Hydrogen from ammonia

A reactor system for production of hydrogen by use of a hydride bed acting upon dissociated ammonia is formed of coaxial annular shells. A central one of the shells includes a hydride bed composed of pelletized material supported on a screen. Inner and outer shells contiguous to the central shell conduct fluid for heat exchange with the hydride bed. Inner and outer diameter of the central shell differ in length by an amount inversely proportional to the square root of the heat adsorption characteristic of the hydride.

FIELD OF THE INVENTION 
The present invention relates to apparatus for the production and delivery 
of hydrogen and to a method for producing and delivering hydrogen. More 
particularly, it concerns an economically feasible method of producing and 
delivering hydrogen on-site to users who require amounts of hydrogen 
intermediate in the range normally provided by the merchant hydrogen 
market. 
BACKGROUND OF THE INVENTION 
Hydrogen use rates in industry vary over a wide range, extending from the 
very small user who consumes one or two cylinders a year or about 0.03 cu 
m/day (1 cu ft/day) to the largest users who consume over 
2.8.times.10.sup.6 cu m/day (10.sup.8 cu ft/day). Over the range of use 
rates, which varies by a factor of 10.sup.8, the cost may vary by a factor 
of about 50. One of the major factors contributing to the large range in 
costs is the cost associated with the delivery of hydrogen. Larger users 
who regularly consume over 2.8.times.10.sup.5 cu m/month (10.sup.7 cu 
ft/month) of hydrogen generally generate their own hydrogen by steam 
reforming methane. Users of less than 2.8.times.10.sup.5 cu m/month 
(10.sup.7 cu ft/month) generally purchase their hydrogen from a merchant 
supplier. These users of merchant hydrogen can be divided into three 
groups depending on their demands namely; small users, intermediate users, 
and large users. Those users in the first of these groups (small users) 
pay primarily for packaging and delivery, with the actual cost of the 
hydrogen to the merchant suppliers representing only a few percent of the 
selling price. This group of users, however, is well served by the 
merchant suppliers since while their unit costs are very high, the 
quantities purchased are small, making their total annual cost too low to 
justify the consideration of alternative sources of supply. The users who 
fall into the third of these three groups (large users) tend to purchase 
liquid hydrogen which can be transported in large quantities at a 
relatively low cost as compared to transportation of gaseous hydrogen. 
Hence, these users are also reasonably well provided for by the merchant 
hydrogen supplier. The second of the three groups described above 
(intermediate users) are generally the most poorly served by the merchant 
hydrogen suppliers. Users in this group, while paying unit costs less than 
the small users, purchase large enough quantities of hydrogen to make it 
desirable to find alternatives to the purchase of merchant hydrogen. In 
particular those users who presently consume 28 to 2800 cu m/day (10.sup.3 
to 10.sup.5 cu ft/day) are the primary (but not exclusive) target of this 
invention. The present invention offers an economically feasible way for 
such users with intermediate requirements to obtain on-site generated 
hydrogen and to avoid high distribution costs associated with the purchase 
of merchant hydrogen. This is accomplished in an apparatus in which 
hydrides are employed to separate hydrogen from a dissociated ammonia gas 
stream. It is made possible by the particular reactor design for 
separating hydrogen and by the form in which the hydrides are used. 
It is known to use hydrides to recover hydrogen from gas mixtures and waste 
gas streams. In U.S. Pat. No. 4,036,944, for example, a process is 
disclosed to recover hydrogen from a gas stream containing a mixture of 
hydrocarbons, and the separation of hydrogen is effected using a hydride 
bed in a tube-shell heat exchanger. The process of U.S. Pat. No. 
