More particularly, the invention relates to a method of optimising the heating efficiency of microwave energy transmitted to a fluid substance in a cavity, wherein the amount of microwave energy reflected from the cavity is minimised by controlling the level of a fluid in the cavity. The invention also relates to an apparatus for optimising the heating efficiency of microwave energy transmitted to a fluid substance in a cavity.

Background of the Invention The use of microwave technology is advantageous for heating substances having suitable dielectric properties. Apart from dissipating electromagnetic energy rapidly and directly into the processed material, microwave technology suits the use of a remotely operated hot cell environment since the heating means can be located outside of the cell.

Over the last two decades several studies have been published on the use of microwave heating for heating fluidised beds, particularly in high temperature applications [Kuester JL (1986): Conversion of cellulosic wastes to liquid hydrocarbon fuels, Vol 5: Microwave heating of fluidized bed reactors-Pyrolysis and calcination applications, (Final report by Arizona State University, Arizona for U. S. Department of Energy) DOE/CS/40202-Tl9-Vol. 5] and [Baysar A (1992) Microwave heating applications of fluidised beds: High-purity silicon production. PhD thesis, Arizona State University].

These studies investigated the operational reliability and feasibility of the process, the equipment developed for it, as well as the interactions between various materials and microwaves at elevated temperatures. Microwave energy can be used effectively to heat materials encountered in the nuclear industry such as oxides, nitrates, sulfates, water, carbon and many other dielectric materials [McGill SL, Walkiewicz JW, Clark AE (1995) Microwave Heating of Chemicals and minerals, United States Department of Interior and Bureau of Mines, Report of investigations RI-9518].

US patentNo 5932131 describes a microwave oven with different phase microwave streams. A microwave oven is disclosed in which the structure of the waveguide is improved such that the variation of the impedance is minimised in spite of the variation of the load. The system involves a series of slot radiators to feed several waves with different phases.

US patent No 6020580 describes a microwave applicator having a mechanical means for tuning. The microwave applicator includes an elongated cylindrical chamber for processing material therein. The cross sectional area of the elongated cylinder can be mechanically adjusted to control and maintain the microwave field uniformity and the resonant mode during processing of the material.

US patent No 6034362 describes a circularly polarised microwave energy feed. The invention discloses a method of matching energy into a enclosure providing circularly polarised energy at constant magnitude but continuously rotating phase.

US patent No 6074533 describes a method and apparatus for optimisation of energy coupling for microwave treatment of metal ores and concentrates in a microwave fluidised bed reactor. This invention applies the same method as WO 98/05418.

US patent No 6080977 describes an apparatus for concentrating salt containing solutions with microwave energy. The invention involves treatment of pourable materials in a cylindrical drum delivering microwave power through the upper portion of the cavity.

To achieve maximum energy transfer to the load, the effective length of the cavity is changed by rising or lowering the drum which is connected to a microwave horn with a specially designed seal.

WO publication No 98/05418 describes a method and apparatus for microwave treatment of metal ores and concentrates in a fluidized bed reactor. The invention discloses a method and apparatus for treating particulate material, comprising a reaction chamber tapering from an upper to lower position. This configuration propagates the microwave energy into the reaction zone where the powder is fluidised.

WO publication No 20024228-A1 describes a microwave applicator for heating thin loads. A specially designed microwave chamber allows applying microwaves to fluidised

US patent No 4855695 describes an automated microwave tuning system for a de- emulsifier. The reflected energy is measured and a phase shifter is controlled to obtain minimum reflection.

US patent No 4967486 describes a microwave-assisted, batch type fluidised bed processor. The fluidised bed processor involves development of randomly-oriented reflected waves within the vessel. This specification claims to provide better operational efficiency than uni-directional microwave processors.

US patent No 5079507 describes an automatic impedance adjusting apparatus for microwave load and an automatic impedance adjusting method therefor. The method involves the use of a three-stub tuner and an impedance measurement system. The measured impedance is used to calculate the stub lengths to achieve impedance at a predetermined value. The stubs of the tuner are adjusted using a motorised mechanism.

US patent No 5250773 describes a microwave heating device. The publication further discloses the use of a cylindrical applicator with a moving upper end to alter the physical size and shape of the cavity to maintain resonance.

US patent No 5399977 describes a microwave power source apparatus for a microwave oscillator, comprising means for automatically adjusting progressive wave power and control method therefor. This patent applies a similar method as that disclosed in US patent No 5079507.

US patent No 5621331 describes an automatic impedance matching apparatus and method. The method involves the use of a multi-stub tuner positioned between the microwave source and the load. The system determines the reflection coefficient associated with the load and scattering parameters of the multi-stub tuner to adjust the stubs until the magnitude of the reflection coefficient is less than a predetermined value.

The stubs of the tuner are adjusted using a motorised mechanism.

US patent No 5837978 describes a radiation control system. The publication discloses automatic control of the location of the microwave antenna in order to maintain the cavity in resonance.

beds. The invention discloses a method of directing microwave energy to a preferential section in the cavity.

