How to Be the Plant Everyone Buys From

Editor’s Note: For 2024, AsphaltPro Magazine allows experts in the industry to share how to expand your operations to the next phase of business. Are you ready to start making your own hot-mix asphalt? Let’s turn to some professionals who have equipment, services, software and tenure to help you expand to mix design, production, hauling and more. This month’s installment looks at the best practices you can employ with the storage silo to offer customers quick, quality asphalt mix for their paving projects.

If you’re in the hot-mix asphalt (HMA) industry, you know one of the game-changing inventions of last century was the storage silo. During his “Asphalt Plant Efficiency” presentation during a World of Asphalt People, Plants and Paving session in Nashville in March 2024, Greg Renegar, the vice president of customer success for Astec Industries, Chattanooga, discussed the benefits of planning ahead with your storage silos in mind.

If your “why” is to provide mix for both customer and in-house crews, you’ll want to plan ahead for overnight storage of appropriate mixes as you build your new plant. During his presentation to the World of Asphalt audience in March, Astec’s Greg Renegar reminded attendees the plant that can start loading out customers first thing in the morning will be the plant everyone flocks to.

As he explained, even if you have older components you’ve been unable to update the past few years, you can operate efficiently if you maintain those parts, tighten up your environmental footprint, and follow best practices, such as optimizing the use of storage silos.

Notice that’s “optimizing” the use of storage silos. Not every mix design is ideal for the suggestions to come, and we’ll talk about those.

“If you are using very low absorption aggregates, it may lead to a high film thickness and the mix is more prone to drain down.”—Steve Jackson

Solve Asphalt Plant Odor Issues

Store It for Fast Startup

Renegar’s presentation included a side-by-side comparison of operations you might be able to share with your production team to showcase what’s optimal and what’s not.

Amazing Producer ABC

Using old technology

  • Starts loading out of prefilled silos at 6 a.m.
  • Starts up the plant at 8:30 a.m.
  • Runs two to three mixes on various jobs, with enough trucks for the day
  • Runs all day with changeovers but no mid-streams
  • Fills the silos at the end of the day for tomorrow’s early customers

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Struggling Producer XYZ

Using new technology

  • Starts making mix at 6 a.m.
  • Runs two to three mixes on various jobs, short of trucks
  • Mid-streams at 8:30 for 45 minutes
  • Runs another 300 tons and finishes for the day!
  • Cleans out
  • Gets a call at 10:15 a.m. for a 150-ton parking lot job
  • Fires back up at 11 a.m., runs 147 tons, then mid-streams while paving foreman figures the last bit needed

One of the two producers in our examples is using new technology for its efficiency and sustainability but isn’t using best planning strategies. Renegar shared plants that start and stop more than three times per shift use up to 20-35% more fuel than they do when they run steadily. These percentages are published in the National Asphalt Pavement Association (NAPA) publication QIP-132.

You can probably monitor the effect of starting and stopping on your own fuel use. By using the storage silo to take up the slack and prevent starts and stops, you keep a steady, even production. Renegar stated Astec’s most successful customers are the ones who use long-term storage capabilities to become more profitable.

Think about it.

Because 95% of breakdowns occur at startup, you have a leg up on the day even if unplanned downtime hits you at first light. You also have a leg up on your competition on the other side of the county if your plant already has mix in the silo while Producer XYZ is still getting fired up.

Renegar explained it this way: “Storage in multiple silos plus planning allows FOB customers to get in and out quickly in the morning. Serving the FOB customers better than your competition will result in more business.”

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Design Your Storage

This isn’t rocket science. But it does require forethought. Renegar cautioned producers on some reasons you might not want to store mix overnight. For example, lack of planning from your customers could result in wasted mix. There’s no point in producing a hundred tons of state mix at 300 degrees if your top five customers will show up wanting a less-pricy mix produced at 340 degrees.

If you don’t have proper heating systems in place, you run the risk of losing mix temperature. There are companies making electric heating elements that can be placed in silo cone packages to take the fear out of overnight storage. These entities might not make the silo itself, but are experts in the manufacture of electric heating components and provide these to OEMs like CWMF Corp., Waite Park, Minnesota, who then assemble the complete silo.

Even with the concern of temperature under control, you want to consider the mix design you’ll store. Renegar listed the “fear of storing polymer” as one of the reasons producers shy away from filling up the silo overnight or over a weekend. Folks in the field echo his concern when it comes to open-graded mixes due to a phenomenon known as drain down. This is when gravity pulls the liquid asphalt cement (AC) away from the aggregate and down toward the silo cone.