4,036,944, like many other processes employing metal hydrides to separate 
hydrogen, is directed primarily towards the recovery of hydrogen from 
industrial waste gas streams. Dissociated ammonia has also been used as a 
source of hydrogen, the hydrogen being separated by diffusion through 
heated palladium. The costs associated with the use of the palladium based 
separation has generally led to the abandonment of this technology in 
favor of the purchase of merchant hydrogen. There are several technical 
and economic differences between the recovery of hydrogen from a hydrogen 
containing industrial waste gas stream and the on-site production of 
hydrogen by a process which starts with a dissociated ammonia feed stream. 
Thus, a technology that best serves one of these applications may not be 
the most appropriate for the other. 
It is an object of the present invention to provide a cost effective 
apparatus and method for providing on-site hydrogen to intermediate sized 
users of merchant hydrogen. A further object is to recover hydrogen from a 
gas stream substantially at atmospheric pressure and from an inexpensive 
readily available gas stream such as a dissociated ammonia stream. Other 
objects and advantages will become apparent from the following description 
taken in conjunction with the accompanying drawing.

THE INVENTION 
In accordance with the present invention, a process and apparatus are 
provided for delivery of hydrogen on-site. In general, the present 
invention contemplates the use of a reactor system for the delivery of 
between 28 to 2800 cu m/day (10.sup.3 to 10.sup.5 cu ft/day) hydrogen 
on-site from dissociated ammonia comprising at least one flow-through 
reactor, said reactor comprising a hydride bed and inner and outer heat 
exchanger shells, said hydride bed being oriented co-axially with respect 
to inner and outer heat exchanger shells; means to provide uniform gas 
flow with low pressure drop through the hydride bed; means for introducing 
dissociated ammonia feed to and means for permitting the flow of waste gas 
or hydrogen from the hydride bed, the hydride bed comprising a hydridable 
material which exothermally and selectively absorbs hydrogen and 
endothermally desorbs hydrogen, said heat exchanger shells being provided 
with circulating fluid and means to supply and extract heat therefrom, 
whereby said reactor when charged is capable of delivering hydrogen 
through desorption of hydrogen from hydridable material contained therein. 
Further in accordance with this invention, the performance of the reactor 
system can be predicted semi-quantitatively by the equation 
EQU F.sup.2 =(D.sub.1 +D.sub.2).sup.2 L.sup.2 .pi..sup.2 K2W.DELTA.T/.DELTA.Ht 
(1) 
where 
F=Hydrogen flow rate, moles/sec. 
D.sub.1 =Diameter of inner wall of the hydride shell, cm 
D.sub.2 =Diameter of outer wall of the hydride shell, cm 
L=Length of hydride bed, cm 
.pi.=Constant=3.14 
K=Thermal conductivity of hydride bed, cal.multidot.sec.sup.-1 
.multidot.cm.sup.-1 .multidot.K.sup.-1 
W=Grams of hydrogen per gram of hydride 
.DELTA.T=Temperature difference between reactor wall and mid-point of the 
hydride bed, .degree.Kelvin 
.DELTA.H=Heat of adsorption of hydride, cal/mole 
t=Charging or dischare time per unit front length, sec. and 
D.sub.2 is related to D.sub.1 by the relationship: 
EQU D.sub.2 =D.sub.1 +8[Kt.DELTA.T/.DELTA.H.rho.W].sup.1/2 (2) 
where .rho.=Density of hydride bed, gm/cc. 
Rearrangement of Equation 2 gives 
EQU t.DELTA.T=0.0156(D.sub.2 -D.sub.1).sup.2 .DELTA.H.rho.W/K (3) 
or if t is in hours 
EQU t.DELTA.T=4.34.times.10.sup.-6 (D.sub.2 -D.sub.1).sup.2 .DELTA.H.rho.W/K 
(4) 
The meaning of the term semi-quantitatively will be understood by reference 
to the subsequent discussion of the experimentally measured performance 
and the comparison of that performance with that predicted by the 
performance model as given by Equation 4. 