Japanese Patent No JP 04184890 describes a microwave heating device with constant voltage standing-wave ratio. Microwave reflection is detected and a motorised compensation device attached to the waveguide is controlled to maintain desired power coupling level.

Japanese Patent No JP 2001033512-A describes a source pull measurement tuner for a microwave transistor. The document discloses an impedance converter that has metal parts that are movable perpendicularly and parallel with respect to central conductors.

Japanese Patent No JP 07078681-A describes a microwave oven having a controller that responds to microwave reflection gathered by a microwave directional coupler. The detected value is used to control the driver system to rotate the antenna.

Japanese Patent No JP 051009667-A describes a microwave matching device that has an automatic impedance matching unit. The system detects a difference in impedance between the generator and the load and it adjusts the extent to which stubs are extended to control the wavelength of the microwaves.

Japanese Patent No JP 03174802-A describes the automatic adjustment of impedance of a microwave load. The document discloses the detection of impedance and adjustment of the impedance at the microwave transmission line. It is particularly designed for plasma applications.

US patent No 4928077 describes a tunable microwave coupler with mechanically adjustable conductors.

US patent No 4324965 describes a microwave heating method and apparatus including adjustable tuning members.

US patent No 4247828 describes a reciprocating piston tuning mechanism for a microwave oscillator.

All the above documents describe the use of one or more of the following three approaches to solve the problem of matching a microwave source to a load: 1. the use of matched loads for custom designed cavities; -2. the use of adjustable short circuits; and 3. the use of stub-tuners to control the frequency of the microwaves.

The first approach does not provide the flexibility required for the continuous tuning of microwaves during the course of the heating process. This approach requires the use of a system which is designed for an average or approximated set of process conditions. It is usually integrated with one of the other two impedance matching approaches.

The second approach requires the incorporation of an adjustable short circuit or a plunger in the calciner cavity. This approach may also involve the changing, by mechanical means, of the shape of the treatment cavity. The approach is unsuitable for high temperatures, for corrosive substances or for hermetically sealed systems.

The third approach involves the use of an automatic motorised impedance matching apparatus (a so-called"stub tuner") in conjunction with an impedance analyser. The device is usually located inside a wave guide. Applicability of this approach is limited by the long response time of the automatic stub tuner. This method of control is for instance too slow to follow the fluctuations in the reflection of microwaves that occur in a fluidised bed.

The known methods of microwave tuning all have the disadvantage that they require elaborative applicator modifications.

There accordingly exists a need for a method and an apparatus for matching or coupling microwave energy transferred from a magnetron to a load, which does not require any substantial modification of the heating cavity or of the microwave transmission system.

Furthermore, because fluidised bed loads are subject to conditions that rapidly change the load characteristics, there exists a need for a method and apparatus for matching the energy transferred to a liquid or fluidised solids bed which lends itself to the use of a relatively short response time, where required.

Object of the Invention It is an object of the present invention to overcome or substantially ameliorate at least one of the above disadvantages.

Summary of the Invention According to a first aspect of the present invention, there is provided a method of increasing the heating efficiency of microwave energy transmitted to a fluid substance in a heating cavity, wherein the method includes the steps of - measuring an increase in microwave energy reflected from the heating cavity; and - adjusting the level of the fluid substance in the heating cavity.

As used in this specification, the term fluid substance is to be taken to include a fluidised bed and any liquid substance or particulate substance which behaves like a liquid. The invention is to be understood as also including in its scope a solid object suspended or submerged in the fluid substance.

According to a second aspect of the present invention, there is provided an apparatus for increasing the heating efficiency of microwave energy transmitted to a fluid substance in a heating cavity, comprising - means for measuring an increase in microwave energy reflected from the fluid substance; and - means for using the increase to adjust the level of the fluid substance.

According to a third aspect of the invention, there is provided a system comprising: - a microwave generator ;

a heater for containing and heating a fluid substance; a microwave wave guide for guiding microwaves generated by the microwave generator to the fluid substance; a detector adapted to detect a parameter associated with the height of said fluid substance in said heater when said system is in operation, and to emit a signal related to said parameter; control means adapted to receive said signal, to compare it with a desired standard and, in the event of a difference existing between the said signal and said desired standard, to issue a control signal for adjusting the level of the fluid substance in the heater.

According to a fourth aspect of the invention there is provided a microwave heating system comprising: - a microwave generator ; - a calciner for containing and heating a fluid substance; a microwave wave guide for guiding microwaves generated by the microwave generator to the fluid substance; - a detector adapted to detect a parameter associated with the height of a level of said fluid substance in said heater when said system is in operation, and to emit a signal related to said parameter; control means adapted to receive said signal, to compare it with a desired standard associated with a desired level of said fluid substance inside said calciner and, in the event of a difference between said signal and said desired standard, to issue a control signal for adjusting the level of the fluid substance so as reduce such difference.

According to a fifth aspect of the invention, there is provided a system comprising : - a microwave generator;

a calciner defining a heating cavity for containing and heating a fluid substance; a microwave wave guide to couple microwaves generated by said microwave generator with said fluid substance; a detector adapted to detect the height of said fluid substance when said system is in operation, and to emit a signal related to said height; means for comparing said signal with a desired standard associated with a desired level of the fluid substance; and means for changing, in the event that said signal differs from said standard, at least one of the height and volume of the fluid substance in said heater whilst maintaining the total length of the said heating cavity substantially constant.

