US20150345085A1

US20150345085A1 - Multiple-entry hot-mix asphalt manufacturing system and method

Info

Publication number: US20150345085A1
Application number: US14/723,163
Authority: US
Inventors: Robert E. Frank
Original assignee: Rap Technologies LLC
Current assignee: Rap Technologies LLC
Priority date: 2014-05-29
Filing date: 2015-05-27
Publication date: 2015-12-03
Also published as: CA2893159A1

Abstract

A hot-mix asphalt manufacturing system for recycled asphalt coated aggregate (RAP) has a rotary dryer and burner operable to heat the dryer to create a maximum gas temperature at a location between first and second ends of the dryer. A first inlet is upstream and a second inlet is downstream of the maximum temperature. A first coarse aggregate is introduced through the first inlet, fine RAP is introduced through the second inlet, and the aggregates are heated and mixed forming a mixture outputted at an outlet. For 100% RAP processes, the first and second aggregates can consist of RAP. For less than 100% RAP processes, virgin aggregate can be introduced through the first inlet, located adjacent the first end of the rotary dryer, and a third inlet can be located between the maximum temperature and the first inlet, with fine and coarse RAP introduced through the second and third inlets.

Description

FIELD OF THE INVENTION

The present teachings relate generally to asphalt manufacturing and, more specifically, to hot-mix manufacturing using recycled asphalt coated aggregate.

BACKGROUND OF THE INVENTION

The asphalt manufacturing industry has in the past used recycled asphalt coated aggregate (commonly called RAP) in place of or in combination with virgin aggregates. However, doing so introduces a number of problems. For example, undesirable hydrocarbon emissions can be created when exposing the asphalt coated RAP to hot combustion gases.
Strategies to heat RAP without creating excessive hydrocarbon emissions have included microwave heating, indirect heating, and most commonly mixing with superheated virgin aggregates. However, such strategies suffer unnecessary restrictions on capacity to heat the RAP and, consequently, limit maximum recycled content or production rate.
U.S. Pat. No. 6,478,461, the content of which is incorporated by reference in its entirety, was previously issued to the inventor and discloses a transportable hot-mix asphalt manufacturing and pollution control system for use with recycled asphalt coated aggregate (RAP). The system disclosed in the '461 patent utilizes a single entry point for aggregate.
U.S. Pat. No. 3,999,743 to Mendenhall, the content of which is incorporated by reference in its entirety, discloses an asphalt-aggregate recycling process using center entry in a rotary dryer to limit radiant and/or convective heat transfer. The system disclosed in the '743 patent teaches a parallel flow dryer with coarse, more difficult to heat material introduced closer to the flame and fine particles furthest away.
The techniques used in the prior art suffer a number of limitations, including inefficiencies realized when using higher percentages of RAP aggregate. Therefore, it would be beneficial to have a superior system and method for hot-mix asphalt manufacturing.

SUMMARY OF THE INVENTION

The needs set forth herein as well as further and other needs and advantages are addressed by the present embodiments, which illustrate solutions and advantages described below.
Traditionally, the use of RAP by many asphalt manufacturers has been limited to around 40% of mixture. However, the present teachings enable the efficient use of RAP up to 100%. A system according to the present teachings provides a number of other benefits beyond the increased use of recycled materials. For example, pollution control is also improved as less heat may be used to heat the RAP and airborne particulates may be substantially reduced. These benefits may result in a simplified pollution control system.
In one embodiment, the system according to the present teachings includes a multiple entry rotary dryer having one or more “center” entries for introducing recycled asphalt coated aggregate (RAP). The system may include a counter flow heating apparatus that provides a parabolic temperature profile in the rotary dryer. The temperature profile may be used to determine desirable locations for introducing coarse and fine aggregate. For example, coarse RAP aggregate may be introduced before (i.e., upstream of) the peak temperature in the rotary dryer whereas fine RAP aggregate may be introduced after (i.e., downstream of) the peak temperature relative to direction of material flow.
Other embodiments of the system and method are described in detail below and are also part of the present teachings.
For a better understanding of the present embodiments, together with other and further aspects thereof, reference is made to the accompanying drawings and detailed description, and its scope will be pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a rotary dryer according to one embodiment of the present teachings.

FIG. 2 is a temperature profile of the rotary dryer according to the system of FIG. 1.

FIGS. 3A-B are schematic views of a center entry collar and bucket for use with the system of FIG. 1.

FIG. 4 is a perspective view of the combustion zone inside a dryer having insulator flights in area of peak air temperature.

FIG. 5 is a side elevation cross-sectional view of a dryer having insulator flights at zone of peak air temperature.

FIG. 6 is a plan view of a pair of insulator flights.

FIGS. 7 and 8 are perspective views from upstream and downstream ends of a combustion zone inside a driver having insulator flights in the area of peak air temperature and having a dam at a downstream end of the zone.

FIG. 9 is a schematic cross-sectional view of a rotary dryer according to an embodiment of the present teachings showing a unitary pollution control system.

