Impact of Feed Spacer Filament Spacing on Mass Transport and Fouling Propensities of RO Membrane Surfaces

Material build-up on membrane surfaces is one of the vital challenges faced by Reverse Osmosis (RO) operations leading to many operational and maintenance issues. To date, several modelling studies dealt with flow behaviour and concentration patterns for crossflow membrane operations. However, the relative fouling propensities of top and bottom membrane surfaces are never addressed in any study for narrow channels filled with ladder type spacers. In the present work, fluid flow patterns through different spacer configurations are visualized using ANSYS FLUENT by varying the dimensionless filament spacing, L (ratio of top or bottom filament spacing and channel height). Results clearly indicated that average shear stress values for the top membrane surface are always higher (3 to 8 times) than bottom membrane surface but yielded approximately similaraverage values of mass transfer coefficient for the two walls, for low to moderate filament spacings of L≤3 (SP22, and SP33) indicating similar fouling propensities of membrane surfaces. Further increase in filament spacing with L≥4 (SP44 and SP66), the average mass transfer coefficient for the top membrane indicated a sharp decline suggesting increased fouling propensity compared to bottom membrane which is not a desirable feature. Among the four spacer arrangements studied, SP44 (with L=4) was found to be the optimal arrangement yielding moderate pressure drop with nearly equal/higher area weighted values of mass transfer coefficient for the two walls and would lead to lower and equal fouling tendencies for top and bottom membrane surfaces respectively

Assessment of Sintering of Fly Ash in a Circulating Fluidised Bed Combustor

This paper describes a comparative study of ash deposition in two 125 MWe circulating fluidised bed combustor power generation plants (CFBC 1 and CFBC 2) undertaken to provide a comparative data for the identification of the possible causes of ash deposition in CFBC 2. Samples of the feed coal, limestone, deposits and ashes were collected from various sampling points within the furnace and from areas of ash deposition. The samples were analysed using a combination of ash chemistry and quantitative X-ray diffraction analysis, optical petrography and QEMSCAN analysis. Although the results indicated that the nature of the superheater deposits in both power plants is similar in terms of mineralogy and texture (dominated by iron oxide and anhydrite occurring as layers), the ash chemistry and mineralogy are both significantly different. The study confirmed that CFBC 2 ashes are unusual in the occurrence of complete sulphation of the decarbonated limestone with no evidence of either the occurrence of intermediate phases such as CaO or the presence of sulphate reaction rims on decarbonated limestone as observed for CFBC 1 ashes and commonly described in the literature. A possible reason for this unusual behaviour is the high sulphur content of feed coal in CFBC 2 leading to complete sulphation of limestone due to an insufficient feed rate. Another possible factor is the greater proportion of fine particles in the milled CFBC 2 limestone which would react completely. This observation is supported by the occurrence of fine anhydrite particles in the CFBC 2 back pass sample and a subsequent increase in grain size in the back end of the boiler, suggesting that winnowing of the fine particles has occurred in the hotter sections of the back pass. This is in contrast to the relatively limited size variation exhibited by anhydrite in the ash samples of CFBC 1

Co-firing of biomass in coal fired boilers – status and lab scale studies

One of the most attractive and easily utilizable renewable energy sources for co-firing technologies is biomass which can be fired up to 20% with coal in existing pulverized coal (pc) fired boilers. When biomass is used as a supplemental fuel in an existing coal boiler, this renewable energy source can provide several benefits including reduced gaseous emissions. However, there are several operational bottlenecks in 1terms of ash related issues. Our studies with European collaborator for nearly decade dealt with ash related issues covering ash transformation mechanisms for both pc-fired boiler environments with the objective of developing a Co-firing Advisory Tool (CAT). Some of the research outcomes to date along with status on co-firing activities are addressed in this paper. The studies towards the development of CAT addressed in this collaborative venture are expected to benefit power utilities in making informed decisions while selecting the renewable energy resources for power generation

Investigations into the Air Heater Ash Deposit Formation in Large Scale Pulverised Coal Fired Boiler

A mineralogical study was undertaken of air heater deposits in a 300 MWe pf boiler located in Western Australia to understand the deposit formation in air heater sections of boiler as an aid to implement possible remediation actions. Several air heater deposit samples were collected in the selected regions of the air heater along with samples of the feed coal, bottom ash and fly ash for comparison of ash chemistry and mineralogy. The deposit samples were examined using a combination of ash chemistry and quantitative X-ray diffraction analysis; the ash samples by bulk chemical analysis to determine the major element chemistry and mineralogy by quantitative X-ray diffraction. Samples of the deposits were also analysed using optical microscopy and QEMSCAN, an automated electron beam image analysis system. Chemical and mineralogical analysis showed that the deposits are unusual in containing high amounts of sulphate, particularly of aluminium and, to a lesser extent, iron. From the analyses it appears that the formation of the deposits is due to the high sulphate content which is acting as a cementing agent. There is an indication that temperature could be another factor in formation of the deposits, with a decrease in temperature leading to the formation of sulphurous acid which then reacts with the reactive glassy amorphous fly ash phase to form the aluminium and iron sulphates. Dew point calculations indicated that this is a possible deposit formation mechanism based upon air heater temperature data obtained from the utility. There was no evidence that unburnt carbon has played a significant role in deposit formation. Large temperature fluctuations resulting from the inherent nature of the operation of the air heater are a significant factor in deposit formation and a practical solution to consider would be the use of an SO2 absorbent placed prior to the air heater

