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Патент USA US3078649

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Feb. 26, 1963
3,078,640
R. M. MILTON
SEPARATION OF SULFUR COMPOUNDS FROM VAPOR MIXTURES
Filed Dec. 18, 1959
3 Sheets-Sheet l
FIG. I.
ZEOLITE A ADSORPTION CAPACITY
For Various Temperature Ratios
(AdsGoZcrAbiena‘vl/mIOs)gd
W°SECUIOAL/DMGPH‘°TRUBNED
5
I;
6
3
m
/
/
I
/
/
OJ
0.2
0.3
0.4
0.5
0.6
0.1
0.8
TEM PERATURE RATIO T2/Tl (Tlond T2 in °K)
INVENTOR.
ROBERT M. MILTON
w A TTORNE)’.
Feb. 26, 1963
3,078,640
R. M. MILTON
SEPARATION OF SULFUR COMPOUNDS FROM VAPOR MIXTURES
Filed Dec. 18, 1959
3 Sheets-Sheet 2
\
IN6
_'
_
w
N
(v amcaz pawmov slums ool/awqmspv sums)
oasuosov swoauvoouom ozuvamvs %.LH9|3M
INVENTOR.
ROBERT M. MILTON
BY
WM
ATTORNE)’.
Feb. 26, 1963
3,078,640
R. M. MILTON
SEPARATION OF SULFUR COMPOUNDS FROM VAPOR MIXTURES
Filed Dec. 18, 1959
5 Sheets-Sheet 3
ZEOLITE A ADSORPTION CAPACITY
For Various Temperature Ro?os
an
a
LIT
232:M0o6m95;wgq“:532.0863
4s|m
m
n
m
m
m
o2
TE
OM. . P
4E
5
EUmmaWTz RonTM 0 . T2G
A
d
BY
o5
o. 7
0.8
INVENTOR.
ROBERT M. MILTQN
.
FI63.
A TTORNE Y.
United States Patent O?ice
8,078,640
Patented Feb. 26, 1963’.
1
2
ecules are excluded. Not all adsorbents behave in the‘
3,078,640
manner of the molecular sieves.
SEPARATION OF SULFUR COMPOUNDS
FROM VAPOR MIXTURES
Such common adsorb
ents are charcoal and silica gel, for example, do not ex-.
hibit molecular sieve action.
Robert M. Milton, Buffalo, N.Y., assignor to Union
'
Zeolite A consists basically of a three-dimensionalv
Carbide Corporation, a corporation of New York
framework of SiO.; and A104 tetrahedra. The tetrahedra.
are cross-linked by the sharing of oxygen atoms so, that
Filed Dec. 18, 1959, Ser. No. 860,526
7 Claims. (Cl. 55-73)
the ratio of oxygen atoms to the total of the aluminum and ,
This invention relates to a method for adsorbing ?uids
silicon atoms is equal to two or O/(Al-l-Si) =2.
More particularly, the invention relates to amethod of
adsorbing hydrogen sul?de with adsorbents of the mo
lecular sieve type. Still more particularly, the invention
relates to a method for preferentially adsorbing hydro
example, an alkali or alkaline earth metal ion.
balance may be expressed by the formula
'
The
and separating a mixture of ?uids into its component parts. ' 10 electrovalence of the tetrahedra containing aluminum is,
balanced by the inclusion in the crystal of a cation, for.
gen sul?de from a vapor mixture containing at least one " 15
member of the group consisting of hydrogen, carbon di
oxide and normal saturated aliphatic hydrocarbons con-,
This
-Al2/ (Ca, Sr, Ba,-Na2, K2) =1
One cation ‘may be‘ exchanged for another‘ by ion"ex-‘"
change techniques which are described below...The spaces
between the tetrahedra are occupied by water molecules"
taining less than nine carbon atoms per molecule.
prior to dehydration.
This separation is advantageous in, for example, sweet
ening the de?ned vapor streamsthereby preventing the. 20 Zeolite‘ A may be activated by heating to effect ‘the.
loss of the water‘of hydration.‘ The dehydration results
deposition of sulfur compounds which can cause plugging
in crystals interlaced with channels of molecular dimen-v
and corrosion of transmission pipes, valves, regulators and
the like. Also, the sulfur compounds may produce un
desirable side reactions with other materials contacting
the vapor mixture.
>
sions that offer very high surface areas for the adsorption
of foreign molecules. These interstitial channels will not‘
25 accept molecules that have a maximum dimension of the
minimum projected cross-section in excess of about 5.5v
A. Factors in?uencing occlusiongby the activated zeolite.‘
A crystals are the size and polarizing power of ‘the inter
stitial cation, the polarizability and polarity of the ‘oc
Broadly, the invention-comprises mixing-molecules, in
a fluid state, of the materials to be adsorbed or separated
with at least partially dehydrated crystalline synthetic
metal-aluminum-silicates, which will be described more
particularly below, and effecting the adsorption of the 30 c'luded molecules, the dimensions and'shape of the'sorb‘ed
the process of the invention is in some respects similar to‘
molecule‘relative to those‘ of the channels, the duration
and severity of dehydration and desorption, and the pres
naturally occurring zeolites. Accordingly, the term “zeo
lite” would appear to be appropriately applied to these
will be understood that the ‘refusal characteristics of‘
adsorbate by the silicate. The synthetic silicate used in.
ence of foreign molecules in the interstitial channels. ‘Itv
zeolite A are quite as important as the adsorptive or posi'-‘
materials. There are, however, signi?cant differences be
tive adsorption characteristics.
