Process Simulation Help Needed

[JonBrady]

Hi,

I am a junior and I am working on a project to design a gas plant using HYSYS and PROMAX. I am still learning and will be glad if some simulation expert can gladly help me with the questions below.

1. When I pass the saturated rich gas through a 3-way separator (in both HYSYS and PROMAX), no liquid is dropped. From textbooks, pressure drop across a vessel in real life is usually between 1 and 5 psi. When I use this pressure drop range, no liquid is dropped. Is this representative of actual separation process - no liquid drops out of the scrubber/separator? Am I on the right track? Or is there something that I am doing wrong? The only way to get any liquid dropout is if I use a higher pressure drop.

2. If this is normal (no liquid dropout from separator), how do I size the liquid pipes? Should I just take the incompressible "liquid" content in the gas and size for that amount (liquid that will be dropped out if I compress the gas)?

3. How do I get the gross and net heating value from HYSYS? I multiplied the molar density (lbmole/ft3) by molar basis heating value (btu/lbmole), but I am getting a ridiculous value. I also tried multiplying the mass density (lb/ft3) by mass basis heating value (byu/lb) and I also get a ridiculous value identical to that obtained using molar basis (62500 btu/ft3 for HHV). I have attached the stream information. The stream is 100% vapor.

Thank you.

Jon...

 

[TD2K]

What is the state of the stream entering the 3 phase separator? These are typically located because you expect to have a 2 or 3 phase feed into the separator and you want to remove the liquids from the gas. It sounds like you have either a saturated feed or a close to saturated feed and you only see liquids when you drop the pressure enough to result in a retrograde condensation so your results are sort of fudged. Separators are also added to ensure you don't get any liquids into processes or equipment that can't tolerate them (such as compressors) so for these, you won't get much liquid and you use another approach to sizing the line. I might look at reducing the liquid level from HLL to LLL in a certain number of minutes. It's a matter of balancing the various effects. A 10" line might drain the vessel in 3 seconds but your lead is unlikely to view that as a reasonable recommendation if say typical services have a 2" or 3" line (which would be my guesstimate).

Sizing the liquid piping is then a matter of establishing a design basis. If it's just a nominal line, a 2" line might be more than sufficient. How much is your feed rate, how much liquid might you condense out, what resulting line size might you need?

I'm going to put on my lead's hat. You really should be talking to your lead engineer when you run into these type of issues. You are using up client hours (I'm assuming) and not using them efficiently. Your lead doesn't expect you to know everything as a junior and there's a balance you need to walk between figuring this out yourself (which is good you are trying) and not being prudent with the client's $. Believe me, I MUCH prefer it when my younger engineers come to me with questions than spending hours trying to figure out something which may not have a nice mathematical basis.

As far as a check of the heating value, if you have heavy materials, the HHV will be high because the heating values of the higher MW HC increase significantly. Take 1 ft3 of the gas. Take the resultant moles in that stream. Convert to mass. Multiply by their heating values and add them all together.

 

[JonBrady]

Thanks, TDK, for the help. You are right, the feed is a saturated vapor feed. When I said that I am a junior, I meant a junior in college, so there is no client's $ involved - it's a school project. My problem with the heating value is that Promax and Hysys give 2 completely different answers and my hand calculation is similar to Promax. Thanks, a lot.

 

[Latexman]

JonBrady,

Student posting is not allowed at Eng-Tips, but you can ask ChE student questions here

http://www.engineering.com/AskForum/aff/31.aspx

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.