MICROWAVE APPARATUS AND METHOD FOR ANALYSIS OF ASPHALT-AGGREGATE COMPOSITIONS

Abstract: A method and apparatus are disclosed for analyzing the aggregate content of asphalt-aggregate compositions. The method includes placing a preheated, preweighed container carrying a sample of an asphalt-aggregate composition into a preheated furnace with the preheated furnace being warmer than the preheated sample, continuing to heat the furnace while drawing air through the furnace at a rate that avoids impeding the heating of the furnace or the sample until the sample in the container reaches its combustion temperature and the combustion of the sample becomes exothermic, initiating a second draw of air around rather than through the furnace to moderate the exterior temperature of the furnace, accelerating the draw through the furnace to increase the rate of combustion of the exothermic reaction, and reweighing the container and sample after combustion is complete. The apparatus includes a furnace, a housing around the furnace, a furnace exhaust in communication with the furnace, a housing exhaust in communication with the housing, means for drawing an airflow through the furnace and into the furnace exhaust, means for drawing a separate airflow through the housing and into the housing exhaust, and a junction between the exhausts that forms a common exhaust from the housing and the furnace.

Description: Microwave Apparatus and Method for Analysis of Asphalt- Aggregate Compositions Field of the Invention The present invention relates to the analysis of compositions formed of asphalt and aggregates such as crushed rock and sand. In particular, the invention relates to a method and apparatus for analyzing the percentage composition of asphalt in such compositions. Background of the Invention Compositions formed of mixtures of asphalt and aggregates materials are one of the most widely used materials for paving roads and highways. Although the term "asphalt" is commonly used to refer to the road material, it more properly applies to the "tar" portion of a mixed composition of tar and aggregate. Thus, the asphalt is a dark brown or black cementitious material, which is solid or semi-solid in consistency, in which the predominating constituents are bitumens which occur in nature or are obtained as byproducts from petroleum refining. Asphalt is a mixture of paraffinic and aromatic hydrocarbons and heterocyclic compounds containing sulfur, nitrogen and oxygen. Asphalt is also referred to as "petroleum asphalt," "Trinidad pitch," or "mineral pitch." Asphalt is a black solid or viscous liquid that has a flash point of about 450° F and an auto-ignition temperature of about 900° F (482° C) and softens from its solid or semi-solid state to a viscous liquid at approximately 93° C. Other typical uses of asphalt include roofing, joint filling, special paints, adhesive and electrical laminates, and hot belt compositions, a dilutent in low-grade rubber products and a number of other applications. These and other properties and uses of asphalt are generally well known in the art and can be found, for example, in Lewis, Hawley's Condensed Chemical Dictionary, 12th Edition (1993). In paving compositions, asphalt is typically present in an amount of about 5% by weight. With respect to such compositions, the percentage of asphalt and the nature and size of the aggregate material (typically rocks and sand) used to make the composition are important for the proper structure and characteristics of the final road structure. For example, typical roads are formed of three layers of asphalt and aggregate compositions. The bottom most layer includes rocks of one inch or greater -2- average size, sand, and the asphalt. An intermediate layer typically includes a composition formed of somewhat smaller rocks, typically 0.5-1 inch in diameter, again with sand and asphalt. Finally, a top layer is usually applied which has the smallest rocks, typically 0.5 inches or less in diameter plus sand and tar. Because the aggregate generally represents more than 90% of a hot asphalt mix, aggregate gradation (i.e., the different particle sizes that are present in the blend) profoundly influences the properties of the hot mix (such as air voids, workability, and the amount of asphalt binder required) and the resulting properties of the pavement (such as stiffness, stability, and durability) (e.g., Aljassar et al., Toward Automating Size-Gradation Analysis of Mineral Aggregate, Transportation Research Record, Issue Number: 1437, pp. 35-42 (1994)). In this regard, research on asphalt- aggregate compositions has become quite detailed, including for example quantifying the influence on resistance to rutting when rounded, smooth, sand-sized aggregate particles are replaced by rough, angular, porous particles while other aggregates and the total gradation remain unchanged (e.g., Perdomo, D. and Button, J.W., Identifying and Correcting Rut-Susceptible Asphalt Mixtures, Final Report, Texas Transportation Institute, Texas A&M University, Texas State Department of Highways & Public Transp, Federal Highway Administration, Report Number: Fhwa/Tx-91/1121-2F;Res Rept 1121-2F;TTI: 2-8-87/91-1121, Pag: 164p), or evaluating the effect of the amount of soil binder (i.e., the smallest aggregate particles) on the engineering properties of asphalt-treated paving materials (Ping, and Kennedy, The Effects of Soil Binder and Moisture on Black Base Mixtures, Texas University, Center for Highway Research, Austin, Texas State Department of Highways & Public Transp Report Number: FHWA/TX-79/08+183-12 Intrm Rpt.;FCP 45C2-352 Pag: 127p (1997)). Aggregate gradation is frequently determined by the well-known and widely used sieve analysis method. Accordingly, as these exemplary references indicate, depending upon the conditions under which a road is used (e.g., traffic patterns and weather conditions), the composition of any one or more of the asphalt layers must be carefully designed and monitored. Additionally, because the aggregate materials are typically taken from local quarries, and the manufacture of the composition is not an exact science, the asphalt and aggregate compositions must be frequently tested, both as they are -3- being made and after they have been applied to a roadway, to make sure that they meet the appropriate requirements. Thus, there exists a need for determining: (1) the weight percentage of asphalt; and (2) the aggregate size distribution in a given sample of an asphalt- aggregate composition. In one conventional method of analysis, the percentage of asphalt is determined by a solvent extraction technique which uses chlorinated hydrocarbons to separate the asphalt from the aggregate materials. Because the solvents are generally considered to raise a hazard to persons who are exposed to their vapors, the solvent testing is becoming more and more disfavored, and indeed is expected to eventually become prohibited under appropriate environmental regulation. In a second conventional method, a weighed sample of the composition is placed into a furnace which is then heated until the asphalt in the composition ignites. The asphalt is then allowed to burn until it is entirely consumed after which the remaining aggregate is weighed. The difference between the starting and ending weight is a measure of the composition of asphalt in the composition. There are, however, at least two problems with this conventional ignition technique. First, in conventional conduction and convection heating, the burning asphalt can carry the combustion to temperatures of up to 900° C, i.e., a state which is somewhat out of control. These extreme temperatures can cause particular problems in the equipment or in handling the hot material or even confining the resulting fire. Second, and perhaps just as important, the extreme temperatures tend to degrade the physical characteristics of the aggregate in the mixture, including its size. Thus, because the aggregate remaining after the asphalt has been burned off is