Steve Jackson, the vice president of plant operations and sustainability for NB West Contracting, Pacific, Missouri, spoke of this phenomenon in stone matrix asphalt (SMA) mixes. “The worst mix that I have seen for drain down is SMA,” Jackson shared. “That is why some agencies are reluctant to remove the cellulose fibers even when you add ground tire rubber or reduce the mix temperature.”

He gave an example. “I remember an SMA project where we filled a silo, and that was all the mix that we made for the night. The first sample, from the bottom of the silo, had high AC and 1.5% air voids. The second sample, toward the top of the silo, had low AC and 7% air voids. We made the mix extra hot because it was going to spend a long time in the silo.” The expensive lesson he shared was having to mill out that tonnage and replace it.

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“SMA, open-graded friction course and other gap-graded mixes are the worst for this phenomenon,” Jackson continued. “They also usually have specified minimum asphalt contents. If you are using very low absorption aggregates, it may lead to a high film thickness and the mix is more prone to drain down. When Joe Schroer (NB West’s construction materials engineer) worked at MoDOT, he evaluated some of those mixes, and started calculating the volume effective binder, and approved some of the SMA mixes with less than the minimum spec requirement AC content of 6.0%.”

In other words, there’s hope for “fixing” the gap-graded mix so it can be stored overnight for quick loadout in the morning, if you’re willing to work with it.

Malcolm Swanson, industry consultant and president of e5Engineers LLC, Chickamauga, Georgia, shared his thoughts. “Coarse graded mixes, SMAs, any mix with little surface area will tend to drain down. That is a major reason for adding fiber to a mix. Fiber adds surface area without changing gradation. The added surface gives the AC a place to hang on.”

“If the state allows the contractor to design their own non-gap-graded mixes, then the mix has the absolute minimum asphalt content, so they are less likely to drain down,” Jackson said. “If there are mixes that specify a minimum asphalt content, then I would be careful. Take a look at the aggregate absorption as well. We typically use aggregates with 1% or lower water absorptions in our high type mixes, these are mixes that we try to drop the mix temperature as low as possible to prevent drain down.”

For producers looking to optimize the use of the storage silos, it’s possible to adjust the mix design and temperature to ensure you have exactly what your customers are looking for first thing in the morning. It might take a little forethought and planning, but the producer who plans ahead is the producer who can optimize all the components for a tight environmental footprint, an efficient operation and a plant that all the customers flock to.

Solve Asphalt Plant Odor Issues

Editor’s Note: For 2024, AsphaltPro Magazine allows experts in the industry to share how to expand your operations to the next phase of business. Are you ready to start making your own hot-mix asphalt? Let’s turn to some professionals who have equipment, services, software and tenure to help you expand to mix design, production, hauling and more. This month’s installment from Ecosorb takes a look at the permit line-item of odor control at the plant to ensure your community relations, sustainability initiatives and production efficiency are working hand-in-hand.

In the asphalt industry, maintaining neighborly relations and curtailing odor complaints before they arise is just as important as maximizing production for every company’s long-term business continuity.

Asphalt binders are complex mixtures of organic compounds produced as a byproduct of petroleum refining. With constantly changing feedstocks, staying on top of blends is critical to controlling odors. In recent years, “opportunity crudes” have become more prevalent industrywide, and it has become necessary to balance the lower cost of these crudes with extra processing and off-gas scrubbing requirements. Another factor to consider is increased equipment maintenance and premature failure because of accelerated corrosion caused by hydrogen sulfide in lower grade crudes.

Each peak in a GC chromatogram represents the presence of a compound, identified and quantified on the x- and y-axes respectively.

During heating, mixing, transfer and application of asphalt binders, odors are often produced from volatile organic compound (VOC) emissions, which can lead to concerns from neighbors and passersby smelling the off-gas. If left unaddressed, concerns can become complaints, which can ultimately pave the way to regulation and operational restrictions.

Odor abatement is a historically difficult undertaking in the asphalt industry because of the complex makeup of bindings and numerous petrol products used in each hot mix. However, plants can now enlist the help of top suppliers with the knowledge and advanced laboratory technologies needed to chemically neutralize odor-causing components. These experts address this issue by identifying the problematic compounds, and by then creating and providing additives specially formulated to neutralize odors from various asphalt mixes.

Neutralization challenges with evolving mixes

Occupying a lower tier on the crude refining food chain, asphalt blends will always vary much more than higher-tier products, such as aviation fuels. Mixes can vary significantly from one season to the next—particularly with opportunity crudes—depending on the oil sources available and how they are processed. As a result, odor neutralization formulations must also adapt to the changing constituents of each season’s asphalt mixes.

