Drying Distillers Grains: Steam Tube Dryer vs. Direct Heat Dryer

Drying Distillers Grains: Steam Tube Dryer vs. Direct Heat Dryer

Consider a drying system to process 20 tons per hour of combined wet cake and syrup produced in an ethanol plant or distillery. At 66% combined feed moisture dried to 10% moisture content, the mass balance would be as follows:

Feed MaterialDischarge Material
Dry Solids13,600 LB per Hour13,600 LB per Hour
Moisture26,400 LB per Hour 1,511 LB per Hour
Total40,000 LB per Hour15,111 LB per Hour

Evaporation:                     24,889 LB per Hour

The heat requirement for drying is approximately 27.2 MMBTU.

Direct Heat Dryer

Using a direct heat dryer system for the process above, the system schematic would be similar to the one shown in Figure 1. Note that the dry material recycle loop is not shown since it would be identical for either dryer.

Drying Distillers Grains Direct Heat Dryer

The direct heat dryer in the example is equipped with a combustion system having a dryer inlet gas temperature of 1,000 F. Based on this system, the dryer exhaust gas discharge volume would be 53,000 ACFM. The system is provided with a flue gas recirculation system. This is included for two reasons:

  • Since the gas is laden with water vapor and combustion products (from having passed through the burner), it lowers the amount of oxygen present in the drum in order to prevent combustion from occurring inside the dryer.
  • It reduces the amount of gas having to go to the thermal oxidizer for destruction of VOC.

    This operating cost of the system is not just the drying but also the cost to operate a thermal oxidizer. This type of system also requires two additional fans, which cause increased horsepower requirements and exponential complexity of process control.

    Steam Tube Dryer

    Using a steam tube dryer system for the process above, the dryer system schematic would be similar to the one shown in Figure 2.

    Drying Distillers Grains Steam Tube Dryer

    Because the steam tube dryer employs steam to do the work, very little air is required. Typically, the dryer is run under slight negative pressure and a small amount of air is vented into the dryer to offset dust from puffing out. Thus, the primary content in the exhaust stream is water vapor instead of air.

    Since water is condensable, the exhaust gas goes through a condensing scrubber where a great deal of the water is collapsed back to liquid phase and removed. The remaining saturated air stream is taken back to the boiler and used as combustion air. The boiler acts as the VOC combustor – potentially negating the need for an RTO.

    Summary

    Employing a steam tube dryer for processing DDGS may initially appear to be a more costly system (because the dryer itself is more expensive, and a boiler is required).

    Yet, for true comparison, the following factors of the overall system also need to be considered:

      • Steam tube dryers do not require heated air to operate.
      • Steam tube dryers do not require recycle gas stream for fire prevention.
      • Steam tube dryers are easily controlled by simple modulation of a steam valve.
      • Steam tube dryers do not require a thermal oxidizer system for most biomass systems. If an RTO is required, it is 75% smaller than with direct heat.
      • Steam tube dryers have an atmospheric environmental impact a fraction of direct heat dryers.
      • Considering the cost of an RTO verses a boiler, the steam tube dryer system may have a smaller initial cost.
      • Since the boiler for a steam tube dryer system may act as a thermal oxidizer, the operating cost is much lower.

    Learn more about When you would need a Steam Tube Dryer vs. a Direct Heat Dryer.

    Contact our Application Engineers to discuss your operation’s specific needs and how we can help you achieve The Lowest Cost per Revolution.

    How Steam Tube Dryers Process Materials

    How Steam Tube Dryers Process Materials

    How Materials are Dried and Processed using Steam Heat transferred through Tubes

    A steam tube dryer is like a rotating shell-and-tube heat exchanger in which medium pressure, saturated steam is charged through a rotary steam joint, into the manifold and then into several heat transfer tubes – typically at a nominal 100-150 psig (and up to 450 psig/500F service for reacting granular solids). Concentric rows of these tubes run the length of the dryer; one to six rows depending on the dryer size and duty requirements.

    steam tube dryer parts

    Steam Chest Design

    A steam and condensate manifold chamber, referred to as a Steam Chest, is mounted on the product discharge end of the dryer. This Steam Chest distributes steam to each of the steam tubes and collects the condensate formed as the steam transfers its latent heat to the material being processed. The steam condenses and the condensate is removed from the dryer and taken back to the boiler. The dryer utilizes only the latent heat of condensation, making the dryer one of the more efficient drying systems in the world.

    steam tube dryer standard steam chest

    The process material is fed into the drum through a screw conveyor and then tumbles over the outside of the tubes. As indirect heat, the thermal load of the saturated steam in the tubes is not in direct contact with the material. The steam remains inside the tubes while the material lies outside of them. The material cascades on the outside of the tubes and condenses the steam on the inside of the tubes, and that conductive and radiant heat is used to dry the material.

