March 2022

Process Optimization

Choosing controlled and optimal parameters for the DHDS unit

Environmental considerations and stringent government regulations drive incessant interest and efforts to decrease diesel sulfur content to ultra-low levels.

Lodhi, A. A., Zishan Engineers Ltd.; Shamshad, M., Energy Enterprise Associates

Environmental considerations and stringent government regulations drive incessant interest and efforts to decrease diesel sulfur content to ultra-low levels. New deep hydrodesulfurization technologies configured to meet diesel product sulfur specifications of less than 10 ppm continue to attract the attention of refiners. The medium-pressure ultra-low-sulfur (ULS) diesel hydtrotreating unit converts sulfur to hydrogen sulfide (H2S) in the presence of hydrogen (H2) and catalysts, with the H2S removed at a later stage. Significant effort is being made to optimize the process and operations to comply with Euro-5 quality specifications for diesel sulfur content and maintain an economic balance.

The improvement in the process requires an understanding of various factors that control outlet sulfur concentration, so that refiners and technology providers can design and operate the hydrodesulfurization unit as a controlled operation to achieve the desired results. This article examines the relationship of catalyst activity and reactor operating factors with conversion rate and sulfur concentration in outlet diesel. Process simulations were performed to analyze how each of the considered variables impact the outlet sulfur content. The results show that an increase in each of the contributing factors (i.e., reaction temperature, H2 purity and catalyst activity) improved the conversion rate of reaction, thus a decline in the outlet sulfur concentration in diesel was achieved. However, optimal parameters must be set to prolong catalyst life and maintain excellent and cost-effective outcomes.

Process scheme

The diesel hydrodesulfurization (DHDS) unit is designed to hydrotreat straight-run diesel from the crude distillation unit and diesel from bottom upgradation units such as the thermal cracker or visbreaker to achieve a target sulfur specification of 10 wppm.

The H2 requirement of the DHDS unit is fulfilled from the reformer (if available in the refinery) and a separate H2 plant. The feed is pumped to a required reaction pressure of 53 barg, combined with the H2 stream, and introduced to a reactor after being preheated by heat exchangers and furnaces to a temperature of 318°C. The reaction is facilitated using a high-activity cobalt-molybdenum (CoMo) catalyst in a fix-bed reactor, where conversion of sulfur to H2S takes place. The reaction is a quenched reaction as the reaction products pass from the heat exchanger and cooler to a cold separator at reduced pressure. The H2-rich vent gas from the cold separator is routed to the amine contactor. Lean H2 gas from the amine contactor is routed to the recycle gas compressor to minimize consumption of makeup H2, while the rest is vented. The liquid from the cold separator is routed to a distillation column where desired desulfurized diesel is obtained as a bottom product.

Case Study feed data

A simulation-based case study was performed to present an analysis of sulfur outlet concentration, with respect to different parameters and recommendations made against each scenario to achieve optimum results. The following feed data was considered and used in the simulation for evaluation: a sulfur concentration in the diesel feed of 1 wt%; a feed flowrate of 32,920 bpd; an operating pressure of 53 barg; and the use of a CoMo catalyst.

Relationship between catalyst activity and sulfur outlet concentration

Catalyst activity is a function of chemisorption of reactants on the catalyst surface, which also affects the rate of conversion of reaction. Coking or carbon deposition, sintering and poisoning of catalysts all refer to physical or chemical deactivation of catalyst active sites, resulting in a decrease in catalyst activity and impacting catalyst performance. As a result, the sulfur content in outlet diesel is high and target specifications are not met. Sometimes, the molecules acting as a catalyst poison get chemisorbed on catalyst active sites, resulting in irreversible changes to the geometric structure of the catalyst’s surface or the chemical nature of active sites. As a result, catalysts must be replaced or regenerated. Therefore, it is important to handle catalysts as per industry standard practice and avoid/minimize carbon deposition and catalyst poisoning.

Carbon formation and deposition on the catalyst can be avoided by providing high partial pressure of H2 in a DHDS reactor, whereas sintering can be avoided by selecting catalyst constituents that have high thermal stability. To preserve catalyst activity, the licensor or technology providers choose catalyst formulation, design, pore size distribution and pellet size as per diesel feed composition.

A case study was performed to analyze the direct impact of catalyst activity on the conversion of reaction and sulfur outlet concentration in the DHDS process.

Case Study results

With increases in catalysts activity, the overall conversion of reaction increases, which utilizes more H2 to convert sulfur to H2S, thus reducing diesel outlet sulfur concentration (TABLE 1). Commercially available hydrodesulfurization catalysts are designed to achieve ULS diesel product. However, catalyst activity must be monitored and assessed for a cost-effective process and operation.

Relationship between H2 purity and sulfur outlet concentration

H2 is added and recycled—with makeup H2—as one of the feed streams to react with diesel in the reactor to convert free sulfur present in diesel to H2S, which will be separated in downstream units, thus producing ULS diesel. If H2 is not recycled, the process economics of the unit becomes infeasible.

H2 partial pressure in the reactor is directly related to the purity of the H2 fed to the reactor as a feed stream (fresh H2 + recycled). The lower partial pressure of H2 leads to a decrease in reaction conversion and higher sulfur concentration in the product. Therefore, it is important to ensure the high H2 purity in the feed stream and the removal of non-H2 components in the recycle stream.

A case study was performed to determine and evaluate the effect of H2 purity on the conversion of reaction and the sulfur concentration in the DHDS product. With the change in H2 purity, other factors also change and have a cumulative impact on sulfur outlet concentration.

Case Study results

The outcomes of the case study indicate the correlation between increasing sulfur outlet concentration in diesel with a decrease in H2 purity (TABLE 2). These results confirm that high H2 partial pressure and purity must be maintained to achieve desired low-sulfur concentration in the DHDS product.

Relationship between reactor temperature and sulfur outlet concentration

Sulfur impurities present in diesel are typically in the form of mercaptans, sulfides, disulfides, cyclo-sulfides and thiophenes. In refinery operations, operators may encounter significant variations in the types of sulfur impurities present in diesel. Since high temperatures drive higher conversion in the DHDS reactor, high-temperature operations are employed to ensure on-spec diesel production even with varying sulfur content in the feed stream.

Activation energy escalates with an increase in reaction temperature due to an upsurge in the number of molecules involved in the hydrogenation reaction. This phenomenon also involves changes in the physical properties of diesel (i.e., an increase of diffusivity and a decrease in viscosity and surface tension), which ultimately promotes the H2 absorption rate in diesel and catalyst pores to reach the active sites where the reaction occurs, thus converting sulfur to H2S.

A case study was performed to analyze the effect of reaction temperature on catalysts (life and deactivation rate) and performance of the DHDS unit with respect to the sulfur outlet concentration of diesel.

Case Study results

The results indicated that increasing the reaction temperature reduces sulfur content in product diesel; however, optimum temperature selection is important to maintain an economical balance between on-spec diesel production and catalyst life and product yield (TABLE 3). Also, high-temperature operation in the DHDS reaction should be monitored and controlled closely, as high reaction temperatures can result in over-cracking or catalyst coking, which reduces catalyst life and yield.

Takeaway

Important operating parameters such as reactor temperature, catalyst activity and H2 purity have a significant impact on outlet sulfur concentration. Understanding the impact of these parameters is essential for selecting optimal parameters to ensure desired product specifications and economic operation of DHDS processes. HP

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