Showing papers by "Sankaran Sundaresan published in 1994"

Turbulent gas‐particle flow in vertical risers

Abstract: The riser reactors used for the catalytic cracking of gas oil use a transported solid catalyst, and their performance can be predicted with confidence only if the physical mechanism that determines the cross-sectional distribution of the catalyst can be identified and modeled. Understanding this mechanism is, therefore, of central importance in predicting the behavior of any system in which solid particles are transported through a duct by fluid, and this is the problem addressed here. Many experimental studies on the cross-sectional distribution of particles in vertical risers have revealed marked segregation of particles. These flows are inherently unsteady with large fluctuations in suspension density. In this article, the authors have analyzed the time-smoothed equations for the motion of dense suspensions to demonstrate the role of these fluctuations on the occurrence of segregation. It is shown that the particles will congregate in regions where the kinetic energy of fluctuations associated with the particles is small. In the context of transport of small particles such as FCC, a simplified model based on the mixture velocity can be constructed. A speculative K-[epsilon] model based on this velocity is analyzed to illustrate the extent of segregation of particles afforded by unsteady fluctuations.

Structural Investigation and Energetics of Mullite Formation from Sol-Gel Precursors

Abstract: The influence of homogeneity of sol-gel precursors on mullite (AleSi2013) formation was examined with a combination of structural and thermodynamic characterization by NMR, X-ray diffraction, and drop solution calorimetry. The most homogeneous gels crystallize to mullite at 980 "C. Gels with distinct silica and alumina phases crystallize to mullite above 1250 "C. Crystallization at 980 "C and the nature of the dominant crystalline phase, spinel or mullite, were related to the abundance of pentacoordinated aluminum atoms in the amorphous phase at 900 "C. The enthalpy of crystallization to mullite from the gels varies from -112 kJ/mol(A&Si2013) for complete mullite formation at 980 "C from a homogeneous gel formed by controlled hydrolysis of TEOS and aluminum isopropoxide, to -33 kJ/mol for mullitization at 1250 "C from a heterogeneous gel made of boehmite in hydrolyzed TEOS. Gels with intermediate mixing formed an alumina-rich spinel phase at 980 "C, which contained 7 wt 5% Si02 as shown by 29Si MAS NMR. Mullite formed at 980 "C is aluminum-rich with an Al/Si ratio of 5, with the excess silica phase separating as amorphous silica. Between 950 and 1200 "C the silica and mullite react increasing the silicon content in mullite crystals. The enthalpy of the substitution reaction, AP+ + liZ vacancy - Si4+ + 1/202-, was determined to be -52 kJ/mol. The enthalpy of formation of mullite from mixtures of a-alumina and quartz varied with the Al/Si ratio and was a minimum at Al/Si = 3.2.

Intercalation of Aliphatic Amines into the Layered Structure of Vanadyl(IV) Hydrogen Phosphate Hemihydrate (VOHPO4.cntdot.0.5H2O)

Abstract: The vanadyl(IV) hydrogen phosphate hemihydrate, VOHPO[sub 4][center dot]0.5H[sub 2]O, is a pyrolytic precursor of the vanadyl(IV) pyrophosphate phase, (VO)[sub 2]P[sub 2]O[sub 7], generally believed to be the active phase in the selective oxidation of n-butane into maleic anhydride. Pyrolytic transformation into the pyrophosphate phase occurs with conservation of a morphology of the material. VOHPO[sub 4][center dot]0.5H[sub 2]O is a layered hydrogen phosphate, where -POH groups form interlayer hydrogen bonds with the water molecules shared by two face-linked vanadyl octahedra. The structure of the hemihydrate is similar to that of [alpha]-zirconium hydrogen phosphate ([alpha]-ZrP), where hydrogen bonds are within the same layer and -POH groups are also pointed into the interlayer space. In contrast to [alpha]-ZrP, where extensive data exist, intercalation chemistry of the layered vanadyl(IV) hydrogen phosphate hemihydrate at present is a terra incognita. This paper reports the results of the first systematic study of VOHPO[sub 4][center dot]0.5H[sub 2]O intercalation with aliphatic amines as a new route to novel vanadyl(IV) phosphate phases. N-Alkylamines have been commonly known as excellent intercalation agents for testing the intracrystalline reactivity of layered oxides. Intercalated alkylamines may also facilitate introduction of thermostable guest molecules, or [open quotes]pillars[close quotes], by ion exchange producing microporous materials whichmore » can modify catalytic and sorptive properties. 9 refs., 6 figs., 2 tabs.« less

Liquid Distribution in Trickle Bed Reactors

Abstract: The quality of liquid distribution at the top of the bed and the manner in which the flows are established can affect the liquid flow behavior in trickle beds profoundly. This, in turn, can impact the rates of chemical reactions. Some recent experimental results highlighting these points are reviewed.

Catalytic process for the production of maleic anhydride

Abstract: The invention relates to vanadium/phosphorus oxide catalyst precursors useful in preparing catalysts useful in the oxidation of hydrocarbons containing 4 carbon atoms to maleic anhydride.