Direct Entry - Process Info
Direct Entry - introduction and background
In non-integrated paper mills most often baled pulp is used to provide a furnish for the paper machines. The process to produce a usable furnish is called direct entry.
Bales of pulp are transported on a conveyor to a pulper. In the pulper white water or filtrate will be mixed with the bales and a rotor in the bottom of the pulper will disintegrate the fibres to a pumpable suspension. In order to reduce the time for filling of the pulper the white water is supplied from a WW dump chest. With this arrangement the WW dump chest can be filled with a constant and moderate flow (=minimizing pump and pipe) while filling of the pulper can be done with a high flow.
The pulper can be either of a low consistency type (4-8%) or a high consistency type (12-15%). The high consistency pulper is preferred when low shear forces are advantageous. This is the case for example when OCC pulp is slushed to reduce disintegration of ink particles.
The pulpers operate in a discontinuous or continuous mode. In a discontinuous pulper white water will be filled to a certain level after which a predefined number of bales will be dropped into the pulper. After a slushing time of 15-25 minutes the pulp suspension can be pumped to a dump chest. When the pulper will be emptied dilution water is added in the bottom of the pulper to give a consistency of 4-5%. There are very small consistency variations in the pulp leaving a discontinuous pulper.
From a continuous pulper the pulp will be pumped all the time. That means that when the level in the pulper drops more white water will be added and a new bales dropped into the pulper. This type of operation will generate consistency variations in the produced pulp. The advantage of a continuous pulper is a somewhat higher production capacity as there will be no stoppage time for filling and draining.
The number of pulpers is depending on the production capacity requirement but often a single pulper has the capacity to serve two different pulp lines i.e. a hardwood and a softwood line.
In the preparation of hardwood pulp only a HD (high density) cleaner will be required beside the consistency controls. The HD cleaner will remove heavier contaminants (stones, bolts etc) from the pulp. There will be intermittent drainage of reject from the cleaner to the sewer. The hardwood pulp will normally not require refining. The consistency of the pulp to the blend chest will be 3,2-3,5%.
In the softwood line the pulp from the softwood dump chest will be consistency controlled to 3,5-4,0% and pumped through a HD cleaner further to refining. The purpose and operation of the HD cleaner is similar to that for the hardwood line. The accept from the HD cleaner will pass a special type 4-way valve and either be directed to the refiner or by-pass the refiner. The specific energy required is depending on the softwood pulp quality and on the paper quality requirements. Normally the required specific energy will be in the range 100-200 kWh/t. To achieve this energy input two or even three refiners in series will be required.
After the refiner(s) there is a recirculation line back to the suction side of the HD cleaner pump. This recirculation line will maintain a minimum flow through the refiner(s) and prevent refiner disc damage at start up and if softwood pulp demand drops (i.e. because a to high level in the blend chest). The consistency of the pulp to the blend chest will be 3,2-3,5%.