LNG LIQUEFACTION PROCESS SELECTION: ALTERNATIVE REFRIGERANTS TO REDUCE FOOTPRINT AND COST Russell H. Oelfke Robert D. Denton Michael R. Miller ExxonMobil Upstream Research Company 1 ABSTRACT This paper describes recent work at ExxonMobil Upstream Research Company to identify novel refrigerants and processes for production of liquefied natural gas (LNG) in large-scale floating and onshore LNG facilities. The goal of the research was to reduce the footprint and cost of baseload LNG plants through the use of non-flammable or reduced flammability refrigerants. Process results are presented and discussed for two novel liquefaction cycles: a mixed refrigerant process and a cascade process. The performance aspects of each process are discussed, and tradeoffs in process complexity and equipment flexibility are considered. 2 BACKGROUND 2.1 Incentives The design of an efficient refrigeration system is of paramount importance for effective and economic LNG production. Several commercially available refrigeration cycles are currently in use at baseload LNG plants. Most of these cycles share a common feature: the use of flammable hydrocarbon refrigerants, such as methane, ethane or ethylene, propane, isobutane, isopentane, and their mixtures (often with nitrogen as well). Facility space is significantly more costly for floating LNG (FLNG) plants than for onshore plants. The use of flammable refrigerants and the requirement for appropriate spacing and other measures to prevent adverse consequences in the event of refrigerant release increases the capital costs of FLNG projects. Requirements for safe refrigerant makeup, handling, storage, and import on to floating vessels also increase FLNG costs relative to onshore plants. Replacing hydrocarbon refrigerants with non-flammable, or reduced flammability, refrigerants offers the potential for reducing the capital costs of FLNG projects. There are also potential cost reductions for onshore LNG plants, as requirements for “buffer zone” land surrounding and within plant sites may be reduced. These economic benefits can only be realized if the liquefaction cycles using the alternate refrigerants have comparable thermal efficiency and equipment requirements relative to the conventional liquefaction processes. 2.2 Alternative Refrigerants In considering alternative refrigerants for baseload LNG projects, we evaluated several classes of refrigerants, including hydrofluorocarbons (HFCs), noble gases, and fluorocarbon refrigerants with reduced greenhouse warming potential. A list of selected conventional and alternative refrigerant candidates appears in Table 1. The table uses the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) “R-code” numbering system, which numerically identifies refrigerants based on their chemical structure.1 The R-codes will be used to refer to specific refrigerants in the balance of this paper. Table 1 also lists the chemical name, chemical formula, molecular weight, normal boiling point, melting point, critical temperature, safety group, and greenhouse warming potential for each of the refrigerants. 1 Table 1. Selected conventional and alternative refrigerants for use in LNG refrigeration cycles ASHRAE NBP MP Crit. T Safety Number Chemical Name Formula MW (°F) (°F) (°F) Group GWP100 R-14 Tetrafluoromethane CF4 88 −198 −298 −50 A1 7,390 R-23 Trifluoromethane CHF3 70 −116 −247 79 A1 14,800 R-41 Fluoromethane CH3F 34 −109 −223 112 A3 92 R-116 Hexafluoroethane C2F6 138 −109 −149 68 A1 12,200 R-32 Difluoromethane CH2F2 52 −62 −213 173 A2L 675 R-410A R-32 / 125 (50 / 50 wt%) — 74 −61 — 160 A1 2088 R-125 1,1,1,2,2-pentafluoroethane C2HF5 120 −56 −153 151 A1 3,500 R-143a 1,1,1-trifluoroethane CH3CF3 84 −54 −168 163 A2L 4,470 R-218 Octafluoropropane C3F8 188 −35 −234 161 A1 8,830 R-1234yf 2,3,3,3-tetrafluoropropene C3H2F4 114 −21 −242 202 A2L 4 R-134a 1,1,1,2-tetrafluoroethane CH2FCF3 102 −15 −154 214 A1 1,430 R-152a 1,1-difluoroethane CH2CHF2 66 −13 −179 236 A2 124 R-1234ze 1,3,3,3-tetrafluoropropene C3H2F4 114 −2 — — A2L 6 R-227ea 1,1,1,2,3,3,3-heptafluoropropane CF3CFHCF3 170 3 −204 215 A1 3,220 R-C318 Octafluorocyclobutane (-CF2-)4 200 21 −40 239 A1 10,300 R-236fa 1,1,1,3,3,3-hexafluoropropane CF3CH2CF3 152 29 −140 257 A1 9,810 R-245fa 1,1,1,3,3-pentafluoropropane CF3CH2CHF2 134 59 −152 309 B1 1,030 R-245ca 1,1,2,2,3-pentafluoropropane CHF2CF2CH2F 134 77 −116 346 B1 693 R-347mcc 1-methoxyheptafluoropropane C3F7OCH3 200 93 −189 328 — 450 R-728 Nitrogen N2 28 −320 −346 −232 A1 0 R-740 Argon Ar 40 −303 −309 −188 A1 0 R-784 Krypton Kr 84 −244 −251 −83 A1 0 — Xenon Xe 131 −163 −169 62 A1 0 R-744 Carbon dioxide CO2 44 −70(s) −109 88 A1 1 R-50 Methane CH4 16 −259 −296 −117 A3 25 R-1150 Ethylene C2H4 28 −155 −273 48 A3 < 25 R-170 Ethane C2H6 30 −127 −297 90 A3 < 25 R-290 Propane C3H8 44 −44 −306 206 A3 < 25 R-600a Isobutane CH3CHCH3 58 11 −255 274 A3 < 25 (CH ) CHCH CH R-601a Isopentane 3 2 2 3 72 82 −256 369 A3 < 25 Notes : 1. The Ozone Depletion Potentials (ODPs) for all components in this Table are 0. All comply with the Montréal Protocol. 