Problems running VM

[Matt64]

I am having problems running our test vapour management column. The column is 2.8 metres high with an internal diameter of 64mm. There are 42 perforated plates in the column. On top of the column we have a reflux condenser and the vapour take off ball-valve sits about 30mm below this.

We measure the temperature above and below the reflux condenser.

The reflux condenser was modified recently as we were getting choking as the vapour tubes were too small, so the condenser now holds 8 x 13.6mm (inner diameter) tubes. Condenser length is about 400mm. I thought that this larger tube design would avoid choking.

Running the column today, the column gets up to about 77°C under the reflux condenser and sits around 25°C above it. Leave the column to get into equilibrium but after about 30 minutes, the temperature above the reflux condenser shoots up and the temperature below drops to 60°C and both temperatures become unstable. Liquid disappears from the bottom plate at this stage. Coolant water was gradually rising but was still at 25°C at this time. Heat input is constant throughout. With liquid passing by the reflux condenser, we switch off. I am guessing that choking occurred again.

When I ran this set-up previously a couple of days ago, with a smaller volume of liquid (for faster warm-up), I completed the equilibrium stage with no problems and opened the vapour take-off valve but even with it fully open, no vapour was exiting until I increased the heat input further. That started the vapour flow but was then too fast and I closed the vapour valve down but had lost liquid at plate 1. So the column became out of balance. No reflux condenser choking on that test run though.

So I am scratching my head and thinking:
- The column is highly sensitive to heat input. Very slightly too much and the reflux condenser is overcome. Slightly too little and there is not enough oomph to get the vapour to exit the port at the top of the column below the reflux condenser. It is very difficult to keep it in balance.
- Maybe perforated plates are not suitable for a 64mm (2.5") column? Maybe packing would allow the column to be more controllable and less sensitive?
- Maybe the 64mm diameter column is too narrow and over sensitive to heat input? Perhaps a larger diameter column would be less sensitive?

Would appreciate any thoughts please as to how I can resolve this.

Thank you!