PH12018000210A1 - A method for producing low sulfur gasoline from straight run naphtha containing sulfur - Google Patents

Abstract

A method for producing low sulfur gasoline from straight run naphtha containing sulfur, comprising the following steps: contacting the straight run naphtha containing sulfur with an inorganic alkaline solution for alkaline wash to remove hydrogen sulfide and most of the mercaptan in the naphtha. The naphtha after the alkaline wash is washed with water and then contacts an upgrading catalyst. The upgrading reaction is carried out under conditions of 200 to 500 oC and 0.2 to 3.0 MPa to obtain a gasoline having a Research Octane Number of 90 or more and a sulfur content of less than 10 ug/g

Description

desulfurizationby alkaline wash and water wash in Step (2) is used as a feed subjected toan upgrading reaction which produces a dry gas, a liquefied petroleum gas and a gasoline product. Since the thioether comprised in the naphtha after desulfurization by alkaline wash is decomposed into hydrogen sulfide in the upgrading reaction and hydrogen sulfide is discharged together with the dry gas, the sulfur content in the upgraded gasoline is less than 10 ug/g.

The upgrading catalyst in Step (3) preferably comprises 10 to 90% by weight of

HZSM-5 molecular sieve and 10 to 90% by weight of alumina, and more preferably comprises 20 to 80% by weight of HZSM-5 molecular sieve and 20 to 80% by weight of alumina.

The above upgrading catalyst may further comprise an oxide of a rare earth element. Preferably, the upgrading catalyst comprises a carrier and 0.1 to 5% by weightoxide of a rare earth element supported on the carrier, calculated on the basis of the carrier, wherein the carrier comprises 10 to 90% by weight of HZSM-5 molecular sieve and 10 to 90% by weight of alumina, and more preferably comprises20 to 80% by weight of HZSM-5 molecular sieve and 20 to 80% by weight of alumina. The content of the oxide of a rare earth element is preferably 0.5 to 2% by weight.

The rare earth element is selected from at least one of a group consisting of La, Ce,

Pr, Nd and Sm. The rare earth element may also be selected from a mixed rare earth.

The molar ratio of silica/alumina of the HZSM-5 molecular sieve in the above catalyst is preferably from 10 to 100, more preferably from 20 to 80.

The above catalyst is prepared by the following method: mixing the HZSM-5 molecular sieve with alumina or alumina precursor, and then adding an acid solution for peptization followed by molding. The molding method can be an extrusion molding or a ball drip molding. After molding, the solid is dried and calcined to obtain a catalyst. Saidacid is preferably nitric acid; the drying temperature for the solid after molding is preferably 100 to 150 °C; and the calcination temperature is preferably 400 to 700°C.

The method for the preparation of the catalyst containing the oxide of a rare earth element is: a carrier which is a catalyst comprising 10 to 90% by weight of

HZSM-5 molecular sieve and 10 to 90% by weight of alumina, more preferably comprising20 to 80% by weight of HZSM-5 molecular sieve and 20 to 80% by weight of aluminais impregnated with a solution of a water-soluble rare earth compound and then dried and calcined. The drying temperature is preferably 100 to 150 °C, and the calcination temperature is preferably 400 to 700 °C. The rare earth compound used for the preparation of the impregnation solution is preferably a nitrate of a rare earth element.

The upgrading reaction of the naphtha in Step (3) may be carried out under hydrogenation or non-hydrogenation conditions. When the upgrading reaction of the naphtha is carried out without presence of hydrogen, the reaction temperature is preferably 250 to 450 °C, the pressure is preferably 0.3 to 1.5 MPa, and the weight hourly space velocity of the feedis 0.1 to 2.0 h™!, preferably 0.3 to 1.0 h'.

When the upgrading reaction of the naphtha in Step (3) is carried out in presence of hydrogen, the reaction temperature is preferably 250 to 450 °C, the pressure is preferably 0.3 to 1.5 MPa, and the weight hourly space velocity of the feed is 0.1 to 2.0 hl, preferably 0.3 to 1.0 h™'. The hydrogen/hydrocarbon molar ratio is from 0.1 to 2.0, preferably from 0.3 to 1.5.

