6 replies to this topic

[kemisage]

Hi all,

 

I am a fourth-year undergraduate student. I am currently taking my Process Design course. I have a carbon dioxide stream (at about 17.5 psia and room temperature) with some water (~2.6 mol%) and nitrogen (<0.2mol%) as impurities. I am trying to remove water (as much as possible). I decided to go for a flash separator operating at the process pressure but I was able to obtain satisfactory separation only at around -70 F.

 

1. Do these conditions for a flash seem practical in the industry?

 

2. How can I cool my CO2 stream to a temperature that low? Do I use a refrigerant? 

 

Thank you for your response.

[Bobby Strain]

Wrong choice. Carbon dioxide and water will form a hydrate at much higher temperature. So you should consider some other alternative. And refrigeration for such low temperature is quite expensive. Normal practice is to use glycol or solid desiccants for drying.

 

Bobby

[kemisage]

Hi Bobby,

I feel so stupid that I forgot about the hydrate formation. I just realized that the simulator didn't have any binary interaction parameters for CO2 and water; it was giving me wrong results! Thank you so much for your response.

So it looks like drying might be the only thing I can do. If you don't mind me asking, how can I decide which drying substance to use? I understand that it can be a hassle to explain it here. I would be grateful if you can point me to a source.

[Pilesar]

Figure out what conditions you need for your product streams. What will you do with CO2 at that low pressure? If you will have to compress the CO2, it may be more practical to remove the water after compression. 

[kemisage]

Hi Pilesar,

CO2 stream comes from a carbon capture plant at 121 kPa but it will be compressed to about 2MPa before it enters the reactor. Can you please expand a bit on why it would be more practical to remove water after compression?

Thank you.

[Pilesar]

In general, the condensing temperature of water is a function of the vapor pressure of water. The vapor pressure of water in your stream can be approximated by 17.5 psia multiplied by 0.026 (the mol% water). You can look this resulting water partial pressure up in a saturated steam table to find the approximate water dew point temperature. If you increase the total pressure of the stream, the water partial pressure also increases and the water dew point temperature increases. With the higher dewpoint temperature, you don't need to cool the stream so drastically to condense water. It is common after compression to have an exchanger to bring the temperature down close to ambient in order to separate additional liquid in a flash drum.

[kemisage]

Thank you for the explanation. I can clearly see your point. I am on Easter break now but I was still thinking about my project since I was uncomfortable with the process I designed. I will make changes to my flowsheet and update you all with what I have done.