Increasingly demanding quality parameters, along with ambitious energy efficiency goals and efforts to better control bottom-line production costs, are driving industrial process engineers to seek viable alternatives to traditional material batch drying.

Drying is one of the most energy intensive processes in any manufacturing setting. Material to be dried is placed on trays in a sealed chamber that is then heated until the drying cycle is completed, and an appropriate moisture content level is achieved in the material. These dryers offer the advantage of being simple to operate: at a basic level they share much in common with a kitchen oven. However, drying in a batch dryer is often uneven, leading to unacceptably high rejection rates.

Thermal processing

Thermal processing, or drying, refers to changing the temperature of a material with the intention of changing its state in some way. Thermal processing equipment is commonly used in manufacturing processes for food, pharmaceuticals, chemicals, mining and other applications.

For example, large quantities of lithium carbonate powder are in high demand by battery manufacturers to power everything from personal electronics to electric vehicles. For these demanding applications, even and consistent drying of the lithium carbonate material is of paramount importance.

Indirect heating is a traditional method of thermal processing. By making use of steam or hot air, this method heats material from the outside or indirectly in ovens or conveyer belts. Materials are exposed to the heating medium for a fixed amount of time during processing.

In batch applications, materials are placed in ovens or industrial heating drums. This allows for varying heating times, measurement of the material during heating, and mixing for a more uniform material temperature.

Batch dryer drawbacks

Traditional batch dryers work well enough but can fall short when it comes to meeting current market demands that require tight quality standards and increasing throughput.

A primary drawback is uneven drying. For example, with thick pieces of material, a conventional dryer may dry only the top portion, leaving the bottom with unacceptable levels of moisture. Two consequences of too much moisture are spoilage and caking.

Neither result is suitable for processes that require high-quality, unspoiled source material as in the battery industry. Nor are the results suitable for the food industry in processes that, for example, dry or decaffeinate coffee beans for consumer markets.

No less important is the amount of extra energy that must be applied to dry a piece of material in a traditional batch dryer. Material drying uses more energy than almost any other industrial process. Hot air must continuously be applied to ensure that the material’s lower portion is properly dried, even after the top portion is sufficiently dry. That means extra energy that not only impacts a facility’s bottom line expenses, but also reduces a process’s energy efficiency.

An even drying solution

Wyssmont’s Turbo-Dryer® addresses these issues with a solution that is designed to gently turn and level the material, enabling it to dry evenly and efficiently while making the most efficient use of energy by reducing drying time and recirculating hot air.

The Turbo-Dryer® consists of a stack of slowly rotating circular trays. The trays are housed in an enclosure in which heated air or gas is circulated by internal fans. Material is fed onto the top tray. After one revolution, the material is wiped onto the next lower tray where it is mixed and leveled. Following another revolution, the material again is wiped to the next tray where the operation is repeated.

Case study

Figure 1. Turbo-Dryer® design. Source: WyssmontFigure 1. Turbo-Dryer® design. Source: WyssmontA case study shows how the technology works in practice. One company uses a process to dry a fine crystalline solid from 8% to 10% moisture to less than 0.5% moisture. The drying had to take place without melting the source material to an unusable fluid. And, the drying process had to ensure positive containment of fumes and dust to avoid air contamination.

To meet the customer’s requirements, a full-sized Turbo-Dryer® was installed. Negative pressure within the dryer of less than 0.1 in/wg prevents escape of dust or fumes. Hot air is circulated through the dryer with the help of fans.

A heat-recovery system captures between 50% and 85% of the exhaust heat and recirculates it to the dryer. This recirculation system means that the heat requirement can be reduced from around 3,000 British thermal units (Btu) per pound of water to 1,600 Btu per pound of water.

As part of the process, a large percentage of the hot exhaust air is recirculated back through a heater and into the dryer. The exhaust air from the system is passed through a dust collector or scrubber to remove any fines from going to the atmosphere. The source material is dried to specification through temperature control that can be maintained within the dryer to a tolerance of +/- 1° F.

A unique design feature in the Turbo-Dryer® is its ability to gently wipe the material that is being dried from one level to another. This wiping action stirs, mixes and levels the material to ensure uniform drying. It also reduces the total drying time to as little as 20% of what would be required by a traditional batch dryer.

The Turbo-Dryer® helps to deliver more consistent drying results with lower energy consumption and even lower rates of rejection as a result of excess moisture and caking.

Wyssmont Company designs, manufactures and services its high quality, reliable Turbo-Dryer® worldwide to the chemical, specialty chemical, pharmaceutical, mineral, food and food additive industries. Founded in 1932, Wyssmont Company has solved many difficult drying problems and offered many benefits to the end user. Wyssmont's test lab requires only a few pounds of wet material per test and the results provide direct scale up to any production capacity. Learn more by clicking here.