3 replies to this topic

[CS Kang]

Hi guys, I have been trying to simulate my optimum number of stages and reflux ratio in HYSYS 8.0 and Pro II softwares. I used fluid package of NRTL for distillation of acetone from acetone-butanol-ethanol-water mixture. I managed to get 99% mol purity of acetone as my distillate when I used vigorous model in HYSYS and PRO II which seemed good. However, it will only work when I specify my reflux ratio to a high avlue of around 9. A friend of mine said it is not feasible to have a reflux ratio of 9 as it is wasting alot of energy. Also the number of stages I had to achieve the purity is around 20 at a certain feed stage and the optimal feed stage depends on the number of trays I input. The confusing thing is if I used on Pro II shortcut, it actually worked with 10 stages and relux ratio simulated is only 0.703 assuming 1.3 times minimum reflux ratio. I can never obtained that in vigorous design in both HYSYS and pro II. Can anyone explain this weird problem? 

[PingPong]

I have no idea what for your problem the required reflux ratio is for 20 stages, but if I found that it is 9 then I would obviously increase the number of stages.

 

In the many years that I do simulations for all kinds of designs I never used a shortcut method. I really don't see the point of that, because it can never be the same as a rigorous method, especially not for non-ideal mixtures, and I have to do it rigorous anyway. Shortcut simulation is a waste of time, in my opinion.

[Pilesar]

The Pro/II shortcut distillation calculations are not very accurate for non-ideal systems as the method makes some simplifying assumptions about the component relative volatilities. Ignore the results from the shortcut unit for your system because they give you no useful information. See the Pro/II Reference Manual for details about the unit calculations and under what circumstances the results may be very poor. 

[CS Kang]

Thanks for the kind replies. I have tried to reduce the reflux ratio to 2 with vigorous simulation and I will obtain around 80% purity. I think probably I have to purify the acetone to 99.5% for commercial use using another separation process. If I can manage to separate the 2nd and 3rd highest component in the mixture which are water and ethanol, I probably can reach 99.5%, sigh... probably have to find molecular sieve method I think...  However, I am very curious ideally, what reflux ratio the industries outside is using? Does any experts or professionals who worked industrially can tell me who is a just a studet now?