WO2005056728A2

WO2005056728A2 - Two-stage hydrotreating process for diesel fuel

Info

Publication number: WO2005056728A2
Application number: PCT/US2004/040093
Authority: WO
Inventors: Jeffrey M. Dysard; Catalina L. Coker; Mark A. Greaney; Peter W. Jacobs; Chuansheng Bai
Original assignee: Exxonmobil Research And Engineering Company
Priority date: 2003-12-05
Filing date: 2004-12-01
Publication date: 2005-06-23
Also published as: WO2005056728A3

Abstract

A process for the two-stage hydrotreating of a diesel feed. A nitrogen removal step is inserted between the two hydrotreating stages. The nitrogen removal step allows the diesel product from the second hydrotreating step to meet desired diesel specifications with regard to sulfur content.

Description

TWO-STAGE HYDROTREATING PROCESS FOR DIESEL FUEL

FIELD OF THE INVENTION

[0001] This invention relates to a two-stage hydrotreating process. More particularly, a nitrogen removal step is inserted between the two hydrotreating stages of a diesel hydrodesulfurization process.

BACKGROUND OF THE INVENTION

[0002] Environmental regulations covering the sulfur content of fuels for internal combustion engines are becoming more stringent with regard to allowable sulfur in fuels. 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 NO_x, SO_x 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.

[0003] 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.

[0004] 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.

[0005] 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.

[0006] It would be desirable to have a diesel HDS process capable of deep hydrodesulfurization to low levels while at the same time maintaining catalyst activity.

SUMMARY OF THE INVENTION

[0007] It has been discovered that hydrotreating of diesel fuels can be improved by a two-stage hydrotreating process including an interstage nitrogen removal step. Accordingly, the process for producing hydrotreated diesel fuels comprises:

(a) contacting a diesel feed in a first hydrotreating stage with a hydrotreating catalyst under hydrotreating conditions to produce a first hydrotreated diesel, (b) removing nitrogen contaminants from at least a portion of the first hydrotreated diesel by contacting the first hydrotreated diesel with a nitrogen removal agent comprising at least one of adsorption, solvent extraction, acid treating or ion exchange, and (c) contacting at least a portion of product from step (b) with a hydrotreating catalyst under hydrotreating conditions.

[0008] Another embodiment for producing hydrotreated diesel fuels comprises: (a) contacting the diesel feed in a first hydrotreating stage with a hydrotreating catalyst under hydrotreating conditions to produce a first hydrotreated diesel, (b) removing nitrogen contaminants from at least a portion of the first hydrotreated diesel by contacting the first hydrotreated diesel with an acidic nitrogen removal agent, and (c) contacting at least a portion of product from step (b) with a hydrotreating catalyst under hydrotreating conditions.

BRIEF DESCRIPTION OF THE DRAWING

[0009] The Figure shows a plot of yield loss vs. product nitrogen obtained after acid treating.

DETAILED DESCRIPTION OF THE INVENTION

[0010] The feed to the present process is a diesel fuel or diesel fuel precursor. By diesel fuel is meant a hydrocarbon fraction boiling in the 102 to 427°C (215 to 800°F) range. The diesel feed- may be untreated or may be previously treated to partially remove heteroatom species or aromatics. The sulfur and nitrogen contents of the feed are generally in the range of less than about 3 wt.% and 0.5 wt.%, respectively, based on feed. In the present process, the diesel feed is hydrotreated in a first hydrotreating stage by contacting the feed with a hydrotreating catalyst and hydrogen under hydrotreating conditions. The terms "hydrotreating" and "hydrodesulfurization" are sometimes employed synonymously.

[0011] 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 metal are non-noble metals including Ni and Co. The amount of metal, either individually or as mixtures ranges from about 0.5 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.

[0012] 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.

[0013] Hydrotreating conditions in the first stage include temperatures of from 200 to 450°C (392 to 842°F), preferably 300 to 400°C (572 to 752°F), pressures of from 1480 to 20786 kPa (200 to 3000 psig), preferably 3549 to 13891 kPa (500 to 2000 psig), liquid hourly space velocities of 0.1 to 10 hr^"1, preferably 0.5 to 5.0 hr , and hydrogen treat gas rates of from 38 to 1780 m /m (200 to 10000 scf/B), preferably 89 to 890 mVm³ (500 to 5000 scf/B).

