A conveyor drying system has proven to be effective in processing fragile, pelleted products such as pet food.

First, a clarification: In part 1, I stated that fluidizing wet agglomerated, extruded or pelletized feed solids is not optimal. To be clear, I was referring to those feed solids that are soft, sticky or have poor particle integrity when wet. Robust agglomerated, extruded and pelletized feeds are effectively dried in fluid bed dryers. As a matter of fact, forming agglomerates by "roll tumble" action is frequently performed in fluid bed systems.

Now, continuing on with my discussion about selecting the optimal dryer, in this second part of a two-part series, I will discuss process and nonprocess considerations.

Points to Ponder

It is necessary to consider some fundamentals in the pursuit of selecting the optimal dryer. Obviously, the first consideration would be capital expenditure. If the value of the product is low and little growth in demand is anticipated, it would be improper to expend vast sums of money on a Rolls Royce drying system. You would consider labor and operating schedules to minimize your costs.

Contrarily, if the product has a high market value and you are supplying a buoyant market, you would have a vested interest in installing the most efficient, rugged, reliable and robust piece of equipment money could buy. You also would want high throughputs and reliable controls. Plant availability and maintainability are supplementary factors that would influence your decisions.

Economics and product viability always will be key factors in determining the type of dryer you select. Periods for return of investments need to be well calculated and markets thoroughly investigated to anticipate future growth, and to allow planning for that growth. This will minimize the requirement for unexpected future expansions or costly plant replacements. Planning to accommodate reasonable expansion by designating strategic logistical areas and oversizing utilities will give you an advantage. Very often, tight space constraints must be overcome. Compact and multistage systems can be designed and implemented to meet your real-estate limitations.

In many instances, applications benefit from using the drying process to transport the material from one area to another. Your requirement in this regard also can be satisfied, bearing in mind that all dryers are required to move, agitate or transport the feed in one way or another. Inherently, certain types of dryers will move the material large distances.

More Pondering: Process

The cost to evaporate moisture is far more significant than the cost to mechanically dewater. This largely is due to the fact that to achieve thermal drying, one has to expend energy on more than just purely reducing the moisture content of the feed. In a direct-drying system, energy in the form of heat must be imparted to the carrier -- typically air. The air is required to transfer the necessary sensible heat and latent heat to the feed material to achieve vaporization. Large portions of the energy costs are associated with heating the air, inefficiencies of heat transfer, and retention of energy in the exhaust air/vapor stream. Similarly, the product retains energy because it leaves the process at elevated temperatures. Therefore, the more moisture you can get out of the product before the dryer, the better.

Recommendations often will lead you in a direction based on the physical properties of the feed material alone. This is poor practice. There are no generalities to the way you can feed a substance into a process based on its physical characteristics. It is a misconception that a slurry will demand a different type of dryer from that required by a free-flowing granule. A more accurate statement would be that a slurry would demand a different type of pre-preparation and feed system than a free-flowing granule. This holds true for most feed-type materials.

A host of difficulties are associated with the feeding of dryers, and many solutions exists. Changing the state of the feed in a pre-process stage may be one method to resolve these problems.