Hydrofining process for efficient preparation of xylene from coal tar

Abstract

This invention belongs to the field of xylene preparation technology, specifically providing a high-efficiency hydrorefining process for preparing xylene from coal tar. The process includes: Step 1: using medium-temperature coal tar as raw material, coking and decarbonizing to separate and collect mixed fractions; Step 2: hydrorefining the mixed fractions in a first hydrorefining unit to separate light components and tail oil; hydrocracking the tail oil in a second hydrorefining unit to separate and collect heavy naphtha; Step 3: mixing the heavy naphtha obtained in Step 2 with the cycloalkane-rich light components and reacting in a catalytic reforming unit to obtain a reforming product with high aromatic potential; Step 4: sending the reforming product obtained in Step 3 into an aromatic separation unit for extraction or distillation to obtain high-purity benzene, toluene, and a mixed aromatic hydrocarbon rich in xylene; further separating the xylene-rich mixed aromatic hydrocarbon through adsorption or distillation to collect the xylene product. This process can improve yield while fully utilizing byproducts.

Description

Technical Field

[0001] This invention relates to the field of xylene preparation technology, and in particular to a hydrogenation refining process for the efficient preparation of xylene from coal tar. Background Technology

[0002] Coal tar is an important liquid product generated during the dry distillation or gasification of coal. Based on the different distillation temperatures, it can be classified into low-temperature coal tar, medium-temperature coal tar, and high-temperature coal tar. Among these, medium-temperature coal tar, produced from medium-temperature dry distillation or the pyrolysis of low-rank coal, has a larger yield and a composition between low-temperature and high-temperature coal tar. It is characterized by a higher content of aromatics, lower levels of phenols, and the coexistence of alkanes and cycloalkanes.

[0003] Existing coal tar to aromatics technology suffers from the following shortcomings: First, the process integration is low. Most processes simply connect hydrotreating and reforming units in series without considering material matching and recycling between units, resulting in low overall aromatics yield. Second, the target product is singular, primarily pursuing total aromatics yield, lacking targeted optimization for the high-value-added product xylene. Furthermore, resource utilization is insufficient during production; heavy aromatics and residual oil are often treated as low-value fuels, failing to be effectively converted into light aromatics. Summary of the Invention

[0004] To address the aforementioned shortcomings, this invention provides a hydrogenation refining process for the efficient preparation of xylene from coal tar, which can improve the yield while making full use of by-products.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a hydrorefining process for the efficient preparation of xylene from coal tar, comprising: Step 1: Using medium-temperature coal tar as raw material, coking and decarbonization are carried out at a temperature of 430~470℃ for 20~40h, and the mixed fraction at 120~450℃ is separated and collected. Step 2: The mixed fraction from Step 1 is hydropurified in the first hydrogenation unit, with the reaction temperature controlled at 330-390℃, pressure at 12-15 MPa, and volume hourly space velocity at 0.4-0.8 h⁻¹. -1 The hydrogen-to-oil volume ratio is 600-800:1, and a light component rich in cycloalkanes and a tail oil are separated. The tail oil is then subjected to hydrocracking in a second hydrotreating unit, with the reaction temperature controlled at 350-390℃, pressure at 12-15 MPa, and volume hourly space velocity at 0.7-1.2 h⁻¹. -1 The hydrogen-to-oil volume ratio is 600-800:1, and heavy naphtha rich in aromatic precursors is separated and collected. Step 3: Mix the heavy naphtha obtained in Step 2 with the light component rich in cycloalkanes and react them in a catalytic reforming unit. Control the reaction temperature at 260-300℃, the pressure at 1.0-2.5MPa, and the volume hourly space velocity at 0.8-1.5h.-1 With a hydrogen-to-oil volume ratio of 500-800:1, reforming products with high aromatic potential content are obtained. Step 4: The reformed product obtained in Step 3 is fed into the aromatics separation unit and extracted or distilled at a temperature of 80-180℃ and a pressure of 0.2-1.0MPa to obtain high-purity benzene, toluene, and xylene-rich mixed aromatics; the xylene-rich mixed aromatics are further separated by adsorption or distillation to collect the xylene product in the 135-145℃ fraction.

