Patent Application
20210001305
The development of regenerable life support systems is critically important for the advancement of NASA's space-exploration projects. In addition to carbon dioxide (CO2) and water vapor (H2O) control, trace-contaminant (TC) removal plays a key role in life support systems, ensuring high quality air for the crew during Extravehicular Activities (EVAs) and also on board spacecraft. This invention addresses the fabrication of structured (monolithic), carbon-based TC sorbents for the space suit used in EVAs. The approach to sorbent fabrication involves the following steps: (1) preparation of the precursor material (polymer) in a desired shape using 3D printing; (2) precursor carbonization (pyrolysis) to produce a carbon monolith; and (3) monolith activation to obtain the desired pore-structure characteristics, the desired surface chemistry (functional groups), and good TC-sorption performance (high sorption capacity and rapid vacuum-regeneration). The subject of this invention is: (1) the demonstration of the use of 3D printing to create polymer monoliths with complex geometries, e.g., honeycomb structures, subsequently converted into effective TC sorbents upon carbonization and activation, while preserving much of their original shape and strength; (2) the demonstration of effective ammonia and formaldehyde removal in the presence of CO2 and humidity; also, the demonstration of rapid sorbent regeneration; and (3) the development of a sub-scale sorbent prototype for use by NASA.
The starting materials for the sorbent-fabrication process are polymer-based precursors that produce microporous carbon upon carbonization (pore size dp<2 nm). The use of the predominantly microporous monolithic carbon is associated with the following benefits: (a) high trace contaminant sorption capacity; (b) low pressure drop; (c) rapid vacuum (pressure-swing) desorption due to thin monolith walls, e.g., in the 0.10-1.0 mm range, and due to the low pressure drop; (d) good thermal management (high thermal conductivity and low adsorption/desorption thermal effects associated with physisorption); and (e) good resistance to dusty environments. The fully vacuum-regenerable sorbent system resulting from this invention is in contrast to the currently used EVA air-revitalization systems, which involve oversized, non-regenerable packed beds of acid-impregnated activated carbon (AC) for TC control. For the sake of clarity, it should be understood that, unless stated otherwise, all references to monoliths made in this specification, in the claims, and in the abstract, are references made to the monoliths produced in accordance with the present invention.
In general, the trace contaminants of primary interest are ammonia and formaldehyde as they are the only ones that are likely to exceed the Spacecraft Maximum Allowable Concentration (SMAC) levels within the space suit (20 ppm for ammonia). It will be appreciated by those skilled in the art that the sorbents described in this disclosure may be used in other applications where trace contaminants need to be removed from a gas environment using sorbents that can be regenerated by exposure to vacuum or a flow of purge gas.
Trace-contaminant removal in spacecraft environments has a long history, and it was reviewed by Paul and Jennings of NASA Johnson Space Center (Paul, H. L. and Jennings, M. A., “Results of the trace contaminant control trade study for space suit life support development,” Proc. 39th Int. Conf. on Environmental Systems (ICES), Savannah, Ga., Jul. 12-16, 2009, SAE technical paper No. 2009-01-2370, SAE International, 2009). It was concluded that “there is currently no technology that is used in any industry that will perform better than activated charcoal for the PLSS application.” Several approaches to carbon regeneration have been attempted (reverse airflow, steam regeneration, and vacuum regeneration), but the challenge of excessive regeneration temperature, and of long desorption time scales, remains to be resolved. In previous work, this problem was addressed through tailoring the porous structure of carbon sorbents, and through the use of oxidative carbon-surface pretreatment (U.S. Pat. No. 9,073,039; Wójtowicz et. al., “Reversible Ammonia Sorption on Carbon for the Primary Life Support System (PLSS),” Proc. 42nd International Conference on Environmental Systems, American Institute of Aeronautics and Astronautics, San Diego, Calif., AIAA-2012-3437, 2012; Wójtowicz et. al., “Adsorption of ammonia on regenerable carbon sorbents,” Proc. 45th International Conference on Environmental Systems (ICES), Bellevue, Wash., Paper No. ICES-2015-179, 2015). In spite of the tremendous progress made (vacuum regeneration possible and the enhancement of sorption capacity as a result of surface oxidation), vacuum regeneration time at room temperature is still substantial (0.25-12 hours). In addition, the manufacture of sorbent monoliths was crude (manually drilled holes), which resulted in large monolith channel wall thickness and significant gas-diffusion resistance. The present invention addresses the above challenges through the use of 3D printing and the polymers that are compatible with 3D printing, and that also produce highly microporous carbons upon carbonization and activation. It should be noted that polyvinylidene chloride (PVDC), which was extensively used in the past (e.g., U.S. Pat. No. 9,073,039; Wójtowicz et. al., 2012; Wójtowicz et. al., 2015; U.S. Pat. Nos. 8,615,812; 8,231,712), is not compatible with 3D printing, mainly due to the release of large amounts of hydrogen chloride upon heating. Therefore, polymers other than PVDC are the subject of the present invention.