The system in accordance with this aspect of the invention may comprise a container for containing a second fluid substance, and said container may be hydraulically coupled to said fluid substance, whereby the level of the fluid substance in the calciner may be controlled by either admitting fluid substance from said container or by discharging fluid substance to said container.

According to a sixth aspect of the invention, there is provided a method of heating a fluid substance, including the steps of : generating microwaves in a microwave generator; - providing a bed of said fluid substance in a heating cavity of a calciner; guiding microwaves generated by the microwave generator to the fluid substance; detecting a parameter associated with the height of said fluid substance in said calciner when said system is in operation; - comparing said signal with a desired standard; in the event of a difference existing between said signal and said desired standard, issuing a control signal for adjusting the level of the fluid substance in said calciner.

According to a seventh aspect of the invention, there is provided a method of heating a fluid substance, including the steps of : generating microwaves in a microwave generator; - providing a bed of said fluid substance in a heating cavity of a calciner ; guiding microwaves generated by the microwave generator to the fluid substance; detecting the height of said fluid substance in said calciner when said system is in operation; emitting a signal related to said height; comparing said signal with a desired standard; in the event of a difference existing between said signal and said desired standard, issuing a control signal for adjusting at least one of the height and volume of the bed of said fluid substance in said calciner whilst maintaining the total length of the said heating cavity substantially constant.

According to an eighth aspect of the present invention, there is provided a method of increasing the amount of microwave energy absorbed by a fluid substance in a microwave cavity, wherein the method includes the steps of locating the substance in the cavity; if necessary, creating conditions within the cavity whereby the substance becomes a fluid substance; - irradiating the fluid substance with microwaves; - measuring the intensity of microwaves reflected by the fluid substance; and adjusting at least one parameter selected from the group consisting of the height of the fluid substance, the diameter of the fluid substance, the volume of the fluid substance, the geometrical shape of the fluid substance, the volume of a free space above the fluid substance, height of the said free space, the diameter of the said free

space, the geometrical shape of the said free space, to reduce the intensity of microwaves reflected by the fluid substance.

According to a ninth aspect of the present invention, there is provided an apparatus suitable for increasing the amount of microwave energy absorbed by a fluid substance in a microwave cavity, comprising: - a microwave generator; - a calciner defining a heating cavity for containing and calcining a fluid substance; - means for irradiating the fluid substance with microwaves; - means for measuring the intensity of microwaves reflected by the fluid substance; and - means for adjusting at least one parameter selected from the group consisting of the height of the fluid substance, the diameter of the fluid substance, the volume of the fluid substance, the geometrical shape of the fluid substance, the volume of a free space above the fluid substance, height of the said free space, the diameter of the said free space, the geometrical shape of the said free space, to reduce the intensity of microwaves reflected by the fluid substance.

The method in accordance with the first aspect of the invention may include the step of, and the apparatus in accordance with the first aspect of the invention may include means for adjusting the level of the fluid in the cavity in such a way that a minimum portion of microwave energy transmitted to the heating cavity is reflected therefrom. It will be appreciated that, when a minimum portion of microwave energy is reflected from the heating cavity, a maximum portion thereof is utilised to heat the substance.

The cavity may be used as a calciner, as a drier or as a chemical reactor.

Alternatively it may be used for the sterilisation of objects, or for any other use in which a substance or an object placed inside the cavity is to be heated.

The substance inside the heating cavity may be a powder, which may be present as a fluidised bed. Conveniently, the powder may be used to indirectly heat a solid object,

which may be suspended in the fluidised bed, or may be covered by it. The bed may comprise a mineral or a material such as aluminium hydroxide which, upon heating, is converted to a dehydrated or dried material or form or which decomposes to form some other material or substance.

The present invention provides a new and useful method for coupling microwave energy in a fluidised bed reactor.

The microwave energy may be generated by a microwave generator. The amount of energy absorbed by the substance or object in the cavity may be controlled by dynamically changing the height of a bed in the cavity so that a maximum portion of the energy emitted by the microwave generator is absorbed by the bed during processing.

Fundamental principles of electromagnetic phenomena dictate that, in a heating cavity, the absorption, by a load, of microwave energy emitted by a microwave source, is directly related to the dimensions and the dielectric properties of the load and the cavity.

In accordance with these principles, the maximum power absorption for a particular material occurs at certain load or bed heights in a heating cavity. There may be several bed heights at which maxima may occur. The maxima may be the same or different.

From a processing point of view, dielectric properties are a function of the temperature and they may vary with variations in the chemical and physical properties of the bed. Apart from temperature dependent dielectric properties, fluidisation of particular materials may also cause changes in the dimensions of the powder bed as a function of temperature (such as thermal expansion) and the flow rate of the fluidisation gas (resulting in an expansion of the bed volume and the formation of larger bubbles).