A Modified Kennard-Stone Algorithm for Optimal Division of Data for Developing Artificial Neural Network Models

This paper proposes a method, namely MDKS (Kennard-Stone algorithm based on Mahalanobis distance), to divide the data into training and testing subsets for developing artificial neural network (ANN) models. This method is a modified version of the Kennard-Stone (KS) algorithm. With this method, better data splitting, in terms of data representation and enhanced performance of developed ANN models, can be achieved. Compared with standard KS algorithm and another improved KS algorithm (data division based on joint x - y distances (SPXY) method), the proposed method has also shown a better performance. Therefore, the proposed technique can be used as an advantageous alternative to other existing methods of data splitting for developing ANN models. Care should be taken when dealing with large amount of dataset since they may increase the computational load for MDKS due to its variance-covariance matrix calculations

Joint Projects and Assessments of Chemical Engineering Units: An Approach to Enhance Student Learning

The ability to apply knowledge of basic science and engineering fundamentals associated with each and every subject learnt in their undergraduate program is an essential attribute of the chemical engineering graduate. Even though principles of chemical engineering are distributed across the units from first to fourth year, a chemical engineer should be able to relate all these principles to solve chemical engineering problems. However, relating these principles and drawing parallels between these subjects is not an easy task unless during their undergraduate study, a chemical engineering student was given training in doing projects involving principles across a variety of units. In view of the above necessity, chemical engineering at Curtin University has implemented combined projects and joint assessments between two units which not only provides an avenue for students to experience relating concepts they learnt from different units, but also reduces the work load for both teaching staff and students. In this paper, two experiences of having combined projects and joint assessments between units in chemical engineering program are presented and discussed

Investigation into the effectiveness of feed spacer configurations for reverse osmosis membrane modules using Computational Fluid Dynamics

© 2016 Elsevier B.V.Reverse osmosis operations for water treatment are usually energy intensive and responsible for most of the product price. Several studies used flow characteristics to compare different geometries of feed spacers, but these cannot completely explain the effectiveness of feed spacers for promoting mass transfer near membranes. A few recent studies introduced a concept (Spacer Configuration Efficacy, SCE) combining mass transfer and energy consumption, but SCE has been applied only to a limited extent. The present study uses 3-dimensional steady state Computational Fluid Dynamics with mass transfer to compare four channels with feed spacer configurations (Ladder-type, Triple, Wavy and Submerged) and an empty plain channel using SCE and other performance measures. In contrast to previous studies, a saturated concentration boundary condition is employed at the membrane surface and optimised meshing of the domain is discussed. Power law correlations for SCE and other performance measures developed from the simulation results enable quick evaluation of the spacers. Results indicated that the assumed saturated solute concentration at the membrane strongly affects the mass transfer coefficient. Based on SCE, the Wavy spacer configuration showed the highest performance for Re>120 among the obstructed geometries considered, while Ladder-type was better for Re<120

Control of ash related problems in a large scale tangentially fired boiler using CFD modelling

The objective of this study is to develop a three-dimensional combustor model for predicting the performance of full-scale tangential fired (TF) boiler and to determine the flow patterns of the gas and coal particles, with an emphasis on formation mechanism of gas flow deviations and uneven temperature on the super-heaters, re-heaters and divisional super-heater sections of the furnace. The importance of these simulations is to identify the locations of optimum additive injection ports to achieve maximum impact of additives in the combustion process to minimize the temperature deviation and reduce ash-related issues. This study is a classic example of numerical investigation into the problem of turbulent reacting flows in large scale furnaces employed in thermal power plants for the remediation of ash deposition problems. Present work also provides an investigation of the influence of number of tripped burners on the characteristics of the flow and thermal fields. Excellent agreement between the simulation results and key boiler design values and available site operation records following full-scale trials indicate that the calculations are reliable. The results obtained from the present work are directly relevant to coal-fired utilities for not only demonstrating the effectiveness of computational fluid dynamics (CFD) based tools in combating operational issues but also provides an alternative to conventional remediation strategies

Control of ash related problems in a large scale tangentially fired boiler using CFD modelling

The objective of this study is to develop a three-dimensional combustor model for predicting the performance of full-scale tangential fired (TF) boiler and to determine the flow patterns of the gas and coal particles, with an emphasis on formation mechanism of gas flow deviations and uneven temperature on the super-heaters, re-heaters and divisional super-heater sections of the furnace. The importance of these simulations is to identify the locations of optimum additive injection ports to achieve maximum impact of additives in the combustion process to minimize the temperature deviation and reduce ash-related issues. This study is a classic example of numerical investigation into the problem of turbulent reacting flows in large scale furnaces employed in thermal power plants for the remediation of ash deposition problems. Present work also provides an investigation of the influence of number of tripped burners on the characteristics of the flow and thermal fields. Excellent agreement between the simulation results and key boiler design values and available site operation records following full-scale trials indicate that the calculations are reliable. The results obtained from the present work are directly relevant to coal-fired utilities for not only demonstrating the effectiveness of computational fluid dynamics (CFD) based tools in combating operational issues but also provides an alternative to conventional remediation strategies.CFD Combustion Industrial furnaces Tangential fired boilers Ash deposition