The approach to TC sorption used in invention is based on physisorption on highly microporous carbon derived from polymers. The pore sizes are close to molecular dimensions, which ensures sufficiently strong van der Waals forces to obtain high TC-sorption capacities. In contrast, most commercial activated carbons contain only a small or modest percentage of microporosity, and this is why their sorption-capacity performance is low or modest, unless enhanced by chemisorption on acidic sites. The fact that the underlying principle for the sorbents of this invention is physisorption rather than chemisorption makes vacuum and thermal regeneration fast and reversible. Also, the high purity of the carbon derived from polymer precursors makes it possible to keep the carbon surface acidity low, which facilitates the reversible TC sorption.
Another important consideration in the design of Trace Contaminant Control Systems (TCCS) is pressure drop. Granular sorbent offers significant resistance to gas flow, which is associated with a high demand for fan power. Thus, there is a great need for an effective TC sorbent that could be regenerated by short exposure to vacuum at low temperatures (under 40° C. for several minutes). A monolithic structure (e.g., a carbon sorbent honeycomb with thin walls) is desired to reduce fan-power consumption and to facilitate rapid vacuum-regeneration. The fabrication and use of monolithic carbon structures for reversible TC sorption/desorption is disclosed in the present specification.
The broad objects of the present invention are to provide novel carbon sorbent monoliths produced from 3D-printed polymer monoliths, provide a method for their fabrication, and to demonstrate the effectiveness of these sorbents in adsorption of trace contaminants, particularly ammonia, as well as rapid vacuum-regeneration of such sorbents. It is also an object of the invention to provide novel carbon monoliths for effectively removing trace contaminants from gaseous environments. Another object of the invention is to provide carbon sorbent monoliths derived from 3D-printed polymer monoliths.
It has now been found that certain of the foregoing and related objects of the invention are achieved by the provision of a method for the removal of trace contaminants from a gaseous environment, comprising the steps: providing a porous, carbon sorbent monolith that is capable of trace-contaminant sorption and desorption; causing a volume of contaminated gas to pass through the sorbent, to thereby effect sorption of trace contaminants from the gas volume; and exposing the sorbent monolith to a vacuum environment, advantageously at ambient temperature, to thereby effect desorption and removal of a substantial portion of the adsorbed trace contaminants therefrom. The carbon sorbent monolith employed is produced by 3D-printing a polymer monolith from a polymer precursor, carbonizing the polymer monolith so as to provide a high-purity carbon monolith, which is exposed to an oxidizing environment under conditions sufficient to produce an effective amount of at least one oxygen species on exposed surfaces, such that the sorbent contains normally at least 0.10, and preferably at least 0.25, and most desirably at least 0.5, weight percent of the oxygen species added during the oxidation (activation) step. Carbon activation also creates internal surface area and porosity, which, together with the oxygen surface species present on the carbon surface, are instrumental in the effective sorption of trace contaminants. The internal surface area is normally higher than 100 m2/g, and preferably at least 300 m2/g, and most desirably at least 500 m2/g.
