Kinetics of Water-Alkaline Hydrolysis of Birch Bark in a Microwave Field

Abstract

Birch bark consists of silver bark and bast. Silver bark contains up to 50 % of extractives and serves as a raw material for obtaining valuable biologically active substances (BAS), including betulin. A distinctive feature of the birch bark structure is a cross-linked polymer, suberin. Its monomers are suberic acids. They have found use in the production of lubricants, oils, insecticides, fungicides, polymers, polyesters, coatings, etc. A common method for extracting suberic acid salts from silver bark is exhaustive hydrolysis with an aqueous or water-alcohol solution of alkali (NaOH or KOH). We have proposed the activation of raw materials during hydrolysis by using an ultrahigh-frequency electromagnetic field (microwave hydrolysis). Isolation of suberin from silver bark is both a chemical and mass transfer process. Salts that are formed during hydrolysis of suberic acids diffuse to the surface of silver bark particles and pass into the hydrolysate. The limiting stage of mass transfer during birch bark hydrolysis is internal diffusion in the pores (mass conductivity). The anisotropy of the silver bark structure complicates the mathematical description of the mass transfer kinetics in the diffusion process. The process of internal diffusion during microwave hydrolysis is characterized by a steady regime starting from the 4th minute. The kinetics of this process and the effectiveness of diffusion in the tangential and longitudinal directions were determined. As the size of birch bark particles increases in both length (tangential direction) and width (longitudinal direction), the rate of the hydrolysis process and the degree of suberin isolation increase. Internal diffusion coefficients during microwave bark hydrolysis were determined. The highest value was obtained for fractions with a particle size of 3–4.5 mm (silver bark); the lowest – for fractions less than 1 mm (bast). The article shows that the small fraction (bast) should be separated and silver bark should be cut along the fiber in order to increase the yield of suberin after birch bark grinding.