The straight-run naphtha of the present invention has a boilingrange of 30 to 185 °C, preferably 35 to 160 °C, wherein the sulfur content is 100 to 1000 pg/g, preferably 100 to 500 ng/g.

The present invention is further described by the following examples. However, the present invention is not limited thereto.

Example 1

Preparation of an upgrading catalyst 80g of HZSM-5 having a silica/alumina molar ratio of 50 and 34g of pseudo-boehmite powder (manufactured by company SASOL with an alumina content of 75% by weight) were uniformly mixed. Then, 34.2g of a nitric acid solution having a concentration of 10% by weight was added. The mixture was molded by extruding, dried at 120°C for 24 hours, and calcined at 550°C for 4 hours to obtain an upgrading catalyst a. Catalyst ahas 75% by weight of HZSM-5 and 25% by weight of y-ALO;.

Example 2

Preparation of an upgrading catalyst 100g of the catalyst prepared according to the method of Example 1 was used as a carrier which was impregnated with 95g of an aqueous solution of Ce(NOs); having a concentration of 3% by weight. The impregnation solution/solid weight ratio was 0.95. After impregnation, the solid was dried at 120°C for 24 hours and calcinated at 550°C for 4 hours to produce an upgrading catalyst b. Catalyst bhas 1.5% by weight of CeO,, calculated on the basis of the carrier.

Example 3

In the following examples,the naphtha is upgraded in accordance with the method of the present invention. (1) alkaline wash 100g of straight-run naphtha A (the composition and properties thereof are shown in Table 1) was added with 200g of an aqueous solution of NaOH with a concentration of 20% by weight and rested for 30 minutes after being uniformly mixed. When the two liquids were stratified, the upper layer of naphtha was separated to produce the naphtha washed with alkaline. The compositionsof the sulfur-containing compounds in the naphtha before and after alkaline wash areshown in Table 2. (2) water wash

The naphtha obtained by the alkaline wash in Step (1) was added with 200g of deionized water, stirred and uniformly mixed, and then allowed to rest for 30 minutes. After the two liquids were completely stratified, the upper layer of naphtha was separated, wherein the Na content was less than0.1 pg/g. (3) upgrading g of catalyst a wascharged into a small fixed bed reactor. An upgrading 10 reaction was carried out with the naphtha washed with water in Step (2) as a feedwithout presence of hydrogen. The reaction temperature was controlled at 360 °C and the reaction pressure was 0.3 MPa. The weight hourly space velocity of the feedwas 1.0 h™. The reaction results are shown in Table 3.

Example 4

The straight run naphtha A was upgraded according to the method of Example 3,except that the concentration of NaOH used in alkaline wash of Step (1) was 15% by weight. The compositionsof the sulfur-containing compounds in the naphtha before and after alkaline wash are shown in Table 2. The results of the upgrading of naphtha after alkaline wash and water washwithout presence of hydrogenin Step (3) are shown in Table 3.

Example 5

The straight run naphtha A was upgraded according to the method of Example 3, except that the concentration of NaOH used in alkaline wash of Step (1) was 10% by weight. The compositionsof the sulfur-containing compounds in the naphtha before and after alkaline wash are shown in Table 2. The results of the upgrading of naphtha after alkaline wash and water wash without presence ofhydrogenin Step (3) are shown in Table 3.

Example 6

The straight run naphtha A was upgraded according to the method of Example 3, except that the concentration of NaOH used in alkaline wash of Step (1) was 15% by weight. The compositionsof the sulfur-containing compounds in the naphtha before and after alkaline wash are shown in Table 2. In Step (3), an upgrading reaction was carried out with the naphtha after alkaline wash and water wash in presence of hydrogen. The reaction temperature was controlled at 360°C and the reaction pressure was 1.0MPa. The weight hourly space velocity of the feed was 1.0 h! with a hydrogen/hydrocarbon molar ratio of 1.0. The reaction results are shown in Table 3.

