Department of Chemical and Mining Engineeering

Permanent URI for this collection

https://repository.udsm.ac.tz/handle/123456789/2

Browse

Now showing 1 - 20 of 108

Bubble Size and Gas-Liquid Interfacial Area Measurements Using Molten Paraffin Waxes in Bubble Columns
(American Institute of Chemical Engineers, 1987) Bukur, Dragomir B.; Patel, Snehal A.; Daly, James G.; Raphael, Matheo L.
Experiments were conducted in 0.05 m ID and 0.23 m ID by 3 m tall bubble columns with different types of molten waxes as the liquid medium and nitrogen as the gas, under processing conditions typical or Fischer-Tropsch synthesis over iron catalysts (i.e. gas velocities up to 0.15 m s, and temperatures between 200 and 270/sup 0/C) to estimate gas liquid interfacial area from measured values of average gas hold-up and Sauter mean bubble diameter. The gas hold-up was estimated from visual observations of the expanded and static liquid heights, and the Sauter was estimated from bubble size measurements obtained by photography and dynamic gas disengagement. The paraffin wax (FT-300) used in the authors' studies is non-coalescing and has a tendency to foam. The amount of foam is greater for runs conducted in the order of increasing gas velocities, than in runs with decreasing velocities. Thus, two values of hold-up are possible and the start-up procedure determines which one will be attained. At higher gas velocities (> 0.05 m/s) the foam disappears and a transition to the slug flow, churn-turbulent regime takes place. Reactor waxes are coalescing in nature and do not produce foam. Despite similar hold-ups for the different waxes at higher gas velocities, the Sauters are significantly different and this is reflected in the specific gas-liquid interfacial areas, with larger values obtained with the paraffin wax compared to values with reactor waxes.
Hydrodynamics of Fischer-Tropsch Synthesis in Slurry Bubble Column Reactors
(1987) Bukur, Dragomir B.; Daly, James G.; Patel, Snehal A.; Raphael, Matheo L.; Tatterson, Garry B.
An improved photographic technique was employed to obtain pictures, for bubble size analysis, in experiments conducted in the Unit AMk2G (511 cm ID, 305 cm tall glass column)] During these experiments measurements of the average gas hold-up were made at 200 and 265oc using FTk300 paraffin wax as the liquid medium and nitrogen as the gas. Additional experiments were performed adding oxygenates, stearyl alcohol and stearic acid, (5 - 10 % by weight) to the wax at a temperature of 265°C. The addition of oxygenates did not have a significant effect on the average gas hold'up.
Hydrocracking Boscan Heavy Oil with Catalysts Containing a Zeolite Component.
(1990) Minja, Rwaichi J. A.
Hydrocracking Boscan Heavy Oil with a Cobalt-Molybdenum/Alumina Catalyst Containing an H-Mordenite Zeolite Component
(1991) Minja, Rwaichi J. A.; Ternan, Marten
Co-Mo/Al2O3 catalysts for hydrocracking heavy oil and residue were modified by adding up to 20 wt % hydrogen-mordenite zeolite. The acidic sites on the external surface of the mordenite crystals were expected to increase cracking reactions. In fact, there was a slight decrease in the +525-degrees-C resid conversion, although vanadium and nickel hydrodemetallization increased as the mordenite content of the catalyst increased. On the other hand, the pseudo turnover frequency for metals removal, i.e, the number of reactions per second per reaction site (or in this case per (nm)2), was greater for catalysts containing greater amounts of mordenite. The catalyst performance was attributed to a combination of two factors. First, both the catalyst bulk density (grams of catalyst per milliliter of reactor volume) and the catalyst specific surface area (m2/g) in pores larger than 3 nm decreased as the mordenite content increased. Hence, a smaller quantity of catalyst could be placed into the reactor and the catalyst that was in the reactor had less surface area per unit mass. Clearly the mordenite changed the structure of the alumina support, which resulted in a net decrease in the effective catalyst surface area. Second, the catalyst acidity, as measured by temperature-programmed desorption of benzofuran, increased as the mordenite component of the catalyst increased. It was concluded that the improved overall hydrodemetallization was caused by both the increased number of acidic sites of the exterior surfaces of the mordenite and the changes in catalyst pore geometry, which improved the rate of diffusion to the catalyst surface.
Effect of H-Mordenite Zeolite as a Component in Co-Mo-Al2O3 Hydroprocessing Catalysts Used for the Conversion of Boscan Heavy Oil
(Elsevier, 1991) Minja, Rwaichi J. A.; Ternan, Marten
