In the second year of the project, the laboratory equipment for dispersing powders in stirred vessels described in IFPRI annual report No. ARR 17-04 and the laboratory methods for measuring the wetting properties were used to investigate the dispersing of aspartame (supplied by DSM) and zeolites (supplied by Unilever Research) in the 10 1 scale. For comparing the pitched-blade turbine used so far with the Rushton type turbine, such a turbine was built and tested using wheat flour, instant tlour and other powders. Experiments were conducted with an unbaffled, partly or fully baffled vessel.
The dispersing behaviour of aspartame and zeolite differs considerably from the skim milk powder investigated during the first year of the project and required new methods for controlling dispersion/solution quality. Aspartame was dispersed in small concentrations (0.5 % wt./wt.). The dissolution process was controlled by simultaneous measurement of the size of the undissolved particles (using laser diffraction) and off-line photometric measurement of the aspartame concentration in the solution.
The main problem with aspartame was its low solubility (= 1 % wt./wt.) and slow rate of dissolution in water. For fast dissolution, a fine powder with accordingly large specific surface is required. This in turn requires that the stirrer be designed and operated in a way to immerge and disperse these fine particles as quickly as possible. The results show that using an unbaftled vessel is the best way to achieve this, but that the particle size distribution of the powder is of greater importance. If the powder is too fine, lumping occurs, diminishing the rate of dissolution.
Zeolites, which are insoluble in water, were used for producing slurries with 50 % wt./wt. solids. The main problem here was to immerge the powder during the second half of the mixing process, when the slurry concentration was already high. This problem had already been predicted by the results of wetting time measurements. Powder dispersion and avoidance of sedimentation, on the other hand, were easy to achieve. Again, an unbaffled vessel turned out to be better suited than a baftled one, where floating layers of unwetted or partly wetted powder prevented complete mixing. In the unbaffled vessel, vortex formation occured even at high solids concentration, aiding in immerging the powder. Additional wetting and immersion experiments indicate that a mixture of 20 % zeolite A4 and 80 % A24 may have much better properties than either of the pure powders.
Although the specific stirring power that can be achieved in an unbaffled vessel is inferior to the specific stirring power for a baffled one, the improved ability to submerge the powder with help of the vortex formed by the rotating liquid proved to be of decisive importance. For hard to disperse powders, however, this may be different. If such a material is to be dispersed, a partly baffled vessel might be the best solution.
Comparison of the pitched-blade turbine and the Rushton turbine showed that both perform equally well at immerging powders in unbaffled vessels, which can be seen by plotting the specific stirring power required for immediate immersion over the desired feed rate of the powder. If a fully baffled vessel is used, the Rushton turbine performs better. Further experiments will focus on partly baffled vessels, in which vortex formation occurs as in unbaffled vessels, while higher energy input may allow better dispersion of critical (lump-forming or gelatinizing) products.
While (agglomerated) skim milk powder could be easily dispersed in baffled vessels as well as in unbaffled ones (see annual report No. ARR 17-04), aspartame, zeolite and wheat flour require unbaftled vessels in which the powder is fed to a vortex. Currently, for scale-up from the laboratory scale to large-scale- vessels the concept of constant Froude numbers can be used as a first approach. For baffled vessels, this is not common as it leads to an impracticably high specific stirring power, but for unbaftled vessels, a reasonable design is easily achieved. Vessel operation is constrained regarding the liquid level, since gassing the liquid should usually be avoided. This problem can be solved by using speed-variable stirrers. The fact that unbaffled vessels can be much easier cleaned is an additional incentive to use this kind of design for industrial applications.
The results from the experiments conducted so far indicate, however, that a complete description of the immersion step would also preferably take into account vortex shape and surface flow pattern (surface shear), which depend on the stirrer type and operating variables, and also powder properties like static wetting time or dynamic wettability. Future work will therefore be directed towards characterizing the flow pattern caused by different stirrer types, characterizing the immersion capability and the dispersing efficiency of such vessels, and towards improving the measurement of dynamic powder wetting.