A modular sub-unit for the production of crystals in a suspension crystallization system includes: a first crystallization segment, at least one further crystallization segment, a first mixing segment, optional further mixing segments, an inlet cap, an outlet cap. The inlet cap and outlet cap are in fluid communication with any crystallization segments and any mixing segments present within the sub-unit. The sub-unit further includes a central rotating axis for providing mechanical energy to the crystallization segments and preferably the mixing segments. The crystallization segments present in the sub-unit are separated from each other by a mixing segment. Also described are a suspension crystallization system which includes the sub-unit and a suspension crystallization process making use of the sub-unit.