Modified Kraft Cooking And Adsorption Performance Of Various Pulps For Formaldehyde Gas

The rapid heating up and black liquor pre-impregnation-compression were applied in the prior stage of birch kraft pulping in this paper.The new technology of modified kraft pulping was developed and the influence of each process parameters of this two kinds of technology were studied.The content of the rapid heating up kraft pulping is that the wood chips was impregnated with cooking liquor at room temperature,then rapidly heated up to about 130? and carried out small heat preservation,and finally heat up to the highest cooking temperature and insulation.The optimum process parameters obtained by the study were:alkali dosage 22%,sulfidity 35%,wood-to-liquor ratio 1:5,cooking liquor immersion temperature 30?,immersion time 30 min;rapid heating-up rate 2.5?/min,rapid heating-up temperature 130?,small holding time 60 min;constant heating-up rate 1.2?/min?the highest temperature 160?,holding time 89 min.Under this condition,the pulp yield was 54.43%,which was 1.54%higher than that of conventional cooking,and the kappa number was 18.7,which was 0.5 higher than that of conventional cooking.The results of black liquid pre-impregnation-compression kraft cooking indicted that black liquid pre-impregnation had great influence on subsequent cooking.The optimum black liquor pretreatment process parameters were determined as follows:wood-to-liquor ratio 1:5,black liquor dosage 66%(relative to total liquor),NaOH dosage 2%,impregnation temperature 110?,impregnation time 30 min;extrusion compression ratio 3:1.After pre-impregnation with black liquor,the chips were softened and high sulfidity increased the delignification selectivity,and the extrusion made the wood chips looser,thus the subsequent cooking became more uniform.The subsequent cooking condition were that alkali charge 20%,sulfidity 35%,the highest temperature 1550C,holding time 120 min.In this condition,the pulp yield was 54.62%,which was 1.73%higher than the conventional kraft cooking;the kappa number was 18.1,which was similar to the conventional kraft cooking;the viscosity was 1222 mL/g,which was 154 mL/g higher than conventional cooking.Because the pulp has abundant microporous structure,it has a certain adsorption effect on VOCs including formaldehyde.This paper investigated the formaldehyde adsorption rates of nine pulps including unbleached and bleached hardwood kraft pulp(KP),unbleached and bleached softwood KP,unbleached bamboo KP,unbleached softwood ammonium sulfite pulp and so on.The formaldehyde adsorption amounts of 10 different pulps under the same conditions were measured and compared with two commercially available activated carbons for adsorption.The results indicate that the saturated adsorption time of formaldehyde of the 9 pulps at room temperature was less than 3 h,and it is usually between 2 h and 3 h.The formaldehyde adsorption rates of the unbleached softwood and hardwood KP were greater than that of the bleached softwood and hardwood KP.In general,the larger the saturated adsorption amount,the faster the adsorption rate.Under the experimental condition and with four-hour adsorption,the unbleached softwood ammonium sulfite pulp had the best adsorption property(213 mg/kg),then the unbleached birch KP and the unbleached eucalyptus KP was the next(179 to 193 mg/kg),while the formaldehyde adsorption amounts of the bleached softwood and hardwood KP were less(135 mg/kg and 126 mg/kg respectively).The unbleached wheat straw soda-AQ pulp and unbleached bamboo high yeild pulp of had the similar formaldehyde adsorption performance as the unbleached wood pulp.Cold alkali impregnation can improve the adsorption performance.The formaldehyde adsorption effect of some pulp can reach to or even over the adsorption effect of some activated carbon for adsorption.