Determination of heat transfer coefficients in animal germplasm cryopreservation processes

N. ZARITZKY; M. V. SANTOS; M. SANSINENA; CHIRIFE, JORGE

Barcelona

Congreso; 10th World Congress of Chemical Engineering; 2017

Cryopreservation is the use of very low temperatures to preserve structurally intact living cells and tissues. The experimental measurement of the temperature versus time is fundamental for calculating cooling rates and for detecting whether the sample has been vitrified or if it has undergone a freezing process in which ice crystals are formed (Baust et al, 2009). One of the most common devices used for cryopreservation of animal germplasm is the polypropylene straw. In the case of cryopreservation of bovine semen the straws are suspended in nitrogen vapor (NV) for dehydrating the cells, prior to being immersed in liquid nitrogen (NL). Little information is available in the literature, about the heat transfer coefficients (h) in these processes. The computational simulation is a valuable tool to obtain h values from the experimental thermal histories by feeding appropriate thermal properties.The objectives of the present work were to determine h coefficients during the cooling of plastic straws (vol = 0.5mL) containing animal germplasm, both in NV and NL from experimentally measured temperature profiles and numerical simulations. When a device is immersed in NL, a film boiling regime is produced due to the large temperature differences between the object and the cryogenic fluid (delta T); nitrogen evaporates in the vicinity of the device, generating an insulating vapor film that delays heat transfer (Leidenfrost effect). When delta T decreases, the regime changs from film to nucleated boiling. In the case of NV, which is in contact with NL at the bottom, stratification of the vapor occurs due to the effect of the temperature on NV density. To predict h values, heat transfer partial differential equations in non-stationary state, applying convective border conditions were numerically solved. A finite element program (Comsol Multiphysics, 2005) was used, with thermophysical properties of the biological fluid (density, thermal conductivity,apparent specific heat) depending on temperature and including phase change of water; this constitutes a complex non linear mathematical problem. Temperature was measured using thermocouples T ( diam