DETAILED DESCRIPTION OF THE INVENTION

The present teachings are described more fully hereinafter with reference to the accompanying drawings, in which the present embodiments are shown. The following description is presented for illustrative purposes only and the present teachings should not be limited to these embodiments.
For purposes of explanation and not limitation, specific details are set forth in order to provide a thorough understanding. In other instances, detailed descriptions of well-known devices and methods are omitted so as not to obscure the description with unnecessary detail.
Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to “a/an/the element, apparatus, component, means, step, etc.” are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. The use of “first”, “second” etc. for different features/components of the present disclosure are only intended to distinguish the features/components from other similar features/components and not to impart any order or hierarchy to the features/components.
The present teachings can be used to increase the use of recycled asphalt content in any number of different systems. For example, direct fired counter flow (CF) rotary dryers, continuous mix plants having separate mixers after dryers, batch mix plants, and CF dryers with external combustion sources, although not limited thereto, may benefit from the present teachings. A continuous mix plant is a plant where aggregates are coated with an asphalt binder and other liquid additives in a rotary dryer after drying. In contrast, a batch mix plant may store heated aggregates and then mix discrete batches with an asphalt binder and liquid additives in a space separate from the dryer. Although specific embodiments are disclosed, one skilled in the art would appreciate that the present teachings are not limited thereto.
RAP is typically recovered from existing road surfaces and other areas by a milling process, or another suitable process, and is broken apart into constituent particles for processing and reuse. RAP may be separated based on particle size that are commonly characterized in two classes: fine and coarse aggregate, which can be distinguished based on their relative sizes. The classification of aggregates as fine or coarse in the asphalt industry is generally known. It is generally accepted that “sands,” either manufactured or natural, are fine aggregates while “stones” are coarse aggregates. For example, fine aggregates may include those that pass through a #4-#8 sieve (e.g., 0.187 to 0.0937 inch sieve opening), where coarse aggregates are larger and may be retained on that sieve.
Fine and coarse aggregates may also be distinguished by differences in their surface area to mass ratios. Fine aggregates may have high surface areas relative to their mass (e.g., a high specific surface area), whereas coarse aggregates may have small surface area to mass ratios (e.g., a low specific surface area). Therefore, fine aggregates may have higher heat transfer rates per unit weight relative to coarse aggregate since heat transfer is proportional to surface area. Similarly, the temperature of fine aggregates may increase more quickly than coarse aggregates at a given differential temperature since fine aggregates have a smaller mass and temperature increase is inversely proportional to particle mass.
It has been found that fine RAP aggregate may heat more rapidly from hot combustion gases than coarse RAP due to fine RAP's large surface area ratio. In a system where fine and coarse aggregate are introduced together, fine aggregates tend to give up heat to coarse aggregate as the two lay commingled in the bottom of a dryer. This exchange may continue as the dryer rotates and the aggregate progresses down the dryer from inlet to discharge outlet. However, this may cause undesirable clumping and adhesion of fine aggregate on the dryer due to the asphalt binder coating the fine aggregate being activated during heating and then being cooled by the relatively colder coarse aggregate to a point where it becomes tacky, causing clumping.
To overcome the above problem, it has been found that it is beneficial to have separate feed locations for fine and coarse RAP when a higher recycled content is desired (e.g., in rotary dryers on batch or continuous mix drum plants). This may be especially so in counter flow dryers where the heat source (e.g., burner) is nearer to the discharge point and directs heat toward the colder inlet end of the dryer.
It has been found that, since coarse RAP requires more time to heat it can be introduced earlier in a counter flow rotary dryer without significant increase in hydrocarbon emissions. The large thermal mass of coarse aggregate serves to moderate the temperature increase of the aggregate. Consequently, coarse RAP can be exposed longer to hotter gas temperatures (and the radiant energy of a burner flame) than fine RAP without visible emissions or increasing hydrocarbon emissions above other asphalt manufacturing processes.
Further, it has been found that fine RAP's high specific surface area enables it to be dried and heated in significantly less time than coarse aggregate. Therefore, it can be introduced behind (i.e., downstream of) the peak temperature profile yet still release surface moisture and reach adequate discharge temperature at high feed rates.
As will be discussed in more detail below, rotary dryers may have a number of distinct zones, depending on plant type. Such zones may be identified by their temperature profiles and/or function, although not limited thereto. A “drying zone” may be optimized to transfer heat from combustion gases to raw material by means of convective transfer to fine aggregate, and conductive transfer from the fine aggregate to the coarse aggregate.
A “combustion zone” may be optimized to facilitate complete combustion of fuels in a space shared with aggregates. To protect the dryer, insulator flights (discussed below) may be provided in the combustion zone (and other zones) to insulate the dryer shell and to convert radiant energy to thermal that is then transferred to the aggregate prior to exiting the combustion zone.
Finally, a “mixing zone” may be downstream from the drying and combustions zones to facilitate mixing asphalt cement binder (or other supplemental ingredients) with the heated aggregate.
