Development of Phase-Change-Based Thermally Modulated Fiber

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Separations

Development of phase-change-based thermally-modulated fiber sorbents Stephen J.A. DeWitt, Héctor Octavio Rubiera Landa, Yoshiaki Kawajiri, Matthew J. Realff, and Ryan P Lively Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04361 • Publication Date (Web): 13 Dec 2018 Downloaded from http://pubs.acs.org on December 14, 2018

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Industrial & Engineering Chemistry Research

Development of phase-change-based thermallymodulated fiber sorbents Stephen J.A. DeWitt, Héctor Octavio Rubiera Landa, Yoshiaki Kawajiri†, Matthew Realff, Ryan P. Lively* School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, GA, 30332, USA *[email protected] Abstract Microencapsulated Phase Change Materials (μPCM) are combined with the Metal-Organic Framework (MOF) UiO-66 and a cellulose acetate fiber support to introduce thermal modulation into CO2 capture devices operating in sub-ambient conditions. μPCM particles are incorporated into sorbent fibers during the fiber spin dope preparation step and are observed to withstand the spinning and subsequent solvent exchange steps with little to no loss of thermal modulating properties as determined by differential scanning calorimetry (DSC). The spinning of this novel sorbent-μPCM fiber sorbent is the first instance of single step spinning of sorbents with thermal modulator. It was found that μPCM weight loading as high as 75 weight percent was attainable while maintaining spinable fibers. Breakthrough adsorption experiments and subsequent temperature profile analysis were collected to compare CO2 breakthrough capacity and heat release for sorbent systems with and without phase change materials incorporated. In adsorption modules with a diameter of 0.455 cm, where heat dissipation through the module wall dominates the global thermal response of the system, modulated fibers showed a 20-25% increase in breakthrough 1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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capacity at short times (CO2 concentration C/C0 = 0.05) as compared to their unmodulated counterparts. Higher breakthrough capacity indicates the phase change material would help manage the heat effects due to the local contact between the μPCM and the MOF. In larger diameter modules (0.7 cm) where wall heat dissipation effects are less dominant than the 0.455cm diameter modules, fibers with “inactive” μPCM (i.e., 50°C below their melting point) show larger sorption-induced thermal excursions and as much as 4x lower capacities at low adsorbate leakage as compared to fibers where the phase change material was active. Through the incorporation of phase change material, the sorbent in the system acts more efficiently, thus potentially driving down adsorption system cost. Keywords: fiber sorbents, structured contactors, Phase Change Materials, heat management, process intensification, CO2 capture

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1. INTRODUCTION The need for energy and cost efficient separations is one key challenge facing society. Consumer products rely heavily on the separation and purification of chemical species; indeed, upwards of 15% of the US’s total energy demand is focused on separations, the majority of which are thermal separations1. Replacement of these thermal separations with more energy efficient processes like adsorption or membranes serves then as a desirable long-term goal for the chemical industry2. Pressure or vacuum-swing adsorptive (PSA or VSA) separations, using highly selective solid sorbents to separate mixtures of gases, are one class of a non-thermal separation. In this approach, beds containing solid sorbent selectively capture one component of a mixture, while allowing the other(s) to flow through. The solid sorbent is then regenerated via reduction of the pressure, i.e., PSA or VSA, and the cycle is repeated. Operation of multiple beds in parallel and proper operation schedules enable continuous conditioning of the feed mixture3, 4. Application of adsorptive separations to large-scale applications such as CO2 capture has been extensively investigated; however, a number of challenges remain such as the cost of large amounts of sorbent to gas pressure drop in industrial-scale beds and effective heat management57

. Focus has been paid in part to developing new sorbent materials exhibiting high operating

capacity and selectivity to help drive down the overall materials cost8. One emergent class of materials are metal-organic frameworks (MOFs). Made up of metal or metal oxo nodes linked together by organic ligands, MOFs have shown great potential as adsorbent materials due to their high surface areas and tunable chemistries. The ability to mix and match metal and linker has created a massive library of structures showing promise for a wide range of adsorption applications9, 10.