As a result of all these changes that occur in the dimensions and dielectric properties of a fluidised bed undergoing physical and chemical changes, the microwave heating of a material in a fluidised bed requires the continuous tuning of the microwave in an effort to maximise the absorption of energy by the bed.

The invention does not require the introduction of a mechanical or an electronic device into the microwave cavity. The invention also does not interfere with the fluidisation process by altering the course of the process.

The invention thus involves the matching of the impedance of a microwave generator to that of the fluidised bed of a powder in sympathy with any changes in the physical, chemical and dielectrical properties of the powder during the course of processing.

It has been found by the inventors that maximum energy transfer can only be achieved at certain bed depths. It was also observed that the optimum bed depth changed due to changing properties of the powder. At elevated temperatures, dimensional changes in the calciner due to thermal expansion of the components also introduce a shift in the microwave energy coupling.

The conventional approach to impedance matching in a fluidised bed is to use a motorised autotuner system which involves placing tuning stubs between the microwave generator and the load. It was observed during the experiments that such a motorised tuner system is not fully capable of maintaining a good match as the above mentioned changes occur simultaneously at the same time in a dynamic fashion.

The present invention thus offers a method of matching a microwave generator to a fluidised bed by adjusting the level of the fluidised powder bed in the heating cavity as the properties of the bed change.

If required, the method or apparatus of the invention may conveniently be used to correct major fluctuations in the level of a fluidised bed heated by microwaves, such as those caused by expansion of the equipment as a result of heating, or those caused by volume changes occasioned by chemical reactions taking place in the bed as a result of an increase in temperature. Alternatively, if required, the method or apparatus of the invention may conveniently be used to correct minor fluctuations in the level of a fluidised bed heated by microwaves, such as those caused by bubbles formed by the fluidisation gas. As another alternative, the method or apparatus of the invention may conveniently be used to correct both major and minor fluctuations in the level of a fluidised bed heated by microwaves.

The present invention does not involve adjustment of the cavity dimensions by any mechanical means.

The apparatus of the present invention may be integrated with a discharge system of the fluidised bed.

The apparatus of the present invention may be combined with an impedance analyser to achieve continuous tuning during the process.

The apparatus of the present invention may be combined with an impedance analyser and 3-stub autotuner to cover the whole"Smith Chart"for tuning capability.

This control may be carried out in an automated manner by using a microwave reflection measurement device or an impedance analyser. The signal from the microwave reflection device or the impedance analyser may be processed with a computer and on or off signals may be sent to the vacuum or pressure control valves.

In another embodiment of this invention the level control means of the invention may be combined with a 3-stub autotuner or with an impedance analyser system. The current invention enables the autotuner system to operate in a smaller range of tuning parameters by adjusting the reflected power to the levels which an autotuner system can handle.

The present invention may be applied in a range of fields, including in the waste, mineral, food, pharmaceutical and chemical processing industries where a microwave- heated fluidised bed is used.

The tuning method according to the invention does not involve any mechanical tuning device in the fluidisation cavity where hot and corrosive gases maybe generated.

According to a tenth aspect of the invention, there is provided a method of increasing the heating efficiency of microwave energy transmitted to a fluid substance in a heating cavity, including the steps of : transmitting microwave energy to the fluid substance in the heating cavity; measuring microwave energy reflected from the heating cavity and the fluid substance; and

-adjusting the level of the fluid substance in the heating cavity to reduce the microwave energy reflected from the heating cavity and the fluid substance. The adjustment may be in response to a measured increase in the amount of said reflected microwave energy.

The microwave energy transmitted through the cavity is related to the microwave energy absorbed by the fluid substance and the microwave energy reflected by the fluid substance. The relationship between the aforementioned variables may be expressed as follows: Q=R+A+T+L Wherein Q = the total microwave energy transmitted to the cavity ; R = the reflected microwave energy; A = the microwave energy absorbed by the fluid substance ; T = the microwave energy transmitted through the fluid substance ; and L = losses.

The method according to the tenth aspect of the invention may be used as an alternative to or in addition to the method in accordance with the first or any other aspects of the invention.

According to an eleventh aspect of the present invention, there is provided an apparatus for increasing the heating efficiency of microwave energy transmitted to a fluid substance in a heating cavity, comprising - means for measuring decrease in microwave energy transmitted through the fluid substance; and - means for using the decrease to adjust the level of the fluid substance.

The apparatus according to the eleventh aspect of the invention may be used as an alternative to or in addition to the apparatus in accordance with the second or any other aspects of the invention.

As a further option, the temperature of the fluid substance may be measured as an indication of the amount of microwave energy absorbed by the fluid substance. The temperature measurement may be used in combination with either of the amounts of microwave energy reflected or transmitted through the bed as a measure of the efficiency of absorption of the microwave energy by the fluid substance.

As an additional option, the voltage of an electric field in the bed or on a side of the bed remote from the microwave generator, may be measured as an indication of the efficiency of absorption of microwave energy by the fluid substance.

