This series looks at 30 questions that should be asked -- and answered -- when preparing the specification of a thermal drying plant. In part 3, consider product-specific questions.

Continuing my discussion, this column is the third in a series that details the questions to be asked - and how their answers are used - to prepare a drying specification.

11. What is the wet product's behavior?

The behavior of the wet product is subtly different than the wet product characteristics. Although behavior also provides information about ways to present the feed to the carrier, it is more focused on materials-handling requirements. Factors such as whether the feed is free flowing, sticky, smeary, friable (breaks up readily), etc., will determine the suitable means of product handling. Also, be sure to note the material's abrasion behavior, which influences mechanical design and affects the selection of the materials of construction.

12. What are the physical characteristics of the dry product?

The ability to provide the desired dry product characteristics will separate "the men from the boys," so to speak. Specifying this requirement may limit your selection of suppliers, but it also will improve your process. All solids dry with their own unique characteristics. For instance, some agglomerates revert to their natural particle size (which can be very small) on drying while some pastes clump, forming clogs. Other products expand, contract, form crystals, etc.

These natural formations can be altered. Should you wish to increase the dry particle's size, for example, you must incorporate the appropriate processing methods such as spray agglomeration, fluidized agglomeration or pelletizing. Dryers can produce flakes, solid and hollow spheres, and other geometrical shapes as needed for other processes or to increase surface areas. Also, dryers can reduce particle size by milling, jet milling, disintegrating or other mechanical abrading technique, and they can be used to classify products during the process.

Specifying these requirements allows the designer to incorporate (or at least account for) them in the design. If they are not considered during the specification stage, you will almost certainly require another piece of process equipment to achieve the desired result.

13. What is the dry product's behavior?

In the same fashion as the wet material specification, the behavior of the dry product is subtly different from its characteristics, and for the same reason - materials handling. Whether the product flows freely, forms bridges and rat holes, or is fibrous or brittle influences the design and should be detailed.

Again, be sure to note the material's abrasive behavior. Frequently, dry materials are handled at higher velocities than wet feeds. Because wear is directly related to velocity, it is important to clearly and accurately define this aspect to ensure suitable selection of the construction materials and appropriate mechanical design. Properly specifying the equipment will reduce maintenance requirements and prevent possible contamination from abraded particles.

14. What are the post-processing operations?

Obviously, post-processing operations that are an integral part of the system must be specified. Cooling the dried product probably is the most common post-process included as a stage in the dryer. If the product must be cooled, the exact process requirements should be defined, including the desired temperature to which the product is to be cooled and the available cooling medium(s). If the designer must determine the cooling requirements or supply the cooling medium system, be sure to state this information clearly.

Other post-drying processes such as sifting, screening, classifying and conveying can be incorporated into the design. For example, should the dried product need to be elevated for a weighing and packaging facility, a pneumatic (flash) dryer can be used to achieve the product lift. But, the designer must know the requirements to implement the options needed.

15. What is the preferred product collection/dust control method?

The mode of product collection is a fundamental characteristic of the dryer's design; however, should you prefer a specific method of collection - say, a bag house over a cyclone - this must be specified. Accommodating specific product collection requirements will impact the system's operating pressures and should be factored in early in the design process. For instance, the pressure drop (differential pressure) across a cyclone, bag house, wet scrubber and electrostatic precipitator (ESP) are all different; likewise, dryer design will differ according to which method is selected. Also, the emission requirements should be stated if the supplier is responsible for meeting them.