Molecular sieve abstract
A method for the preparation of a carbon molecular sieve capable
of separating acid gases and fluorocarbons of the formula C.sub.a
H.sub.b X.sub.c F.sub.d, wherein a is from 1 to about 6 b is from
0 to about 13 c is from 0 to about 13 d is from 1 to about 14
and X is a halogen; on the basis of shape selectivity and size exclusion.
A precursor resin is heated at about 0.2.degree. C. per minute to
about 500.degree. C. Then the resin is soaked at about 500.degree.
C. for about 6 hours in flowing inert gas.
Molecular sieve claims
We claim:
1. A process for the preparation of a carbon molecular sieve capable
of separating acid gases and fluorocarbons of the formula C.sub.a
H.sub.b X.sub.c F.sub.d, wherein a is from 1 to about 6 is from
0 to about 13 c is from 0 to about 13 d is from 1 to about 14
and X is a halogen; comprising the steps of:
a) heating an oxygen containing precursor resin at about 0.1.degree.
to about 2.degree. C. per minute to about 400.degree. C. to about
800.degree. C. and
b) soaking to about 400.degree. C. to about 800.degree. C. for
at least 1 hour in flowing inert gas.
2. A process as claimed in claim 1 wherein the carbon molecular
sieve has a pore size of about 4.5 to about 5.5.ANG..
3. A process as claimed in claim 1 wherein the precursor resin
is selected from the group consisting of polyacrylonitrile, phenol
formaldehyde resin, polyvinylidene chloride, polyfurfuryl alcohol
and a mixture thereof.
4. A process as claimed in claim 1 wherein the precursor resin
is polyfurfuryl alcohol.
5. A process as claimed in claim 1 wherein the fluorocarbon is
CF.sub.3 CH.sub.2 F.
6. A process as claimed in claim 1 wherein the inert gas in helium.
7. A process as claimed in claim 1 wherein the steps a) and b)
are
a) heating the precursor resin at about 0.1.degree. to about 2.degree.
C. per minute to about 450.degree. C. to about 600.degree. C. and
b) soaking to about 450.degree. C. to about 600.degree. C. for
about 4 to 6 hours in flowing inert gas.
8. A process as claimed in claim 1 wherein the steps a) and b)
are
a) heating the precursor resin about 0.1.degree. to about 2.degree.
C. per minute to about 500.degree. C. and
b) soaking to about 500.degree. C. for about 6 hours in flowing
inert gas.
9. A process for the purification of CF.sub.3 CH.sub.2 F by the
removal of HF in the azeotropic mixture, on the basis of shape selectivity
and size exclusion comprising a bed of molecular sieving carbon
prepared as in claim 1 which is contacted with the mixture at approximately
150.degree.-250.degree. C. and at 1 to 5 atm pressure and with a
gas hourly space velocity between 100 and 1000.
10. A carbon molecular sieve made by the process of claim 1.
11. A carbon molecular sieve for the separation of acid gases from
fluorocarbons said sieve comprising a precursor resin selected from
the group consisting of polyacrylonitrile, phenol formaldehyde resin,
polyvinylidene chloride, polyfurfuryl alcohol and a mixture thereof,
and said sieve having a pore size of about 4.5 to about 5.5.ANG.,
said fluorocarbons having the general formula C.sub.a H.sub.b X.sub.c
F.sub.d wherein a is from 1 to about 6 b is from 0 to about 13
c is from 0 to about 13 and d is from 1 to about 14 and X is a
halogen.
12. In a process for the separation of acid gases from fluorocarbons
wherein the fluorocarbons have a general formula of C.sub.a H.sub.b
X.sub.c F.sub.d wherein a is from 1 to about 6 b is from 0 to about
13 c is from 0 to about 13 and d is from 1 to about 14 and X
is a halogen, the improvement being in the use of a carbon molecular
sieve comprising an oxygen containing precursor resin and having
a pore size of about 4.5 to 5.5.ANG..
13. In the process as claimed in claim 12 wherein a is from 2
to about 4.
14. In the process as claimed in claim 12 wherein a is 2.
15. In the process as claimed in claim 12 wherein b is from 0
to about 6.
16. In the process as claimed in claim 12 wherein b is 2.
17. In the process as claimed in claim 12 wherien X is chlorine,
fluorine or bromine.
18. In the process as claimed in claim 12 wherein X is chlorine.
19. In the process as claimed in claim 12 wherein c is from 0
to about 6.
20. In the process as claimed in claim 12 wherein c is 0.
21. In the process as claimed in claim 12 wherein d is from about
2 to about 6.
