The subject invention relates to a method of removing volatile organic compounds such as fluorocarbons from air and other gases. The subject invention specifically relates to the use of carbon-based adsorbents to treat or purify gas streams or to recover hydrocarbons or other organics from a gas stream. The carbon-based adsorbents used in the subject process are carbon molecular sieves produced by a novel method of manufacture in which a substantially oxygen-free cross-linked polymer is carbonized under oxygen-free conditions to yield a final shaped adsorbent particle.
It is well known that carbon-based materials function well as adsorbents for a wide variety of organic compounds. Numerous grades and brands of charcoal and activated carbon, also referred to as active carbon, are sold commercially. These materials are known to be useful in solvent recovery as described in Volume 4 of the Kirk-Othmer Encyclopedia of Chemical Technology and in an article "Vapor-phase adsorption cuts pollution, recovers solvents" at page 58 of the Dec. 31, 1979 edition of Chemical Engineering. This utility has led to a significant amount of activity in developing methods to produce better carbon molecular sieves as represented by the references cited below and the material referred to in these references.
U.S. Pat. Nos. 4,540,678; 4,627,857; and 4,629,476 issued to R. F. Sutt, Jr. are believed relevant for their general teaching in regard to the preparation of carbon molecular sieves. hhese references describe the preparation of carbon molecular sieves from carbon sources such as coal or coconut char. The references also indicate that it is known to produce the sieves by treating the feed material in a substantially oxygen-free environment during a heating or calcining step. According to these references, the pore distribution may be adjusted through the treatment with a pore blocking substance such as a large molecular weight hydrocarbon although this material is not employed in all embodiments described in these three references.
U.S. Pat. Nos. 4,046,709 issued to N. Yuki and 4,420,415 issued to N. Yuki et al. are believed relevant as they describe the production of carbon molecular sieves from polymeric precursor materials similar to that which may be employed in the subject invention. These references describe the utilization of a vinylidene chloride copolymer as a precursor material. The copolymer is carbonized for the removal of its chlorine component and the resultant char is pulverized. Sintering agents such as coal tar pitch and binder materials such as cellulose are then added to pulverized product and the mixture is shaped. The thus shaped bound material is then subjected to a high temperature carbonizing treatment to yield the finished material.
U.S. Pat. No. 3,960,768 issued to W. Ripperger et al. is also believed relevant for its teaching in regard to the production of carbon molecular sieves from polymerized halohydrocarbons by the elimination of hydrogen halide during a heating procedure. The reference indicates appropriately shaped end products can be obtained by using raw materials having a corresponding shape. The teaching at column 3 of this reference indicates the preshaped starting materials shrink into the corresponding shapes.
The last cited reference teaches a specific method of heating the polymer in order to avoid the problem associated with the prior methods. More specifically, in the prior methods such as that of U.S. Pat. No. 4,420,415 cited above the polymer would tend to produce a foam-like highly porous carbon upon heating and removal of hydrogen chloride. The resultant material was too soft and porous to be utilized effectively in commercial applications. For this reason, it is necessary to grind the foam-like powder and employ a binder to form articles having a desired finished shape.
There have been references in the literature to carbon molecular sieves which have a hydrophobic nature. For instance, this is described in the article at page 512 of Volume 271 of Nature (1978) by E. M. Flannigen et al.
USPTO Backgrounds
USPTO Backgrounds