Published December 2012
Acrylic acid monomer has historically been used primarily to make a combination of acrylate esters for the coatings industry, and superabsorbent polymer for the absorbents industry (baby diapers, adult incontinence products, wire and cable waterproofing, formulation thickener, firefighting). Of the 6.3 MM metric tons of acrylic acid produced globally in 2012, approximately two-thirds is ester-grade material, while one-third is glacial (polymer)-grade material. As the absorbents industry has evolved, the industry has required higher purity of product. Current industry specifications for polymer-grade acrylic acid purity are 99.6% by weight. Due to the high reactivity of acrylic acid monomer at elevated temperature, fractional distillation requires very high dosage rates of inhibitor (usually MEHQ) to prevent auto polymerization, and still encounters operating problems associated with polymer accumulations. Auto-polymerization explosion hazards of acrylic acid are also a major process safety concern in plant design and operation, as experienced by Nippon Shokubai in Japan in 2012. As a result, the industry seems to have settled on melt-phase falling film crystallization as the preferred method of acrylic acid purification to glacial-grade specifications.
Sulzer ChemTech (Winterthur, Switzerland) has developed the dominant crystallization technology for producing glacial-grade acrylic acid monomer. In this study, we present the technology that we believe Sulzer uses, and we estimate both the process design parameters and the corresponding project economics for converting 35 thousand metric tons per year of ester-grade acrylic acid (roughly 90% purity) to glacial acrylic acid using a combination of dynamic falling film crystallization and static suspension crystallization at subambient temperatures using melt-phase techniques.