2 replies to this topic

[Trop]

I am a recent graduate and have started working in industry. My manager has asked me to investigate the capacity of a distillation column that is used for ethanol/water separation. This column is very old and was modified from its original design for this process about 25 years ago, so there is no manufacturer information to design data available. The column has a diameter of 6.5ft and consists of 66 trays. The condenser sits above the column ~80ft off the ground and the reflux is gravity fed.

I have been asked to determine the maximum amount of ethanol the column can produce. Currently, they have only been able to achieve 14gpm, but from my calculations the column should be able to reach 20+gpm. I have attached my excel sheet that shows my work. I basically used the column diameter to calculate the vapor flow at the top of the column. From this I determined the reflux and distillate flows.

There are several things I believe might be attributing to the discrepancy. The first is the trays. The column consist of sieve trays with a 1 ft spacing, a 8-12in downcomer, and holes only covering about 50% of the tray, so my estimated tray efficiency of 70% might be incorrect. Also, the top of the column does not have a reflux drum instead after leaving the condenser the reflux flows directly into the column until the column temperature drops below the set point and product begins going to storage until the temperature increases. It is my thought that the use of a reflux drum could add stability to the column, which experiences swings in the reflux/ draw rate. Lastly, if my top vapor flow is correct the condenser is significantly undersized for the process.

I would like to know if I am on the right track, since it has been awhile since I have dealt with column sizing. Let me know if I need to provide more information. Any help would be much appreciated.

Thanks,
T

Attached Files

•  Column Capacity.xls   17KB   124 downloads

[katmar]

Unfortunately I do not have time to check all your calculations at the moment, but in order to narrow down your focus on where the problems lies you need to find out what happens when they try to push the column to obtain more than 14 gpm. The failure mode will tell you where the bottleneck lies.

The capacity of an ethanol recovery column is very sensitive to the top strength. If you are able to back off your top spec by a few tenths of a percent you will increase the capacity dramatically.

[Trop]

Thanks for the response that is something I am currently looking into. I have not personally seen the column pushed past 14gpm, but from what I have been told the column temperature starts to decrease and when more steam is added the column pressures up. This is an atmospheric column, so the pressurization makes me curious if there is a vent issue as well. I am new to real world distillation, and there are a lot of things I am still trying to work out and understand, so I welcome your continual help.