Saving the Environment and Bottom Lines by Reducing Emissions

One of the largest sources of emissions is from the creation of volatile organic compounds (VOCs), by thermally damaging product in the dryer itself.
By Becky Long | May 20, 2019

Industrial dryers tend to have gaseous and particulate emissions. One of the largest sources of emissions is from the creation of volatile organic compounds (VOCs), by thermally damaging product in the dryer itself. Smaller infeed particles can be lost through the cyclones or secondary cleanup.

Why should you care? If a dryer is large enough, the U.S. EPA requires secondary cleanup, anyway. You should care because you could be losing a lot of money. If your dryer processes 800,000 metric tons at 45% moisture content (MC) wet basis, you would expect 488,889 metric tonnes at 10% MC wet basis to come out. If you are thermally damaging product and driving off VOCs, however, that is not what you will get. If you damage 1% of the dry solids, which is relatively easy to do, your loss is on the order of $750,000. If you are damaging 4% of your dry solids, still not a big stretch, that is almost $3 million worth (Figure 1).

If you ignore thermal damage and only look at particulate emissions, you have a similar situation. Again, you expect be getting out 488,889 metric tonnes per year, but if your cyclones are 98% efficient, you are blowing away almost $1.5 million every year (Figure 2). Particulate emissions can be partially recovered in a baghouse, but most plants have gone away from them due to the fire risk.

It’s important to conceptualize what happens in a dryer. Everyone has a clothes dryer at home, so people tend to think it is a relatively simple piece of equipment. Imagine buying brand new, fluffy towels. Then, you wash and dry them a couple times. They produce a lot of lint! Your dryer is thermally damaging the towels. We can envision the lint as VOCs, and eventually, you will end up with thin, scratchy, not-so-fluffy towels. Just think—you want to sell fluffy towels, not thin towels. 

Dryers that are thermally damaging product typically have a harsh drying environment. A harsh drying environment would be similar to putting a burger on the grill too early, with charcoal that is too hot. The result is a burger that’s crispy and burnt on the outside, and still cold and red on the inside. Most dryer operators will decide that product like this is actually underdried, not thermally damaged, and will turn up the temperature, leading to a good chance of overdrying. In a harsh drying environment, there will never be a happy medium.

This brings us to the discussion of bound moisture vs. free moisture. Consider water droplets on a leaf to be free moisture, while the moisture in the leaf itself can be considered bound moisture. Evaporating free moisture off the surface of a product is relatively easy and straight forward, but evaporating the bound moisture is not. It is the method employed to evaporate the bound moisture that determines level of thermal damage.

A harsh drying environment is a low wet bulb drying environment. The wet bulb temperature (Twb) is the temperature that water evaporates at, as opposed to the dry bulb temperature (Tdb), which is what’s read on a thermometer. If children are playing in the sprinklers in the summer, their wet shirts will feel much cooler on a dry day than on a humid day. The temperature felt would be considered the wet bulb temperature.

When a drying system heats atmospheric air, the best-case wet bulb temperature that can be reached at the product inlet of the dryer is a maximum of about 80 degrees Fahrenheit in the summer (much lower in the winter).  As soon as that a particle enters the hot drying gases, well over 80 degrees F, the surface or free moisture flashes dry, leaving a dry cellulose layer, which just happens to be a great insulator. The particle continues traveling along with the hot gases trying to penetrate the insulating cellulose layer, which causes the unprotected shell of the particle to overheat and become thermally damaged.

The heat from the drying gases eventually penetrate the cellulose boundary, and water vapor begins to travel from the inside of the particle to the outside, forming a protective vapor barrier to allow more efficient heat transfer into the particle, and for the bound moisture to finally evaporate.

The goal is to get the protective vapor barrier around the particle immediately, instead of letting it become thermally damaged. Water droplets bouncing around on a hot skillet, or dipping a hand into liquid nitrogen, would demonstrate the Leidenfrost Effect. There is a temporary vapor barrier formed to protect the water droplets or hand. Creating a protective vapor barrier around the product is exactly how to prevent thermal damage in a dryer. Accomplish this by raising the wet bulb temperature, increasing the water vapor content of the drying gases. When the particle enters, instead of the surface moisture immediately reaching the wet bulb temperature, it has to be heated to about 170 degrees F, the new wet bulb temperature. Because the insulating cellulose layer is never formed in this instance, heat is transferred much more efficiently into the interior of the particle, without thermal damage. The water is able to move freely from the interior of the particle to the exterior and be evaporated.

The first line of defense against particulate emissions is mechanical separation. There are many rotary drum dryer systems out there that send all the product through the cyclones. Even with cyclones at 98% separation efficiency, a lot of money is being wasted. Using a classifying outlet hopper before the cyclones can improve overall separation efficiency, even if it decreases the actual separation efficiency of the cyclones. If the outlet hopper removes 85% of the product from the gas stream before going to cyclones, less product is being sent to the cyclones. That action likely drops cyclone separation efficiency because they are getting less product, but if 95% separation efficiency is maintained, only about $550,000 will be lost, rather than close to $1.4 million, making a net gain of $900,000.

There are ways to increase cyclone separation efficiency if it is not possible to add a classifying outlet hopper before the cyclones. Adding a drop or catch box at the bottom of the cyclone cone is important to stop the particles from spinning, but the most crucial thing that can be done is placement of an airlock at the outlet of every cyclone, or below every cyclone drop box, and ensuring they are well maintained, with rotors replaced every year or two. No cyclone is identical and there will always be differences in pressure, so without an airlock, or with worn airlocks, one cyclone can pull through a conveyor or chute, into another cyclone, and bring already-separated product back out.

Not only are emissions bad for the environment, they are bad for your bottom line. Thermal damage combined with particulate loss can easily be on the order of $1 million to $4.5 million worth of losses. We like to say “don’t let moisture dampen your profits,” but we could also say “don’t let thermal damage combust your profits,” and “don’t let particulate emissions blow away your profits.”

Author: Becky Long
Dryer Design Engineer, Thompson Dryers