Gotowa bibliografia na temat „CHEMICAL TECHNIQUE”

Ultrasound has found application in chemical processing control using both low power, high frequency monitoring techniques and high power, low frequency process enhancement approaches. In many cases, standard ultrasonic systems are retrofitted to a process and while these produce efficiency improvements, the design of bespoke systems may offer more potential. In particular, this Thesis has considered two techniques used in the biomedical field; harmonic imaging and high intensity focused ultrasound (HIFU) and has translated these into ultrasonic transducers for use in an industrial process control system. Traditional ultrasound monitoring techniques are based on operation in the linear domain and are used to monitor chemical processes by measurement of material acoustic velocity, attenuation or based on spectral analysis. Both active and passive methods have been reported for application in this industrial sector. One issue is the presence of multiple reflections in the received ultrasonic signal which can mask the signals of interest from the load medium. This Thesis has considered a new ultrasonic monitoring approach using a combination of both linear and nonlinear spectral components. This was applied to high-throughput products and a dual frequency transducer designed and fabricated to acquire the ultrasonic backscattered signals in both the fundamental and second harmonic frequency regimes. The additional information provided by the harmonic device enabled discrimination between shampoo and conditioner products with the same density, but different molecular weights. HIFU transducer array designs are then considered for high power, low frequency chemical process enhancement applications. Typical applications of high power ultrasound use single or multiple discrete transducers to insonify a process. These are effective, but inflexible in the delivery of the ultrasonic field. The application of a HIFU array would provide control of the high power focal region in the load medium, which offer advantages to industry. Two transducer array approaches have been considered in this Thesis based on piezoelectric composite configurations. Three HIFU arrays based on the 1-3 piezocomposite have been fabricated to operate between 200-400kHz and fully characterised to evaluate their high power performance. A second transducer configuration was based on a novel 2-2 piezocomposite with a 2 layer stacked configuration. Simulation of this transducer design illustrated its potential for high power applications, although a number of fabrication issues resulted in the manufactured array not operating at full capacity. Importantly, the transducer configurations developed in this Thesis are shown to induce cavitation through the standard aluminium foil test.

In order to evaluate the electron microprobe (EMP) method for chemical dating of monazite, we chemically analyzed selected suites of monazite grains that were previously dated by standard U/Pb isotopic methods at three laboratories each equipped with a sensitive high resolution ion microprobe (SHRIMP). Representing diverse igneous and metamorphic lithologies, these grains yielded conventional isotopic ages ranging in age from Neoarchean to Devonian. Chemical dating was performed at Virginia Tech using a Cameca SX-50 EMP in which the analytical routines and settings were specifically optimized for monazite geochronology, including correction of analytical peaks for all major spectral interferences and correction of peak intensities for local background emission. Placement of cross-grain analytical traverses was based on backscattered electron (BSE) images together with wavelength-dispersive (WD) generated X-ray maps for Y, Th, U, and Ca, which revealed the internal compositional complexity of each grain. Shorter EMP traverses were selected adjacent to each SHRIMP pit in order to provide the best possible comparison of ages obtained by the two dating methods. Synthesis protocol for key elemental measurements (Y, Th, U, and Pb) was developed utilizing the 1Ï elemental errors associated with individual analyses, providing an objective approach for data synthesis. Analytical dates were either accepted or excluded based on analytical and spatial justifications. Isotopic dating techniques utilize three independent age calculations, provided the sample is old enough to have accumulated sufficient 207Pb (i.e., â ¥~1000 Ma). Similarly, the chemical dating method can utilize two independent age calculations (i.e., Th/Pb and U/Pb) and a U-Th-Pbtotal centroid age in Th/Pb vs. U/Pb space, verified independently against the calculated Th* or U* CHIME ages. Across the entire 2,200 m.y. age range represented by the sample set, the chemical ages calculated from the EMP data chemical ages are internally consistent (within 2Ï error) with the previously measured SHRIMP isotopic ages, except in one case where bulk mixing of discrete age domains within an ablation pit led to an isotopically discordant apparent age. Overall, this study illustrates that EMP chemical dating (1) represents both an accurate and precise primary method for dating monazite from igneous and polymetamorphic terrains; (2) provides superior spatial resolution for obtaining meaningful ages from small and/or irregular domains of discrete age that may be irresolvable or misinterpreted by other dating techniques that sample larger volumes; and (3) illuminates the geological meaning of isotopically discordant monazite ages obtained using conventional methods with lower spatial resolution (e.g., SHRIMP).<br>Ph. D.

The self-cycling fermentation (SCF) technique was used to grow three species of the genus Streptomyces which have been described as producing antibiotics. Application of this technique led to reproducible growth patterns for filamentous organisms. Control of the system using the CO$ sb2$ concentration in the off-gas was achieved under conditions which prevented monitoring of the dissolved oxygen. It was also possible to use the SCF system with either defined or complex media.<br>Production of tetracycline by S. aureofaciens was obtained on an iron-deficient medium. The production rate was as high as 0.272 g tetracycline/g sucrose/h. This was comparable with that reported for tetracycline in the patent literature. Growth patterns of the SCF when tetracycline was present in the fermenter were erratic, demonstrating the negative effect of the antibiotic on the growth of the producer organism. Further growth in a secondary fermenter system resulted in additional production over a period of about fifteen hours. In these experiments, tetracycline exhibited characteristics of a siderophore.