typically measured to determine whether it is of the proper size, the size degradation resulting from the conventional ignition test leads to a certain inaccuracy in measuring the sizes of aggregate in any given sample. As another problem, if the aggregate contains carbonate compounds, the excessive heat can drive off carbon dioxide, thus changing both the chemical and physical characteristics of the aggregate. Finally, the conventional ignition techniques tend to cause a loss of "fine" aggregates—literally blowing them away-so that such fines are neither sized nor weighed, further acerbating the accuracy problem. -4- Accordingly, the need exists for a method of determining the amount of asphalt in an asphalt-aggregate combination which avoids the use of environmentally disfavored solvents, which more carefully controls the combustion and which avoids the breakdown in the aggregate materials that tends to result in improper sizing following such testing. Therefore, it is an object of the present invention to provide a method and apparatus for analyzing asphalt aggregate compositions that has the potential to be generally faster, more efficient, and more accurate than prior techniques. Object and Summary of the Invention The invention meets this object with a method of analyzing asphalt- aggregate compositions in which sufficient microwave radiation is directed from a microwave source to a sample of an asphalt-aggregate composition to ignite the asphalt in the composition and to thereafter entirely combust the asphalt in the sample. In preferred embodiments the microwave radiation is moderated as is the oxygen available to the sample to maintain the temperature of the ignited composition within the controllable range. In further embodiments of the invention, the weight of the sample is measured before and after the combustion of the asphalt, and the remaining aggregate is sized to determine it particle size distribution. In another aspect, the invention comprises an apparatus for analyzing the asphalt content of an asphalt-aggregate composition. The apparatus comprises a source of microwave radiation, a cavity in communication with the microwave source, a sample holder in the cavity for holding a sample of an asphalt-aggregate composition during the exposure to microwaves from the source, thermal insulation between the sample holder and the remainder of the cavity and means for minimizing or eliminating any undesired combustion products generated by the burning asphalt. In another aspect, the invention is a method of analyzing the aggregate content of asphalt-aggregate compositions, in which the method comprises heating a furnace with a preweighed container carrying a sample of an asphalt aggregate composition therein while drawing air through the furnace at a rate that avoids impeding the heating of the furnace or the sample until the sample in the container reaches its combustion temperature and the combustion of the sample becomes exothermic. At that point the method comprises accelerating the draw through the furnace to increase -5- the rate of combustion of the exothermic reaction until the exothermic reaction is complete, and thereafter re-weighing the container and sample. In another aspect, the step of heating the furnace comprises heating the furnace while drawing air through it at the rate that avoids impeding the heating of the furnace or the sample until the sample reaches a first predetermined steeping temperature, a which point the draw through the furnace is accelerated to increase the rate of combustion of the sample. In another aspect, the method comprises heating a sample of an asphalt- aggregate composition to combustion in a furnace, drawing an airflow into, through and out of the furnace to promote the combustion of the asphalt in the composition; drawing a separate airflow past, but not through, the furnace to help moderate the exterior temperature of the furnace without interfering with the combustion therein, and blending airflow that has exited the furnace with the airflow that has passed to the exterior to thereby moderate the temperature of the furnace exited airflow. In its apparatus aspects, the invention comprises a cavity for holding an asphalt-aggregate sample, means for introducing microwave radiation into the cavity, materials in the cavity for absorbing microwave radiation in converting the microwave radiation into heat, an afterburner in gasflow communication with the cavity, a first fan for drawing air through the cavity and through the afterburner, and a second fan for drawing air around the cavity to moderate the external temperature of the cavity and its immediate environment. In another aspect, the furnace system comprises a furnace, a housing around the furnace, a furnace exhaust in communication with the furnace, a housing exhaust in communication with the housing, means for drawing an airflow through the furnace and into the furnace exhaust, means for drawing a separate airflow through the housing and into the housing exhaust, and a junction between the two exhausts that forms a common exhaust from the housing and the furnace. The foregoing and other objects and advantages of the invention and the manner in which the same are accomplished will become clearer based on the following detailed description taken in conjunction with the accompanying drawings in which: Brief Description of the Drawings Figure 1 is a schematic cross-sectional representation of the apparatus according to the present invention; and Figure 2 is a top plan cross-sectional view of the apparatus taken along line 2- 2 of Figure 1 Figure 1 is a cross-sectional schematic view of the invention; Figure 2 is a front perspective view of a commercial embodiment of the invention; and Figure 3 is a rear perspective view of the embodiment illustrated in Figure 2. Detailed Description In the first embodiment, the invention is a method of analyzing asphalt- aggregate compositions comprising directing sufficient microwave radiation from a microwave source to a sample of an asphalt-aggregate composition to ignite the asphalt in the composition and to thereafter entirely combust the asphalt in the sample. Because one of the objects of the invention is to determine the weight percentage of asphalt in the composition, the method preferably further comprises weighing the sample before and after combustion to provide the data needed to calculate weight loss and percentage asphalt. These are, of course, very straightforward calculations and are usually taken by measuring the difference between the starting and the finishing weight, dividing the difference by the starting weight of the sample, and then expressing the answer either as a decimal fraction or a percentage. As noted above, one of the most important characteristics of an asphalt- aggregate composition is the particle size distribution of the aggregate materials. Thus, the method of the invention further comprises sizing the aggregate after combustion of the asphalt. In particular, the nature of the combustion process according to the invention provides a less distorted and frequently undistorted aggregate for which the sizes and size distribution can be more accurately determined. In preferred aspects of this embodiment, the method comprises directing microwave radiation from the source to the sample until the composition ignites, then moderating the microwave radiation directed to the sample and the amount of oxygen available to the sample to maintain the temperature of the ignited composition within a controllable range, and then reducing the gaseous combustion products substantially to carbon dioxide and water vapor. In order to prevent the ignited asphalt from burning out of control, the step of moderating the microwave radiation preferably comprises measuring the temperature of the sample during combustion and then moderating the amount of microwave radiation applied based on the measured temperature. It will be understood that the temperature can be measured