Production of Low Color Middle Distillate Fuels

CATEGORY: DIESEL

PATENT

Production of Low Color Middle Distillate Fuels

United States Patent Application 20120000817

Inventors: Shih, Stuart S. (Gainesville, VA, US)

Application Number: 13/169170

Publication Date: 01/05/2012

Assignee: ExxonMobil Research and Engineering Company (Annandale, NJ, US)

Abstract:

In a process for producing a low color diesel and/or kerosene fuel, a middle distillate feed can be supplied to a reactor having at least one first catalyst bed containing a first desulfurization and/or isomerization catalyst and at least one second catalyst bed containing a decolorization catalyst downstream from the first catalyst bed(s). The feed can be reacted with the hydrogen in the presence of the first catalyst at a temperature from about 290° C. to about 430° C. to produce a first liquid effluent, which can be cooled by about 10° C. to about 40° C. with a quench medium and cascaded to the at least one second catalyst bed. The cooled first liquid effluent can then be reacted with hydrogen in the presence of the decolorization catalyst at a temperature from about 280° C. to about 415° C. to produce a second effluent having an ASTM color less than 2.5.

FIELD OF THE INVENTION

This invention relates to a process for reducing the color level of middle distillate fuels, such as diesel fuel and kerosene.

BACKGROUND OF THE INVENTION

Color, sulfur level and cloud point are important product specifications for diesel fuel and kerosene. For example, changes in the U.S. regulatory system in 2006 reduced the maximum sulfur levels in diesel fuels from 500 wppm (S500) to 15 wppm (S15). Similarly, the color specification required for most refinery diesel pools is less than 2.5 ASTM color units, which means that many refineries limit the color for desulfurized diesel to less than about 2.0 ASTM color units. In addition, depending on season and geography, diesel fuels and kerosene may be required to have a cloud point of less than −27° C.

Generally, a multi-stage hydrotreating process is used to reduce the sulfur and color levels of diesel fuel to the required values. For example, U.S. Pat. No. 6,103,104 discloses a process in which a middle distillate petroleum stream is hydroprocessed in two or more first temperature stages operated at a temperature from about 360° C. to about 450° C. The reaction product of the first temperature stage(s) is(are) then quenched to a temperature from about 260° C. to about 350° C., stripped of H2S, NH3 and other dissolved gases, then sent to the second temperature stage which is operated at the quenched temperature range, whereby color bodies produced in the higher temperature first stage are hydrogenated in the second stage.

Chinese Published Patent Application No. CN 1824736 discloses a method for preparing diesel oil with improved color property and ultra-low sulfur content by deep hydrodesulfurization and hydrotreating comprising (1) carrying out deep hydrodesulfurization to produce a hydrocarbon oil with a boiling point of 200-400° C. in the presence of a catalyst (including CoMo as active metal) at 330-380° C. under 40-48 kg/cm2 with a liquid hourly space velocity (LHSV) of 0.1-2.0 hr−1, and (2) hydrotreating in the presence of catalyst (including NiMo as active metal) at 230-320° C. under 40-80 kg/cm2 with a LHSV of 4-10 hr−1. The resultant diesel oil has a sulfur content not more than 10 ppm and a Saybolt color index not less than 10.

U.S. Pat. No. 6,652,735 discloses a process for isomerization dewaxing of a hydrocarbon feed, such as a diesel oil, which includes contacting the hydrocarbon feed with a large pore size, small crystal size, crystalline molecular sieve, such as zeolite beta, and an intermediate pore size, small crystal size, crystalline molecular sieve, such as ZSM-23 or ZSM-48, to produce a dewaxed product with a reduced pour point and a reduced cloud point.

To date, the chemistry involved in the formation of color bodies in diesel fuels has not been well understood, although it is known to be generally related to prior desulfurization and/or isomerization at high temperature. As a result, the maximum operating temperature for many desulfurization and isomerization units is limited by the color specification of the product rather than by catalyst activity, thereby limiting cycle length.

