5 replies to this topic

[Olaniyi]

Hi Members
I have a question. I have always struggled with the concept of a partial draw-off tray. How do you design for a partial draw-off. Does the partial draw off mean you are trying to draw off a portion of the fluid or e.g. a phase e.g. drawing off water in a water/condensate mixture. How would the design of a partial drawoff tray look?

Thanks for any answers!!

Ogeds

[katmar]

In my experience a draw off is always done as either liquid only or vapour only. I have not seen anyone (deliberately) take off a mixed phase draw.

A liquid draw off is usually done by installing a chimney tray, or a normal tray with a recessed pan. The liquid height on a standard tray is usually not enough to ensure a flooded take off.

A vapour take off should be at a tray with an increased tray spacing above it or there is a risk of entraining liquid into the vapour. An advantage of a vapour take off is that it can be piped to a condenser at a higher level to allow liquid to flow by gravity to a point that is higher than the original take off point.

[Olaniyi]

Thanks Katmar
However I'm still unclear about how to differentiate between partial and total draw-off?

[katmar]

Other than the top vapour take off, a standard column will only use partial draw offs.

The only time that I have used a total liquid draw off was where I wanted to scrub a vapour stream with two liquid treams in series. The column was built so that the vapour could enter the column at the base (as usual) and pass all the way up the column. The first liquid was fed to the top tray and taken out with a total draw off chimney tray half way down. The second liquid was fed just below the chimney tray. Effectively this is two separate columns, but for convenience built into one shell.

I suppose you could also have total draw offs where you have an intermediate condenser or an intermediate reboiler, but I have no personal experience with such a situation. However, the condenser or reboiler would have to condense or vapourise only part of the stream fed to it or it would mean no vapour would reach the top of the column (in the case of the intermediate condenser) or no liquid would reach the base of the column (in the case of the intermediate reboiler). In my experience a partial draw off was used to feed the intermediate condenser, and the condenser totally condensed the vapour diverted to it.

[paulhorth]

Ogeds,

A partial liquid draw can be taken from a sump below a downcomer. There needs to be some flow control on the drawoff, and the excess liquid in the downcomer which is not drawn off will run on to the next tray and remain in the column. Clearly it is possible to draw too much liquid by maloperation, leaving the column dry. That situation would have to be avoided by operational practice. A similar draw can be used to take water out from a tray as a sidedraw, by allowing the sump to provide some phase separation from hydrocarbon liquid.. This is not ideal, and usually some hydrocarbon will also be drawn off, which will need to be returned to the column from a receiving drum. The system has to be operated by controlling the draw flowrate and by sampling the draw fluid.

Paul

[Chesps]

Partial draw simply means, the portion of total liquid coming down from the downcomer of tray above is drawn out of column for some purpose and remaining portion goes to the next tray as usual.

Allow minimum liquid holdup above drawoff nozzle (You can find it in Distillation Design by H Z Kister) for trapped vapour separation also to avoid vapour going into draw if there is not sufficient liquid level above nozzle. For these reasons the draw sumps have some depth below the tray deck.

The draw rate is generally level (draw sump) controlled to avoid tray going dry.

The draw line generally is few pipe diameters straight vertical to allow some more intrapped vapour disengangement. The line is sized using gravity line sizing criteria.

The draw tray has more tray spacing than normal trays of the column based on the sump depth, nozzle size etc.