Carbon Dioxide Removal by Ionic Liquid Sorbent …...VLE = vapor liquid equilibrium Wt % = weight percent I. Introduction HE Carbon Dioxide by Ionic Liquid Sorbent (CDRILS) system

48th International Conference on Environmental Systems ICES-2018-17 8-12 July 2018, Albuquerque, New Mexico

Copyright © 2018 Honeywell International Inc.

Carbon Dioxide Removal by Ionic Liquid Sorbent

(CDRILS) System Development

Stephen F. Yates1 and Rebecca J. Kamire2

Honeywell Aerospace Advanced Technology, Des Plaines, IL, 60017

Phoebe Henson3 and Ted Bonk4

Honeywell Aerospace Defense and Space, Glendale, AZ, 85308

Using a liquid absorbent like an ionic liquid eliminates many of the disadvantages of solid

adsorbent systems for carbon dioxide (CO2) removal from air in deep space missions. Systems

built around a liquid absorbent avoid complicated valve networks to switch between

absorbing and desorbing beds. Continuous flow processing delivers an even flow of product

carbon dioxide and has the potential to provide a more robust system overall. Ionic liquids are

particularly desirable for space applications since they are non-volatile, non-odorous, and

have high oxidative stability. The CDRILS system pairs hollow fiber membrane contactors

with ionic liquid absorbent to provide rapid, continuous CO2 capture and recovery of pure

CO2 from the liquid.

Significant progress has been made in the development of the CDRILS system for use in

life support applications. Membrane contactors have been designed that provide high surface

area without allowing escape of the liquid, and the long-term reliability of both contactors and

ionic liquid has been assessed. Using measured CO2 and water capacities and mass transfer

coefficients, alternative system designs have been evaluated to identify those that maximize

performance while minimizing weight, volume and power consumption. Because water is

strongly absorbed by most ionic liquids, water management is a key focus in designing the

closed-loop system. Determination of optimized operating conditions and the optimum system

design will allow scale up of lab-scale experiments to a full-size unit capable of removing 4.16

kg/day of CO2.

Nomenclature

BMIM Ac = 1-butyl-3-methylimidazolium acetate

BMIM BF4 = 1-butyl-3-methylimidazolium tetrafluoroborate

CDRA = Carbon Dioxide Removal Assembly

CDRILS = Carbon Dioxide Removal by Ionic Liquid Sorbent

CMS = Carbon Dioxide Management System

CO2 = carbon dioxide

DGA = diglycolamine

DTA = differential thermal analysis

ECLSS = Environmental Control and Life Support System

EMIM Ac = 1-ethyl-3-methylimidazolium acetate

EMIM BF4 = 1-ethyl-3-methylimidazolium tetrafluoroborate

H2O = water

ISS = International Space Station

LTL = Low Temperature Loop

MEA = monoethanolamine

MFC = mass flow controller

1Research Fellow, Engines and Air Management, 50 E. Algonquin Road, Des Plaines, IL. 2Scientist III, Engines and Air Management, 50 E. Algonquin Road, Des Plaines, IL. 3Systems Engineer II, Human Space Advanced Development, 19019 N. 59th St., Glendale, AZ. 4Engineer Fellow, Space Applications, 19019 N. 59th St., Glendale, AZ.

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International Conference on Environmental Systems

ppm = parts per million

RH = relative humidity

TGA = thermogravimetric analysis

VLE = vapor liquid equilibrium

Wt % = weight percent

I. Introduction

HE Carbon Dioxide by Ionic Liquid Sorbent (CDRILS) system was first described in 2016,1 and more completely

described in 2017.2 It is a new alternative approach to the removal of carbon dioxide from the air in an occupied

environment that relies on the use of ionic liquid sorbents instead of solid adsorbents. The concept is simple; the air

to be treated is first contacted with an ionic liquid that has a specific affinity for carbon dioxide over the other air

components, removing the carbon dioxide (Figure 1). This loaded ionic liquid is then pumped to a separate contactor

in which the pure carbon dioxide is recovered for delivery to a downstream process such as a Sabatier reactor. The

use of a liquid sorbent brings with it a number of significant advantages, including simplicity of operation and ease of

maintenance. Unlike solid adsorbents, which must be returned to a gravity environment for replacement at end of life,

liquid sorbents offer the possibility of replacement in zero gravity. Because the sorption and desorption steps are

physically separated, less oxygen or nitrogen will contaminate the product carbon dioxide, and each contactor can be

optimized separately. The complicated network of valves and controls needed for solid adsorbent systems is

eliminated. Because the same ionic liquid can be cycled back and forth between absorption and desorption rapidly, a

lower inventory of this liquid is needed, and the size and weight of the overall system is reduced. We have screened

various ionic liquids and identified a subset that have high CO2 capacity, reasonable viscosity, and excellent stability.

Because these are ionic liquids, they have very low vapor pressure, which prevents loss of working volume, simplifies

separation of product gases from the absorbent, and reduces exposure of the crew to the liquids. These liquids are

generally considered to be safe, and a review based on the Safety Data Sheet (SDS) by NASA JSC Toxicology3

provided our ionic liquid with a preliminary toxicity hazard level of 0, the lowest rating. A toxicological analysis

based on testing is needed to fully understand the toxicity of the liquids.

The idea for CDRILS stemmed from our review of the liquid sorbent CO2 removal systems used in submarines.

These systems have been in use for a long time, and are considered reliable and effective. They have not been used in

space applications because the sorbent liquid, monoethanolamine, has toxicity and odor challenges that make it

unsuitable for a closed space environment. Like all amines, monoethanolamine is air oxidized over time, generating

additional volatile air contaminants of concern. We reasoned that replacing the amine with an ionic liquid would retain

most of the advantages of this system, but remove many of the problems. At first, we retained a direct liquid contact

system similar to that in submarines, generating a spray of ionic liquid that efficiently removes carbon dioxide from

air. Addition of a centrifugal contactor allowed the system to be used in zero gravity. However, generating a mist with

the appropriate gas/liquid ratio at the scale required for a space environment proved to be complicated, and we

identified an alternative approach using hollow fiber membrane contactors suitable for gravity independent operation.

