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Journal ArticleDOI

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

01 Jan 2001-Chemical Engineering and Processing (Elsevier)-Vol. 40, Iss: 1, pp 71-81

AbstractThe 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.

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Citations
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Journal ArticleDOI
TL;DR: For the first time, the molecular mechanism and structure of liquid crystalline materials as a function of concentration, temperature, and frequency is explained.
Abstract: 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.

16 citations

Journal ArticleDOI
TL;DR: 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.
Abstract: 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.

6 citations

Journal ArticleDOI
Abstract: O-chlorobromobenzene and some other impurities are easily introduced during the synthesis of p-chlorobromobenzene, which results in the higher impurity content and lower product purity. What’s more, p-chlorobromobenzene and o-chlorobromobenzene are isomers of each other, which brings the high similarity of their physical and chemical properties, thus the conventional separation methods do not work efficiently. A melt crystallization was carried out in this work to explore the separation and purification process of chlorobromobenzene. The influences of the cooling rate, final crystallization temperature, and crystal growth time were examined systemically based on the following criteria: the distribution coefficient, crystallization yield and purity. At last, the optimal process conditions for the separation of p- and o-chlorobromobenzene were reached at the cooling rate of 0.2 °C/min, the final crystallization temperature of −2 °C, and the crystal growth time of 5 h. The final yield was 62.74%, the purity was 99.15%, and the distribution coefficient was 0.0166, which indicated that the optimal process conditions can effectively separate the mixture of o-chlorobromobenzene and p-chlorobromobenzene. The above results indicated that the melting crystallization process was an efficient separation and purification method for chlorobromobenzene.

2 citations

Journal ArticleDOI
Abstract: 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.

2 citations

Journal ArticleDOI
Abstract: 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.

2 citations


Cites background from "Kinetics of melt crystallization of..."

  • ...According to Radhakrishnan and Balakrishnan [3], the basic characteristic of PFC process is normally described by the rate of crystal growth, in which this rate is mainly determined by heat and mass transfer at solid-liquid interface....

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References
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Journal ArticleDOI
Chie Gau1, R. Viskanta1
Abstract: Etude experimentale de l'ecoulement de convection naturelle dans la phase liquide et de son influence sur le mouvement de l'interface liquide-solide et sur le transfert de chaleur au cours de la fusion et de la solidification de gallium a haute purete sur une paroi verticale

438 citations


"Kinetics of melt crystallization of..." refers background or methods in this paper

  • ...The correlation has also been compared with the data of Hale and Viskanta [4], who studied the freezing and melting of n-octodecane in a rectangular enclosure (again with Ste*=0 and C0%=0) with slightly better agreement than with the data of Gau and Viskanta [7]....

    [...]

  • ...Gau and Viskanta’s [8] experimental data on the solidification of pure gallium on a vertical wall with initial superheat is also shown in Fig....

    [...]

  • ...In using the data of Gau and Viskanta [8] with the present...

    [...]

  • ...In using the data of Gau and Viskanta [8] with the present correlation, C0% is set equal to 0 (pure substance)....

    [...]

  • ...Gau and Viskanta’s [7] experimental data on the solidification of Lipowitz metal (an alloy of Bi, Pb, Sn and Cd) in a rectangular enclosure with no initial superheat and at its eutectic composition (Ste*=0 and Co%=0) is shown in Fig....

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Journal ArticleDOI
Abstract: We Present a mathematical model for the region of dendritic or cellular growth which often forms during the solidification of alloys. The model treats the region of mixed phase (solid and liquid) as a continuum whose properties vary with the local volume fraction of solid. It is assumed that transports of heat and of solute are by diffusion alone, and the model is closed by a condition of marginal equilibrium. Results are obtained for the unidirectional solidification of an alloy from a plane wall. The spatial variations of solid fraction are highly suggestive of the types of morphology that can occur, and a wealth of different structures are found as the physical parameters are varied. Although the model ignores gravity entirely, the results can be applied to the solidification from below of an alloy which is initially less dense than its eutectic. Predictions for the growth rate of the mixed-phase region agree well with existing experimental measurements of ice growing from aqueous salt solutions.

272 citations


"Kinetics of melt crystallization of..." refers background in this paper

  • ...Huppert and Worster [9] stated that the crystal layer height varied with the square root of the growth time....

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  • ...The parameter g is given by Huppert and Worster [9] as cC0 F(g) 1−F(g) n rmCpm F(emg) + rsCps G(esg) n = rsCps(Tl(C0)−Tw) G(esg) − rmCpm(T0−Tl(C0)) F(emg) (24) where, F(x)=P1/2xex2erfc(x) (25) and G(x)=P1/2xex2erfc(x) (26) The interface concentration is obtained from the following equation F(g)= Ci−C0 C0 (27) and subsequently the interface temperature is obtained from the phase diagram....

