A water draw off vessel works by the liquid density/specific gravity difference (Buoyancy) between the water and the other liquid phase.  I am assuming that you are separating a two phase system from a distillation tower made up of water and hydrocarbon (HC).  Water usually has a specific gravity greater than the HC so if the mixture is let set calmly in a container (Vessel or Accumulator Tank) it will form two layers, water on the bottom and HC on the top.  The water can be drawn off of the bottom and the HC returned back to the distillation tower or to the product receiver.

Depending where the two phase mixture is formed in your distillation tower the water draw vessel may be designed slightly differently but the specific gravity difference principle for separating the fluids is the same. 

If the water is to be drawn off at the top of the tower after the overhead condenser the water/HC separation may take place in the reflux accumulator designed with a water collection sump (Boot) on the bottom of the accumulator where the water will collect.

If the water is to removed from the tower at an intermediate point a side stream from the tower will have to be taken from the tower and placed in a tank where the phases can separate and the water drawn off of the bottom of the tank and the HC returned to the tower.

The sizing of the separator tank (Decanter) is based upon residence time.  The greater the specific gravity difference between the fluids the lower the need resonance time and conversely the smaller the specific gravity difference the larger the tank.  Some bench tests can give you the bubble separation rate and enable you to determine the resonance time needed.  Be sure that the downward velocity in the separator is significantly less than the rising or settling rate of the bubbles.  You may need to put a bubble coalescer in the tank to make sure or speedup the rate at which water or HC bubbles coalesce into a continually layer.  Depending upon the rate at which the bubbles coalesce the time for the bubbles to coalesce maybe the controlling parameter.  This is the same problem that occurs in liquid/liquid extraction towers.

You will also have to control the interface between the two phase layers with a level control.  There are several different ways to monitor the interface and make the water draw.  The simplest being a sight glass and a manual valve.  This method is good for low amounts of water that may only need drawn off once a day or once per shift.  For higher amounts of water the draw with have to be automated.  If this is the case, I suggest you contact an instrument supplier to automate the draw off process.

Do not forget that even though that the phases are separating because of their low mutual solubility’s that some of each phase even after separation is dissolved in the other.  If the water is being discarded to the environment it should be checked for BOD etc. and may need treating before discarding.

If this is a trayed distillation tower and you need to make an intermediate draw off, you may need to modify the tray to make a small amount of draw.  If the amount of draw of liquid is small enough it may be possible to take it out of the tray down-comer.  If the amount of liquid draw is large or if it is a packed tower you may have to install a collector tray to remove the liquid from the tower.

I suggest Perry’s Chemical Engineering Handbook and H.Z. Kister’s book “Distillation Operations” as references for getting started.  I don’t know the size of your project or how involved it is but if it is critical and involves a lot of modifications to the existing distillation system I suggest that you contact an engineering consulting company familiar with this type of work because there are many practical details that need considered which their experience should cover.  Many qualified engineering companies and equipment suppliers are listed on Fractionation Research, Inc (FRI) Web site:  www.fri.org. Once there, just click on Members.