Design and optimization of the continuous separation process for lignin and alkali by ‘Extended Separation Volume’ method

This work is to design and optimize the 4column-simulated moving bed unit to separate and recovery alkali and lignin from black liquor. Since both alkali lignin and NaOH are the main products, we proposed the ‘extended separation volume’ methodology to construct a four-dimensional separation region (Q_I×Q_II×Q_III×Q_IV) to design the operating conditions. Special attention was paid to investigate the influence of flow-rates in each zone on the performance parameters of these two products. The results show besides Q_II and Q_III, Q_I and Q_IV can affect the concentrations and productivities significantly and have to be taken into account. Afterwards, the Response Surface Methodology, conducted by Design-Expert® was chosen to deal with the multi-objective optimization problem. The unique optimal operating conditions were finally obtained with the concentration of 74.7% and 48.1% of feed solution, and the productivity of 3.9 and 0.7 g/(L·h)⁻¹ for alkali lignin and NaOH. Considering the actual operation, the productivity of 2.5 and 0.446 g/(L·h)⁻¹ were obtained by experiment of internal actual operation point.