3 replies to this topic

[kasiagaspar]

I need to simulate a column with two reboilers, but do not know what option is the best one. How to do it? I use PRO/II.

2 heat exchangers in the bottom seems to be not a good idea, i guess.

[Milutin]

For mass and energy balance simulation there is no difference if you have one or two reboiler.
Two parallel reboilers on the column bottom is not uncommon solution in process industry.

Regards,

[abhi_agrawa]

May I add, that distillation columns with a side reboiler are also not very uncommon. I have seen several Ethylene Fractionators (they separate Ethane and Ethylene), Propylene Fractionator (to separate Propane and Propylene) with a side reboiler. Sometimes, you may have a column with side condensers also (although, it does not seem to be a very good scheme).

In PRO-II you do not simulate this as two reboilers, rather you specify a duty on the stage where the reboiler is located, or you specify it as a pumparound.

You may also have two reboilers in the bottom of the column. This is usually the case when the duty of the reboiler is too high to have a single equipment. In such a case you can simulate it as one reboiler in PRO-II. There are also cases, when due to the application required, you may have a spare reboiler also. In such a case, the spare reboiler operates when there is a certain kind of process upset. The best example for such a system is heat integrated columns, where the condenser of one column acts as the reboiler for the other.

Hope this helps,
abhishek

[Madhavan]

I need to simulate a column with two reboilers, but do not know what option is the best one. How to do it? I use PRO/II.

2 heat exchangers in the bottom seems to be not a good idea, i guess.

If you want to simulate with two reboilers and also use the heat exchanger module to rate/ design the reboiler, you may simulate with any of the following options:
1) as two heaters attached to the column at the bottom stage
2) simulate the reboilers as external units (column module without reboiler option) with draws and feed connected to the column. This is usually more cumbersome and more difficult to converge.
3) simulate it as a single reboiler and use pseudo-streams from column bottom splitting into two using splitter unit and feed it to the heat exchanger unit. Here you have to make sure that the return streams are same as those calculated in the internal column calculations.

If the split is not even (if the reboilers/piping is not identical, this is likely to be the case), the simulation is slightly more complicated as you have to decide the split based on the pressure balance.