Asphalt contractors are always looking for ways to control moisture during the asphalt production process. As highway agencies demand increased value for their money and expect suppliers to guarantee the longevity of their products, an interest in moisture control additions for plants is increasing.
The decision to include moisture measurement and control, or moisture compensation, is split into two main areas. One is cost reduction and the other quality. These benefits are applicable to both batch- and continuous-type processes but in very different ways.
Cost reduction benefits for batch-type operations are mainly gained by optimizing the drying cycle of the materials. A number of batch process plants have seen benefits from installing a very simple system utilizing one moisture sensor measuring in the blended material ahead of the drier. This is used for burner control and for manual adjustment of the feed rates. If the moisture increases the fine material is fed slightly faster to ensure that the bins are evenly filled for each of the aggregate sizes.
See FIGURE ONE
For continuous plants the cost benefit is primarily derived from ensuring that the correct amount of binder (asphalt cement and additives) is used to make the desired volume of finished product. Once properly interfaced to the Plant control system you can automatically control the amount of binder by measuring the moisture of the materials on the cold feed belt and adjusting the belt speed to ensure that the varying grades of materials are delivered in the correct quantities (ratios) into the burner. The wetter the material, the faster the belt must run to deliver the correct amount and maintain consistent material ratios (and vice versa). Product yield is therefore maximised for the amount of asphalt cement used, thus greatly reducing the amount of excess binder needed in the process. Cost savings can be immediately seen in the reduced amount of binders and additive’s required. In a continuous system, a sensor would be installed underneath both the coarse and fine materials bins to ensure the correct blend of materials are delivered to the burner.
See FIGURE TWO
Hydronix sensors will reliably give moisture results to +/- 0.2%. Customers have quoted potential savings in material as high as $45,000 on a single 100,000 ton production run if they are able to continually control and adjust for moisture to this accuracy. As with the batch process, if the dry materials are kept constant, a correlation between the moisture level of the material and the energy required to dry the material may be made and hence the efficiency of the burner monitored. There are two reasons why a burner would require more energy to achieve the same moisture target, one that the starting moisture level was higher, the other is that the burner has become inefficient and requires a service or possibly it is cost effective to replace the unit altogether for the latest technology. The exact efficiency of the burner may now be monitored.
The cost of mistakes and poor quality
The drive for quality has largely been a result of specifiers requesting that contractors be 'bonded.' This is a term used to describe the agreement between the contractor and the customer to guarantee the asphalt product for periods of up to 15 years. Previously only the suppliers of concrete roads were bonded in such a manner. The practice of contracting the supplier with stricter terms, however, is becoming more widespread as highway agencies are increasingly considering the entire life cycle cost and looking for long-term value. The outcome of this is to place greater demands on the supplier to ‘guarantee’ product quality and indemnify the highway agency against ongoing maintenance costs.
Measuring the moisture of the materials during production not only ensures that the maximum yield may be achieved but can also highlight problems upstream with material suppliers or storage. Additional moisture in the material increases the weight, so the producer will purchase less material when buying by weight, or require additional energy to dry the material if poorly stored. Perhaps more critically it ensures that the ratios of the materials are maintained exactly as the design stipulates. This ensures consistent product density which is a critical ingredient for quality and durability for any asphalt product.
While some highway agencies stipulate that moisture is controlled during production others more frequently ‘bond’ their supplier leaving the supplier to be self-motivated to produce to the highest possible specification. With the stakes rising for the contractors with the cost implication of poor quality, many are now looking to retrofit moisture control systems to existing plants. This leaves an opportunity for forward thinking and innovative suppliers of asphalt plant automation and machinery to bring to market new solutions for their customers to ensure peace of mind as each project is completed.
Moisture is clearly not the only issue the asphalt producer has to contend with to ensure they meet quality control requirements within tight economic 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.