Note: Descriptions are shown in the official language in which they were submitted.
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SHIPPING CONTAINER FOR STORING MATERIALS AT
CRYOGENIC TEMPERATURES
Thi8 invention relate6 to container~ for
storing materials at cryogenic temperatures and more
particularly to an open to atmosphere ~hipping
containeL adapted to hold a supply of liquid
nitrogen for refrigerating a ~tored biological
product during transportation from one location to
another over a relatively long time period.
Backaround of ~hi6 Invention
The shipment of heat-sensitive bio-systems,
as for instance ~emen, vaccines, cultures of
bacteria and viruses at optimal temperature levels
between about 78K and lOOK, poses a 6eries of
difficulties. Th~ vials or "straws~', in which the
biologicals are hermetically sealed, mu~t be kept
continuously at near liquid nitrogen temperature to
preserve the viability of the biological product.
But since the boiling point of liquid nitrogen at
ambient pre6~ure is 77.4K (-320.4F) the cryogen
holding vessel (refrigerator) must remain open to
the atmosphere to vent the boiled-off gas and thus
avoid a dangerou~ pressure build-up in6ide. Fo~
this reason open-to-atmosphere liquid nitrogen
ve6~els are used for refrigeration. It is obvious
that 6uch vessel6 must be kept upright at all times
to prevent 6pillage of the cryogen. This condi~ion
i8 difficult to control during a long shipment
unless an attendant accompanies the ves~el on the
trip which is rarely a feasible option.
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To overcome the difficulties a6sociated
with the shipment of biologicals at cryogenic
temperature a fihipping container was developed in
which the liquid nitrogen i8 retained in a solid
porous mass by adsorption, capillarity and
absorption. Based upon this development a patent
is6ued to R. F. O'Connell et al. in 1966 as U.S.
Patent No. 3.238~002. The shipping containe~
described in this patent is of a double-walled
con6truction to provide a vacuum space around the
inner vessel which hold6 the liquid nitrogen. The
vacuum space is filled with a multilayer in~ulation
to reduce heat t~ansfer by radiation. An adsorbent
and a getter are part of the system to maintain
vacuum integrity. The inner vessel is filled with
the solid porous mas6 which, when saturated with
liquid nitrogen, will hold the cryogen by
adsorption, and capillarity as well as by
ab60rption, similar to a sponge "holding" water. In
the center of the porous filler core one or more
voids are provided to hold the vials containing the
biologicals.
The solid components of the porous mass
described in V.5. Patent 3,238,003 are silica
(~and), quick-lime, and a small amount of inert heat
resistant mineral fibers such as asbestos. The
porous mass i8 formed ~tarting with an aqueous
filurry of the filler components which is poured into
a mold and then baked in an autoclave under
precisely controlled equilibrium conditions of
pressure and temperature.
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The components undergo a chemical reaction
~orming a porous ma~s of calcium silicate~,
reinforced by inert fibers. The evaporated water
leaves inside the dried out 601 id structure
microscopic void~, of complex geometry, sometimes
referred to as "poresl', which comprise on the
average 89.5% of the appacent ~olid volume. Since
the resulting mass is incompressible the mold must
either provide the mas6 with a shape confor~ing to
the inner vessel of the stocage container or it muct
be machined to size. The porous mass is filled with
liquid nitrogen by submerging it in a liquid
nitrogen bath until it i8 ~aturated. The filling
operation for a conventional two liter container
housing a sand-lime pocous mass matrix take6 about
twenty-fouL hours.
The baked sand-lime porous mass i8
intrinsically hydrophilic. Because of this property
moisture must be periodically driven out of the
porous mass matrix to prevent the accumulation of
trapped water. If this i& not done, the trapped
water will turn into ice crystals every time it is
exposed to liquid nitrogen and eventually will crack
the brittle microstructure of the filler. This ~ay
be preYented by periodically heating the porou6
structure to above 100C after several fill and wacm
up cycles.
