13 replies to this topic

[Matteo Giorgio Marrano]

Goodmorning,
in a new work i have to follow the design of a dehydrator/desalter pakage.

Our client gave us the PFD of the plant: the layout of the equipments is slightly different from other desalter package that i've seen.

The pacakge is divided in
1)2nd stage separator
3)dehydrator vessel
3)3rd stage separator
2)desalter vessel

The 1st stage separator, not present in the pfd, is out of the battery limit.

The oil is mixed with water and goes in the 2nd separator in wich the liquid phase (approx 300000 kg/h) is separed from the vapour phase (9000 kg/h).
The liquid then passes by gravity to the dehydrator vessel and then goes to another separator (3rd) in wich the liquid phase is separed another time from the vapour phase.
The liquid coming from the bottom of the second separator goes by gravity to the desalter vessel an then exit from the system.

You will find the pfd that i drawned yesterday.

My question is this:
As i understand, the scope of the two separator vessels is to separe the vapour phase from the liquid one. I think that another function is to give to the dehydrator/desalter a constant feed flowrate, right?
For that reason, do i have to design that separators considering a specific time of residence?

In my mind, i think that the vessel should be horizontal, with the liquid height at approx half of the vessel, and with some devices that contributes to separate the vapour phase from the liquid one like deflectors on the inlet and demisters on the outlet of the vapour..

Is it right?

edit: i've found this guide on the web, i think it will be usefull..do you think so?

http://www.red-bag.c...r-vessel-sizing
thanks
Matteo

Attached Files

•  pfd.bmp   2.1MB   161 downloads

[Matteo Giorgio Marrano]

Goodmorning

Hi, during the afternoon i tried to size the vessel...this is what i calculated:

Input data:

-liquid flowrate: 410 m3/g (density 758.2 kg/m3)
-vapour flowrate: 649 m3/h (density 13.3 kg/m3)

Hypotesis:
-residence time: 10 minutes (i could choose from 5 to 15 minutes from the guide posted above, referring to feed to column, reactors, heaters)
-height of LLL: 0.2 m
-% of liquid in the vessel : 60%
-ratio L/D: 2.5

Calculation:
From my calculation this is what i found:

Holdup volume: 68 m3

DIAMETER: 3.86 m
LENGHT: 9.66 M

height of LLL:0.2m
height of HLL:2.3 m

max allowable vapour velocity: 0.6 m/s
velocity of vapour trough the cross sectional area of the vessel : 0.04 m/s


Do you think i made a good calculation?Are my assumptions good or not?
In detail, i don't know if the time of 10 minutes is ok or i have to choose a lower value due to the fact that i have to separe "only" 10000 kg of vapour from 300000 kg of liquid.
Maybe the esperience gives more info to decide..

thanks
Matteo

[Art Montemayor]

Matteo:

Your submitted PFD is terrible! I don't know who drew it but it is very difficult to read or interpret because of the horrible black background, the small fonts, lack of standard instrumentation IDs (what does "LDC" mean?), etc., etc.

Look at the Excel PFD I am submitting. I had to generate this drawing during my lunch hour and didn't have enough time to include all the instrumentation and valving you show. However, since it is only a PFD, it will suffice.

My most important comment is that the flow shown is not logical. Note that when I depict the relative liquid levels in different colors and level locations, that the need for a third separator is not logical. There is NO GAS TO SEPARATE in the third separator. In order to generate and liberate any dissolved gas contained in the crude exiting the decantation vessel, you would need a heater. You don't show one, so there is no gas.

If you want, you can include your instrumentation and other details on this Excel drawing, but even if I do say so myself - my sketch is a lot more readable and it doesn't give you a headache.

Please supply temperatures, pressures and heat loads on this PFD. Then we can discuss in detail - and even revise and/or add to it.

You should learn to exploit all of Excel's engineering features in order to round out your skills and abilities. (Compare the number of kilobytes in your file and in mine!).

 Matteo__s_Crude_Oil_Separation.xls   42KB   197 downloads

[Matteo Giorgio Marrano]

Your submitted PFD is terrible! I don't know who drew it but it is very difficult to read or interpret because of the horrible black background, the small fonts, lack of standard instrumentation IDs (what does "LDC" mean?), etc., etc.

Hi art, glad to hear you...
I made this pfd copying the one that our client submitted (a big oil company in dubai)...It is drawn in autocad and all PFD and PID that i've seen here in italy is drawn like the one i build.
Tomorrow i will send you the pdf that our client sent us, you will see that is equal to mine...

Look at the Excel PFD I am submitting. I had to generate this drawing during my lunch hour and didn't have enough time to include all the instrumentation and valving you show. However, since it is only a PFD, it will suffice.

Here we make all the pfd's and PID with autocad..Obviusly i made the mistake doing the "rude" screenshot from my pc. The purpose of that pfd is to print them in A3-A2-A1 sheets, that's why it is small...
Every colour line will generate different thickness of the line during the printing, that's why i use different colour for process lines,instrument lines etc..

My most important comment is that the flow shown is not logical.

This is what you will find where i will send you (privatly of course) the pdf submitted to us; there is also a 25 pages detailed mass balance...

