Refrigerants for the 21st Century
11. Identifying Alternatives
| IDENTIFYING CFC ALTERNATIVES BASED ON CORE CONCEPTS OF ATMOSPHERIC CHEMISTRY, ENTHALPIES OF FORMATION, CHEMICAL BONDS AND RADICALS |
Researchers interested in comparing the impact of various
fluorinated compounds on O3 developed a ranking scale known as the
ozone depletion potential
(ODP).
Arbitrarily, CFCl3 (CFC-11) was set at 1.0. Any compound
with a value less than 1.0 should be less damaging to O3.
Conversely, any compound with an ODP greater than 1.0 should cause more damage.
|
Compounds were compared to CFC-11 based on their enthalpies of formation, atmospheric reactivities and the total number of C-Cl bonds. The following table shows the ODP's of several commercially important fluorocarbons. |
The importance of atmospheric chemistry is seen by comparing the ODP's of CFC-115 and
HCFC-22. Both have one C-Cl bond. However, HCFC-22 has an 85% lower ODP. Since the bond
strengths of C-H, C-Cl and C-F are all high, the difference in the ODP's lies in the
ability of the prevalent OH radical to attack the two compounds. The driving force is the
enthalpies of formation of the products from OH attack:
| Bond | Bond Strengths, KJ/mole | Product from OH Attack | Enthalpies of Formation (g), KJ/mole |
| C-H | 412 | HOH | -241.82 |
| C-Cl | 338 | HOCl | -92.05 |
| C-F | 484 | HOF | -98.32 |
Obviously, water (written HOH here)
formation is strongly favored over HOCl or HOF. As a result, CFC-115 must await attack by
some very energetic species, such as high intensity uv or the oxygen radical (O) found
primarily in the stratosphere.
In selecting O3-compatible alternatives for CFC's, then, two
molecular concepts are to be used:
This is in addition to maintaining the desirable physical and physiological
properties already ascribed to the CFC's.
.
Principal Investigator Laurence Peterson; Project Director Matthew Hermes;
Author of this module William Gumprecht.
Fluorocarbon Alternatives