BACKGROUND OF THE INVENTION [0002] Acid extraction of petroleum feeds using dilute mineral acids to remove nitrogen compounds has been known for many years. The use of concentrated acids has typically been avoided due to sludge formation that may contaminate further processing equipment as well as product. In order to address the corrosive problems posed by mineral acids, a caustic wash has been employed. This creates disposal problems of salts formed.

[0003] Different alternatives to acid extraction using mineral acids have been proposed for removing nitrogen compounds from feeds. These include extraction with carboxylic acids, solvent extraction and the use of ion exchange resins. More recently, hydrotreating using catalysts resistant to nitrogen and sulfur poisoning have been employed to remove both nitrogen and sulfur contaminants from petroleum feeds.

[0004] An important incentive to improve hydrotreating processes has been environmental regulations covering the sulfur content of fuels for internal combustion engines. These regulations are becoming more stringent with regard to allowable sulfur in fuels.

[0005] One important class of fuels is diesel fuels, and diesel fuels are of environmental concern due to their potential contribution to regulated species such as NOX, SOx and particulate matter. It is anticipated that the current diesel fuel standard of 500 wppm sulfur content that is applicable in many countries will be lowered to 50 wppm or less.

[0006] A common way for reducing the sulfur content of diesel fuels is by hydrodesulfurization (HDS) which converts sulfur-containing species to hydrogen sulfide. The HDS process is controlled by the nature of the catalyst employed and by reaction conditions within the reactor or reactors including pressure, temperature, space velocity and flow direction.

[0007] As the requirements for sulfur content of diesels reach more stringent levels, the demands on catalysts continues to increase. In order to remove sulfur to low levels, one must typically make reaction conditions more severe. However, increasing the severity under which the HDS operates typically results in decreased catalyst life as well as product degradation. It is known that nitrogen contaminants act as poisons towards most hydrotreating catalysts.

[0008] One method for removing polar compounds such as nitrogen compounds prior to HDS is to use adsorbents. It is also known to pretreat feeds using solvent extraction to remove contaminants.

[0009] It would be desirable to control conditions for hydrotreating in order to run the hydrotreating reaction to accomplish HDS at favorable pressures without incurring appreciable catalyst activity or product yield losses.

SUMMARY OF THE INVENTION [0010] The process according to the invention relates to a method for acid extracting a nitrogen-and sulfur-containing diesel feed and hydrodesulfurizing the diesel feed which comprises: (1) contacting the diesel feed with sulfuric acid having an acid concentration of from 80 to 98 wt. %, based on acid, in an extraction zone to produce an acid treated feed, provided that the ratio of acid to feed in the extraction zone is from 1: 200 to 1: 3, (2) separating the acid treated feed into a nitrogen lean diesel having a nitrogen removal of at least about 60% over nitrogen present in the feed and a nitrogen rich acid soluble oil/acid mixture, provided that the ratio of nitrogen lean diesel to acid soluble'oil is at least about 5: 1, and (3) hydrodesulfurizing the nitrogen lean diesel with a hydrotreating catalyst at hydrotreating conditions including hydrogen pressures of from 100 to 800 psig provided that the conversion of diesel in step (3) to lower boiling product is less than 10 wt. %, based on nitrogen lean diesel, and sulfur removal is greater than 95%, based on sulfur in the diesel feed.

[0011] Another embodiment of the invention relates to a process for acid extracting a nitrogen-and sulfur-containing diesel feed and controlling the hydrogen pressure for hydrodesulfurizing the diesel feed which comprises: (1) contacting the diesel feed with sulfuric acid having an acid concentration of from 80 to 98 wt. %, based on acid, in an extraction zone to produce a sulfuric acid treated feed, provided that the ratio of acid to feed in the extraction zone is from 1: 200 to 1: 3, (2) separating the sulfuric acid treated feed into a nitrogen lean diesel having a nitrogen removal of at least about 60% over nitrogen present in the feed and a nitrogen rich acid soluble oil/acid mixture, provided that the ratio of nitrogen lean diesel to acid soluble oil is at least about 5: 1, (3) hydrodesulfarizing the nitrogen lean diesel and the unextracted diesel feed with a hydrotreating catalyst at hydrotreating conditions including 3 or more different hydrogen pressures, (4) measuring the activity credit at each hydrogen pressure which is defined as the following ratio: catalyst activity credit = hydrodesulfurization rate of nitrogen lean hydrocarbon hydrodesulfurization rate of unextracted hydrocarbon feed (5) plotting the catalyst activity credit against hydrogen pressure, (6) determining slopes between pairs of adjacent pressure points, and (7) determining the pressure at which the slope is less than 1.5 lpsig 1.

BRIEF DESCRIPTION OF THE DRAWINGS [0012] Figure 1 is a graph showing the HDS rate as a function of pressure for two different concentrations of nitrogen in a feed.

