Comparing Atmospheric Stability Versus HVACR Equipment Chemical Stability of New Low GWP Olefin Based Refrigerants.
The HVACR industry has witnessed many changes in refrigerant chemistries with the advent of chlorofluorocarbons (CFCs) and subsequent transitions to the use of both hydrochlorofluorocarbons (HCFCs) and HFCs in equipment. The chemical nature of these compounds have also widely evolved, moving from the less stable, ozone depleting CFCs, to highly stable yet GWP HFCs and HCFCs. The introduction of HFOs, hydrochlorofluoroolefins (HCFOs) and hydrochloroolefins (HCOs) provides another era in refrigerant chemistry, and as with prior transitions uncertainties exist due to lack of understanding. This unfamiliarity has driven some confusion in understanding how a chemical can be designed to dissipate in the atmosphere in days yet be expected to provide 20 plus years of stability and reliability in HVACR equipment.
Chemical reactions in the atmosphere are all around us and complex but are not apparent to the population since humans have evolved in the presence of these atmospheric reactions. One well known example of atmospheric chemical reactions is the formation of smog. Smog is an unnatural, visible indication of the chemical reactivity in the atmosphere. The interaction of anthropogenic pollutants such as nitrogen oxides (produced from fossil fuel combustion) and hydrocarbons with sunlight leads to the production of photochemical driven smog. This smog is composed of small visible airborne particles and ground level ozone which are noxious to humans, and therefore the prevention of such pollution has been a priority of government air quality regulations.
Refrigerant fugitive emissions through production of the chemicals, manufacturing of HVACR equipment, leaks from equipment in the field and at decommissioning are a few ways where refrigerants can enter the atmosphere. The atmospheric life of these refrigerants along with a whole host of other chemicals have been studied, continue to be studied and have been published in the many United Nations climate reports. In addition to atmospheric life, the global warming potential (GWP) of these materials is determined based on their relative measure of how much heat a chemical in gas form traps in the atmosphere. The GWP of a certain gaseous chemicals is a comparison of the amount of heat trapped by a similar mass of carbon dioxide with carbon dioxide being equal to 1. The GWP of a gas depends on three factors: atmospheric lifetime of the species, the absorption of the infrared radiation by a given species and the spectral location of its absorbing wavelengths. A high GWP correlates with a large infrared absorption and a long atmospheric lifetime. A gas has the most effect if it absorbs a large band of infrared wavelengths where the atmosphere is fairly transparent. In addition another complicating factors is as a chemical breaks down in the atmosphere, additional products can form that have higher infrared absorption or radiative force than the parent material. The GWP of the parent material is an accounting of all the GWPs of the potential breakdown species so studying the detailed reactions has been active endeavor. Atmospheric lifetime is defined when one half of the chemical remains in the atmosphere if no additional chemical would be added to the atmosphere. GWP for a given time period is the integration of the amount of radiative force times the amount of time in the atmosphere.
A GWP for gaseous chemical is calculated over a specific time period, typically 100 or 20 years, is expressed as a factor of carbon dioxide. Carbon dioxide is defined to have a GWP of 1 over all lifetimes. Other time periods are reported, like 20 years, which additionally adds to the confusion. Thus a gas with a high infrared radiation absorption force but also a short lifetime will have a large GWP on a 20 year scale but a much smaller GWP on a 100 year scale. For example, methane has a reported atmospheric lifetime of 12.4 years with GWP values reported of GWP20 of 84 and GW[P.sub.100] of 28 (IPCC, 2013). Another example of a very short lived refrigerant, like R1234ze (E), has a reported atmospheric lifetime of 16.4 days and GW[P.sub.20] of 4 and GW[P.sub.100] of less than 1. Even though the atmospheric lifetimes are drastically different, years versus nearly single days, the radiative efficiency are drastically different with methane being 0.000363 watts/[meter.sup.2] (0.00012 btu-hr/[ft.sup.2]) per part billion of chemical concentration compared to R1234ze(E) equal to 0.04 watts/[meter.sup.2] (0.013 btu-hr/[ft.sup.2]) per part billion. The difference in radiative forcing is a factor of approximately 110 times higher for R1234ze(E) than methane, but the atmospheric life of methane is approximately 54 times higher than R1234ze(E). Methane is a global warming gas of concern because of the high release rates, multiple sources (naturally occurring, man-made and from animals) and long atmospheric life.