Thus, the invention provides a method of increasing the heating efficiency of microwave energy transmitted to a fluid substance in a heating cavity. The increase in heating efficiency can be achieved in a variety of alternative or complementary ways. One is by measuring an increase in microwave energy reflected from the heating cavity, and by using the increase to control the level of the fluid substance in the heating cavity. Another is by measuring a decrease in microwave energy passing through the fluid substance, or alternatively or additionally, a voltage in an electrical field or a temperature inside or on any side of the cavity, or a combination thereof, and by using such measurement (s) either alone or in combination with the measure of the reflected microwave energy to adjust the level of the fluid substance in the heating cavity.

Brief Description of the Drawings A preferred form of the present invention will now be described by way of example with reference to the accompanying drawings wherein: Figure 1 is a schematic illustration an apparatus in accordance with the present invention; Figure 2 is a diagrammatic representation of the hydraulic behaviour of fluidised beds connected in accordance with the present invention;

Figure 3 is a graphic representation showing the variations, over time, of reflected power and bed temperature of the apparatus shown in Figure 1, when four different powders, designated A, B, C and D were calcined; Figure 4 is a graphic representation showing the variations, over time, of reflected power and differential pressure between the riser and the calciner shown in Figure 1 Figure 5a is a graphic representation showing the variations, over time, of reflected power and bed temperature of the apparatus shown in Figure 1, when a powder designated powder B was heated in the bed; Figure 5b is a graphic representation showing the variations, over time, of reflected power and bed temperature of the apparatus shown in Figure 1, when a different powder was calcined in the calciner and the calcination involved the decomposition of nitrates in the powder; and Figure 6 is a graphic representation showing the variations, over time, of reflected power and forward power of the apparatus shown in Figure 1, when the bed height was initially adjusted.

Figure 7 is a graphic representation showing the variations, over time, differential pressure and powder height in the riser, when the powder is transferred from the riser to the calciner.

Figure 8 is a graphic representation showing the variations, over time, of reflected power and the bed height when the bed height is increased, the reflected power being a function of the bed height; Detailed Description of the Preferred Embodiments Figure 1 shows an apparatus 10 in accordance with the invention comprising a fluidised bed reactor 12, also referred to as a calciner in this specification, which is manufactured from a cylindrical stainless steel pipe with an inside diameter of 108 mm and having a length of 410 mm. The dimensions of the reactor cavity were chosen to permit propagation of the TEIls mode of microwaves at 2450 MHz. The calciner 12 is

provided with a distributor plate 14 disposed above a plenum chamber 16 at the bottom of the calciner 12 defining a heating cavity. The distributor plate 14 is manufactured from sintered porous stainless steel which is 10 mm thick. Fluidised powder is supported on the distributor plate 14 at the bottom of the calciner 12. Fluidisation gas 18 is fed into the calciner 12 from the bottom thereof through the distributor plate 14. Reaction products and the gas stream are evacuated from the calciner 12 through an off-gas line 20.

A microwave generator 22 which is capable of providing a continuously variable microwave power output of 0. 6. to 6 kW at a frequency of 2450 10MHz, is operatively connected to the top end of the calciner 12. A portable magnetron-circulator assembly 24 is connected to the calciner 12 via a six-port vector reflectometer 26, a stub tuner 28 and an air filled aluminium WR340 rectangular to circular transition waveguide 30. The reflectometer 26 is capable of accurately measuring both modulus and phase of the complex reflection coefficient of a load under full-power operating conditions of the microwave generator 22. The reflectometer 26 is optimised for a frequency of 2450 MHz and can also measure incident, reflected and absorbed power, as well as operational frequency of the microwave generator 22. Display and logging software allows the complex reflection coefficient to be graphically displayed in real-time on a Smith chart and all of the microwave measurements are saved to a computer 23 at a rate of two data points per second.

The waveguide 30 is connected to the calciner 12 with a transition section 31. The transition section 31 is designed to transform the TEIO mode of the microwaves to a linear polarised TE, lp mode of the circular heating cavity of the calciner 12.

Microwaves are introduced axially into the calciner 12 from the top end as is shown in Figure 1. A water-cooled quartz window 32 is provided at the rectangular end of the waveguide 30 to prevent the fluidising gas 18 and any powder carried therein from entering into the microwave transmission system. The total pressure drop through the fluidised powder bed and the distributor 14 as well as the temperature of the bed and the wall of the calciner 12 are measured and down loaded to the computer 23. The temperature of the bed is measured with a shielded K type thermocouple (not shown) which may be inserted to a depth of 5 mm into the bed through the side-wall of the

calciner 12 at a location about 25 mm above the distributor 14. The thermocouple may be placed perpendicular to the direction of the electric field to eliminate interference therewith.

A riser 34 is provided adjacent the calciner 12 and is in fluid communication with it through a connecting tube 36. The riser is provided with inlets 38 and 40 for fluidisation gas at respectively its bottom and its side where an L-valve is provided. The riser also has an outlet 42 at its top end, for discharging fluidisation gas, and, if required, powder.

Powder may be introduced into the riser 34 through any one or more of the tube 36 and the outlet 42.

An off-gas filter 44 is provided to separate powder and dust from the fluidisation gas, before it is discharged to the atmosphere through the off-gas 20.

