Hi everybody,
This is a summary regarding dry air.
An updateable copy of this summary is available at
http://nmrwiki.org/wiki/index.php?title=Dry_air/gases_for_analytical_instruments.
I want to thank for help all who responded: L S Mombasawala, Kurt
Wollenberg, Chris Rithner, Deryck Webb, Jerry Hirschinger, George
Furst, Martha Morton, Charles Amass, David VanderVelde, Martine
Ziliox, Jane Strouse, Carlos Steren, Valerie Robertson, Dave Scott.
Contents:
A) Dry nitrogen/air solutions
(1) Boil-off nitrogen;
(2) PSA dryers
(3) Membrane dryers <---- new membrane dryers go to -73C dewpoint!
(4) Molecular sieve desiccant dryers
(5) Refrigeration dryers
(6) Note on using auto-drain before the dryer.
B) Compatible tubing materials
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A) Dry nitrogen/air solutions:
(1) Boil-off nitrogen. Very dry, dew point probably around -200C if
(i) the evaporator chamber in the source tank is free of ice (ii)
delivery tubing made of material that does not allow diffusion of
water across the walls (see part B). Best results are achieved when
using big industrial-size outdoor tanks. Portable tanks (~200L) can
once in a while come with a wet evaporation chamber
if tanks are left open while they are still cold by whoever handled
the tank before. Frost will blow out and may even be visible as white
particles flying along. The evaporation chamber can even take some
hydraulic fluid if filling is done incorrectly.
(2) PSA dryers (pressure-swing adsorption). Models with dew point
-73C are available.
These work by using difference in kinetic rate of adsorption of gases
by some surface, e.g. activated alumina or carbon (alumina is used in
dryers). Typically there are two towers - one in adsorption cycle,
another in desorption. Separation happens during the adsorption cycle.
Desorption happens spontaneously when the pressure is reduced during
the desorption cycle.
As a side-effect PSA dryers generate gas with a swinging
concentration of oxygen. I think in somebody had previously mentioned
that change in O2 concentration can swing lock level as well. Large
ballast tanks can even out those oxygen level oscillations. Deryck
Webb
uses two 240L tanks (see some of Deryck's pictures at
http://nmrwiki.org/wiki/index.php?title=NANUC_air_drying_setup)
Alumina powder is abrasive, so it's a good idea to install 0.1 micron
filter after the dryer.
(3) Membrane dryers. Hankison offers a model with dew point -40C.
Balston's new model UDA300 gives -73C - same
as earlier PSA systems! Shop around, may by you can get by with just
the membrane dryer!
The membrane in these dryers has pore size less then mean free path of
the molecule. In this regime (Knudsen diffusion) flow is proportional
to inverse of square root of the molecular mass. So smaller molecules
like hydrogen and water leak through the walls of the membrane faster,
larger molecules keep going along the tube made of the membrane
material. These dryers are probably the most robust, as the required
maintenance is minimal.
(4) Molecular sieve desiccant dryers. Similar to PSA, but separation
works by selective absoption of water by molecular sieves.
Regeneration requires heating - so there is electricity consumption
and sieves need to be periodically replaced. These have been
supplanted by the PSA technology and possibly now by membrane dryers.
(5) Refrigerated dryers. These are not popular currently as they
continuously consume energy, refrigeration equipment has moving parts,
is
costly, and contains freons. Achievable dew points are not as low as
with PSA,membrane and molecular sieve dryers.
(6) One person mentioned troubles with liquid water getting into the
dryer column. These can be completely prevented with an automated
drain placed before the dryer. Many new dryers are equipped with the
auto-drain traps.
B) Compatible tubing materials: (i) copper; (ii) possibly high density
polyethylene; polyurethane tubing won't work as it is permeable to
water. Use plastic tubing with caution. Thanks for the tip to Martine
Ziliox, Clemens Anklin, Charles Amass, Valerie Robertson.
Best regards,
Evgeny.
--
Evgeny Fadeev, Ph.D.
Director, BioMolecular Spectroscopy Facility
1212 Natural Sciences 1
University of California Irvine
Irvine, CA 92697
telephone: 949-824-5842
www.physics.uci.edu/~biomolenmr
nmrwiki.org
Received on Tue Dec 02 2008 - 13:59:18 MST