Example 7 (1) alkaline wash 100g of straight-run naphtha B (the composition and properties thereof are shown in Table 1) was added with 200g of an aqueous solution of NaOH with a concentration of 15% by weight and rested for 30 minutes after being uniformly mixed. When the two liquids werestratified, the upper layer of naphtha was separated to produce the naphtha washed with alkaline. The compositions of the sulfur-containing compounds in the naphtha before and after alkaline wash are shown in Table 2. (2) water wash

The naphtha obtained by alkaline wash in Step (1) was added with 200g of deionized water, stirred and uniformly mixed, and then allowed to rest for 30 minutes. After the two liquids were completely stratified, the upper layer of naphtha was separated, wherein the Na content was less than 0.1 pg/g. (3) upgrading 10 g of catalyst a was charged into a small fixed bed reactor. An upgrading reaction was carried out with the naphtha feed washed with water in Step (2) without presence of hydrogen. The reaction temperature was controlled at 360 °C and the reaction pressure was 0.3 MPa. The weight hourly space velocity of the feed was 1.0 h™. The reaction results are shown in Table 3.

Example 8

The straight run naphtha B was upgraded according to the method of Example 7,except that the concentration of NaOH used in alkaline wash of Step (1) was 20% by weight. The compositions of the sulfur-containing compounds in the naphtha before and after alkaline wash are shown in Table 2. The results of the upgrading : of naphtha after alkaline wash and water wash without presence of hydrogen in

Step (3) are shown in Table 3.

Example 9

Straight run naphtha B was upgraded according to the method of Example 7,except that the concentration of NaOH used in alkaline wash of Step (1) was 20% by weight. The compositions of the sulfur-containing compounds in the naphtha before and after alkaline wash are shown in Table 2. The results of the upgrading of naphtha after alkaline wash and water wash without presence of hydrogenusing catalyst b in Step (3) are shown in Table 3.

Example 10

Straight run naphtha C was upgraded according to the method of Example 7, exceptthat the concentration of NaOH used in alkaline wash of Step (1) was 20% by weight. The compositions of the sulfur-containing compounds in the naphtha before and after alkaline wash are shown in Table 2. The results of the upgrading of naphtha after alkaline wash and water wash without presence of hydrogen using a catalyst b in Step (3) are shown in Table 3.

Comparative Example 1 10g of catalyst a wascharged into a small fixed bed reactor. Straight run naphtha feed A was directly fed into the reactor without alkaline wash and water wash. The weight hourly space velocity of the feedwas controlled at 1.0 h!. The reaction was carried out without presence of hydrogen at temperature of 360 °C and pressure of 0.3 MPa. The reaction results are shown in Table 3.

Comparative Example 2 10g of catalyst a wascharged into a small fixed bed reactor. Straight run naphtha feedB was directly fed into the reactor without alkaline wash and water wash. The weight hourly space velocity of the feed was controlled at 1.0 h™'. The reaction was carried out without presence of hydrogen at temperature of 360 °C and pressure of 0.3 MPa. The reaction results are shown in Table 3.

Comparative Example 3 10g of catalyst b was charged into a small fixed bed reactor. Straight run naphtha feed C was directly fed into the reactor without alkaline wash and water wash. The weight hourly space velocity of the feed was controlled at 1.0 h™. The reaction was carried out without presence of hydrogen at temperature of 360 °C and pressure of 0.3 MPa. The reaction results are shown in Table 3.