22. In the process as claimed in claim 12 wherein d is 4.
Molecular sieve description
BACKGROUND
Hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs)
are considered to be useful alternatives to traditional chlorofluorocarbons
(CFCs) in a variety of application from refrigeration, and cleaning,
to blow molding of polymers. These compounds have shorter atmospheric
residence times and decompose at lower altitudes than CFCs, because
of the presence of reactive carbon-hydrogen bonds. This behavior
is less deleterious to the upper atmosphere since much less ozone
is decomposed by chlorine radicals for HCFCs and since there is
no ozone loss for HFCs.
Although these compounds are beneficial to the environment they
are more difficult to synthesize and to purify than the CFCs. This
difficulty arises from the propensity of HFCs and HCFCs to hydrogen
bond with other organics and acid gases, especially HF, a common
by-product of HFC and HCFC synthesis. These hydrogen bonding effects
result in azeotropes and pinch-points in the vapor liquid equilibria
(VLE) of mixtures of these compounds.
Pinch-points and azeotropes complicate standard approaches to separations
of product streams. Pinch-points require larger columns with higher
numbers of trays (plates) and azeotropes cannot be separated by
standard distillation. Distillation must be done by extraction in
two column systems, or the azeotrope must be "cracked"
at low temperatures using cryogenic methods. In every case the cost
of separation increases considerably with the complexity or size
of the unit operation or operations to be employed.
The theoretical concepts dealing with the mechanisms of absorptive
separations are well known to those trained in the art. Good discussions
are provided by R. T. Yang, Gas Separation by Adsorption Processes,
Butterworths, Boston (1987) and D. M. Ruthven, Principles of Adsorption
and Adsorption Processes, Wiley, New York (1984).
In comparing the relative merits of a standard distillative separation
versus a less standard absorptive separation certain criteria need
to be met in order for the latter to be more efficient than the
former. In most cases distillation is the most cost effective and
simplest operation employed for separation. However, criteria can
be established and met under some circumstances that suggest the
preferential use of absorption technology. These criteria have been
reviewed in a number of places and are familiar to anyone trained
in the art. G. E. Keller, R. A. Anderson and C. M. Yon, "Adsorption",
in Handbook of Separation Process Technology, R. W. Rosseau, Ed.
Wiley, New York, (1987), pp. 644-696 have done a particular service
to the field by clearly enumerating these criteria, as has G. E.
Keller III, "Gas-Adsorption Processes", in Industrial
Gas Separations, ACS Sym. Series 223 T. E. Whyte, Jr., C. M. Yon,
E. H. Wagner, Eds., ACS, Washington (1983), pp. 145-171 in an earlier
work. In particular these authors state that adsorptive separations
should be considered as an alternative to distillation, when the
relative volatility between the key components to be separated is
between 1.2 to 1.5 or less--as is the case for azeotropic mixtures--and
when separations by distillation require multiple columns or cryogenic
operation. Of course these criteria require that a suitable sorbent
can be identified which carries out the separation efficiently and
economically.
The choice of a suitable sorbent is also made on the basis of the
separation mechanism it imparts on the mixture. This is important
for the following reasons. If the product stream to be separated
is high in concentration of less valuable, and less strongly adsorbed
component, then a simple equilibrium driven separation is ideal.
The minor component builds in concentration on the sorbent surface
until full capacity is reached, the feed is swung to a second column
and finally it is regenerated to strip off the pure, high value
component. Here the sorbent bed can be relatively small with low
capital expenditures, and the cycle time can be long with low operating
and process costs. Activated carbon, silica or some other sorbent
can be suitable for such a simple process approach.
Similarly, if a stream rich in a value-added component is contaminated
by one or more minor components, then a straightforward equilibrium-driven
separation over a standard sorbent can be used, provided the impurities
are more strongly sorbed than the major component.
However, this may not always hold. Specifically, if one has a stream
which is contaminated by impurities, and the mixture is difficult
to separate by distillation--according to the criteria mentioned--and
if the impurities are not more strong sorbed than the major component,
then a simple equilibrium separation may not be feasible. The reason
is that in order to carry out the separation, the carbon bed would
have to be too large to be economical. Process costs would be expected
to scale with the bed size.
In this type of situation, which can often be the case with HFC
and HCFC product streams, a different sorptive mechanism over a
non-standard sorbent can be considered. Once again the key, as pointed
out generally by Keller et al. (1987), is the choice of a suitable
sorbent. It is often true that the more strongly "held"
molecule is also larger, if all other factors are roughly equal.
When this is true then "shape-selective" separations can
be used. Here the separation is driven by the differential rates
of uptake of components in the mixture on a molecular sieving sorbent.
At one extreme the pore structure of the sorbent completely restricts
access of one or more components of the mixture to the inner sorbent
surface. At the other extreme the separation is based on different
fluxes into the sorbent, which in turn are based on different diffusivities
for different sized components.
