Effects of extended cooking times in the production of eucalytus pulps

Transcription

1 Effects of extended cooking times in the production of eucalytus pulps Miguel A. Osses, Env. Mgr, Valdivia Pulp Mill /Arauco Chile, Abstract Trying to explain the industrial behavior of Eucalyptus pulps produced with extended cooking times, a laboratory study was carried out to investigate thise kind of pulps and correlate the results with the pulp mill performance. Long cooking time pulps are produced in a mill when some of the production areas stop and there is no enough capacity for storage in the high density towers of unbleached or bleached pulps. Eucalyptus globulus and Eucalyptus. nitens chips in the proportion of 80/20 were obtained directly from the pulp mill wood yard. Chips where cooked in a conventional kraft batch digestor using a laboratory MK System. Pulps produced with different cooking times where evaluated and O2 delignified to obtain a constant kappa. The results showed that long cooking time pulps, Presented significant degradation, loosing chemical yield and affecting the production cost because of higher wood consumption, black liquor presented higher solids content. For line 1 of Arauco pulp mill, it can affect pulp production because the recovery boiler is deficient in black liquor burning capacity. In spite of the pulp degradation, measured through the viscosity loss, the strength properties were not affected, but the Gurley porosity increased significantly. This behavior could explain a lot of the pulp drainage problems in the rotary filters, presses and Fourdrinier when these pulps are involuntarily produced. Finally the production cost of these pulps increase directly with the retention time, because de higher wood consumption and higher chemicals bleaching. Key-Words: Eucalyptus kraft cooking, Effects of long cooking times Introduction Celulosa Arauco y Constitución S. A. produces in Chile around three millions tons per year of market pulps. The main experience of the company was producing softwood pulps (Radiata Pine). After a program of eucalyptus plantations of E. globules and E. nitens, the wood was used to produce market bleached kraft pulps. Line 1 in the Arauco pulp mill was decided and adapted for this. Some production trials were also done, but at the beginning a lot of production problems occurred traduced in a discontinue operation and with pulps remaining more time in the digestors, because the pulp tank were totally full. To understand the behavior of eucalyptus pulps produced with these involuntary long retention time in conventional kraft cooking, laboratory pulps were produced at kappa 15 constant, using E. globulus and E. nitens industrial chips coming from the wood yard in the proportion of 80/20. The pulps were delignified up to kappa around 10 and bleached with the short ECF sequence D0EpD1, that is used in the line 1 of the Arauco pulp mill in Chile. The mains objectives were to study the cooking yield variations, solids black liquor production, changes in the pulps viscosity, in the drainage, bleachability and finally to use these results for a better comprehension of the production problems with these pulps like poor filtration in the washer filters and foudrinier, presses and the big quantity of breaks. Also the economics effects were evaluated.

2 Experimental The alkali charge and time to temperature of 165 ºC were maintained constant, changing only the time to the cooking temperature. For cooking trials was was used a 5 liter MK System digester with indirect heating. Were used times of 25, 60, 120,180 and 240 minutes. The pulps obtained with kappa 15 were delignified up to kappa 10 and bleached with the short sequence of D0EpD1 up to 90 % ISO. In the cooking the following analysis were done. Kappa, screened yield, rejects at slots of 0,005 mm width, viscosity, hexenuronics acid, hemicelluloses, brightness and black liquor solids. For O2 delignification and bleaching were determined the normal and standards parameters for these experiences and showed in the following tables. All the analysis were conducted using TAPPI standards. Results and discussion The following tables show the results of the different laboratory experiences for wood chips, brown pulp, delignified and bleached pulps. Also an estimation of the increased production cost is presented, mainly focused in the higher consumption of wood and chemicals for bleaching, when these kind of pulp are produced. Wood chips characteristics Table 1 Some basic characteristics of pulps from E globulus y E nitens parameter Unidad E. globulus E. nitens Insoluble lignin % bms Celullose % bms Hemicelullose % bms extractables % bms 1, ,5 Basic density kg/m 3 s Screened yield % bms Fiber lengh mm 0,9 1,0 0,6 0,7 vessels vessels/gr Here is possible to see de big difference between the two Eucalyptus globulus has much lower insoluble lignin content and higher cooking yield. Table 2 - Chemical characteristics of mixed wood Component M1 M2 mean Component M1 M2 mean Glucans 46,7 46,1 46,4 Arabinanos 0,3 0,2 0,3 Xilan 14,6 14,3 14,5 Soluble lignin 4,3 4,5 4,4 Galactanes 1,0 1,2 1,1 Insoluble lignin 21,2 20,8 21,0 Mananes 1,9 2,1 2,0 Lig Total 25,5 25,3 25,4 Extraíbles 1,7 1,8 1,8 These values are refered to 80/20 E Globulus y E Nitens mixture.

