Particle transport in fouling caused by kaolin-water suspensions on copper tubes

Particulate fouling tests were carried out using kaolin-water suspensions flowing through an annular heat exchanger with a copper inner tube. The flow rate was changed from test to test, but the fluid temperature and pH, as well as the particle concentration, were maintained constant. In the lower range of fluid velocities (<0.5 m/s), the deposition process seemed to be controlled by mass transfer. The corresponding experimental transport fluxes were compared to the predictions obtained with several models, showing that diffusion governed particle transport. The absolute values of the mass transfer fluxes and their dependences on the Reynolds number were satisfactorily predicted by some of the models

The role of wall deposition and re‐entrainment in swirl spray dryers

A new experimental method is outlined to study fouling in spray dryers and similar devices. In essence, it makes the deposits traceable so that one can quantify the material that comes off the walls, how long it remains there and how the deposits agglomerate with particles in the air. This paper investigates a countercurrent swirl spray dryer of detergent and provides sound evidence that fouling is a dynamic process: clusters form and break at the walls renewing an active layer of deposits. Remarkably, the wall generates >20% of the product and most of the large granules, and increases drastically the residence time of the powder. The assumptions of current numerical models are clearly invalid (i.e. particles rebound at the wall or deposit indefinitely). Several re-entrainment mechanisms and their times scales are identified in this work, and accordingly, a new general framework to describe fouling in spray dryers is proposed

Particle aggregation in large counter-current spray drying towers: Nozzle configuration, vortex momentum and temperature

This work investigates particle growth in a counter-current swirl detergent dryer, operating with a single nozzle, at a range of nozzle heights, air drying temperatures, TA, and superficial air velocities, UA, which were selected to enhance or inhibit particle aggregation in the dryer. The growth kinetics are discussed paying special attention to the impact of the cycle of deposition and re-entrainment of material from the wall deposits. All cases lead to substantial aggregation and mono-modal product size distributions. The operation at low UA and high TA, (i.e. low momentum) does not inhibit growth as one would expect from a lower particle concentration and faster heat and mass transfer, conditions which would lead to less particle collisions resulting in growth. In contrast, generation of aggregated particles > 850 μm is promoted, suggesting that a change in the erosion behavior of particles from the wall due to a reduction in energy of particle impacts. As a result of lower stresses, erosion is suppressed and clusters remain at the wall for longer, what allows them to sinter and be re-entrained at larger sizes. In contrast, increasing the momentum of the continuous phase by operation at low TA and high UA inhibits particle growth, particularly in the production of the largest sizes > 850 μm. In this case the rate and energy of impacts to the wall increases, this leads to higher disruptive stresses on the wall deposits, thus, reducing the size of the clusters re-entrained. In summary, this work describes aggregation mechanisms in swirl detergent dryers operated with single nozzles, suggesting that, contrary to expectations, wearing of deposits rather than air-borne contacts may be a key contributor to the enhancement or inhibition of growth

Deposition and wear of deposits in swirl spray driers: The equilibrium exchange rate and the wall-borne residence time

Wall deposits are omnipresent in many particle technology operations, such as spray drying and granulation processes. Their impact has been long recognized and is typically considered detrimental to the process performance. Counter-current spray drying units, such as those used in the manufacture of detergents, make use of strong swirling flows which leads to substantial multi-layered deposits at the walls. However their relation to the rest of the process has never been studied in detail. The work presented here discusses the generation of the structure at the walls and how it interacts with the air-borne population of particles. A tracer experiment is outlined that permits one to track the release of material from the deposits and differentiate the origin of aggregates in the product, as being generated either by the atomization or through the erosion of deposits. Monitoring of the wall and analysis of the release rate of the tracer has permitted identification of a dynamic equilibrium between the rates of deposition and re-entrainment, and quantification of the exchange rate above 12-20% of the full production. The age of the re-entrained material, indicative of the time the granules remain resident at the wall, appears much higher than expected for an air-borne trajectory. Such observations suggests that the description of the spray drying process exclusively in terms of the air-borne condition of particles is incomplete, at least in relation to the manufacture of detergents in swirl assisted systems. Focus should be paid to the wall-borne condition of the product, for it is seen to govern the generation of large granule sizes and dominate the residence time and drying kinetics of a significant part of the product

Fine water sprays for fire protection A halon replacement option

SIGLEAvailable from British Library Document Supply Centre- DSC:q93/06164(Fine) / BLDSC - British Library Document Supply CentreGBUnited Kingdo