Zeolites can be expected to show shape selective separations because
of their molecular-sized pores. A variety of examples are provided
in the literature (Yang, 1987), Ruthven (1984). Although useful
for some separations of HFC and HCFC containing mixtures, they are
not useful for the separations of mixtures that contain acid gases,
especially HF since they are detrimentally reacted with and consumed
by the acid.
An alternative to zeolites are the carbon molecular sieves (CMS)
which are shape and size selective, but are not consumed by acid
gases. Commercial applications of carbon molecular sieves are typified
by the recovery of pure nitrogen from air in a pressure swing adsorption
process. Although oxygen and nitrogen differ in size by only 0.2
.ANG., the separation is efficient. This arises from the fact that
the rate of transport of oxygen into the carbon sieve pore structure
is markedly higher than that of nitrogen. Hence, the kinetic separation
works, even though the equilibrium loading levels of O.sub.2 and
N.sub.2 are virtually identical, and therefore would not provide
any separation. These effects have been considered by Yang (1987).
Carbon molecular sieve-based separations of fluorocarbons have
been investigated for particular separations. S. F. Yates, U.S.
Pat. No. 4940824 reports that carbon molecular sieve can be used
for the removal of vinylidene chloride from HCFC-141b. In a separate
disclosure, U.S. Pat. No. 4940825 Yates reports that dichloroacetylene
is separated or removed from HCFC-141b and/or vinylidene chloride
over a carbon molecular sieve with a mean pore size of 4.2-4.5 .ANG..
In both cases the examples indicate that the carbon molecular sieve
strongly adsorbed the impurity molecules, thereby stripping them
from the feed. In none of the examples was a CMS material regenerated
or shown to be regenerable. Similarly, in S. F. Yates, U.S. Pat.
No. 4906796 (1990) teaches that R-1122 (2-chloro-11-difluoroethylene)
can be substantially removed from HFC-134a (C.sub.2 F.sub.4 H.sub.2)
by selective sorption of the R-1122 into either CMS or 5A zeolite,
with the latter preferred. Here also the key to the separation is
the strong absorption of the impurity molecules into the pore structures
of either the CMS or the zeolite.
For each of the cases mentioned it was noted that the carbon molecular
sieves utilized were prepared from polymeric precursors that did
not contain oxygen. This goes back to the teaching of Chang in U.S.
Pat. No. 4820681. This patent describes the methodology for the
synthesis of the CMS material used in each of the subsequent process
patents. Particularly important to these applications is the use
of a cross-linked polymer precursor which is oxygen free. An example
is a polymer made from vinylidene fluoride (PVDF) and crosslinked
with divinyl benzene.
SUMMARY OF THE INVENTION
This invention involves carbon molecular sieves suitable for the
kinetic separation of hydrogen fluoride azeotropes. This invention
provides a new composition of matter for a carbon molecular sieve
for the separation of acid gases and fluorocarbons of the general
formula, C.sub.a H.sub.b X.sub.c F.sub.d where a is from 1 to about
6 b is from 0 to about 13 c is from 0 to about 13 d is from 1
to about 14 and X is a halogen. The carbon molecular sieve is made
from a precursor and has a pore size of about 4.5 to about 5.5 .ANG..
One aspect of the invention is a method for the preparation of
a carbon molecular sieve capable of separating acid gases and fluorocarbons
of the formula C.sub.a H.sub.b X.sub.c F.sub.d, wherein a is from
1 to about 6 b is from 0 to about 13 c is from 0 to about 13
d is from 1 to about 14 and X is a halogen; on the basis of shape
selectivity and size exclusion, comprising:
a) heating a precursor resin at about 0.1.degree. to 2.degree.
C. per minute to about 400.degree. C. to about 800.degree. C. and
b) soaking at about 400.degree. C. to about 800.degree. C. for
at least 1 hour in flowing inert gas.
Another aspect of the invention is a process for the purification
of HCF-134a (CF.sub.3 CH.sub.2 F) by the removal of HF in the azeotropic
mixture, comprising a bed of molecular sieving carbon prepared as
stated above in contact with the mixture at a temperature in the
range of 150.degree.-250.degree. C. and preferably approximately
200.degree. C. and at about 1 to about 5 atm pressure and with a
gas hourly space velocity between about 100 to about 1000.
Another aspect of this invention, is to use a precursor that does
not have to be oxygen free.
Another aspect of this invention, is to develop a method of purifying
C.sub.a H.sub.b X.sub.c F.sub.d.
The invention can be practiced with an azeotropic mixture that
can be separated over large pore, activated carbons at an economical
cost.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing the HFC-134a isotherm on coconut charcoal.
FIG. 2 is a graph showing the breakthrough curves of HF and HFC-134a
on Calgon BPL Carbon.
