Richard W. Evitts

Growth kinetics of Chlorella vulgaris and its use as a cathodic half cell.

The kinetics of growth of the algal species Chlorella vulgaris has been investigated using CO(2) as the growth substrate. The growth rate was found to increase as the dissolved CO(2) increased to 150 mg/L, but fell dramatically at higher concentrations. Increasing the radiant flux also increased growth rate. With a radiant flux of 32.3 mW falling directly on the 500 mL culture media, the growth rate reached up to 3.6 mg of cells/L-h. Both pH variation (5.5-7.0) and mass transfer rate of CO(2) (K(L)a between 6h(-1) and 17 h(-1)) had little effect on growth rate. Growing on glucose, the yeast Saccharomyces cerevisiae produced a stable 160 mV potential difference when acting as a microbial fuel cell anode with ferricyanide reduction at the cathode. The algal culture was observed to be a workable electron acceptor in a cathodic half cell. Using an optimum methylene blue mediator concentration, a net potential difference of 70 mV could be achieved between the growing C. vulgaris culture acting as a cathode and a 0.02 M potassium ferrocyanide anodic half cell. Surge current and power levels of 1.0 microA/mg of cell dry weight and 2.7 mW/m(2) of cathode surface area were measured between these two half cells.

A Microbial Fuel Cell with a Photosynthetic Microalgae Cathodic Half Cell Coupled to a Yeast Anodic Half Cell

Abstract A recently developed photosynthetic cathodic half cell has been coupled to a fermentative anode to produce a complete microbial fuel cell. The photosynthetic organism Chlorella vulgaris was employed as the electron acceptor in the cathodic half cell with the yeast strain Saccharomyces cerevisiae serving as the electron donor in the anodic half cell. Maximum power was observed at 90 mV and a load of 5000 Ω, giving a power density of 0.95 mW/m2 of electrode surface area. The effect of changing culture conditions on fuel cell operation is reported. A maximum open circuit voltage of 315 mV was achieved with the addition of supplemental glucose to the anodic half cell. Enriching the feed air bubbled into the cathode half cell with 10% CO2 increased the open circuit voltage by a factor of 1.5 compared to using pure air as the feed gas. Since photosynthetic microbial growth at the cathode of the MFC consumes CO2, this complete microbial fuel cell is CO2 neutral and produces both electricity and ethanol as valuable energy products.

Effect of the Crevice Gap on the Initiation of Crevice Corrosion in Passive Metals

Abstract Past research into the mechanism governing the time to active crevice corrosion—the incubation period—of a passive metal crevice has produced theoretical models coupled with the B-dot model, the Debye-Huckel limiting law, and other activity models to correct for nonideal behavior at moderately high concentrations. In this research, the transport model of Watson and Postlethwaite is coupled with the ionic interaction model of Pitzer to predict the effect of the crevice gap on the iR drop and chemical activity of the crevice solution. Two cathodic reactions, crevice external oxygen reduction and crevice internal hydrogen ion reduction, are assumed to balance metal dissolution. To validate the model, the experimental Type 304 (UNS S30400) stainless steel crevice of Alavi and Cottis is simulated. Model predictions improve upon predictions of past models and match observations of this experimental work within experimental uncertainty. The effect of crevice gap on a titanium crevice immersed in 0.5 M a...

Corrosion of Carbon Steel and Corrosion-Resistant Rebars in Concrete Structures Under Chloride Ion Attack

Corrosion of reinforced concrete is the most challenging durability problem that threatens reinforced concrete structures, especially structures that are subject to severe environmental conditions (i.e., highway bridges, marine structures, etc.). Corrosion of reinforcing steel leads to cracking and spalling of the concrete cover and billions of dollars are spent every year on repairing such damaged structures. New types of reinforcements have been developed to avoid these high-cost repairs. Thus, it is important to study the corrosion behavior of these new types of reinforcements and compare them to the traditional carbon steel reinforcements. This study aimed at characterizing the corrosion behavior of three competing reinforcing steels; conventional carbon steel, micro-composite steel (MMFX-2) and 316LN stainless steel, through experiments in carbonated and non-carbonated concrete exposed to chloride-laden environments. Synthetic pore water solutions have been used to simulate both cases of sound and carbonated concrete under chloride ions attack. A three-electrode corrosion cell is used for determining the corrosion characteristics and rates. Multiple electrochemical techniques were applied using a Gamry PC4™ potentiostat manufactured by Gamry Instruments (Warminster, PA). DC corrosion measurements were applied on samples subjected to fixed chloride concentration in the solution.

Effect of Ionic Interactions on the Initiation of Crevice Corrosion in Passive Metals

In the crevice corrosion process, oxygen reduction occurs faster than diffusion into an occluded crevice, causing deoxygenation of the crevice solution. Once oxygen is depleted in the crevice, oxygen reduction can only occur on the metal surface outside of the crevice. The cations produced by metal dissolution are hydrolyzed, which causes the pH of the crevice solution to drop. This increases the rate of metal dissolution, forming an autocatalytic coupling that causes concentrated electrolyte solutions to form in the crevice. The focus of this paper is the prediction of the effect of nonideal solution behavior on crevice corrosion using the ionic interaction model of Pitzer coupled with an electrolyte mass-transport model. This mathematical model was used to simulate the type 304 stainless steel crevice corrosion experiment of Alavi and Cottis. The results are in excellent agreement with experimental observations. The model was then applied to simulate the crevice corrosion initiation period of a titanium crevice. Comparison of the predictions to those generated via an ideal solution crevice corrosion model indicates that interionic forces draw chloride ions into the crevice and hinder the transport of hydrogen ions out of the crevice.

