Continuous salt precipitation and separation from supercritical water. Part 3: Interesting effects in processing type 2 salt mixtures

[Display omitted] ► Continuous salt separation and precipitation from supercritical water. ►Separation behavior of salt mixtures from supercritical reaction mixtures depends on their phase behavior. ►Salt separation is crucial for hydrothermal biomass conversion processes. ►This study provides unexpected results from processing type 2 salt mixtures. Using a modified continuously operated laboratory plant which had been constructed in our group to visualize the salt separation from supercritical water using neutron radiography we investigated the continuous salt separation and recovery from supercritical water for four different ternary salt and water mixtures containing either a type 1 and a type 2 salt or a mixture of two type 2 salts. As the mixture of Na 3PO 4/K 2SO 4 (both type 2 salts) may form a mixture of K 3PO 4/Na 2SO 4 (type 1/type 2) by permutation of the ions during precipitation from supercritical water, the separation performance of the K 3PO 4/Na 2SO 4 mixture containing the same concentration in sodium, potassium, sulfate, and phosphate as the Na 3PO 4/K 2SO 4 mixture was investigated as well. Both mixtures showed the same trend in the separation performance exhibiting a maximum in the separation efficiency in the temperature range studied, and showing a preferential separation of sodium and phosphate or potassium and sulfate ions depending on the temperature of the separator vessel. The mixtures of Na 3PO 4/Na 2SO 4 and Na 2SO 4/K 2SO 4 were “real type 2 salt mixtures”, i.e. mixtures in which the permutation of the ions does not lead to the formation of a type 1 salt in the respective mixture. The salts from the Na 3PO 4/Na 2SO 4 mixture could not be recovered as a concentrated brine. This salt mixture behaved as would be expected for the respective single type 2 salt solutions of either Na 3PO 4 or Na 2SO 4. Interestingly, a concentrated brine could be recovered for the Na 2SO 4/K 2SO 4 mixture with the salt recovery in the brine effluent showing a maximum at a separator setpoint temperature of 450–460 °C. Beyond this temperature the salt mixture became “sticky”, thus exhibiting the behavior as would be expected for a type 2 salt mixture.