Semester of Graduation

Fall 2019

Degree

Master of Science (MS)

Department

The Cain Department of Chemical Engineering

Document Type

Thesis

Abstract

As a consequence of seasonal eutrophication and human input, a vast hypoxic area termed The Dead Zone develops every year in the Gulf of Mexico (GOM) during summer along the Louisiana coastline characterized by vertical seawater density-stratification with oxygen concentrations less than 2 mg.l-1 at the seafloor. It poses a threat to bottom-dwelling faunae and their environment which has negative ecological and economic consequences. This project aims to mitigate hypoxia by employing mechanical impellers placed at strategic water depths and locations in the Gulf. Enhanced transport of oxygen results by mixing oxygen-enriched seawater at the surface, downward into the deeper oxygen-depleted water below.

This concept is supported and verified by laboratory experiments involving the mixing of targeted water layers below the water surface using a lab-scale impeller in a standard fish tank containing simulated density-stratified water. The vertical position of the impeller is determined by the Constant Flux Chemical Transport Model, which is derived from the classical advection-diffusion model, a well-established science and chemical-engineering-based approach. It is based on the presence of several horizontal density stratified layers positioned vertically throughout the water column. It is a modification of the traditional eutrophication-type mathematical model for use in the GOM shelf and when combined with the field-measured oxygen profiles, it can be used to identify specific “choke points” contributing to the resistance of vertical oxygen transport. The fish tank propeller mixing experiments resulted in the disruption of the density stratification, thereby enhancing vertical oxygen transport throughout the water column. On average, layer mixing for 30 minutes at an impeller rotation of 70 rpm resulted in stratification destruction and uniform oxygen concentration profile at the propeller position in the column. The constant flux model is a theoretical and mathematical approach proving useful in developing the solution to hypoxia by identifying mixing propeller vertical placement depth for destroying oxygen choke points. Though limited in number, field-size studies in lakes suggest that propeller with low power units can effectively move oxygenated surface water downward.

Committee Chair

Thibodeaux, Louis J

DOI

10.31390/gradschool_theses.5040

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