[0014] The product from the first stage hydrotreating is preferably stripped to remove light ends, ammonia and hydrogen sulfide, and then conducted to a nitrogen removal zone. The nitrogen removal may be accomplished by < contacting said first stage product with a nitrogen removal agent which may be at least one of adsorption, solvent extraction, acid treating and ion exchange, preferably acid treating. In adsorption, the first stage hydrotreated product is contacted with adsorbent to remove polar compounds such as nitrogen heterocycles. Adsorbents include activated alumina, activated carbon, molecular sieves, silica gel, silica-alumina, ion exchange resins, hydrated alumina and clays, and these adsorbents may be used singly or in combination in a single column or in multiple columns. The adsorbents are then desorbed by an elution solvent. Elution solvents are typically chosen based on the nature of the adsorbent and may include naphtha, kerosene, diesel, gas oil and mixtures thereof, ketones, alcohols, ethers and mixtures thereof. The hydrotreated product may then be separated from solvent by known methods such as distillation.

[0015] The first-stage hydrotreated product may also be solvent extracted. The solvent extraction process selectively dissolves the aromatic components, including nitrogen heterocycles, into the extract phase while leaving more paraffinic components, i.e., the diesel fuel, in the raffinate phase. Typical solvents for solvent extraction include phenol, furfural and N-methyl pyrrolidone. By controlling the solvent to oil ratio, extraction temperature and method of contacting hydrotreated diesel to be extracted with solvent, one can control the degree of separation between extract and raffinate.

[0016] The preferred method for removing nitrogen compounds from the first stage hydrotreated product is acid treating. In acid treating, the first stage hydrotreated product is contacted with acid. The acid may be fresh acid or may be acid which has been used previously but still retains significant acidity, such as spent alkylation acid. The acid is preferably a mineral acid, more preferably a strong mineral acid, most preferably sulfuric acid. For sulfuric acid, the acid concentration is preferably 80-96 wt.%, more preferably 85-91 wt.%, based on acid. In the case of sulfuric acids that contain organic contaminants from previous use, the effective sulfuric acid concentration is calculated based upon the sulfuric acid and water contents only. For example, a typical spent alkylation acid may contain 87 wt.% sulfuric acid, 6 wt.% water and 7 wt.% organic matter. The effective sulfuric acid concentration of this acid is 93 wt.%. Addition of water to the spent alkylation acid may be used to adjust the acid strength. The contacting method can be dispersive such as an agitator to mix components or nondispersive. Nondispersive methods may be used to facilitate separation of aqueous acid phase from the organic feed phase. Nondispersive contactors are disclosed in U.S. Patent No. 3,992,156. After separation of the aqueous and organic phases, the low-nitrogen organic phase is preferably conducted to a settler to achieve further separation by reducing acid carryover to the hydrodesulfurization stage.

[0017] The separated organic phase from the settler is preferably contacted with a caustic solution to neutralize acid carryover and acid dispersed in the organic phase. The contacting can be dispersive or nondispersive. The separated organic phase containing hydrotreated feed is preferably transferred to a settler or other separation device to remove any caustic solution carryover, may be water- washed and is then dried and may also be clay and/or salt treated to remove any remaining water or other contaminants. The dried organic phase product from the nitrogen removal zone is then conducted to a second hydrotreating stage. The catalyst for the second hydrotreating stage may the same or different from the catalyst for the first hydrotreating stage. The catalyst for the second hydrotreating stage may also include noble metal Group VIII, preferably Pt, Pd or mixtures thereof. The second stage hydrotreating conditions may be the same or different from the first stage hydrotreating conditions. The general hydrotreating condition ranges are the same for both hydrotreating stages. The present process allows for more severe reaction conditions in the second stage due to contaminant removal, especially nitrogen contaminants, as such contaminants are known to adversely affect maintenance of catalyst activity.