There is NO GAS TO SEPARATE in the third separator. In order to generate and liberate any dissolved gas contained in the crude exiting the decantation vessel, you would need a heater. You don't show one, so there is no gas.

If you want, you can include your instrumentation and other details on this Excel drawing, but even if I do say so myself - my sketch is a lot more readable and it doesn't give you a headache.

sorry!i have to post it differently..

Please supply temperatures, pressures and heat loads on this PFD. Then we can discuss in detail - and even revise and/or add to it.

yes, tomorrow morning i will do it..

You should learn to exploit all of Excel's engineering features in order to round out your skills and abilities. (Compare the number of kilobytes in your file and in mine!).

i don't like to use excel very much for drawings.."
For drawings we use autocad; for calculation sheet i use mathcad that is more "goodloking" that excel, specially to write formulas.
see you tomorrow

[Matteo Giorgio Marrano]

Goodmornig,
Art, this is a screenshot of the process description, as you can see the two separator are needed and their function is described. It is a jpeg...

As i told yesterday, there is also a 25 pages mass balance that confirm that also in the 3rd separator there is a flow (of 5000 kg/h) of vapour..

Attached you will find also the new pfd that i made this morning: it is a .pdf so you can print in A3 if it is too small in A4..
As you see the 3rd separator is at a pressure of 3.5 barg while the 2nd is at 12.8 barg, that's why there is the vapour also in the 3rd separator.

I think now it's cleaner, it's the same drawing but saved differently:)

ciao

Attached Files

•  process_description.JPG   203.9KB   64 downloads
•  pfd_matteo.pdf   174.17KB   122 downloads

[Matteo Giorgio Marrano]

Goodmornig,
Art, this is a screenshot of the process description, as you can see the two separator are needed and their function is described. It is a jpeg...

As i told yesterday, there is also a 25 pages mass balance that confirm that also in the 3rd separator there is a flow (of 5000 kg/h) of vapour..

Attached you will find also the new pfd that i made this morning: it is a .pdf so you can print in A3 if it is too small in A4..
As you see the 3rd separator is at a pressure of 3.5 barg while the 2nd is at 12.8 barg, that's why there is the vapour also in the 3rd separator.

I think now it's cleaner, it's the same drawing but saved differently:)

ciao

Any comments?Art?

[Art Montemayor]

Matteo:

I don't communicate on these Forums in .pdf or in Cad format because I can't expect other engineers to have these or any other special program to read the files. Also, USA professional engineers never work in Cad. For Cad drawings we have professional designers who specialize in that. You don't require an engineering degree to work in Cad. That's why I don't have need for Cad.

I cannot markup or make calculations on a .pdf format. It wasn't meant for that.

That is why being "Catholic" (universal), the excel spreadsheet is more readily practical, fast, and available to everyone with a PC - even an Apple.

Your description of the PFD is not correct. Vessel 21-V3 is not a dehydrator. It is another Desalter and is in series with the 2nd Desalter, 21-V5. Now that you show all the rest of the process with the temperatures and pressures I can see what is being attempted. You are merely expanding down, getting rid of the associated gas through adiabatic expansion in 3 steps. However, why is it that there is no heating of the crude? The Desalter works best with hot crude - and so does the flashing off of the associated gas. Who is the engineering designer for this process? Has a simulation run been done to show that heatup of the crude is not needed?

[djack77494]

Your description of the PFD is not correct. Vessel 21-V3 is not a dehydrator. It is another Desalter and is in series with the 2nd Desalter, 21-V5. .... However, why is it that there is no heating of the crude?

Matteo,
I concur with Art's assessment. If 21-V3 were intended to remove residual moisture from the crude, why would you be adding water upstream? It is, as Art says, the first of two stages of desalting. Your configuration is quite unusual with unexpected separators in the middle of your desalting operation. And, again as Art has noted, your temperatures are too low for effective desalting. Where is this facility? Surely it is not in a crude distillation unit, so I'm assuming it is located at a production facility. Please obtain more complete information about the intent of this facility so that we might better help you in realizing that intent.

[Matteo Giorgio Marrano]

Matteo,
I concur with Art's assessment. If 21-V3 were intended to remove residual moisture from the crude, why would you be adding water upstream?

i don't know guys,the description is taken from the design basis of the project...It is an upgrade on an existing facility.

Where is this facility?

it is an oil extraction site in Iraq, in nahar bin Ummar Oil field.

Surely it is not in a crude distillation unit, so I'm assuming it is located at a production facility. Please obtain more complete information about the intent of this facility so that we might better help you in realizing that intent.

tomorrow i will give you more details (now it's 22.43 in the evening)..

Art i will reply to you tomorrow..

[Matteo Giorgio Marrano]

I don't communicate on these Forums in .pdf or in Cad format because I can't expect other engineers to have these or any other special program to read the files. Also, USA professional engineers never work in Cad.

really?here in italy we used autocad also during the university, in example to develop pfd's, P&I's and drawing equipment's sketch...

