Process for drying CH.sub.2 F.sub.2 refrigerant utilizing zeolite

Difluoromethane (R32) is of current interest as a partial replacement for chlorodifluoromethane (R22) refrigerant heretofore widely used in vapor compression refrigeration systems. R32 has, however, proved to be more reactive than is desirable with the zeolite A adsorbent-desiccant compositions used in such systems to prevent corrosion and freeze-up problems. The sodium cation form of a zeolite having the crystal structure of zeolite B is found to be an effective adsorbent for that purpose and to be significantly less reactive with the R32.

FIELD OF THE INVENTION 
The present invention relates in general to the removal of water from 
refrigerant mixtures containing difluoromethane (HFC-32) and more 
particularly to the treatment of such mixtures employed as circulating 
refrigerant streams of refrigeration systems to sequester water and 
decomposition products of the refrigerant as a means for avoiding 
freeze-ups and corrosion. The treatment comprises adsorption of these 
impurities on a zeolitic molecular sieve having the crystal structure of 
zeolite B. 
BACKGROUND OF THE INVENTION 
In view of the now well-established relationship between 
chlorofluorocarbons (CFC's) released into the atmosphere and the depletion 
of the earth's ozone layer, considerable attention is being directed to 
finding effective substitutes for these once widely used compounds. It 
appears that the worst offenders are the fully halogenated CFC's which 
contain chlorine. These compounds are relatively unreactive with other 
compounds in the lower atmosphere and thus are able to diffuse into the 
stratosphere intact and be decomposed by ultraviolet radiation to form, 
inter alia, chlorine compounds which readily react with ozone. On the 
premise that it is the chlorine constituent of the CFC's which ultimately 
reacts with and destroys the ozone molecules, and in the interest of 
approximating as closely as possible the physical properties of the CFC's 
already in use, the proposed substitutes have in general been HCFC's 
containing lesser proportions of chlorine or fluorocarbons containing no 
chlorine at all. For example, dichlorodifluoromethane, widely used under 
the trademark Freon 12 as a refrigerant in household refrigerators, in 
automotive units and in commercial freezers and display cases, has been 
replaced in many instances by 1,1,1,2-tetrafluoroethane (also known as 
R-134a) or by chlorodifluoromethane (also known as R22 or HCFC-22). 
Because R-134a is not miscible with many commonly used lubricants, 
mixtures of R-134a and R22 have been proposed for systems employing 
lubricants soluble in R22. See U.S. Pat. No. 5,198,139, Bierschenk et al, 
in this regard. In the recent past, over 90% of the chlorodifluoromethane 
and about a third of the dichlorodifluoromethane manufactured was utilized 
in air-conditioning and refrigeration. 
With increasing recognition of the seriousness of atmospheric ozone 
depletion, stricter limitations on the future use of any 
chlorine-containing refrigerant continue to be imposed. One of the most 
suitable replacements for R22 in stationary refrigeration systems is a 
non-flammable mixture of the HFC compound difluoromethane, also known as 
R32, with other halocarbons or halohydrocarbons such as R134a and R125 
(C.sub.2 HF.sub.5). One such mixture has been proposed which consists of 
60% R32 and 40% R125. Another proposed mixture consists of 30% R32 , 10% 
R125 and 60% R134a. A significant problem in making this substitution 
arises from the fact that R32 is more reactive than R22 with the zeolite A 
commonly employed as an adsorbent-desiccant in the circulating refrigerant 
stream to protect against freeze-ups and corrosion of the refrigeration 
unit. 
SUMMARY OF THE INVENTION 
It has now been discovered that the sodium cation form of an activated 
zeolitic molecular sieve which has the crystal structure of zeolite B and 
a framework Si/Al.sub.2 molar ratio of at least 2.5, and preferably at 
least 5.0, possesses the requisite selectivity and adsorption capacity for 
effectively removing water from R32 refrigerant and exhibits a relatively 
low reactivity with the HFC under the conditions encountered in use in a 
refrigerant recycle stream such as employed in a stationary refrigeration 
unit. Of the various cation forms of zeolite B, the sodium form is found 
to have the most favorable combination of properties necessary for 
effective R32 drying. These properties include pore openings small enough 
to significantly limit, though not totally prevent, the passage of R32 
molecules, a relatively large capacity for water vapor adsorption and the 
ability to withstand the thermal and hydrothermal stresses of being 
incorporated into engineered agglomerate forms. It also has the distinct 
commercial advantage of being the cation form of the as-synthesized 
zeolite B. 
