Earlier this week, we had the end-of-year display of student engineering projects. There were lots of posters put up to browse around over tea, several interesting large objects such as pieces of electric cars, and many fascinating talks given by the students.
One of the most enjoyable talks was given by student Timothy Walmsley, concerning a study on the sticking of milk powder in spray dryers. To convert milk into powder form it is sprayed into a dryer; the milk solids remain and fall to the bottom but the water content is removed; the result is something that is easily packaged and transported.
However, there’s a bit of a problem. Anyone who uses milk powder at home will have realised it’s sticky. I think it’s horrible stuff – it gets itself all over the place, then you can’t get rid of it. I learned on Tuesday that sticky milk powder is one ‘phase’ of powder – there is also a non-sticky phase, and there is a phase transition that separates the two. (Just like ice and water are two different phases of water – and there is a freezing/melting transition between the two.) If the temperature is high enough milk powder isn’t sticky, but if it drops too much we can expect the sticky form. Just what this temperature is depends on other conditions, such as relative humidity, but we are talking something vaguely around 70 Celcuis. Timothy has done a nice series of experiments studying this sticky transition and thinking about its application to what goes on inside a dryer.
Here’s an example of the issue – as I understood from the talk, the exhaust air exits at about 80 degrees Celcius. That’s warm, and there’s a lot of energy that is going to waste here. Why can’t we recover some of it?
The problem is that a little bit of milk powder finds its way into the exhaust. If we take heat from the exhaust (e.g. with a simple heat exchanger) we cool the exhaust, and we get into the non-sticky-to-sticky phase transition territory. That means that the particles of powder would start sticking onto the inside of the exhaust pipe (or so I understood) which would cause problems. It’s a bit like the problem with ice forming in the outside unit of your heat pump in cold weather – the pump has to enter a defrost phase to get rid of it – and in the spray dryer exhaust pipe something similar might have to happen if heat is to be recovered from it.
Overall, a fascinating talk about a phase transition that I wasn’t aware of.