Pressure and Solidosity Profiles in Cake Filtration

Filtration is an important unit operation in theseparation of liquids from solids. The aim of this work was toinvestigate filtration properties of lignin sludge obtained in theproduction of fuel ethanol from spruce wood. In order to validate anew methodology of measuring and calculating filtration properties,three model materials were used: calcium silicate, kaolin andtitanium dioxide. Simultaneous direct local measurements of pressureand solidosity were performed in filtration experiments.Constitutive equations relating solidosity-solid compressivepressure, permeability-solidosity and specific filtrationresistance-solid compressive pressure were fitted to experimentaldata. Generally, good fits were obtained. It was found that theconcentration and pH of kaolin suspensions influenced theconstitutive relationships.Constitutive equations obtained by local measurementsduring filtration of calcium silicate and kaolin suspensions weretested in a model describing compressible cake build-up. Excellentagreement was obtained for the calculated cumulative filtratevolumes and pressure profiles compared to validation experiments forboth materials. For kaolin, the calculated solidosity profiles andcake height evolution deviated from experiment due to a poor fit ofconstitutive equations to local solidosity at low solid compressivepressures. The agreement was good, however, for calcium silicate,which builds less compressible cakes.The lignin suspensions obtained in the production ofethanol from spruce wood had high specific filtration resistances(1013-1014 m/kg). However, a lower specific resistancewas obtained on heating to boiling temperature; the positive effectremained after cooling. Experiments on the expression and washing ofenzymatically hydrolyzed pretreated spruce wood indicated that it ispossible to obtain a high monosaccharide yield and a high dry solidcontent if the filter cakes are expressed at low pressures, washedand then expressed at a high pressure.Processed spruce wood obtained from the production ofethanol was fractionated using liquid-liquid extraction. 5-14% ofthe lignin was found in the aqueous phase as water soluble lignin