In accordance with one aspect of the invention the reactor system provided 
which is capable of a continuous delivery of hydrogen over the capacity of 
hydrogen absorption of the hydridable material in the hydride bed, said 
reactor system comprising at least two flow-through reactors capable of 
operating in tandem, said reactors being operationally connected so that 
at any one time one reactor in the system is capable of hydrogen delivery 
through desorption from hydridable material contained therein. By 
appropriate operational connection, one reactor can be capable of hydrogen 
delivery through desorption of hydrogen from hydridable material contained 
therein while another reactor absorbs hydrogen from the dissociated 
ammonia stream connected thereto. 
In the reactor system of the present invention, it is possible to lower the 
cost of hydrogen delivery for users requiring 28 to 2800 cu m/day 
(10.sup.3 to 10.sup.5 cu ft/day) by a factor of about 2 to 1.1, 
respectively. 
It is noted that the reactors used are "flow-through" reactors. In the 
context of the present invention, a flow-through reactor is one in which 
during the charging cycle a waste gas stream having a hydrogen 
concentration less than that of the inlet feed stream is exhausted from 
the reactor thereby preventing a build-up of the non-hydrogen component(s) 
of the feed stream. The heat transfer characteristics of the flow-through 
reactor of this invention are at the heart of its performance. The heat 
flow is interrelated to flow rate, pressure and recovery. These variables 
operate to establish the effective hydrogen absorption pressure. This, in 
turn, determines the peak temperature within the pellet bed, which 
relative to the coolant temperature establishes the temperature 
difference, .DELTA.T, causing heat flow. The flow-through reactor of this 
invention requires no high pressure containment or components so that it 
can be relatively large without being excessively massive. It has a high 
surface-to-volume ratio to provide good heat transfer, and it is easily 
scalable on the basis of hydrogen delivery requirements and a relatively 
simple performance model based on heat transfer considerations. 
Coordinated reactors can be designed to act alternately so that while one 
unit is absorbing hydrogen, the other is desorbing hydrogen. More than two 
units can be used with appropriate timing. For example, if the discharge 
time (DC) is 30 minutes and the charging time (C) is 60 minutes, then with 
3 reactors cycled as shown below, a continuous flow can be maintained with 
one reactor delivering hydrogen while 2 are being charged: 
______________________________________ 
Time Reactor 1 Reactor 2 Reactor 3 
______________________________________ 
T + 0 30 60 90 120 150 180 
##STR1## 
##STR2## 
##STR3## 
______________________________________ 
In this way hydrogen can be continuously provided. The heat necessary for 
desorption can be provided in part by the heat given off during the 
absorption phase and by an external source. For example, hot fluid, e.g. 
hot water, can be used to provide required heat. In the same way cold 
fluid, e.g. cold water, can be provided to the heat exchanger to aid in 
the absorption phase. 
One aspect of the present invention involves the use of suitable hydridable 
material for extraction of hydrogen from the ammonia stream. Many 
compositions are known which are capable of absorbing and desorbing 
hydrogen. In the present system, the hydridable materials must have an 
ambient temperature pressure plateau of less than one atmosphere, they 
must not be subject to poisoning by ammonia at the levels of concentration 
of ammonia in the dissociation gas stream, and they must not be subject to 
nitride formation under the operating conditions of the present invention. 
Hydridable materials useful in the practice of the present invention are 
those metals and alloys which can react with hydrogen in a chemically 
reversible manner to form hydride compounds. One class of hydridable 
materials is definable by the formula AB.sub.x wherein A is selected from 
the group of rare earth metals (including yttrium) and calcium, 
substitutable in an amount up to about 0.3 atom with a wide variety of 
metals, B is selected from the group of nickel and cobalt substitutable in 
an amount of up to about 1.5 atoms by a wide variety of metals and x is a 
number between 3 and 8. Another class of hydridable materials comprises 
pure or substantially pure elements from the group of magnesium, titanium, 
vanadium and niobium. A still further class of hydridable materials 
comprises AD.sub.m alloys where A is one or more rare earth metals 
including yttrium, D is one or both of cobalt and nickel or a mixture 
thereof with one or more of the elements Fe, Cu and Mn and m satisfies 
1/3.ltoreq.m&lt;3. Still further examples of hydridable materials are RMg and 
RNiMg (where R=rare earth) series alloys disclosed in U.S. Pat. No. 