[0006] As a further improvement of the present invention, the benzene product in the 75~85℃ fraction is collected after separation in step 4.

[0007] As a further improvement of the present invention, toluene from the 105-115℃ fraction is collected after separation in step 4.

[0008] As a further improvement of the present invention, after separation in step 4, C9+ heavy aromatics with a boiling point higher than 140°C are collected and part of them are returned to the catalytic reforming unit in step 3 or sent to the lightening unit.

[0009] Part of it is returned to the catalytic reforming unit in step 3 or sent to the lightening unit to improve the overall xylene yield.

[0010] As a further improvement of the present invention, the catalytic reforming unit in step 3 adopts a continuous reforming or fixed-bed reforming process and uses a highly selective platinum-based or platinum-tin bimetallic catalyst to improve the yield of C6~C8 aromatics.

[0011] As a further improvement of the present invention, the catalysts used in the hydrorefining and hydrocracking units in step 2 are supported Ni-Mo and Ni-W catalysts, respectively, and their supports are modified alumina or molecular sieves.

[0012] As a further improvement of the present invention, after separation in step 2, high-quality white oil from the 260~355℃ fraction and a small amount of clean diesel fuel components from the 180~360℃ fraction are also collected as by-products.

[0013] As a further improvement of the present invention, after adsorption separation or distillation separation in step 4, the raffinate oil obtained is rich in alkanes, and it is returned to the hydrocracking unit in step 2 for chain breaking treatment, or used as a high-quality ethylene cracking feedstock.

[0014] The beneficial effects of this invention are: 1. By optimizing the coking decarbonization temperature and time, asphaltenes and heavy components in the feedstock were effectively removed while retaining suitable aromatic precursor structures. Hydrorefining and hydrocracking were carried out in steps, ensuring sufficient enrichment of aromatic precursors such as cycloalkanes. The catalytic reforming unit employed optimized reaction conditions to maximize the dehydrogenation and aromatization of cycloalkanes. The overall xylene yield could be further improved, achieving high-value utilization of coal tar resources.

[0015] 2. This invention employs a process combining extraction and precision distillation in the aromatic hydrocarbon separation unit. Separation is carried out under certain temperature and pressure conditions, resulting in improved purity of benzene, toluene, and xylene products. This meets the high-specification market requirements of polyester grade, solvent grade, etc., and significantly improves the economic efficiency of the process.

[0016] 3. Simultaneously, it can co-produce high-quality white oil and clean diesel components. The separated raffinate is rich in alkanes and can be returned to the hydrocracking unit to be converted into aromatic precursors or used as a high-quality ethylene cracking feedstock; C9+ heavy aromatics can be returned to the reforming unit or enter the disproportionation unit to achieve light processing. Through multi-stage recycling, resources are fully utilized. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0018] A hydrogenation refining process for the efficient preparation of xylene from coal tar includes: Step 1: Medium-temperature coal tar is fed into a coking tower. Delayed coking is carried out under conditions of controlled coking temperature of 430~470℃ and time of 20~40h. The oil and gas produced during the coking process are separated by a fractionation tower, and the mixed fraction at 120~450℃ is collected as feedstock for the hydrogenation unit.

[0019] Step 2: Step 201: The mixed fraction from Step 1 is fed into the first hydrogenation reactor (hydrogenation refining reactor) for hydrogenation refining. The reactor is packed with a supported Ni-Mo catalyst, with modified alumina or molecular sieve as the support. The reaction temperature is controlled at 330-390℃, the pressure at 12-15MPa, and the volume hourly space velocity at 0.4-0.8h. -1 With a hydrogen-to-oil volume ratio of 600-800:1, light components rich in cycloalkanes and tail oil are separated.

[0020] Step 202: The tail oil obtained from S201 is hydrocracking in a second hydrotreating reactor (hydrocracking reactor). The reactor is packed with a supported Ni-W catalyst, with modified alumina or molecular sieve as the support. The reaction temperature is controlled at 350-390℃, the pressure at 12-15MPa, and the volume hourly space velocity at 0.7-1.2h. -1 The hydrogen-to-oil volume ratio is 600-800:1. Heavy naphtha rich in aromatic precursors is collected in the 70-180℃ fraction, high-quality white oil in the 260-355℃ fraction, and a small amount of clean diesel components in the 180-360℃ fraction are produced as by-products.

