DEvice for the study of Critical LIquids and Crystallization - High Temperature Insert-Reflight (DECLIC HTI-R) - 01.09.14
Science Objectives for Everyone
DECLIC High Temperature Insert (HTI) studies water near its critical point, the point beyond which water loses its distinction between liquid and vapor and begins to behave as a dense gas. Salt tends to precipitate out from water at temperatures and pressures beyond its critical point. Understanding this behavior will assist designers in building extended-life and low-maintenance supercritical water oxidation (SCWO) reactors that will provide for more environmentally friendly waste management systems and reduce operating costs of power plants that use supercritical water for its working fluid.
Science Results for Everyone Information Pending
Centre National d'Etudes Spatiales, Toulouse, , France
Sponsoring Space Agency
National Aeronautics and Space Administration (NASA)
Human Exploration and Operations Mission Directorate (HEOMD)
ISS Expedition Duration:
September 2012 - March 2014
Previous ISS Missions
The DECLIC HTI-R investigation is a follow-on investigation to the precursor investigation, HTI that was designed to observe fundamental phenomena (e.g., “piston effect”) in pure water at near-critical conditions.
The DECLIC HTI-R investigation attempts to observe and understand the phenomena associated with salt precipitate formation, agglomeration, and transport in water at very high pressures where liquids and vapors no longer co-exist (i.e., critical point of water).
The investigation determines the “critical end point” (i.e., the shift in critical point due to the presence of salt) and provides detailed observations of precipitate agglomeration and transport processes. DECLIC's optical components and precise thermal control capabilities, consisting of a high resolution camera, light scattering laser, and thermoelectric heater/sensor assemblies allow for detailed observations and precise thermal control of fluid temperatures and controlled gradients at near critical conditions.
Results from this investigation provide observations that can enhance the fundamental understanding of the mechanisms that govern salt precipitation and transport in supercritical fluids. This understanding is necessary in order to address one of the biggest technological hurdles, salt deposition and corrosion, currently limiting widespread application of supercritical water oxidation in waste management and resource reclamation systems.
To date there has been no systematic investigation into the various transport mechanisms of precipitants in “supercritical” (i.e., fluid at temperatures and pressures beyond its critical point) salt-water in the absence of gravity. Phenomena such as the onset of precipitation in a supercritical water/salt mixture, the diffusive transport of particles in the presence of temperature gradients, the transport response to rapidly changing surface temperatures, and the “trans-critical” (i.e., as fluid transitions from sub-critical to supercritical) nature of a water/salt mixture in the presence of rapidly changing heat fluxes are some of the phenomena that will be studied in this investigation. In near-critical conditions, the solubility of salt varies inversely with temperature; i.e., as temperature increases salt saturation levels decrease so that a salt concentration that is soluble at near-critical temperature results in precipitation at supercritical temperatures.
The presence of this secondary phase shifts the critical point of water and involves the precipitation phenomena, which may occur in either one or both of heterogeneous and homogeneous modes. Heterogeneous precipitation tends to occur on nucleation sites of surfaces immersed in the fluid (heater rod or a heated wall). Homogeneous precipitation occurs in the bulk of the fluid once saturation conditions are met.
It is be possible to study two basic phase transformations of in the binary mixture in the absence of gravity driven convection: (i) the gas-liquid phase separation crossing the critical temperature of the mixture; (ii) the salt precipitation and its transport behavior in the presence of what are normally secondary forces and modes of diffusion (i.e., apart from Fickian diffusion considered above), such as thermophoresis (i.e., the movement of particles in a thermal gradient from high to low temperatures or Soret-Dufour effects). This is of interest because these secondary transport mechanisms may become dominant in microgravity or reduced gravity conditions (e.g., lunar or Martian surfaces). As such, it is important to understand these phase transformation processes in reduced gravity, particularly as it relates to deposition rates on critical surfaces.