Although three zones are discussed herein, the present teachings are not limited thereto, and one skilled in the art would appreciate that a system could utilize more or fewer zones without deviating from these teachings.
In one embodiment, the air temperature profile in the combustion zone may be roughly parabolic, going from the material discharge temperature (e.g., ˜300 F) behind the flame (in a counter-flow dryer) to a maximum temperature at a midpoint of the combustion zone along the dryer length (e.g., ˜1400 F-2000 F) and then gradually back down to lower inlet temperature (e.g., ˜280 F) at the first inlet. Fine RAP may be introduce after the maximum temperature point while coarse RAP may be introduced at a point where it will pass through that maximum. Coarse RAP may also be introduced in the drying zone, but this may result in increased hydrocarbon emissions that potentially could require treatment to comply with air pollution regulations.
Referring now to FIG. 1, shown is a schematic cross-sectional view of a rotary dryer according to one embodiment of the present teachings.
As shown, a hot-mix asphalt manufacturing system may comprise a rotary dryer 100 adapted to receive ingredients of hot-mix asphalt and to perform a drying and heating process on such ingredients, although not limited thereto.
Preferably, the rotary dryer 100 is a counter-flow rotary dryer. The term “counter-flow” is understood to mean that the materials being dried in the rotary dryer 100 generally have a flow stream (i.e., are conveyed) in one direction, whereas the hot combustion gases and/or by-products of the drying process flow in an opposite direction. As shown, the rotary dryer 100 may include a burner 102 (or other heat source) and a rotatable drum 104 having a first end 106, which is elevated, and a second end 108, which is lower than and opposite the first end 106. The burner may provide a flame with a conical shape, although not limited thereto. Material may be heated with hot gases traveling upstream in a direction opposed to the material flow stream. The rotary dryer 100 may include a drying zone 120, combustion zone 122 and mixing zone 118, discussed further below.
In an alternative embodiment, a separate mixing/coater may be used. This includes batch type plants having a separate batch tower and pugmill mixer that combine aggregates with fresh asphalt. A separate external continuous coater may be either a paddle type mixer or rotating drum.
An external combustion source may also be used. In this case, the flame is removed from the rotary dryer. Combustion may take place in a stationary space typically lined with refractory or similar heat resistant materials where radiant energy is converted into thermal energy (e.g., hot gases) before entering the rotary dryer.
A first inlet 110 for “new” aggregate (e.g., fresh or virgin aggregate) may be located at or adjacent the first (elevated) end 106 of the rotatable drum 104. What is meant by “new” or virgin aggregate is aggregate that has not been (or essentially has not been) reclaimed or recycled from a previous asphalt mixture, for example from an existing road surface. A second inlet 114 for fine RAP may be located at an intermediate position between the first inlet 110 and the second end 108. A third inlet 112 may be used for coarse RAP and may be located at an intermediate position between the first inlet 110 and the second inlet 114. Specifically, the third inlet 112 can be downstream of the first inlet 110 and upstream of the second inlet 114. A discharge outlet 116 for the hot-mix asphalt manufactured by the rotary dryer 100 can be located at or near the second end 108. One skilled in the art would appreciate that this configuration is exemplary and the present teachings are not limited thereto.
For hot-mix asphalt processes using at or near 100% recycled asphalt coated aggregates (RAP) (or when virgin aggregates are commingled with coarse RAP), the aggregate may be introduced through the first inlet 110 and fine RAP may be introduced through the second inlet 114, in which case the third inlet 112 could be unused or omitted. However, it may be preferable to have the first, second and third inlets 110, 114, 112 for conventional processes using less than 100% RAP so new material may be introduced at the first inlet 110, with fine and coarse RAP introduced at the second and third inlets 114, 112, respectively.
In a system using less than 100% RAP, fresh (virgin) primary aggregates may be brought to the first inlet 110 and introduced into the rotatable drum 104 using a conventional conveyor system (e.g., a belt-type conveyor). The fine and coarse recycled ingredients may be introduced through the second and third inlets 114 and 112, respectively, by a RAP collar or another suitable method. For process using 100% RAP and/or less than 100% RAP, the coarse and fine aggregates may be about 64% and about 36% of the total RAP content, respectively, although not limited thereto.
Supplemental ingredients (e.g., additives) can be introduced into the mixing zone 118 to be mixed with the coarse and fine aggregate after they have substantially completed drying and heating in a drying zone 120 and combustion zone 122, although not limited thereto. Supplemental ingredients may include, for example, fresh or recycled asphalt cement, recycling agents, fibers, polymers, fillers, and/or combinations thereof, although not limited thereto.
In operation, the primary ingredients of the hot-mix asphalt may be received through the inlets 110, 112, 114 for passage through the rotatable drum 104 toward the second end 108. At the same time, combustion gases from the burner 102 may flow substantially from the burner 102 toward the first end 106 to heat and dry the ingredients. Downstream of the mixing zone 118 (with respect to ingredient flow), the hot-mix asphalt manufactured by the rotary dryer 100 may be allowed to drop through the outlet 116 and onto a hot-mix conveyor system.
Referring now to FIG. 2, shown is a temperature profile 150 of a rotary dryer 100 according to one embodiment of the present teachings. As shown by the temperature profile 150, three zones, namely, a drying zone 120, combustion zone 122, and mixing zone 118 may be identified by their relative positions and temperatures, although not limited thereto. For example, as heat leaves the burner 102 it may produce a parabolic air temperature profile 150 in the rotary dryer 100, with the highest (peak) temperature found in a middle of the combustion zone 122. The temperatures shown in FIG. 2 are exemplary in nature and not limiting.
