Note: Descriptions are shown in the official language in which they were submitted.
10770f~
The field of art to which the invention pertains
is solid-bed adsorptive separation with an adsorbent com-
prising a zeolite. More specifically, the claimed inven-
tion relates to an improved process for the separation
of cresol isomers by employing a solid crystalline alumino-
silicate adsorbent which selectively removes para-cresol
from the feed mixture. The para-cresol is then recovered
from the adsorbent through a desorption step which em-
ploys a desorbent material containing an alcohol.
It is known in the separation art that certain
crystalline aluminosilicates can be used to separate hy-
drocarbon species from mixtures thereof. In particular,
the separation of normal paraffins from branched chained
paraffins can be acccmplished by using the type A zeolites
which have pore openings from 3 to 5 Angstroms. Suc~ a
separation process is disclosed for example in U.S. Patents
2,986,589 and 3,201,491. These adsorbents allow a sep-
aration based on the physical size differences in the
molecules by allowing the smaller or normal hydrocarbons
to ~e passed into the cavities within the crystalline
aluminosilicate adsorbent, while exciuding the larger or
branched chain molecules.
U.S. Patents 3,265,750 and 3,510,423 for example
disclose processes in which larger pore diameter zeolites
such as the type X or type Y structured zeolites can be
used to separate olefinic hydrocarbons.
The type X or type ~ zeolites have additionally
been employed in processes to separate individual hydro-
carbon isomers. In the process described in U.S. Patents
3,558,730; 3,558,732; 3,626,020; and 3,686,342 for
example, they are used to separate desired
_ _
107706~
xylene isomers; in U.S. Patent 3,66B,267 they are used to
separate pa-ticular alkyl substituted naphthalenes.
More specifically, U.S. Patent 3,014,078 teaches
the separation of cresol isomers by emp~oying an adsor-
bent consisting of a crystalline zeolitic metallo alumino-
silicate to selectively adsorb a cresol isomer from a
feed mixture thereby producing a rich adsorbent. In the
preferred mode of operation, the adsorbed isomer is then
removed by contacting with a displacement exchange fluid.
A preferred displacement exchange fluid is phenol although
other materials which may be employed include ethers,
aromatic hydrocarbons, and paraffin hydrocarbons.
The present invention relates to an improved pro-
cess for separating for the separtion of cresol isomers.
In particular we have found that employing a desorbent
material comprising an alcohol to remove the selectively
adsorbed para-cresol isomer from the zeolitic adsorbent in-
creases the selectivity of the adsorbent for para-cresol
with respect to the other cresol isomers thereby permittins
a more efficient separation with a higher purity extract
stream recovered from the process.
It i5, accordingly, a broad ob,ective of our inven-
tion to provide an improved process for the separation of
para-cresol from a feed mixture containing para-cresol and
at least one other cresol isomer.
In brief summary, our invention is, in one embodi-
ment, an improved process for separating para-cresol from
a feed mixture containing para-cresol and at least one
107706~
other cresol isomer which process comprises: (a) con-
tacting at adsorption conditions said feed mixture with
a crystalline aluminosilicate s~lected from type X struc-
tured and type Y structured zeolites containing one or
more selected cations at the exchangeable cationic sites
there~y selectively adsorbing para-cresol from said feed
mixture; and, (b) contacting said adsorbent with a de-
sorbent material at desorption conditions to remove the
adsorbed para-cresol therefrom; characterised b~ employ-
ing a desorbent material comprising an alcohol which issoluble in the feed mixture at adsorption and desorption
conditions and which has an average boiling point subs-
tantially different than that of the feed mixture.
Other embodiments and objects of the present in-
li vention enc~mpass details about feed mixtures, adsorbentsdesorbent materials, and operating conditions all of
which are hereinafter disclosed in th- following discus-
sion of each of these facets of the present invention.
The process of this invention provides an improved
alternative to the separation of para-cresol from mixed
cresols than by toluene sulfonation and caustic fusion.
Para-cresol finds specific use, for example, as a star-
ting material in a manufacture of butylated hydroxy toluene,
a widely-used antioxidant.
Preferred adsorbents which can be used in the ad-
sorptive separation of cresols are certain crystalline
aluminosilicates or molecular sieves including both the
natural and synthetic aluminosilicates. Such crystalline
107706~
aluminosilicates have cage structures in which the alumina
and silica tetrahedra are intimately connected in an open
three dimensional network. The tetrahedra are cross-linked
by the sharing of oxygen atoms with spaces between the tet-
rahedra occupied by water molecules prior to partial ortotal dehydration of this zeolite. The dehydration of the
zeolite results in crystals interlaced with cells having
molecular dimensions. Thus, the crystalline aluminosilicates
are often referred to as molecular sieves and separations
performed with molecular sieves are generally thought to
take place by a physical "sieving" of smaller from larser
molecules appearing in the feed mixture. In the separation
of cresol isomers, however, the separation of the isomers
apparently occurs b~^ause of differences in electro-chemical
attraction of the different isomers and the adsorben~ rather
than on pure physical size differences in the isomer mole-
cules.