[0021] As a further explanation of this embodiment, supported Ni-Mo and Ni-W catalysts can enhance the removal capacity of sulfur and nitrogen impurities from feedstocks and improve the saturation selectivity of aromatics.

[0022] Step 3: The heavy naphtha obtained in Step 2 is mixed with a light component rich in cycloalkanes and reacted in a catalytic reforming unit. The catalytic reforming unit adopts a continuous reforming or fixed-bed reforming process and uses a highly selective platinum-based or platinum-tin bimetallic catalyst to improve the yield of C6-C8 aromatics. The reaction temperature is controlled at 260-300℃, the pressure at 1.0-2.5MPa, and the volume hourly space velocity at 0.8-1.5h. -1 A hydrogen-to-oil volume ratio of 500-800:1 is used to obtain reforming products with high aromatic hydrocarbon potential.

[0023] Step 4: The reforming product obtained in Step 3 is fed into the aromatics separation unit. Sulfolane is used as the extractant, and extraction or distillation is carried out under the conditions of 80-180℃ and 0.2-1.0MPa to separate non-aromatics (raffinate oil) and obtain mixed aromatics.

[0024] As a further explanation of this embodiment, the obtained raffinate oil is rich in alkanes, which is returned to the hydrocracking unit in step 2 for chain breaking treatment, or used as a high-quality ethylene cracking feedstock.

[0025] The mixed aromatics are fed into a distillation column for precision fractionation. The first distillation column collects the 75-85℃ fraction to obtain benzene. The second distillation column collects the 105-115℃ fraction to obtain toluene, which is then partially or entirely sent to the disproportionation and alkyl transfer unit to increase xylene production. The third distillation column collects the 135-145℃ fraction to obtain mixed xylene products (including ortho-, meta-, and para-xylenes). The bottom fraction of >140℃, mainly C9+ heavy aromatics, is collected from the bottom of the distillation columns, and a portion of it is returned to the catalytic reforming unit in step 3 or sent to the light processing unit to improve the overall xylene yield.

[0026] Example 1 This embodiment provides a hydrorefining process for the efficient preparation of xylene from coal tar, the specific operation of which is as follows: Step 1: Coking Decarbonization: Medium-temperature coal tar (density 0.98 g / cm³, moisture 3.5 wt%, ash 0.08 wt%) produced by a certain plant is fed into the coking tower. The coking temperature is controlled at 450℃ for 30 hours, undergoing delayed coking. The oil and gas produced during coking are separated in a fractionation tower, and a mixed fraction at 120~450℃ is collected as feedstock for the hydrogenation unit. The aromatic potential content of this mixed fraction is approximately 45 wt%.

[0027] Step 2: Hydrorefining and Cracking The mixed fraction from step 1 is mixed with hydrogen and fed into the first hydrogenation reactor (hydrogenation refining reactor). The reactor is packed with a Ni-Mo / Al₂O₃ catalyst. The reaction temperature is controlled at 360℃, the reaction pressure at 13.5 MPa, and the volume hourly space velocity at 0.6 h⁻¹. -1 The hydrogen-to-oil volume ratio is 700:1. The reaction products are separated by a high-pressure separator and a fractionation tower to obtain a light fraction (<180℃ fraction) rich in cycloalkanes and a tail oil (>180℃ fraction).

[0028] The tail oil is fed into the second hydrotreating reactor (hydrocracking reactor). The reactor is packed with a Ni-W / molecular sieve catalyst. The reaction temperature is controlled at 370℃, the reaction pressure at 14.5 MPa, and the volume hourly space velocity at 1.0 h⁻¹. -1 The hydrogen-to-oil volume ratio was 700:1. The reaction products were separated, and the heavy naphtha rich in aromatic precursors (cycloalkanes content of about 38 wt%) in the 70-180℃ fraction was collected.