tween the synthetic and natural silicates. To distinguish
Although there are a ‘number of cations that may be
the synthetic material used in the method of the inven
present in zeolite A it is preferred to formulate or.syn-.
tion from the natural zeolites and other similar synthetic
thesize the sodium form of the crystal since the reactants
silicates, the sodium-aluminum-silicate and its derivatives
taught hereinafter to be useful in the process of the in 40 are readily available and water soluble. The sodium "in
the sodium form of zeolite A may be easily exchanged
vention will be designated by the term “zeolite A.” While
for other cations as will be shown below. Essentially
the structure and preferred method of making zeolite A
the preferred process comprises heating a proper mix
will be discussed in some detail in this application, addi~
tional information about the material and its preparation
ture in aqueous solution of the oxides, or of materials‘.
whose chemical compositions can be completely. repre
may be found in an application ?led December 24, 1953,
Serial No. 400,388, now US. Patent 2,882,243.
sented as mixtures of the oxides, NaZO, A1203, SiOz and
_
It is the principal object of the present invention to
provide a process for the selective adsorption of hydrogen
' H2O, suitably at a temperature of about 100° ‘C. for
periods of time ranging from 15 minutes to 90 hours or
‘longer. The product which crystallizes from the hot
sul?de from ?uids. A further object'of the invention is
to provide a method whereby hydrogen sul?de 'may be 50 rnixt-ure is ?ltered off and washed With distilled 'water
adsorbed and separated by crystalline synthetic metal
until the effluent wash water in equilibrium with the zeo
aluminum-silicate from ?uid mixtures with other ,7 mol
' lite has a pH of from about 9-to 12. The material, after
ecules.
.
activation, is ready for use as a molecular sieve.
I
In the drawings,
'
‘ "
‘ Zeolite A may be distinguished from other zeolites and
FIG. 1 is a graph showing the amount of sulfur com 55 silicates on the basis of its X-ray powder diffraction pat
tern. The X-ray patterns for several of the ion yex—
pounds adsorbed versus the temperature ratio T2/T1 for
changed forms of zeolite A are described below. ‘Other
various forms of zeolite A;
FIG. 2 is a graph showing the amount ofC; through C8
characteristics that are useful in identifying zeolite A are;
its composition and density.
‘ > ~
~
normal saturated aliphatic hydrocarbons adsorbed versus
the temperature ratio Tz/Tl for various forms of zeolite 60 The basic formula for all crystalline zeolites where‘
“M” represents a metal and “)1” its valence may be repre-v
A; and
~
FIG. 3 is a graph showing the amount of carbon dioxide
sented as follows:
‘ '
adsorbed versus the temperature ratio T2/T1 for various
forms of zeolite A.
’
I
~
Certain adsorbents, including zeolite A, which selec-.
tively adsorb molecules on the basis of the size and shape
of the adsorbate molecule are referred to as molecular
sieves. These molecular sieves have a sorption area avail
M2O2Al2O3:XSlO2:YHzO
65
In general a particular crystalline zeolite will have values
The value X
for a particular zeolite will vary somewhat since the
. for X and Y that fall in a de?nite range.
aluminum atoms and the silicon atoms both-occupy esi
able on the inside of a large number of uniformly sized
pores of molecular dimensions. With such an arrange 70 sentially equivalent positions in the lattice. Minor vari
ations in ‘the relative’ numbers of these atoms do not
ment molecules of a certain size and shape enter the pores
signi?cantly alter the crystal structure or physical prop‘
and are adsorbed while larger or differently shaped mol
3,078,640
3
erties of the zeolite. =For zeolite A, numerous analyses
Form of Zeolite A
have shown that an average value for X is about 1.85.
The X value falls within the range 1.85 $0.5.
The value of Y likewise is not necessarily an invariant
for all samples of zeolite A particularly among the vari
ous ion exchanged forms of zeolite A. This is true be
cause various'exchangeable ions are of different size, and,
Percent of
Exchange
Sodium ______________________________ _ .
Lithium
since there is no major change in the crystal lattice di
100
__
Density,
gJce.
l. 99i0.1
65
1. 92i0. 1
Potassium . _ _ . . . . . _ _ . . .
. _ _ _ _ _ . ..
95
2. 08:1;0. 1
Cesium _ _ _ . _ _ _ . . . . . _ _ _
_ _ . l _ _ _ --
31
2. 265A). 1
Magnesium- _-_
75
2. 04=t;0. 1
Calcium ______ _.
93
2. 05:};0. 1
Thallous ............ ._
80
about 3.36
mensions upon’ ion exchange, more or less space should
In making the sodium form of zeolite A, representa
be available in the pores of the zeolite A to accommo~ 10
tive reactants are silica gel, silicic acid or sodium silicate
date water molecules. For instance, sodium zeolite A
as a source of silica. Alumina may be obtained from
was partially exchanged with magnesium, and lithium,
activated alumina, gamma alumina, alpha alumina,
and the pore volume of these 'forms, in the activated
alumina trihydrate, or sodium aluminate. Sodium hy
condition,‘ measured with the ‘following results:
15 droxide may supply the sodium ion and in addition assist
in controlling the pH. Preferably the reactants are water
soluble. A solution of the reactants in the proper pro
portions is placed in a container, suitably of metal or
glass. The container is closed to prevent loss of water
Na .......................................... _.
0
5.1
Mg
-.
-_- .
75
'5. 8 20 and the reactants heated for the required time.
