The Caltech Photooxidation Flow Tube reactor: design, fluid dynamics and characterization

Abstract

Abstract. Flow tube reactors are widely employed to study gas-phase atmospheric chemistry and secondary organic aerosol (SOA) formation. The development of a new laminar-flow tube reactor, the Caltech Photooxidation Flow Tube (CPOT), intended for the study of gas-phase atmospheric chemistry and SOA formation, is reported here. The present work addresses the reactor design based on fluid dynamical characterization and the fundamental behavior of vapor molecules and particles in the reactor. The design of the inlet to the reactor, based on computational fluid dynamics (CFD) simulations, comprises a static mixer and a conical diffuser to facilitate development of a characteristic laminar flow profile. To assess the extent to which the actual performance adheres to the theoretical CFD model, residence time distribution (RTD) experiments are reported with vapor molecules (O 3 ) and submicrometer ammonium sulfate particles. As confirmed by the CFD prediction, the presence of a slight deviation from strictly isothermal conditions leads to secondary flows in the reactor that produce deviations from the ideal parabolic laminar flow. The characterization experiments, in conjunction with theory, provide a basis for interpretation of atmospheric chemistry and SOA studies to follow. A 1-D photochemical model within an axially dispersed plug flow reactor (AD-PFR) framework is formulated to evaluate the oxidation level in the reactor. The simulation indicates that the OH concentration is uniform along the reactor, and an OH exposure (OH exp ) ranging from ∼ 10 9 to ∼ 10 12 molecules cm −3 s can be achieved from photolysis of H 2 O 2 . A method to calculate OH exp with a consideration for the axial dispersion in the present photochemical system is developed.

Assessing the degree of plug flow in oxidation flow reactors (OFRs): a study on a potential aerosol mass (PAM) reactor

“…Finally, we compare results from tracer tests and CFD for the base case. We compare this approach to that of previous studies by Lambe et al (2011a), Huang et al (2017), andSimonen et al (2017), which to the best of our knowledge are the only other studies to date that report experimentally derived RTDs in OFRs. We do not provide predictive configurations for the PAM reactor because there are many avenues different groups can take depending on their focus, and this study is central to the current design.…”

“…Such a low spacetime, while increasing the degree of plug flow, would result in a potentially significant loss of semivolatile or low-volatility gases. Additionally, other modes of transport such as convective effects (vertical mixing for non-isothermal conditions) could become more apparent, as revealed by Huang et al (2017) for the Caltech photooxidation flow tube (CPOT) reactor. As mentioned earlier, a detailed phenomenological modeling study of RTDs in the WU-PAM is beyond the scope of this study; however, at more conventional spacetimes, it would be helpful to visualize hydrodynamics to assess what contacting patterns and state of mixing the reactor exhibits.…”

“…Lambe et al describe RTDs for the two volumes using the axial dispersion model (ADM; Taylor, 1953Taylor, , 1954a, which is based on modeling plug or laminar flow with axial dispersion of material. Generally, as also stated by Huang et al (2017), the ADM is valid for regions where the radial Péclet number (Pé r ) is less than ∼ 4 times the aspect ratio (length of reactor divided by its cross sectional area), or if Pé r is greater than √ 48 (Aris, 1956;Taylor, 1954b). Both the PSU-PAM OFR and the WU-PAM OFR meet these requirements under typical flow rates (see Supplement,Sect.…”

“…Furthermore, our simulations are limited to isothermal conditions, and therefore cannot predict buoyancy effects that could explain spread in the RTD at low flow rates (or low Reynolds numbers; Fig. 3f), as observed by Huang et al (2017). Lambe et al (2011a) modeled the Pennsylvania State University PAM (PSU-PAM) reactor using a compartmen- Lower-frequency data for panel (e) were due to instrument repair, and temporarily set on longer averages.…”

“…Unfortunately, low levels of conversion and high wall losses seen in these large reactors did not allow simulated exposures that exceeded a day at most, which is just a short glimpse into the average 2-week lifespan of an atmospheric aerosol (Seinfeld and Pandis, 2006). Due to such limitations, oxidation flow reactors (OFRs) with short spacetimes (ratio of reactor volume to the volumetric flow rate) are being developed (Cazorla and Brune, 2010;Ezell et al, 2010;George et al, 2007;Huang et al, 2017;Kang et al, 2007).…”