3 Table 3 Cooks of eucalyptus mixture kappa 14,5 12,5 11,9 11,6 10,7 Screened yield % bms 56,1 55,9 54,8 53,9 53,4 Rejects % bms 0,2 0,01 0,001 0,001 0,001 Brighness % ISO 41,2 40,0 39,2 38,1 36,0 viscosity cm3/gr BL solids % 19,3 19,8 20,2 20,4 20,6 BL ph 12,9 12,5 12,3 11,7 11,5 Residual alkali gr/lt 6,7 3,6 1,4 0,0 0,0 Glucans % bps 80,4 80,1 80,2 80,4 80,4 Xilan % bps 17,1 16,9 16,9 17,1 17,3 Galactans % bps Manans % bps and Vicosa University As was espected with same alkali charge, changing only the cooking time were obtained pulps with lower kappa, lower yield, lower brightness, lower viscosity, showing the unwanted of this practice that is possible to have in a mill, when there are operations problems after cooking stage. Galactans and manans were totally dissolved because initial ph was similar for all the cooks. Table 4 Pulps delignification results O2 pressure bar 4,5 2,0 2,0 2,0 2,0 Temperature C kappa in 14,5 12,5 11,9 11,6 10,7 kappa out 9,8 10,1 10,0 9,9 10,1 Deslignification % 32,4 21,1 16,0 14,7 6,5 Brighness % ISO 55,6 49,3 48,0 44,7 39,9 viscosity cm 3 /gr Viscosity drop cm 3 /gr

4 Talble 5 - Bleaching results Do stage time min Temperature C Multiple chlorine - 0,24 0,24 0,24 0,24 0,24 ClO 2 charge Kg/Bdt 9,0 9,2 9,1 9,0 9,3 Ep stage NaOH charge kg/bdt 11,5 11,5 11,5 11,5 11,5 H 2 O 2 charge kg/bt 5,2 5,2 5,2 5,2 5,2 kappa - 2,1 2,1 2,5 2,5 2,5 D1 stage ClO 2 charge kg/bdt 2,1 6,4 9,0 11,0 112,0 Viscosity cm 3 /kg Brighness % 90,1 90,1 90,2 90,3 89,3 ClO 2 residual kg/bdt traces traces 0,2 0,6 0,9 Total consumptions Chlorine dioxide kg/bdt 11,1 15,6 18,1 20,0 21,3 Soda caustic kg/bdt 11,5 11,5 11,5 11,5 11,5 Hidrogen peroxide kg/bdt 5,2 5,2 5,2 5,2 5,2 This table clearly shows the difficult for bleaching of the pulps produced with high cooking time. The main chemical as the chlorine dioxide increases almost the double. Table 6 Refining of unbleached pulps PFI 30 SR Tear mnm 2 /g 8,5 9,4 9,6 9,5 9,4 Burst kpa/g 4,5 5,2 5,9 6,3 5,5 Tensile Nm/g TEA J/g 1,1 1,4 1,7 2,0 1,7 Porosity (Gurley) seg/100ml 7,0 10,0 13,0 18,0 21,5 Elongation % 2,1 2,5 2,7 3,1 2,7 Ref energy consump Wh 6,0 12,5 15,0 22,5 15,0 Specific volume cm 3 /g 1,6 1,5 1,5 1,5 1,5 Water absortion mm 51, ,5 33 Opacity % 95,9 96,0 97,5 97,0 97,0 Fiber length mm 0,89 0,89 0,87 0,83 0,85

5 Table 7 Refining of bleached pulps PFI rev to 30 SR Tear mnm 2 /g 9,7 9,8 10,0 9,8 10,0 burst kpa/g 5,9 5,9 5,7 5,3 5,3 tensile Nm/g , TEA J/g 2,2 2,2 2,1 2,0 1,9 Porosity Gurley seg/100ml 9,0 9,0 9,9 8,0 8,0 Elongation % 3,9 3,7 3,5 3,5 3,4 Refining energy Wh 21,0 35,0 36,0 37,0 36,0 Specific volume cm 3 /g 1,49 1,46 1,43 1,43 1,41 Water absortion mm Opacity % ,8 69,5 69,5 Fiber lenght mm 0,78 0,77 0,77 0,78 0,78 Table 8 Estimated Wood consumptions Cooking yield % 55,2 52,7 52,3 51,9 51,5 O2 delig. yield % Bleaching yield % Final yield % 53,0 50,6 50,2 49,8 49,5 Wood consumption tms/bdt 1,89 1,98 1,99 2,01 2,02 Wood consumption m3/bdt 3,912 4,097 4,129 4,161 4,193 Wood consumption US$/BDt 154,9 162,3 163,5 164,8 166,0 Source: calculations from cooking yield results Table 9 Estimated bleaching and final cost ClO2 kg/bdt 11,1 15,6 18, ,3 NaOH kg/bdt 11,5 11,5 11,5 11,5 11,5 H2O2 kg/bdt 5,2 5,2 5,2 5,2 5,2 H2SO4 kg/bdt Bleaching cost US$/BDt 39,0 49,1 54,7 58,9 61,8 Final cost US$/BDt 193,9 211,3 218,2 223,7 227,9 Increasing of cost % 0 9,0 12,5 15,4 17,5 Conclusions The obtained results show the problems that is possible to have, when the pulps are produced with extended cooking times and batch conventional technique. The main direct problem are the following: higher wood consumption and associated cost, because the lower yield, less brightness, lower viscosity, higher chemical consumption in bleaching. The lower porosity of unbleached pulps can help to understand the drainage problems in the rotary vaccum filters used in the unbleached and bleached washing stage. The strength properties of bleached pulps remain almost without changes.

6 Acknowledgements To the research laboratory people of the Arauco pulp Mill in Chile, they needed to do twice these lab experiences, because the laboratory pulps were lost with the big 2010 hearthquake.