49th International Conference on Environmental Systems ICES-2019-394 7-11 July 2019, Boston, Massachusetts Development and Characterization of a LiOH Air Regeneration Model in Thermal Desktop® Cheyn L. Worn1 Aerodyne Industries, Houston, TX, 77058 John F. Keener.2 Jacobs Engineering, Houston, TX, 77058 Orion post landing activities use a lithium hydroxide (LiOH)-based scrubber for carbon dioxide removal and so a robust Thermal Desktop® model for these activities requires a LiOH reaction computational algorithm. The chemical sequesters CO2 from air through a series of exothermic and endothermic reactions that consume and produce water vapor. NASA and its contractors developed a chemical kinetics based LiOH/CO2/H2O reaction modeling subroutine in the 1970’s and 1980’s. This work details adapting that LiOH code for a Thermal Desktop® model in preparation for Orion post-landing simulations. Nomenclature Units s = second min = minute hr = hour °C = degrees Celsius K = Kelvin J = Joule mol = mole cm = centimeter m = meter m3 = meters cubed g = gram kg = kilogram cm/s = centimeters per second 3 m /hr = cubic meters per hour kg/hr = kilograms per hour Pa = Pascal °F = degrees Fahrenheit °R = degrees Rankine BTU = British Thermal Unit lbmol = pound-mole in = inch ft = feet lbm = pound mass cfm = cubic feet per minute pph = pound mass per hour psi = pounds per square inch 1 Thermal Analysis Engineer, 2224 Bay Area Blvd./JE-5EA. 2 Thermal and Environmental Engineer, 2224 Bay Area Blvd./JE-5EA. *Trade names and trademarks are used in this report for identification only. Their usage does not constitute an official endorsement, either expressed or implied, by the National Aeronautics and Space Administration. Symbols G = correlation factor H = volumetric molar concentration h = forced convective heat transfer coefficient m = mass ṁ = mass flow rate M = species molecular weight Q = heating rate R = reaction rate RGas = gas constant T = temperature u = natural convective heat transfer coefficient V̇ = volumetric flow rate V0 = gas velocity X = molar ratio Xk = rate constant ∆t = time step η = reaction efficiency ρ = density Acronyms and Chemical Species NASA = National Aeronatics and Space Administration JSC = Lyndon B. Johnson Space Center ECLSS = Environment Control and Life Support System CAMRAS = Carbon dioxide And Moisture Removal Amino Swing bed LiOH = lithium hydroxide chemical species and technology LiOH.H2O(s) = solid, lithium hydroxide monohydrate chemical species Li2CO3 = lithium carbonate chemical species CO2 = carbon dioxide gas H2O(g) = water vapor Subscripts 1 = property pertaining to the reaction of lithium hydroxide and water vapor 2 = property pertaining to the reaction of lithium hydroxide hydrate and carbon dioxide 3 = property pertaining to the reaction of lithium hydroxide and carbon dioxide Bed = cartridge bed Cab = cabin C = Li2CO3 species related property CO2 = CO2 gas related property E = Poly-ethylene plastic related property f = property at end of time step G = gas H = LiOH.H2O(s) species related property H2O = water vapor related property i = property at initiation of time step in = property at cartridge inlet L = LiOH species related property pg = purge 2 International Conference on Environmental Systems I. Introduction xploration Mission 2 (EM-2) will be the first manned iteration of the National Aeronautics and Space E Administration’s (NASA) planned Orion missions. The Space Launch System (SLS) will launch the Orion and its crew from the Earth. They will circle the Moon and then return to Earth, splashing down in the Pacific Ocean. Once the Orion has splashed down, the crew must remain inside the cabin breathing from ship’s environmental control and life support system (ECLSS) for at least two hours. In flight, the Orion normally uses amine-based swing-bed (CAMRAS) units to scrub carbon dioxide (CO2) and water vapor (H2O(g)), but this technology requires a vacuum to purge those scrubbed gasses. This facet of their design limits them to operation in vacuum/space only and so when the Orion splashes down in Earth’s ocean, the CAMRAS units will not be able to operate. NASA plans to use lithium hydroxide based scrubbers (LiOH) for this final segment of the mission. LiOH technology has a long history of usage at NASA, being the primary carbon dioxide scrubber in the Apollo and Shuttle spacecraft. Lithium hydroxide chemically consumes carbon dioxide and water vapor in a series of reactions that ultimately produce lithium carbonate (Li2CO3) and excess water vapor. These reactions unfortunately also release a substantial amount of energy that transfers to the regenerated air and the cabin that surrounds the cartridge; this and the chemical consumption provide an interesting challenge for thermal ECLSS simulations. NASA’s Johnson Space Center (JSC) and its contractors developed a LiOH simulation code for the Shuttle starting in the 1970’s and concluding in