Patent Application
20110110177
The present invention relates to the field of asphalt manufacturing and, in particular, to a reclaimed asphalt pavement (RAP) pre-heating system for pre-heating RAP being conveyed into asphalt manufacturing modules during the asphalt manufacturing process.
The creation and maintenance of millions of miles of roads depend on asphalt production. In today's industry, there is great emphasis on using recycled or reclaimed asphalt pavement (“RAP”). It reuses existing materials, rather than requiring the quarrying of new aggregate materials. It also has the advantage of already having some asphalt content, thus lowering the amount of asphalt needed to make a new finished product. In these respects, it is both economical and green. Using RAP presents challenges not present with traditional asphalt production, however. Specifically, although the RAP must be heated to at least 212° F. for the moisture to be released from the aggregate, as RAP contains asphalt, it can only be heated to a certain extent before the asphalt ignites around 375-400° F. As traditional asphalt production generally involves aggregate materials such as rock and sand, before the asphalt is introduced to the mix, there was not such a need for temperature control. Demand for asphalt that is 25-50% RAP is common now, but given the challenges inherent in using RAP, it is difficult to raise the percentage above 15%-25%. As demand for, and interest in, RAP grow, production systems with better heat control are necessary.
The raw material for roadway asphalt, known in the industry as Hot Mix Asphalt (“HMA”), is usually prepared at a batch or drum plant. In addition to the asphalt oil itself, HMA includes an aggregate, which is typically a mixture of sand, small rocks, and other filler material, such as shredded rubber tires, or may be RAP that is crushed into small pieces. This aggregate used in the manufacturing process invariably has moisture entrapped therein, which must be removed before the asphalt oil is added.
Conventional HMA plants include a conveyor belt upon which aggregate is conveyed into a rotating drying drum, which may include impellers to lift the aggregate to assist in the drying process. The drum is typically rotated and heated to a very high temperature. Heating is typically accomplished by firing an oil burner and using a fan located adjacent to the drum to direct a flow of hot air into the drum. The aggregate is tumbled in the hot air flow by the rotation of the drum, and by the impellers lifting the aggregate and dropping it into the air stream, essentially drying and heating the aggregate. Once the aggregate is heated to a desired temperature, typically in the range of 120-180° C., the aggregate is sufficiently dried and a flow of hot liquid asphalt is introduced to the aggregate, and mixed therewith, producing the finished HMA.
The process employed by conventional HMA plants is effective at drying and heating the aggregate and generally produces an acceptable end product. However, this process has three substantial drawbacks.
First, the burners used to heat the aggregate during the drying and pre-heating process use an enormous amount of fuel, which is costly both in terms of purchasing the oil and in terms of controlling the emissions produced thereby. Therefore, the longer these burners are forced to run, the greater the expense of producing the HMA. Unfortunately, even with the use of impellers to mix the aggregate during drying, bulk drying of the entire batch of aggregate at one time is inefficient and results in the burners being fired for a significant period of time to effect drying, resulting in a significant amount of expensive fuel being used and causing unnecessary emissions.
Second, the longer the drying process takes, the fewer batches of HMA that may be produced. Because the equipment used in HMA production is very expensive, and because the demand for HMA is such that all batches produced by a given plant would be readily sold, increasing the rate at which batches of HMA may be processed will greatly increase the profits for HMA manufacturers.
Third, the amount of aggregate used in each batch produced by the HMA manufacturing process is typically measured by the weight of the aggregate in the drying drum. Therefore, variations in the moisture content of the aggregate can cause the amount of aggregate to be too low. Thus, the manufacturer is forced to either live with these variations, resulting in batch-to-batch inconsistencies of the HMA produced, or to add more wet aggregate to the drum, which further increases the amount of fuel used and drying time.