[0013] Figure 2 is a graph showing activity credit as a function of pressure for two different concentrations of nitrogen in a feed.

[0014] Figure 3 is a graph showing activity credit as a function pressure for two different concentrations of nitrogen in a feed using another catalyst.

DETAILED DESCRIPTION OF THE INVENTION [0015] The feed to the present process is a diesel fuel feed or diesel fuel precursor containing nitrogen and sulfur contaminants. By diesel fuel feed or diesel fuel precursor is meant a hydrocarbon boiling in the 102 to 427°C (215 to 800°F) range. Boiling point determinations are made by standard methods such as ASTM D 86. The diesel fuel feed may be untreated or may be previously treated to partially remove heteroatom species or aromatics. Examples include virgin distillates in the diesel boiling range, refined or partially refined stocks with respect to heteroatom reduction, aromatics reduction or both, processed stock such as hydrotreated stocks to partially remove heteroatoms, aromatics or both, or blended stocks wherein different diesel feeds are blended, combined with additives or both.

Jet fuels may also be included within the meaning of diesel fuel feed.

[0016] The diesel feed is contacted with sulfuric acid. The acid may be fresh acid or may be acid that has been recycled. The sulfuric acid concentration is 80- 98 wt. %, preferably 85-91 wt. %, based on acid. The ratio of sulfuric acid to diesel feed is in the range from 1: 200 to 1: 3, preferably 1: 100 to 1: 3. The contacting method can be dispersive such as an agitator to mix components or nondispersive.

The nondispersive method is preferred to facilitate separation of acid phase from the diesel feed phase. Nondispersive contactors are disclosed in U. S. Patent No.

3,992, 156.

[0017] The acid treated diesel mixture is then conducted to a separation zone to achieve at least a partial separation of acid and organic phases. The separation zone is preferably a settler. Settlers are phase separation devices and are known in the art. Settlers may include coalescing media. Coalescing media include physical devices or chemical agents as aids to phase separation. Physical devices are preferred. The acid treated diesel mixture is separated into a nitrogen lean diesel having a nitrogen removal of at least about 60% over nitrogen present in the diesel feed, and a nitrogen rich acid mixture containing acid soluble oils. The ratio of nitrogen lean diesel to acid soluble oil is least about 5: 1.

[0018] The separated acid phase, i. e. , the spent acid phase may be combined with fresh make-up acid and recycled back'to be contacted with fresh feed. The separated nitrogen lean diesel phase from the settler is preferably contacted with a caustic solution to neutralize acid carryover and acid dispersed in the diesel phase.

The contacting can be dispersive or nondispersive with the nondispersive method being preferred. The caustic neutralization step may, however, be omitted. If a neutralization step is employed, the separated nitrogen lean diesel phase is preferably transferred to a settler to remove any caustic solution to minimize caustic carryover, and is then dried to remove any remaining water. If no neutralization step is employed, the nitrogen lean diesel phase is preferably conducted to a settler to achieve further separation and reduction of acid carryover to the hydrodesulfurization stage.

[0019] In the present process, the nitrogen lean diesel phase is hydrodesulfurized in a hydrotreating stage by contacting the feed with a hydrotreating catalyst and hydrogen under hydrotreating conditions. The terms"hydrotreating"and "hydrodesulfurization"are sometimes employed synonymously.

[0020] Hydrotreating catalysts are those containing at least one Group VIB metal (based on the Periodic Table of the Elements published by the Sargent-Welch Scientific Company) and at least one Group VIII metal on an inorganic refractory support material. Preferred Group VIB metals include Mo and W and preferred Group VIII metals are non-noble metals including Ni and Co. The amount of metal, either individually or as mixtures, ranges from about 0.2 to 35 wt. %, based on catalyst. In the case of mixtures, the Group VIII metals are preferably present in amounts of 0.5 to 5 wt. % and the Group VIB metals in amounts of from 5 to 30 wt. %. The hydrotreating catalysts may also be bulk metal catalysts wherein the amount of metal is 30 wt. % or greater, based on catalyst.

[0021] Any suitable inorganic oxide support material may be used for the hydrotreating catalyst. Non-limiting examples of suitable support materials include: alumina, silica, silica-alumina, titania, calcium oxide, strontium oxide, barium oxide, magnesium oxide, carbon, zirconia, diatomaceous earth, lanthanide oxides including cerium oxide, lanthanum oxide, neodynium oxide, yttrium oxide and praesodynium oxide, oxides of chromium, thorium, uranium, niobium and tantalum, tin oxide, zinc oxide, and aluminum phosphate. Preferred supports are alumina, silica, and silica-alumina. More preferred is alumina.

