12 replies to this topic

[Root]

Hi,

Can any one shed some light on Reboiler out let temp control (COT) by PDIC instead of TIC. Reboiler put let is two phase flow and product IBP is 100^0C and FBP 170^0C.
More over the orifice is used an eccentric orifice and hole in bottom side and flow in line is two phase.
To view the sketch please open attachment.
Thanks
Toor

Attached Files

•  Naph.xlsx   12.99KB   39 downloads

Edited by Toor, 05 July 2010 - 08:19 AM.

[Zauberberg]

Toor,

It would be really good if you can post a sketch/P&ID of this system, including all the control loops. The way you have posted your question doesn't give the minimum level of details required for accurate answer - we can only guess.

[Art Montemayor]

Toor:

Can you make our life more pleasant and tranquil by being specific and using fully identified nomenclature? For example,

Why is “out let temp control” = COT and not TOC (Temperature Outlet Control)?

Is “PDIC” = Differential Pressure Indicating Controller?

Is “Reboiler put let” = Reboiler outlet?

Is “product IBP” = product Indicated Boiling Point?

Is “FBP” = Final Boiling Point?

Instead of writing “100^0C”, why don’t you use our engineering editor and use 100 ^oC? It is so much clearer.

Do you mean to say: “Moreover, the orifice used is an eccentric orifice.”?

If you have to use acronyms, then please identify or define them. Different countries as well as different languages and cultures have different meanings and different ways to define engineering acronyms. Simply be specific. And there is nothing more internationally specific than a detailed, accurate engineering drawing or P&ID.

[Zauberberg]

The best answer I can give without knowing any details - just a blind guess - is that the PDIC controller in the reboiler return line is there to prevent excessive pressure drop (= excessive heat input) which causes liquid level inside the tower to rise, because higher driving force is required to overcome pressur drop. If the level increases above a certain limit, flooding will occur.

[djack77494]

Toor,
I'm making somewhat different assumptions from Zauberberg or Art, so my approach is quite different. I assume COT means Coil Outlet Temperature. That is often an important operating parameter when dealing with a multipath fired heater, so I assume your Reboiler fits that catagory. IBP and FBP should be Initial and Final Boiling Points. What are you separating? That could be useful to know. How many parallel paths are there throug the reboiler? Anyway, I do not see how this system might work. It tends to be wildly optimistic to think you can measure a two phase flowrate, especially at the outlet of a fired heater. I don't care what any vendors may say, this is a situation where I'd have to see it to believe it. And you're doing it with an eccentric orifice and differential pressure??? Wow, all I can say is "No Way". There are just so many ways this can go wrong. If you've successfully applied this technology, then please let me know the specifics, because I'd sure like to know how it's done.
Doug

Edited by djack77494, 06 July 2010 - 04:45 PM.

[Root]

Doug,

I have operated two plant, one in Saudi Arabia and another in Qatar. UOP is using this control philosophy and in their operating manual they mentioned, where delta "T" is less in reboiler outlet there they are using this technology.
In my assumption, maybe they are using where, vaporization is more (may 80%). my reboiler is four pass and collected in one point and eccentric orifice is installed almost 8 meter away from common outlet. I'm confused why they used eccentric orifice while there is no sallery type fluid in this service.
Toor

[djack77494]

Toor, you have not provided any additional useful information, so I really cannot add anything to my previous comments. I'd suggest you supply a sketch showing the system in more detail. I am not feeling real comfortable with some of the parameters, but I've worked before with UOP and feel they tend to be top-notch in this stuff.

[Root]

Dear Doug,
Please open attached file and view Splitter PFD.
Thanks
Toor

Attached Files

•  Over Flash flow.xlsx   237.14KB   57 downloads

[iyer]

Toor

Hey, Listen I have some doubts on this arrangement. Now Since it is a fired heater, I would like to know if it a natural draft or a balanced draft heater. In case it is a balanced draft heater, then there ought to be an air to fuel ratio controller, which would control the fuel gas flow etc. etc. However in order to prevent the flame from extinguishing, there is normally a pressure indicator downstream of the fuel gas control valve. A high signal selector is installed between the fuel gas flow need and this pressure, in fact you may reach a point where the Coil outlet temperature cannot be reduced due to this low pressure condition existing and u need to cut a burner. Hence I do not think the PDIC (Differential pressure indicator and controller) as indicated by the schematic will work for a balanced draft heater.
Now in the case of a natural draft, my question would be what happens if the fuel gas density were to change. Basically the schematic shows that there is no importance to coil outlet temperature, which in my opinion kills the purpose of the heater.
My thought is there is something wrong with the schematic, I would also like to know if the heater is running on manual or auto/cascade as designed.
Again these are my thoughts and I may be wrong. As you said you have direct experience with this arrangement.

Edited by iyer, 10 July 2010 - 10:50 PM.

[Zauberberg]

It's the first time I see such temperature/heat input control scheme and probably the best idea is to discuss this subject and ask for more information from UOP.

[Root]

iyer,

Firstly, you must show respect and don't mention "hey!!!". 2nd this scheme is perfectly working and this is my 2nd plant operation in middle east with this control scheme.

Now, I will come your questions. this is a natural draft heater and temperature is being controlled through PDIC and TI they give for APC control. Yes, we are running this scheme auto cascade control and TI have no role to control the temperature. Moreover, in UOP available literature, they mentioned where delta T is small there, they are applying this control scheme.

Last, I'm not checking the skill or knowledge but I want to know why they applied this scheme here and I have seen more operation where delt T is very very small or some time we need to bypass heaters also.

Thanks
Toor

[djack77494]

Toor,
Your sketches and the full discussion shed much light on your situation. It makes much more sense to me now (though I am still speculating on how the system will work). My suspicion is that the Naphtha Splitter bottoms have a fairly narrow boiling range, or some other characteristic that would make the logical choice (temperature) not well suited for use as the control variable. If you think some about this, temperature will not be a "robust" control variable - one that is or closely mirrors what you want to control and has a large change in value as you move away from the desired control range. Perhaps the designers said that a more useful indicator of column performance could be derived from knowing how much of the reboiler pumparound is actually vaporized. Measure and control that, and just use the heater outlet temperature as an interesting variable to monitor. Continuing this line of thought, it's pretty difficult to measure two phase flow as I alluded to earlier. What to do? Well, how about using pressure differential as a substitute parameter. Not perfect, but probably not too bad either.

Well, my guess might be right on (like I hope) or it might be "out in left field" (please indulge my American colloquialism). The only sure way to know is to ask UOP how the system is intended to operate. Good luck in coming to peace with this issue.

[Zauberberg]

I agree, things look much more clear now although the PDC-type of control seems to be quite exotic for column reboilers - at least from my experience. If the boiling range is quite narrow (that shouldn't be expected for a naphtha splitter, though - we usually separate C6+ components in the bottom from C5- components in the overheads), controlling temperature will not result in having stable process. On the other hand, employing PDC-type of control probably relates to the quanity of vapor in the reboiler discharge line (and thus the DP across orifice), as we normally want to control the quantity of vapor generated by the reboiler.

However, I wonder how this thing works at different column throughputs because different feed rates call for different duties (assuming all other things being equal) and therefore different pressure drops across the orifice, or putting it this way - different set point values on the PDIC controller.