Molecular sieve abstract
A primary molecular sieve drying bed is regenerated by circulating
a hot inert gas through the heated primary bed to desorb water held
on the bed. The inert gas plus water vapor is then cooled and passed
through an auxiliary molecular sieve bed which adsorbs the water
originally desorbed from the primary bed. The main advantage of
the regeneration technique is that the partial pressure of water
can be reduced to the 10.sup.-9 atm. range. This is significant
in certain CTR applications where tritiated water (T.sub.2 O, HTO)
must be collected and kept at very low partial pressure.
Molecular sieve claims
What I claim is:
1. An improved method for removing tritiated water from a fuel
recovery system of a controlled thermonuclear reactor while maintaining
same at a partial pressure of water (P.sub.W) not greater than 10.sup.-8
atmospheres, comprising the steps of: providing the fuel recovery
system with a molecular sieve drying bed through which a gas containing
tritiated water is circulated to remove a substantial portion of
the tritiated water from said gas and regenerating the molecular
sieve drying bed; the step of regenerating the bed comprising: circulating
a hot inert gas through the molecular sieve drying bed to desorb
tritiated water held on the bed, passing the inert gas plus tritiated
water vapor through a cooling means where it is cooled to a temperature
below about 25.degree. C. (77.degree. F.), directing the thus cooled
inert gas plus water vapor through an auxiliary molecular sieve
bed which adsorbs the water vapor from the inert gas thereby drying
the inert gas, and heating the thus dried inert gas for recirculation
thereof through the first-mentioned molecular sieve drying bed.
2. The method defined in claim 1 wherein the inert gas and water
vapor is cooled to not more than about -20.degree. C. (-4.degree.
F.), and wherein the partial pressure of water (P.sub.W) is reduced
to about 10.sup.-9 to 10.sup.-10 atmospheres.
3. The method defined in claim 1 wherein the steps of regenerating
the primary molecular sieve bed can be carried out by selecting
appropriate combinations of the residual loading of water and the
operating temperature for a particular application, wherein the
partial pressure of water in the moist inert gas is reduced to at
least about 10.sup.-9 atm.
4. The combination of a fuel-recovery system of a controlled thermonuclear
reactor, which utilizes tritiated water and a primary molecular-sieve
drying bed, and an apparatus, which regenerates said primary bed
and maintains a partial pressure of water (P.sub.W) not greater
than 10.sup.-8 atmospheres in said primary bed; said primary bed
being constructed such that a gas containing tritiated water is
circulated therethrough and functions to remove a substantial portion
of the tritiated water from the gas and constructed to have an inlet
and an outlet; said regeneration apparatus including an auxiliary
molecular-sieve bed having an inlet and an outlet, a cooler positioned
flowwise intermediate said primary bed and said auxiliary bed and
flow connected to said outlet of said primary bed and to said inlet
of said auxiliary bed, a heater positioned flowwise intermediate
said auxiliary bed and said primary bed and flow connected to said
outlet of said auxiliary bed and said inlet of said primary bed,
and means for circulating a heated inert gas sequentially through
said primary bed, said cooler, and said auxiliary bed so that the
heated inert gas desorbs tritiated water held on said primary bed
creating a moist inert gas which is cooled and passed through said
auxiliary bed which adsorbs the tritiated water from the moist inert
gas thereby drying the inert gas which is again heated and circulated
through said primary bed thus regenerating said primary bed and
maintaining therein a partial pressure of water (P.sub.W) no greater
than 10.sup.-8 atmospheres.
5. The apparatus defined in claim 4 wherein said means for circulating
an inert gas comprises a blower positioned intermediate said auxiliary
bed and said heater.
6. The apparatus defined in claim 4 wherein said inert gas is
selected from the group composed essentially of helium, argon, gas
not adsorbed by the molecular sieve bed, and mixtures thereof.
7. The apparatus defined in claim 6 wherein said primary and auxiliary
bed contain molecular sieve materials composed essentially of crystalline
alumino-silicates having a zeolite structure.
8. The apparatus defined in claim 4 wherein said moist inert gas
has a temperature of not lower than about -20.degree. C. and the
partial pressure of water (P.sub.w) is about 10.sup.-9 -10.sup.-10
atmospheres.
Molecular sieve description
BACKGROUND OF THE INVENTION
The invention described herein was made in the course of, or under,
Contract No. W-7405-ENG-48 with the Energy Research and Development
Administration.