The oxygen species may be produced by exposing the sorbent to air, oxygen, carbon dioxide, steam, ozone, hydrogen peroxide, nitric acid, or mixtures thereof, or to another suitable oxidizing environment. In many instances, the oxidizing environment will most desirably comprise air at a temperature in the range 150° C. to 400° C., with an exposure time of at least 5 minutes; preferably, the temperature range will be 250° C. to 325° C. and the exposure time will be 24 to 300 hours. When the oxidizing environment is other than air the temperature and exposure time will be varied so as to produce equivalent levels of surface oxidation. In any event, the carbon of which the carbon sorbent monolith consists will normally be substantially free from mineral contaminants in elemental or molecular state.
The polymer precursor used in the method will desirably be selected from the group consisting of polyether ether ketone (PEEK), polyetherimide (PEI), and polycarbonate (PC), and mixtures thereof, and most desirably it will comprise at least 70 weight percent of PEEK. Furthermore, the polymer precursor will desirably comprise reinforcement in the form of fibers, preferably carbon fibers, for improved shape retention during carbonization and enhanced strength. In preferred embodiments, the majority of pores of the sorbent will not substantially exceed 2 nanometers in size.
The carbon sorbent monoliths utilized in the trace contaminant removal method may advantageously be produced by constraining the polymer monoliths during the fluid stage of carbonization (polymer melting) using structural supports to prevent the loss of shape of polymer monoliths during carbonization. In a preferred embodiment, the structural supports comprise of vertically positioned stainless-steel dowel pins as well as top, bottom, and side support plates. In another preferred embodiment, the structural supports comprise a side support plate as well as pins extending from top and bottom support plates in what can be termed a “bed-of-nails” assembly.
Other objects of the invention are attained by the provision of a method for the production of a sorbent monolith, and by the provision of a sorbent monolith so produced, comprising the steps: 3D-printing a polymer monolith from a polymer precursor; carbonizing the polymer monolith so as to produce a high-purity carbon monolith; and exposing the high-purity carbon monolith to an oxidizing environment under conditions sufficient to produce at least one oxygen species on exposed surfaces, the sorbent containing at least 0.10, and preferably at least about 0.25, weight percent of the oxygen species added during the oxidation (activation) step. The nature of the polymer material used, the carbonization conditions employed, the oxidizing conditions used, and the characteristics of the sorbent may be as hereinabove and hereinafter described.
Three polymers were initially chosen, and then one of them was down-selected for further work, and the following criteria were used for precursor selection:
To ensure that the selected polymers were indeed compatible with 3D printing, commercially available 3D printing filaments were chosen. They were ordered and received from 3DXTECH Additive Manufacturing of Grand Rapids, Mich., some of them reinforced with ˜10 wt % carbon fiber (CF): polyether ether ketone (PEEK and PEEK/CF), polyetherimide (PEI/CF), also known as Ultem, and polycarbonate (PC and PC/CF). PEEK powder, supplied by Goodfellow USA of Coraopolis, Pa., was also used in some experiments.
A standard laboratory tube furnace was used for carbonization, and several heating profiles were utilized, some of them with a single heating rate of 5 K/min, others with hold times at about 500° C. The final carbonization temperature used was 800° C., and nitrogen was used as a carrier gas. In several experiments, polymer/carbon-fiber filament samples were rapidly inserted into the tube furnace preheated to 450° C., 500° C., 550° C., and 600° C., and held at the above temperatures for ˜30 min. It was found that the carbonization conditions did not have a strong effect on shape retention for the PEEK polymer. For this reason, unless indicated otherwise, only results obtained using a heating rate of 5 K/min are reported. Carbon activation was carried out in a flow of air at 325° C. to a burn-off of ˜20 wt % using a tube furnace.
