Investigation of pulp dewatering – relating freeness to filtration properties
For Sweden the forest sector is of great importance and contributes significantly to the Swedish export. One of the most important products is pulp, which can be used for production of packing material, printing paper and tissue paper. Pulp is also produced in several other countries (e.g. Finland, Canada and Brazil) and the world production is around 180 million tons during 2010. There are several different methods used for pulp production, both mechanical and chemical methods can be used. Among the chemical methods the kraft process is the most common. One important product based on pulp is paper. Paper is formed by dewatering a mix of pulp and additives, to achieve a proper formation the starting pulp needs to be severely diluted, consistency as low as 0.1 % is required to form a paper of a good and even quality. The formation of the paper takes place on a wire section, where large amounts of water are removed by filtration. Further dewatering is then performed in the pressing section and finally the paper is dried. In the industry the dewatering properties of the pulp are measured using simple equipment measuring the freeness of the pulp, normally no filtration properties are measured. The goal for this study has been to investigate pulp dewatering behaviour and relate freeness measurements to measured filtration properties. The fibres used in this study are produced in a kraft process based on softwood material To investigate how the filtration properties change with the dewatering rate, the pulp was treated in a PFI-mill to change the structure on the pulp and thus the dewatering rate. The dewaterbility was measured using two industrial methods Canadian standard freeness (CSF) and Schopper-Riegler (SR). The filtration resistance of the pulp was also measured at 1 bar filtration pressure. A model for CSF from the literature was used to compare a theoretical relation between freeness values and specific filtration resistance and the relation found from the filtration experiments in this study. The range of the freeness value of the processed pulp was from 695 ml to 125 ml for CSF, which corresponds to a specific filtration resistance of 9×10 9 m/kg to 7.7×10 12 m/kg at 1 bar filtration pressure. The model for CSF indicated lower filtration resistances than the ones measured in the experiments. An important explanation for this is the difference in filtration pressure and with the following difference in compression of the filter cake. Table of
Characterization of the fiber–water separation process through a suction box of a single-wire pilot paper machine
Abstract During paper forming on a single-wire paper machine, i.e. a Fourdrinier or a hybrid paper machine, the pulp suspension is dewatered on a moving wire mainly through vacuum assisted devices (suction boxes). The dewatering process is supposed to occur by dead-end filtration under constant pressure as long as the fiber mat (filter cake) remains fully saturated with water. A physical model was written based on the kinetics of deposition of a compressible fiber mat and a drainage model for the filtrate. After some mathematical arrangements, a model similar to Hermans and Bredee’s formulation [1] was obtained. It states that, at a constant suction pressure, the amount of filtrate per unit surface area is proportional to the square root of suction time. Our approach also yields an equation that is similar to the empirical drainage model used in the paper industry, which links the drained flow rate to a power law of the suction pressure and of the dwell time. By identification of the values of the coefficients, it is then possible to analyze the compressibility of the fibrous mat. For purpose of validation, experimental investigations were performed on one slotted suction box of a Fourdrinier pilot paper machine. The suction pressure was varied from 0.5 to 3.0 kPa and the dwell time was changed by successively increasing the number of slots from 1 to 6 slots. The flow rate of filtrate was measured in each case in order to determine the corresponding specific amount of filtrate. Result analysis confirmed that the specific volume of filtrate varies as the square root of the dwell time. In addition, we could calculate the basis weight of the mat deposited on the wire upstream of the suction box and on the suction box itself, the average specific filtration resistance which was 10 10  m kg −1 for the studied case. The methodology developed in this study can be transposed to other dead-end filtration processes.
Formation of unexpected ions from a first-generation polyamidoamine dendrimer by use of methanol: an artefact due to electrospray emitter corrosion?
We report the formation of unexpected ions during the analyses of a first-generation polyamidoamine dendrimer in negative ion mode using an ion trap equipped with an electrospray ionisation source. These surprising ions corresponded to an increase of 12 m/z units over those expected. The formation of the unexpected ions was dependent on the tuning of the solution flow rate and the capillary high voltage. In addition, measurements of unusual value of the current suggested that a reaction was occurring in the corona plasma. The influence of methanol in this phenomenon was demonstrated by using CD(3)OH in the sample preparation. We propose two structures to explain the observed adduct based on the results of MS(2) experiments and by referring to previous work dealing with 12 m/z units addition. We showed that a corona discharge caused by alterations taking place to the electrospray capillary emitter was the origin of these unexpected ions. Finally, we discuss the mechanism involved in the formation of the ions and we propose means to control such artefacts.