FORM 2
THE PATENTS
ACT, 1970 (39 of 1970)
COMPLETE SPECIFICATION
(See Section 10; rule 13)
TITLE
PROCESS FOR PREPARING ULTRA LOW SULFUR DIESEL FUEL HAVING
IMPROVED COLOUR PROPERTIES
APPLICANT
SK CORPORATION
99 Seorin-dong
Jongro-ku Seoul 110-110
Korea Nationality: Korea
The following specification particularly describes
the nature of this invention and the manner in which it is to be performed

PROCESS FOR PREPARING ULTRA LOW SULFUR DIESEL FUEL HAVING
IMPROVED COLOR PROPERTIES
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for preparing ultra low sulfur diesel fuel having improved color properties using two-step hydro-catalytic reactions. More particularly, the present invention is directed to a process for preparing ultra low sulfur diesel fuel having a sulfur content of 10 ppm or less, capable of effectively suppressing an increase in sulfur content caused during hydrotreatment, which is carried out for improving the color properties of the product, subsequent to ultra deep hydrodesulfurization (HDS).
2. Description of the Related Art
In order to prevent the contamination of air environments and the development of global warming, various alternative energy sources have been developed. However, in the automotive fuel sector, gasoline and diesel fuels, which have been mainly consumed to date, are expected to continue to be used in the future. Moreover, the demand for high quality automotive fuels is increasing, as well.
Since the year 2000, desulfurization technologies containing advanced hydrodesulfurization(HDS) catalysts and novel techniques using adsorption or solvent extraction have been vividly developed for the production of ultra low sulfur diesel fuel having a sulfur content of 50/10 ppm or less. In this respect, most of petroleum refiners have preferred processes of maximally utilizing the already existing HDS
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equipments. Further, ultra low sulfur diesel fuel having the sulfur content of 50 ppm (or 10 ppm) or less will be widely employed both in most of developed countries and in Korea around 2005 through 2010. For example, the sulfur content for fuel marketed in the European Union in 2005 is regulated to 50 wppm or less. In addition, according to the U.S. Environmental Protection Agency (EPA) regulation scheduled to go into effect in June 1, 2006, the sulfur content in diesel fuel is required to decrease to 15 wppm or less. Moreover, a similar regulatory standard for low sulfur diesel fuel is also expected to take effect in many European countries and some Asian countries. Particularly, the standard for fuel of category 4 of WWFC (World Wide Fuel Charter) requires a sulfur content of 5-10 ppm. Therefore, a deep desulfurization for realizing standard of sulfur content of 10 ppm or less is becoming more urgently needed.
However, in the case where the conventional deep desulfurization is adopted, the color of diesel fuel inevitably deteriorates during the desulfurization. Although the USA and European countries take no notice of such color deterioration, research into methods of maintaining the color of ultra low sulfur diesel fuel to at least the present level has been additionally conducted, considering the color of diesel fuel has been strictly controlled for a long time in Japan and Korea. In this regard, the conventional techniques have proposed the control of reaction conditions of HDS or the introduction of a hydro treatment for color improvement as a subsequent step of HDS, in order to improve the color of diesel fuel. Such conventional techniques are as follows:
Japanese Patent Laid-open No. Hei. 6-100870 discloses a method of contacting diesel produced from petroleum, having 0.5% or less sulfur and a boiling point of 150-400°C, with hydrogen under conditions of a temperature of 200-300°C and pressure of 20-45 kg/cm in the presence of a hydrotreating catalyst, to achieve a Saybolt color index of-10 or more.
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U.S. Patent No. 5,316,658 discloses a two-step hydrotreating process using oil having 0.1-2.0 wt% sulfur and a boiling point of 150-400°C as feedstock, in which the first step functions mainly to decrease the sulfur content, and the hydrotreatment, as the second step, serves to improve the color properties.
