Versatility of Drying Bulk Materials

Drying of bulk materials is a critical step in the process industries. Applications can range from drying food products to preparation of active pharmaceutical ingredients, and production of raw materials for Li-batteries. The need to rapidly dry materials under controlled conditions, without damage or degradation, has evolved into many process dryer designs through the years. Long after the earliest method of drying using the sun and wind, manufacturers now maintain more control to reduce process time by taking advantage of several proven approaches.

Many sophisticated, optimized processes are available. Current technologies provide benefits specific to product requirements and application, capacities, conditions, time, and space available.

What is Drying?

What is drying? Simply put, it’s the physical process by which volatile liquids or solvents are separated or evaporated to yield a solid product. Moisture or solvent content from solids or powders are most commonly evaporated by application of thermal energy or heat, mixing, and vacuum. Drying operations often follow mechanical separation or removal of solvent or water from solid products. However, while simply stated, the practice of removing moisture from products has many challenges. Care must be taken not to damage, degrade, or chemically change the product. For example, in the manufacture of Li-batteries residual moisture or solvent can reduce or created problems in coating electrodes. Drying operations are often used after a precipitation or crystallization operations prior to tableting or filling or subsequent admixing operations.

Benefits of drying often include reduced packing, storage, and logistics cost, increased shelf life, and elimination of bacterial contamination.  

Examples of Types of Dryers Related to Source of Energy

- Spray drying: Involves pressurized feed of a liquid slurry through an atomizing nozzle into a high-velocity hot gas causing rapid evaporation of the moisture. Drying is almost instantaneous but often requires further drying steps to remove residual bound moisture.

- Freeze drying (lyophilization): involves freezing and drying under vacuum, allowing ice to change from a solid to a vapor without passing through the liquid phase. It is typically used for temperature-sensitive materials such as vaccines or enzymes, is characterized by freezing of the product solution, followed by water or moisture sublimation drying, and ending with secondary drying by heating. (Perry’s 9th Edition) 

- Oven drying or tray dryers: Heating product by convection in a chamber to volatize moisture at constant temperature allowing liquid to evaporate. Oven dryers are often equipped with a fan or turbine encouraging the drying process that heats and dries products. The designs are equipped with movable trays. Oven dryers are very gentle in the treatment of product but are susceptible to long process times.

- Microwave drying: Use of microwaves creating heat to convert liquid in to vapor to dry product. Albeit effective some of the drawbacks include degradation or changes in the product quality due to damage caused by microwaves.

- Conical (agitated) dryers: Widely used in the battery, pharmaceutical food, nutraceutical, and chemical industries, material is placed in a kettle equipped with a helical agitator. As the material is stirred encouraging surface renewal, indirect heat and vacuum are applied to the kettle. Heat transferred into the material converts the liquid to vapor. Vacuum is applied to the kettle to reduce the boiling temperature of the moisture or solvent facilitating effective drying of the product at reduced boiling temperature of the liquid phase.

There are many other dryer configurations such as tunnel, rotary drum, tumbling, falling film, and band dryers. These are not discussed here but are also widely practiced in industry. (Perry’s 9th Edition)

Of the examples, conical (agitated) dryers are one of the most versatile and widely deployed drying techniques in industry and are reliably scaled from bench top to commercial scale equipment, from 2 to more than 4,000 l.

The theory behind conical dryers is utilization of thermal energy and vacuum to lower the boiling point of the moisture or solvent to dry materials. Indirect thermal energy is supplied through a jacketed vessel and agitator blade. Vacuum is supplied by an external vacuum pump or system allowing heating and drying below the atmospheric boiling point of the moisture or solvent contained in the solids. The agitator, typically a ribbon or helical blade, promotes “gentle mixing and renewal of the solids” in the dryer for effective product turnover promoting moisture renewal without affecting the product. Typical feed materials including wet solids cakes, slurries or pastes, and fluids.

The types of moisture that are removed in conical dryers include:

- Free fluid

- Adhesive fluid on the solid particle surface

- Free fluid inside the macro and micro capillaries (flow of liquid thru the solid interstices) over the surface of the solid

- Crystalline fluid (moisture bound to the molecule)

Application examples include drying of crystalline API materials, food products, or wet cakes discharged from spray dryers or filtration equipment. 

In the Li-ion battery industry, conical dryers are used extensively in the mining, synthesis, and preparation of raw materials. Various lithium compounds such as lithium chloride, lithium carbonate, lithium hydroxide monohydrate, and lithium hydroxide are important materials. One such example is the drying of lithium hydroxide monohydrate to separate water from Li hydroxide. Other examples include drying of precursors and active materials after synthesis, filtration, and wash steps in the preparation of NiCoMn materials.

An important step in the implementation or commercialization of a conical dryer is the technical feasibility, and proof of concept.  Criteria such as the initial product condition including cake density, viscosity or fluidity of the infeed product, type of moisture and content, vapor pressure curve of the moisture, characteristics of the cake or solids (porosity, homogeneity, hygroscopic properties), and final target quality and product characteristics are all very important.

It is important to point out that dryer manufacturers often have test facilities or trial instruments where applications can be evaluated prior to commercialization or scaling. Often development in a bench or pilot scale dryer is extremely helpful. A drying curve can be prepared for the product evaluating % moisture (initial and final) as a function of time with respect to various temperatures and vacuum conditions

For each and every product, there is a representative “drying” curve that describes the drying characteristics for a product at specific temperature, drying gas velocity, and pressure conditions. This curve is referred to as the drying curve for a specific product. Variations in the curve will occur principally in rate relative to temperature, pressure, and the dryer design.

The curve is extremely valuable in understanding the thermodynamics associated with the drying of each unique product during three phases of drying:

1) Initial period where sensible heat and or vacuum is transferred to the product and contained moisture

2) Constant rate period where free moisture persists on the surfaces of the solids and the evaporation is relatively constant as moisture content reduces.

3) Falling rate period where migration of moisture from the inner interstices of each particle to the outer surface becomes the limiting factor in drying rate. 

In addition to the above aspects the user has to also consider peripheral or support equipment for dryer installation such as heating source and vacuum source, solvent recovery, management or filtration and handling of solids, and of course safety related to operators and site.

Conclusion

In conclusion, dryers--especially conical agitated dryers--are widely used in the chemical, energy, pharma, and food process industries across many markets. Careful consideration and attention to detail in partnership with the vendor will usually lead to successful implementation.  

Lee Holliday is process sales director, Joseph Porcelli is regional sales manager, and Daniel Nenno is systems sales engineer with IKA Works Inc. For more information, call 910-452-7059 or visit www.ikaprocess.com/en.

References

1. Perry’s Chemical Engineering Handbook, 9th Edition
2. IKA Works Inc., Staufen, Germany
3. Processheating.com