Chemical Engineering Use of Catalyzed Sulfite Oxidation Kinetics for the Determination of Mass Transfer Characteristics of Gas-Liquid Contactors

A critical review and experimental verification of the correct use of the dynamic method for the determination of oxygen transfer in aerated agitated vessels to water, electrolyte solutions and viscous liquids

Venue, Cambridge, Massachusetts 02139 Oxygen-transfer enhancement has been observed in the presence of colloidal dispersions of magnetite (Fe3O4) nanoparticles coated with oleic acid and a polymerizable surfactant. These fluids improve gas-liquid oxygen mass transfer up to 6-fold (600%) at nanoparticle volume fractions below 1% in an agitated, sparged reactor and show remarkable stability in high-ionic strength media over a wide pH range. Through a combination of experiments using physical and chemical methods to characterize mass transfer, it is shown that (i) both the mass transfer coefficient (kL) and the gas-liquid interfacial area ( a) are enhanced in the presence of nanoparticles, the latter accounting for a large fraction of the total enhancement (80% or more), (ii) the enhancement in kL measured by physical and chemical methods is similar and ranges from 20 to 60% approximately, (iii) the enhancement in kL levels off at a nanoparticle volume fraction of approximately 1% v/v, and (iv) the enhancement in kLa shows a strong temperature dependence. These results are relevant to a wide range of processes limited by the mass transfer of a solute between a gas phase and a liquid phase, such as fermentation, waste treatment, and hydrogenation reactions.

Enhancement of Oxygen Mass Transfer Using Functionalized Magnetic Nanoparticles

A simple and convenient method for measuring K(L)a in large-scale fermentors was proposed. This method was based on the measurement of the dissolved oxygen concentration under steady state conditions established by an equivalency of the sulfite ion feed and chemical oxidation rates. This method had the following advantages: It was a steady state method, and so it was not necessary to consider the response lag of a dissolved oxygen probe and the response lag due to gas phase mixing in fermentors. The oxygen content of the effluent gas in this measuring system was nearly the same as that of the sparged air. Therefore, it was possible to use the oxygen partial pressure of the sparged air for the calculation of the driving force of oxygen transfer. The detailed information on the kinetics of sulfite oxidation was not necessary, because the dissolved oxygen concentration in steady state was not influenced by sulfite oxidation rates. The K(L)a measurement was finished in as short a period as 150 s, even in a fermentor with a volume of 10 m(3). Since the amount of Na(2)SO(4) accumulation in the test fermentors was very small because of the quick measurement, the K(L)a values obtained by this method were applicable to the electrolyte-free system. Furthermore, we could discharge the used liquid from the fermentors into a drain without any pretreatment due to the low salt concentration.

A simple Na2SO3 feeding method for KLa measurement in large‐scale fermentors

Bread doughs are fermented for very short periods of time with a range of 30 minutes to 4 hours. They are inoculated with 300 × 106 cells per gram and there is little or no yeast growth during the fermentation. In contrast, wine, beer, and distiller’s mashes are fermented for periods ranging from several days to several weeks. Inoculation levels are in the range of 2–10 × 106 cells per milliliter and there is a 5- to 10-fold multiplication of yeast cells during the fermentation. In addition, yeast cells may be recycled for use in succeeding batches of beer or wine fermentations. Baker’s yeast cannot be recycled because the yeast is killed during the baking process. Consequently, the production of baker’s yeast can be carried out on a very large industrial scale, and since the latter part of the nineteenth century, baker’s yeast has been produced by companies that specialize in its production.

Baker’s Yeast Production

Oxygen absorption enhancement in a sodium sulfite solution was studied in the absence and presence of copper catalyst both for absorption across the liquid surface in a stirred cell and for absorption from individual bubbles rising through a turbulent liquid. The enhancement factor was determined from the ratio of oxygen and argon mass transfer coefficients, measured under identical experimental conditions in the same batch of liquid. It has been found that the oxygen absorption is not chemically enhanced, as long as the mass transfer coefficient, kL0, is high enough, i.e., higher than the value 1.4 × 10−4 m sec−1 for the sulfite solution we used. An analysis of our data as well as literature data indicates that the sulfite system is poorly suited for studies of the volumetric mass transfer coefficient of physical absorption (kL0a) in fermentors, inasmuch as oxygen absorption can be chemically enhanced while the degree of enhancement depends on the operating conditions of batch aeration, as well as on the concentration of trace impurities with catalytic effects upon the sulfite solution used.

P. Beneš

Papers

Enhancement of oxygen absorption into sodium sulfite solutions