Optimized drying process helps Isolava to reduce energy consumption and improve end product quality
Isolava was founded in 1963 and has been part of the Knauf Group since 1990. The company manufactures plasterboard blocks for the construction industry. The most energy-intensive part of the production process is drying, where the wet blocks are loaded into trolleys and run through a four-stage drying oven. Previously, only thermometers were used for process control.
At the end of 2017, Isolava launched a project to optimize the drying process under the leadership of project engineer Peter Vaneyghen. The first step was to install moisture sensors in the dryer in order to better understand the dynamics of the drying process.
Understanding complex processes
The aim of an industrial drying process is to ensure that the moisture content of the product is as uniform as possible. To optimize the drying process, it was crucial for Isolava to understand the drying dynamics of gypsum blocks. However, there is no simple and reliable way to measure the moisture content of a gypsum block before drying is complete.
The drying of a solid is a mass transfer process, i.e. the moisture evaporates from the object into the environment. The environment inside the oven is controlled by: gas burners to generate warm air, ventilation systems to remove excess moisture and fans to distribute the drying air evenly between the product surfaces.
The process begins with heating the products to oven temperature. Next comes the constant drying speed phase, in which the solid contains enough water to form a liquid surface. Finally, no free moisture remains on the product surface during the decreasing drying speed phase. The movement of moisture from the material to the surface dries the product.
In addition to the typical drying mechanism of a hygroscopic product, gypsum has another variable that must be taken into account: It consists of various salts, some of which are water-soluble. During the process, the dissolved salt reaches the surface of the block, where it crystallizes as the water evaporates.
"We examined the microstructure of the dried blocks with an electron microscope and found that uncontrolled drying in the early stages of the process can lead to a lower drying rate in the later stages when the capillaries of the material are closed," explains Peter Vaneyghen.
"In addition to a slower drying speed, this can also be observed as a discoloration of the product," he continues.
Savings through optimization
"We realized that we could modulate the final drying phase and reduce moisture fluctuations in the end product by drying less in the initial phases of the process and controlling the water reduction. With this in mind, a new, advanced control scheme was implemented in which temperature and moisture are measured," explains Peter Vaneyghen.
The results are impressive: the consumption of natural gas alone has fallen by around 20% and the annual savings for all ten dryers add up to hundreds of thousands of euros. In addition, the final moisture content of the end product has been stabilized despite the speed fluctuations when moving the blocks through the dryers.
"Drying is an interesting and fascinating subject for me. It is worthwhile to find a working solution that has been sought after for a long time. However, this requires intensive and thorough work."
Peter Vaneyghen also points out that the right measurement technology has played an important role in the success of the new solution. The Wielsbeke plant currently operates 40 Vaisala HMP7 humidity and temperature probes with Indigo201 transmitters.
"From a serviceability point of view, the interchangeability of the probe is a major advantage. If a sensor needs to be calibrated, it can simply be replaced without having to disconnect the transmitter."
Active project management
The next step is to optimize the drying of Isolava's plasterboard products. Vaisala dew point transmitters with high temperature tolerance are currently being tested for this purpose.
Intelligent, Indigo-compatible probes for efficient drying processes
The Vaisala HUMICAP® HMP7 is an interchangeable humidity and temperature probe with Modbus RTU output that is compatible with Vaisala Indigo series transmitters. The probe utilizes the latest generation Vaisala HUMICAP® R2 sensor, which provides excellent accuracy and long-term stability at temperatures up to 180 °C.
The Vaisala DRYCAP® DMP6 is designed for moisture measurement in industrial drying applications at very high temperatures up to 350 °C. The high temperature stability is achieved by a passive cooling system that dissipates heat from the probe and reduces the internal temperature to an optimum range for the sensor.