7 replies to this topic

[despy]

Hello everybody,

I've started to work in a chemical company which uses packed column distillation as a seperation technique and i am trying to understand the system.

- First of all when i was student, i didnt learn the effect of bottom circulation rate for a distillation column. If i always use constant steam amount in reboiler i should always vaporize the same amount of bottom content. So what is the effect of circulation rate(reboiler flow rate)? What will change if it is 100.000 kg/h or 500.000 kg/h?

- Second question: we changed the packing types so our pressure drop in the column decreases. However after changing them we face a problem. Top temperature starts to increase when we increase the feed flowrate (by the way feed flow is all vapor), so control system increases the reflux rate and after that bottom temperature drastically drops. When bottom temp. decreases control system automatically increases the reboiler steam rate, again resulting in top temperature increase.

As a result we were expecting to increase column capacity but now we can operate with less feed rate than before. Our capacity drops. I suspect about flooding but i dont know what to do.

That's all, thanks already...

P.S: Quality of top product and bottom product is still the same

Edited by despy, 10 December 2012 - 12:46 PM.

[Arpit_Jain]

Despy,

Answer to your first question:
As the Reboiler duty is same, increasing the flow rate will lead to decrease in reboiler outlet temperature which will lead to decrease in column temperature.

Experts, please correct me if I am wrong.

Regards,
Arpit

[kuldeep singh]

answer to you second question:
if your top temperature is increasing means no vapour is reaching at the top. this might be the case of column chocking or flooding.
can u tell me what mixture are u using in your packed column???

Regards
Kuldeep

[Arpit_Jain]

@Kuldeep,

this is not the case of column choking or flooding as in this case pressure drop is decreasing and moreover if vapours are not reaching at the top it will lead to fall in temperature.

Regards,
Arpit

[despy]

Pressure drop is also increasing while the top temp. is increasing. After we increase the reflux rate pressure drop, top temp. and also bottom temp. decreases. Pressure drop and top temp. decreases are normal but bottom temp decreases sharply...

[Arpit_Jain]

Despy,

here you say that pressure drop decreases and now you are saying pressure drop increases. Please frame question properly and don't pour in random things.

- Second question: we changed the packing types so our pressure drop in the column decreases.

[kuldeep singh]

@arpit.
just think of a batch distillation, if we are continously supplying the heat to the reactor jacket the most volatile component will go at the top. top temp adjusts according to the super saturated temperature of that volatile component but in the case of @despy if he increases the reflux rate the pr drop, top temp and bottom temp decreases as in texts if you are increasing the reflux there wiil be no effect on the bottom temp provided your heat input is constant, the top temp decreases upto the supersaturated vapour temp and becomes constant.

Regards
Kuldeep

[S.AHMAD]

despy
1. For any distillation column, the pressure is already fixed during design stage. Therefore, for products quality control you only have top and bottom temperature as the controlling parameters.
2. The top temperature is the dew point of the distillate. The temperature is controlled by reflux. Higher top temperature indicates more bottom product in the distillate. On the other hand, lower temperature indicates more distillate at the bottom and distillate yield is expected to be lower. Bottom yield increases.
3. The bottom temperature is the boiling point of the bottom product. It is normally controlled above the reboiler or at the reboiler outlet by regulating the heating medium e.g. steam. This means that you must have the right temperature profile in the column for steady operations.
4. If you would like the members to help you further, send operating data before and after the changes with the distillation control scheme.

Edited by S.AHMAD, 13 December 2012 - 02:15 AM.