Thermodynamic and mechanistic studies on recovering phenol crystals from dilute aqueous solutions using pervaporation–crystallization coupling (PVCC) system

Abstract

High purity phenol crystal was recovered from dilute aqueous solution via a novel pervaporation–crystallization coupling (PVCC) system, which used a two-stage condenser to collect the permeate. The first stage was used as a crystallizer of phenol and the second stage was to collect remaining permeate that was not trapped in the first stage. Due to the low equilibrium pressure of phenol in the crystallizer operated at low temperatures (e.g., −9°C) and excellent membrane permselectivity for phenol, a large proportion of phenol vapor leaving from membrane surface was condensed as high purity phenol crystals in the first condenser (crystallizer). The crystallization mechanism of phenol under vacuum condition was discussed. Phenol crystals were obtained when the temperature of the crystallizer was maintained in the range of −7 to 19°C. Phenol crystals at a purity of 99.8% were recovered in the crystallizer cooled at −9°C under a permeate pressure of 280Pa, and the quantity of phenol collected as crystals accounted for 75.9–96.1% of the total amount of phenol permeated through the membrane for a broad feed concentration range of 220–10,100ppm at an operating temperature of 80°C. Phase equilibrium analysis revealed that water vapor would also be precipitated when the temperature was below a certain point at a given pressure. Thus, the crystallizer should be operated at an appropriate temperature in order to prevent water condensation in the phenol crystallizer.