The effect of H-mordenite zeolite as a component in Co-Mo-Al2O3 hydroprocessing catalysts has been studied. Catalysts containing up to 20% H-mordenite were used for hydrocracking Boscan heavy oil at 13.9 MPa. Although the acidic sites on the external surface of the zeolite crystals were expected to increase cracking reactions, little effect on conversion was observed. As the H-mordenite content of the catalyst increased, the bulk density and the specific surface area of the catalysts decreased substantially. When the reaction results were expressed on the basis of constant residence time and constant catalyst surface area there was an increase in the reaction parameter with increasing H-mordenite content of the catalyst. It was also found that coke deposition increased with the increasing H-mordenite content of the catalyst. These two observations suggest that H-mordenite caused an increase in the number of acidic sites in the catalyst. The results indicate that catalysts with H-mordenite would produce greater conversions than catalysts without H-mordenite, if the extrudates could be prepared in such a way that the catalyst bulk density does not change when the H-mordenite is added.
Isoelectric Precipitation of Sunflower Protein in a Tubular Precipitator
(Wiley, 1995) Raphael, Matheo L.; Rohani, S.; Sosulski, F.
Isoelectric precipitation of sunflower protein was carried out in a 10-m long, 6-mm internal diameter glass tubular precipitator. The effects of feed flow rate, protein concentration in the feed stream, and volumetric feed ratio of precipitant (HCl aqueous solution) to protein solution on solid protein recovery and particle size distribution were studied. The dispersion range of the tubular precipitator was modelled to predict the experimental results. Calculated initial growth rates of protein particles were found to: increase with increases in feed flow rate and protein concentration in the feed stream, and decrease with increases in volumetric feed ratio.
Isoelectric Precipitation of Sunflower Protein in an MSMPR Precipitator: Modelling of PSD with Aggregation
(Elsevier, 1996) Raphael, Matheo L.; Rohani, S.
Isoelectric precipitation of sunflower protein was carried out in a 273 ml MSMPR precipitator. Experimental results showed a bimodal particle-size distribution (PSD) of protein particles when the solids concentration or the mean residence time was low. Increasing the solids concentration and the mean residence time transformed the bimodal PSD to a unimodal PSD. Protein particle growth by turbulent collision mechanism and breakage by shear mechanism were modelled using an approach similar to Glatz et al. A.I.Ch.E. J.32, 1196–1204 (1986). The model results showed that the breakage of large aggregates results in the birth of two daughter fragments. Also at high solids concentrations the particle growth rate was linear with respect to particle size. At low solids concentrations the growth rate constant was larger than the breakage rate constant and vice versa at high solids concentrations.
On-Line Estimation of Solids Concentrations and Mean Particle Size Using a Turbidimetry Method
(Elsevier, 1996) Raphael, Matheo L.; Rohani, S.
On-line measurement of solids concentrations was performed using a turbidimetry method. Four different samples (PVC, sand, protein and KCl particles) with solids concentrations up to 10 wt.% were used in this study. At higher solids concentrations the measured light intensity approached zero. The extrapolated Beer-Lambert's equation in polynomial form, ln (transmission) as a function of ln(solids concentration), was found to best fit the experimental data. For protein particles with mean sizes less than 50 μm the fifth or sixth order polynomial equation was required to give the best fit (regression coefficient greater than 0.98). Whereas, larger particles were best fit using Beer-Lambert's equation with the mean particle size as one of the parameters. Transmission data from samples with unimodal particle size distribution (KCl samples) were used to estimate the optical parameters of the KCl suspension. With known optical parameters and on-line turbidity and solids concentrations the mean particle size of the flowing suspension was estimated. The calculated and experimental mean particle sizes are within ± 10%.
Protein in a Tubular Precipitator
(1997) Raphael, Matheo L.
Recovery and Kinetics Study of Isoelectric Precipitation of Sunflower Protein in a Tubular Precipitator
(1997) Raphael, Matheo L.
Oilseed sunflower is one of the major vegetable oil sources. The residue after the extraction of oil (defatted meal, DM) contains a substantial amount of nutritious protein which is recoverable by extraction and precipitation methods. For effective recovery of the solid proteins from the solids-liquid suspension, it is imperative to have large particles with a narrow spread in their size distribution and high solids concentration. In this research, up to 97 % w/w and 65 % w/w of the proteins in the laboratory and industrially defatted sunflower meals, respectively, were extracted using aqueous alkaline solutions at pH 10.0. Up to 84 % of these soluble proteins were recovered as solids when the pH was lowered from 10.0 to 4.0 (isoelecmc pH) using aqueous HCI acid. The amount of proteins recovered as solids decreased with decreasing concentrations of the acid.
Modified Polysulphide (Aq) Pulping Of Softwood
(1997) Minja, Rwaichi J. A.; Kleppe, Peder J.; Karlsen, Trond
