Distribution of sodium chloride and sodium hydroxide between steam and water at dry-out in an experimental once-through boiler

Abstract

A steady state model is developed for the partition of a solute between an evaporating annular film and steam assuming a finite coefficient of mass transfer for solute in steam. It is concluded that a thin film of concentrated solution is formed at dry-out which penetrates along the tube wall beyond the point at which tube wall temperature starts to rise. The liquid film beyond dry-out is at the equilibrium concentration determined by the temperature of the tube wall, and its maximum penetration downstream is determined by the feed water concentration and steam flow parameters. The model is tested by determining the hold-up of NaCl and NaOH in a small horizontal once-through tube from a record of the Na concentration in the effluent steam. The tube was operated at pressures between 6·9 and 17·8 MN/m 2 with NaCl feed concentrations between 0·3 and 28 ppm Na and NaOH concentrations between 1 and 10 ppm Na. Under all conditions, solute is held up in the tube as dry-out is established and recovered quantitatively when wet-wall conditions are restored. The variations in the effluent concentration recorded for different feed concentrations are interpreted in terms of an equilibrium liquid film penetrating to the point of salt deposition or to the end of the tube. Reasonable agreement is found between measured and predicted solute concentrations in the effluent steam.