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One such MOF, UiO-66—made up of Zr6O4(OH)4 nodes and terephthalic acid linkers—has drawn extensive interest for its outstanding chemical and thermal stability. UiO-66’s stability, high surface area (BET surface areas of approximately 1100 m2/g are typical), and well-developed synthesis procedure make it an interesting material to probe adsorption device performance11-14. Indeed, its high water stability15 allowed it to be spun into a fiber morphology and analyzed for its application to remove trace mercaptan and other sulfur species from natural gas11. Parallel to work creating novel sorbent materials, consideration has been paid to improving adsorption processes by combating the deleterious effects of pressure drop and sorption enthalpy and by improving mass transfer via the use of structured contactors5, 16. Production of such scalable contactors, e.g., fibers, monoliths, or laminate sheets, has the potential to enable higher operating velocities, lower pressure drops and the possibility of containing or removing sorption enthalpy from the bed. Indeed, for propylene-propane separations, structured contactors have shown considerable promise for reducing the sorbent and size requirement of the adsorption system, in great part due to thermal management17. Enabling the efficient use of new sorbent materials with high capacities will require engineered systems to contain and manage heat, as the amount of sorption enthalpy released scales directly with operating capacity and adsorbate flow rate. With this in mind, some authors have considered strategies to manage or remove this heat 18-24. Heat released during adsorption and withdrawn during desorption, termed sorption enthalpy, causes a broadening of the adsorbate breakthrough front due to the temperature increase. This inefficiency in the sorption dynamics leads to additional sorbent being required to reach separation targets. Management of the heat to improve sorption dynamics may serve as one approach to drive down sorbent requirements for adsorptive separations. Moreover, developing new thermal management techniques is certainly highly relevant in the area of CO2 capture, where high flow rates and


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pronounced sorbate-sorbent interactions (i.e., high sorption enthalpies) are the norm. These effects tend to be exacerbated at the industrial scale, where convection to the surroundings is no longer able to manage the heat release25. UiO-66 has many desirable properties as a model material, but at ambient conditions UiO-66 lacks a sufficiently high operating capacity for CO2 and other gas species desired to study heat effects with lab scale experiments. This limitation can be overcome through operation at sub-ambient temperatures and elevated pressures, where UiO-66 displays higher capacities (Figure S1). Achieving these sub-ambient conditions and elevated pressures—while counter-intuitive from a conventional post-combustion CO2 capture perspective—has been shown to be an economical approach for conditioning an incoming flue gas26-29. The development of such a process scheme is not the focus of this article, but congruent operating conditions are used for the experiments discussed here. Methods for managing sorption-induced heat generation use a variety of factors, ranging from bed geometry and sorbent structure to regeneration methodology and utility availability. To simplify the discussion, we propose broadly categorizing different heat management methods based on the nature of the management strategy (i.e., “active” versus “passive”) and the nature of the contact with the sorbent (i.e., “internal” versus “external”), as visualized in Figure 1. Distinguishing between active and passive strategies may be done by drawing a system boundary around the sorbent bed (black dotted lines in Figure 1). If any material streams besides those involved directly in adsorption (i.e., the stream containing the adsorbate) cross the system boundary, the management method is defined as “active.” Conversely, a passive method will only have adsorbate streams cross system boundaries; i.e., no heat transfer fluids cross the boundary. Distinguishing between internal and external methods involves drawing a boundary around an individual sorbent 5 ACS Paragon Plus Environment


contactor (pellet, monolith, laminate, fiber) inside the bed (green dotted lines). If the management method is located inside the boundary, regardless of which pellet or other structure inside the bed is selected, the method can be classified as “internal.” As an example, a traditional heating jacket on a zeolite pellet bed would be considered active external thermal management. A heating jacket with tubes running through the bed would still be considered active and external because not all sorbent structures experience the same effect of the strategy.

Internal

Active

External

Jacketed Vessel

[24]

Hollow Fiber Sorbents

[19]

Sorbentloaded porous polymer matrix

Passive

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µPCM

Melt Freeze

PCM packets

[36]

This work

Figure 1. Different methods of heat management in adsorption-based systems. Black dotted lines indicate system boundary of sorbent bed used to determine internal vs external management approach. Dotted green lines indicate the sorbent contactor boundary used to determine whether the strategy is active or passive.


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For thermal management in PSA systems, passive thermal management methods may be preferred to simplify the installation and operation of the unit. Incorporation of a phase change material (PCM) into the sorbent bed to trap sorption enthalpy has garnered interest in the patent literature3033

, as well as several recent process modeling studies34, 35. Conceptually, the phase change material

works as a thermal ballast, melting at or near the operating temperature of the adsorption device, and storing the heat of adsorption in the heat of fusion of the PCM. Upon depressurization and desorption, heat is withdrawn from the PCM, solidifying it, thus keeping the system close to the desired operating temperature. This coupled adsorption-melting / desorption-freezing cycling should help, in principle, to manage effectively thermal fronts generated by the sorption process in the bed. In traditional pellet systems, large containers of phase change material are loaded into the bed with the sorbent pellets to help contain the heat released; however, the heat transfer efficiencies in such systems may be poor, so that the additional cost of the PCM may not always be justified. In a study looking at phase change material application in a CH4 storage system, Toledo et al. found that spheres of PCM allowed for more rapid pressurization and managed the heat near the PCM, but could not capture the heat from further away36. Several authors have considered how to improve the distribution of PCM throughout the adsorbent bed30-35. Regardless of approach, incorporation of PCM results in a natural tradeoff: the volume the PCM takes up in the bed cannot be used for sorbent21. The use of smaller PCM capsules has been considered before, but the use of microencapsulated phase change material (μPCM) has been dismissed in traditional packed beds as the small size leads to excessive pressure drop or may become entrained in the flow37. To circumvent this PCM distribution problem, Lively and co-workers38 used zeolite-NaXcontaining hollow fibers packed in a shell-and-tube configuration to distribute sorbent and PCM 7 ACS Paragon Plus Environment


