Research Repository :: Browsing by Author "Minja, Rwaichi J. A."

Browsing by Author "Minja, Rwaichi J. A."

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Analysis of Different Techniques for Improving Performance of Flat Plate Solar Collectors for Fruits Drying
(2014-06) Bakari, Ramadhani; Minja, Rwaichi J. A.; Njau, Karoli N.
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.
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.
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.
Effect of Diffusional Mass Transfer on the Performance of Horizontal Subsurface Flow Constructed Wetlands in Tropical Climate Conditions
(2011) Njau, Karoli N.; Gastory, L; Eshton, Bilha; Katima, Jamidu H. Y.; Minja, Rwaichi J. A.; Kimwaga, Richard; Shaaban, M.
The effect of mass transfer on the removal rate constants of BOD₅, NH₃, NO₃ and TKN has been investigated in a Horizontal Subsurface Flow Constructed Wetland (HSSFCW) planted with Phragmites mauritianus. The plug flow model was assumed and the inlet and outlet concentrations were used to determine the observed removal rate constants. Mass transfer effects were studied by assessing the influence of interstitial velocity on pollutant removal rates in CW cells of different widths. The flow velocities varied between 3-46 m/d. Results indicate that the observed removal rate constants are highly influenced by the flow velocity. Correlation of dimensionless groups namely Reynolds Number (Re), Sherwood Number (Sh) and Schmidt Number (Sc) were applied and log-log plots of rate constants against velocity yielded straight lines with values β = 0.87 for BOD₅, 1.88 for NH₃, 1.20 for NO₃ and 0.94 for TKN. The correlation matched the expected for packed beds although the constant β was higher than expected for low Reynolds numbers. These results indicate that the design values of rate constants used to size wetlands are influenced by flow velocity. This paper suggests the incorporation of mass transfer into CW design procedures in order to improve the performance of CW systems and reduce land requirements.
Effect of Glass Thickness on Performance of Flat Plate Solar Collectors for Fruits Drying
(Hindawi Publishing Corporation, 2014) Bakari, Ramadhani; Minja, Rwaichi J. A.; Njau, Karoli N.
This study aimed at investigating the effect of thickness of glazing material on the performance of flat plate solar collectors. Performance of solar collector is affected by glaze transmittance, absorptance, and reflectance which results into major heat losses in the system. Four solar collector models with different glass thicknesses were designed, constructed, and experimentally tested for their performances. Collectors were both oriented to northsouth direction and tilted to an angle of 10° with the ground toward north direction. The area of each collector model was 0.72 m2 with a depth of 0.15 m. Low iron (extra clear) glass of thicknesses 3 mm, 4 mm, 5 mm, and 6 mm was used as glazing materials. As a control, all collector performances were analysed and compared using a glass of 5 mm thickness and then with glass of different thickness. The results showed that change in glass thickness results into variation in collector efficiency. Collector with 4 mm glass thick gave the best efficiency of 35.4% compared to 27.8% for 6 mm glass thick. However, the use of glass of 4 mm thick needs precautions in handling and during placement to the collector to avoid extra costs due to breakage.
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.
Energy Sources, Utilization and Energy Access in Tanzania
(2010-01) Minja, Rwaichi J. A.
The energy sources in Tanzania include: bio energy (mainly biomass), electricity, petroleum oil (imported), natural gas, coal, solar and wind. Among these sources, biomass, hydro, wind and solar are renewable sources of energy. As a result of high capital costs and technologies associated with harnessing solar and wind energies, these have been utilized to a very small scale in Tanzania. Some of the potential hydropower sources have been developed for production of main grid electricity, but due to climate change, which has lead to drought in some years, electricity production from the installed hydropower plants sometimes goes below the production capacity. Other electricity production sources are from natural gas, petroleum oil and coal. Bio energy resource (mainly fuel wood and charcoal from both natural forest and plantations) accounts for 93% of total energy consumption. The challenge which lies ahead is how to increase clean energy access. The paper explores the current energy status in Tanzania, energy access and problems associated with energy access and initiatives in place.
Experimental Analysis of Air Flow Patterns in Perfomance of Flat Plate Solar Collectors
(Academic Journals, 2015) Bakari, Ramadhani; Minja, Rwaichi J. A.; Njau, Karoli N.
Solar drying is one of the promising methods of reducing post-harvest losses in rural areas. Studies have shown that, heat transfer mechanisms in a solar collector influences the performance of solar dryers. This study aims at improving heat transfer in flat plate solar collectors by designing different air flow patterns inside the collector. Three flat plate solar collectors were constructed by using Pterocarpus timber (Mninga) and tested for their effect in heat transfer at various flow patterns. Three different flow patterns namely: single duct front pass, double duct parallel flow and double duct counter flow were designed and tested. Experimental results show that collector efficiency of single duct front pass, double duct front pass and double duct counter flow were 30.6, 36.1 and 38.2% respectively. It was found that, double duct flow gives improved performance compared to single duct flow due to the increased heat transfer area. In additional, double duct counter flow showed superior performance compared to double duct parallel flow due to extended heat transfer area and the advantage of air preheating at the inlet which reduces heat losses through glazing. Through this study, it was concluded that, solar collector designs with double duct counter flow can improve collector performance for up to 8.3% compared to single duct front pass.
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.
Hydrocracking Boscan Heavy Oil with a Como/Al Sub 2 O Sub 3 Catalyst Containing an H-Mordenite Zeolite Component
(2009-01) Minja, Rwaichi J. A.; Ternan, Marten
