become apparent from the following detailed description, taken in combination with the appended drawings, in which:

[0028] FIG. 1 is a cross-section of a microwave pyrolysis system comprising a microwave pyrolysis reactor, a coupler and a stub tuner, in accordance with an embodiment;

[0029] FIG. 2 is a cross-sectional view 2 of a stub tuner assembly of FIG. 1 said cross-section being orthogonal to the longitudinal axis of the waveguide;

[0030] FIG. 3 is a cross-sectional side view of a stub tuner assembly of FIG. 1 along the outside of the stub tuner assembly;

[0031] FIG. 4 is a cross-sectional side view of the stub tuner assembly of FIG. 1 along the longitudinal axis of the stubs;

[0032] FIG. 5 is an isolated view of FIG. 4;

[0033] FIG. 6 is a perspective cut-away side view of the stub tuner assembly of FIG. 1.

DETAILED DESCRIPTION

[0034] FIG. 1 illustrates one embodiment of a microwave pyrolysis system 10 running on a 915 MHz microwave source for example. System 10 comprises a reactor or vessel 12, a coupler 14 and a stub tuner 16 assembly. It should be understood that the stub tuner assembly 16 is connectable to the source of microwaves or microwave generator (not shown) either directly or via a microwave waveguide. Reactor 12 is configured for performing chemical and/or physical reactions therein under the action of microwave energy.

[0035] It should be understood that the shapes, dimensions, inlets and outlets of reactor 12, coupler 14 and stub tuner assembly 16 are merely examples and may vary. For example, while coupler 14 and stub tuner assembly 16 are shown as configured to be connected substantially orthogonally to the longitudinal axis of reactor 12, it should be understood that other embodiments may be possible. The relative proportions of reactor 12, coupler 14 and stub tuner assembly 16 are for illustrative purposes and merely exemplary.

[0036] In another embodiment (not shown) there is no coupler 14, in other words no physical interface between the cavity defined within the reactor and the rectangular waveguide of stub tuner assembly 16. However, the absence of a coupler may make the microwave pyrolysis system 10 inadequate for performing chemical reactions that involve multiphase environments (solid, gas and/or liquid) that need to be contained inside reactor 12. Because of the absence of any physical barrier, the solid, gas and/or liquid may interact with the microwaves to produce hot spots, arcing (hot plasma) and failure of stub tuner assembly 16.

[0037] Since stub tuner is subjected to a high microwave power density and high electric field, the tendency towards arcing and hot spot production is high. Therefore, installation of an adequate coupler 14 is preferred to minimize arcing and failure of stub tuner assembly 16.

[0038] Before use, coupler 14 is assembled to reactor 12 by appropriate means such as nuts and bolts. In an embodiment, coupler 14 has a widened diameter in relation to stub tuner 16 and provides a transfer from a rectangular tubular waveguide shape to a generally cylindrical shape.

[0039] Reactor 12 is provided with appropriate inlets and outlets for the material to undergo microwave pyrolysis. An example of material level fill line 66 is shown. In some embodiments reactor 12 is hermetic and adapted to work under vacuum or pressure. Reactor walls 18 can be double-walled or otherwise jacketed to allow for a cooling or heating of the reactor walls 18.

[0040] In the illustrated embodiment, stub tuner assembly 16 is used for impedance matching and guiding the microwaves emitted by the microwave generator (not shown) up to coupler 14. Coupler 14 is used for propagating the microwaves coming from stub tuner assembly 16 into reactor 12. Reactor 12 is configured for receiving therein a product to be pyrolyzed which is heated by microwave heating.

[0041] Stub tuner assembly 16 will now be described in further detail by reference to FIGS. 1 to 6.

[0042] As illustrated, stub tuner assembly 16 comprises a waveguide cavity 20 enclosed within a microwave waveguide and on which three plunger (stub) housings 22 are attached. A lock nut 24 allows each plunger 26 to be fixed in a certain position within housings 22 and prevents leakage of microwave by ensuring good electrical contact between the plunger threaded section 46 and the housing screw section 48. A secondary lock nut 32 locks the first lock nut 34. While traditional designs would have a plunger threaded section of around ⅛″, the illustrated embodiment proposes a section of roughly >1.0″.

[0043] Advantageously, the axial distance between plungers 26 is a function of the wavelength 2, of the microwave field and typically is λ/3.

[0044] Typically, the depth that plunger 26 can reach in the waveguide cavity 20 is typically no more than λ/4.

[0045] In an embodiment, a cooling fluid is circulated inside each plunger 26 using a dual flow rotary union 35 which allows the cooling fluid to circulate inside plunger 26 while allowing rotation of the plunger 26 for tuning. The rotation of plunger 26 allows displacement of plunger 26 in the axial position going from a totally out position to a position that lands approximately in the middle of the waveguide cavity 20.

[0046] Plunger casing 36 allows containment of the microwave and the gap between the plunger 26 and the plunger casing 36 allows for complete or near complete electrical choking of the microwaves. Plunger 26 is a hollow shaft 40 with a cooling tube diameter proportioned at about (λ/6) where λ is the wavelength, to allow circulation of the cooling fluid inside plunger cooling tube 38.

