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D. M. D. Drummond, M. T. M. Rodrigues, I. E. Grossmann and R. Guirardello

Brazilian Journal of Chemical Engineering

the drying process (Figure 1). Recently, there has been
significant improvement in the operation of presses to
improve economy by increasing the water removal in
the press section and, at the same time, keeping or
improving the characteristics of the web of paper.

This work considers the novelty of minimizing the

replacement of the felts, by using an optimal sequence
of paper production, in order to improve the water
removal in the press section. The main goal is to
optimize the change of felts in a paper machine, while
determining the optimal sequence of the production for
different types of paper, the felts to be replaced, and the
optimal cost of production. The objective for
optimizing the press section is to obtain the best
sequence of production of the paper reels and the time
of replacement of felts, increasing the water removal in
the press section. In this way, less water needs to be
removed in the drying section, where the energy costs
are higher. A comparison of the model results with
actual industrial values is presented.

PAPER MACHINE DESCRIPTION

Until the 18

century, paper sheets were made

manually. The laborer immersed a screen, attached
to a wood frame in a tank containing a fiber
suspension, forming a leaf that was subsequently
dried in air. In 1799, Nicolas Robert invented a
machine that made possible the formation of a sheet
of paper of infinite length. It was made of wood and
had a suspended fabric screen, in which the fiber
suspension was sparged. Robert, due to financial and
technical difficulties, could not go ahead with his
project and was obliged to sell the patent to the
Fourdrinier brothers, who did not have much success
as well. In 1818, Donkin successfully built, in
Germany, the first machine of this type. In it, the
mass remained in agitation and fell on the screen by
gravity. After being pressed, the leaf was rolled up at

the end of the machine. The modern machines, based
on the one invented by Robert, are constituted of
many independent sections, each one with its
function and proper characteristics. Almost all the
sections of a paper machine can be remodeled and
improved, with the exception of the width that is
fixed. In general, the parts of a machine to
manufacture paper are: the wire section, the press
section and the drying section (Figure 1). The
removal of water starts by gravity, is followed by
suction and pressing, and finishes by evaporation.
Currently, due to increasing energy costs, efforts are
concentrated on the drying.

The amount of water removed in each section

varies according to the kind of paper used, the
operating conditions, and the machine settings. In
general, as shown in Figure 1, the wire section
reduces the amount of water from 99% to around
80%, the press section reduces it from around 80% to
around 50%, and the drying section reduces it from
around 50% to 5%.

PRESS SECTION AND FELTS

The high replacement frequency of the felts in the

press section has received increased attention in the
last few years, considering that they are very
expensive. Also, the amount of water that can be
removed by pressing the paper before it enters the
drying section embodies the largest potential savings
in the total cost of water removal. The drying section
is responsible for around 78% of the cost of water
removal in the web of paper, while about 10% of the
cost is in the wire section and about 12% of the cost
is in the press section. These values depend on both
the unitary costs in each section and the amount of
water removed in each one.

The press section is shown in Figure 2, in which

four different felts are shown.

Drying section

Wire section

Press section

Drying section

Wire section

Press section

Figure 1: Sections of a paper machine (with paper humidity, given as mass water/mass
cellulose): entering the wire section (99%), entering the press section (80%), entering
the drying section (50%) and leaving the paper machine (5%).