4 replies to this topic

[MSUChemE]

Hi, I'm having trouble sizing a flash drum for an amine contactor system. The issue is that the company recommended a residence time of 10 minutes, but the vapor coming off the top is very small compared to the liquid loading. When I use the sizing equations (even with a literature value for K) I get a diameter of 0.2 feet.
Any suggestions?
Thanks~!

[Zauberberg]

Residence time of 10 minutes accounts for the liquid (amine) phase, not for the vapor phase. Therefore you have to design this vessel for 10 minutes residence time based on the design amine flow, and also by taking into account the NLL (normal liquid level) inside the vessel since not the entire vessel volume is utilized for phase separation.

One standard available online could provide you with some more guidelines: http://217.174.18.60...pr/e-pr-880.pdf

In addition to that, you can use the "Search" option and browse the content of ChE forums. There has been a lot of topics dedicated to rich amine flash vessels.

[Art Montemayor]

OK. So you are coming up with an impractical or ridiculous result.

But we don't know why, anymore than you do. We don't have your work or any explanation on how you generated the result, so we have no positive advice or comments to add.

Why don't you take the time and effort to present your work, showing clearly what you are doing step-by-step? Engineers always check each other's work - especially calculations. It isn't about "trust"; it's all about professionalism. ALL engineering calculations are subject to being checked.

What "company" are you referring to? You posted in the Student Forum, so I assume you are a student. What "literature" are you using?

[MSUChemE]

I apologize for my vagueness.
The project is for a senior design course and was originally given to us by a local refinery. We were given a sour gas stream with about 1400 ppm H2S in 6MMSCFD. We need to reduce this to 4 ppm using MDEA. We've been having a bit of a communication issue with our mentor from the refinery so we've been doing a lot of internet and text resources (Amine Basic Practices Guidelines, papers from Bryan Research and Engineering Inc, GPSA, and this website). Using Pro ii software we determined a lean amine flow of almost 2.3 MMSCFD. This is then put into a flashdrum. The vapor flow off the top was determined to be 1 kmole/day. The estimated K value was from the GPSA volume 2, (0.35 for the K multiplied by a factor between 0.6-0.8 since it's an amine), but with the low flow of vapor coming off the top, the diameter came out to be around 0.2 feet. The 10 minute residence time was suggested by the company.
I think I'll just keep trucking through. Sorry

[Art Montemayor]

MSUChemE:

Thank you for the response explaining the background and the basic data. Now things start to clear up about how to better help you in your query.

I can’t check your results if I don’t have a credible Amine flow rate into your flash drum. As Zauberberg states, “you have to design this vessel for 10 minutes residence time based on the design amine flow”. I don’t believe your statement, “we determined a lean amine flow of almost 2.3 MMSCFD”. That just can’t be true!!! Everyone and his brother measures LIQUID flow rates in totally different units. What you report is a “standard” unit measurement for GAS flow rate – and not liquid flow rate. Is this another “communication issue”? Can this be the cause of your ridiculously small flash drum results? Let’s get back on track by you checking this out and feeding us the correct basic data.

Give us the stream conditions and compositions related to the 3 streams in your heat and mass balance around the flash drum:

• The liquid feed amine stream;
• The overhead vapor stream;
• The bottom liquid amine stream.
  
  You are using a good simulation program for amines. There should be no problem with PRO II, GPSA, and the BRE information. You are using good tools. Something else is going bananas.