Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB
Fraunhofer Institute for Interfacial Engineering and Biotechnology IGB

Principle of superheated steam drying

Drying in industrial production

In the production, treatment and processing of solid matter, drying is often an essential process step. In many cases, drying requires a large amount of the energy consumed in the whole process chain, and takes up a lot of time and space.

This proves to be disadvantageous as drying is an inevitable part of the process for manufacturers and often means no rise in value. It is used, for example, to preserve products (durability, stability of food, sludge etc.) or to reduce weight (waste or building materials, sludge etc.). In some cases, the water within a material has to be extracted to prepare it for a following stage in the production process (e.g. automotive industry, plastics and textile industry, foodstuffs).

In the drying process, moisture is extracted from solid matter by evaporation. For this purpose, energy must be charged to the material.

This energy-input can be performed by:

  • Convection – by means of a carrier gas, e.g. hot air, superheated steam or an inert gas
  • Conduction – hot surfaces
  • Radiation – microwaves, infrared, solar

Beside heat transfer, mass transfer also plays an essential role in drying. Usually, industrial drying processes combine various effects and methods in order to obtain a particularly suitable drying result for a specific product.

Guidelines for process design are defined by the required quality of the product and its specific sensitivity. The incorporation of secondary engineering processes – such as preparatory and after-treatment of the product, materials-hoisting and conveying, flow control, etc. – is essential for obtaining successful drying results.

Properties of superheated steam and advantages

Due to the superior heat transfer properties of superheated steam to air (higher thermal conductivity and heat capacity at the same temperature) and no resistance to diffusion of the evaporated moisture in its own vapor, higher drying rates can be achieved with superheated steam. This not only has a positive impact on the short process time, but also results in more homogenous drying. Thus, superheated steam dryers are usually more compact in design and involve less investment costs compared with hot air dryers. Moreover, it is feasible to reuse the energy of excess steam for other purposes in the facility. In such cases, high overall energy efficiency can be achieved for the entire process chain.

The higher thermal conductivity and heat capacity of superheated steam compared to hot air results in a considerably enhanced heat transfer not only to the material to be dried, but also to contaminating microorganisms. This effect leads to an inactivation of the microorganisms and to a hygienization of products. Despite the high operating temperature, degradation rate of product compounds especially in foodstuff is relatively low due to the short retention time.

 

Principle of superheated steam drying

The material to be dried is introduced to the superheated steam atmosphere where it is heated up convectively after which its moisture evaporates. This heat transfer process is enhanced effectively, since superheated steam has a high heat capacity and thermal conductivity. Due to its low viscosity, fast penetration into the material is facilitated. As a consequence, this drying principle is especially effective for materials with a porous structure and results in a short retention time in the drying process. As the evaporation heat is supplied to the material from superheated steam, the steam atmosphere is cooled down. The moisture that is carried off vaporizes and becomes excess steam, which is discharged from the drying room in order to regulate the stratification layer. The superheated steam is recirculated and reheated in a closed loop. In this way, the temperature can be kept constant and the steam remains superheated.

By exploiting the substantial difference in density between air and steam as well as through proper handling of the material to be dried, any conveying technology can be applied for drying with superheated steam.

Advantages

  • No airlocks and sluices necessary
  • Up to 50 percent lower energy consumption and up to 80 percent reduction in drying time compared with hot-air drying
  • 90 percent of the supplied latent heat can be recovered
  • Compact plant and lower investment costs

For sensitive products such as foodstuffs, drying with superheated steam achieves good results in spite of the high temperature required. Due to the short drying phase, a degradation of the substances in the foodstuffs or browning as a result of enzymatic reaction is only observed to a small extent. At the same time, superheated steam drying at temperatures over 120 °C results in the hygienization of products.