Intensification of Gas Absorption in a Downward Flow Microbubble Bioreactor

Author

Manizheh Ansari, CUNY City CollegeFollow

Date of Award

2019

Document Type

Dissertation

Department

Chemical Engineering

First Advisor

Sanjoy Banerjee

Keywords

Bioreactor, Gas Transfer, Intensification, Production, Turbulence

Abstract

Bioreactors are of interest for value-upgrading of stranded or waste industrial gases, such as CO, CH₄, or syngas. Process economics requires reduction of reactor cost and size while maintaining high production rate via rapid delivery of gas feedstock to the liquid phase which in turn requires a high volumetric mass transfer rate (). One strategy to achieve this goal is to increase interfacial area density, a, to >3000 m² m^-3, while minimizing use of energy. Here we show a novel reactor column design that uses a micro-jet array to break feedstock at ambient pressure into a downward microbubble flow. The performance of this device exceeds known technology in terms of volumetric gas transfer energy efficiency defined as reactor divided by reactor power density (achieving values near 0.34 s^-1 m³ kW^-1) while simultaneously achieving high values near 0.7 s^-1. A suite of hydrodynamic measurements reveal the character and mechanisms affecting reactor performance, specifically that variations in bubble and velocity properties are homogenized radially by turbulence, whereas axial gradients in turbulence are strong. Axial gradients in bubble diameter are negligible if small concentrations of surfactant additives are used (concentrations of order 10^-4 M) due to suppression of bubble coalescence. The key to increased and energy efficiency appears to be the bubble breakage mechanism by the liquid jets in the reactor, which produces highly sheared liquid layers at the jet edges, where bubbles break up and then rapidly move away after being broke up. At low or moderate gas injection rates, the liquid phase is always continuous with bubbles dispersed in it, while at the highest gas injection rates a gas-continuous layer forms at the top of the reactor underneath the pore plate. However we hypothesize that the bubble breakage mechanisms are still similar in both cases. We present measurements from four reactor heights (height-to-diameter ratios of 12, 9, 6 and 3) of , total interfacial area a, liquid residence time distribution, energy consumption, turbulence parameters, bubble breakup size, and bubble size distribution. A physical model for predicting is validated, and suggests that is governed by different hydrodynamics at different locations below the micro-jet array: 1) “entrance effects” due to Higbie (1935) type penetration mechanisms due to the creation of ‘new’ surfaces as bubbles are broken up and formed dominate at short distances, 2) turbulent convective mechanisms on the liquid side of the bubble interfaces (Calderbank & Moo-Young 1961) dominate at intermediate distances, and finally 3) convective effects due to the bubble rise velocity (Clift 1978) dominate at large distances. A persistent eddy is always found below the reactor’s pore plate, and appears to be an inescapable phenomenon related to the Coanda Effect. Recent advances in the understanding of molecular and hydrodynamic effects are applied to our data, including the effects of sodium dodecyl sulfate (SDS) and potassium chloride (KCl). Here we make experimental measurements of bubble breakup, bubble size distribution, and surface tension in the microbubble reactor with various concentrations of SDS and KCl. We find that surface tension is not the leading physical force that controls bubble breakage and coalescence rate, and neither surface tension nor critical Weber number predict the effect of additives on bubble size distribution. The presence of surfactants is found to increase the of the bubble dispersion, contrary to intuition. Our measurements of bubble breakup by the micro-jet array shows no effect from surfactant concentrations, thus we ultimately explain the effect of surfactants as a reduction in the rate of coalescence that leads to smaller average bubble size in the bulk locations of the reactor, which in turn leads to higher . Generalized energy efficiency equations for bubble production are derived and explained.

Recommended Citation

Ansari, Manizheh, "Intensification of Gas Absorption in a Downward Flow Microbubble Bioreactor" (2019). CUNY Academic Works.
https://academicworks.cuny.edu/cc_etds_theses/875