An investigation into parameters affecting crystal purity of urea in a stirred tank and an oscillatory baffled crystallizer

Confidential industrial trials showed that higher purity crystals were produced in an oscillatory baffled crystallizer (OBC) comparedto a stirred tank crystallizer (STC) when operating at similar conditions. This trend is verified in this work using urea as the testcompound that is different from that was employed in industrial tests. We have observed that a higher supersaturation level atnucleation coupled with a lower nucleation temperature was measured in the STC at all investigated conditions compared to thatin the OBC. This led to a higher nucleation rate in the STC, consequently producing smaller crystals. Crystal size distributions andimaging analyses suggest that these smaller crystals were more likely to form agglomerates in the STC, trapping either motherliquor or impurity or both and leading to the lower purity observed when compared to that in the OBC.