5 replies to this topic

[EddyHyip]

I came across an example in a text book. It is the distillation of styrene and ethylbenzene. I can attempt the question but actually I am puzzled... why is the reflux ratio so high? It is 13. The thumb of rule is to have reflux ratio below 8 (or 5 if I'm not wrong) and the optimum should be around 1.2-1.5 of min reflux ratio.

Can someone explain/ teach me why (under what circumstances) high reflux ratio is desirable than low reflux ratio? I know high reflux ratio will increase energy consumption but lowers the number of tray. Is there any specific condition that need to use high reflux ratio?

Your input is important. I am so curious to find out something I don't know (or failed to dig any info on the net/ the same text book)

Thank you in advance

[Clayton]

I am not too familiar with this process but I am assuming that the RR is high because of the purity of the tops required. You may be able to get that purity by adding more stages but mechanically distillation columns are limited to a certain height.

[SSWBoy]

Put simply, the two components you've listed have very low relative volatility. A quick check from wiki shows the difference in boiling point is approximately 5 centigrade (similar to propylene-propane which have RR of 12-15). To actually get the two components to separate NEEDS that much reflux.

"Rule of thimb to have reflux ratio below 8" Largely i expect this is true, except in the case of superfractionators (other common examples if you want to research this any more, are Ethylene-Ethane and Propylene-Propane)

"Optimum is between 1.2-1.5" I would err on the lower end of this scale, this 'optimum' is simply a balance of operating v capital costs and differs from service to service, for instance the cost of refrigeration for a demethaniser may mean you build a larger tower to start off with or conversely if you have a debutaniser with the overheads condensed against CW it may be cheaper to use a smaller tower and accept higher operating cost (which of course will be far lower compared to refrigerant)

Off the top of my head i can't really think of any reason why you would opt for a high reflux ratio if a lower reflux ratio would do the job (this is of course assuming all things being equal between the high/low reflux case). As you point out you would just needlessly be consuming energy.

[Art Montemayor]

SSWBoy:

Great, practical, experienced field response and explanation to a basic academic query. Your explanation and examples should go a long way towards lighting the those for those resourceful Chem E students reading this thread.

You deserve the appreciation and gratitude from EddyHyip and others whose engineering knowledge you have greatly increased. I hope you you find the time to visit us frequently in the future. There are countless other students who hunger for the basic know-how you demonstrate.

[Neelakantan]

hi
as SSWBoy has mentioned, the particular case in study REQUIRES a high reflux ratio for separation due to their close boiling condition.

and he has rightly said it is the capital vs operating costs that drive the reuqired reflux ratio otherwise.

the following pdf file link shall be useful in understanding how it is done;
http://www2.tku.edu....e/4-2/4-2-4.pdf

regards
neelakantan

[Jiten_process]

Can someone explain/ teach me why (under what circumstances) high reflux ratio is desirable than low reflux ratio? I know high reflux ratio will increase energy consumption but lowers the number of tray. Is there any specific condition that need to use high reflux ratio?

I just would like to add little more though others have already explained your query.

Sometimes there may be constraint of height of column. Many a times such columns are installed within the building and for lower reflux you may have to provide large no. stages which inturn may increase the height of the column. So to acomodate column within the building you may have to provide high reflux.

In our case we have designed one packed column for seperation of DMF (dimethyl formamide)-WATER mixture where we have kept Reflux ratio as 11. Our constraint was something different, these column was actually designed for DMSO (dimethyl sulfoxide)-water recovery where we have kept RR as 4. Our client wanted to use this same column in future for DMF-WATER. So after rating the column we found that we will have to keet reflux as 11.

So such is the case sometimes, however theoratical as others have explained i dont think there is any problem, practically cost (operating/capital) ofcourse plays very important role.