U.S. Patent No. 6,264,827 discloses a two-step hydrotreating process using petroleum distillate having a cetane number, a sulfur content and a boiling point in specific ranges as feedstock, in which the first hydrotreatment functions to increase the cetane number via ring-opening by hydrogenation and decrease the sulfur content, and the second hydrotreatment acts to eliminate unstable materials from the first step (having a polycyclic aromatic structure) which cause the color deterioration of diesel and the generation of sludge. That is, in the above two-step hydrotreating process, the first hydrotreatment is intended to increase the cetane number of oil to at least 45 and decrease the sulfur content to 350 ppm, and the second hydrotreatment serves to increase the storage stability of oil without changes in the cetane number and in the sulfur content realized in the first hydrotreatment.
However, the three above-mentioned two-step processes are problematic in getting simultaneously the specification of sulfur content requiring l0ppm or less and its color properties because the reaction conditions (in particular, liquid hourly space velocity (LHSV)) of the first step of HDS actually fall outside of the operation window for ultra deep desulfurization of 10 ppm or less. As well, in the above prior arts, the possibility of exceeding the sulfur standard (that is, 10 ppm) due to the increase in the sulfur content caused by the recombination of sulfur occurring upon the second step of hydrotreatment, which is intimately related to the first ultra deep desulfurization conditions as mentioned below, is never discussed, or no attempts to solve such a problem have been made. That is, the conventional two-step process, comprising
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hydrodesulfiirization and hydrotreatment, target the preparation of diesel fuel having the sulfur content of hundreds of ppm, where an increase in sulfur content by ones of ppm is not considered problematic in improving color properties. However, it is regarded as an significant problem in the ultra deep desulfurization technique involving strict control of the sulfur content to 10 ppm or less. Therefore, there is an increasing demand for the preparation process of ultra low sulfur diesel fuel, capable of greatly improving color properties while controlling the sulfur content to 10 ppm or less, and utilizing the already existing deep desulfurization equipments to the fullest.
SUMMARY OF THE INVENTION
Leading to the present invention, intensive and thorough research into preparations of ultra low sulfur diesel fuel, carried out by the present inventors aiming to avoid the problems encountered in the related prior arts, led to the specific operation window required for the process of preparing ultra low sulfur diesel fuel, capable of improving the color properties while utilizing the already existing deep desulfurization equipments to the fullest, by solving the problem in which the standard for sulfur content of 10 ppm or less is not realized due to the increase in the sulfur content caused by the recombination of sulfur upon the second step of hydrotreatment of the conventional two-step process (involving hydrodesulfurization followed by hydrotreatment without separation of H2S).
Accordingly, an object of the present invention is to provide a process for preparing ultra low sulfur diesel fuel having the sulfur content of 10 ppm or less and improved color properties.
Another object of the present invention is to provide a process for preparing
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ultra low sulfur diesel fuel, which is conducted under operation conditions capable of simultaneously achieving both color improvement and deep desulfurization performance while maximally utilizing the already existing deep desulfurization equipments.
In order to accomplish the above objects, the present invention provides a process for preparing ultra low sulfur diesel fuel having improved color properties, comprising steps of:
(a) subjecting hydrocarbon oil having a boiling point of 200-400°C to deep HDS under conditions of reaction pressure of 40-80 kg/cm2 , a reaction temperature of 330-380°C, and LHSV of 0.1-2.0 hr-1 in the presence of a catalyst, to obtain a product of HDS; and
(b) subjecting the product of HDS to a hydrotreatment under conditions of reaction pressure of 40-80 kg/ cm2 , a reaction temperature of 230-320°C, and LHSV of 4-10 hr-1 in the presence of a catalyst,
in which the operation conditions of said steps (a) and (b) to obtain the diesel fuel having a sulfur content of 10 ppm or less and a Saybolt color index of 0 or more are that (i) in said step (a), when the reaction temperature is 360°C or less, LHSV is set within a range sufficient for decreasing the sulfur content in the product of HDS to 10 ppm or less; or (ii) in step (a), when the reaction temperature is higher than 360°C, LHSV is set within a range sufficient for decreasing the sulfur content in the product of HDS to 10 ppm or less, and in said step (b), the reaction temperature is further limited to 280-320°C.