    The latent heat is efficiently drawn from the steam (about 850 BTUs per pound) and transferred through the tube wall and into the material – which drives off its moisture. Our dryers execute this process at a consistent temperature – providing a very uniform heating of the material particles.

    The water condensed out of the steam is continuously removed from the steam chest through a stationary siphon and cycled back to the boiler for regeneration. And the dried product discharges through ports in the shell at the steam chest end of the dryer.

    Pipe Manifold Design

    pipe manifold

    For certain applications, an alternative to the fabricated Steam Chest design is needed. For example, the Louisville Dryer Pipe Manifold design is a available based on customer preference. The Pipe Manifold is comprised of concentric rings of pipe, one for each row of steam tube in the dryer, set on different planes, which are connected to a center steam and condensate distribution chamber with steam supply pipes, and condensate return pipes. Each row of steam tubes extends through the dryer and is welded to its corresponding manifold ring.

    The Steam Advantage

    • Steam Tube Drying is thermally very efficient and friendly as heat loss through the cylinder wall is minimal since the steam tubes are fully enclosed by the dryer.
    • This indirect heat system is also much safer for processing temperature-sensitive organics and volatile chemicals which could catch fire even at low temperatures.
    • The exhaust gas from the dryer is about 80 % less than what is required for direct heat dryers, which significantly reduces the size and cost of the air pollution control equipment.
    • Using saturated steam enables us to maintain an inert environment – which is safer for your materials and production/maintenance team.
    • Since Steam Tube Dryers process under lower temperatures and without gases present in a direct-fired dryer, your material will not get scorched or burnt as much as they might from using Direct Heat Dryers.
    • Thus, your material will maintain or receive better coloring, and its bypass proteins will be protected by not being overheated (which is important for digestion of Distillers Dried Grains by livestock).
    Steam Tube Dryers Process

     

    Steam Tube Dryer Videos

    To view how our Steam Tube Dryers process temperature-sensitive, volatile or corrosive materials, watch this video.

    This segment is one in our new video series on Steam Tube Dryers:

    • WHY choose our custom Steam Tube Dryers for processing Distillers Dried Grains (DDGS) and other temperature-sensitive materials?
    • HOW do our Steam Tube Dryers process materials?
    • WHAT components comprise our Steam Tube Dryers?
    • WHERE in a Steam Tube Dryer is the material processed?
    • HOW LONG do our Steam Tube Dryers operate efficiently, safely and profitably?
    • WHO is Louisville Dryer Company?

    Green Drying

    Using clean steam rather than products of combustion, Louisville Steam Tube Dryers are one of the most ecologically friendly drying technologies. As indirect heat dryers, they use the latent heat from clean low-temperature saturated steam rather than hot gases from fossil fuel-fired combustion systems. This highly efficient heat-transfer design also makes Louisville Steam Tube Dryers one of the most thermally efficient drying technologies.

    The residual non-condensable gases left inside of the steam tubes once the steam has condensed are vented through a flexible connection attached to a common Vent Header that is mounted at the feed end of the dryer. This Vent Header has thermostatically controlled vent valves which allow the cooler non-condensable gases to be efficiently vented as they accumulate while containing valuable steam energy inside the steam tubes.

    Since the Steam Tube Dryer utilizes the latent heat of steam to drive the drying process, only a small amount of sweep air is required to remove the water vapor driven off the product generated in the drying process. This is usually less than about 30% of the exhaust gas required for a direct heat dryer for the same process parameters.

    Central Discharge CS Seal Steam Tube Dryers

    The gap between the stationary Feed and Discharge housings at either end of the Steam Tube Dryer is sealed to keep ambient air from leaking into the dryer. Various sealing technologies are used depending on the process parameters. These seals may be as simple as tensioned woven fabric belts and as sophisticated as machined packed and inert gas purged seals (e.g. solvent extraction and other vapor capture applications).

    Louisville Dryer Company is an ASME Division I Design and Manufacturing Center

    Our Steam Tube Dryers are custom engineered for each specific application. The steam side component is a pressure vessel, designed and manufactured in conformance with the latest edition of ASME Section VIII, Division I. They are stamped and registered with the National Board. Materials of construction range from carbon steel to various nickel alloys and duplex stainless steels.

