For more
than four decades a chlorofluorocarbon known as R22 (chlorodifluoromethan) has
been the refrigerant of choice for application in freezers, refrigerators and
air-conditioners. Yet, in the early 70s, Rowland and Molina showed that the
chlorine of chlorofluorocarbons and hydrochlorofluorocarbons is released when
the molecules are exposed to ultraviolet radiation in the stratosphere. A free
chlorine atom in the stratosphere can act as catalyst for breaking down many
stratospheric ozone molecules [180]-[182].
Consequently, in 1987 the Montreal Protocol was signed by almost all industrial
countries. The Montreal Protocol is an international agreement on the phase-out
of refrigerants based on chlorofluorocarbons. The Montreal Protocol is carried
out in national laws of all participating countries like the "Clean Air
Act, Title VI" in the United States [183]
or the "FCKW-Halon-Verbotsverordnung" in Germany [184].
In consequence,
national laws [185],[186]
regulated the substitution of the chlorofluorocarbons and hydrochlorofluorocarbons
by hydrofluorocarbons, which do not deplete the ozone layer. Among these substitutes,
the hydrofluorocarbon R134a (1,1,1,2-tetrafluoroethane) plays the most important
role. According to the "Alternative Fluorocarbons Acceptability Study"
[187]
the portion of R134a among the worldwide sold refrigerants for air-conditioners
and refrigerators increased from 0% to 31% in the period from 1990 to 2000.
In the same time, the portion of the hydrochlorofluorocarbons decreased from
95% to 64% whereby in 2000 practically no hydrochlorofluorocarbons except of
R22 were sold for the use in refrigerants and air-conditioners.
This
gradual substitution process of the refrigerants and the resulting diversity
of the refrigerants found in old freezers, refrigerators and in air-conditioners
prevent a reuse of the refrigerants. At the moment, the recycling process is
a downcycling process whereby the refrigerants are converted to hydrogen chloride
and hydrogen fluoride. A high quality recycling is not possible since the refrigerants
are sometimes returned with no labels or wrong labels and since the refrigerants
are often mixtures originating from different refrigerators or even were used
as blends like the so-called R400 and R500 series. On the other hand, an analysis
of the refrigerants of each refrigerator by a laboratory is too expensive. This
results in the need of a detection method, which enables a fast, durable and
economical on-line quantification of different mixtures of refrigerants. As
sensor setups can come up to these requirements, data sets were recorded to
examine how feasible an application of sensor setups for the quantification
of R22 and R134a in mixtures in air is. Additionally several polymers were investigated
to find an optimal polymer respectively an optimal combination of polymers for
the discrimination of the refrigerants R22 and R134a (see chapter
3).
figure 14: 3D ball and sticks molecular models of the refrigerants R22 and R134a.
The
gas mixtures of R22, R134a and air were generated using a gas mixing station
with computer-driven mass-flow controllers (MKS, Munich, Germany).
A 4-way valve before the cell ensured that the path length was the same for all
analytes. Dry synthetic air was used as carrier gas. All measurements were
performed at a constant flow rate.
Ph. D. Thesis