Control of wet granulation

It is outside the scope of this chapter to discuss production aspects of agglomeration processes. The readers are referred to literature on instrumentation for the control of wet granulation, for example the review by Kristensen and Schaefer (1987) which includes a chapter on end-point control of wet massing methods and Werani (1988) showing the benefits of power consumption measurements for production purposes. However, a few comments on end-point control are given in order to illustrate their relation to agglomeration mechanisms.

Leuenberger etal. (1979) investigated the liquid requirements during granulation in low shear mixers and provided an equation expressing the optimum amount of liquid as a function of granule porosity, feed equilibrium humidity and density of feed and liquid. Experiments on lactose-corn starch mixtures demonstrated that the liquid requirement can be predicted from measurements of the tapped density of the dry feed material (Bier etal., 1979; Leuenberger etal., 1979; Leuenberger, 1982). The predicted amount of liquid agreed well with granulation experiments in planetary and Z-blade mixers.

A variety of instrumentation has been proposed for control of liquid volume and process time. Such instrumentation should measure the changes in the rheological properties of the mass. The potential of torque measurements for detecting the stages of wet granulation processes has been well documented, as has the measurement of the bending moment on a probe inserted into the agitated mass. A simple mixer torque rheometer has recently been proposed by Hancock etal. (1991) to study the rheological behaviour of moistened masses and to assess binder-solid interactions. A similar idea was presented by Alleva and Schwartz (1986).

Figure 20 shows a power consumption profile recorded during addition of a povidone solution to a lactose-corn starch mixture in a planetary mixer. Similar profiles have been recorded for wet granulation in high shear mixers (Leuenberger etal., 1990). The profile reflects the characteristic phases of the agglomeration process (Leuenberger, 1982). In phase I, the powder is dry mixed and the amount of liquid is insufficient to create pendular liquid bondings. The power consumption remains unchanged. At a certain point, phase II, a sharp increase in power consumption is seen

or time t fig. 20 Power consumption profile recorded during liquid addition to a lactose-corn starch mixture in a planetary mixer. Reproduced from Leuenberger etal. (1979) with permission from the authors.

or time t fig. 20 Power consumption profile recorded during liquid addition to a lactose-corn starch mixture in a planetary mixer. Reproduced from Leuenberger etal. (1979) with permission from the authors.

because the amount of liquid now suffices to establish liquid bondings. In phase III, the power consumption profile levels off while the liquid phase fills up the intragranular voids and coarser granules are produced. In phase IV, the granules are filled with liquid. The power consumption drops when reaching S5 which corresponds to 100% saturation.

Leuenberger etal. (1979), using different scale planetary mixers, showed that the estimated values of S3 and S4 are proportional to the batch size and that the liquid required to run a robust process lies in the range S3-S4. The amount of applied liquid can be normalized as follows (Leuenberger etal., 1990):

+1 0

Post a comment