BACKGROUND OF THE INVENTION Historically, chlorofluorocarbons have been widely used in various capacities such as refrigerants, foam blowing agents, cleaning solvents and propellants for aerosol sprays. In recent years, however/, there has been pressure to avoid their use due to their adverse effect on the ozone layer and their contribution to global warming.

Consequently, attempts are underway to find suitable replacements which are environmentally acceptable. The search for suitable replacements has centered generally on hydrofluorocarbons (HFCs) which do not contain chlorine. The hydrofluorocarbon difluoromethane (HFC-32) is of particular interest as one such replacement. Difluoromethane has an ozone depletion potential of zero and a very low global warming potential.

A widely-used method for preparing hydrofluorocarbons involves the fluorination of chlorinated starting materials. Unfortunately, fluorination of chlorinated staring materials usually results in the formation of unwanted, chlorinated by-products.

For example, production of HFC-32 tends to produce a variety of chlorinated methane by-products including chlorodifluoromethane (HCFC-22), dichlorodifluoromethane

(CFC-12), and chlorofluoromethane (HCFC-31). While distillation effectively removes many chlorinated impurities from an HFC product stream, some chlorinated impurities, particularly HCFC-31, cannot be removed readily through conventional distillation. Nevertheless, HCFC-31 must be removed to extremely low levels, for example, below 10 ppm, because it is highly toxic and tends to react with the desired HFC product.

Therefore, there is a need to remove chlorinated methane impurities, particularly HCFC-31, from a product stream more effectively then through distillation. The present invention fulfills this need among others.

DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS The present invention provides for the removal of chlorinated methane impurities from a product stream using a zeolite adsorbent. It has been found that zeolite adsorbents, which have been traditionally used for drying and purifying acid product streams, selectively adsorb chlorinated methanes. Thus, removal of chlorinated impurities may be effected readily and selectively from a variety of product streams by using commercially-available and inexpensive zeolite adsorbents.

One aspect of the present invention is a process for removing a chlorinated methane impurity from a product stream by contacting the impure product stream with at least one zeolite adsorbent.

The process of the present invention has been found to be particularity effective in adsorbing a chlorinated methane impurity having the formula:

CHClyX,0) wherein each X is an independently selected halogen; y> 1 and w+y+z=4.

Preferably X is fluorine. In a more preferred embodiment, the chlorinated methane impurity is selected from the group consisting of chlorofluoromethane (HCFC-31), dichloromethane (HCC40), chlorodifluoromethane (HCFC-22), chlorotrifluoromethane (CFC-13), dichlorodifluoromethane (CFC-12) and combinations of two or more thereof. In the most preferred embodiment of the invention, the chlorinated methane impurity is HCFC-31.

The zeolite adsorbent also is particularly effective in selectively adsorbing chlorinated methanes over halogenated compounds (other than the chlorinated methane impurity described above). In a preferred embodiment, separation is effected between a chlorinated methane of formula (1) and a halogenated compound having the followingformula: CACI,X\(2) wherein: each X'is an independently selected halogen other than chlorine, and 1 > n 10 ; n> p; k 21 ; and 2n+2=m+p+k.

More preferably, n < 3, p=0, and X'is fluorine, and, even more preferably, n =1. In the most preferred embodiment, the product stream comprises HFC-32.

With respect to the particular zeolite used, it has been found that to achieve the

preferred selective adsorption of a chlorinated methane over the halogenated compound described above, a zeolite adsorbent having a nominal pore size of about 2 to about 8 A is preferred, while a pore size of about 4 to about 6 A is more preferred.

In the most preferred embodiment, the pore size is about 5 A.

Suitable adsorption rates of have been achieved using units of adsorbent having a bulk density of about 30 to about 70 lb/ft3 and a unit density of about 60 to about 100 lb/ft3. Preferably, the units have a bulk density of about 40 to about 60 Ib/ft3 and a unit density of about 65 to about 90 lb/ft3, and, more preferably, a bulk density of about 40 to about 50 Ib/ft3 and a unit density of about 70 to about 80 lb/ft3. In the most preferred embodiment, the units have a bulk density of about 45 Ib/ft3 and a unit density of about 72 lb/ft3.

The configuration of the units of adsorbent may vary providing that the physical parameters above are met. For example, it has been found that pellets of zeolite adsorbent perform satisfactory. The zeolite pellets range in size preferably from about 1/64 to about 1/4", and, more preferably, from about 1/32"to about 1/8". Most preferably, the zeolite has nominal size of 1/4".

The particular chemical composition and the degree of hydration of the zeolite adsorbent may vary providing it has the physical properties described above.

