When a panel of industry quality control experts was asked what automated measurements would add value to asphalt production, "moisture content on belts/in bins" and "automatic gradation and moisture measurement from belt sampling" came in the top three.
Do these sound like familiar measurements that would also add value to your asphalt production? The increasing cost of raw materials and energy has made asphalt producers look into their production process in much greater detail than in the past.
Proven in-line technologies are now available that can provide a very real economic benefit and allow greater process control - reducing costly on-site testing and off-line forms of control.
The necessity for more exact process control is further reinforced by moves toward more plant/process certification. Instrumentation has become a major part in allowing asphalt producers to differentiate themselves from others through repeatable quality, reliability and efficiency.
This differentiation and 'getting it right the first time' can be the difference between winning a job and losing an important customer. Increased quality control and assurance has also been reflected by developments in national standards. In the United States, NCAT (National Center for Asphalt Technology) carried out an investigation into improving HMA production technologies and in their conclusions recommended the on-line belt-sampling of materials and moisture content compensation.
Sources of variation in moisture and its effects
Moisture content can change in aggregates stored in cold feed bins, in the RAP (if used) and in the finished HMA or other "warm" asphalt.
By measuring moisture in the incoming materials, the finished asphalt product can have its moisture controlled in a proactive real-time manner.
A change in moisture of the raw materials due to rainfall or dry weather will have a very large impact in the grading of the aggregates being loaded into the dryer and therefore on the quality of the asphalt produced. It also changes the energy consumption of the drier considerably and can result in under-yielding.
In practice, fine aggregates can hold considerably higher moisture than coarser gravels, and therefore it has been found that the greatest benefit in moisture measurement is usually achieved in aggregates with particle sizes lower than 10mm.
Field trials have proven that by using Hydronix moisture sensors, for example, measurement accuracies of +/- 0.2% are achieved and that these more than exceed the industry requirement.
Batch plant operations
Uniform cold feeding of aggregates from the cold bins is necessary for several reasons, and moisture affects all of these:
- Large moisture variations in the aggregates in the cold feed can lead to moisture content changes in the HMA
- Moisture variations can lead to aggregates leaving the dryer at different temperatures. This can lead to incorrect temperatures of the materials in the pugmill or mixer
- Moisture variations can lead the dryer to work inefficiently, over-drying already dry aggregate or outputting insufficiently dry material
- Inconsistent feeding of aggregates from the cold bin can, in the extreme, lead to differential feeding of the hot bins resulting in some hot bins overfilling while others remain low on materials
By measuring moisture at the base of the cold bins or on the cold bin discharge belts, and integrating this measurement into a process control system, it is possible to achieve a real-time solution to the issues above. Benefits include:
- Ensuring that the correct grading of materials is discharged
- Adjusting the dryer rate taking into account the moisture content of the aggregates being fed, resulting in energy savings
- Having the correct aggregate temperature, which leads to optimum adhesion to binder
Without using moisture management, the following can occur:
- Unknown aggregate dry weights
- Possible different moisture contents in different aggregates if left uncompensated
- Dryer rate is adjusted by the exhaust temperature in a retroactive manner
- Occasional overflow and/or insufficient material in hot storage bins due to variations in the wet aggregate fed to the dryer
- Possibility of moisture variation in the finished product
The benefits of using moisture management for batch plant operations include:
- Hydro-Probe II sensors measure moisture and feed data into the system
- Variable belt speeds can be adjusted in real time to ensure the correct dry weight proportioning of each aggregate
- Real time dryer rate optimization for moisture contents of aggregates inside the burner
- Consistent hot storage bin levels
- Improved final product with moisture consistency
Continuous (drum) plant operations
In the production of asphalt by drum mixing, aggregate gradation is controlled at the cold feed, therefore ensuring the cold feed¡¦s accuracy is essential.
Installing moisture sensors at the belt weighers or at the base of the cold feed bins allows real-time feed adjustment ensuring that the correct dry weight of aggregates is loaded into the drum mixer, resulting in:
- Increased yield and reduced asphalt binder usage through correct aggregate gradation
- The burner rate can be adjusted to suit the moisture content of incoming materials, allowing efficiency monitoring
- Energy savings
- Control of final HMA moisture content
- Reduction in inadequate adhesion between asphalt binder and aggregates
Without using moisture management, the following can occur:
- Unknown dry weight in aggregates results in incorrect asphalt grading
- Occasionally incorrectly graded asphalt
- Possible over drying of aggregates
The benefits of using moisture management for continuous drum plant operations include:
- Real time "dry weight" grading
- Reduced asphalt binder usage
- Burn rates adjusted in real time avoiding over drying
- Well-graded asphalt with correct HMA moisture content
Warm asphalt operations
Warm asphalt production techniques, including low-energy asphalt and foamed asphalts, require a higher degree of accuracy in aggregate gradation to ensure asphalt quality comparable with other HMAs.
