Article pubs.acs.org/IECR
Improvement of Commercial Gas Mask Canisters Using Adsorbents Enhanced by Sintered Microfibrous Networks Donald R. Cahela* and Bruce J. Tatarchuk Center for Microfibrous Materials Manufacturing (CM3) Department of Chemical Engineering, Auburn University, 212 Ross Hall, Alabama 36849, United States S Supporting Information *
ABSTRACT: We used a U.S.A. military respirator test with dimethyl methylphosphonate (DMMP) to compare a commercial gas mask canister to composite bed designs. These comparisons were conducted at a constant volume of 320 cm3, equal to the total volume of the adsorbent and HEPA elements in a commercial gas mask canister, and fifty percent improvement in performance rating was experimentally demonstrated. In a second comparison, a 3-fold increase in gas life of a commercial canister with a 1.5 cm deep bed of activated carbon was experimentally demonstrated by adding a backup layer to a fully packaged commercial canister. An equation describing irreversible physical adsorption occurring in a two-layer bed and an iterative inverse solution is presented for design of composite beds utilizing polishing sorbents. An overall adsorption rate constant model is presented to predict adsorbent bed performance. A generally applicable axial dispersion model was developed for application to adsorbents enhanced by sintered microfibrous networks.
1. INTRODUCTION 1.1. Background. Gas mask canisters typically employ a bed of adsorbent and a pleated high efficiency particle aerosol (HEPA) filter. The adsorbent bed is designed to provide desired protection from chemical and biological agents with a tolerable breathing resistance. The adsorbent bed in this case has to be of a minimum depth to prevent immediate breakthrough of a contaminant challenge referred to as the critical bed depth (CBD). Additional bed depth provides useful adsorbent filter gas life. A more advanced composite bed design is produced by adding a backup layer with higher adsorption rate constant. This backup layer is only required to polish the effluent from the first layer, so it can have a much smaller capacity than the first layer. The higher adsorption rate backup layer is referred to as a polishing sorbent. 1.2. Adsorbents Enhanced by Sintered Microfibrous Networks (AESMN). A novel microstructured carrier technology, AESMN,1−7 utilizes smaller particulates than typical packed beds with higher external surface area to provide enhanced external and interparticle mass transfer rates while minimizing axial dispersion effects. An example scanning electron microscopy (SEM) of an AESMN composed of 55− 88 μm BPL carbon particulates entrapped in a mesh of 2, 4, and 8 μm sintered nickel fibers is shown as Figure 1. Using AESMN in a composite bed as a polishing layer would allow design of gas mask canisters with larger cross sectional areas and thinner beds to reduce breathing resistance. Microfibrous entrapment of small particulates has produced high contacting efficiency adsorbents7 suitable for applications demanding high levels of contaminant removal. Voidage in these materials can be adjusted from 98% down to values similar to a packed bed and they are made by a wet-lay/paper making process resulting in high uniformity of basis weight. Adsorbent particles as small as 20 μm can be entrapped in sintered microfibrous networks while minimizing pressure drop penalties by tailoring voidage © 2014 American Chemical Society
Figure 1. SEM of 55−88 μm Calgon BPL carbon entrapped by 2, 4, and 8 μm sintered nickel fibers.
to ca. 80%. AESMN are sometimes figuratively described as a “frozen fluidized bed” since particles in the material are held in place by a sintered network of fibers at voidages similar to that experienced in a fluidized bed. The uniform distribution of microfibers produced by the wet lay papermaking process used to manufacture these materials minimizes channeling in these materials. Adsorbents and catalysts enhanced by sintered microfibrous networks have been manufactured using metal, polymer, and ceramic fibers. Each type has advantages for different application requirements. Fluidization limitations in air lay methods limit competing polishing sorbents to larger particles than used in AESMN made by wet lay processes. Also, materials produced by air lay processes do not exhibit uniformity of basis weight. Testing of several of these air-laid Received: Revised: Accepted: Published: 6509
December 16, 2013 March 19, 2014 March 24, 2014 April 2, 2014 dx.doi.org/10.1021/ie404222d | Ind. Eng. Chem. Res. 2014, 53, 6509−6520
Industrial & Engineering Chemistry Research
Article
Nickel fibers in 4 and 8 μm diameters, 3 mm in length, were purchased from Intramicrometer Metals Division. Blotter paper with a mixture of hardwoods and softwoods was purchased from Georgia-Pacific. Bicomponent polymer fibers were obtained from Invista: Type 105, 6 mm length in both 1.5 and 3.0 denier (13 and 19 μm diameter). These polymers have a sheath of specialized linear low density poly-ethylene (LLDPE) on a core of polypropylene (PP). Cruwik SYN produced by Crucible Chemical Company and Chemfloc 4512 obtained from ACCO Unlimited Corporation was also used in the polymeric AESMN papermaking process. 2.2. AESMN Fabrication Methods. During the fabrication process for polishing sorbents PS1 and PS2 described in Table 2, traditional wet-lay papermaking techniques3−7 are used to
materials8 for contacting efficiency showed they were less effective than AESMN. PICA activated carbon particulate (ACP) AESMN have been tested for gas phase adsorption of hexane.9 1.3. Composite Adsorbent Beds Utilizing AESMN Polishing Sorbents. The pressure drop through a constant adsorptive capacity of material decreases with the area squared, so enlarging the cross sectional area of a filter greatly reduces the pressure drop. A porous media permeability equation10,11 (PMPeq) accurately estimates pressure drop for the entire range of possible values of bed voidage, including values above 80% in the regime where the Kozney-Carman12 or Erguns’ equation13 produces low estimates.
