We are looking at using compressed air to remove liquid from a transfer line. I'm having some problems working out how much air we will require for this (and hence whether it will work).

The line in question runs between a delivery tanker truck and a storage tank. After delivery, there is the requirement to drain the hose and transfer pipe so that the hose can be disconnected by the operator without the risk of spills or contact with the liquid. As stated, the idea is to connect air to the hose near the truck and blow the liquid through the hose and pipe into the storage tank.

Details:
Fluid: hydrofluosilicic acid (SG ~1.2, viscosity ~6.5 Cp)
Transfer line: consists of 4m hose connected to 10m of pipe.
Hose: DN50, length 4m
Pipe: DN50, lenth 10m, uPVC
The transfer line (hose/pipe*ball valve) will be full of hydrofluosilicic acid after truck delivery.
The tank inlet is 3m above the truck delivery point.

The issue: How much air is required/velocity/pressure to empty the hose/pipe?

The fundamental issue: Is there a better way to solve the problem of draining the hose - manual handling is the issue (acid ph ~1) - currently hose is drained manually into a drain, but they don't particularly like this method...

I've started by assuming a liquid droplet size of 5mm, and performing a mass balance on it (force from air pressure - force of gravity - force of drag). In this case air at 700 kPa looks like it will lift the liquid the required 3m up into the tank. I'm still not confident though.

It would be great to hear from anyone who has advice or comments on how to go about this - whether it be some fluid mechanics or practical advise.

I think you will have to look at some flow cases using 2-phase flow correlations.  The air to liquid ratio makes a difference in flow pattern.  Too little air and the air will flow as bubbles at the top of the line, more air will tend to flow at the top of the line separately from the liquid at the bottom, even more air might develop a foam flow, still more air can create mist flows.  When cleaning a liquid line with air, you may experience all the above at one time or another.

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