either in or near the sample and that the resulting measurements will provide essentially equivalently useful information. Given the rapid interaction of microwaves with materials, and the fact that microwave power can be immediately stopped, as opposed to convection or conduction heating which tend to continue until thermal equilibrium is reached, the method provides an increased amount of control over asphalt ignition and combustion reactions than has previously been available. Stated in somewhat simplified fashion, the combustion can be started and stopped more quickly than with conventional techniques and devices. In preferred embodiments, the interaction between the microwaves and the asphalt- aggregate composition is not the sole source of heating. Instead, the method further comprises placing one or more susceptors within the furnace that also absorb microwave energy and convert it into heat. The susceptors are formed of one or more materials that will both absorb microwaves and convert them into heat, with silicon carbide (SiC) being an efficient choice for the material. Preferably, the SiC susceptors are present in an amount, and in selected positions, sufficient to raise the temperature inside the furnace to at least about 540°C, even in the absence of a sample. In a preferred embodiment of the method, the SiC susceptors are placed in the furnace and microwaves are applied to the susceptors until they heat the furnace to about 540°C, after which the sample is added. The size and number of susceptors is also selected as to optimize the control of the combustion reaction after ignition, and to help complete the later stages of the combustion process after the rapidly-burning components of the composition have been consumed. In a preferred embodiment, the invention further comprises the method of controlling the airflow to the sample to thereby moderate the oxygen available and control the combustion following ignition. In preferred embodiments, the method comprises measuring the temperature of the burning sample and controlling the airflow in response to the measured temperature. In this regard, the airflow can be used to either accelerate or decelerate the combustion. Because asphalt is a mixture of hydrocarbons with different properties, its initial ignition tends to generate a rapid combustion of the highly volatile and highly flammable portions, followed by a slower and more deliberate combustion of the remaining portions. Accordingly, airflow can be decreased to moderate the rapid combustion and increased to encourage the slower combustion. In other aspects of the method, the step of reducing the byproducts comprises carrying out a follow up combustion step on the gaseous byproducts to more completely reduce them to carbon dioxide and water vapor. The follow-up combustion step is typically carried out in an afterburner which will be described in more detail with respect to the apparatus aspects of the invention. For control purposes, the microwave power that reaches the sample can be moderated by moderating the amount of microwave produced in the source, or by moderating their passage between the source and the sample. In another aspect, the invention comprises a method of analyzing composition road building materials which comprises weighing a sample of an asphalt-aggregate composition, directing sufficient microwave radiation from a microwave source to a sample of an asphalt-aggregate composition to ignite the asphalt in the composition, thereafter continuing to direct microwave radiation to the sample until the asphalt in the sample is entirely combusted, weighing the sample after the asphalt has been entirely combusted, and sizing the aggregate. The sample to be measured is typically selected from existing road materials, portions of asphalt aggregate compositions that have just been produced, and portions of asphalt-aggregate compositions that are being produced; i.e., during the production process. In typical embodiments, the step of sizing the aggregate comprises sieve analysis (e.g., U.S. standard sieve sizes), but it will be understood that any appropriate sizing technique can be used with the present invention and that the invention provides the advantage of maintaining the aggregate in its original size following combustion much more successfully than have prior ignition techniques. -9- In another aspect the invention comprises an apparatus useful in carrying out the method of the invention. In Figure 1, the overall apparatus is broadly designated at 10. The apparatus comprises a source of microwave radiation which is illustrated schematically at 11. The source is typically a magnetron because of its generally well-understood characteristics and reasonable cost. Those familiar with the generation of microwaves, however, will recognize that any appropriate microwave source could be incorporated, including klystrons, solid-state devices, and other microwave generators. Because the operation of these is generally well known and not otherwise limiting of the present invention, they will not be discussed in detail herein. It will also be understood that the source 11 is in communication with a cavity which is illustrated as the housing 12 in Figure 1. In most embodiments, the source 11 will communicate with the cavity 12 through a waveguide, again in a manner well understood to those of ordinary skill in this art. The housing 12 is typically formed of metal and defines the cavity 13 and provides a shield against the emission of microwave radiation from the cavity 13 when the cavity is receiving microwaves from the source 11. The particular metal and structural details of the housing 12 can be easily selected by those of ordinary skill in this art and without undue experimentation. A sample holder 14 is positioned in the cavity for holding a sample 15 of the asphalt-aggregate composition that is being analyzed during exposure to microwaves from the source 11. The sample holder should be large enough to hold the necessary sample size desired for testing, and is preferably heat resistant and made of a material that is either transparent or minimally absorbent of microwave radiation and that can withstand the high temperatures generated after the sample 15 ignites. Typical materials can include stainless steel, aluminum, ceramic materials or combinations of these materials. Preferably, the sample holder 14 is perforated to permit a freer movement of air through the asphalt-aggregate mixture. Additionally, depending upon the circumstances and other factors, the sample holder can rotate on a turntable (not illustrated in this embodiment). In the illustrated embodiment, the apparatus includes a balance 16 for measuring the weight of the sample 15 in the sample holder 14 before, during and after combustion. The balance 16 includes the balance pan or platform 17, and the supports 20 that connect the pan 17 to the balance mechanism -10- 16. Appropriate balance mechanisms and the manner of incorporating them into microwave devices are generally well understood in this art and can be selected and incorporated without undue experimentation. Additionally, those familiar with weighing heated objects in ovens will recognize the presence and effects of convection air currents, and will likewise be aware of the need to incorporate an appropriate correction factor into the weight calculations. The invention further comprises thermal insulation shown in the form of the three solid polygons 21, 22, and 23 in Figure 1. The thermal insulating material completely surrounds the sample in the cavity 13 and forms the furnace in which the sample can ignite and burn without spreading to the ambient surroundings. As Figure 1 illustrates, the thermal insulation and can be formed from a number of individual pieces, and it will be understood that there is at least one more piece covering the front and the back portions of the cavity 13 and that is not shown in Figure 1 because it would otherwise obstruct the view of the remaining