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