Determination of drying rate of citric acid for the design of a dryer

As part of a Final Career Project in Chemical Engineering to obtain citric acid by fermentation of concentrated residual whey followed by fluidized bed drying, this presentation addresses the characterization and drying behavior of citric acid crystals. A particle size analysis of the solid (ASTM mesh sieves 3.36, 2.38, 2.0, 1.68 and 1.41 mm; 250 mm) on a 21.603 g sample, subjected to mechanical vibration for 10 min. Fractions (F_i), are the mass retained over total mass. The average diameter was determined D_sp=(∑(F_i/D_pi))^-1 , where D_pi is the average diameter per mesh. The majority fractions corresponded to D_s (mm) of 2.87 (32%) and 0.83 (24%). D_sp=1.56 mm, with an average spherical surface area of 3.84x10^-6 m² and mass of 3.32x10^-3 g per sphere. In the drying process, 21.95 g of citric acid conditioned to 5% wet base were placed in a horizontal flow tray dryer in a monolayer at 68 °C and an air velocity of 0.8 m/s, and dried until constant weight. The drying rate was defined as N_a2=367.9-3.3627×W, where (g) mass of dry solid, (m²) area exposed to drying, W(g/g) moisture on a dry basis, θ(min) drying time. The temporal variation of water mass per g dry solid (∂W/∂θ) was measured experimentally; m_s was determined by drying in a convective oven at 105 °C to constant weight. Analysis of the drying curves showed two periods: constant drying rate (N_a1 8.8 g/m²-min) and falling rate (N_a2) modelled as N_a2 =367.9-3.3627×W within the range from a critical moisture content of 0.033 g water/g dry solid to an equilibrium of 0.0091 g water/g dry solid). The citric acid crystals were approximated as sphere particles; to estimate the surface area for mass transfer and drying rate, key variables that were used in the design of the drying equipment for the scale.