5 replies to this topic

[Amanda Chan]

Hi,
I've got a question regarding the pressure of the condenser and reboiler in a vacuum distillation column. Is it possible for me to assume that the condenser and reboiler pressures are roughly about the same? (Since I'll be using packings and pressure drop across each tray is small). My feed is at 1000Pa. In industry, column pressure is about 2000-3000 Pa. Since it wouldnt be possible for my feed to enter the column if the pressure of the stage at which the feed is entering is higher than my feed pressure, can I assume that my column pressure is 1000 Pa? Feed is 0.4515 vapour at a temperature of 198.7degC. Dew point is calculated to be 184.39degC and bubble point 208.82degC.
Thanks.

[Amanda Chan]

I've got another question. Is it weird to have both packings and trays in a vacuum distillation column?

[Technical Bard]

The pressure of your vacuum column will depend on the method used to generate the vacuum (ejectors, pumps) and the condensing temperature after the (thermo)compressor.

If you need more pressure in the feed, put in a pump.

The pressure at the top and bottom of the column will be different. Although packings and trays have low pressure drops, at vacuum conditions they are definitely not trivial and need to be considered. Also, operating at 1 kPa(abs) versus 2-3 kPa(abs) means your actual vapour volumes will be 2-3 times larger and require larger equipment.

The goal in vacuum distillation is to operate at as high a pressure as you can without temperature or separation efficiency becoming a problem.

[katmar]

Amanda, are you sure the pressure is <3000 Pa? This is extremely low for an industrial sized column. I assume you are not talking of a lab column since you are talking of trays. Btw it is unusual to have mixed trays and packing, but not weird - it certainly is done. At these very low pressures you will require such a large diameter to keep the pressure drop for the vapor low that it will make it difficult to properly distribute the liquid and wet all the packing.

Even when specially designed for vacuum conditions the pressure drop across a tray will typically be 100 to 200 Pa, so you will not be able to get many of these into your column without over pressurising your base.

In atmospheric (or pressure) columns in preliminary designs it is often assumed thatthe pressure is constant throughout the column but I agree with Technical Bard that you need to be more rigorous in a vacuum situation.

[Amanda Chan]

Dear Technical Bard and katmar,
thanks lots for ur reply. I'm currently doing vacuum distillation column as my major equipment for my final year project. I should have asked earlier. Dateline is near and I cant change things now. =( My feed is 0.45 vapour so I cant put a pump in front. And since it's the 3rd distillation column in a series of distillation columns, HYSYS predicted that the pressure would be about 1000 Pa. First column is at 5000 Pa and 2nd column is at 3000 Pa. And yes, I'm aware that my vapour flows are rather large. By using the method in Kister, my column diameter was calculated to be 1.68m. Is that large?
I'm just asking this out of curiosity: Suppose I want to operate my column at a higher pressure, say, 3000 Pa and my feed has 0.4515 vapour at 1kPa. Is it possible to cool the feed stream to liquid and then pump the pressure up to 3000 Pa? Is there any other ways to tackle this problem?

[katmar]

1.68 m is a very normal size for a column - trayed or packed - so you have no problems there.

The 0.4515 vapour fraction troubles me. How do you get a partially vapourised feed? If this is the third column in a train then presumably this feed stream is a product stream from one of the ealier columns. It is (mechanically) very difficult to control the take-off from a column to be at a specific vapour fraction. The take-off nozzle would normally be positioned to ensure that the take-off stream is all liquid or all vapour.

I presume this feed stream originates as a liquid stream from an upstream column, and when you lower the pressure it partially vaporises. But if it starts off as a liquid then you can pump it before you drop the pressure and allow it to vapourise. This means you can pump it to a higher pressure - thus avoiding the vapourisation.

Your option of condensing and pumping the feed is also feasible. One of the things that makes process engineering so interesting is that there is very rarely only one right answer. There are many ways of doing most things and each option will have its benefits and disadvantages. If you can give us a bit more detail on why you are dealing with this partially vapourised feed, and where it comes from, we can probably give you more relevant advice.