An automated gas-sorption system ASiQwin (manufactured by Quantachrome Instruments of Ashland, Va.) was used for collecting and processing nitrogen-isotherm data for carbon sorbents. Prior to adsorption-isotherm measurements, each sample was outgassed under vacuum at 300° C. for at least 3 hours. Nitrogen-adsorption isotherms were determined at 77 K, and these data were used to perform the following analyses: (a) Brunauer, Emmett, and Teller (BET) surface area; (b) pore volume; (c) Dubinin-Radushkevich (D-R) micropore surface area and micropore volume; and (d) pore-size distribution of micropores using the Density Functional Theory (DFT).
The ammonia and formaldehyde sorption capacities were determined using the testing system shown schematically in
The system incorporates a Fourier transform infrared (FTIR) multi-gas analyzer, which is used for ammonia, formaldehyde, carbon dioxide, and water quantification. Using mass flow controllers, ammonia/nitrogen, CO2/oxygen, and formaldehyde/nitrogen gas mixtures are blended to achieve the desired gas concentrations. The flow of formaldehyde is generated by placing a permeation tube within a flow of nitrogen. For humidifying the gas stream, a portion of the nitrogen flow is routed through a water bubbler, using fine needle valves for adjustment. During testing, the final mixture is first directed through a sample bypass line, to establish the baseline trace contaminant and humidity conditions. The gas is then re-directed through the sample “cell” for the sorbent adsorption testing. The sample cell consists of a glass tube that contains the sorbent sample. It is mounted in a vertical orientation with the gas inlet at the top of the cell so that gas flow is in a downward direction.
The 18 mm diameter multi-channel carbon sorbent monoliths are wrapped in Teflon tape and then inserted into a 22 mm diameter glass tube. The sorbent monolith height is typically 0.6 cm. The Teflon tape assures a snug and reasonably gas-tight fit between the foam sample and the quartz tubing. For the granular sorbents, about 0.200 g of sieved sample (+45-20 mesh) is loaded into a 5.5-mm i.d. glass tube and held in place using ceramic wool on both ends, resulting in a carbon bed length of about 25 mm. The gas flow rate used is 1 L/min.
Gas-concentration data are collected using the FTIR analyzer at one minute intervals. The experimental procedure involves monitoring the trace-contaminant breakthrough curves (outlet concentration versus time) and terminating the adsorption measurement when the trace-contaminant concentration reaches at least 90% of the inlet concentration (after breakthrough).
For vacuum regeneration experiments, the sample cell is removed from the test stand and installed in a high vacuum chamber pumped by a turbomolecular pump (base vacuum of about 10−6 Torr). After the vacuum regeneration, the sample cell is re-installed on the test stand and the trace-contaminant adsorption is measured again to determine the regenerated capacity.
Samples of PC, PEI, and PEEK filaments were used in preliminary screening experiments to determine, qualitatively, the degree to which a given material could retain its shape upon carbonization.
Results of carbonization experiments performed using PEEK filaments, with and without carbon-fiber reinforcement, are shown in
The following conclusions can be made on the basis of the above carbonization tests:
Ammonia and formaldehyde sorption and sorbent regeneration properties were later found to be excellent for the PEEK-based carbon, as discussed below. Consequently, carbon fiber reinforced PEEK (PEEK/CF) was selected for further work.
The solid model of the 1 mm square channel monolith, generally designated by the numeral 10 in
A polymer monolith 3D-printed using PEEK/CF filament with a target channel wall thickness of 0.25 mm is shown in
Carbon fiber reinforced PEEK monoliths were carbonized, and it was found that shape retention was good for 0.5-mm thick monoliths, but problematic in the case of 3.0-mm thick monoliths. In order to understand the mechanism of polymer melting and carbonization during heat treatment, an experiment was designed in which the polymer sample was heated by an external furnace, while being visually observed through a mirror. It was found that PEEK monolith melting and fusion of channels took place in the temperature range 350-385° C., with swelling observed at about 450° C. A carbonization run was also performed using a thermogravimetric analyzer (TGA), which showed that the onset of devolatilization, indicated by sample weight loss, occurred well above 500° C. Therefore, it was concluded that melting happened before devolatilization, and this is why measures needed to be taken to prevent the loss of monolith shape at the melting stage.
