Tesi sul tema "Deactivation"

Hydrodesulfurization is an important part of the hydrotreating process. More stringent regulations on the quality of fuels bring new requirements to the catalytic processes. The removal of sulfur has become a key issue in the oil refining and this work aims to address several aspects of the process.

Kinetic studies of the hydrodesulfurization reaction over conventional (molybdenum-based) and new (Pt/Y-zeolite) catalysts are reported. The hydrodesulfurization of both the real oil (light gas oil from Statoil Mongstad refinery) and model compounds (thiophene and dibenzothiophene) over a NiMo/γ-Al₂O₃ catalyst were studied. In a high-pressure study of the light gas oil, substituted alkyl-DBTs were found to be the most difficult to desulfurize and the order of reactivity was found to be DBT > 4-MDBT > 4,6-DMDBT. Steric hindrance together with electronic effects were identified as possible reasons for this behavior. The difference in reactivities of the individual compounds was found to decrease with the increasing reaction temperature. A gas chromatograph equipped with the atomic emission detector (GC-AED) was used for the analysis of the individual components of the oil.

The initial deactivation and the steady-state kinetics were studied during the HDS of thiophene at atmospheric pressure. Unpromoted Mo/γ-Al₂O₃, CoMo/γ-Al₂O₃, NiMo/γ-Al₂O₃, and phosphorus modified NiMo/γ-Al₂O₃ were used for the deactivation study, while NiMo/γ-Al₂O₃,CoMo/γ-Al₂O₃, and Pt/Y-zeolite (with three different pretreatments) were used for the steadystate study. Several experiments related to the deactivation of Mo/γ-Al₂O₃ and NiMo/γ-Al₂O₃ catalysts prepared with the chelating agent (NTA) were also performed and NTA was found to have no significant effect on the activity of the catalysts.

In the deactivation study, a fast initial decrease in the activity was observed on all the catalysts. However, nickel promoted catalysts were found to be more resistant to deactivation than unpromoted ones. The presence of phosphorus slightly increased the activity of the catalyst towards the thiophene HDS, but had no effect on the deactivation behavior. Several methods to regenerate the catalyst were investigated. During the resulfiding experiments, a difference between Mo/γ-Al₂O₃ and NiMo/γ-Al₂O₃ was observed. Deactivation of the Mo catalyst was more severe with increasing temperature, while for the NiMo catalyst the opposite behavior was observed. Carbon deposition on catalysts followed the similar trend: More carbon was observed on the Mo catalyst at higher temperatures, while the opposite is true for NiMo. The restoration of the activity of NiMo was complete, while the reactivation of the Mo catalyst was only partial. The results from the reactivation experiments with pure H₂ and inert gas (helium) suggest that several mechanisms of the restoration of activity exist: Resulfiding of the desulfided active sites, hydrogenation and removal of the deposited carbonaceous species, and the desorption of the reactants and products from the active sites of the catalyst. Based on the observed results, the higher hydrogenation activity of nickel is assumed to be the reason for the behavior. Hydrogenation causes the faster removal of the deposited carbonaceous species and this leads to the conclusion that the desulfiding of the active sites and the adsorption of the reaction species is significantly less pronounced on the NiMo/γ-Al₂O₃ catalyst.

Characterization studies show differences between standard and NTA-based catalysts. The higher amount of carbon on the NTA catalysts is attributed to the presence of the carboncontaining precursor - NTA. The changes in the surface area and the pore volume were observed only during the sulfiding process. In the case of standard catalysts the surface area and the pore volume decreased, while for the NTA-based catalysts the opposite is true. No change in the surface area and the pore volume with the increasing time on stream indicates that the deactivation is not due to structural changes of the catalyst. The amount of sulfur was found to be constant during the time on stream for all the catalysts.

In the steady-state study of the HDS of thiophene, CoMo and NiMo catalysts were found to be equally active. The activity of the Pt/Y-zeolite catalyst was found to be comparable to conventional catalysts when based on the amount of active material, but a fast deactivation was observed. The product selectivities during the HDS of thiophene were found to be the same for all standard catalysts, but slightly different for the Pt/Y-zeolite catalyst. This was attributed to a higher hydrogenation activity of the Pt/Y-zeolite catalyst.