Although the ingcedients used in
manufacturing the sand-lime pocous ma~ are
relatively inexpensive (deionized water, sand,
quick-lime and inert fibers, as for example
asbesto6~ the finishing operations in handling a
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solid porous ma6s are very expensive due to the high
labor co6ts involved and the elaborate safety
precautions required. It is noe economically
feasible to cast the porous filler in a cryogenic
holding vessel. Elaborate safety precautions are
indispensable when handling substances like asbe~tos
fibers and noxiou6 dust. In addition, the thermal
energy co~t is very high for the manufacturing
procesc of the sand-lime filler ma6s.
Alternative sy~tems for retaining liquid
nitrogen in a container through a combination of
adsorption, absoeption and capillarity have in the
past being investigated by those skilled in the
art. The u~e of high porosity blocks, artificial
stone~, bricks and light papers made from cellulose
fibers such a~ towels and bathroom tissues have been
studied and, in general have been dismissed as
inferioL compared to the use of the sand-lime porous
mass matrix due primarily to their low porosity.
The average porosity of the sand-lime porous matrix
is 89.5% whereas the porosity of a matrix fabricated
from any of the aforementioned materials is below
60%. More recently block in~ulation material
composed of hydrous calcium silicate has been used
as the adsorption matrix. Such material is closer
in porosity to the sand-lime porou~ ma~6 composition
but al~o has ~ost of the shortcomings of the
sand-lime porous mass composition. The porosity of
the filler matrix determines for a gi~en size
shipping container its liquid nitrogen capacity.
The porosity and rate of evaporation are the most
important characteristic~ of a liquid nitrogen
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~torage container for trangporting a product at
cryogenic temperature~. A ~torage container using a
sand-lime porous ma~6 matrix has an average 5 day
holding time based on an evaporation rate of .33
liters per day and a liquid capacity of 1.6 liter6.
Accordingly, the act has long sought a less
expen6ive and much more efficient liquid nitrogen
adsorption sy~tem as an alternative to the 6torage
sy~tems in present use.
Obiects of the Invention
It i8 therefore, ~he principle object of
the present invention to provide a low cost
refrigerated container for transporting bio-systems
at cryogenic temperatures.
It is another object of the pre~ent
invention to provide a refrigerated container for
shipping a bio-system over a long holding period
during which time the bio-system is sustained in
~uspended animation at cryogenic temperatures.
It i8 yet another object of the present
invention to provide a low cost refrigerated
container having a liquid nitrogen adsorption matrix
which has a high average holding capacity and i6
intrinsically hydro-neutral.
A still furthec object of the pre6ent
invention i~ to pzovide a refrigerated con~ainer
having a liquid nitrogen adsorption matrix which ha6
a higher ad60rptivity than state of the art liquid
nitrogen adsorption matrices and which will fill to
capacity in a sub~tantially reduced time period.
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Summary of the Invention
The present invention provides a structure
for holding a liquified gas such as liquid nitrogen in
adsorption and capillary suspension comprising a core
permeable to liquid and/or gaseous nitrogen having a
cavity extending therethrough and a liquified gas
adsorption matrix composed of a mass of randomly
oriented microfiber particles having a diameter in a
range of between 0.03 to 8 microns in relatively close
engagement with one another surrounding said core as a
homogeneous body.
Brief Description of the Drawings
Further objects and advantages of the
present invention will become apparent from the
following detailed description of the invention when
read in conjunction with the accompanying drawings of
which:
Figure 1 is a front elevational view, in
section, of the shipping container of the present
invention;
Figure 2 shows a preferred insertion technique
for forming the micro-fibrous adsorption matrix within
the inner vessel of the cryogenic shipping container of
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Figure 1 before the bottom end of the inner vessel is
attached;
Figure 3 is a partial view of Figure 2 showing
the inner vessel after the fibrous adsorption matrix
has been formed and the bottom end attached; and
Figure 4 is a perspective view of the micro-
fibrous adsorption structure of Figure 1 formed as a
self-supported structure by an alternate manufacturing
process.
Description of the Preferred Embodiment
The invention is illustrated in the
preferred embodiment of Figure 1 which shows a shipping
container 10 having a self supporting outer shell 12
surrounding an inner vessel 13. The inner vessel 13 is
suspended from the outer shell 12 by a neck tube 14.