Figure 1 is a summary comparing the atmospheric life of groups of halogenated refrigerants to their GWP as published in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change commonly referred to as AR5. Adding to the confusion in the industry, climate change reports are published every 3 years and GWP of select compounds are updated based on the latest measurements with many organization citing GWP values from earlier climate change reports. As you can see in Figure 1, the GWPs of various halogenated chemicals have a well behaved relationship of atmospheric life to reported GWP values. This is because of the radiative force of various halogenated chemical are relatively similar. Other compounds, like hydrocarbons, have a different relationship since the radiative forcing is much less than that of halogenated chemicals. In the end, both atmospheric life, concentration in the atmosphere (leaks and emissions) and radiative force need to be considered when evaluating new refrigerants chemistry for global warming impacts.
ATMOSPHERIC CHEMISTRY REACTIONS OF OLEFINS REFRIGERANTS
Atmospheric chemistry describes chemical processes that occur in the atmosphere (Manahan, 2013). Atmospheric chemistry is a broad and complex science with chemical reactions that occur in the local air environment to tens of kilometers above the earth surface. The atmosphere we live and breathe in is a sea of gas compounds, composed mostly of nitrogen, oxygen and water vapor, with complex chemical reactions continuously occurring, aided by solar radiation unseen to the human eye. The atmosphere is resource for all life on the planet with molecular [O.sub.2] for most organisms, C[O.sub.2] as a photosynthesis source for plants and other organisms used to synthesize biomass, and N2 which serves as a source of nitrogen for essential components of protein and other biochemical reactions. The atmosphere is a dynamic mixture of gasses, and with the presence of natural and anthropogenic gas emissions a great variety of chemical reactions can occur within our atmosphere. These reactions, as they pertain to olefins and haloolefins, will be discussed in this section.
The chemical feature of all haloolefins is the presence of a carbon-carbon double bond. This carbon-carbon double bond, in most cases, is more reactive than saturated carbon-carbon, carbon-hydrogen, and carbon-halogen (fluorine, chlorine, bromine, iodine) bonds. This increase in reactivity of the carbon-carbon double bond drives the atmospheric lifetimes of haloolefins to be significantly shorter than CFC, HCFC, and HFC molecules. Understanding the atmospheric reactions with these various carbon bonds determines the appropriate bonding features to include in a molecule to optimize it for atmospheric life. Analogously, understanding the appropriate bonding features to include in a molecule can be used to optimize a chemical for its use as a refrigerant in HVACR equipment. This balance of designing a chemical for its environmental impacts and use as a refrigerant in HVACR has been one source for the innovation occurring in the HVACR industry for the past 30 years.
Photochemical reactions are chemical reactions in which energetic photons from ultraviolet solar radiation break chemical bonds in the atmosphere oxygen molecules (O2) to produce O atoms as well as other ions. These photochemical reactions can also include reactions with water in the atmosphere to form hydroxyl radicals (OH*). The importance of OH radicals in atmospheric reactions was discovered in the 1970s and now is recognized as a crucial intermediate in local air quality and smog formation. The olefin refrigerants are designed to take advantage of the OH radical chemistry in the local air environment and react quickly to form new chemical species that can be removed by the natural water cycle. Halocarbons are removed by the atmosphere by two general mechanisms; reactions with OH radicals and photolysis (Wallington, et al., 1994). The reaction of OH radicals is very rapid and as result dominates the atmospheric removal mechanism for haloolefins. As an illustration, Wallington gives the example of the reactivity of a saturated halocarbon HFC-245cb (C[F.sub.3]C[F.sub.2]C[H.sub.3]) and the haloolefin HFO-1234yf (C[F.sub.3]CF=C[H.sub.2]). With respect to reactions with OH radicals, HFO-1234yf is approximately 2000 times more reactive than HFC-245cb. The author goes on to state that unsaturated haloolefins react with other gas phase oxidants present in the atmosphere such as O3, Cl atoms and NO3 radicals but these reactions are of limited significance compared to that with OH radicals.
