Paper mills with medium and low production are often
equipped with machinery that performs well, though machinery can
be new or quite old. Machinery of different size and age in principle
work well and respond to the mill's production needs.
Mills generally buy new machines to increase production.
They are often added to the existing flow system. This procedure,
which may meet immediate needs, is not always satisfactory in
terms of the homogeneity of the whole system. Often the performance
of a new machine can be limited by the existing equipment in terms
of energy, capacity, maintenance, etc.
Unfortunately often, and particularly in this period
of economic constraint, the possibility of making new investments
is limited and may be postponed. Potential machine upgrades are
not carried out even when there is a strong case for them. For
example, the installed base can be modified with future expansion
in mind. Usually, in production lines, particularly in the stock
preparation area, where different fibres are used, different elements
do not match. Often the existing machines were selected oversized
foreseeing future production increases.
The same is true for the process parameters, eg
pulping times in the pulper, delays caused by low levels in the
dump chest, consistencies too low that request greater pumping
energy, etc.
How to proceed? First of all it is useful if not
essential to carry out a process evaluation, updating the schematics
to show the reality of the current configuration, reviewing structural
dimensions of individual elements in the line.
It is also worth carrying out statistical simulations
to establish the maximum and minimum parameters and to know how
efficiently the system is working.
The comparison between the maximum possibilities
and the "real requested ones" can highlight the potential
optimization and the possible resource savings.
In the following table we synthesize those that,
according to our experience, could be some of the interventions
and improvements that can be carried out in a typical process
of the stock preparation for the tissue paper production:
REAL EXAMPLES
By using the approach outlined in the Table, the mill described
below optimized its process with a consequent reduction in energy
consumption. The mill collected data from its DCS and from laboratory
tests on the final product and carried out the following modifications/optimizations.
Pulper area: speed regulation with inverter to reduce total pulping
power. Diversification of pulping energy requirement according
to LF and/or SF. A saving of 30% for pulping has been achieved.
Refiners: No-load-power reduction by rotation speed
and disk diameter reduction; recirculation reduction using check
flow valve; use of SR transmitter in line. The result has been
a saving of about 100 kW (28%) for each refiner in applied specific
energy. Approach-flow area: Systematic search of the consistency
increase in the headbox, without influencing formation, to obtain
improvements in the energetic-volumetric fan-pump consumption.
In the future it is foreseen that thick stock will
be injected directly into the fan-pump suction to increase the
turbulence and then to work with higher consistencies.
Process water system: Reduction in water consumption
through increased use of the clarified waters and through use
of filtered and purged water to replace fresh water (reduction
to 4 l/ kg of tissue paper).