The invention relates to regeneration of molecular sieve drying
beds, and particularly to a regeneration technique using an auxiliary
molecular sieve bed whereby the partial pressure of water can be
reduced to a very low level.
Molecular sieves (complex aluminum-silicates) are widely used for
drying gases. When the drying bed reaches its adsorption capacity,
it is regenerated by passing a hot inert gas through the bed to
drive off (desorb) the adsorbed water. In these prior systems, a
circulating inert gas is heated and passed through the "saturated"
molecular sieve bed to desorb the adsorbed water. The moist gas
is then cooled below its dew point and passed through a liquid-gas
separator. The separated water is drained, and the dry gas phase
is recirculated through the system. This prior type of regeneration
system is capable of reducing the partial pressure of water (P.sub.w)
in the drying bed to about 10.sup.-7 atm., when it is returned to
normal service. While this is adequate in most cases, certain critical
applications occur where P.sub.w must be maintained at much lower
levels. For example, in certain controlled thermonuclear reactor
(CTR) applications tritiated water (T.sub.2 O, HTO) must be collected
and P.sub.w maintained in the range of 10.sup.-9 atm. This P.sub.w
level is a factor of 100 lower than that obtainable with conventional
systems for regenerating molecular sieve dryers. Thus, a need exists
in the molecular sieve drying field for a regeneration technique
capable of maintaining the P.sub.w at the 10.sup.-9 or lower level.
SUMMARY OF THE INVENTION
The present invention is an improved system for regenerating molecular
sieve drying beds, which basically involves utilizing an auxiliary
molecular sieve bed whereby enhanced drying of the inert gas lowers
the P.sub.w to 10.sup.-9 and possibly 10.sup.-10 atmospheres, thereby
filling the above-mentioned need. Thus, the invention has particular
CTR applications where tritiated water (T.sub.2 O, HTO) must be
collected and kept at very low partial pressure.
Therefore, it is an object of this invention to provide an improved
technique for the regeneration of molecular sieve drying beds.
A further object of the invention is to provide a molecular sieve
drying bed regeneration system utilizing an auxiliary drying bed.
Another object of the invention is to provide regeneration of molecular
sieve drying beds wherein the partial pressure of water can be reduced
to at least the 10.sup.-9 atm. range.
Another object of the invention is to provide regeneration of molecular
sieve drying beds utilized in CTR applications where tritiated water
must be collected and kept at very low partial pressure.
Another object of the invention is to provide a means whereby tritiated
water can be readily recovered after its storage at very low partial
pressures, in order to permit further processing of the valuable
tritium.
Another object of the invention is a means of regenerating molecular
sieve drying beds to extremely low residual loadings of tritiated
water while concurrently maintaining the tritiated water recovered
thereby on an auxiliary molecular sieve bed in an essentially solid
(adsorbed) state, thereby minimizing the possibility of accidential
release as liquid or vapor of the dangerous, radioactive tritiated
water.
Other objects of the invention will become readily apparent to
those skilled in the art from the following description and accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of the prior art system;
FIG. 2 is a schematic view of the regeneration system of the invention;
FIG. 3 graphically illustrates a specific regeneration cycle using
the FIG. 2 system; and
FIG. 4 schematically illustrates a fuel recovery system for a CTR
utilizing the invention.
DESCRIPTION OF THE INVENTION
The invention is directed to an improved method and apparatus for
regenerating molecular sieve drying beds. The main advantage of
the regeneration system of the invention is that the partial pressure
of water can be reduced to the 10.sup.-9 atm. range or lower, which
is of particular importance in controlled thermonuclear reactor
(CTR) applications where tritiated water (T.sub.2 O, HTO) must be
collected and kept at very low partial pressure.
As pointed out above, molecular sieves (complex aluminum-silicates)
are widely used for drying gases, and when the drying bed reaches
its adsorption capacity it is regenerated by passing a hot inert
gas through the bed to drive off or desorb the adsorbed water. FIG.
1 illustrates a conventional system for regenerating a molecular
sieve bed. A circulating inert gas is heated by the heater assembly
and passed through the "saturated" molecular sieve bed,
as indicated by the flow arrows, to desorb the adsorbed water in
the bed, whereafter the moist inert gas is then cooled by the cooler
assembly to below its dew point and passed, as indicated by the
flow arrow, through a liquid-gas separator. The separated water
is drained from the bottom of the separator and the dry gas phase
is recirculated through the molecular sieve bed via a blower and
the heater, as indicated by flow arrows.