2. The GWP100 is the relative 100 year Greenhouse Warming Potential, with CO2 as “1”. 3. The Safety Group is an ASHRAE designation, “A” meaning Occupational Exposure Limit (OEL) above 400 ppm allowed, “B” meaning the OEL is below 400 ppm. A number of “1” indicates non- flammable, “2” means slightly flammable, and “3” means highly flammable. An “L” suffix indicates very low flame propagation speed. Safety and environmental considerations helped define the list of candidate refrigerants. The primary safety criteria were flammability and toxicity. ASHRAE maintains a safety classification scheme that designates both the flammability and toxicity of refrigerants.1 Initial alternative refrigerant candidates were mostly within toxicity class ‘A’ (essentially non-toxic) and completely nonflammable (class ‘1’). Environmental regulations also influenced the selection of refrigerants. Laws governing the Ozone Depletion Potential (ODP) of refrigerants were enacted by many governments following the Montréal Protocol.2 It was discovered in the 1970s that chlorinated and brominated halocarbons deplete the ozone layer via free radical chain reactions.3 Nevertheless, halocarbons that contain fluorine as the only halogen were found not to react 2 with ozone. Therefore, compounds containing chlorine and bromine were excluded from the set of candidate refrigerants. 2.2.1 Hydrofluorocarbons (HFCs) Hydrofluorocarbons (HFCs) are a class of refrigerants that first came into widespread use in the 1990s for small-scale applications (relative to baseload LNG) in response to regulations phasing out the previous generation of chlorofluorocarbon (CFC) refrigerants.2 Many HFCs are non-flammable, non-toxic, and non- reactive. Moreover, the HFC family has a wide range of normal boiling points (NBPs) that are similar to the NBPs of the hydrocarbon refrigerants used in most natural gas liquefaction processes. The Kyoto Protocol specifically identifies HFCs and perfluorocarbons (PFCs) as greenhouse gases.4 One measure of the greenhouse warming effect of a compound is its global warming potential over 100 years, or GWP100. The GWP100 is defined as the equivalent mass of CO2 needed to have the same greenhouse effect as releasing a unit mass of refrigerant over a 100-year period.4 Most HFCs have relatively high global warming potentials. The chemical stability of many HFCs is the main cause for their greenhouse potential. Some countries, in conjunction with carbon taxation schemes, have begun to tax refrigerants on the basis of 5 GWP100. 2.2.2 Noble Gases Most of the noble gases—in particular argon, krypton, and xenon—are candidates for alternative LNG refrigerants. These noble gases are non-toxic, non-flammable, non-ozone-depleting and have no greenhouse potential. Xenon and krypton are of particular interest, as their NBPs are in the appropriate range to cover the colder portion of the LNG cooling curve. 2.2.3 Refrigerants with Lower Greenhouse Impact Adding hydrogen atoms to HFC molecules increases their rates of atmospheric degradation, but also tends to increase their flammability and reactivity. Of the refrigerants identified in Table 1, the heavily fluorinated methane derivatives R-14 and R-23 have high GWP100 values (7,390 and 14,800, respectively) and no flammability. R-32, which has two hydrogen atoms, has a significantly lower GWP100 of 675, but is classified as 2L (weakly flammable). R-41, with only one fluorine and three hydrogen atoms, has an even lower GWP100 of 97, but greater flammability (class 3 — flammable). HFCs can include unsaturated bonds, in which case they are designated hydrofluoroolefins (HFOs).6 HFOs tend to be more reactive and flammable due to the presence of unsaturated bonds, but they also degrade more rapidly in the environment. HFOs were developed in anticipation of EU regulations limiting the GWP100 6 of automotive air-conditioning refrigerants to values less than 150. For example, R-1234yf (GWP100 = 4) was designed as a replacement for R-134a (GWP100 = 1,430). Despite its ASHRAE classification as slightly flammable (2L), R-1234yf has been approved by the Society of Automotive Engineers for use in vehicle air conditioners.6 Oxygen atoms can also hasten degradation of HFCs. Hydrofluoroethers (HFEs) and fluorinated ketones 7 have been developed as low-GWP100 cleaning solvents, refrigerants, and fire suppressants. In particular, R- 347mcc (C3F7OCH3) may be a promising warm end component for an LNG mixed refrigerant due to its desirable combination of properties, including complete non-flammability, low MP and GWP100, and high NBP.7 3 METHODS 3.1 Refrigerant Selection When selecting a refrigerant for a particular service, the NBP and the melting point (MP) are important. It is desirable for the refrigerant vapor pressure to be greater than atmospheric pressure throughout the entire 3 cycle to avoid vacuum conditions in the chillers.