Table 1 naplia feed A [Bc] distillation range, °C CC point with 12 101 50 %volumedistillated 107 ! final boiling poin hydrocarbon composition, % by weight ] cycloalkane aromatic hydrocarbon sulfur content,ug/g CC] merceplan sulfur | 108 | a | 257

Cs mercaptansulfor | 100 | 19% | 238

CCymercaptanslfer | 8 | 15 | 19 (ioethert disulfide uli

Table 2 oxamplerumber [3 [4 [5 Je [7 [8 [9 [10 naphtha feed A [A [aA |A |B [B |B [C concentration of NaOH used in total sulfur 153 | 153 | 153 [153 [289 |289 28 415 sulfur content 9 before 21 alkaline mercaptan sulfur 108 | 108 108 | 211 {211 1 257 wash, u ; ; ge | (thicether+disulfide) | 0 |, 4s 4s |78 |78 | 78 | 158 sulfur sulfur content eit [Gamo [111 [2 [1 Js [1 [1 fo wash, ug/g | CeCymercaptan 1 4 J10 [4 [13 17 sulfur (thioethertdisulfide) | os | 4s 145 [45 [78 |78 |78 | 158 sulfur

Table3

Com | Com | Com parat | parat | parat example ive | ive | ive number 3 4 > 7 10 Exa | Exa | Exa mple | mple | mple 1 2 3 naphtha 1 4 | A | A cl A C feed upgrading catalyst non | non | non non | non | non | non | non- | non- | non- conditions | -hy |-hy |-hy | hyd | -hy |-hy |-hy |-hy |hydr | hydr | hydr of dro | dro [dro |rog [dro |dro [dro |dro |ogen | ogen | ogen upgrading | gen | gen | gen [ena | gen | gen | gen | gen | ation | ation | ation reaction | atio | atio | atio | tion | atio | atio | atio | atio n |n n n n n n gasoline yield, % 71. 71 71. | 71. | 73.1 73.1 72. | 70. 714 | 733 | 705 by weight 3 3 3 8 1 1 0 5

Research

Octane

Number ” 7 o> % > oy > 90.1 | 903 | 90.2 (RON) of gasoline benzene content in gasoline, % by volume aromatic hydrocarb oncontent . 31. | 31. | 31. | 30. | 34. | 34. | 35. | 30. in 2 > 5 7 3 3 6 5 31.2 | 34.8 | 30.5 gasoline, % by volume

Olefin content in gasoline, | 2323123201919 2721] 23 19 | 2.1 % by volume sulfur contentin | 58 | 92 | 110 gasoline,

A METHOD FOR PRODUCING LOW SULFUR GASOLINE FROM

STRAIGHT RUN NAPHTHA CONTAINING SULFUR

Technical field

The present invention relates to a method for producing gasoline from a straight run naphtha containing sulfur, and, specifically, a method for producing clean low sulfur gasoline from a straight run naphtha containing sulfur.

Background of the technology

Naphtha, particularly straight-run naphtha from the atmospheric distillation process of crude oil, is not suitable for direct use as a gasoline component as it generally contains much sulfur and has a relatively lowoctane number. Conventional methods for desulfurization of straight-run naphtha include methods of hydrogenation and alkaline wash. The advantage of the method of hydrogenation is that the sulfur content in the naphtha can be reduced to a relatively low level.

However, the disadvantages lie in the need to set up a device for the hydrogenation treatment of naphtha and the need for more hydrogen. Thus the method has a higher investment and running cost. The method of alkaline wash can reduce the sulfur content in naphtha at a lower investment and running cost. However, it can only remove all the hydrogen sulfide and most of mercaptan sulfur in naphtha but fails to remove the thioether and disulfidein naphtha.

At present, conventional processing of naphtha concerns the process of hydrodesulfurization of the naphtha before increasing the octane number of naphtha by reforming and isomerization, thereby producing gasoline blending components having low sulfur and a high octane number. The processing of straight run naphtha using this combined process has advantages to obtainproducts having a high octane number and a low sulfur content, and disadvantages of high investment and processing costs, and a higharomatic hydrocarbon content in the \

gasoline products.

CN201210035100.9 discloses a process for desulfurization of naphtha at normal temperature, wherein the device comprises a settling tank and a desulfurization reactor. The volume of the settling tank is 2 to 3 times that of the desulfurization reactor. The desulfurization reactor is a fixed bed desulfurization reactor,the bottom of which is filled with a molecular sieve or an inert material of a ceramic ball. The desulfurizing agent is loaded on the molecular sieve or the inert material of a ceramic ball. The molecular sieve or the inert material of a ceramic ballis loaded at a height between 1/10 and 1/3 of the loading height of the desulfurizing agent. The naphtha enters the settling tank for settlement and dehydration. The water generated by the settling tank is discharged via the drain valve at the bottom.

