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Analysis of primary drying of poly-γ-glutamic acid during vacuum freeze drying

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Abstract

This paper analyzes primary vacuum freeze drying of poly-γ-glutamic acid (γ-PGA), a biomaterial, by considering the variation in tortuosity under different operating conditions. The results indicate that lower heating-plate temperatures and thinner sample thicknesses increase tortuosity, but the influence of heating-plate temperature on tortuosity is relatively small compared to that of sample thickness. A tortuosity model reflecting mass-transfer characteristics of the pore structure in a dried γ-PGA sample was derived, and the drying process was numerically analyzed via a moving-grid system and the finite-difference method. Numerical simulation results using the developed drying model matched well with the experimental results, with an error of less than 10 %. Decreasing heating-plate temperature or increasing sample thickness increases the time required to complete primary drying. In particular, increases in sample thickness result in greater changes in drying time than those in heating-plate temperature. The sample thickness range that resulted in the fastest drying rate was approximately 14–19 mm depending on heating-plate temperature. Lower heating-plate temperature resulted in smaller optimum sample thickness. The maximum drying rate in this study was observed at a heating-plate temperature of −5 °C and a sample thickness of 19 mm.

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Abbreviations

C 2 :

Constant for structure [-]

C p :

Specific heat capacity [kJ/kgK]

C :

Concentration of water [kg water/kg solid]

D :

Diffusivity in a binary mixture [m2/s]

L :

Sample thickness [mm]

M :

Molecular weight [kg/kmol]

N :

Mass flux [kg/m2s]

P :

Pressure [Pa]

R :

Universal gas constant [kJ/kmolK]

T :

Temperature [K]

X :

Position of boundary layer [mm]

K :

Knudsen diffusion coefficient [m2/s]

k :

Thermal conductivity [kW/mK]

kD1, kD3 :

Bulk diffusivity constant [m2/s]

kD2, kD4 :

Self diffusivity constant [m3s/kg]

t :

Time [s]

ΔH v :

Enthalpy of sublimation [kJ/kg]

x :

Position

ρ :

Density [kg/m3]

ε :

Porosity [-]

T :

Tortuosity [-]

d :

Dried layer

f :

Frozen layer

t :

Total flux

w :

Water vapour

in :

Inert gas

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Acknowledgments

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2019R1F1A1062125).

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Authors and Affiliations

  1. Department of Mechanical Engineering, Graduate School, Kookmin University, Seoul, 02707, Korea

    Si-Ye Byun & Ji-Su Kang

  2. School of Mechanical Engineering, Kookmin University, Seoul, 02707, Korea

    Young Soo Chang

Authors
  1. Si-Ye Byun
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  2. Ji-Su Kang
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Corresponding author

Correspondence to Young Soo Chang.

Additional information

Si Ye Byun received Master’s degree from the Kookmin University in Korea, regarding optimizing thermal system. She is now working as a Vehicle DeVelopment Engineer at Renault Samsung Motors.

Ji Su Kang received Master’s degree from the Kookmin University in Korea, regarding optimizing thermal system. She is now working as a Water Resource Engineer at K-water, Korea Water Resources Corporation.

Young Soo Chang received Ph.D. from the Seoul University in Korea. He is a Professor in the School of Mechanical Engineering, Kookmin University, Korea. His research interests include heat transfer, two-phase flow and HVAC system.

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Byun, SY., Kang, JS. & Chang, Y.S. Analysis of primary drying of poly-γ-glutamic acid during vacuum freeze drying. J Mech Sci Technol 34, 4323–4332 (2020). https://doi.org/10.1007/s12206-020-0922-9

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