1,721,052 research outputs found
Anaerobic biodesulfurization of thiophenes
No full textDistillates from crude oil such as diesel and fuel oil may contain significant amounts of dibenzothiophenes and their alkylated derivatives, containing organically bound sulfur. Combustion of those fossil fuels leads to the release of polluting sulfur dioxide into the atmosphere, where it causes 'acid rain'. Due to stricter environmental legislation and depletion of crude oil reserves with low organic sulfur contents, effective desulfurization processes are becoming increasingly important. For instance: beginning in 2005 the maximal allowable sulfur content in gas oil in the European Community will be 0.005 wt.%. Currently, the refining industry applies the energy intensive physico-chemical hydrodesulfurization (HDS) process in order to reduce the sulfur content. Due to the high costs and inherent chemical limitations associated with HDS, biodesulfurization of hydrocarbon streams might represent an attractive complementary method to obtain sufficiently low sulfur levels. Bacteria require relatively mild process conditions (pressure and temperature) and bacterial enzymes are very selective in converting target molecules.The objective of this thesis was to develop an anaerobic biodesulfurization process. The thesis is build up around the reductive desulfurization reaction presented below.Dibenzothiophene (DBT) is converted under anaerobic conditions to biphenyl and sulfide with the concomitant conversion of reduction equivalents. The caloric value of the fuel molecule is retained and the sulfur is removed specifically.Chapter 1 presents a general introduction on physico-chemical and microbiological methods to desulfurize organic sulfur compounds.In Chapter 2 the DBT mass transfer rate within hydrocarbon droplets is compared to aerobic DBT conversion rates. The apolar DBT must diffuse to the hydrocarbon/water interface where bacteria prevail. The calculated values for the DBT mass transfer rate were compared to those found for aerobic DBT conversion rates, as reported in the literature. Temperature dependent data (ranging from 20 up to 60°C) of viscosity, density, and interfacial tension of various hydrocarbon distillates were incorporated in the model. The model simulated the DBT diffusion in hydrocarbon droplets as obtained in a stirred tank reactor. Based on these calculations, we estimated that the mass transfer rate of DBT within the hydrocarbon droplet to the hydrocarbon/water interface is at least a factor 10 to 10 4higher than the specific DBT conversion rates. However, the presence of a high specific surface area is essential to enhance the surface contact between bacteria and the hydrocarbon phase.The availability of a suitable biomass is crucial to develop this new bioprocess. In Chapter 3 a screening method is described to enrich biomass from mixed bacterial populations obtained from oil-polluted environments. The enriched cultures were able to grow in the presence of thiophenes as the sole electron acceptor. A proof of principle was obtained; the formation of sulfide and biphenyl from dibenzothiophene was shown conclusively. Also thiophene and benzothiophene depletion with concomitant sulfide formation was observed. However, apart from sulfide no thiophene nor benzothiophene desulfurization products could be demonstrated. The main problem during consecutive enrichments was the loss of biological activity after transferring the desulfurizing biomass. A mixed population was present and the active desulfurizing biomass was easily overgrown by acetogenic bacteria. Therefore, it was attempted to isolate the desulfurizing bacteria. The isolation procedure resulted in the availability of highly enriched cultures able to desulfurize thiophenes when cultivated using a selective medium with H 2 as electron donor and limiting amounts of bicarbonate and acetate (1 mM each).Based on process considerations H 2 gas is the most suitable electron donor for the reductive desulfurization process. In Chapters 4 and 5 attention is paid to the mass transfer rate of H 2 in a gas/water/hydrocarbon three-phase system using n -dodecane as model solvent. Because vigorous foam formation occurs when H 2 gas is directly added to a n -dodecane in water dispersion, it was proposed to saturate the n -dodecane with H 2 gas prior to disperse it into the water phase. Experiments to determine the H 2 mass transfer coefficients involved using physical methods are described in Chapter 4. The H 2 mass transfer coefficients between the gas and the n -dodecane phase ( k d ) and between the gas and the water phase ( k w ) were determined using a dynamic method by following the pressure decline in time, whilst the overall H 2 mass transfer coefficient between n -dodecane and water ( k dw ) was determined using a steady state method. The value for k dw was assessed using tritium-hydride (T-H instead of H-H) as the tracer. The effects of the temperature (30, 40 and 50 oC) and salt concentrations (0-250 mM) were studied. The value for k w [(9.7 ± 0.2) x 10 -5ms -1at 30ºC] was found to be a factor 3.3 higher than for k d [(2.89 ± 0.12) x 10 -5ms -1at 30ºC] because of the lower viscosity of water. No effect was found for the presence of salts (up to 250 mM NaCl) on the k w -value. The k dw -value determined in the steady state experiments at 30ºC was (5 ± 0.6) x 10 -6ms -1which is 19.4 times smaller than the above-mentioned k w -value. The considerable smaller value for k dw must be attributed to the additional mass transfer resistance introduced by the second liquid phase. Calculations of the maximal attainable H 2 flux revealed values of 0.016 x 10 -3mol/m 2s and 3.9 x 10 -3mol/m 2s for a n -dodecane/water and gas/water system, respectively. Therefore, the specific surface area between n -dodecane and water is the determining parameter for sufficient H 2 mass transfer. In Chapter 5, the H 2 mass transfer is described further using a bioreactor equipped with a nozzle to create very fine n -dodecane droplets. The specific surface area is dependent on the maximum attainable hold-up of n -dodecane and the diameter of the droplets. These parameters were studied in a model system consisting of n -dodecane and water supplemented with NaCl. The use of the nozzle resulted in droplets with a Sauter mean diameter of only 10.3±0.9mm. The droplet size was found to be independent of the applied pressure drop over the nozzle. The hold-up of n -dodecane in the aqueous medium is clearly dependent on the sodium ion concentration. The hold-up decreases rapidly (from 0.14 to 0.04) with increasing sodium ion concentrations due to coagulation; from 94 mM onwards the hold-up becomes 0.04. The application of n -dodecane droplets as carrier phase for H 2 mass transfer was demonstrated in batch tests for biological sulfate reduction. During operation of the bioreactor, biomass attached to the rising n -dodecane droplets and eventually flotated from the system.In addition biological steady state experiments were performed with hydrogenotrophic sulfate reducing bacteria to determine the H 2 mass transfer coefficient for a n -dodecane/water system ( k dw ). A value of (4.0±0.24) x 10 -6ms -1was found, which is close to the values found in the experiments using tritium hydride. Final calculations showed that the volumetric H 2 mass transfer rate (mol/m 3s) from n -dodecane to water can be comparable to values found for gas lift reactors, thus the high specific surface area that can be created by applying a nozzle can overcome the lower value of the H 2 flux (mol/m 2s) to a large extent.Chapter 6 addresses the role of sulfide on anaerobic biodesulfurization. The presence of increased sulfide concentrations is undesirable because it is expected that sulfide will inhibit the DBT conversion. Therefore, insight in the partitioning of gaseous hydrogen sulfide (H 2 S) over a three-phase gas/water/hydrocarbon system is required. The partitioning of H 2 S over a gas/water/ n -dodecane system is described. Experimental results matched well with the model predictions. The effect of the presence of an extra hydrocarbon phase ( n -dodecane) notably decreased the total sulfide in the water phase and the H 2 S fraction in the gas phase. The hydrocarbon phase serves as a sink for H 2 S molecules and by scrubbing the H 2 S in a separate process step ( e.g. during H 2 saturation) the sulfide concentration can be lowered to favor the anaerobic biodesulfurization
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
- …