    Steam Tube Dryer Louisville Dryer

    We have completed thousands of installations processing hundreds of materials around the world and pioneered many of the technologies applied to today’s Direct Heat Dryers and coolers, Indirect Heat Dryers and calciners, Steam Tube Dryers and Water Tube Coolers.

    Materials processed include organic and inorganic chemicals, petrochemicals, grains, metals, aggregates, and waste byproducts. If the material is granular solid, it probably has been processed in a Louisville Dryer.

    Contact our Applications Engineers today to begin finding your custom solution.

    Solutions for a Wet Corn Milling Plant

    Solutions for a Wet Corn Milling Plant

    Customer’s Problem Dryer had Material Build-up and Tube Failures

     

    When a prominent wet corn milling plant contacted Louisville Dryer Company for solutions to their drying problems, our process engineers promptly met with their production staff. Loss of their drying capability resulted in a loss of revenue for their company, a problem we were eager and fully equipped to help solve.

    This processor had multiple dryers which were demonstrating broad swings in processing rates. The primary dryer in question was a rotating tube bundle dryer. The dryer had a stationary housing with a tube bundle that rotated inside the casing.

    When they fired up their steam bundle, material would build up in the unit causing variations in retention timewhich resulted in some under-drying and some overdrying of their product. The issue was exacerbated by two catastrophic failures of the tube bundle. Such unplanned outages can cost hundreds of thousands of dollars in repairs and millions of dollars in lost revenue.

    Louisville Dryer Solutions for Wet Corn Milling Plant

    Louisville Dryer Co. provides effective and profitable drying solutions for Wet Corn Milling processors

    Using custom mixing protocols to match their needs, we developed a successful recipe for their recycle stream. Along with our performance guaranty, the company engaged us to build a replacement steam tube dryer for their plant.

    Steam tubes on dryers engineered by Louisville Dryer Co. process material with precise accommodation of temperature, pressure and speed requirements

    Steam tube dryers generally rotate at a slower speed and operate at a lower temperature than other rotary dryers. Within steam tube dryers, the material tumbles gently around the tubes which rotate with the shell. Once we custom fabricated their dryer, with Louisville Dryer field engineers on-site for installation and startup, the dryer ran exactly as we had designed and predicted.

    This is but one of hundreds of examples of our commitment to high quality, long-lasting customer solutions using our proven Application Engineering, Custom Design, Project Management, Manufacturing, Installation and Commissioning services.

    Why are Consistent Drying Temperatures Important for Wet Corn Milling?

     

    Drying conditions influence corn wet-milling performance as well as the physiochemical and functional properties of corn wet-milled fraction. The effect of the drying process on the quality of starch, gluten, germ, and fiber recovered during the wet-milling process help to develop more accurate engineering and economic models. Drying temperature combined with the initial moisture content of the grain has a significant impact on the wet-milling performance and the quality of recovered materials.

    We are Ready to Help you Solve your Mission-Critical Challenges

     

    At Louisville Dryer Company, we stand ready for jobs that are out-of-the-ordinary. Each project creates new challenges and opportunities for us to provide custom-engineered solutions to complex problems.

    We have vast experience in processing materials including organic and inorganic chemicals, petrochemicals, grains, metals, aggregates and waste byproducts.

    Louisville Dryer Co. is an ASMA Division I design and manufacturing center

    Louisville Dryer Company is qualified as an ASME Division I design and manufacturing center. We have completed thousands of installations, processing hundreds of materials around the world, and pioneered many of the industrial drying technologies. We focus on:

    Let us know how we can assist you and your company.

     

    Contact Us Today

    Why you Should Button up your Plant with Dryer Seals

    Why you Should Button up your Plant with Dryer Seals

    Dryer Seals are Key to Efficient Production 

    During the cold of winter when you are running your furnace to heat your home, would it be HVAC-efficient, environmentally helpful or financially wise to leave some of your windows open? Of course not.

    Yet for closed controlled systems on a much grander scale, hundreds of industrial direct heat dryers are operated every day without being effectively sealed. Drum Seals which are either absent, poorly designed or damaged often contribute to excessive gas flow through the drum due to air leaking in. Such exhaust gas leaks in the rotary drying process increase fuel consumption (to heat the excess air), increase material carry-out (due to higher gas velocity), place a heavier burden on air pollution equipment and increase operating expenses.