A con
K‘
i
95
4
venient and preferred procedure for preparing the reactant
Ca .......................................... __
,93
5
mixture‘ is to make an aqueous solution containing the
sodium aluminate and hydroxide and add this, preferably
The average value for Y thus determined for the'fully
with agitation, to an aqueous solution of sodium silicate.
hydrated sodium zeolite A was 5.1; and in varying con
ditions of hydration, the value of Y can vary ‘from 5.1 25 The system is stirred until homogeneous or until any gel
which forms is broken into a nearly homogeneous mix.
to essentially zero. The maximum value of Y has been
After this mixing, agitation may be stopped as it is un
found in 75% exchanged magnesium zeolite A, the fully
necessary to'agitate the reacting vmass during the forma
hydrated form of which has .a Y value of 5.8. In gen
tion and crystallization of the zeolite, however, mixing
eral an increase in the degree of ion exchange ‘of the
magnesium form of zeolite A results in an increase in the 30 during formation and crystallization has not been found to
be detrimental. The initial mixing of ingredients is con
Y fvalu'e; Larger values, up to 6, may be obtained with
Ion Exehanged Form of Zeolite A
more fully ion exchanged materials.
Percent
Na Ions
Replaced
‘
Value of Y
veniently done at room temperature but this is not essen
'
In ‘zeolite A synthesized according to the preferred pro
tial.
'
In the synthesis of zeolite A, it has been found that the
cedure, the ratio Na2O/Al2O3 should equal one. But if
all of the excess alkali present in the mother liquor is 35 composition of the reacting mixture is critical. The
not washed out of the precipitated product, analysis ‘may
crystallizing temperature and the length of time the
show a ratio greater than one, and if the washing is car
crystallizing temperature is maintained are important
variables in determining the yield of crystalline material.
ried too far, some sodium may be ion exchanged by
hydrogen, and the ratio will drop below one.’ Thus, a
typical analysis for a thoroughly washed sodium zeolite
Under some conditions, for example too low a tempera
40 ture for too short a time, no crystalline materials are pro
duced. Extreme conditions may also result in the pro
A ‘ is 0.99Na2O:1.0Al2O3:1.85SiO2:5.1H2O. The ratio
duction of materials other than zeolite A.
Nam/A120, has varied as much as 23%. The composi
' The sodium form of zeolite A has been produced at
tion for zeolite A lies in the range of
‘
MO
El a
45
100‘? C., ‘essentially free from contaminating materials,
from reacting mixtures whose compositions, expressed
as mixtures of the oxides, fall within either of the follow
ing ranges.
where “M” represents a metal and “n” its valence.
' Thus the formula for zeolite A may be 'written as
follows:
50
‘“
1.0 =1: 0.2M 2 O:Al;0;:1.85 i 0.5Si0z2YH2O
F
In this formula f‘M” represents a metal, “n” its valence,
Range 1
Sim/A1203 __________________________________ __
NaiO/SiO; __________________________________ __
HgO/NauO .
. ._.
.____
Range 2
0.5-1. 3
1. 0-3. 0
1. 3-2. 5
0. 8-3. 0
35-200
35-200
and ‘.‘Y” may be any value up to 6 depending on the
' When zeolite A has been prepared, mixed with other
chemical analysis. When other materials as well as water
tivity, characteristic of zeolite A are present in the prop
identity of the metal and the degree of dehydration of 55 materials, the X-ray pattern of the mixture can be repro
duced by a simple proportional addition of the X-ray pat
the crystals.
terns of the individual pure components.
The pores of zeolite A are normally ?lled with water
Other properties, for instance molecular sieve selec
and in this case, the above formula represents their
are in the pores of zeolite A, chemical analysis will show 60 erties of the mixture to the extent that zeolite A is part
of the mixture.
a lower value of Y and the presence of other adsorbates.
The adsorbents contemplated herein include not only
The presence in the pores of non-volatile materials,
the sodium form of zeolite A as synthesized above from
such as sodium chloride and sodium hydroxide, which
a sodium-aluminum-silicate-water system with sodium as
are not removable under normal conditions of activation
at temperatures of vfrom 100° C. to 650° _C. does not 65 the exchangeable cation but also crystalline materials ob
tained from such a zeolite by partial or complete replace
signi?cantly alter the crystal lattice or structure of zeolite
ment of the sodium ion with other cations. The sodium
A although it will of necessity alter the chemical compo
cations can be replaced at least in part, by other ions.
These replacing ions can be classi?ed in the following
zeolite A were determined by the ?otation of the crystals 70 groups: metal ions in group I of the periodic table such as
potassium and silver, and group II metal ions such as
on liquids of appropriate densities. The technique and
calcium and strontium with the exception of barium.
liquids used are discussed in an article entitled “Density
Other cationic metal zeolites are too complex in their
of Liquid Mixture” appearing in Acta Crystallographica,
preparation for use in the present invention.
1951, vol. 4, page 565. The densities ofseveral such
crystals are as follows:
75. The spatial arrangement of the aluminum, silicon, and
sition.
The apparent density of fully hydrated samples of
5
3,078,640
oxygen atoms which make up the basic crystal lattice of
the zeolite remains essentially unchanged by partial or
complete substitution of the sodium ion by other cations.
The X-ray patterns of the ion exchanged forms of the
zeolite A show the ‘same principal lines at essentially the
same positions, but there are some differences in the rela_
gt.
tive intensities of the X-ray lines, due to the ion exchange.
