Developments in drying have been largely refinements of existing technology. These refinements are multidirectional and include refinements in design, control, materials and application. Other developments have been due to the emergence of new requirements such as those of the wood plastic composite (WPC) market, which needs very dry wood flour at relatively high rates.

Design aspects include both electromechanical and process. The advent and availability of two- and three-dimensional computer-aided design (CAD) packages have allowed for the development of "better" designs, easier fabrication and closer tolerances on equipment. Items such as part lists, cut sheets, details and even geometrical developments for fabricated items can be generated from the software application.

This CAD technology allows models to be created and mechanically debugged using electronic simulation media and interference checks. The drying portions of facilities can be integrated into the overall facility by simply incorporating the subsystem model into the master or overall model.

Take Control

Field instruments can acquire exact operating data -- real-time -- and log this data into trending software. This data can take the form of temperatures, pressures, airflow rates, material-flow rates, energy usage, currents and pretty much anything you can ever need. It is expensive to include this type of instrumentation and acquisition system in the scope, but it allows for real-time modeling, leading to a better understanding of the process and hence improved designs.

In my humble opinion, the greatest development in drying, or any processing system for that matter, over the past 10 years has been in the controls arena. Dryers now have flexibility and precision control that could not have been conceived 20 years ago and was prohibitively expensive 10 years ago.

Programmable logic controllers (PLCs) have become simpler to program, have become more affordable and allow for far more complex control than early PLC systems. Gone are the days of relay-logic panels with solid-state controllers, especially for large or complex machines.

Together with dramatically improved and varied field instruments, taking control of the equipment has become routine -- not that there was no control before. Nested control loops allow multiple variables to be monitored and controlled so that system responses are predictable. Analog control using self-tuning proportional, integral and derivative (PID) loops has removed the mystique of gaining fine-tuned control. For example, one can monitor and modulate the feed rate based on current to a motor, while controlling the inlet temperature based on the discharge product moisture.

Human-machine interfaces (HMIs) have become intuitive. Meaningful data for both digital (on/off) as well as analog (real-time values) are displayed on appropriate screens, and alarms (with descriptions) for any condition can be displayed. Web-enabled control systems allow management to monitor operations remotely, so your facility in Johannesburg, Mich., can be monitored while vacationing in Johannesburg, South Africa.

The most significant development related to controls has been in direct labor or operator numbers and associated costs. Constant operator intervention is no longer required to "nurse" the machine to full potential. Well-designed control systems maintain the operation and peak performance, leading to higher operating efficiencies with fewer operators.

Another benefit to the controls arena in the last 10 years is the availability and relatively low expense of AC variable- frequency drives (VFDs). Big motors such as those used on rotary dryer drums, exhaust fans and other mechanical devices can be controlled and modulated based on a measured process variable.

Materials Improve

Machinery or specific components are being manufactured from so called "high tech" or composite materials such as ceramics, graphite, carbon fibers and even Kevlar. These materials have enormous benefits for their intended advantage over regular materials. Bags for dust collection have improved and also offer fabrics for high temperature, corrosion resistance and wear resistance.

Build a Better Mousetrap

For example, historically, mineral concentrates have been dried using rotary dryers. These are inefficient in this application and produce large agglomerates of various sizes with relatively high moisture. This is problematic for post-drying operations, but the direct users such as mines and other post-processors have lived with it. In the last 10 years, flash dryers have become the preferred solution for this application, providing bone-dry, free-flowing product that is ideal for the post-operation. Kudos to the company that had the vision to propose this, and to that individual who had the nerve to try something new!

Other examples of non-traditional solutions to long-standing problems can be cited for the mining, food, cereals, pigment, chemical, and paper and pulp industries. Hats off to all who are innovators.

There have been a few new ideas in drying technology that have sparked my interest. While pulse combustion is not a new technology, it has been developed over the last few years to a point where it is economically and commercially feasible, reliable and safe. I predict that it will grow in the years ahead and find a comfortable niche.

Microwave drying is exciting. I don't know how many of you have tasted an apple chip that has been dried on a microwave dryer. They are remarkable! The cost of these dryers is still extremely high and therefore is really only suited to high-value products. They have been able to overcome challenges associated with microwaves and vacuum, resulting in machines that are commercially viable.

Host-media-bed technology for fluid bed dryers permits fluidization of products that have a wide particle size distribution or are very fragile. The product is supported on a bed of host material while enjoying the benefits of fluid bed drying. Once dry, the product is separated from the host material, which is recycled back into the system.

A Japanese company presented a dryer it called a slurry media dryer that I found rather interesting. It is a combination of a spray dryer, a drum dryer, a fluid bed dryer and a flash dryer. The dryer consists of what is essentially a static fluid bed that is fluidizing ceramic (zircon) balls. Slurry is atomized (spray dryer) onto the hot balls, forming a skin and drying on the ball, much like a drum dryer. As the balls move, vibrate and impact each other, the dry product flakes off and is conveyed out of the dryer into a cyclone (flash dryer), where the final drying is performed.

It has not been a mind-blowing 10 years to some. To others, each subtle nuance of improvement was a significant milestone in drying technology.

Did I miss anything? Let me know.

Links

• Drying Files: Complete Column Archives
• Drytech Engineering