Regenerable device for scrubbing breathable air of CO.sub.2 and moisture without special heat exchanger equipment

The device concerns the circulation of cabin air through canisters which absorb and adsorb carbon dioxide, together with excess moisture, and return the scrubbed air to the cabin for recirculation. A coating on an inert substrate in granular form absorbs and adsorbs the impurities at standard temperatures and pressures, but desorbs such impurities at low pressures (vacuum) and standard temperatures. This fact is exploited by making the device in a stack of cells consisting of layers or cells which are isolated from one another flow-wise and are connected to separate manifolds and valving systems into two separate subsets. A first subset may be connected for the flow of breathable air therethrough until the polyethyleneimine of its cells is saturated with CO.sub.2 and H.sub.2 O. During the same period the second subset of cells is manifolded to a vacuum source. After the first period the first subset is re-valved to connect it to the vacuum system to draw off the collected impurities, while the newly purified subset B is connected to the breathable air of the cabin. These phases or half cycles are repeated, making it unnecessary to collect and discard any used-up materials. The cells of the device are separated from one another by sheets of a lightweight but impervious metal which is both physically strong and a good conductor of heat. To assist in the heat transfer process, a high conductivity foamed metal is disposed throughout the polyethyleneimine bed of each stratum, such foamed metal being formed into blocks which preferably touch both the upper and lower parting sheets defining the cell, to transfer heat to both the superior cell and the inferior cell so that the stack of cells operates essentially isothermally.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention lies in the field of life support systems, and more 
specifically concerns the systems which affect and control the air 
breathed by human beings within an enclosure. It has broad application to 
any "air conditioning" system which includes a suction or vacuum 
subsystem, and has particular application to spacecraft and other 
interplanetary and planet-orbiting vehicles, operating in a vacuum or 
rarefied atmosphere. Even more specifically, the invention deals with 
methods and means for reducing the concentration of carbon dioxide 
(CO.sub.2) in a flowing breathable airstream, and also removing excess 
moisture from the airstream, and at the same time not affecting the 
control of the temperature of said airstream. 
2. Description of the Prior Art 
One known technique for scrubbing CO.sub.2 out of an airflow is by 
filtering the air through a chemisorbent material such as lithium 
hydroxide (LiOH). While this technique is feasible under certain 
circumstances, it has the major disadvantage that the LiOH filter is not 
regenerable; a permanent, non-reversable reaction takes place, with the 
result that the material is consumed and must be discarded and replaced 
with fresh material. This disadvantage is particularly obnoxious in a 
space craft, as it exacts a considerable weight penalty; the larger the 
crew, the more filter material must be packed into the ship at launch 
time. Also, the use of a non-regenerable filter material limits the space 
mission in various other respects, e.g., the duration of the flight and 
the other equipment which must be limited to make room for the consumable 
filter material. 
In the same prior art system, it has been common to remove excess moisture 
from the breathable air by passing it through a condensing heat exchanger. 
Again a weight penalty is paid, and in addition there is a waste of 
energy. Some means is desired to make the CO.sub.2 scrubber material 
usable over and over again and, as a side benefit, to extract H.sub.2 O at 
the same time. It is also desirable not to be obliged to supply heat to 
one part of the system or to remove it from another, as each of these 
steps implies the need for additional equipment and materials, and such 
equipment and materials extract another weight penalty. 
SUMMARY OF THE INVENTION 
In the present invention a structure which might be called a heat 
exchanging canister is provided, and such canister is stratified into a 
multiplicity of layers or cells which are stacked on top of one another so 
that no air or other fluid can flow from one cell to the next, but which 
are separated from one another only by metallic barriers or parting sheets 
which permit and even encourage the flow of heat, in a direction normal to 
the airflow. Each cell is approximately parallelepiped in shape, and 
airflow between a pair of opposed surfaces is promoted by defining such 
surfaces with very fine wire screen, fine enough to encourage airflow but 
at the same time preventing the flow of a fine granular material which 
occupies most of the space within each cell. Such material is 
approximately of the consistency and granule size of a fine, dry sand, 
e.g., 30 - 40 mesh U.S. Sieve Series, but is considerably less dense and 
has a much greater surface area. 