This is now our preferred approach, and the results in this paper pertain to this approach.

T

Figure 1: General CDRILS schematic.

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II. Ionic Liquids for CO2 Removal and Water Management

While the purpose of the CDRILS system is carbon dioxide removal and recovery, the handling of humidity and

water has a major effect on the system design. For a crew of four, the CDRILS system must remove 4.16 kg/day of

carbon dioxide from the air.4 Assuming a CO2 partial pressure of 2 torr, this requires, at a minimum, processing 644

L/min of air. If this air has a relative humidity of 50%, the air would also contain 10.5 kg/day of water. Essentially all

sorbents that have been shown to remove carbon dioxide also have an affinity for water, and the air in an occupied

environment will have at least 50% relative humidity. In particular, ionic liquids are quite hygroscopic, and will readily

remove essentially all the water vapor from the incoming air. Fortunately, co-sorption of water with CO2 does not

have a very negative effect on the ionic liquid capacity or mass transfer, so long as water concentrations in the liquid

do not become extreme. However, water management becomes important since water desorption must equal water

absorption to avoid dilution of the liquid.

To select the water concentration to be used in our ionic liquid, we must understand the effect of water on key

performance properties, such as CO2 capacity, since this directly influences the gas/liquid ratio required for effective

removal of the CO2 from the air. We also need to understand the effects of water concentration on viscosity, since

viscosity has a major effect on the liquid phase mass transfer coefficient. In addition, we recognize that the heat of

absorption of water in our ionic liquid is significant, and that changes in temperature as a result of absorption or

desorption can play a significant role in performance.

Ionic liquids are salts, generally comprised of an anion and organic cation, that are liquid at their temperature of

use. They have effectively zero vapor pressure, eliminating odors and reducing the likelihood of contaminating the

purified air. They are generally nontoxic, and have sufficient stability to resist deterioration. Ionic liquids typically

contain relatively large organic cations (quaternary ammonium, imidazolium or phosphonium compounds) and any

of a variety of anions, both of which can be tailored to obtain desired characteristics. As a result of this versatility,

there are many to choose from and they are often called “designer solvents.” Also, they are relatively easy to make,

and major chemical manufacturers have begun to make major classes of ionic liquids available in large quantities.

Honeywell has chosen to focus on two ionic liquids for the CDRILS application: 1-butyl-3-methylimidazolium acetate

(BMIM Ac) and 1-ethyl-3-methylimidazolium acetate (EMIM Ac), due to the good CO2 capacities, high safety, high

stability, and low viscosity of these liquids. However, with many ionic liquids to choose from and frequent

introductions of ionic liquids with new properties, a higher capacity, lower viscosity ionic liquid may be chosen in the

future to decrease the size of the system.

A. Ionic liquid capacity vs. water content. Based on the work of Stevanovic,5 it is known that the capacities of

BMIM Ac and EMIM Ac decrease with increasing water content, and with increasing temperature. However, these

studies were performed at a CO2 partial pressure of 20-180 torr, quite a bit higher than the 2 torr partial pressure of

greatest interest for life support applications. We therefore needed to run experiments at 2 torr for use in our scrubber

model to lend understanding to our scrubber and stripper performance studies. We ran these studies in two ways. In

the first approach, we used thermogravimetric analysis in a somewhat complicated experiment. We began by heating

a sample of EMIM Ac in the TGA under nitrogen to a temperature of 70°C to remove as much water as possible from

it. We cooled to the temperature of interest, and then introduced water into the nitrogen stream to achieve a

concentration of 0.0-1.7 mmol/mol. The ionic liquid sample increased in weight by 0.1-11.5% as expected then

plateaued. The humidified nitrogen was then replaced with a custom mixture of nitrogen containing 2600 ppm of CO2

at the same humidity. A weight increase again occurred. From the weight gain on introduction of water, we can obtain

the water concentration in the ionic liquid, and from the further weight gain on introduction of CO2, we can obtain the

CO2 capacity of the ionic liquid at the measured temperature and water content.

This approach is complicated and relies on the assumption that the capacity of the liquid for water is unchanged

when CO2 is present. It also suffers the concern that water might not be completely desorbed in the first step, putting

the water concentration values in question. For example, the water content for each of the two TGA samples without

added humidity is plotted as “zero” in Figure 2b, though trace residual water remained in the samples. As a result, a

second method was also developed. In this approach, we used a lab-scale scrubber module comprising 120 hollow

fibers in a module 36 cm long. Ionic liquid was circulated through this system by pumping it through the tube side of

the module and then back into the feed reservoir (Figure 2a). Both the module and the ionic liquid were controlled at

the same temperature. At first, feed nitrogen was humidified to 0.5-5 mmol/mol by bubbling a portion of the stream

through water. This humidified nitrogen flowed through the shell side of the contactor and was monitored for humidity

and CO2 concentration at the outlet. The purpose of this step was to equilibrate the liquid with the humidity in the air

and to desorb any CO2 present in the liquid. Once the CO2 and water in the liquid were in equilibrium with the

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humidified nitrogen stream, the feed was switched to nitrogen containing 2600 ppm of CO2 at the same humidity. CO2

was then absorbed, as measured by the CO2 monitor, until the ionic liquid became equilibrated to this feed, and the

concentration in the product air became the same as the feed. By integrating the difference between the feed CO2

concentration and the product CO2 concentration over this interval, the CO2 capacity could be obtained. The final

water concentration in the ionic liquid was confirmed by Karl Fischer titration.