[0022] The hydrotreating catalysts are low acidity catalysts. By low acidity catalyst is meant that the acidity is controlled such that the conversion of diesel to lower boiling products, i. e. , products boiling outside the diesel range is less than 10 wt. %, based on diesel. The acidity of the catalyst is controlled by adjusting known parameters such as the nature of support material, nature of the hydrogenation metal, and dispersion of metal on the support. Hydrotreating conditions may also be adjusted to favor lower conversion to lower boiling products. In addition, the hydrotreating catalysts achieve a sulfur removal of at least about 95 wt. %, based on sulfur in the diesel feed.

[0023] Hydrotreating conditions include temperatures of from 200 to 450°C (392 to 842°F), preferably 300 to 400°C (572 to 752°F), pressures of from 790 to 5617 kPa (100 to 800 psig), preferably 1135 to 4783 kPa (150 to 700 psig), liquid hourly space velocities of 0.1 to 10 hr-1, preferably 0.5 to 5.0 hr~1, and hydrogen treat gas rates of from 38 to 1780 m3/m3 (200 to 10000 scf/B), preferably 89 to 890 m3/m3 (500 to 5000 scf/B).

[0024] The present process includes a method for controlling the pressure under which hydrotreating or hydrodesulfurization (HDS) takes place. For any given catalyst system, there is an optimal pressure for the HDS process, i. e. , increases above the optimal pressure result in no or minimal improvements in hydrodesulfurization of the feed. In order to determine this optimal pressure, hydrodesulfurization rates are measured under at least three different hydrogen pressures, preferably under 6 different hydrogen pressures, the other reaction parameters being held constant. The hydrogen pressure range over which the HDS takes place (for rate constant measurements) may range from 100 to 800 psig (790- 5617 kPa), preferably 150 to 700 psig (1135-4783 kPa). The rate constants for the HDS are preferably measured at approximately equal intervals of pressures within the overall pressure range. The pressure increment between adjacent pressures is about 50 to 300 psig, preferably 75 to 250 psig. A hydrodesulfurization rate constant is then determined at each different pressure.

Assuming 1.5 order reaction kinetics, the rate constant is determined by measuring the concentration of sulfur in the product as a function of time until product sulfur line-out is achieved. The HDS rate constants are determined from the equation: KHDS = LHSV x (WSp, uct-l/S&ed) X 100 where LHSV is the liquid hourly space velocity, Sproduct is the concentration of sulfur in the product and Sseed is the concentration of sulfur in the feed. The rate constants can be determined by standard computer programs such as Microsoft Excel or Kaleidagraph@. The rate constant for the hydrocarbon feed prior to the <BR> <BR> extraction zone, i. e. , the unextracted feed, is also determined for a base line value.

Figure 1 illustrates a plot of HDS rate as a function of pressure for two different concentrations of nitrogen in feed.

[0025] A catalyst activity credit at each different hydrogen pressure used to measure HDS rate constants is then determined. The catalyst activity credit is defined as follows: Catalyst activity credit = HDS rate of nitrogen lean hydrocarbon HDS rate of unextracted hydrocarbon feed [0026] A plot of catalyst activity credit vs. pressure is then made. Figure 2 demonstrates the effect of pressure on catalyst activity credit for two feeds having different nitrogen concentrations. A visual inspection of Figure 2 shows that there is minimal additional activity credit above a hydrogen pressure of about 400 psig.

This then represents the approximate pressure at which the hydrodesulfurization reaction should take place in order to maximize the HDS rate without the need to increase pressure. Increased hydrogen pressure typically results in increased costs so it is desirable to run at the minimum hydrogen pressure consistent with maximizing the rate of reaction to maximize yields of hydrodesulfurized product.

This hydrogen pressure will vary according to the concentration of nitrogen in the feed. If the desired product sulfur has not been achieved at this pressure, then other conditions such as space velocity and temperature can be modified, but more preferably the nitrogen content of the extracted feed should be decreased, if possible. These alternatives, rather than pressure increase, are more desirable for reactors capable of withstanding low pressures only.

[0027] This invention can also be used as a method for predicting the pressure required to achieve the desired product sulfur while maintaining all other conditions constant for an acid-extracted hydrocarbon, when the rate constants of the unextracted hydrocarbon are already known at different pressures. After the activity credit reaches a plateau, the known activity credit and the known reaction rate of the hydrocarbon prior to extraction can be used to calculate the hydrodesulfurization rate and the product sulfur of the extracted feed.

