You also need to know the liquid level within the drum. with that and the drum diameter and length you can calculate the volume of liquid in the drum. Divide by your flow rate and you have your residence time.
That's what I thought but I wasn't sure if it was the volume of the tank of liquid..... one more thing I only have the flow out the drum...can I use it
You need to know, or assume, a LL in the flash drum, say 30% of the T-T length for a vertical drum. Knowing the geometry of the vessel estimate the volume occupied by the liquid and proceed as SeanB told you.
