SINGLE AND DUAL-STAGE HYDROTREATING CAN IMPROVE DIESEL CETANE

Pending diesel aromatics specifications can be achieved by one of two methods, say Seck Leong Lee and Menno de Wind of Akzo Chemicals BY.

The researchers reported on the use of these methods-single and dual-stage hydrotreating-at American Chemical Society's national meeting, April 5-10, in San Francisco.

With both the U. S. and Europe soon expected to reduce diesel fuel emissions by tightening fuel specifications, SOx emissions will be controlled by limiting sulfur to 500 ppm or less. For control of particulate emissions, specifications on aromatics content, or the related cetane index (CI), will be set.

In most existing diesel hydrotreaters, where process conditions have been selected to achieve hydrodesulfurization (HDS) with minimum hydrogen consumption, CI improvement is limited.

A refiner requiring additional CI improvement has two alternatives for significantly increasing the cetane index of his diesel fuel:

Single-stage, severe hydrotreatment using a special base-metal catalyst that Simultaneously removes sulfur and nitrogen
Dual-stage hydrotreatment using a noble metal catalyst in the second stage after removing sulfur and nitrogen in the first stage.

The choice of options, according to Lee and de Wind, depends on many factors, including the sulfur and nitrogen content of the feed and the ease with which these can be removed at "normal" diesel hydrotreating conditions.

EXPERIMENTAL

The cetane quality of a diesel fuel is a function of its aromatics content.1 Thus, to gain insight into how the cetane index can be altered, an understanding of how the aromatics composition changes under different hydrotreating conditions is essential.

To study the chemical pathway of aromatics saturation, a straight-run fight diesel was treated under a range of operating conditions over a NiMo catalyst specifically designed for diesel hydrogenation. The resulting products contained varying aromatics levels.

Feed and product aromatics were analyzed using FIA (ASTM D1319) and high-pressure liquid chromatography (HPLC). The total hydrogen content of the product oils was measured using low-resolution proton nuclear magnetic resonance (NMR).

The results of this study indicated the following chemical pathway for the hydrogenation of diesel:

Diaromatics --

monoaromatics -- saturates

The conversion of monoaromatics to saturates is the rate-determining step. Diaromatics, however, can be converted under relatively mild conditions of hydrotreatment.

CETANE INDEX

To investigate the change of CI with product aromatics content, a diesel blend consisting of 75 vol % straightrun heavy gas oil, 15 vol % light cycle oil (LCO), and 10 vol % visbroken gas oil was studied.

A plot of the CI vs. product aromatics is shown in Fig. 1. At low severities of hydrotreatment, there is a large initial decrease of the diaromatics, accompanied by a large initial increase in CI. Total aromatics content at this severity, however, remains virtually constant.

A subsequent decrease in total aromatics (caused by the saturation of the monoaromatics) causes a more gradual increase in CI.

Thus, under mild hydrotreating conditions, diesel feedstocks containing substantial amounts of diaromatics (e.g., LCOs) will give a relatively large increase in CI compared to a diesel feedstock with the same total aromatics content, but containing predominantly monoaromatics.

This is coupled with the fact that, under these mild processing conditions, the kinetics for monoaromatics saturation are not favorable.

AROMATICS SATURATION

Pilot plant tests were performed on light diesel from a North Sea crude at a number of temperatures, at constant LHSV, and at a hydrogen partial pressure of about 50 bar. Two catalysts were compared:

Ketjenfine 752, the newest generation of CoMo catalyst designed for deep HDS of middle distillates
Ketjenfine 852, a NiMo catalyst specially developed for single-stage hydrogenation.

Results are shown in Figs. 2 and 3. Increasing the temperature from 250 C. to 360 C. produces a decrease in product FIA aromatics and an increase in CI. At higher temperatures, aromatics increase and CI decreases.

At about 360 C., the minimum aromatics content is reached, resulting in a CI improvement of approximately 2.5 points for the NiMo catalyst and 1 point for the CoMo catalyst.

The activity of the NiMo catalyst for aromatics saturation was higher than that of the CoMo catalyst under all conditions tested. Product sulfur contents for both catalysts were less than 50 ppm at 250 C. and less than 5 ppm at temperatures above 300 C.

Therefore, under normal HDS conditions, the cetane improvement of a light diesel is limited to typically 1-3 points. A catalyst specifically designed for cetane improvement can achieve a greater increase than the most active CoMo catalysts normally used in HDS units.

SEVERE SINGLE STAGE

As discussed, medium-pressure hydrotreating can only marginally improve CI. The required aromatics saturation can, however, be achieved by treating the diesel at high pressure (80 bar) and moderate temperatures (340-390 C.) using a NiMo or NiW catalyst.

Under these conditions, sulfur and nitrogen removal occur very rapidly in the same step.

Lee and de Wind compared two high-activity NiMo catalysts, showing that, when using a dedicated aromatics saturation catalyst, CI can be increased substantially in a single stage, high-pressure hydrotreatment (Fig. 4). This is of particular interest for heavy gas oil and cracked stocks high in sulfur and nitrogen.

TWO-STAGE PROCESS

In two-stage hydrotreatment, the feedstock is first subjected to deep hydrotreating conditions to remove sulfur and nitrogen. Deep hydrotreating, in this case, means the use of normal hydrotreating pressures and temperatures, but at low LHSV (

The product from the first stage is then hydrogenated over a noble metals catalyst at low temperatures and pressures to saturate the aromatics.

In another test, light and heavy diesel were hydrotreated over Ketjenfine 752 in the first stage. Properties of these diesels are given in Table 1.

Unlike mild hydrotreating as previously described, deep hydrotreating to remove sulfur and nitrogen resulted in larger cetane improvements for both feedstocks.

For the heavy diesel, the degree of deep hydrotreating required to obtain the low nitrogen and sulfur in the product was higher than that for the light diesel. This is reflected in the larger aromatics removal, density decrease, and cetane index increase in the hydrotreated product for this feedstock.

Further, because the diaromatics content of the heavy diesel is higher than that of light diesel, the initial CI improvement is caused by the saturation of these diaromatics.

In the second step of this process, the products were reacted over a noble metals (Pt/Pd) catalyst at 30-40 bar and 260-300 C. Final product properties are given in Table 2.

Overall results show that, with the heavy feedstock, the improvement in CI is higher in the first stage than in the second stage. For the light feedstock, the CI is only 55.5, although practically all the aromatics have been removed.

For the heavy diesel, the CI is significantly higher but the aromatics removal is lower (74.5%). This is because the CI is a calculated value (ASTM D976) based on the oil density and the temperature at ASTM 50% recovery.

The CI is not only dependent on the aromatics content (which predominantly influences oil density), but also on the distillation range. Hence, in the case of light diesel, the effect of total aromatics removal (density decrease) does not outweigh the effect of lower ASTM 50 vol % point, compared to the heavier feedstock.

Lee and de Wind say the two-stage process is preferred in cases where the diesel feedstock is light. Deep sulfur and nitrogen removal are easier in the first stage, and very low aromatics (98% aromatics removal) are achievable in the second stage.

The product, which is low in sulfur, nitrogen, and aromatics, is an excellent blending stock for the diesel pool.