- Feed viscosity directly influences atomisation performance, droplet size distribution and downstream powder characteristics
- Inline viscosity monitoring provides real-time insight into concentrate consistency, enabling faster process adjustments and improved dryer stability
- Used alongside solids measurement, viscosity monitoring helps reduce process variability, minimise fouling and support more consistent powder quality
Spray Drying Manufacturing Process Overview
Milk powder is produced by the process of “spray-drying”, which is widely used in the food and chemicals industries for making products as diverse as baby foods, coffee, soup mixes and dyestuffs.
A process diagram depicting a typical spray-drying process:
The liquid product concentrate is pumped to the atomiser, which reduces the liquid to a spray of droplets of a controlled size, generally from 10 to 500 micrometres depending on the type of product. The droplets are sprayed into a stream of hot gas, which is usually dried air, unless the liquid is oxygen-sensitive or a flammable solvent such as ethanol, when nitrogen is used.
The tiny droplets lose moisture very rapidly in the drying air, after which the dry powder is separated from the moist air in cyclones by centrifugal action. Pre-concentrating the liquid improves the efficiency of the relatively expensive drying stage and thereby increases productivity.
The physical properties of the products are intimately associated with the powder structure generated during spray drying. It is possible to control many of the factors that influence powder structure in order to obtain the desired properties, but to do so effectively demands continuous on-line monitoring of the feed liquid in order to keep the product within specification.
Manual sampling and measurement introduces delays that can cause lengthy start-up and shutdowns and uncertainties that lead to poor batch-to-batch comparability.
While solids concentration remains the primary control parameter in many evaporation systems, viscosity provides additional insight into product rheology and behaviour during atomisation. Together, these measurements can provide a more complete understanding of process performance than either parameter alone.
The efficiency and consistency of spray drying are strongly influenced by the degree of atomisation. Feed viscosity is one of the key parameters affecting droplet formation, droplet size distribution and ultimately the drying performance and powder characteristics.
Inline Viscosity Monitoring Solution
An inline viscometer is therefore ideally suited to monitoring the concentrated feed entering the spray dryer, providing continuous insight into a critical process variable that directly influences atomisation performance and powder quality.
There are three stages in the powdered milk process where XL7 viscometers are typically installed:
- Post-evaporator recirculation loop
- Atomiser feed line
- Pre-evaporator feed line
Stage (i) suits a standard XL7 with Tri Clamp fitting. This is elbow mounted in the evaporator pipeline. Stage (ii) and (iii) require high pressure variants.
For one installation, a custom Ø10mm sensor was supplied to suit an Ø18mm pipeline, to provide critical insight into the concentrate prior to atomisation. The viscosity of the concentrate can provide valuable insight into product consistency and can be correlated with factors such as solids concentration, particle size distribution and final powder characteristics.
Importantly, measurements are unaffected by changes in the flow rate or by the amount of suspended solids in the concentrate, which could be as high as 40%. The single solid-rod sensor has a hygienic clean-in-place design, presenting minimal possibilities of fouling and eliminating “dead” spaces where microorganisms could propagate.
Before acquiring the XL7 viscometer, one company had tried to use conductivity as a measure of milk viscosity, but found that the technique was unreliable and inaccurate. Now, with the XL7 providing continuous, real-time monitoring of milk viscosity, production staff can make instant adjustments to maintain product quality.
“We use the online viscometer to measure product interfaces and percentage solids. The live measurement means we can minimise yield losses.” Robin Sandman, Engineering Manager, H.J. Heinz.
Ready to learn more?
Contact Hydramotion to discuss your spray-drying application requirements. Or visit the following links to learn more about Hydramotion's inline viscometers for spray-drying.