Accordingly, in a vapor compression refrigeration process wherein a 
refrigerant fluid comprising difluoromethane is cycled within a closed 
system and is alternately vaporized and condensed in the known manner to 
produce cooling, the present invention provides an improvement which 
comprises incorporating within the closed system containing said 
difluoromethane and in contact therewith as a desiccant the sodium cation 
form of an activated zeolitic molecular sieve having the crystal structure 
of zeolite B, said zeolite B having a framework Si/Al.sub.2 molar ratio of 
at least 2.5, and preferably having at least 75 percent of its 
AlO.sub.2.sup.- framework tetrahedral units associated with sodium 
cations. More preferably, substantially the entire cation population are 
sodium cations.

DETAILED DESCRIPTION OF THE INVENTION 
Ideally a purified and dried refrigerant fluid, after having been sealed in 
a refrigeration unit, would continue to circulate through the compressors, 
Joule-Thompson nozzles, cooling coils, etc., for the life of the unit 
without causing any corrosion or freeze-up problems. In practice, however, 
the system can rarely be so thoroughly sealed, or the components so 
thoroughly dried before sealing, to prevent water and other contaminants 
from entering the sealed system, and these materials must be removed or 
sequestered to avoid the development of the aforementioned problems. 
Conventionally the contaminants are rendered innocuous by adsorption on a 
suitable adsorbent inserted into the sealed system in contact with the 
circulating refrigerant stream. In the case of halocarbon refrigerants the 
contaminants of greatest concern, in addition to water, are attributable 
to the degradation products of the refrigerant molecules themselves. 
Halogen acids, notably HCl, can form and cause corrosion. In some 
instances, the adsorbent composition itself can be a reactant in the 
chemical reactions which result in the production of corrosive products. 
Zeolitic molecular sieves generally exhibit this property and, 
accordingly, it is necessary to select the particular zeolite adsorbent in 
view of the physical and chemical properties of the refrigerant involved 
to minimize harmful reactions. Since essentially all of the active sites 
of a zeolite are reachable only by molecules which can enter the internal 
cavities of the crystal structure through its uniform pore system, it is 
advantageous to employ a zeolite whose pore openings admit water and other 
small impurity molecules, but exclude molecules of the refrigerant. Thus, 
a commonly used adsorbent for refrigeration systems is a highly exchanged 
potassium cation form of zeolite A having pore diameters of about 3 
Angstroms. The effective pore diameters can be further reduced, to a 
slight degree, by controlled steaming. A potassium cation exchanged (40%) 
form of zeolite A, i.e., zeolite 3A, has been found to be quite effective 
in drying R-134a and R22 , for example. 
R-32, however, is both smaller in molecular size and more polar than R22 by 
virtue of the substitution of a hydrogen atom for the chlorine atom in 
chlorodifluoromethane. It is also more reactive than R22 with constituents 
in the lower atmosphere and thus, advantageously, is less likely to escape 
unreacted into the stratosphere. It is, by the same token, more reactive 
with zeolites, including zeolite 3A, having pores large enough for R32 to 
enter. The greater polarity of R32 also means that the partial blocking of 
zeolite pores by cation exchange techniques is less effective in excluding 
the R32 from the inner cavities of the zeolite crystal structure. 
The sodium cation form of a zeolite having the crystal structure of zeolite 
B, and having a framework Si/Al.sub.2 molar ratio of at least 2.5, is 
found to be highly effective as a desiccant-adsorbent when employed in R32 
refrigerant streams. This zeolite composition has the advantage of having 
a very small effective pore diameter and, particularly in those forms 
having a higher Si/Al .sub.2 molar ratio, a lower overall framework charge 
compared with zeolite A. These properties tend to limit access to the pore 
system of the R32 refrigerant and lessen the reaction with the R32 
molecules to produce reaction products corrosive toward the refrigeration 
system and destructive of the adsorbent itself. The adsorption capacity of 
the zeolite for water is, on the other hand, entirely adequate for its 
intended use. For example a NaB composition having a Si/Al .sub.2 molar 
ratio of 5.0 is able to adsorb 19.5 weight percent water vapor at 
25.degree. C. and 4.6 torr water pressure after activation at 400.degree. 