Figure 2b combines the results from both experimental methods. They are in reasonable agreement with each

other, and show that the capacity of the ionic liquid for CO2 decreases with increasing water concentration. This data

is useful in selecting the initial water content to be used in the scrubber. We must recognize that in the CDRILS

scrubber, both CO2 and water are simultaneously absorbed. Thus, the capacity of the ionic liquid for CO2 at the point

where it enters the contactor will be higher than at the point where it leaves the contactor. From Figure 2b, we see that

this will have significant effect on capacity. Further, the Figure demonstrates that elevated temperature in the stripper,

aided by reduced pressure on the shell side of the module, can be used to provide driving force for desorption. We

estimate the heat of desorption of CO2 from EMIM Ac containing 5% water using the Clausius-Clapeyron equation

to be about 23 kJ/mol.

B. Ionic liquid separation efficiency for carbon dioxide over nitrogen and oxygen. The CDRILS system is

capable of higher separation efficiency for carbon dioxide over the other permanent gases than current approaches

involving solid adsorbents. This is not because the ionic liquids themselves are more selective; for example, the

selectivity of the 5A zeolites used in the Carbon Dioxide Removal Assembly (CDRA) for CO2 over other gases is very

high. Instead, it comes from the system design. In a system with solid sorbents, there will always be a point in the

cycle in which the bed of sorbents must be switched from adsorption mode to regeneration. At this point the bed is

still filled with air. The designer is presented with poor choices; one must either pump this air into the regeneration

stream, contaminating the CO2 product, or begin regeneration by pumping this air back into the cabin, losing

performance. CDRILS does not have this issue, since the scrubber and stripper are physically separated.

Since only the ionic liquid is transported from the scrubber to the stripper, the ratio of nitrogen or oxygen to carbon

dioxide at the outlet of the stripper will simply be the ratio of the solubilities of these gases in the liquid, after

accounting for differences in partial pressure of the gases in the cabin air. The values in Figure 2b correspond to the

solubilities of carbon dioxide under our experimental conditions. Solubilities of nitrogen and oxygen have not yet

been measured under the same conditions, but we can refer to the literature for comparison values. Solubilities of

nitrogen and oxygen have not been measured in EMIM Ac, but have been measured in the very similar EMIM

tetrafluoroborate (EMIM BF4) and BMIM tetrafluoroborate (BMIM BF4). Since neither acetate nor tetrafluoroborate

have any specific affinity for nitrogen or oxygen, the solubilities of these gases in these liquids should be similar.

Jacquemin et al.6 reported the Henry’s law constant for oxygen in BMIM BF4 at 293K to be 1552 bar, and for nitrogen,

1646 bar. Finotello et al.7 reported a value of 3850 bar for nitrogen at 313K for EMIM BF4. Estimates based on these

literature values and the CO2 capacity data in Figure 2b indicate that CDRILS will meet the CO2 purity requirements

for the Sabatier reactor feed. Experiments are underway to measure the solubilities of nitrogen and oxygen in the

acetate-based ionic liquids in our lab.

Figure 2a: Lab-scale scrubber schematic. Figure 2b: Capacity of EMIM Ac for CO2 as a

function of water content.

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C. Ionic liquid affinity for water. Just as we

require capacity data for the absorption of CO2 by

the ionic liquid, we also require water capacity data.

Our ionic liquid does not form a 1:1 complex with

water, but like most ionic liquids is very

hygroscopic. Vapor liquid equilibrium (VLE) data

for water in the water-EMIM Ac system has been

measured by Römich,8 but the lowest measured

value at 20°C corresponded to a water mole fraction

of 0.687, and at 60°C, a mole fraction of 0.494.

Lower values are expected in our system, so

experimental measurements were made. The

experiment was, in fact, the same experiment as

those to measure CO2 capacity, since in each

experiment air with a known concentration of water

in air was introduced, and after equilibrium was

reached, water concentrations were measured by

Karl Fischer titration. Experiments were completed at 20, 35, 50 and 65°C. Figure 3 compares our experimental values

with those obtained from exponential extrapolation of the Römich vapor pressures versus water mole fraction and

temperature. Good agreement was observed between our extrapolations of the literature data and our experimental

values.

It is also important to consider the heats of

absorption of both CO2 and water in our design. As

we have already shown, the capacity of the ionic

liquid to CO2 is sensitive to temperature, so we control

the temperature of the ionic liquid as it enters the

scrubber. However, absorption of water and CO2 is

exothermic, so the temperature of the ionic liquid will

rise as absorption occurs, potentially reducing the

effective capacity. Similarly, in the stripper,

desorption of water and CO2 will cool the ionic liquid,

limiting how much CO2 and water can be removed.

We can use the Clausius-Clapeyron approach to

estimate the heat of absorption of water in EMIM Ac

directly from the Römich data by using the fitted

curves to interpolate between experimental values and

allow direct comparison of vapor pressures at the

same liquid water concentration, but different

temperatures. Figure 4 shows how the heat of vaporization varies with liquid phase water concentration.

As an experimental check on this approach, we also measured the heat of solution of water directly using a

combined thermogravimetric analysis-differential thermal analysis (TGA-DTA) approach. Using samples of EMIM

Ac containing various initial concentrations of water, we heated these to 95°C and held them at this temperature.

Water evaporated from the ionic liquid, and the weight change resulting from this evaporation was measured by TGA.

Simultaneously, by DTA, we measured the heat required to keep the sample at the required temperature. Knowing

both the quantity of water evaporated, and the heat required, we were able to measure the heat of desorption as a

function of the liquid phase water concentration. Figure 4 shows these results. We observe that at higher water

concentrations, the measured heat of desorption is less negative than the value from the Clausius-Clapeyron approach,

but as the liquid phase water concentration decreased, the heat of desorption sharply decreased. This reflects the

difference between loosely coordinated water and water in solvation shells closely associated with the cation and

anion. This result also emphasizes the difficulty in completely dehydrating a sample of ionic liquid. Driving the water

concentration below about 2 wt% (mole fraction 0.165) requires supplying a much higher heat of solution, and is

correspondingly difficult. We have chosen to use our ionic liquid with water concentrations between 5 and 10 wt%

(0.33 and 0.5 mole fractions).

