Continuous salt precipitation and separation from supercritical water. Part 3: Interesting effects in 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 Na3PO4/K2SO4 (both type 2 salts) may form a mixture of K3PO4/Na2SO4 (type 1/type 2) by permutation of the ions during precipitation from supercritical water, the separation performance of the K3PO4/Na2SO4 mixture containing the same concentration in sodium, potassium, sulfate, and phosphate as the Na3PO4/K2SO4 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 Na3PO4/Na2SO4 and Na2SO4/K2SO4 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 Na3PO4/Na2SO4 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 Na3PO4 or Na2SO4.Interestingly, a concentrated brine could be recovered for the Na2SO4/K2SO4 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.

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► 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.