THE DEVELOPMENT OF FLOCCULATION EFFECTS IN FILTER THEORY

A flocculation model of filtration was applied to estimate flocculation conditions in filter beds. The model was derived on the basis of hydraulic parameters and defined in terms of the mean velocity gradient in filter beds. The velocity gradient allows us to establish the effect of filter media density, which has not been yet described quantitatively. Presented are also relations between media density and the optimal filtration rate and filter bed depth. It was found that floes compressibility together with some other hydraulic parameters should be considered important when determining the filtration capacity of the beds. Development of the flocculation effects in filtration theory leads to the optimization of filter designing.     

Rigid poly(vinyl chloride) (PVC) gelation in the brabender measuring mixer. I. Equilibrium state between sliding,breaking, and gelation of PVC

This article discusses the first part of the Brabender typical torque process curves. Rigid poly(vinyl chloride) (PVC; Polanwil S‐61) was processed at various temperatures between 160 and 200°C with blade speeds in the range of 5–40 min^?1. The morphological changes of the processed compound was characterized by scanning electron microscopy and optical microscopy, and the degree of gelation was estimated on the basis of DSC thermograms. The values of torque minimum strongly depends on temperature. Shear rate affects mainly the time of reaching the minimum torque. The grains are disintegrated into the nonuniform particles and aggregates of the dimensions between 20 and 4 μm. The significant variation of the gelation degree was found for the sample produced in the same conditions and unloaded from Brabender chamber at minimum of torque. It was found that the value of the minimum of torque described momentary equilibrium state between sliding of breaking grains and increasing degree of PVC gelation. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 966–971, 2004     

HDS and HDN activity of molybdenum and nickel–molybdenum catalysts supported on alumina–titania carriers

Alumina–titania supports containing 5–50 wt.% of TiO₂ were prepared by coprecipitation method using inorganic precursors (sodium aluminate and titanium chloride). DTA-TGA, XRD, SEM, TPD_NH3, and IR spectroscopy were used to characterise these materials. The study shows that the promoting effect of nickel on the HDS activity of molybdenum catalysts supported on Al₂O₃TiO₂ is significantly lower than that for molybdenum catalyst supported on Al₂O₃, and depends on the TiO₂ content. The SEM results show that in the case of rich Al support (20 wt.% of TiO₂) molybdenum was aggregated on the external surface of the catalyst, whereas it was uniformly dispersed on the external surface of alumina. Results also show that molybdenum is preferably supported on aluminum oxide. Application of Al₂O₃TiO₂ oxides enhances the HDN activity of nickel–molybdenum catalysts. The highest HDN efficiency was obtained for the NiMo/Al₂O₃TiO₂ catalyst containing 50 wt.% of TiO₂. HDN activity was found to depend on protonic acidity and anatase content.     

The influence of wood hardness and logging operation on coupling forces exerted by lumberjacks during wood harvesting

Petrol chain saws commonly used in forestry cause mechanical vibration, which may lead to the development of non-specific disorders in upper extremities of the chain saw operator, referred to as hand-arm vibration syndrome (HAVS). Progress of HAVS depends on the intensity of mechanical vibration transmitted throughout the body, which is directly proportional to coupling forces applied to the vibration tool. The aim of the study was to measure coupling forces exerted by lumberjacks on chain saws and find correlation between force magnitude, hardness of cut wood and kind of logging operation.Coupling forces exerted by workers with right and left hands were measured by means of hydro-electronic force meter. All measurements were done during harvesting wood in real work conditions.Maximal temporary forces exerted by woodcutters reached 275 N. The smallest average forces of 27 N were registered while limbing. During felling and cross-cutting chain saw operators exerted larger forces, reaching 50 N.The findings of this study suggest that coupling forces used by woodcutters during logging depend on wood hardness and kind of logging operation.

Relevance to industry

This study shows the relationship between coupling forces, wood hardness and technique of cut which are further expected to enhance our knowledge on the assessment of vibration exposure. Nowadays, understanding how changes in harvesting technique affect the magnitude of coupling force, should lead to improvements in ergonomic design of the tool and the workplace.     

Low-energy arc welding with gas shielding – the effect of material and technological conditions on joint quality and the emission of impurities in the workplace

Connecting modern extra-lightweight materials requires the use of technology which cannot be provided by traditional gas-shielded welding methods. It is, therefore, necessary to develop variants of the arc welding method, which would deliver a very small amount of heat to the material but which would still ensure that the resulting joints would be tough. coldArc welding is a modification of the MIG/MAG method, which meets those requirements. With this method, all interventions in the process are made directly at the source of the power supply, without mechanical intervention in the feeding of the solder wire. As such, it is possible to use standard holders, and this method may also be successfully used in manual welding.     