In hydrated form, the preferred crystalline alumino-
silicates generally encompass those zeolites represenLed by
the formula below:
M2/nO :A1203 :WsiO2 YH2
where "M" is a cation which balances the electrovalence of
the tetrahedra and is generally referred to as,an exchang-
eable cationic site, "n" represents the valence of the
cation, "w" represents the moles of SiO2, and "y" represents
the moles of water. The cations may be any one of a number
of cations which will hereinafter be described in detail.
Adsorbents comprising the type X structured and
_5_
~07706~
type Y structured zeolites are especially preferred for
the adsorptive separation of the cresol isomers. These
zeolites are described and defined in U.S. Patents 2,882,
244 and ~,120,007 respectively. The preferred type X and
type Y structured zeolites contain from 4 to 8 wt.
water on a volatile-free basis.
Adsorbents contemplated herein include not only the
common sodium form of the type X and the type Y zeolites
but also crystalline materials obtained from such zeolites
by partial or complete replacement of the sodium at the
exchangeable cationic sites with one or more other speci-
fied cations.
The cations which may be placed upon the zeolite
include cations selected from, but not limited to, the
Group IA, Group IIA, and Group IB metals of the Periodic
Table of Elements. Specific cations which show a pre-
ferential selectivity for para-cresol with respect to
other cre~sol isomers include lithium, sodium, potassium,
rubidium, cesium, beryllium, maqnesium, calcium, strontium,
barium, silver, manganese, cadmium, and copper. Where the
above cations are used, para-cresol would be the preferen-
tially adsorbed component of the feed mixture. In the pro-
cess of this invention we have found that an adsorbent com-
prising a type X or type Y zeolite containing barium or
potassium as a selected single cation at the exchangeable
cationic sites is particularly preferred.
Type X or Type Y zeolites containing the following
combinations of cations have also been shown to be suitable
for para-cresol separation. These cations include potassium
10770~;~
and barium, potassium and beryllium, potassium and man-
ganese, rubidium and barium, cesium and barium, copper
and cadmium, copper and silver, zinc and silver, and cop-
per and potassium, with the barium and potassium co~bi-
S nation being preferred. A particularly preferred adsor-
bent is one comprising type X or type Y zeolite containing
barium and potassium at the exchangeable cationic site, in
a weight ratio of barium to potassium of from 1 to 100.
When singular cations are based exchanged upon a
zeolite the singular cations can comprise anywhere from 5
up to 75 wt. ~ on a relative volatile free basis of the
zeolite depending upon the molecular weight of the material
exchanged upon the zeolite. It is contemplated that when
single ions are placed upon the zeolite that they may be
on the zeolite in concentrations of from about 1~ to abou
100% of the original cations present (generally sodium) up-
on the zeolite prior to its being ion-exchanged.
When two or more cations are placed upon the zeolite
there are two parameters in which one can operate in order
to effectively produce a zeolite having the maximum selec-
tive properties. One of the parameters is the extent of
the zeolite ion exchange which is aetermined by variables
such as the length or ion-exchange times, ion-exchange
temperature, and cation concentration. The other paramater
is the ratio of individual cations placed on the zeolite.
In instances in which the cation pairs comprise a Group IIA
metal and a Group IA metal the weight ratio of these two
components upon the zeolite can vary anywhere from about
1~7706~ .
less than one up to about one hundred dcpending upon the
molecular weight of the Group I IA or Group IA metal.
In the process of this invention we have additionally
found that a small amount of water on the adsorbent is bene-
ficial to promote relatively sharp isomer separation and toprevent "tailing" of one cresol isomer into another. The
preferred range of water on the adsorbent is from 3 to 8 wt.
% LOI (loss on ignition) at 600C. This desired range can
be maintained by intermittent or preferably continuous water
addition to the process.
The ortho-, meta-, and para-cresols are commonly
obtained by the distillation of coal tar. ,An unpurified
mixture of tle three isomeric cresols is known as "tri-
cresol" or"cresylic acid". Since the amounts obtainable
by this source may not equal the demand, they can also be
produced from the toluidines by the diazo reaction or
more frequently by toluene sulphation and caustic fusion.
Proper selection of reaction conditions favours the pro-
duction of para-cresol. Since the boiling points for ortho-,
meta-, and para-cresol are respectively 191.5C., 202.8C.,
and 202.5C., it can be seen that ortho-cresol can be re-
covered by fractionation but because of their close boil-
ing points meta- and para-cresols cannot. The separation
of meta- and para-cresol thus is ideally suited to separation
by selective adsorption with a solid adsorbent.