elements. Figure 2 illustrates these pieces at 44 and 45. In preferred embodiments, the thermal insulation is heat resistant, microwave transparent and has a low thermal conductivity. Preferred materials both withstand the heat generated by ignition and combustion and also keep the heat from being transmitted to the housing 12 or to the ambient surroundings. Preferred materials include an open-cell quartz, and quartz or borosilicate glass fibers. The insulation and the structure it produces are quite similar to those referred to as "muffle" furnaces (e.g., U.S. Patent No. 5,318,754 which is commonly assigned with the present invention) and will not otherwise be described in detail herein. As set forth in the background portion of the specification, the asphalt portion of an asphalt- aggregate composition is formed of a wide variety of hydrocarbon materials and thus results in a similarly wide variety of combustion byproducts. According to the present invention, it has been determined that a preferred method of eliminating these undesired combustion products is to include an afterburner which in Figure 1 is illustrated as the tubular furnace 24. The tubular furnace 24 is heated (typically to about 1000° C) by any conventional process such as electric resistance heating, and is insulated from the ambient surroundings by the insulating material 25 which can be the same as that which forms the thermal insulation inside the cavity. In preferred embodiments, the apparatus includes means for directing the combustion -11- products from the cavity 13 to the afterburner 24. In one preferred embodiment, the means for directing the combustion products to the afterburner include one or more openings 26 in the cavity in fluid (i.e., airflow) communication with the sample holder 14 and the sample 15, a duct 27 in communication with the downstream portion of the afterburner 24, and a fan schematically illustrated at 30 associated with the duct 27 and downstream from the afterburner 24 for drawing ambient air into the cavity 13 and for concurrently drawing the afterburner products away from the afterburner 24. In a particularly preferred embodiment, the invention will further comprise an additional opening 31 in the housing 12 outside of the thermal insulation 21, 22, 23 and leading to an additional duct 32 and fan 33 for drawing an additional portion of air through the cavity and through the housing. Using these fans, the amount of air being drawn through the cavity, and thus available for combustion of the sample 15, can be more carefully controlled. As noted above, during the early portions of combustion following ignition, the airflow may preferably be minimized to keep the combustion from getting out of control. Later in the process, when the combustion is moving more moderately or even relatively more slowly, the airflow can be increased to speed that part of the process along. Accordingly, in preferred embodiments of the invention, the apparatus includes a temperature sensor 34 preferably positioned inside of the thermal insulation 21, 22, and 23 for determining the temperature in the portion of the cavity 13 inside of the thermal insulation, and thus serving to provide an appropriate means of monitoring the temperature of the sample 15 during combustion. If desired, the temperature sensor 34 could be placed entirely within the sample, but it has been determined that because the temperature within the boundaries of the insulation is closely representative of the combustion temperature, there is no particular need in most circumstances to directly contact the sample 15 with the temperature sensor 34. The temperature sensor 34 can be selected as desired based on a number of engineering criteria, but will typically comprise of a thermometer, a thermocouple or an optical temperature measuring device. It will be understood that the particular device used to measure the temperature is in no manner limiting of either the apparatus or method of the present invention. -12- In preferred embodiments, the apparatus further includes means shown as the controller 35 for controlling microwave power or the airflow applied to the sample 15 based on the temperature detected by the temperature sensor 34. As schematically illustrated in Figure 1, the controller can be connected to the fan 30 through the circuits schematically shown as 36, or to the fan 33 through the circuits schematically illustrated in 37. The controller is connected to the probe, 34 through the circuit illustrated schematically at 40, and to the microwave source through the circuit schematically illustrated at 41. Figure 1 illustrates that the apparatus can be used to control the application of microwave power to the cavity by moderating the power at the source 11 , or where desired or necessary, the microwaves can be moderated by moderating their passage from the source 11 to the cavity 15. Figure 1 illustrates such moderating means at 42 and the controller is connected to these through the circuit illustrates at 43. The operation of controllers to produce output signals in response to input information such as temperature as well understood in the electronic and computer arts and will not otherwise be described in detail. Exemplary controllers and their method of operation are set forth, for example, in Dorf, The Electrical Engineering Handbook, Second Edition, CRC Press (1997). Similarly, means for moderating the passage of microwaves based on controlling them between the source and their destination are generally well understood, with a particularly newer and unique method being set forth in commonly assigned U.S. Patent No. 5,796,080, for "Microwave Apparatus for Controlling Power Levels in Individual Multiple Cells." Figure 2 illustrates the positioning of the silicon carbide susceptors referred to above. Consistent with Figure 1, Figure 2 shows the housing 12 and the insulation 21 and 23 that forms a portion of the furnace cavity 13. Figure 2 also shows two additional pieces of insulation 44 and 45 that were not visible in the view of Figure 1, along with the sample holder 14 and sample 15. Figure 2 further illustrates the silicon carbide susceptors 46 and 47 the number and position of which, as noted above, can be selected as desired or necessary to heat the sample. In preferred embodiments, the size and number of the silicon carbide susceptors are sufficient, when subjected to microwave radiation from the source, to raise the temperature inside the furnace cavity to, and maintain the temperature at, about 540°C even in the absence of any sample. -13- Because asphalt-aggregate compositions can vary so widely depending on a variety of factors, objective determinations of post-combustion aggregate quality can be difficult. Nevertheless, the appearance of the post-combustion aggregate can provide a qualitative measure of the success of the method in preserving the aggregate as closely as possible to its pre-combustion condition. In this regard, the post- combustion samples from asphalt-aggregate compositions analyzed according to the present invention give every indication of being in better— i.e. close or identical to pre- combustion— condition that samples analyzed using more conventional ignition techniques. It is thus expected that any objective evaluation of such post-combustion samples will similarly demonstrate the advantages of the present invention in properly preserving the aggregate. In summary, the invention provides a more controllable method of carrying out an ignition and combustion analysis of asphalt-aggregate compositions and does so in a manner that minimizes or avoids the degradation of the aggregate that is commonly observed in conventional ignition and combustion testing. Figure 3 is a cross-sectional schematic diagram that illustrates additional aspects of the method and apparatus of the invention. The overall apparatus is broadly designated at 110 and includes a cavity 111 which is generally defined by the surrounding ceramic fibrous material 