2. EXPERIMENTAL DETAILS 2.1. Materials. Several 3 M cartridge FR C2A1 gas filters were purchased for testing to provide a baseline for comparison. Glass beads 0.5 mm diameter used in some of the pressure drop measurements were purchased from BioSpec Products. ASZM-TEDA and BPL ACP 12 × 30 mesh size fraction were obtained from Calgon Carbon Corporation. A portion of the BPL carbon was ground and sieved to 60 × 140 mesh sizes for preparation of PS2. Coconut shell based activated carbon 60 × 140 mesh size fraction was purchased from PICA U.S.A. Inc. Measurements on PICA U.S.A. ACP were performed by Kalluri.9,14 Measurements on BPL carbon and ASZM-TEDA were done using the same instrument. Plots of nitrogen isotherm data and pore size distribution for each of the ACP are given in the Supporting Information. Packed bed density of 12 × 30 mesh size ASZM-TEDA was measured using a 250 cm3 graduated cylinder and 60 × 140, 60 × 100, and 80 × 140 mesh size range ACP was measured using a 50 cm3 graduated cylinder. The PICA U.S.A. ACP was ground and sieved to the size ranges indicated in Table 4 for use in PS1 and PS3. Particle density of neat ACP was calculated using the density of graphite (2.267 g/cm3)15 as indicated by the following equation: 1 1 = + vpore ρp ρgraphite (1 + L M)
Table 2. Description of Adsorbent Bed in C2A1 Canister and Three AESMN Polishing Sorbents
carbon type mesh range total area density, g/cm2 bed length, cm fiber type voidage ads. bed cap., g/cm3 (DMMP) surface area avg. diam., μm
(1)
Table 1. Properties of Activated Carbon Particulates BPL
ASZM-TEDA
PICA U.S.A.
1165 0.758 0.834 0.909 0.486 0.417
913 0.595 1.082 0.644 0.632 0.410
1121a 0.671a 0.899 0.746 0.537 0.403
PS2
PS3
PICA U.S.A.
BPL with AZM-TEDA
PICA U.S.A.
12 × 30 1.58
80 × 140 0.148
60 × 140 0.094
60 × 100 0.064
2.5 N.A. 0.4159 0.1730
0.470 nickel 0.7649 0.0606
0.350 nickel 0.8718 0.0427
0.296 polymer 0.7699 0.0405
620
46
29
69
process micrometer diameter metal fibers, activated carbon and blotter paper into performs. The resulting paper product is subsequently heated in a hydrogen sintering furnace at 1000 °C for 30 min, which removes the cellulose and causes the μm size nickel fibers to sinter-bond at their junctures, entrapping the ACP. Diameter for pyrolyzed cellulose binder in PS1 and PS2 materials was previously estimated.11 A third type of AESMN was prepared from bicomponent polymer, pulp and ACP. Preform was made by wet lay process followed by sintering at 130 °C with flowing air for 30 min. Luna17 describes preparation of polymeric AESMN more fully. Properties of the polymeric AESMN are given in Table 2 labeled as PS3. Diameter of cellulose fibers is from a report.18 A more detailed version of Table 2 is shown in the Supporting Information as Appendix B. 2.3. Pressure Drop Measurements. Pressure drop through beds of ACP were measured to determine particle shape factors. A glass tube 1″ inner diameter made in three sections that are clamped together with O-ring seals was used for measurements on packed beds. This tube had 6 mm threaded glass fittings for connections. A stainless steel screen located below the O-ring seal was used to support the ACP beds. Pressure drop was measured using a DP Cell from Omega Engineering, Inc. model PX-154−010DI. Air flow rate was controlled using ALICAT, Inc. mass flow controllers, rated for either 20 or 50slpm air. For measurements on smaller particles, 6.00 g of 0.5 mm glass beads was loaded on top of the bed of ACP to distribute the air before it entered the ACP. Pressure drop measurements on the AESMN materials were taken using a sample holder composed of two 6″ flanges with 1.0″ diameter opening with a layer of foam on each flange to compress the edges of the samples.