[0018] The conditions in the second stage may be adjusted according to product properties desired. For example, it is preferred to do deep hydrodesulfurization (deep HDS) in the second stage. In order to accomplish deep hydrodesulfurization to lower sulfur concentration to levels of less than 500 wppm sulfur, reaction conditions are adjusted according to the desired sulfur content of the product from the second stage hydrodesulfurization. The level of hydrodesulfurization (HDS) can be controlled by at least one of temperature, hydrogen pressure and space velocity. Deep hydrodesulfurization to achieve lower sulfur concentration in the final product is generally accomplished by adjusting pressures, temperatures and space velocities within the general hydrotreating ranges set forth above. The precise conditions of pressure, temperature and space velocity are determined by the final sulfur concentration desired in the diesel product. If the second stage contains a noble metal catalyst, it is preferred that the hydrogen used for hydrotreating contain amounts of sulfur and nitrogen which will not result in deactivation of the second stage catalyst and the product from the first stage hydrotreating be stripped to remove ammonia and hydrogen sulfide.

[0019] In the process, the feed is first hydrotreated by conventional hydrotreating. Here, the feed and hydrogen are heated and contacted with a hydrodesulfurization catalyst in a reactor. A heat exchanger is used to cool down the hydrodesulfurized feed, which then runs through a flash drum where the hydrogen is separated from the feed. The hydrogen is recycled back, preferably after passing through the H₂S scrubber. The hydrodesulfurized feed is heated by another heat exchanger and then runs through a stabilizer, where all light ends are removed. The hydrodesulfurized feed with light ends removed is cooled down by third heat exchanger. The hydrodesulfurized feed and sulfuric acid go into a drum where they are contacted with a nondispersive contactor and then separated. The acid is pumped out from the bottom of the drum, combined with make-up sulfuric acid and recycled back to the drum. The acid-washed hydrodesulfurized feed is preferably transferred to a settler to remove any acid carryover. The settler is optional and may be eliminated if desired. The acid- washed hydrodesulfurized feed then goes into another drum together with a caustic solution to neutralize any accompanying acid. The contacting with caustic solution is preferably done with a fiber film contactor. After separation, the caustic solution is pumped out from the bottom of the drum, combined with make-up caustic solution and recycled back to the drum. As before, the neutralized hydrodesulfurized feed is preferably (but not necessarily) transferred to a settler to remove any caustic solution carryover. After separation, the neutralized feed is cooled by a heat exchanger before going into the salt dryer, which removes any remaining water. The dried feed is hydrodesulfurized in a second stage with conventional hydrotreating, as described above or may be preferably deep hydrodesulfurized to lower sulfur to very low concentrations. A final diesel product may be obtained from the second stage.

[0020] The product from the second hydrotreating stage is a diesel fuel having reduced nitrogen and sulfur content. The amount of nitrogen in the diesel product from the second stage ranges from 0 to 1 wppm, based on diesel product. Sulfur concentrations in the product from the second stage are less than 500 wppm, preferably less than 50 wppm, more preferably less than 15 wppm, based on diesel product. The second stage product may be used as a diesel fuel, combined with additives or used as a blend stock.

[0021] The following non-limiting examples serve to illustrate the invention.

Example 1. HDS-Nitrogen Removal-HDS

[0022] Typical sulfur and nitrogen contents of a diesel feed requiring two hydrotreating stages are 1.67 wt.% S, and 714 wppm S, respectively, as shown in Table 1. Hydrotreatment of this feed at liquid hourly space velocity (LHSV) of 0.23 hr^"1, temperature of 338°C (640°F), hydrogen pressure of 3548 kPa (500 psig), 75% H₂, 303 m3/m³ (1700 scf/B) yielded a product containing 605 wppm S and 440 wppm N. Acid treatment with 96% sulfuric acid, at a 1.5 - 2.5 % treat rate using a nondispersive method produced a feed that contained 584 wppm S and 9.3 wppm N. After neutralization with a caustic wash, the neutralized feed was further hydrotreated at LHSV of 0.7 hr^"1, temperature of 335°C (635°F), 3617 kPa (510 psig), 100% H₂, 178 m³/m³ (1000 scf/b) to yield a product containing 15 wppm S. Without a nitrogen removal step, the stage 2 hydrotreatment required a higher pressure of 5203 kPa (740 psig) at the much lower space velocity of 0.25 hr^"1. Table 1 HDS-Nitrogen Removal-HDS

The conditions assumed are: 0.23 hr^"1, 500 psig, 75% H₂, 1700 scf/b, 640 °F. ^bThe conditions assumed are: 0.7 hr^"1, 635 °F, 510 psig, 100% H₂, 1000 scf/b.