The temperature profile 150 may be used to determine preferable locations for the inlets 112, 114 (shown in FIG. 1). Virgin material may be introduced at the furthest upstream point in the material flow stream, for example, at the first inlet 110 (shown in FIG. 1). Coarse RAP may preferably be introduced downstream of the first inlet 110 and upstream of the peak temperature gradient in the dryer, in or adjacent to the upstream end of the combustion zone, for example, at the third inlet 112 (shown in FIG. 1). Fine RAP may then preferably be introduced downstream of the peak temperature gradient in the dryer, in or adjacent a downstream end of the combustion zone, for example, at the second inlet 114 (shown in FIG. 1), which is downstream of the third inlet 112.
In one embodiment, the second inlet 114 may be roughly 4 feet upstream from an end 103 of the tubular extension 105 of the burner 102 (shown in FIG. 1) anchoring the flame, which may be about 10 feet from the discharge outlet 116. The second inlet 114 may be within the combustion zone (as shown), outside of the combustion zone (e.g., in the mixing zone) or even as a separate material feed to the batch mixer, although not limited thereto. The third inlet 112 may be roughly 6 to 10 feet upstream from the end 103 of the tubular extension 105 of the burner 102, and the first inlet 110 may be about 24 feet from the end 103 of the tubular extension 105, for a dyer having a length of about 34 feet. Asphalt cement and/or supplemental additives may be introduced about 6 to 8 feet upstream from material discharge outlet 116 for a unified dryer/mixer as shown, although not limited thereto. Alternatively or additionally, it may be added at the batch mixer or part of external coater.
New material (e.g., for conventional <100% RAP processes) is preferably introduced at the first inlet 110 and, by the time the new material reaches the coarse RAP entry point, for example third inlet 112, the new material may preferably be at a desired mix discharge temperature that can range from 250-330 F, although not limited thereto. By the time the coarse RAP/new material mix (or coarse RAP for 100% RAP processes, etc.) reaches the fine RAP entry point, the coarse RAP/new material may preferably be at a desired mix discharge temperature that can range from 250-330 F, although not limited thereto. The temperature ranges disclosed herein are exemplary in nature and not limiting.
Where RAP includes one or more intermediate gradation(s) of aggregate between fine and coarse RAP (for example, where RAP includes fine, medium and coarse gradations of aggregate), the intermediate gradations(s) of RAP can be introduced through the fine and/or coarse RAP inlets, or through additional inlet(s), specific to the intermediate gradations(s) of RAP, which may be located between the inlets for fine and coarse RAP.
Benefits of the multiple entry system include the ability to produce mixes where all or a majority of aggregate is provided by a RAP feed. Introducing coarse RAP upstream of the peak temperature in the dryer and directly heating the coarse RAP in the combustion zone 122 may reduce temperature requirements of fresh aggregate that otherwise must be superheated (e.g., 100 to 500 degrees F. above desired discharge temperature). Increased usage of RAP, and specifically increased usage of RAP fines, reduces the requirement for new (virgin) fine material containing ultra-fine material (e.g., minus 70 micron) that is easily made airborne and carried into a bag house. Ultra-fine material in RAP may be bound by a RAP binder to larger particles and not easily made airborne. Thus, pollution control may be simplified.
More thorough heating and drying of RAP aggregates results from longer time in the dryer. Internal moisture in the center of the aggregate is released in the dryer. This is different than that shown in the prior art, where interior moisture remains in high RAP mixes which then tend to cool after discharge due to evaporation of the interior moisture. According to the present teachings, more gradual heating of RAP aggregates results because RAP lays in the bottom of the dryer with aggregate close to mix discharge temperature. This also results in less damage to the RAP binder otherwise caused by exposure to superheated aggregates. For example, the prior art requires superheating fresh aggregate (e.g., to over 800 F for only 40% recycled content). The RAP binder can be damaged at these temperatures.
There is better transfer of RAP binder to fresh aggregates using a system according to the present teachings. Heating in the combustion zone while mixed with fresh aggregate increases mixing energy to distribute the RAP binder evenly. This reduces spatial variations in binder content between recycled and fresh aggregates.
The use of colder, fine RAP aggregate may be used to cool combustion gases. Also, adding fines separately from coarse aggregate may significantly reduce buildup of RAP fines in the dryer. Both fine and coarse RAP may be heated more thoroughly before being mixed with fresh binder material. In addition, a RAP binder may be better distributed onto fresh uncoated particles reducing spatial variation in binder content of finished product, although not limited thereto
The system according to the present teachings is also safer since the probability of all entry feeds running empty simultaneously is much lower than for systems described in the prior art where RAP aggregate is introduced at a single location. Multiple entry feed provides a redundancy so the material in the dryer doesn't overheat. There may be no need to superheat new material (e.g., to 800 F). Excessive hydrocarbon fumes from fresh asphalt binder are created if superheated aggregate reaches the mixing zone without being cooled by RAP feed. Hydrocarbon fumes create risk of fire in the dryer or bag house when fumes are ignited as they pass through the combustion zone. This is a problem in the prior art with high RAP mixes and counter flow dryers when the sole RAP feed in a single-feed system is interrupted. A redundant feed also decreases the risk of fire from ignited hydrocarbon fumes because redundant RAP feeds are less likely to fail in unison.
