Reboiler Duty 1

[peteeng]

I am trying to figure what is the right and practical reboiler size for a stabilizer column. I have simulated the Stabilizer Column with a feed/Bottom Exchanger. At steady State the Reboiler Duty required is 5.0 MMBtu/hr. At start up condition (Feed/Bottom Exchanger duty set to zero), the Reboiler Duty is 15.0 MMBtu/hr. How much excess duty above 5.0 MMBtu/hr is reasonable?

 

[pmureiko]

The reboiler you design for normal operation will have a fixed area of your choosing. The actual duty will be a function of actual U (overall heat transfer coefficient) and dTlm (Log mean delta T). I'd expect the actual reboiler duty during startup to be higher because of a higher dTlm. As the column warms up the dTlm will decrease, reducing duty, but that is alright because the column needs less heat input as it approaches start up conditions. Couple this with design factors when sizing the reboiler, and you may find it big enough for normal and startup conditions.

Also important is to make sure items like control valves and steam traps are sized for startup and have turndown for normal operation.

 

[peteeng]

pmureiko,
Thank for your reply.
If the control valve is sized for the start up condition, Should the reboiler needs to be bigger because the larger amount of heat medium (approximately 3 times) is required due to higher pressure drop? Although the heat transfer area may be sufficient because of higher LMTD and higher heat transfer coefficient due to higher heat medium velocity.

 

[TD2K]

In the case you described, the reboiler duty would be 3x normal duty at startup if you sized it this way. What is the impact on the reboiler operation after startup when you need only 1/3 the duty? What about at reduced column throughput? Will the reboiler still work at these conditions? If you put a 60% column turndown case on top of that, the reboiler will be very lightly loaded.

I'm assuming at the 3x duty case that you are starting up is with the column at full rates. Would you actually do this or would you start up at reduced rates and then increase feed as capacity is available?

There's another factor you may want to consider when setting the size of the reboiler design duty. If the feed/bottoms exchanger becomes fouled, what will the site do? Is the column shut down totally so the exchanger can be cleaned? Do you have multiple shells so you can take one out and clean the other at reduced heat transfer? Do you plan on just reducing rates in this configuration until the exchanger is cleaned and back in service.

All of these can be factors in setting the design duty. There isn't a straight forward answer but usually, I've seen minimal additional duty added to exchangers for start up conditions, definitely not adding in 200% additional area. Startup is usually handed by running at reduced rates. However, there is a cost associated with that and that may support adding in additional area.

 

[peteeng]

TD2K,

Thank you for your comment. I guess you are right, during start-up, one would normally start at a much reduced rate, therefore the design at normal operation would be sufficient plus of course the standard design margin.