Each granule is preferably made of an inert substrate or core on which is 
added a coating of active material, the core material being selected 
primarily from those having multiple reentrant contours, i.e., a large 
surface area, such as the "Amberlite" XAD7 a polymeric adsorbent marketed 
by Rohm and Haas Company, which in the indicated size range has a surface 
area in the neighborhood of 450 square meters for each gram of material, a 
truly remarkable surface area. The coating applied to the core granule has 
a minimal thickness, and is selected primarily from those which will 
adsorb and/or absorb CO.sub.2 at standard temperature and pressure, but a 
later time will readily release the lightly-bound gas when the pressure is 
reduced to a relatively low vacuum. It is something of a bonus if the same 
granules will also remove H.sub.2 O and give it up under the corresponding 
sets of circumstances, and in fact the inventor has found that this 
actually occurs, with the preferred sorbent materials. 
A particularly suitable active coating material is a polyethyleneimine, one 
such material being made and marketed by the Dow Chemical Company under 
the designation "PEI18, " an amber colored substance which is a viscous 
liquid at standard temperature and pressure. Finished granules as thus 
coated have a bulk density of 0.378 grams per cubic centimeter (cc), and 
flow as readily as water, easily and completely filling any canister cell 
into which they are poured, even though the canister may be structurally 
subdivided into a multiplicity of tiny pores. 
The technique for coating the PEI18 polyethyleneimine on the XAD7 has been 
developed by applicant's corporate employer, and the resulting granules or 
bulk material is presently designated "HS-C." 
The use of somewhat similar granules for analogous purposes is not entirely 
unknown to the prior art, a particular application having been disclosed 
in U.S. Pat. No. 3,659,400, issued May 2, 1972, to Frank L. Kester and 
assigned to United Aircraft Corporation, the former name of the inventor's 
employer's parent company. This reference described a regenerable CO.sub.2 
sorption -- desorption device utilizing granules of polyethyleneimine on a 
substrate of a polymerized divinylbenzene, the material being alternately 
exposed to cabin air for sorption and vacuum for desorption. The reference 
is completely silent, however, about structure disclosed in the present 
application for retaining the active granules and structure for the 
transfer of heat between layers or beds of the material. As indicated in 
the reference, the exact nature of the sorption process is not known, but 
is believed to be a combination of adsorption and absorption. 
The heat transfer structure of the present invention is probably more 
important than any other, particularly in view of the fact that the 
inventor's companion application, Ser. No. 688,855, filed concurrently 
herewith, discloses and claims structure and methods substantially 
identical with those herein, the exception being that such companion 
application makes use of a somewhat more conventional heat exchange 
structure, namely metal fins. In the present application, the inventor 
discloses and claims combinations which include foamed metals as the heat 
exchange medium. Such foamed metals are cut into blocks of the appropriate 
dimensions with very reasonable tolerances, and do a very adequate job of 
transferring heat liberated in a cell operating in the sorbing mode to its 
upper and lower neighbors operating in the desorbing mode. At the same 
time, they do not interfere with the filling of and retention by the cell 
with the active granules, as such granules may be poured into the metal 
sponge to fill all the small spaces therein, as completely as water would 
do. All that is necessary to finish a cell is to cover two opposed 
surfaces with a pair of fine metal screens for the inflow and outflow of 
air, such screen having openings which will not permit the granules to 
leak out or act as plugs. 
It should also be noted that such foamed metal or metal sponge has such 
good mechanical strength as to permit two significant improvements. One, 
they may be made longer than metal fins, i.e., in the heat flow direction. 
Two, they provide a homogenous matrix completely across the internal space 
between the outer walls of the casing or canister of the entire device, 
making it possible to use very thin parting sheets to separate one cell 
from another. The real structural strength lies in the metal sponge, while 
the parting sheets act only as seals.

DETAILED DESCRIPTION OF THE DRAWING 
The module 10 shown in FIG. 1 consists of a canister 12 having the four 
vertical manifolds 14, 16, 18, and 20 integrally connected to the canister 
at its four vertical edges. For convenience in assembly, each of the four 
manifolds is shown terminating in a pair of flanges 22 just above and 
below the canister body. Module 10 is structurally self-sufficient as 
shown, although of course brackets may be needed if the module is to be 
secured to a wall or bulkhead. 
