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Propylene Storage Tank In Crude Oil Refinery


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#1 Afshin445

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Posted 18 October 2008 - 11:38 AM

Dear All,

I am working in basic design for Tankage area in crude oil refinery project.
We have three spherical propylene storage tanks for storage of propylene product, produced from FCC unit.
The liquid propylene from storage tanks routed to loading arms for loading by trucks and vessels.

In this regard, we have two way for storage of propylene in spherical tanks.

Case 1: First way is storage of propylene in high pressure and amb. temp. (The vapour pressure of propylne in 50 C (max. amb. temperature) is about 21 bara ). In this case, operating pressure should be about 24 bara (3 bar margin) and design pressure about 26.5 bara (10% overdesign).
In this case we can send propylene to flare via a PCV with set pressure of 24 bara, the PCV will
be open when pressure excess 24 bara and lower 24 bara is closed.

Case 2- The second way is using boiloff compressor and after cooler and valve for storage of propylne in temperature and pressure lower than case-1 in spherical tanks. In this case design pressure in spherical tank is lower than case-1 and we don't need tank area flare but process is more complex.

I don't know which case is practical and normally used in crude oil refineries contain FCC unit.

I appriciated if anyobody with same experince could advice me in this regard.

Best Regards



#2 bot

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Posted 07 November 2008 - 05:43 AM


Hey Afsheen,

Not an exact reply but I hope that the below would pose some help.
I would simply go with option number 2 i.e. installing a vapour recovery system for propylene.

Though, we don’t do Propylene but we do have propane system here on site & for which, we have a dedicated vapour recovery system.
The storage pressure for propane is around 3 - 4 barg (Tsat = 1°C pure), we normally have storage temp in the range of around -2 to 2°C. The ambient temperature remains below 15 round the year so we don’t have to press our vapour recovery system too hard. Sphere with volumes of around 2500 to 4000m³, doesn’t stay feasible to have design pressure around 30 bars. By the way, our spheres have design pressures around 4.1 bars – relieving into flare head once the pressure reaches Psets.

Coming back to your query, Our FCCU Off gas specifically C3/C4 (C3/C4 Olefins, propane, propylene 30-40%, butane, butylenes] is stored around 4.0bars [Tsat =7°C] with temp range around 5-15°C. Before it enters Alky, its pressure is boosted up to 27bars via high differential head pumps.

I wonder what storage volume you are looking at – but do consult with a mechanical engineer before choosing a way forward.

regards,
bot.



#3 fallah

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Posted 07 November 2008 - 08:20 AM


Dear Afshin,

I have not any experience in propylene storage,but:

As may you know, the process similar to case 2 (storage in Atmospheric saturated conditions e.g. for propane at around 1 bara and -42C,recovery of boil-off gas by dedicated compressors and finally returning the recovered gas to storage tanks) now is working in many oil&gas plants for Propane storage.

Regards

#4 Art Montemayor

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Posted 07 November 2008 - 08:21 AM


Afshin:

I believe that you have not told us exactly what you are proposing. You state: "We have three spherical propylene storage tanks for storage of propylene product, produced from FCC unit."

I believe what you really have is a PROPOSAL to install 3 spherical storage tanks for Propylene and you are assigned to determine the optimum storage conditions that can be achieved successfully. Am I correct? If not, then please explain how you can have existing storage tanks that have already been fabricated and are being looked at as being applied at different pressures and temperatures. Storage tanks are not fabricated first and then subjected to different process conditions. The process conditions are SET and FROZEN first, and then the tanks are fabricated to meet those design conditions.

You also fail to tell us the SIZE of the spheres. This is very important - just as the design conditions and this size is linked to the process design conditions throught an OPTIMIZATON PROCESS.

Saturated Propylene can be stored at a variety of process conditions. The best and most recommended condtions are the optimum conditions. And YOU are supposed to find out what those conditions are taking into consideration what every engineer must evaluate:

Your site process requirements;
Your location's ability to fabricate the size of storage vessels you require;
Your local cost of energy;
Your local Cost of Capital;
Your local cost of operating such facilities;
Your local requirement of minimum Return on Investment.

There are other conditions that management can impose on your optimization - but only you can find out what those are. It is a routine situation that every engineer lives with from day-to-day: try to design the "optimum" process for your environment. You are making erroneous assertions when you state that you don't need a flare when you store Propylene at a lower, refrigerated pressures.


bot is wrong in recommending any type of storage system without first requiring ALL the basic data and then analyzing all your alternatives as well as all your constraints. In short, no one besides you or your local colleagues knows the exact optimization requirements for your site. You are the sole judge of that.







#5 bot

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Posted 07 November 2008 - 08:32 PM

QUOTE (Art Montemayor @ Nov 7 2008, 09:21 AM) <{POST_SNAPBACK}>
bot is wrong in recommending any type of storage system without first requiring ALL the basic data and then analyzing all your alternatives as well as all your constraints. In short, no one besides you or your local colleagues knows the exact optimization requirements for your site. You are the sole judge of that.blink.gif

Art,
I believe you are referring above to this. unsure.gif


QUOTE
Sphere with volumes of around 2500 to 4000m³, doesn’t stay feasible to have design pressure around 30 bars.


Art,
I will run a quick feasibility crack down on this for different regions of the world. Spheres with Design pressure around 30 bars sure look unrealistic (& expensive too).


#6 Afshin445

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Posted 08 November 2008 - 08:26 AM


Dear Bot,

Thank you for your good information. I think the main difference between our site is Maximum Amb.
Temperaure, The Maximum Amb. Temperature in our site is about 50C but in your site is lower 15 C.
In other hand I don't understand you stored FCC off gas in Spherical Tanks or Low pressure Tank with Boil off recovery.

Regards,
Afshin

#7 Afshin445

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Posted 08 November 2008 - 08:45 AM

Dear Art,

Thank you for your complete answer. You right, the three spherical storage tanks has been selected in conceptual stage. We are in Baisc Design stage now.The capacity of each tank is about 4200 cubic meter with 20 m diameter. In this time, I don't have any idea about economical parameters in detail, but project is executed in Middle East, where energy cost is too low.

In other hand, if we don't use boil off recovery system the process will be very easier than if we use. In other refinery close to our project, the designer don't use Boil off recovery system for LPG storage tanks and case-1 has been chosen by process designer for those tanks

Regards,
Afshin

#8 Andrei

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Posted 05 December 2008 - 09:27 AM

Afshin,

According to the practice in the area I work the spheres were recommended to be designed in a certain way in the conceptual phase. You are called to confirm or not that recommendation during basic engineering. And your resolution still will not be final. Don't be afraid to recommend changes if you see fit. But be responsible, unnecessary changes will only increase the design costs and annoy your project manager. You can do changes till a contract with the manufacturer is stamped, in some cases even after that. As long as the equipment was not fabricated you can change, the paper will withstand a lot of changes.





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