We have found that the selectivity of adsorbents
comprising type X and type Y zeolites for para-cresol with
respect to the other cresol isomers is strongest when the
--8--
10770~;~
feed mixture to be separated contains concentrations of
para-cresol and one or more other cresol isomers of up to
15 vol. % each. Apparently because of the relatively
high acidity of cresols, selectivity of the adsorbent for
any cresol isomer diminishes as the cresol concentration
in the feed mixture increases. Separation of a desired
isomer by selective adsorption takes place, it is theorized,
because of a rather delicate acidity~basicity difference
between the desired isomer and the adsorbent. At cresol
isomer concentrations higher than 15 vol. % each of para-
cresol-and at least one other cresol isomer this difference
diminishes. The feed mixtures may contain as diluents
materials which are generally less selectively adsorbed
~if at all) in this adsorption system than ary of the
cresol isomers and in which the cresols are soluble. As
one example, hereinafter described liquid desorbent ma-
terials can be employed as diluents to achieve the proper
concentrations of cresol isomers in the feed mixture.
To separate para~cresol from a feed mixture con-
taining para-cresol and at least one other cresol isomer
the mixture is contacted with an adsorbent comprising
a crystalline aluminosilicate and the para-cresol is more
selectively adsorbed and retained by the adsorbent while
the other cresol isomers are relatively unadsorbed and are
removed from the interstitial void spaces between the
particles of adsorbent and the surface of the adsorbent.
The adsorbent containing the more selectively adsorbed
para-cresol is referred as a "rich" adsorbent--rich in the
1077Q6~
more selctively adsorbed para-cresol.
The more selectively adsorbed feed component is
commonly referred to as the extract component of the feed
mixture, while the less selectively adsorbed component is
referred to as the raffinate component. Fluid streams
leaving the adsorbent comprising an extract component and
comprising a raffinate component are referred to, respec-
tively, as the extract stream and the raffinate stream.
Thus, the raffinate stream will contain as raffinate com-
ponents all of the feed mixture isomers except para-cresol
and the extract stream will contain para-cresol as the ex-
tract component.
Although it is possible by the process of this in-
vention to produce high purity (98% or greater) para-cresol
at high recoveries, it will be appreciated that an extract
c~mponent is never completely adsorbed by the adsorbent, nor
is a raffinate component completely non-adsorbed by the ad-
sorbent. Therefore, small amounts of a raffinate component
can appear in the extract stream, and, likewise, small a-
mounts of an extract component can appear in the raffinatestream. The extract and raffinate streams then are further
distinguished from each other and from the feed mixture by
the ratio of the concentrations of an extract component and
a specific raffinate component, both appearing in the par-
ticular stream. For example, the ratio of concentration ofthe more select~vely adsorbed para-cresol to the concentration
of less selectively adsorbed meta-cresol will be highest in
the extract stream, next highest in the feed mixture, and
--10--
1077064
lowest in the raffinate stream. Likcwise, the ratio of
the less selectively adsorbed meta-cresol to the more selec-
tively adsorbed para-cresol will be highest in the raf-
finate stream, next highest in the feed mixture and the
lowest in the extract stream.
The adsorbent can be contained in one or more cham-
bers where through programmed flow into and out of the cham-
bers separation of para-cresol is ef~ected. The adsorbent
will preferably be contacted with a desorbent material which
is capable of displacing the adsorbed para-cresol from the
adsorbent. An extract stream comprising para-cresol and de-
sorbent material will tllen be withdrawn from the adsorbent
and the desorbent material separated thereby leaving high
purity para-cresol. Alternatively, the para-cresol could
be removed from the adsorbent by purging or by increasing the
temperature of the adsorbent or by decreasing the pressure of
the chamber or vessel containing the adsorbent or by a com-
bination of these means.
~he adsorbent may be employed in the form of a dense
compact fixed bed which is alternatively contacted with the
feed mixture and a desorbent material (herinafter described
in more detail). In the simplest embodiment of the inven-
tion the adsorbent is employed in the form of a single
static bed in which rase the process is only semi-continuous.
A set of two or more static beds may be employed in fixed-
bed contacting with appropriate valving so that the feed
mixture is passed through one or more adsorbent beds while
the desorbent material is passed through one or more of the
other beds in the set. The flow of feed mix-
ture and desorbent material may be either up or down through
--11--
10770~
the desorbent. Any of the conventional apparatus employedin static bed fluid-solid contacting may be used. Counter~
current moving-bed or simulated countercurrent moving-bed
liquid flow systems, however, have a much greater sep-
aration efficiency than fixed adsorbent bed systems and aretherefore preferred. In the moving-bed or simulated moving-
bed processes the adsorption and desorption operations are
continuously taking place which allows both continuous pro-
duction of an extract and a raffinate stream and the continual
use of feed and desorbent streams. One preferred process-
ing flow scheme which can be utilised to effect the process
of this invention includes what is known in the art as the
simulated moving-bed countercurrent system. The general op-
erating sequence of such a flow system is described in U.S.
Patent 2,985,589. This patent generally described the pro-
cessing sequence involved in a particular simulated mo~ing-
bed countercurrent solid-fluid contacting process. The
processing sequence generally described in that patent is
the preferred mode of operating the separation process dis-
closed herein.
