Filtration Solutions Disk Drive Filtration Technical Note
Using Adsorption Isotherms to Find the Best Activated Carbon to Remove Volatile Organic Contaminants Andrew J. Dallas, Ph.D.,* Lefei Ding, Jon Joriman Donaldson Co., Inc; Corporate Technology, 9250 W. Bloomington Fwy., Bloomington, MN, 55431
Experimental: Toluene, an important environmental contaminant, was chosen as one of our probes of the pore structure and specific polar/nonpolar interactions between contaminants and activated carbon surfaces. Adsorption isotherms were measured over the toluene concentration range of 5-35,000 ppm using gravimetric methods. A schematic of the experimental setup is given in Figure 1.
Thermostated Microbalance
GS
MFC1
Mixer
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DA
Figure 1: Schematic of adsorption isotherm experiment. GS = toluene; MFC1/2 = mass flow controllers; DA = dilution air. Toluene adsorption isotherms were measured for six activated carbons manufactured from two different natural materials. Their physical properties are given in Table 1. Results and Discussion: The toluene adsorption isotherms for all six activated carbons are given in Figure 2. The shape of the adsorption isotherm for Carbon A and B relative to the other four carbons reveals several subtle adsorption differences exist between the two natural starting materials.
Table 1: Carbon properties as measured by nitrogen BET. Sample Carbon A Carbon B Carbon C Carbon D Carbon E Carbon F 100.0
BET Surf. Area (m2/gr) 1150 1639 1530 1022 995 800
Pore Vol. (ml) 0.91 1.17 0.74 0.48 0.49 0.39
Micropore Area (m2/gr) 72 142 984 825 800 662
Carbon A Carbon B Carbon C Carbon D Carbon E Carbon F
90.0 Weight Toluene Adsorbed (%)
Introduction: Chemical filters containing activated carbon have been used in hard disk drives for many years. Activated carbon is typically used, due in part to its unsurpassed ability to remove volatile organics and to control humidity levels within the drive environment. However, only recently has it become apparent that understanding the pore structure, pore size distribution, and surface chemistry of activated carbon is critical in designing chemical filters for the removal of organics at low concentrations.1-4 Since these factors can be critical to the performance of chemical filters, we have developed several methods to probe how they influence the thermodynamics and kinetics of adsorption.5 In this technical note we discuss our methods of measuring organic contaminant adsorption isotherms and how this information is used to make the appropriate choice of activated carbon for specific applications.
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Figure 2: Toluene adsorption isotherms on activated carbons. Most adsorbent testing in the industry has been performed at high concentrations since minimal time and resource commitments are required. As Figure 2 illustrates, at high organic concentrations, high surface area, or a large pore volume, are necessary to provide maximum capacity. However, Figure 2 also reveals this observation does not hold as the organic concentration decreases to levels encountered in high purity applications. As the toluene concentration decreases, a crossover is observed in the relationship between capacity and surface area.1,5 At low organic concentrations, pore structure, microporosity and surface chemistry become more important than total surface area. The kinetics of adsorption can also be extracted from the experimental raw data. The toluene adsorption rate constant, k (s-1) was calculated using a linear driving force mass transfer model and is given as a function of concentration in Figure 3.6 For brevity we have only provided the low concentration kinetic results. As the toluene concentration decreases, so does its
* e-mail:
[email protected]; phone #: 952-887-3318; FAX: 952-887-3937
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shape, interconnectedness, and chemistry of these pores must play a role in toluene adsorption on these activated carbons.
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Figure 3: Toluene adsorption rate constant on activated carbon at low concentration. rate of adsorption. An increase in the microporosity of activated carbon also leads to slower rates of adsorption of toluene. However, for the four carbons that contain significant amounts of microporosity, the rate constant does not follow any simple micropore or surface area relationship. Therefore, the
Summary: Both the equilibrium capacity and dynamics of organic adsorption play a critical role in choosing the best activated carbon adsorbent for an application and for designing the most efficient chemical filter. For disk drive chemical filters, the chosen activated carbon(s) must strike the preeminent balance between the low concentration capacity and the kinetics of adsorption. Organic adsorption isotherms, along with an understanding of the pore structure, pore size distribution, and chemistry allow these decisions to be easily made. References: 1. K. L. Foster, et al.; Chem. of Materials, 1992, 4, 1068-1073. 2. C. L. Mangun, et al.; Carbon, 1997, 36, 123-131. 3. H. Tamon, et al. ; Carbon, 1996, 34, 741-746. 4. T. J. Bandosz, et al.; Langmuir, 1996, 12, 6480-6486. 5. A. J. Dallas, et al.; in preparation. 6. N. J. Foley, et al.; Langmuir, 1997, 13, 2083-2089.
* e-mail:
[email protected]; phone #: 952-887-3318; FAX: 952-887-3937