[0029] Step 3: Catalytic Reforming The heavy naphtha obtained in step 2 was mixed with a portion of a light component rich in cycloalkanes (adjusting the aromatic potential content to above 50 wt%) and fed into a catalytic reforming reactor. The reactor employed a continuous reforming process and was loaded with a platinum-tin bimetallic catalyst (such as commercially available PR-9 catalyst). The reaction temperature was controlled at 280℃, the reaction pressure at 1.5 MPa, and the volume hourly space velocity at 1.2 h⁻¹. -1 The hydrogen-to-oil volume ratio is 600:1. After light hydrocarbons are removed from the reaction product in a stabilizing tower, reformed gasoline with a high aromatic content (approximately 85 wt%) is obtained.

[0030] Step 4: Aromatic hydrocarbon separation The reformed gasoline obtained in step 3 is fed into the aromatics extraction unit. Sulfolane is used as the extractant, and liquid-liquid extraction is carried out at a temperature of 120°C and a pressure of 0.5 MPa to separate non-aromatics (raffinate) and obtain mixed aromatics.

[0031] The mixed aromatics are fed into a distillation column for precision fractionation. The first distillation column collects the fraction at 75-85℃ to obtain benzene product with a purity of 99.8%, accounting for 3.2 wt% of the medium-temperature coal tar feedstock. The second distillation column collects the fraction at 105–115 °C to obtain toluene product with a purity of 99.6%, representing 5.1 wt% of the medium-temperature coal tar feedstock. The third distillation column collects the 135-145℃ fraction to obtain a mixed xylene product (including ortho, meta, and para xylenes) with a purity of 99.5% and a yield of 12.8 wt% of the medium-temperature coal tar feedstock. The bottom of the column collects the >140℃ fraction, which is mainly C9+ heavy aromatics and has a yield of 4.5 wt% of the medium-temperature coal tar feedstock.

[0032] In addition, in step 2, the fractionation tower also collects high-quality white oil products from the 260~355℃ fraction, with a yield of 8.3 wt% of the medium-temperature coal tar in the feedstock; and a small amount of clean diesel components from the 180~360℃ fraction, with a yield of 6.1 wt% of the medium-temperature coal tar in the feedstock.

[0033] Comparative Example 1 A conventional medium-temperature coal tar processing technology was used as a comparative example. The raw materials were the same as in Example 1, with the following differences: (1) The coking decarburization temperature was adjusted to 410℃ for 50 hours; (2) Hydrorefining and hydrocracking are combined into one hydrorefining unit with a reaction temperature of 320°C and a pressure of 12 MPa. No special cracking of the tail oil is carried out. (3) The temperature of the catalytic reforming reaction was adjusted to 250℃ and the pressure to 3.0MPa; (4) Aromatic hydrocarbon separation only involves simple distillation, without extraction and precision fractionation.

[0034] Results: The final xylene product yield was only 5.7 wt% of the medium-temperature coal tar feedstock, and the total yield of benzene and toluene was about 4.2 wt%. A large amount of aromatic potential was lost in the diesel and tail oil components.

[0035] Example 2 This embodiment is basically the same as Embodiment 1, except that in step 3, the C9+ heavy aromatics obtained in step 4 are returned to the inlet of the catalytic reforming unit, mixed with fresh feedstock, and reformed. The recycling ratio is 15 wt% of the fresh feedstock.

[0036] Results: By recycling heavy aromatics to make them lighter, the total yield of xylene increased to 14.2 wt% (accounting for medium-temperature coal tar feedstock), and the discharge of C9+ aromatics decreased to 2.8 wt%, significantly improving the yield of high-value-added light aromatics.

[0037] Example 3 This embodiment is basically the same as Example 1, except that in step 4, all the separated toluene is sent to the disproportionation and alkyl transfer unit, where it reacts with some C9+ aromatics under the conditions of reaction temperature 240°C and pressure 2.8 MPa, and is further converted into xylene and benzene.

[0038] Results: The single-pass conversion rate of the toluene disproportionation unit was 45%, and the selectivity was 96%. The final total xylene yield increased to 16.5 wt% (as a percentage of the feedstock medium-temperature coal tar), the benzene yield increased to 5.8 wt%, and the external sales volume of toluene decreased significantly.

[0039] Example 4 This embodiment is basically the same as Embodiment 1, except that the raffinate (rich in alkanes) extracted and separated in step 4 is returned to the inlet of the hydrocracking reactor in step 2 and mixed with the tail oil for hydrocracking. The recycling ratio is 80% of the raffinate production.