Figure 3: Vapor liquid equilibrium for water in EMIM Ac.

.

Figure 4: Heat of desorption of water from EMIM Ac.

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D. Effect of water on ionic liquid viscosity. Viscosity is an important property for the ionic liquid because of its

effect on mass transfer. As we explained in greater detail previously,2 mass transfer of either CO2 or water is expected

to be dominated by mass transfer in the liquid phase. Liquid phase mass transfer is strongly influenced by viscosity,

making reducing the viscosity an important priority. Since water absorption inevitably results in at least some water

concentration in the ionic liquid, we use this water diluent to our advantage as a means of reducing the ionic liquid

viscosity.

Stevanovic5 and Almeida9 have separately reported viscosity measurements for BMIM Ac and EMIM Ac as a

function of temperature and water concentration, so we limited our viscosity measurements to the temperature and

water concentration ranges of interest for our application. Measurements were made using samples of each liquid to

which water was added to the final concentrations listed in Figure 5. A Brookfield CV-II+ viscometer was used at a

shear rate of 28 s-1. We separately observed that the viscosities we measured were insensitive to shear rate over the

range 1.4 – 132 s-1. We observe that viscosity decreases with increasing water concentration and with increasing

temperature. At comparable conditions, BMIM Ac is more viscous than EMIM Ac. The effect of CO2 content on

viscosity is negligible in the presence of water, which is favorable for CO2 capture in narrow hollow fibers where

dramatic increases in viscosity could be problematic. These measured values are useful in estimating mass transfer

coefficients for comparison with experimentally measured values.

E. Effect of water concentration in ionic liquid on scrubber performance. Using the laboratory test stand

shown in Figure 2a, we have also investigated the effect of water concentration on the rate of mass transfer of CO2

and water from the gas side to the liquid side of the membrane. All experiments were run using 2600 ppm CO2 in

nitrogen as the feed, at various inlet gas temperatures and relative humidities; the CO2 concentration remaining in the

Figure 5: Effect of water content and temperature on viscosity of EMIM Ac and BMIM Ac.

Figure 6: Effect of water content in EMIM Ac, relative humidity, and temperature on CO2 and H2O permeate

flux.

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gas stream was measured after a single pass through the scrubber. Figure 6 shows the effect of the water concentration

in the ionic liquid on CO2 mass transfer, expressed as permeate flux. We observe that, under all the conditions tested,

mass transfer rate decreased with increasing water content. Increasing temperature increased the rate, as expected, but

changing the relative humidity of the gas stream from 8% to 36% had very little effect. This trend with water

concentration is similar to the trend for capacity, but is the opposite of what would be expected based on ionic liquid

viscosity. Water mass transfer was also measured by the same technique and also showed a trend for decreased mass

transfer with increasing water concentration.

Using capacity data and this mass transfer data, we have the basic data needed to scale up the scrubber process

from lab-scale to the anticipated full scale. The results of this scale up calculation are reflected in designs to be

discussed later in this paper. Since the flux of water into the ionic liquid is almost a factor of 10 higher than the flux

of carbon dioxide, it is the flux of carbon dioxide that determines the size of the scrubber and stripper modules. The

water concentration will increase down the length of the contactor, resulting in reduced capacity and reduced mass

transfer at the outlet. These relationships have been used as we have developed scaling models for the CDRILS system.

III. Reliability of Ionic liquids and Membrane Contactors

Use of a liquid sorbent for CO2 removal is well-known in terrestrial applications and is routine in industrial use.

Ionic liquids are not used in these large systems because they are new and expensive, and do not offer enough

advantages over cheaper solutions. In a space environment, where the lack of appreciable vapor pressure is a major

advantage, and the partial pressure of CO2 is much lower, ionic liquids become much more attractive. The use of

membrane contactors with ionic liquids is relatively new, and solves many of the problems associated with older

generation liquids like aliphatic amines. In particular, the membrane-ionic liquid interface is well understood.10

Nonetheless, since this is a new technology, it is important to understand its reliability. A number of long term stability

experiments are underway in our laboratory to improve our understanding of the life of components in the CDRILS

system, and are currently incomplete. In this section, we describe studies to establish the limits of stability for

materials, and provide in-progress results from the long stability experiments.

A. Ionic Liquid Stability. The stability of the ionic liquid sorbent over the long working life of the CDRILS

technology is a key advantage for this system. In use, the ionic liquid is continuously contacted with air from the

cabin, and is also cycled between ambient temperature and 50-60°C. Both thermal and oxidative stability are therefore

quite important. Liquid-based CO2 sorption systems using aliphatic amines like monoethanolamine have been widely

reported, but these are unsuitable for use in an ECLSS application because of the short life of the liquid, and its

potential to generate air contaminants including ammonia and aldehydes.11 The observation of a similar loss of

ammonia from the solid amine SA9T shows how this degradation mode results in similar impacts on the cabin air

quality.12 Limited information is available for thermal stability for ionic liquids. Loss of the liquids by evaporation is

minimal due to their extremely low vapor pressure. Cao et al.13 studied EMIM Ac and BMIM Ac by thermogravimetric

analysis, and reported onset temperatures for thermal decomposition of 221°C and 216° respectively, but this refers

only to rapid decomposition. Little data is available at lower temperatures. As a result, we have begun studies in which

EMIM Ac is sparged with dry air, both at room temperature and at 60°C, to confirm that it will remain stable under

these conditions. A sample of EMIM Ac has been sparged with dry air for five months, and a second sample has been

Figure 7: ATR-FTIR spectra before and after aging EMIM Ac under bubbling air at 25°C and 60°C.

