Design and engineering of biogas plants - The Biogas: Science, Production and Applications - page 197

Table 8.2 Mechanical agitators

Type Description Application field

Submersible
propeller
agitator

High-speed agitator with two- or
three-blade propeller, installed
horizontally or inclined. Height of
agitator is adjustable. Up to 1500
rpm

CSTR digesters with
temperature up to 408C (Eder
and Schulz, 2006). Can be used
as secondary agitator (e.g. to
destroy layers)

Rod mixer High-speed agitator installed at

the ceiling or sidewall of the
digester. The motor stays outside
the digester. Up to 1000 rpm

CSTR digesters. In case of
large propellers and low rpm,
it is also appropriate for higher
solid contents

Paddle
agitator

Low-speed agitator composed of
a central driving shaft with
paddles, which turn transversal to
the flow direction. No current is
produced. Up to 20 rpm

Plug-flow and CSTR digesters.
Appropriate for high solids
concentration and fibrous
substrates

8.2.8 Agitators

Agitation of the digestion material is important for distributing the
substrates, micro-organisms and heat; it also helps to drive out gas bubbles
and avoid the formation of floating or settling layers. Agitation is done at
intervals, with the length and frequency of the intervals being determined for
each plant. In the beginning, agitation should be done in long and frequent
intervals. Gradually, it can be reduced while observing the performance and
eventual layer formation. Models can be used for simulating flow behaviour
inside the digester. Computational fluid dynamics (CFD) programs can be
used to predict particle movements as a function of digestion substrates and
certain plant parameters.
There are three main forms of agitation techniques - mechanic, hydraulic
and pneumatic.

￿ ￿ ￿ ￿ ￿ ￿

. Mechanical agitators frequently used are propellers or paddles that

cause mixing by their own rotational movement. They are distinguished
by their shape, rotation speed and diameter. While small and fast
propellers work for liquid substrates, larger and slower paddles are used
as dry matter content increases. The most common types of mechanical
agitators are submersible propeller agitators, rod mixers and paddle
agitators. Their characteristics and application fields are described in
Table 8.2. The disadvantage of mechanical agitators is their sensitivity
to abrasion. Serious damage can be caused by unadapted feedstock,
such as substrates with high sand content, metallic or glass residues.
Repair to the agitators is laborious because accessing the damage