Temperature of the air stream exiting the dryer also may remain uniform as RAP percentage changes. The prior art suffers significantly higher dryer exit gas temperatures as the percentage of RAP increases and fresh aggregate is superheated. Superheating reduces heat transfer efficiency of the dryer due to lower temperature difference between aggregate and gases. Accordingly, there may be more efficient dryer operation according to the present teachings because less heat is lost through ductwork, dryer shell and exit gas temperature.
The present teachings may be used to retrofit conventional asphalt manufacturing systems to increase RAP use up to 100%. Such systems may already have an existing entry for RAP (e.g., a collar) which may be used for RAP, although not limited thereto. Alternatively or additionally, coarse RAP may be added in the first inlet for 100% RAP processes, or for processing using less than 100% RAP, a coarse RAP inlet could also be added in accordance with the present teachings.
In preexisting counter-flow dryers used for RAP processes, a single RAP entry is typically located between the combustion and mixing zones. To convert such systems, the retrofit can include shortening the tubular extension 105 of the burner 102 to move the end 103 of the tubular extension (and thus also the combustion zone) downstream such that the existing RAP entry is in the combustion zone, downstream of the peak dryer temperature, making the system suitable for introduction of fine RAP aggregate according to the present teachings. If required, the asphalt cement/supplemental additives (AC) injection point can be moved downstream to maintain the AC injection point in a shortened mixing zone. Further, an inlet for coarse RAP can be added in the combustion zone, upstream of the peak dryer temperature, as described herein.
Inlets for introducing fine RAP aggregate and coarse RAP aggregate are discussed herein. One skilled in the art would appreciate that material introduced through the inlet for fine RAP aggregate may be solely fine RAP. Alternatively, the material may be primarily fine RAP or may consist essentially of fine RAP. Thus, the material introduced through the inlet for fine RAP may include components other than fine RAP, such as other gradations of RAP aggregate and/or virgin aggregate, processed shingles, or other components. Similarly, material introduced through the inlet for coarse RAP aggregate may be solely coarse RAP, or alternatively may consist essentially of or may be primarily coarse RAP, including other gradations of RAP and/or virgin aggregate, or other components.
Referring now to FIGS. 3A-B, shown are schematic views of a collar and bucket for use with the system of FIG. 1. As shown, a special design may be used for a RAP collar 200 for introducing fines to a dryer. This may provide straight transfer from the exterior of the dryer to the interior perpendicular to the axis of the dryer.
Interior buckets 202 may create a one-way door to keep the aggregate in the dryer. They may empty onto insulator flights that protect the dryer shell from peak temperatures (e.g., +2000 F). Segmented flights may line the dryer shell at the combustion zone. Aggregates may pass over the top of the insulator flights.
Referring now to FIGS. 4-6, shown is an inside view of a dryer 300 having a plurality of insulator flights 310 lining an interior of the dryer shell 320. The insulator flights 310 are designed to shield the dryer shell 320 from heat generated by the heat source (e.g., the burner), and to minimize heat loss to the ambient environment.
The insulator flights 310 can be segmented to allow for individual removal and replacement as required and can be installed side by side and end to end, overlapping one to another to completely cover (i.e., line) the interior of the dryer shell 320, over a potion the interior or the entire interior. Multiple circumferential bands of insulator flights 310 can be arranged in a longitudinally adjacent, closely abutting arrangement to effectively line an area of the dryer shell 320.
In one embodiment, insulator flights may be installed in the combustion zone of the rotary shell dryer. This is where fuel from the burner completes combustion and provides volume for hot gases to expand and fuel access to oxygen. However, insulator flights could be installed in all zones or any particular portions of the dryer, as desired. Installing them throughout the length of the dryer may help protect the dryer from excessive wear and tear caused by aggregate.
Preferably, the insulator flights 310 are formed from steel plates (for example ⅜″ thick) and are attached to the interior of the dryer shell 320 so that they rotate with the dryer. The insulator flights 310 are spaced radially inwardly from the shell 320 of the dryer 300 forming an annular gap 330 between the insulator flights 310 and the shell 320.
Insulation 340, such as ceramic insulation or other suitable insulation, is disposed in, and preferably substantially fills the gap 330. The presence of the insulation 340 in the gap 330 serves the insulation and protective purposes discussed herein, and, in addition, serves to prevent aggregate and other materials from entering the gap 330 between the insulator flights 310 and the dryer shell 320. Thus, the insulation 340 may prevent the annular gap 330 between the interior surface of the dryer shell 320 and the insulator flights 310 from filling with fine aggregate, which can negatively impact mix gradation.
The insulation 340 can provide increased thermal efficiency and help prevent heat transfer to the dryer's outer shell and supporting structures. However, external insulation (not shown) on the exterior of the dryer 300 could also be used for better thermal efficiency. Thus, the use of the insulator flights 310 may also enable the use of an additional insulation layer outside of the dryer shell 320 to reduce heat loss to the atmosphere without risk of excessive steel temperatures in the dryer shell 320 (e.g., <600 F) at the combustion zone.