4,126,242, titanium-vanadium-manganese and titanium-vanadium-iron alloys 
disclosed in U.S. Pat. No. 4,111,689; titanium-manganese alloys as well as 
conventional Ti-Ni and Ti-Fe alloys disclosed in U.S. Pat. No. 4,144,103 
and conventional Zr-Ni Mg-Ni and Mg-Cu alloys disclosed in U.S. Pat. No. 
4,110,425. Those skilled in the art will appreciate that other alloy and 
intermetallic compound systems involving two, three, four, five and even 
more elements are also known to form chemically reversible hydrides and 
are included within the ambit of the term hydridable material for purposes 
of the present specification and claims. The following are examples of 
suitable hydridable materials: Fe.sub.0.8 Ni.sub.0.2 Ti, ZrCr.sub.0.5 
Fe.sub.1.5, LaNi.sub.4.7 Al.sub.0.3, MNi.sub.4 Al, (where M is 
mischmetal), LaNi.sub.4 Cu, and La.sub.1-x Ca.sub.x Ni.sub.5 (where 
x.ltoreq.0.5). 
It is well known that one problem associated with the hydrides is that they 
expand considerably on absorption of hydrogen. Merely using a smaller 
volume of hydrides in the reactor and leaving room for expansion will not 
solve this problem since this would cause poor gas-to-hydride contact in 
the reactor. Another difficulty is that the hydrides continuously 
decrepitate in use and would lead to excessively high pressure drop across 
the reactor bed as well as loss of the fine particles. Further, handling 
of the very fine powder is dangerous because it can be pyrophoric. To 
obviate such problems, the hydrides in the present system are pelletized. 
And it is another aspect of the present invention that the pelletized 
hydrides when used in the reactor described meet the following 
requirements: (1) pelletization does not adversely reduce the performance 
of the reactor during either the hydriding or dehydriding phase; (2) the 
pellets are stable under thermal cycling over the required temperature 
ranges; (3) pelletization does not reduce significantly heat transfer in 
the hydride bed; (4) the pellet and material can accommodate the volume 
changes of the hydride and can contain the fine particles; and (5) the 
pellets have a size and shape such that the pressure drop in the 
flow-through reactor is very small. 
Suitable pellets for use in the present process and a process for 
pelletization of hydridable materials have been disclosed by Bernstein et 
al in U.S. patent application Ser. No. 226,454 filed on Jan. 19, 1981. The 
use of pellets in combination with ballast material has also been 
disclosed in U.S. patent application Ser. No. 11,194 filed Feb. 12, 1979. 
A particularly suitable pelleted hydride is shown in the examples of the 
present invention. 
To aid in the understanding of this invention reference is made to the 
schematic flow-through reactor shown in FIG. 1. The reactor (10) is an 
insulated cylindrical shell (11) containing a cylindrical housing (12) for 
pelletized hydridable material, which material constitutes the hydride bed 
(13). The cylindrical housing (12) containing pelletized hydride is 
oriented co-axially with respect to inner shell (14) and outer shell (15), 
in which inner and outer shells a heat exchange fluid can be circulated. 
Support screen (16) for the pelletized hydride are located at the bottom 
of cylindrical housing (12). Inlet means (17), a conduit for delivery of 
dissociated ammonia to the hydride bed (13), contains valve (18). Outlet 
means (19), containing valve (20), is a conduit which serves for the 
passage of the effluent hydrogen product stream or waste gas stream from 
the hydride. Inlet means (21) and outlet means (22) are conduits which 
serve, respectively, for entry and exit of a heat exchange fluid into and 
from outer shell (15). By means not shown, heat exchange fluid is 
delivered to the inner shell (14) at conduit means (23) and exits shell 
(14) at conduit means (24). Pressure in the hydride bed (13) is measured 
by pressure gauge (25). Relief valve (26) is provided to prevent 
overpressurization of the hydride bed (13). 