Calgon Carbon Corporation (CCC) BPL ACP is loaded with 29 wt % of metal oxides and 5 wt % triethylene diamine (TEDA)16 to produce ASZM-TEDA. Particle density is used to calculate the volume fraction of particulates in packed bed or AESMN. Particle voidage is computed as pore volume multiplied by particle density. Properties of ACP used in this study along with computed particle density, particle voidage and bed voidage for packed beds of 12 × 30 mesh particulates are given in Table 1.
surface area, m2/g pore volume, cm3/g particle density, g/cm3 particle voidage packed bed density, g/cm3 packed bed voidage
PS1 ASZMTEDA
a
References 9 and 14, permission to reuse N2 isotherm data was obtained. 6510
dx.doi.org/10.1021/ie404222d | Ind. Eng. Chem. Res. 2014, 53, 6509−6520
Industrial & Engineering Chemistry Research
Article
2.4. DMMP Breakthrough Measurements. All gas life tests using DMMP were performed by Battelle Memorial Institute-Aerosol Science under a subcontract. The challenge was generated by flowing air through a bubbler containing DMMP. Challenge concentration was measured using an acetone scrubbing bubbler and analysis of the solution by gas chromatography with flame ionization detector. Challenge was diluted to required concentrations in air. PA-260 detector, with detection limit of 0.0005 mg/m3 for DMMP, analyzed effluent from the sample holder. The streams were controlled by mass flow controllers and the system had pumps pulling on the exit of each stream. System effluent was filtered by carbon canisters before release into the fume hood. 2.5. Modification of CBA/RCA Canister with AESMN Polishing Sorbent. A commercial chemical, biological and aerosol riot control agent (CBA/RCA) canister was modified by replacing the exit coarse particle filter with a layer of PS3. The polymer polishing sorbent was incorporated into prototypes of commercial canisters with 1.5 cm deep beds of ASZM-TEDA, referred to as CBA/RCA-PS. One canister was a commercial CBA/RCA canister, 54 cm2, and one was the same product modified by addition of a layer of polymeric polishing sorbent (CBA/RCA-PS) replacing the fine particle filter at the canister outlet. MSA manufactured these canisters on a production line.
Figure 2. Diagram of components in 320 cm3 constant volume analogs to the C2A1 canister. (A) Three layers of Hollinee E200 (fixed thickness, 0.66 cm), (B) ASZM-TEDA CCC bed (variable: area, thickness, volume), (C) polishing sorbent with 60 × 140 mesh AZMTEDA (fixed thickness, 0.35 cm).
the polishing filter concept using a constant thickness of PS2 and face velocities to correspond to various filter areas. The required thickness of Hollinee E200 aerosol filtration media for 99.97% particulate filtration is less than the thickness of the pleated glass fiber media in the C2A1, so a thicker bed of ASZM-TEDA could be used than that in the C2A1 at the face velocity of 9.8 cm/s. We planned two other sets of breakthrough tests using ASZM-TEDA bed depths of 1.0 cm and 0.5 cm. The corresponding areas and face velocities are shown in Table 5. Table 5. Design of Tests for Constant Volume (320 cm3) Analogs to C2A1 Canister
3. COMPOSITE BED ADSORBENT DESIGN METHODS 3.1. Test Plan for Demonstration of Polishing Filters Analogs in Same Volume as C2A1 Canister. In the composite bed adsorbent concept, a polishing sorbent is placed behind a conventional packed bed of sorbent particles. The dense packed bed provides high adsorption capacity while the polishing sorbent provides high contacting efficiency to achieve low breakthrough thresholds while nearly completely utilizing the total adsorptive capacity. The C2A1 gas mask canister is used as a baseline for evaluation of composite bed designs. Conditions for the standard U.S. Military gas mask canister test employing DMMP19 are shown in Table 3. A C2A1 canister was disassembled and measurements are shown in Table 4.
C2A1 cross sectional area, cm2 face velocity, cm/s Hollinee E200 thickness, cm ASMZ-TEDA bed thickness, cm polishing sorbent, cm
50 3000