The insulator flights 310 have a substantially rectangular base portion 350 which is spaced radially inwardly from the shell 320 of the dryer and which is preferably curved to complement or follow the curvature of the dryer shell 320. In particular, when installed, an axis of curvature 352 of the base portion 350 is preferably substantially the same as (i.e., substantially collinear with) an axis of curvature of shell 320 of the dryer 300. The insulator flights 310, and can have a longitudinal length of about 4 feet and a can have a circumferential width (i.e., arc length) of about 1.5 feet.
The insulator flights 310 have a circumferential overlapping portion 360 preferably integrally formed with the base portion 350 and extending from a rotationally downstream longitudinal edge 370 of the base portion 360. The circumferential overlapping portion 360 is angled to project radially inwardly from the base portion 360 sufficiently such that it overlaps and contacts a radially inward surface of the rotationally downstream adjacent insulator flight 310. In particular, the circumferential overlapping portion 360 of the insulator flight 310 contacts and overlaps a radially inward surface of the rotationally upstream longitudinal edge 380 of the adjacent insulator flight 310, forming a substantial seal therebetween. As depicted, all of the insulator flights 310 overlap as described herein such that the insulator flights 310 form a substantially circumferentially contiguous lining for the dryer shell 320 such that, during operation and rotation of the dryer 300, material flows over each successive insulator flight 310 in a circumferential direction.
The insulator flights 310 also overlap in the longitudinal direction. Preferably, the insulator flights 310 have a longitudinal overlapping portion 390 substantially covering a seam between longitudinally adjacent insulator flights 310. The longitudinal overlapping portion 390 can be fixed (e.g., welded or integral) to a radially inward surface of one of the adjacent insulator flights 310, for example a longitudinally downstream edge 400 of a longitudinally upstream insulator flight 310. Thus, in the area in which the insulator flights 310 are present, they can form a substantially longitudinally contiguous lining for the dryer shell 320 so that, during operation and rotation of the dryer 300, material flows over each successive insulator flight 310 in the direction of the material flow stream in the dryer 300.
The insulator flights 310 are preferably fixed to the dryer shell 320 in a manner that allows substantial thermal expansion and contraction of the insulator flights 310 during operation cycles, thereby avoiding substantial thermal stress. Preferably, the insulator flights 310 are held in place by a pair of compression plates 410, 420 bearing on or attached to a radially inward surface of the base portion 350, for example at longitudinally upstream and downstream portions of the rotationally downstream longitudinal edge 370 of the base portion 350. Each compression plate 410, 420 can be mounted on a clip base 430 affixed to and projecting radially inwardly from the dryer shell 320. The opposite longitudinal edge of the insulator flights 310, specifically the rotationally upstream longitudinal edge 380, is preferably indirectly connected to the dryer shell 320 by being trapped or clamped down by the circumferential overlapping portion 360 of the rotational upstream adjacent insulator flight 310 as described herein.
Preferably, the insulation 340 disposed in the gap 330 is compressed between the insulator flights 310 and the dryer shell 320 and is resilient such that it exerts radially inward pressure on the radially outward surface of the insulator flights 310 to effectively trap each insulator flight 310 between the associated compression plates 410, 420 and the circumferential overlapping portion 360 of the adjacent insulator flight.
To construct a system according to the present teachings, the insulation 340 may be laid on the inside of the dryer shell 320 in an uncompressed state and then the insulator flights 310 may be attached to the dryer shell 320 over the insulation as discussed herein. During such installation, the insulator flights 310 may compress the insulation 340 to a thickness of about 25%-75%, or preferably about 50%, of the uncompressed thickness of the insulation 340. For example, for a gap 330 of about 2 inches, insulation 340 having an uncompressed thickness of about 2.5 inches to about 8 inches, or preferably about 4 inches, would be suitable.
The insulation 340 may be dense mineral insulation, preferably ceramic fiber, which is able to withstand the high temperatures in the dyer 300 and in particular in the combustion zone of the dryer 300.
The dyer 300 can include a plurality of lifting flights 500 to enhance mixing and heating of materials within the dryer 300. The lifting fights 500 can be affixed to the base portions 350 of the insulating flights 310 and can be oriented in a longitudinal direction as shown. The lifting flights 500 can have a stem portion 510 affixed to and projecting radially inwardly from the insulator flight 510, and can have a hook portion 520 extending circumferentially in the direction of rotation of the dryer shell 320 from a radially inward end of the stem portion 510, forming a substantially-L shape.
The insulator flights 310 and lifting flights 500 are preferably disposed at regular circumferential (i.e., angular) intervals, for example in about 9 intervals (or about every 40 degrees) for small diameter dryers, or one per insulator flight segment.
The lifting flights 500 can be tapered such that they decrease in size in a direction toward the heat source for the dryer 300, to gradually reduce the amount of material that the lifting flights 500 carry in areas closer to the flame initiation (e.g., the end 103 of the tubular extension 105 of the burner 102). Preferably one or both of the height of the stem portion 510 (in a radial direction) and the length of the hook portion 520 (in a circumferential direction) can gradually decrease in a direction toward the heat source.