Self-cycling fermentation is a new technique used to cultivate synchronous populations of cells. This work used the technique in the production of citric acid from a strain of Candida lipolytica. Application of this technique resulted in extremely stable, reproducible patterns of growth and induced a high level of cell synchrony. Synchrony was maintained even throughout long periods of extended nutrient starvation. Thus, self-cycling fermentations can be operated with cycle times significantly longer than the doubling time of the microorganism. Use of this technique led to a full order of magnitude increase in the specific biomass production rate compared to literature values. This biomass was found to be suitable for the production of citric acid in a second stage reactor. Although this preliminary work with the second stage reactor was not optimized, the specific citric acid production rate obtained was comparable to rates found in the literature.

Microreactor technology is becoming an increasingly active research field in terms of chemical reaction engineering and process intensification. An important feature of microreactor technology is the requirement of a catalyst layer.

The present study was aimed at an investigation of the use of the pore reduced cement (PRC) technique on the fabrication and properties of calcium aluminate-based refractories. Physical and chemical performances were compared with those of conventionally prepared (unpressed) refractory castables. The influence of firing on two types of calcium aluminate cements; (Ciment Fondu and Secar 80), mixed with aggregates such as firebrick and alpha-alumina, were observed. Most of the pressed samples based on Ciment Fondu cracked and melted during firing to 1250^oC. Pressed samples based on Secar 80, however, tended to crack less with increased tabular alumina content. An appropriate weight proportion of tabular alumina and Secar 80 was found to be 70 to 30. The physical properties of Secar 80 blended with (i) tabular aluminas (20-μm and 250-μm) only and (ii) round (RMA325) and 250-μm tabular alumina were investigated before and after firing to 1350^oC. Pressed tabular alumina mixes containing various percentages of aggregate showed very little change in bulk density (3000 kg/m³) and flexural strength (23 MPa), but after firing, flexural strengths increased considerably as the fine tabular alumina content increased (up to 50 MPa). Microstructural evidence indicated that the interlocking structure occurring from the crystallisation of hexagonal plates of calcium hexaluminate including partial sintering of fine alumina grains could be the origin of the increased strength on firing. Although unpressed samples show similar structures, they are more porous. On firing, unpressed samples tended to expand (0.43 to 0.64%) less than pressed samples (1.32 to 1.71%) because the phase transformation from CA₂ to CA₆occurred which in pressed samples leads to expansion whereas in unpressed products, expansion is accommodated within available porosity. Abrasion resistance increases as strength increases. Fracture toughness increases with the coarse tabular alumina content increases. Similar improvements in bulk density and flexural strength are observed when round alumina (~ 44μm) and tabular alumina (250-μm) are blended. The thermal shock resistance of pressed samples was also found to be better than for unpressed samples as shown by the higher relative strength after the test. The chemical durability of unpressed and pressed samples based on the slag test was investigated. Pressed samples resist slag penetration better than the unpressed samples because of the smaller pores and reduced pores connectivity. Otherwise, chemical interaction between cement and slag was similar. In conclusion, refractory products based on the PRC technique have shown considerable improvements in both physical and chemical performance, especially green strengths which are substantially higher than for conventional castables, making them less susceptible to handling damage prior to service, i.e. during transport and installation.