While the prior known regeneration systems are capable of reducing
the partial pressure of water (P.sub.w) in the drying bed to about
10.sup.-7 atmosphere (atm.) and is thus adequate for most applications,
certain critical applications occur where the P.sub.w must be maintained
at much lower levels, such as 10.sup.-9 atm., a factor of 100 lower
than obtainable with the conventional system of FIG. 1.
FIG. 2 illustrates the regeneration system of the present invention
and basically involves replacing the liquid-gas separator of the
FIG. 1 system with an auxiliary molecular sieve bed. The enhanced
drying of the inert gas obtainable with the auxiliary bed lowers
the P.sub.w to 10.sup.-9 to 10.sup.-10 atmospheres. Thus, CTR applications
using tritiated water (T.sub.2 O, HTO) which must be collected and
kept at very low partial pressure can effectively incorporate the
improved regenerating system.
As shown in FIG. 2 the improved regeneration system comprises a
primary molecular sieve bed 10 having an inlet connected to an outlet
of a heater 11 and an outlet connected to an inlet of a cooler 12
via tubing indicated at 13 and 14 respectively, with an auxiliary
molecular sieve bed 15 connected to the inlet of heater 11 via a
blower 16 and tubing 17 and 18 and connected to the outlet of cooler
12 via tubing 19. Thus, as indicated by the flow arrows and legends,
inert gas is circulated by blower 16 through heater 11 where it
is heated and directed through primary bed 10 to desorb or pickup
water held on the bed, whereafter the inert gas plus water (water
vapor) is cooled in cooler 12 and passed through auxiliary bed 15
which adsorbs the water originally desorbed from the primary bed,
the dry gas phase from auxiliary bed 15 is recirculated by blower
16 through the system. The inert gas may be composed of helium,
argon, air, other gases not adsorbed by the molecular sieves, or
mixtures thereof.
A specific regeneration cycle using the FIG. 2 system is shown
in FIG. 3 which is a set of water adsorption isotherms for Linde
type 5A (manufactured by Union Carbide Corporation) molecular sieve
type pellets, which are composed essentially of crystalline alumino-silicates
having a zeolite structure. The cycle of the primary bed is at the
lower left; the cycle of the auxiliary bed is above and to the right.
If the primary bed has a maximum loading of 0.5 wt. % water, regeneration
proceeds as follows:
1. The primary bed is heated to 315.degree. C. (600.degree. F.),
raising the P.sub.w to .about.5.times.10.sup.-4 atm.
2. Inert gas, such as helium, heated to a temperature of about
315.degree. C., circulating in the system passes through the primary
bed and picks up water (desorption), whereafter it is cooled to
at least 25.degree. C. (77.degree. F.), and gives up water on the
auxiliary bed (adsorption).
3. The auxiliary bed can pick up water to a loading of .about.10
wt. %. The limit of this system is that the minimum P.sub.w of the
hot primary bed (point B in FIG. 3) must always be greater than
the maximum P.sub.w of the cold auxiliary bed (point A in FIG. 3)
during regeneration. Otherwise, there would be no driving force
for transfer of water vapor from the primary bed to the auxiliary
bed.
4. When the effluent is cooled to 25.degree. C. (77.degree. F.),
the primary bed can dry to as low as P.sub.w =.about.2.times.10.sup.-8
atm.
5. The auxiliary bed can be regenerated by conventional means,
such as shown in FIG. 1. If its moist effluent is cooled to a dew
point of 0.degree. C. (32.degree. F.), the auxiliary bed will desorb
to .about.1.1% residual loading (point C in FIG. 3), during its
regeneration cycle.
Residual (not recoverable) loadings in the primary bed are kept
to 0.2 wt. %. Residual loading in the auxiliary bed is higher, but
this is more than offset by the auxiliary bed's smaller size. If
"moist" gas (from a fuel recovery system, or other associated
system) entering the primary bed for normal drying can be chilled
to, say -20.degree. C. (-4.degree. F.), the P.sub.w can be reduced
to less than 2.times.10.sup.-9 atm., and possibly as low as 2.times.10.sup.-10
atm. This low value for P.sub.w would be adequate for even the most
critical applications, such as the CTR tritiated water collection
mentioned above.
By inspection of FIG. 3 it is readily apparent that for any given
operating temperature, P.sub.w can be reduced by operating at a
reduced residual water loading; conversely, for any given residual
water loading, P.sub.w can be reduced by operating at a lower temperature.