As will be evident from FIG. 2, the canister is generally rectangular 
parallelepiped in shape, having the usual six surfaces in three parallel 
pairs consisting of upper and lower closure plates 24 (bottom not shown), 
side walls 26 and 28, and end closure plates 30 and 32. These six wall 
members (preferably of aluminum in spacecraft use) are joined and sealed 
together to form what would be, were it not for the manifolds and other 
openings to be discussed below, a leaktight box. 
It should first be mentioned that the canister 12 is divided into a number 
of horizontally stratified layers or cells, numbered 34, 36, 38 and 40 
from top to bottom in FIG. 2, and that each cell is provided with a pair 
of integral filler necks 42 (shown only in FIG. 3) into each of which is 
jammed a Neoprene plug 44 held in place by a cap screw 46 screwed into 
threads on the inner surface of neck 42. These necks are used when 
charging or emptying the cells with the sorbent granular material 50, 
about which more below. It will be noted from FIGS. 2 and 3 that each of 
the cells is defined by one or more horizontal parting sheets 52 which are 
coextensive with the upper and bottom closure plates, i.e., each parting 
sheet 52 extends completely between side walls 26 and 28, and between end 
closures 30 and 32, and is brazed or otherwise secured to these wall 
members so that each cell is flow-isolated from both of its neighbors in 
the stack; no air nor granular material can flow up to its neighbor on the 
floor above, nor drop through a parting sheet to its lower neighbor. 
These same figures, in particular FIG. 3, which is a horizontal section 
through the next-to-top cell 36, make clear that of the four manifolds 
only a polygonally opposed pair are flow connected to any one cell. Thus 
in cell 36 the flow may follow the direction indicated by the arrows, 
entering from manifold 18, passing through the internal convergent space 
54, through the fine screen 56, across the bulk granular material 50, 
between the screens, and exiting through a second screen 56, divergent 
internal passageway 58, and manifold 14. Each manifold is secured in a 
leaktight fashion to canister 12, and it should be noted regarding cell 36 
that the other pair of manifolds 16 and 20 are blocked from this cell by 
the end walls 30 and 32. 
Screens 56 are preferably made of a very fine aluminum screen, the openings 
of which are just small enough but not smaller than that required to 
prevent the passage through or blockage by the granules 50. The openings 
in each screen are preferably very closely spaced, so that the pressure 
drop over the screens will be minimal. A suitable material is expanded 
AA1145 aluminum in a 0.005 inch thickness, having openings of 0.009 inch 
diagonally spaced with 30 percent open area. 
FIGS. 2 and 3 show a canister in which the upper layer 34 and bottom layer 
40 are only about half as thick as the center cells 36 and 38. This is for 
heat transfer purposes, as the outside cells transport heat only to one 
neighbor each, whereas the interior cells 36 and 38 transport heat both 
upward and downward, to two neighbors apiece. The granular sorbent 
material 50 is not a particularly good heat conductor, so most if not all 
of the heat flow is through the foamed metal or metal sponge 61 and 63 
which occupies the same compartments as the granular material 50 but 
cannot easily be shown because of the difficulty of illustrating these two 
intermingled masses as separate items. The granules are, of course, a 
comminuted material whereas the foamed metal is a reticulated cohesive 
mass of metal wires all joined together and defining a multiplicity of 
interconnected spaces. (A chemist frequently speaks of a solvent as a 
"continuous" phase and a solute as a "discontinuous" phase, but in the 
present instance both phases are continuous, as might be true of a mixture 
of two liquids such as water and grain alcohol.) It should be noted from 
FIGS. 2 and 3 that the metal sponge may completely fill the air passages 
54 and 58 (made in separate triangular pieces from the block filling the 
space between the two screens 56), as illustrated. This is actually the 
preferred construction, as the strength of the metal sponge makes it 
possible to use thinner parting sheets 52 than is possible with the 
unsupported parting sheets used with the fin type heat transfer structure 
of the concurrently filed application of the present inventor. 