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throughout the bed. In the shell-and-tube modules, a feed gas of CO2 and He were fed through the bore of the fibers, while the PCM packing in the interstitial space surrounding the fibers in the module helped contain the sorption enthalpy release. They found the PCM was able to keep the fibers almost entirely under isothermal conditions. In these thermally modulated fiber sorbents, sorption performance was enhanced, with increased CO2 capacities and sharper breakthrough fronts. The work described here details the first of a kind production of novel fiber sorbents containing both sorbent particles and microencapsulated phase change material within a fiber. The intimate contact between the phase change material and sorbent particles inside the fiber should allow for internal, passive control of the sorption enthalpy released during the adsorptiondesorption process, making for more efficient sorption dynamics. The reduced effect of heat generation on the breakthrough profiles will lead to more sorption occurring prior to breakthroughs. After proof-of-concept spinning of composite fibers, dynamic column CO2 breakthrough experiments operating at sub-ambient conditions and elevated pressures reveal the effects of heat management on the sorbent’s dynamic performance. Dynamic column breakthrough capacities at low sorbate leakage are compared between fibers with and without temperature relevant phase change material included to demonstrate the effect of thermal management. The benefits of the intimate contact between the sorbent and phase change material are probed in nearisothermal small diameter beds, while larger diameter beds illustrate how the microcapsules can manage effectively heat generation in conditions closer to an adiabatic regime, typically found in industrial-scale adsorption-based processes, e.g., PSA/VSA.


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2. EXPERIMENTAL SECTION 2.1 Materials Cellulose Acetate (CA, 50,000 MW, Sigma Aldrich), and polyvinylpyrrolidone (PVP, K30 Mw 40,000, TCI) were used as the polymers for the formation of fiber sorbents. The metal-organicframework UiO-66 (particles size 0.8-1 μm forming irreversible agglomerates of 3-5 μm as confirmed by SEM) was purchased from Inmondo Tech and activated using standard procedures 13

. Dimethylformamide (ACS Grade, Alfa Aesar) was used to wash the received UiO-66. All

polymers and solid additives were dried at 80 °C overnight to remove any residual moisture. Nmethyl-2-pyrrolidone (NMP), methanol, hexanes, (ACS Grade, VWR) were used as solvents for the polymer/spinning dope and subsequent solvent exchange steps, as well as to probe μPCM stability during spinning. Commercial microencapsulated phase change material, microPCM -30D and microPCM 28D (Microtek Laboratories, Inc.) were purchased as a suitable material for this work, as its size (~20 microns) is reasonable for spinning and their respective phase change temperatures occur within the region of interest for this application. The purchased microencapsulated phase change material is composed of a melamine formaldehyde shell enclosing an organic phase change material made up of mixtures of branched chain hydrocarbons. Simulated dry flue gas with a helium tracer (12.5% CO2, 12.5% He, 75% N2) was purchased from Matheson Gas. Research grade Nitrogen, used for bed purging and pressurization, was purchased from Air Gas. 2.2 μPCM Characterization Microencapsulated phase change material solvent and spin stability characteristics were tested before spinning to determine the feasibility of incorporation of these materials into a spin dope and subsequent survival of the solvent exchange process. First, small samples of μPCM were soaked



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for 24 hours in solutions of pure NMP, water, methanol, and hexanes, vacuum filtered and dried for 24 hours at 100 °C in a vacuum oven. A control sample, unexposed to solvent, was also placed in the vacuum oven to confirm effects experienced were due to interaction with solvent only. Samples were then sputter coated, and scanning electron microscopy (SEM) was performed to confirm any physical changes to the phase change material. Differential scanning calorimetry (DSC) was also performed and analyzed to determine any loss of phase change behavior in the materials. 2.3 Fiber Formation and Characterization A standard spinning dope contained polymer (CA), additive (PVP), solvent (NMP), nonsolvent (deionized water), sorbent (UiO-66) and in some cases microencapsulated phase change materials (μPCM). The polymers, sorbent, and phase change materials were all dried overnight at 80 °C before use. In polymer dopes where μPCM was being incorporated it was first added to 80% of the required NMP/water for the dope and sonicated using a 100 W sonication horn. The dispersion alternated between sonication (5x for 30 seconds at 25% intensity followed by 30 seconds off) and stirring via impeller (5 minutes) for 1 hour until the suspension was well dispersed. The adsorbent, UiO-66, was then added to the dispersion and sonicated/stirred using the same procedure outlined above for 1 hour. A “Prime” dope, prepared the day prior, made up of 20% of the required polymer and the remaining 20% of the solvent/nonsolvent was combined and mixed overnight on a roller, was then added to the sorbent-solvent dispersion and underwent stirring and sonication cycles for 30 additional minutes. Finally, the remaining polymer was added to the solution and stirred for 4 hours until the dope was of continuous consistency, and all polymer had been incorporated forming a ready-to-spin dope. The final spinning dope was rolled for an additional 24 hours to eliminate


