S. R. Egorova

Specific features of the phase transition of gibbsite into boehmite under hydrothermal treatment of floccules in an aqueous suspension

Effect of the conditions in which floccules of gibbsite are hydrothermally treated on the phase composition of products of its dehydration products, obtained at T = 180–210°C and P = 1.0–1.9 MPa was studied. The phase transition of gibbsite into boehmite occurs by the dissolution–deposition mechanism upon delamination of gibbsite crystals along the (001) plane to give a multitude of layers with thicknesses of 20–100 nm and cracks in between, with widths of 10–50 nm. In the dissolution of gibbsite, [Al(OH)4]– anions pass into solution and react with protons of hydroxy groups on the (001) planes of gibbsite, with the subsequent nucleation of boehmite and growth of its crystals. The crystallization of coarse boehmite particles favors formation of nonporous floccules. Boehmite particles form no strong crystallization bonds with each other, which impairs their abrasion resistance.

On the nature of phase conversions and transformations in porous system in hydrothermal processing of χ-Al2O3 into boehmite

Study of the influence exerted by conditions of hydrothermal treatment of χ-Al2O3 on the phase composition and porous system parameters of the resulting products at T = 150–200°C and P = 0.5–1.5 MPa demonstrated the products formed in hydrothermal treatment of χ-Al2O3 are the bayerite and boehmite phases formed simultaneously in parallel pathways. Bayerite crystals have a needle-like shape and length of about 10 nm. 3D boehmite crystals are formed as parallelepipeds with edge size exceeding 200 nm in an alkaline medium at pH 8.0–9.2 and as 2D particles having the shape of rhombic plates with edge size of 80–500 nm and thickness of 20–100 nm at Ph 4.0–6.0. The crystallization of coarse boehmite particles favors formation of coarse mesopores and a low-porous system. A full phase transition of χ-Al2O3 to boehmite occurs at 180–200°C in 180–240 min.

Effect of the design of a feedstock injection device in a fluidized-bed reactor on the efficiency of the reaction using the dehydrogenation of iso -paraffins in a fluidized chromia–alumina catalyst bed as an example

Mathematical modeling is performed for the operation of two units of industrial chemical fluidized-bed reactors with different gas feedstock injection devices, i.e, three toroidal rings with nozzles in unit 1 and a false bottom with nozzles distributed over it in unit 2. Efficiency is analyzed (using the target product (iso-butylene) yield) for the operation of the two units over 4 months under industrial conditions and revealed the higher efficiency of unit 2. To dedetrmine the reasons for different product yields in the two units, a numerical solution is found by mathematical modeling to obtain characteristic pictures of catalyst particle concentrations and temperature fields in these units. It is concluded that unit 2 is characterized by a more uniform and dense distribution of the catalyst along with more uniform heating of the reactor. Pictures of the principal catalyst circulation flows are plotted to explain the considerable difference between the catalyst concentrations and gas temperature fields. Based on the numerical solution, the operational efficiency of the two units is subjected to comparative analysis, which showed good agreement with the results from an analysis of industrial reactors. The approach used in this work could be used in designing new units and optimizing existing units.

Effect of the nature of silicon oxide structures on the activity of an alumina-chromium catalyst in the reaction of iso-butane dehydrogenation

The formation of silicon oxide structures in the composition of an alumina-chromium catalyst for the dehydrogenation of iso-butane is studied by means of nitrogen adsorption, XRD analysis, solid-state 29Si NMR spectroscopy, temperature-programmed desorption of ammonia, and UV-Vis and Raman spectroscopy. It is established that 0.5–1.2 wt % silicon was distributed on the catalyst surface in the form of Si(OSi)4 structures. As the silicon content was increased to 2.2–3.6 wt %, Si(OSi)3(O-) structural elements were present on the surface in addition to Si(OSi)4. The formation of silicon oxide structures on the catalyst surface was responsible for an increase in the concentration of Cr(III) ions and a decrease in the surface acidity; the activity and selectivity of the catalysts in the reaction of iso-butane dehydrogenation increased.

Effect of high-temperature treatment on the properties of an alumina-chromium catalyst for the dehydrogenation of lower paraffins

The crystal and pore structures of a microspherical alumina-chromium catalyst calcined at 800–1100°C were studied using a set of currently available physicochemical techniques (X-ray diffraction, lowtemperature nitrogen adsorption, diffuse reflectance UV-vis spectroscopy, Raman spectroscopy, and EPR spectroscopy); the state of its active component and the catalytic properties in isobutane dehydrogenation were examined. As the calcination temperature was increased from 800 to 900–1000°C, the properties of the catalyst were improved as a result of the formation of Cr2O3 clusters in an optimum amount and a decrease in the surface acidity of the catalyst due to the dehydroxylation and phase transformations of the aluminum oxide support. Calcination at 1100°C was accompanied by a decrease in the yield of isobutylene as a result of the formation of inactive macrocrystalline chromium (III) oxide and a chromium species inaccessible to reacting molecules; this chromium species was encapsulated in closed pores as the constituent of a solid solution of α-Al2O3-Cr2O3.

