Separation of halogenated compounds

Details Separation of halogenated compounds Separation of halogenated compounds

2003-03-03

2004-11-23

Cintins, Ivars C. (Department: 1724)

Liquid purification or separation

Processes

Ion exchange or selective sorption

C095S142000, C570S179000

Reexamination Certificate

active

06821436

ABSTRACT:

FIELD OF INVENTION
This invention relates generally to a method for the separation of halogenated compounds, and, more specifically, to a method of removing the toxic impurity chlorofluoromethane (HCFC-31) from a product stream of difluoromethane (HFC-32).
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 (ODP) of zero and a very low global warming potential (GWP).
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 readily be removed 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 relates to the identification of a commercially-available polymer adsorbent that removes chlorinated methane impurities from a product stream. Although polymeric absorbents are known to remove organics from air and water (see, e.g., Dow Chemical Company, Polymeric Adsorbent XUS 43493, Technical Bulletin 3.03 (hereby incorporated by reference)), it has been found unexpectantly that the adsorbent of the present invention is particularly suitable for selectively adsorbing chlorinated methanes over halogenated compounds. In particular, the adsorbent of the present invention adsorbs chlorinated methanes, such as HCFC-31, but not hydrofluorocarbons, such as HFC-32.
One aspect of the present invention is a process of using a polymer adsorbent to remove a chlorinated methane impurity from an impure product stream comprising a halogenated compound other than the chlorinated methane impurity. In a preferred embodiment, the polymer adsorbent has a pore size distribution characterized by a cumulative porosity as a function of the log of pore diameter greater than that of activated carbon. In another preferred embodiment, the adsorbent comprises a matrix of at least one cross-linked styrenic polymer having a total porosity of at least about 0.8 cc/g, an average pore diameter of about 30 to about 60 Å, and a BET surface area of at least about 900 m
2
/g.
The process of the present invention has been found to be particularity effective in adsorbing a chlorinated methane impurity having the formula:
CH
w
Cl
y
X
z
  (1)
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 (HCC-40), 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.
In a preferred embodiment, separation is effected between a chlorinated methane of formula (1) and a halogenated compound having the following formula:
C
n
H
m
Cl
p
X′
k
  (2)
wherein:
each X′ is an independently selected halogen other than chlorine; and
n, m, p, and k are integers with the provisos that 1≧n≧10; n>p; k≧1; 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.
It is believed that pore distribution of the adsorbent may play a significant role in the selectivity described above. (The scope of the invention, however, should not be limited by any particular theory of adsorption). As used herein, “pore distribution” is a linear relationship between cumulative porosity and the log of the pore diameter. The preferred adsorbent of the present invention has pore distribution characterized by a higher cumulative porosity as a function of the log of pore diameter than that of activated carbon. In a more preferred embodiment, the pore size distribution is characterized by a cumulative porosity as a function of the log of pore diameter of no less than about 0.43 cc/g. Still more preferably, the cumulative porosity as a function of the log of pore diameter of no less than about 0.45 cc/g. The linear relationship of cumulative porosity to the log of pore diameter can vary, although a portion of the relationship is characterized by an exponential increase in cumulative porosity.
In a preferred embodiment, the absorbent comprises a matrix of at least one cross-linked styrenic polymer having a total porosity of at least about 0.8 cc/g, an average pore diameter of about 30 to about 60 Å, and a BET surface area of at least about 900 m
2
/g. More preferably, the total porosity is about 1.1 cc/g, average pore diameter is about 35 to about 55 Å, and BET surface area is at least about 1000 m
2
/g. Still more preferably, the total porosity is about 1.1 to about 1.2 cc/g, the average pore diameter is about 40 to about 50 Å, and the BET surface area is at least about 1100 m
2
/g. In the most preferred embodiment, the total porosity is about 1.16 cc/g, the average pore diameter is about 46 Å, and the BET surface area is about 1100 m
2
/g.
It has been found that polymeric adsorbents having relatively low moisture content tend to outperform equivalent adsorbents having relatively high moisture content. Accordingly, in a preferred embodiment, the moisture content is no greater than about 30% by weight, more preferably, no greater than about 10% by weight, and, even more preferably, no greater than about 5% by weight.
The configuration of the units of adsorbent may vary providing that the physical parameters above are met. It has been found, however, that spherical beads achieve the desired results. In a preferred embodiment, the beads have a diameter from about 10 to about 70 mesh, and, more preferably, from about 20 to about 50 mesh. Suitable results have been obtained using an adsorbent having an apparent density of about 0.20 to about 0.80 g/cc. Preferably, the apparent density is about 0.30 to about 0.70 g/cc, and, more preferably, about 0.34 g/cc.
Particular preferred and commercially-available polymeric adsorbents useful in the present invention includes DOWEX OPTIPORE 493 Series (available through Dow Chemical, Midland, Mich.), especially V493, which is described in detail in Dowex Optiore Adsorbents, Fluidized Properties of Dow Polymeric Adsorbent, Form No. 177-01731-597ORP (May 1997), herein incorporated by reference.
In the process of the invention, the product stream is contacted with the zeolite by passing the product stream over a fixed bed of polymeric absorbent in either the liquid or vapor phase. It has been found, ho

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