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air bubbles left from the preparation. Dope compositions used in this work can be found in Table 1. Table 1. Dope compositions of fibers spun in this work Unmodulated -no μPCM

Modulated

Unmodulated -μPCM

(Wt%)

(Wt%)

(Wt%)

CA

7.5

7.7

7.5

PVP

6.0

6.1

6.0

NMP

56.4

57.8

56.6

H2O

7.5

7.6

7.5

UiO-66

22.6

10.4

11.2

μPCM -30C

0

10.4

0

μPCM 28C

0

0

11.2

Dope Component

The nonsolvent phase inversion technique known commonly as “dry-jet wet-quench spinning” was used to create the monolithic fibers formed for this work39-42. In short, the spin dope is extruded through a spinneret through a small air gap and into a water quench bath to initiate phase inversion of the polymer, producing a porous polymer matrix containing sorbent UiO-66 and microencapsulated phase change material. Fibers were collected on a take-up drum, removed from the drum using a razor blade and placed in a deionized water bath for three days (water was exchanged once a day). Fibers not containing phase change material were then solvent exchanged through three consecutive 30 minute soaks in aliquots of methanol followed by an additional three 30 minute soaks in hexanes. Fibers containing μPCM did not undergo this solvent exchange step due to the instability when μPCM came into contact with methanol (discussed in Section 3.1). Fibers were then hung to dry in a fume hood for one hour to remove residual solvent then placed in a vacuum oven and dried at 110 °C overnight. 11 ACS Paragon Plus Environment


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Table 2. Spinning parameters used in current work

Spinning Characteristics Flow Rate (mL/hr)

200-1000

Air Gap (cm)

5

Spinneret temp (°C)

50

Bath Temp (°C)

50

Take-up rate (m/min)

10-30

Fibers were characterized using scanning electron microscopy (SEM, Hitachi SU8230 cold-fieldemission scanning electron microscope), thermogravimetric analysis (TGA, TA Instruments TGA500), and cryogenic N2 physisorption (Microtrac Belsorp-max). Differential scanning calorimetry (DSC, TA Instruments Q-200) was used on fiber states containing μPCM to confirm relative loading of μPCM in composite fibers. 2.5 Breakthrough and Thermal Profile Collection Fiber sorbent modules were created for “small” and “large” diameter beds by filling ¼” ( 0.455 cm ID) and 3/8” (0.700 cm ID) Swagelok® tubings of set lengths 25.4 cm in length for small beds and 12.7 cm for large beds with 12 (1050±10 μm diameter) and 28 (800±10 μm) diameter fibers, respectively. The shorter length for the larger diameter bed is a result of the space occupied by the axial thermocouple probe installed in these modules. The resulting beds had a tight packing of fibers (62-65% packing fraction). Insertion of an axial thermocouple (type K, Omega) down the center of the 3/8” bed allowed for the collection of temperature profiles along three points of the bed’s length. Schematics and pictures of the modules are given in the supplemental information. Dynamic column breakthrough experiments were performed in a custom built PSA-100 (L&C Systems) displayed schematically in Figure S3 (Supporting Information). Built within a sub12 ACS Paragon Plus Environment

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ambient cabinet, the system allows for excellent temperature control when performing multicomponent competitive adsorption experiments, pressure-swing-adsorption cycles, and collection of temperature profiles. Mass flow controllers (Parker Porter), located outside of the sub-ambient cabinet, were used to control the flow of purge and simulated flue gas throughout the experiments. System pressure is maintained using a custom programmed backpressure controller, where the transducer (Honeywell) is located inside the cabinet, and the flow controller (Parker Porter) is outside the cabinet. Temperature profiles in the “large” modules were monitored using software internal to the system that tracks the temperature of as many as six thermocouples installed within the system at a sampling rate of 1 Hz. Fiber modules were dried overnight in flowing N2 at 50 °C before exposure to simulated flue gas. Complete removal of water was confirmed when the concentration of water in the outlet was found to be zero under transient measurement by mass spectrometry (Pfeiffer Omnistar) for thirty minutes. Competitive fixed bed breakthrough experiments were performed with simulated dry flue gas (12.5% CO2, 12.5% He, 75% N2) at a variety of flow rates and temperatures to investigate sorption kinetics and the effects of temperature modulating phase change material. Once the beds were devoid of water, they were pressurized to 16 bar under 100 sccm flow of dry nitrogen and cooled to the lowest target temperature. Once at the desired operating pressure of 16 bar (2 bar CO2), the flow was adjusted to the target flow rate of the experiment and allowed to idle under that flow for 5 minutes. At the start of the experiment, a valve downstream of the N2 flow was closed and the valve downstream of the simulated flue gas opened (~1 second), and the effluent composition was monitored using mass spectrometry. In this procedure, He acts as the inert tracer gas, N2 as the carrier gas, and CO2 as the adsorbate of interest. After the experiment was complete, the bed 13 ACS Paragon Plus Environment