The top ends of the riser 34 and the calciner 12 are operatively connected to a vacuum/pressure control system 46. The computer 23 is operatively connected to a pneumatic control unit 48 which in turn has been set up so that either a vacuum or a pressure can be admitted from the vacuum/pressure control system 46 to either the riser 34 or the calciner 12.

In use, a desired level for a fluidised bed of powder in the calciner is determined as described in Example 1. The bed of powder is fluidised at the desired bed height inside the calciner 12. One of several heights may be used, as is apparent from the examples that follow below. The powder is heated by operating the microwave generator 22, and by guiding the microwaves generated by it by means of the waveguide 30 to the fluidised bed in the calciner 12. As the temperature of the powder starts increasing, the calciner starts expanding and the fluidisation conditions inside the calciner 12 start changing, inter alia as a result of the physical properties of the fluidisation gas changing, chemical reactions that may take place in the powder, etc. This causes the level of the bed to fluctuate. Because of the fluctuations in the level of the calciner, the amount of microwave energy that is not absorbed and is reflected by the fluidised bed fluctuates widely, as can be seen from Figures 3,4 and 6. The reflected microwave energy is measured by the reflectometer 26, which is reported to the computer 23, which in turn sends a signal to the pneumatic control unit 48. The pneumatic control unit 48 causes

either vacuum or pressure to be admitted to the riser or the calciner so as to cause the level of the fluidised bed in the calciner to be adjusted to the predetermined desired value.

In this way, the reflected power is reduced and the absorbed power is increased.

Figure 2 shows a schematic presentation of the control system in accordance with the invention, as applied to a microwave heated fluidised bed.

The system is analogous to a U-tube containing a liquid. A pressure PB acts on the surface of the liquid in the left-hand side of the U-tube, whilst a pressure PA acts on the surface of the liquid in the right-hand side of the U-tube, the pressure PAbeing higher than the pressure PB. The higher pressure on the right hand side of the U-tube causes the liquid to have a higher level in the left-hand side of the U-tube than in the right-hand side thereof. The difference in level is designated by AP. The relationship between PA and PB can be stated as follows: PA= PB + AP Thus, by applying a pressure of PB + AP to the surface of the liquid in the right hand side of the U-tube, a change in the levels of the surfaces of the liquid in the U-tube as shown may be brought about. This involves a lowering of the level of the liquid in the right hand side of the U-tube and a raising of the level of the liquid in the left-hand side thereof. Conversely, by applying a vacuum of similar magnitude as AP to the left hand side of the U-tube, the level of the surface in the left-hand side of the U-tube may be increased.

In a similar manner, the interconnected fluidised bed of the calciner and the fluidised bed inside the riser/downcomer act as a hydraulic pendulum and allow the transfer of the powder between the riser and the calciner in a controlled manner by the application of pressure or vacuum to either one. of the two, preferably to the riser, to cause a desired change in the levels of both the riser/downcomer and the calciner.

When power reflected from the fluidised bed of the calciner exceeds a predetermined value, a slight vacuum or pressure is applied to the riser by either operating a vacuum control valve communicating with the riser, in order to reduce the pressure in

the riser or by operating a pressure control valve communicating with the riser, in order to increase the pressure in the riser. Applying vacuum or pressure to the riser thus results in the transfer of particulate material from the fluidised bed reactor into the riser or else from the riser into the fluidised bed reactor. Either application changes the level of the bed in the fluidised bed reactor which in turn causes a change in the amount of microwave power reflected from the calciner. If the reflection of the microwave power increases, the application of pressure or vacuum is reversed in the riser until the minimum reflected microwave power is observed. Then the pressure or vacuum application is maintained constant to stop the powder flow between the riser and the fluidised bed reactor, thereby maintaining the levels in both at the same height. This procedure is repeated during the course of the calcination process in the calciner, whenever an increase is observed in the reflected microwave power.

Although the system was designed to be operated batch-wise, the scope of the invention is by no means restricted to batch processes. On the contrary, continuous processes are intended to be included in the scope of the invention. Because continuous processes are preferably run under steady state conditions, the invention may advantageously be applied to achieve such conditions.

For purposes of a continuous process, the calciner may be fed through the riser by maintaining a substantially constant level inside the riser under steady state conditions, in which event the finished product exiting from the calciner should do so through a special exit from the calciner or alternatively through an overhead cyclone from where a portion of the overheads may be returned to the calciner and a portion may be discharged as product. Alternatively, the calciner may be fed through a special feed line and the level may be controlled by maintaining a level inside the riser that is similar to the level inside the calciner under steady state conditions. The finished product may exit from the calciner through the connection with the riser, from where the finished product may be discharged using conventional means for discharging. The latter method offers the advantage over the former method in that any material returned from the riser for purposes of level control in accordance with the invention, will be closer to the operating temperature than when material has to be passed to the calciner in the former method.

The following non-limiting examples further illustrate the application of the method and apparatus of the invention.

