Method for densely packing molecular sieve adsorbent beds in a PSA system

The improvement in the separation of gaseous mixtures by pressure swing adsorption (PSA) is disclosed. The interparticle voids in kinetically-selective PSA adsorbent beds are filled with fine particles of kinetically-selective adsorbent. The ratio of the average diameter of the coarse adsorbent particles to the average diameter of the fine particles, the size of the fine particles themselves and the percent of volume of the fine particles in the bed are all critical to optimum PSA performance.

This invention relates to gas enrichment utilizing pressure swing 
adsorption techniques, and more particularly to an improved pressure swing 
adsorption process wherein the efficiency of the adsorbent bed is improved 
by packing with coarse and fine kinetically-selective molecular sieve 
particles. 
BACKGROUND OF THE INVENTION 
The use of adsorption techniques to separate a gaseous component from a 
gaseous stream was initially developed for the removal of carbon dioxide 
and water from air. Gas adsorption techniques are now conventionally 
employed in processes for the enrichment of hydrogen, helium, argon, 
carbon monoxide, carbon dioxide, nitrous oxide, oxygen and nitrogen. Gas 
enrichment utilizing at least one, typically two, adsorption vessels in a 
cycling pressurized relationship is commonly referred to as pressure swing 
adsorption (PSA). 
A typical PSA process for enriching a gas, for example nitrogen from air, 
employs at least one, generally two or more, adsorption beds filled with 
molecular sieve material, each being subjected to two or more, generally 
four, distinct processing steps in each cycle. In a first step of the 
cycle, one adsorption bed is pressurized with concomitant nitrogen 
production while the other bed is regenerated, such as by venting. In a 
second step, often referred to as pressure equalization, the adsorption 
beds are placed in fluid communication, thereby being brought to an 
intermediate pressure. In a third step, the first adsorption bed is 
regenerated, sometimes with a countercurrent flow of product-quality gas 
to enhance the regeneration (referred to as "purge"), while the second bed 
is pressurized with concomitant nitrogen product. The last step of the 
cycle is pressure equalization between the beds. During such pressure 
swings, pressure conditions in the adsorption beds typically vary from 
about 15 psig to 120 psig in a process employing carbon molecular sieves 
for nitrogen production and somewhat lower pressure ranges in processes 
employing crystalline zeolites for producing oxygen. 
Although pressure swing adsorption (PSA) techniques have been refined to 
some degree, PSA still suffers certain disadvantages inherent in being a 
cyclic process. For example, in the process of separating a strongly 
adsorbed component from a weakly adsorbed product component of a gaseous 
mixture, the purge step of the PSA cycle serves the desirable function of 
removing the strongly adsorbed component from the sieve, but is also 
accompanied by an undesirable loss of the product component which is 
contained in the interparticle voids of the bed. The interparticle voidage 
of a typical adsorbent bed is about forty percent of the total bed volume 
and, therefore, losses from this source can be significant. 
The problem is substantially alleviated and overall performance of the PSA 
process markedly improved in accordance with the present invention by 
combining certain percentages of fine particulate kinetically-selective 
molecular sieve with conventional coarse particles thereof to achieve an 
optimum volume ratio of comparatively coarse and fine 
kinetically-selective sieve particles in the bed. A very significant 
enhancement in yield can be achieved by using these beds in PSA processes, 
such as nitrogen enrichment. 
It is known to combine in a vessel coarse and fine particles intended for 
adsorption of a material. Ma, U.S. Pat. No. 3,757,490, discloses such a 
particle mix in a system intended for solid-liquid chromatographic 
separations. The particles utilized by Ma are all active adsorbent 
particles and are relatively close in size range in that ninety percent by 
weight have a diameter within ten percent of the average diameter of all 
particles. Ma is also concerned only with a solid-liquid system which is 
markedly different from a PSA gas separation system. Most important, the 
particles of adsorbent in Ma's system are equilibrium selective as opposed 
to kinetically selective as will be discussed below. 
More recently, Greenbank in European Patent 0 218 403 discloses a dense gas 
pack of coarse and fine adsorbent particles wherein the size of the 
largest fine particles is less than one-third of the coarse particles and 
sixty percent of all particles are larger than sixty mesh. Although not 
specifically stated, it is evident from the examples that these 
percentages are by volume. This system is designed primarily for enhancing 
gas volume to be stored in a storage cylinder. It is mentioned, however, 
that it can be utilized for molecular sieves. There is nothing in this 
application, however, which would give insight into the fact that 
significantly enhanced PSA efficiency could be obtained by combining 
coarse and fine particles of kinetically-selective sieve material in a 
single bed. It has been found in accordance with the present invention 
that, within certain limits as will be defined, a mixture of coarse and 
fine kinetically-selective sieve particles will unexpectedly give enhanced 
PSA performance. 
