(698f) Filtration of Compositionally Heterogeneous Aerosols Using Micro- and Nano-Fiber Media

Subject Areas (Order of preference): (a) Separations, (b) Particle Technology, (c) Environmental TopicsDeadline for submissions: May 9, 2016, Word Limit: 1500

Filtration of compositionally heterogeneous aerosols using micro- and nanofiber media

Saptarshi Chattopadhyay1, Junli Hao1*, Koli Taghizadeh2 and Gregory C Rutledge1# 1Chemical Engineering Department,2Center for Environmental Health Sciences Massachusetts Institute of Technology

*-Presenting author, #Corresponding author. Tel.: +1 617 253 0171; Fax: +1 617 324 3127, E-mail address: rutledge@mit.edu (G. C. Rutledge)

Key words:

Particle, Condensation, Mobility Diameter, Maximum Penetrating Particle Size, Composite Filter, toxicity, Kelvin Equation, Vapor Pressure

Abstract:

Many naturally occurring aerosols exhibit compositional heterogeneity as a function of particle diameter (dp). They are formed by condensation from mixtures of volatile gases with different vapor pressures. Tobacco smoke1, coal gas2 and combustion products3 are examples of aerosol particulates whose chemical compositions vary with particle size. Typically, the portion of chemical compounds with higher boiling point increase with dp in such aerosol particulates 4. Because some of the higher boiling point (BP) compounds are toxic to human health5, a method for selectively removing these compounds is desired. Particulate aerosol filters have a wide range of applications targeting human health and environment, among them are respirators, high volume air samplers, engine exhaust filters, and clean room filters. A good filter medium is characterized by low particle penetration (fraction of particles present downstream of the filter medium) and small pressure drop. Theoretically and empirically, particle penetration is found to be a function of dp, permitting a filter to be selective with respect to incoming particle sizes. The aerosol size at which the probability of particle penetration through the filter is maximal is called the most penetrating particle size (MPPS)6. This work seeks to capitalize on our previous research on size-selective filter media and filtration design solutions7, for the purpose of reducing the transmission of high BP constituents in particulate aerosols. Our work focuses on producing model aerosols exhibiting compositional heterogeneity with dp, and application of selective filter media to reduce the transmission of higher BP constituents of the aerosol particulate phase.

In this study, we first generated novel aerosol surrogates for naturally occurring heterogeneous aerosols. A commercial Sinclair-La Mer type8 condensation mono-dispersed aerosol generator (CMAG) was used for this purpose. Condensation of vapor on the surface of salt particles, acting as nuclei, was controlled by varying the vapor pressure and the concentration of nuclei6. Many different hydrocarbons have been studied by Japuntich D.A. (1991)9 to produce either solid particles (made of steric acid or carnauba wax etc) or liquid particles (made of sebacate oil, phthalate oil etc) using CMAG. However, the CMAG has been used so far to condense only single component vapor on a salt nucleus. We generated bi- component oil aerosol particles, whose compositional heterogeneity is a function of dp. We generated aerosols comprised of two miscible oils: bis(2-ethylhexyl)sebacate (DEHS) and

diethylhexyl phthalate (DEHP), with BP of 212 °C and 383 °C, respectively. The objective was to investigate if the fraction of higher boiling oil (DEHP) is preferentially enriched within aerosols with larger dp. Controlled dp distribution can be achieved in the range of ~100 nm to

~500 nm by regulating the temperature at which the vapor is generated (saturator temperature or Tsat) in CMAG. We chose to compare the overall compositions of aerosols generated at different Tsatâ??s as a way to obtain the composition-size correlation. Sizing of aerosol particles was done using a differential mobility analyzer (DMA) and condensation particle counter (CPC). For composition analysis, aerosols were collected on a glass fiber filter and assayed using Gas Chromatography Mass Spectrometry (GC-MS). The mass of oils in the aerosols was estimated from the GC-MS peak areas using a calibration correction. The mass ratio of DEHS to DEHP was then related to the mean diameter at the corresponding Tsat. Mathematical models based on the Kelvin equation were employed to understand the formation of these heterogeneous aerosols. In the second part of this work, nanofiber-microfiber composite filter media were developed for size-selective separation of aerosol particulates. In a previous study7 we examined performances of commercial microfiber filters (MFc) and cellulose acetate (CA) nanofiber filters as stand-alone uniform filter media, and demonstrated how the MPPS of CA nanofiber filters could be regulated by tuning the fiber properties. In this study, we deposit nanofibers on one face of MFc to obtain improved filter performance of nanofiber media without incurring large pressure drop. The nanofibers were generated by the electrospinning process using a solution of cellulose acetate in acetone and N,N-dimethylacetamide mixture. For measurement of separation selectivity, aerosols downstream of the filter media were collected by a glass fiber filter for

composition analysis in GC-MS.

Our results showed that the ratio of DEHP to DEHS exhibited an almost linear dependence on dp at smaller dp range, but the increase leveled off as the mean dp becomes larger. Nonetheless, these observations accord with the existing theory in general, which predicts an increase in the fraction of higher BP compounds in aerosol particles of larger dp. Combined with applications of size-selective filter media, this finding demonstrates the potential of compositionally selective separation for particulate aerosols through air filtration.

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