The dehydrated naphtha enters a fixed bed desulfurizationreactor for desulfurization reaction. The water accumulated at the bottom of the desulfurization reactor is dischargedvia a drain valveset on the bottom of the desulfurization reactor. The method can prevent the water fromremaining in the naphtha and damaging the strength of the desulfurizing agentin the reaction. The desulfurizing agent can be continuously applied for more than one year.

CN201610473218.8 discloses a process for the production of aromatic hydrocarbonsfrom naphtha. The combined process steps comprise: a hydrodesulfurization unit, an aromatization unit, an extraction unit and a distillingunit. The process conditions of the aromatization unit include a catalyst circulation rate of 260 kg/h and a loading ratio of the catalyst in the first reactor to the catalyst in the second reactor of 1:1. The catalyst is a zeolite catalyst. The combination device used in the combined process comprises a hydrodesulfurization device, an aromatization device, an extracting device, and a distilling devicein order according to the process for the production of aromatic hydrocarbons from naphtha. The zeolite catalyst used in the process has good sulfur resistance and nitrogen resistance. The feedstocks do not need deep processing, the aromatic hydrocarbon yield is not limited by the potential content of the aromatic hydrocarbon in the naphtha, and thefeedstocks do not need pre-fractionation.

CN02805032.0 discloses a method for simultaneouslyfractionating and treating full boilingrange naphtha. First of all, the full boilingrange naphtha is separated into light boiling range naphtha, intermediate boiling range naphtha and heavy boiling range naphtha by simultaneous thiolation or selective hydrogenation. The naphtha of the intermediate boiling range which contains thiophene and thiophene with thiol boiling range, diolefins or mixtures thereof may be subjected to the second thiolation or selective hydrogenation, depending on the composition thereof. Then the naphtha is delivered to a hydrotreating reactor or the whole intermediate stream can be directly delivered to the hydrotreating reactor. The bottom product is subjected to simultaneous hydrodesulfurization and fractionation. The combination of the overhead distillate and the bottom product is sent to the hydrotreating reactor.

The effluent from the hydrotreating reactor can be combined with the light boilingrange naphtha to form new full boilingrange naphtha which comprises total sulfur substantively less than the feed. Treating the naphtha component in this method is to meet the high standard for sulfur removal and remove the sulfur compoundswhile retaining the olefinadvantageously.

CN200710120343.1 relates to a method for hydro-upgrading of catalytic crackinggasoline, which cuts the full range of the gasoline into light fraction and heavy fraction, wherein the cutting point is 60 °C ~ 80 °C. The mercaptan in the light gasoline fraction is removed by alkaline wash. The heavy gasoline fraction and hydrogen are subjected to catalytic hydrogenation for desulfuration, denitrification and olefin saturation reaction. The reaction effluent or the reaction effluent from which hydrogen sulfide is removed contacts with an octane number recovery catalyst for isomerization and aromatization and oligomerization. The hydrogenated product is separated to produce a light hydrocarbon and a gasoline fraction. The hydrogen-rich gas at the top of thegas and liquid separation tankis recycledvia a hydrogen sulfide removal tank. The light hydrocarbons at the top of the stabilizing columnreturn to the gas and liquid separation tank to be separated again. The conditions for the hydrotreating reaction include hydrogen partial pressure of 1.5 to 3.0 MPa, reaction temperature of 250 to 320 °C, liquid hourly space velocity of 3.0 to 5.0 hl, a hydrogen/oil ratio of 200 to 500 Nm *m * . The sulfur content in the product gasoline is <100 ppm. The octane number keeps the same and thegasoline yield is up 1098.5 wt.%.