The lifting flights 500 of adjacent circumferential bands of insulator flights 310 are preferably aligned in a longitudinal direction such that, together, they form a plurality of contiguous longitudinal lifting flights 500 (e.g., 9). The configuration of lifting flights 500 of adjacent bands of insulator flights 310 can be complementary such that a smooth and continuous taper (from a larger size to a small size) is formed in the stem and/or hook portions 510, 520 of the lifting fights 500 from an end furthest from the heat source to an end closest to the heat source. Specifically, the height and length of the stem and hook portions 510, 520 of abutting portions of adjacent lighting flights 500 can have generally the same size and shape such that a smooth transition is formed between the adjacent lifting flights 500.
Insulator flights 310 may have a number of operational advantages. For example, they may maximize radiant heat transfer to the aggregate and may allow radiant energy to penetrate deep into the aggregate. Thermal energy of the insulator flights may be conducted to the aggregate in contact with the steel surface. As a result, aggregate surface moisture may be dried more quickly, minimizing temperature increase of asphalt coating on RAP particles. Robust energy transfer by radiation and conduction may lower temperature of combustion gases passing through the dryer to the opposite end. This may advantageously minimize volatilization of hydrocarbons from asphalt coated ingredients.
Insulator flights 310 may also serve to protect the dryer from premature aging due to high heat and wear, although not limited thereto. Accordingly, they may be placed throughout a dryer (e.g., more than the combustion zone). When placed throughout a dryer the compressed ceramic insulation reduces heat loss through the dryer shell, improving overall thermal efficiency of entire plant rendering delicate external insulation unnecessary. The insulator flights 310 and lifting flights 500 may help to fix the aggregate and prevent “sliding,” which increases wear on the inside of the dryer. Insulator flights 310 can be replaced when worn out, saving the expense of having to replace the dryer.
Referring to FIGS. 7 & 8, the rotary dryer 300 can also include a dam 530, for example at the low (i.e., downstream) end of the combustion zone, to maintain optimum depth of the aggregate independent of production rate. The dam 530 can be in the form of annular wall projecting radially inwardly from the dryer shell 320, and radially inwardly of the adjacent insulator flights and lifting flights, for example about 6-8 inches. Openings or gaps 532 in the dam 530 may allow the associated zone (e.g., the combustion zone) to empty completely when the feed stops, for example at the end of a production run. For example, the dam 530 can have two gaps 532 diametrically opposed from one another (e.g., 180 degrees apart). However, other numbers and configurations of gaps are contemplated.
Referring to FIG. 9, a system and method according to the present teachings can enable the use of a preferred pollution control system with 100% RAP mixes and high percentage RAP mixes (>40%). High percentage RAP mixes, and particularly 100% RAP, have at times been disfavored because they can result in more hydrocarbon emissions than non-RAP. To address this, prior systems have been developed to employ indirect heating so that the small amount of hydrocarbons volatilized during heating may be incinerated by the indirect process heater. However these systems are thermally inefficient. The present system may result in less dust as compared to systems using no or a lower level of RAP because less or no new (e.g., virgin) fine aggregate (typically having high levels of dust) may be required.
The present system can have a direct fired rotary dryer 600 followed by one or more pollution control device(s) 602 operable to treat emissions from a direct fired rotary dryer 600 so that hydrocarbons and particulates are substantially removed from the emissions before they are released into a surrounding environment.
The pollution control device 602 can include an inertial separation zone 604, followed by a cooling zone 606, which is followed by a fiber bed filter 608. The inertial separation zone 604 is operable to remove relatively large airborne particulates from the emissions, for example particulates from about 150 to about 50 microns (100 to 325 mesh). The cooling zone 606 is operable to cool the emissions from the rotary dryer 600 enough that such the emissions achieve a temperature that is compatible with the fiber bed filter 608, for example to a temperature less than about 180 degrees Fahrenheit. The fiber bed filter 608 is configured to provide coalescent filtration of emissions from the rotary dryer 600. The coalescent filtration may be achieved by subjecting the emissions from the rotary dryer to Brownian diffusion filtration, although not limited thereto. Fiber bed filters suitable for use in the system are commercially available, for example, from Air Clear.
The system may be part of a unitized portable system for RAP. By unitized it is meant that the pollution control 602 may be on the same chassis as the dryer 600 and heating mechanism 610 (e.g., burner). Reduced particulate emissions help to enable a compact, transportable design. A portable system may be preferable during the winter season when conventional plant maintenance is taking place. It may also be preferable for remote regions with large distances between conventional plants or where specialty contractors may require small capacity systems to produce “make safe” material.
In the portable unitized system, the dryer may be either parallel or counter flow and may have two RAP feeds as disclosed herein, and the burner may be direct fired or external. The pollution control may comprise a fiber bed coalescing filter, which may include disposable particulate filter media and/or adiabatic cooling water spray.
While the present teachings have been described above in terms of specific embodiments, it is to be understood that they are not limited to these disclosed embodiments. Many modifications and other embodiments will come to mind to those skilled in the art to which this pertains, and which are intended to be and are covered by both this disclosure and the appended claims. It is intended that the scope of the present teachings should be determined by proper interpretation and construction of the appended claims and their legal equivalents, as understood by those of skill in the art relying upon the disclosure in this specification and the attached drawings.

