TITLE: REFRIGERANT SEPARATION
FIELD OF THE INVENTION
This invention relates to the separation of chlorofluorocarbons (CFC*s) and
hydrochlorofluorocarbons (HCFC's) and hydrofluorocarbons (HFC's) from mixtures
of various components using selective adsorbents.
BACKGROUND OF THE INVENTION
Certain refrigerant mixes are difficult to separate. For example, the boiling
point of R12 (CC12F2) is -29.8°C at atmospheric pressure and the boiling point of R22
(CHC1F2) is -40.8°C. An azeotrope forms with composition 97.9% R12 and 2.1 %
R22 by weight and complete separation of the two by distillation is extremely
difficult. Further, fractionating columns typically required for separation are long and
expensive. Accordingly, mixtures of R12 and R22 are usually regarded as
"unprocessable".
DISCLOSURE OF THE INVENTION
There is therefore a need for a method for separating chlorofluorocarbons
and/or hydrochlorofluorocarbons from mixtures thereof.
According to a broad aspect of the present invention there is provided a
method for separating a chlorofluorocarbon, a hydrochlorofluorocarbon or a
hydrofluorocarbon from a gaseous mixture of chlorofluorocarbons,
hydrochlorofluorocarbons and/or hydrofluorocarbons comprising the steps of passing
the mixture through a bed of adsorbent, the adsorbent being selected so as to
preferentially adsorb the chlorofluorocarbon, hydrochlorofluorocarbon or
hydrofluorocarbon from the mixture to thereby produce a gaseous mixture depleted of
the chlorofluorocarbon, hydrochlorofluorocarbon or hydrofluorocarbon, optionally
followed by the step of desorbing and recovering the adsorbed chlorofluorocarbon,
hydrochlorofluorocarbon or hydrofluorocarbon.
PREFERRED EMBODIMENT OF THE INVENTION
Suitable adsorbents include aluminosilicate clays, activated silica, activated
alumina, activated carbon, selective zeolites or molecular sieves. The particular
adsorbent used is typically selected on the basis of molecular size and differing dipole
moments of the components to be separated.
When molecular sieves are used, preferably the molecular sieve is a
crystalline, metal aluminosilicate with a three-dimensional interconnecting network of
silica and alumina tetrahedra having the structural formula:
Mx/n[(A102)x(Si02)y].wH20
wherein n is the valence of the cation M which can be sodium, potassium, lithium,
calcium, barium, strontium, a rare earth metal such as cesium or lanthanum, ammonia
or hydrogen, w is the number of water molecules per unit cell, x and y are the total
number of tetrahedra per unit cell, and y/x have a value of from 1 to 200 or greater.
The size of the entrance to the molecular sieve cages can be varied by using
different cations. For example, large cavities can be formed using small divalent
cations and small cavities result when large monovalent cations, such as potassium,
are used. A Type A molecular sieve having the structure Na12[(A102)12(Siθ2)12] H2θ
will have cavities of approximately 4 Angstrom unit in diameter. Substitution of a
portion of the sodium for another metal such as potassium results in a molecular sieve
having a cavities of approximately 3 Angstrom unit diameter. Conversely,
substitution of a portion of the sodium for calcium results in a molecular sieve having
cavities of approximately 5 Angstrom unit diameter. Suitable molecular sieves for use
in the invention can be obtained, for example, from the Davison Chemical Division of
W. R. Grace & Co of Baltimore, Maryland. United States of America and sold as 3 A.
4A, 5 A and 13X sieves.
In the present invention, the molecular sieves generally behave as a selective
physical adsorbent such that a chlorofluorocarbon. hydrochlorofluorocarbon or
hydrofluorocarbon smaller than the cavity entrance enters the internal structure and is
held there by physical forces of the Van der Waals type.
For activated carbon, alumina and silica, because of their large surface area
(500 grams of carbon providing about 500,000m of surface), adsoφtion occurs on the
surface by weak bonding of the Van der Waals type between the chlorofluorocarbon.
hydrochlorofluorocarbon or hydrofluorocarbon and the adsorbent surface, the surface
of the adsorbent being non-polar and having an affinity for non-polar adsorbents.
Where the molecular size of the chlorofluorocarbon, hydrochlorofluorocarbon or
hydrofluorocarbon is small enough, they are also capable of penetrating the
micropores of the activated adsorbent.
With any adsorbent of the invention, as chlorofluorocarbon,
hydrochlorofluorocarbon and hydrofluorocarbon compounds are inherently polar
because of the asymmetric distribution of atoms which have differing
electronegativities, the Van der Waals forces are augmented by an ion dipole
interaction and the adsorbing force is appreciably strengthened.