Contents of the invention

The object of the present invention is to provide a method for producing low sulfur gasoline from a straight run naphtha containing sulfur. The method which produces a gasoline with a high octane number and effectively removes the sulfur therein at the same timeresults in a clean gasoline product with a high octane number.

The present invention provides a method for producing low sulfur gasoline from a straight run naphtha containing sulfur, comprising the following steps: (1) contacting the straight run naphtha containing sulfur with an inorganic alkaline solution for alkaline wash to remove hydrogen sulfide and most of mercaptan in the naphtha, (2) washing the naphthawith water after the alkaline wash in Step (1), (3) contactingthe naphtha which is washed with water in Step (2)with an upgrading catalyst,wherein the upgrading reaction is carried out under conditions of 200 to 500 °C and 0.2 to 3.0 MPa to obtain a gasoline having a sulfur content of less than 10 pg/g.

The method of the invention increases the octane number and decreases the sulfur content of naphtha by firstly removing all hydrogen sulfide and most of mercaptan sulfur in the straight run naphtha by alkaline wash and then upgrading the naphtha.

In the meanwhile, the sulfur-containing compounds such as thioetherand disulfide in the naphtha undergo a cracking reaction during theupgrading reaction to be converted into hydrogen sulfide, thereby further reducing the sulfur content of the gasoline product.

Specific embodiments

In the method of the invention, hydrogen sulfide and a part of the mercaptan sulfur in the straight run naphtha are firstly removedby alkaline wash, and afterbeing washedwith water thenaphtha is subjected to an upgrading reaction so as to increase the octane number of the naphtha. In the meanwhile, the sulfur-containing compounds such as thioetherand disulfide in the naphtha undergo a cracking reaction during the upgrading process to beconverted into hydrogen sulfide and thus are removed. Hence, a gasoline withan extremely low sulfur content and a high octane number is produced. In the method of the invention,the desulfurization reaction and the upgrading reaction are simultaneously carried out, thereby simplifying the desulfurization process and reducing the running cost. The

Research Octane Number (RON) of the obtained gasoline product isincreasedup to 90 or more and the sulfur content is reduced to a level of10 pg/g or less.

Step (1) of the method of the present invention is the alkaline wash of naphtha, wherein an inorganic alkaline solution is required for the alkaline wash. The inorganic alkaline solution is contacted with the naphtha for alkaline wash so that hydrogen sulfide and most of mercaptan in the naphtha are dissolved in the alkaline solution and removed.Most of mercaptan removed by said alkaline wash refers to the mercaptan of Cs (i.e., carbon atom number isS or less). The removal rate of Cs mercaptan is preferably up to 95.0 % by weight or more, more preferably more than 98 % by weight or more. The C¢-Cimercaptan can be removed in the subsequent upgrading reaction. The inorganic alkali is preferably sodium hydroxide or potassium hydroxide. The concentration of the inorganic alkaline solution is preferably 5 to 20% by weight, more preferably 10 to 20% by weight.

The time of contacting the inorganic alkaline solution with the naphtha for alkaline wash is preferably 5 to 60 minutes, more preferably 10 to 30 minutes.

In the alkaline washing step (1),the straight run naphtha containing sulfurcan be injected into an alkaline wash tank which is filled with an inorganic alkaline solution. The naphtha is mixed and contacted with the inorganic alkaline solution in the alkaline wash tank. The mixing method is preferably stirring. After being stirred and contacted for a period, the naphtha and the inorganic alkaline solution are stratified. All hydrogen sulfide and most of the mercaptan are reacted with the inorganic alkali to generate a water-soluble product which is dissolved in the alkaline solution. The naphtha is discharged from the top of the alkaline wash tank.

Thus, the naphtha from which hydrogen sulfide and most of the mercaptan have been removed is obtained.

Step (2) of the method of the present invention is to wash the naphtha with water after desulphurization by alkaline wash in order to remove the residual alkaline liquor in the naphtha. The amount of the inorganic alkali contained in the naphtha after washing with water is less than 0.1 ug/g, calculated on the basis of the metal amount in the inorganic alkali. If the naphtha is subjected to alkaline wash using a sodium hydroxide solution in Step (1), the Na content in the naphtha after water wash is less than 0.1 ug/g. The time for washing the naphtha with water after alkaline wash in Step (1) is preferably 5 to 100 minutes, more preferably 10 to 60 minutes.