One broad embodiment of this process is a process
for separating para-cresol from a feed mixture comprising
para-cresol and at least one other cresol isomer which
process generally employs the operatin~ sequence descrlbed
in U.S. Patent 2,985,589 and which comprises the steps of:
contacting the feed at adsorption conditions with a par-
ticular zeolitic adsorbent thereby selectively adsorbing
para-cresol; withdrawing from the adsorbent bed a stream
comprising less selectively adsorbed components in the feed;
-12-
10770~4
contacting the adsorbent at desorption conditions with a
desorbent material to effect the removal of para-cresol
from the adsorbent; and, withdrawing from the adsorbent a
stream comprising desorbent material and para-cresol.
S Adsorption and desorption conditions for adsor~-
tive separation processes can generally be either in the
liquid or vapour phase or both but for cresol separation
processes employing zeolitic adsorbents all liquid-phase
operations are preferred because of the lower temperature
requirements and the slightly improved selectivities
associated with the lower temperatures. Adsorption con-
ditions will include temperature within the range of from
38C to 260C and will include pressures in the range from
1 to 35 atmospheres. Pressures higher than 35 atm~spheres
do not appear to affect the selectivity to a measureable
amount and additionally would increase the cost of the pro-
cess. Desorption conditions for the process of the inven-
tion shall generally include the same range of temperatures
and pressures as described for adsorption operations. The
desorption of the selectively adsorbed isomer could also be
effected at subatmospheric pressures or elevated temperatures
or both or by vacuum purging of the adsorbent to remove the
adsorbed isomer but this process is not directed to these
desorption methods.
The desorbent materials which can be used in the
various processing schemes employing this adsorbent will
vary depending on the type of operation employed. The term
"desorbent material" as used herein shall mean any fluid
substance capable of removing a selectively adsorbed feed
component from thc adsorbent. In the swing-bed system in
-13-
1C~77064
which the selectively adsorbed feed component is removed
from the adsorbent by a purge stream, desorbent materials
comprising gaseous hydrocarbons such as methane, ethane,
etc., or other types of gases such as nitrogen or hydrogen
may be used at elevated temp~ratures or reduced pressure
or both to effectively purge tlle adsorbed feed component
from the adsorbent.
However, in processes which are generally operated
at substantially constant pressures and temperatures to
insure liquid phase, the desorbent material relied upon
must be judiciously selected in order that it may dis-
place the adsorbed feed component from the adsorbent with
reasonable mass flow rates without itself being so strong-
ly adsorbed as to unduly prevent the extract component
from displacing the desorbent material in a following
adsorption cycle.
Desorbent materials which can be used in the pro-
cess of this invention should additionally be substances
which are easily separable from the feed mixture that is
passed into the process. Indesorbing the preferentially
adsorbed component of the feed, both desorbent material
and the extract component are removed in admixture from
the adsorbent. Without a method of separation such as
distillation of these two materials, the purity of the
extract component of the feed stock would not be very
high since it would be diluted with desorbent. It is
therefore contemplated that any desorbent material used
in this process will have a substantially different average
boiling point than that of the feed mixture. T'ne use of
10770~;~
a desorbent mat~rial having a substantially different
average boiling point than that of the feed allows sep-
aration of desorbent material from feed components in
the extract and raffinate streams by simple fractionation
thereby permitting reuse of desorbent material in the pro-
cess. The term "substantially different" as used herein
shall mean that the difference between the average boil-
ing points between the desorbent material and the feed
mixture shall be at least 8C. The boiling range of the
desorbent material may be higher or lower than that of the
feed mixture, although for the process of tnis invention it
is preferred that desired desorbent material have a boiling
range less than that of the feed material.
The prior art has generally chosen phenol as the
preferred "displacement exchange fluid" or de,orbent ma-
terial for separation processes employing an adsorbent
comprising cyrstalline aluminosilicate to separate the
cresol isomers. Other materials which have been recognised
by the prior art are ethers, aromatic hydrocarbons, and
paraffin hydrocarbons. Such desorbent materials are best
suited to adsorptive separation processes generally char-
acterised as equilibrium adsorptive type operations. How-
ever, in processes characterised by less ~han equilibrium
adsorption, we have discovered that there is a distinct
advantage in employing a desorbent material comprising an
alcohol and that this advantage results in an improved pro-
cess for the separation of cresol isomers by selective
adsorption on a zeolite-containing adsorbent.
-15-
10770ti~ ~
The term "equilibrium adsorption" as used herein
shall mean that there is essentially no competitive ad-
sorption of the adsorbent of both desorbent material and
an extract com~onent of the feed mixture during the pro-
cess adsorption step or steps, Equilibrium adsorptionessentially takes place in the sequence of steps in which
a feed stream which does not contain any desorbent material
is first passed through a zeolitic adsorbent bed until the
effluent stream which passes out of the adsorbent after
contact therewith is essentially of the same composition
as the material fed to the adsorbent bed indicating no,net
transfer of material between the adsorbed material within
the adsorbent and the feed stock surrounding the adsorbent.