C. A similarly activated zeolite B is found to adsorb nil difluoromethane 
at 500 torr pressure and 25.degree. C. The adsorption capacity for water 
for other cationic forms is in general the same as for NaB, but many of 
the various cation forms of zeolite B cannot be activated fully or at 
temperatures above about 200.degree. C. without suffering a loss of 
crystallinity. For example, the Zn.sup.++ -, Mg.sup.++ - and Li.sup.30 
-exchanged forms of NaB, even at relatively low levels of ion-exchange, 
become x-ray amorphous when activation is attempted even at a temperature 
as low as 250.degree. C. By the term activation is meant the removal of at 
least a major proportion of the water of hydration present in the crystal 
void space as a consequence of synthesis. Further, the pore diameters of 
other alkali metal cation forms of zeolite B are large enough to freely 
admit R32 into the inner cavities of the zeolite structure, thus 
permitting an unacceptable degree of reaction between the zeolite and the 
R32. 
The zeolite adsorbent generally referred to as zeolite B is in fact a 
series of microporous crystalline aluminosilicates whose crystal 
structures have the same chemical bonds but have differences in bond 
angles due to displacive transformations, i.e., changes through rotation 
of structural sub-units. The crystal framework is remarkably flexible, and 
the displacive transformations produce marked changes in the lattice 
constants. At least eight members of the series have been identified and 
denominated as B.sub.1 through B.sub.8. The composition denominated 
B.sub.1 has a body-centered cubic structure and is the same as the 
composition denominated as Na--P.sub.1 by Barrer et al [J. Chem. Soc., 
1521-28 (1959)]. The B.sub.2, B.sub.3, and B.sub.6 members have 
body-centered tetragonal structures with varying degrees of displacive 
transformation of the body-centered cubic structure of B.sub.1. Members 
B.sub.4 and B.sub.7 resemble the tetragonal primitive structure 
characteristic of the phase denominated Na--P.sub.t by Taylor and Roy [Am. 
Mineral. 49, 656-682 (1964)]. Member B.sub.8 was formed by drying B.sub.1 
in air at 120.degree. C. This partially dehydrated form exhibits major 
x-ray diffraction lines which are displaced considerably from the lines of 
the B.sub. pattern. Upon rehydration the B.sub.8 member reverts to the 
B.sub.1 structure. The x-ray powder diffraction patterns of these series 
members are shown in drawings. 
The syntheses of zeolite B in the various forms referred to hereinabove are 
disclosed in detail in the Taylor and Roy literature article cited above 
and in U.S. Pat. No. 3,008,803, issued Nov. 14, 1961, to R. M. Milton, and 
also in the W. C. Beard literature article in "Advances in Chemistry, 
Series 101," American Chemical Society, Wash. D.C., pgs. 237-249 (1971). 
These publications are incorporated by reference herein. 
In view of the foregoing, the zeolite adsorbent utilized in the process of 
the appended claims is an activated microporous aluminosilicate zeolite 
which has a Si/Al.sub.2 molar ratio of at least 2.5 and in its fully 
hydrated state has an x-ray diffraction pattern the major d-spacings of 
which are 
______________________________________ 
Relative 
d, .ANG. 
Intensity 
______________________________________ 
7.10 .+-. 0.1 
S-VS 
4.97 .+-. 0.1 
M-S 
4.10 .+-. 0.1 
S-VS 
3.18 .+-. 0.1 
S-VS 
______________________________________ 
In terms of I/I..times.100, the relative intensities designated VS (very 
strong), S (strong) and M (medium) are within the ranges (80-100), (60-79) 
and (40-59), respectively. 
For use as a desiccant-adsorbent in R32 refrigerant streams the zeolite B 
crystals are agglomerated into engineered forms to avoid entrainment in 
the stream and plugging of orifices and conduits and abrasive damage to 
the refrigeration system. While compaction to create self-bonding of the 
crystal particles is possible, it is advantageous to utilize binder 
materials to create agglomerates of high attrition resistance. It has been 
determined, in this regard, that the choice of binder material can be an 
important factor in inhibiting the reactivity of the R32 with the 
adsorbent agglomerates. For example, clays exhibiting significant degrees 
of basicity react with the R32 in essentially the same manner as the basic 
NaB zeolite. Ideally the clay binder should be neither basic nor acidic 
and require modest calcination temperatures to be set. Unfortunately no 
commercially available clay has been found to possess all of these 
properties. Without wanting to be bound by any particular theory, it is 
possible that R32 reacts with basic zeolitic aluminosilicates, such as 
zeolite NaB, according to the following equations: 
EQU CH.sub.2 F.sub.2 +Na(--Si--O--Al--).fwdarw.(CHF.sub.2).sup.- 
+H(--SiOAl--)1. 
