Heat transfer and operating conditions for freeze concentration in a liquid–solid fluidized bed heat exchanger

Abstract

Freeze concentration involves concentrating liquids partially by freezing water out of the solution thus leaving behind a concentrate rich in its solutes. It is an expensive process that could benefit from alternative approaches that are more cost-effective. In this paper, freeze concentration is studied inside a fluidized bed heat exchanger (FBHE), which is a cheaper alternative to the conventionally used scraped surface heat exchanger (SSHE) in a freeze concentration process. A pilot scale vertical fluidized bed heat exchanger was designed and constructed in the University of Auckland to test the feasibility of this process using stainless steel (SS) particles for ice removal. Experiments were conducted with NaCl solutions to determine the effect of NaCl concentration and bed porosity on the stability of operation. Operating stability improved with increasing NaCl concentration and decreasing bed porosity proving the feasibility of this equipment to freeze concentrate liquid solutions at certain conditions in the range of parameters tested. A long-term freeze concentration experiment was carried out in which freeze concentration in the solution was confirmed via freezing point depression (FPD). FBHE can be used to freeze concentrate NaCl liquids more cost-effectively than SSHE at stable conditions within the range of tested parameters.