The original DECLIC-HTI investigation insert returned to the ground with the STS-133/ULF-5 Shuttle flight. The original cell, containing pure water, is being replaced in the DECLIC HTI-R investigation by a similar cell containing a dilute salt-water mixture. This reflight cell will be integrated into the same HTI, which will be subsequently referred to as the HTI-Reflight (HTI-R). The main objective is to study salt precipitation phenomena close to the critical temperature and in the presence of a temperature gradient. This investigation is referred to as the DECLIC HTI-R investigation, using the refurbished insert from the earlier pure water investigation, is intended to investigate the behavior of an aqueous mixture of a Type 2 salt (i.e., sodium sulfate, Na2SO4) in the vicinity of the water-salt mixture’s critical point. Of particular interest are the point at which precipitation occurs, the rate of precipitate agglomeration, the precipitate’s spatial distribution, and the precipitate’s predominate transport mechanism in the presence of salinity and temperature gradients
From a technical point of view, the DECLIC hardware uses two ISS program-provided lockers comprised of the lower locker called the ELectronic Locker (ELL), which houses power supplies, data handling and central regulation electronics for operation and control. This lower locker contains all necessary electrical and electronic systems that permit the facility to operate in an autonomous mode or with telescience interactions from the scientific team at the dedicated user center. The upper locker is the EXperiment Locker (EXL). It contains the DECLIC optical bench that receives the investigations cartridge insert which contains the specific scientific material being examined. This optical bench contains all optical and opto-electronic sensors that are necessary to perform measurements at low or high rate of acquisition.
The High Temperature Insert Reflight (HTI-R) investigation is operated by the Central Regulation Electronics (CRE), located in the EXL of the DECLIC instrument. The CRE controls several functions such as: running thermal control algorithms (step-by-step procedures for solving a problem); making a precision acquisition of temperature sensors used by the thermal control algorithm; supplying accurate electrical voltages that are width modulated by insert electronics for heating elements and TEC (Thermal Electric Coolers); controlling power to the heating elements; and managing the safe status of the cartridge insert (i.e., to prevent overheating).
In many supercritical water applications, a solid phase consisting of inorganic salts exists which arises from chemical reactions during oxidation or from impurities in non-reacting systems. The presence of salts can severely limit the lifetime of system components due to its corrosive behavior and tendency to build depositions on critical thermal control surfaces, plug valves, and flow passages, etc. Precipitation occurs near-critical and supercritical regimes due to an order of magnitude decrease in the dielectric constant of water compared to its value under room conditions. This leads to a recombination and eventual precipitate agglomeration from dissolved salts.
The investigation proposes to follow a specific program, which is based on the selection of the pressure/temperature/composition of the water/salt binary mixture, includes the following three over-arching objectives:
1. Observe and quantify the critical point of the liquid-gas phase transition.
2. Observe and quantify the onset of precipitation in the supercritical homogeneous phase as a function of temperature.
3. Observe and quantify the transport process of the precipitate in the presence of temperature and/or salinity gradients.
The DECLIC-HTI experiments (e.g., Supercirtical Water Mixture) provide new insight into fluid management in space, including potential organic waste treatment for future interplanetary missions.
The Supercritical Water Mixture (SCWM) experiment performed in the DECLIC facility will provide information to enable the development of new (SCWO) reactors for use on Earth. These new reactors will be designed to exploit separation mechanisms that take place when precipitated salts are transported across regions where water transitions from supercritical to subcritical phases and will rely on internally generated heating from the oxidation of organic material in wet waste streams. It is expected that these new reactors will significantly reduce operating costs and extend SCWO technology beyond its currently limited application base and benefit future spinoffs in clean energy, waste disposal and water reclamation applications.
After the DECLIC HTI-R cartridge insert is installed by the crew, the DECLIC runs automatically following a timeline of sequences that are commanded by the payload control center (CADMOS in Toulouse, France). The duration of each sequence is typically 20 days, and the total program duration using the HTI-R insert is about 60 days. The data transmitted to ground by real time telemetry allows the investigation team to monitor the health and status. Yet, due to telemetry bandwidth limitations the entire data set cannot be downlinked and is therefore copied to Removable Hard Disks Drives (RHDDs) for future return to the ground. The data is then transferred and stored at the CADMOS operational center, and available for the research teams to process and review.
The ISS crew installs the DSI-R hardware into the DECLIC facility and then powers the facility on. Crew participation is not required during investigation runs except for the changing out of full Removable Hard Disk Drives (RHDDs). A typical timeline of an individual Test Sequence lasts about 10-15 days and consists of slowly raising the fluid temperature to near critical conditions and then a period of thermal cycling of the fluid cell above and below the critical temperature (~374°C). Due to the very high compressibility of near critical fluids close to their critical points, and the need to precisely control any thermal overshoots it is necessary to slowly approach the critical temperature (Tc) and any departures above and below need to be carefully performed. The high degree of accuracy associated with the HTI-R thermostat inside DECLIC allows calibrated temperature steps of better than 1 mK, at high temperatures around Tc, to be preformed. The duration of each temperature step is then calculated relative to the temperature distance from Tc. Temperature gradients of as much as 5°C across the fluid cell will be established and held for long periods of time in order to observe the phenomena of interest.
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