Example 1: Establishing desirable level (s) of fluidised bed in calciner The microwave reflection behaviour of the system shown in Figures 1 and 2 was studied for various precursor powders as a function of bed height. In this example 1, four different types of ceramic powders with varying dielectric properties were investigated.

The bulk density of the powders and the experimental conditions are given in Table 1. The expansion of the fluidised bed of powders were also measured under the gas flow conditions which are given in Table 1.

Table 1. Density of the fluidised powder beds and operating conditions of the calciner.

Powder Bulk density Fluidisation gas Powder feed Mass of the bed Forward power [gcm'] Oowrate rate [g] [W] <BR> <BR> [m'h-'] [gs]<BR> A 0. 85 0. 70 0. 60 1720 870 B 1.15 0.70 1.84 1290 870 C 2.38 3. 06 0.52 1940 660 D 1.63 3.06 0.34 2090 660 Each powder had a particle size of 15 to 150 microns and was fluidised with nitrogen gas.

The dielectric constant for each of the powders was calculated. It was found that Powder D had the highest dielectric constant, whilst Powder A had the lowest dielectric constant, with Powder B being in between. A dielectric constant was not available for Powder C.

Fluidisation of each powder was carried out at a gas velocity of approximately four times the minimum fluidisation velocity required for the respective powder. This was done to ensure bubbling fluidisation conditions were maintained for each powder. The powders were continuously fed into the calciner cavity using a vibratory feeder at a constant feed rate whilst fluidisation gas was turned on. The approximate feed rates for each of powders A, B, C and D were 0.6, 1. 84, 0.52 and 0.34 gs'1, respectively. The reflected microwave power from the calciner was continuously measured as the fluidised

powder level in the reactor was gradually increased under constant forward power level and gas flow rate conditions. Voltage standing wave ratios (VSWR) were calculated from the forward and reverse power readings.

All three stubs of the tuner were retracted during the tests to eliminate any interference of the tuner with the microwaves reflected from the calciner.

In microwave heating technology, the degree of match between a load and a generator is characterised by the voltage standing wave ratio (VSWR). Hence, VSWR versus bed height plots shown in Figures 5a and 5b can be used to examine optimum matching conditions of a particular powder in a microwave heated fluidised bed.

Forward and reverse microwave power measurements revealed that each powder demonstrated a characteristic and repeatable VSWR pattern with increasing bed height.

VSWR and bed temperature data as a function of the height of each fluidised powder is shown in Figures 3 (a), (b), (c) and (d).

In Figures 3 (a), (b), (c) and (d), the major declines in VSWR values with increasing bed height are numbered as 1 through 4 in each of the separate graphs for the four different powders. VSWR versus bed height plots of the four powders depict similar patterns, as can be seen in Figures 3 (a), (b), (c) and (d). The positions of the four major declines with the lowest VSWR values, are summarised in Table 2.

Table 2. The bed heights of the tested powders where four major declines in VSWR values are observed.

Bed height [cm] Powder 1st Decline 2nd Decline 3rd Decline 4tb Decline in in VSWR in VSWR in VSWR VSWR A 4.6 8.1 13 20 B 3.5 6.5 8.3 12.4 C 1.2 3.7 6.5 10.3 D 1. 3 3. 7 6'. 7 10. 7

Bearing in mind that the dielectric constant for Powder A was the lowest and the dielectric constant for Powder D was the highest of the four powders, Table 2 reveals that, for each powder, the positions of the declines in VSWR values exhibit a considerable shift towards lower bed heights with increasing dielectric constant of the fluidised powder bed. As is shown in Table 2, all four declines in VSWR values of powders C and D are observed at very similar bed heights.

All powders demonstrated fluctuations in the amplitude of the VSWR plot as the depth of the bed increased.

Although the tests started at ambient conditions, the temperature of the powder bed gradually increased due to transitional low values of VSWR. Figures 3 (a), (b), (c) and (d) show that, for each powder, the temperature response of the fluidised bed is very sensitive to coupling between the microwave source and the bed. This behaviour can be seen from the bursts in the temperature plots, which occur concurrently with the rapid declines in VSWR as the bed height increased.

Microwave heating of powder B in the fluidised bed calciner was demonstrated by stopping the powder feeding into the calciner as soon as the second decline in VSWR value was observed. A temperature and VSWR plot of this run is shown in Figure 3a. As is shown in Figure 3b, the second minimum in VSWR occurred at approximately 6.5 cm of fluidised bed height which is equivalent to approximately 600 g of powder.

Microwave reflection measurements of powders with different dielectric properties demonstrated similar VSWR patterns as the bed height changes. However, considering the bed density and dielectric constant of each powder, the overall peak pattern of the VSWR plot shifted towards the lower bed heights with the increased permittivity of the fluidised bed. This behaviour may be used as a tool for monitoring the calcination reactions by which the properties of the bed change.

The VSWR values demonstrated fluctuations with higher amplitude with increasing bed depth and powder density, which could be attributed to the increased size of the bubbles in deep and dense beds, respectively. Therefore, the changes in the amplitude and

frequency of the VSWR may be utilised as a monitoring method for the fluidisation dynamics and powder properties.

