HSC CHEMISTRY 2015​.

  Alternatives                       

 

HCFC:

Hydrochloroflurocarbon, haloalkanes in which only some of the hydrogen atoms have been replaced by chlorine and fluorine.

These compounds have a lower ozone depletion potential (ODP) than CFC’s as they are more reactive due to the H–C bonds which are susceptible to break down by free radicals thus decomposing the molecule. This decomposition occurs in the troposphere, however these reactions are quite slow thus some molecules will diffuse into the stratosphere where ozone depletion occurs.

 

Disadvantages:

• Contain chlorine which as a free radical is the catalyst for ozone decomposition. Therefore although its ODP is much less than that of CFC’s it still has a capacity to destroy ozone, and so are only considered a temporary solution.

• Greenhouse gas: HCFC’s absorb the heat radiation from the Earth’s surface and trap heat in the atmosphere, significantly contributing to global warming.

• Some HCFC’s corrode the inner wall of refrigerators

• Some HCFS’s are explosive

• Toxicities of HCFC’ are unknown.

 

HFC:

Hydroflurocarbon, haloalkanes in which only some of the hydrogen atoms have been replaced by fluorine.

These compounds also contain C–H bonds thus are decomposed in the troposphere but do not contain H–Cl which can decompose to form Cl∙ free radicals in the stratosphere. Thus they have an even lower impact on ozone depletion as their ODP is zero.

 

Disadvantages:

• Greenhouse gas: HFC’s absorb the heat radiation from the Earth’s surface and trap heat in the atmosphere. HCF’s GWP can be up to 1200 times more damaging than CO2.

• Cost- the cost of a HCF replacement can be up to 4 times the cost of the original CFC.

• Efficiency – HFC’s are often less efficient than the CFC they are replacing.

• Different cooling characteristics – HCF’s have different cooling characteristics thus require redesigned air conditioner units, with larger evaporators and condensers.

 

Changing Trends:

Since the international agreement of 1992 which included, ending

Halon use by 1994, ending HCFC production by 1996 and phasing

out HCFC use by the early 21st century, measurements of Chlorine

and Ozone concentrations have noted changing trends.

Since 1997 Chlorine concentrations have begun a steady decline

following a dramatic increase in the years prior.

Additionally Ozone percentages have begun an increasing trend

reaching its highest point since 1991 in 2004, as shown in figure 1.

 

 

 

Evaluating CFC replacements:

To be effective replacements HCFC’s and HFC’s must first and foremost have as little ODP and GWP. In addition to this they must satisfy the requirements of the application they will be fulfilling whilst only minimally decreasing the quality of performance. Cost is also a factor; the replacements must be of a reasonable price so that they can realistically be used by consumers, however they are expected to be more expensive than CFC’s.

 

HCFC’s: HCFC’s are partially decomposed in the troposphere thus has a lower ODP than CFC’s, however it still forms Cl∙ in the stratosphere thus decomposing ozone. It also has a higher heat retention that CO2 thus has a damaging GWP. In addition some HCFC’s are explosive or corrosive with unknown toxicity thus not suitable for many applications, such as refrigerants. Therefore HCFC’s are not an effective replacement for CFC’s.

 

HFC’s: HFC’s do not contain Chlorine thus have a ODP of zero and won’t lead to ozone depletion. However it has a GWP of up to 1000 times that of CO2 thus contributes significantly to global warming. HFC’s are non-flammable but has different cooling properties to CFC’s and requires equipment modification. Therefore HCF’s are a moderately effective CFC replacement, however its GWP is a detrimental disadvantage which must be addressed for it to be wholey effective. 

 

References:

Figure 1)

NASA, (2007), Ozone Data [ONLINE]. Available at:http://www.nasa.gov/images/content/156217main_ozone-20060830-graph-browse.jpg [Accessed 05 March 15].

 

The Montreal Protocol       

 

The Montreal Protocol is an international agreement to phase out the use of ozone destroying chemicals, focusing on certain haloalkanes (such as CFC’s and HCFC’s). This treaty was signed by 46 countries before its launch in 1989 and by 2004 143 nations had joined the treaty.

 

Targets:

The treaty follows a timeline of targeted chemical phase outs as follows:

• Halons phased out by the end of 1993 (2010 in Developing countries)

• CFC’s phased out by the end of 1995 (2010 in Developing countries)

• HCFC’s

• 60% reduction by 2010

• 90% reduction by 2015 (developing countries freeze at 2015 base levels in 2016)

• Total phase out by 2020

• Developing Countries total phase out by 2040

 

The Montreal Protocol has been an effective agreement mostly due to the availability of viable replacement compounds such as HCFC’s and HFC’s. Due to this most of the involved countries have been reaching (or at least attempting to reach) their targeted reduction targets. This has led to CFC concentrations decline or have levelling off. As HCFC’s also contribute to ozone-depletion they have also been marked as a targeted decrease avoiding additional issues.

However due to the Global warming profile of the replacements there is increasing danger of human activity changing the climate thus there is need to link both ozone and climate protection.

 

CFC Replacements:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Due to their inertness and insolubility CFC’s it will take time for all of it to leak into the stratosphere and decompose thus will remain in the atmosphere for some time.

 

Since the introduction of the Montreal Protocol NASA has noted a gradual decrease in the concentration of Cl∙ in the stratosphere and Ozone concentrations have begun to increase. In addition the ozone thinning over the Antarctica has ceased and remained at the same thickness for the last three years.

This is due to CFC concentrations levelling off or decreasing, Halon concentrations (still used in fire extinguishers) rate increase slowing.