In order to optimise the drying process many users will require continuous information on the product’s moisture content during processing.
Although this ensures the drying process does not continue for longer than necessary it can, in practice, be hard to achieve and, in many cases, is not necessary.
By measuring the water content of the incoming and outgoing air, operators can control the maximum allowable withdrawal of water in a given time period. This allows control engineers to easily detect of the end of the drying process.
If you imagine the huge cost of running an industrial drier, even shaving a few minutes off a drying time will equate to significant savings and a positive environmental impact. And the benefits don’t just stop at energy savings. Over-drying can be hugely damaging, even dangerous, for many products. Paper, provides a useful example. Over-drying paper will damage it and cause dust, which represents a very serious explosion hazard.
What is drying?
From a general point of view drying means the withdrawal of a liquid from a solid. It is an important step to stabilise the consistency of a solid and improve its handling quality. If you are trying to transport a substance then too much moisture content will cause it to agglomerate and cause blockages. This is particularly important in the chemical and pharmaceutical industries where it is common for powders to be transported along pipelines with compressed air. Too much moisture in a powder and it will stick together in much the same way sugar or salt does when left for a long time.
Roughly speaking there are mechanical, chemical and thermal methods for drying. From a process point of view, the most popular method, to achieve low values of product moisture in a solid, is the thermal method.
The energy required to dry a product is strongly dependent on its equilibrium moisture (humidity) or water present on the surface of a solid. Therefore, the overall water content is less relevant.
This is because chemically bound water cannot be withdrawn, either partially or totally, without fundamentally changing the product. As a result it is more practical to set up a drying process while considering the water quantity that can be withdrawn from a solid. This value can easily be obtained by determination of air humidity.
As a general rule a large range of air humidity equals a much smaller range of water content. This offers a better determination of the value of equilibrium humidity and, in most cases, results in more precise process control.
The dryer enthalpy is the energy necessary to dry a product and is significantly influenced by equilibrium relative humidity (ERH), also known as water activity (aw). This is the point at which a product is neither gaining or losing water. By way of example, imagine a saturated sponge in a room; as the water evaporates from it there will be a point when the sponge and the room reach the same level of humidity.
With pure water the aw equals one and the drying, or vaporising enthalpy, is 2300 - 2500 kJ/kg. As the aw value decreases, the drying enthalpy increases. This is because the water to be evaporated is absorbed further into the product to be dried (capillary bonds). For an aw value of 0.1, the drying enthalpy is approximately 4000 to 5000 kJ/kg. This explains why removing the last remaining water content is more costly.
The time required to remove equal moisture quantities increase when lowering their starting levels. Capillary (or diffusion forces), which determine the movement of water, have an increasing impact. The heat required to evaporate the existing moisture must be applied to the product to be dried. In practice the water vapour quantity evaporating from the dryer content is continuously carried away from the surface of moist solids by a moving air stream.
The drying speed when evaporating is directly proportional to the product surface and the water vapour pressure drop between the product to be dried and the dryer air. Basically, the vapour pressure above a product to be dried increases the same as above water.
If the temperature of the product to be dried is increased, its vapour pressure and the vapour pressure drop between the product and surrounding air is increased too.
The water vapour pressure drop represents one of the most significant control parameters in a drying process. Other influencing parameters are air temperature and flow rate, vapour pressure, air quantity per time unit, evaporating surface and sorption properties of the product to be dried.
The measurement method
To determine the water quantity withdrawn from the product during drying you measure the water content (g/kg) in the dryer inlet and outlet air
Click on the links below to learn about humidity measurement in the drying process.
Article written by Peter Meuller, product manager of Rotronic AG with contribution from Robin Farley, managing director of Rotronic UK. Edited by Anna Mitchell