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The Source for Dry Processing and Bulk Handling Technology

Indirect Heating Improves Drying Efficiency

February 2, 2009
Farah Salaria

Farah Salaria
Solex Thermal Science Inc.

The drying of various bulk solids such as oilseeds, chemicals, plastic pellets, catalysts, bio solids, and fertilizers is critical for downstream processing and storage. It prevents caking, agglomeration, spoilage, and degradation of material. Controllability of the drying process is important in order to compensate for changes in the feedstock. Excessive drying of solids is undesirable and results in weight loss, whereas too much moisture can lead to degradation and numerous quality issues during storage and also incur penalties at sale. Varying weather and process conditions call for more-efficient and flexible methods of drying. Also, solids have different limits on temperatures to which they can be heated and varying ideal moisture content for storage.

Natural drying is not always an option, and many variations of the air-drying method have been used. The drying process is governed by typical factors like ambient conditions outside, relative humidity, temperature, grain texture, sensitivity to heat, moisture content, and toughness of the solid particle.

Conventional methods for drying free-flowing bulk solids typically use high volumes of hot air that are blown through the bed of solids. Use of hot air as the heating media, as well as for moisture removal, limits the efficiency of this technique. Large volumes of air must be heated for heat transfer, and as the air picks up moisture from bulk solids the temperature of the air drops and the air reaches saturation. Large volumes of air are required and the larger the volume of air used the more energy that is expended; much of this energy is lost up the stack.

Many drying techniques have thermal efficiencies as low as 30 %. Also, the distribution of air through the bulk solid beds and silos is often not ideal, which leads to nonuniform heat transfer and drying.

It has been proven that the use of indirect heat through hot water or steam can radically reduce the volume of air, consumption of energy, and emissions. If heat is available from a waste heat source, this can further add to the energy efficiency of the drying process. The plate heat exchanger design incorporating cross airflow for removal of moisture uses indirect heating of the solids through plates. This reduces the demand for a large volume of hot air and keeps the air temperature constantly high, which enables the air to carry increased amounts of moisture before reaching saturation. Plate bank heat exchanger technology can be used for fertilizers, sugar, chemicals, plastics, food grains, etc.

This technology is unique in the sense that it combines the science of heat transfer and thermal modeling with the knowledge of mass-flow characteristics of bulk solids. The result is a thermally efficient technology that provides precise predictability, control, and flexibility of performance. Moreover, consistent product quality is achieved as the equipment is designed for uniform heat transfer, ideal mass flow, and uniform drying throughout the solids bed. The technology is easily adaptable to varying products as long as the bulk solids flow freely under gravity.

Plate bank heat exchanger technology dryers are recommended for any manufacturing facility looking to gain green points for environmentally friendly technology use. Installing said dryers, as preconditioners or residual dryers, will reduce emissions and load on otherwise energy-intensive dryers, thus improving the overall energy efficiency of the facility and ultimately increasing production capacity.

Farah Salaria is vice president, product development for Solex Thermal Science Inc., Calgary, AB, Canada. She has extensive knowledge in the area of plate heat exchangers and centrifugal separation equipment, and has worked in oil and gas, water and waste treatment, as well as the chemical and fertilizer industries. Salaria holds a degree in chemical engineering and is also a member of the Association of Professional Engineers, Geologists, and Geophysicists of Alberta (APEGGA) and is on the Industrial Water Quality Committee for the Water Environment Federation.