In addition to HMA, a number of companies have recently developed formulations for Warm Mix Asphalt (WMA) to replace traditional HMA. WMA is manufactured using a process similar to BMA, but uses different formulations of aggregate and asphalt additives that are each mixed at lower temperatures than HMA. Switching from HMA to WMA reduces the amount of fuel used in the manufacturing process, as it is heated to lower temperatures than HMA, and allows the use of certain additives that cannot withstand the temperatures required during the production of HMA. Further, WMA has been found to reduce the curing time of the asphalt, allowing a shorter period of time between laying the asphalt surface and allowing the pavement to be used.
A number of tests on WMA have produced encouraging results. However, a recent report by the National Center for Asphalt Technology cautioned that the moisture content of the mix is an important consideration and cites the potential for moisture damage due to too much water left in close content with the aggregate. Thus, there is a need for a means for pre-drying the aggregate used in WMA production in order to reduce the chance of moisture damage.
At least two systems currently exist for drying particulate matter, such as aggregate, using a heating element over a conveyor belt. The first system is disclosed in U.S. Pat. No. 4,136,964 to Swisher. This is an apparatus for simultaneously mixing and conveying particulate material, the apparatus comprising a housing having an input end and an output end disposed vertically higher than the input end, means for feeding the particulate material into the input end of the housing, a conveyor disposed within the housing and having a plurality of lifting surfaces provided with perforations therethrough so that a portion of the particulate material being lifted by each lifting surface descends through the perforations and is mixed with particulate material being lifted by lifting surfaces disposed therebelow, and, means for discharging the mixed particulate material from the output end of the housing.
The housing of the mixer/conveyor is provided with at least one opening through the upper wall section thereof to facilitate the introduction of heated exhaust gases produced by an associated burner assembly connected thereto. Each burner assembly is comprised of a housing having suitable refractory material lining the inner surfaces thereof, and an oil or gas fired burner of the conventional construction. Hydrocarbon fuel for the burner will be supplied in a conventional manner from a suitable source of fuel, while combustion-supporting air is preferably supplied by a blower assembly via an air duct. An adjustable draft, exhaust damper assembly should also be connected to the output end of the housing to facilitate control of the pressure in the mixer/conveyor as well as to direct the heated exhaust gases exiting from the housing.
Although capable of drying particulate matter, this system, and its burner assembly in particular, is ill suited for portability. First, the mixer/conveyor and burner assembly are bulky because they are completely enclosed so that the heated exhaust gases may flow across the material to be dried. This bulk cannot be diminished to provide better portability without thwarting the burner assembly's heating capabilities. Moreover, doing so, especially in the field, would cause the introduction of the exhaust gases into the atmosphere. Second, in part because of the system's bulk, it is difficult to set up and break down, which is a key aspect of portability. Given the size and weight of the various assemblies that make up the system, a crane is necessary to position the assemblies. A crane is another large vehicle, requiring a skilled operator, which would have to come out to the site, greatly increasing the cost and carbon emissions of using the system.
The second currently existing system for drying particulate matter, such as aggregate, using a heating element over a conveyor belt is disclosed in U.S. Pat. No. 5,557,858 to Macaluso et al. This system is an infrared wood product dryer apparatus including an enclosure structure defining an interior, a conveyor assembly configured for conveying a particulate material along a material flow path through the interior substantially between an inlet and an outlet, an array of infrared radiant energy sources configured for exposing the material to infrared radiant energy while it is conveyed along the path, and a series of agitators configured for agitating the material in order to increase the exposure of the material to the infrared radiant energy. A gas recirculation assembly is provided to direct a heated interior gas onto the material in order to convection-dry the material. An exhaust assembly reduces the moisture content of the interior gas by drawing a quantity of the gas from the dryer so that fresh gas having a lower moisture content may be drawn into the dryer.
This system is also capable of drying particulate matter, but is also ill suited for portability. First, as a practical matter, the inlet to which material to be dried is introduced to the system is at the top of the system, so that material must be lifted up to be dried. This lifting up will likely require an added mechanical element, for the system to be used in the field. The more elements necessary for the system's use, the more costly and difficult it becomes to use in the field. Moreover, portable systems need to be able to be driven around on a truck or other vehicle, and this system is too tall for that type of transportation. Any system that is going to be driven on roads needs to be fairly flat so that trucks can get through tunnels and other low passages while hauling the system. This system has at least three stacked layers of conveyor belts and heaters within the enclosure structure, making it quite tall, and thus unwieldy for road transportation.
