Elsevier

Chemical Engineering Journal

Volume 378, 15 December 2019, 122176
Chemical Engineering Journal

Two-component model for catalyst deactivation

https://doi.org/10.1016/j.cej.2019.122176Get rights and content

Highlights

  • Equations on asphaltene and coke concentrations in the catalyst pellet is studied.

  • Model depends on long-time of pellet deactivation and short-time of Fick’s diffusion.

  • The model is non-homogeneous: characteristics of a catalyst depend on space and time.

  • There is good correspondence between experimental data and obtained coke concentration.

Abstract

Texture evolution of alumina hydrotreating catalyst was theoretically modeled using geometrical characteristics calculated via Monte-Carlo methods and methods of the graph theory. In the prescribed model deactivation was specified by monotonic increase of alumina grain radius, which imitated deposition of coke and metal species onto the pore surface. A novel two-component nonhomogeneous model was designed based on the diffusion of asphaltene molecules inside the cylindrical pellet according to Fick’s law and deposition of the coke particles from heavy oil on the surface of the catalyst. The model resulted in non-monotonous coke distribution with a distinct maximum in the inner region of the pellet, which, nevertheless, shifts to the center at later deactivation stages due to the higher hydrogenation performance of pellet border region. Model can be applied to describe profiles of coke depositions during deactivation process for a pellet with an arbitrary boundary.

Introduction

Running out of conventional light oils make petroleum industries turn to heavy oils with high viscosity, density and content of impurities to cover our needs in liquid fuel. Among catalytic technologies for heavy oil upgrading, hydroprocessing in a fixed-bed reactor still remains the most widely used [1], [2].

One of the main problems in this approach is a rapid catalyst deactivation by coke and metal deposits. Asphaltenes – high-molecular aromatic Ni, V, S, N-containing nanoparticles with a size from 1.5 to 10 nm [3], [4] – are found in heavy oils in large quantities (5–15 wt%) responsible for poisoning of acidic sites, formation of coke species and, finally, pore plugging [5]. To prolong catalyst lifetime, texture of the catalyst should be optimized. Enlarging the pore size in the range 30–200 nm reduces diffusional limitations and prevents rapid plugging of pore cavities with coke species. Nevertheless, too high pore size (>200 nm) is not preferred as well, since it leads to significant decrease in surface area, making them ill-suitable for good catalytic performance.

Deactivation of hydroprocessing catalysts has been a subject of numerous theoretical works. Hydroprocessing reactions in such multicomponent systems as heavy oils were usually described by lumped kinetics with the reaction orders from 0.55 to 2.0 [6]. Hydrodemetallization (HDM) and hydrodeasphaltenization (HDAsph) reactions were often represented by simplified first-order kinetics assuming Freundlich or Langmur-Hinshelwood adsorption [7]. The most widely used theory to calculate diffusion of asphaltene in porous structure was quasi homogeneous model [7], [8], which is based on continuity equations: the transport of solute – spheres with Brownian and hydrodynamic properties – is described by Fick law. Given a steady state case, in which the reagent flow to the pellet is compensated by the reagent consumption according to the reaction, the following expression could be formulated:dCdt=DC-kC=0

Catalyst porous structure is defined through porosity ε and tortuosity τ, which are composed into the effective diffusion coefficient Deff according to equation:Deff=DbulkετfRmolRporewhere Dbulk is a combination of molecular and Knudsen diffusitivity in a straight pore, f(Rmol/Rpore) is a power or exponential dependence originated from the Renkin’s equation, which is valid at Rmol/Rpore < 0.5 [9]. This function reflects both partition of the solute concentration inside and outside porous structure and hydrodynamic drag experienced by the molecule as it moves through the pore. Using this equation, effective diffusion coefficient was estimated to be in the range 10−10–10−12 (m2/s) in various porous materials [7].

Among the reported deactivation mechanisms one could mention surface poisoning [10], competitive adsorption [11], uniform pore size reduction and plugging [12], pore plugging by lumps of metal and coke deposits [13]. Percolation theory [14], [15] explicitly incorporates porous structure into description of deactivation and takes into account pore connectivity, unlike the pseudo-homogeneous model.

The present paper aims at modeling catalyst deactivation and calculating the concentration profiles of coke depositions within the catalyst pellet. The model includes construction a porous material using Poisson distributed grains, solving Fick’s diffusion equation to obtain the concentration profile for given porous structure. Deactivation was modeled by reduction of pore size through monotonic increase of alumina grains radii. In the theoretical model it was proposed that coke depositions appear uniformly on the surface of the spherical alumina grains. The suggested model is not homogeneous: the geometric characteristics of the porous media depend on the position inside the catalyst pellet. The evolution of the asphaltene and coke concentration profiles in the cylindrical catalyst pellet during deactivation was simulated.

Section snippets

Model description

The model is two-component and describes the diffusion of asphaltene molecules inside the cylindrical pellet and deposition of the coke particles from heavy oil on the surface of the catalyst. The deactivation process is described via two distribution functions of the asphaltenes Casph and the coke depositions Ccoke, depending on the position inside the pellet and the stage of deactivation. At each stage of deactivation the following system is considered:ΔCasph=Ψ·Casph(a)ΔCcoke=-Λ·Casph(b)

HereΔ=

Results and discussion

Deactivation of the catalyst was defined through evolution of the textural parameters. A consecutive increase in the alumina grain size leaded to the following trends in porosity, tortuosity and specific surface area (Fig. 1). The textural parameters obtained are consistent with those of the real catalysts. System of alumina grains was modeled as a random packing of the Poisson distributed spheres of diameter 10 nm, the packing for mesoporous catalyst in a cube with 1 µc edge contains

Conclusion

A novel two-component deactivation model was designed based on the diffusion of asphaltene molecules inside the cylindrical pellet and deposition of the coke particles from heavy oil on the surface of the catalyst. The modeling of porous catalyst was carried out by a set of Poisson distributed grains. Texture evolution during deactivation was specified through the increase of the catalyst grain diameters. Model describes the evolution of asphaltene and coke concentration profiles during the

Acknowledgements

The present work was supported by RFBR and Government of the Novosibirsk region according to the research project N17-43-543303. Ya. V. Bazaikin was supported by the Program of fundamental scientific researches of the SB RAS No. I.1.2., project No. 0314-2019-0006.

References (18)

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