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Reboiler Design For Vacuum Distillation Column


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#1 anaskw

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Posted 23 August 2006 - 06:58 AM

Hi everyone!

What kind of reboiler should i choose for a column that opperates whith a slight vacuum? (about 0,2bar at bottom stage) ... Is it possible to use a kettle reboiler pumping the fluid through it? or should i consider a different reboiler configuration?

Thanks a lot!

//Ana

#2 Art Montemayor

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Posted 23 August 2006 - 10:40 AM

anaskw:

I've often wondered why chemical engineering students seem to have a lot of problems in selecting and applying Unit Operations equipment in their projects. A lot of the questions I get seem so obvious and easy to me, but to some students the applications seem to mystify or confuse them.

There is nothing difficult or complex about making a good engineering judgment on which type of reboiler to select or to apply in a given or specified service. The main solution to this type of problem is purely common sense - there are no secret equations, graphs, mathematics, or even thermodynamics involved. And thank goodness! You have enough problems getting through your courses as it is, without adding more grief to your tasks. What I recommend you do is just sit down with a cup of coffee and relax, thinking about what the application involves. Have a sketch pad and a pencil with you and start sketching how you imagine the equipment will be joined or assembled in real life. Of course, this assumes that you have paid good attention to the drawings and fotos of basic equipment in your text books and manuals and studied how they are built and how they work. You should know each type of major reboiler and how it is built and assembled.

Make a note of the fluid and the processes characteristics - such as viscosities, pressures, temperatures, flammability, toxicity, corrosiveness, cleanliness, impurities, condensibles, non-condensibles, etc. Now narrow down the list to those characteristics that seem to be giving you a design problem (or could give you one). In your case, you seem to have done that job already and have idenfified a partial vacuum as an area to be particularly concerned with. I believe you are right: you should be very concerned with the low availability of positive pressure inside your column and reboiler and any pressure drops you must overcome to make your process work. If you have reached this conclusion, you have done a good job of pre-design conceptualization and are now on a firm, good design base.

A partial vacuum will basically cause you three major design problems if you are not careful in selecting the proper type of equipment:

1) Partial vacuum operation requires particular attention to have very low pressure drops within your column/reboiler; this usually means that your overhead piping is going to be larger than the usual - sometimes this can go into the "Big Bore" class of piping - 20" OD and larger. I've used as big as 48" overhead piping in one application.

2) The next area of concern is that your vessels should be mechanically designed to sustain a FULL VACUUM (FV). This means that large diameter vessels will cost more due to the heavier wall required and the reinforcment also needed.

3) The need to evacuate the bottoms liquid product with a pump necessitates that you ensure a good NPSHa (available Net Positive Suction Head) for your bottoms pump. This usually means you should have a positive liquid level height well above the suction flange of your pump (usually a minimum of 2 meters).

In your application I believe point (2) is what should concern you. A kettle reboiler will work fine in a vacuum application - without a pump-around system as you describe. The only thing to be aware of is that the kettle is a type that has the largest vessel diameter of all the reboiler types. Therefore, you will have to pay more for the shell design.

A vertical thermosyphon type of reboiler is usually better suited for vacuum applications because of the inherent design of having the process fluid in the tubes - and not in the shell. The tubes can easily take a vacuum mechanical design without too much added cost.

As I said above, I see no need for a pump-around system on your reboiler - unless there are other process characteristics that you haven't made known that would mandate such a special reboiler design. A heavy, viscous fluid might require that type of design; but you haven't told us that.

Another type of reboiler that adapts well to vacuum operation is a tube bundle (usually U-tube) that is inserted directly into the sump of the distillation column. If your reboiler duty is relatively small, this is a quick and easy way to resolve the application. Plus, it's the cheapest of all reboiler types. However, if you will require maintenance and the ability to remove the bundle, you will have operability problems down the road.

Those are the main thoughts I would convey to you and the type of pre-design thinking and planning that you should be doing. You are wise to consider the question of how to go about this process and to ask those of us who have already gone down that path many prior times. That is good engineering practice: always ask those who may have the experience and know-how. That is a quick, efficient, and accurate way to learn.

I hope I've helped you out.

#3 anaskw

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Posted 23 August 2006 - 03:29 PM

Thanks a lot!!!! you certainly did help me out. I´m following your advice... sitting here with a cup of coffee thinking about my reboiler...

once again, thanks for taking the time to answer!
Ana

#4 process equipment

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Posted 29 August 2006 - 04:30 PM

Dear ana,

I was wondering how you design your column in the first place and for what purpose. Like many companies decide to build more vacuum destillation column nowadays, I like to know what are the criteria to desgn such a column. And of course all the equipment around it, like condenser, reboiler etc.

From university we learn this from the books, unfortunately not all the details. In the real world this is different...

Can you share this project with me please..

Thanks

PE




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