Practical Assessment of Electronic Water Treatment for the Prevention of Fouling

Fouling or scaling, i.e., the occurrence of deposits on tubes, heat exchangers, and other devices, is a common problem in industrial and domestic processes. Many anti‐fouling techniques exist, both chemically and non‐chemically. The efficiency of non‐chemical methods is still controversial, although the use of these devices is already widespread in industry. In this contribution, a case study is presented in which the operation of such a device is demonstrated. The electronic water treatment device under study was connected to a 2 m‐long copper tube immersed in a water bath with a temperature of 65 °C. A short‐term experiment revealed that the device did not alter the water chemistry and that locally, fouling was prevented by the device. However, the potential for fouling further downstream was not reduced. A long‐term experiment further confirmed the findings of the short‐term experiment showing that locally, the device has an anti‐fouling action as a significant increase in weight was observed when the device was switched off and a significant decrease in weight was observed when the device was switched on.     

Fouling of tube bundles under pool boiling conditions

This paper aims to provide experimental heat transfer results for boiling of CaSO₄ solutions on the outside of tube bundles, and also to shed some light into the mechanisms which influence heat transfer fouling under pool boiling conditions. Heat transfer coefficients for three heater rods with an identical diameter of 10.67 mm on a 35 mm vertical pitch have been measured. The independent variables bulk concentration and heat flux have been varied from 0.8 to 1.6 g/L and 33 to 300 kW/m², respectively. The experimental results show that the mechanisms of fouling on the middle and top heater substantially differ from those at the bottom heater, due to the dominant effects of bubble impingement on the heat transfer surfaces. In particular, the competing effects on the degree of wall superheat and the supersaturation of the boiling liquid play a significant role. Spalling effects are more pronounced on the upper heaters where thin, friable deposits were found with low density and adherence, particularly at higher heat fluxes and lower concentrations. In terms of tube bundle efficiency, the best results were found for lower concentrations.     

Quantitative measurement of atomic sodium in the plume of a single burning coal particle

The release of volatile sodium during coal combustion is a significant factor in the fouling and corrosion of heat transfer surfaces within industrial coal-fired boilers. A method for measuring the temporal release of atomic sodium from a single coal particle is described. Laser absorption was used to calibrate laser-induced fluorescence measurements of atomic sodium utilising the sodium D1 line (589.59 nm) in a purpose-designed flat flame environment. The calibration was then applied to planar laser-induced fluorescence measurements of sodium atoms in the plume from a single Victorian brown coal particle (53 mg) suspended within the flat flame. The peak concentration of atomic sodium was approximately 64.1 ppb after 1080.5 s, which appears to correspond to the end of char combustion. To our knowledge this is the first in situ quantitative measurement of the concentration field of atomic sodium in the plume above a burning particle. A simple kinetic model has been used to estimate the rate of sodium decay in the post-flame gases. Comparison of the estimated and measured decay rates showed reasonable agreement.     

Engineering of an MBR supernatant fouling layer by fine particles addition: a possible way to control cake compressibility

For membrane bioreactors (MBR) applied to wastewater treatment membrane fouling is still the prevalent issue. The main limiting phenomena related to fouling is a sudden jump of the transmembrane pressure (TMP) often attributed to the collapse of the fouling layer. Among existing techniques to avoid or to delay this collapse, the addition of active particles membrane fouling reducers (polymer, resins, powdered activated carbon (PAC), zeolithe…) showed promising results.Thus the main objective of this work is to determine if fouling can be reduced by inclusion of inert particles (500 nm and inert compared to other fouling reducers) and which is the impact on filtration performances of the structuring of the fouling. Those particles were chosen for their different surface properties and their capability to form well structured layer.Results, obtained at constant pressure in dead end mode, show that the presence of particles changes foulant deposition and induces non-compressible fouling (in the range of 0.5-1 bar) and higher rejection values compared to filtration done on supernatant alone. Indeed dead end filtration tests show that whatever interactions between biofluid and particles, the addition of particles leads to better filtration performances (in terms of rejection, and fouling layer compressibility). Moreover results confirm the important role played by macromolecular compounds, during supernatant filtration, creating highly compressible and reversible fouling.In conclusion, this study done at lab-scale suggests the potential benefit to engineer fouling structure to control or to delay the collapse of the fouling layer. Finally this study offers the opportunities to enlarge the choice of membrane fouling reducers by taking into consideration their ability to form more consistent fouling (i.e. rigid, structured fouling).     

Implications of enhancing critical flux of particulates by AC fields in RO desalination and reclamation

This paper discusses fouling in reverse osmosis in terms of the critical flux of foulants and the fouling mechanisms based on hydraulic resistance and loss of driving force due to cake-enhanced osmotic pressure (CEOP). In many cases CEOP is the dominant effect and this is exacerbated by the use of constant flux processing. The implications of increasing critical flux are described and the potential benefit of using AC field gradients to do this is illustrated for a model foulant.     

