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.

The flow behaviour of shallow fluidised beds

The flow behaviour of shallow gas-fluidised beds was studied. experimentally using a rotational viscometer, and an inclined open channel. Initially, tests were carried out with the viscometer in order to establish qualitative trends in the flow properties of a variety of materials over a wide range of fluidising conditions. Also, a technique was developed which enabled quantitative viscosity data to be extracted from the experimental results. The flow properties were found to be sensitive to the size, size-range and density of the fluidised material, the type of distributor used, and the moisture content of the fluidising gas. Tests in beds up to 120 mm deep showed that the fluidity of the bed improves with reduction in depth; and indicated a range of flow behaviour from shear-thinning to Newtonian, depending chiefly on fluidising velocity .. Later, an apparatus was built which provided for a steady, continuous flow of fluidised material down an inclined open channel of 3m length x 0.15m square, up to a mass flowrate of 10 kg/s (35 ton/hr). This facility has enabled data to be obtained that is of practical value in industrial applications; which is otherwise difficult in view of the present limited understanding of the true mechanism of fluidised flow. A correlation has been devised, based on analogy with laminar liquid flow, which describes the channel flow behaviour with reasonable accuracy over the whole range of shear-rates used. 1he channeI results indicated that at low fluidiising velocities the flow was adversely affected by settlement of a stagnant layer of particles on to the distributor, which gave rise to increased flow resistance. Conversely, at higher fluidising velocities the resistance at the distributor appeared to be less than at the walls. In view of this, and also because of the disparity in shear-rates between the two types of apparatus, it is not possible as yet to predict exactly the flow behaviour in an open channel from small-scale viscometer tests.

Measuring the flow properties of small powder samples using an avalanche tester

The feasibility of using a small-scale avalanche tester to measure the flow properties of pharmaceutical lactose powders was examined. The modes of behaviour observed in larger systems were displayed and showed a clear distinction between angular, free-flowing particles and more spherical particles of similar flow characteristics. Angular Lactohale LH100 particles showed slumping behaviour at a rotational frequency of 0.33Hz which disappeared at higher frequencies. Spherical lactose powder with a similar flow function to LH100 only showed rolling behaviour under the same conditions, as did more cohesive powders LH200 and LH300. Further investigation of the LH100 data using Fast Fourier analysis showed that the slumping frequency was 1/10th of the rotational frequency.