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pressure was reduced to atmospheric pressure, and then exposed to dynamic vacuum (~ 10-4 bar) for at least 5 minutes to remove the majority of the adsorbed CO2 from the bed. The bed was then re-pressurized to 1 bar, and dry N2 was flowed as a purge gas over the bed for at least 5 minutes. The bed was then pressurized to the operating pressure with N2, and the effluent was sampled using mass spectrometry to confirm there was no remaining CO2 or He. A new experiment was started once the effluent stream was found to contain the only N2 for at least 5 minutes of operation Calculated breakthrough and pseudoequilibrium capacities were used to understand the effects of incorporation of μPCM into the fibers. As heat effects in adsorptive breakthrough will broaden the breakthrough profile, so then more capacity at low adsorbate “leakage” can be indicative of heat being managed. Capacities were calculated using the area bound by the He and CO2 concentration profiles from the effluent breakthrough curves collected by the mass spectrometer. As the Helium acts as an inert tracer, it captures the dead volume of the system, allowing for quick calculation of sorbent capacity. The product of area bound by the two curves at any time after He breakthrough and the inlet molar flow rate of the adsorbate species one can calculate the amount of CO2 adsorbed at that given time43. For comparisons of capacity near breakthrough, where the effects of sorption enthalpy are most prevalent, we used the capacity at 5% leakage of CO2. To confirm the loading of sorbent in the fibers was as expected the capacity at 95% leakage or “pseudoequilibrium” capacity was used. 3. RESULTS AND DISCUSSION 3.1 Solvent Stability of μPCM To spin microencapsulated phase change material into a fiber sorbent contactor, the stability of the μPCM in the spinning solvents must be confirmed. The as-delivered μPCM particles were spherical with a polydisperse size distribution and a maximum diameter of ~20μm, as illustrated 14 ACS Paragon Plus Environment

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in Figure 2a. DSC analysis on the as-delivered μPCM gave a heat of fusion of 148 J/g, as shown in Figure 2b. DSC profiles at very slow temperature ramp (0.1°C/min) are depicted in Figure S5, to show the freezing characteristics with minimal conduction effects. The stability of the μPCM particles in conditions relevant for solution spinning was tested. When determining the feasibility of spinning non-polymer fillers, it is vital to confirm the stability of the filler in the solvents utilized in the spinning process. For all solvents, differential scanning calorimetry showed no significant loss in capacity indicating the μPCM has good solvent stability. SEM analysis on the solvent treated μPCM showed no noticeable structural change due to treatment, with the exception of methanol. A number of the examined microcapsules of the methanol treated μPCM showed structural collapse (Figure S6). Although the DSC signaled phase change performance was retained, we removed the relatively standard methanol solvent exchange step from the fiber fabrication process (this step is typically included to prevent the collapse of small pores beneath the skin layers of asymmetric hollow fiber membranes). This step appears to be unnecessary for fiber sorbents of cellulose acetate, as the fibers produced in this work exhibit none of the usual signs of collapsed pore structures because of avoiding the methanol exchange.

b)

a)

Heat Flow (mW)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2

Hfreezing= 148.0 J/g

1

Hfreezing= 51.2 J/g

0 -1 Out of ContainerPCM PCM-Sorbent Fibers

-2

20 μm

-3 -50

-40

-30

-20

-10

0

Temperature (Degree C) Figure 2. a) Scanning Electron Microscope (SEM) images of neat out of container μPCM b) DSC profile of out of container μPCM (black) and modulated composite fiber.



