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[Pradeepbarola]

Refer to the attached drawing for more details .

 

In this distillation column acetone is to be separated from a mixed feed coming at ambient temperature. It is a packed column with mellapack packings. Steam is used in the reboiler (thermo syphon) for heating. Details of these streams are

Feed: 600 kg/hr (72% acetone, 17.5% water & 10.5% aniline)

Steam: 600 kg/hr, 3 bar-g

                Boiling point of acetone is 56 degC. I want to use the steam condensate coming from the reboiler for preheating the feed upto acetone boiling point(without vaporization).From the calculations it comes out ~203kg/hr of condensate is required and a plate heat exchanger can be used for this.

But since I have 500 kg/hr condensate available, it can be used to vaporize the feed to some extent.

Questions:

1.       How do we enter the partially vaporized feed into the column i.e. will any changes be required in the existing piping and pumping system.

2.       Location of PHE: should it be kept below reboiler level to avoid flooding of reboiler or 3 barg pressure is sufficient to have continuous flow.

3.       Location of feed tray: since the feed is near saturation or vaporized w.r.t  acetone, I thing feed tray location can be lowered further(refer pic. y>x) and it might increase the product purity as well.

Attached Thumbnails

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[PingPong]

There is not enough information for outsiders to answer your questions properly, but my first impression is that installing a preheater is not worth the trouble.

 

What kind of duties and temperatures are we talking about here?

For example: what is the column bottoms temperature, what is the real steam pressure inside the reboiler, what is the temperature of the condensate from the reboiler?

 

What is the condensor duty?

 

The reboiler takes 600 kg/h steam so that duty will be something like 400 kW.

 

How much duty will the preheater do, even if it takes 500 kg/h condensate? 30 kW ?

 

In general: when you increase the preheat to a column, the optimum feed stage location is lower, the reboiler duty drops somewhat, and the condensor duty increases somewhat. In other words: part of the additional preheat duty goes to the condensor, and only part of the preheat lowers the reboiler duty.

 

When you start vaporising part of the feed, the feed nozzle on the column may be to small, and the internals in the column may not be suitable to handle and separate a mixed phase feed. As we see no drawing of the column, we cannot tell whether there will be a problem with a mixed phase feed.

 

Lowering the feed location in an existing column is very difficult.

 

All in all: is it really worth the trouble for a small saving (probably less than 5 %) on reboiler steam consumption?

Edited by PingPong, 12 September 2013 - 04:45 AM.