BRIEF DESCRIPTION OF THE DRAWINGS
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FIG. 1 schematically illustrates a process for preparing ultra low sulfur diesel fuel, according to the present invention;
FIG. 2 illustrates one of well-known assumption related to the reason why the color of a product of deep desulfurization is deteriorated;
FIG. 3 illustrates the sulfur content (ppm), a Saybolt color index, and an A-value of the product of HDS, varying with LHSV at the fixed reaction temperature (370°C), in Example 1 of the present invention;
FIG. 4 illustrates the sulfur content (ppm) of the product of HDS as a first step and the product of a hydrotreatment as a second step, varying with LHSV of HDS at the fixed reaction temperature (370°C), in Example 2 of the present invention;
FIG. 5 illustrates the sulfur content (ppm) of the product of HDS as the first step and the product of a hydrotreatment as the second step, varying with the temperature of HDS at the fixed LHSV (0.94 hr-1), in Example 2 of the present invention;
FIG. 6 illustrates the results of chromatography of sulfur compounds contained in three samples (360 R2-1, 385 Rl-1, and 385 R2-1), in Example 3 of the present invention;
FIG. 7 illustrates the relationship between LHSV and temperature which enables to obtain the operation window required to achieve a sulfur content of 10 ppm or less upon HDS as the first step, depending on the degree of recombination of sulfur caused during the hydrotreatment as the second step, in a two-step process for preparing ultra low sulfur diesel fuel comprising HDS and a hydrotreatment; and
FIGS. 8 and 9 illustrate the Saybolt color index and the degree of color improvement, and the sulfur content of the product of HDS and the product of a hydrotreatment, respectively, varying with the temperature of the hydrotreatment for
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color improvement of the product obtained under the conditions (LHSV: 0.94 hr-1, and reaction temperature: 370°C) of deep desulfurization, in Example 4 according to a two-step process for preparing ultra low sulfur diesel fuel (sulfur content: 10 ppm or less) comprising hydrodesulfurization and hydrotreatment.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, a detailed description will be given of the present invention in conjunction with the accompanying drawings.
In the present invention, a process in which a hydrotreating or hydrofinishing reactor is directly connected to a reactor for deep desulfurization, without separation of H2S, is applied in order to improve the color qualities, in particular, a Saybolt color index of diesel fuel, and preferably to exhibit no fluorescent color.
FIG. 1 schematically illustrates the embodiment of the present process.
As shown in this drawing, as feedstock 1, hydrocarbon oil having a boiling point of 200-400°C, and more typically 220-380°C, is used. Such hydrocarbon oil includes various oil fractions discharged from petroleum refining processes, for example, straight-run gas oil, pyrolytic oil, catalytic cracking oil, etc. Preferably, straight-run gas oil is used. Typically, the above-mentioned feedstock contains sulfur in an amount of 2.0 wt% or less and nitrogen in an amount of 400 wppm or less.
The feedstock 1 is transferred into a first reactor 11 (ultra deep HDS reactor) to undergo deep desulfurization in the presence of a catalyst and hydrogen. Subsequently, the product 2 resulting from the deep desulfurization is transferred into a hydrotreating reactor 13 for a second step, without separation of H2S generated by the desulfurization. As such, the product 2 is preferably cooled using a heat exchanger 12.
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This is because a subsequent hydrotreatment is conducted at a lower temperature than is the deep desulfurization. Thus, in the above drawing, a flow 3 of the cooled product of the first step serves as an inlet flow 3 of the hydrotreatment of the second step. In this regard, the first step is chiefly used to decrease the sulfur content to a level of deep desulfurization, whereas the hydrotreatment as the second step is mainly used to improve color properties. Specifically, since the deep desulfurization is carried out under severe conditions such as a high temperature and a long retention time in a reactor, the color properties of the resulting diesel fuel product are drastically deteriorated. That is, as shown in FIG. 2, during the desulfurization, an alkyl-dibenzothiophene compound contained in the feedstock would be converted into stabilized radicals, which are then polymerized to produce small amount of polycyclic aromatic products, undesirably deteriorating the color properties (in particular, Saybolt color index). Hence, the subsequent hydrotreatment is conducted to solve the problem of color deterioration, and in particular, this step is performed without the separation of H2S from the desulfurized product in terms of economic benefits.
