BOILING RATE, HEAT TRANSFER AND VISCOSITY OF TECHNICAL SUGARCANE LIQUORS

摘要

Incidents of extremely slow sugar boiling were experienced in 2002 by Louisiana mills following periods of prolonged wet weather. Based on the testing at Audubon that followed these observations, they were ascribed to severely reduced heat transfer rates in the vacuum pans. Surprisingly, though, this was found to be unrelated to viscosity effects or polysaccharide levels but rather due, to all indications, to deposition on the heat transfer surface during sugar boiling of a water soluble layer of as yet unknown chemical nature. These presumed deposits were found to be pH-sensitive, and small additions of hydrochloric acid were effective in restoring normal boiling. In the continued testing in 2003, boiling heat transfer coefficients of cane molasses were determined in a pilot vacuum pan under identical conditions to those in 2002, viz. 15.2 kPa.a (4.5"Hg) absolute pressure, 69 kPa.a (10 psi.a) steam pressure, and 80% to 90% dry solids content. With some deviations, a linear relationship of the form HTC = A x (%RDS) - B between the boiling heat transfer coefficient and dry solids content was found satisfactory, unlike in 2002 when inverted S-shaped curves were required to adequately describe the rapid drop of HTC in the 79-81% RDS range for the refractory materials. The consistency of the same molasses as used in the HTC measurements was measured over the 80-90% dry solid content and an equation for the overall heat transfer coefficient as a function of the molasses viscosity (cP) at standard conditions (50℃, 2/sec) HTC (W/m2℃) = 75176 (μ)~(-0.5295) was developed. Surprisingly, molasses viscosity at standard conditions did not show any correlation to its dextran or starch content. It is suspected that this observation is related to an incomplete hydrolysis of dextran by dextranase under industrial conditions and the nature of the monoclonal antibody assay. Of the three main cations, no correlation was found between viscosity of molasses and its K and Mg levels, and only a limited one with its Ca level.