2.11.21 | Panu Tikka

Transport phenomena in kraft (sulphate) cooking process

Wood substance is a very complicated porous structure consisting of lignin bound wood fibers. The structure is porous in many different ways and different dimensions. Biggest natural cavities are in parenchyme cells and tracheid fibers lumen (macro pores). Fiber walls contain much smaller porosity between fibrilles and other cell wall structure elements (micro pores). In cooking about 50 % of wood matter dissolves most rapidly in the early part of cooking greatly increasing porosity.  

Cooking process starts by steaming with the purpose to remove air from the cooking reactor (easy displacement) and from the wood chip solid material (slow difficult gas diffusion). Simultaneously chips are pre-heated. Impregnation of alkaline cooking liquid (white liquor often mixed with some spent black liquor) is a mass transfer process by two mechanisms: penetration of liquor to steam&gas filled cavities and diffusion in liquid filled wood structure. These mass transfer phenomena vary a lot depending on wood species, direction of transfer in wood, pressure, temperature and liquor properties. 

We were initializing and leading a 15 years period of research on these issues and their interdependence s with pulp industry, machinery manufacturers and Helsinki University of Technology. Quite new on-line weight measuring reactors and uni-directional mass transfer testing equipments were constructed and used along own design cooking reactors to mimic industrial process conditions and their gradients.

In very short brief:

  • Good thorough steaming (= air removal)( 100 oC; 30 min) is an absolute prerequisite for efficient impregnation liquor transfer. 
  • Liquor penetration follows successfully after complete steaming and applying some over pressure. 
  • Diffusion is always slow and dangerous for cooking uniformity: The diffusion front of alkali starts to react immediately and looses active alkali. Therefore temperature in impregnation should be rather low to prevent alkali depletion. 
  • The desired delignification and other temperature dependent cooking reactions should start after complete alkaline impregnation. If this is fulfilled then the cooking process can be selected and designed quite freely – the cooking uniformity is already guaranteed in the impregnation front-end of the process 

The research results have been published and available in the below selected 12 publications:

  • Malkov, S., Kumzin, V.A., Baltakhinov, V.P. and Tikka, P., Efficiency of chip presteaming – result of heating and air escape processes. Nordic Pulp and Paper Research Journal, Vol 17/No.4/2002. 
  • Malkov, S., Tikka, P., and Gullichsen J., Towards complete impregnation of wood chips with aqueous solutions. Part 1: A retrospective and critical evaluation of the penetration process. Paperi ja Puu-Paper and Timber, Vol.85/No.8/2003. 
  • Malkov, S., Tikka, P., Gullichsen, J., Towards complete impregnation of wood chips with aqueous solutions. Part 2: Studies on water penetration into wood chips. Paperi ja Puu, 83(2001):6, p 468-473.  
  • Malkov, S., Tikka, P., Gullichsen, J., Towards complete impregnation of wood chips with aqueous solutions. Part 3. Black liquor penetration into pine chips.  Paperi ja Puu, 83(2001):8, p 605-609. 
  • Malkov, S., Tikka, P., Towards complete impregnation of wood chips – new research enables effective processes. TAPPI 2002 Fall Technical Conference, Sept. 9-11, San Diego, CA, USA. 
  • Malkov, S., Kumzin, V.A., Baltakhinov, V.P. and Tikka, P., Modelling the process of liquid penetration into softwood chips. Journal of Pulp and Paper Science, Vol.29/No.4/2003. 
  • Määttänen, M. &Tikka, P., Determination of phenomena involved in impregnation of softwood chips. Part 1: Method for calculating the true penetration degree. Nordic Pulp and Paper Research Journal 27(2012):3, p. 550-558. 
  • Määttänen, M., & Tikka, P., Penetration of water and black liquor into overthick and treated wood chips. Paperi ja Puu – Paper and Timber 90(2008):2, p. 34-39. 
  • Määttänen, M. &Tikka, P., Determination of phenomena involved in impregnation of softwood chips. Part 2: Alkali uptake, alkali consumption and impregnation yield. Nordic Pulp and Paper Research Journal 27(2012):3, p. 559-567. 
  • Määttänen, M., & Tikka, P., Determination of the contribution of penetration, diffusion and alkali consumption in the impregnation of Eucalyptus grandis chips. Appita Journal 66(2013):4, p. 313-321. 
  • Pokki, J.-P., Laakso, V., Tikka, P., Aittamaa, J., Specific Permeability of Wood to Water Part 1: Longitudinal Specific Permeability of Steamed, Impregnated, and Kraft-Cooked Wood Industrial & Engineering Chemistry Research 49(2010):5. p. 2144-2154. 
  • Pokki, J.-P.,  Laakso, V., Tikka, P., Aittamaa, J., Specific Permeability of Wood to Water Part 2: Perpendicular Specific Permeability of Steamed, Impregnated, and Kraft-Cooked Wood. Industrial & Engineering Chemistry Research 49(2010):5, p. 2155-2160.