Nov-2023

Novel reactor for three-phase hydroprocessing applications (RI 2023)

Hydroprocessing treats heavy hydrocarbon feedstocks with hydrogen to create high-quality fuels in a heterogeneous, exothermic process.

Vinod Kumar, Abdul Quiyoom, Pranab K Rakshit and Ravi Kumar V Corporate R&D Centre
Bharat Petroleum Corporation Ltd

Article Summary

It involves hydrogen reacting with hydrocarbons to produce desired fuels and lubricants. This typically occurs adiabatically with temperature control through intermediate quenching. The conventional method employs a three-phase packed bed reactor known as trickle bed reactors (TBRs) under high pressure and temperature. This process removes hetero-atoms (S, N, metals), saturates unsaturated hydrocarbons (olefins, aromatics), and cracks heavier molecules to obtain desired quality products.¹

During hydroprocessing, as reactions progress, hydrogen sulphide (H₂S) and ammonia (NH₃) accumulate, reducing hydrogen partial pressures and also inhibiting the reactions (see Figure 1). Elevated concentrations of these compounds hinder desulphurisation, denitrogenation, and other saturation reactions, affecting catalyst acidity and conversions. This often requires high-severity operation and increased catalyst inventory to meet quality specifications.

In the TBRs, a high gas-to-oil ratio is used, which leads to undesired vaporisation of hydrocarbon feed and lighter products during the reaction,² as shown in Figure 2.

In addition, a higher gas/oil ratio and longer mean flow path for gas leads to increased pressure drop. Further, gas phase hold-up increases along the length as gaseous products are generated. This results in inefficient utilisation, insufficient catalyst wetting, and higher catalyst requirements for given throughput and desired product quality and yields.^3,4 Further, the increased residence time of intermediate products such as naphtha and diesel in the reactor leads to more gas production and excessive hydrogen consumption. Dry spots are also formed in the catalyst bed in the process, which leads to underutilisation of catalyst in the reactors. To ensure the desired property of treated hydrocarbon, the hydroprocessing consists of multiple stages to overcome thermodynamic equilibrium, increasing the operational cost.⁵

Cross Flow Reactor (CFR)
The CFR is a three-phase gas-liquid-solid reactor with several advantages over conventional TBR.⁶ A schematic representation is shown in Figure 3. In CFR, liquid feed flows downward through the fixed catalytic bed. Gaseous components such as H₂S and unreacted H₂, from the catalytic reactions, flow in the radial direction and are continuously removed from the catalyst bed, which limits the extent of product inhibition. As the gases must travel only the catalyst bed radius, the pressure drop across the bed is significantly reduced. Also, introducing H2 uniformly across the length of the catalyst bed enables maintaining a high partial pressure of H₂ throughout the reactor. It also leads to an increased rate of hydroprocessing reactions and a reduction in both catalyst deactivation and gas-to-oil ratio requirements. A patent US9914104 has been granted on the reactor technology.⁶

Diesel Hydrodesulphurisation (DHDS)
DHDS is a process in which H₂ is used for removing sulphur from the diesel stream at high temperature (300-350ºC), high pressure (40–60 bar), and high gas-to-oil ratio (around 400 Nm³/m³). Application of CFR in the DHDS process has been simulated using an in-house-developed 2D mixing cell network (MCN)-based kinetic model^7,8 for an industrial-scale reactor. Figure 4 shows that when CFR is used, it maintains similar temperature profiles across the reactor length without using a quench. Moreover, it can provide the same reactor outlet temperature (~370ºC), which is directly linked to the reactor conversions like TBR even when the gas inlet temperature is much lower (26ºC). This saves 15-20% of energy for gas heating, which proves the process is energy efficient.

Conclusions
Hydroprocessing is typically carried out in a TBR and requires severe operating conditions. There are various disadvantages to using TBRs, such as high feed vaporisation, high pressure drop, and high product inhibition. A novel reactor (CFR) has been conceptualised and studied in detail to overcome the drawback of the TBRs.

It has been established that the CFR will result in higher conversion without any quench stream. The fruits of the technology can be reaped in many forms per the requirements and concerns of the process, such as reduction in energy requirements, reduction in gas-to-oil ratio, reduced pressure, and higher reactor throughput. The CFR is energy efficient, saving 15-20% of energy for hydroprocessing applications.

This short article originally appeared in the 2023Refining India Newspaper, which you can VIEW HERE