Preferably, the zeolite has the following formula: M2, : AI, 03 : aSi02 : bH20 (3) where: M is an alkali or alkaline earth metal ion;

v is the valence of M (usually 1 or 2); a is about 1 to about 3; and b is about 0 to about 4.

More preferably, M is selected from potassium (K) (v=1), sodium (Na) (v=1), and calcium (Ca) (v=2), a is about 2, and b is about 0. Even more preferably, M is Ca.

The manufacture of zeolites is well known and is disclosed, for example, in U. S. Patent Nos. 5,248,491,5,192,522,4,406819,4,386,012, and 4,372,876, herein incorporated by reference.

Particular preferred and commercially-available zeolite adsorbents useful in the present invention include AW-300 and AW-500 (available through UOP, Inc., Des Plaines, 11) AW-500 is most preferred.

In the process of the invention, the product stream is contacted with the zeolite by passing the product stream over a fixed bed of zeolite adsorbent in either the liquid or vapor phase. It has been found, however, that more effective removal of chlorinated methane impurities is achieved using a vapor-phase product stream. The bed should be packed tightly to ensure that very little, if any, vapor stream"breaks through"and passes through the bed without contacting the adsorbent sufficiently to promote adsorption. Selection of the pellet size and bed shape may be varied within a broad range and may be determined according to known principles, and, particularly, to provide the preferred densities described above. Various other techniques known in the art also may be used for contacting the product stream with the zeolite pellets, including, for example, fluidized or moving beds of zeolite pellets.

The hourly space velocity of the product stream over the zeolite adsorbent may be varied within a wide range. Generally, the product stream is passed over the active carbon with a gas hourly space velocity of about 5 to about 1000h-l, and preferably with a gas hourly space velocity of about 10 to about 500h-l, although the gas hourly space velocity may be much greater or much lower than this if desired. A corresponding liquid hourly space velocity for liquid phase operation is about 1 to about 30h'1, and, again, this velocity may be more or less if desired.

The conditions under which the process of the present invention is conducted may be varied widely and generally depends upon the equipment available. The temperature at which the vapor phase process is conducted is typically between about -50 and about 100°C, more conveniently between about 0 and about 50°C, and even more conveniently at about room temperature. The pressure will be dependent to some extent upon whether liquid or vapor phase contacting is chosen and the operation temperature, although an operation pressure between about 0.1 and about 30 bar is generally suitable. Preferably, the process is conducted at about atmospheric pressure or slightly below to avoid the use of specialized equipment.

The bed of zeolite adsorbent will require regeneration to desorb the chlorinated impurity when its adsorption capacity has been filled. One skilled in the art will appreciate that the adsorption capacity is filled when break through is experienced or when adsorption levels of the vapor stream are below a desired level and it is more economic to stop the process and regenerate rather than recycle the vapor stream or reduce throughput to reduce the concentration of contaminants therein. Regeneration

may be performed by passing a gas stream, typically nitrogen or air, over the bed of zeolite adsorbent at elevated temperature, for example, from about 150 to about 400°C. Such regeneration is well known in the art and is discussed for example in the product literature associated with the adsorbent AW-300 and AW-500, A Report on Acid-Resistent Molecular Sieve Types AW-300 and AW-500, herein incorporated by reference.

According to the process of the present invention, chlorinated methane impurities can be effectively removed from a product stream with high selectivity. The process of the present invention is particularly well suited for removing HCFC-31 from a product stream comprising HFC-32. For example, it has been found that the process of the present invention can be used to purify a vaporized product stream having a space velocity of no greater than about 100 ho 1 in a tube packed with adsorbent of the present invention over a period of no greater than about 4 hours to result in a purified product stream containing less than 10 ppm of HCFC-31.

The following examples serve to illustrate the invention EXAMPLE 1 A 0.5 inch diameter by 19 inch long Teflon tube was packed with UOP'S AW- 500 Molecular Sieve adsorbent to a bed height of approximately 16.5 inches. The tubes were sealed with glass wool on top and bottom of the bed so that the bed could not move. The bottom of the tube was fitted with a connection to accommodate a feed of an impure product stream comprising HFC-32 and 549 ppm of HCFC-31 at a rate of 13.8 g/hr. The top of the tube was fitted with connections so that the product stream could be collected as it exited into cold traps. The chlorofluoromethane (HCFC-31) concentration was down to 1 ppm after one hour of flowing the feed gas through the bed.

EXAMPLE 2 Example 1 was repeated, except flow rate of the feed gas mixture was at a rate of 6. 20 g/hr. The chlorofluoromethane (HCFC-31) concentration was down to 12 ppm after four hours of flowing the feed gas through the bed.