EQU Dealumination 2. 
EQU (CHF.sub.2).sup.- +CH.sub.2 F.sub.2 .fwdarw.F.sub.2 HC--CFH.sub.2 +F.sup.- 
3. 
EQU Al.sup.+3 +6F.sup.- .fwdarw.(AlF.sub.6).sup.-3 4. 
EQU Si.sup.+4 +6F.sup.- .fwdarw.(SiF.sub.6).sup.-2 5. 
Presumably similar reactions occur between CH.sub.2 F.sub.2 and the various 
clay compositions commonly used as binder material. These reactions do 
not, however, occur with equal facility among the various clays. For 
example, in an experimental procedure wherein liquid CH.sub.2 F.sub.2 was 
contacted with a sample of avery clay [ideal formula Al.sub.2 (Si.sub.2 
O.sub.5) (OH).sub.4 ] and a sample of attapulgite clay [ideal formula 
Mg.sub.5 Si.sub.8 O.sub.20 (OH).sub.2.8H.sub.2 O] at 75.degree. C. and the 
vapor pressure of R32 at that temperature (.about.53 atmospheres) for 7 
days, post-treatment analysis of the avery clay indicated a 0.67 wt. % 
fluoride content versus a 1.23 wt. % fluoride content for the attapulgite. 
Sepiolite and halloysite appear to resemble attapulgite and avery clay, 
respectively, in their reactivity toward CH.sub.2 F.sub.2. The firing 
temperatures necessary to achieve adequate bonding exceed 550.degree. C. 
for halloysite and avery clays. These temperatures cause undue thermal and 
hydrothermal degradation of the NaB. The significantly lower firing 
temperature required for attapulgite makes it a preferred binding material 
for NaB agglomerates. 
EXAMPLE 1. 
In order to verify the suitability of zeolite B for use in the present 
process, a refrigerant/desiccant compatibility test was carried out in 
which a mixture of liquid R32 and a polyolester lubricant were contacted 
with NaB in a stainless steel bomb. In carrying out the test, 15 grams of 
1/16" activated pellets of NaB zeolite having a Si/Al.sub.2 ratio of 5.0 
bonded with 20 weight percent attapulgite clay were first added to the 
bomb followed by the injection of 1 gram of the lubricant and lastly by a 
charge of 10 grams of the R32 refrigerant in the liquid phase. Air was 
evacuated from the bomb after adding lubricant but before charging with 
refrigerant. The temperature of the sealed bomb was raised to 75.degree. 
C. and retained in that state for 7 days. Thereafter the NaB pellets were 
recovered and adsorbed R32 refrigerant was removed from the zeolite pores 
by first grinding the zeolite particles, permitting the ground solids to 
hydrate in open air for a period of about 11 days. The residual fluorine 
content, believed to be in the form of fluoroaluminate and fluorosilicate, 
is a measure of the degree to which the R32 reacted with the NaB 
adsorbent. It was found by chemical analysis that the fluorine content was 
about 1200 ppm (wt.). This value is quite favorable when compared with 
attapulgite-bonded beads of zeolite NaA (1.23 wt. % F.) and 
halloysite-bonded beads of zeolite KA (0.6 wt. % F.) when tested under 
essentially the same conditions. 
EXAMPLE 2. 
The ability of NaB to adsorb water present in low concentrations of R32 was 
determined under static conditions by contacting at 125.degree. F. 
(51.7.degree. C.) samples of NaB, preloaded with water, with liquid phase 
R32. In each instance the R32 water composition was maintained in contact 
with the zeolite for 20 hours at the test temperature. It was determined 
by analysis that the water loading on the zeolite following contact was 
6.7 Wt. %, 10.4 wt. % and 16.3 wt. %. Analysis of water content of R32 was 
3 ppm (wt), 166 pm (wt), and 308 ppm (wt), respectively.