Dewatering parameters in a screw press and their influence on the screw press outputs

Abstract

A Thune SP23 screw press dewatering parameters were studied. The dewatering efficiency was affected more by the rotational speed and the pulp properties. The counterpressure affects dewatering near the discharge end, and it was observed to influence the outlet consistency and filtrate flow rate of Kraft, which has much longer fibres and fewer fines compared to TMP and BCTMP.The feed stock freeness and consistency are very important variables to consider in the screw press performance. The freeness reflects the degree of drainage, which is an important parameter to consider when optimising the screw press, while the feed consistency is a parameter of the fibre-fibre contact degree. The pulp properties, especially the fines content and fibre flexibility are also two very important parameters that affect the screw press performance.This study was to provide an insight of the screw press performance and to show the complex effect of the operational parameters on the dewatering characteristics. Using three different pulps, Kraft and TMP softwood fibres and a BCTMP hardwood fibres, we have shown that the fines content and fibre properties are two dominant properties that should be highly considered when operating a screw press.

Dewatering saturated, networked suspensions with a screw press

“…The fluxes of both the solid and liquid phases are writtenû s andû f , respectively, where the hat decoration denotes dimensional variables. We neglect any inertial effects: the compaction due to the contracting geometry is relatively slow, with a pore-scale Reynolds number for the wood pulp pressing of [12] of around 10 −4 . We also neglect gravity: the ratio of the buoyancy force to a characteristic measure of the network stress p * is (ρ s − ρ f )gr b /p * , where ρ s and ρ f are the densities of the (incompressible) solid and fluid constituents respectively, g is the gravitational acceleration, andr b is the basket radius; for the wood pulp press of [12], this ratio is always less than 0.05, although gravitational effects may be more important for other materials, particularly if the press is unsaturated.…”

“…So that the analysis of the model remains as general as possible, we shall derive the model for the general constitutive relation (5) before considering the model predictions for limiting behaviour of the constitutive relation for materials with and without a solid bulk viscosity. For direct comparison to experimental data, we shall then restrict the constitutive relation to those appropriate for the particular cases of paper-making fibre suspension pressing [12] and clay pressing [11]. On solid surfaces with normal vector n we apply the no-penetration boundary condition n •û s = n •û f = 0 on both the solid and fluid phases.…”

“…To leading order in δ, equations (16) and (18) now indicate that the total stress −p + P is independent of r and ξ, while the remaining stress equation 17, which indicates that −p + P varies slowly with ϕ, becomes redundant in this limit, being decoupled from the the remaining equations of motion (11)(12)(13)(14)(15). These can be reduced at leading order to…”

“…We also make comparisons to experimental data for materials with or without a rate-dependent solid stress. The results for the case with rate-dependent solid stress are compared with measurements using an industrialscale paper-making fibre suspension press performed by [12]. In the case without an assumed rate-dependent solid stress, we compare the theoretical predictions with the results of Shirato et al [11], who explored the dewatering of a clay slurry in a laboratory-scale press.…”

“…Because the basket is relatively rough but the shaft and flight are smooth, the associated drag forces prevent the suspension from rotating around the Fig. 1 Sketch of the geometry of a Thule (Voith) SP23 screw press as given in [12] and Appendix B.1, and the two regions of the corresponding idealized model. In detail, (a) shows a plan view.…”