11/6/2008
Eckart V:
Physico-Chemical Life Support Subsystems
V.1 Atmosphere
Management (CO2, O2, N2, TCC, THC, FDS)
Objectives
Atmosphere Revitalization and Control Subsystem (ARCS)
Functional Requirements: Provide continuous monitoring, control and revitalization for:
Ύ CO2 removal / concentration / venting / reduction
Ύ O2 provision for metabolic needs
Ύ N2 provision / Gas Storage for buffer / leakage
Ύ Total Cabin Pressure / Ventilation and Convective Cooling
Ύ Gas Storage (solid, high pressure or cryo - buffer for EVA repress, leakage makeup, emergency)
Ύ Trace Contaminant Control (TCC)
Ύ Temperature and Humidity Control (THC)
Ύ Fire Detection and Suppression (FDS)
Potential Trade Factors (pp. 179-180)
Thermodynamic efficiency
Degree of material closure
Power
Heat rejection
Consumables
Reliability
Interaction with other systems
Safety (including temperature and byproducts)
Robustness
Multiprocessing
Crew time
Start up / shut down ops
Operation in degraded or rescue mode
Moving parts
Noise
TRL
ESM (system, power, heat, consumables, fuel)
Cost to develop
Cost to manufacture
Launch / resupply costs
Operational and Maintenance costs (crew time and spare parts, tools - replace vs. repair)
adsorption - The accumulation of gases, liquids, or solutes on the surface of a
solid or liquid
absorb - To take something in through pores or interstices
sorption - The process in which one substance takes up or holds another by either absorption or adsorption
Lithium Hydroxide (LiOH)
Requires ~2 kg to remove 1 kg CO2, used in various spacecraft, including shuttle
Sodium, potassium and barium hydroxides CO2 goes into solution and forms carbonic acid, then sodium carbonate, and finally calcium carbonate (side note: Biosphere II had a problem with formation of calcium carbonate in the internal concrete structure that inhibited CO2 accumulation in the atmosphere, initially suggesting leakage.)
Alkali and alkaline earth metal
superoxides solid chemicals that provide O2 and scrub CO2 when exposed to
water vapor. Potassium superoxide (KO2) produced on industrial scale, used on
early
Regenerable Processes
zeolites or metal ion alumino-silicates to collect CO2
silica used to remove H2O (H2O is preferentially absorbed if present)
4-bed: two stage, one system removing H2O / absorbing CO2, other in desorbing cycle
4-bed molecular sieve (4BMS), Skylab, needs predry step and bake out at 478K (205C)
2-bed: one absorbing, one desorbing
2-bed molecular sieve (2BMS) less mature, no need for desiccant
Solid Amine Water Desorption (SAWD) (Figure V.3, p. 184)
Similar to 2BMS but uses solid amine, solid amine degrades over time unlike Zeolites, and requires hygiene quality water which vents to vapor in the spacecraft, but desorption takes place at cabin pressure rather than needing vacuum as the MSs require to vent CO2, CO2 can be compressed into an accumulator, penalized for steam generation and heat rejection.
Electrochemical Depolarization Concentrator (EDC)
(Figure V.4, p. 189)
EDC reacts H2 and O2 with CO2 inside an electrochemical cell. Anode side outlet produces high concentration CO2 with some H2. Cathode contains air with low CO2 concentration that is returned to the cabin. EDC also generates DC electrical power similar to an H2-O2 fuel cell.
Adv. Operates in continuous of cyclic mode, variable CO2 removal controlled by current,
Disadv. Requires O2 consumption, produces water vapor, potential safety hazard due to presence of hydrogen.
Air Polarized Concentrators (APC)
Similar to EDC but does not require hydrogen for CO2 removal, safer than EDC, but becomes a power consumer
Osmotic filtering, requires large area to remove small amount of CO2, TRL low
Electroactive Carriers bind to CO2 in a reduced form and release in oxidized state, TRL low
Metal Oxides limit lifetime due to expansion and contraction cycles, 140C needed to regenerate, water needed for absorption process
Carbonate potassium carbonate binds reversibly to CO2, desorption requires 150C, lifetime limits due to gradual consumption
Ion-Exchange Electrodialysis uses an ion exchange resin reacting with CO2 to form carbonate ions, electrical field causes carbonate ions to migrate from absorbing cell to concentrating cell
Regenerable adsorbents technologies: liquid amines, aqueous alkaline electrolytes, molten carbonate electrolytes
Other biochemical/biological methods: enzymes, biological methods, phototropic organisms like certain bacteria, algae and plants.