SUMMARY OF THE INVENTION 
There is provided a means of significantly enhancing the performance of a 
pressure swing adsorption system by packing the adsorbent beds thereof 
with kinetically-selective molecular sieve adsorbent from about ten to 
fifty percent by volume of fine particles and from about fifty to ninety 
percent by volume of coarse particles, wherein the ratio of the average 
diameter of the coarse particles, or pellets, to the average diameter of 
the fine particles is from about five to about fifteen to one and the fine 
particles are greater than 200 mesh, preferably forty to sixty mesh.

DETAILED DESCRIPTION OF THE INVENTION 
In order to appreciate the unexpected nature of the present invention, it 
is initially necessary to understand the basic differences between 
equilibrium adsorption and kinetic adsorption. For prospective, an 
adsorbent which behaves as a true molecular sieve performs "steric" 
separation. Separation based on a large difference in the adsorption rate 
between or among components of a gaseous mixture to be separated is known 
as "kinetic" separation. Separation based on the difference in the 
equilibrium amount of the adsorption an adsorbent provides is 
"equilibrium" separation. All adsorbents have equilibrium adsorption 
capabilities, but they are of widely varying effect on a given process. 
Zeolites, for example, are capable of performing steric separation, but 
most commercial applications thereof, including air separation are based 
on equilibrium separation. Carbon molecular sieve (CMS), on the other 
hand, separates air in a pressure swing adsorption (PSA) unit kinetically 
since, for a CMS, the amount of nitrogen adsorbed at equilibrium is almost 
equal to the amount of oxygen adsorbed. However, the adsorption rate for 
oxygen is much greater that of nitrogen. It is, therefore, possible to 
produce purified nitrogen in a PSA unit utilizing a CMS by adsorbing large 
quantities of oxygen under nonequilibrium conditions. Certain adsorbents, 
such as modified 4A zeolites, will produce nitrogen at short cycles, i.e. 
under 20 seconds and enriched oxygen at longer cycle times. This 
represents the transition from kinetic to equilibrium separation. 
The present invention is directed to a PSA system wherein the adsorbent 
beds contain a particular mixture of coarse and fine kinetically-selective 
adsorbent material, i.e. molecular sieve. The size relationship between 
the coarse and fine adsorbent particles, as well as the ratio of percent 
by volume of each type of material, are critical parameters in improving 
PSA efficiency. 
Pressure swing adsorption (PSA) is a known process which can be 
advantageously employed to selectively adsorb various components of 
readily available feed gas mixtures, thereby separating and purifying a 
desired product gas. For example, PSA can be advantageously used to 
separate nitrogen from air. Other applications of PSA include the 
separation and purification of hydrogen present as a major component of a 
feed gas mixture also containing carbon dioxide as a selectively 
adsorbable component, commonly together with one or more additional minor 
components to be removed as undesired impurities, such as nitrogen, argon, 
carbon monoxide, and the like. 
The PSA process, in general, can be carried out using any 
kinetically-selective adsorbent material having a selectivity for one or 
more components of a gaseous mixture. Suitable adsorbents include certain 
zeolite molecular sieves and activated carbon. Zeolite molecular sieve 
adsorbents are generally preferable for the separation and purification of 
hydrogen contained in mixtures with carbon dioxide and the like. The 
preferred material utilized to separate nitrogen from air is an activated 
carbon having pores which have been modified so that oxygen molecules are 
kinetically selectively adsorbed from nitrogen molecules commonly known as 
carbon molecular sieve (CMS). A preferred CMS material is prepared 
according to the process described in U.S. Pat. No. 4,458,022. 
A basic two-bed PSA process is shown schematically in FIG. 1. In FIG. 1, 
the valves controlling flow of feed into the system, product withdraw and 
waste gas venting from adsorbent beds A and B are numbered 1 through 10. A 
full cycle on a conventional PSA unit as shown in FIG. 1 is as follows: 
______________________________________ 
Step Bed A Bed B 
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1 --Bed Pressure equalization -- 
2 Pressurization and Product 
Vent to Atmos- 
release pheric Pressure 
3 Constant feed and Product 
Vent to Atmos- 
release pheric Pressure 
4 --Bed Pressure equalization -- 
5 Vent to atmospheric pressure 
Pressurization and 
product release 
6 Vent to atmospheric pressure 
Constant feed and 
product release 
______________________________________ 
Typical timing and value positions are shown in FIG. 2 using a cycle time 
of 120 seconds and 100 psig product pressure. 