Scale formation on polypropylene and copper tubes in mildly supersaturated tap water

This paper presents an experimental study of scaling of polypropylene and copper tubes exposed to flowing tap water over an eight week period. The study was motivated by the recent interest in switching from copper to polymeric materials in heat exchangers and absorbers used in solar water heaters. The chemistry of the tap water is controlled to provide mildly supersaturated conditions at pH equal to 8 and 40 °C. Supersaturation is 5.0 with respect to calcite and 7.9 with respect to hydroxyapatite. These experimental conditions are a departure from the common practice of accelerated testing with distilled water highly saturated with respect to calcium carbonate and thus better represent conditions expected in solar systems. The microstructure and composition of the scale as it grows are examined with scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and ion chromatography. The results show scaling in tap water is slower than scaling in distilled or deionized water with a single supersaturated component. In the case of copper, prolonged exposure to tap water oxidizes the surface of the tube and subsequently affects scaling. The oxidized copper surface is more susceptible to calcium phosphate scaling and this tendency results in the higher scaling rate for copper than for polypropylene in water containing phosphate. The scaling rates for polypropylene and copper tubes are 0.8 × 10⁻¹⁰ and 1.7 × 10⁻¹⁰ (mol Ca)/(m²s), respectively.     

Experimental and CFD studies of fluid dynamic gauging in duct flows

Experimental and computational fluid dynamics (CFD) studies of the technique of fluid dynamic gauging (FDG) in duct flows have been performed. Experiments were performed using a stainless steel gauging nozzle located on the centreline of a Perspex duct of square cross section. The test fluid was water, flowing through the duct at 0.0077-0.74 m/s (Re_duct 116-11 100). The success of the experiments was confirmed by the results of Tuladhar et al. [2003. Dynamic gauging in duct flows. Canadian Journal of Chemical Engineering 81, 279-284].For the first time, CFD has been applied to simulate FDG in an imposed flow for steady, incompressible, laminar flows. Experimental data and simulation results agreed to within 6%, supporting the validity of both the experiments and the assumptions underpinning the simulation. CFD simulations predicted the stresses beneath the lip of the nozzle and confirmed the practical working range of the gauge (0.1<h/d_t<0.25). This is a major achievement, proving that CFD can be used to model this flow-FDG accurately, which is valuable for future work in this area, namely fouling in food, crude oil and cross-flow membrane systems.     

Slagging and fouling characteristics of seam 32/33, Panian coalfield, Semirara Island, Philippines

Twenty samples of seam 32/33, the main seam of Panian coalfield in Semirara Island, Antique Province, Philippines, were collected from a borehole drilled at the northeastern edge of the coalfield. The samples were analyzed to characterize the coal geochemistry of the seam and understand why the coals of Semirara Island exhibit a high tendency for slagging and fouling despite its low average ash content. Analysis of the slagging and fouling characteristics of this seam is important because it supplies five electric power utilities and several cement plants in the Philippines. Proximate analyses and vitrinite reflectance measurements designate the rank of the seam as sub-bituminous C, based on ASTM coal classification. H/C versus O/C ratios indicate that the seam is made up largely of huminite, denoting early stages of coalification. Chemical analysis of the ash reveals high contents of Na, Mg, Fe, Ca, Ba and Sr. The strongly negative correlation of these elements with the ash content indicates an organic affinity of the chemical elements of the seam. Owing to enrichment in alkali and alkali-earth elements, slagging and fouling indices indicate that the seam has medium to high propensity for slagging and a severe tendency for fouling. The detrimental characteristics of coal feedstock from Panian mine is mitigated by modifications to the boiler design and operational conditions and by blending with coals imported from Indonesia, China and Australia.     

Evaluating non-stick properties of different surface materials for contact frying

The paper describes, characterises and validates the construction of an experimental rig for making contact frying experiments under controlled conditions. The construction enables a controlled fouling of different coatings on steel and aluminium plate under realistic frying conditions, in order to evaluate non-stick and cleaning properties of the coatings. In accordance with industry standards pancake was selected as the food model for the non-stick properties. The performance of different frying surfaces (stainless steel, aluminium, PTFE (polytetrafluoroethylene) and three ceramic coatings with two different levels of smoothness) were tested at different frying temperatures and rated by a standardised rating procedure. The subjective rating assessment was validated by measuring the force of adhesion. The performances of the surfaces were reproducible and significantly different to be used for screening of new surface coatings for contact frying tested in frying experiments at the same temperature. In contrast, conventional testing in a convection oven could not distinguish between these surfaces. Comparative tests of the ceramic coatings showed that surface roughness had a distinct effect on their non-stick properties, so that the smoother surfaces gave a higher force of adhesion between pancake and surface.     

Simulation and analysis of a sugarcane juice evaporation system

The production of sugar and alcohol is the main objective of the sugarcane processing industry. The evaporation of sugarcane juice has a high energetic cost and is usually performed in multiple-effect evaporators. The loss of performance during operation due to fouling makes the process more complex. In this study, modeling, simulation, validation, and analysis were performed for a sugarcane juice industrial evaporation system (IES) composed of a falling film evaporator followed by three short vertical-tube evaporators arranged in parallel. The IES model was developed using a commercial process simulator and validated with data from the plant. The IES had marked performance losses in the first 14 days of operation, mainly due to fouling in the first effect, with a 30% decrease in the evaporation rate.