Ion exchange of the sodium form of zeolite A (which
for convenience may be represented as NazA) or other
6
large to enter the pore system of the zeolite freely, but
are not large enough to be excluded entirely, there is a
finite rate of adsorption and the amount adsorbed will
vary with time. In general, the recorded data represents
the adsorption occurring within the ?rst one or two hours,
and for some borderline molecules, further adsorptionl
may be expected during periods of ten to ?fteen hours.
Washing techniques, different heat treatments and the
crystal size of the sodium zeolite A powder can cause
forms of zeolite A may be accomplished by conventional 10 very'appreciable differences in adsorption rates for the‘
ion exchange methods. A preferred continuous method
borderline molecules.
is to pack zeolite A into a series of vertical columns with
The calcium and magnesium exchanged zeolite A have
suitable supports at the bottom; successively pass through
molecular sieve adsorptive properties characteristic of
the beds a Water solution of a soluble salt of the cation
to be introduced into the zeolite; and change the ?ow
from the ?rst bed to the second bed as the zeolite'in the
?rst bed becomes ion exchanged to the desiredsextent.
materials with larger pores than exist in sodium zeolitev
A. These two forms of divalent ion exchanged zeolite
A behave quite similarly and adsorb all molecules ad
sorbed by sodium zeolite A plus' some larger molecules. ,
To obtain hydrogen exchange, a water solution of an
There are numerous other ion exchanged forms of
acid such as hydrochloric acid is effective as the exchang
zeolite
A such as lithium, ammonium, silver, zinc, nickel,
ing solution. For sodium exchange, a water solution of 20 hydrogen
and strontium. In general, the divalent ion
sodium chloride is suitable. Other convenient reagents
exchanged materials such as zinc, nickel and strontium
are: for potassium exchange, a water solution of potas
zeolite A have a sieving action similar to that of calcium
sium chloride or dilute potassium hydroxide (pH not over
and magnesium zeolite A, and the monovalent ion ex-_
about 12); for lithium, magnesium, calcium, ammonium,
changed materials such as lithium and hydrogen zeolite
nickel, or strontium exchange, water solutions of the 25 A behave similarly to sodium zeolite A, although some
chlorides of these elements; for zinc exchange, a water
di?erences exist.
‘A
solution of zinc nitrate; and for silver exchange, a silver
Another unique property of zeolite A is its strong
nitrate solution.
While it is more convenient to use Water
preference for polar and polarizable molecules, provid-f
soluble compounds of the exchange cations, other solu
ing of course that these molecules are of av size and
tions containing the desired cations or hydrated cations 30 shape
permitting them to enter the pore system of the
may be used.
zeolites. This is in contrast to charcoal and silica gel
Among the ways of identifying zeolite A and distin
which show a main preference based on thevolatility
guishing it from other zeolites and other crystalline sub
of the adsorbate. A selectivity for polar and polarizable
stances, the X-ray powder diffraction pattern has been
molecules is not new among adsorbents. Silica gel ex:
found to be a useful tool. The X-ray powder diffraction
hibits some preference for such molecules, but the extent
lines of zeolite A are set forth in the previously referenced
of this selectivity is so much greater with zeolite A that
US. Patent 2,882,243 to R. M; Milton, incorporated here
separation processes based upon this selectivity become
in ‘by reference.
_The zeolites contemplated herein exhibit adsorptive
Zeolite A shows a selectivity for adsorbates,.provided
properties that are unique among known adsorbents. The
that‘
they are small enough to enter thev porous network
common adsorbents, like charcoal and silica gel, show
of the zeolites, based on the boiling points of the, ad
adsorption selectivities based primarily on the boiling
sorbates, as well as on their polarity and polarizability.
point or critical temperature of the adsorbate. Activated
For
instance, hydrogen which has a low boiling point is
zeolite A on the other hand exhibits a selectivity 2based on
feasible.
.
_
,_
.
the size and shape of the adsorbate molecule. Among 45 not strongly adsorbed at room temperature.
A further important characteristic of zeolitev A is its
those adsorbate molecules whose size and shape are such
property of adsorbing large amounts of adsorbates' at
as to perrmit adsorption by zeolite A,'a very strong pref:
low adsorbate pressures, partial pressures or concentra
erence is exhibited toward those that are polar and polar~
tions. This property makes zeolite A uniquely useful in
izable. Another property of zeolite A that contributes
the more complete removal of adsorbable impurities from
to its unique position among adsorbents is that of adsorb
ing large quantities of adsorbate either at very low pres
sures, at very low partial pressures, or at very low con
centrations. One or a combination of one or more of
gas and liquid mixtures. It gives them a relatively high
adsorption capacity even when the material being ad;
sorbed from a mixture is present in very low» concentra-f
tions ,and permits the'e?icient recovery of minor‘c'om-i
thesethree adsorption characteristics or others can make
zeolite A useful for numerous gas or liquid separation 55 ponents of mixtures; This characteristic is all the more
important since adsorption processes are most frequently
processes where adsorbents are not now employed. The
used
when the desired component is present in low “con;
use of zeolite A permits more e'l?cient and more economi
centrations or low partial pressures. High adsorptions
oal operation of numerous processes now employing other
at low pressures or concentrations on zeolite A are illus~
adsorbents.
Common adsorbents like silica gel and charcoal do 60 trated in the following table, along with some compara'é
tive data for silica gel and charcoal.
not exhibit any appreciable molecular sieve action where
as the various forms of zeolite A do.