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3.2 Fiber Sorbent Characterization Confirmation of the capability of μPCM to be spun into composite fibers requires proof of concept spinning of the material. Small-scale extrusions of μPCM-loaded spin dopes through a 16-gauge needle were carried out to confirm the survival of the microcapsule under simulated stresses of spinning. SEM micrographs of the fibers produced are shown in the supplemental material, showing the microcapsule spheres survive the spinning process with little to no change in physical structure. DSC analysis was used to determine the weight loading of the μPCM within the fiber by taking the fraction of the heat of fusion (determined by integrating the fiber-μPCM DSC curve) relative to the heat of fusion of the neat μPCM. The DSC results for 50 and 75 weight % μPCM syringe fibers are shown in Figure S7 and compare well with the target loadings (i.e., observed loadings of 51.8 wt% and 78 wt% were estimated via DSC). In both cases, the onset temperatures of freezing and melting lag behind that of the neat μPCM. This lag is expected as the fiber has additional thermal resistance due to the polymer matrix. The μPCM particles were co-spun with UiO-66 to create composite fiber sorbents with localized thermal modulation. By varying the flow rate of the dope as well as the take-up rate fiber diameters ranging from 500-1200 μm were produced. The desired weight fraction of UiO-66 and μPCM in the fiber matrix was set such that all of the heat released upon CO2 adsorption into UiO-66 at -30 °C (the onset temperature for freezing and melting of the μPCM) could be captured by the heat of fusion of the μPCM. Figure 3 shows the morphological structure of the μPCM-UiO-66-CA fibers spun using the conditions noted in Tables 1 and 2. Figure 3a) shows the microcapsules are well distributed throughout the fiber, and the fiber appears to be highly porous. Figure 3b) presents a magnified view of the composite structures, revealing that the MOF crystals are distributed throughout the fiber matrix and importantly adjacent to the phase change material particles. This


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adjacency is critical for improving heat transfer performance, as it suggests that significant decreases in the thermal resistance of the system relative to other arrangements that would place the sorbent relatively far (i.e., >100 µm) from the PCM. Confirmation of fiber sorbent loading was carried out via TGA and BET analysis. This analysis, along with DSC to confirm μPCM loading, and additional SEM images can be found in the Supplemental Information.

a)

b)

UiO-66

μPCM 500 μm

10 μm

Figure 3. SEM images of cellulose acetate fibers containing both μPCM and the MOF UiO-66. (a) low magnification image showing the distribution of μPCM throughout the fiber. (b) magnified image showing the relative location of the two fillers.

3.3 Dynamic Breakthrough Analysis Fixed bed dynamic breakthrough experiments can be used to probe the effects of phase change material incorporation on sorption dynamics. In the 0.455 cm inner diameter modules described in section 2.5, fixed bed dynamic breakthroughs were performed and analyzed. In these low diameter modules, the effects of heat dissipation through module walls tends to dominate the heat effects, thus dampening the magnitude of adsorption-induced thermal excursions in the bed (in essence, the heat is rapidly removed from the system through the metal walls of the module, so there is intrinsic global thermal modulation due to the small scales of the experiment). This effect is advantageous for studying the μPCM–MOF composite fibers, as differences in sorbent



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performance can be ascribed to local thermal modulation effects (i.e., near the MOF surfaces within the fiber walls). The CO2 breakthrough profiles for the “Modulated” and “Unmodulated-no μPCM” fiber sorbents are shown in Figure 4. The flow conditions, 16 bar, 200 sccm, and 238 K, were selected to serve as an illustrative case of flows where the nature of the thermal modulation alone might be considered. It is important to note that when analyzing these breakthrough profiles, the high flow rate (200 sccm) and face velocity (122 cm/min) makes comparisons to other sorbents based on time to reach saturation challenging. The pseudoequilibrium capacities of the sorbent in the two fiber states were calculated to be ~5.5 mmolCO2/gUiO-66 , which matches well with the sorption isotherms for UiO-66 at these conditions. For a detailed comparison between equilibrium uptake values of different CO2 capture adsorbents for CO2 capture, the interested reader is directed to excellent reviews by Choi et al. and by Samanta et al. 8, 44. This temperature was selected based on differential scanning calorimetry of the microencapsulated PCM at a low ramp rate (Figure S5, Supporting Information), showing the majority of the freezing behavior occurs prior to 238K, while the onset of melting behavior occurred around 243 K. The breakthrough time for the modulated fibers is shorter than that of the unmodulated fibers. Shorter breakthrough time is expected, since the amount of available sorbent in the modulated fibers is only half the amount in the unmodulated fibers, as per Table 1. The influence of the phase change material on sorption dynamics can be confirmed by analyzing the shape of the breakthrough profiles, which will be sharpened by local containment of sorption enthalpy. We confirmed an 80% sharper CO2 breakthrough (slope of CO2 breakthrough calculated at C/C0=0.5, where C is the effluent concentration and C0 is the inlet concentration respectively)


Page 19 of 31

in the case of the modulated fibers. Similar breakthrough profile comparisons collected at other flow rates and temperatures can be found in Figures S10 and S11. The differences in CO2 breakthrough capacities in the modulated and unmodulated cases at 5% leakage (C/C0=0.05) are a useful probe to capture the dynamic performance of the adsorption bed. Figure 5 highlights the breakthrough capacities calculated at 5% CO2 leakage for both modulated and unmodulated without PCM fibers normalized by the MOF loading over the range of tested flow rates at two temperatures: one near the melting point of the μPCM, and the other away from it.

