Christer Sandberg

Effects of chip pretreatment and feeding segments on specific energy and pulp quality in TMP production

Abstract Increased wood softening and refining intensity have earlier been utilized to improve refining efficiency in mechanical pulping. We have evaluated a combination of increased softening by low dose sulphite chip pretreatment and increased intensity by feeding segment design in a TMP line for production of high quality printing papers. Norway spruce wood chips were preheated, compressed in an Impressafiner and impregnated with water or sodium sulphite solutions (Na2SO3 charges 3.6 and 7.2 kg/t). Chips were refined in two parallel 68” double disc refiners using two different refining conditions: standard bidirectional segments at normal production rate (9 t/h) and feeding segments at increased production rate (11.1–12.1 t/h). The feeding segments enabled a 30 % increase in production rate. Refining with feeding segments at 12.1 t/h production rate combined with chip pretreatment with 3.6 kg/t sodium sulphite reduced the specific energy 360 kWh/t (19 %) compared to refining with standard segments and no pretreatment. Pulp properties were similar for the two configurations. The combination of feeding segments and chip pretreatment with water reduced the specific energy 180 kWh/t (9 %). Implementation of most of the technology presented has reduced the electrical energy use for the mill by approximately 80 GWh/year.

Process intensification in mechanical pulping - OPEN ACCESS

Process intensification is a term used in the chemical process industry for major improvements in the process design leading to radical changes in process complexity, equipment size and efficiency. ...

Mill evaluation of an intensified mechanical pulping process - OPEN ACCESS

Mill-scale demonstration of a process concept inspired by Process Intensification (PI) principles was performed in Holmen Paper Braviken mill, Norrkoping, Sweden. The intensified process consists o ...

Low dosage sulfite pretreatment in a modern TMP-line

The effects of low dosage sulfite pretreatment combined with modern high consistency double disc refining were evaluated for production of thermomechanical pulp in a mill scale trial using Norway spruce wood at the Braviken paper mill (Holmen Paper AB, Sweden). Spruce wood chips were mechanically pretreated in an Impressafiner before impregnation with different dosages (0-1.2%) of sodium sulfite (Na2SO3) at pH 9. Approximately 23% of the added sulfite was converted to sulfonate groups in pulp, resulting in a sulfonate content of 0-0.28% (as Na2SO3). The low dosage sulfite addition increased tensile index, elongation, density, brightness and decreased shive content, light scattering and light absorption coefficients when compared at equal specific energy consumption (SEC). The increase in tensile index was proportional to dosage of sulfite. Further analyses showed that low dosage sulfite addition did not affect the distribution of the Bauer-McNett fractions nor the fibre length for pulps refined with equal SEC. However, the low dosage sulfite addition increased fibre delamination/internal fibrillation (D/IF). With the addition of 1.2% Na2SO3, it was possible to produce pulp with a tensile index of 47 Nm/g using ~320 kWh/bdt (~15%) lower refining energy, compared with pulps produced without sulfite addition.

Effect of flow recirculation on pulp quality and energy efficiency in low consistency refining of mechanical pulp

The effect of pulp flow recirculation on energy efficiency and pulp properties development in low consistency refining of TMP has been studied. Trials were made with TwinFlo 58 refiners in two mills in Sweden. The refiners were operated at constant specific refining energy but different flow conditions. No effect of recirculation was seen on refining energy efficiency, pulp quality or fibre wall delamination/internal fibrillation. At high degree of recirculation though, a somewhat larger fibre length reduction was seen. This means that a large degree of recirculation can be used to attain a high specific energy in one stage low consistency refining, without significant negative effects on fibre properties. If a high specific energy input is desirable, it is more cost effective to install one large refiner with recirculation compared to several small without recirculation in series. When recirculation was increased by means of increased flow through the refiner, the outlet pressure dropped. The lower pressure drop over the refiner probably decreased the internal recirculation between rotor and stator, which could explain the observation that there was little effect on fibre development by recirculation. This means that at certain conditions the distribution in fibre treatment might not increase so much when the outer recirculation is increased, since the internal recirculation might be reduced simultaneously.

Low-consistency refining of mechanical pulp in the light of forces on fibres

The aim of this investigation was to find new approaches to evaluate the performance of low-consistency refiners. Data from a paper mill producing TMP from Norway spruce was used in order to find a possible way to calculate the power split between the zones in a TwinFlo refiner. An assumption of equal amount of fibres captured between overlapping bars was found successful in order to develop equations for the power split. The equations predicted equal power in both zones at equal disc gaps. The power was found to increase approximately linearly with decreasing disc gap over the range, 0.1-0.2 mm. The power split was essential to know for calculating refining intensities expressed as specific edge load and forces on fibres in the two zones. The reduction in fibre length was about 5% at 0.17 mm disc gap or at 0.03 N forces on fibres or at 0.7 J/m specific edge load. Disc gap, forces on fibres and specific edge load was found to predict fibre shortening approximately equally upon changes in power and flow rate through the refiner.