In conventional PSA processes, the adsorbent columns are packed with 
adsorbent material, e.g. carbon molecular sieve (CMS), in pellet or bead 
form. The pellet form of CMS as is commercially available can have an 
average diameter of from 1 to 7 mm, preferably 2,5 to 3 mm. The packing of 
a bed with such CMS material results in a certain amount of void volume 
which is the space between the pellets or beads where no material will 
fit. The gas that fills the void volume does not interact with the sieve 
material and, therefore, adversely affects the efficiency of the unit in 
several ways. The impure gas in the void space will combine with product 
gas thereby reducing productivity Interstitial gas in the void spaces must 
be vented during regeneration thereby producing an increased amount of 
vent gas and, if a purge step is utilized, requiring a greater quantity of 
purge gas to clean the bed. If the purge gas is product quality, this 
represents an additional loss in efficiency. 
An adsorbent capable of kinetic adsorption can also perform equilibrium 
separation simply by lengthening the cycle time. The length of time 
necessary for an adsorbent to adsorb 50 percent of its capacity for a 
particular gas species is known as the "adsorption half-time". If the 
process cycle time is much greater than the adsorption half-time for the 
gas component most slowly adsorbed, there will be an equilibrium 
separation. If the cycle time is shorter, the separation will be kinetic. 
In order to kinetically separate a gas mixture containing two main 
components, such as air, there must be a significant difference in their 
adsorption half-times. For an equilibrium separation, there must be a 
significant difference in the equilibrium adsorption constants. These two 
ratios are not necessarily compatible. 
Mixing coarse and fine particles of an adsorbent will improve an 
equilibrium separation of a fluid mixture by reducing the void volume in 
the bed. Since the particles have the same particle density, the mixture 
will have a higher density than either the coarse or fine component alone. 
Consequently, increasing the packing density of the adsorbent in an 
equilibrium separation will improve the separation as is the case with Ma 
U.S. Pat. No. 3,757,490. The magnitude of the bulk density increases with 
the difference in size ratio of the particles. Therefore, greater size 
ratio of coarse to fine particles will improve equilibrium separation. 
This is true up to the point where the reduction in void volume produces 
adverse hydrodynamic effects and a deterioration in the process. 
For a kinetic separation, however, other factors come into consideration 
which negate the benefit of reduced voidage. Reducing the particle size of 
kinetic adsorbents increases their speed and, as a consequence, reduces 
half-times. Therefore, for normal PSA cycles, reduction in particles size 
of the adsorbent brings the separation closer to equilibrium. Since a 
kinetic adsorbent such as CMS cannot perform air separation at equilibrium 
conditions, it would be expected that, as particle decreases, kinetic 
separation becomes more difficult. 
These considerations would be expected to hold true for a mixture of coarse 
and fine kinetically-selective adsorbent since, as the difference in 
average particle size of the mix increases, the difference in the 
disparity in half-times also increases. An increase in the variance 
between kinetic half-times is known to lead to a loss of performance for 
kinetic separations. This is true even when the particles are uniform in 
size and the variance is caused by other factors. All other factors being 
constant, it would be expected, therefore, that mixing coarse and fine 
particles of a kinetically-selective adsorbent would adversely affect the 
separation, i.e., exactly the opposite of what would be expected from the 
effect on an equilibrium separation. 
Although it is to be expected that kinetic separation would always lead to 
a deterioration in performance when coarse and fine kinetically-selective 
adsorbent particles are combined, it has been found in accordance with 
this invention that there exists a size range of such a mixture where the 
separation is improved over that of the coarse component alone. The change 
in performance is increased with increasing fine particle size moving from 
a performance less relative to the coarse particles alone to an eventual 
performance improvement. 
In a PSA unit wherein the fine particles in the adsorbent bed are too 
small, fluidization of the particles within the bed will take place due to 
the large pressure changes. The fluidized fines exert a grinding action 
which reduces the size and effectiveness of the coarse particles and 
reduces the fines themselves to dust. Those skilled in the art recognize 
that dust is very undesirable in a PSA operation both as a product 
contaminant and because of detrimental effects, e.g. plugging on valves, 
analytical instrumentation and the like. 