However, when the operation conditions of the HDS reactor are very severe for deep desulfurization, even though the sulfur content satisfies the standard of 10 ppm or less through the deep desulfurization, it is increased again during the hydrotreatment and thus falls outside of the above standard. According to the analysis of the reaction product, it is confirmed that new sulfur compounds different from the type of sulfur compounds remaining after the deep desulfurization have been formed during the hydrotreatment. This phenomenon is presumed to occur due to the recombination of H2S compounds that are fed into the hydrotreating reactor along with the first HDS product.
According to the present invention, an oil fraction 4 obtained via the two-step
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process has 10 ppm or less sulfur and a Saybolt color index of 0 or more (preferably, 10 or more). In addition, the A-value is not decreased upon the hydrotreatment as the second step. As such, a Saybolt color index showing transparency is determined according to ASTM D-4625, and the A-value showing a fluorescent color index refers to the value a among the values of L/a/b obtained using a Minolta colorimeter. The A-value of the present invention is preferably in the range from about -12 to 0.
The present invention is based on the unexpected finding that an increase in sulfur content upon the hydrotreatment subsequent to the deep desulfurization is intimately related to the conditions of the deep desulfurization. That is, the recombination of sulfur is believed to originate from the undesirable reactive hydrocarbons formed under the conditions of the deep desulfurization. Therefore, this problem is overcome by specifically setting the reaction conditions of the deep desulfurization and the hydrotreatment. Consequently, in the present invention, a typical two-step process scheme is adopted, and the reaction conditions are specifically limited at each step to achieve color improvement while realizing the intended sulfur-content standard in the final product.
According to the present invention, the first step of HDS is conducted under the conditions of pressure of about 40-80 kg/cm , and preferably about 50-70 kg/cm2 , a temperature (that is, weight average bed temperature: WABT) of about 330-380°C, and preferably 350-370°C, and LHSV of about 0.1-2.0 hr-1, preferably about 0.3-1.5 hr-1 and more preferably about 0.3-1.0 hr-1 in the presence of a catalyst. In addition, the H2/oil ratio is about 150-1000 Nm3Vkl, and preferably about 200-500 Nm3/kl.
The HDS is carried out in the presence of a catalyst typically used for deep desulfurization. Such a catalyst includes a first metal component such as Mo and a second metal component selected from among Co, Ni, W and combinations thereof on
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a porous refractory support (e.g., γ-alumina, silica, zeolite, and combinations thereof). Based on the total weight of the catalyst, the first metal component is used in an amount of about 10-30 wt%, and preferably about 12-20 wt%, and the second metal component is used in an amount of about 2-10 wt%, and preferably about 2-7 wt%. The catalyst suitable for deep desulfiirization is a CoMo-based catalyst (e.g., the molar ratio of Co/[Co+Mo] is about 0.1-0.5). In addition, the catalyst may have the shape of a particle, cylinder, tablet, etc., as known in the art, and the type of reactor is exemplified by a fixed-bed reactor or a fluidized-bed reactor. The product of HDS typically has a Saybolt color index of about -40 to -20 and an A-value of about +20 to -20.
In the present invention, the second step of hydrotreatment is conducted under
the conditions of pressure of about 40-80 kg/cm2 9 and preferably about 50-70 kg/cm2 , a temperature of about 230-320°C, and preferably 280-320°C, and LHSV of about 4-10 hr-1, and preferably about 4-7 hr-1 in the presence of a catalyst. In addition, the H2/01I ratio is about 150-1000 Nm3/kl, and preferably about 200-350 Nm3/kl. The catalyst, the shape of catalyst and the type of reactor used for the hydrotreatment are similar to those used in the HDS process mentioned above. In the hydrotreatment, a NiMo-based catalyst (e.g., the molar ratio of Ni/[Ni+Mo] is about 0.1-0.5) is preferably used.