FIG. 3 is a graph showing the analysis of the breakthrough data
for HFC-134a provided a mixture of the equilibrium isotherm under
the conditions in Example 2.
FIG. 4 is a graph showing the HFC-134a isotherm on Carbosieve G.
FIG. 5 is a typical chromatogram of this invention.
FIG. 6 is a graph showing an example of the reverse order of elution
according to this invention.
DESCRIPTION OF THE INVENTION
This invention provides a new composition of matter for carbon
molecular sieve for the separation of acid gases like HF, HCl and
others from fluorocarbons. The fluorocarbons can be of the general
formula: C.sub.a H.sub.b X.sub.c F.sub.d wherein "a" is
from about 1 to about 6 preferably from about 2 to about 4 and
even more preferably 2; "b" is from 0 to about 13 and
preferably from 0 to about 6 and even more preferably 2; "c"
is from 0 to about 13 and preferably from 0 to about 6 and more
preferably 0; "X" is a halogen, preferably chlorine, fluorine
or bromine, and even more preferably chlorine; and "d"
is from 1 to about 14 and preferably from about 2 to about 6 and
more preferably 4. The carbon molecular sieve has a pore size of
about 4.5 to about 5.5 .ANG.. The carbon molecular sieve is made
from a precursor. The preferred examples of the precursors that
can be used include PAN (polyacrylonitrile), PFR (phenol formaldehyde
resin), PVDC (polyvinylidene chloride), PFA (polyfurfuryl alcohol)
or any combinations of the above.
Another aspect of the invention is a method for the preparation
of a carbon molecular sieve capable of separating acid gases and
fluorocarbons of the formula C.sub.a H.sub.b X.sub.c F.sub.d, wherein
a is from 1 to about 6 b is from 0 to about 13 c is from 0 to
about 13 d is from 1 to about 14 and X is a halogen; on the basis
of shape selectivity and size exclusion, comprising:
a) heating a precursor resin at about 0.1.degree. C. to 2.degree.
C. per minute to about 400.degree. C. to about 800.degree. C., preferably
from 450.degree. C. to about 600.degree. C., and more preferably
about 500.degree. C., and
b) soaking at about 400.degree. C. to about 800.degree. C., preferably
from about 450.degree. C. to about 600.degree. C., and more preferably
at about 500.degree. C. for at least 1 hour in flowing inert gas.
The preferable time would be for about 4 to about 6 hours and more
preferably for about 6 hours. The preferable inert gas is helium.
The process for the purification of HCF-134a (CF.sub.3 CH.sub.2
F) by the removal of HF in the azeotropic mixture, comprising a
bed of molecular sieving carbon prepared as stated above in contact
with the mixture at approximately 150.degree.-250.degree. and preferably
at 200.degree. C. and at about 1 to about 5 atm pressure and with
a gas hourly space velocity between about 100 to about 1000.
Among the more difficult separations in the field of alternate
fluorocarbons processing is the removal of HF from HFC-134a, since
these form a strong azeotrope (97% HFC-134a, 3% HF). Although the
separation can be done cryogenically, it is a less than an optimal
solution to the process problem. Sorptive separations are of interest
as an alternative means to purifying HFC-134a. As shown in the examples
below, the azeotropic mixture can be separated over large pore,
activated carbons. The basis for the separation on these materials
is that the larger HFC-134a molecule is more strongly sorbed than
the HF molecule, which under the conditions tested is virtually
nonsorbing. Even though this provides a useful separation, the holdup
of the more valuable organic molecule is in practical terms problematic,
since a very large carbon bed would be required with long cycle
times. This would translate into high capital and operating costs.
An alternative to the equilibrium-driven separation is a kinetic
one based on shape selective effects. Medium and small pore zeolites
are in the correct size domain, presumably, to do the separation
on the basis of the different molecular size of HF and HFC-134a.
However, because of their facile consumption by HF, the zeolites
are--in native form--unsuited for this separation, or any other
for that matter which involves a mixture containing HF. Carbon molecular
sieves are materials that as a class have zeolite-like shape selective
properties, but which are not reacted in a consumptive manner with
HF and other acid gases.
The first two examples are demonstrations of equilibrium separations
of the azeotrope over coconut charcoal and Calgon BPL carbon, derived
from bituminous coal. The third example demonstrates that the separation
can be made on Carbosieve G. Despite its name this carbon behaves
as the first two and displays an equilibrium-driven separation.
Examples 4 and 5 deal with the synthesis of a carbon molecular sieve
that is able to provide a kinetic separation of the azeotrope. In
a bed of this material the order of breakthrough or elution is the
reverse of that obtained with activated carbon, since the smaller
HF molecule is held-up for longer mean residence times than the
HFC-134a. |