Development of strategies for continuous desalination of weak black liquor based on phase-behaviour analysis

This paper investigated different strategies to prevent salt deposition during weak black liquor (WBL) desalination under hydrothermal liquefaction conditions based on phase behaviour analysis. Phase equilibria of model salt solutions replicating the WBL composition were studied by high-pressure differential scanning calorimetry (HP-DSC) and different strategies to induce the Type 1 salt behaviour were applied. Two strategies led to an optimised salt separation: adjusting the concentrations of Type 1 salts, NaOH and NaHS, present in the WBL, and selectively exchanging carbonate from the model salt solution by replacing it with hydroxide. These strategies were tested in PSI’s continuous salt separation test rig, considering the recovering of salts. Although increasing the concentration of the Type 1 salts (NaOH and NaSH) impacted the precipitation temperature and extent of the salts, the strategy was not able to prevent it entirely. The HP-DSC results confirmed the efficiency of carbonate replacement by hydroxide since the Type 1 behaviour was observed after exchanging 75% of the carbonate. In the presence of organics, some salt accumulation and overflow into the desalinated stream occurred, which was addressed by optimising the brine flow rate.