[0040] Results: The alkanes in the raffinate underwent chain scission and isomerization reactions in the hydrocracking unit, generating more light components and isoalkanes, which further improved the yield of heavy naphtha and the content of aromatic precursors. Finally, the xylene yield stabilized at 13.1 wt%.

[0041] In summary, the present invention achieves efficient conversion of medium-temperature coal tar into high-value-added xylene by optimizing the depth of coking decarbonization, staged hydrocracking and refining, matching suitable reforming conditions, and precise aromatic separation. Compared with the conventional process in Comparative Example 1, the xylene yield can be increased by approximately 1.5 to 2.8 times, while simultaneously producing high-purity benzene, toluene, white oil, and other products.

[0042] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above-described embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. A hydrorefining process for the efficient preparation of xylene from coal tar, characterized in that, include: Step 1: Using medium-temperature coal tar as raw material, coking and decarbonization are carried out at a temperature of 430~470℃ for 20~40h, and the mixed fraction at 120~450℃ is separated and collected. Step 2: The mixed fraction from Step 1 is hydropurified in the first hydrogenation unit, with the reaction temperature controlled at 330-390℃, pressure at 12-15 MPa, and volume hourly space velocity at 0.4-0.8 h⁻¹. -1 The hydrogen-to-oil volume ratio is 600-800:1, and a light component rich in cycloalkanes and a tail oil are separated. The tail oil is then subjected to hydrocracking in a second hydrotreating unit, with the reaction temperature controlled at 350-390℃, pressure at 12-15 MPa, and volume hourly space velocity at 0.7-1.2 h⁻¹. -1 The hydrogen-to-oil volume ratio is 600-800:1, and heavy naphtha rich in aromatic precursors is separated and collected. Step 3: Mix the heavy naphtha obtained in Step 2 with the light component rich in cycloalkanes and react them in a catalytic reforming unit. Control the reaction temperature at 260-300℃, the pressure at 1.0-2.5MPa, and the volume hourly space velocity at 0.8-1.5h. -1 With a hydrogen-to-oil volume ratio of 500-800:1, reforming products with high aromatic potential content are obtained. Step 4: The reformed product obtained in Step 3 is fed into the aromatics separation unit and extracted or distilled at a temperature of 80-180℃ and a pressure of 0.2-1.0MPa to obtain high-purity benzene, toluene, and xylene-rich mixed aromatics; the xylene-rich mixed aromatics are further separated by adsorption or distillation to collect the xylene product in the 135-145℃ fraction.

2. The hydrorefining process for efficient preparation of xylene from coal tar according to claim 1, characterized in that, After separation in step 4, the benzene product from the 75-85℃ fraction is collected.

3. The hydrorefining process for efficient preparation of xylene from coal tar according to claim 1, characterized in that, After separation in step 4, toluene from the 105-115℃ fraction is collected.

4. The hydrorefining process for efficient preparation of xylene from coal tar according to claim 3, characterized in that, After separation in step 4, C9+ heavy aromatics with boiling points above 140°C are collected and partially returned to the catalytic reforming unit in step 3 or sent to the lightening unit.

5. The hydrorefining process for efficient preparation of xylene from coal tar according to claim 1, characterized in that, In step 3, the catalytic reforming unit adopts a continuous reforming or fixed-bed reforming process and uses highly selective platinum-based or platinum-tin bimetallic catalysts to improve the yield of C6-C8 aromatics.

6. The hydrorefining process for efficient preparation of xylene from coal tar according to claim 1, characterized in that, The catalysts used in the hydrorefining and hydrocracking units in step 2 are supported Ni-Mo and Ni-W catalysts, respectively, and their supports are modified alumina or molecular sieves.

7. The hydrorefining process for efficient preparation of xylene from coal tar according to claim 1, characterized in that, After separation in step 2, high-quality white oil from the 260-355℃ fraction and a small amount of clean diesel fuel from the 180-360℃ fraction are also collected as by-products.

8. The hydrorefining process for efficient preparation of xylene from coal tar according to claim 1, characterized in that, After adsorption separation or distillation separation in step 4, the resulting raffinate is rich in alkanes. It is returned to the hydrocracking unit in step 2 for chain breaking treatment, or used as a high-quality ethylene cracking feedstock.