Therefore, there is a need for a system for producing asphalt from RAP that effectively and safely dries and/or pre-heats RAP for use in HMA or WMA production.
The present invention is a system and method for producing asphalt from RAP. In its most basic form, the system for creating asphalt from RAP includes a conveyor belt in communication with a source of RAP, at least one infrared chamber, a source of fuel, at least one mixer disposed between the infrared chamber and the conveyor belt, and a drum or batch heater or other blending device. The conveyor belt is preferably manufactured of a rubberized material that is adapted to convey the aggregate material at a predetermined rate. The infrared chamber, or chambers, is/are disposed in substantially parallel relation to the conveyor belt at a distance sufficient to allow infrared heating of the aggregate material. The fuel source is preferably a gaseous fuel, such as propane or natural gas, which is in communication with the infrared chamber. The mixer is dimensioned and disposed relative to the conveyor belt so as to mix the RAP during pre-drying.
In operation, the conveyor belt conveys the RAP from its source and under the infrared chamber at a predetermined, but adjustable, rate to a terminal end of the conveyor belt. The fuel flows to the infrared chamber, which burns the fuel causing the infrared chamber to emit infrared radiation therefrom. The infrared radiation contacts the top surface of the aggregate that is conveyed on the conveyor belt and acts to heat the top portion of the RAP proximate to the infrared chamber. The mixer then mixes the RAP such that heated and unheated RAP are mixed together, allowing the unheated RAP proximate to the conveyor belt to rise to the top surface proximate to the infrared chamber. The infrared radiation from the infrared chamber then heats the mixed RAP to effectively pre-heat and pre-dry the RAP.
The preferred heating system includes at least three, and preferably eight infrared chambers. The preferred infrared chambers include a solid aluminum top cover. Moisture escapes out of the upper end of the infrared chambers in the gaps between the chambers where the mixers are placed, as discussed below. The conveyor belt is preferably inclined so as to act as a chimney. In an alternative embodiment, the top portion of the infrared chamber is manufactured of expanded metal, which allows for the escape of such moisture. In still other embodiments, the chamber includes a plurality of holes formed therethrough to allow such moisture to be vented. Each infrared chamber includes a plurality of infrared converters.
The preferred heating system also includes at least one igniter assembly in communication with the infrared chamber. The igniter assembly is adapted to ignite the fuel within the infrared chamber so that it may be burned and turned into infrared radiation. The preferred embodiment utilizes multiple, individual igniter assemblies, which are each in communication with a single infrared chamber and are independently controlled by the control box. Each igniter assembly is itself in communication with the source of fuel, and includes two igniter rods, two sparker transformers with mounting plates, one flame sensor, a main gas valve, and a pressure switch. The rods are in communication with the infrared converters of the infrared chamber, so that the ignited fuel may travel into the converters. Having individual igniter assemblies for each infrared chamber, rather than a single igniter used to ignite the fuel within each of the chambers, is preferable as it allows individual infrared heaters to be turned off. This provides greater control over the heating of the system, which is particularly important when using RAP.
The mixers are placed between the infrared chambers so as to mix the RAP as it travels along the conveyor belt. The infrared chambers are preferably disposed in a row over the conveyor belt such that each infrared chamber is disposed relative to an adjacent infrared chamber so as to form a gap therebetween. One mixer is preferably disposed within each gap. One mixer is also preferably disposed at the end of the conveyor belt, after the last infrared chamber.
In the preferred embodiment for use with RAP, the mixer is a rotary mixer assembly. This preferred mixer is a bolted roller with 5″ diameter disposed across the conveyor belt. The roller includes six evenly spaced bolts protruding from cross sections spaced 2″ apart down the length of the pipe, with every other set of bolts being offset from the sets of bolts on either side of it. In the preferred embodiment, the mixers are individually controlled as to how fast they rotate, and thus mix the aggregate.
