5 replies to this topic

[dehn0045]

I am working with and trying to understand an ammonia-water distillation column. I know, it sounds academic, but trust me that it isn't. The feed is 10% ammonia and 90% water and is preheated by a feed/bottoms exchanger to 325 degF. The column is 18 trays(numbered 1 to 18 from bottom to top, bubble cap on the top and valve trays on the bottom). Pressure is controlled at 240 psig. The overhead condensers are regular shell/tube exchangers with cooling water on the tube side, the pressure on the column is controlled by the condenser duty. The bottoms composition is >99.9% water, with a bottoms temperature of 403 degF and a tray 5 temperature controlled at 392 degF. The overheads composition is unknown, I realize that this is a crazy statement and I am working to get a sample. The reflux to distillate ratio is approximately 3:1.

OK, now for the problem. The overheads vapor temperature is 150 degF, whereas the reflux drum temperature is 115 degF. Where I start to get confused is the Tray 18 liquid temperature is 115 degF, Tray 16 liquid is 125 degF, and Tray 14 is 200 degF. The feed comes in on Tray 10.

It doesn't make any sense that the overhead vapor is hotter than the liquid on the trays below. The only explanation that I can come up with is a rediculously poor tray efficiency.

Note -- I verified the column temperatures with an IR gun (and my hand where applicable -- I know this is not advised but I am kind of old-school that way) via inspection ports

I looked through all of the literature that I have available and could not find anything on this topic.

Does anyone have any experience working with a column that has similar issues? Any suggestions are welcome.

[siretb]

I have had a similar case, years ago, with a trayed (sieve tray) column distillating a wide boiling mixture.
Due to a limited efficiency, I had, from memory, it's now old, the vapors leaving the top stages were some 30°C (~50F) hotter than the liquid. Efficiency was not that poor.
So you are not crazy.
The temperature difference (liquid-vapor) got small when moving towards the bottom. I bet it will be the same in your case.

If you have access to a good process simulator, It can calculate for a given geometry the expected temperature or each phase. Do that if you can.
I think that ammonia water is such a wide boiling point mixture that it aggravates the phenomena.

[AKsrey]

We have several columns with profiles showing regions of depressed temperatures. In our case, they are caused by regions with two liquid phases. In one case, the cause is an intermediate boiler that accumulates.

[Maabdolat]

Hi....

kindly tell me why hysys does not calculate no. of stages automatically in distillation column, ? in order to design distillation column in hysys , does i have to first calculate no. of stages seperately by McCabe theile method.... please clear out my confussion....
regards,

[VMR]

Hi....

kindly tell me why hysys does not calculate no. of stages automatically in distillation column, ? in order to design distillation column in hysys , does i have to first calculate no. of stages seperately by McCabe theile method.... please clear out my confussion....
regards,

Dear Maabdolat,

You can calculate number of stages manually (McCabe theile, short-cut method,etc) and use it as initial value and follow on running simulator. Than on you have to manipulate number of trays, pressure, and so on to converge the column. You can easily converge a column by controlling known parameters such as reboiler duty, condenser duty, so on.

[ChemG]

Hi All:
You can calculate the min no of trays required using the HYSYS shortcut distillation column.Simulate and look in Trays Tab,you will find it.You will need to provide minimal information for this.Try it!Thanks.