118 that is insulating with respect to conductive heat transfer and substantially transparent to microwave radiation. Although the term "furnace" can be applied to the entire apparatus 110, the term "furnace" is also used to describe the ceramic fibrous material 118. The particular use will be evident herein based on the context. The ceramic fibrous material 118 and the furnace structure it produces are quite similar to those referred to as "muffle" furnaces (e.g., U.S. Patent No. 5,318,754 which is commonly assigned with the present invention) and will not otherwise be described in detail herein. The cavity 111 holds an asphalt aggregate sample 112 which in preferred embodiments is maintained in a container 113. Means illustrated as the magnetron 114 and the waveguide 115 introduce microwave radiation into the cavity 111. Although microwave energy can be used to heat the sample 112 directly, in preferred embodiments, other materials illustrated as the susceptors 116, typically formed of a -14- material such as silicon carbide, are placed in the furnace to absorb microwave radiation and convert it into heat energy which in turn heats the sample 112. The apparatus 110 further comprises an afterburner 117 which in preferred embodiments comprises a tubular furnace, the nature and operation of which are generally well understood and will not be otherwise described in detail herein. The afterburner serves to complete the combustion of certain of the byproducts of the initial combustion of the sample 112 in the cavity 111. A first fan 120 draws air through the cavity 111, into a microwave choke 119, and through the afterburner 117. A second fan 121 draws air around, but not through the cavity 111 to moderate the external temperature of the cavity 111 and its immediate environment. The choke 119 is formed of a material that blocks microwave radiation (usually a metal) and has a length to diameter ratio that prevents microwaves from escaping into or through the afterburner. The length and diameter of the choke are selected based upon the wavelength of the microwaves produced by the source, using relationships that are well understood in this art. Because the asphalt-content aspects of an the asphalt-aggregate analysis is generally based on weight loss, in preferred embodiments, the apparatus 110 further comprises a scale 122 in the cavity 111 for monitoring (preferably continuously) the weight of a sample 112 in the cavity 111. In preferred embodiments, and as illustrated in Figure 3, the scale has a pan portion 123 in the cavity 111, and a series of supports 124 that extend downwardly from the pan through openings 125 in the furnace material 118. In this arrangement, many of the mechanical and electrical components of the scale can be positioned outside of the cavity 118 thus protecting them from the relatively harsh conditions, both of temperature and materials, that are generated in the cavity 111 during operation of the furnace. In the preferred embodiments, the support openings 125 also provide the entry point for airflow from the external surroundings into the cavity 111 and then to the afterburner as just described. In preferred embodiments, the apparatus further comprises a temperature sensor 126 in the cavity 111 for monitoring the temperature of the sample. In these preferred embodiments, the first and second fans 120,121 are controllable variable speed fans. In these embodiments, the apparatus further comprises a controller 127 -15- that moderates the draw of the first fan 120, the second fan 121, or both, based upon the sample temperature measured by the temperature sensor 126. As will be described herein with respect to the method aspects of the invention the ability to control the airflow based upon the particular stage of heating, combustion, or cooling, can speed the analysis considerably. The temperature sensor can be selected as desired based on a number of engineering criteria, but will typically comprise a thermometer, a thermocouple or an optical temperature measuring device. It will be understood that the particular device used to measure the temperature is in no manner limiting of either the apparatus or method of the present invention. Controllers and their method of operation are generally well-understood in this art and will not be described in detail herein. Exemplary devices and techniques are set forth, for example, in Dorf, The Electrical Engineering Handbook, Second Edition, CRC Press (1997). Because this particular embodiment of the invention uses microwave heating, it further comprises the housing 130 shown in Figure 3 that is formed of a material, typically metal, that surrounds the cavity 111 and the ceramic furnace material 118 to shield the external surroundings from the microwaves that are propagated into the cavity 111 by the magnetron 114 and the waveguide 115. Any materials that will suitably shield microwaves are appropriate for the housing 130, but metal is typically used under most circumstances. The housing 130 also defines a ventilation opening 131 through which air can flow into the space between the housing 130 and the furnace ceramic material 118. In this embodiment, the second fan 121 is positioned to draw an airflow into the space between the housing 130 and the cavity 121. Because this airflow starts from an ambient source, and does not pass through the combustion chamber, it generally serves to moderate the high temperatures on the exterior of the cavity generated by the combustion taking place inside. As in the case of the choke 119, the opening 131 (which is not drawn to scale) is either of a size that prevents microwaves from escaping therethrough, or includes an appropriately sized choke of its own. Figure 3 further illustrates a furnace exhaust pipe 132 that is in communication with the afterburner 117 and the first fan 120, and a housing exhaust pipe 133 that is in communication with the housing 130 and the second fan 121. The exhaust pipes -16- 132 and 133 merge at a junction 134 to form a common exhaust designated by the arrows 135 for both of the fans. In preferred embodiments, the apparatus 110 includes an ambient air stem 136 in communication with the furnace exhaust pipe 132 which helps draw additional ambient air that blends with the furnace exhaust to help reduce its temperature. Figure 3 also illustrates that in the preferred embodiment, the housing exhaust pipe 133 has portions that are large enough for a smaller portion of the furnace exhaust pipe 132 to be positioned therein in generally concentric fashion (when the pipes are circular). In this manner, the generally cooler air that the second fan 121 draws into the housing 130 and through the housing exhaust pipe 133, cools the generally much hotter gaseous byproducts from the furnace 118 and the afterburner 117 that travel through the furnace exhaust pipe 132. Thus, the invention provides an apparatus and method for drawing one airflow through the furnace 118 and into the furnace exhaust 132 while concurrently drawing another, separate airflow through the housing 130 and into the housing exhaust 133, and then joining the exhausts to form a common exhaust from both the housing 130 and the furnace 118. It will be understood that a single fan, if positioned appropriately downstream, can draw the separate airflows, but in preferred embodiments the two separate fans 120 and 121 are incorporated. In the preferred embodiments, the fans 120 and 121 are each located upstream from the junction 134 (with downstream referring to the exit portions of the exhaust) although other arrangements in the positioning of the fans are appropriate provided the airflows are drawn in the manner of the invention. Figures 4 and 5 show some of these same elements in a more realistic and less schematic illustration. In Figure 4 the apparatus 110 includes the overall housing 130, the housing for the scale 122, the afterburner 117, the furnace exhaust pipe 132 , and the common exhaust 135. Figure 4 also illustrates a control panel 