Progress Report for Molten Ammonium Salt Hydrates as Carbon Dioxide Absorbents for Environmental Control and Life Support Systems

Tammy Schwab (Paul A. Flowers, faculty advisor) 
 

Department of Chemistry and Physics, University of North Carolina at Pembroke, Pembroke, North Carolina 28372-1510

Abstract

      Continued study of molten tetramethylammonium fluoride tetrahydrate (TMAF) was examined.  Setup of scrubber assembly and procedures were used from previous study slight variations.  The calibration of the apparatus showed a linear plot.  Results of CO₂ sorption experiment shows near 100% removal.

Introduction

      Environmental control and life support systems are required elements of any habitats or vehicles used for manned exploration of sea or space.  Among the functions of these systems is the removal of carbon dioxide.  Carbon dioxide removal is typically achieved by sorption methods, either chemical (chemisorption) or physical (physisorption), and a variety of strategies have been developed to date. Lithium hydroxide has long been used and remains the most commonly employed chemisorption reagent.  Despite the historical and continued usage of this system, the sorption reaction is difficult to reverse and LiOH scrubbers are thus essentially nonregenerable, prohibiting their use on prolonged missions.  Certain molten quaternary ammonium salt hydrates possess interesting CO₂ absorption properties including large absorption capacities and rapid desorption. 

Experimental

Reagents

      Tetramethylammonium fluoride tetrahydrate (Fluka), nitrogen, argon, and carbon dioxide were used as received from the vendors.

Apparatus

      The diagram of the setup and connections is shown in Figure 1.  The scrubber assembly is shown in Figure 2.  An Isco 260D syringe pump whose outlet was fed to a Rheodyne XXXX loop injector was used to pump gas mixtures through FTIR.  The sample loop ports of the injector were connected to a small scrubber assembly (see detail in Figure 2) containing the TMAF absorbent.  The scrubber apparatus was fabricated from a 7 mm fritted tube connector (porosity D, Ace Glass, Inc.) and included 9.5 mm rubber septa (Supelco, Inc.) to accept gas inlet and outlet tubes and an additional septum with a short segment of tubing placed near the outlet to serve as a foaming baffle. The overflow apparatus is fabricated with 11 mm rubber septa (Hewlett Packard).  All gas connections between the carbon dioxide tank and the loop injector were made using either 1/8" or 1/16" stainless steel tubing.  The loop injector's outlet port was connected to an infrared flow cell (BioRad Model GC/C 32) using 1/16" steel tubing (exiting the injector) sealed with epoxy (Torr Seal) to 0.3 mm flexible glass capillary tubing (Agilent) to accommodate the flow cell's inlet port.  The flow cell was interfaced to a BioRad FTS40 FTIR spectrometer. 
 

Figure 1. Block diagram of the experimental apparatus used for carbon dioxide absorption experiments.  Solid lines represent steel or glass tubing connections. 
 
 
 

Figure 2. Detailed illustration of the scrubber assembly depicted in Figure 1.

Procedure

Calibration

      First flush the syringe with nitrogen to make sure it is void of other gases.  The syringe pump acquires the gases by purging gas into a plastic bag and then inserting inlet tubing for the syringe pump to extract the gas.  Then flushing the IR detector with nitrogen in inject mode and measuring a reference spectrum.  This was accomplished by connecting the nitrogen gas directly to overflow apparatus by a syringe and plastic tubing as in figure 3. 
 

Figure 3. Flushing diagram. 
 

Make a CO₂ mixture of some concentration in the syringe pump by using a plastic bag and allow gases to mix thoroughly.  Flow CO₂ mixture through IR detector at 5 ml/min and monitor absorption of CO₂ until constant and record an absorbance spectrum versus the nitrogen reference spectrum.

CO₂ Sorption Experiment

      For CO₂ sorption experiments, the syringe pump is again flushed with nitrogen, the IR detector is flushed with nitrogen as described in calibration procedure, and a reference is recorded.  CO₂ mixture is then made in syringe pump and allow gases to mix thoroughly.  The scrubber assembly is loaded with a weighed quantity of TMAF.  The salt is then heated until molten.  The CO₂ mixture is pumped through the IR detector in the load mode (bypassing the scrubber assembly) at 5 ml/min until constant absorbance is observed.  Then the kinetic software of WIN-IR is started with a flow rate of 1 ml/min to record an absorbance spectra for every three minutes and for the first 2 scans in load mode and then thereafter in the inject mode.  

