The word asphalt comes from the Greek asphaltos, meaning secure. The Romans changed the word to asphaltus and used the substance to seal their baths, reservoirs, and aqueducts. The Roman highway called the Appian Way, built in 244 BC, was made of polygonal top stones and lava. This surface was called pavimentum from which we derive the modern term pavement.

Today, hot mix asphalt (HMA) is the most common road surfacing material used in the United States. Between 450,000,000 and 500,000,000 tons of hot mix asphalt are produced annually in the U.S. alone. HMA is comprised of sand and various sizes of crushed rock, called aggregate, which are mixed together with liquid asphalt cement derived from petroleum that acts as a binder. This mixture results in a highly durable compound that can withstand tremendous weights and extremes in weather before it develops weaknesses.

Paving the Way to the Future
At the turn of the century, the typical asphalt “plant” would travel by locomotive from city to city, whereas the modern hot mix asphalt plant is comprised of several components, each of which performs a specific task in the production of the asphalt mixture, including feeder bins, conveyor systems, drum dryers and mixers, storage silos, and air filtration systems. Today, engineers are using sophisticated computer modeling to design all features of an asphalt plant to include rock-drying burners, emission filtrations and duct flow systems.

Historically, asphalt manufacturing was a relatively low-tech enterprise and engineers relied on the trial and error of physical tests to develop plant designs. However, advances in computer simulation technologies have allowed industry leader Astec, Inc. to make extensive use of computer modeling to optimize plant design to meet the demand for more energy-efficient and ecologically friendly plants.

Andrew Hobbs, an engineer for Astec, uses computational fluid dynamics (CFD) and discrete element modeling (DEM) to model fluid and aggregate flow in key areas of Astec’s equipment. He explains a typical “Continuous Mix” process: A set of bins meter quantities of aggregate of differing sizes onto a conveyer system that transports the aggregate to a large drum dryer. When all ambient moisture has been removed, the aggregate is dropped into another mixing chamber where the liquid asphalt cement is added. The drying process is essential, as the liquid asphalt cement will only adhere properly if the aggregate is completely dry. Lastly, the mix is conveyed into 300-400 ton insulated storage silos. Mix enters the silos at approximately 300-350o Fahrenheit. The temperature of the mix must be kept above 250oF or the product will be compromised; cooling must only occur after the mix has been laid to ensure good road characteristics and integrity.

The Road Least Traveled
Astec, which holds over 70 patents worldwide, recently turned to CEI’s EnSight software to post-process a coupled CFD and DEM simulation of aggregate inside the drum dryer, complete with natural gas combustion. “DEM was needed to solve for each individual particle and its position, or each piece of rock’s interactions with other pieces of geometry,” says Hobbs. “Within the dryer, both flame and rock are present; we wanted to see if the material had any effect on the flame shape.” The steady state material flow inside the dryer was first simulated using software called EDEM™, developed by DEM Solutions Ltd. Then the discrete element model was coupled to a CFD simulation of the fluid zone and the natural gas flame created in FLUENT by using a coupling utility provided by DEM Solutions. The resulting coupled solution indicated that the presence of the material bed does not adversely affect the flame shape inside the dryer.

The challenge was how to visualize the results from both the CFD and DEM simulation. “If you couple FLUENT and DEM, there’s no way to look at the results simultaneously. Without a way to visualize the coupled solution, you cannot see the whole picture,” explains Hobbs. Each program has its own built-in visualization tools, but neither can read the results of the other.

Because EnSight can visualize results from many different software programs, including CFD and DEM, and can load multiple results at one time, “EnSight lets you read in the results from each solver and see them simultaneously,” he said. This ability enabled him to make calculations, images, movies, and even 3D scenes with the combined results, including velocity of rock particle, as well as velocity of fluid domain. Peter Weitzman, Vice President of Americas at DEM Solutions, adds, “What’s nice about EnSight in conjunction with EDEM™ is that now you can visualize solids and fluids together and see how they interact.”

Share the Road
Other CEI products, namely the EnLiten software, play a significant role in asphalt plan design and computer modeling at Astec. Hobbs explains, “Prior to using EnLiten, I would send 2-D pictures but needed to call or e-mail my colleagues with descriptions of my analysis. EnLiten lets me take results from the simulation and it puts them in a generic file format to send out to colleagues. They can open up the files and view in 3-D, which is vastly superior. The models can be manipulated and turned – complete with my comments and direction to points of interest.”

Comparative analysis of the effect of the rock material bed (simulated using the DEM - Fluent coupling) on a natural gas flame inside a counter flow aggregate drum dryer. The upper two quadrants show the flame shape and velocity pathlines with the material bed present and the lower two quadrants show the same with in an empty drum.

With EnLiten’s easy file-sharing, communication can be rapid and accurate – and much less expensive. Astec often has engineers working abroad or on-site, which can sometimes complicate information sharing. As a matter of fact, Hobbs works from England and must coordinate his research with corporate headquarters in Chattanooga, Tennessee. Remarking on the efficiency and ease he has been able to realize with CEI’s products, Hobbs says, “Any of our 50 engineers can access EnLiten files on temporary assignment just as easily as if they were at the home office, and EnLiten allows for sharing in a realistic and useful manner. We also use EnVideo to make animations of all the DEM simulations. It’s a nice easy-to-use utility.”

The Road Never-Ending
Since asphalt’s historic debut in Babylon, the design and composition has undergone tremendous evolution, but it seems unlikely the essential product will face replacement or obsolescence any generation soon. These days, industry engineers rely on computer modeling to design efficient plants that can meet the current strict emissions regulations, as well as consumer recycling demands, with 73 million tons of asphalt (more than twice the combined weight of glass, aluminum, paper, and plastics) recycled through their plants each year.

In the same way that roads play a vital role in connecting people and business, EnSight allows industry leaders to continue to lead the way by connecting its engineers with the information they need to innovate. Weitzman states, “The quality of visualization that comes from EnSight can be used to showcase the quality of our EDEMTM product. Together with EnSight, customers can access very detailed and powerful visualizations of real world models.”

No matter how much technology advances the asphalt industry, however, it can only sweeten the experience of the modern American road trip. Convertible top down, wind blowing through our hair, we glance at the rear-view mirror and see the ribbon of black that represents where we’ve been. Eyes on the road, we can only wonder where we’ll go.

Flame shape and flow characteristics of one of Astec's premix burners called the Phoenix Talon. (The upper left shows an isosurface of ppm of CO defining the flame shape. The upper right shows velocity vectors on a midplane in the burner nose. The lower left shows contours of temperature on the burner nose, and the lower right shows velocity contours on a midplane leaving the burner nose.)

Comparative analysis of the effect of the rock material bed (simulated using the DEM - Fluent coupling) on a natural gas flame inside a counter flow aggregate drum dryer. The upper two quadrants show the flame shape and velocity pathlines with the material bed present and the lower two quadrants show the same with in an empty drum.