Counterflow Rotary Dryer for 100% RAP Asphalt Processing
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Solution Overview
Problem
The use of Reclaimed Asphalt Pavement (RAP) in asphalt paving mixtures is limited due to the release of volatile, flammable materials at high temperatures, which poses a fire risk and inefficiencies in heating processes, restricting the percentage of RAP that can be utilized.
Innovation Solution
A method where RAP is heated and dried separately in a counterflow rotary dryer, with only hot air from a burner contacting the RAP, and effluent gases are treated through a wet scrubber and coalescing filter to minimize flammable gas exposure and risk, allowing up to 100% RAP usage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If RAP is heated to high temperatures to dry and process it, then the RAP can be effectively dried and prepared for reuse, but volatile flammable materials are released which create fire hazards and limit the percentage of RAP that can be used
Solution Approach 1:
The heating process is segmented into two distinct zones: a veiled zone where RAP is heated by hot air without direct flame contact, and a non-veiled zone near the burner where no RAP is present. This spatial segmentation allows high-temperature heating while preventing flame-RAP contact that would cause fire hazards.
Solution Approach 2:
Hot air acts as an intermediary medium to transfer thermal energy from the burner to the RAP indirectly. The burner heats air, and this hot air then circulates through the veiled zone to dry the RAP without the flame directly contacting the material, thus eliminating fire hazards while maintaining effective heating.
2Reliability
If virgin aggregate is superheated and mixed with RAP to heat and dry the RAP, then the RAP can be dried, but a large quantity of virgin aggregate must be heated which is not energy efficient
Solution Approach 1:
The invention extracts and removes the unnecessary step of heating virgin aggregate as a means to dry RAP. Instead, RAP is dried directly in the veiled zone using hot air circulation, eliminating the energy-wasting step of heating large quantities of virgin aggregate while maintaining effective RAP drying.
Solution Approach 2:
RAP serves its own drying function by being directly exposed to hot air circulation in the veiled zone. The system is designed so that RAP particles are lifted, dropped, and tumbled in the hot air stream, allowing the RAP to dry itself without requiring virgin aggregate as a heating medium.
3Power
If the burner flame is allowed to contact RAP in the dryer, then heating efficiency may be improved, but the flammable volatile materials in RAP can ignite causing fires and damage to the plant
Solution Approach 1:
Different zones within the dryer are given different qualities: the veiled zone has hot air circulation suitable for heating RAP, while the non-veiled zone near the burner has no RAP present. This local differentiation allows the burner to operate at high temperature for efficiency while ensuring RAP never contacts the flame, preventing ignition of flammable volatiles.
4Productivity
If higher percentages of RAP are used in asphalt mixtures, then the need for virgin aggregate and new binder is reduced, but the risk of fire from volatile materials increases
Solution Approach 1:
RAP is pre-dried and heated in the veiled zone before being discharged from the dryer. This preliminary heating and drying action removes moisture and prepares the RAP for high-temperature processing in subsequent mixing operations, enabling higher RAP percentages to be used safely by eliminating the fire risk associated with improper drying.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables the safe and efficient use of higher RAP percentages by reducing off-gas ignition risks and energy consumption, while protecting plant equipment from potential fires through a fire damper system and efficient effluent treatment.
Implementation Method 1
the dryer circulates hot air through the RAP to heat and dry the RAP
Implementation Method 2
a wet scrubber system which both removes most of the particulates from the effluent and cools the effluent
Implementation Method 3
a coalescing filter to remove the hydrocarbon off-gases and some of the water before the effluent is exhausted from the plant
Data Source
AI summary
A method for producing asphalt using a high percentage of RAP (recycled asphalt pavement) in the total aggregate raw material. A counterflow rotary drum dryer is used to dry the raw material. The extent of the burner flame in the dryer is limited such that the RAP does not come into direct contact with the flame. The hot gases leaving the dryer pass to a wet scrubber.