A Model for Purging a 15,000 ton LAR TPC tank

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A Model for Purging a 15,000 ton LAR TPC tank

• Richard Schmitt
• Fermi National Accelerator Lab March 13, 2006

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Topics

• Large flat bottom tanks
• Purging goal
• Calculation results
• Examples
• Plans

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Flat Bottom Tank

• Commonly used for largest LNG (liquefied natural
  gas) storage
• Well known technology
• Excellent safety record
• Reasonable cost
• Can not withstand vacuum

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50 m diameter
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Air Purging Goals

• Purge to less than 10 ppm O2
• Liquid Argon arrives with 1 ppm oxygen and is
  purified
• Warm gas contain 0.1 of the contamination that
  must be removed
• Purge in a reasonable length of time

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Analysis Calculations

• ANSYS-CFX, finite volume analysis
• Hand calculations
• Ficks law
• Boundary layer
• Perfect mixing

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Uniform Inlet, Single Outlet

• Tank Diameter 24.5 m
• Tank Height 24.5 m
• Total flow 8.5 m3/minute
• Tank volume change every 22 hours
• Isothermal at 20C

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One volume change
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• Concentration after nine volume changes
• Center of tank is in upper left

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Single Outlet Conclusions

• Residual air is left in the outer, upper part of
  the tank
• Average oxygen concentration is 300 ppm
• A single outlet is inadequate

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Local inlet model

• Inlet spacing 2.1 m
• 100 inlets
• Model height 2.4 m

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Distribution plot

• One ppm reached in 30 hours at 2.4 m elevation
• Some mixing occurs

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Red 3200 to 3600 ppm oxygen Orange 2800 to 3200
ppm oxygen
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Hand Calculations

• As a rough check of ANSYS analysis
• Diffusion calculations per Ficks Law
• Boundary layer diffusion
• Perfect mixing

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Purging Examples

• LNG tank purging
• SELEX Ring Imaging Cherenkov
• Small test tank

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LNG tank purging

• One inlet/one outlet
• Purged with nitrogen at room temperature
• Typically achieve 1 oxygen in one day, one
  volume change
• This rough number compares well with the single
  outlet model
• Initially outlet is all air
• later oxygen concentration drops rapidly

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SELEX RICH

• Purged with Neon
• 44 m3 volume, horizontal cylinder
• Purge headers along the top and bottom
• Purge rate 0.5 m3/minute
• Outlet reached 2 ppm oxygen in 7 hours

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Small tank

• Vertical, cylindrical tank
• 1.5m diameter, 4.5m3 volume
• Mean velocity 1.2m/hour
• Dispersed inlet, single outlet

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Small tank model early results
0.3 hours
One hour
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one volume change
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Small Tank - outlet
Ten volume changes
Supply argon contains 8.5 ppm oxygen
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Planning

• Analyze more details with ANSYS
• Importance of other contaminants, nitrogen,
  water, carbon dioxide
• Test 10 m diameter water tank with distribution
  and venting pipe system
• Palladium catalyst vs. venting cost comparison