Improved impregnation efficiency and pulp yield of softwood kraft pulp by high effective alkali charge in the impregnation stage

Abstract

Abstract:A pulp yield increase up to 2% can be achieved by impregnation with a liquor containing 2 M effective alkali (EA) concentration instead of 1 M. The yield increase is due to higher cellulose and glucomannan contents in the pulp, which can be rationalized by less yield loss by peeling, as impregnation is more effective at an elevated EA level. A rapid loading of chips with alkali can be realized due to a high diffusion rate. When the temperature becomes higher in the cooking stage, enough alkali is available for delignification reactions without the risk of alkali depletion in the chip core, so that the delignification is more homogeneous.

New drum-chipping technology for a more uniform size distribution of wood chips

“…Although pulping processes differ in sensitivity to uneven distribution of chemicals during impregnation and uneven temperatures (Rydholm 1965), all fibers in the wood need to get their share of the chemicals and the heat during the cooking process. Therefore, many recent studies have addressed the impregnation of wood chips in kraft pulping, the most common chemical pulping method (Malkov et al 2002;Malkov et al 2003;Wedin et al 2010;Brännvall and Bäckström 2016;Tavast and Brännvall 2017;Brännvall and Reimann 2018). Poor wood chip quality can result in more shives in the pulp, or even in digester shutdown.…”

Consequences in a softwood kraft pulp mill of initial high alkali concentration in the impregnation stage

“…The alkali consumptions during HAI impregnation are based on previous studies (Brännvall andBäckström 2016, Brännvall 2018) as is the alkali consumption in the cooking stage at a kappa number of approx. 40 (Brännvall and Bäckström 2016). A higher alkali consumption during the impregnation stage is assumed to lead to less alkali consumed in the cooking stage, as previously shown (Andrews et al 1983, Tolonen et al 2010, Tavast and Brännvall 2017, Brännvall and Bäckström 2016.…”

“…The process was reported to reduce kappa number variations and gives higher screened pulp yield. The initial effective alkali concentration however, has been reported to be only 0.8 M (Wizani et al 1997), which is not as high as 1.7-2.0 as proposed for efficient impregnation (Brännvall and Bäckström 2016, Brännvall and Reimann 2018, Brännvall 2018. The alkali concentration of the white liquor and the moisture content of the chips limit the maximum value of the initial alkali concentration achievable in industrial conditions.…”

“…Diffusion rate can be increased by increasing the alkali concentration in the impregnation liquor. This has been shown to lead to a faster and more thorough impregnation (Gullichsen et al 1995, Määttänen and Tikka 2012, Brännvall and Bäckström 2016, Brännvall and Reimann 2018 and resulting in a higher screened pulp yield (Gullichsen et al 1995, Brännvall and Bäckström 2016, Brännvall 2018). An even impregnation by high alkali concentration has been shown to improve screened pulp yield by decreasing the extent of alkaline hydrolysis and thereby the extent of secondary peeling (Brännvall 2018), which is the main reason for yield loss of cellulose (da Silva and van Heiningen 2015, Paananen and Sixta 2015).…”

“…An even impregnation by high alkali concentration has been shown to improve screened pulp yield by decreasing the extent of alkaline hydrolysis and thereby the extent of secondary peeling (Brännvall 2018), which is the main reason for yield loss of cellulose (da Silva and van Heiningen 2015, Paananen and Sixta 2015). Performing impregnation at high alkali concentration can also enhance yield by improved retention of cellulose and glucomannan (Brännvall and Bäckström 2016) as the stopping reaction is enhanced in comparison to the peeling reaction at elevated alkali concen-trations (c. f. Lai andOntto 1979, Paananen et al 2010). In the EnerBatch® process, white liquor (WL) together with spent impregnation liquor is used (Wizani et al 1992).…”