COMPUTER SIMULATION OF HYDRODEMETALLATION IN FIXED-BED REACTORS

An isothermal model for hydrodemetallation (HDM) of crude oils in catalytic fixed-bed reactors is proposed. This model involves a consecutive reaction mechanism, which is capable of accounting for particle deposit profiles with interior maxima. Consistent with the fact that HDM catalysts are conglomerates formed by precipitation, the porous catalyst itself is modeled as randomly overlapping spheres of equal size. The metal is deposited as growing metal sulfide crystallites on the inner surface of the catalyst. These crystallites originate from a certain number of randomly scattered nuclei and increase in size as the deposition proceeds. The random sphere model for the catalyst and the deposit provides the changes in the catalyst pore structure—local porosity and surface area.

The mass transport within the domain of the particle is due to restricted liquid diffusion, since the diameter of the metal bearing compound (porphyrin) and the intermediate are comparable to the pore size. The diffusion restrictions taken into account are the enhanced drag imposed on a molecule by adjacent pore walls and steric partitioning.

Since the deposition process is much slower than diffusion and reaction, the pseudo-steady-state assumption can be justified. The equations of conservation for mass are solved by orthogonal collocation on finite elements. Based on this solution technique a computer simulation program of HDM is designed that allows two modes of operation: constant temperature and constant conversion. The simulation program “SIMULA” is highly flexible with regard to reaction kinetics, catalyst structure, reactor design, and operating conditions. In comparison to a base case with uniform activity, the effect of intraparticle (radial) and bed (axial) activity profiles on the conversion rate is discussed. For the case investigated, a radial distribution of activity higher at the center of the particle than at the edge can increase catalyst life by 25%, but axial distribution was less successful.     

HYDRODEMETALLATION CATALYST DEACTIVATION—EFFECT OF METALS ACCUMULATION ON HYDROGENATION AND HYDROGENOLYSIS ACTIVITIES

The deactivation of hydrodemetallation catalyst was investigated in diffusion-free conditions using a model oil system. The hydrodemetallation activity of a CoMo/γ-alumina catalyst was studied using nickel or vanadyl etioporphyrins as model compounds and the hydrodesulfurization and hydrogenation activity using dibenzothiophene and naphthalene. The experiments were carried out over sulfided catalysts at 320°C and 4.83 MPa total pressure. The catalyst activity for nickel etioporphyrin hydrodemetallation was maintained at least up to 50% metal loading. The hydrogenolysis activity decreased with increasing nickel and vanadium on the catalyst but the hydrogenation activity increased. The changes in activity were attributed to the different activities of the various sulfide phases present in the catalyst system.     

THERMIDOR: A new model for combined simulation of operations and optimization of catalysts in residues hydroprocessing units

Atmospheric or vacuum residues can be upgraded under high hydrogen pressure into valuable distillates and low sulfur fuel oils using several types of hydroprocessing units. Starting from such highly asphaltenic feeds, with the goal to achieve current very low sulfur targets (<0.3%) in fuels for export or subsequent upgrading by catalytic cracking technologies, fixed bed technologies like the HYVAHL process are still the most efficient. This paper discusses some recent developments of HYVAHL, aiming at improved desulfurization levels and operating times. These improvements were helped by the predictions of a newly developed mathematical model called THERmal Monitoring for Isoperformance Desulfurization of Oil Residues (THERMIDOR). This model simulates the HYVAHL process operation along time on stream, taking into account the complex associations of guard bed materials and catalysts including particle size, activity, pore size and shape grading effects. A crucial non trivial achievement in THERMIDOR is the realistic representation of the two main catalyst deactivation mechanisms in residues hydroprocessing: coke and metals deposition. The model shows clearly the synergy between dedicated HDM (“chesnut bur” type, larger mesopores, and macropores) and HDS (higher surface area and activity) catalysts. A very well defined optimum partition of the two catalysts is predicted and found in excellent agreement with the typical optimum determined from pilot plant experiments, and further implemented in commercial plants. The simulation results are successfully compared with representative operational data from a typical HYVAHL plant and reveal new insights on the complex catalyst deactivation phenomena involved in residue hydroprocessing.     

HYDRODEMETALLATION CATALYST PELLETS WITH NONUNIFORM RADIAL PORE SIZE DISTRIBUTION

Hydrodemetallation catalysts typically become inactive by pore mouth plugging, leaving a portion of the catalyst in the interior unutilized. By designing the pore size distribution so as to accommodate larger amount of metal deposit, improved performance can be attained. In this work the effect of nonuniform radial pore size distribution on catalyst performance is explored theoretically. Under the constraints of fixed total internal surface area and local void fraction, it is shown that catalysts with linear radial pore size distribution have larger metal deposit capacity. Furthermore, plugging of the pore mouth is delayed and therefore catalyst lifetime also increases.     

添加剂对渣油加氢脱金属性能的影响

以不同种类和不同含量高芳香性的催化裂化产物重循环油(HCO)和轻循环油(LCO)作为添加剂,考察了其对常压渣油加氢脱金属反应(HDM)性能的影响。结果表明,HCO和LCO添加剂的加入,可改善渣油在反应过程中的扩散性能,促进渣油中沥青质的溶解,从而提高渣油的HDM性能,其对加氢脱镍(HDNi)性能的提高更优于加氢脱钒(HDV)。同时发现,加入高芳香性添加剂后,渣油HDM过程中S、N和CCR的脱除率均有提高,反应产物的相对分子质量分布明显向低相对分子质量方向移动;随着LCO添加剂含量的增加,渣油的HDNi、HDV性能逐步提高,但当LCO质量分数增加到20%后,其提高渣油HDM性能的作用稍有下降。     

OPTIMAL DESIGN FOR THE RESIDUAL OIL HYDRODEMETALLATION IN A FIXED BED REACTOR

A restricted diffusion model was used to investigate the optimal design for the residual oil hydrodemetallation in a fixed bed reactor. Based on the total lifetime activity, the optimal catalyst pore sizes and their corresponding optimal division locations in the bed were determined. The results indicate that the total amount of demetallation could be significantly improved by using a multi-layers reactor and the nonuniform activity catalysts. In addition, the influence of the Thiele modulus on the optimal designs were illustrated.