SUMMARY OF THE INVENTION

It has been discovered that the color of diesel fuel and kerosene can be specifically dependent on the 3+ ring aromatic content of the fuel. Based on this knowledge, a new process for producing low color fuels has been developed in which an additional catalyst bed can be added to the desulfurization/isomerization reactor for color control or the last bed in the desulfurization/isomerization reactor is converted for color control. This final bed can be operated at a temperature close to, but lower than, that of the preceding catalyst beds in the reactor. Either a recycle gas or the liquid diesel product can be used to quench the effluent from the desulfurization/isomerization bed, so that the color control bed can be operated at the lower temperature. This color control catalyst bed can re-saturate at least some of the 3+ ring aromatics generated by the desulfurization/isomerization catalyst beds. With such an arrangement, the main desulfurization/isomerization bed(s) can be operated over a broader temperature range as the catalyst ages, thereby prolonging the catalyst cycle length while simultaneously meeting the color specification.

One aspect of the invention relates to a process for producing a diesel and/or kerosene fuel of low color, the process comprising: (a) supplying a middle distillate feed having a first cloud point and hydrogen to a reactor having at least one first catalyst bed containing a first desulfurization and/or isomerization catalyst and at least one second catalyst bed downstream of the first catalyst bed(s) and containing a decolorization catalyst; (b) reacting the feed with the hydrogen in the presence of the first catalyst in said at least one first catalyst bed at a temperature from about 290° C. to about 430° C. to produce a first liquid effluent having a second cloud point; (c) cooling the first liquid effluent by about 10° C. to about 40° C. with a quench medium; (d) cascading the entire cooled first liquid effluent to said at least one second catalyst bed; and (e) contacting the cooled first liquid effluent with hydrogen in the presence of said decolorization catalyst at a temperature of about 280° C. to about 415° C. to produce a second effluent having an ASTM color of less than 2.5 and a third cloud point.

Another aspect of the invention relates to a process for producing a low color and low sulfur diesel and/or kerosene fuel, the process comprising: (a) supplying a hydrogen and middle distillate feed having a first cloud point and containing at least 0.03 wt % sulfur to a reactor having at least one first catalyst bed comprising a first desulfurization catalyst and at least one second catalyst bed downstream of the first catalyst bed(s) and comprising a decolorization catalyst; (b) reacting the feed with hydrogen in the presence of the first catalyst in said at least one first catalyst bed at a temperature from about 315° C. to about 430° C. to produce a first liquid effluent comprising 15 wppm or less sulfur and having a second cloud point; (c) cooling the first liquid effluent by about 15° C. to about 35° C. with a quench medium; (d) cascading the entire cooled first liquid effluent to said at least one second catalyst bed; and (e) contacting the cooled first liquid effluent with hydrogen in the presence of said decolorization catalyst at a temperature from about 300° C. to about 415° C. to produce a second effluent comprising 15 wppm or less sulfur, having a third cloud point, and having an ASTM color of less than 2.5.

Still another aspect of the invention relates to a process for producing a low color diesel and/or kerosene fuel having a low cloud point, the process comprising: (a) supplying a middle distillate feed having a first cloud point and hydrogen to a reactor having at least one first catalyst bed comprising a first isomerization catalyst and at least one second catalyst bed downstream of the first catalyst bed(s) and comprising a decolorization catalyst; (b) reacting the feed with hydrogen in the presence of the first catalyst in said at least one first catalyst bed at a temperature from about 285° C. to about 420° C. to produce a first liquid effluent having a second cloud point that is at least 10° C. less than that of the first cloud point; (c) cooling the first liquid effluent by about 10° C. to about 20° C. with a quench medium; (d) cascading the entire cooled first liquid effluent to said at least one second catalyst bed; and (e) contacting the cooled first liquid effluent with hydrogen in the presence of said decolorization catalyst at a temperature from about 275° C. to about 400° C. to produce a second effluent having a third cloud point that is at least 10° C. less than that of the first cloud point and having an ASTM color of less than 2.5.

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