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Low Pressure Cryogenic Air Separation Plant


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#1 jamesgrg

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Posted 15 April 2014 - 10:07 PM

Does any body have an idea about working parameters of low pressure cryogenic air separation plant. We have a ASP from universal boschi and we are not able to get the stable purity of LOX.How to get the satble purity? As it is low pressure ASP, we have 2 rotatory screw compressors with discharge pressure of 7 bar and gas bearing expansion turbine with brake. We are not able to get 30-35% oxygen in rich liquid beacause when tempreture decreases pressure also decreases and unable to maintain pressure at bottom column which should be 4 to 5 bar as per manufacturers specification. Any kind of information will be highly appreciable.

#2 Bodhisatya

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Posted 16 April 2014 - 12:34 AM

(a) A sketch of the entire Plant running and how Expansion turbine is configured (Whether it's Lachmann Cycle or Claude Cycle) will be appreciated.

 

(b) Why have you not been able to maintain the HP Column pressure?  Is the Pressure of HP Column increasing or decreasing?  Can you confirm PL purity that acts as reflux to LP Column?  Also, are you drawing more PL and sending less reflux to HP column?  This might lead to reduced washing down of Oxygen into RL.  Also how are measuring RL purity?  Do you have an analyser there ?

 

© What do you mean by stable purity?  What is the expected purity of LOX and how much are you getting?

 

Looking forward for your replies.

 

Bodhisatya.



#3 jamesgrg

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Posted 16 April 2014 - 09:04 AM

Air is compressed upto 7 bar and after purification, it goes to main heat exchanger. Where it is cooled against cold nitrogen stream and turbine outlet stream. Turbine outlet stream goes to atmosphere after heated up with process air. After heat exchanger air is throttle via expansion valve( air throttle valve). We have 2 expansion valve, through which rich liquid and poor nitrogen liquid (96% as per design) are feed on the LP column. It is double column with reboiler- condenser. We have oxygen percentage analyser to monitor the purity of rich liquid which is currently approx. 50% in HP column . As we try to further cool down to get the value of 30-35% O2 in rich liquid, pressure is dropping as temperature drops. Poor nitrogen reflux valve is almost close to give sufficient reflux to HP column. As per design, plant is capable to produce 99.6% LOX but still we are in 93-96%(varying) with 50-55% in rich liquid. During LOX mode, there is no nitrogen production. It has been observed that the column pressure are also fluctuating within the time interval.

#4 Art Montemayor

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Posted 16 April 2014 - 12:34 PM

Low pressure or high pressure air feed – it all comes down to the Linde double column operating at the same pressures, temperatures, and compositions as it was designed to work at.  The difference between high and low pressure air feed is equal to the refrigeration requirement being achieved in a different manner.  You can either use:

  1. Direct Joule-Thomson expansion;
  2. Reciprocating expansion engine;
  3. Turboexpander.

I am accustomed to operating the high pressure (bottom) column at 70 – 75 psig and running the crude liquid oxygen in the HP sump at approximately 40-45% O2 to obtain 99.6% LOX in the LP column.  These conditions may vary depending on the designer-fabricator of the process.  In this case, Boschi in Italy is the licensor of your process and Universal Boschi in New Delhi, India is the fabricator.  Please refer to the attached document.  If your unit has operated satisfactorily at initial startup and has now gone bad, this may be due to:

  1. You are operating it in the wrong manner;
  2. You have something wrong or damaged in the columns – such as collapsed trays, leaks, etc.

In my opinion your best path towards a resolution is to make a serious contact with the vendor (Universal-Boschi, in this case) and explain what has happened to your unit since it was left operating satisfactorily and request their valued recommendations and assistance.  I do not believe this Forum can help you as much as the designer/fabricator.

 

Agreeably, if you are located in Equatorial Guinea, you have an expensive problem if the designer/fabricator has to send someone out to your site.  But that is the nature of this type of proprietary designed equipment.  Only the original designer/fabricator has the real, correct information and background that can resolve the situation.  Besides, I assume your unit is under some warranty of performance and operation.

Attached File  Low Pressure Air Separation Plants - Universal Boschi.pdf   666.25KB   54 downloads



#5 jamesgrg

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Posted 16 April 2014 - 12:56 PM

Thank you Mr. Art for your valuable suggestion.

#6 Bodhisatya

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Posted 17 April 2014 - 02:02 AM

 We have oxygen percentage analyser to monitor the purity of rich liquid which is currently approx. 50% in HP column . As we try to further cool down to get the value of 30-35% O2 in rich liquid, pressure is dropping as temperature drops. Poor nitrogen reflux valve is almost close to give sufficient reflux to HP column. 

 

 (A) Are you saying that RL is now having a Composition of 50 %,instead of usual 30 %?If yes,perhaps the vapor fraction of Incoming Air to Column has reduced.Do you notice any difference in the Inlet Air Temp to Column as against the design value?

 

(B) Are you trying to say PL flow to LP Column is almost negligible now,why are not maintaining it to the required spec?

 

Depending upon on your answers ,the troubleshooting will progress,If Art sir's assertion of Column internal damage is not true.

 

Bodhisatya



#7 jamesgrg

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Posted 17 April 2014 - 03:11 AM

If we increase the air feed What will be the effects without changing valve position of RL & PL? Compressor is in full load and if we change the feed then there will be pressure drop at turbine side, which should be constant and at design value.

#8 Bodhisatya

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Posted 17 April 2014 - 03:31 AM

Can you please confirm Air inlet temp to Column ,is it same as design or less?.

 

What about turbine loading?Is it loaded as per spec or overloaded?

 

Bodhisatya.



#9 jamesgrg

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Posted 17 April 2014 - 04:01 AM

Air inlet temp is -167ºC at 5 bar press and turbine is gas bearing type with brake, of inlet 5 bar and outlet 0.3 bar. Temp drop across turbine inlet(-130ºc) and outlet(-175ºc) is 45ºc.

#10 Bodhisatya

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Posted 19 April 2014 - 08:11 AM

 As we try to further cool down to get the value of 30-35% O2 in rich liquid, pressure is dropping as temperature drops. Poor nitrogen reflux valve is almost close to give sufficient reflux to HP column. 

 

 

(a) How come further cooling of the plant will give u 30-35 % O2 in RL?

(B) Why is the PL valve to LP column not as per the Design spec?The LP column might be starving of reflux.

 

My gut feeling says the turbine is overloaded,U need to warm up the Cold box a bit by reducing turbine load.Also requesting you to send the temp profiles and flow rates of all incoming and outgoing streams across Main Heat Exchanger,so that I can check the vapor fraction of Air Feed to HP Column.

 

Bodhisatya.






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