Preferably, the method of the invention is used to separate R22 from
R12/R22 mixtures using a 5 Angstrom molecular sieve to trap the R22 and result in
pure R12 product or is used to separate R12 from R12 R22 mixtures using an
activated carbon adsorbent to adsorb the R12 and result in a pure R22 product.
Preferably, when carrying out the invention, a mixture of refrigerants is
typically first analysed (usually by chromatographic methods) to determine its
components. The molecular size and configuration as well as the polarity for each
molecular component is then calculated from the available listings of covalent and
Van der Waals radii and dipole moments and adsorbents, including molecular sieves
with the appropriate cavity sizes and Al :Si ratios, are then chosen for selective
adsoφtion. The adsorbent is usually in the form of small beads (molecular sieves), or
in the form of a powder (activated adsorbents), which is packed in a fixed bed through
which the mixture is passed under a pressure drop between the top and the bottom of
the column.
It may be necessary to use more than one adsorbent to separate numerous
components thus giving rise to a number of fixed bed columns which can be run in
series or in parallel. Different adsorbing media can also be placed in the same column.
The number of columns used will depend on the molecular parameters of the
component mixture. Preferably a number of columns are run in parallel so as to
enable a continuous operation whereby columns alternate between adsoφtion and
desoφtion i.e. two or more adsorbent beds are employed and operate on similar cycles
but are sequenced to be out of phase with one another, so that when one bed is on its
adsoφtion step another bed is on its desoφtion step, and vice versa.
Preferably the invention is conducted under vacuum or at normal pressure
and temperature. The appropriate conditions used is however dependent on the
effectiveness of the adsorbent at the relevant pressure and temperature, the
concentration of the chlorofluorocarbon, hydrochlorofluorocarbon or
hydrofluorocarbon to be separated and the actual retention time in the adsorbent. The
adsorbed refrigerant can be desorbed or regenerated by heating and the use of a stream
of purge gas. The preferred method of regeneration is collection, typically under
vacuum, using radiofrequency excitation of the polar adsorbed molecules.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described by way of example only with reference
to the accompanying drawing and examples in which the separation of R12 and R22
from mixtures thereof was analysed and in which:
Figure 1 is a schematic flow diagram of an apparatus for performing the method.
EXAMPLE 1
Prior to performing the method of the invention, an experiment was
conducted to ascertain what size of molecular sieve should be used in order to separate
a mixture of R12 and R22. A 3 Angstrom sieve was found to have a cavity too small
to accept either component. A 10 Angstrom sieve was found to adsorb both
components. A 5 Angstrom sieve was found to selectively adsorb R22 as indicated by
the following Examples 2, 3 and 4.
EXAMPLE 2
A mixture consisting of 16.59% of R12 and 83.32% of R22 was passed
through a column of 5 A zeolite beads and the emission gases were analysed by gas
chromatography. There was an instant reduction in the concentration of R22 present
in the mixture exiting the column, from 83.32% to 0.08%. The gas emitted from the
top end of the column consisted of 99.51% R12. A small amount (0.069%) of Rl 15
(CC 1 F2CF3) was evident within the Rl 2. The adsoφtion rate of the R22 followed the
Langmuir isotherm adsoφtion curve.
The unexpected adsoφtion of the R22 molecules in the 5 A cavities of the
zeolite can be partially explained in terms of the higher polarity of the smaller
CHC1F2 molecule. Both CC12F2 (R12) and CC1F2CF3 molecules are larger and less
polar than CHC1F2 (R22). The dipole moments are 0.61, 0.52 and 1.42 Debye units
respectively.
EXAMPLE 3
400g of pure R22 and 4Kg of pure R12 were mixed in an evacuated lOKg gas
bottle to give a final weight of 14.09Kg. The weight/weight ratio in the liquid was
initially 10:1 (R12.R22) but a G.C. run showed the gaseous composition to be
58.38%R12 and 41.62%R22 due to the greater volatility of R22. An apparatus as
shown in Figure 1 was used and comprised a stainless steel tube 1 (96cms long and 8
cms in diameter) packed with 1.44Kg of 5 Angstrom zeolite beads 2 obtained from
Davison Chemical Division of W.R. Grace & Co. The apparatus was evacuated and
the mixture was passed from the gas bottle 3 into the bottom of the tube through a
filter 4. The mixture was allowed to travel through the zeolite bed 2 and the gases
exiting the column 1 were condensed in a specimen container 5 immersed in solid
carbon dioxide. The weight of the gas bottle was monitored 6 throughout so that the
amount of gas entering the tube could be measured. The components of the gas
exiting the tube was monitored using a gas chromatograph 7.