A desorbent material is then passed through the bed of ad-
sorbent to displace the selectively adsorbed components OL
the feed. In this type sequential operation there is no
desorbent material in contact with the adsorbent when ad-
sorption operations are completed (any desorbent initially
present on the adsorbent is displaced by feed components).
The term "less than equilibrium adsorption" shall
mean that there is this competitive adsorption of desorbent
material and an extract component during the process ad-
sorption step. In continuous simulated or actual coun-
tercurrent liquid flow systems in which an extract com-
ponent of the feed is continuously and selectively ad-
sorbed from the feed mixture by a solid adsorbent, there
are zones in which there is essentially a simultaneous con-
tacting of the adsorbent during adsorption with a mixture
comprising desorbent material and the feed mixture. The
`
-16-
10770~i;'l ~
presence of feed and desorbent material in admixture
creates a condition where there is a competitive ad-
sorption of the adsorbent of both desorbent material and
the selectively adsorbed component of the feed mixture.
In most continuous countercurrent solid-fluid sep-
aration processes, the solid adsorbent contacts the feed
mixture in what is generally referred to as an adsorption
zone. The feed and solid adsorbent countercurrently con-
tact each other with the adsorbent passing out of the ad-
sorption zone containing an extract component of the feed
and some desorbent within the solid adsorbent. The solid
adsorbent is eventually contacted with desorbent material
in a desorption zone. The desorbent material displaces an
extract component from the solid adsorbent and allows a
l; mixture of desorbent and extract component of the feed to
be removed from the process as an extract stream. The
extract stream eventually passes to a separation means where-
in the desorbent material is separated from the extract com-
ponent giving a stream enriched in an extract component of
20 the feed. The solid adsorbent aft~r being contacted with -
the desorbent in the desorption zone, continues to flow
in a countercurrent direction in relation to the fluid flow
in thesystem and eventually is recontacted witn the feed in
the adsorption zone for the adsorption of the extract com-
ponent of the feed by the solid adsorbent. Between the ad-
sorption zone and desorption zone are located the flushing
or rectification zones which by carefully controlled pre-
ssure drops and liquid flow rates prevent the raffinate or
extract streams from contaminating each other. The material 1 r
-17-
107706~
contained in the flushing or rectification zones gen-
erally contains desorbent material. The desorbent ma-
terial in the flushing or rectification zones flushes a
raffinate material carried by the solid adsorbent back
into the adsorption zone and eventually ends up con-
tacting the adsorbent in the adsorption zone substantially
the same time the feed mixture contacts the solid adsor-
bent in the adsorption zone. The desorbent material which
contacts the adsorbent in the adsorption zone causes competi-
tive adsorption between it and the extract component offeed. The presence of desorbent material during the ad-
sorption step can affect selectivity of tne adsorbent for
the extract component.
We have found that improved separation is obtained
in an adsorption process for separating cresol isomers in
which the desorbent material is present while adsorption of
para-cresol takes place by employing a desorbent material
comprising an alcohol. Specifically, we have found that
employing such a desorbent material increases the selec-
tivity of particular adsorbents for para-cresol with re-
spect to other cresol isomers and also increases the rate
of desorption of para-cresol from the adsorbent. The
exact mechanism by which this occurs is not fully under-
stood but it is thought that the alcohols modify the acidity/
basicity relationships between the cresol isomers and the
adsorbent.
Alcohols which can be used in the process of this
invention shall broadly be those which satisfy these two
criteria: they shall be soluble in the feed mixture used in
-18-
10770f~
th~ process at adsorption and desorption conditions andthey shall have an average boiling point substantially
different than that of the feed mixture. Preferably the
alcohols will be derivatives of normal paraffins or cy-
S cloparaffins. Although secondary and tertiary alcoholsare suitable for use in the process of our invention,
primary alcohols are more preferred because they do not
readily dehydrate to form olefins. Even more preferred
are those primary alcohols which have boiling points less
than, rather than higher than, that of the feed mixture.
These particular primary alcohols are preferred because
the larger chain higher-boiling primary alcohols tend to
behave more like normal paraffins and thus their ability
to modify the adsorbent characteristics is diminished.
Thus as indicated in Table No. 1 below primary alcohols
having from one to and including seven carbon atoms per
molecule will be the preferred primary alcohols. Of
these l-hexanol is particularly preferred. Primary al-
cohols having greater than seven carbon atoms per molecule
which generally have boiling points greater than any of
the cresol isomers are not as desirable for use as de-
sorbent materials for this process.
Mixtures of alcohols with hydrocarbons such as
paraffins or aromatics are also effective as desorbent ma-
terials in the process of this invention. Such hydrocarbonsshall be those which are soluble in both the alcohol and
the feed mixture at adsorption and desorption conditions
and, like the alcohols employed, shall have average boil-
ing points substantially different than that of the feed
--19--
10770~ .
mixtures. The paraffins can include straight or branched
chain paraffins or cycloparaffins which meet these two
criteria. Particularly preferred aromatics are benzene,
toluene, and the xylenes. Typical concentrations of the
alcohol in mixtures of an alcohol and a hydrocarbon can be
from a few volume percent up to near 100 vol. % of the
total desorbent material mixture but such concentration
preferably will be within the range of from 25 vol. % to
75 vol. % of the mixture.
