PEP Review 2011-12 CONTINUOUS SAP PRODUCTION VIA DOUBLE KNEADER REACTOR By Anthony Pavone (November 2011) ABSTRACT 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. Capacity (mt/y) 40 80 160 Investment ($ million) Total Fixed Capital 112.8 174.6 270.4 Production Cost ($/mt) At 100% Capacity 3,388.0 3,245.9 3,145.1 SRI Consulting, IHS Inc. PEP Review 2011-12
A private report by the Process Economics Program Review No. 2011-12 CONTINUOUS SAP PRODUCTION VIA DOUBLE KNEADER REACTOR by Anthony Pavone November 2011 Menlo Park, California 94025
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CONTENTS INTRODUCTION... 1 VALUE PROPOSITION... 2 SCOPE OF REPORT... 2 SAP INDUSTRY STATUS... 3 SAP CHEMISTRY AND PROCESS TECHNOLOGY... 5 SAP Product Chemistry... 5 SAP Polymerization Chemistry... 6 Choice of Solvent... 9 Conventional, Commercial SAP Manufacturing Sequence... 10 Reaction Initiators... 12 Reaction Start-Up... 13 Reaction Kinetics... 13 Cross-Linking Agents... 14 Chain Transfer Agents... 15 Chelating Agents... 15 Dispersing Agents... 15 Residual Monomer Conversion... 16 Reactor Cooling... 17 Neutralization... 17 Reactor Product Washing... 17 Extractable Polymer Content... 17 Polymer Gel Size Reduction... 18 Polymer Drying... 18 Solvent Recovery... 18 Polymer Milling... 18 DESIGN BASIS FOR BASF PROCESS TECHNOLOGY... 19 SAP Product Specifications... 21 BASF KNEADER-REACTOR PROCESS TECHNOLOGY... 22 SRI Consulting, IHS Inc. iii PEP Review 2011-12
CONTENTS (Concluded) Rationale for Continuous SAP Production... 22 First Commercial Migration to Continuous SAP Processing... 23 BASF Migration to Continuous SAP Processing... 23 BASF Continuous Horizontal Kneader-Reactor SAP Technology... 25 Chemicals Mixed and Reacted... 25 Conditions under Which Reaction Occurs... 26 Means for Managing Exothermic Heat of Reaction... 26 SAP Reactor Apparatus... 27 Composition of SAP Reactor Product... 29 Post Reactor Processing... 30 BASF KNEADER-REACTOR PROCESS DESIGN... 30 Block Flow Diagram... 31 Process Flow Diagram and Process Description... 32 Material Balance... 33 Equipment List... 37 PRODUCTION ECONOMICS... 38 Itemized Capital Cost Estimate... 38 Total Fixed Capital Cost... 40 Variable Production Costs... 42 Fixed Production Costs... 43 Measures of Profitability... 45 REFERENCES... 46 SRI Consulting, IHS Inc. iv PEP Review 2011-12
FIGURES 1 SAP Producer Global Market Share... 4 2 Posted U.S. Historic SAP Prices... 4 3 Chemical Structure of Polyacrylic Acid... 5 4 Structure of Partially Neutralized Polyacrylic Acid with Cross-Linking Agents... 5 5 Collapsed (Dry) and Expanded (Gel) State of Water Soluble Polymer Chains... 6 6 Free Radical Polymerization of Acrylic Acid Monomer... 7 7 Impact of ph on SAP Absorption Rate... 8 8 The Commercial Manufacture of Superabsorbent Polymer Block Flow Diagram... 10 9 Polyacrylic Acid Relative Reaction Rate as a Function of ph... 14 10 Use of N 2 Sealed Endless Screw Conveyors... 20 11 BASF Continuous Reactor SAP Process Block Flow Diagram... 24 12 BASF Horizontal Continuous SAP Reactor (USP 4,769,427)... 28 13 LIST AG Kneader-Reactor Internals (USP 6,039,469)... 28 14 LIST AG Kneader-Reactor Shaft Attachments (USP 6,039,469)... 29 15 LIST AG Kneader-Reactor Vessel (USP 6,039,469)... 29 16 Kneader Reactor Block Flow Diagram... 31 17 Continuous SAP Production via Double Kneader Reactor (1 of 2) Process Flow Diagram... 47 18 Continuous SAP Production via Double Kneader Reactor (2 of 2) Process Flow Diagram... 49 SRI Consulting, IHS Inc. v PEP Review 2011-12
TABLES 1 Major SAP Producers... 3 2 Taylor Screen Size versus Particle Size... 19 3 Representative SAP Product Specifications... 21 4 BASF Patents for SAP Continuous Horizontal Kneader Reactor... 25 5 Producing Superabsorbent Polymer by Continuous SAP Kneader-Reactor Polymerization... 30 6 Stream by Stream Material Balance... 34 7 Equipment List with Duty Specifications... 37 8 Itemized Capital Cost Estimate... 39 9 Total Fixed Capital Cost Estimate... 41 10 Unit Pricing Used in Economic Analysis... 42 11 Continuous Process SAP Unit Variable Costs... 43 12 Assumptions for Fixed Cost Calculations... 43 13 SAP Production Cost Estimate... 44 14 SAP Profitability Estimate... 45 SRI Consulting, IHS Inc. vi PEP Review 2011-12