Manufacturers, therefore, cannot always rely on the same odor-mitigating additives from one season to the next, even when producing the same end product. Additionally, since crude sources and refining processes vary by region and supplier, the nature and intensity of odors can differ even between batches of asphalt binders. Other factors for odor mitigation assessment include the temperature at which the plant is operated, geographical attributes—such as hills and valleys—humidity, temperature, wind speed and direction, and proximity of neighbors.

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There are many potential VOC emission sources during the processes of refining the feedstock, manufacturing the mix, and storing the finished product. This is especially notable during the storage of bulk asphalt in a heated tank, in addition to stack emissions during production. Loading asphalt from one vessel to another—such as from the silos into trucks, and from trucks into a paver hopper—is another frequent odorous phase.

VOC concentrations increase significantly at higher temperatures. Below 150°C, few detectable VOCs are produced, but above this point, emissions increase. VOC prevalence is also dependent on the surface area of asphalt exposed to air over asphalt-coated aggregates. VOCs also increase substantially when these asphalt-coated aggregates are agitated.

Ecosorb’s vapor phase delivery system is useful for mitigating odors in airborne particulate matter at a baghouse.

Research-driven and plant-based solutions

Asphalt production variability spawns the need for adaptive and sometimes customized blends. Although there is no one-size-fits-all solution, there are general formulations that bolster odor mitigation efforts for a variety of asphalt mixes. These general formulations are added to the mixes to neutralize several odor-causing constituents, like hydrogen sulfide, mercaptans and general hydrocarbons. Specialty suppliers update their additives from year to year based on aggregate samples taken at asphalt plants to maintain effectiveness.

Although general formulations are efficacious for many mixes, some plants require custom blends, and expert suppliers can help with these needs as well. In these situations, asphalt samples are taken from the plant during multiple stages of production. These samples are then studied in a lab using gas chromatography mass spectrometry (GC-MS) to determine the precise molecular makeup of odors, even those present in minute amounts.

A GC chromatogram is a visual output of the data recorded by the detector, and it is presented as a plot of detector response along the y-axis, versus retention time along the x-axis (See the graph in this article).

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Each compound detected appears as a single peak on the graph, with the corresponding retention time value used for identification. Once the odor-causing compounds are identified, scientists develop a formulation using plant oils to molecularly neutralize the odors when the additive is combined with the mix.

This results in versatile and cost-efficient solutions—specially crafted for maximum effectiveness in each application—with each solution leveraging customized concentrations of plant oils, biobased surfactants and water to eliminate odors. These additives are designed to be safe, non-toxic and biodegradable, and to be applied during any phase of the asphalt lifecycle: manufacturing, storage, transportation and use. When used in refineries, hot mix plants, transportation systems and paving operations, they are blended directly into the asphalt mix (Figure 3).

These additives are also useful for mitigating odors and blue smoke when dispersed via vapor phase during the capture of particulate matter in a baghouse.

Airborne vapor phase dispersion is also commonly deployed to control odors at storage terminals, where asphalt is kept prior to delivery.

Multi-mix odor neutralization with a single additive

One longtime Ecosorb end-user customer—a new and reclaimed asphalt pavement plant producing 1,500-2,000 tons of hot mix asphalt each day for contractors and construction firms—relied on a basic odor-mitigating additive for years. However, recent asphalt mix variability spurred the need for a specially targeted formulation. The different mix sources and grades began producing pungent odors in the areas surrounding the plant, and the standard additive was no longer effective.

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The plant shipped samples of three different asphalt mixes to Ecosorb’s lab to assess the levels of hydrogen sulfide, mercaptans and general hydrocarbons in each. The data from one asphalt mix, before and after treatment at 150°C is shown in Table 1.

Although the levels were different in each mix, the team of scientists formulated a single blend to neutralize odors in all three mixes, eliminating the burden of correctly matching different additives with a specific mix. The plant added this single blend to all its mixes, which reduced airborne sulfurous release by over 90%.

Ecosorb uses GC-MS instrumentation to identify odor-causing substances in asphalt mixes, and then develops custom plant-based formulas to neutralize odors.

Effective eco-products enhance odor abatement

By adding plant-based odor removers to asphalt mixes, manufacturers can effectively mitigate odors using safe, environmentally friendly and cost-efficient methods. These custom formulations do not mask smells, but instead mitigate them by breaking down and neutralizing odor-causing molecules in the mixes.

These mitigation techniques empower asphalt manufacturers to redirect their time from odor control and complaint handling, to maximizing production and overcoming the steady stream of challenges posed by evolving feedstocks.

Laura Haupert, Ph.D., is the chief scientific officer for Ecosorb, where she leads research and development, regulatory, safety and quality control. She earned her B.S. in chemistry from Manchester College and Ph.D. in physical chemistry at Purdue University, working with bond energies of solvated clusters. Haupert also completed her post-doctoral research at Purdue.