Continuous deep desulfurization method and system for liquefied petroleum gas

Abstract

The invention relates to a continuous deep desulfurization method and system for liquefied petroleum gas, and the method comprises the following steps: (1) contacting the liquefied petroleum gas with extraction alkali liquor for alkali extraction to obtain a first liquefied petroleum gas product and thiol-containing alkali liquor; (2) contacting the thiol-containing saline solution with oxidizing gas to obtain a material to be separated; (3) separating the material to be separated to obtain residual oxidizing gas and disulfide-containing alkali liquor; (4) carrying out reverse extraction on the disulphide-containing alkali liquor to obtain a disulphide-containing selective solvent and regenerated alkali liquor; (5) carrying out fractional distillation on at least part of the first liquefied petroleum gas product, and carrying out C5 fraction distillation on the mixed C4 or above fractions to obtain C5 fractions; the selective solvent in the step (4) comprises a C5 fraction and a part of disulfide-containing selective solvent. According to the method, the sulfur content of the liquefied petroleum gas is remarkably reduced, the effective utilization rate of the alkali liquor for extraction is greatly increased, and the C5 fraction in the liquefied petroleum gas is converted into the gasoline fraction to be utilized.

Description

Technical Field

[0001] The invention relates to the technical field of hydrocarbon material refining, and in particular to a continuous deep desulfurization method and system for liquefied petroleum gas. Background Art

[0002] Liquefied petroleum gas (LPG) is an important petroleum refining product. LPG mainly comes from catalytic cracking, delayed coking, atmospheric and vacuum, hydrocracking, continuous reforming and other devices. Its main components are C3 and C4 hydrocarbons, with a small amount of C2 and C5 hydrocarbons. The sulfides it contains are mainly hydrogen sulfide and mercaptan sulfur, and a small amount of other low-boiling sulfides. The uses of LPG include use as civil or automotive fuel, and as a chemical raw material, such as a raw material for extracting commercial propane and commercial propylene, as a raw material for methyl tert-butyl ether (MTBE) for synthetic gasoline anti-knock agents, a raw material for preparing composite gasoline, and a raw material for preparing alkylated gasoline, etc. LPG usually needs to be desulfurized and refined before it can be used as a target product. In the national, industry, group and enterprise standards related to liquefied petroleum gas and liquefied petroleum gas derivatives, the requirements for sulfur content and hydrocarbon composition are different. For example, the national standard GB11174-2011 requires that the sulfur content of civil liquefied petroleum gas should not exceed 343 mg / m 3 , the content of C5 and above components (volume fraction) shall not exceed 3.0% or 2.0%; the national standard 19159-2012 requires that the sulfur content of automotive liquefied petroleum gas shall not exceed 50 mg / kg, and the content of C5 and above components (volume fraction) shall not exceed 2.0%; the national standard GB / T7716-2014 requires that the sulfur content of superior polymer-grade propylene shall not exceed 1 mg / kg; when methyl tert-butyl ether (MTBE) is used as a gasoline blending component, its sulfur content is required to be no more than 10 mg / kg (i.e. 10 μg / g).

[0003] At present, the industrial refining method for desulfurization of liquefied petroleum gas mainly includes the continuous process of hydrogen sulfide extraction for the purpose of removing hydrogen sulfide and mercaptan extraction for the purpose of removing mercaptans. Hydrogen sulfide extraction mostly adopts alcohol amine extraction, and mercaptan extraction mostly adopts alkali solution extraction. The alcohol amine extraction is to contact the liquefied petroleum gas with an alcohol amine compound (such as methyldiethanolamine, MDEA) solution and absorb hydrogen sulfide in the liquefied gas. The alcohol amine solution after absorbing hydrogen sulfide is continuously used after desorption treatment. The alkali solution extraction is to contact the liquefied petroleum gas after hydrogen sulfide is removed by alcohol amine extraction with an inorganic alkali solution and absorb mercaptans in the liquefied gas. The alkali solution after absorbing mercaptans is recycled after regeneration treatment. The liquefied petroleum gas after the removal of hydrogen sulfide and mercaptans becomes a refined product.