Claims

What is claimed is:

1. A method of manufacturing asphalt using recycled asphalt coated aggregate (RAP), comprising:

providing a rotary dryer having

a first end and a second end, and a material stream flow toward the second end;

a burner operable to heat the rotary dryer and material therein, and to create a maximum gas temperature inside a volume of the rotary dryer at a location between the first and second ends,

a first inlet upstream of the location of the maximum temperature,

a second inlet downstream of the location of the maximum temperature, and

an outlet adjacent the second end of the rotary dryer;

selecting a first aggregate and a second aggregate, the first aggregate being at least primarily coarse aggregate and the second aggregate being at least primarily fine RAP;

operating the rotary dryer and during the operation

introducing the first aggregate into the rotary dryer at the first inlet,

introducing the second aggregate into the rotary dryer at the second inlet, and

heating and mixing the first and second aggregates forming a mixture; and

outputting the mixture from the rotary dryer at the outlet.

2. The method of claim 1, wherein the first aggregate is at least primarily coarse RAP.

3. The method of claim 1, wherein the second aggregate consists essentially of fine RAP.

4. The method of claim 3, wherein the first aggregate is at least primarily coarse RAP.

5. The method of claim 4, wherein the first aggregate consists essentially of coarse RAP.

6. The method of claim 5, wherein the second aggregate consists of fine RAP.

7. The method of claim 6, wherein the first aggregate consists of coarse RAP.

8. The method of claim 1, wherein:

only coarse RAP is introduced into the rotary dryer at the first inlet; and

only fine RAP is introduced into the rotary dryer at the second inlet.

9. The method of claim 1, further comprising:

providing the first inlet adjacent the first end of the rotary dryer, and the first aggregate being at least primarily coarse virgin aggregate;

providing a third inlet between the first inlet and the location of the maximum temperature;

selecting a third aggregate, the third aggregate being at least primarily coarse RAP; and

during the operation, introducing the third aggregate into the rotary dryer at the third inlet.

10. The method of claim 9, wherein the second aggregate consists essentially of fine RAP.

11. The method of claim 9, wherein the third aggregate consists essentially of coarse RAP.

12. The method of claim 9, wherein the first aggregate consists essentially of coarse virgin aggregate.

13. The method of claim 9 wherein the second aggregate consists of fine RAP.

14. The method of claim 9, wherein the third aggregate consists of coarse RAP.

15. The method of claim 9, wherein the first aggregate consists of coarse virgin aggregate.

16. The method of claim 9, wherein

only virgin aggregate is introduced into the rotary dryer at the first inlet; and

only RAP is introduced into the rotary dryer at the second and third inlets.

17. The method of claim 16, wherein

only fine RAP is introduced into the rotary dryer at the second inlet; and

only coarse RAP is introduced into the rotary dryer at the third inlet.

18. The method of claim 9, wherein coarse RAP aggregate comprises approximately 55% to 75% of a total RAP content in the mixture, and fine RAP aggregate comprises approximately 25% to 45% of the total RAP content in the mixture.

19. The method of claim 1, wherein the mixture comprises at least 40% RAP.

20. The method of claim 1, wherein the mixture comprises at least 75% RAP.

21. The method of claim 1, wherein the mixture comprises at least 95% RAP.

22. The method of claim 1, wherein the rotary dryer comprises a counter flow dryer.

23. The method of claim 1, wherein the rotary dryer is in a continuous mix plant.

24. The method of claim 1, further comprising:

providing a batch asphalt plant having a mixer separate from the rotary dryer;

introducing the mixture output from the rotary dryer into the mixer;

introducing a supplemental ingredient into the mixer; and

operating the mixer forming a second mixture.

25. The method of claim 1, wherein the selecting comprises classifying fine aggregate as aggregate that fits through a #4 sieve and classifying coarse aggregate as aggregate that does not fit through a #4 sieve.

26. The method of claim 1, wherein the selecting comprises classifying fine aggregate as sands and classifying coarse aggregate as stones.

27. The method of claim 1, further comprising a pollution control device having a fiber bed filter, the pollution device treating emissions from the rotary dryer.

28. The method of claim 1, wherein the maximum temperature of the rotary dryer is less than 1500 F.

29. The method of claim 9, wherein a temperature profile of the rotary dryer defines

a drying zone adjacent the first end and having a material temperature in a range of 50-350 F;

a combustion zone between the drying zone and the second end and having an air temperature in a range of 700-1500 F;

a mixing zone between the combustion zone and the second end and having a material temperature in a range of 250-350 F; and

wherein the second and third inlets are located in the combustion zone.

30. The method of claim 29 wherein a supplemental ingredient is introduced into the rotary dryer in the mixing zone.

31. The method of claim 30 wherein the supplemental ingredient comprises asphalt cement and/or recycling agents.

32. The method of claim 1, wherein the rotary dryer has a dam at a lower end of the combustion zone that maintains a depth of aggregate inside the rotary dryer while it rotates.

33. The method of claim 1, further comprising:

placing insulation on the inside of the rotary dryer; and

attaching insulator flights to the rotary dryer radially inwardly of the insulation, the insulator flights lining at least a portion of the inside of the rotary dryer.

34. The method of claim 33, wherein the insulator flights comprise overlapping rectangular metal segments.

35. The method of claim 33, wherein the insulation comprises ceramic insulation.

36. The method of claim 33, further comprising:

attaching lifting flights to the insulator flights, the lifting flights being operable to agitate aggregate as the rotary dryer rotates.

37. The method of claim 33, wherein the insulator flights are located substantially throughout the rotary dryer between the first and second ends.

38. The method of claim 33, wherein the rotary dryer has no external insulation.

39. Use of the method for manufacturing asphalt of claim 1 in a portable unitized mix plant.

40. The method of claim 1, wherein the burner has a tubular extension with an end; and the providing a second inlet comprises shortening the tubular extension so that the second inlet is between the end of the tubular extension and the location of the maximum temperature.

41. The method of claim 9, wherein the providing a third inlet comprises affixing a collar to the rotary dryer.