A study of fluidised bed granulation
An experimental study of fluisised bed granulation is presented. Preliminary experiments establish the main variables in the granulation process and enable a systematic experimental programme to be devised. In this programme, either glass powder or porous alumina are used as the bed material. The effects of the fundamental fluidised bed granulation parameters on particle growth are established from a series of batch experiments conducted in a 0.15m diameter, glass-walled bed. Together with measurements of the physical properties of the product granules and the binder solutions (carbowax or benzoic acid, in methanol) and with supporting studies of the bed structure (in the form of X-ray photography and measurement of temperature profiles), this information is also used to propose a mechanism of particle growth. Two types of product granule are identified, agglomerates which consist of two or more, and usually several, initial particles; and layered granules, which consist of single initial particles with dried feed material adhering to the surface. The effects of varying the excess fluidising gas velocity, the binder concentration and the initial particle size are quantified. A bed can be prevented from quenching (a defluidisation phenomenon leading to the failure of the process) by increasing the fluidising gas rate. For a given bed particle/binder combination, successively higher excess gas velocities allow an otherwise quenching bed to be operated firstly so as to produce agglomerates and subsequently layered granules. Similar effects are observed with increases in initial bed particle size and decreases in binder concentration. A particle growth mechanism is proposed in which the initial stages of both the desirable particle growth (irrespective of the type of granule) and of bed quenching, are considered to be exactly the same. Beyond the initial formation of liquid bonds between adjacent bed particles, the strength of the inter-particle bridges (which is a function of the binder material and relates to growth) and the extent of fluid drag and inertial forces on particles (which are functions of gas velocity and particle size respectively, and relate to granule breakdown) determine the equilibrium granule form and size. However, if the particles are porous the above mechanism may break down since the liquid can enter the pores and not be available for the initial formation of liquid bonds.