|Difference between (A) the conventional method and (B) the approach described in the paper. Credit: ACS, Jarullah et al. Click to enlarge.|
Researchers at the University of Bradford (UK) are proposing a method for increasing the yield of middle distillates (such as car fuel, jet fuel, and diesel fuel) from the refinery by applying catalytic hydrotreating (HDT) to the full crude oil, rather than the now common application of hydrotreating processes to oil fractions a (i.e., after the separation of crude oil into its fractions, such as gasoline, kerosene, and light and heavy gas oils).
In a study reported in a paper in the ACS journal Energy & Fuels, the team found that their process showed greater yield of desirable middle distillates compared to the yield produced by conventional methods and, consequently, a decrease in the yield of less-desirable reduced crude residue (RCR). They also noted that the specifications of RCR produced by the direct hydrotreating of crude oil are better than the specifications of RCR produced by conventional processes; levels of sulfur, nitrogen, metals, and asphaltene are much lower in comparison to the contents of RCR produced by conventional methods, allowing for the production of good fuel oils.
HDT employs hydrogen at high reaction temperatures and pressures, along with a high-activity hydrotreating catalyst. The compounds that have high molecular weight in the oil feedstock will be cracked and saturated with H2 to yield distillate fractions with increasing hydrogen/carbon (H/C) ratio and decreasing impurities. In other words, HDT increases the quality and quantity of distillate fractions, they note, adding that applying hydrotreating processes on separate oil fractions as is now common is fairly easy.
However, the crude oil hydrotreating process is regarded as a large and difficult challenge because crude oil contains several compounds and complex structures, in addition to multiple phases and the presence of the asphaltenes that contain a large amount of sulfur and metals (which close the active sites of the catalyst).—Jarullah et al.
In this study, multiple reactions were carried out in a continuous flow isothermal trickle-bed reactor (TBR) using Iraqi crude oil as a feedstock and commercial cobalt-molybdenum on alumina (Co-Mo/γ-Al2O3) as a catalyst for a range of reactor temperatures, hydrogen pressures, and liquid hourly space velocities (LHSVs), with a constant hydrogen/oil ratio. The crude oil hydrotreated at the best operating conditions was distilled into light naphtha (LN), heavy naphtha (HN), heavy kerosene (HK), light gas oil (LGO), and reduced crude residue (RCR) to compare the yield and properties of these derivatives to the same fractions produced by conventional methods.
They attributed the increase in the percent of the yield fractions via their method to converting of heavy compounds and long molecules that are concentrated in heavy fractions (such as RCR) to light compounds as a result of hydrotreating of crude oil before the distillation process. When conventional processes are carried out for each fraction separately, the heavy compounds and long molecules are deposited at the bottom of the atmospheric and vacuum distillation column. Hydrotreating those using normal operations and conditions is difficult, they note.
HDT of crude oil is one of the toughest and challenging tasks in the petroleum refining industry that has not been reported widely in the literature. Petroleum refining industries undergo continuous changes in the schemes of processing various crudes to achieve the market request for middle distillates with the desired properties, and the increase in middle distillate quantities can improve the refinery economics substantially as a result of transportation demand. HDT has the ability to increase the yield of distillate fractions and simultaneously reduce the contents of impurities (such as S, N, V, Ni, and Asph).—Jarullah et al.
Aysar T. Jarullah, Iqbal M. Mujtaba and Alastair S. Wood (2011) Improvement of the Middle Distillate Yields during Crude Oil Hydrotreatment in a Trickle-Bed Reactor. Energy Fuels, Article ASAP doi: 10.1021/ef101327d