On the concentration of RO-brines by seeded horizontal tube falling film evaporation (HTFE)

Abstract

Horizontal tube multiple effect has proved to be reliable, energy- and capital cost efficient in a number of medium sized seawater desalination plants. We ourselves studied extensively the stacked version, especially the brine distribution by sieve trays, optimal stack design with respect to low vapor side pressure losses and the efficient removal of non condensibles. It seemed of interest whether this design could be used for the concentration/crystallisation of brines from brackisch water RO plants, i.e. whether it would be operated in the supersaturated range with respect to components like calciumsulfate, magnesium hydroxide and silica. For large brackish water desalination plants far inland the concentration of the RO-brines has become essential since the waste of about 20% of the water cannot be afforded anymore and since the area required for the large solar evaporation ponds is often not available. Scaling of evaporator heat transfer surfaces can be controlled in this concentration range only by either a seeding or a fluidized bed technique. This paper discusses modes of operation of a horizontal tube evaporator with sieve tray brine distribution in case of seeding. Experimental results on minimum wetting rates will be presented.