Compressive Dewatering. Dewatering caused by high mechanical pressure follows a different mechanism

Dewatering caused by high mechanical pressure follows a different mechanism

than outlined above for drainage, because the fiber web’s compressibility needs to

be appropriately considered. Published investigations made in respect of pressing

have focused on the phenomena in roll nips of paper machines, but have also a

general relevance for compressive dewatering.

There are basically two models of wet pressing – the decreasing-permeability

model and the limiting-consistency model. The decreasing-permeability model

attributes the decrease in water removal as pressing progresses to an increasing

flow resistance caused by narrower flow channels within the fiber web. The liquor

remaining in the web after pressing is related to the initial consistency, the press

impulse, a permeability factor and a compressibility factor [10–12]. The tenet of

the limiting-consistency model is that for any dewatering system, there is a maximum,

or limiting, consistency that can be reached regardless of how long a dewatering

force is applied. The limiting consistency is a function of the applied pressure,

initial consistency and permeability at a given compression [13].

As the fiber web is compressed, the increase in consistency goes along with a

reduction in web thickness. It has been shown that, under a roll-nip-like stress

pulse, the compaction of the web continues beyond the peak stress, though at a

low rate [8]. When the stress is removed, the web expands again and is susceptible

to rewetting. It is therefore essential for efficient compressive dewatering to leave

as little pressate as possible next to the pulp network at the end of the pressing

action, so that such pressate cannot be sucked back into the pulp by the expanding

web.

5.2.4