This is shown in
the tables following in the speci?cation, for typical sam
pics of the adsorbents. In these tables the term “Weight
percent adsorbed” refers to the percentage increase in
Tempera- Pressure
Weight; Percent‘ Adsorbed ',
Adsorbate ture(° O.) (mm.Hg)
“
,
NazA
CaA MgA
the weight of the adsorbent. The adsorbents were ac
tivated by heating them at a reduced pressure to remove
adsorbed materials. Throughout the speci?cation the
activation temperature for zeolite A was 350” C. and
the pressure at which it was heated as less than about
0.1 millimeter of mercury absolute unless otherwise speci
?ed. Throughout the speci?cation, unless otherwise in
dicated, the pressure given for each adsorption is the
pressure of the adsorbate at the adsorption conditions.
O02 ______ --
g
H28 ______ __
009 ...... -_
25
1.0
25
25
80
750
5.0
25
0.5
8.5
12.7
6.6
25
11
10.4
21.0
15.2
25
198
723.7
20.3
25.9
0
50
17
-
5.0
5.3
15.0 10.5
18.9; 24.4
15.0
22.2
Charcoal
"
' *
Silica
Gel. .
.......... .
Any cationic
........ form
-_
of000zeolite
21.8 A having a pore size of
‘In borderline cases where adsorbate molecules are too 75 at least 4 Angstroms is suitable for practicing ‘the present
3,078,640
invention.‘ Smaller pore sized forms, such as potassium
zeolite A do not admit hydrogen sul?de and the mer
captans.
The adsorption capacity of adsorbents usually decreases
with increasing temperature, and while the adsorption
8
That is, the T; values for these examples were obtained
from the preceding portion of the speci?cation and the
T2 values were read from the vapor pressure tables in
“Industrial and Engineering Chemistry,” vol. 39, page
517, April 194-7.
TABLE C
capacity of an adsorbent at a given temperature may be
su?icient, the capacity may be wholly unsatisfactory at
a higher temperature. With zeolite A a relatively high
capacity may be retained at higher temperatures.
Ion Form Zeolite A
Wt.
Temperature,
Pressure,
Percent
° K.
mm. Hg
1128 Adsorbed
Zeolite A may be activated by heating it in either air, 10
TI/Tl
T]
a vacuum, or other appropriate gas to temperatures of
as high as 600° C. The conditions used for desorption
of an adsorbate from zeolite A vary with the adsorbate,
0.5
11
198
0. 5
11
O. 5
11
198
5. 0
12
5.0
12
50
12
50
150
237
2. 7
1. 6
0. 15
5. 0
2. 7
12
50
by either raising the temperature or reducing the pressure,
partial pressure or concentration of the adsorbate in
contact with the adsorbent or a combination of these
steps is usually employed. Another method is to dis
place the adsorbate by adsorption of another more
strongly held adsorbate.
The present process for separating hydrogen sul?de
from certain vapor mixtures depends upon two related
properties 'of zeolite A with respect to the adsorbed
phase. The ?rst property is the selectivity of the in~
ternal surfaces of the crystal towards these strongly polar
compounds as compared to normal saturated aliphatic
hydrocarbons, hydrogen and carbon dioxide. As previ
ously discussed and illustrated by the tables, zeolite A
is capable of adsorbing all of these compounds based on
8. 5
16. 4
23. 7
l2. 7
21. 0
6. 6
15. 2
25. 9
11.1
14. 3
5. 8
7. 9
12. 0
2. 2
5. 0
7. 1
9. l
8.1
0.8
0. 5
10. 0
4. 7
8.1
11. 0
298
298
298
298
298
298
298
298
298
298
348
348
348
423
423
423
423
298
423
423
298
348
348
348
T2
133
157
191
133
157
133
157
191
150
158
150
158
174
158
174
191
196
144
136
127
150
144
158
174
0. 45
O. 53
0. 64
0. 45
U. 53
0. 45
0. 53
0. 64
0. 49
0. 52
0.43
0. 46
0.50
0.37
0.41
0. 45
0. 46
0. 47
O. 32
0. 30
0. 40
0. 41
0. 46
0. 50
a consideration of the zeolite A pore size and critical
An inspection of Table C will reveal that it includes
molecular dimensions of the compounds. For example, 30
adsorbate temperatures from 25° C. to 150° C. and ad
the pores of zeolite A are su?iciently large and in fact
sorbate pressures 0.5 mm. Hg to 237 mm. Hg. It was
do receive methane, ethane, propane, hydrogen and car
unexpectedly discovered that hydrogen sul?de, methyl
bon dioxide molecules.
mercaptan and ethyl mercaptan, all being freely adsorbed
Based on these considerations, one skilled in the art
would logically conclude that zeolite A would not possess 35 on zeolite A exhibit the same temperature ratio Tz/Tl
any particular selectivity for hydrogen sul?de in prefer
ence to the other constituents of the present vapor mix
ture. Contrary to these expectations, it has been dis
relationship to weight percent of sulfur compound ad
sorbed. That is, for a given T2/T1 value, the weight per
cent adsorbed will be the same for all of the previously
de?ned sulfur compounds. The present invention utilizes
lectivity for the enumerated sulfur compounds to the sub 40 this relationship in combination with the previously dis
cussed polar compound selectivity property of certain
stantial exclusion of normal saturated aliphatic hydrocar
crystalline
zeolitic molecular sieves to provide a novel
bons, hydrogen and carbon dioxide. One reason for this
covered that zeolite A possesses an extremely strong se
selectivity is the highly polar nature of hydrogen sul?de
separation process.
