10 replies to this topic

[Elshan]

Hi,

 

I've been working on my school project for the past week or so and I can't figure out my problem with the Vacuum Distillation Column. It is to generate separation of Cold Lake, Alberta crude oil. I need to separate Naphtha, LGO, HGO, and Vacuum Residue. Also, the Vacuum Residue should be at around 360 degC. 

 

I simulated CDU and it worked fine. But I can't converge my VDU unit. And I don't know where I'm going wrong.

 

List of unsure points:

1) The stripping stream T and P

2) VDU unit specifications (Temp, Pressure, number of trays,...)

3) The Pump Around system (Not sure if I've done it right)

4) Design Specs for the column (which ones I need)

 

I have already searched this site's and other sites' forum sections but no luck.

 

 

I have attached the file. Please take a look at it and let me know if you need any info.

 

Note: The VDU in question is T-100 column. I separated from other units so I can focus on its streams only.

 

Thank you.  

Attached Files

•  VDU - HYSYS.zip   308.72KB   192 downloads

[AlertO]

Hi Elshan

unfortunately, my Hysys version is older than yours. So i cann't open and see what you have filled into your model. However, i try to help you in general things.

stripping steam - using superheated steam which has slightly higher pressure than the column bottom, but has 10-15 degC lower than the column bottom.

For others, i think the main important input is to input the matching between the reasonable fluid cut temperature and pressure especially at flash zone separating HVGO and Vac residue.(this depends on you VDU feed).

[Pilesar]

I do not have Hysys loaded and cannot review your file.

   Simulating a vacuum column by using only the results of a topped crude simulation is tricky. For your CDU, you do not need much definition of the heavy cuts so you do not need to include many cutpoints in the heavy area. Those heavy components that you lumped together for the CDU simulation are exactly where you need more definition in the vacuum column. Make sure your vacuum column simulation has enough cut points where the products streams transition or your distillation will never converge.

   Do you have the product specs and product flow rates for your vacuum column? If so, then you first adequately define the product streams in your simulation. This sounds backwards, but then you combine all the product streams to form the feed stream! Do not worry if you do not match the top temperature of your vacuum column -- the important results are the quantity and specifications of your other product streams.

   A useful reference if you can find it: "Refinery Process Modeling" by Gerald Kaes (ISBN-10: 0967927404)

[Elshan]

Thank you both fro your replies. 

 

I apologize about not saying the version of HYSYS used.

 

I tried my best to take some screen shots. I have attached the ZIP file.

 

Meanwhile I'll work on it more based on your suggestions.

 

Also, I have that book Pilesar!  It's a very useful book.

 

Thank you again.  

Attached Files

•  VDU - HYSYS.zip   542.68KB   126 downloads

[PingPong]

I don't understand what you are doing. Using a vacuum column at 0.01 bar in overhead to separate naphtha from residue?W

 

What is the feed to your VDU ?

 

In the real world an atmospheric column (CDU) is used to obtain naphtha, kero LGO and HGO. The atmospheric residue (TBP cutpoint ~360 ^oC) from the atmospheric column bottoms is sent to a VDU to obtain some more HGO, and LVGO and HVGO. Remaining vacuum residue usually has a TBP cutpoint in the range of 560 - 590 ^oC, depending on the required spec for the HVGO (depends on what HVGO is used for).

 

What you are doing is very strange (to put it mildly).

[Elshan]

Oh sorry, I should have clarified this before. I have a DRU (Diluent recovery unit) to separate my diluent. For simplicity reasons, I chose Naphtha. So then I'm sending the atm. residue to the VDU, to separate the rest of the Naphtha from the condenser, LGO and HGO from the sides, and the heavy hydrocarbons (540+C) in the bottoms. So our project involves DRU and a VDU.

And about your concern about the pressures, are they too low? I found some reference files and they were using 0.02bar at the top, and 0.05bar at the bottom of the VDU.

Thank you

[PingPong]

Your drawing showed naphtha product in the VDU overhead, which obviously will never condense at 0.01 bar, but apparently that is not really naphtha but the heavy part of the diluent.

 

Figure 3.PNG gives assay data, but that is for the whole crude blend that is fed to the CDU.

What is the composition in pseudocomponents (or TBP curve) for the atmospheric resudue feed to the VDU?

[Elshan]

I attached BPC for the feed stream into the VDU. 

 

And about the pressures, I see your point now. That's why my temperature cut at the top of the column was higher. So I should clarify, the diluent used in our project is Naphtha. So I should increase the pressure? What pressure range would you recommend along the column if the products are suppose to be Naphtha, LGO, HGO and vacuum residue?

 

Also, would you recommend a separation "train" of VDUs?  So, use 2 or 3 VDU's to separate the components. 

 

Please let me know if you need more info. 

Attached Files

•  22.PNG   97.7KB   21 downloads

[PingPong]

I attached BPC for the feed stream into the VDU.

That curve in 22.PNG seems to be the same as the table in 3.PNG, which in my opinion must be the crude oil blend (feed to the CDU), not the atmospheric residue feed to the VDU.

 

Nobody in his sound mind would sent such feed to a VDU.

[Elshan]

K PingPong, are u here to help or insult? "Nobody in his sound mind"? I'm a student and so in the learning process. Without talking me down and telling me that I'm wrong, you can help me understand why the conditions don't make much sense. So please try and explain, and not question my intelligence. If you can't let go of your negativity, then please leave this forum and let others help.
Oh and by the way, I managed to converge the column today with my initial operating conditions. Good thing I didn't listen to you.

[PingPong]

I was not insulting you personally, but merely emphasizing that what you list must be the crude oil feed to the CDU, not the atmospheric residue feed to the VDU. Nobody would send a feed containing components with a boiling point starting at 34 ^oC.

 

That you manage to converge a simulation with the wrong feed and/or specs does not prove anything. A converged soluition is not by definition a correct one.

 

As you do not seem to be listening at all, I will not bother you anymore, and wish you good luck with whatever it is that you are doing.