State of the Art in Dewatering Processes During Paper Manufacture

Abstract

The European paper and board industry has currently to pursue an important investment effort in order to improve its competitiveness and to meet the challenge of an increasing product demand both in quantity and quality.

Papermaking is essentially based on water removal from a wet fibers and fillers network formed on a continuous wire, and then mechanically dewatered in the press section and thermally dried in the drying section.

The different dewatering devices of the machines have to be improved in order to work with higher energy efficiency, at higher speed and at constant or improved paper quality.

The ratio of the energy costs for removing 1 T of water at the press section or at the drying section is presently: 1 for 6.

Due to the fact that most paper machines are dryer limited, an increase of the sheet dryness after the press section of 1 %, brings in practice marginal gains of 2.1 % in production rate, which means savings in the range of 0.64 billion ecus/year in the E.E.C. This clearly stresses the interest of developing mechanical dewatering methods in paper industry.

Recent developments have led to the design of long nip presses of various type with an achievable exit dryness of the paper sheet changing from about 45 % to about 55 % in a 10 year period.

Further progress includes the use of a certain amount of heat transfer at the press level:

• Steam boxes, used for heating the incoming sheet, are in current use in industry now,

• Hot presses (in the range of 100°C) give good results on slow machines manufacturing heavy weight papers.

The present developments in course in the world concern two main techniques :

• Impulse-drying , using a roller heated at temperatures in the range of 150 to 500°C.

• Press-drying, with temperatures ranging from 150 to 250°C.

The main draw-backs of these techniques are sheet delamination, sheet sticking to the roll and paper color changes.

Together with the continuing technological adaptation of these techniques, further progress in the field of sheet dewatering in paper manufacture, require now the refinement to the knowledge of the basic phenomena which take place in the pressing nip, in the porous media (paper and felts) and in the press itself.