This series looks at 30 questions that should be asked -- and answered -- when preparing the specification of a thermal drying plant. In part 2, consider questions about the type and quantity of moisture to be removed as well as the pertinent temperatures for the process.

This second in a series of columns continues to detail the questions to be asked - and how the information is used - when preparing a drying specification.

5. What is the nature of moisture to be removed?

The moisture to be removed - water or solvent - must be defined. This provides design information (specific heat capacity, for example) that will be used in the system calculations. For water, specific data for the operation can be found in published tables.

If the moisture to be removed is a solvent, however, the following information should be provided: solvent name, formula, specific heat capacity, vapor pressure, boiling point, temperature of vaporization, and molecular weight. You also should specify whether you wish to recover or treat the solvent. Solvent recovery frequently is performed by condensation, so cooling media will need to be added to the system. Or, if the exhaust must be treated (scrubbed, flared, etc.), this must be specified completely.

6. What is the initial moisture in the wet product?

The feed moisture must be defined as a percentage moisture, and be sure to define the basis of this information: mass (weight) or volume. Defining the moisture by weight is preferred. Frequently, the product feed will have a range of moisture content. If so, the maximum and minimum levels should be defined to allow for extreme-case calculations.

7. What is the desired moisture in the dry product?

Also, the desired final moisture must be defined as a percentage moisture; again, be sure to indicate the basis of this information. In addition, if there is an acceptable variance in the moisture level, it should be included. This enables the designer to optimize the system and will affect how he accounts for variations in feed moisture and seasonal fluctuations. Finally, if the product has an equilibrium moisture content that is above the desired final moisture content (i.e., the product is hygroscopic), the dry product must be protected against re-hydrating to the natural equilibrium moisture content. While this can be achieved by blanketing the product with a dry gas, it should be noted in the specification.

8. What is the feed temperature?

Temperature of wet product as fed will influence the design. Most products are fed at atmospheric temperatures. But, if a product is kept indoors in a conditioned environment that is above the ambient outdoor temperature, the operation's energy requirements will be reduced. Often, the material to be dried flows from or follows another process that increases the temperature of the feed. Using this preheated material also will reduce energy requirements. Overall, reducing the process's energy requirements will minimize both the plant cost (it will be smaller) and the operating costs (less energy will be expended).

9. What are the product's temperature limits?

Both the wet and dried products' temperature limits - that is, the temperature limit of the feed, not the air, above which product quality is impaired - should be described. Knowing this temperature in advance allows the designer to implement methods to prevent this damage from occurring. For example, if the product oxidizes at high temperatures, using an oxygen-depleted or oxygen-free carrier when the process operates at a higher temperature would minimize the risks.

10. What are the characteristics of the wet product?

This is possibly one of the most important aspects of the specification. Drying, and effective drying system design, are as much art as science. The art of drying technology is developing the methods to effectively present the feed to the carrier - maximizing the exposed surface areas, constantly presenting new surfaces to the carrier, and controlling the particles in the process.

Moisture content alone tells little about the material's physical characteristics: A vegetable can have a moisture content as high as 95% (by weight), but it remains a solid throughout the dehydration process. Other materials are liquids with as little as 20% moisture (by weight). Defining whether the feed is slurry, pasty, lumpy, granular, etc., will greatly assist the designer.

Describing the process step directly before the drying stage will better illustrate to the designer the expected form. For instance, if the previous process is membrane filtration, the designer can more realistically visualize the resulting filter cake if it were specified as "membrane pressure filtration" rather than simply "filtration." The description will help determine whether the feed can be atomized, fluidized, pelletized, pre-formed or other processes to better expose the feed to the carrier.