We have developed a new methodology for characterizing the solvent cluster structures which occur in a pure supercritical fluid in its compressible regime. This methodology takes advantage of the time scale separation which exists between collective-cluster and individual-solvent–atom motions in order to classify atoms according to their ‘‘instantaneous’’ local environments. The resultant picture is of a fluid having density inhomogeneities on a mesoscopic length scale—i.e., clusters and cavities. Calculation of partial radial distribution functions shows that atoms residing in different density domains have very different equilibrium structural properties, information which is not available from the usual total radial distribution function. For example, for a 2-dimensional Lennard-Jones fluid at a reduced temperature Tr=1.06 the nearest neighbor coordination number in a high density domain is 4.2, whereas in a low density domain it is only 1.0. We have also found that, for such clustering fluids that in a finite volume system there is an ensemble independent [within terms of 𝒪(1/N)], nonstructural long-range correlation which arises from an excluded volume effect. This long range correlation enables us to determine the average domain size, volume fraction and density for both the high and low density domains.

The kinetics of transesterification of Pongamia oil using methanol at 60°C were studied. The forward as well as the reverse rate constants of all three steps involved in the transesterification of Pongamia oil are reported for the first time. Among the forward rate constants, the one governing the conversion of TG to DG was the highest and the one for DG to MG was the lowest. A distinct feature of the present work is the direct estimation of the equilibrium constants of all three steps by measuring the concentrations of TG, DG, and MG at very long reaction times. This reduced the number of parameters to be determined from the kinetic data by one-half, thereby leading to more accurate estimation of the rate constants. The equilibrium constant of the final step involving the conversion of MG to methyl ester and glycerol was at least an order of magnitude greater than that of the first two reaction steps. A detailed comparison was made with kinetic parameters reported in literature. The trend in the relative magnitudes of the rate constants appears to be unique to Pongamia oil.

The kinetics of the transesterification of vegetable oil is known to follow a three-step reaction mechanism. The third step involves the transesterification of MG. In this study, the transesterification of MG obtained from crude Pongamia oil was achieved with methanol in the presence of KOH as the catalyst. A MG/methanol ratio of 1:10 was used at different temperatures (30, 45, 55, and 60°C). 1 H NMR was used to monitor the progress of transesterification. The study revealed that the kinetics of this reaction followed a reversible second-order model, with a good fit obtained for all temperatures except 30°C. This result is explained as arising out of the importance of transport effects at low temperatures. The forward rate constant increased with an increase in temperature, whereas the reverse rate constant showed a decreasing trend, suggesting that the proposed reverse reaction was not an elementary step.

No abstract

No abstract