After 170g of the mixture had passed through the bed, the gas exiting the bed
contained 100% R12. After 400g of the mixture had passed through the bed, the gas
exiting the bed contained 99.98% R12 and 0.02% R22. The specimen container was
removed and replaced with a second specimen container. The experiment was
continued and after 800g of the mixture had passed through the bed, the gas exiting
the bed contained 98.46%R12 and 1.54%R22. Finally after 91 Og of the mixture has
passed through the bed, the gas exiting the bed contained 90.229%R12 and
9.771%R22. It is clear that at this point the molecular sieve bed had adsorbed its
capacity of R22 (approximately 130g i.e. approximately 10% w/w adsoφtion). The
first specimen was weighed and analysed and contained 1 lOg of gas consisting of
99.875%R12 and 0.125%R22. The second specimen was also weighed and analysed
and contained 11 Og of gas consisting of 94%R12 and 6%R22. As 130g of R22 was
adsorbed, by calculation it can be determined that 182g of pure R12 exited the column
before saturation of the bed and consequently the total weight of processable gas in
order to obtain a 100% separation, was 312g.
EXAMPLE 4
An experiment was conducted in the same apparatus as in Example 3 except
that the mixture contained 33.63 %R 12 and 66.36%R22. Accordingly it was expected
that the zeolite would adsorb 130g of R22 and produce 60g of R12. The condensed
gas was weighed as it was collected into the cooled specimen holder and when 80g of
gas had been collected the contents were analysed using a gas chromatograph and
found to contain 99.9%R12 and 0.08%R22. A further 60g of gas was collected in a
second specimen tube and found to contain 99.668%R12 and 0.332%R22.
The experimental design of the apparatus could be improved by filling the
entire tube with zeolite and sampling the gas exiting the top of the tube thereby
providing immediate detection of the exiting gases and preventing the build up of a
buffer zone of a R12/R22 mix. Further noting a fixed adsoφtion rate for the zeolite,
the amount of exiting gas can be controlled so as to ensure a particular purity of
gaseous product and would enable one to conduct the method in a number of parallel
beds, the beds being alternated when the molecule sieve bed becomes saturated.
EXAMPLE 5
Experiments were conducted in the same apparatus as in Example 3 except
that instead of the column being packed with zeolite beads, the column was packed
with a bituminous based activated charcoal. A first experiment was conducted to
determine whether preferential adsoφtion occurs between R12 and R22 and the
surface of the activated carbon. Firstly R22 was passed through the column at a
pressure of 200kPa. A vigorous exothermic reaction was observed during which the
2% water previously adsorbed in the column was emitted as steam and R22 was
adsorbed to an extent of 24.21% w/w. R12 was then fed into the column, and pure
R22 began to emerge from the top of the column. This continued until all the R22 in
the column was quantitatively displaced by the R12, at which point pure R12 emerged
from the top of the column. R12 was also adsorbed in the column to an extent of
44.7% w/w.
In a second experiment the activated carbon was saturated with R12, and R22
was fed into the bottom of the column. Pure R12 emerged from the top of the column
decreasing with time to 0% with a concomitant increase in R22 concentration to
100%. At the conclusion of the experiment, not all of the R/12 was displaced by the
R22, the column containing at the conclusion of the experiment 489g of R12 and 21 lg
of R22.
These experiments indicate that activated carbon surfaces have a preferential
adsoφtion for R12 in R12/R22 mixes. However, the adsoφtion is dynamic, rather
than the static capture mechanism that occurs with zeolites. It is thought that the Van
der Waal's forces may be due to overlap between the vacant 3d orbitals of σ bonded
chlorine atoms and the partially filled 2p orbitals of graphitic carbon. This interaction
cannot occur with bonded fluorine atoms due to the absence of d orbitals in the second
shell. The experiments indicate that the selectivity shown by activated carbon is
related to the degree of chlorination of the CFC. an observation in keeping with the pi
bonding theory.
The invention is useful for separation of mixtures of CFC's, HCFC"s and/or
HFC's.
Although the invention has been described with reference to separation of
dichlorodifluoromethane R-12 and hydrochlorodifluoromethane R-22 it should be
appreciated that the invention can be embodied in many other forms and can be used
for the separation of other mixtures of CFCs, HCFC's and HFC's such as mixtures of
any of R-12, R-22, chlorotrifluoromethane R-13, fluorotrichloromethane R-l 1,
trichlorotrifluoroethane R-l 13, dichlorotetrafluoroethane R-l 14,
dichlorotrifluoroethane R-l 23, dichlorofluoroethane R-141B, tetrafluoroethane R-
134A, pentafluoroethane R-125 and heptafluoropropane R-227. The other
halogenated alkane based refrigerants, such as HFC's and iodo substituted derivates
are also embraced by this invention.