Table No. 1
Normal Boiling Points of Selected Primary Alcohols
normal boiling point, C
Methanol 64.7
Ethanol 78.5
l-Propanol 97.2
l-Butanol 117.7
l-Pentanol 138
l-Hexanol 157.2
l-Heptanol 176
- l-Octanol 195
l-Nonanol 213
l-Decanol 231
ortho-cresol 190.8
para-cr~sol 201.1
meta-cresol 202.8
benzene 80.1
toluene 110.6
ortho-xylene 114.4
meta-xylene 139.1
para-xylene 138.3
The improvement that results from employing a de-
sorbent material comprisin~ an alcohol can be better under-
25 stood by brief reference to certain adsorbent properties I ;
which are necessary to the successful operation of a selec- !
tive adsorption process. It will be recognised that im-
provements in any of these adsorbent characteristics will
.
-20-
` 107706~
result in an improved separation process. ~mong such
characteristics are: adsorptive capacity for some volume
of an extract component per volume of adsorbent; the
selective adsorption of an extract component with respect
to a raffinate component and the desorbent material; suf-
ficiently fast rates of adsorption and desorption of the
extract component to and from the adsorbent; and, in in-
~tances where the components of the feed mixture are very
reactive, little or no catalytic activity for undesired
reactions such as polymerization and isomerization.
Capacity of the adsorbent for adsorbing a specific
volume of an extract component is, of course, a necessity;
without such capacity the adsorbent is useless for ad-
- sorptive separation. Furthermore, the higher the adsorbent's
capacity for an extract component, the better is the adsor-
bent. Increased capacity of a particular adsorbent makes
it possible to reduce the amount of adsorbent needed to
separate the extract component contained in a particular
charge rate of feed mixture. A reduction in the amount of
adsorbent required for a specific adsorptive separation
reduces the cost of the separation process. It is im-
portant that the good initial capacity of the adsorbent
be maintained durin~ actual use in the separation process
over some economically desirable life.
The second necessary adsorbent characteristic is
the ability of the adsorbent to separate components of the
feed; or, in other words, that the adsorbent ~ossess ad-
sorptive selectivity for one component as compared to a-
nother component. Some adsorbents demonstrate acceptable
-21-
10~7064
capacity but possess little or no selectivity. Silver ni-
trate on silica gel for instance possesses a large capacity
for cresols but little selectivity for one isomer with re-
spect to another. Relative selectivity can be expressed
not only for one feed mixture component as compared to `¦
another but can also be expressed between any feed mixture
component and the desorbent. The relative selectivity,
(B), as used throughout this specification is defined as
the ratio of two components of an adsorbed phase over the
ratio of the same two components in an unadsorbed phase at
equilibrium conditions.
Relative selectivity is shown as Equation 1 below: {
Selectivity = (B) = ~ol. percent C/vol. percent
~ol. percent C/vol. percent ~ UA
where C and D are two components of the feed represented in
volume percent and the subscripts A and U represent the
adsorbed and unadsorbed phases respectively. The equili-
brium conditions were determined when the feed passing over
a bed of adsorbent did not change composition after contact-
ing the bed of adsorbent. In other words, there was no nettransfer of material occurring between the unadsorbed ar.d
adsorbed phases. ¦
Where selectivity of two components approaches 1.0
there is no preferential adsorption of one component by the
adsorbent with respect to the other;they are both adsorbed
~or non-adsorbed) to about the same degree with respect to
the other. As the (B) becomes less than or greater than 1.0
there is a preferential adsorption by the adsorbent for one
component with respect to the other. When comparing the
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1077~
selectivity by the adsorbent of one component C over com-
ponent D, a (B) larger than 1.0 indicates preferential ad-
sorption of component C within the adsorbent. A (B) less
than 1.0 would indicate that component D is preferentially
adsorbed leaving an unadsorbed phase richer in camponent :
C and an adsorbed phase richer in component D. Desorbent
materials ideally would have a selectivity equal to about 1
or slightly less than 1 with respect to an extract component.
Employing a desorbent material comprising an alco-
~0 hol increases the selectivity of the adsorbent for a para-
cresol with respect to the other cresol isomers thereby
permitting sharper separation of the isomers and improving
the process.
The third important characteristic is the rate of
exchange of the extract component of the feed mixture ma-
terial or, in other words; the relative rate of desorption
of the extract component. This characteristic relates
directly to the amount of desorbent material that must be
employed in the process to recover the extract component
from the adsorbent; faster rates of exchange reduce the a-
mount of desorbent material needed to remove the extract
component and therefore permit a reduction in the operating
cost of the process. With faster rates of exchange, less
desorbent material has to be pumped through the process and
separated from the extract stream for reuse in the process.
We have found that the use of a desorbent material comprising
an alcohol also increases the transfer rates besides im-
proving the selectivity for para-cresol.