For Cad drawings we have professional designers who specialize in that. You don't require an engineering degree to work in Cad. That's why I don't have need for Cad.

really strange!When i worked in ENI i worked also whit UOP (american), ALBEMARLE, and other international companies. When there where a new project, like the new Isomerization by UOP (i worked in Venice Refinery) UOP gave us many pfd made by autocad in pdf format...We spoke with UOP trough these PDF and autocad files...

That is why being "Catholic" (universal), the excel spreadsheet is more readily practical, fast, and available to everyone with a PC - even an Apple.

i repeat, it sound's very strange for me the fact taht in use you don't use autocad...

Your description of the PFD is not correct. Vessel 21-V3 is not a dehydrator. It is another Desalter and is in series with the 2nd Desalter, 21-V5.

as i told before, the description is taken from design basis and the pfd is the one given by our client--

Now that you show all the rest of the process with the temperatures and pressures I can see what is being attempted.

Who is the engineering designer for this process? Has a simulation run been done to show that heatup of the crude is not needed?

tomorrow i will give you details....
I have a simulation of that process made with HYSYS,including the mass and heat balance; it's 25 pages long so i think that they do their work...If you want i can send it to you..

see you tomorrow for more details..

[djack77494]

Matteo,
I want to offer a few quick comments here. For all the LARGE engineering companies I have worked for, the company neither expected nor wanted their engineers to work with Autocad. Generally all the transmitted drawings were produced using Autocad, but the engineers could merely view and print the drawings, not edit them. (There are several programs available for these purposes. They allow one to open, view, and print, and nothing else.) This is like a pdf file. We use pdf files for archival purposes, but we do not try to edit them. When engineers feel compelled to produce a sketch, they can do so the old fashioned way, using pencil and paper, or (in my situation) we can use the less capable Visio software, or we can use Excel. I think Art's point is that Excel is to be preferred since it is the closest thing we have to a universal and editable software that is still capable enough to do the job.

Your design basis must be in error. The first step of an oil dehydration operation would not be to disperse water through your oil. Please respond to this assertation if you wish to dispute it. The concept simply defies logic. I still strongly contend that this must be a Desalter.

[Matteo Giorgio Marrano]

Matteo,
I want to offer a few quick comments here. For all the LARGE engineering companies I have worked for, the company neither expected nor wanted their engineers to work with Autocad.

maybe it is a typical italian way, as also in university we have to work with autocad.

Generally all the transmitted drawings were produced using Autocad, but the engineers could merely view and print the drawings, not edit them.

depending on where you work:
1)if you work in a refinery it doesn't matter
2)if you work for an engineering company that BUILD and CONSTRUCT all the items and piping etc, i think an engineer should know how to build a pfd and a p&i, this is my modest point of view...
I work as a project engineer even if i'm a chemical engineer..

When engineers feel compelled to produce a sketch, they can do so the old fashioned way, using pencil and paper, or (in my situation) we can use the less capable Visio software, or we can use Excel.

i use for that autocad, maybe this is a different way to work..

I think Art's point is that Excel is to be preferred since it is the closest thing we have to a universal and editable software that is still capable enough to do the job.

this is clear and i completely agree with you, i only think that autocad is better to buildup pfd and drawings...

Your design basis must be in error. The first step of an oil dehydration operation would not be to disperse water through your oil. Please respond to this assertation if you wish to dispute it. The concept simply defies logic. I still strongly contend that this must be a Desalter.

the jpeg that i posted was taken from the basic engineering given by the iraq company, have they written wrong thing?maybe with "dehydrator" they mean "first stage desalter", this is the first thing that i thought. In fact, most of desalters that i've seen are two stages, in this project there is that strange thing of the separators between the two stages...

If you want i can send you the basis engineering and the 25 pages mass balance, in order to comprehend if it is only a tranlation mistake or the iraq-men have done a well done work..

if you have any question i'm here, i can send also the original pfd...

matteo

[djack77494]

2)if you work for an engineering company that BUILD and CONSTRUCT all the items and piping etc, i think an engineer should know how to build a pfd and a p&i, this is my modest point of view...

I DO work for an engineering company that Designs, Builds, and Constructs. Also, I am intimately familiar with PFD's and P&ID's; in fact I've probably spent 25 work years working mainly on just these drawings. I certainly hope that I know how to build a PFD and a P&ID. All of the PFD's and P&ID's that I work with are generated using AutoCAD. So I think we must have some errors in communication. My only point is that as an engineer working for my company, I am expected to use red, green, blue, and yellow pencils to communicate what modifications I want done on these drawings. My designer then takes my pencil mark-ups and uses AutoCAD to make these changes on the "official" drawings, which I am asked to let him do.

[pezhman]

hi everybody
yes its true the vessel is dehydrator because of the high water
cut (maybe) in feed stream they forced to use electrostatic field
the reason for injacting dilution water before first stage is to
reduce salt content in addition of dehydration and increasing
the efficiency. point that dehydartion and desalting are two
diffrent way desalting is use for salt content and dehydrator
is for water cut .but the temp is too low thay can use electrostatic
fierd heater treater instead of using this vessel or use the new technology
of aibel viec (vessel internal electrostatic coalescer www.aibel.com) company that can use internal of separator with low temp.