EXPERIENCE 

• Over 50 patents issued world wide
• Over 60 years of experience in innovation, design, fabrication, operation, and evaluation of dryer systems
• First company to successfully install exhaust gas recycle on a dryer system
• Developed low drying inlet temperature with low air-pollution concept & technology
• First to comply with a strict air-pollution regulation without any secondary abatement equipment
• Can dry with near theoretical energy requirements
• Developed the modern center-fill and related technology that allows suspension low air pollution, low energy dryer
• Developed the first practical in-process thermal oxidation system called an integrated Thermal Oxidizer (ITO)
• We build dryers to last; some TDC dryers have been in operation for over 40 years.

Research and Development 
TDC has a firm commitment to research and development.  In 1955 TDC began working on the first heavy industrial dryer drum design.  In 1965 TDC setup two alfalfa dehydrating plants, each with two identical drying systems.  TDC would keep one dryer as the standard while experimentation was conducted on the other.  If the experiment was successful, the improvements would be applied to one drying system at the other plant.  If it was then successful at both plants, the improvement was applied to all the drying systems.   Then, the next round of experimentation would begin again.  Our research and development has lead to our heavy industrial designed dryer system and many other innovations resulting in over 50 patents issued worldwide.  Some of the more notable innovations or technologies to come from our R&D are: 

 

Exhaust Gas Recycle Rotary Material Handling Steam Purge Ball Buster Flights Integrated Thermal Oxidizer Biomass Burners A variety of flighting packages for different velocity drying systems

 

Efficiency
Efficiency is extremely important to dryer performance.  TDC has learned that an efficient drying system is both a safe and environmentally friendly one.  The efficiency of a drying system is completely dependent on its proper use of thermal energy and rotary material handling. 

In 1976 TDC wrote “Proper Thermal Energy Application,” a paper that describes the drying process.  The paper shows that Exhaust Gas Recycle (EGR), also known as Flue Gas Recycle, along with Rotary Material Handling (RMH) are paramount to proper drying.  Proper applications of EGR and RMH have radically changed the drying industry.  Most of the drying industry now uses a form of EGR. 

A rotary drying system requires a specific ratio of mass of gas to mass of material for stable pneumatic conveying.  In the past this was always done with excess air.  The drying and combustion gases typically represent about one-third of the gases going through the dryer.  The excess air for stable pneumatic conveying is roughly two-thirds of the gases going through the dryer.  The excess air must be heated from ambient temperatures to the drying system exit temperatures and then rejected. This process requires 20% to 40% more energy for drying than a system with EGR.  EGR takes some of the outlet gases and recycles them back through the inlet of the dryer to make up stable pneumatic conveying.  The EGR gases are recycled at the same temperature and composition as the exit gases.  When the drying system is at steady-state the gases are primarily water vapor and products of combustion.

Another gain TDC has made in dryer efficiency is tied again to EGR and RMH.  EGR allows the product to be dried at a lower temperature.  This decreases the thermal damage done to the product.  When a particle incurs thermal damage some of the mass of the particle is driven off of the product as volatile organic compounds (VOC) or more simply, smoke.  TDC discovered that avoiding this thermal damage by using EGR can produce as much as 9% more product mass than a dryer without EGR.

RMH is a specialized technology that controls the flow and placement of the materials to be dried throughout the dryer drum in the drying process.  It is imperative that the flow and placement of the material to be dried falls within the amount of energy in that area, as this limits the width of the drying temperature profile curve within the drum and the chances of thermally damaging the material.  The narrower the drying temperature profile curve is, the less chance there is of thermal damage occurring to the material being dried or of air pollutants being generated.

RMH involves the internal flighting system which is made up of both shell and center flighting.  TDC designs special flighting systems for different materials.  We have flighting systems that range from tyne flighting for the OSB industry that can handle wood strips 0.035-inch thick by 3-inch wide by 24-inches long at the rate of 50 to 60-tons per hour, to the T-flighting for smaller particles, to the tray designs for high velocity drying.  In all, we have more than six different types of flighting systems used in various configurations for the wide variety of materials we handle.  TDC has EGR and RMH technology and experience to make your drying system work.

Emissions
During TDC’s experimentation we learned about air quality control and emissions.  Originally, the drying systems in the alfalfa dehydrating industry emitted from 5% to 20% of the dry weight mass they were processing.  TDC pioneered low emission drying systems, circa 1969, with a system that passed the stiff Bay Area Air Pollution Control District Process Weight Standard without any secondary cleanup equipment.  We learned how to transfer the thermal energy to the material being dried without causing thermal damage.  TDC has never failed an initial air pollution test, and TDC has never failed an air pollution test when the equipment was in proper operating condition and run under designed criteria.  A TDC Dryer System has passed the required EPA tests for ethanol plants including the EPA Method 25A test on a near 10-MGPY ethanol plant without a thermal oxidizer.  TDC did this with a drying process that uses our Exhaust Gas Recycling and Rotary Material Handling technologies.  