A New Technique to Determine Convection Coefficients With Flow Through Particle Beds

A new method for determining the heat transfer coefficient for air flowing steadily through beds of particles is presented. In this technique, a step change in the inlet air temperature is applied to a small test bed and temperature distributions in the bed and at the air outlet are sampled over a short time period. The convective heat transfer coefficient is determined using data from the convective heat transfer process in the bed where the analysis includes the partial differential equation that describes the transient energy storage in the particles within the bed. The analysis is performed for a short time duration when the temperature distribution in the particle bed is almost linear along the axis of the bed. This time period permits the most accurate determination of the heat transfer coefficient using the data. Using beds of spherical particles a new correlation is developed for the Nusselt number versus the Reynolds number (5<Re dh <280) and includes the uncertainty bounds. This new correlation compares well with correlations developed by some other researchers for similar spherical particle beds.

Microbial fuel cell with a polypyrrole/poly(methylene blue) composite electrode

Different config urations of anodic and cathodic half-cells were incorporated into a microbial fuel cell to determine the effectiveness of a composite electrode. This novel composite electrode consisted of poly(methylene blue) and polypyrrole electrodeposited onto a stainless steel electrode. The novel electrode/immobilized mediator was incorporated into a microbial cathodic half-cell that relied on the microalgae Chlorella vulgaris for photosynthesis, and was a net reducer of carbon dioxide. Similar microbial cathodic half-cells were also examined using electrodes fabricated from graphite and graphite deposited with methylene blue. Results from using these three different electrodes in the microbial cathodic half-cell were examined and compared with the results from others. The electrode using the novel immobilized mediator demonstrated the highest short circuit current density of 65 mA/m 2 when compared with other C. vulgaris systems. Different anodic half-cells were also incorporated into the microbial fuel cell and tested. Anodic half-cells tested included a microbial half-cell containing Saccharomyces cerevisiae and one containing no microbial material and based on purely chemical constituents. In the case of the microbial anodic half-cell, different electrodes, including the novel immobilized mediator/electrode, were tested. It was found that the anodic half-cell performed better with a soluble mediator than an immobilized mediator/electrode. In the case of a fuel cell where both the anodic and cathodic half-cells are microbial, our results demonstrate better performance than previous systems by using a soluble mediator in the anodic half-cell with an immobilized mediator in the cathodic half-cell.

New method for calculating charge density in electrochemical systems

Abstract A new model for calculating the influence of charge density on electrolyte mass transport has been developed. This model is derived from Poissons equation for charge density and is implemented as an algebraic charge density correction. Use of this method greatly improves the computational efficiency of electrolyte mass transport modelling by avoiding the solution of Poissons equation. The new correction was used in the simulation of a moving boundary experiment. The predicted moving boundary velocity matched experimental results. Furthermore, the performance of this model was compared with the operator splitting algorithm of Evitts and Watson. Use of the new charge density model resulted in improved computational efficiency, numerical stability and accuracy.

A hybrid differencing scheme for mass transport in electrochemical systems

Purpose – To present a new hybrid differencing scheme for the numerical solution of an electromigration‐diffusion equation. The value of this work is evidenced by demonstrated improvement in the simulation of the Fu and Chan experiment when using the hybrid scheme.Design/methodology/approach – A hybrid differencing scheme is developed which is based upon the solution of the pseudo‐steady state electromigration‐diffusion equation. In this scheme, a weighting parameter is calculated that varies the relative influence of the upwind node (relative to the direction of electromigration). This scheme significantly enhances the accuracy of electrochemical system mass transport models.Findings – The hybrid scheme was compared to the upwind scheme. Use of the new hybrid scheme improved the accuracy of the model predictions by as much as 87 percent compared to the upwind scheme. However, use of the new scheme also increased the simulation time by between 6 and 43 percent. Deviations from electroneutrality and the pr...

Effects of Heat Loss/Gain on the Transient Testing of Heat Wheels

Heat wheels are used in ventilation systems to provide indoor thermal comfort by recovering considerable amount of sensible energy from exhaust airstream. The transient single step test is a new testing method developed to determine the sensible effectiveness of heat wheels. In practice, heat loss/gain may create large uncertainty in the sensible effectiveness obtained through the transient testing. In this study, the transient analytical model in the literature is extended to account for heat loss/gain effects in the transient testing. The results state that in particular operating conditions, the sensible effectiveness can be affected by more than 10% due to heat loss/gain. A new testing facility is developed to investigate the effects of heat loss/gain on the sensible effectiveness through transient testing of a small-scale heat exchanger. After decoupling heat loss/gain effects from transient test data, less than 2% difference was observed in the sensible effectiveness while supply and exhaust flow rate was small (Re 37.5 °C or Re > 600. An empirical correlation was proposed based on the transient test data that correlates the sensible effectiveness with the heat capacity rate ratio. Comparing the results of proposed correlation with literature, less than 2% difference was observed at the heat capacity rate ratio of greater than 0.5 after the heat loss/gain effects were decoupled from transient test data.