|Pulp is a mass of fiber obtained from wood (rags of cotton or linen, waste paper etc.). Wood pulp is derived from softwoods and hardwoods trees. It is composed of cellulose fiber (50%), lignin and hemicellulose (50% left).|
Pulping process is carried out in large digesters where wood chips are mixed with white liquor (such as sodium sulfate or sodium hydroxide) and cooked at very high temperature and under high pressure. Through this process cellulose fiber is separated from lignin and hemicellulose. Cellulose is a natural binder and is used as main material in papermaking.
Instrumentation for wood pulping step:
|Pictures 1 and 2: Pressure measurement in the digester|
Picture 3 : Gauge or vessel transmitter with flush
|Black liquor is used during pulping process. It contains lignin residue, hemicellulose and chemicals.|
While it is toxic, black liquor can be used as biofuel : the steam obtained after the black liquor went into the recovery boiler serves as fuel for turbo-generator. It can also be used for drying paper or for extracting cooking chemicals.
Instrumentations for chemical recovery :
|Picture 1 : Gauge pressure transmitter with remote seals specific coating and extension|
Picture 2 : Flushing ring with ball valve
Picture 3 : Flushing ring
Picture 4 : Level transmitter with extension remote seal and specific lenght
|Recovered papers are mixed with pulp in the pulper. Screening removes large contaminants. During deinking the addition of chemical separate the hydrophobic particles (inks) from the hydrophilic particles (paper pulp).|
The mixture is then aerated in the flotation cell and the froth attached to the hydrophobic particles is collected. Finally, the pulp is bleached using chemicals.
Instrumentation for pulping from recovery paper :
|Picture 1 : Flow measurement on bleachng tower|
Picture 2 : Remote seal with ceramic coating
Picture 3 : Flush level transmitter with ball valve for controlling stock discharge stock
Picture 4 : Level measurement on pulper
|The pulp slurry (stock + additives) is laid out to form into sheet then drained and dried. Then the paper may be coated with pigment to increase the strength and pressed with roller to be more glossy.|
During forming, press and dryer sections, the pulp slurry is drained on mesh wire to form wet fiber web (80% humidity). The web is squeezed between two press felt (55% humidity) and heated to dry (8% humidity).
During calendering the paper surface is smoothed and coated. Finally it is wound up, cut, packaged and shipped.
Instrumentation for the final paper making step :
Pictures 1 and 2 : Level and pressure measurement in headbox using flush diaphragms.
Difficult process operating (high humidity level - high temperature)