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sparged at 60°C for one month. The samples have been compared with control samples by ATR-FTIR and by

elemental analysis. Figure 7 shows the ATR-FTIR results as an overlay. We observe that the only detectable difference

between the spectra before and after aging is the band at 3400 cm-1 corresponding to water that has been absorbed

from the air by the liquid. Table 1 shows combustion analysis results for carbon, hydrogen and nitrogen for the same

samples. We observe a systematic discrepancy between the experimental values and those predicted from the

stoichiometry of EMIM Ac, but very little change has so far been observed for the chemical composition of our liquid.

This experiment is ongoing.

To better understand oxidative stability, we completed a comparison of BMIM Ac and EMIM Ac with

representative amines, monoethanolamine (MEA) and

diglycolamine (DGA) using cyclic voltammetry. The

ionic liquids were used as mixtures containing 10%

water, and the amines with 35% water and 0.1 M sodium

hydroxide. Cyclic voltammetry is done in an

electrochemical cell; the voltage is swept and the current

is observed. Figure 8 shows the results of this analysis.

Focusing on the oxidative side of the voltammogram, we

see that little oxidation occurred until a voltage higher

than 1.5 V vs. Ag where water is oxidized to make

molecular oxygen. A small oxidative wave at 1.3 V and

reductive wave at 0.2-0.3 V are likely due to

contamination of the samples with chloride, as also

observed by Ni et al.14 In contrast, irreversible oxidation

waves for DGA and MEA peak at 0.81 and 0.55 V,

respectively. The irreversible oxidation of the amines at

low potentials where the ionic liquids are nonreactive

demonstrates the improved oxidative stability of the

ionic liquids over the amines.

B. Membrane Contactor Stability. The hollow fiber membrane contactors used for both the scrubber and the

stripper have similar construction. Specialized hydrophobic fibers are used. In a laboratory-scale module, 50-4,000

fibers may be used, while in a full-scale module, the number of fibers used can be >50,000. These fibers are secured

at either end by an epoxy tubesheet which serves as the seal between the shell and tube sides of the membrane, and

are enclosed in a rigid shell. We were interested in the durability of these materials with prolonged exposure to the

ionic liquid.

To evaluate durability of the modules to exposure to ionic liquid the test stand in Figure 2a was again used. Three

lab-scale modules with 48 membrane fibers each were aged with ionic liquid inside the fibers at room temperature

and tested for CO2 removal performance after 1, 15 and 31 days of exposure. Four additional modules were aged at

60°C and tested at the same intervals. The averages and standard deviations of the performance of the modules are

Table 2: Performance of lab-scale membrane

contactors after aging in contact with ionic liquid.

Days of aging

Average % CO2 removal

Aged at room temperature Aged at 60°C

1 20.6 ± 0.4 21.5 ± 0.6

15 22.9 ± 0.3 21.4 ± 0.5

31 21 ± 1 21.2 ± 0.5

Table 1: Combustion analysis for carbon, hydrogen and nitrogen

for EMIM Ac after extended sparging with dry air. Temperature (°C) Time (days) C H N C/N ratio Remainder

25 0 57.7 8.68 17.1 3.37 16.5

25 160 57.1 8.66 17.1 3.34 17.1

60 0 57.6 8.65 17.2 3.35 16.6

60 16 57.6 8.64 17.1 3.37 16.7

60 30 57.9 8.64 17.2 3.37 16.3

Stoichiometric for EMIM Ac 56.5 8.29 16.5 3.43 18.8

Figure 8: Cyclic Voltammograms for EMIM Ac and

BMIM Ac as mixtures with water, and DGA and MEA

as mixtures with water and 0.1 M sodium hydroxide.

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reported in Table 2 and show no significant difference in performance over the period tested at either temperature,

which indicates the stability of the fibers to the ionic liquid. Longer term aging experiments are ongoing.

To determine the durability of the epoxy tubesheet

with prolonged exposure to ionic liquid, we have begun

an experiment in which we measure the effect of the

liquid on mechanical strength. The results after 15 and

30 days of exposure are reported here, but the study is

ongoing. Samples of the tubesheet both filled with

hollow fibers and without hollow fibers were molded

into 1/8” × 1/2” × 3” bars, and immersed in EMIM Ac

at either room temperature or 60°C. A control set was

stored for similar lengths of time without ionic liquid

exposure. Five samples for each set of conditions were

tested using a short beam shear test (ASTM 2344) to

determine the effect of exposure on the strength of the

composite. The mean and standard deviations are shown

in Figure 9. We observed no statistically significant

difference between the control sample and the exposed

samples, and did not observe any trends in strength as a

function of duration of exposure. Based on the current

results, there appears to be no reason to believe that the

tubesheet is negatively affected by ionic liquid

exposure.

In a separate experiment, we are interested in

whether ionic liquid might leak through a stripper

module when exposed to vacuum for an extended

period. As we have noted before, due to the hydrophobic

nature of the porous membrane fibers, the ionic liquid is

not expected to penetrate the pores from the liquid side

to the gas phase, while gas phase components like CO2

or water can permeate easily. To confirm this

expectation, we built the simple leak experiment shown

in Figure 10. A bundle of 750 hollow fibers was potted

in a module with a 1” diameter. The fibers were open to

an ionic liquid reservoir on the top of the module and

sealed on the bottom. The apparatus was heated at 60°C

with vacuum of 10 torr pulled on the shell side of the

fibers. After 2 weeks of testing to date, there is no detectable ionic liquid on the shell side of the fibers. This

corresponds to a negligible leak rate of <1.6 x 10-7 g m-2 s-1.

