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Vertical Thermosyphon Reboiler Replacement

Vertical Thermosyphon Reboiler Replacement

(OP)
Hello, I am working on a project to replace a feed flash exchanger to a column. This exchanger will be increasing in surface area from 1000 ft2 to 1400 ft2. This exchanger is also used as a vertical thermosyphon reboiler during campaigns about one day per month when the tower processes a different feed to a different nozzle on the tower. The remainder of the time it is a once through feed flasher to the tower.

My question is: The increase in surface area and heat duty is only required for the predominant mode of operation (feed flasher) where we will be processing more feed. When we switch modes to use the exchanger as a thermosyphon reboiler, I want to ensure no issues arise. The elevation relative to the tower, liquid draw sump, liquid feed line, and reboiler return will not be changed, i.e the height of the exchanger will be the same and the increase in surface area will be accomplished by increasing shell diameter. The reboiler and outlet piping will be increasing in diameter. The feed required to the reboiler during this mode will be the same as it is today. The friction loss through the entire network will be decreasing, so I should still be able to process the same feed? Likely at a lower hot fluid mass flow.

Is there anything else I should be paying attention to with regard to ensuring the thermosyphon mode still works?

Thank you,
D

RE: Vertical Thermosyphon Reboiler Replacement

I assume a process engineer has modeled this exchanger in a program such as HTRI, and run simulation of the various operating conditions. If not, that should be done. Check the recirculation rate and the outlet vfrac. The circulation rate will probably be approximately the same since you haven't changed the relative elevations. The additional surface area is going to result in a lower steam side pressure. You'll need to be sure that this reduced steam-side pressure is still high enough to remove the condensate. Otherwise you'll have control problems due to repeated cycles of condensate building up inside the reboiler until the pressure rises high enough for it to get out.

RE: Vertical Thermosyphon Reboiler Replacement

In thermosyphon flow mode, recirculation flow will increase, since you have lower dp on the process side for the previous design flow and transfer line return temp to the column will be kept constant. To go back to the previous heating rate, you have 2 choices : (a) throttle the feed liquid flow to the thermosyphon and maintain current TC setpoint, or (b) allow unrestricted flow into the thermosyphon and lower the TC setpoint. Option b may result in unstable flow in the transfer line due to lower vfrac, hence would presume some throttling of feed flow would be preferable.

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