David T. Allen

Energy-water nexus for mass cultivation of algae.

Microalgae are currently considered a potential feedstock for the production of biofuels. This work addresses the energy needed to manage the water used in the mass cultivation of saline, eukaryotic algae grown in open pond systems. Estimates of both direct and upstream energy requirements for obtaining, containing, and circulating water within algae cultivation systems are developed. Potential productivities are calculated for each of the 48 states within the continental U.S. based on theoretical photosynthetic efficiencies, growing season, and total available land area. Energy output in the form of algal biodiesel and the total energy content of algal biomass are compared to energy inputs required for water management. The analysis indicates that, for current technologies, energy required for water management alone is approximately seven times greater than energy output in the form of biodiesel and more than double that contained within the entire algal biomass. While this analysis addresses only currently identified species grown in an open-pond system, the water management requirements of any algae system will be substantial; therefore, it is critical that an energy assessment of water management requirements be performed for any cultivation technology and algal type in order to fully understand the energy balance of algae-derived biofuels.

Catalytic Hydroprocessing of Chlorinated Benzenes

The catalytic hydrodechlorination reactions of hexachlorobenzene and all of its dechlorinated intermediates were examined in a differential microflow reactor. A presulfided NiMo/γAl2O3 catalyst and a reaction temperature of 325°C were used in all of the experiments. The dechlorination rate data and product selectivities for hexachlorobenzene and other chlorinated benzenes demonstrate that reaction steps frequently involve removal of multiple chlorine atoms. Not all possible intermediates are formed; all of the observed intermediate dechlorination steps proceed at comparable rates. A reaction mechanism based on an arenium ion intermediate is proposed to explain observed selectivities.

Flow characteristics of four commonly used mechanical heart valves

The in vivo and in vitro fluid dynamic performance of 4 mechanical heart valves was reviewed: Starr-Edwards silicon-rubber ball valves (models 1200/1260 aortic and 6120 mitral valves), Björk-Shiley tilting disc valves (standard spherical model, modified and unmodified convexo-concave [60 degrees and 70 degrees C-C] models), the Medtronic-Hall (Hall-Kaster) tilting disc valve and the St. Jude Medical bileaflet valve. These valves were chosen because of their past or present popularity in clinical use and because they encompass most of the basic mechanical valve designs used during the past 2 decades. The flow measurements reported include in vivo and in vitro mean pressure drop, cardiac output or cardiac index, regurgitant volume, effective orifice area and performance index.

Fourier Transform Infrared Spectroscopy of Aerosol Collected in a Low Pressure Impactor (LPI/FTIR): Method Development and Field Calibration

A method for determining the loadings of organic and inorganic functional groups in size segregated ambient aerosol has been developed and demonstrated. The method uses a Hering Low Pressure Impactor (LPI), equipped with ZnSe impaction surfaces, to sample the aerosol. The aerosol samples are analyzed directly, without extraction, using transmission infrared spectroscopy. The resulting spectra of aerosol size fractions are used to determine loadings of sulfate ion, nitrate ion, aliphatic carbon, carbonyl and organonitrate groups, using calibration factors developed in field and model compound studies. This paper describes, in detail, the data interpretation and calibration methods used to obtain these functional group loadings from the infrared spectra. The functional group loadings, particularly the size distributions of organic compound classes, provide new insights into the composition of atmospheric aerosol.

Fourier transform infrared analysis of aerosol formed in the photo-oxidation of isoprene and β-pinene

Abstract The chemical composition of smog-chamber aerosol generated during the photo-oxidation of isoprene and β-pinene was probed using infrared (i.r.) microscopy interfaced with a low pressure impactor. The low-pressure impactor employed ZnSe impaction surfaces which allowed direct analysis, with no extraction, using i.r. microscopy. The low detection limits of this technique, coupled with direct sample analysis, permitted the chemical analysis of aerosol generated at low to moderate hydrocarbon and ozone loadings. The quantitative analysis of the i.r. spectra is limited, in part, by the absence of calibration standards, however, it is clear that the biogenic aerosols generated in this work were liquid mixtures containing ketone, aldehyde, alcohol and organonitrate functional groups. Molar loadings of each of these functional groups were estimated for nine smog-chamber experiments. For aerosol formed in isoprene photo-oxidation, aldehyde and ketone groups dominated (1.8 groups per average molecule) while for aerosol formed in β-pinene photo-oxidation, alcohols and ketones dominated (a combined 2.7 groups per average molecule).

In vitro flow dynamics of four prosthetic aortic valves: a comparative analysis.

The velocity fields downstream of four prosthetic heart valves were mapped in vitro over the entire cross-section of a model aortic root using laser Doppler anemometry. THe Björk-Shiley 60 degrees convexo-concave tilting disc valve, the Smeloff-Cutter caged ball valve, the St. Jude Medical bileaflet valve, and the Ionescu-Shiley standard bioprosthesis were examined under both steady and pulsatile flows. Velocity profiles under steady flow conditions were a good approximation for pulsatile profiles only during midsystole. The pulsatile flow characteristics of the four valves showed variation in large scale flow structures. Comparison of the valves according to pressure drop, shear stress and maximum velocities are also provided.