The above considerations led to the idea of using support structures in the form of smooth, stainless-steel dowel pins, designated by the numeral 15 in
Since the dowel pins used in early experiments (see
Since the use of individual support structures (pins) for each channel is tedious, an improved concept was put forward, in which two “beds of nails” (i.e. arrays of pins affixed on a support), generally designated by the numeral 26 and shown in
The assembly shown in
Two samples of PEEK/CF-based sorbents were characterized with respect to their surface area, pore volume, and pore-size distribution using nitrogen-adsorption isotherms at 77 K. Both carbon sorbent monoliths were found to have similar pore-structure characteristics, and results for one of them are discussed below.
As expected, the carbon sorbent activated to ˜18% weight loss (burn-off) is mostly microporous (70% microporosity), i.e. having mainly pores smaller than 2 nm (20 Å), which is clearly seen in the pore-size distribution plot (
X-Ray Photoelectron Spectroscopy (XPS) analysis was used to gain insights into the carbon surface chemistry and the relative changes produced by carbon activation and exposure to ammonia. The following three PEEK/CF samples were analyzed for relative amounts of carbon, oxygen, and nitrogen, and also for the functional groups present on the carbon surface:
Results are shown in Table 1, and they indicate that carbon activation introduces oxygen functionalities onto the carbon surface, and also that carbon exposure to ammonia results in the formation of nitrogen functionalities on the surface. The speciation of nitrogen functionalities for sample C+A+NH3 is given in
Ammonia Sorption on Granular Sorbents—Several PEEK/CF granular sorbents were prepared using the carbonization and mild-oxidation (activation) techniques described by Wójtowicz et al., 2012. Samples of these carbons, which had different degrees of oxidation burn-off, were used to determine the equilibrium ammonia sorption capacity at room temperature. These data were then compared with results of previous work on PVDC-based sorbents (Wójtowicz et al, 2015), and this comparison is shown in
Regeneration of Sorbents Supported on Reticulated Carbon Foam—A test sample was produced by impregnating reticulated carbon foam with pure PEEK powder, followed by carbonization and low temperature oxidation, as described by Wójtowicz et al., 2012. After the initial ammonia-sorption test, regeneration was performed by exposing the sample to high vacuum (ultimate vacuum ˜5×10 Torr) for 6 hours. Slightly better than 50% regeneration was achieved (
Ammonia Sorption and Sorbent Regeneration for Carbon Monoliths—Vacuum regeneration of ammonia sorbent is a critically important feature of the sorbent monolith of this invention. Trace contaminant sorption on high-purity carbons that have not been impregnated with any acids, and that are essentially free of mineral impurities, is governed mostly by physical adsorption (physisorption) rather than irreversible, or almost irreversible, chemisorption, which dominates trace contaminant sorption on acid-treated or mineral-matter containing carbons. For this reason, little or no loss of sorption capacity is expected in the instant sorbents following initial cycles of ammonia adsorption-desorption. In contrast, acid-treated carbons normally show little or no recovery of their original sorption capacity after the first chemisorption event (Paul and Jennings, 2009).
As expected, no loss of sorption capacity beyond the first adsorption-regeneration cycle was observed in a series of experiments involving a PEEK/CF carbon sorbent monolith which was subjected to repeated ammonia adsorption-desorption cycles (
Data presented in
The Effect of Humidity on Ammonia Sorption—An additional experiment, i.e. Cycle 5, was performed using the same sorbent discussed in the previous section. This time, the gas composition was the same as in Cycle 4, except a relative humidity of 40% was used. Data in
Formaldehyde Sorption—Formaldehyde adsorption on a PEEK-based carbon monolith was carried out, and it was found that, under conditions used in this work, the breakthrough never occurred, which means that all the formaldehyde present in the inlet gas got adsorbed. The test had to be aborted after more than 1,000 minutes (˜17 hours) of excellent sorbent performance.
