Sulfur Dioxide Transport through Aqueous Solutions

Author: Roberts, Daryl Lynn

Year: 1979

Degree: Dissertation (Ph.D.)

Advisor: Friedlander, Sheldon K.

Committee Member: Unknown, Unknown

Option: Chemical Engineering

DOI: 10.7907/1zqn-mh71

Abstract

A theory for the rate of SO2 transport through aqueous solutions has been developed based on measurements of the steady-state flux of SO2 through aqueous films of known composition. Conditions in most of the experiments were typical of flue gas desulfurization (FGD) scrubbers.

Aqueous films, in the form of agar gels or soaked polyethylene filters, were mounted in a diffusion cell. Humidified nitrogen with a known mole fraction of SO2 (ySO2,o) flowed through one side of the cell. Clean, humidified N2 flowed through the other side, sweeping away SO2 passing through the films. Films of water and solutions of neutral and alkaline sodium salts (0.1-2.0M NaCl, 10-3-3.0M NaOH, NaHSO3, or Na2SO3) were studied with ySO2 = 10-4, 2·10-4, 5·10-4, and 10-3.

In alkaline solutions, the transport by sulfur-containing ions was up to 1300 times larger than the contribution of dissolved SO2 molecules. The chemical reactions of SO2 that form these ions have previously been regarded as instantaneous (i.e., at equilibrium). Equilibrium theory, however, over-predicts the observed flux through alkaline solutions by up to a factor of seven. A new non-equilibrium boundary layer analysis (NEBLA) has been developed to account for this deviation. This analysis treats the boundary layers as regions where the conversion from molecular SO2 to sulfur-containing ions (and vice-versa) is frozen. The NEBLA is the first analysis to predict the flux for a system that has non-linear kinetics and is far from the limits of both zero and instantaneous chemical reactions.

For water and NaCl solutions, data agreed with an analytical expression for the flux based on the equilibrium approximation. This expression accounts for the (diffusion-induced) potential gradient. The potential gradient increases the effective diffusivity of HSO3- by approximately 73%. With this assist, HSO3-; is responsible for 80% to 95% of the net flux. The agreement of the water and neutral solution data with equilibrium theory shows that the deviation from equilibrium in the alkaline solutions is a pH effect.

Because the transport theory has been developed for a one-dimensional film, it can be most easily adapted to the film model that is employed in the absorption literature. In this way, the theory can be used to predict absorption rates of SO2 in FGD scrubbers and by bodies of water, rain, plumes, lung mucus, and plants.

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