In operation dissociated ammonia is fed through inlet (17) to flowthrough 
reactor (12) containing a pelletized hydridable material, i.e., the 
hydride bed (13), e.g., at a packing density of about 1.1 to 5 g/cc, 
depending on the hydride. Cooling means, e.g., water circulating through 
outer shell (15), through inlet (21) and out outlet (22) is circulated 
then through the inner cooling shell (14) through inlet (23) and out 
outlet means (24). When hydrogen from the stream is fully absorbed by the 
hydrides, as detected (detector not shown) for example, by the 
concentration of hydrogen in the effluent stream from outlet (19), the 
dissociated ammonia stream is switched to a companion reactor (not shown), 
and the valve (18) is closed. The water in the inner and outer cooling 
shells is heated during the absorption of hydrogen by the hydrides. 
Auxiliary heating means (not shown) is used to raise the temperature of 
the water if necessary and hydrogen is discharged from the hydrides 
through outlet (19), heat being absorbed from the water. The absorption of 
hydrogen is effected in the second reactor during desorption of hydrogen 
in the first reactor. 
It can be seen that by having a plurality of flow-through reactors, a 
continuous system for hydrogen delivery can be designed. 
In order to give those of ordinary skill in the art a greater appreciation 
of the invention the following illustrative examples are given. 
In the examples given below the pellets are formulated with LaNi.sub.4.7 
Al.sub.0.3. At 0.3 formula atoms aluminum, the capacity of the hydride is 
quite high, about 0.8 hydrogen/metal atom (H/M) at 20 psia. The absorption 
and desorption plateau pressures of about 3.5 psia at 10.degree. C. and 60 
psia at 90.degree. C., respectively, are attractive in relation to the 
hydrogen recovery efficiency and available thermal inputs, both in the 
laboratory and commercially. 
The alloy was pelleted with a silicone rubber binder (General Electric 
Stock No. GE 2567-012 clear.). The following procedure was employed in the 
preparation of the pellets. The hydridable alloy is first hydrided and 
dehydrided several times to produce a -325 mesh powder. The powder is 
stabilized by slow exposure of the hydrided material to air. The 
stabilized powder is then pelleted by mulling it with silicone rubber in 
the amount of 5% silicon rubber to 95% alloy until uniform, and the mulled 
mixture is pressed in a 1/2".times.4" die (1/4" thick) at 30,000 lbs., 
cured in a 100.degree. C. oven for 16 hours and then cut to 1/4" cubes 
(pellets). 
The reactor was functionally the same design as that shown in FIG. 1, 
however there were some differences to provide some of the flexibility 
required for test and evaluation. The reactor was constructed of 0.318 cm 
stainless steel cyclinder walls fixed to flanged end plates 1.27 cm thick. 
The end plates were bolted to 1.25 cm thick manifold plates with O-ring 
seals. Gas was distributed through a chamber below the porous support 
platform. This platform was sealed to the walls with silicone rubber to 
prevent channeling. The reactor was designed for pellets of lower density 
than the pellets actually used. As a result only about 30 percent of the 
available heat transfer surface was in contact with the pellet bed. Thus, 
in comparing the experimental performance with the model, the actual bed 
length was used rather than the length of the reactor. 
The absorption and desorption tests were run in an essentially isobaric 
manner at nominally constant temperature of the cooling (heating) water. 
The operating mode was selected to provide the most direct basis for 
comparisons and extrapolation of performance trends rather than those 
which would be most suited for ultimate use of the apparatus. 