It is also necessary that the adsorbent possess
-23-
1()770~
little or no catalytic activity toward any reaction such
as polymerization or isomerization of any of the feed com-
ponents. Such activity might effect adsorbent capacity or
selectivity or product yields or all of these, but in the
adsorptive separation of cresol isomers with a zeolite-
containing adsorbent this is generally not a problem.
In order to test various adsorbents and desorbent
materials with a particular feed mixture to measure the
adsorbent characteristics of adsorptive capacity and selec-
tivity and exchange rate a dynamic testing apparatus isemployed. The apparatus consists of an adsorbent chamber
of approximately 70 cc. volume having inlet and outlet
portions at opposite ends of the chamber. The chamber is
contained within a temperature control means and, in addi-
tion, pressure control equipment is used to operate the
chamber at a constant predetermined pressure. Chroma-
tographic analysis equipment can be attached to the outlet
llne of the chamber and used to analyse the effluent stream r
leaving the adsorbent chamber.
A pulse test, performed using this apparatus and
the following general procedure, is used to determine
selectivities and other data for various adsorbent systems.
Theadsorbent is filled to equilibrium with a particular
desorbent material by passing the desorbent material through
the adsorbent chamber. At a convenient time, a pulse test
of feed containing known concentrations of cresol isomers
all diluted in desorbent material is injected for a dur-
ation of several minutes. For convenience a known con~
centration of a non-adsorbcd tracer compound may be
-2~-
1~7706~ ~
included in ~he feed. Flow of desorbent material is re-
sumed, and the tracer (if one is employed) and the cresols
are eluted as in liquid-solid chromatographic operation.
The effluent can be analysed by on-stream chromato~raphic
S equipment and traces of the envelopes of corresponding
component peaks developed. Alternativeli~, effluent samples
can be collected periodically and later analysed separately
by gas chromatography.
From information derived from the chromatographic -~
10 traces adsorbent performance can be rated in terms of ca- ¦
pacity index for para-cresol, selectivity for para-cresol
with respect to the other cresols and rate of desorption
of para cresolby the desorbent. The capacity index is char- I ;
acterised by the distance between the centre of the para-
15 cresol peak envelope and the tracer peak envelope or some
other known reference point such as volume of desorbent
pumped. It is expressed in terms of the volume in cubic
centimetres of desorbent pumped during this time interval.
Relative selectivity, (B), for para-cresol with respect to f
20 the other cresols is characterised by the ratio of the
distance between the centre of the para-cresol peak envel-
ope and the tracer peak envelope (or other reference point)
to the corresponding distances for the other cresol isomers.
The rate of exchange of para-cresol with the desorbent can
25 be characterised by the width of tile para-cresol peak
envelope at half intensity. The narrower the peak width,
the faster the desorption rate.
To further evaluate promising adsorbent systems and
to translate this type of data into a practical cresol
-25-
1077064
separation process requires actual testing of the best sys-
tem in a continuous countercurrent liquid-solid contacting
d'evice.
The general operating principles of such a device
S have been previously described and are found in U.S. Patent
2,985,589. A specific laboratory-size fluid-solid contact-
ing apparatus utilising these principles is described in
U.S. Patent 3,706,812.
The improved process of this invention for sep-
arating para-cresol from a feed mixture containing para-
cresol and at least one other cresol isomer, which was
demonstrated by pulse tests, was confirmed by actual con-
tinuous testing in the continuous device.
The examples below illustrate first the selectivity
characteristic of adsorbents comprising zeolites which makes
possible a process for the adsorptive separation of cresol
isomers and second the improvement in that characteristic,
as well as in the rate of desorption, which results in the
improved process of our invention. The examples are pre-
sented to further illustrate the process of the present in-
vention and are not intended to limit the scope and spirit
of the invention.
The examples present pluse test results obtained
with an adsorbent comprising type X crystalline alumino-
silicate which contained barium and potassium cations atthe exchangeable cationic sites within the aluminosilicate.
Results for one pulse test which employed an adsorbent com-
prising a type X crystallinc aluminosilicate containing
calcium cations at the exchan~eable cationic sites are
-26-
10770~4
.
also include~ for comparison. The first-mentioned ad-
sorbent was essentially totally ion-exchanged, con-
tained a weight ratio of barium oxide to potassium oxide
of about 3.3, and was approximately 20-40 mesh particle t
5size. Analyses of this adsorbent are shown in Table No.
2 below.
Table No. 2
ADSORBENT ANALYSES
Volatile Matter (LOI @ 900 C.) 6.43
Si0 (volatile free) wt. ~ 42.1
A12~ (volatile free) wt. % 28.3
Na 03(volatile free) wt. % 2.0
K2~ (volatile free) wt. % 6.1
BaO (volatile free) wt. % 20.3
Si02/A1203 2.53
The latter adsorbent was commercially-available Linde 10X
Molecular Sieves of approximately 20-40 mesh particle size.
EXAMPLE I
In this example the adsorbent comprising the type X
zeolite containing barium and potassium cations described
above was placed in the testing unit and a pulse test was
conducted in the following manner.
