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Aspen Hysys - Recovery Of Propionaldehyde

distillation propionaldehyde

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#1 clifford.v

clifford.v

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Posted 22 September 2020 - 02:37 PM

A good day to all of you. I am working on an assignment about the recovery of propionaldehyde. The feed contains 4.89% CO2, 4.14% water, 6.02% ethane, 84.09% propionaldehyde, and balance is trace of H2, CO and ethylene (all in molar basis). The feed is at 30 degree C and 620 kPa. I use Aspen Hysys for simulation and the fluid package is UNIQUAC.

 

My task is to recover as much propionaldehyde as possible, and also, get rid of all the light gases from propionaldehyde. First, I tried to use a flash column. However, I could not get rid of CO2 and ethane from propionaldehyde.

 

Next I tried to use a reboiled absorber. I ended up with 93% recovery of propionaldehyde, but there were still CO2 and ethane in propionaldehyde.

 

I then tried to use a distillation column with a partial condenser. I ended up with 98% recovery, and the bottom product only had propionaldehyde (more than 95 mol%) and water. This is very good and is what I want. However, I have a conceptual question here. What does this distillation column separate? What is the cut? 98% of the water and propionaldehyde from the feed go to the bottom products. The light gases go to the overhead vapour stream. As for the condensate stream, it contains propionaldehyde (87.68 mol%), water, CO2 and ethane. It seems to me that propionaldehyde is the major component in both distillate and bottom product, which does not seem right. It is as if I were separating propionaldehyde from itself.

 

Can you please advise me on this issue? Even though I can do the assignment, I feel that there may be something wrong with my understanding. Many thanks in advance.

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Edited by clifford.v, 22 September 2020 - 02:51 PM.


#2 Pilesar

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Posted 22 September 2020 - 08:08 PM

By your description, you are recovering 98% of propionaldehyde from an impure feed stream as a high purity product mixed with water. Were you given the overhead temperature? A colder condenser would remove more hydrocarbons from the vapor stream. What good is the liquid distillate product? What would happen if you refluxed all the overhead liquid back to the column? 
 
I will give some general comments -- no longer addressing the particulars of your simulation. Design the separation to meet the specifications. Typically you do not just put a stream to a column to see what comes out. For a design, you begin with product specs desired and then find out what process is required to achieve the separation. Changing parameters such as pressure, number of stages, reflux ratio affect column performance. Under some conditions, hydrocarbons and water may form two liquid phases allowing the liquid phases to be separated in a decanter. Some components may be difficult to separate cleanly by distillation. Some real-life columns generate a purity stream and a dirty mixed stream containing a relatively large quantity of the desired 'pure' component. The dirty mixed product stream might be recycled to a reactor loop upstream or perhaps sold as a cheaper product, burned as fuel, or incinerated as waste. Economics matter to help determine whether additional separation effort is justified. 


#3 clifford.v

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Posted 22 September 2020 - 10:49 PM

 

By your description, you are recovering 98% of propionaldehyde from an impure feed stream as a high purity product mixed with water. Were you given the overhead temperature? A colder condenser would remove more hydrocarbons from the vapor stream. What good is the liquid distillate product? What would happen if you refluxed all the overhead liquid back to the column? 
 
I will give some general comments -- no longer addressing the particulars of your simulation. Design the separation to meet the specifications. Typically you do not just put a stream to a column to see what comes out. For a design, you begin with product specs desired and then find out what process is required to achieve the separation. Changing parameters such as pressure, number of stages, reflux ratio affect column performance. Under some conditions, hydrocarbons and water may form two liquid phases allowing the liquid phases to be separated in a decanter. Some components may be difficult to separate cleanly by distillation. Some real-life columns generate a purity stream and a dirty mixed stream containing a relatively large quantity of the desired 'pure' component. The dirty mixed product stream might be recycled to a reactor loop upstream or perhaps sold as a cheaper product, burned as fuel, or incinerated as waste. Economics matter to help determine whether additional separation effort is justified. 

 

Many thanks for your reply. I was not given the overhead temperature, but I set it at 40 degrees C to enable the use of cooling water. Lower temperature would require chilled water for cooling. My distillate contained mostly propionaldehyde and light gases. Normally, when I do other exercises, I would first decide the cut. For example, it would be A/BCD or AB/CD or ABC/D. For this exercise, I just wanted to test the distillation column because the flash column and reboiled absorber did not work. To my surprise, the distillation column met my needs. However, I cannot tell what is the cut here.

 

As per your suggestion, I tried to reflux all the overhead liquid back to the column, and it also worked. 






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