The most important characteristic of the present invention is to assure an operation window capable of realizing the color improvement while effectively suppressing the increase in sulfur content upon the hydrotreatment as the subsequent step related to the conditions of deep desulfurization, in addition to the above-mentioned basic process conditions. Therefore, the present invention proposes two approaches to solve the recombination phenomenon of sulfur caused upon the hydrotreatment.
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In the case of the typical desulfurization of gas oil in the presence of the above catalyst, the conditions of deep desulfurization to the sulfur content of 10 ppm or less have the reaction temperature and LHSV in the reactor as determined by Equation 1 below:
Equation 1
Reaction Temperature (BAT, @10 ppm) = A x Ln (LHSV) + B
As such, the correlation coefficients A and B depend on the catalyst, the pressure and the F^/oil ratio. Based on the CoMo catalyst used in the following examples of the present invention, when the partial pressure of hydrogen (reaction pressure) and the H2/oil ratio are set to 52 kg/cm2 and 300 Nm3 /kl, respectively, A = 17.156 and B = 367.84 are applied, thus obtaining the operation window for deep HDS shown in FIG. 7. Although the operation window may vary slightly with parameters such as the partial pressure of hydrogen, the H2/oil ratio, and the properties of feedstock, it may be applied without special problems under the basic process conditions as mentioned above.
The deep desulfurization characteristics of FIG. 7 are obtained in consideration of the degree of generation of reactive hydrocarbons causing the recombination of sulfur upon the subsequent hydrotreatment. Accordingly, (i) when the deep desulfurization is conducted in the stable ultra deep desulfurization region (I), the generation of reactive hydrocarbon is suppressed and the intended standard for sulfur content is met without further limitation of hydrotreatment conditions. Further, a Saybolt color index may be improved to 10 or more. To this end, the temperature of deep HDS should be decreased to 360°C or less. In addition, in order to realize the standard for sulfur content of 10 ppm or less, LHSV should be appropriately adjusted depending on the decreased reaction temperature. As in FIG. 7, in the case of (i),
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LHSV of deep desulfurization is preferably in the range of about 0.3-0.7 hr-1. Hence, it is preferred to avoid any circumstances where the reaction temperature should be increased to exceed 360°C due to high LHSV based on the borderline of 10 ppm of FIG. 7 or due to the desulfurization that is difficult to conduct for particular reasons such as aging of the catalyst or change in properties of feedstock even though LHSV is sufficiently low.
However, unavoidable circumstances may inappropriately occur under the conditions of deep desulfurization as the first step. Thus, in the present invention, in the case (ii) where the deep desulfurization is conducted at a reaction temperature higher than 360°C, allowing the recombination of sulfur. Then, the conditions of the subsequent hydrotreatment are further limited. Thereby, even though the color improvement is slightly sacrificed, the problem of falling outside of the standard for sulfur content due to the recombination of sulfur may be solved. Referring to the regions depicted in FIG. 7, the case of (ii) satisfies the sulfur standard of 10 ppm or less of II (mild recombination region), III (severe recombination region) and IV (unstable ultra deep desulfurization region). Preferably, the operation windows corresponding to the regions II and III are selected, and more preferably the region II is selected. As such, it is preferred that LHSV be sufficiently adjusted in consideration of the reaction temperature to meet the standard for sulfur content of 10 ppm or less. In FIG. 7, in the case of (ii), LHSV of deep HDS is preferably in the range of about 0.7-1.2 hr-1.