[0028] A more quantitative approach to determining the most desirable maximum pressure for running the HDS reaction is to determine the slopes of adjacent pressure points in the catalyst activity credit vs. pressure curve. Again referring to Figure 2, the data points in the Figure can also be plotted using Microsoft Excele or programs such as Kaleidagrapho. The slopes between adjacent pressure points are calculated by the plotting program. The slope is defined as the change in activity credit over the change in pressure (measured in kpsig units). The pressure of the first point of a pair of adjacent points with a slope equal to less than 1.5 kpsig-1 is preferred, as long as all pairs of adjacent points with slopes higher than 1.5 lcpsig 1 are at a lower pressure. As the change in catalyst activity credit with increasing pressure decreases, the slope may approach 0. As noted above, the minimum number of pressure points is 3. Increasing the number of pressure points will facilitate identification of the pressure at which the slope becomes less than about 1.5 kpsig'\, [0029] In a preferred embodiment, the process of the invention is described as follows. The feed and sulfuric acid are fed to a drum where they are contacted in a nondispersive contactor and then phase separated. The acid phase is pumped out from the bottom of the drum, combined with make-up sulfuric acid and recycled back to the drum. The remaining acid-washed feed phase is preferably transferred to a settler to remove any acid carryover. The settler is optional and may be eliminated if desired. The acid-washed feed then preferably goes into another drum together with a caustic solution to neutralize any accompanying acid. After separation, the caustic solution is pumped out from the bottom of this drum, combined with make-up caustic solution and recycled back to the drum. As before, the neutralized feed is preferably (but not necessarily) transferred to a settler to remove any caustic solution carryover. After separation, the neutralized feed is optionally cooled by a heat exchanger before going into the salt dryer, which removes any remaining water. The dried feed is hydrodesulfurized in a hydrotreating stage, under hydrotreating conditions as described above.

[0030] The following non-limiting examples serve to illustrate the invention.

Example 1 [0031] A typical diesel containing 1.72 wt. % sulfur and 392 wppm nitrogen was treated with concentrated sulfuric acid in a separatory funnel in a 20: 1 hydrocarbon-to-acid ratio. After the acid was separated, the hydrocarbon layer was washed with a 10% solution of sodium hydroxide. Drying of the hydrocarbon was achieved with magnesium sulfate. The extracted hydrocarbon contained 1.3 wt. % sulfur and 7 wppm nitrogen. Both the extracted and the unextracted hydrocarbons were hydrotreated at 625°F (329°C), 1 hr~1 LHSV, and 2000 scf/b (356 m3/m3) at 160 psig (1205 kPa) and at 400 psig (2859 kPa). At the higher pressure of 650 psig (4583 kPa), the temperature was raised to 650°F (343°C) and the space velocity was raised to 2 hr-1. These changes are not necessary and will not change the activity credits, since activity credits are significantly more dependent on the nitrogen content and the pressure than on other parameters. The 1.5 rate constants were calculated and are shown in Figure 1.

[0032] The catalyst activity credit at each hydrogen pressure was then determined, according to: Catalyst-activity credit = HDS rate of nitrogen lean hydrocarbon HDS rate of hydrocarbon feed [0033] Figure 2 shows the results. It is clear by visual inspection that a significant improvement in activity credit was obtained when the pressure was raised from 160 psig to 400 psig. On the contrary, no significant additional activity credit was obtained after the pressure was raised from 400 psig to 650 psig.

Consequently, the pressure of 400 psig is the preferred hydrotreating pressure. The product sulfur at this condition was 22 wppm. In order to obtain the desired less than 15 wppm sulfur, the temperature could be raised (to approximately 635 °F), the space velocity could be decreased (to approximately 0.8 hr 1), or more preferably, more nitrogen may be removed from the extracted feed to the extent possible as long as the yield loss is not significantly increased.

Example 2 [0034] In the example above, the slopes between adjacent points were calculated. The slope of the line passing through points at pressures 160 and 400 psig is equal to (3.4-1. 9)/ (0. 4 lcpsig-0. 16 kpsig) = 7.5 kpsig 1. The slope of the line passing through points at pressures 400 and 650 psig is equal to (3.7- 3. 4)/ (0. 65 kpsig-0. 4 lcpsig) = 1. 2 kpsig 1. The slope of the second set of points is less than 1.5 kpsig-1, so the pressure of 400 psig is preferred.

Example 3 [0035] Hydrocarbons from Example 1 were hydrotreated with a different catalyst under the same conditions given in Example 1. The HDS rates and the activity credits were calculated as explained in Example 1. Figure 3 shows the plot of the activity credit as a function of pressure. The slope of the line passing through points at pressures 160 and 400 psig is equal to (3.7-1. 7)/ (0. 4 kpsig-0.16 kpsig) = 8.0 kpsig. The slope of the line passing through points at pressures 400 and 650 psig is equal to (3.6-3. 7)/ (0. 65 lcpsig-0. 4 kpsig) =-0. 4 lcpsig-l. The slope of the second set of points is less than 1.5 kpsig'\ so the pressure of 400 psig is preferred.