The atomisation of detergent slurries

A study was made to determine the conditions under which the optimum droplet size distribution (ie., narrowest size range with a minimum of fines and over-sized agglomerates), is generated in sprays from centrifugal pressure nozzles. A range of non-Newtonian detergent slurries were tested but the results are of wider application and parallel work was undertaken with water, ionic solutions and chalk slurries. Six centrifugal pressure nozzles were used and the drop-size distributions correlated as a function of fluid properties, pressure, fiowrate, feed temperature, and nozzle characteristics. Measurements were made using a Malvern Particle Size Anayser slung across a specially-designed transparent tower section of approximately 1.2m diameter in order to reduce obscuration caused by the spray and improve existing droplet sizing techniques. The results obtained were based upon the Rosin-Rammler distribution model and the Size Analyser provided a direct print-out of size distribution and the parameters characterising it. A Spraying System nozzle, AAASSTC8-8, produced the optimum spray distribution with the detergent slurry at a temperature of 60°C whilst operating at 1200 psi. With other fluids the Delevan 2.2SJ nozzle produced the optimum spray distribution operating at 1200 psi but with the Spraying Systems nozzles there was no clear-cut optimum set of conditions, ie. the nozzle and pressure varied depending upon the fluid being sprayed. The mechanisms of liquid sheet break-up and droplet dispersion were investigated in specially-constructed, scaled-up, transparent nozzles. A mathematical model of centrifugal pressure nozzle atomisation was developed based upon fundamental operating parameters rather than resorting to empirical correlations. This enabled theoretical predictions to be made over a wide range of operating conditions and nozzle types. The model predictions for volumetric fiowrate, liquid sheet length and air core diameter showed good agreement with the experimentally determined results. However, the model predicted smaller droplet sizes than were produced experimentally due to inaccuracies identified in the initial assumptions.

Flowback of solids through distribution plates of gas fluidized beds and associated phenomena

This work is concerned with a study of certain phenomena related to the performance and design of distributors in gas fluidized beds with particular regard to flowback of solid particles. The work to be described is divided into two parts. I. In Part one, a review of published material pertaining to distribution plates, including details from the patent specifications, has been prepared. After a chapter on the determination of the incipient fluidizing velocity, the following aspects of multi-orifice distributor plates in gas fluidized beds have been studied: (i) The effect of the distributor on bubble formation related to the way in which even distribution of bubbles on the top surface of the fluidized bed is obtained, e.g. the desirable pressure drop ratio ?PD/?PB for the even distribution of gas across the bed. Ratios of distributor pressure drop ?PD to bed pressure drop at which stable fluidization occurs show reasonable agreement with industrial practice. There is evidence that larger diameter beds tend to be less stable than smaller diameter beds when these are operated with shallow beds. Experiments show that in the presence of the bed the distributor pressure drop is reduced relative to the pressure drop without the bed, and this pressure drop in the former condition is regarded as the appropriate parameter for the design of the distributor. (ii) Experimental measurements of bubble distribution at the surface has been used to indicate maldistribution within the bed. Maldistribution is more likely at low gas flow rates and with distributors having large fractional free area characteristics (i.e. with distributors having low pressure drops). Bubble sizes obtained from this study, as well as those of others, have been successfully correlated. The correlation produced implies the existence of a bubble at the surface of an orifice and its growth by the addition of excess gas from the fluidized bed. (iii) For a given solid system, the amount of defluidized particles stagnating on the distributor plate is influenced by the orifice spacing, bed diameter and gas flow rate, but independent of the initial bed height and the way the orifices are arranged on the distributor plate. II. In Part two, solids flowback through single and multi-orifice distributors in two-dimensional and cylindrical beds of solids fluidized with air has been investigated. Distributors equipped with long cylindrical nozzles have also been included in the study. An equation for the prediction of free flowback of solids through multi-orifice distributors has been derived. Under fluidized conditions two regimes of flowback have been differentiated, namely Jumping and weeping. Data in the weeping regime have been successfully correlated. The limiting gas velocity through the distributor orifices at which flowback is completely excluded is found to be indepnndent of bed height, but a function of distributor design and physical properties of gas and solid used. A criterion for the prediction of this velocity has been established. The decisive advantage of increasing the distributor thickness or using nozzles to minimize solids flowback in fluidized beds has been observed and the opportunity taken to explore this poorly studied subject area. It has been noted, probably for the first time, that with long nozzles, there exists a critical nozzle length above which uncontrollable downflow of solids occurs. A theoretical model for predicting the critical length of a bundle of nozzles in terms of gas velocity through the nozzles has been set up. Theoretical calculations compared favourably with experiments.

Spray drier simulation and air flow pattern studies

The literature pertaining to the key stages of spray drying has been reviewed in the context of the mathematical modelling of drier performance. A critical review is also presented of previous spray drying models. A new mathematical model has been developed for prediction of spray drier performance. This is applicable to slurries of rigid, porous crust-forming materials to predict trajectories and drying profiles for droplets with a distribution of sizes sprayed from a centrifugal pressure nozzle. The model has been validated by comparing model predictions to experimental data from a pilot-scale counter-current drier and from a full-scale co-current drier. For the latter, the computed product moisture content was within 2%, and the computed air exit temperature within 10oC of experimental data. Air flow patterns have been investigated in a 1.2m diameter transparent countercurrent spray tower by flow visualisation. Smoke was introduced into various zones within the tower to trace the direction, and gauge the intensity, of the air flow. By means of a set of variable-angle air inlet nozzles, a variety of air entry configurations was investigated. The existence of a core of high rotational and axial velocity channelling up the axis of the tower was confirmed. The stability of flow within the core was found to be strongly dependent upon the air entry arrangement. A probe was developed for the measurement of air temperature and humidity profiles. This was employed for studying evaporation of pure water drops in a 1.2m diameter pilot-scale counter-current drier. A rapid approach to the exit air properties was detected within a 1m distance from the air entry ports. Measured radial profiles were found to be virtually flat but, from the axial profiles, the existence of plug-flow, well-mixed-flow and some degree of air short-circuiting can be inferred. The model and conclusions should assist in the improved design and optimum operation of industrial spray driers.