IV. The CDRILS System Architecture

A. Benefits of a Liquid System Architecture over a Solid System Architecture. The CDRILS system is capable

of higher CO2 removal rates than systems using solid adsorbents while using lower volume and weight. This benefit

results from the high surface area of the hollow fiber membrane contactors, and the increased efficiency in sorbent

utilization that comes from a liquid system. In a system with solid sorbents, the sorbent bed fills to capacity and then

must be switched from absorption mode to regeneration mode. Compromises are inevitable, since, as the bed nears

saturation, it becomes less effective, but if the switch to regeneration is made earlier, less CO2 has been adsorbed.

Temperature and pressure swings introduce more inefficiencies, since the sorbent beds take time to come to

appropriate temperature and pressure, and during the transition, are neither efficiently adsorbing or desorbing.

Switching from adsorption to desorption also presents an opportunity for air contamination of the CO2 product stream

and downstream Sabatier system, since, at the switch over time, the bed is air-filled.

In a liquid system, scrubbing and stripping occur separately, and thus, absorption and desorption each occur at

their most optimal temperature and pressure. The optimal temperature for stripping is lower than for zeolites, so less

heating power is required. The estimated heat of desorption of CO2 from EMIM Ac of 23 kJ/mol is significantly lower

Figure 9: Short beam shear strength for tubesheet

samples aged at 25°C and 60°C (average ± standard

deviation from 5 samples).

Figure 10: Fiber leak test.

60°C

Ionic liquid reservoir

P

vent

Collection of any leaked liquid

<10 torrT

10


than the 52 kJ/mol required for 5A zeolites.15 Continuous flow of liquid allows for insertion of a recuperative heat

exchanger between the scrubber and stripper that dramatically reduces the heating and cooling requirements further.

Also resulting from continuous flow, the capacity of the sorbent is never reached. The system is instead operated at a

steady state below sorbent capacity where CO2 capture is most effective. Concerns with introduction of oxygen and

nitrogen when switching between beds in solid systems are avoided in CDRILS because the vacuum pump maintains

the vacuum pressure in the stripper alone and is never exposed to cabin air. The benefits of these design factors on

size, weight, power and cooling are discussed further in Section VI.

A. Utility of adding dehumidification to CDRILS architecture. It is possible for a CDRILS system (Figure 1)

to operate without any additional equipment, but it imposes demands on the compressor/condenser system to handle

the water load. Consequently, we are evaluating alternative designs that take advantage of membrane dehumidification

to simplify downstream steps and decrease the size and volume of the overall system. Figure 11 shows one such

approach. In this approach, the cabin air, containing CO2 and water, is introduced into a scrubber via a dust filter and

a blower. The scrubber takes the form of a membrane contactor in which the air passes, in a countercurrent direction,

over a packed array of hollow fibers through which ionic liquid is pumped. Both CO2 and water are absorbed by the

liquid, leaving dry clean air to be returned to the cabin. The loaded liquid passes through a heat exchanger and then a

heater to bring it to the regeneration temperature. Only a modest temperature rise is needed for our ionic liquids so the

liquid temperature entering the stripper is about 60°C. In the stripper, a vacuum is applied, removing both the water

and the CO2 from the liquid. Cooling for condensation of the stripped water and cooling of the liquid for return to the

scrubber are provided by an external coolant loop similar to the Low Temperature Loop (LTL) on ISS.

As we noted above, our ionic liquid sorbent will contain, by design, some water, and the pressure in the stripper

will be adjusted to return the ionic liquid to this target concentration by removing the water absorbed in the scrubber.

This water must be removed from the product stream, since the specification for water in the CO2 product is that it is

below saturation level at an ambient temperature of 65°F.16 Compression of the product stream will increase the vapor

pressure of water to well above the saturation vapor pressure, and condensation will occur. Cooling is required, since

the heat of vaporization will be released in this process, and a water separator will be required to remove this water at

zero gravity. In Figure 11, we add a membrane dehumidifier to this process. Nafion membrane dehumidifiers are

becoming increasingly well developed, and Paragon17 has recently demonstrated how useful such a dryer can be in a

space environment. In this application, the dehumidifier is introduced between the water separator and the outlet of

the CDRILS process as a polishing step to further reduce the humidity in the CO2 product. A sweep gas is generated

by taking a small quantity of the inlet stream and dropping the pressure through a needle valve before introducing it

to the permeate side of the membrane. Since this stream then is connected to generate a sweep gas for the stripper, it

has reduced pressure due to its fluid connection to the compressor. Water stripped from the CO2 in this polishing step

will then be combined with water removed from the ionic liquid and removed.

Another alternative design is shown in Figure 12. This design preserves the basic architecture of CDRILS, but

moves the membrane dehumidifier to the front. Thus, inlet air from the blower passes into the dehumidifier, where

most of the humidity is removed. It then enters the scrubber, where CO2 and more water is removed. The product air,

now purified and very dry, becomes the sweep gas for the dehumidifier. The humidity that was removed from the inlet

air is transferred to this stream, re-humidifying it. This stream becomes the product air stream.

Figure 11: CDRILS system design with membrane dehumidifier post-processing.