Shortly after employing EGR on our own dryers, TDC discovered that the EGR also greatly lowered the amount of NOx emissions.  The proper introduction of EGR around the burner flame has reduced NOx on high NOx generation burners used in drying systems with NOx emissions of over 90 parts per million down to around 8 parts per million.  We have also mixed EGR gases with the combustion air to lower the combustion temperature also minimizing NOx formation.

TDC is committed to low emissions as demonstrated by the following story.  Our Topeka alfalfa dehydrating plant was the first in the nation to meet a strict air quality requirement without secondary clean up using EPA method five (the front half).  This was considered to be a clean air plant by the EPA.  We were not convinced that the emissions were truly clean, so we collected further data that indicated our drying systems were emitting from 4% to 9% of the dry weight of material that they were processing as non-condensable VOCs.  This was many times greater than the desired EPA limits at that time.  While the EPA tested for condensable VOCs emissions, their test did not measure non-condensable VOCs.  Although the plant met EPA method five standards, it was not truly clean.  We showed the EPA this data back in 1970, but at that time they had no way to accurately test for non-condensable VOCs.  At the time TDC knew this eventually would be an issue.  In 2002, Gopher State Ethanol in St. Paul, MN had a severe odor problem.  The plant apparently had passed all of the EPA method five tests.  However, because of the odor and complaints from local residents, the Minnesota Pollution Control Agency used the new EPA method 25A test which measured all VOCs.  This revealed that the plant was emitting around 1,100-lbs/hr of VOCs.  This number was significantly greater than the 22-lbs/hr or less required by law when tested by the EPA test method five (the back half), which only captured the condensable VOCs. The EPA realized through testing of several other plants that other ethanol plants with dryers had a similar non-condensable VOCs emission problem.  This was not the case on our dryers as we have understood this problem since 1970 and have created a drying process to minimize VOCs.

Knowing that all VOCs would eventually become a problem, TDC pioneered Low Temperature Drying for rotary drying systems.  With EGR the water vapor content increased significantly which also increased the specific heat of the drying gases.  This allows the thermal energy to be transferred to the product at a lower temperature.  Lower temperature drying harms the product far less than high temperature drying.  With the increased efficiency and the high specific heat drying gases, we were able to dry at 100°F to 400°F lower temperatures thereby decreasing the generation of VOCs.  We also developed center fill flighting that allowed the drying gases velocity to be increased through the dryer drum.  The increased volume of gases going through the drum at the same evaporation rate allowed more EGR which lowers the inlet temperature.  Ultimately, we were able to decrease the inlet temperatures from near 1,800°F to the 600°F to 900°F range.  The combination of EGR, RMH, low temperature drying, high water vapor drying, and hiding the high temperature radiation from the drying process has created conditions whereby VOC generation is very minimal.   Putting this into prospective, TDC has passed the EPA Method 25A test on an ethanol plant about half the size of Gopher State Ethanol without a thermal oxidizer. 

The proper use of modulating EGR is an extremely powerful tool to use in a drying process.  TDC developed an Integrated Thermal Oxidizer (ITO) for our drying systems.  The ITO uses a split modulating EGR flow to the dryer’s systems vertical combustion chamber.  The top part of the modulating EGR split flow directs the (vent) gases which would normally go to the atmosphere but now go to the burner system on the top part of the combustion chamber or thermal oxidizer section.  Here the vent gases are raised to the required oxidation temperatures and retained for specific time periods.

The middle part of the combustion chamber is designed for the oxidized gases to be pulled out of the combustion chamber through heat recovering heat exchangers and then vented to the atmosphere.  The second part of the split EGR flow passes through the heat exchangers recovering the thermal energy.  The lower part of the combustion chamber injects the recovered energy into the combustion gases which becomes the drying gases.  Stand-alone oxidation systems typically use an additional 6% to10% more energy than is required for drying.  The TDC ITO system uses about 3% more energy than is required for drying. 

Safety
At TDC, we take pride in being among the safest drying systems in the world.  We have diligently researched the causes of fires and explosions in drying systems in the past, and have utilized our findings in our designs to prevent such disasters from taking place.  We design our systems without refractory, if possible, to minimize any residual heat in the system in case of an upset condition.  We have also developed modulating EGR, which uses low oxygen, high water vapor content drying, making fires and explosions very unlikely.