In another embodiment, the mixer is a series of ramps that are disposed proximate to the conveyor belt. Each ramp has a ramp surface that is disposed at an angle from the plane formed by the conveyor belt and is dimensioned to allow the asphalt aggregate to be pushed up the ramp by aggregate that is in contact with the conveyor belt and to tumble back onto the belt, effectively mixing the aggregate. This embodiment is not preferred because the preferred incline of the conveyor belt leads to damming of the aggregate material at the mixers.
In another embodiment, the mixing is a result of multiple conveyor belts that are spaced to allow the aggregate material to tumble from one conveyor belt to the adjacent conveyor belt. Once tumbled, the aggregate material is further mixed by moving through the tines of a mixer as described below. This embodiment is not preferred as the multiple conveyor belts, each with separate mechanics, drive up the cost of the system.
In some embodiments, the mixers include a plurality of tines forming a plurality of spaces therebetween, and each is dimensioned and disposed within each gap such that each tine of one mixer is aligned with a space of adjacent mixers. In this manner, the aggregate is thoroughly mixed rather than just having the areas adjacent to the tines mixed. This embodiment is not preferred for use with RAP because it has been found that it does not release moisture from the aggregate as well as the mixers mentioned above.
In other embodiments, the mixer includes a channel, and a plurality of tines rotatably attached to the channel. The tines are in communication with at least one spring and are disposed in sufficiently close proximity to the conveyor belt so as to contact the aggregate material conveyed by the conveyor belt. The spring allows the tines to flex while preventing them from becoming entangled with the conveyor belt. In this embodiment, the tines may be joined together into a rake and a single spring is used to maintain the tines in position. However, in other embodiments, each tine is independent from the other tines and is in communication with its own spring. These embodiments are also not preferred for being less efficient at moisture release.
In the preferred embodiment, an area at the end of the conveyor belt includes multiple smaller mixers disposed between the conveyor belt and the infrared chambers. This area is preferably the length of two infrared converters down the conveyor belt, but may be less or more area of the conveyor belt. The small mixers are preferably similar to the preferred bolted rollers discussed above, but smaller in diameter and the length of the protruding bolts. These small mixers may be directly adjacent to one another, rather than distanced by the length of an infrared chamber. They may also be periodically spaced between the larger mixers that are spaced between each infrared chamber. The speed at which the small mixers rotate is also preferably controllable, and is preferably faster than the speed of rotation of the large mixers. These mixers protect against ignition of the asphalt within the RAP by ensuring even distribution of heat throughout the aggregate when the RAP is at its hottest at the end of the conveyor belt.
The preferred embodiment includes further heat control measures to safeguard against asphalt ignition. Specifically, the second to last space above the conveyor belt for an infrared chamber is preferably occupied not by an infrared chamber, but by a heat reflector that will reflect back the heat of the aggregate, but not introduce additional heat. There is also preferably a thermometer or other heat sensor between the heat reflector and the last infrared chamber that indicates the temperature of the aggregate at that point. The last infrared chamber will be activated if the aggregate is not near temperatures at which the asphalt is likely to ignite. Alternatively, it will remain dormant, without adding additional heat to the aggregate if the aggregate is near temperatures at which the asphalt is likely to ignite. Although this is the preferred embodiment, it is understood that all spaces may be filled with infrared chambers, and that heat reflectors may be substituted for one or more heat chambers in any of the positions along the conveyor belt. Finally, at least the last infrared chamber at the end of and above the conveyor belt, but preferably all infrared chambers above the length of the conveyor belt are adapted to move up and down to increase or decrease the distance between the infrared chambers and the conveyor belt. Furthermore, as discussed above, as each infrared chamber includes a designated igniter, it is possible to turn each individual infrared chamber on and off as desired.
The preferred embodiment of the present invention also includes a drum heater disposed at the terminal end of the conveyor belt such that the dried RAP is deposited into the drum heater. The drum heater may be substituted with a pugmill. For HMA, the drum heater adds liquid asphalt as needed to the dried and heated RAP and mixes all to form new asphalt. For WMA, the drum heater adds the requisite additives and mixes the all to form new asphalt. It is preferred that the drum heater include additional external heaters around the exterior of the drum heater to provide additional heat in the asphalt producing process.