Thus, to obtain inert gas having a given P.sub.w, there exist several
combinations of operating temperature and residual water loading
which will be satisfactory. The best combination for a particular
application depends on factors such as size, economics, etc.
FIG. 4 illustrates an application of the FIG. 2 regeneration system
in a fuel-recovery system of a CTR utilizing tritiated water (T.sub.2
O). The CTR fuel recovery system of FIG. 4 may, for example, be
that of a mirror hybrid fusion-fission reactor described in report
UCRL-51797 by R. W. Moir et al, University of California, Lawrence
Livermore Laboratory, released for public distribution on Oct. 10
1975.
Referring now to FIG. 4 the fuel recovery system associated with
a reactor 20 will process, as follows, helium containing both T.sub.2
O and T.sub.2 taken downstream of a power cycle heat exchanger 21
at .about.300.degree. C. via tubing 22:
1. The gas is cooled to -20.degree. C. by a main regenerative heat
exchanger 23 and refrigerator heat exchanger 24.
2. Essentially all T.sub.2 O is removed by a primary molecular
sieve bed 25.
3. The gas is then reheated to 600.degree. C. by the main regenerative
heat exchanger 23 and a makeup heat exchanger 26.
4. The gas then passes through a vanadium diffuser 27 wherein part
of the T.sub.2 diffuses through a bank of permeable vanadium tubes,
as indicated by arrow 28 and legend T.sub.2.
5. The final effluent from diffuser 27 is returned to the main
helium stream via tubing 29.
To prevent fluidization and attrition of the sieve material of
the FIG. 4 system, the primary molecular sieve bed 25 requires,
for example, an area of 5.2 m.sup.2. The bed contains 720 kg of
type 5A sieve pellets, giving a depth of 20 cm. In this short depth,
gas flow is still highly turbulent, and good drying is insured.
By operating at -20.degree. C. up to a maximum T.sub.2 O loading
equivalent to 0.5 wt. % H.sub.2 O, the bed will dry to an exit partial
pressure of .about.2.times.10.sup.-9 atm. of T.sub.2 O which is
necessary to permit proper operation of the vanadium diffuser 27
for recovery of T.sub.2 gas.
The primary bed 25 can be regenerated to a residual loading that
is equivalent to .about.0.2 wt. % H.sub.2 O. This bed will have
a total T.sub.2 O inventory equivalent to 1200 g of T.sub.2 of
which 720 g can be recovered and 480 g always stays on the sieve
(residual). The bed capacity is equal to 172 days recovery of T.sub.2
O for this example. T.sub.2 O (and H.sub.2 O) adsorbed on any molecular
sieve bed is held in an essentially solid state, as opposed to a
liquid or gaseous stage as in other stages of the process.
To regenerate the molecular sieve bed 25 to a 0.2 wt. % residual
loading, an auxiliary molecular sieve bed 30 is added along with
a heater 31 cooler 32 and blower 33 so as to form a regeneration
system similar to that illustrated in FIG. 2. Helium effluent at
315.degree. C. from the primary bed 25 is cooled to 25.degree. C.
and passed through the auxiliary bed 30. The bed 30 contains, for
example, 20 kg of type 5A molecular sieve pellets, and the loading
is allowed to rise to .about.12 wt. %. The bed 30 in turn, is regenerated
at 315.degree. C. in a manner similar to that shown in FIG. 1 and
well known to those skilled in the art, and its effluent chilled
to -25.degree. C. to collect the T.sub.2 O. When the auxiliary bed
30 is not in use, its residual T.sub.2 content will be 33 g. Appropriate
portions of these several systems will be valved off in a manner
readily apparent to those skilled in the art whenever another system
is in operation.
It has thus been shown that the present invention provides an improved
method and apparatus for regenerating molecular sieve drying beds,
with the main advantage being that the partial pressure of water
(P.sub.w) can be reduced to the 10.sup.-9 to 10.sup.-10 atm. range
which is of significance in certain CTR applications where tritiated
water must be collected and kept at very low partial pressure.
While the invention has been described with specific reference
to the regeneration of molecular sieve drying beds, the improved
regeneration system is generic and can be applied to drying agents
other than molecular sieves.
Fusion reactor components previously known as Controlled Thermonuclear
Reactor (CTR) Systems are now also being referred to by many in
this field as Magnetic Fusion Energy (MFE) Systems.
While particular embodiments of the invention have been illustrated
and described, modifications will become apparent to those skilled
in the art, and it is intended to cover in the appended claims all
such modifications as come within the spirit and scope of the invention. |