Broadband dielectric spectroscopy has been used to analyze the temperature, frequency, and concentration dependences of the molecular dynamics of a nematic liquid crystal (5CB) mixed with the nonpolar solvent benzene. Differential scanning calorimetry measurement has been also performed to confirm the phase transitions of 5CB/benzene mixtures. The phase transition temperatures (crystalline to isotropic phases) thus obtained have been described very accurately from the temperature-dependent relaxation strength, the relaxation time, and the symmetric shape parameter of the relaxation function obtained from the fitting procedure. Two relaxation processes reflecting overall rotations around the short and long molecular axes are observed in both the nematic and isotropic phases. In the crystalline phase, the former process with the longer relaxation time disappeared, and latter process with shorter relaxation time shows a discontinuity at the freezing temperature. The relaxation process with shorter relaxation time obtained in the crystalline phase is larger than that obtained in the nematic phase because of the large restrictions in the crystalline phase. For the first time, we have precisely explained the molecular mechanism and structure of liquid crystalline materials as a function of concentration, temperature, and frequency.

Broadband dielectric spectroscopy of a nematic liquid crystal in benzene.

A new melt crystallization process for efficient purification of chlorobromobenzene

Differential scanning calorimetry and broadband dielectric spectroscopy measurements were performed to investigate the phase transitions in nematic liquid crystal (LC)/benzene mixtures. Noticeable changes of the crystal-isotropic phase transition and the phase transition enthalpy were observed. We also estimated the number of unfreezable benzene molecules from the LC/benzene mixtures. The crystal-isotropic phase transition temperature was described very accurately from the temperature-dependent relaxation strength, the relaxation time, and the symmetric shape parameter obtained from the fitting procedure. Surprisingly, the abnormal behavior of the isotropic phase was observed in LC/benzene mixtures that suggested the presence of local structure in the mixture, which set off the dipole moments along the long axis. The interaction between the LC molecules was also discussed from the tau-beta diagram.

Phase transition and abnormal behavior of a nematic liquid crystal in benzene.

A simulated moving-bed reactor (SMBR) for the production of p-xylene is studied as part of a proposal to modify an existing aromatics complex in order to increase the production of benzene and p-xylene. The proposed complex includes a single-stage crystallization unit to further purify high p-xylene streams from a selective toluene disproportionation and said SMBR units. Mass balances for two cases are estimated, namely, the current flow of reformate fed to the complex and a twofold flow. Results show significant increases of benzene and p-xylene for both cases compared to the current production of the complex.

Revamping an Existing Aromatics Complex with Simulated Moving-Bed Reactor for p-Xylene Production

Behaviours of ice crystal growth at two different operating parameters namely coolant temperature and circulation time were investigated for progressive freeze concentration (PFC) of glucose solution through a vertical finned crystallizer (VFC). Two determinant parameters which are ice production rate (mu), and water recovery (WR) were used to illustrate the behaviours of ice crystal growth in this study. From the result, higher ice production rate (mu) and water recovery (WR) were achieved at lower coolant temperature. On the other hand, longer circulation time resulted in lower ice production rate (mu), but at the same time increased the water recovery (WR). The maximum ice production rate (mu) and water recovery (WR) attained through this study were 1.522 gm-2s-1 and 51.131 %, respectively.

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Behaviour of Ice Crystal Growth in a Vertical Finned Cylindrical Freeze Concentrator

The kinetics of melt crystallization of binary eutectic forming organic mixtures in non-flow systems with cooling from below for the preferential crystallization of one component alone have been studied experimentally. The effect of subcooling, superheating and the initial concentration of the melt on the crystallization kinetics have been studied. Based on the experimental data, a correlation for the instantaneous height of the crystal (volume fraction of the solidified mass) has been proposed. The experimental results were also analyzed based on a model from the literature, which considered simultaneous heat and mass transport. The analysis shows that in the present experiments, the melt crystal interface was very close to the initial melt composition, i.e. concentration equilibrium was maintained. This is attributed to the very low cooling rates used, even though the thermal diffusivity is much greater than the mass diffusivity of the melt.

K.B Radhakrishnan

Papers

Kinetics of melt crystallization of organic eutectic forming binary mixtures in non-flow systems