    Furthermore, these leaks reduce product quality, fuel efficiency and even production capacity. (Common causes for seal failures are abrasion/contamination, shell expansion/contraction, excessive shell run-out, poor maintenance and product overflow.)

    The Problem with Air Leaks

    Rotary drums, dryers and kilns serve a crucial role in the processing of hundreds of various consumer products including bulk solids and chemicals.  Since dryers utilizing heat transfer in these controlled systems operate with internal temperatures much higher than ambient air, any outside air allowed to leak into* the vessel will reduce the operating temperature and cool the gas stream, thus reducing the overall effectiveness and cost-efficiency of the system.

    Furthermore, unintended changes in the processing temperature alter the intended chemical reactions – which negatively affects the state and quality of the end-product.

    * Note: When rotary dryers are operated as designed to be under slight negative pressure, out-gassing will be reduced, i.e. leakage of heat and combustion gases from the vessel will be minimized.

    Specific Examples of How Air Leakage Affects Dryer Efficiency

     

    The 1” gap around the end circumference of this drum shell can cause an air leakage of approx. 5,400 ACFM, which places an unnecessary/avoidable significant burden on the baghouse and fan, resulting in loss of 10-15% of production capacity.

    A 1" gap can result in a leakage of 5,000 to 6,000 ACFM

     

    The air leakage allowed by the 1 ½” gap (from missing seals) on this 8 ft. diameter drum is equivalent to a 1’ 9” square hole (= 3.1 sq. ft.) in an exhaust duct!

    Air leakage in an Industrial Dryer

    Poorly maintained (or absent) seals contribute to poor performance and loss of efficiency

    Cracked Dryer Seals
    Cracked dryer seals

    The Louisville Dryer Solution

    Correctly sealed dryer
    Properly sealed dryer

    We button up your plant with high-quality durable seals

    To close the gap between our rotating vessels and their stationary discharge housing, we custom-design and install seal systems. Our application engineers emphasize that properly designed and maintained seals are a significant contributor to the system’s overall efficiency and can thus provide a relatively prompt ROI for plant owners/managers who invest in them.

    If your direct heat dryer does not have a seal system – or is missing leaves/sections of its existing seal, contact us soon to request a diagnostic inspection and solution recommendation from our engineers.

    Properly sealed dryer

    Benefits which your plant could realize from installing/maintaining a proper seal system for your rotary drum/dryer:

    • Decreased production expenses
    • Lower maintenance costs (from reduced gas volume/velocity and fan wear)
    • Saved MBTUs
    • Raised flame temperature
    • Increased production capacity

    Types of Dryer Seals

    When it comes to seals, clearly one size does not fit all. Louisville Dryer Company can offer many different types of drum sealing devices specifically designed to meet the needs of the application process – considering factors including operating temperatures, material in process, dryer size and rotational speed.

    When you would need a Steam Tube Dryer vs. a Direct Heat Dryer

    When you would need a Steam Tube Dryer vs. a Direct Heat Dryer

    Choosing the right drying system is a significant investment and critical to your process. It is essential to know the variations between the different dryer options. The heat source is one of the most common differences. Read on and consider the following reasons why we often recommend a steam tube dryer over a direct heat dryer.

     

    Steam Tube Drying vs. Direct Heat Drying: What’s the Difference? 

     

    Steam Tube Drying  

    Steam tube dryers perform as if it were a rotating tube with a shell heat exchanger. The drum is fitted with a high-pressure chamber that distributes steam into tubes running the length of the drum. Because its applications are for low-temperature processes, the equipment required for thermal destruction is reduced or eliminated. 

     

    Steam Tube Dryers Are the Most Efficient Industrial Dryer

    The most efficient of all the dryers, the steam tube dryer has very little air being exhausted – making the stack loss significantly reduced. The air pollution control system is also much smaller in a steam tube dyer and, since only a minor amount of air is used to sweep out the water vapors, the exhaust volume is a fraction compared to that of a direct heat dryer.

    Steam Tube Dryers are also one of the easiest dryer types to operate. Once the steam pressure is set, the unit will take what it needs to do the job. Think of it as having a set-it-and-forget-it technology.

    A steam tube dryer is used extensively in processing high-moisture organic materials such as distiller’s grain, gluten feed from the wet milling industry, oilseeds, and a variety of sludges.

    steam tube dryer

     

    How Does an Industrial Steam Tube Dryer Work?

    Material is fed into the drum and tumbles over the outside of the inner tubes. Inside those tubes, steam is collapsing to condensate. The latent heat from the steam—about 850 BTUs per pound—provides the heat energy for drying. The condensate from the steam is removed through a rotary joint. It is then taken back to the boiler for regeneration.

