HICOSYS for process control and optimization of spray dryers
When spray drying, the goal is to have a dry and constant product at the end of the process. However, keeping your product consistent with dry product specifications can be difficult. In some scenarios, the product can become sticky due to a combination of moisture and temperature; weather has a major influence on the drying process. The NIZO, the Dutch Food Research Center, tested our HICOSYS analyzer for the process control and efficiency of spray drying.
Moisture content rises and falls in powder drying
Spray dryer operators must be aware of how humid weather can cause problems during the spray drying process. As the moisture content in ambient air rises and falls, so too does the moisture content in the powder. And without amending the dryer’s concentrate flow and/or air temperature, the moisture content will exceed accepted specifications and the powder will become sticky. A widespread problem during spray drying is the fouling of the dryer due to stickiness of the product, especially for products with a high carbohydrate content. Fouling results in shorter running time between cleaning procedures, and in the worst-case scenario, lumps of powder blocking the dryer. The point at which a product becomes sticky due to temperature can be determined at lab-scale, and can be represented for commercial operating conditions.
Direct and accurate measurement of the absolute humidity of the outlet air is highly preffered
Until now, there have been limited methods to compensate for moisture variations in the ambient air. CFD (Computational Fluid Dynamics) calculations show that in a spray dryer, there is a jet of hot and dry air just below the air inlet, and in the remainder of the dryer the air conditions are very close to external conditions. The spray drying rate is limited by water diffusion within the particle. The outer shell of the particle dries very quickly and reaches moisture content equilibrium with the surrounding air after a short drying time, while the inner of the particle remains wet.
The balanced condition of the outer shell with the outlet air will determine if the product is sticky or not. Therefore, it’s important to know the temperature and humidity of the outlet air. The temperature can be measured easily, but measuring the humidity is more problematic. Direct and accurate measurement of the absolute humidity of the outlet air is highly preferred. As well as detecting CO for pre-warning for fire and explosion, the Hobré HICOSYS provides an accurate measurement of the absolute humidity of air (± 0,3 g/kg).
Testing the HICOSYS on the pilot of spray dryer
The Hobré HICOSYS was installed and tested on the pilot spray dryer by NIZO food research. The multi stage dryer (NIRO-250) was equipped with both an internal fluid bed and an external vibro-fluid bed, and several sample probes were also mounted at separate locations i.e. at the inlet and outlet of the dryer.
The DrySPEC-tool was fed online with real time data from the pilot spray dryer. Five tests were performed with whole milk powder to simulate changes in weather conditions by injecting a controlled flow of steam at the main inlet of the tower. In each test, dryer temperature conditions proposed by the DrySPEC-model were set. This resulted in a lower capacity, but no stickiness issues were encountered.
The tests demonstrated the benefit of adapting the dryer’s conditions when inlet air conditions change. By adjusting the dryer’s settings when monitoring air humidity, the dryer’s capacity of whole milk powder production can be increased up to 22%. The complete testcase and results can be found in the white paper.
The test concluded that an accurate measurement of the absolute humidity of the inlet and outlet air is needed during changing weather conditions. Undesired changes in the absolute humidity of the outlet air will become visible, and immediate action can be taken to avoid stickiness issues. The HICOSYS, in combination with the DrySPEC-tool, can be very useful in identifying the adjustments needed to return to an optimal state and avoid stickiness and powder moisture.
This combination creates a more consistent quality throughout maximum production capacity. More accurate control of the powder’s stickiness also makes it acceptable to operate closer to the tip of the nose of the stickiness-curve without risking lumping and blocking of the dryer. In some cases, energy costs may also decrease. Infant formula powders are more sticky than whole milk powder, and experience has taught us that an outlet air humidity no higher than approximately 30 g/kg can be applied. For products like these, the differences in dryer capacity are even higher.