Hello Everyone
As many of you have helium recovery or are looking into it, I wanted to post
what we have learned about our gas purity. In the beginning of our journey
we could not get any information about what the purity of the gas to expect
at the various stages of a helium recovery system. Vendors did not have any
information and this community did not have any information at the time either
as this was almost 5 years ago. I put the important numbers first, because
this got long.
Summary:
What we have learned: Normal boil off comes into the gas bag at 0.03-0.04%
impurities, when a fill is being done that drops to 0.00% for the duration
of the fill and a short time after it will be less than 0.03% impurities. If
we have had several fill or transfer events to purge the bag, it will read
0.1 % impurities for a time but as the bag fills and the gas sits we can
see impurities up to 1.6% on the regular. The Bauer does clean a lot
of this up and so as the compressed gas comes back up from the storage
cylinders it can be 0.1 to 0.45% impurities on the regular. Again the more
we do fills or transfers the better these numbers are. These are more normal
day to day numbers.
Back story of our system build, what we tried, and testing:
Let me give a brief summary of our system layout. There were 8 NMRs and 1 EPR
to be connected to our recovery system. These systems are spread out over 3
floors and the entire length of our building with the recovery room on the
top floor. This means gas could have to travel up 4 floors to get to the
recovery room and the gas bag. We have a 900 scf bag that is pumped out by
a Bauer compress to the basement where we have 12 high pressure gas cylinders.
The basement is 6 floors below the bag, so the gas is pumped down and
pressurized and then it comes back up to the recovery room at 30 PSI where
we use a precision regulator to maintain 15 PSI.
We used 1in and 2in stainless steel diesel exhaust fluid tubing (DEF-tubing
to connect the systems. This amounted to almost 900 ft of tubing installed
covering the height of the building 3 times and the length of the building
3 times. DEF-tubing is somewhat flexible and comes on spools so it can be
cut in the field and put together with compression fittings and standard pipe
fittings. We were able to make continuous long runs minimizing connections
compared to running rigid tubing such as copper or stainless steel.
Even at the planning stages we were thinking about monitoring purity. We have
made attempts to monitor gas purity of our incoming gas and outgoing gas from
the bag. But this was with an O2 sensor system in line; they were just not
sensitive enough. These units work for valve control but not for purity
monitoring.
We have always worried about gas purity because we went with a LN2 purification
system that has no monitor for when it is full. We did get information
from the company that it can handle 0.2% impurities at a gas flow of 10 L/min
for 60 days before it would need to be regenerated. Even at 0.4% impurities
we are not pushing the trap's capacity because we are not in production 24/7.
Usually we are only running 4 to 12 hours a day.
When we first installed the system we were having to change out this LN2 trap
every 48 hrs. We were told as the liquifiers efficiency went down it was
an indication the trap was getting full. The liquifier was losing efficiency
quickly and changing the trap helped. After much trouble shooting with the
company's help, we found that the Bauer had some major leaks. This troubleshooting
was possible because we bought a binary gas analyzer from Stanford Research Systems
( link (
https://www.thinksrs.com/products/bga244.html ) ).
This unit can be set to hundreds of gas combinations. We installed this unit on
the 15 PSI gas lines coming from the high pressure storage tanks to the LN2
Purification system. We see that our incoming gas from the high pressure
storage cylinders varies between 0.12 to 0.4% depending on how long the gas
sits in the gas bag and if we have done a transfer from the liquifier to a
transport dewar or a magnet fill. We have had this monitor in line since
late 2020.
This Binary gas analyzer is just too restrictive at ⅛” to flow
the low pressure gas into it from other parts of the recovery system. We have
tried other methods to monitor our gas coming into the bag from the NMRs and
EPR and the gas coming out of the bag. Thermal conductivity sensors proved to
be too fragile and the calibration process was painful. We could not get
ultrasonic sensors to work well inside the 2in tubing either. We tried several
methods of pulling gas out and passing it into the binary gas analyzer but
with mixed results.
Finally we did find a method that works, we put small vacuum pumps ( from amazon
(
https://www.amazon.com/gp/product/B0BF5ZJB29/ref=ox_sc_act_title_13?smid=A65F4E2EXH4SX&th=1 ) )
into the 2in tubing (KF-50 Tee or cross work well). These pumps are not air tight
so having them in the tubing accounts for that. They have an inlet and outlet port
so we hooked the outlet up to a KF plate with a quick disconnect on the outside.
This line is then connected to a binary gas analyzer. There is one on the gas line
going into the gas bag and one on the line coming out of the gas bag. An arduino
controls which one is on, so we know which line we are sampling from. Gas is
returned to the bag no matter which line is being sampled.
What we see when we have an EPR running with Bruker Gen 1 CryoFree system.
We have a Bruker EPR that has a cryogen free system for reaching 4K. First
we changed out the diaphragm pump to a small rotary vane pump because we knew
it would put less air into the system. When this system is started up it is
purged with UHP helium gas for 1hr before the cool down is started. There is
a O2 sensor controlling an actuated valve so as the purge starts and the O2
level drops the valve is turned to let the pump exhaust into the helium
recovery system.
During purging the impurities do go up. We have seen them as high as 1.5 % and
this does indicate a problem with the EPR. Typically we will see up to 0.7 %
during the start of the purge and then when the system is cold these will
drop back down to a more normal level of 0.03-0.04%.
Introducing a leak:
One test we did with our system was to put a small orifice in line on one of
the NMR’s boil-off lines. We purchased a 0.01” orifice form
McMaster-Carr, this was the smallest orifice we could get at the time. This
was left inline for 45 min before we took it out. Thinking that we were not
seeing anything because our values did not change for incoming impurities.
However about 1 to 1.5 hours later things started to increase. The impurities
just take that long to get pushed up to the recovery room. We do not know
what the normal total flow is in the 2” DEF tubing but obviously
it is not too fast. This tubing is also corrugated so we envision the
impurities at the bottom of the DEF tubing having to be pushed along going
in and out of the corrugations. Anyway we did see these impurities for the
better part of the day and they maxed out at 1.5% impurities. So yes this
setup will see basically a pin hole leak!
Gregory P. Wylie, Ph.D
NMR Facility Manager
Texas A&M University
Department of Chemistry
P.O. Box 30012
College Station, TX 77843-3255
gpwylie_at_tamu.edu
979.458.0705 (voice)
979.845.4719 (fax)
706.206.0007 (cell)
http://nmr.tamu.edu
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Received on Tue May 28 2024 - 13:28:16 MST