The temperature and flow rate of the cooling (heating) water was controlled 
by an external bath circulator and cooler. Gas flows was measured using 
electronic mass flow meters with 100 liters/minute sensors. In experiments 
involving hydrogen separation from a mixed gas, electronic flow meter 
sensors were used on both the inlet and outlet gas streams. The output 
from these sensors were coupled to three readout units to indicate 
directly the 75% H.sub.2 +25% H.sub.2 inlet flow, the H.sub.2 absorption 
flow, and the N.sub.2 -enriched outlet flow. 
The performance of the cylindrical flow-through reactor for the separation 
of H.sub.2 from dissociated ammonia was evaluated in three parts. The 
first was the operation of the reactor in pure H.sub.2 to provide base 
line data for later comparisons. The second part was the operation in a 
75% H.sub.2 +25% N.sub.2 mixed gas made to simulate dissociated ammonia. 
The third part was the comparison of the experimental results to the 
performance model outlined above. 
PURE HYDROGEN OPERATION 
A series of absorption and desorption tests were run with pure hydrogen. 
The experiments were carried out at a nominal temperature of 25.degree. C. 
They showed the increasing rate of absorption and temperature excursion as 
the charging pressure is increased. The peak temperature at each pressure 
was quite close to that predicted by the Van't Hoff relation for the 
alloy. These test results are given in the Table and in FIG. 2. 
HYDROGEN SEATION 
Hydrogen separation was evaluated using a 75% H.sub.2 -25% N.sub.2 premixed 
gas to simulate dissociated ammonia. Separations were performed using 
three different combinations of controlled operating parameters. Two tests 
were performed at a nominal temperature of 25.degree. C. and were 
structured to give slow and fast operation which correspond to high and 
medium H.sub.2 recovery efficiencies. After these two tests, a pure 
H.sub.2 adsorption was run to determine if there had been any adverse 
effect on the base line performance due to the mixed gas operation. This 
was followed by a third separation test run at a nominal temperature of 
5.degree. C. These test results are given in the Table and in FIG. 2. 
TABLE 
______________________________________ 
No.Test 
Test .degree.C..DELTA.T, 
Hourst*, 
t*.DELTA.t 
##STR4## .vertline.%.vertline.Deviation 
______________________________________ 
1 A 50 1.75 87.5 26.6 23.3 
2 B 44 2.15 94.6 19.5 17.1 
3 B 41 2.40 98.4 15.7 13.8 
4 A 37 2.50 92.5 21.6 18.9 
5 A 37 2.90 107.3 6.8 6.0 
6 A 39 3.10 120.9 6.8 6.0 
7 C 30 4.40 132.0 17.9 15.7 
8 A 26 4.50 117.0 2.9 2.5 
9 C 23 5.10 117.3 3.2 2.8 
10 A 22 5.40 118.8 4.7 4.1 
11 B 25 5.60 140.0 25.9 22.7 
12 C 22 5.60 123.2 9.1 8.0 
13 C 16 8.40 134.4 20.3 17.8 
Average 114.1 12.2 
______________________________________ 
A = Absorption TestsUHP-H.sub.2 - 
B = H.sub.2 Separation Tests75% H.sub.2 + 25% N.sub.2 
C = Desorption TestsUHP-H.sub.2 - 
*t = Time for absorption or desorption to 95% of capacity 
The separation test results show overall that good H.sub.2 separation was 
achieved, and that it was in reasonable accord with the performance 
anticipated from the pure hydrogen base line data. Separation efficiencies 
for the tests fell between 65 and 70%. 
REACTOR MODEL 
The performance model previously described predicts from Equation 4 that 
for a given system 
EQU t.DELTA.T=C (5) 
where the constant, C, in Equation 5 is given by 
EQU C=4.34.times.10.sup.-6 (D.sub.2 -D.sub.1).sup.2 .DELTA.H.rho.W/K (6) 
when t is in hours. 