This behaviour may be attributed to the development, in deeper beds, of bigger bubbles, combined with a higher degree of turbulence inside the bed and on the surface of the bed. Because microwaves penetrate into the bed and interact with the interfaces between the bubbles and the bulk powder, measurable changes in the transmission and standing wave patterns of microwaves are expected.

Example 2: Reflected power as a function of the powder level in a calciner.

In another run, powder A was gradually charged into the calciner through the riser at a rate of 4.3 g/s. Microwave heating was applied. The powder height and reflected power were measured. The results are shown in Figure 4. Figure 4 illustrates that the reflected microwave power decreased suddenly at certain bed heights, such as, approximately, at 6.7, 17.6 and 27.5 cm. The experiment revealed that such minimums could be achieved at various bed heights.

Example 3: Reflected power and bed temperature as a function of time: Powder B.

In a further run, Powder B of Example 1 was heated to approximately 760°C by means of microwave energy at a microwave energy level of 600 W. The bed temperature and VSWR were measured and plotted in Fig Sa.

As is shown in Figures 5a, a bed temperature of 760°C can easily be achieved by stopping the powder feeding as soon as the second minimum in VSWR is observed and by continuing to apply heat.

Example 4: Reflected power and bed temperature as a function of time: Powder containing nitrates In still a further run, a powder containing nitrates was heated to approximately 760°C by means of microwave energy. The bed temperature and VSWR were measured and plotted in Fig 5b causing volatile conditions changing the bed depth, inter alia as a result of nitrates decomposing during heating.

As is shown in Figure 5b, a bed temperature of 760°C can also easily be achieved, even for the decomposing powder. However, because both the dielectric constant of the powder and the height of the fluidised bed were functions of temperature, the 3-stub tuner was used in an effort to manually maintain VSWR low during the course of heating. As can be seen from the wide fluctuations in the reflected power, this was not very successful. There was no level control of the powder in the calciner.

Example 5: Reflected power and bed temperature as a function of time: Powder containing nitrates Example 4 was repeated using the nitrate containing powder. However, this time the level of the powder in the calciner was controlled at the desired height in the calciner, by maintaining a level of and fluidising the same type of powder in the riser, and by controlling the level of the powder in the calciner by increasing or decreasing the pressure in either the calciner or the riser in accordance with the invention, and as is also shown in Figure 1. Forward and reflected power were measured as a function of time, and are graphically illustrated in Figure 6. It is clear from Figure 6 that the reflected power can be controlled to be at a minimum and the absorbed power to be at a maximum, despite volatile conditions in the calciner caused by the decomposition of the nitrates.

Example 5 shows that a high heating rate can be achieved by choosing a certain bed height at which VSWR demonstrates a minimum value and by controlling the bed height.

When the correct bed height was not controlled, as in Example 4, the coupling between the bed and the generator could not be achieved with an automated 3-stub tuner at the same degree of accuracy as in the case of Example 5.

Figure 6 also shows that the amount of reflected power sometimes increases suddenly due to disturbances in the height of the fluidised bed, but the level control in accordance with the invention rapidly responds to correct the level, and thus the reflected power.

Although only some embodiments of the invention have been described in relation to microwave heated fluidised bed systems, it should be understood that the invention is not limited to that field and that the principles of the invention may be applied in many other

fields. Other fields include the microwave treatment of liquids or slurries in closed containers.

Example 6: Differential pressure and powder height in the riser as a function of time In another run, powder A was gradually charged into the calciner through the riser by applying a pressure gradient between the riser and the calciner, which is the only driving force to transfer the powder from one vessel to the other. The transfer rate of the powder was measured as a function of time by using a glass riser section which was marked with a metric length scale. The differential pressure between the riser and the calciner was also measured during powder transfer operation. Both data, the differential pressure and the powder level in the riser, is shown in Figure 7 as a function time.

Figure 7 shows that the transfer rate of the powder between the riser and calciner and the differential pressure are in close correlation in a linear relationship with the same slope value and therefore the transfer rate of the powder can be monitored by measuring the differential pressure between the riser and the calciner during transfer operation.

Since the bulk density of the powder and the diameter of the riser are known the linear powder level plot can be used to calculate powder transfer rate in grams per second. <BR> <BR> <P>Riser crosssectionxTotal powder height transferred<BR> Transfer_rate = x Bulk density<BR> Total _ transfer_ tlme Since the differential pressure plot is in liner character and has the same slop value with the powder level plot, the differential pressure plot can directly be utilised to measure the transfer rate of powder.

Therefore, numerical value of the slope of the differential pressure plot is equal to: Total _ powder _ height-transferred Total _ transfer _ time This value was placed in"Transfer rate"equation to calculate the powder transfer rate.

Since the diameter of the calciner and the bulk density of the powder were known, the powder transfer rate was used to calculate the height of the powder in the calciner at any moment of transfer operation in accordance with the following equation:

Transfer_rate x Time<BR> Powder_height_in_calciner =<BR> Bulk-density x Calciner_cross section_area When the powder height in the calciner is plotted as a function of transfer time, a linear plot for powder height is obtained and is illustrated in Figure 8.