The present invention combines the previously dis
as compared with the other possible constituents of the
cussed
properties of zeolite A in such a manner that a
45
vapor mixture.
novel process is provided for separating hydrogen sul
The second interrelated property of zeolite A which
?de ?rom a vapor mixture containing at least one mem
is utilized by a preferred form of the present invention
ber selected from the group consisting of hydrogen, car
is the relationship of the boiling point or vapor tension
bon
dioxide and normal saturated aliphatic hydrocarbons
characteristics of an individual ?uid or clearly related
type of ?uid to the capacity of the crystalline zeolite to 50 containing less than nine carbon atoms per molecule.
In its broadest form, the process consists of contacting the
adsorb the ?uid at a given temperature and pressure.
vapor mixture with a bed of at least partially dehydrated
More speci?cally, it has been discovered that a rela
zeolite A adsorbent material having a pore size of at
tionship exists between the amount of ?uid adsorbed and
least about 4 Angstroms, thereby adsorbing the hydrogen
the temperature ratio T2/ T1 where T1 is the temperature
in degrees Kelvin during adsorption, assuming that the 55 sul?de. The 1hydrogen sul?de-depleted vapor mixture
is then discharged from the crystalline zeolite A bed.
temperature of the ?uid and the adsorbent are in equi
It is to be understood that the expression “pore size,"
librium. T2 is the temperature in degrees Kelvin at
as used herein refers to the apparent pore size, as dis
which the vapor pressure of the ?uid is equal to the par~
tinguished from the effective pore diameter. The appar
tial pressure or vapor tension of the ?uid in equilibrium
with the zeolite adsorbent. Stated in another way, T2 is 60 ent pore size may be de?ned as the maximum critical
dimension of the molecular species which is adsorbed
the temperature at which the vapor pressure of the ad
by
the zeolitic molecular sieve in question under normal
sorbate is equal to the partial pressure of the adsorbate
conditions. Maximum critical dimension may be de
during adsorption. T2 is actually the dew point of the
?ned as the diameter of the smallest cylinder which will
adsorbate at the adsorption conditions.
This relationship is clearly shown in FIG. 1 which 65 accommodate a model of the molecule constructed us
ing the best available values of bond distances, bond an
is a plot of the weight percent of sulfur compounds ad
gles, and Van der Waal radii. Effective pore diameter
sorbed versus the temperature ratio T2/ T1 for both mono
is de?ned as the free diameter of the appropriate sili
valent and divalent cationic forms of zeolite A having
cate ring in the zeolite structure. The apparent pore
a pore size of at least about 4 Angstroms. The tested
adsorbate in all cases was hydrogen sul?de, but it has 70 size for a given zeolitic molecular sieve will always be
larger than the eifective pore diameter.
been found that methyl and ethyl mercaptan behave simi
The previously described contact between zeolite A
larly. The following table is a summary of the‘ data
adsorbent material and the vapor mixture is preferably
from which FIG. 1 was prepared, the data for the ?rst
e?ected under conditions such that the temperature ratio
eight examples having been assembled from tests de
scribed in more detail in other parts of the speci?cation. 75 T2/ T1 with respect to the inlet end of the bed and with
3,078,640
respect to at least (ne of the enumerated sulfur com
pounds of the vapor mixture is between 0.31 and 1.0,
where T1 is the adsorption temperature and is less than
873° K., and T2 is the temperature at which the one
sulfur compound has a vapor pressure equal to its par
tial pressure in the vapor mixture. The lower limit of
0.31 for the temperature ratio T2/ T1 is ?xed by the dis
covery that below this value there is a smaller percentage
change in adsorption capacity per unit change in the
10"
a vapor pressure equal to the partial pressure of the
compound over the zeolite A bed at the end of the re
generation. It will be understood by those skilled in
the art that at least two adsorbent beds may be provided,
with one bed on adsorption stroke and the» other bed on
regeneration stroke. The respective flows are then pe-'
riodically switched when the ?rst ‘bed becomes loaded
with the adsorbate, so that the latter is placed on regen
eration stroke and the second bed is placed on-stream.
temperature ratio. In contrast, above 0.31 there is a 10
The continuous process is most e?iciently performed
if T1, the adsorption temperature, is less than 616° K.
change in the temperature ratio. Stated in another way,
but higher than 283° K., for previously stated reasons.
if it is desired to obtain a certain incremental adsorbate
Also, for maximum e?‘iciency during the adsorption
loading at a speci?ed adsorption temperature with a
stroke, T2 the dew point ofthe hydrogen sul?de of the
given feed stream, it would be necessary to ‘increase the 15 vapor mixture, is below 499'0 K. During the regenera
pressure of operation by a greater percent if the tem
tion stroke, T1 is preferably above 283° K. and below
larger percentage change in adsorption capacity per unit
perature ratio is below 0.31 than if it is maintained
616° K., also for the previously discussed reasons.
above this value in accordance with the invention. Also,
It will be understood by those skilled in the art that
the temperature ratio of 0.31 corresponds to a bed load
the temperature ratio may be adjusted by well-known
ing of about 0.5 weight percent adsorbate and if the tem 20 methods as for example heating the bed by direct or
perature ratio were reduced below this value, a larger
indirect heat transfer, employing a purge gas, or by draw
adsorption bed would be required with its attendant
ing a vacuum .on the bed during the regeneration stroke.
higher investment and operating expenses.