From the data given in Table I and plotted in FIG. 2 one finds a value of C 
equal to 114.1 hr.multidot.K with a maximum deviation of 23.3% and an 
average deviation of 12.2%. For the LaNi.sub.4.7 Al.sub.0.3 alloy pellets 
used as the hydridable material in the prototype reactor .rho.=3.2 gm/cc, 
W=0.0114 gm of hydrogen per gm of hydride and .DELTA.H=7200 cal/mole. The 
reactor bed thickness, .DELTA.r, was 6.0 cm, thus D.sub.2 -D.sub.1 
=.DELTA.D=2.DELTA.r=12 cm. Employing these values in Equation 6 gives a 
value for K of 1.44.times.10.sup.-3 cal.multidot.cm.sup.-1 
.multidot.sec.sup.-1 .multidot.K.sup.-1. The best independent evaluation 
of the thermal conductivity of the pellets gave a value for K of 
1.5.times.10.sup.-4 cal.multidot.cm.sup.-1 .multidot.sec.sup.-1 
.multidot.K.sup.-1. Thus the overall quality of the agreement between the 
behavior of the flow-through reactor predicted by the performance model 
and the experimentally observed behavior, demonstrates the utility of the 
performance model in semi-quantitatively predicting the bahavior of a 
flow-through reactor designed in accordance with the principles described 
above. 
The general agreement between test data and the reactor model establishes 
the utility of the model for predicting effects of process or system 
variations. For example, the original concept aim was that of a system 
with a one hour cycle time for a 10.degree. C. temperature difference. 
Several variations of the reactor size are shown in FIG. 2, with reactor 
bed thickness, .DELTA.r, ranging from 6 cm to 2 cm. Objectives may be 
accomplished without change in any other dimension of the reactor. This is 
possible because the void volume above the pellet bed is about 3 times the 
volume of the bed itself. 
Other alternatives are also possible. For example, if a hydride bed 
thickness of 2 cm presented any problem, a value of 3 cm could be 
considered along with certain other changes. At 3 cm bed thickness, a 
2.5-hour cycle time would be required for a 10.degree. C. temperature 
difference. A one-hour cycle time could be achieved if the temperature 
differences was increased to 25.degree. C. This could be done by 
increasing the system pressure (if possible) or by lowering the plateau 
temperature of the alloy by 15.degree. C. (corresponding to roughly a 
factor of 2 decrease in the isotherm pressure). 
Another factor implicit in the model is that the bed functions relatively 
uniformly over its entire length. If a breakthrough front becomes well 
defined, then the model only applies in the region of the front. This 
could be accounted in the model, to a first approximation, by modification 
of the time term such that: 
##EQU1## 
This effect could become significant as the bed length increases. The 15 
cm bed length used in the demonstration test series showed only slight 
evidence of non-uniform bed operation. However, decreasing the bed 
thickness to 3 or 2 cm, as in the above examples, would increase the 
length of 30 or 45 cm. At these lengths, non-uniform bed operation is more 
likely and may require introduction of Equation (7) into the model. The 
effect will be to increase the cycle time predicted by the model. 
The above considerations are based on retention of the thermal conductivity 
and density of the pellets as tested. If the pellet formulation were 
changed, then other system variations might also be considered or 
required. The important fact, however, is that considerable system 
flexibility exists which encompasses a practical range of operating 
conditions and which can be tailored and predicted based on the results of 
this demonstration reactor. 
PELLET STABILITY 
The silicone-bonded pellets were removed from the reactor following 
completion of the test series. The pellet bed and individual pellets were 
examined physically for signs of structural breakdown. No evidence was 
found for particulate formation, settling, or expansion. Any of these 
changes might lead, with continued cycling, to packing and increasing back 
pressure within the bed. Indeed, in these preliminary tests all the 
obvious characteristics of the pellets appeared to be unchanged from their 
initial condition. 
Although the present invention has been described in conjunction with 
preferred embodiments, it is to be understood that modifications and 
variations may be resorted to without departing from the spirit and scope 
of the invention, as those skilled in the art will readily understand. 
Such modifications and variations are considered to be within the purview 
and scope of the invention and appended claims.