We have found that specific ranges for particle size for the fines, the 
size ratio of the fines to the coarse particles, and the percent by volume 
ratio of the fines to the coarse particles in the bed, are all essential 
to the optimum performance of the PSA unit. There are disadvantages in 
utilizing fine particles of too small as well as too large dimension. With 
both particles, the critical size dimension is the average diameter as 
defined by a weight averaging technique. This critical factor is the 
extruded diameter for commercial pellets or the geometric average of the 
screen openings for the upper and lower mesh sizes for special or 
irregularly-shaped fines. The fine particles according to the present 
invention are greater than 200 mesh, i.e. all particles will be retained 
on a U.S. Standard Mesh 200 sieve. When utilizing commercially available 
CMS pellets having an average diameter of 2.5 to 3 millimeters, for 
example, the fines should preferably have a particle size of -40/+60 U.S. 
Standard Mesh, i.e. all particles will pass a U.S. Standard Mesh 40sieve 
and be retained on a U.S. Standard 60 Mesh sieve. This is equivalent to a 
particle size of from about 250 to 375 microns. It will be appreciated by 
those skilled in the art that these dimensions are exemplary and represent 
an optimum range for the diameter of the coarse particles. 
The relative size ratio of the coarse to fine particles is likewise 
critical in achieving optimum PSA performance for a kinetically-selective 
adsorbent material, i.e., CMS. We have found that the ratio of the 
critical dimension of the coarse particles, i.e. the average diameter of 
commercial CMS pellets, to the average diameter, as defined herein, of the 
fine particles should be between about 5:1 and 15:1, preferably between 
about 6.6:1 and 12:1. While these ratios are generally applicable, those 
skilled in the art will appreciate that there are practical size 
limitations of commercially available CMS or other adsorbents and that 
these, in turn, dictate the size limitations of the fine particles to be 
combined therewith. 
The final criterion to be considered in achieving an adsorbent bed packing 
for optimum PSA performance is the volume ratio of fine to coarse 
particles in the bed. It will be appreciated that the volume ratio and 
weight ratio will be approximately the same when the fine particles are 
comminuted coarse adsorbent material. Hence, it is preferred to express 
the percentage fill in terms of volume. It has been found that the percent 
by volume of fine particles in the bed should be from about 10 to 50, 
preferably from about 38 to 42, and most preferably about forty. 
The following Examples further illustrate this invention, it being 
understood that the invention is in no way intended to be limited to the 
details described therein. In the Examples, all parts and percentages are 
on a volume basis and all temperatures are in degrees Celsius, unless 
otherwise stated. 
EXAMPLE 1 
A series of experimental runs was conducted utilizing a conventional PSA 
unit as illustrated in FIG. 1, and a range of cycle times of from 90 to 
480 seconds according to the flow diagram as shown in FIG. 2. The 
adsorbent beds contained approximately 2 liters of commercial 2.5 mm CMS, 
Kuraray Chemical Company, density 0.664 g/ml. The 2.5mm refers to the 
average diameter of the pellets. 
A second series of runs was conducted under the same conditions utilizing a 
bed packing of about sixty percent of the commercial CMS material and 
about forty percent of communited CMS material having a particle size 
range of -40/+60 mesh, i.e. all particles will pass a 40 mesh U.S. 
Standard sieve and be retained on a 60 mesh U.S. Standard sieve, density, 
0.749 g/ml. The ratio of the average diameter of the CMS material to the 
average diameter of the communited fines was about 10:1. At steady state, 
product purity was ninety-nine percent. The results are given in Table I. 
All runs were at 100 psig pressure. 
TABLE I 
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Cycle Time Spec. Prod. Mass Sp. Prod. 
Yield 
Seconds Liter/Hr. m.sup.3 /Tonne/Hr. 
Percent 
______________________________________ 
Commercial CMS 
480 45.76 68.92 55.45 
480 48.27 72.69 57.14 
360 57.90 87.20 55.59 
Commercial CMS 
360 58.31 87.82 53.53 
180 74.79 112.64 50.53 
180 75.25 113.33 52.08 
120 82.23 124.74 47.43 
120 83.41 125.62 46.30 
90 85.96 129.46 41.86 
60% Commercial CMS/40% Fines (-40, +60 Mesh) 
480 49.52 66.11 57.84 
480 50.50 67.43 62.43 
360 66.60 88.92 56.72 
360 69.14 92.31 54.94 
360 70.07 93.55 55.81 
180 82.22 109.77 42.24 
180 90.00 120.16 53.96 
180 95.19 127.09 53.89 
180 99.23 132.48 55.39 
180 96.72 129.13 52.64 
120 106.26 141.87 49.52 
120 111.44 148.80 49.20 
90 105.53 140.89 46.41 
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The data in Table I demonstrate improved results for all cycles utilizing 
the coarse/fines mixture. The greatest degree of improvement was produced 
using cycle times between 120 and 180 seconds. The improvement in yield 
and mass specific product obtained is considered commercially significant.