In a later paper, Wallington also provides the example for the proposed atmospheric chemical mechanisms and pathways of decomposition of R1234yf (Wallington, et al., 2014). Halogenated carbonyl compounds are removed from the atmosphere via wet and dry deposition on a time scale of days to weeks. Hydrolysis of halogenated carbonyl compounds gives acid and C[O.sub.2] products. Another important reactions with halogenated compounds is the potential formation of trifluoroacetic acid (CF3C(O)OH) or commonly referred to as TFA which is a target pollutant for some halocarbons. TFA is a naturally occurring material contained in low concentrations in saltwater and is part of the natural environmental chemistry, especially along the sea coasts. The extra TFA formed by the breakdown of halocarbons is of concern and has been heavily studied, debated and found to be not a concern. This paper will not discuss the details and debate around TFA other to note that some haloolefins form TFA as they breakdown and others do not form TFA. Table 1 is a summary for various HFOs and HCFOs (Wallington, et al., 2014). Note that R1234ze(E) and both isomers of R1233zd do not form TFA, but rather acids of hydrofluoric acid and formic acid for R1234ze(E) and hydrofluoric and hydrochloric acids for the isomers of R1233zd. Formic acid is a naturally occurring acid used by fire ants and hydrofluoric and hydrochloric acid will be neutralized by soils and other features and will be mineralized in the water and soil sinks.
The atmospheric chemical reactions of haloolefins cannot occur within HVACR equipment because of the lack of sufficient oxygen, sources of solar radiation and significant sources of atmospheric oxidation products. While residual air and moisture can be present in HVACR systems, they are not present in sufficient concentrations to be a significant source of refrigerant breakdown. Furthermore, residual moisture and refrigerant breakdown products can be removed by the existing filter drier systems. Low pressure centrifugal chillers, using R11, R123, R514A, R1233zd(E), operate under lower than atmospheric situations and air can be drawn into these systems. These systems employ automatic purge systems and removal of the air and moisture drawn into these systems occurs within minutes. In addition, the amount of air drawn into these systems is very small, on the order of 10's of milliliters (multiples 0.6 cubic inches) of air volume at 1 atmosphere pressure (14.7 psia), which does not contain sufficient reactive oxygen species to be an issue.
HVAC&R HERMETIC SYSTEM CHEMISTRY REACTIONS OF OLEFIN REFRIGERANTS
Unlike atmospheric chemistry interactions with refrigerants, HVACR refrigerant system chemistry is a less dynamic process due to limited interactions with air and moisture and no presence of solar radiation. HVACR systems are dark, air and moisture free with temperature being the primary accelerant. For these systems the interaction of the refrigerant with lubricant and system materials becomes the primary concern. Good refrigerant stability in HVACR systems is defined as the ability to perform over a wide range of temperatures, in the presence of materials (necessary for the system operation), with limited deterioration. New refrigerants must have an inherent level of thermal stability before being considered for use in HVACR systems. Once the general thermal stability of the refrigerant alone is established, it is then subjected to a wide range of materials and potential systems containments, like residual air, water and processing chemicals. The refrigerant must not be adversely affected by these materials. Various known and possible refrigerant reaction pathways will be discussed in the following sections.
Historically Understood Reduction and Disproportion Reactions of Refrigerants
Two types of reactions appear to be common with CFCs, HCFCs and to a much lesser amount with HFCs in the presence of lubricant. These chemical reactions are the reduction of the refrigerant by lubricant species and disproportionation of refrigerant. An example of these reactions are shown in Equation 1 and 2 below for R22. The primary refrigerant breakdown mechanism for CFC and HCFC are reduction reactions which involve a direct interchange of chlorine and hydrogen atoms between the refrigerant and the oil (Spauschus and Doderer, 1961). Measurement of the breakdown products formed in the reaction of the refrigerant with the lubricant has become the industry standard method of judging whether a material in a system is acceptable for use. The reduction reaction does not always chlorinate the lubricant and hydrochloric acid can be produced as well.