The neck tube 14 connects the open neck 15 of the inner
vessel 13 to the open neck 16 of the outer shell 12 and
defines an evacuable space 17 separating the outer
shell 12 and the inner vessel 13. A neck tube core 18
is removably inserted into the neck tube 14 to reduce
heat radiation losses through the neck tube 14 as well
as to prevent foreign matter from entering into the
inner vessel 13 and to preclude moisture vapors from
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buildîng up highly objectionable frost and ice barriers
inside the neck tube 14. The neck tube core 18 should
fit loosely within the neck tube 14 to provide
sufficient clearance space between the neck tube 14 and
the neck tube core 18 for assuring open communication
between the atmosphere and the inner vessel 13.
The evacuable space 17 is filled with insulation
material 19 preferably composed of low emissivity
radiation barriers, like aluminum foil, interleaved
with low heat conducting spacers or metal coated
nonmetallic flexible plastic sheets which can be used
without spacers. Typical multilayer insulation systems
are taught in U.S. Patent Nos: 3,009,600, 3,018,016,
3,265,236, and 4,055,268. A plurality of frustoconical
metal cones 20 may be placed around the neck tube 14 in
a spaced apart relationship during the wrapping of the
insulation in order to improve the overall heat
exchange performance of the storage container 10
following the teaching of U.S. Patent No. 3,341,052.
To achieve the required initial vacuum condition
in the evacuable space 17, the air in the evacuable
space 17 is pumped out through a conventional
evacuation spud 31 using a conventional pumping system
not shown. After the evacuation has
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been completed the spud 21 is hermetically sealed
under vacuum in a manner well known in the art
using, for example, a sealing plug and cap ~not
~hown).
An adsorbent Z2 is located in the vacuum
space 17 to maintain a lo~ ab601ute pres6ure of
typically le~s then 1 X 10-4 torr. The ad60rbent
22 may be placed in a retainer 23 formed between the
shoulder 24 and the neck 15 of the inner vessel 13.
The retainer 23 ha6 a ~ealable opening 25 through
which the adsorbent 22 i~ inserted. The ad~orbent
22 is typically an activated charcoal or a zeolite
sueh as Linde 5A which i~ available from the Union
Carbide Corporation. A hydrogen getter 26 ~uch as
palladium oxide (PdO) or silver zeolite may also be
included in the vacuum ~pace 17 foc removing
residual hydrogen molecules. To those skilled in
the art it is apparent that other locations. as well
as methods of placement of the adsorbent and the
hydrogen qetter, are feasible.
The inner vessel 13 contains a
micro-fibrou6 6tructure 27 for holding liquid
nitrogen by adsorption and capillary 6uspension.
The micro-fibrou6 structure 27 Gomprise~ a permeable
cylindrical core 28 and a liquid nitrogen ad60rption
matrix 30 composed of a homogeneous mass of randomly
oriented ~hort particles of inorganic fibers e.g.
qlass quartz or eeramic of very 6mall diameter. The
micro-fibrous structure 27 is shown in longitudinal
cross section in Figure 2 at the manufacturing
stage. The upper cylindrical portion 32 of the
inner ve6sel 13, hermetically ~ealed between and
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permanently attached to the neck tube 14, as well as
the permeable cylindrical core 28 are placed in a
loosely fitting relationship over a columnar
extension 38 of a suppo t device 39. The
cylindrical portion 32 with the attached necktube 14
are placed for this operation upside-down, that is,
the unattached nec~tube end is facing downwards.
The open space between the tubular core 28 and the
inner surface of the cylinarical portion 32 i6
filled with an aqueou6 micro-fibrous slurry 40 of
inorganic fibers pceferably glas~ poured from a
mixing vat (not shown) at such a rate that the water
41 rom the in-pouring slurry 40 is free to flow
through the opening6 29 in the core 28 as well as
down thsough pas6ages 42 in the columnar support 3æ,
leaving the moist semi-solid micro-fibrous glass
re6idue to form the homogeneou6 body of the
ad60rption matrix 30. The slurry influx i6 stopped
when the level of the body 30 reaches the rim 34.