Also, during the adsorption stroke, the ratio may be ad
The upper limit of 1.0 for the temperature ratio should
justed for favorable operation by varying eitheror both
not be exceeded, because if the adsorption temperature 25 the temperature and the pressure.
is equal to or less than the dew point, condensation of
The many advantages of the invention are illustrated
the sulfur compound will occur, thereby essentially elim
inating the sieving action of the zeolite A adsorbent.
by the following examples.
quent loss of adsorption capacity and reduction in pore
feed mixture comprising 0.035 mole fraction HZS, 0.035
Example I
The broad upper limit of 873° K. for T1 is due to the
30
It is desired to remove hydrogen sul?de from a meth
fact that above this temperature, the crystal structure
ane-ethane vapor stream provided at 600 p.s.i.a., the
of zeolite A will be disrupted or damaged with conse
size, thereby fundamentally changing its adsorptive char
mole fraction ethane and the remainder methane. The
mixture is to be passed through a bed of sodium zeolite
acteristics. The present adsorption process is most ef
?ciently performed if T1, the adsorption temperature, is 35 A at a temperature of 218° C. (425° F.). The hydrogen
sul?de-loaded‘ zeolite A bed is to be regenerated at the
less than 616° K. but higher than 283° K. This is for
same temperature under a vacuum pressure su?icient to
the reason that above such range, the hydrocarbon con
obtain a loading of 1.5 weight percent adsorbate at the
stituents of the vapor feed stream in contact with zeolite
end of the regeneration stroke.
'
A will tend to isomerize, crack, aromatize and polymer
The potential capacity of the bed to adsorb hydrogen
ize, all of which will clog the pores and cause loss of 40
sul?de at the bed inlet section may be determined as
capacity of zeolite A molecular sieve. Also, to employ
follows: Since the partial pressure of hydrogen sul?de at
adsorption temperatures below about 283° K., a refrig
the inlet end will be 21 p.s.i.a., T2 will be 221° K., as
erating system would be required which increases the
extrapolated from the previously referenced vapor pres
complexity and expense of operation. Also, for maxi
sure table. Accordingly Tz/Tl will be 221/4911 or 0.45.
mum e?‘iciency, T2 the dew point of one of the sulfur
This temperature ratio will provide a loading of 7.0
compounds of the ?uid mixture is preferably below 499°
weight percent hydrogen sul?de on the zeolite A ad
K. corresponding to the critical temperature of ethyl
sorbent as determined by a reading of the FIG. 1 graph.
mercaptan. This is to more effectively utilize the adsorp
The potential capacity of the adsorbent bed inlet end for
tive capacity of zeolite A.
The present invention also contemplates a process for 50 methane and ethane may be ‘determined in a similar man
ner. That is, T2 for ethane is 193° K., so that T2/T1
continuously separating hydrogen sul?de from a vapor
will be 0.39. Refer-ring now to FIG. 2, which is a'plot
mixture containing at least one member of the group
of the weight percent of normal saturated aliphatic hy
drocarbons adsorbed versus the temperature ratio T2/T1,
containing less than nine carbon atoms per molecule,
hydrogen and carbon dioxide. 'Ihis continuous process 55 this corresponds to a potential loading of only about 0.6
weight percent ethane. For methane, T2 is 186° K., so
includes two steps, an adsorption stroke and a regenera
that T2/T1 will be 0.38. Referring again to FIG. 2,
tion stroke. The adsorption stroke is the same as the
consisting of normal ‘saturated aliphatic hydrocarbons
previously described adsorption where the temperature
the potential loading is only about 0.5 weight percent
ratio T2/T1 is between 0.31 and 1.0, and the broad range
for T1 is less than 873° K. In the regeneration stroke,
at least part of the adsorbed hydrogen sul?de is removed
by subjecting the zeolite A adsorbent to conditions such
that the temperature ratio T2/ T1 at the end of the regen
methane. The adsorption stroke can be terminated when
the amount of hydrogen sul?de reaches the maximum
tolerable concentration.
_
'
-
On regeneration, the end loading is to be 1.5% ad
sorbate, which corresponds to a T2/T1 value of 0.35.
If a pressure swing cycle is to be employed, T1 is ?xed
eration stroke with respect to at least one of the ad
sorbed sulfur compounds, is less than the temperature 65 at 491° K. so that the dew point T2 must change and be
equal to 174° K. or —99° C. This corresponds to a hy
ratio at the end of the adsorption stroke. Also, the dif
drogen sul?de vapor pressure of 50 mm. Hg, which should
ference in total adsorbate loading between the ends of
be the desorption pressure of the cycle. Thus, regenera
the adsorption and regeneration stroke is at least 0.1
tion is accomplished by maintaining a constant adsorption
weight percent for increased e?iciency of the overall con
tinuous process. A lower differential adsorbate loading 70 bed temperature but drawing a vacuum on the system.
would entail prohibitively large adsorber units. During
the regeneration stroke, T1 is the regeneration tempera
Example II
It is desired to remove hydrogen sul?de from a
ture and is less than 873° K. for the broad range, and
methane-hydrogen stream provided at 200 p.s.i.a., the
T2 is the temperature at which the hydrogen sul?de has 75 feed mixture comprising 0.00005 mole fraction HZS, 0.12
3,078,640
11'
12
2. A process for separating hydrogen sul?de from a
vapor mixture containing hydrogen sul?de and at least
one member of the group consisting of hydrogen, carbon
dioxide and normal saturated aliphatic hydrocarbons con
taining less than nine carbon atoms per molecule which
comprises contacting said vapor mixture with a bed of at
least partially dehydrated crystalline zeolite A adsorbent
mole vfraction methane, 0.0005 mole fraction propane,
the remainder being hydrogen. This mixture is to be
passed through a bed of sodium zeolite A at 25° C. The
hydrogen sul?de~loaded zeolite A bed is to be regener
ated at elevated temperature using the feed mixture as
a purge gas to obtain a loading of 0.5 weight percent at
the end of the stroke.