A review of the polysulphide (PS) liquor generation methods has been made. By preparing some PS liquors which simulate future possibilities, pilot scale PS-(AQ) pulping studies were conducted. The studies were carried out in a recently acquired pilot plant which can simulate both conventional cooking techniques as well as the latest pulping developments. Two types of pulps were made, type 1 were liner board grade pulps and type 2 were bleachable grade pulps. Type 1 pulps were made by using up to 20% green PS-liquor in conventional PS-AQ pulping. Type 2 pulps were made by adding all PS in the impregnation stage of ITC simulated cooks and applying either NaOH (Soda liquor) or kraft liquor (WL) as the major alkali source.
Improving the Pulp Yield by Using Ps/Aq And/Or Two Stage Oxygen Delignification
(1998) Minja, Rwaichi J. A.; Moe, Størker T.; Kleppe, Peder J.
The effect of polysulfide (PS) and anthraquinone (AQ)addition on kraft cooking is investigated together with oxygendelignification of the high-yield PS/AQ kraft pulps. Additionof polysulfide and AQ increases pulp yield. The yield benefitis considerably lowered when cooking to kappa numbersbelow 27-28. Oxygen delignification to about 55%delignification preserves the yield increase attained by PS/AQaddition. For two-stage extended oxygen delignificationbeyond 55% kappa decrease , the yield increase is lowered.Peroxyacetic acid “activation” of the pulp followed by asecond oxygen stage also lowered the yield significantly.From the data presented, optimal yield for bleached pulpwould be reached when cooking with PS/AQ addition tokappa numbers between 40 and 45, followed by oxygendelignification to kappa numbers around 22. This could thenbe followed by bleaching with more selective bleachingchemicals like ozone and/or chlorine dioxide.
The Possibilities to Apply Polysulphide-Aq Pulping in Kraft Mills
(1998) Kleppe, Peder J.; Minja, Rwaichi J. A.
Developments in polysulphide pulping during the last 35 years are reviewed with emphasis on the successful PSpulping experience at the Peterson kraft mill in Moss, Norway. New and economical PS generation technology not disturbing the Na/S-balance in kraft mills will enhance application of PS-pulping world wide. Multi-stage PSpulping may improve the yield increase for a given PSsulphur charge to a digester system. Especially PS-black liquor impregnation followed by kraft cooking with conventional white liquor is interesting. Drying of fresh wood chips before PS-AQ pulping has a negative effect on obtainable yield increase. The paper and board making properties of polysulphide-AQ pulps are comparable to kraft pulps from a given wood source. PS-AQ paper grade pulps seem to preserve their paper making properties by recycling better than kraft pulps containing less hemicelluloses.
Carbohydrate Analysis of Pulps Using Enzymatic Hydrolysis and High Pressure Liquid Chromato- Graphy
(1999) Syverud, Kristin; Moe, Størker T.; Minja, Rwaichi J. A.
A revised method for analysis of carbohydrates in pulps is presented. The method utilizes commercially available enzyme preparations for enzymatic hydrolysis of the pulp, followed by acid hydrolysis for complete cleavage of oligomers that are still present. Quantification of monosaccharides in the hydrolysate was performed by HPLC using a Pb-sulphonate/polymer column and deionized water as the mobile phase. Integration of the chromatograms was performed using least squares fitting of partially resolved peaks by commercial software. The method requires moderate operator skills and low investment costs for specialized analytical equipment.
Sunflower Protein Precipitation in a Tubular Precipitator
(Wiley, 1999) Raphael, Matheo L.; Rohani, Sohrab
Isoelectric precipitation of sunflower protein was carried out in a 20 m long, 6 mm internal diameter glass tubular precipitator. Effects of feed flow rate, protein concentration in the feed stream, and mean residence time on particle size distribution (PSD) were studied. The population balance equation (PBE) for the tubular precipitator was solved using the orthogonal collocation multiple shooting method. A simulated annealing method was used to determine the precipitation kinetic parameters for the nucleation rate, growth rate, aggregation rate, and breakage rate from the experimental data. Due to the lack of experimental data on the kinetics of sunflower protein precipitation in the literature, the extracted kinetics from our experiments were used to evaluate the predictive capability of the present model for experiments whose results had not been used to derive the kinetics. The model predictions of the particle size distribution along the tubular precipitator showed better agreement with the experimental data for large particles in comparison to the small particles (< 10 μm).
Estimation of Nitrogen Requirement in Peat and Perlite Biofilters Removing Hexane from Air
(Springer Netherlands, 2001) Kibazohi, Oscar; Anderson, William A.; Moo-Young, M.