The matrix 30, con6isting of a qua6i-infinite number
of randomly oriented inorganic micro-fiber
particles, typically about 3mm to lOmm in length, i8
then, her~etically sealed in6ide the inner vessel 13
by welding the bottom 33 around the circumference of
34, a6 ~hown in the partial cro6s-6ectional view of
Figure 3. A curved bottom plate 33 is used to
provide an ullage 43 between the matrix 30and the
bottom en~ of the inner vessel 13 to enable the
liquid nitrogen to readily permeate the matrix 30
axially as well as radially. The re6idual moi6ture
in the matrix 30 can be removed by the application
of moderate heat trai6ing the temperature to about
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Cl~ ~OCE,~ se
70C) and by simultaneou~ application of a ~E~
6l vacuum of about 20 to 150 torr.
However, to those skilled in the art it i~
apparent that other processe6 can be u~ed foc the
manufacture of the matrix 30. One of them, very
similar to the one described above, would consist
for example, of a long cylindrical mold made up of
two longitudinal hemi-cylindrical halves that could
be separated from each other for easy removal of the
molded product. The inside diameter of the mold
would be the same as the inside diamete~ 11 of the
inner vessel 13 in Figure 1. The permeable core
would be an appropriate tubing, matching in length
the hemi-cylindrical mold. The void between the
inside of the mold and the outside of the permeable
core would be filled with an aqueous micro-fibrous
slurry and treated afterwards in a similar fashion
as the individual matrix shown in Figure 2. The end
product of such an operation would be a long
cylindrical semi-fini~hed micrQ-fibrous adsorptive
body 30 surrounding a permeable core 2a which would
then be cut into pieces of appropriate length to
form a structure 27 a~ shown in Figure 4
corresponding exactly to the structure 27 of Figure
1. Since the microfibrou~ structure 27 is the same
identical reference characters have been used in
describing the alternate method~ of manufacture.
The pre-fabricated and pre-cu~ structure 27 of
Figure 4 would then be in6erted into the upper
cylindrical section 32 of the inner vessel 13 of
Figure 1. The open bottom would then be closed
u~ing a curved bottom plate 33 which may be welded
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around the periphery 34 as explained earlier in
connection with Figure 3 leaving an ullage 43
between the bottom plate 33 and the structure 27.
This ullage 39 may readily be avoided leaving no
open space 43 if 80 desired.
Although one does not ordinarily as60ciate
glass with characteristic6 ~uch as 6pongine66 and
poro6ity, it has been di6covered in accordance with
the present invention that reasonably compacted
glass fiber6 possess high capacity for holding
liquid nitrogen by adsorption and capillary
suspension provided the gla66 fibers in forming the
web are of very small diameter. The liquid nitrogen
is held in the micro-fibroufi matrix 30 by molecular
adsorption to the enormous aggregate area of the
micro fiber6, as well as by capillary su6pen~ion
made possible by the microscopic intra-fibrous voids
between individual fibers. It i6 therefore of
importance that the diameters of the glass fibers be
as small as pos~ible with the preferred range from
.03 to 8 microns. The body of micro-fiber qlass
particles ~hould preferably be formed without using
any rigidizing binder6 or cemen~6. The structural
6tability of the felt-like body is effected
primarily by intra-fibrou6 friction. Substantially
binderless inorganic micro fibers in diameter~
ranqing from 0.3 to 8 micron are commercially
available from e.g., the ~anville Corporation and
Subsidiaries, Denver, Colorado and Owen6-Corning,
Toledo, Ohio. The glass fiber6 u6ed in this
invention are composed of boro6ilicate glass with
the glas6 fibers ranging from .5 to .75 microns in
diameter.
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The core 2a is preferably of tubular
geometry having a central void 31 into which the
biological product is to be placed during shipment.