     

    Direct Heat Drying

     

    How Does A Direct Heat Rotary Dryer Work?

    A direct heat rotary dryer uses a hot gas which is induced into a rotary drum. Its heat source may be as simple as a steam coil in low-temperature applications, or it can have a burner for higher temperatures. The burner may combust into a chamber or it may fire directly into the dryer drum. We use the term direct-fired when there is no combustion chamber.

    direct heat dryer

     

    Direct heat dryers typically work best when the burner system can operate with a minimal amount of excess air or near stoichiometric balance of the gas and air. This allows the dryer to minimize the air required to carry the thermal energy.

     

     

    frac sand dryer

     

    Wet material is fed into the same end of the drum (parallel flow) or the opposite end of the drum (counterflow) and is picked up and showered (or veiled) into the hot gas stream. The thermal energy is transferred from the hot gas to the material, which heats the solids and the water, ultimately evaporating the water. The primary heat transfer is by convection.

     

    frac sand dryer

    If your product is not affected by heat from the burner flame or the products of combustion, then a direct-fired unit will provide an efficient drying solution through the full utilization of all three heat transfer methods: radiation, convection, and conduction.

    Heat-sensitive materials such as biomass and organic products could be processed with either a direct heat dryer (with an air heater to control the temperature) or a steam tube dryer.

     

     

    Why Choose a Steam Tube Dryer over a Direct Heat Dryer? 

     

    Developed more than 100 years ago, the Louisville Steam Tube Dryer has always been a quality choice for drying high moisture organic by-products such as brewers’ and distillers’ spent grain. Today, this same dryer technology serves thousands of applications. 

     

    What Materials Does an Industrial Steam Tube Dryer Process?

    A steam tube drying system is our recommended choice for drying and processing inorganic and organic chemicals and other bulk solid materials including, but not limited to: 

    • Hemp
    • Dried distillers grains (DDG, DDGS)
    • Wet corn (fiber, grits, meal)
    • Wood chips
    • Soda ash
    • Paper mill sludge
    • Oil Seed (soy, sunflower, canola)
    • Soy meal
    • Chemical products
    • Petrochemical products (TA, PTA)
    • Lithium processing
    • Copper concentrate

    All our dryers, including the steam tube system, are built in our state-of-the-art factory using proven technologies. Our in-house design team of engineers have vast experience in every type of process and industry; thus, our customers always receive a drying system that is custom fit for their operation. 

     

    5 Benefits of A Steam Tube Drying System  

     

    #1: More Cost-Effective

    Louisville Steam Tube Dryers have been proven superior to other types of indirect-heat dryer installations in which hot gas, rather than steam, is used as the heat source. We provide solutions with the Lowest Cost per Revolution throughout our industries. Maintenance savings alone have often prompted the replacement of other dryer systems with the steam tube dryer.

     

     

    #2: Better Quality

    A steam tube dryer is classified as an ASME vessel. Thus, the requirements for its construction are more stringent than for other dryers. 

     

     

     

    #3: More Efficient 

    The steam tube dryer generally operates at a lower temperature than other dryers and rotates at a slower speed. Material tumbles gently around tubes that rotate with the shell, eliminating the friction forces that other types of dryers require.

     

     

     

    #4: Easier Maintenance

    Louisville Steam Tube Dryers enjoy a longer lifespan than most other dryers and require very little maintenance.

     

    #5: Environmentally Friendly

    Steam Tube Dryers are environmentally friendly. Since the steam tube dryer utilizes the latent heat of steam to drive the drying process, only a small amount of sweep air is required to remove the water vapor driven off the product generated in the drying process. Typically, the method uses less than about 30% of the exhaust gas required for a direct heat dryer

     

    Purchasing A Steam Tube Dryer 

     

    Because choosing the right drying system is a major investment and critical to your process, you should be sure you are getting the best equipment available. The Louisville Dryer Company manufacturing facilities hold esteemed accreditation by the American Society of Mechanical Engineers (ASME) and The National Board of Boiler and Pressure Vessel Inspectors

    We extend personal attention and service to each customer, ensuring that their rotary processing equipment needs are efficiently met. If you are interested in learning more about the Louisville Dryer Company Steam Tube Drying System and its applications, please reach out directly to Hank Lawson, our Senior Process Engineer. He would be happy to listen and discuss the best options available for your application.

     

    Call (800) 735-3163 or fill out our online contact form to get started.