137, the first fan 120, and a third fan 140 which is used to cool the electronic portions of the device rather than the airflow from the furnace or the housing. As in many microwave devices, the housing 130 includes a door 140 for providing access to the furnace therein, as well as a window 141 that permits portions of the interior to be visible from the exterior of the housing 130. Figure 4 also -17- illustrates a basic power switch 142 and connections for exchanging data (e.g. parallel and serial connectors) 143. Figure 5 illustrates many of the same elements as 3 and 4, but from a rear perspective view. In addition to those elements already recited with respect to Figure 4, Figure 5 shows the ambient air stem 136 and the housing exhaust pipe 133. The second fan 121 is generally interior to the housing in these embodiments, and thus not visible in Figures 4 or 5. Figure 5 also shows a power connector 144 for providing current to the device from a typical commercial or industrial line. The apparatus aspects of the invention compliment the method. In its basic aspects, the method of the invention comprises heating a furnace with a preweighed container carrying a sample of an asphalt-aggregate composition therein while drawing air through the furnace at a rate that avoids impeding the heating of the furnace or the sample until the sample in the container reaches its combustion point and the combustion of the sample becomes exothermic. At that point, the draw of airflow through the furnace is accelerated to increase the rate of combustion of the exothermic reaction until the exothermic reaction is complete. Thereafter, the container and sample are re-weighed. With respect to weighing the sample, it will be understood both from the apparatus aspects of the invention and from the techniques described herein that in preferred embodiments, the container and sample are continuously weighed during the heating and combustion processes. As further set forth herein, after the aggregate has been cooled, it is preferably sized to provide the useful information about the aggregate that is the desired end product of the overall analysis. As described with respect to the apparatus aspects, the method preferably further comprises initiating a second draw of air around rather than through the furnace to moderate the exterior of the furnace. In typical operation, the method comprises weighing the sample and its container before placing it in the furnace and also preheating the sample and container prior to placing them in the furnace. In the preferred embodiments, the furnace is preheated to a temperature higher than the preheated temperature of the container and sample. In typical embodiments, the container and sample are heated to a temperature of about 150° C prior to placing -18- them in the furnace, while the furnace is typically heated to a temperature above about 540° C prior to placing the container and sample therein. As in the apparatus aspects of the invention, the step of heating the furnace preferably comprises directing microwave radiation at microwave absorbent materials in the furnace that convert the microwave energy into heat. In another aspect, the step of heating the furnace while drawing air therethrough can comprise heating the sample until it reaches a first predetermined setpoint temperature and then accelerating the draw through the furnace to increase the rate of combustion of the sample. In this embodiment, the method preferably also comprises initiating the second draw of air around rather than through the furnace when the temperature reaches a second predetermined setpoint. In the most preferred embodiments, the second predetermined setpoint temperature is lower than the first predetermined setpoint temperature so that the second draw of air around the furnace is typically initiated prior to the acceleration of the draw through the furnace. As exemplary, but not limiting temperatures, the external draw of air through the housing is initiated when the temperature of the sample and container reach about 480° C, and the furnace draw is accelerated when the temperature reaches about 500° C. The temperature is monitored throughout the process, and preferably continuously. In another aspect, the method comprises heating a sample of the asphalt- aggregate composition in the furnace, drawing an airflow into, through, and out of the furnace to promote the combustion of the asphalt in the composition, drawing a separate airflow around and past, but not through, the furnace to help moderate the exterior temperature of the furnace without interfering with the combustion therein, and blending the airflow that has exited the furnace with the airflow that has passed the exterior to thereby moderate the temperate of the furnace exited airflow. In this embodiment, the method preferably further comprises blending an ambient airflow with the furnace-exited airflow, and thereafter blending again with the exterior airflow. As in the prior embodiments, the step of drawing the airflow out of the furnace preferably comprises drawing the airflow and gaseous combustion products from the furnace into the afterburner prior to blending it with the exterior airflow. -19- In operation to date, the apparatus and method of the invention appear to reduce the time required to conduct the combustion portion of an asphalt-aggregate analysis by at least about 40%. As set forth in the parent application, one of the advantages of faster analysis of asphalt-aggregate compositions is the ability to get information about the composition as soon as possible after the composition has been sampled. In that manner, any changes required in the aggregate mix or the aggregate asphalt composition can be made as quickly as possible. As noted above, one of the most important characteristics of an asphalt- aggregate composition is the particle size distribution of the aggregate materials. Thus, the method of the invention further comprises sizing the aggregate after combustion of the asphalt. In particular, the nature of the combustion process according to the invention provides a less distorted and frequently undistorted aggregate for which the sizes and size distribution can be more accurately determined. In typical embodiments, the step of sizing the aggregate comprises sieve analysis (e.g., U.S. standard sieve sizes), but it will be understood that any appropriate sizing technique can be used with the present invention and that the invention provides the advantage of maintaining the aggregate in its original size following combustion much more successfully than have prior ignition techniques. Because asphalt-aggregate compositions can vary so widely depending on a variety of factors, objective determinations of post-combustion aggregate quality can be difficult. Nevertheless, the appearance of the post-combustion aggregate can provide a qualitative measure of the success of the method in preserving the aggregate as closely as possible to its pre-combustion condition. In this regard, the post- combustion samples from asphalt-aggregate compositions analyzed according to the present invention give every indication of being in better— i.e. close or identical to pre- combustion— condition that samples analyzed using more conventional ignition techniques. It is thus expected that any objective evaluation of such post-combustion samples will similarly demonstrate the advantages of the present invention in properly preserving the aggregate. In summary, the invention provides a more controllable method of carrying out an ignition and combustion analysis of asphalt-aggregate compositions and does -20- so in a manner that minimizes or avoids the degradation of the aggregate that is commonly observed in conventional ignition and combustion testing. In the drawings and specification, there have been disclosed typical embodiments of the invention, and, although specific terms have been employed, they have been used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.