Formation of Phases and Porous System in the Product of Hydrothermal Treatment of χ-Al2O3

The presence of χ-Al2O3 resulting from thermal decomposition gibbsite as part of alumina catalysts is unfavorable because of its acid characteristics. One of the available techniques of χ-Al2O3 removal is crystallization under hydrothermal conditions into boehmite, which is a main precursor of active γ-Al2O3. The influence of products of the hydrothermal treatment of χ-Al2O3 obtaining in result of thermal decomposition gibbsite under T = 150–200 °C, P = 0.5–1.5 MPa and pH = 4.0–9.2 were studied. The hydrothermal treatment products in these conditions are gibbsite and boehmite phases which are formed coincidently by parallel ways. In the alkaline medium at pH = 8.0–9.2 three-dimensional parallelepiped boehmite crystals with the edge length > 200 nm are formed, at pH = 4.0 two-dimensional rhombic-shaped plates with thickness 20–100 nm and with the edge length ~ 80–500 nm are formed. Crystallization of coarse boehmite particles promotes the formation of large and closed mesoporous.

Pilot tests of the microspherical aluminochromium KDI-M catalyst for iso-butane dehydrogenation

Results from pilot tests of microspherical aluminochromium KDI-M catalyst mixed with IM-2201 in a large-scale unit (Nizhnekamskneftekhim) for iso-butane dehydrogenation are discussed. Compared to KDI catalyst, its modified analogue KDI-M is more active and selective; the optimized grain-size composition and mechanical strength ensures higher yields of iso-butylene and longer nonstop operation (up to 400 days) of the reactor unit.

NUMERICAL SIMULATION OF HEAT AND MASS TRANSFER PROCESSES IN LARGE-SCALE FLUIDIZED BED COMPLEX STRUCTURE APPARATUS AS AN EXAMPLE OF THE REACTOR OF ISOPARAFFINS DEHYDROGENATION

In the chemical industry are widely used fluidized bed apparatus. The advantage of them is the high speed of heat and mass transfer between components of the reaction, which are in different aggregation states. Studies of large-scale apparatus are hindered big sizes and plurality of structural elements. Often such apparatus operate at high temperatures (500900 C), which further complicates the study. In this paper we consider a fluidized bed reactor block intended for the dehydrogenation of isobutane. In numerical simulation of fluidization was extended Eulerian-Eulerian approach. Differential equations that describe the hydrodynamic and thermal processes in the field of computational model of the reactor were solved in ANSYS Fluent CFD for axisymmetric unsteady flow scheme. At full simulation of the unit of the reactor in differential equations for the mass fraction of components of the gas mixture is necessary to consider changes related with chemical reactions. In the model used for this purpose it is necessary to add terms to the equations of mass transfer and absorption of heat depending primarily on the gas temperature and catalyst concentration. In this paper we’ll restrict considering the minimum number of components of the reaction (raw materials – isobutane, product isobutylene). For a given chemical reaction is written User Defined Functions (UDF). The influence of the ambient gas, the catalyst and the time step on the progress of chemical reaction in the volume element is studied. Numerical calculations were carried out, due to them circulating streams in the apparatus, the temperature field distribution of the catalyst and the conversion of the feeding gas-raw were analyzed.

Mathematical modeling of changes in the fractional composition of dehydrogenation catalysts in a fluidized-bed reactor

A mathematical model describing the destruction of catalyst particles during operation in an industrial fluidized-bed reactor with allowance for the crushing and abrasion of particles is proposed. Differences between the mechanical properties of IM-2201 and KDI catalysts used in the dehydrogenation of iso-butane to iso-butylene at PAO Nizhnekamskneftekhim are established. Using KDI catalyst with modification of the cyclone group and retention of the equilibrium distribution of particles according to size in a reactor is shown to provide a more than a 2.5-fold reduction in catalyst consumption.

Study of factors affecting the erosive wear of equipment for dehydrogenation units in fluidized beds of microspherical chromia-alumina catalysts under industrial operating conditions

The main problems associated with the operation of microspherical treating-type chromia-alumina catalysts with increased strength during isoparaffin dehydrogenation are discussed. The erosive wear of the walls of overflow pipelines when using a mixture of treating-type KDI and conventional IM-2201S catalysts and ways of solving the problem are emphasized. It is found that the main reason for an increase in erosive wear is the greater momentum of catalyst particles due to a higher mean particle size and gas transport rate; upon transitioning from IM-2201S to a mixture of IM-2201S and KDM (70 : 30), the mean particle size of the equilibrium catalyst grows from 68 to 74 μm. The optimum size range of a high-strength catalytic system in which the activity does not increase over time is calculated with a lower rate of transport gas injection while keeping the number of particles 20–40 μm in size at 20–30 wt %. Pilot batch production of high-strength catalyst in the optimum size range is recommended in order to shift units for the industrial dehydrogenation of isobutane to the use of treating-type KDI catalyst without the addition of IM-2201S.