Ammonia Sorption under Rapid-Cycling Conditions—The large ammonia equilibrium sorption capacity demonstrated by PEEK-carbon monoliths (up to 20 mg NH3 per gram of sorbent) is certainly an attractive feature of this technology. The fact that multiple adsorption-desorption cycles can be performed without sorption-capacity degradation (except for Cycle 1, as discussed above) is also encouraging. What these experiments do not reveal, however, is desorption kinetics, which are of paramount importance if the preferred mode of operation is rapid cycling with a frequency of several minutes.
In order to evaluate the suitability of PEEK-carbon monoliths for rapid cycling pressure swing operation, the following experiment was carried out. A carbon monolith derived from PEEK/CF was exposed to a flow of 20 ppm ammonia over more than 18 hours to reach the state of complete sorbent saturation with ammonia. The sorbent was then subjected to rapid sorption-desorption cycles, with each adsorption and desorption step taking 5 minutes. The gas flow rate was 1.0 L/min, and the carbon weight was 0.25 g. Vacuum regeneration (desorption) was performed using a roughing pump capable of providing only ˜0.35 Torr vacuum at the sample location during the pumpdown. This of course was far from ideal, but the objective of this experiment was to see if the sorbent's response was fast enough, even though its sorption performance may have been reduced due to the poor vacuum.
Results are shown in
The effect of improved vacuum quality on sorbent performance was addressed in a separate experiment, and results of the fast cycle regeneration test that was conducted under improved vacuum conditions are shown in
General observations are as follows:
Although not extensively tested over hundreds of cycles, the currently available data demonstrate the feasibility of using PEEK-carbon monoliths for pressure-swing operation with cycle times of a few minutes.
One of the main advantages of the monolithic structure is a low pressure drop, which also means a low fan-power requirement. To get an idea about the magnitude of the pressure drop across the sorbent monolith, calculations were performed using the approach described by Cybulski and Moulijn (“Monoliths in Heterogeneous Catalysis,” Catalysis Reviews, 36(2), 179-270, 1994). Input information for the calculations is summarized below.
Under the above conditions, calculations showed that the pressure drop was immeasurably small. Although no meaningful measurements of such a low pressure drop could easily be performed, pressure-drop measurement at much higher flow rates were carried out. The flow resistance was determined using a small laboratory system previously described (Wójtowicz et al., 2012). The gas flow rate was measured by a flow meter, and pressure gauges were used to determine pressure upstream and downstream of the sorbent element.
Experimental results presented in
One of the shortcomings of granular carbon sorbents currently used for TC control is attrition and the release of fine particles, which may take place, for example, due to vibrations occurring during spacecraft launch. It is expected that carbon monoliths will exhibit better resistance to vibrations. A preliminary evaluation of the mechanical integrity of a prototype test unit subjected to vibration was performed using a laboratory sieve shaker (CSC Scientific model no. 18480). The test cell was clamped directly to the shaker stage and shaken for a period of 5 minutes at an intermediate setting of 5 on the sieve shaker. No evidence of damage to the carbon monolith, such as fracture or shedding, was found. Although the above test is not directly linked to any standard testing methodology, it nonetheless provides evidence of mechanical robustness of PEEK-derived carbon sorbent monoliths.
Thus, it can be seen that the above-stated objects of the invention are attained by the method and product described in the foregoing specification and defined by the appended claims.
This application claims the benefit of United States provisional applications Nos. 62/870,718 and 62/871,106, both bearing the foregoing tide, and filed on Jul. 4, 2019 and Jul. 6, 2019, respectively, the entire specifications of which is incorporated hereinto by reference thereto.
The United States Government has rights in this invention under the following contract: National Aeronautics and Space Administration contract No. 80NSSC18P1961.
| Number | Date | Country | |
|---|---|---|---|
| 62870718 | Jul 2019 | US | |
| 62871106 | Jul 2019 | US |
