An Overview of Rotary Tube Furnaces for Advanced Materials Processing

Interview conducted by Stuart MilneMar 3 2015

Brian Fuller, Sales Engineer with Harper International, talks to AZoM about their rotary tube furnaces and the benefits they bring to the industry.

Could you please provide a brief introduction to the industry that Harper International works within and outline the key drivers?

Harper International engineers and manufactures advanced thermal process systems for a variety of different industries. These are custom and complete systems for various applications. Each system is unique and specific to our end users’ process.

We work within a variety of advanced material industries including technical ceramics, microelectronics, aerospace and composites, nuclear catalysts, solar and energy devices, and powdered metals.

Could you briefly explain the basic technique of how rotary furnaces work and why they are used?

Indirect fired rotary furnace is a continuous thermal system which applies heat to the incoming process material via multiple thermal control zones. The power to the system can be applied by gas or electric heating. The heat is transferred from the heat source to a rotating tube which contains the process materials and process atmospheres. Finally, the heat transfers from the tube wall to the bed of material which is being processed.

The process tube is supported at an angle, and rotated. This action causes the material to continuously convey through heating and cooling sections. The rotary system is a complete system with feeding, heating, cooling, and provides a very efficient method for processing powders.

Rotary tube furnaces are primarily used because of their efficiency in heat transfer and mass transfer for powders. If you compare it to other types of equipment, it has one of the highest thermal efficiencies and lowest operating costs.

How does this differ from direct-fired rotary equipment and other forms of continuous, high temperature systems?

Direct fired rotary equipment applies heat in a different method. Typically, it will have a single burner mounted at the discharge of the rotating tube, and this creates a flame inside the process environment, which extends a certain length into the tube, and it directly heats the process material. However, direct fired equipment severely limits the temperature control and atmosphere control, which limits it to certain industries. When Harper looks at an application, we're really looking for materials that require atmosphere control and very precise temperature control as well.

This is primarily created by the powder mixing with itself and with the atmosphere as the tube rotates. Compared to other thermal processing equipment, the rotary furnace is highly efficient for gas-solid reactions because of the heat and mass transfer.

Typically, the heat or mass transfer are rate-limiting steps for powder processing. That's a big advantage that the rotary furnace has over static powder beds.

How can the type of powdered material and process effect the design used?

One typical use for rotary furnaces is powders that require frequent exposure to fresh process gas, such as the reduction of a metal oxide with hydrogen.

This reaction involves water, and the water vapor is heavier than the hydrogen which means it normally sits at the bottom of the reactor where the powder bed is. The objective then is to remove the water from the powder bed and expose it to fresh hydrogen. So, by using internal features in the tube, you can continuously expose the powder bed to fresh hydrogen. There are several other types of tube internals which can be used depending on the process.

Other special features may be required for powders that are sticky or cohesive or that have exothermic reactions.

How can mixing and conveying technologies offer improvements in a rotary system?

The mixing technologies can dramatically increase the volume utilization of the rotary system. This is achieved by decreasing the time requirement in the case of processes in which the time requirement for the reaction is limited by either heat or mass transfer. The mixing technologies in a rotary furnace can dramatically reduce this, thus giving the end user, a smaller furnace that will meet their requirements.

Even the simplest rotating tube is conveying the material, but there's always some random motion of the powder. Each particle can have a distribution of time spent in the reactor. Devices can be installed in a rotary furnace which tightly control the dwell time at temperature. An example of this is by installing an Archimedes' screw spiral inside the tube, each and every particle will see the same time/temperature profile.

How can using a multi atmospheres system help maximize the utility of the process?

There are several processes which require a series of different environments to complete the process. An example of this is temperature, whereby if you take an example of calcining an ore, water can be evolved at a low temperature, and as you increase the temperature, more CO2 will be evolved. These temperature profiles are routinely used in several types of furnaces, but a more complex change of environments would involve different atmospheres.

The traditional way to achieve this is by installing two completely separate furnaces and operate them in series, but if these multiple atmospheres can be achieved in just one furnace, then the investment and likely the operating cost can dramatically be reduced.

What industries typically use these methods?

There are many examples of industries that can benefit from using these concepts. Unfortunately, today it's not routinely used. Harper is pioneering some methods to more accurately control and create two different atmosphere environments within a single furnace. It’s quite a difficult method with a rotary furnace because you can only access the environments on the entrance and exit sides. Any industry that has a condensable off gas coming from the product can utilize this. You remove the condensable gas while it's still up at a high temperature. This method can also be used by any industry that has an off gas that inhibits further reaction.

How does Harper’s flexibility to build custom furnaces help your customers meet their needs?

Our customers come to Harper to create thermal processing solutions. On their end, our customers also have customers, which are putting ever-increasing materials, properties, specs, and purity requirements onto them. Together, we work with the material producers to understand the process and customize a solution. Harper has a strong team of engineers, scientists, and PhDs that work on understanding the process, and have some advanced material production experience before coming to Harper. This enables our customers to work with us and meet their specifications.

Are there any recent case studies that you are particularly proud of?

We recently put into operation a unique rotary system for U.S. Demil, LLC. The goal of the project was to safely and greenly decommission munitions that have expired and were being stored by various governments for safekeeping. Together, we partnered with them to develop the process and equipment specifications through work in our Technology Center. We were able to exactly control the time that each individual munition spends in the reactor. Now, at some point, the munition releases some energy, so it was important to design the internal features of the tube to keep the munition in a specific spot in the reactor, and not have it randomly move to the next temperature control zone. It was a very unique rotary system which fully met the requirements set out by our customer.

Where can our readers learn more?

They can visit our website here.

About Brian Fuller

Brian Fuller, Sales Engineer at Harper International, received his Bachelor’s Degree in Chemical Engineering from the University of Colorado Boulder. Since joining the company 7 years ago, Brian has collaborated with several customers through our Ignite™ program to develop new commercial solutions for advanced materials. Brian’s passion is for learning and understanding customer’s unique technical processes.