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Nitrogen Purging.


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#1 muzziman

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Posted 29 October 2009 - 11:50 PM

Hi, I am working on a plant design project at my university. I am supposed to design a nitrogen purge system. I haven't been able to find much literature on it. Are their any useful resources that I may be able to use?

The nitrogen purge system is for a de-propanizer system (distillation facility). The things I am unsure about are:
- whether to use compressed N2 gas or liquefied N2. The plant site I am currently studying does have enough space to have a compressed N2 gas storage vessel but it would add to the total capital cost. While buying liquefied N2 (cryogenic) is relatively expensive. How is it generally done in the industry?

- the setup i.e. what would be a suitable way to distribute it in the system? i.e. having a single pipeline connecting it to the main feed line or having multiple N2 feed lines at various locations in the system
- suitable pressure to feed N2 gas at into the system other info will also be helpful. According to an author, a suitable pressure to carry out the purging is approx. 70 psig.

- what precautions can be undertaken to avoid reverse flow of N2?
Positive inputs would be appreciated.

#2 Qalander (Chem)

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Posted 30 October 2009 - 03:32 AM

Dear muzziman Hello/Good Afternoon,
I have witnessed at our previous(employers) Petroleum refinery;was

Nitrigen bottles(large sized bullets) for temporary storage at site.

Small sized compressors were empoyed to maintain system pressure(s) as per needs.

Liquid Nitrogen was shipped via tank trucks by local vendors and a local VIE(Vacuume Insulated Evaporator) was employed to convert into gaseous state.

Term Purging is most usually 'meant'& 'considered' for gases only;i.e. inert gas,steam, nitrogen or air purging.

For Liquids I understand"flushing' is the Term employed.

Nitrogen source could be Crygenic distillation of air or some other methodes commercially viable and feasible for the quantums involved.

Hope thses are useful in showing you with a way forward.



#3 muzziman

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Posted 30 October 2009 - 11:14 PM

Thanks for your reply. Some questions that come to my mind:

Would it not rather be efficient to purchase large scale cylinders (that can withstand high pressures) to store liquid nitrogen on site. From what I understand regarding your explanation, you used to receive liquid nitrogen and then store it on site by converting it into gaseous phase using a compressor. Isn't this adding to the cost a lot?

Also, doesn't liquid nitrogen convert to gas once it is exposed to standard temp and pressure conditions? If so, why use a VIE to convert it to gaseous state? Its sort of confusing. If you could clear these things up, I would appreciate it.

Thanks.


Dear muzziman Hello/Good Afternoon,
I have witnessed at our previous(employers) Petroleum refinery;was

Nitrigen bottles(large sized bullets) for temporary storage at site.

Small sized compressors were empoyed to maintain system pressure(s) as per needs.

Liquid Nitrogen was shipped via tank trucks by local vendors and a local VIE(Vacuume Insulated Evaporator) was employed to convert into gaseous state.

Term Purging is most usually 'meant'& 'considered' for gases only;i.e. inert gas,steam, nitrogen or air purging.

For Liquids I understand"flushing' is the Term employed.

Nitrogen source could be Crygenic distillation of air or some other methodes commercially viable and feasible for the quantums involved.

Hope thses are useful in showing you with a way forward.



#4 Qalander (Chem)

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Posted 31 October 2009 - 10:16 AM

Thanks for your reply. Some questions that come to my mind:

Would it not rather be efficient to purchase large scale cylinders (that can withstand high pressures) to store liquid nitrogen on site. From what I understand regarding your explanation, you used to receive liquid nitrogen and then store it on site by converting it into gaseous phase using a compressor. Isn't this adding to the cost a lot?

Also, doesn't liquid nitrogen convert to gas once it is exposed to standard temp and pressure conditions? If so, why use a VIE to convert it to gaseous state? Its sort of confusing. If you could clear these things up, I would appreciate it.

Thanks.



Dear muzziman Hello/Good Afternoon,
I have witnessed at our previous(employers) Petroleum refinery;was

Nitrigen bottles(large sized bullets) for temporary storage at site.

Small sized compressors were empoyed to maintain system pressure(s) as per needs.

Liquid Nitrogen was shipped via tank trucks by local vendors and a local VIE(Vacuume Insulated Evaporator) was employed to convert into gaseous state.

Term Purging is most usually 'meant'& 'considered' for gases only;i.e. inert gas,steam, nitrogen or air purging.

For Liquids I understand"flushing' is the Term employed.

Nitrogen source could be Crygenic distillation of air or some other methodes commercially viable and feasible for the quantums involved.

Hope thses are useful in showing you with a way forward.


Dear

The Cylindrical bullets indicated in my above post, are only high pressure cylindrical shaped vessels.But these are placed on proper civil foundation(saddles) and duly clamped.

As regards any high pressure cylinder storage of gases there are numerous hazards associated which are and Must be taken care-off.