The role of water is essential to achieve the desired "foaming" effect when mixed with the bitumen, allowing the system to uniformly coat all the aggregates.
Hydronix equipment allows real-time and on-line moisture measurement of the aggregates before they are fed into the pugmill, mixer or drum.
With a feed-forward automation system, producers can correct aggregate weights in real-time. For techniques that use the water in the aggregates themselves for the foaming effect, information about the quantity of water being fed into the mixer or pugmill can also be known.
Without using moisture management, the following can occur:
- Unknown dry fine aggregate weight loaded into the mixer
- Unknown water in fines added into the mixer
- Potential for incorrect foaming effect
- Fine aggregate gradation variation
The benefits of using moisture management for warm asphalt operations include:
- Real time, automatic, variable speed belt adjustment to ensure the correct "dry weight" of fine aggregates fed into the mixer
- Correct moisture information to allow the desired "foaming" effect
- Real time data for water addition into the mixer
Return on investment
It has been found that an increase in 1% moisture in a ton of aggregate can result in an additional 0.16 gallons of fuel being consumed to evaporate it.
At 6% moisture, one gallon of fuel is required to dry 1 ton of aggregate. Once dry, 3/4 of a gallon of fuel is required to heat the aggregate to 270 degrees F, meaning that more energy is used in drying the aggregate than in heating it.
In practice, the dryer or drum¡¦s exhaust temperatures are already measured and used for feeding back information into the burner control. However online moisture measurement allows real-time dryer/drum adjustments relating to the material inside, not the material already discharged.
This reduces the process control time lag and ensures that the correct dry weights are being loaded into the dryer resulting in savings and quality improvement.
If a plant runs 300 tons of asphalt per hour at 6% moisture, it will be using 317 gallons per hour of fuel to dry the aggregate before it is heated up to the correct temperature.
If the moisture content then changed from 6% to 5% without the dryers being adjusted for one hour, that would equate to 475 gallons of fuel being wasted in that one hour.
Were this to be the case, moisture measurement equipment would pay for itself in approximately 30 production hours. A reduced process control time lag continuously improves the efficiency of the plant.
In most cases moisture measurement systems will pay back within the first 3 to 6 months in operation.
Moisture measurement can also be used to evaluate the dryer¡¦s performance. Should the exhaust temperatures and incoming moistures not match (exhaust temperature increases with reducing moisture contents or vice-versa) there may be a requirement to tune the dryer.
There are other economic effects to moisture measurement. By knowing the correct moisture content in the aggregates, it is possible to maximise the efficiency of the drying process to reduce over drying and wasted production time, therefore increasing the yield of the plant.
Installation of a moisture sensor
A wide range of digital moisture measurement sensors can be used for installations in the base of the cold feed bins, on the feed belts, or alternatively on loading "plates."
It has been found in practice that the first two positions are generally the easiest installation locations.
Integration with an asphalt plant¡¦s control system is simple, via the use of standard analog outputs. Full communication with the control system is also available via digital comms (RS485/RS232) and the latest asphalt plants are now using Hydronix Ethernet Adapters for communication with sensors.
Calibration consists of finding the linear correlation between the sensor¡¦s "unscaled" reading (always 0 in air and 100 in water, allowing easy sensor swap-out if required) and the moisture content in a particular aggregate.
This is done by simply running material past the sensor¡¦s face and taking a reading of the sensor¡¦s unscaled value.
A sample of the material is taken and then oven dried to find its moisture content as per the applicable standard.
Once the above procedure has been carried out for a few points over the moisture range of the aggregate, the sensor will accurately read moisture - online and in real time.
Why invest in moisture control?
Moisture is clearly not the only issue the asphalt producer has to contend with to ensure he meets quality control requirements within tight economical constraints. However it is a simple improvement that brings immediate, quantifiable returns.
Accurate moisture measurement and control is now well established and proven to directly reduce costs as well as ensuring a consistent quality product improving competitiveness. ƒÞ
Information provided by Hydronix Ltd.. For more information, visit www.hydronix.com.