Some embodiments of the heating system of the present invention include at least one hygrometer and/or thermometer for determining the moisture content and/or temperature of the aggregate at least at the start of the pre-heating process. In such embodiments, the hygrometers and/or thermometers are preferably in communication with a conveyor control, which slows the conveyor belt or speeds up the conveyor belt based upon the amount of moisture within the aggregate. Thus use of such a control is preferred when the system is used in connection with WMA manufacturing as it allows for careful control of the amount of moisture while maximizing the speed of the process in instances where there are minimal amounts of moisture within the aggregate.
The preferred heating system includes a control box in electrical communication with the infrared chamber and the source of fuel. The control box includes controls for controlling several system functions, including the operation of each individual igniter assembly, the flow of fuel from the source to the infrared chambers, blower motor operation, conveyor belt speed control, large mixer rotation speed controls, small mixer rotation speed controls, and infrared chamber elevation controls.
The preferred heating system also includes at least one blower motor in communication with the control box, the source of fuel, a source of air, and the infrared chamber. The blower motor is controlled by the control box and is adapted to mix fuel and air together and force the mixture of fuel and air into the infrared chamber. The use of a blower motor is preferred as it allows the infrared chamber to consistently produce a greater amount of heat than may be produced by relying upon the pressure from the source of fuel.
The system and method of the present invention are very well suited for use with RAP. As described above, several heat controlling measures, such as adjustable elevation of the infrared chambers, individual controls for each infrared chamber, temperature sensors, additional mixers, and the strategic substitution of heat reflectors for infrared chambers in certain places make these systems much safer to use with RAP. As temperature is well controlled, the possibility of igniting the asphalt inherent in the RAP is lessened, if not eliminated. Moreover, the preferred mixer for these systems and the preferred positioning of the conveyor belts at an incline allow for optimal moisture release from the RAP, which commonly has high moisture content at the outset.
Therefore, it is an aspect of the invention to provide a system for safely and effectively pre-heating and pre-drying RAP during asphalt production.
It is a further aspect of the invention to provide systems capable of controlling the heat during RAP drying so that the asphalt in RAP does not ignite.
It is a further aspect of the invention to provide a system capable of reaching the optimal temperatures for asphalt production.
It is a further aspect of the invention to provide a reflector used in connection with heaters in order to focus heat downward onto the RAP.
It is a further aspect of the invention to provide a system and method to reduce the risk of moisture damage due to the presence of excess moisture in RAP used in the production of WMA.
These aspects of the invention are not meant to be exclusive and other features, aspects, and advantages of the present invention will be readily apparent to those of ordinary skill in the art when read in conjunction with the following description, appended claims and accompanying drawings.
Referring to
The system 10 includes a conveyor belt 12 in communication with a source (not shown) of the RAP 22. The conveyor belt 12 has a first end 11, where RAP 22 is first deposited onto the conveyor belt 12, and a terminal end 13, where the dried and heated RAP 22 leaves the conveyor belt 12 and is deposited into asphalt producing module 70. The conveyer belt 12 preferably takes the form of conveyor belts currently used to transport aggregate material in conventional HMA and WMA manufacturing processes. The belt may be manufactured of a non-combustible material, such as steel, but is preferably a composition belt manufactured of a rubberized material. Such a material is preferred due its gripping properties and price. The conveyor belt 12 is adapted to convey the RAP 22 at a predetermined rate from the source to the asphalt production module 70. The conveyor belt 12 is preferably in an inclined position, up to a four to one incline, to facilitate the chimney like aspects of the aluminum top portion 77 (shown in
The system 10 also includes at least one infrared chamber 14. In the embodiment of
The infrared chambers 14 are in communication with a source of fuel 16, preferably propane but natural gas may also be used. When natural gas is used, the gas must be introduced to the infrared chambers 14 at a higher pressure than when propane is used, but no modifications to the hardware of the system are necessary to use natural gas instead of propane. In the embodiment of
The preferred blower motor 25 also includes a shut off valve (not shown) to shut off the flow of fuel 16 to the infrared chambers 14. A control box 18, which is in electrical communication with the blower motor 25 and a source of power 50, preferably controls the operation of the blower motor 25. Source of power 50 is capable of powering the control box 18 and the mechanical elements of system 10. As described with reference to other embodiments, the control box 18 may also include other controls, such as ignition controls, speed controls for the conveyor belt 12 and mixers 20, 19, and elevation controls for the infrared chambers 14. The use of a blower motor 25 and control box 18 is preferred as it allows the infrared chamber 14 to consistently produce a greater amount of heat than may be produced by relying upon the pressure from the source of fuel 16 alone. However, in other embodiments, both the blower motor 25 and control box 18 are eliminated and the flow of fuel 16 to the infrared chambers 14 is controlled via a manually operated valve (not shown), which opens to allow fuel 16 to flow solely via pressurization from the fuel source and is closed to shut off flow completely.