[0047] In an embodiment, plunger 26 can be provided by more than one cooling tube so as to achieve more cooling.

[0048] Plunger 26 is constructed of a cast or machine material such as metal including steel, copper, aluminum, alloys any thereof or combination materials. In one embodiment, the plunger 26 can be coated with a low electrical loss material, such as silver.

[0049] Plunger cooling tube 38 is connected to the cooling fluid inlet 42 and forces cooling fluid to enter the tip of the plunger 26 and leave at the top of the plunger trough the rotary union 35 and fluid outlet 44. One of skill in the art will appreciate that the flow direction of the cooling fluid could also be reversed and that each plunger 26 may advantageously be connected in series to the cooling fluid flow.

[0050] In an embodiment the cooling fluid is circulated in an open circuit, for example by use of municipal water. In another embodiment, the cooling fluid is circulated in a closed circuit and connected to a refrigeration exchanger (not shown).

[0051] In an embodiment, to prevent arcing between the edge of plunger 26 and plunger casing 36, the minimum plunger length inside the plunger casing 36 is initially set to about one quarter of the internal height of waveguide and adjusted as required for proper impedance matching.

[0052] Thus, when impedance mismatch is observed between a load and a source, the stub tuner system of the present invention is used to compensate for the complex portion of the impedance. To this end, plungers 26 are inserted and adjusted in and out of a waveguide cavity 20 to affect the overall system's impedance and ensure that the complex component of the impedance is near 0 (horizontal on a Smith chart) to maximize the power transmitted in the microwave reactor 12.

[0053] Thus, when impedance mismatch is observed between a load and a source, the stub tuner assembly, plungers 26 can be displaced and adjusted deeper into the waveguide. In some embodiments additional plungers 26 as described above may also need to be added to further increase the inductance of the system.

[0054] It has been observed that dissipative (resistive) energy losses arising from inductive current in the plungers 26 generate heat and that the amount of heat dissipation that is required through is proportional to the depth of the plungers and therefore when high level of mismatch is recorded, larger amount of heat is generated and more cooling is consequently required.

[0055] In order to maintain the plungers 26 at constant temperature, the amount, flowrate and nature of the cooling fluid flowing in cooling tube 38 through hollow shaft 40 is chosen.

[0056] In one embodiment, the cooling fluid is constantly circulated by a circulating pump (not shown) and monitored by flowrate and/or temperature sensors (not shown) to relay this data to a controller (not shown). The flowrate and/or temperature of the cooling fluid is appropriately controlled and adjusted in real-time by the controller.

[0057] In an embodiment, the controller operatively links the source of fluid with a cooling device (not shown) for adjusting the temperature of the fluid to a desired temperature prior to entering cooling tube 38. In a further embodiment, the controller also operatively links the source of fluid to a variable speed circulating pump (not shown). Thus, desired temperature and flow rate of the cooling fluid may be chosen and in a preferred embodiment adjusted in real-time so as to cool the plunger 26 while the stub tuner 16 is in operation.

[0058] In one embodiment, each plunger 26 may be provided with a plurality of cooling tubes 38 circulating the cooling fluid. In the same or other embodiments, the length and diameter of cooling tubes 38 may be configured to generate more cooling towards the tip of the plunger 26 which is in the waveguide cavity 20. In some embodiments the cooling tubes 38 may be fluidly connected together so that a single inlet and a single outlet may be present.

[0059] In one embodiment, the stub tuner assembly further comprises at least one temperature sensor for sensing the temperature of plungers 26 via measurement of the cooling fluid as in exits plungers 26. In the same or another embodiment, the reactor 12 is provided with at least one flow sensor for sensing the flow of the temperature control fluid. It should be understood that the temperature sensor(s) and/or the flow sensor(s) can be installed at any adequate location to measure the temperature and/or the flow rate of the temperature control fluid, respectively.

[0060] In order to allow for moving the plungers in and out, dual fluid rotary unions 35 are installed at the tip of each plungers 26 which allows the plungers to be screwed in and out freely while being cooled down by the circulating cooling fluid.

[0061] Also, when high levels of impedance mismatch are observed, plungers 26 are usually moved inward deeper by screwing into the waveguide cavity 20 and out from casing 36.

[0062] To further prevent arcing between the end of the plunger and the lower end of the waveguide cavity, the present invention is using a plunger housing 22 with longer straight section than conventional arrangements so as to keep at least a section of at least about five times the distance between housing 22 and plunger 26. This serves to confirm that the microwave electrical field is completely or nearly completely choked and prevents from having electrical fields present in the waveguide cavity 20. Without this extended overlapping section, the plungers may leak out microwave into the waveguide cavity and create unwanted arcing.

[0063] The embodiments of the invention described above are intended to be exemplary only. The scope of the invention is therefore intended to be limited solely by the scope of the appended claims.