According to the experiments by the present inventors, the color improvement (that is, de-coloring) performance of the hydrotreating or hydro finishing is increased in proportion to the decrease in both LHSV and temperature. On the other hand, the degree of recombination of sulfur in the level of sulfur content of about 5-15 ppm is
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increased as the temperature is lowered. Hence, when the production of reactive hydrocarbon is unavoidable under the conditions of deep HDS, even if color improvement is slightly sacrificed, the hydrotreatment is conducted with the adjustment of the temperature of the hydrotreating reactor to about 280-320°C, and preferably about 280-300°C. Thereby, the production of products exceeding the standard of 10 ppm may be suppressed while maintaining desired color properties. In particular, when the hydrotreatment temperature does not exceed 300°C in the case of (ii), a Saybolt color index of 10 or more may be achieved. Further, even though the temperature is increased to 320°C, a Saybolt color index of 0 or more may be obtained.
A better understanding of the present invention may be obtained in light of the following examples which are set forth to illustrate, but are not to be construed to limit the present invention.
The sample used in Examples is straight-run gas oil, the properties of which are as shown in Table 1 below. The reaction experiment was conducted in a manner such that two reactors respectively charged with a deep HDS catalyst and a de-coloring catalyst were connected to each other and then the deep desulfurization performance and color improvement were measured depending on changes in LHSV and temperature under conditions of continuous flow. The corresponding catalyst and experimental conditions are set forth in Tables 2a and 2b below.



EXAMPLE 1
The sulfur content and color index of the product of deep HDS varying with LHSV at a fixed temperature of 370°C are given in FIG. 3. As shown in this drawing, as LHSV is decreased, the sulfur content in the product is reduced to 5 ppm or less but the color properties are further deteriorated.
EXAMPLE 2
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The changes in the sulfur content before and after the hydrotreatment (LHSV: 5, and reaction temperature: 260°) for color improvement, depending on the conditions of deep HDS, were compared. FIG. 4 shows the sulfur content before and after the hydrotreatment, varying with LHSV of the desulfurization at the fixed temperature (370°C). In addition, FIG. 5 shows the sulfur content before and after the hydrotreatment, varying with the reaction temperature of the HDS at the fixed LHSV (0.94 hr-1).
From FIG. 4, as LHSV among the conditions of the HDS is decreased, the sulfur content in the flow to be fed into the hydrotreating reactor is reduced. However, the sulfur content is increased again during the hydrotreatment. When the hydrotreatment is applied to improve the color properties, it is confirmed that the sulfur content in the final product is hardly affected by the changes in LHSV of HDS.
According to FIG. 55 when the reaction temperature among the conditions of deep HDS is increased to 370°C, the sulfur content is greatly decreased. However, the sulfur content seldom changes at the higher temperature. This phenomenon has been already known in the field of deep HDS. In this regard, in the case of conducting the hydrotreatment subsequent to the HDS at 370°c or more, it should be noted that the sulfur content of 10 ppm or less is quite difficult to achieve.
Taking into account the above results, the hydrotreatment is greatly affected by specific process conditions, in particular, temperature and LHSV, of deep HDS. In the case of directly connecting the deep HDS (the first step) to the hydrotreatment for color improvement (the second step), the production of ultra low sulfur diesel fuel of 10 ppm or less is confirmed to be difficult.
In this way, compared to the inlet flow (that is, feedstock of hydrotreatment) into the hydrotreatment directly connected to the deep HDS in the deep HDS region of
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10 ppm or less, the outlet flow (that is, hydrotreated product) from the hydrotreatment has the higher sulfur content, and the reason therefor should be examined.
EXAMPLE 3
By observing whether any type of sulfur compound is increased during the hydrotreatment, whether increase in sulfur compound is based on measurement error or generation of another sulfur compounds can be examined. To this end, the analysis of sulfur species gas chromatography is conducted, and chromatograms for three samples are shown in FIG. 6. As can be seen in FIG. 6, it is confirmed that the increase of sulfur content during the hydrotreatment stems from the generation of new sulfur compounds, rather than measurement error.
The chromatogram of a 360 R2-1 (S: 22 ppm) sample subjected to desulfurization at 360°C and then hydrotreatment corresponds to a chromatogram of sulfur compounds in a conventionally desulfurized product. Those indicated by Peaks C and D are 4-methyl dibenzothiophene and 4,6-dimethyl dibenzothiophene as refractory sulfur compounds.