The inhibition effect of other sulfur compounds and aromatics on the high-pressure hydrodesulfurization of dibenzothiophene (DBT), the so-called “matrix effect” was studied. Thiophene and DMDS have the same inhibiting effect on the total conversion of DBT, but differences exist in the effect on the selectivities of the products at low concentrations. The results indicate that the inhibiting effect of H₂S on the direct desulfurization route is stronger than the effect of thiophene on the hydrogenation pathway. In the study of aromatics, both toluene and naphthalene affect the total conversion of DBT. Naphthalene was found to be a much stronger inhibitor and inhibits mainly the direct desulfurization pathway, while the hydrogenation route is more affected by the presence of toluene.

This thesis studies dynamic modelling, estimation and optimization of the methanol synthesis with catalyst deactivation. Conversion of natural gas is of special interest in Norway for both economic and political reasons. In June 1997 Statoil opened a methanol plant at Tjeldbergodden. It is among the largest in the word with a capacity of 830 000 metric tons per year, which equals 1/4 of Europe's capacity. Methanol is today mainly used as a building block to produce chemical intermediates, and it is also a promising fuel for fuel cells.

The overall theme for this thesis is to find the optimal operation of the methanol synthesis when undergoing catalyst deactivation by dynamic optimization. There has been an increase in method development, tool development and industrial use of dynamic optimization in the past decade. Few realistic, large-scale applications with nonlinear, first principle models, exist apart from this thesis.

A rigorous pseudo-steady state model of the total methanol synthesis loop is developed. The model has been verified against a design flow sheet and to some extent, validated against process data. Overall, good agreement was found.

Optimal operation policy of the methanol synthesis is found by dynamic optimization. Both the reactor and the reactor system with recycle are studied, and it is shown that the total reactor system with recycle must be considered to find the optimal operating policy. It is also shown that a heterogeneous reactor model gives different optimal policy and more accurate results than a pseudohomogeneous reactor model. Optimization of the loop leads to USD 3 165 000, or 0.75 per cent, increased profit over four years compared to a selected reference case with a constant operation policy. Optimal operation policy is compared with an operating procedure recommended for the Tjeldbergodden methanol plant. The calculated optimal operation policy gives higher profit. However, there are two important advantages of optimization: The ability to find the optimal operation if some of the variables in the optimization problem change, and the ability to track changes in the process by model updating and repeated optimization.

During modelling and optimization, it became evident that a good industrialscale deactivation model is needed for the methanol synthesis. The sensitivity in the dynamic optimization of the methanol synthesis is analysed with regards to the deactivation model by a first order error propagation - approach. It is found that 20 per cent standard deviation in the deactivation parameters is suffcient for optimization purposes.

A deactivation model for the methanol synthesis catalyst is estimated from historic process data from a methanol plant. A model on the generalized power law form was successfully fitted to process data from a limited period of time. The estimated model is of second order. No measurable effect of water was found, probably because the variations in the feed compositions were too small. The model parameters found are confidential, and are not valid for the total catalyst lifetime because the deactivation process is fast in the beginning and slower after some time. It is necessary to use data from a longer period of time to obtain a model that is valid over the total catalyst lifetime.

The work presented in this thesis serves as a framework for the implementation of dynamic optimization in the control system of a methanol synthesis plant. The dynamic optimization should be implemented in the optimization layer of the control system with feedback to update the activity. A new catalyst deactivation model should be estimated whenever the commercial catalyst type is changed. A shrinking horizon algorithm with repeated optimization is proposed.

The main contribution in this thesis is a realistic, large-scale case study on modelling, estimation and dynamic optimization. Several researchers have studied optimal operation of fixed bed reactors experiencing catalyst deactivation. This work adopts an approach which is more realistic. A rigorous model of the total reactor system with recycle, with varying model parameters and thermodynamic properties is used. A thorough analysis of the process is enployed to formulate the optimization problem. The actual time varying control variables in the reactor system, the recycle rate and coolant temperature, are optimized with regards to an economic objective, and path constrains on the reactor temperature are considered. An optimal operation strategy for the methanol synthesis has not been published before. Similar studies have probably been performed in industry without being published.