Reduction: R22 + lubricant +catalyst --> R32 + other species (1)
Disproportionation: R22 +catalyst --> R23 + chloride + other species (2)
The reduction reaction involves the exchange of a hydrogen atom from a lubricant molecule for the chlorine atom in the R22. Each molecule of R32 represents the decomposition of one molecule of R22. When the chlorine is released, it can then form aggressive chlorine based species, such as hydrochloric acid, and attack metals and other construction materials. Typically, a lubricant sample can be analyzed for total acid number to determine how much refrigerant breakdown has occurred in systems as well as analyzing the refrigerant by gas chromatography for the formation of reduction products. The reduction product of R12 is R22; the reduction product of R11 is R21. The amount of reduction products that form is dependent on lubricant concentration, catalyst used, and temperature. Table 1 summarizes the primary breakdown products of various CFCs and HCFCs in the presence of lubricant.
Hermetic System Reactions with Olefin Refrigerants
Olefin refrigerants introduce a complicated chemistry situation with the addition of the double bond and with the molecules, in general, being more complex with the addition of the 3rd or 4th carbon molecules over the refrigerant they targeted to replace. For example, R11 (1 carbon molecule) was replaced by R123 (2 carbon molecule) and R123 is being replaced by a combination of molecules and mixtures, including R1233zd(E) (3 carbon molecule) and R514A which is a blend of R1336mzz(Z) (4 carbon molecule) and R1130(E) (2 carbon molecule). R12 (1 carbon molecule) was replaced by R134a (2 carbon molecule) and R134a is being replaced by combination of R1234ze(E) (3 carbon molecule) and R1234yf (3 carbon molecule) or blends of R1234ze(E) and R1234yf with other HFCs.
Reduction Reactions with Olefin Refrigerants
Unlike the saturated halogenated refrigerants, olefin based refrigerants have been studied and few refrigerant reduction reactions with the lubricant have been observed in sealed glass tube studies. (Majurin et al., 2014, Rohatgi et al., 2012, Fuji taka, 2010). The reduction products of the various olefins would require the elimination of fluorine or chlorine from the molecule and substitution by hydrogen atom across the double bond. Table 3 gives summary of potential reduction products if they would occur with various olefin refrigerants. R1336mzz(Z) does not possess a fluorine on the double bond so a reduction reaction and elimination of a halogen will not occur in this case.
Stereoisomer Rearrangement Reactions with Olefin Refrigerants
Unlike the saturated halogen refrigerants which are free to rotate into different orientations in three-dimensional space, olefin refrigerants contain a carbon double bond that does not allow the molecule to rotate freely. This double bond can lead to the formation of different isomers (stereoisomers) with different properties yet the same chemical formulation. Refrigerant nomenclature specifies when there are different stereoisomers with the designation of (E) and (Z) in the formula. In general, if substituents are on the same side of the double bond, a "Z" prefix is added to the name and if on the opposite sides an "E" prefix is added to the name. The prefixes are German names of zusammen, meaning together and entgegen meaning opposite. Figure 2 gives an example of the two stereoisomers for R1234ze. The "Z" isomer has the hydrogen atoms on the same side of the double bond while the "E" isomer has the hydrogen atoms on the opposite side. Interesting, this small change in stereochemistry results in a rather large boiling point difference of 51.7[degrees]F (28.7[degrees]C) with R1234ze(E) having a boiling point of -2.2[degrees]F (-19[degrees]C) and R1234ze(Z) having a boiling point of 49.5[degrees]F (9.7[degrees]C).
Stereoisomer rearrangement processes has been well studied and documented, but limited information is published with the new olefin refrigerants which have the potential for stereoisomer rearrangement. R1233zd(E), R514A and R1234ze(E) are used in chillers today with no reported stereoisomer rearrangement issues. R514A is a blend of R1336mzz(Z) and R1130(E) and both these molecules could stereo-rearrange to form R1336mzz(E) and R1130(Z) respectively. R1233zd(E) and R1234ze(E) could rearrange to form R1233zd(Z) and R1234ze(Z), respectively. The authors have completed many internal and published studies looking for the potential for stereoisomer rearrangement with all these refrigerants. Small amounts of stereoisomer rearrangement has been detected at accelerated temperatures for some of the above materials. Figure 3 summarizes the stereoisomer rearrangement data for R1233zd(E), R1234ze(E) and R514A and compares these values to published reduction reaction studies for R11, R12, R123, R22, R32, R134a and R125. As you can observe, stereoisomer rearrangement reaction rate is very low and not a significant concern with reactions rates being on the order of R22 reduction reaction stability and well below R11, R12 and R123 reduction reactions. All these refrigerants were acceptable for use and gave good reliability in HVACR products.
