Increasing pulp yield in kraft cooking of softwoods by high initial effective alkali concentration (HIEAC) during impregnation leading to decreasing secondary peeling of cellulose

Abstract

Pulp yield can be improved by a more homogeneous delignification of the chips, achieved by improved impregnation prior to the cooking stage. Complete and efficient impregnation is obtained by increasing the diffusion rate by means of an impregnation liquor with a high initial effective alkali concentration (HIEAC). In the present study, the effect of HIEAC in the impregnation was evaluated and compared to a reference impregnation procedure and a prolonged impregnation. After the various impregnation scenarios, the alkali concentration was always adjusted to the same level in the beginning of the cooking stage. Impregnation with a HIEAC resulted in yield improvements by 1–1.5% units, due to a higher cellulose yield and possibly also to higher yield of glucomannan. The HIEAC with an even alkali distribution within the chips prior to the cooking stage resulted in a more uniform delignification carbohydrate degradation. Yield increase obtained by uniform delignification is due to both decreased shives content as well as less secondary peeling.

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

“…The value for the moisture content in wood, MC, is set to 50 %. A 1.5 %-unit yield increase is assumed for the HAI cases compared to the reference, based on earlier studies of HAI, where impregnations with high [OH − ] init (1.7 and 2.0 M) were compared with lower [OH − ] init (1.3 and 1.0 M) (Brännvall andBäckström 2016, Brännvall 2018). The yield increase is valid in the kappa number range of 35-50.…”

“…The yield increase is valid in the kappa number range of 35-50. 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).…”

“…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).…”

“…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). 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).…”