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To improve the performance of yeast surface-displayed Rhizomucor miehei lipase (RML) in the production of human milk fat substitute (HMFS), we mutated amino acids in the lipase substrate-binding pocket based on protein hydrophobicity, to improve esterification activity. Five mutants: Asn87Ile, Asn87Ile/Asp91Val, His108Leu/Lys109Ile, Asp256Ile/His257Leu, and His108Leu/Lys109Ile/Asp256Ile/His257Leu were obtained and their hydrolytic and esterification activities were assayed. Using Discovery Studio 3.1 to build models and calculate the binding energy between lipase and substrates, compared to wild-type, the mutant Asp256Ile/His257Leu was found to have significantly lower energy when oleic acid (3.97 KJ/mol decrease) and tripalmitin (7.55 KJ/mol decrease) were substrates. This result was in accordance with the esterification activity of Asp256Ile/His257Leu (2.37-fold of wild-type). The four mutants were also evaluated for the production of HMFS in organic solvent and in a solvent-free system. Asp256Ile/His257Leu had an oleic acid incorporation of 28.27% for catalyzing tripalmitin and oleic acid, and 53.18% for the reaction of palm oil with oleic acid. The efficiency of Asp256Ile/His257Leu was 1.82-fold and 1.65-fold that of the wild-type enzyme for the two reactions. The oleic acid incorporation of Asp256Ile/His257Leu was similar to commercial Lipozyme RM IM for palm oil acidolysis with oleic acid. Yeast surface-displayed RML mutant Asp256Ile/His257Leu is a potential, economically feasible catalyst for the production of structured lipids. [ABSTRACT FROM AUTHOR]

The toroidal thermosyphon is widely used in solar water systems, nuclear reactors and geothermal energy systems, etc. The flow stability has been the main research topic but little attention was paid to the influence of boundary conditions. An experiment was performed on a copper torus. The Lorenz-like chaotic flow was not observed in clear contrast to that in the usual glass torus. Numerical simulation showed that the axial heat conduction in the tube wall deformed the heat flux distribution. A one-dimensional model generally formulated the boundary condition effects on the Hopf bifurcation. The model suggested that the metal wall could efficiently eliminate temperature perturbation, decrease the driving force and hence stabilize the global flow. The model was also used to analyze the effect of torus tilt angle on flow stability, whose conclusion agreed with previous experiment. [Copyright &y& Elsevier]

• The MSTEs consisting of tubes with an inner diameter of 1.6 mm are experimentally investigated as the pre-cooler of S-CO 2. • The effect of baffles on the heat transfer and pressure drop performance of the MSTEs is analyzed. • The accuracy of different correlations of the supercritical CO 2 heat transfer coefficient is verified. • The pressure drop of CO 2 in the MSTEs is tested and the pressure loss coefficient is fitted. The characteristics of heat transfer and pressure drop of supercritical CO 2 and water in microtube heat exchangers with and without baffles are studied experimentally. The tube bundle of the microtube heat exchanger consists of 37 stainless steel tubes with an inner diameter of 1.6 mm and an outer diameter of 2 mm. The length of the tube is 500 mm. The experiments are divided into hot water–cold water heat transfer experiments and supercritical CO 2 –water heat transfer experiments. The results of hot water–cold water experiments are shown to produce a large discrepancy with the conventional heat transfer correlation and new correlations of water convective heat transfer coefficient in the shell side are fitted according to the experimental results. The comprehensive performance of the heat transfer and pressure drop of the microtube heat exchanger with baffles is better than that without baffles. The results of CO 2 –water experiments show that the prediction of the design program based on Dang's heat transfer correlation of supercritical CO 2 is relatively in better agreement with the experimental results than others. The overall pressure drop of CO 2 in microtube heat exchangers is small. However, the pressure drop at the inlet and outlet of microtube heat exchangers is larger than that of conventional shell and tube heat exchangers. The pressure loss coefficient is modified according to the experimental results. [ABSTRACT FROM AUTHOR]

• Heat transfer behavior between the supercritical carbon dioxide and water in the microtube heat exchanger is numerically investigated. • The effects of pressure, mass fluxes and buoyancy on the heat transfer of CO 2 flowing at low reynolds numbers are obtained. • The flow patterns of water in the shell side influence the secondary flow intensity of CO 2. • Separate correlations should be used to get a better prediction of heat transfer due to different flow forms of CO 2 in the microtube heat exchanger. A numerical study on the heat transfer between supercritical carbon dioxide and water in the microtube heat exchanger at low mass fluxes of CO 2 is reported in this paper. The inner diameter of the tube used in the heat exchanger was 1.6 mm. Four sections with different heat transfer performance are found during the heat exchange process between CO 2 and water. The operating pressure, mass fluxes of CO 2 and water, buoyancy effect are considered to analyze the heat transfer characteristics of sCO 2. The heat transfer coefficient is found to be the maximum when the temperature is near pseudocritical point and the peak value increases when the pressure is getting closer to the critical point. The heat transfer coefficient increases with the increase of mass flux of CO 2. The buoyancy effect is found to have a significant influence on the heat transfer at the different locations of the tube. The secondary flow of CO 2 results in the enhancement of heat transfer on the top side of the tube and weakens the heat transfer on the bottom side. Different flow patterns of water in the shell side are compared in this study. The counter-current flow achieves the best heat transfer effect, while the downward flow is slightly better than the upward flow on the total heat transfer due to the different intensity of secondary flow. Existing different correlations should be used to get a better prediction of heat transfer depending on the state of CO 2 in the microtube heat exchanger, especially when CO 2 flows at a low mass flux. [ABSTRACT FROM AUTHOR]

To improve the performance of yeast surface-displayed Rhizomucor miehei lipase (RML) in the production of human milk fat substitute (HMFS), we mutated amino acids in the lipase substrate-binding pocket based on protein hydrophobicity, to improve esterification activity. Five mutants: Asn87Ile, Asn87Ile/Asp91Val, His108Leu/Lys109Ile, Asp256Ile/His257Leu, and His108Leu/Lys109Ile/Asp256Ile/His257Leu were obtained and their hydrolytic and esterification activities were assayed. Using Discovery Studio 3.1 to build models and calculate the binding energy between lipase and substrates, compared to wild-type, the mutant Asp256Ile/His257Leu was found to have significantly lower energy when oleic acid (3.97 KJ/mol decrease) and tripalmitin (7.55 KJ/mol decrease) were substrates. This result was in accordance with the esterification activity of Asp256Ile/His257Leu (2.37-fold of wild-type). The four mutants were also evaluated for the production of HMFS in organic solvent and in a solvent-free system. Asp256Ile/His257Leu had an oleic acid incorporation of 28.27% for catalyzing tripalmitin and oleic acid, and 53.18% for the reaction of palm oil with oleic acid. The efficiency of Asp256Ile/His257Leu was 1.82-fold and 1.65-fold that of the wild-type enzyme for the two reactions. The oleic acid incorporation of Asp256Ile/His257Leu was similar to commercial Lipozyme RM IM for palm oil acidolysis with oleic acid. Yeast surface-displayed RML mutant Asp256Ile/His257Leu is a potential, economically feasible catalyst for the production of structured lipids.