T.A.G. Langrish

OBSERVATIONS OF FLOW PATTERNS IN A SPRAY DRYER

ABSTRACT Experiments have been performed on a pilot scale, cylinder-on-cone spray dryer fitted with a vaned-wheel atomiser to observe air flow patterns, with and without water spray. A combination of tufts, smoke streams and a laser light sheet, was used to provide information about regions of recirculation, flow stability, spray trajectories and wall deposition. While atomiser-induced swirl dominated the flow patterns under typical operating conditions, some instability was observed, although different in type, for situations with and without atomiser rotation. Clockwise eddies were observed to form and collapse between the wall and the strongly anti-clockwise swirling core created by anti-clockwise atomiser rotation. Without swirl, large portions of the recirculation zones at the walls were observed to have a weak tendency to change randomly between clockwise, anticlockwise and chaotic behaviour.

COMPARISON OF RESIDENCE TIME MODELS FOR CASCADING ROTARY DRYERS

ABSTRACT The predictions of the models of Matchett and Baker (1988) Saeman and Mitchell (1954) and Friedman and Marshall (1949) for the solids residence time in rotary dryers have been compared wit...

The development and validation of a system model for a countercurrent cascading rotary dryer.

ABSTRACT An overall system model for a countercurrent rotary dryer has been developed with the ullimale aim of assessing controller pairings in these dryers. This model is based on heat and mass balances within dryer regions combined with two subsidiary models, one describing the equipment (which determines particle transport and heat transfer)and the other describing the behaviour of the material (the drying kinetics). Six partial differential equations have been set up to evaluate six state variables: solids moisture content, solids temperature, gas humidity, gas temperature, solids holdup and gas holdup as functions of time and rotary dryer length. A control-volume method has been used to reduce the six partial differential equations with respect to time and the length of the rotary dryer to six ordinary differential equations in time. The drying model has been implemented in the SPEEDUP flowsheeting package (with FORTRAN subroutines) The model has been validated by fifteen experiments-in a pilot scale countercurrent-flow rotary dryer (0.2m in diameter and 2m in length)

An Improved Drying Achedole for Australian Ironbark Timber: Optimisation and Experimental Validation

ABSTRACT A continuous schedule for the drying of Australian ironbark timber has been optimised using non-linear model-predictive control techniques. Initially, an experimental study was carried out using a conventional schedule to dry nine 600 mm long × 250 mm wide × 25 mm thick boards in order to obtain information on the drying behaviour and the extent of timber cracking. A diffusion model accurately fitted the average moisture contents observed when using this conventional drying schedule. The fitted coefficients in the diffusion model were used to optimise the drying schedule with the aim being to keep the strain below 0.02 mm/mm and the surface moisture above 7%. The resulting optimised schedule set gentler conditions at the start of drying and more aggressive ones towards the end than the conventional schedule. The new schedule was tested in an experiment using the same number of boards from the same tree, and was found to reduce the number of small and medium-sized cracks to less than 25% of the nu...

A novel method for the production of crystalline micronised particles.

The aim of this study was to develop a method for converting an amorphous drug to a crystalline form to enhance its stability and inhalation performance. Spray-dried amorphous salbutamol sulphate powder was conditioned with supercritical carbon dioxide (scCO(2)) modified with menthol. The effect of menthol concentration, pressure, temperature and time on the characteristics of the resulting salbutamol sulphate powder was investigated. Pure scCO(2) had no effect on the physical properties of amorphous salbutamol sulphate; however, scCO(2) modified with menthol at 150bar and 50 degrees C was efficient in converting amorphous drug to crystalline form after 12h of conditioning. The average particle size of powders decreased slightly after the conditioning process because of reducing agglomeration between particles by increasing surface roughness. Emitted dose measured by the fine particle fraction (FPF(emitted)) of amorphous salbutamol sulphate was enhanced from 32% to 43% after conditioning with scCO(2)+menthol and its water uptake was significantly decreased. This study demonstrates the potential of scCO(2)+menthol for converting amorphous forms of powders to crystalline, while preserving the particle size.