material, said material being capable of adsorbing mole
The potential capacity of the bed to adsorb hydrogen
clues that have a maximum dimension of the minimum
sul?de at the bed inlet section may be determined as
follows: Since the partial pressure of hydrogen sul?de 10 projected cross-section up to about 5.5 Angstroms, and
thereafter discharging the hydrogen-sul?de-depleted va~
at the inlet end will be 0.01 p.s.i.a., T2 will be 134° K.,
por stream from said bed.
as determined from the previously referenced vapor pres
3. A process ‘for separating hydrogen sul?de from a
sure table. Accordingly, T2/T1 will be 134/298 or 0.45.
vapor mixture containing hydrogen sul?de and at least
This temperature ratio will provide a loading of 7.0
weight percent hydrogen sul?de on the zeolite A ad 15 one member of the group consisting of hydrogen, carbon
dioxide, methane and ethane and at least one member
sorbent as determined by a reading of the FIG. 1 graph.
of the group consisting of normal saturated aliphatic
The potential capacity of the adsorbent bed for methane,
hydrocarbons containing more than two but less than
propane and hydrogen may be determined in a similar
nine carbon atoms per molecules and molecules with a
manner. That is, T2 for propane is 152° K. so that T2/T1
will be 0.51. Referring again to FIG. 2, this corresponds 20 maximum dimension of the minimum projected cross
section larger than about 5.5 Angstroms which comprises
to a potential loading of about 4.0 weight percent propane.
contacting said vapor mixture with a bed of at (least par
However, as previously discussed, zeolite A will adsorb
tially dehydrated crystalline zeolite A adsorbent material
hydrogen sul?de to the substantial exclusion of propane
‘having a pore size of at least about 4 Angstroms, and
due to the former’s greater polarity. For methane, T2
is 116° K. so that T2/T1 will be 0.39. Referring to FIG. 25 thereafter discharging the hydrogen sul?de-depleted va
por stream from the bed.
2, the potential loading is only about 0.6 weight percent
4. A process for separating hydrogen sul?de from a
methane.
vapor mixture containing hydrogen sul?de and molecules
With regard to hydrogen, it has been determined ex
with a maximum dimension of the minimum projected
perimentally that when T2/ T1 is 0.07, the hydrogen load
ing on zeolite A is zero. Thus, in the present example, 30 cross-section larger than about 5.5 Angstroms and at
least one member of the group consisting of hydrogen,
T2=32° K. and T2/ T1 is 0.11 so that the hydrogen load
carbon dioxide and normal saturated aliphatic hydro
carbons containing less than‘ nine carbon atoms per mole
cule which comprises contacting said vapor mixture with
percent adsor-hate, which corresponds to a T2/T1 value of
a bed of at least partially dehydrated crystalline zeolite A
CO
Or
0.31. If a temperature swing-purge cycle is to be em
adsorbent material, said material being capable of adsorb
ployed, T2 is 134'’ K. as previously determined and T,
ing molecules that have a maximum dimension of the
must be 433° K. or 160° C. Thus, regeneration is
minimum projected cross-section up to about 5.5 Ang
achieved by heating either the adsorbent and/or the
stroms and thereafter discharging the hydrogen-sul?de
inlet gas mixture su?iciently to provide a regeneration
temperature of 160° C. while using the inlet gas for 40 depleted vapor stream from said bed.
ing is negligible.
On regeneration, the end loading is to be 0.5 weight
5. A process as described in claim 1 wherein the vapor
purging.
mixture contains hydrogen sul?de and methane.
Although the preferred embodiments have been de-.
scribed in detail, it is contemplated that modi?cations
6. A process as described in claim 1 wherein the vapor
mixture contains hydrogen sul?de and ethane.
of the process may be made and that some features may
7. A process is described in claim 2 wherein the vapor
mixture contains hydrogen sul?de and normal saturated
be employed without others, all within the spirit and
scope of the invention as set forth herein.
aliphatic hydrocarbons containing less than nine carbon
When the inlet vapor mixture contains C02, the poten
tial capacity of the zeolite A adsorbent for this constituent
is similarly determined by reference to the vapor pres
atoms per molecule.
References Cited in the ?le of this patent
sure tables ‘and FIG. 3.
This is a continuation-in-part application of copending
application Serial No. 400,385, ?led December 24, 1953,
UNITED STATES PATENTS
in the name of R. M. Milton, now abandoned.
What is claimed is:
m U!
1. A process for separating hydrogen sul?de from a
vapor mixture containing hydrogen sul?de and at least
one member of the group consisting of hydrogen, carbon
hydrated crystalline zeolite A adsorbent material having
a pore size of at least about 4 Angstroms, and thereafter
discharging the hydrogen sul?de-depleted vapor stream
from the bed.
Christensen et al. _____ _._ Dec. 31, 1957
OTHER REFERENCES
“Molecular-Sieve Action of Solids” by R. M. Barrer,
Quarterly Reviews, London Chemical Society, vol. III,
1949, pages 293 to 320.
dioxide, methane and ethane which comprises contacting
said v-apor mixture with a bed of at least partially de
2,818,449
60
“Examine These Ways To Use Selective Adsorption,”
Petroleum Re?ner, vol. 36, No. 7, July 1957, pages l36~
140.
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