Apparatus for manufacturing asphalt paving composition as well-known. Stated broadly, asphalt plants include a means for heating and drying virgin aggregate and a means for mixing the heated and dried aggregate together with liquid asphalt to form a paving composition. Optionally, recycleable asphalt pavement (commonly referred to as "RAP") is also included in the mix. The RAP must be heated sufficiently to melt the asphalt therein so that the components of the RAP can become thoroughly intermixed with the virgin aggregate and liquid asphalt.
Asphalt plants can be divided into two broad categories: batch plants and continuous-mix plants. In a batch plant, a quantity of virgin aggregate is heated and dried and dumped into a mixer along with a proportional quantity of liquid asphalt. The batch of aggregate and liquid asphalt is then thoroughly mixed and discharged into a storage bin so that the next batch can be prepared.
By far the preferred type of plant today is the continuous-mix plant. In a continuous-mix plant, ingredients are continuously being introduced into the plant, and asphalt paving composition is continuously being discharged from the plant, rather than manufacturing the asphalt paving composition in batches. Since materials are continuously being introduced, the proportions of the components in the mix must be controlled by controlling the relative rates at which the various components are introduced into the plant, rather than by merely controlling the relative quantities of the various components. Continuous-mix plants generally fall into one of two categories. In the first type of continuous-mix plant, virgin aggregate is heated and dried in a drum dryer. The heated and dried aggregate is then discharged into a separate mixing device, such as a pugmill. Liquid asphalt is then introduced into the mixer along with the aggregate and is thoroughly mixed, the resulting asphalt paving composition being discharged from the other end of the mixer.
In a second type of continuous-mix plant, known as a "drum mixer," the drying and mixing processes are both carried out in a single rotating drum. Virgin aggregate is introduced into the upper end of the rotating drum. A burner mounted in the upper end of the drum heats the air flowing through the drum, and the aggregate is heated and dried as it is tumbled through the heated airflow in the upper end of the drum. Liquid asphalt is introduced into the drum at a point sufficiently removed downstream from the burner so that the liquid asphalt will not smoke. The heated and dried aggregate and the liquid asphalt are then mixed in the bottom portion of the drum, and the asphalt paving composition is discharged out the lower end of the drum. Air removed from the drum is typically ducted to a dust-collection system, such as a baghouse, wet-washer, or cyclone separator.
RAP can be included as a component of the asphalt mix in either of these two varieties of continuous-mix plants. In the first type of continuous-mix plant comprising a dryer and separate coater, the RAP is introduced either into the lower end of the dryer at a point sufficiently removed from the burner that the asphalt in the RAP does not smoke excessively; or, the RAP can be introduced into the mixer along with super-heated aggregate, the heat from the aggregate melting the asphalt in the RAP so that the components of the RAP can be thoroughly intermixed with the aggregate and liquid asphalt. In the case of a drum mixer, the RAP is introduced into a mid-point of the drum, either radially through ports in the circumference of the drum or axially from the lower end of the drum. The RAP is introduced into the drum mixer at a point where the temperature is sufficiently high to melt the RAP but not so high as to cause the asphalt in the RAP to smoke excessively. The aggregate, melted RAP, and liquid asphalt are then mixed in the lower portion of the drum, with the asphalt paving composition being discharged from the bottom end of the drum.
One of the primary difficulties in designing asphalt plant apparatus concerns the need for providing a high temperature zone for heating and drying the virgin aggregate while shielding liquid asphalt and RAP from such high temperatures as would cause the asphalt to smoke or burn. The drum dryer and separate coater has obvious advantages in this respect, since the liquid asphalt and RAP need not be exposed within the same vessel used for heating and drying the aggregate. However, such designs suffer the disadvantages of higher cost, both in manufacturing two separate components and in the energy costs of driving the dryer and a separate coater. Further, these designs are typically bulkier and hence less portable than drum mixers. Conversely, drum mixers are less expensive to operate and more portable. However, the liquid asphalt and RAP must be exposed within the same vessel used to heat and dry the aggregate. Thus, the design must provide a drum of sufficient length that the liquid asphalt and RAP can be introduced at a point removed from the intense heat of the burner while still providing sufficient exposure within the drum to afford adequate opportunity for mixing. If the drum is made too short, either capacity will suffer, or the liquid asphalt and RAP will smoke excessively and create a pollution problem.
A further problem with prior-art drum mixers is that they are not thermally efficient. In a prior art drum mixer, since asphalt cannot be exposed in the proximity of the flame, all materials must move through the drum in the direction away from the flame. Since the flow of material is thus moving in the same direction as the flow of heated gases moving through the drum, such an arrangement is known as "parallel flow" or "concurrent flow." While a parallel flow design keeps asphalt from coming too close to the flame and burning or smoking, such a design is less efficient in drying damp aggregate than a "counterflow" arrangement, where the materials flow toward the burner counter to the direction of the airflow through the drum. In a concurrent-flow dryer, the aggregate is exposed to the highest temperatures while it is still cold and damp. By the time the aggregate begins to get heated, it has moved down the drum into a cooler zone. In contrast, as aggregate is heated in a counterflow dryer, it moves into a higher temperature zone, and it is exposed to the highest temperatures immediately prior to being discharged from the drum. Thus, a counterflow arrangement is thermodynamically more efficient. Prior-art drum mixers thus suffer the disadvantage of drying aggregate in a parallel flow drying zone and thus sacrifice the thermal efficiency of a concurrent flow design.
An additional problem with prior art drum mixers is that of exposing liquid asphalt to steam and high temperatures present within the drum mixer. As aggregate is dried in the upper end of the drum, steam is generated. Contact with this steam can strip light end hydrocarbons from the liquid asphalt. The stripped light end hydrocarbons can condense on the walls and filters of the baghouse, clogging the filter elements and reducing plant capacity. Oil-soaked bags further lead to a higher probability of baghouse fires. Finally, the light end hydrocarbons can be released through the stack and into the atmosphere, creating pollution problems. The problem of stripping light end hydrocarbons becomes even more pronounced when recycling used asphaltic pavement, since the softer liquid asphalts used to compensate for the harder asphalt in the RAP will have more light end hydrocarbons.
Accordingly, there is a need to provide an improved apparatus for manufacturing asphalt paving composition which avoids the problem of exposing liquid asphalt and RAP to high temperatures which can cause smoking or burning.
There is a further need to provide an improved apparatus for manufacturing asphalt paving composition which efficiently segregates the drying and mixing functions.
There is also a need to provide an improved apparatus for manufacturing asphalt paving composition which protects liquid asphalt against exposure to steam which can strip light end hydrocarbons from the liquid asphalt and cause pollution problems.