Continuous SAP Production via Double Kneader Reactor

Superabsorbent polymer (SAP) is a partially neutralized homo-polymer of glacial acrylic acid (with internal and surface cross-linking) that has found widespread commercial use as a liquid absorbent in baby diapers, adult incontinence products, feminine hygiene products, electric cable wrap protection, fire fighting, planting soils, and as a thickener in cosmetics and coatings. As global demand for SAP has increased to 2 million metric tons per year in 2011, production plants have gotten larger in capacity, bulk customers have demanded more consistent product composition, and buyers have naturally demanded product at lower cost. In response to these market inputs, the major SAP producers (Evonik, Nippon Shokubai, BASF) have developed process technology to convert the manufacturing operation from bulk production to continuous production.

As glacial acrylic acid is polymerized to produce SAP, it forms a sticky, wet gel with very high viscosity that is impossible to pump, As a result, historically, batch polymerization was conducted in shallow, horizontal trays on a flat conveyor belt that was processed through a modest temperature air furnace to complete the conversion of monomer to polymer, and conduct the separate unit operations for internal and surface cross-linking. While in process, the SAP gel was sliced into manageable chunks (1–3 cm) to enhance the drying operation. Once dry, the SAP chunks are pulverized to a fine powder product with an average size range of 200–800 microns. In this review we develop an alternate, continuous SAP production process that relies on BASF's patent estate for kneader-reactor technology. The technology employs a horizontal, kneader-reactor design by LIST AG (Switzerland) whose internals contain two or more interleaved and counter-rotating shafts along the horizontal axis of the reactor. The shafts are fitted with blades for slicing the SAP gel into small particles, and T or L shaped arms for kneading the polymer gel such that it flows with a nearly plug flow profile through the reactor operating between 70–100°C, and 500 millibar operating pressure. Estimated capital investment and production costs are summarized below.