An ascending aortic aneurysm is a pathologic enlargement of the ascending portion of the aorta. Comorbidities of dilation include aortic valve disease and connective tissue disorders. If the ascending aorta exceeds a threshold diameter, open heart surgery is recommended. This is a traumatic procedure and the recovery is demanding. As our population ages and with improved technologies to diagnose the disease, the number of cases will increase every year. Understanding the mechanics of ascending aortic tissue will help cardiac surgeons make timely decisions on when to intervene. There are many ways to characterize the mechanical properties of aortic tissue. In this study, we used biaxial and uniaxial tensile testing with an optical tracking system to record the Green-Lagrangian (Green strain) strain. Engineering and true stiffness values were calculated and compared along with patient characteristics. Aortas were classified by valve type as healthy, tricuspid, bicuspid type 1 and bicuspid type 2. The results show that diseased tissue does behave differently than healthy tissue indicating that a local remodeling does occurs to the aortic wall. There are also differences in the mechanics between the types of diseased valves suggesting that valve type also affects the way the aortic wall responds to the disturbed hemodynamic environment. Correlations between stiffness and patient characteristics show that no matter which experimented technique or method of stiffness calculation is used, relationships are generally conserved. The only difference is the magnitude of the elastic modulus. The conclusions drawn from the data would not change whether biaxial or uniaxial experiments were performed. However when comparing engineering and true stiffness, only 7/12 covariances were similar and therefore the conclusions are inconsistent.<br>Un anévrisme de l'aorte ascendante est un agrandissement pathologique de l'aorte ascendante. Les comorbidités de dilatation incluent les maladies de la valve aortique et du tissu conjonctif. Si l'aorte ascendante dépasse un diamètre seuil, la chirurgie à cœur ouvert est recommandée. Il s'agit d'une procédure traumatique et la récupération est exigeante. Comme notre population vieillit et que les technologies pour diagnostiquer la maladie se sont beaucoup améliorées, le nombre de cas décelé va augmenter chaque année. Comprendre les mécanismes du tissu aortique ascendant aidera les chirurgiens cardiaques à prendre les bonnes décisions au bon moment. Il y a plusieurs façons de caractériser les propriétés mécaniques du tissu aortique. Dans cette étude, nous avons utilisé les essais de traction biaxiale et uniaxiale avec un système de suivi optique pour enregistrer la souche Green-Lagrange (souche verte). Valeurs de rigidité d'ingénierie et de vrai ont été calculées et comparées avec les caractéristiques des patients. Les aortes ont été classées en 4 types de valves : bonne santé, tricuspides, bicuspides de type 1 et bicuspides de type 2. Les résultats montrent que les tissus malades sont différents des tissus en bonne santé qui indiquent qu'un remodelage local se produit sur la paroi aortique. Il existe aussi des différences dans le mécanisme des différents types de valvules qui suggèrent que le type de valve affecte également la façon dont la paroi aortique répond à l'environnement perturbé hémodynamique. Les corrélations entre les caractéristiques de rigidité et le patient nous montrent que quelque-soit la technique ou la méthode de calcul utilisée pour la rigidité, les relations sont généralement conservées. La seule différence est la grandeur du module d'élasticité. Les conclusions tirées de ces données ne changeraient pas, peu importe le type d'expérimentation effectué; biaxiale ou uniaxiale. Cependant, lorsque l'on compare la rigidité d'ingénierie et de vrai, seulement 7 covariances sur 12 sont similaires et les conclusions sont contradictoires.

Gas absorption with chemical reaction is an important unit operation in the chemical and petroleum industries for the selective removal of components from industrial gas streams. Apart from choosing absorption media, the most difficult problems facing the design engineer are the sizing and performance prediction of the absorption tower due to the scarcity of fundamental design data, especially when novel absorption media and/or packings are used. The solubility of carbon dioxide in 2 and 3 M solutions of 2-amino-2-methyl-1-propanol (AMP), which is a newly introduced absorbent, was determined at 20, 40, 60 and 80 °C and for CO₂ partial pressures ranging from approximately 1 to 100 kPa. The results were interpreted with a modified Kent-Eisenberg model which predicted the present and previous experimental results well. The absorption capacities of AMP and monoethanolamine (MEA) solutions were also compared. Detailed concentration and temperature measurements were reported for the absorption of carbon dioxide from air into NaOH, MEA and AMP solutions. A full-length absorber (0.1 m ID, packed with 12.7 mm Berl Saddles up to heights of 6.55 m) was used. It was operated in countercurrent mode and at 30 to 75 % flooding velocities which are typical for gas absorber operations. The following ranges of operating conditions were employed: superficial gas flow rate 11.1 to 14.8 mol/m² s; superficial liquid flow rate 9.5 to 13.5 m³/m² h; feed CO₂ concentration 11.5 to 19.8 %; total absorbent concentration 1.2 to 3.8 kmol/m³; liquid feed temperature 14 to 20 °C; total pressure 103 kPa. The measurements for the CO₂-NaOH and CO₂-MEA systems were compared with predictions from a previously developed mathematical model. Generally good agreement was obtained except at high CO₂ loadings of MEA solutions. Compared with MEA, AMP was found to have superior CO₂ absorption capacities and inferior mass transfer rates. A new procedure, called the Pilot Plant Technique (PPT), for designing gas absorbers with chemical reactions has been developed. The PPT is primarily intended for designing absorbers for which fundamental design information is lacking. It is based on the premise that full-length absorption columns can be sized by making a minimum number of tests using a small-scale pilot plant. Two special features of the PPT are (i) the details of hydrodynamic parameters (i.e. mass transfer coefficients, effective interfacial area and liquid hold-up) and the physico-chemical information of the system (e.g. reaction mechanism, reaction rate constants) need not be known and (ii) complex calculations are avoided. Using the PPT to size the height or to predict the performance of a given full-length absorber, the specific absorption rate, which is the essential information, can be measured directly using the pilot plant model (PPM) column if both columns have the same hydrodynamic conditions. This can be achieved by using the same type and size of packing in the PPM and the full-length columns and ensuring that the end and wall effects are negligible. The PPM column must also be operated at the same superficial fluid velocities as those of the full-length column. The specific absorption rate was then obtained from the gradient of the fluid composition profile along the PPM column. The validity of the PPT was demonstrated by determining the height and predicting the performance of the full-length column in which carbon dioxide was absorbed from air by aqueous solutions of NaOH and AMP at various operating conditions; good agreement was obtained.<br>Applied Science, Faculty of<br>Chemical and Biological Engineering, Department of<br>Graduate