Cryogenic freeze of water and CO2 ice and dry ice
STS RCRS
The Regenerable CO2 Removal
Subsystem (RCRS) consists of two beds filled with a proprietary Hamilton
Sundstrand solid chemical (called HSC) that absorbs CO2. The two beds alternate between one having
cabin air circulate absorbing through it while the other is vented to vacuum
(desorbing CO2 out). A system of valves configures the bed either into the
adsorbing mode or desorbing mode. The
RCRS is preferable to using LiOH because the HSC does not lose potency (or any
measurable potency that is significant enough to make a difference) over the
length of an extended duration mission for the orbiter (>16 days)
eliminating the need for refills and associated crew time for servicing (as the
LiOH system requires), and providing for some net launch mass savings.
CDRA
Carbon Dioxide Removal Assembly, used in US Lab on ISS, regenerable CO2 removal system
VOZDUKH
Used in Russian Service Module on ISS, regenerable CO2 removal system
CO2 reacts with H2 at high temp (700-1000K) in presence of a catalyst (activated steel wool, nickel, nickel/iron), output is solid carbon, water and heat. Gasses are heated and compressed before entering catalyst bed. Exothermic load on thermal system. Single pass reduction efficiency <10%, so must be run in recycle mode. Recycled gas mixture contains CO2, CO, CH4, H2 and H2O vapor. Solid carbon deposits on catalyst, so cartridge must be periodically replaced. Optimum operating temp changes as carbon builds up on catalyst. Dual temp (low/high) reactors may increase efficiency.
Less ESM (size and power) than Bosch and higher TRL, but produces methane (CH4) as a primary byproduct. CO2 reacted with H2 at high temp (450-800K) in presence of ruthenium catalyst producing methane and water. CO2 ΰ CO, then CO ΰ CH4. Exothermic above 450K self sustaining to 866K, then reversible endothermic. CO2 conversion ~98%, so net O2 loss over time. Single pass conversions up to 98%
Advanced Carbon-Formation Reactor System (ACRS)
Consists of a Sabatier reactor, gas/liquid separator to remove H2O from methane, and a carbon formation reactor to reduce CH4 to C and H2. Reduced consumables, but impractical operating temp (1144K)
Solid oxide electrode reduces CO2 AND produces usable O2 as byproduct. Problems lie in high temp (1140K) ceramic-ceramic seals. Low TRL. Solid C deposits on electrode. Increased O2 achieved by increasing DC voltage to electrolyzer.
-- see above CO2 removal
Shuttle
Liquid oxygen
Used for breathing and electrical power production
Cryogenic
Thermally insulated, double walled vacuum annulus tanks
-176 °C
High pressure - 5 MPa
Heaters maintain pressure
Up to 5 spherical tanks (+4 more for EDO- OV105)
Tank volume = 320 liters
Tank mass = 98 kg
O2 mass = 354 kg / tank
Inconel 718 inner and 2219 Aluminum outer shells
Regulates at PPO2 = 20.3 to 23.8 kPa
ISS
Gaseous oxygen
On US Airlock
High pressure cylindrical tanks
2 external, V = 428 liters at 18.6 MPa
Provide total of 15,664 liters O2 at 1 Atm
In Service Module
High pressure portable tanks
1 internal, V = 20 liters at 31 MPa
Provide total of 640 liters O2 at 1 Atm
from H2O
Static Feed Water Electrolysis (SFWE) (Fig V.9, p 203)
Produces O2 and H2, which can be fed to fuel cell for electricity. Yields max of 0.74 kg O2 per kg CO2. Requires DC power and heat rejection. Uses potassium hydroxide (KOH). Water vapor can be further processed by coupling to electrolysis dehumidifier to eliminate need for downstream vapor/O2 separation.
Solid Polymer Water Electrolysis (SPWE) (Fig. V.10, p 204)
Also electrolyzes water using solid polymer electrolyte. Can function at a range of pressures to deliver desired output. Continuous or cyclic operation.
Water Vapor Electrolysis (WVE) (Fig. V.11, p. 204)
Electrolyzes water vapor directly from the air. H2 produced as byproduct. Can help reduce load on humidity separation.
O2 extracted directly from air (e.g. in a plant growth chamber or on surface of Mars) by binding with organo-metallic compounds such as hemoglobin. Low TRL.