SCALE-UP OF CASCADING ROTARY DRYERS

Abstract Until now most of the design methods for cascading rotary dryers have been either empirical or purely theoretical. A theoretical model is presented which simulates the operation of both cocurrent and countercurrent rotary dryers. It relies on pilot plant and bench scale tests to determine the values of parameters which describe respectively the transport of solids through the dryer and the drying rate of the feedstock. A procedure is outlined for using the model to scale up from these pilot-plant and bench-scale tests to full-scale dryers.

IMPROVING THE QUALITY OF TIMBER FROM RED BEECH (N. FOSCA) BY INTERMITTENT DRYING

ABSTRACT The effect of cycling the external conditions on the timber quality and drying time during seasoning has been studied for red beech (Nothofagus fusca), a difficult-to-dry New Zealand hardwood, inalaboratorybatchdryer. Intermittent drying at anair temperature of 45°C and a wet-bulb depression of 10°C takes longer to yield a given moisture content than continuous drying under the same conditions, but produces timber without honeycomb or severe case-hardening as observed in continuous drying. The experiments also indicate that the timber can tolerate a higher air temperature (55°C) in intermittent drying at the same wet-bulb depression with less case-hardening and without showing honeycomb compared with continuous drying at 45°C, having a similar total drying time. Intermittent drying at a slightly higher dry-bulb temperature (60°C) and a much higher wet-bulb depression (18°C) gives a similar degree of case-hardening to that found in samples dried continuously at a dry-bulb temperature of 45°C.

Modelling the temperature profiles within boards during the high-temperature drying of Pinus radiata timber: the influence of airflow reversals

Profiles of the surface and centre temperatures in the high-temperature drying of Pinus radiata have been predicted as a function of time for heartwood and sapwood boards, using a receding evaporative plane model, which incorporates equations for local heat and mass balances, internal and external heat transfer, and internal moisture fluxes due to bound-water diffusion and flow of water vapour and liquid water. The model predictions have been compared with independent measurements of the temperatures at the surface and centre of sapwood, heartwood and mixed-wood boards. The simulated and measured temperature profiles are in good agreement except that the outer temperatures are predicted to rise more swiftly when the evaporative plane starts to recede in the wood, while the inner temperatures are estimated to rise more sluggishly once the first period is over. However, the maximum discrepancies in these three cases are less than 6°C, and may be due to difficulties in getting accurate measurements of local temperatures when these are changing. This model is used to predict local temperature and moisture-content profiles in high-temperature kiln drying of this timber for conditions with airflow reversals. Variations have been reported in the external mass-transfer coefficients in the streamwise direction which lead to differential drying of the boards. The differences in the extent of drying across each board can be reduced by reversing the airflow periodically. Airflow reversals every 4 h are adequate to give a high degree of uniformity in the final temperature and moisture-content profiles for heartwood after 24 h of drying, and may be sufficient for sapwood if a small degree of variation is acceptable. Flow reversals every 8 h yield essentially the same result.

Nonlinear model predictive control of timber drying

There is considerable economic incentive for improved timber drying in terms of reduced total drying time, lower wastage levels and improved product quality. This paper presents simulation results in two areas pertinent to improved timber drying productivity. First, it is shown that timber drying can be posed and solved as a nonlinear model predictive control problem. Recognition of two distinct phases (drying to a set average moisture content, followed by flattening of the moisture content profile) reduces the amount of computation required. It is then shown that the initial moisture content and an on-line measurement of a timber stacks weight provide sufficient information to design a closed-loop control system which achieves significant improvements (relative to standard drying schedules) in both total drying time and final product quality.

A Comparison of Drying Time and Timber Quality in the Contihuous and Cyclic Drying of Australian Turpentine Timber

ABSTRACT Continuous- and cyclic-dryins experiments have been carried out on an Australian hardwood timber; tuipentine. to compare the drying timesand the final quality of the timberproduced by these techniques. More severe drying conditions have been used in the the active drying Periods for cvclic drvins than for continuous drvins in order to hchieve similar drying Cimis. Fourteen boards of approximate original dimension 45 mm wide by 25 mm thick by 750 mm long, sealed at each end using an epoxy mastic coating, have been dried in each experiment. with end-matched samples being used to reduce the effect of different growing locations. The two cyclic-drying experiments differed in the use of high-humidity conditions in the relaxation period for the first experiment and ambient conditions in the relaxation period for the second experiment. The total drying time for the second cyclic-drying experiment was equal to that for continuous drying, while the drying time for the first cyclic-drying experiment was 20...