Wastes as raw materials

Post consumer waste, industrial scrap and unwanted by-products from manufacturing operations should not be viewed as wastes. Rather, they are raw materials, but these raw materials are often significantly underutilized. One of the challenges of the emerging discipline of Industrial Ecology will be to identify productive uses for materials that are currently regarded as wastes, and one of the first steps in meeting this challenge will be to understand the nature of industrial and post consumer wastes. More than 12 billion tons of industrial waste (wet basis) are generated annually in the United States (EPA, 1988a,b; Allen and Jain, 1992). Municipal solid waste, which includes post-consumer wastes, is generated at a rate of 0.2 billion tons per year (EPA, 1990). When these material flows are compared to the annual output of the top 50 commodity chemicals (0.3 billion tons per year; Chemical and Engineering News, 1991), it is apparent that wastes should not be ignored as a potential resource. While these comparisons between waste mass and the mass of commodity products make apparent the magnitude of industrial wastes, just considering mass flows can be somewhat misleading. The extent to which industrial wastes could serve as raw materials is dependent not only on the mass of the waste stream, but also on the concentration of resources in the wastes. As shown in Figure 1, the value of a resource scales with the level of dilution at which it is present in the raw material. Resources that are present at very low concentration can only be recovered at high cost, while resources present at high concentration can be recovered economically. Thus, to understand the extent to which wastes can be used as raw materials, both waste stream flow rates and waste stream composition must be determined. The primary goal of this paper will be to evaluate the flow rates and concentrations of valuable resources in waste streams and determine the extent to which materials currently regarded as wastes might be used as raw materials. The results of this examination of waste stream compositions and flow rates will demonstrate that 0 many materials currently discarded in waste streams may be economically recoverable based on current market prices 0 deficiencies in waste stream data limit the range of materials and waste streams that can be included in industrial ecology studies. Mapping the flows of more than 12 billion tons of industrial and post-consumer waste …

Chlorine chemistry in urban atmospheres: a review

Environmental context Atmospheric chlorine radicals can affect the chemical composition of the atmosphere through numerous reactions with trace species. In urban atmospheres, the reactions of chlorine radicals can lead to effects such as increases in ozone production, thus degrading local and regional air quality. This review summarises the current understanding of atmospheric chlorine chemistry in urban environments and identifies key unresolved issues. Abstract Gas phase chlorine radicals (Cl•), when present in the atmosphere, react by mechanisms analogous to those of the hydroxyl radical (OH•). However, the rates of the Cl•-initiated reactions are often much faster than the corresponding OH• reactions. The effects of the atmospheric reactions of Cl• within urban environments include the oxidation of volatile organic compounds and increases in ozone production rates. Although concentrations of chlorine radicals are typically low compared to other atmospheric radicals, the relatively rapid rates of the reactions associated with this species lead to observable changes in air quality. This is particularly evident in the case of chlorine radical-induced localised increases in ozone concentrations. This review covers five aspects of atmospheric chlorine chemistry: (1) gas phase reactions; (2) heterogeneous and multi-phase reactions; (3) observational evidence of chlorine species in urban atmospheres; (4) regional modelling studies and (5) areas of uncertainty in the current state of knowledge.

Reactions of methylene and ether bridges

The reactions of model compounds containing methylene and ether bridges were examined, in the temperature range 350–400 °C. Aromatic displacement reactions, in which atomic hydrogen displaced the bridges, and homolysis via a keto-enol intermediate were the dominant modes of bridge decomposition. The aromatic displacement reactions of methylene-bridge compounds occurred with activation energies of ≈10 kcal mol^(−1) and with A factors of 10^(10.5–12) Is^(−1) m^(−1). The A factors vary with the size of the rings and the substituents on the rings connected by the methylene bridges. The activation energy is relatively independent of these effects. When phenolic substituents are present, methylene bridges can unimolecularly decompose via a keto-enol tautomerism, and the rate of product formation is solvent-dependent. Reactions for ether bridge decomposition were more complex and the rate parameters encompassed a much larger range of values.

Optimal time-space tradeoff in probabilistic inference

Recursive Conditioning, RC, is an any-space algorithm for exact inference in Bayesian networks, which can trade space for time in increments of the size of a floating point number. This smooth trade-off is possible by varying the algorithms cache size. When RC is run with a constrained cache size, an important problem arises: Which specific results should be cached in order to minimize the running time of the algorithm? RC is driven by a structure known as a dtree, and many such dtrees exist for a given Bayesian network. In this paper, we examine the problem of searching for an optimal caching scheme for a given dtree, and present some optimal time-space tradeoff curves for given dtrees of several published Bayesian networks. We also compare these curves to the memory requirements of state-of-the-art algorithms based on join-trees. Our results show that the memory requirements of these networks can be significantly reduced with only a minimal cost in time, allowing for exact inference in situations previously impractical. They also show that probabilistic reasoning systems can be efficiently designed to run under varying amounts of memory.