Cereals processing technology

Wheat, corn and coarse grains milling 43
3.7.1 Modelling of mill processes
Oxford dictionary definition of simulation: ‘an imitation of the conditions
of a situation’.
The optimisation of processes requires understanding the operations that make
up the process. In the context of the milling process this means understanding
the behaviour of materials and machine operations and being able to assign
mathematical models to this behaviour.
The two principal operations in milling are grinding and sieving. Thus this
section documents the work carried out in modelling grinding and sieving
processes, in addition to the work performed on the basic principles of the
fracture of granular materials.
3.7.2 Fracture modelling
The subject has been the focus of work by numerous authors including Hook et
al. (1984), Thuy (1994) and Moss et al. (1994). Most research concentrates on
the impact of the process on the quality characteristics of the resultant flours,
rather than on the mechanical efficiency of the process.
Thuy’s (1994) model used Bingham’s plastic theory to develop a model of
grain deformation whose parameters can be determined experimentally.
Parameters such as expected energy requirement and thrust force generated
could be predicted for a grain sample. Hook et al. (1984) determined the
relationship between air humidity and temperature, and flour milling and quality
characteristics. Moss et al. (1994) examined the microstructure of grains and
were able to predict the various milling characteristics such as cleavage pattern
and relative sieving performance from micrographs. Thus a tool for predicting
milling performance was developed which could be used in wheat breeding
programmes.
Scanlon and Lamb’s (1993, 1995) papers take the process of fracture
mechanics to a much more detailed level and use matrix mathematics as well as
physical material properties to understand the mechanisms of particle fracture.
This material is of fundamental importance in understanding the macro process
of milling.
3.7.3 Roller mill models in milling
There are many mill types in use in mills, but roller mills dominate the flour
milling process. Consequently roller mills dominate the literature on the subject.
The mechanics of roller mills
The complex machine that is a roller mill may be represented for analytical
purposes by a simple two component schematic. The essential elements of the
scheme are two cylindrical rollers almost touching each other along their full
length. Because the rollers are perfect cylinders and contact is made along the