Continuous salt precipitation and separation from supercritical water. Part 2. Type 2 salts and mixtures of two salts

Using a continuously operated laboratory plant for the catalytic hydrothermal gasification of biomass featuring a supercritical water salt separator we investigated the separation performance of three different binary type 2 salt–water mixtures and three ternary salt–water mixtures that consisted either of two type 1 salts or two type 2 salts dissolved in water. It turned out that a concentrated salt brine could not be recovered at the salt separator for the binary type 2 salt–water mixtures of Na2CO3, Na2SO4, and K2SO4. These salts precipitate as solids from supercritical water and thus lead to salt deposits inside the salt separator vessel. rnary mixtures of two type 1 salts dissolved in water (KH2PO4–K2HPO4–H2O and three different mixtures of NaNO3–K2CO3–water) exhibited a separation performance similar to the binary solutions of type 1 salts that were discussed in Part 1 of this article. However, the mixtures showed separation performances that were different from the corresponding single salt solutions. also possible to recover a concentrated brine when feeding solutions containing the two type 2 salts Na2CO3 and K2SO4. For these mixtures a certain amount of the type 1 salt K2CO3 might form in supercritical water leading to salt separation efficiencies up to 95% for these mixtures.