The desorbent material employed was 15 vol. ~ phe-
nol in toluene. The feed mixture utilised contained 5 vol.
~ each of para-cresol and meta-cresol in desorbent material.
Ortho-cresol was omitted from the feed mixture in order to
simplify the test and focus on the para/meta selectivity
which is the most critical selectivity because of their
close boiling points. Additionally from previous experi-
ments it had been determined that the meta- and orth-isomers
behave in substantially the same manner. Since desorbent
material was a part of the feed mixture, adsorption of
-27-
107701 i~ j
para-cresol took place in the presence of, and in compe-
tition with, desorbent material, and; therefore, selec-
tïvities were obtained at less than equilibrium condi-
tions. The desorbent was placed in a 70 cc adsorbent
column which was maintained at a constant temperature of
about 150C. with constant moderate pressure during the
entire operation. A Waters Automatic Fraction Collector
was connected to the effluent end of the chamber to sample
the effluent every 2.1 minutes.
Desorbent was first pumped through the adsorbent r
chamber at approximately 1 cc~min at about 150C. A 4.7
cc feed pulse was then cut into the system via an injection
loop and the effluent was periodically sampled in the man- ¦
ner indicated above. A measured void volume of 43.0 cc
was used for the 70 cc adsorbent column and sampling was
started after 40 cc of desorbent was p~mped beginning at
feed injection. The individual effluent samples were stop-
pered after collection and analysed separately by Gas
Chromatography (GC). A digital integrator was used to ob-
tain peak areas and the count produced (x 10 ) was plot-
ted versus cc of desorbent from time of feed injection,
for both para- and meta-cresol. This plot produced an
envelope for para- and meta-cresols similar to those ob-
tained when using on-stream GC analysis. The cresol samples
take 50 minutes to elute from the analytical GC column.
The selectivities were calculated by measuring the
cc of the desorbent pumped from the measured 43 cc void
~ volume to the midpoints of the individual para- and meta-
cresol envelopes at one-half the peak heights. The ratio
-28-
1~)770~
of these volumes represent the relative selectivity of
para-cresol with respect to meta-cresol.
Reproducible data from this test gave a relative
para-cresol to meta-cresol selectivity of 1.53 which -
demonstrates the characteristic of the adsorbent which
makes the adsorptive separation process possible.
EXAMPLE II .
In this example three pulse tests A, B, and C
were performed; tests A and B with the adsorbent des-
cribed above and used for Example I and test C with anadsorbent comprising a type X zeolite having calcium
at the exchangeable cationic sites. The results are shown
in Table No. 3 below.
-29-
~ . ~
r
107706~ 1
~, W
-
W W ~
U~
I I U~
3 o
~5
X ~D~D ~
X X r~
.
. C
~D '
3 W ~3
O o O o O o ~D
Ln
C ~ C ~ C
r~ u~ C~
X tD X ~D ~ ~D O ~
~s ~ ~ ~3
Y o "-
O O 1- ~ ~D ~
_ ~ ~h t-
r
U~ f3
~D ~
O 1- 1_ It fD ~S
ID n
~D CO W Ul ~ r~
~I o ~- 1~-
~' ~
~`
ID--
U~ ~
~o
~ .
o ~ ~t
tD
, ,_ ~P~ o
~I ~
Ul
o o O O
ID ~h
~ D ~
~ ~ C
rt ~ I~
~a -- P~
1077~)t;4
The test procedure and cquipment used was the same
as that described in Example I.
For test A the desorbent material was 30 vol. ~ phe-
nol in toluene and the feed mixture contained 5 vol. % each
of para-cresol and meta-cresol in desorbent material. The
relative selectivity of the adsorbent for para-cresol with
respect to meta-cresol in the presence of this desorbent
material was 1.37 and the para-cresol peak envelope width
at half height was 17Ø
The same adsorbent was employed for test B but the
desorbent material was 50 vol. % hexanol in toluene. Now
the relative selectivity of the same adsorbent for para-
cresol with respect to meta-cresol in the presence of this
desorbent material was 1.8 or an increase of 31% over that
of test A. The para-cresol peak envelope width at half
height had decreased from 17.0 obtained for test A to 11.0
for test B indicating a faster rate of desorption of para-
cresol for test B. The comparison of the results of test
A and B, therefore indicates the improvements in the ad-
sorbent characteristics of selectivity and transfer rateand hence in the separation process itself when a desorb-
ent material comprisin~ an alcohol is employed.
Test C was conducted with the same desorbent ma-
terial as was used for test B but a different adsorbent was
~mployed. l'he adsorbent used comprised a type X zeolite
containing calcium cations at the exchan~eable cationic
sites. With the same desorbent material, hexanol-l in
toluene, this adsorbent exhibited essentially no selec-
tivity for either cresol isomers thus indicatin~ that this
adsorbent, while suitable for use in the prior art cresol
separation process, is not suitable for use in the cresol
isomer separation process of this invention.
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