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Ethylene Storage - Ldpe Plant


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#1 RicardoPT

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Posted 03 October 2020 - 06:33 AM

Hello, I'm a master chemical engineering student from Portugal and i'm taking a course that consists on doing a low density polyethylene factory engineering design ( flowsheet, mass and energy balance, etc) and i came across some questions, mainly on the ethylene storage:

 

1) Which tank should i choose to storage liquefied ethylene, taking into account that LDPE production is 400kt/year? I know that ethylene is mainly stored in three types of tanks: bullet cylindrical, sphere and fixed roof tank at atmospheric pressure, but which one is the most suitable and the easiest for such high capacity needs?

 

2) I'm assuming that the ethylene transportion to the factory is via pipeline from a close-by cracker, but which are the pressure and temperature of such transportation by pipeline? 

 

3) Should i do purification steps to the ethylene? I know that for polymerization, the ethylene specifications are very tight, like for example 2ppm of water and 5 ppm of oxygen, but should i assume that the ethylene from the foreign cracker fulfill this specifications, or should i add some purification steps, and which?

 

 

Thank you.


Edited by RicardoPT, 03 October 2020 - 06:33 AM.


#2 Bobby Strain

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Posted 03 October 2020 - 08:54 AM

Too bad that you must waste your valuable time with such an effort. This sort of thing does not belong in the ChE academic program.
Since it is all academic, you can assume that you need no ethylene storage. The cracker will provide an uninterruptible supply via pipeline. And assume a "black box" purifier.

 

Good luck.

 

Bobby



#3 Francesco_Chem_Eng

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Posted 29 October 2020 - 09:11 AM

Hi,

 

the problem is typically found in petrochemical complex design when dealing with intermediate storages. Assume that the cracker has an availability of 8000 hour per year while the LDPE plant has a higher availability of 8200 h/y. It is reasonable that you want the cracker shutdown does not imply to cease production in the downstream LDPE. The intermediate storage of ethylene is then for 8 days. Say one week equal to 8,200 tonnes.

 

The storage type is linked to safety considerations, plot space availability and CAPEX vs OPEX. Full containment refrigerated tanks and mounded bullet are both adequate solutions for safer design. The trade off between CAPEX and OPEX should be evaluated to select the most convenient option. Storage tanks are the best option for high inventories. I would select a full containment tank for this hold-up.

 

Inside the steam cracker unit, the ethylene is usually purified upstream the cold section of the plant. In the cold section ethylene is separated from the other compounds like hydrogen, methane, ethane, etc. Ethane-Ethylene splitter is usually designed to produce polymer grade ethylene. You can assume ethylene is already on spec for the downstream polymerisation unit.

 

Regards,

Francesco






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