Revamps 2023 Issue

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 esca- lating overall energy consumption. Various measures can be taken to enhance the energy efficiency of

Utility consumption of various hydrotreater feed streams

Utility

Naphtha 200-350

Middle distillates

Resid

Fuel, MJ/mt

300-500

300-800

Electricity, kWh/mt

5-10

10-20

10-30

Steam, kg/mt

10-60

60-150

Cooling water, m 3/mt (ΔT=10ºC)

2-3

H₂, kg/mt

1-15

10-100

Table 1

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 hydrotreat- ing various feed streams. Naphtha Naphtha hydrotreaters are primarily found as a pretreat- ment step for feed to catalytic reformers and isomerisation units. Both hydrodesulphurisation (HDS) and hydrodeni- trogenation (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 pre- treatment, 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-pres- sure steam can be utilised in other processing units. To reduce hydrogen consumption within the unit, opti- mising 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

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 separa- tor 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. Resid The wider use of heavy crudes compels refiners to convert more atmospheric resid, vacuum resid, and deasphalted oil into cleaner fuels. The primary interest lies in the hydro- treated resid, which serves as an improved feed for RFCC or hydrocracking. However, upgrading resid feeds to meet ultra-low sulphur standards requires operating the resid hydrotreater under more severe conditions and may involve multiple processing steps, resulting in increased energy, hydrogen consumption, and higher refinery CO₂ emissions. For instance, fixed-bed reactors processing heavy feeds, like resids, incur high energy consumption due to excessive pressure drop, catalyst deactivation, and the need for tem- perature adjustments. Protective measures, like external fil- tration and guard beds, help, but energy efficiency remains a significant challenge. Renewable feedstocks To diminish the carbon intensity of liquid fuel products, a significant solution might entail implementing a phased

Revamps 2023