Single Pot Granulator

A mixer/granulator that dries granules in the same equipment without discharging is commonly called a single pot (Figure 1).The granulation is done in a normal high shear processor; however, care must be taken to avoid the formation of lumps as they cannot be broken down before drying.

Figure 1 A Typical Single Pot Set-Up

There are various options for drying in single pots. The traditional heat source comes from the dryer walls, which are heated; the boiling temperature and vacuum are used to reduce and remove vapours.The heat transfer is related to the surface area of the dryer walls and the volume of product treated. Therefore, this direct heating method is only effective for small scale use.

Introducing stripping gas into the pot allows large scale operation. A small quantity of gas is introduced in the bottom of the equipment, which passes through the product bed, improving the heat flow from the wall into the product.The gas also improves the efficiency of vapour removal. However, as the heated wall is the only source of drying energy, linear scale-up is not possible.This problem is exacerbated if the material to be processed is heat sensitive (as this limits the wall temperature); if water is used as a granulation liquid (it has a high boiling temperature under vacuum and a high heat of evaporation); and if used for larger-scale production (the surface/volume ratio deteriorates as the volume increases).

Microwave energy can be used to overcome these limitations.This provides a further source of energy and has the additional advantage, with organic solvents, that only pure organic vapours must be treated on the exhaust side, and not a mixture of solvent and large volumes of process gas, as would be required in most other wet granulation technologies.