3 replies to this topic

[gopal.panjwani]

I'm working on a methanol project. From a coal based feed we are producing methanol after final seperation with water via distillation. For distillation column reboiler i'm using LP steam.

Can anybody suggest what is the optimal trade off between reboiler duty & no of stages?

Also how to ensure to avoid any chances of film boiling in steam reboiler, at at flow sheet stage when reboilers are not designed.

[Art Montemayor]

Is this a real project, or are is it a student problem?

 

I don't know what you mean by "optimal" tradeoff between reboiler duty & no. of stages.

 

You avoid any film boiling by not using pool boiling - such as a kettle reboiler.  But you can use a kettle by controlling the heat flux below a level where there is no film boiling.  Usually this is 15,000 Btu/hr-ft² or less.

[matt.o]

I have a few suggestions:

1.  Make sure you have the right property model for your components.  I'm not too familar with MeOH and water, but I'm pretty sure it's non-ideal and has an azeotrope.

2.  To select the right heat duty/column height trade off, you'll need an NQ curves analysis.  This will optimize the feed point and generate the condenser and reboiler duties over a range of equilibrium stages.  Aspen Plus can do this for you in Radfrac.

[Pilesar]

Even with infinite trays, there will be a minimum boil up ratio and reflux ratio. Determine the minimum reflux ratio to meet your specs. Increasing the reflux ratio from the minimum reduces the number of stages needed. Your optimal reflux ratio will be about 1.2 times the minimum reflux ratio. Start with that as a ballpark figure to determine the stages needed. For a more precise optimum, you can then perform a sensitivity analysis around that point to see its effect on capital and operating costs.

 

Film boiling can be avoided by not having too large a temperature difference at the reboiler. But if the temperature difference is too small, your reboiler will be unreasonably expensive. You may be able to adjust your column pressure so the reboiler process-side temperature is about 50F cooler than the saturated LP steam, but try not to increase the pressure so much that you have to use refrigeration at the condenser.