Results and Discussion

Calibration 

      The plot of the calibration of the apparatus is shown in Figure 4.  Absorbance was measured at 2350 cm^-1.  Numbers for absorbance are rounded to the 1000^th place and for the CO₂ concentration it is rounded to the 100^th place. 
 

Figure 4. Calibration curve for carbon dioxide.

CO₂ Sorption Experiment

Figure 5. Absorbance vs. Time plot for 1.14% CO₂.

      Figure 5 is the first run of CO₂ sorption experiments.  The salt was used in solid state and not heated until molten.  The flow rate for this run was higher than the others; it was 5ml/min.  This run was done with Argon gas instead of Nitrogen gas and 1.1435 g of TMAF was used.  The run was also much shorter than the others; it was only for 60 minutes.  The run started out with a high absorbance and then when bubbled through the salt it went to low levels.  One reason the absorbance went quickly back up to high levels of absorbance is that the CO₂ gaseous mixture was pumped through the salt at a flow rate of 5 ml/min and did not give the salt enough time to absorb the CO₂. 
 

Figure 6. Absorbance vs. Time plot for 1.10% CO₂.

      Figure 6 indicates a run done with 1.10% CO₂ gaseous mixture with 1.1435 g of TMAF, the same salt used for the first run but is heated until molten and with a flow rate of 1 ml/min.  The run also was done with Argon gas instead of Nitrogen gas.  The graph has a hole in the data because the data was lost for that period of time between 56 minutes and 154 minutes.  The plot shows that the data started out at the constant CO₂ absorbance and is then switched over at 9 minutes to inject mode so that the CO₂ gaseous mixture can bubble through and then shows a low absorbance.  The absorbance starts to rise and then comes back down at around 150 minutes. 
 

Figure 7. Absorbance vs. Time plot for 0.99% CO₂.

      Figure 7 shows the plot for 0.99% CO₂.  The run used Argon instead of Nitrogen and 1.5067 g TMAF.  On this run the CO₂ gaseous mixture was not bubbling through right away so the pressure was increased on syringe pump to initially force the gas through and then was switched back to 1 ml/min.  The CO₂ absorbance started out at high in load mode and then switched at around 20 minutes to inject mode resulting in low absorbance.  The absorbance began to again rise from near zero levels and level out between 0.2-0.3. 
 

Figure 8. Absorbance vs. Time plot for 1.26% CO₂.

      Figure 8 shows the run for absorbance versus time for 1.26% CO₂ gaseous mixture reacting with 1.3256 g TMAF and Argon gas was used instead of Nitrogen gas.  The gaseous mixture did not bubble through right away so the pressure was again increased until bubbling through and then switched back to 1 ml/min.  The plot shows that it started out with a constant high absorbance and when switched over to inject mode around 12 minutes resulting in a low absorbance near zero.  At around 70 minutes the absorbance begins to rise and then level off.  
 

Figure 9. Absorbance vs. Time plot for 1.04% CO₂.

      Figure 9 shows the absorbance versus time plot for 1.04% CO₂ gaseous mixture.  Argon gas was used instead of Nitrogen gas and 1.2612 g TMAF was used.  The plot shows a high absorbance of CO₂ around ~1.2 and then drops off when switched to inject mode at around 13 minutes.  The plot has a small peak at around 60 minutes. 

Other Remarks

      There was a problem with the WIN-IR Kinetic software.  The software claims that it records a spectrum every three minutes but after observation the software recorded two spectrums every 6 minutes.  The salt, when the CO₂ gaseous mixture is being bubbled through causes the salt to become foamy; therefore causing the gas mixture to cover more surface area.  For 0.5 g of TMAF to be completely saturated by a 1% CO₂ gaseous mixture, it would take 3.7 L.  At 1 ml/min it would take around 62 hours for saturation if the salt absorbed all the CO₂ passing through it.