2 replies to this topic

[ashn0tray]

Hi guys

I'm currently designing a sub-critical CO2 extraction plant and having trouble with depressurizing the liquid CO2 into gas method. I need to separate mixture of liquid CO2 with ethanol and some other by products by decreasing the pressure from 100 bar to 1 bar which make the co2 turn into gas . What I have already on my mind is by putting pressure relief valve on a large volume tank to stored the mixture so pressure decreases. Is pressure vessel necessary? any recommendation? it would be better if the CO2 gas is not exhausted to the atmosphere because I like to recycle it instead.

Thanks

[Art Montemayor]

ashn0tray:

Although you don’t mention it, I have to assume you are a student and that your problem is an academic assignment. Am I correct? I need to know this information and more if I am to help you resolve the questions you ask. As most students, you are using general, layman terms for pressure - which are misleading and problems for engineers. You fail to tell us WHAT type of pressure you are citing. Is it gauge or absolute?

I understand that you are probably extracting ethanol and other organic products using liquid CO2 and want to separate the CO2 from the mixture by converting it into a gas and subsequently recompressing and cooling it so that you can re-employ it in the extraction process again. Am I correct?

You don’t tell us the temperature of the 100 bar(g?) mixture, so I am forced to assume that it is at ambient temperature of 27 oC. If my assumptions are correct, then the expansion of the liquid CO2 will yield a mixture of: solid CO2 “snow”, CO2 vapor, and frozen ethanol + other organic products at approximately 1 bar(g) and -75 oC. Approximately 70% of the CO2 expanded will be converted into vapor at -75 oC and 30% will be solid CO2 at the same temperature.

If you have already taken a Thermodynamics course, you should be able to understand how I was able to predict the results of the expansion. From these predicted results, you can appreciate that you have a serious problem on your hands in trying to recover the ethanol + other organic products. Additionally, you got another problem in that approximately 30% of the liquid CO2 expanded is now a SOLID – and consequently can’t be recirculated, nor even handled in an efficient manner. To make things even worse, the -75 oC temperature is beyond the tolerance levels of most steels and you will probably have to warm it up in order to recompress it to the original 100 bar(g?).

You certainly require a pressure vessel as the expansion chamber. I recommend sizing it for 3 barg as a design pressure and operate it at 1 bar(g?) – if that is your process requirement. I would not put any "pressure relief valve" on the system and expect it to carry out what you propose. Please furnish us with an Excel Schematic Sketch of the process you are proposing so that we can detail out all the correct and proper ways you can piece it together.

Confirm that I’ve assumed everything correctly or correct the information we should be using. Then, we can discuss what you want to do and what you can do in order to keep within the Scope of Work that you have been assigned.

Await your reply.

[ashn0tray]

Thanks Art for really nice reply

sorry for long reply, I was having uni. Yup, this is my assignment problem. About the CO2, is it possible to get 100% or at least 90% conversion to gas??? I was thinking to have several tank in series to decrease the pressure from 100 bar-75-50-25 n then 1 bar???Or maybe manipulate the flow rate into a big tank?

Also I have another question. Is it possible to have a flow rate of 32000 L/hr in industry??? From my material balance, I got this number but i felt a bit weird since its really huge. Is it possible to get a solubility data of ethanol in CO2 (sub critical liquid)?

Thank you for your reply