Pressure and Solidosity Profiles in Cake Filtration

Doctoral thesis, 2005

Filtration is an important unit operation in the separation of liquids from solids. The aim of this work was to investigate filtration properties of lignin sludge obtained in the production of fuel ethanol from spruce wood. In order to validate a new methodology of measuring and calculating filtration properties, three model materials were used: calcium silicate, kaolin and titanium dioxide. Simultaneous direct local measurements of pressure and solidosity were performed in filtration experiments. Constitutive equations relating solidosity-solid compressive pressure, permeability-solidosity and specific filtration resistance-solid compressive pressure were fitted to experimental data. Generally, good fits were obtained. It was found that the concentration and pH of kaolin suspensions influenced the constitutive relationships. Constitutive equations obtained by local measurements during filtration of calcium silicate and kaolin suspensions were tested in a model describing compressible cake build-up. Excellent agreement was obtained for the calculated cumulative filtrate volumes and pressure profiles compared to validation experiments for both materials. For kaolin, the calculated solidosity profiles and cake height evolution deviated from experiment due to a poor fit of constitutive equations to local solidosity at low solid compressive pressures. The agreement was good, however, for calcium silicate, which builds less compressible cakes. The lignin suspensions obtained in the production of ethanol from spruce wood had high specific filtration resistances (1013-1014 m/kg). However, a lower specific resistance was obtained on heating to boiling temperature; the positive effect remained after cooling. Experiments on the expression and washing of enzymatically hydrolyzed pretreated spruce wood indicated that it is possible to obtain a high monosaccharide yield and a high dry solid content if the filter cakes are expressed at low pressures, washed and then expressed at a high pressure. Processed spruce wood obtained from the production of ethanol was fractionated using liquid-liquid extraction. 5-14% of the lignin was found in the aqueous phase as water soluble lignin.