Example 2. Nitrogen Removal-HDS-HDS

[0023] Figure 1 shows a plot of yield loss vs. product nitrogen obtained after acid treating the feed from Example 1. Thus, the feed that is treated with sulfuric acid at a 4% treat rate and at a yield loss of 2.7% will result in a 21 lwppm N feed. Because of the high treat rate and yield loss, some significant amount of sulfur is expected to be lost. In this example, a 15% sulfur yield loss is assumed, as this will account for approximately 1.7 wt.% of the total yield loss.

[0024] The 211 wppm N feed is hydrotreated at the same conditions used in reactor 1 of Example 1 : 0.23 hr^"1, 338°C 640°F, 3548 kPa (500 psig), 75% H₂, 303 m3/m³ (1700 scf/b). The literature relationship HDS oc [N]^{" 15} predicts that a 211 wppm N feed will yield an activity that is 1.2 times the activity of a 714 wppm N feed. The experimental relative catalyst activity (RCA) observed for the 714 wppm feed is 29.3. Thus, the predicted RCA for the 211 wppm N feed is 35.2 yielding a product containing 345 wppm sulfur. The product nitrogen, based on 1st order HDN is expected to be 131 wppm. This hydrotreated diesel feed is further hydroprocessed at the same conditions used in reactor 2 of example 1: 0.7 hr^"1, 335 °C (635°F), 3617 kPa (510 psig), 100% H₂, 178 m m³ (1000 scf/b).

[0025] The relationship of HDS oc [N]^{"0 15} predicts that a 131 wppm N feed will yield an activity that is 0.67 times the activity of a 9.3 wppm N feed. The experimental RCA observed for the 9.3 wppm feed is 43.2. Thus, the predicted RCA for the 131 wppm N feed is 28.9, which corresponds to a product sulfur of 14.1 wppm. This is the same sulfur level as that obtained for the HDS-nitrogen removal-HDS configuration, but at a significantly higher yield loss. Therefore, in order to obtain a significantly lower product S in a nitrogen removal-HDS-HDS configuration, an even higher yield loss would have to be taken. These results are summarized in Table 2.

Table 2 Nitrogen Removal - HDS - HDS

The conditions assumed are: 0.23 hr^"1, 500 psig, 75% H₂, 1700 scf/b, 640 °F. The conditions assumed are: 0.7 hr^"1, 635 , 510 psig, 100% H₂, 1000 scf/b. ^QAssuming approximately a 15 wt.% S loss. ^dPrediction based on shake tests.

Claims

CLAIMS:

1. The process for producing hydrotreated diesel fuels which comprises: (a) contacting a diesel feed in a first hydrotreating stage with a hydrotreating catalyst under hydrotreating conditions to produce a first hydrotreated diesel, (b) removing nitrogen contaminants from at least a portion of the first hydrotreated diesel by contacting the first hydrotreated diesel with a nitrogen removal agent comprising at least one of adsorption, solvent extraction, acid treating or ion exchange, and (c) contacting at least a portion of product from step (b) with a hydrotreating catalyst under hydrotreating conditions.

2. A process for producing hydrotreated diesel fuels which comprises: (a) contacting a diesel feed in a first hydrotreating stage with a hydrotreating catalyst under hydrotreating conditions to produce a first hydrotreated diesel, (b) removing nitrogen contaminants from at least a portion of the first hydrotreated diesel by contacting the first hydrotreated diesel with an acidic nitrogen removal agent, and (c) contacting at least a portion of product from step (b) with a hydrotreating catalyst under hydrotreating conditions.

3. The process of any preceding claim wherein the hydrotreating catalyst includes at least one Group VIB metal and at least one Group VIII metal on an inorganic refractory support material.

4. The process of any preceding claim wherein the Group VIB metal is at least one of Mo or W and the Group VIII metal is at least one of Ni or Co.

5. The process of any preceding claim wherein the hydrotreating conditions include temperatures of from 200 to 450°C, pressures of from 1480 to 20786 kPa, liquid hourly space velocities of 0.1 to 10 hr^"1, and hydrogen treat gas rates of from 38 to 1780 m³/m³.

6. The process of any preceding claim wherein the nitrogen removal agent is acid treating.

7. The process of any preceding claim wherein the acid is sulfuric acid.

8. The process of any preceding claim wherein hydrotreated product from step (c) has a sulfur concentration less that 500 wppm, based on product.

9. The process of any preceding claim wherein.the hydrotreated product from step (c) has a sulfur concentration less than 50 wppm.