Condenser

Cooler Heater

Cabin Air: N2,O2, CO2, H2O,contaminants

Clean CabinAir: N2, O2

Finalfilter

CleanIonic

Liquid Storage

Ionic Liquidwith CO2, H2O

Liquid water

CO2

CABIN

CO2 STORAGE / SABATIER

WATER STORAGE

CO2, H2O

Trace CO2, watervapor

Dustfilter

Blower

CompressorClean

Ionic Liquid

HOLLOW FIBER

MEMBRANE SCRUBBER

HOLLOW FIBER

MEMBRANE STRIPPER

MEMBRANE DEHUMIDIFIER

HX

Pump

Waterseparator

Needle valve

LTLLTL

Pump

11


These alternative designs have both advantages and disadvantages. For both, adding a component adds complexity,

component weight and volume. If the dehumidifier is placed before the scrubber, then it must be sized to the flow rate

of the inlet air. This flow rate is determined by the requirement that 4.16 kg/day of CO2 is removed, and the efficiency

of the scrubber. Removing humidity before the air reaches the scrubber simplifies control of the water content in the

ionic liquid, and reduces both the heat generated by absorption of water in the scrubber, and the heat required for

stripping this water. Since, in the original design, the stream exiting the stripper was at least 70% water vapor,

removing this water vapor before the scrubber reduces the flow rate of this stream, allowing the compressor and

condenser to be downsized. It does not change the pressure required in the stripper, since this is determined by the

chosen water concentration in the ionic liquid. Reducing the compressor and condenser size reduces weight, volume,

power for the compressor, and cooling for the condenser.

Placing the dehumidifier after the compressor and condenser has different effects. In this location, it is sized for

the product stream, which is at least fifty-fold lower in flow rate than the inlet stream. By supplementing the water-

removal capability of the condenser, it reduces the load on this unit. Splitting the product stream reduces the efficiency

of CO2 recovery at the dehumidifier, but the produced sweep gas improves the efficiency of CO2 recovery at the

stripper. It also increases reliability, since variations in the temperature of the coolant loop will no longer directly

result in variations in the water content of the product stream.

Discussion of two air impurities is important to any discussion of the use of Nafion in a life support application,

ammonia and carbon dioxide. Ammonia18 is a known bioeffluent that contaminates air on the international space

station and is expected on any manned mission. Since the Nafion polymer is strongly acidic, ammonia can react with

it in an acid-base equilibrium reaction, and this has been shown to negatively affect performance of the membrane.19

When the dehumidifier is placed after the CDRILS scrubber and stripper, it is less likely ammonia will have any effect

on the Nafion membrane, since it will be very soluble in the ionic liquid and will likely be removed along with water

before the gas stream reaches the dehumidifier. If instead, the dehumidifier is placed before the CDRILS scrubber,

then it will encounter ammonia at bioeffluent concentrations. Determination of the equilibrium loading under these

conditions, and the effect on steady state performance should be performed. In either placement, lower concentrations

of ammonia will contact the Nafion membrane than in amine-based systems because ionic liquids do not degrade to

produce ammonia.

While the selectivity of Nafion membranes for water over air components is well established, there are reports20

that this membrane is significantly more permeable to CO2 when hydrated. To evaluate whether this is a potential

concern, we measured CO2 mass transfer through the membrane from a humid air stream. A stream of air containing

2 torr CO2 and 12-15 mmol/mol humidity (% relative humidity) was flowed through the lumen of Nafion fiber, while

dry nitrogen was flowed countercurrent through the shell. Under flow and pressure conditions where up to 66% of the

humidity was transferred to the dry stream, no CO2 transfer was observed within the detection limits (<0.02 torr loss

in CO2 partial pressure). At rates below these limits, CO2 loss through the Nafion membrane is not a performance

concern.

Figure 12: Alternative CDRILS design with membrane dehumidifier pre-processing.

Cooler

Heater

Cabin Air: N2,O2, CO2, H2O,contaminants

Clean Cabin Air:N2, O2, H2O

Finalfilter

CleanIonic

Liquid Storage

Ionic Liquid withCO2, trace H2O

CO2

CABIN

SABATIER

WATER STORAGE

CleanIonic Liquid

Dustfilter

Blower

Compressor

HOLLOW FIBER

MEMBRANE SCRUBBER

HOLLOW FIBER

MEMBRANE STRIPPER

HX

Pump

Water separator

LTL

LTL

MEMBRANE DEHUMIDIFIER

Liquid water

Condenser

Pump

12


B. Compression of CO2 product and water separation. The

stream exiting the outlet of the stripper will contain carbon dioxide

and water. If no dehumidification is used, this stream will contain

70-80 mole % water, while if dehumidification is used the water

concentration will be less than 10 mole %. Compression of this

stream to near cabin pressure is required, and will increase the vapor

pressure of water vapor to past saturation. Condensation will remove

much of the water for delivery to the waste water system.

Compression of mixtures of steam and CO2 requires a compressor

that is tolerant of a two-phase mixture, with a relatively high

compression ratio. One example of such a system is the two-stage

pump currently used in the Honeywell Carbon Dioxide Removal

Assembly (CDRA), shown in Figure 13. A compressor like this may

be used to achieve this high compression ratio.

V. Estimated System Size, Weight, Power and Cooling

A. Scale-up. The CDRILS unit requires fabrication of hollow fiber membrane contactors for the scrubber and

stripper, a blower, heat exchanger, compressor and condenser. Many of these parts are already available from prior

ECLSS programs, and can be used with small modification, or can be easily designed and fabricated. The membrane

contactors are the heart of the technology, and are built by assembling a large bundle of hollow fibers in an appropriate

shell and potting the two ends to create the tubesheet. Honeywell has experience in scaling this process to full scale,

and has developed a manufacturing process for full scale

modules for a different application. Figure 14 shows

modules during preparation, and a high magnification

view of the tubesheet. Applying the experience we have

gained in scale up for that program makes us confident

that our lab-scale unit can be scaled up with minimal

difficulties.

Based on the capacity and mass transfer data

presented earlier in this paper, and based on the

assumption that CDRILS has a single pass efficiency of

80%, Honeywell estimates that a CDRILS unit sized for

the CO2 output of a crew of four will require a scrubber

with a membrane area of 258 m2, and a stripper with a

membrane area of 214 m2. For both the scrubber and the

stripper, our-scale up approach is based on the use of

equilibrium capacity data to establish a linear isotherm,

and the use of the experimentally determined mass

transfer coefficients to establish the height of a transfer

Figure 13: Two-stage pump for CDRA.

