Pulp screening and washing - process info
Pulp screening and washing - introduction and background
In the pulp screening system knots and shives (parts of chips which are not defibrated in the cooking process) are removed from the pulp.
Other impurities like coloured spots originating from pulp wood (resin and bark) and dirt from foreign material like sand, plastic, rubber and rust are also removed in this process.
Screening can be divided into three parts; knot separation (deknotting), fine screening and reject treatment.
It is common to combine deknotting and the first screening stage in a so called combi-screen (knotter/primary screen). Accepts from the first screening stage are usually screened in two additional stages. For the deknotting and fine screening pressure screens are normally used. These are operated filled with liquor and under pressure.
Centrifugal cleaners or a pressure screen for sand removal are used in the end of the screen room. Rejected knots from the first stage are dewatered, washed and sent to re-cooking or rejected from the system.
Screen rejects from the last screening stage are dewatered, washed and sent back to the previous stage or are rejected from the system.
The purpose of the pulp washing is to separate the spent cooking liquor with dissolved wood substances (= black thin liquor) from the pulp. The washing process shall use as little wash liquid as possible since the spent liquors shall be evaporated and burnt in the recovery boiler.
The wash liquid is usually the clean fractions of condensate from the evaporation plant and/or fresh hot water. The liquid is applied as wash liquid on the last washer in the “closed” part of the fibre line and is then used in a counter current mode through the whole washing and screening to finally be used as wash liquid in the bottom of the digester before being evaporated and burnt in the recovery boiler.
The pulp washing can be done in the bottom of the digester (long Hi-Heat zone older digesters, short bottom diffusion zone newer digesters), in an atmospheric diffuser washer (AD), in a pressure diffuser washer (PD), wash filter, wash press, compaction baffle filter or in a drum displacement washer. These listed washers are the most common ones, but there are others. Usually a combination of different washing units is used to obtain a suitable washing system.
All washers are based on the principle of displacement, to displace foul liquor with cleaner liquor from the pulp. Some washers may also include the principles of dilution, dewatering and pressing.
The principle for the digester bottom diffusion zone and the diffuser washer is that the wash liquor is slowly displacing the liquor in the pulp web in the washing equipment. The Atmospheric diffuser can be designed with one or two washing stages.
In the wash filter the diluted pulp enters the filter vat and a pulp web is formed on the filter drum. The liquor is sucked through the pulp web by vacuum (dewatering), which is generated in a drop leg.
Wash liquor is sprayed on the pulp web on the filter to displace the original liquor.
Compaction baffle (CB) filters and drum displacement (DD) washers are based on the same principles as wash filters (vacuum filters).
However, one main difference from a process point of view is that the pulp can be fed at a higher inlet pulp consistency. Further, both the CB filter and the DD washer are operated under pressure instead of vacuum to achieve initial dewatering and displacement. The DD washer can be designed for one, two or three washing stages in the same piece of equipment.The pulp enters a wash press at a wide range of consistencies, degrees of dilution.
The pulp is dewatered on the press roll(s). Wash water is displacing the pulp web on the rolls.
The pulp is then pressed to high pulp consistency in the press nip between two rolls.
Two principles for presses are dominating the market, those with the same diameter on the press roll (twin roll) or one large press roll with a smaller press roll.
The pulp feed to the press is done in different ways and is vendor dependant.
It can be done via a press vat or through direct feed with distribution screws or through a so called paraformer.
A common configuration of washing and screening in an old fibre line is Hi-Heat washing, washing in an atmospheric diffuser (AD) on top of a storage tower for pulp of medium consistency, followed by deknotting and screening, and then washing in two filters in series. A common configuration in a new fibre line is limited washing in the digester bottom, washing in a pressure diffuser (PD), pulp storage at medium consistency, deknotting and screening, washing in wash presses or DD washers in two stages.
In the flow sheet a probable retrofit of an old line with increased capacity is shown and hence including a variety of washers: limited washing in the digester bottom (shown in digester flow sheet), PD washer, AD washer on top of a storage tower for pulp of medium consistency, deknotter and screens, filter and wash press. DD washers are included as an example in oxygen delignification.
Control valves only listed for Knotter/Primary screen No 1 and control valves common for Screen No 1 and No 2.