The naphtha washed with alkaline obtained in Step (1) can be sent to a water wash tank in Step (2). After the naphtha is mixed and contacted with water in the water wash tank for a period, the naphtha and water are stratified. The residual alkaline liquor in the naphtha is dissolved in water. The naphtha from which the residual alkaline is removed is obtained after the naphtha washed with water is discharged from the top of the water wash tank.

In Step (3) of the method of the present invention, the naphtha after

Claims (14)

Claims

1. A method forproducing low sulfur gasoline from a straight run naphtha containing sulfur, comprising the following steps: (1) contacting the straight run naphtha containing sulfur with an inorganic alkaline solution for alkaline wash to remove hydrogen sulfide and most of mercaptan in the naphtha, (2) washing the naphtha with water after the alkaline wash in Step (1), (3) contacting the naphtha which is washed with water in Step (2)with an upgrading catalyst, wherein the upgrading reaction is carried out under conditions of 200 to 500 °C and 0.2 to 3.0 MPa to obtain a gasoline having a sulfur content of less than 10 pg/g.

2. The method according to Claim 1, characterized in that the inorganic alkali in Step (1) is sodium hydroxide or potassium hydroxide, and the inorganic alkaline solution has a concentration of 5 to 20% by weight.

3. The method according to Claim 1, characterized in that the time for alkaline wash of the naphtha using the inorganic alkaline solution in Step (1)is 5 to 60 minutes.

4. The method according to Claim 1, characterized in that an amount of inorganic alkali contained in the naphtha after the washing with water in Step (2) is less than

0.1 ng/g, calculated on the basis of a metal amount in the inorganic alkali.

5. The method according to Claim 4, characterized in that the amount of inorganic alkali contained in the naphtha after the washing with water in Step (2) is less than

0.1 pg/g, calculated on the basis of the Na content.

6. The method according to Claim 1, characterized in that in Step (2), the time for washing the naphtha with water after alkaline wash in Step (1)is 5 to 100 minutes.

7. The method according to Claim 1, characterized in that the upgrading catalyst in Step (3) comprises 10 to 90% by weight of HZSM-5 molecular sieve and 10 to 90% by weight of alumina.

8. The method according to Claim 1, characterized in that the upgrading catalyst in Step (3) comprises a carrier and 0.1 to 5% by weight of an oxide of a rare earth element supported on the carrier, calculated on the basis of the carrier, wherein the carrier comprises 10 to 90% by weight of HZSM-5 molecular sieve and 10 to 90% by weight of alumina

9. The method according to Claim 7 or Claim 8, characterized in that molar ratio of , the silica/alumina of said HZSM-5 molecular sieve is from 10 to 100.

10. The method according to Claim 8, characterized in that said rare earth element is selected from at least one of a group consisting of La, Ce, Pr, Nd, and Sm.

11. The method according to Claim 10, characterized in that said rare earth element is selected from a mixed rare earth.

12. The method according to Claim 1, characterized in that the upgrading reaction of the naphtha in Step (3) is carried out without presence of hydrogen, wherein the reaction temperature is 250 to 450 °C, the pressure is 0.3 to 1.5 MPa, and the weight hourly space velocity of the feed is 0.1 to 2.0 h™.

13. The method according to Claim 1, characterized in that the upgrading reaction of the naphtha in Step (3) is carried out in presence of hydrogen, wherein the reaction temperature is 250 to 450 °C, the pressure is 0.3 to 1.5 MPa, the weight hourly space velocity of the feed is 0.1 to 2.0 hl, and the hydrogen/hydrocarbon molar ratio is from 0.1 to 2.0.

14. The method according to Claim 1, characterized in that the said straight-run naphtha has a boiling range of 30 to 185 °C, wherein the sulfur content is100 to 1000 ng/g.