9 replies to this topic

[Geetanjali]

Hi Everyone,

 

What are the most popular (industrial) methods for separating propane from a mixture of light gases like hydrogen, methane, ethane, butane, and C5? Note: 90% of the feed stream is composed of propane.

 

I know about 2 processes :

 

1. Chilling process: here, you have the upstream CO2 removal, then chilling train (cold boxes) and finally downstream separation with demethanizer, deethanizer, depropanizer, debutanizer, etc.
2. Membrane separation

Is there any other method that is used favorably in industry/refineries?

 

 

Note: I am modelling a process in Aspen plus where plastic feedstock (LDPE polymers) reacts with H₂ in a hydrocracker (fixed bed, with catalyst) to result in 85% gases and remaining are C5-C12 liquids. Out of the gaseous stream, 90% is formed of propane. Rest is hydrogen, methane, ethane, butane, C5. The unreacted H₂ from the hydrocracker is recovered (about 85%) separately in a PSA unit . The rest of the H₂ gas along with other light gases move to process section where I recover Propane with high purity. 

 

I have followed the chilling process (method 1 above) for my work (attached snapshot of the Aspen Plus flowsheet I have created). But I was just wondering if there is something I don't know that is the go-to way for this type of separation.

 

Thanks for your response. This group is very useful. 

 

Best,

Geetanjali

Attached Files

•  Propane separation Aspen Plus.PNG   213.51KB   1 downloads

Edited by Geetanjali, 09 March 2023 - 03:55 PM.

[Pilesar]

HD5 propane is the highest purity propane grade normally sold with 90% propane composition. Define what spec you actually need to meet. You may be able to knock out enough liquids with a bit of cooling and a knockout drum.

[Geetanjali]

Thank you for your answer. This is useful. 

 

I was aiming for a >99 wt% purity. But I read about HD5 propane being the highest quality propane. In that case I may not need to have all these extensive unit operations. What you are suggesting (to knock out liquids with a bit of cooling and a KO drum) makes absolute sense. I can sell the propane product as-is either as HD5 or HD10 (depending on the propane % in the stream) too. This would save me a lot of capital and energy costs. 

 

Thanks. 

Geetanjali

Edited by Geetanjali, 09 March 2023 - 05:03 PM.

[Bobby Strain]

I am curious to know which university you attend. Would you share that with us?

 

Bobby

[Geetanjali]

Hi Bobby,

 

I have sent you a message.

 

Best,

Geetanjali

[abhi_agrawa]

Geetanjali,

 

I would configure things differently:

• Depending on CO2 specifications in feed and 99% Propane, CO2 removal may or may not be required.
• What type of refrigeration have you assumed to be available?
• Would tend to simplify the chilling - what is stream 604 pressure and temperature?
• Demethanizer, followed by Deethanizer is a good idea thermodynamically, but did you try to look at Depropanizer first scheme?
• It seems that the Demethanizer is configured at high pressure, did you look at low pressure Demethanizer?

These are just some pointers.

 

Hope they help,

Abhishek

[Geetanjali]

Hi Abhishek,

 

Thanks for your time in understanding my problem. 

 

1. CO2 in the incoming stream is about 0.37% by weight. This looks small but is it okay to ignore this and not have CO2 removal step. I read earlier that having CO2 stream pass through these distillation columns (like demethanizer, deethanizer etc.) might not be a good idea as it leads to problems downstream. However, do you think this is a small % by weight and need not be separated?
2. I have used ethylene, propane refrigerants and have built a refrigeration cycle that exchanges the hot and cold process streams  to cool or heat the streams. Have outlined the region in the attached snapshot (blue region that includes three cold boxes - I have outlined one cold box in black containing a refrigeration cooler, plate and fin heat exchanger, knock out drum). 
3. Stream 604 is at 38 bar and 60 °C. Please see the attached snapshot. Have underlined it in red color.
4. Nope I haven't looked at Depropanizer first. I will try that now.
5. I intentionally kept the demethanizer at high pressure becuase in one of the earlier discussions in the group, ti was suggested that keeping the demethanizer at high pressure might be less energy intensive than otherwise. 

 

Additionally, I actually tried to just flash the stream 602B in a two phase flash drum (Oper pressure = -80 C and Oper. pressure = 23 bar). I got >90% propane in the top phase (vapor) and rest going to the bottom phase as a liquid stream. I am re-thinking to revise the flowsheet in this case. I think I can sell the top stream as HD5 propane stream. See the light green color outlined section.

 

Thanks so much 

 

Best,

Geetanjali

Attached Files

•  Revised Aspen Plus Process Flowsheet.PNG   319.49KB   0 downloads

[Pilesar]

Look up 'liquefied petroleum gas' (LPG.) If LPG is a possible product, the C4s would not need to be separated from the C3s. Refrigeration is expensive. Your flow scheme is similar to what you might see in an olefins plant but ethylene and propylene are more valuable than alkanes. 

[Geetanjali]

Thank you, this is a great suggestion. 

[abhi_agrawa]

Geetanjali,

 

• Can you allow CO2 with H2/Ch4 mixture? If yes, then does enough CO2 go with H2/CH4 to meet final product specifications? What are your product specifications? What is your feed?
• Why did you not consider low pressure demethanizer?
• A Cold Box is very different from you snapshot. It has multiple streams.
• Stream 604, currently at 39 bar pressure. What would happen is you increase this to say 50 bar? Does your refrigeration requirement increase or decrease? What is the minimum temperature you need to meet your target recovery?

Abhishek

 

 

 

Hi Abhishek,

 

Thanks for your time in understanding my problem. 

 

1. CO2 in the incoming stream is about 0.37% by weight. This looks small but is it okay to ignore this and not have CO2 removal step. I read earlier that having CO2 stream pass through these distillation columns (like demethanizer, deethanizer etc.) might not be a good idea as it leads to problems downstream. However, do you think this is a small % by weight and need not be separated?
2. I have used ethylene, propane refrigerants and have built a refrigeration cycle that exchanges the hot and cold process streams  to cool or heat the streams. Have outlined the region in the attached snapshot (blue region that includes three cold boxes - I have outlined one cold box in black containing a refrigeration cooler, plate and fin heat exchanger, knock out drum). 
3. Stream 604 is at 38 bar and 60 °C. Please see the attached snapshot. Have underlined it in red color.
4. Nope I haven't looked at Depropanizer first. I will try that now.
5. I intentionally kept the demethanizer at high pressure becuase in one of the earlier discussions in the group, ti was suggested that keeping the demethanizer at high pressure might be less energy intensive than otherwise. 

 

Additionally, I actually tried to just flash the stream 602B in a two phase flash drum (Oper pressure = -80 C and Oper. pressure = 23 bar). I got >90% propane in the top phase (vapor) and rest going to the bottom phase as a liquid stream. I am re-thinking to revise the flowsheet in this case. I think I can sell the top stream as HD5 propane stream. See the light green color outlined section.

 

Thanks so much 

 

Best,

Geetanjali