Figure 14: A. Honeywell developmental hollow fiber module 16 cm in diameter

containing 66,000 400 µm fibers. B. Tubesheet of developmental module.

BA

Figure 15: Predicted vs. experimental CO2 removal

efficiency for the lab-scale experiments. The line has a

slope of 1.

13


unit. The operating line is then determined by our selection of the desired capture efficiency and ionic liquid flow rate,

and we calculate the number of transfer units, and hence the membrane area. The robustness of this approach is

demonstrated in Figure 15, in which we compare the model predictions vs. experimental results from experiments

performed with varying flow rates and water contents of liquid and gas streams, temperatures, and module sizes using

our scrubber test stand. The line on the graph has a slope of 1.

B. CDRILS system volume and weight. Based on the 258 m2 of area required for the scrubber, as discussed in

the section above, an internal bed volume of 1.736 ft3 is required for the scrubber. Likewise, similar calculations were

made for the stripper, giving an internal volume of 1.44 ft3. The weight of the scrubber and stripper’s internal bed area

and the ionic liquid inside is expected to be 30.5 kg and 25.3 kg, respectively.

Figure 16 shows a 3D model of a CDRILS system

sized to fit within 2 middeck lockers. This system’s

scrubber and stripper are split into 8 scrubber modules

and 8 stripper modules. In addition, the figure shows

major components including the compressor-condenser,

blower, reservoir of ionic liquid, and various heat

exchangers. The full CDRILS system is expected to

require around 4 middeck lockers and weigh around 190

kg, although improvements are being made to reduce

this further.

Solid sorbent systems such as the CDRA system

desorb CO2 intermittently due to their alternating

sorbent bed architecture, and thus require use of the CO2

Management System (CMS) to integrate with the

downstream Sabatier system. The CMS has a volume of

3.3 ft3, weight of 75 kg, and power requirement of 288

We.16 However, since the CDRILS system generates

product CO2 at a constant rate, the CMS is not

necessary. Instead, CDRILS can provide the required 10

psi CO2 inlet pressure of the Sabatier reactor directly.

Use of the CDRILS system, instead of a solid sorbent

system and the CO2 Management System, would not only save spacecraft operators volume, weight, power and

cooling, but also save logistics and integration costs.

C. CDRILS system power and cooling. As with other CO2 removal systems, heat is required in order to desorb

the CO2 and water, and cooling is required to bring the sorbent back to its optimum state for absorption. Based on the

capacity data reported above and on measured mass transfer coefficients, Honeywell has determined that the optimum

temperatures for absorption and desorption are 35 and 60°C, respectively. Efficient desorption at 60°C is achieved

under reduced pressure, as discussed above. The heating power and cooling required to cycle the ionic liquid stream

flowing at 1 kg/min between these temperatures would typically be 1384 We and 1384 Wh, respectively. However,

use of a moveable liquid sorbent, rather than a static solid sorbent, enables heat to be recuperated between absorption

and desorption. A recuperative heat exchanger of reasonable size is expected to bring the total heating power and

cooling down to 218 We and 218 Wh. With the other components, a total power of 740 We and total cooling of 419

Wh is expected for the CDRILS system.

D. Reliability and longevity. The CDRILS system offers many benefits in reliability. Unlike zeolite- or solid

amine-based sorbent systems, which generate dust or off-gas ammonia, our ionic liquid is contained within

membranes. The negligible vapor pressure along with the high stability discussed above means that the sorbent will

not evaporate or degrade significantly over the course of a mission, and likely can be used for more than one three-

year mission. Extended testing is underway to demonstrate the longevity of the liquid on the mission timescale. If the

ionic liquid were to experience degradation, it could be replaced during a mission with minimal upmass. The mass of

ionic liquid in the entire CDRILS system is expected to be less than 20 kg. Additionally, if the ionic liquid were to

degrade, the flowrate could be increased so as to fail more gracefully at a minor cost in power for the liquid pumps.

The stability of the hollow fiber membranes is anticipated to be similarly high, and, due to the modular approach,

membranes could also be replaced during a mission.

Figure 16: 3D model of the CASIS CDRILS system.

14


VI. Next Steps and Conclusions

Honeywell is committed to continued development of the CDRILS system as a high potential alternative to solid

adsorbent systems for CO2 removal. Our active laboratory program is focused on increasing the efficiency of the

scrubber and stripper and gathering data to refine our scale up model. Larger modules will soon be built, and scale-up

to a half to full-person unit should be complete by the end of 2019. Meanwhile, we are working to design, prototype,

and optimize a vacuum pump and condenser that can sufficiently handle the water exiting the stripper and are

performing trade studies to decide whether we’d like CDRILS to perform humidity removal as well, or if we’d like to

use a membrane dehumidifier upfront of the scrubber. Honeywell plans to launch the first microgravity demonstration

of the CDRILS system on ISS in 2020 through CASIS.

The CDRILS technical approach, using an ionic liquid carbon dioxide sorbent and hollow fiber membrane

contactors has been shown to be an effective method for controlling carbon dioxide. The approach is capable of

maintaining a carbon dioxide partial pressure of 2 torr and provides high purity carbon dioxide, free of nitrogen and

oxygen. Using high surface area membrane contactors, and efficient countercurrent flow, the size and weight of a

CDRILS system is significantly less than for solid adsorbent alternatives, and proposed designs promise to be durable

and easily maintained. Research to quantify the reliability and long term stability of components of the system are

underway, and preliminary data shows no signs of deterioration over 30 days for the ionic liquid or the contactor.

15


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Documents

Carbon Dioxide Removal by Ionic Liquid Sorbent …...VLE = vapor liquid equilibrium Wt % = weight percent I. Introduction HE Carbon Dioxide by Ionic Liquid Sorbent (CDRILS) system