Nov-2023

Technical solutions for hydrotreating

Revamps and turnarounds allow time to upgrade hydroprocessing units to make refineries both profitable and sustainable

Amy Hearn, Diana Brown, Brian Yeung, Patrick Christensen and Tom Yeung
Hydrocarbon Publishing Company

Article Summary

Hydrotreating is one of the most energy-intensive processes in a refinery and is usually near the top of the list for throughput capacity. As a result of these two factors, hydrotreating is viewed as a prime area for energy savings in a refinery.

The refining industry is facing increasing pressure to reduce its carbon footprint in its internal operations according to Scope 1 as defined by the Greenhouse Gas Protocol. As part of this effort to address climate scrutiny, global refiners are implementing a range of decarbonisation strategies aimed at reducing their GHG emissions and transitioning to a low-carbon energy system.

There are two key drivers behind the carbon footprint of a refinery, namely energy consumption and GHG emissions from unit operations. In terms of energy consumption, processes demand different amounts of electricity, steam, and fuel, which are considered major sources of GHG emissions. Figure 1 shows how much steam, fuel, and electricity are used by various refinery processes, according to the US Environmental Protection Agency (EPA).

A study by the US EPA identified combustion – from energy use – as the leading contributor to GHG emissions, with 63.3% of total shares. It is followed by FCC unit coke burn-off (23.8%), hydrogen plants (5.8%), flaring (2.5%), and sulphur plants (1.8%), with the remaining emissions from others (3.1%). Based on energy intensity and hydrogen consumption, very high throughput capacity hydrotreating is a key unit for refiners to establish short- and long-term strategies that specifically address emissions from refinery internal operations as profitability and sustainability are key goals.

Conventional feedstocks
Catalytic hydrotreating can take on several different roles in a refinery. Depending on the feed processed and catalyst used, this process is utilised to remove sulphur, nitrogen, metals, and aromatics from different hydrocarbon streams. Because of this wide range, it will be necessary at times to discuss specific hydrotreating applications for processing naphtha, middle distillates, and resid, although some generalisation will occur. The unit requires energy to heat the feed stream and to power the flow of materials. It also indirectly uses a significant amount of energy due to the consumption of hydrogen.

Hydrotreaters are one of the major energy consumers in a refinery, mainly because they lack adequate heat integration. Product hydrotreating makes up 10% of the refinery’s energy consumption. In specific applications, it is important to improve the energy usage of middle-distillate hydrotreaters, as these units are key in meeting stringent fuel specs now and in the future.

Wider use of heavy crudes means that refiners will be faced with converting greater amounts of atmospheric resid, vacuum resid, and deasphalted oil into cleaner and lighter products, including fuels. In resid hydrotreating, fuel, electricity, and H₂ consumption are higher compared to middle-distillate hydrotreating due to the more severe operating conditions required in resid hydrotreating.

Table 1 compares the utilities consumption of hydrotreating various feed streams.

Naphtha
Naphtha hydrotreaters are primarily found as a pretreatment step for feed to catalytic reformers and isomerisation units. Both hydrodesulphurisation (HDS) and hydrodenitrogenation (HDN) reactions are carried out in this type of hydrotreater as well as saturation reactions for the unstable hydrocarbons.

Commercially, a network of dividing wall columns (DWCs) can be used in place of traditional distillation columns in a naphtha hydrotreater to boost overall unit energy efficiency and profitability. DWCs are said to reduce the number of columns needed, lowering equipment costs and plot space requirements compared to traditional distillation columns.

Middle distillates
The hydrotreating process for middle distillates serves to remove sulphur and olefins from hydrocarbon fuels. Similar to other hydrotreating methods, energy is primarily utilised in several aspects: heat exchangers for feedstock pretreatment, process heaters, high-pressure steam to power compressors and the main pump, low-pressure steam for additional process heating, and electricity to operate pumps and fans. This process allows for the generation of both low- and medium-pressure steam. While the low-pressure steam is consumed within the process, the medium-pressure steam can be utilised in other processing units.

To reduce hydrogen consumption within the unit, optimising the feedstock blend directed to the unit can be a viable solution, especially if other diesel hydrotreaters are present on site or planned. This optimisation can lower the blend’s aromatics content.

Middle distillate hydrotreaters represent a significant energy consumer in a refinery, mainly due to the lack of sufficient heat integration. In fact, hydrotreating products account for approximately 10% of a refinery’s total energy consumption. As more stringent fuel specifications are anticipated in the future, the usage of middle distillate hydrotreaters is expected to increase, further escalating overall energy consumption.

Various measures can be taken to enhance the energy efficiency of middle-distillate hydrotreaters. For instance, enlarging the surface area of the feed preheat exchanger and installing a hot separator in front of the stripper can be effective. The increased surface area enables better heat transfer from the effluent to the feed, leading to improved heat recovery. As a result, the feed furnace can operate more efficiently, providing increased throughput for the unit. Although implementing a larger surface area requires a greater initial investment and a longer payback period, it reduces furnace duty and saves more energy in the long run.

Removing the heat exchanger and employing a hot separator before the stripper in hydrotreating has advantages. The hot effluent stream goes to the stripper, while the remainder cools against the air before reaching the cold separator. This practice reduces energy costs by €885K ($978K)/yr, though with a higher initial investment of €3.3MM ($3.65MM) and a 3.7-year payback period. In comparison, increasing the heat-exchanger surface area by 67% costs €2.3MM ($2.5MM) with a payback period of 4.6 years. However, the hot separator raises the separation temperature and lowers hydrogen content in the recycle gas, shortening catalyst life and increasing processing costs.⁵

In particular, the hydrotreating of high-aromatic light cycle oil (LCO) from FCC units consumes a significant amount of hydrogen. The primary hydrogen consumer is the aromatics in the feed, which decrease in density and increase in cetane level during saturation.