#5 muzziman

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Posted 05 November 2009 - 10:20 PM

Dear Qalander,
Thank you for your input. I think you are right about using compressed N2 gas vs liquefied as liquid N2 seems to cost a lot more and has hazards associated with it. If I were to use compressed nitrogen gas, is there any advantage of using multiple vessels vs. one big vessel? Also, correct me if I am wrong: the pressure the vessel is kept at is about 3000 psig and the temperature is kept around 70F right..?

Also, when buying nitrogen, the cost varies based on its purity. I would be using it to purge an LPG->Propane conversion plant. Is there a way to determine how pure the N2 should be? Because, if I don't need 99.9% pure N2, I may save on operational cost.

If you have any further advice for me regarding the design of the N2 system, that would also be appreciated. Thanks.



Thanks for your reply. Some questions that come to my mind:

Would it not rather be efficient to purchase large scale cylinders (that can withstand high pressures) to store liquid nitrogen on site. From what I understand regarding your explanation, you used to receive liquid nitrogen and then store it on site by converting it into gaseous phase using a compressor. Isn't this adding to the cost a lot?

Also, doesn't liquid nitrogen convert to gas once it is exposed to standard temp and pressure conditions? If so, why use a VIE to convert it to gaseous state? Its sort of confusing. If you could clear these things up, I would appreciate it.

Thanks.



Dear muzziman Hello/Good Afternoon,
I have witnessed at our previous(employers) Petroleum refinery;was

Nitrigen bottles(large sized bullets) for temporary storage at site.

Small sized compressors were empoyed to maintain system pressure(s) as per needs.

Liquid Nitrogen was shipped via tank trucks by local vendors and a local VIE(Vacuume Insulated Evaporator) was employed to convert into gaseous state.

Term Purging is most usually 'meant'& 'considered' for gases only;i.e. inert gas,steam, nitrogen or air purging.

For Liquids I understand"flushing' is the Term employed.

Nitrogen source could be Crygenic distillation of air or some other methodes commercially viable and feasible for the quantums involved.

Hope thses are useful in showing you with a way forward.


Dear

The Cylindrical bullets indicated in my above post, are only high pressure cylindrical shaped vessels.But these are placed on proper civil foundation(saddles) and duly clamped.

As regards any high pressure cylinder storage of gases there are numerous hazards associated which are and Must be taken care-off.


Edited by muzziman, 06 November 2009 - 12:56 AM.


#6 Qalander (Chem)

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Posted 09 November 2009 - 06:58 AM

Dear
The concept of "purging and its extent" varies probably

industry to industry and

application to application; therefore a general guideline is very difficult as such.

However what we saw/practiced in relation to hydrogen and light hydrocarbons containing circuits vacuum used to be pulled on the under purging system for gas-freeing;and broken with nitrogen gas& system pressurized up to the requisite benchmark, maintained for certain period,again vacuum used to be pulled,broken with nitrogen gas& system pressurized up to the requisite benchmark, maintained for certain period.

This was repeated at the least three times; G.C.(Gas chromatography) done to establish the purged out or further purging needs, occasionally problems may be faced.

Such problems could arise from lack of adequate isolation(s) i.e. not with blind plate(s),spades;where mandated.

Hope this is going to be helpful in deciding the way forward,since specifically what your case is may or may not mandatorily need nitrogen purge exactly as described above.

In certain plant(s) if moisture and/or high temperature exposure is not an issue steam(purging) followed by nitrogen purging is considered as low cost resolution.

Still one has to be extremely cautious and analytical considering all merits or de-merits of any system considered to be applicable.

#7 VikingUK

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Posted 18 November 2009 - 07:28 AM

Hi, I am working on a plant design project at my university. I am supposed to design a nitrogen purge system. I haven't been able to find much literature on it. Are their any useful resources that I may be able to use?

The nitrogen purge system is for a de-propanizer system (distillation facility). The things I am unsure about are:
- whether to use compressed N2 gas or liquefied N2. The plant site I am currently studying does have enough space to have a compressed N2 gas storage vessel but it would add to the total capital cost. While buying liquefied N2 (cryogenic) is relatively expensive. How is it generally done in the industry?

- the setup i.e. what would be a suitable way to distribute it in the system? i.e. having a single pipeline connecting it to the main feed line or having multiple N2 feed lines at various locations in the system
- suitable pressure to feed N2 gas at into the system other info will also be helpful. According to an author, a suitable pressure to carry out the purging is approx. 70 psig.

- what precautions can be undertaken to avoid reverse flow of N2?
Positive inputs would be appreciated.


You won't find any guidelines on N2 purging because it varies from plant to plant depending on size and type.
A nitrogen generator (simple forced membrane) is a good way to provide purge gas which is only needed at low pressures typically <5 barg.
The N2 needs to go in the systems at various points to ensure good flow. Non return valves should be fitted dependent but if you are fitting flow controllers these will usually have one built in. As for flow rates it really depends on size and equipment. 60 litres/hour is good enough for simple continuous purging eg a flare line.




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