Some aspects of adsorption in displacement washing of pulp
There are many aspects on how to improve the paper pulping process. In the washing sequence improvements to decrease the discharge to the environment can be achieved by closure of the liquid system and recycling liquid from other parts pulp process.
The objective of the pulp washing operation is to remove the liquor containing organic material such as dissolved lignin, and also inorganic material including the so called non-process elements. Cations, for example iron, manganese, magnesium etc. fall under the category non-process elements. These cations are very important to remove in the washing sequence since they cause problems during bleaching by decomposing the bleaching chemicals. The cations also cause filtration failure and incrust formation in the evaporator system. The washing should be performed with minimum amount of water to avoid diluting the liquor that is later evaporated before it is burnt in the recovery boiler.
To investigate various aspects of pulp washing a simulation model is helpful. Using a process simulator such as ASPEN Plus the different steps in a displacement drum washer can be described. In order to take account for the sorption of cations ions on the pulp fibres, blocks with an equilibrium model has to be included. The equilibrium model is encoded using FORTRAN. In the literature different models have been presented to take account for the sorption effects, the Langmuir isotherm, the Donnan model and the distribution coefficient model. In the presentation a comparison between these models is be made and how the simulation results are be influenced. Comparing the three adsorption models, The Donnan model was found to need the most data for the specific set of pulp and ions, second in that aspect is the Langmuir isotherm, and least amount of data are required for the distribution constant model. Results from the simulation show that the influence of the sorption model and thus also the choice of model is important only in washing stages where the concentration of ions is very low.
One way to decrease the water consumption and at the same time decrease the release of process water to the environment is to use condensate as washliquid. Condensates are produced in the cooking section and in the evaporator system. Depending on the origin of condensates they contain different amount of impurities, methanol, COD and sulfite substances for example. Sulfite substances have a very strong odour even at low concentration levels and can therefore cause problems both in the process and in the final product. Condensates do, however, have very low concentration of cations and are therefore suitable as washliquid.
To investigate the possibility of using condensate as washliquid washing experiments were performed on bleached and unbleached pulp. The two pulps were washed with four condensates with different concentration of COD. The washing results were consistently good but some unexpected results were obtained for the case where the condensate with the highest concentration of COD was used. The characteristic of the pulp was then investigated by an adsorption study including manganese, calcium and magnesium.
Keywords: pulp, displacement washing, adsorption, cations, distribution model, Donnan model, Langmuir isotherm.