Approaches to chemical alternatives assessment (CAA) for the substitution of hazardous substances in small- and medium-sized enterprises (SMEs)

Six alternatives assessment frameworks have been reviewed for specific features that might affect the implementation of alternatives assessment and cause regrettable substitution. These features are: the assessments included, the assessment flowchart structure, the inclusion of the assessors with limited resources (e.g., SMEs) in terms of resource intensity, the tools and methods included or guided to, and the indicators. The purpose of this review was to point out the existing important differences among the frameworks and also to stress the possibility of future improvements for the application of frameworks in SMEs. In general, it has been determined that, although there are similar features (e.g., hazard assessment methods) among the reviewed frameworks, there are also serious differences that might affect the assessment outcome, such as the use of physicochemical properties, the scope of life cycle thinking, and decision methods. These differences are caused by the exclusion of particular assessments, as well as the differences among the assessment methods used and the flowchart structure of the framework that incorporates these assessments. Ideally, the frameworks should give the same results under the same circumstances. Also, frameworks usually ignore the follow-up stage of the alternatives assessment, which is an important shortcoming of the frameworks. Common approaches, such as the exclusion of assessments or the use of the sequential elimination method seem to be a temporary solution to the existing problem of the implementation of these frameworks by SMEs. Common principles and methods should be in place to be able to minimize those differences among frameworks toward an optimized framework that enables assessors with limited resources to conduct a comprehensive assessment that is necessary to avoid a regrettable substitution.     

Ion exchange kinetics in removal of small ions. Effect of salt concentration on inter- and intraparticle diffusion

Kinetics of ion removal from wastewaters by ion exchange on a commercial synthetic zeolite has been investigated. Dynamic breakthrough profiles have been registered and exploited to determine ion exchange equilibria and mass transport resistances.A simplified dynamic model based on the equation of solid-film driving force has been employed to determine coefficients of mass transport kinetics in the presence of different solutions of regenerating salt. Two diffusional resistances have been found to govern the mass transport kinetics: interparticle diffusion (external film diffusion) and the solid phase diffusion within micropores of the zeolite crystals. At low solution concentration interparticle mass transfer was dominant in the mass transport mechanism, whereas solid phase diffusion dominated at higher concentration. The influence of the salt concentration on mass transfer kinetics has been analyzed using the Nernst-Planck and the Maxwell-Stefan models.The kinetic and isotherm coefficients determined were used to simulate the cyclic process of ion removal: ion exchange and the column regeneration.To verify analysis a generalized dynamic model has been used accounting for directly all the diffusion resistances.     

Welding aluminium alloys using low arc energy MIG method in terms of reducing pollutant emissions

Abstract

A comparison between the Charpy-V (CV) test, widely used in steel characterisation, and the dynamic fracture toughness K_1d found on precracked Charpy testpieces, is carried out.

The experimental procedure, not yet standardised, used to determine the dynamic fracture toughness K_1d (instrumented precracked charpy test or IPC test) is first described.

The experimental procedure, not yet standardised, used to determine the dynamic fracture toughness K_1d (instrumented precracked charpy test or IPC test) is first described.

The conceptual differences between dynamic fracture toughness K_1d and CV impact strength, the limits of CV testing in evaluating the toughness of steels, and finally the advantages of adopting the IPC test and its possible applications in the field of welding are then successively discussed.     

Removal mechanisms of dew via self-propulsion off the gecko skin

Condensation resulting in the formation of water films or droplets is an unavoidable process on the cuticle or skin of many organisms. This process generally occurs under humid conditions when the temperature drops below the dew point. In this study, we have investigated dew conditions on the skin of the gecko Lucasium steindachneri. When condensation occurs, we show that small dew drops, as opposed to a thin film, form on the lizard''s scales. As the droplets grow in size and merge, they can undergo self-propulsion off the skin and in the process can be carried away a sufficient distance to freely engage with external forces. We show that factors such as gravity, wind and fog provide mechanisms to remove these small droplets off the gecko skin surface. The formation of small droplets and subsequent removal from the skin may aid in reducing microbial contact (e.g. bacteria, fungi) and limit conducive growth conditions under humid environments. As well as providing an inhospitable microclimate for microorganisms, the formation and removal of small droplets may also potentially aid in other areas such as reduction and cleaning of some surface contaminants consisting of single or multiple aggregates of particles.