This series looks at 30 questions that should be asked -- and answered -- when preparing the specification of a thermal drying plant. In part 4, look at the chemical characteristics of both the product and the moisture to be driven off.

This column is the fourth in a series to detail the questions to be asked - and how the information is used - when preparing a drying specification.

Chemical Characteristics

The material's chemical characteristics should be clearly specified so the parameters can be incorporated into the design.

16. Is product hygroscopic, thixotropic or deliquescent?

These factors affect how the feed, product and/or exhaust are treated with respect to handling and equipment selection.

17. Does product undergo physical changes while drying?

The specification should state whether the product becomes sticky, softens, discolors, etc., during the drying process. These changes will need to be interpreted and accommodated, or prevented from occurring in the design. For example, an alternate carrier such as nitrogen or CO2may be required to promote effective operation. In this case, a recycling system probably would need to be incorporated to allow cost-effective operation.

18. Is any water of hydration (crystallization) to be driven off?

If the product to be dried involves a reaction, whether the reaction is exothermic or endothermic, be sure to note this in your specification. If the reaction is exothermic, the energy generated by the reaction can be used to promote drying and reduce fuel usage. Obviously, if the reaction is endothermic, more energy will be required.

19. What is the chemical formula?

Should the process involve a reaction, it is helpful to the designer to see the chemical formula of the compound before and after drying. Also, provide dehydration vs. temperature data with heat of dehydration.

20. What is the specific heat of the solids?

The specific heat of the solids should be provided to calculate the energy requirements for the sensible energy transfer.

21.What is the bulk density?

The product's bulk density -- both wet and dry -- must be specified. This information is used to confirm geometrical aspects such as bed depth, percentage fills, feeding device sizes, etc.

22. What is the particle size distribution?

Knowing the particle size distribution of the solids allows the designer to establish the dust collection system's design parameters and determine the suitability of the feed for various processes. For example, if solids in the suspension of a slurry have a wide size distribution, the slurry may not be suitable for atomization. Consistent particle size offers benefits in materials handling. Ultra-fine particles should be dried with low air velocities and/or high collector efficiencies. Heavy particles can enjoy higher velocities and higher drying rates.

To some extent, dryer type selection will be based on these properties or characteristics. For example, a fine particle offers more surface area per unit mass than a coarse particle. This promotes a more effective moisture release, especially if the particle can be exposed to the carrier effectively. Other factors such as fluidization velocity and conveying velocity are a function of the particle size and bulk density.

23. Is the product explosive?

If the product is explosive, you must specify the Kst value and the applicable design code such as NFPA stated. Also, specify the area and occupancy classification such as Class 2, Division 2, Group G.

General Characteristics

Once the product- and process-specific questions have been asked, there are a few general questions to address.

24. What is the heat source?

The heat source for drying - steam, gas, oil, electricity or other - should be specified. Usually, this is a function of availability or preference and is dependent on costs or logistics. This specification should include all pertinent aspects such as the supply pressure, calorific values, sulfur content, etc.

Some heat sources require auxiliary equipment for appropriate operation. For example, heavy fuel oils require heat tracing to reduce the viscosity and enable flow. Coal-burning applications require solid-fuel and ash-handling systems. If the vendor is to provide these ancillary systems, include them in the specification up front. They are best detailed in the battery limits of supply. Also, specify whether other off-gases and recuperative sources are available.