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Gravity-dependent transport phenomena in zeolite crystal growth

Zhang, Hao
http://rave.ohiolink.edu/etdc/view?acc_num=case1060021149

Abstract Details

1992, Doctor of Philosophy, Case Western Reserve University, Mechanical Engineering.
It has been thought that large zeolite crystals with perfect quality can be grown in microgravity environments due to avoidance of particle settling and elimination of natural convection. However, there has been no detailed quantitative study in the literature to determine the nature and extent of these effects. We investigated the related transport phenomena to see if they play important roles in zeolite crystal growth. Natural convection due to temperature and concentration gradients is analyzed by estimating thermal and solutal Grashof numbers. The results suggest that the effect of natural convection on zeolite syntheses is negligible even on the earth. Moreover, several different effects caused by sedimentation are studied herein. The shear stress at the growing surface due to settling and buoyancy driven natural convection is shown to be many orders of magnitude smaller than that due to vessel shaking. Therefore, there is no reason to believe that the shear stress caused by particle settling and natural convection has a significant effect on the sizes of resultant zeolite crystals. Furthermore, it is commonly thought that because of crystal settling a nutrient-poor crystal sediment is formed at the bottom of the reactor where growth terminates. However, the settling itself m ay help transfer more nutrient to the growing surface than diffusion alone can, thereby accelerating the growth rate. Zeolite crystal growth is a unique process which usually takes place within an amorphous solid gel plus an aqueous solution. It is found herein that, under normal gravity, a white opaque gel portion containing amorphous solid gel, crystals and aqueous solution shrinks to the bottom of the hydrothermal reactor due to depletion of the flocculated gel particles. A correlation for the shrinkage is derived which agrees very well with experimental observations. Also, a non-dimensional parameter is suggested as a criterion for the occurrence of gravity-caused secondary nucleation which has been believed to result in smaller crystal size. In addition, zeolite crystal growth under microgravity is discussed in terms of under what conditions reduced gravity might be beneficial based on our analyses. The gel structure and behavior in the absence of a gravitational force is preliminarily explored, which suggests the mass transfer rate will be reduced at μ-g
Simon Ostrach (Advisor)
123 p.
Engineering, Aerospace
Zeolite crystals; Transport phenomena

Recommended Citations

Citations

  • Zhang, H. (1992). Gravity-dependent transport phenomena in zeolite crystal growth [Doctoral dissertation, Case Western Reserve University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=case1060021149

    APA Style (7th edition)

  • Zhang, Hao. Gravity-dependent transport phenomena in zeolite crystal growth. 1992. Case Western Reserve University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=case1060021149.

    MLA Style (8th edition)

  • Zhang, Hao. "Gravity-dependent transport phenomena in zeolite crystal growth." Doctoral dissertation, Case Western Reserve University, 1992. http://rave.ohiolink.edu/etdc/view?acc_num=case1060021149

    Chicago Manual of Style (17th edition)

case1060021149
708
© 1992, all rights reserved.
This open access ETD is published by Case Western Reserve University School of Graduate Studies and OhioLINK.