AU1348000A - Refrigerant separation - Google Patents

Description

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AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

V

.r ORIGINAL

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fr Name of Applicant: Actual Inventors: Address for Service: Invention Title: Colcard Pty. Ltd.

Neville Charles Stephenson and Barry Branscombe Walker and Colin William Spencer BALDWIN SHELSTON WATERS MARGARET STREET SYDNEY NSW 2000 'REFRIGERANT SEPARATION' Details of Original Application No. 49336/96 dated 22 MAR 1996 The following statement is a full description of this invention, including the best method of performing it known to me/us:- File: 26862AUP00 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 5 of various components using selective adsorbents.

BACKGROUND OF THE INVENTION Certain refrigerant mixes are difficult to separate. For example. the boiling point of R12 (CC is -29.8 0 C at atmospheric pressure and the boiling point of R22 (CHC 1F) is -40.8 0 C. An azeotrope forms with composition 97.9% RI 2 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".

-2- 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 15 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 ot silica and alumina tetrahedra having the structural formula: 02),(Si0)].wHz0 -3wherein 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 S cations and small cavities result when large monovalent cations, such as potassium, are used. A Type A molecular sieve having the structure Na 2 [(A10 2 12 (Si0 2 12 ].wH 1

O

will have cavities of approximately 4 Angstrom unit in diameter. Substitution of a

C

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 S 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 15 W. R. Grace Co of Baltimore. Maryland. United States of America and sold as 3A.

4A. 5A 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), adsorption occurs on the surface by weak bonding of the Van der Waals type between the chlorofluorocarbon.

-4hydrochlorofluorocarbon 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 Selectronegativities, 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.

15 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 adsorption. 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 adsorption and desorption 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 adsorption step another bed is on its desorption 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 15 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: -6- 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 5A 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 R 15 (CCIF:CF 3 was evident within the R12. The adsorption rate of the R22 followed the Langmuir isotherm adsorption curve.

The unexpected adsorption of the R22 molecules in the 5A cavities of the zeolite can be partially explained in terms of the higher polarity of the smaller CHCIF, molecule. Both CC1,F(R12) and CCIFCF 3 molecules are larger and less polar than CHC 1 F, (R22). The dipole moments are 0.61. 0,52 and 1.42 Debye units respectively.

-7- EXAMPLE 3 400g of pure R22 and 4Kg of pure R12 were mixed in an evacuated 10Kg 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 SDavison 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 o amount of gas entering the tube could be measured. The components of the gas exiting the tube was monitored using a gas chromatograph 7.

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15 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 9 10g 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 adsorption). The -8first specimen was weighed and analysed and contained 1 10g of gas consisting of 99.875%R12 and 0.125%R22. The second specimen was also weighed and analysed and contained 1 10g 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%R12 and 66.36%R22. Accordingly it was expected io 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.

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15 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 adsorption 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.

-9- EXAMPLE 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 adsorption occurs between R12 and R22 and the surface of the activated carbon. Firstly R22 was passed through the column at a S 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 4 44.7% w/w.

In a second experiment the activated carbon was saturated with R12. and R22 15 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 adsorption for R12 in R12/R22 mixes. However, the adsorption 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 a 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 10 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- 11.

trichlorotrifluoroethane R-l 13. dichlorotetrafluoroethane R- 114.

dichlorotrifluoroethane R-123, dichlorofluoroethane R-141B. tetrafluoroethane Ro* 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.

Claims (9)

1. A method for separating a chlorofluorocarbon, a hydrochlorofluorocarbon or hydrofluorocarbon from a gaseous mixture of chlorofluorocarbons. hydrochlorofluorocarbons and/or hydrofluorocarbons comprising passing the mixture through a bed of adsorbent, the adsorbent being selected so as to preferentially adsorb the chlorofluorocarbon, hydrochlorofluorocarbon or hydrofluorocarbon from the S. 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 10 or hvdrofluorocarbon.

2. The method of claim 1 wherein the adsorbent is selected from aluminosilicate clays, activated silica, activated alumina, activated carbon, selective zeolites or molecular sieves. eoo.*

3. The method of claim 1 or 2 wherein the adsorbent is a molecular sieve and is a crystalline, metal aluminosilicate with a three-dimensional interconnecting network of silica and alumina tetrahedra and having the structural formula: 10),(Si0),].wH,0 wherein n is the valence of the cation M which can be sodium, potassium. lithium. calcium, barium, strontium, a rare earth metal. 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. -12-

4. The method of any one of claims 1 to 3 wherein the adsorbent is selected on the basis of the molecular size and differing dipole moments of the components to be separated. The method according to any one of the proceeding claims wherein the adsorbent is a

5 Angstrom molecular sieve or activated carbon and R12/R22 mixtures are separated.

6. The method according to any one of the preceding claims wherein prior to separating the halocarbons, the mixture is analysed to determine its components, the adsorbent being selected on the basis of the molecular size and configuration as well 10 as the polarity for each molecular component.

7. A method of anyone of claims 1 to 6 comprising conducting the method under a pressure gradient.

8. A method of anyone of the preceding claims wherein more than one adsorbent is used either in a single column or in separate columns, the separate columns being run in series or in parallel.

9. The method of any one of the preceding claims wherein the halocarbon is desorbed by heating and passing a purge gas through the bed. The method of any one of the preceding claims wherein halocarbon is desorbed by radiofrequency excitation under vacuum. DATED this 21st Day of January, 2000. COLCARD PTY. LTD. Attorney: PAUL G. HARRISON Fellow Institute of Patent Attorneys of Australia of BALDWIN SHELSTON WATERS