the 1980’s1-3. They developed the model from reaction chemical kinetics and refined its output to match data with a reaction efficiency term. This paper details adapting those old codes to simulate LiOH-based cabin air regeneration with Cullimore & Ring Technologies Thermal Desktop/FloCAD®4. The LiOH code integrates as a FORTRAN subroutine that calculates reaction, mass change, and heating rates. A simple Thermal Desktop/FloCAD® model with constant, cabin-like inlet conditions characterizes simulated reaction results and byproducts and then examines the effects of altering air inlet temperature and composition on bed performance. This work will ultimately serve detailed post- landing Thermal Desktop models for the Orion spacecraft. II. Theory There are two primary parts to this model – the LiOH subroutine and Thermal Desktop/FloCAD® implementation. The following sections will first delve into the reaction calculations behind the LiOH subroutine, introduce the Thermal Desktop/FloCAD® model, and finally discuss cartridge properties. A. The LiOH Subroutine The LiOH subroutine, given inlet conditions, computes reaction rates, the heat of reaction, and the amount of carbon dioxide sequestered/water vapor produced. The following sections discuss the theory behind the calculations performed and chemical processes simulated in the subroutine. The subroutine’s calculations have two foundations. Reaction chemical kinetics form the model’s primary foundation1. The Davis efficiency model2,4 forms the second. Reaction Calculations Lithium hydroxide sequesters carbon dioxide from the inlet airflow through a series of chemical reactions, three of which are used in this model: LiOH H2O(g) LiOH.H2O(s) (1) 2LiOH.H2O(s) CO2 Li2CO3 3H2O(g) (2) 2LiOH CO2 Li2CO3 H2O(g) (3) In Reaction (1), lithium hydroxide forms a hydrate with water vapor from the inlet gas stream. This reaction is reversible, however, depending on the incoming air stream’s humidity; if it is greater than a moisture equilibrium within the bed, the hydrate will disintegrate back into water vapor and lithium hydroxide. Reaction (1) is exothermic when it forms the hydrate and endothermic in the opposite way. In the second two reactions, dry or hydrated lithium hydroxide reacts with air stream carbon dioxide to produce lithium carbonate and water vapor. Reaction (2) is always endothermic and Reaction (3) is always exothermic. 3 International Conference on Environmental Systems The model depends only on binary collisions of the species. The following equations estimate the reaction rates: (i) * R1 * Xk1 * X M *H H 2O H H 2O (1) (i) R2 * Xk CO2 * X H * HCO2 *V /V0 (2) (i) R3 * Xk CO2 * X L *HCO2 *V /V0 (3) Where: R1,2,3 – reaction rates, lbmol/hr (mol/s). 3 3 -2 3 Xk1 – rate constant, 2.4*10 ft /hr (1.89*10 m /s) XkCO2 – rate constant, 15.2 (regardless of units) XL – ratio of the moles of LiOH at the initiation of the time step to the moles of LiOH at the beginning of the simulation XH – ratio of the moles of LiOH.H2O at the initiation of the time step to the moles of LiOH at the beginning of the simulation XC – ratio of the moles of Li2CO3 at the initiation of the time step to the moles of LiOH at the beginning of the simulation V̇ – volumetric flow, ft3/hr (m3/s) V0 – gas velocity at cartridge inlet, cm/s. Please note that this velocity must be in units of cm/s regardless of overall unit system chosen. ε – exponent for V0, unitless. 0.5 – XC 3 3 HCO2(i) – concentration of carbon dioxide in the air at the cartridge inlet, lbmol/ft (mol/m ) 3 3 HH2O(i) – concentration of water vapor in the air at the cartridge inlet, lbmol/ft (mol/m ) 3 3 HH2O(*) – equilibrium concentration of water vapor, lbmol/ft (mol/m ). The equilibrium between LiOH.H2O(s) and H2O(g) determines the direction of Reaction (1); demanding equality between the vapor pressure (Clapeyron equation) of the LiOH.H2O(s) and incoming H2O(g) yields the equilibrium concentration as: * (i) (i) H H 2O exp22.075 13,135.4/TBed /RAir *TAir (4) 3 2 2 o RAir – gas constant for air, 10.73 ft -psi/lbmol-˚R (8.313 kg-m /mol-s -K) o TBed(in) – LiOH bed temperature at the initiation of the time step, degrees Rankine (note – to use Kelvin, multiply the bed temperature by 5/9). o TAir(in) – air inlet temperature, degrees Rankine (Kelvin) XM – number of moles of Reaction (1)’s driving component; this too is determined by the equilibrium between LiOH.H2O(s) and H2O(g). When the inlet water vapor concentration exceeds the equilibrium concentration (calculated by Equation (4)), the bed absorbs water vapor and XM will be XL; otherwise, the hydrated LiOH dissociates and XM will be equal to XH. Finally, symbol η represents the Davis reaction efficiency in percent: 1 exp * X L (5) expG 1 X L / 2*lnV0 (6) Unitless term XL and factors µ and G define the efficiency calculation. µ is an intermediate step (Equation (6)) and G is a unit-less correlation factor that must be coalesced from real data.