Primary furnish can be pulped either in individual
batches or in continuous operating mode. Most
pulpers for defibering primary fibers today
operate in batch mode. This is notably the
case for throughputs below 200 tons/day, or
for complex mixtures of different primary fibres
associated with frequent changes of product
and/or colour.
Batch pulping involves unproductive
work cycles such as filling and emptying of
the pulper vat. A considerable amount of energy
and time is consumed that would otherwise be
available for the actual pulping process. The
production capacity of batch pulpers is therefore
lower than that of continuous pulpers. Another
drawback of batch pulpers is stock consistency
fluctuations, due to dilution during the emptying
and flushing
phases.
Continuous pulping, suitable for throughput
above 200 tons/day, consumes about 40% less
energy compared with the batch mode. Moreover,
with a suitable control strategy, the stock
consistency remains constant and simplifies
the overall stock preparation process.
Optimal agitation is indispensable during every
pulping phase and only as much energy should
be applied to the suspension as is actually
required for the defibering process.This means
that, in batch pulping, the suspension surface
motion can almost cease when the stock consistency
is high at the end of each pulping phase.
On the other hand, there must be enough agitation
in a continuous pulper to draw the bales into
the rotor vortex – on
no account must they be allowed to settle at
the bottom of the pulper. The optimal agitation
energy can be defined by the coefficient of
power to volume [kW/m3], which varies according
to application and operating mode.
Good mixing of the suspension requires frequent
rotor contact with the furnish bales,and in
practice this is assessed visually. If for
example a bale remains on a concentric circulation
path for too long in the pulper, it does not
contact the rotor frequently enough. As a result,
mixing is inadequate and so is pulping.
To ensure intensive mixing, most pulpers today
have baffles on the vat wall to break up the
flow pattern by diverting the mainly rotational
flow inward toward the rotor. The drawback
is that flow interference entails loss of energy.
This applies not only to the baffles, but also
to the pulper bottom design. Voith has been
carrying out development work on both these
components to make optimal use of flow energy
for pulping.
Asymemetrical Motion
Voith’s new IntensaPulper incorporates
two important features that reduce energy use
| • |
The
rotor is eccentrically
arranged in
the cylindrical
vat. |
| • |
The transition
from the pulper
bottom to the
vat walls is
flow-optimized
with a double
cone design. |
An eccentrical
rotor arrangement
in the pulper vat
results in good
mixing thanks to
asymmetrical flow.
The IntensaPulper
rotor is therefore
installed off-centre.
Since this alone
optimizes mixing,
no energy-wasting
baffles are required
and more energy
is available for
pulping.
An
optimized transition from the horizontal pulper
bottom to the vertical vat walls enables further
exploitation of flow energy for pulping.
In
the new IntensaPulper the transition from the
pulper bottom to the vat walls is optimized
with a specially designed double cone bottom.
The two cone angles are precisely determined
to simulate a low-loss torospherical profile
optimally diverting the flow generated by the
pulper rotor.
Performance tests on a 20 m3 IntensaPulper,
operating in batch mode, showed a 26% reduction
of specific energy consumption [in kWh/ton]
compared with the conventional pulper previously
used. Furthermore, production output was 7%
higher with the same defibering quality.
In absolute figures, this means that an IntensaPulper
with an output of 100 tons/day saves about
175,000 kWh per year. This energy savings reduces
operating costs significantly.
In continuous operating mode, specific energy
savings with the newly developed IntensaPulper
are similar. Particularly for large throughput
capacities, the resultant cost savings can
be substantial.TW