Biofiltration experiments to remove hexane from air were conducted in column reactors packed with peat, perlite and their mixture. The particle size of the solid medium ranged from 1.70 to 4.75 mm, and the average empty bed superficial velocity was 20 m/h. To achieve and maintain a high rate of hexane removal, addition of nutrient solution was necessary. Adding a nutrient solution of a commercial fertilizer containing 1000 g of nitrogen for the first two weeks, followed by a weekly addition of 280 g of nitrogen per m3 of filter bed was found to be effective in maintaining a high hexane removal rate between 20 and 30 g/m3.h. The hexane removal rate decreased gradually to less than 15 g/m3.h in 50 days due to an accumulation of biomass in the reactors. Pressure drop, which varied depending on the type of packing, also increased drastically to maximum values of 120 Pa/m for 100% perlite and 2930 Pa/m for the mixture. For long-term operation and low energy cost, prevention of biomass accumulation and maintenance of low pressure drop is essential. When the frequency of nutrients addition was reduced excessive biomass growth, and increase in pressure drop with time were controlled. Our observations showed that addition of a nitrogen source of approximately 1 kg of nitrogen per m3 of filter bed for the first and second weeks, and every 30 days (approximately) resulted in an extended life and slightly lower hexane removal. The columns packed with peat and the mixture showed a better hexane removal than the column packed with perlite alone. However, the column packed with perlite had the lowest pressure drop.
Arsenic Adsorption Capabilities of Soil-Bentonite Mixtures as Buffer Materials for Landfills
(2002-01) Minja, Rwaichi J. A.; Ebina, Takeo
Removal and fixation of As (III) and As (V) from aqueous solution by soil/bentonite mixtures were studied to develop reliable clay liners for waste landfill sites. One of two soils such as, Masatsuchi. soil (weathered granite) and Murram soil (pumice) was used as a main body of the liner, and Wyoming bentonites were mixed with the soils because of its superior impermeability. As the result of batch experiments, it was shown that large part of As was removed by Masatsuchi soil without pH buffer, andth e result was better than those of Murram soil. Both soils appeared to have best adsorption of As (V) and As (III) within pH ranges of 3-6.5 and 7-9.5 at where the dominant species in the aqueous solution were H2AsO4- and H2AsO3-, respectively. A long-term leak test showed that Masatsuchi soil-Wyoming bentonite combination had the ability to remove all the As (III) from 4 ppm aqueous solution for more than 100 days.
Development and Testing of a Probe to Monitor Gas-Side Fouling in Cross Flow
(Taylor & Francis, 2002-05) Temu, Abraham K.; Næss, Erling; Sønju, O. K.
A probe to monitor gas-side fouling in cross flow has been developed and tested in laboratory experiments. The probe is suitable for monitoring both the heat flux and the mass accumulated on the front and rear side of a cylinder in cross flow. A procedure for converting the mass accumulation measurements to thermal resistance is developed. Experimental results obtained using the mass accumulation method are compared to data obtained by heat flux measurements, and show good agreement in terms of the thermal fouling factor. It is also shown that the test results are in good agreement with other experimental results found in the literature. The probe is well suited for distinguishing the thermal effect (thermophoresis) from the other forces contributing to fouling. It is also useful for nonfouling heat transfer studies, providing average heat transfer coefficients on the front and rear sides of a cylinder.
Correlation of Hydraulic Conductivity of Clay–Sand Compacted Specimens with Clay Properties
(Elsevier, 2004) Ebina, Takeo; Minja, Rwaichi J. A.; Nagase, Takako; Onodera, Yoshio; Chatterjee, Abhijit
Hydraulic conductivity of compacted specimens consisting of 1 of 16 clay samples and siliceous sand/natural soils was measured using permeameters with flexible wall cells. Clay samples studied were eight natural bentonites, one purified montmorillonite, five standard clay samples, one synthetic hectorite, and two cation-exchanged bentonites. The clay content to whole solid was fixed at 11.5% w/w, and the mixtures were rammed in a mold to form disk compactions. A higher hydraulic conductivity was observed with increase in the Ca/Na ratio of clay. Hydraulic conductivity was also affected by bentonite content of the clays and the mineral content. A relation between the hydraulic conductivity and the characteristic index properties of the clays such as chemical concentration, methylene blue adsorption capacity (MB), and so on were elucidated, and then a polynomial expression was derived to reproduce measured hydraulic conductivity data. This relation was then utilized to estimate hydraulic conductivity of blended bentonites from characteristic properties of the component bentonites.
Major Observations, Lessons and Recommendations
(2004) Mwamila, Burton; Diyamett, Bitrina; Temu, Abraham K.; Trojer, Lena; Bångens, Lennart

Browse

Browsing Department of Chemical and Mining Engineeering by Issue Date

Results Per Page

Sort Options