Co-Mo/AlâOâ 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 +525Â°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)Â²), 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 (mÂ²/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.
Hydrocracking Boscan Heavy Oil with Catalysts Containing a Zeolite Component.
(1990) Minja, Rwaichi J. A.
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.
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.
Modifying Plant Oils for Use as Fuel in Rural Contexts Tanzania: Techno-Economic Analysis
(2014) Mlay, Happiness; Katima, Jamidu H. Y; Minja, Rwaichi J. A.
Techno-economic analysis of a small-scale Modified Plant Oil (MPO) production plant that has an annual production capacity of 15,072,741 kg of MPO (batch process) was carried out to estimate the capital and operating costs of a plant. The analysis was done by using a computer model that was designed and simulated with an aid of SuperPro Designer (Version 4.32) software. The specified feedstock was crude Jatropha oil (JO) and the main product was MPO. The major processes involved were degumming, neutralisation and blending. Degumming involved the removal of gums or phospholipids, and two methods were used: water degumming and acid degumming, whereas blending involved mixing of degummed or purified JO with natural gas condensate (NGC) modifier to lower the viscosity of JO. From techno-economic analysis of the process, it was found that the total capital investment of a plant was about US $ 10,222,000 and the predicted unit production cost of MPO was US $ 1.315/kg at a value of US $ 1.0/kg of JO. The economic feasibility of MPO production was found to be highly influenced by the price of feedstock, which contributed about 95% of the total annual production cost. The relationship between plant throughput and unit cost of producing MPO showed that unit production cost was very sensitive to production rate at low annual throughputs. The MPO cost showed a direct linear relationship with the cost of JO, with a change of US $ 0.50/kg of MPO in MPO cost in every change of US $ 0.50/kg of JO in JO price. The process technology simulated was found to be economically viable and can be implemented in rural setting, taking into consideration Tanzania’s rural situation.
Plant Oil/Fuse Oil Blends as Alternative Fuels in Low-and Medium Speed Diesel Engines
(2015-04) Mlay, Happiness; Katima, Jamidu H. Y; Minja, Rwaichi J. A.
Fuel blending is one of the methods of lowering plant oil’s viscosity, which is the major parameter that leads to serious problems on diesel engine performance in terms of operation and durability. A study was carried out on fuel properties of Jatropha (JO) and Croton (CROT) seed oils blended with a distillery by-product, fusel oil (FO) (modifier) of between 20% and 50% (v/v) at room temperature (30 ± 3˚C). The effect of FO composition in the resulting blend on the kinematic viscosity and fuel combustion properties of the blends was analysed. The results were compared to the properties of crude plant oils (POs) and diesel fuel (grade-4D) specifications. The blends were found to be compatible over a wide range of compositions and exhibited much lower viscosity and better combustion properties than crude plant oil (PO) samples. The optimum proportions for a blend’s viscosity to meet the specifications for diesel fuel (grade 4-D) were found to depend on the modifier characteristics; and that 20% (v/v) FO was enough to produce the desired liquid biofuel blend. The characteristics of blends presented in this work matches well with the characteristics of fuels for low- and medium-speed diesel engines. This implies that the PO/FO blends can be used as alternative fuels in low- and medium speed diesel engines.
Plant Oils Blends as Alternative Fuels for Low -and Medium Speed Diesel Engines: Natural Gas Condensate Utilization
(2015-04) Mlay, Happiness; Katima, Jamidu H. Y.; Minja, Rwaichi J. A.
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.
Pumice Soil: A Potential Wetland Substrate for Treatment of Domestic Wastewater
(IWA Publishing, 2003) Njau, Karoli N.; Minja, Rwaichi J. A.; Katima, Jamidu H. Y.
Laboratory and fieldwork studies were carried out to evaluate the potential of pumice soil for use as a wetland substrate in wastewater treatment. The composition of pumice soil was analysed by x-ray fluorescence (XRF) and x-ray diffraction (XRD) techniques. Adsorption kinetic studies were carried out in a semi-batch recycle system. Fieldwork tests were carried out on Subsurface Flow Constructed Wetland (SSFCW) cells planted with Phragmites mauritianus and Vetiveria zizanioides. The results have shown that pumice soil composition contains among other elements Al, Ca, Fe and Mg, which are positive indicators for phosphorus adsorption. The main minerals observed by XRD were augite, hematite, and sodium titanium silicate. Phosphorus adsorption kinetics have shown that phosphorus is adsorbed on pumice soil following first order kinetics and the adsorption was highly influenced by mass transfer. Approximately 3% of the phosphorus was removed by plant uptake.
The Use of Toddalia Asiatica (L) Lam. (Rutaceae) in Traditional Medicine Practice in East Africa
(Elsevier, 2008) Orwa, Jennifer A.; Jondiko, Job I.O.; Minja, Rwaichi J. A.; Bekunda, Mateete
Toddalia asiatica (L) Lam. (Rutaceae) has been used by traditional health practitioners in East Africa for management of diseases, however, theextent of its usefulness has not been established to date. Fieldwork for this study was carried out in the Lake Victoria Basin between March andSeptember 2006. The purpose was to collect ethnomedical information that will serve as a basis for further studies to establish current and potentialmedicinal uses. The ethnomedical information was obtained through interviews using semi-structured questionnaires. Consultative meetings werealso conducted with traditional health practitioners and other members of the communities in Kenya, Uganda and Tanzania.Results of this study show that Toddalia asiatica is collected in the wild, prepared mostly as decoctions or concoctions and administered orally.It is used for the management of a number of disease conditions. The most frequently cited diseases were stomach problems (78%) followedby malaria (25%). Cough (22%), chest pain (13%), food poisoning (8%), sore throat (7%), were also mentioned among other disease conditionstreated. Validation studies of therapeutic claims will be carried out at a later date.© 2007 Elsevier Ireland Ltd. All rights reserved.

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