A 385 Rl-1 (S: 7 ppm) sample corresponds to the feedstock of the hydrotreatment following the desulfurization at 385°C. As with this sample, almost none of the peaks of sulfur compounds shown for the 360 R2-1 are seen, and only small peaks similar to noise are confirmed. However, in the case of 385 R2-1 Cau (S: 12 ppm) subjected to the hydrotreatment, new peaks A, B and E are detected at the positions different from peaks shown for 360 R2-1 or 385 Rl-1.
Thus, without the separation of H2S, in the case where the desulfurized product is directly connected to the subsequent hydrotreatment, although trivial side-reactions, causing the sulfur content to increase by 3-7 ppm compared to the feedstock of the
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conventional color improvement process, do not generate significant problems in satisfying the standard of the level of hundreds of ppm, they are confirmed to entail fatal problems in the ultra deep HDS region requiring the sulfur content of 10 ppm or less.
Selection of Optimal Operation Conditions
The results of Examples 1 to 3 are compared with LHSV and reaction temperature enabling the formation of the sulfur content of 10 ppm or less under typical conditions (partial pressure of hydrogen: 52 kg/cm2, and the H2/01I ratio: 300 Nm3/kl) of deep desulfurization of gas oil using a CoMo-based catalyst. Thereby, as shown in FIG. 7, stable operation regions able to produce ultra low sulfur diesel fuel having high quality color can be obtained via a two-step process including deep HDS and a hydrotreatment.
As mentioned above, the region I corresponding to the stable ultra deep HDS functions to effectively suppress the recombination of sulfur due to the presence of reactive hydrocarbon during the subsequent hydrotreatment, and thus may confer somewhat flexibility to the hydrotreatment. However, there may be circumstances in which the operation cannot but be conducted in only the regions II to IV, for example, circumstances in which the deep desulfurization cannot but be conducted in the region causing the recombination of sulfur (for example, due to the long operation period of time). In such cases, when the hydrotreating temperature is increased, the side-reactions increasing the sulfur content in the diesel fuel may be lowered, while the color improvements may be slightly sacrificed.
EXAMPLE 4
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The color improvement effect and the changes in the sulfur content of the product obtained under the conditions of deep desulfurization (LHSV: 0.94 hr-1 reaction temp.: 370°C) were observed while varying the temperature among the conditions of hydrotreatment.
As discussed above, although LHSV affects the color improvement during the hydrotreatment, it seldom affects the sulfur content in the product. Thus, the influence of hydrotreatment temperature on the sulfur content was evaluated. As such, LHSV of the hydrotreatment is 5 hr-1. The results are given in Table 3 below and FIG. 8.

As is apparent from Table 3, an increase in hydrotreating temperature exceeding 260°C may be provided as another means for preventing a phenomenon falling outside of the intended sulfur standard upon the production of ultra low sulfur diesel fuel of 10 ppm or less. However, color improvement based on a Saybolt color index decreases in proportion to the increase in the temperature.
In this regard, as shown in FIGS. 8 and 9, the hydrotreating temperature required to suppress the increase in the sulfur content upon the hydrotreatment while maintaining a Saybolt color index of 10 or more under the conditions of deep HDS as mentioned above is confirmed to range from about 280-300°C, and the temperature capable of maintaining a Saybolt color index of 0 or more is determined to be in the range of about 280-320°C.
As described hereinbefore, the present invention provides a method of
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preparing ultra low sulfur diesel fuel having improved color properties. Although the conventional two-step process may improve, through hydrotreatment, the lowered color properties of a diesel originating from ultra deep desulfurization for the sulfur content of 10 ppm or less to the considerable extent, it undesirably causes a problem of falling outside of the 10 ppm standard for sulfur content due to side-reactions upon the hydrotreatment. However, the diesel fuel preparation process of the present invention can effectively solve the above problem. In particular, since the present process involves specific adjustment of the operation window of each step, both color improvement and ultra deep desulfurization can be simultaneously achieved while maximally utilizing the already existing equipments, thus generating economic benefits.
Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.