CO2 Breakthrough Profiles (200sccm, 238K, 16 bar)

1.0 0.8

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.6

Helium - PCM Fibers Helium - No PCM Fibers CO2 - PCM Fibers

0.4

CO2 - No PCM Fibers

0.2 0.0 0

100

200

300

400

500

600

Time (seconds) Figure 4. CO2 breakthrough profiles collected for “Modulated” and “Unmodulated-no µPCM” UiO-66 fiber sorbents. Profiles were collected at 238K, 16 bar total pressure, and 200 sccm (mL/min) flow rate. Profiles were smoothed using a 5-point Savitzky-Golay method in Origin Pro 2016.

When operating at 228 K—a temperature requiring large local thermal excursions to reach the melting conditions of the phase change material—no increase in sorbent CO2 capacity at 5% leakage is noted (Fig. 5a). In these small diameter beds, the similar capacities are indicative that rapid heat dissipation through the walls is not allowing the fibers to reach the elevated temperatures



Page 20 of 31

required to melt the PCM. With the heat being removed before the phase transition temperature of the μPCM can be reached locally, the two beds operate approximately the same when normalized by the amount of MOF, even with double the amount of adsorbent being present in the no-μPCM case. At 238 K, where the PCM is active with only small local thermal excursions, the breakthrough capacity of the sorbent at 5% gas leakage increases by at least 20% across all flow rates investigated (Fig. 5b). The increase in sorbent capacity at low leakage in the modulated fibers near the phase change temperature, but not at lower temperatures, indicates that the μPCM is capable of capturing locally released heat, improving the sorption dynamics of the system. As the operating temperature moves further from the phase change temperature, the heat of adsorption can be removed effectively via heat dissipation through the module wall, so the phase change material never has the opportunity to be effective. This effect would only be present when the heat transfer distances between the phase change material and sorbent are very short.


Page 21 of 31

Sorbent Capacity at 228 K 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0

Unmodulated no PCM Modulated 0 100 200 300 400 500 600 700 800 900

Breakthrough Capacity (mmol/gUiO-66)

b)

a) Breakthrough Capacity (mmol/gUiO-66)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Sorbent Capacity at 238 K

1.8 1.6 1.4 1.2 1.0 0.8 0.6

Unmodulated no PCM Modulated

0.4 0.2 0.0

0 100 200 300 400 500 600 700 800 900

Flow Rate (sccm)

Flow Rate (sccm)

Figure 5. Sorbent capacity at 5% adsorbate leakage of fibers containing μPCM as compared to those fully loaded with UiO-66 over a range of flow rates, capacities are normalized per gram of sorbent (a) 228 K, significantly below point (b) 238K, near phase transition temperature

3.4 Heat Front Analysis The incorporation of phase change material into the fiber sorbent contactor should reduce the amount of heat transferred to the gas phase in the adsorption process. Through the incorporation of an axial thermocouple (Figure S2) in a larger diameter bed (0.7 cm ID), the temperature profile of different breakthroughs may be collected. Working with the larger diameter was necessary for insertion of the thermocouple, and helps to reduce the effect of heat leakage through the walls. The two fiber states used in the large bed operation (Modulated-μPCM and Unmodulated-μPCM) contain approximately the same weight loading of UiO-66 sorbent, but different kinds of μPCM materials, i.e., different phase-transition temperatures. This additional degree of control for comparison was desired in the case closer to adiabatic conditions of larger diameter modules to isolate the effects of the μPCM more effectively. The use of ambient temperature μPCM in the “unmodulated” case kept the sensible heat of the fibers constant in both samples, and the similar loading of sorbent kept the expected equilibrium heat generated by breakthroughs consistent.



Similar experiments to those described in the small module case were carried out with the larger diameter modules, with internal data logging software used to monitor the temperature in the bed. Different velocities and operating temperatures were used to vary the heat generation rate and amount of heat required to trigger the onset of the phase change material. The temperature at the center of the bed (TC-2) during breakthrough of both the modulated PCM fibers and the unmodulated PCM fibers across five flow rates (100, 200, 300, 400, and 800 sccm) are given in Figure 6. As the flow rate of gas through the bed increased from 100 sccm to 800 sccm the peak temperature of the gas increased and the thermal wave propagates through the bed more quickly. This behavior was consistent for both Modulated-μPCM and Unmodulated-μPCM fibers, showing the microencapsulated phase change material at current loading was not capable of containing all of the sorption enthalpy produced. Comparing the modulated fibers to those unmodulated at the fastest feed condition, 800 sccm, (Figure 6 inset) the peak amplitude of the thermal wave is approximately 40% in the unmodulated fiber case as compared to the modulated 3.5