Table V.4, p. 181
-- see above: electrolysis, superoxides
The replaceable cartridge is a thin-walled cylinder of stainless steel, which contains a briquette of an oxygen-bearing product. The briquette is composed of three parts: two main tablets and an igniter tablet, onto which a flash igniter is installed. This flash igniter is struck by the firing pin when the assembly is activated. One cartridge yields 600 L of oxygen. The contents of the cartridge take 5 - 20 minutes to decompose at a reaction temperature of 450 - 500°C (842 - 932°F). The temperature of the outer surface of the ТГК may reach 50°C (122°F). The oxygen generated by this process is then cooled, scrubbed of odors, mixed with station air, and released into the station atmosphere. To guarantee the cooling of a spent cartridge the fan is not turned off any earlier than 1½-2 hours after the start of the process. The operation of the ТГК fan is monitored by MCC-M via telemetry. Certain components of the ТГК require periodic replacement. The harmful contaminant filter with the striking mechanism is changed periodically, after processing 100 cartridges. The dust collector is replaced upon instructions from the ground (roughly after every 20 cartridges).
Nitrogen lost via leakage and food consumption.
Simplest design solution
Subject to ~0.05 kg tank mass and ullage for each kg N2 provided. Nitrogen can be stored at less mass in form of hydrazine (~0.2 kg penalty / kg N2H4)
Thermal catalytic dissociation of hydrazine (OMS/RCS thruster fuel). Exothermic, requires cooling. H2 separation and NH3 breakdown into N2 and water
N2 from ammonia. Endothermic reaction, requires heat. Low TRL
Pressure Monitoring
and Control
Absolute pressure monitors
Pressure change monitors
Air flow monitors
Regulators
Positive and negative pressure relief valves
Detect all trace gasses by MW. Quantified by retention time. MS adds resolution to processed GC output plots.
IR-active substances produce an adsorption spectrum, which provides a signature of constituents in the air. Compounds with similar structures can not be discriminated. Hydrogen and chlorine are not IR active.
Ion Trap Mass Spec / Mass Spec (MS/MS)
Uses tandem MS to perform both the separation and identification of analytes (substance being analyzed) based on MW and analyzed by ion detector. Lots of potential, low TRL.
Monitor ppO2 with redundant sensors.
CFU analyzer, including VBNC organisms
Trace Contaminant Control (TCC)
Sample multi-stage system given in Fig. V.12, p. 31
Gasses (from crew
metabolism, off-gassing, leakage/spills)
Convert to harmless forms by non-consuming methods. Oxidation.
Particulate filters
Dust and particulates
Higher MW contaminants. Couple with oxidizer for increased effectiveness. Consumable, non-regenerable. Regeneration of charcoal bed produces undesirable byproducts.
Lower MW gasses not absorbed by charcoal or converted by oxidation.
Oxidize contaminants not readily absorbed.
Reactive Bed Plasma (RBP)
Regenerable system under development. Oxidizes contaminants at low temp (394K) using plasma and catalyst. Also acts as electrostatic precipitator for particulate removal. Potential for long-life, universal contamination control system.
Used to treat water, but input air can be treated as well. Eliminates need for stand alone TCC. Low TRL
US TCC System on ISS
Removes products of off gassing
Alcohol, aldehydes, aromatics, esters, hydrocarbons, keytones, sulfides, inorganics, etc
Components
Activated charcoal bed
Catalytic oxidizer
LiOH bed
Russian Harmful Impurities Removal System on ISS
Absorbs similar contaminants to TCCS
Heats to drive impurities out of sorbent bed
Vents to vacuum
20 day cycle
Cool air and remove humidity
Membrane Technologies
Hydrophobic/hydrophilic concepts
Smoke Detectors or Flame
Detectors
Sense particulate products of combustion
Photoelectric (obscuring, scattering or condensation) (Figs. V.18&19, p. 216-217)
Ionization detectors (Fig V.21, p. 217)
Sense gas products of combustion
Spectroscopy
Chemical Analysis
Sense energy emissions (heat radiation UV, vis or IR) from the fire (Fig V.22, p. 218)
Circulation requirements
False alarms undesirable
CO2, N2 (Halon), Foam (Russians) or water
Concerns of impacting cabin air (O2 displacement)
Two fire detection methods are currently used in manned spacecraft. Both are placed in ventilation systems to detect the presence of particulate smoke (Friedman et al. 2000).
Orbiter: Ionization Smoke Detectors ionize particles in the air, when smoke interferes alarm goes off
20 false alarms in first 20 years with no fires
5 potential fire incidents, none of which set off alarms
(Friedman et al. 2000).
ISS: Photoelectric Smoke Detectors detect the forward scattering of IR light by the smoke
By September 2005, 9 false alarms with no potential fire causing incidents ISS
(Von Jouanne 2005).