Oligomerization or Polymerization Reactions with Olefin Refrigerants
The presence of a double bond can lead to the potential for oligomerization (joining of two or a few monomer refrigerant units for example dimers, trimers or tetramers of the based refrigerant unit) or polymerization (large numbers of monomers joined together) of the olefin refrigerant. No reported occurrences have been documented in literature under typical HVACR system chemistry situations.
Nucleophilic Substitution Reactions with Olefin Refrigerants
In organic chemistry, nucleophilic substitution is a fundamental class of reactions in which an electron rich nucleophile selectively bonds or attacks the positive or partially positive charge of an atom or group of atoms to replace a leaving group. Nucleophilic substitution across a halogenated double bond is possible but hydrogen, fluorine, chlorine atoms and trifluorocarbonyl group attached to the double bond are not good leaving groups. In addition, the formation of hydrofluoric or hydrochloric acid as the result of limited reduction reactions with olefins is possible, but unlikely to occur because these acids are quickly neutralized by system materials or the filter drier component before they can reach significant concentrations to initiate the reaction.
It is confusing to many that a refrigerant with high atmospheric reactivity can have low reactivity in HVACR equipment. It is not apparent to many that the atmosphere we live in and breathe in is a sea of gas compounds, mostly composed of nitrogen, oxygen and water vapor, with complex chemical reactions aided by various types of solar radiation typically unseen by ourselves since humans have evolved in the presence of these reactions. One atmospheric reaction that is visual and not naturally occurring but gives an indication of the chemical reactivity of the atmosphere is the formation of smog. The importance of OH radicals in atmospheric reactions was discovered in the 1970s is now recognized as a crucial intermediate in local air quality and chemical reactivity. The olefin refrigerants are designed to take advantage of the OH radical chemistry in the local air environment and react quickly to form new chemical species that can be removed by the natural water cycle. The reaction of OH radicals is very rapid and as result dominates the atmospheric removal mechanism for haloolefins that results in their removal in days not in years like the CFCs, HCFCs and HFCs.
Unlike atmospheric chemistry interactions with refrigerants, HVACR refrigerant system chemistry is less dynamic chemistry situation with essentially no interactions with air, moisture and solar radiation present. HVACR systems are dark, air and moisture free with temperature being the primary accelerant with interaction of the refrigerant with lubricant and system materials being the primary concerns. These new olefin refrigerants have been evaluated in laboratory testing under highly accelerated temperature conditions and they have shown a variety of chemical reactions. These reactions have been minor and result in acceptable stability for these olefin based refrigerants in HVACR products.
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Table 1: Summary of Various Olefin Refrigerants and Their Final Products Refrigerant Breakdown Product R1234yf TFA, CO2, HF R1234ze(E) C[O.sub.2], Formic Acid, HF R1216 TFA, C[O.sub.2], HF R1233zd(E) C[O.sub.2], HF, HCl R1233zd(Z) C[O.sub.2], HF, HCl Table 2: Summary of Various Refrigerant and Their Primary Breakdown Product Refrigerant Breakdown Product R11 (CF[Cl.sub.3]) R21(CHF[Cl.sub.2]) R12 (C[F.sub.2][Cl.sub.2]) R22 (CH[F.sub.2]Cl) R22 (CH[F.sub.2]Cl) R32 (C[H.sub.2][F.sub.2]) R123 (C[F.sub.3]CH[Cl.sub.2]) R133a (C[F.sub.3]C[H.sub.2]Cl) Table 3: Summary of Various Olefin Refrigerants and Potential Reduction Products Refrigerant Breakdown Product R1233zd(E) R1243zf R1336mzz(Z) Not Applicable R1234yf R1243zf R1234ze(E) R1243zf R-1130(E) R-1140
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|Title Annotation:||global warming potential|
|Author:||Kujak, Steve; Sorenson, Elyse|
|Publication:||ASHRAE Conference Papers|
|Date:||Jan 1, 2018|
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