7 replies to this topic

[distillationman]

I'd like to hear from those who have experience with reboilers operation if there are any problems/inconvenients in using a reboiler with a 100% more area than the necessary... There will be a steam control valve in steam line controlling the column pressure.

I intend to use a kettle in these conditions.

Regards,

[srfish]

Distillationman,

Too much excess surface can create surging problems. Years ago I saw a case where the excess surface was enough in the kettle where the boiling liquid would flash enough to uncover most of the bundle then liquid feed would surge into the kettle. This surging created problems downstream. But just how much excess surface creates this surging point I don't know. As I remember there was about 100% excess surface.

[Robert Montoya]

Dear friend, in a kettle type exchanger when there is a decrease in the area, the following happens:
- decreases the pressure drop and the velocity of tube side, in this case the steam. This causes an increase in the fraction of steam in the output, however if the steam is subcooled water to the cooling circuit can be handled. However, this drawback can be overcome "condemning the tubes that are necessary."
- With respect to the shell side, due to the same geometry in Kettle type, should not be changes in the pressure drop in fact HTRI simulations under these conditions with no changes seen in the pressure drop, but if seen decreasing heat transfer coefficient on the side of the shell decreases slightly.
This is very important that the pressure drop on the side of the shell is kept as minimal possible because there is a significant effect on the hydraulic of the distillation column.
From my point of view would be appropriate for validity with the new flows and corroborate the information shown above.
Regards.

[katmar]

If you have excess area the steam control valve will close to decrease the condensing pressure of the steam, thus decreasing the temperature driving force. The problem with this situation is that the pressure in the coil can get too low to drive the condensate to where ever it is designed to go, and the condensate flow can stall and you start to lose heat transfer surface.

Eventually the steam valve will open wide to increase the pressure (and temperature) to compensate for the decreased area and once the condensate starts flowing again you can have a situation where you suddenly have excess area with hot steam and this can result in the surging type behavior described by srfish.

If you pay attention to the condensate removal system and ensure no condensate backup in the coil then the steam valve will be able to find its equilibrium position and you should be OK.

[Propacket]

I always wonder about the effect of extra surface area on the performance of kettle type reboilers. I am often confronted with turn downs of kettle type reboilers down to 15% of the design flow. This definitely leads to enormous surface areas. We procure kettle type reboilers from a renowned heat exchanger manufacturing company and the fact is that we have never received anything alarming from that heat exchanger vendor regarding turn down of the reboiler. The performance of the kettle type reboiler has been guaranteed at as low as 15% turndown.

Additional inputs from the respected members are requested to resolve this issue.

Edited by Art Montemayor, 27 April 2012 - 09:34 AM.

[katmar]

P.Engr - you (and your supplier) are correct in that there is no reason why the kettle type reboiler cannot function with a large turndown ratio. There is unlikely to be any problem with the reboiler itself. What I was pointing out above is that you have to be careful with the design of the condensate system. If the steam (and therefore the condensate) systems can work at the low pressure that would be required to compensate for the excess area available then the coil is unlikely to give problems.

[Robert Montoya]

I definitely think that the heat transfer will not affect the operation of the reboiler because the pressure drop on the side of the shell is almost nil. My concern is the hydraulics of the vapors that go to the fractionation tower. During normal level process in the kettle should cover the exchanger tubes and at least be the same level of liquid in the column. But if the kettle must be set the normal operating level in the column taking care of the waves that can be formed and can cause damage to the trays within the column.

Edited by roberdani12, 27 April 2012 - 07:03 PM.

[distillationman]

Thank you so much for your help!
I'll pay close attention to condensate removal system.
I have just found in spirax sarco website a good description of stall in heat exchangers.
The suject of this description is in total agreement with katmar's advice.

http://www.spiraxsar...s-and-stall.asp