TDC uses the modulating EGR during upset conditions to help reduce the possibility of a fire or explosion by keeping the oxygen content low in the drying gases.  If steam is available, we put it in to pressurize the system and keep the air out.  This process has been patented by TDC and is a major safety consideration for safe dryer system operation in any upset condition where over temperature, fires, and explosions are a possibility. 
 
Should a problem occur and a fire does break out in the system, we provide ways to shut the system down safely.  We force the system to go to full modulating EGR to minimize the danger by keeping the oxygen content low.  Then we use our patented “Steam Purge” and regulated air venting to clear the system of the combustible material and explosive gases as safely as possible.

In addition to incorporating innovative safety features into our new system designs, TDC can also improve the safety of your current drying system.  For example, we have taken another manufacturers’ dryer system which had fires and/or explosions almost every time the system shutdown and we modified that system to where it has run for over three years to date, with every conceivable type of upset condition, without a single fire or explosion. 

There are three critical requirements to operate a dryer system without fires:  inert atmosphere, uniform product, and uniform material handling.  The most important of the three is an inert atmosphere.  If the drying atmosphere is always less than 10% oxygen (O2) content, dry basis, it is theoretically impossible for a fire and or an explosion to exist.  The remainder of the drying gases is made up of carbon dioxide (CO2), nitrogen (N2), and water vapor (H2O) of which ideally the water vapor content is in excess of 50% by volume. 

It is very difficult for a fire or an explosion to occur when conditions have less than 10% oxygen content and at least a 184°F.

For a drying system to operate without fires and explosions we need a system which can continually meet these three critical requirements through all phases of operation including:

• Starting the equipment
• Starting the process
• During the process
• During the shutdown the process
• During upset conditions
• Turning off the equipment. 

To achieve system safety the following criteria must be met:

1. The combustion air must be controlled from 5% to 40% excess air throughout the Dryer System’s operating range within a modulating exhaust gas recycle environment.
2. The Dryer System’s equipment and controls must maintain a near neutral dryer inlet pressure and it must have a low pressure drop across the rigid, industrial grade dryer drum.  The drum’s seals must be nearly airtight and perfectly round.
3. The whole system, including support equipment, must be sealed up throughout the Dryer System’s life.
4. The Dryer System’s control system must maintain critical pressures throughout the systems operating range.
5. The Dryer System must have a material handling system that produces consistent uniform product dispersion inside the dryer drum.  Any changes to the operating parameters must occur gradually throughout the operating range.
6. The Dryer System must have the availability of steam for steam purge.  Steam is used to displace air and other unwanted gases and combustible solids to quickly create an inert atmosphere under any condition.
7. A continuously uniform product from the start to finish that is designed so that any changes to the product occur gradually throughout the operating range.

TDC’s designers are highly knowledgeable and skilled at creating these critical requirements and conditions to minimize fires on start up, shutdowns, and upset conditions throughout the operation.  Please contact one of our Dryer Specialists for more information on how to improve your plant’s safety.

Product Quality

TDC’s Rotary Material Handling, Exhaust Gas Recycle, Control System, and other critical parts of our design not only make the system safer, they also insure some of the best product in the ethanol industry.  Distillers Dried Grains (DDGS) used to look dark brown due to thermal damage and VOCs being driven off in the drying process.  TDC changed that and was the first to dry DDGS to the brilliant yellow-gold color that is now the ethanol industry standard.  We did this with Broin and Associates, now Poet, when they first started at the Scotland and Aberdeen, SD plants.

Durability, Reliability and Flexibility
TDC’s defines heavy industrial design as having no stresses in the fabricated parts that are known to exceed the cyclic endurance limits or the static yield strength of the materials of fabrication during normal operation.

There are three grades of dryer systems that are used in industry today: agriculture, light industrial, and heavy industrial.  In an 8,400-hr per year operation an agriculture dryer drum will have fatigue cracks in the structure in 1 to 3 years of operation.  A light industrial dryer drum will have fatigue cracks in the structure in 3 to 6 years of operation.  A properly operated and maintained heavy industrial dryer drum will not have fatigue cracks in the structure after 10 years of operation. 

It is critical that the drum maintains its structural integrity and alignment throughout its life to maintain the roundness needed for the seals and other requirements to continue the drying system’s efficiency, reliability, and durability.  Only a heavy industrial design can accomplish this for an 8,400-hr/yr plant.

