7 replies to this topic

Hello.

I am a student at Tromsø University College.
Currently studying to for a bachleor degree in process technology.

I have a little problem.
How do I size a separator when there are no liquids in the inlet streams?
It is a 2-phase vertical separator.

And then I cannot size the valve on the Liquid Outlet stream.

6500 kPa pressure in
1500 kmole/h in.

regards
KurtO

[Art Montemayor]

Kurt:

Welcome to the Forum. It's a pleasure to have someone who lives and studies in such a beautiful Artic environment - further north than even Sweden.

I don't think you have a problem. What you say you have is a pure gaseous fluid (probably natural gas) and, as such, you can't apply a 2-Phase separator logically because you only have one phase. If you have no liquids in the inlet stream, there is nothing to separate.

That is how you have described the "problem". However, could it be that what you really have as a problem is a Flash Separator? Now, that is a totally different story. A Flash Separator is a vessel that receives the product (a liquid + gas mixture) that results when you adiabatically expand a high pressure gas fluid (such as natural gas) from a high pressure (such as 6500 kPa) to a lower pressure. The adiabatic, free expansion causes rapid cooling and condensation of some of the components in the gas and they normally have to be separated. That is why a Flash separator is employed. Could this be what you are doing? If so, then that explains the need to use the Hysys simulator. Normally simulators are not used to design a 2-phase separator. In fact, all Hysys will tell you is the resultant composition and temperature of the produced mixture. The design of the separator is done normally on a spreadsheet - much as I've shown on the workbook I have uploaded on another thread.

Await your reply.

Art.

Thank you for the reply.

It is nice and beautiful here where I live and study. Hopefully you will be abel to see some of it. Tromsø might apply to be the host of the 2018 Olympic Games.
And might I add how much I enjoy this site and all the help I get from reading threads here.

Actually. I really dont know if it is a flash separator or not. It is not given in the task.
I am attending a course in hysys here at my school, and I am a novice when it comes to hysys.
I have been given a ready steady state pfd, and told to make it ready for dynamic simulation. I have not expirienced or done any calculations on a separator without the presens of a liquid phase.

Can I size the vessel without the liquid phase?


The pressure in the vessel is set to 6500 kPa. So there are no adiabatic expansion.
This one is the first of two separators. The gas is chilled before it enters the next one, and I get the presens of a liquid phase, and I sized the separator.


KurtO

[Art Montemayor]

Kurt:

You have been instructed to "make it (PFD) ready for dynamic simulation". What you want to look at in a PFD is to spot and identify where, in your process, you are going to undergo physical (Unit Operations) and chemical (Unit Processes) CHANGES. A pure gaseous fluid going into a vessel and subsequently exiting that vessel as the same gaseous fluid and with only a pressure drop to show for the ingress and outgress is not worthy of being simulated. The simulation is going to show nothing of any importance - except for a slight pressure drop. The vessel is acting as nothing more than "a wide spot in the line" and nothing more. There is no logical reason for sizing because there is no need.

However, if you need a pulsation dampner that is another subject. That is going to require a program beyond a simulator like Hysys. For natural gas conditioning and treatment, I see no reason for including a separator - unless you have failed to recognize another need or reason within the PFD.

I hope this helps resolve your dilemma.

[joerd]

Art is completely right in his reply, and I would encourage you to challenge your professor who gave you the task in a similar fashion, and ask him/her what the separator is for.
Sometimes, there might be a separator to protect equipment, such as a compressor, while normally you would not expect any liquids. In that case, you can size the separator assuming that there is indeed some liquid, but you'll have to guess at its properties. You might, for example, take the same liquid as you have used in sizing the next separator, and then use the Souders-Brown equation to find the diameter. If you use Art's workbook, the other dimensions will be fairly easy to set as well.

[Art Montemayor]

Well, just when I thought I had helped a student with my best thoughts, what happens? The realization about being a member of a good Engineering Forum suddenly hit me in the face.

Thanks to Joerd, one of the basic and very common-sensical aspects of engineering design has been injected in a timely fashion. I completely failed to bring up the point that he so importantly mentions: If you don’t challenge the proposal, you might miss out on an important purpose within the scope of work that simply wasn’t mentioned or communicated correctly. This kind of stuff happens all the time and experienced engineers are always looking out for it. It could very well be that someone has taken the precaution of supplying needed and required protection for a compressor. This is a very important aspect and I failed to mention it. I am reminded of the quote from John M. Campbell that I use in my workbook on vapor-liquid separators:

“Do not be naïve. Most published capacity curves are for ideal conditions. Use them cautiously. Consider a factor like pulsating flow from a well head or a two-phase line. The liquid enters in “heads”. During this time the gas rate is low. Between “heads” the gas rate is normal but little liquid enters. Using some average 24 hour gas and liquid rate to size the separator would be criminal, or at least unprofessional. There must be sufficient capacity to handle properly the instantaneous rates expected.”

Joerd is precisely on target. Challenge your professor; insist on having a complete description of the scope of work and explicit instructions on what is needed exactly. If a separator is being asked for, fine. But insist on having a detailed reason for its inclusion. Without a good reason, you can’t design accordingly. North Sea natural gas coming onshore has to have land separators that protect land facilities from the liquid slugs coming with the gas sporadically or without a good reason. Sometimes low spots in the subsea pipeline allow for the accumulation of liquid and subsequently this liquid is “slugged” downstream. This is why the use of slug catchers is so critical in the natural gas industry. The design of a slug catcher is totally different from that of a vapor-liquid separator, but Joerd’s point should be well taken. Always challenge when you suspect that all of the scope of work is not being communicated correctly. There might well be a valid reason for using a separator.

This is such an important engineering point, and it isn’t taught in universities – nor is it written in books. Yet, it will hit you directly straight on the very first days you spend as a professional engineer. If this point is understood by those students reading this thread, then it will have more than justified the value of the entire thread because it is a part of the practical learning experience of an engineer.

Thank you for your replys Art and Joerd

I asked my professor. And you were right Joerd. The separator is there to handle liquids and small slugs if they should appear. But he was reluctant to say anything more.
So I will use the same liquids that I got from the second separator and use for the first one and calculate the diameter.

Thank you again for the help, and I will look for Art's woorkbook.

KurtO.

[fantomus]

Hey all,

I read your posts and I was hoping you can help me with sizing of slug catcher. I have information about the compositions of the gas, the pipeline diameter, pressure, temp etc. I need to simulate flow lines using Hysys and then calculate how much slug is being formed so I can decide a size for a slug catcher if its required.