    [...]

  • ...Worster [5] defined a criterion to avoid supercooling and subsequent linear instabilities at the interface (i.e. non-flat and non-planar interface) ks\ km 1− [1− (l/jCps)(D/as)(1/C0)] (2) where, j is the slope of the liquidus line and C0 is the initial composition of the melt....

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  • ...Furthermore, Worster [5] has pointed out that the bulk temperature and the solute fields are statistically stable to convective motion if the system is cooled at the lower horizontal boundary and if the less dense component is crystallized....

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  • ...In order to use the above solutions due to Huppert and Worster [9], the parameter g was determined from Eq....

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Journal ArticleDOI
01 Apr 1985-Nature
Abstract: The fundamental physics and fluid dynamics of a solidifying two-component system cooled from below is analysed quantitatively Theoretical results for the rate of growth of the solid are in good agreement with experimental measurements on crystallizing aqueous solutions The results are applicable to the solidification of a wide variety of binary alloys

183 citations


"Kinetics of melt crystallization of..." refers background or methods in this paper

  • ...Huppert and Worster [9] stated that the crystal layer height varied with the square root of the growth time....

    [...]

  • ...The interface concentration is obtained from an equation due to Huppert and Worster [9] for (D/a) 1 and is given by 1+ cC0 (Tl(C0)− Tw) n−1 = Ci−C0 C0 (10)...

    [...]

  • ...The parameter g is given by Huppert and Worster [9] as cC0 F(g)...

    [...]

  • ...In order to use the above solutions due to Huppert and Worster [9], the parameter g was determined from Eq....

    [...]

  • ...Huppert and Worster [9] solved these equations and the results are...

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Journal ArticleDOI
Chie Gau1, R. Viskanta1
Abstract: The role of natural convection on solid-liquid interface motion during melting and solidification of Lipowitz metal in a rectangular cavity was studied. The measurements of both temperature distributions and temperature fluctuations were used as a qualitative indication of the natural convection flow regimes and structures of melt during phase transformation. The measured and predicted solid-liquid interface positions during solidification from below and above, as well as melting from above and below, show reasonably good correspondence. The suppression of natural convection in the Lipowitz metal, which is not taken into account in the model, leads to a slower rate of melting and a higher rate of freezing than that predicted.

109 citations


"Kinetics of melt crystallization of..." refers background or result in this paper

  • ...The correlation has also been compared with the data of Hale and Viskanta [4], who studied the freezing and melting of n-octodecane in a rectangular enclosure (again with Ste*=0 and C0%=0) with slightly better agreement than with the data of Gau and Viskanta [7]....

    [...]

  • ...4 between the present data and the data of Hale and Viskanta [4]....

    [...]

  • ...Gau and Viskanta’s [8] experimental data on the solidification of pure gallium on a vertical wall with initial superheat is also shown in Fig....

    [...]

  • ...The correlation has also been compared with the data of Hale and Viskanta [4], who studied the freezing and melting of n-octodecane in a rectangular enclosure (again with Ste*=0 and C0 %=0) with slightly better agreement than with the data of Gau and Viskanta [7]....

    [...]

  • ...In using the data of Gau and Viskanta [8] with the present correlation, C0% is set equal to 0 (pure substance)....

    [...]

Journal ArticleDOI
Abstract: Solid-liquid phase-change heat transfer has been studied experimentally and analytically in several different materials (e.g. stearic acid, sodium phosphate dodecahydrate, sodium sulfate decahydrate and n-octadecane which have been suggested as candidates for latent-heat-of-fusion thermal energy storage materials. Solid-liquid interface motion during freezing and melting from above as well as below has been determined in a rectangular test cell suitable for photographic observations. Comparison of experimental data for n-octadecane with predictions based on Neumann and other analyses which account for natural convection heat transfer at the solid-liquid interface show that natural convection in the liquid must be accounted in the prediction of phase-change boundary motion for unstable situations which arise during melting from below and solidification from above.

104 citations


"Kinetics of melt crystallization of..." refers result in this paper

  • ...These were compared with the data of Hale and Viskanta [ 4 ], whose experiments were conducted using n-octadecane....

    [...]

  • ...Good agreement can be observed in Fig. 4 between the present data and the data of Hale and Viskanta [ 4 ]....

    [...]

  • ...The correlation has also been compared with the data of Hale and Viskanta [ 4 ], who studied the freezing and melting of n-octodecane in a rectangular enclosure (again with Ste* 0 and C 0% 0) with slightly better agreement than with the data of Gau and Viskanta [7]....

    [...]