Agglomeration of Particle Systems in Fluidised Beds

Publication Reference: 
08-04
Author Last Name: 
Tardos
Authors: 
G I Tardos
Report Type: 
ARR
Research Area: 
Powder Flow
Publication Year: 
1987
Publication Month: 
12
Country: 
United States

Work performed on agglomeration of particle systems in fluidized beds at both low temperatures (granulation) and high temperatures (sintering) is The research described in Part I and Part II of this report,. respectively. reported on was performed during the period December 1986 - November 1987. carried out from December 1987 to September 1988 is also included. A special section in Part I (7.2) contains Future work for both projects to be three years program starting in September 1988 for the low temperature granulation project only.

In Part I of this report, containing the description of theoretical and experimental work on granulation in fluidized beds, it is demonstrated that the viscosity of the binder (in addition to other properties such as surface tension, wetting, etc.) is a very important characteristic which in final analysis determines the morphology and strength of the formed granules. After a short review of theoretical and experimental procedures-relating to liquid bridge strength given in section 3, the newly developed "dynamic bridge apparatus" is described (see section 4) and its capabilities are shown. The effects of the Capillary number, viscosity, bridge volume, etc. on the' strength of an axially strained pendular bridge and comparison of theoretical and measured values are given. It was demonstrated that under conditions of low Reynolds and Capillary numbers, large bridge volumes and favorable wetting of the solid surface the theoretical and experimental data are in good agreement. It was also clearly shown (see section 5) that binders which show a high rate of strengthening with time as the solution becomes more concentrated, i.e., the viscosity increases, yield agglomerated granules as a final product from granulation while binders for which the strengthening rate is moderate or low, yield layered granules but no agregates. This result was also predicted from a simple experiment using the dynamic bridge apparatus. Additional experiments with different binders exhibiting a wide range of properties in both the dynamic bridge apparatus and in the fluidized bed granulator are given in the Appendix (see section 10).

In Part II of the report which contains work on agglomeration due to high temperature sintering, it is shown that there is a strong correlation between elongation-contraction behavior of a powder sample in the proposed future work for an additional dilatometer and the agglomeration of the same powder when fluidized. The minimum sintering temperature of m&y different materials as determined in the dilatometer was shown to correspond to the temperature at which the material will defluidize. It was also found however that, although the correlation mentioned above holds for'incipient sintering, there seems to be no direct relation between rates -of deformation in the dilatometer at temperatures beyond sintering and actual sintering rates in the fluid bed. It is therefore often necessary that both dilatometer and fluidized bed tests be performed on the same powder before a final conclusion can be drawn. A detailed description of both experimental methods mentioned above are given in sections 2 and 3 respectively, while results and discussion of different test materials are given in section 4.. This section also contains a case study of a powder undergoing a chemical reaction which induces agglomeration. It was found that if the product of the chemical reaction causes agglomeration the controlling factor in the process is the conversion rate; this type of agglomeration behavior can not be detected in the dilatometer.

Section 5 in Part II contains a critical review of existing models of agglomeration; this section also includes some directions for the development of a more realistic theoretical model. The need for some basic knowledge of the magnitude of the break-up forces in a fluidized bed is also discussed. Finally, the Appendix to Part II contains a copy of a paper on high temperature sintering presented at the annual AICHE meeting in November of 1987.