It should be understood that the invention is not
li~ited to a ~inqle void 31. ~ultiple voids 31 may
be readily formed using muleiple cores 2~ and
arranged in any desired pattern or geometry. The
core 28 can be of any material composition, e.g.,
metal or plastic that will remain structually ~table
and retain its form after being repeatedly subjected
to cold shocks at liquid nitrogen temperature~. To
maintain the lowest po~sible temperature within the
cavity 31 the core 28 must be permeable to the
nitrogen gas that boil~ off from the liquid nitrogen
stored in the glass fiber matrix 30. The
permeability of the core can be provided by forming
the core 2~ fLom a perforated sheet rolled into a
tube or using a porou6 sintered tube without
apparent hole6. Where perforations are u~ed, ehe
holes 29 in the wall of the core 28 must be small
enough to prevent any loose fiber particles from
passing across the core wall 28 into the storage
cavity 31 containing the biological product.
The storage container 10 of Figure 1 is
preferably assembled starting with the inner ve6sel
assembly 13 of a two piece construction, having an
upper cylindrical section 32 with an open end bottom
34, a lower section 33, and then neck tube 14
permanently attached by way of the open neck 15 to
the cylindrical section 32, employing any acceptable
joining method.
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The adsorption/storage system 27,
comprisinq the homogeneous micro-fibrous matrix 30
and the permeable core 28, in coaxial alignment with
the neck tube 14, make up the inner container
as6embly 13.
The outer shell 12 is al80 of a two piece
construction with an upper cylindrical section 35
and a lower bottom ~ection 36. The inner ves~el 13
i8 inserted into the upper section 35 before the two
sections are joined to each other. Where a wrapped
compo~ite insulation system is used, the inner
ve6sel i~ first wrapped with the layers of
insulation preferably using the hea~ exchange cones
20 before the inner ves~el 13 is inserted into the
upper 6ection 35. The adsorbent 22 is placed in~ide
the ad~orbent retainer 23 before the insulation is
applied. The upper 6ection 35 may have ccimped end
37 to facilitate attachment of the lower section
36. Before the two sections 35 and 36 are velded
together to from a unitary structure, the getter
composition 26 i6 placed inside the vacuum space
17. Instead of circumferential crimping as shown in
34 and 37 of Figure 1 other means of alignment of
mating cylindcical components can be u~ed, e.g. bute
welding with a back-up ring or tack welding in a jig.
The liquid capacity of the micro-fibrous
matrix with randomly oriented fiber par~icles is
determined by the apparent volume of the matrix and
its "porosity". The design volume being 2,400 cm
and the "porosity" of the microfibrous adsorption
medium having a mean value of 92%, the mean liquid
capacity of such a cryogenic ~torage container is
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found to be 2,400 cm x 0.92 = 2,208 cm or
about 2.2 liters.
This then is the design figure for the
amount of liquid nitrogen to be held within the
micro-fibrous matrix by ad~orption and capillarity
without deainage or ~pillage.
In service, the liquid nitrogen, held in
the matrix, keeps evaporating due to the unavoidable
heat inflow from ambient resulting from the
temperature gradient between ambient and liquid
nitrogen. Eventually all ~he cryogen is bound to
boil off completely, leaving the &torage compartment
for the temperature sensitive product without
refcigeration. Considering this ci~cumstance, which
in essence is a race between the hold time of the
storage container and the shipping time of the
product, the rate of evaporation is the most
important characteristic of a shipper-refrigerator.
The evaporation rates of containers of this
invention have a mean value of 0.084 liter/day.
This low evaporation rate makes it possible to
achieve a mean holding time of:
2.2 liters . 26 days
0.084 liter/day
compared to 5 day~ for the state-of-the-art
shipper~. In other words, a shipper/refrigerator of
this invention will provide the required near liquid
nitrogen temperature inside its seorage compartment
to maintain bio-systems in the state of suspended
animation thcoughout a maximum of 26 days of
transportation, regardless whether the shipper is
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standing upright. laying on the ~ide. or even
Up6 ide-down.
The invention as de6cribed in accordance
with the preferIed embodiment 6hould not be
construed as limited to a specific configuration for
the core and adsorption matrix in definin the
micro-fib~ous structure. For example, the core may
have a plurality of voids defined, for example.
within a ~ubular framework with the void~ separated
by partitions extending from a solid control po~t to
the outer tubula~ wall of the core. In such case
only the outer tubular wall of the core must be
permeable to ga~eou~ nitrogen.
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