Flow characteristics of aqueous salt solutions for applications in supercritical water oxidation

Abstract

Investigations were made on the flow characteristics of aqueous salt solutions in a tube lab scale apparatus with low flow velocities (0.8–65 cm/s), and a tube bench scale apparatus with high flow velocities (45–196 cm/s), at high temperatures (350–600 °C) and pressures (25 MPa). Cation and anion recovery rates were determined by measuring inlet and outlet ion concentrations of the heated tubes. In the lab scale apparatus, NaCl, KCl, and CaCl 2 solutions were used at concentrations ranging from 0.1 to 10 wt.%. For NaCl solutions, both cations and anions could be almost completely recovered in the effluent under conditions of 400 °C, up to 10 wt.% concentration without pressure fluctuations and plugging. For NaCl solutions of 1.0 wt.% at 450 and 500 °C, plugging occurred within 30 and 15 min, respectively. For 0.1 wt.% NaCl solutions at 500 and 600 °C, salt plugging did not occur, but pressure fluctuations were observed. For 1.0 wt.% NaCl solutions under conditions of 500 °C and flow velocities ranging from 18 to 65 cm/s, plugging occurred in all cases. On the other hand, when KCl and CaCl 2 solutions (10 wt.%) were supplied under conditions of 600 °C and small flow rates (4.5 g/min), these salts could almost be completely recovered in the effluent without supply pump pressure increases or fluctuations for 4 h operation. The differences observed in the plugging characteristics among salts are most likely due to the phase equilibria of the water–salt systems. In the bench scale apparatus, the plugging characteristics of NaCl, KCl, K 2CO 3 and KHCO 3 solutions were examined under conditions of 600 °C, 25 MPa, and concentrations of 0.1 and 1.0 wt.%. In these tests, salt solutions were heated to test conditions without mixing with supercritical water and supplied at high flow rate conditions. Tests with 1.0 wt.% NaCl solutions caused plugging during heating to 600 °C. For KCl, K 2CO 3 and KHCO 3 solutions, all salt solutions could be recovered in the effluent without the pressure increase and fluctuations. Mixtures of these salts could also be recovered. Salt deposition tests of NaCl, KCl, and CaCl 2 were carried out using a batch autoclave at 500 °C and it was found that NaCl tended to agglomerate while KCl and CaCl 2 tended to disperse and coat the vessel walls. From the tests made in this research, it can be concluded that flow characteristics of salt (NaCl, KCl, K 2CO 3, KHCO 3 and CaCl 2) at 400 °C did not present any severe operational problems. At temperatures of 450–600 °C and 25 MPa, however, all salts examined except NaCl could flow without pressure fluctuations or plugging. We conclude that bases such as KHCO 3, K 2CO 3, or KOH are suitable for use as neutralizing agents in SCWO processes, whereas the base NaOH is unsuitable for many conditions due to precipitation characteristics.