Claims: -21- Claims: 1. A method of analyzing asphalt-aggregate compositions comprising directing sufficient microwave radiation from a microwave source to a sample of an asphalt-aggregate composition to ignite the asphalt in the composition and to thereafter entirely combust the asphalt in the sample. 2. A method according to Claim 1 comprising: directing microwave radiation from a source to a sample of an asphalt- aggregate composition until the composition ignites; moderating the microwave radiation directed to the sample and the oxygen available to the sample to maintain the temperature of the ignited composition within a controllable range; and reducing the gaseous combustion byproducts substantially to carbon dioxide and water vapor. 3. An analysis method according to Claim 2 wherein the step of moderating the microwave radiation comprises measuring the temperature of the sample during combustion and then changing the microwave radiation applied based on the measured temperature. 4. An analysis method according to Claim 2 wherein the step of reducing the byproducts comprises carrying out a follow-up combustion step on the gaseous byproducts. 5. A method according to Claim 2 wherein the step of moderating the oxygen comprises decreasing the airflow to discourage overly rapid combustion and thereafter increasing the airflow to encourage slower combustion. 6. A method according to Claim 5 comprising measuring the temperature of the burning sample and controlling the airflow in response to the measured temperature. -22- 7. A method according to Claim 1 comprising: weighing a sample of an asphalt-aggregate composition; directing sufficient microwave radiation from a microwave source to a sample of an asphalt-aggregate composition to ignite the asphalt in the composition; thereafter continuing to direct microwave radiation to the sample until the asphalt in the sample is entirely combusted; weighing the sample after the asphalt has been entirely combusted; and sizing the aggregate. 8. A method according to Claim 7 and comprising weighing a sample selected from the group consisting of existing road materials, portions of asphalt-aggregate compositions that have just been produced, and portions of asphalt-aggregate compositions that are being produced. 9. A method according to Claim 7 wherein the step of sizing the aggregate comprises sieve analysis. 10. A method according to Claim 7 comprising: drawing air through the furnace at a rate that avoids impeding the heating of the furnace or the sample until the sample in the container reaches combustion and the combustion of the sample becomes exothermic; and accelerating the draw through the furnace to increase the rate of combustion of the exothermic reaction until the exothermic reaction is complete. 11. An analysis method according to Claim 10 and further comprising initiating a second draw of air around rather than through the furnace to moderate the exterior temperature of the furnace. 12. An analysis method according to Claim 10 comprising the steps of weighing and preheating the container and sample and thereafter placing the container and sample in the furnace. -23- 13. An analysis method according to Claim 12 comprising preheating the furnace to a temperature higher than the preheated temperature of the container and sample prior to placing the container and sample in the furnace. 14. An analysis method according to Claim 10 wherein the step of heating the furnace comprises directing microwave radiation at microwave-absorbent materials in the furnace that convert the microwave energy into heat. 15. An analysis method according to Claim 10 comprising weighing the container and sample while the container and sample are in the furnace and while the furnace is being heated. 16. An analysis method according to Claim 10 and further comprising sizing the aggregate after combustion of the asphalt. 17. A method according to Claim 10 comprising: placing a preheated, preweighed container carrying a sample of an asphalt- aggregate composition into a preheated furnace with the preheated furnace being warmer than the preheated sample; continuing to heat the furnace while drawing air through the furnace at a rate that avoids impeding the heating of the furnace or the sample until the sample in the container reaches its combustion temperature and the combustion of the sample becomes exothermic; initiating a second draw of air around rather than through the furnace to moderate the exterior temperature of the furnace; accelerating the draw through the furnace to increase the rate of combustion of the exothermic reaction; and reweighing the container and sample after combustion is complete. 18. A method according to Claim 17 and further comprising monitoring the temperature of the furnace during the heating, drawing and accelerating steps.; -24- 19. An analysis method according to Claim 18 comprising initiating the second draw of air when the temperature reaches a second predetermined setpoint. 20. An analysis method according to Claim 19 wherein the second predetermined setpoint temperature is lower than the first predetermined setpoint temperature so that the second draw of air around the furnace is initiated prior to the acceleration of the draw through the furnace. 21. A method of analyzing the aggregate content of asphalt-aggregate compositions, the method comprising: heating a sample of an asphalt-aggregate composition to combustion in a furnace; drawing an airflow into, through, and out of the furnace to promote the combustion of the asphalt in the composition; drawing a separate airflow around and past but not through the furnace to help moderate the exterior temperature of the furnace without interfering with the combustion therein; and blending airflow that has exited the furnace with the airflow that has passed the exterior to thereby moderate the temperature of the furnace-exited airflow. 22. An analysis method according to Claim 21 and further comprising blending an ambient airflow with the furnace-exited airflow. 23. An analysis method according to Claim 22 comprising blending the ambient and furnace-exited airflows, and thereafter blending with the exterior-passed airflow. 