8 4

Increasing Flow Rate

T-Tinitial (0C)

3.0

10

2.5 2.0 1.5 1.0 0.5

T-TInitial (0C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

0.0

2 6

-0.5 0

200

400

600

800

Time (Seconds)

0 4

Unmodulated Fibers 2 0

Modulated Fibers 0

500

1000

1500

2000

Time (Seconds) Figure 6. Temperature profiles collected at different flow rates from 100-800 sccm comparing modulated fibers (bottom) and unmodulated fibers (top). (inset) overlaid comparison of 800sccm flow rate thermal profiles for fibers with (green) and without (blue) thermal modulation.


Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


case. Higher peak amplitudes were found across all flow conditions. With near equal weight loading of UiO-66 in both fiber states, “Modulated” and Unmodulated-no μPCM,” under identical conditions, if no thermal modulation were present, it would be expected that the thermal profiles. Breakthrough results similar to those in Figure 5 for these experiments, along with results for 0.7 cm diameter without an installed thermocouple are reported in Figures S13-18. While there is still the question as to whether the mass transfer front is fully developed in the samples with the thermocouple installed, it is worth noting that in all cases the modulated fibers show 20-30% improvement in breakthrough capacity. CONCLUSIONS

Incorporation of phase change material into sorbent systems has been proposed as a process intensification approach for minimizing adsorption-induced thermal excursions in rapidly cycled pressure swing adsorption systems. μPCM-UiO-66 composite fiber sorbents were produced, the first instance of phase change material and sorbent being incorporated simultaneously into a single structured contactor. Composite fiber sorbents were successfully spun containing two separate solid fillers, UiO-66 sorbent, and microencapsulated phase change material as thermal ballast, well distributed throughout a cellulose acetate porous support. In this work, the applicability of microencapsulated phase change materials for thermal front modulation was explored and subsequently applied to fiber sorbent contactors for use in the sub-ambient CO2 capture. μPCM were found to have desirable freezing and melting behavior and required chemical and thermal stability for spinning into fiber sorbent contactors. Dynamic gas breakthrough experiments show an increase in breakthrough time per sorbent mass loading and sharpening of CO2 breakthrough profiles, as characterized by the sorbent capacity at five percent adsorbate leakage. In low diameter 23 ACS Paragon Plus Environment


Page 24 of 31

modules, an improvement of 20-25% CO2 capacity per gram of UiO-66 was noted. Thermal fronts observed in this study show a reduction in heat released to the flowing gas and peak intensity of thermal fronts when microencapsulated phase change materials are incorporated into the fiber bed and operate at temperatures near its melting point. This work has presented a highly scalable method of incorporating passive heat management strategies into adsorption-based processes. Compared to previous methods of phase change material incorporation, this method provides a single step approach (fiber spinning) while overcoming expected pressure drop challenges when using microencapsulated phase change materials. The intimate contact between phase change material and sorbent particles provides a significant improvement in sorbent efficiency even in cases where the bed is operated nearly isothermally. This work focuses on operation at sub-ambient conditions, but a wide range of μPCM melting temperatures are available commercially, opening up the possibility of a wide variety of applications to sorbent- μPCM fiber contactors for adsorption processes. Optimization of relative loadings of μPCM and sorbent, further analysis of freezing/melting kinetics over single and multiple cycles, and durability of μPCM over long-term cycling offer opportunities for greater understanding of system performance. Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Subambient CO2 Isotherms of UiO-66, Module design and construction information, Definition and sample calculation of pseudoequilibrium capacity using helium tracer, additional characterization (DSC, SEM) of microencapsulated phase change materials, SEM, DSC, TGA, N2 Physisorption of μPCM and μPCM-UiO-66 fibers, Breakthrough comparisons of 0.455 and 0.7 cm modules at a variety of temperatures, bed lengths. 24 ACS Paragon Plus Environment

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AUTHOR INFORMATION Corresponding Author Email: [email protected]

Present Address: Department of Materials Process Engineering, Nagoya University, Nagoya, Japan

ACKNOWLEDGEMENTS The authors would like to thank Brian Pimentel for his helpful comments and Matthew Orr and Meisha Shofner for sharing the use of their differential scanning calorimetry system. The authors acknowledge the U.S. Department of Energy through grant DE-FE0026433 for financial support. Any options, findings, conclusions or recommendations expressed herein are those of the author(s) and do not necessarily reflect the views of the DOE.

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For Table of Contents Only:

Sorption/ desorption enthalpy

Sorbent-loaded porous polymer matrix

Melt Freeze

µPCM Fiber Module


Development of Phase-Change-Based Thermally Modulated Fiber

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