Various forms of foamed metal may be utilized, important characteristics 
being low weight, high thermal conductivity and a modicum of structural 
strength. A particular foamed aluminum found eminently suitable for 
embodiments of the present invention is known as Duocel, a product of 
Energy Research and Generation, Inc., of Oakland, California. This 
material may be cut into usable blocks as thick as 2 inches (5.08 cm.) in 
the heat transfer direction, and has a compression strength of 800 psi. It 
has the form of reticulated network of spaces of duodecahedral 
configuration in which the spaces are all interconnected and the metal 
defining the edges of the spaces form a continuous wire network, the wires 
having an average diameter of 0.033 cm. while the spaces average about 0.2 
cm. in diameter. The result is a cake or sponge of tough metal in which 
the metal occupies only about 4% of the available volume, and yet conducts 
exothermic energy so well from a sorbing cell to its desorbing neighbors 
that the stack of cells operates essentially isothermally, maintaining an 
approximately constant mean average temperature and requiring neither the 
addition or removal of heat from the stream of cabin air. At the same 
time, almost all of the volume of each cell is available for the sorbing 
material which scrubs or purifies the air. 
The nature of the foamed metal blocks 61 and 63 can perhaps be seen better 
in the somewhat enlarged end view of the FIG. 4. In this figure the sponge 
metal blocks next-to-top cell 36 and next-to-bottom cell 38 are shown, 
together with the three parting sheets 52, and the screens 56. This view 
shows one center block 61 and two edge blocks center of the foamed metal 
in each cell, the blocks differing from one another only in orientation. 
Directing the reader's attention now to FIGS. 5 and 6, it will be seen that 
the designation 80 has been used to symbolize an enclosure inhabited by 
air-breathing organisms, e.g., the room of a home or office, an automobile 
or the cabin of a spacecraft. In each figure a very heavy line has been 
used to designate the path through which the cabin air is forced by the 
fans or blowers 82. As indicated by the "Legend" forming a part of FIG. 5, 
which legend is also applicable to the other figure, a lighter weight line 
is used to indicate the flow path to space vacuum or a vacuum pump, while 
a dashed line indicates a pipe or conduit which has been rendered inactive 
by the closing of a valve during one phase of the overall cycle. Since 
there are only two phases per cycle, a phase is synonomous with the term 
"half-cycle." 
Starting in the lower left corner of FIG. 5, for example, it will be seen 
that during the half-cycle represented by this figure breathable air 
passes into conduit 84, is blocked by closed valve 85, but passes through 
open valves 86 into conduit 88, is blocked by closed valve 89 but passes 
through conduit 90 into canister 12 via manifold 18, entering, e.g., cells 
36 and 40 of the prior figures. The air passes through these cells and is 
purified by them, and leaves the canister by way of manifold 14 to enter 
conduit 92. Thereafter it is blocked by closed valve 93 but passes through 
open valve 94 to enter conduit 96, passes through open valve 98 (blocked 
by closed valve 99) into conduit 100 and finally is blown through fans 82 
and conduit 102 back to the cabin or other enclosure 80. 
During the same half-cycle as is represented by FIG. 5, flow commences in 
both directions from the center of cells 34 and 38, and indicated by the 
opposed arrows 104 and 105 within canister 12. Part of the air (including 
some oxygen, which becomes waste or "ullage") is sucked, along with 
CO.sub.2 and H.sub.2 O, to the right in the figure, exits the canister by 
manifold 16 into conduit 106, passes through open valve 107, conduit 108, 
conduit 110 and conduit 112 to the space 81 which symbolizes space vacuum 
or the intake of a vacuum pump. At the same time some of the air and 
contamination products pass to the left, as indicated by flow arrow 104, 
passing out of the canister by manifold 20 and into conduit 114. It then 
flows through open valve 116 and conduit 118 to join the other flow in 
conduit 112 to vacuum. During this half-cycle conduit 120 is inactive, in 
the sense that nothing flows through it. 
A detailed description of the flows indicated in FIG. 6 would be redundant, 
so the inventor will only point out that all the valves have been reversed 
in position, closing for the second half-cycle every valve that was open 
for the first, and vice versa. The effect of this synchronized switching 
is to now direct the flow of breathable air through cells 34 and 38 of the 
canister, which have been freshly purified as a result of their exposure 
to vacuum during the first half-cycle. At the same time, the vacuum 
purification means is switched for the second half-cycle to cells 36 and 
40, to begin the job of drawing off the CO.sub.2 and H.sub.2 O absorbed 
and adsorbed on the granules detained in these cells during the first 
half-cycle.