24. An analysis method according to Claim 21 wherein the step of drawing the airflow out of the furnace comprises drawing the airflow and gaseous combustion products into an afterburner prior to blending with the exterior airflow. -25- 25. An analysis method according to Claim 21 wherein the step of heating the sample comprises directing microwave radiation into the furnace and to a susceptor that absorbs the microwave radiation and converts it into heat. 26. An analysis method according to Claim 21 and further comprising sizing the aggregate after combustion of the asphalt. 27. An apparatus for analyzing the asphalt content of asphalt-aggregate compositions, said apparatus comprising: a source of microwave radiation; a cavity in communication with said microwave source; a sample holder in said cavity for holding a sample of an asphalt-aggregate composition during exposure to microwaves from said source; and thermal insulation between said sample holder and the remainder of said cavity for defining a furnace within said cavity. 28. An asphalt analyzing apparatus according to Claim 27 and further comprising: a susceptor within said cavity furnace for converting microwave radiation into heat; and means for minimizing or eliminating any undesired combustion products generated by the burning asphalt. 29. An asphalt analyzing apparatus according to Claim 27 wherein said sample holder further comprises a balance for measuring the weight of the sample in said sample holder, and further comprising a temperature sensor for determining the temperature within said thermal insulation. 30. An asphalt analyzing apparatus according to Claim 29 and further comprising means for controlling the microwave power applied to the sample based on the temperature detected by said temperature sensor. -26- 31. An asphalt analyzing apparatus according to Claim 28 wherein said combustion byproduct minimizing means comprises: an afterburner in fluid communication with said cavity; and; means for directing the combustion products from said cavity to said afterburner. 32. An asphalt analyzing apparatus according to Claim 31 wherein said directing means comprises: an opening in said cavity in fluid communication with said sample holder; a duct in communication with the downstream portion of said afterburner; and a first fan associated with said duct and downstream from said afterburner for drawing ambient air into said cavity and the afterburner products away from said afterburner. 33. An asphalt analyzing apparatus according to Claim 32 comprising means for moderating the airflow in response to the measured temperature. 34. An asphalt analyzing apparatus according to Claim 32 wherein said afterburner comprises a resistance-heated tubular furnace, and wherein said thermal insulation is heat resistant, microwave transparent and has a low thermal conductivity. 35. An asphalt analyzing apparatus according to Claim 34 wherein said insulation comprises an open cell quartz. 36. An asphalt analyzing apparatus according to Claim 28 wherein said susceptor comprises silicon carbide. 37. An asphalt analyzing apparatus according to Claim 28 comprising a plurality of susceptors in said cavity furnace. -27- 38. An asphalt analyzing apparatus according to Claim 27 wherein said sample holder is perforated to permit air to flow more freely though said sample during combustion. 39. An apparatus according to Claim 32 and further comprising a second fan for drawing air around said cavity to moderate the external temperature of said cavity and its immediate environment. 40. An analysis apparatus according to Claim 39 wherein said first and second fans are controllable variable speed fans. 41. An analysis apparatus according to Claim 40 and further comprising respective controllers that moderate the draws of said first and second fans based upon the sample temperature measured by said temperature sensor. 42. An analysis apparatus according to Claim 39 comprising a housing surrounding said cavity. 43. An analysis apparatus according to Claim 42 wherein said second fan is positioned to draw an airflow between said cavity and said housing. 44. An analysis apparatus according to Claim 39 wherein said cavity is formed of a material that is insulating with respect to conductive heat transfer and substantially transparent to microwave radiation. 45. An asphalt analysis furnace system that enhances the airflow and combustion of asphalt-aggregate compositions, said furnace system comprising: a furnace; a housing around said furnace; a furnace exhaust in communication with said furnace; a housing exhaust in communication with said housing; -28- means for drawing an airflow though said furnace and into said furnace exhaust; means for drawing a separate airflow though said housing and into said housing exhaust; and a junction between said exhausts that forms a common exhaust from said housing and said furnace. 46. An analysis system according to Claim 45 wherein said furnace drawing means and said housing drawing means comprises a single fan downstream from said junction. 47. An analysis system according to Claim 45 wherein said furnace drawing means comprises a first fan in communication with said furnace exhaust upstream from said junction and said housing drawing means comprises a second fan in communication with said housing exhaust upstream from said junction. 48. An analysis system according to Claim 45 wherein said furnace comprises a source of microwave radiation and a susceptor for absorbing the microwave radiation and converting it into heat. 49. An analysis system according to Claim 45 and further comprising an afterburner between said furnace and said furnace exhaust. 50. An asphalt analysis furnace system according to Claim 45 comprising: a first fan that pulls air though said furnace; a second fan that pulls air between said furnace and said housing, but not through said furnace; a furnace exhaust pipe in communication with said first fan; a housing exhaust pipe in communication with said second fan; and a junction between said exhaust pipes that forms said common exhaust. -29- 51. A furnace system according to Claim 50 wherein said furnace comprises a source of microwave radiation and a susceptor that absorbs microwave radiation and converts it into heat. 52. A furnace system according to Claim 50 comprising an ambient air stem in communication with said furnace exhaust pipe.

Inventors: BARCLAY DAVID ALLAN, REGIMAND ALI, BARCLAY, DAVID, ALLAN, REGIMAND, ALI
Applicants: BARCLAY DAVID ALLAN, CEM CORP, REGIMAND ALI, BARCLAY, DAVID, ALLAN, CEM CORPORATION, REGIMAND, ALI
Application year: 1999
Main classification: G01N31
IPC codes: G01N1, G01N5, G01N31, G01N33