Flexing is a major problem in rotary dryers where durability and reliability are required.  The drum will change shape and be wider than it is tall in any position during its rotation.  An exaggerated way to think about it is to imagine a water balloon rolling across the floor.  12-foot diameter stainless steel drums are common in industrial drying.  If a stainless drum flexes over 0.250-inch while it rotates, meaning it is always 0.250-inch wider than it is tall, it will have serious fatigue cracks in the structure in one to three years.  Drums must flex less than 0.125-inches to last the 10 years required for a heavy industrial design status.  TDC knows how to stop flexing in our system and in other manufacturers’ systems.  We use a heavy drum endplate design coupled with a system of radial arm tension supports throughout the length of the drum to maintain roundness.  We pre-stress these members so that when the drying system is operated at normal temperatures with the difference of thermal expansion between the radial arm tension supports and the dryer drum shell, the structural members operate with near neutral stresses.  Our system design takes into account the complications caused by thermal expansion to insure proper sealing throughout the system.

The roundness of the rotating surfaces, tracks and seals and the alignment of the trunnion wheels are critical for durability, reliability, and flexibility in a low air infiltration drying system.  It is essential to grind the tracks and seals properly.  TDC has developed a special case of centerless grinding where we field grind the drum’s tracks nearly perfectly round.  TDC has taken drums with track roundness and seal problems, that when measured with a dial indicator between the trunnion wheels showed 0.300-inch to 0.400-inch runout and ground them to 0.003-inch runout.  This becomes important as rolling surfaces on any rotating drum’s tracks will have wear and periodically the trunnion wheels and drum’s tracks must be resurfaced by grinding.  TDC provides the service that grinds these surfaces in the field with those tolerances.  We also do the same for the seal rings on the dryer drum.  We even design the trunnion bases and trunnion wheels so that we can grind them this way with the precision required while the drying system is operational.  Only grinding between the trunnion wheels can get the tracks to the desired results.

Exact trunnion wheel alignment is critical.  The dryer drum must stay in the same location for the seals to work.  TDC aligns the drum and trunnion wheels in five planes.  The trunnion bases must be level with each other, all the trunnion wheels must be at the same elevation, the trunnion wheels must be level, the drum must be level, and the trunnion wheels must be aligned within 0.003-inch of each other.  The drum and trunnion wheels must be aligned with the required precision to track properly.

The type of trunnion wheel bearing chosen is critical because if a bearing fails, the wheel or bearing must be removed and then reinstalled, changing the alignment.  Self aligning bearings with over 100,000-hrs of expected life should be used.  These will fit the bases without undue stress caused by bearing misalignment.  TDC has, as an option, alignment bars as a secondary alignment system on the dryer drum.  These are added after a TDC drum alignment.  The bars sit in front of each trunnion wheel.  TDC supplies a special fitting and micrometer device that will measure to 0.001-inch from the trunnion wheel to the bar.  If that wheel or bearing is changed for any reason the plant maintenance can always go back to the original settings and have nearly perfect alignment.

TDC’s adaptable bolt-in shell and center flighting is unique.  This allows relatively quick flighting configuration changes in the field.  The ability to change the configuration of the flights to meet the needs of the material being handled is very important, especially on new products or processes.  We know enough about RMH that with bolt-in flighting we have the ability to change the conditions in the dryer drum and make the process work.  We continuously work to improve our flighting systems and the customer has the ability to benefit from this in the field.  The industry is constantly changing.  Changes to the process occur in all industries especially when the cost of doing business changes, like now with high energy costs.  With bolt-in flighting the internals of the drum can be changed for peak efficiency and production easier than when the flights are welded in.  Welding in the flighting may be easier and cheaper but then your system is not as adaptable to a changing environment.

Industries Served
TDC works with several industries drying a variety of products such as:

 

Ethanol TDC has dried wet cake to make distillers dried grains with solubles (DDGS) for the ethanol industry as a feed product.

 

Forest Industry In board plants as well as Pulp and Paper plants in the forest industry we have dried many different products.  In the board industry we have dried wood for particle board, wafer board, and strands for OSB.  In the Pulp and Paper industry we have dried sludge, hog fuel, and forest residual products for fuel.  We have also worked with sawmills and other forest processing plants to use wood wastes as fuel and to pelletize wood for wood pellet burning stoves.

Sugar Industry
In the sugar industry we have dried bagasse for use as a fuel as well as sugar beet pulp for feed.  TDC has designed and repaired drying systems in the molasses industry.

All of these products have lead to the development of their own dryer drum flighting packages.  TDC has several different types of flighting packages we use to properly handle and dry various feedstocks.  To see more applications, click here.

 

Alfalfa		Baggasse		Hog Fuel		Paper Mill Sludge