At least one large mixer 20 is disposed between the infrared chambers 14 and the conveyor belt 12. The large mixer 20 is dimensioned and disposed relative to the conveyor belt 12 so as to mix the aggregate material 22 during pre-drying. The large mixer 20 contacts the aggregate material 22 and mixes the hot top layer with the cool lower layer to form a substantially homogenous mixture. The infrared chambers 14 and heat reflector 93 are disposed so as to form a gap 39 therebetween. A large mixer 20 is preferably disposed within each gap 39. A final large mixer 20 is disposed at the terminal end 13 of conveyor belt 12. The large mixer 20 is described in more detail with reference to
Small mixers 19 are preferably included under the heat reflector 93 and infrared chamber 14 closest to the terminal end 13 of the conveyor belt 13, but may be spread throughout the length of conveyor belt 12. The small mixers 19 are preferably similar in both shape and function to the large mixers 20 discussed above and in more detail with reference to
In operation, the conveyor belt 12 conveys the RAP 22 from its source and under the infrared chambers 14 at a predetermined rate. The fuel 16 flows to the infrared chambers 14, which burn the fuel 16 causing the infrared chambers 14 to emit infrared radiation therefrom. The infrared radiation contacts the top surface of the RAP 22 that is conveyed on the conveyor belt 12 and acts to heat the top portion of the RAP proximate to the infrared chambers 14. Large mixers 20 then mix the RAP 22 such that the heated and unheated layers of RAP 22 are mixed together, allowing the unheated RAP 22 proximate to the conveyor belt 12 to rise to the top surface proximate to the infrared chamber 14. The infrared radiation from the infrared chamber 14 then heats the mixed RAP 22 to effectively and safely pre-heat and pre-dry the RAP 22.
System 10 also includes asphalt producing module 70, which is preferably a drum mill 72, but may also be a pug mill. Asphalt producing module 15 may add at least one additive to the pre-heated and pre-dried RAP 22 that is deposited into asphalt producing module 15 by conveyor belt 12. The additive may be heated liquid asphalt for HMA production or the requisite additives for WMA production. The additive addition may be automated. The same asphalt producing module 15 may be used for either WMA or HMA. Asphalt producing module 15 then produces new asphalt from the RAP.
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Although the present invention has been described in considerable detail with reference to certain preferred versions thereof, other versions would be readily apparent to those of ordinary skill in the art. Further, although the heating system was developed for use in connection with aggregates used as paving materials, it is readily adapted for use with aggregates used for other purposes, such as livestock feed, or the like. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.
This application is a continuation in part of co-pending U.S. Non-Provisional patent application Ser. No. 11/805,021, filed on May 22, 2007, and claims the benefit of priority of U.S. Provisional Patent Application Ser. No. 60/802,360, filed on May 22, 2006.
| Number | Date | Country | |
|---|---|---|---|
| 60802360 | May 2006 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11805021 | May 2007 | US |
| Child | 12924115 | US |
