The hot filter-press - The new technology of vapour pressure dewatering

When separating suspensions containing solvents or toxic substances, the unification of filtration and drying processes within one single apparatus is especially desirable. Contrary to conventional methods, the dewatering technique presented here has the advantage of producing a dry granular bulk. (V) under bar apor (P) under bar ressure (D) under bar ewatering (VPD) involves mechanical differential pressure dewatering of a saturated bulk in conjunction with contact drying. VPD is based on a strong heating of a saturated bulk from one side. On this side the bulk liquid heats up and evaporates, causing considerable expansion of volumina and pressure build-up. As with difference pressure dewatering, the liquid is displaced and pushed out of the bulk material. For the technical application of the VPD, a filter press was modified so that every cake chamber of the filter press was divided by a heating plate. Thus, the new chamber was formed by a heating plate and a membrane filter plate covered by a filter cloth. During the first step of the dewatering the suspension is conventionally pumped into the still unwarmed filter press by a piston pump. During a second step the heating plates are heated and VPD sets in. For glass beads (x(50,3)=28 mu m) the purely mechanical displacement of the bulk liquid exceeds 80%, for sewage sludge it is below 10%. Other dewatering targets range between these values.