[0004] Refined LPG can be further fractionated to obtain light hydrocarbon products with different carbon numbers or even different isomers. In order to obtain low-carbon olefins that meet the sulfur content requirements, synthesize low-sulfur methyl tert-butyl ether, and produce composite gasoline or alkylated gasoline, refineries generally require that the sulfur content of refined LPG is no more than 10μg / g or even less than 2μg / g.

[0005] As a traditional method for refining hydrocarbon materials, alkali extraction is widely used in the desulfurization and refining of low-carbon light hydrocarbon materials such as natural gas, liquefied petroleum gas, light gasoline, naphtha, alkanes, olefins, etc. containing acidic sulfides. The method is to contact the hydrocarbon fluid with alkali solution, and the acidic sulfides mainly composed of mercaptans in the hydrocarbon stream react with the alkali solution to form mercaptides and are removed from the hydrocarbon stream.

[0006] If the alkali solution containing mercaptide salt is directly discharged, it is neither economical nor in line with environmental protection requirements. Usually, an oxidation method is used to oxidize the mercaptide salt into disulfide so that the alkali solution containing mercaptide salt can be regenerated (see U.S. Patent No. 2853432). Specifically, the method injects air and an oxidation catalyst into the used alkali solution to oxidize the mercaptide salt dissolved in the alkali solution into disulfide, thereby regenerating the alkali solution. However, the alkali solution after oxidation and regeneration contains disulfide. If it is circulated for the extraction of hydrocarbon streams, the hydrocarbon streams will absorb the disulfide and make it difficult to reduce its sulfur content or even increase it. For this reason, it is necessary to separate the regenerated alkali solution from the disulfide therein by sedimentation separation and / or organic hydrocarbon solvent back extraction so that the regenerated alkali solution can continue to be used for removing mercaptans in the hydrocarbon stream and the sulfur content in the hydrocarbon stream can be effectively reduced, thereby greatly reducing the discharge of spent alkali.

[0007] The existing alkaline solution extraction desulfurization technology includes the following basic steps in succession: (1) extraction, (2) oxidation, (3) phase separation, and (4) back extraction. In the extraction step, the alkaline solution (including the regenerated alkaline solution) contacts a hydrocarbon stream including liquefied petroleum gas containing mercaptans, and reacts with the mercaptans to form mercaptides, which are dissolved in the alkaline solution; in the oxidation step, the alkaline solution containing mercaptides from the extraction step is mixed with the injected oxidizing gas and oxidation catalyst to oxidize the mercaptides in the alkaline solution to disulfides, thereby regenerating the alkaline solution, wherein the oxidation catalyst is a well-known metal phthalocyanine, such as sulfonated cobalt phthalocyanine and polyphthalocyanine cobalt. Usually, the metal phthalocyanine catalyst is intermittently or continuously injected into the oxidation step along with fresh alkali solution (commercially available metal phthalocyanine catalyst is usually in the form of powdered solid or liquid mixture); in the phase separation step, the regenerated alkali solution from the oxidation step and the mixture of disulfide and oxidizing gas are separated by sedimentation, on the one hand, to release the excess oxidizing gas, and on the other hand, to separate the alkali solution from the disulfide aggregated into phases, so as to achieve the purpose of removing the disulfide. Due to the small density difference between disulfide and alkali solution, it is usually difficult to aggregate the disulfide into phases and completely separate and remove it in the phase separation step of sedimentation due to the relatively short sedimentation time. For this reason, the back extraction step is applied. In the back extraction step, the alkali solution from the phase separation step of sedimentation is mixed with an organic hydrocarbon solvent so that the remaining disulfide in the alkali solution is back extracted into the organic hydrocarbon solvent so that the regenerated alkali solution and the disulfide-containing solvent are separated from each other, and the regenerated alkali solution treated by back extraction is returned to the extraction step for continued use. In order to enhance the desulfurization effect of hydrocarbon streams, organic liquid additives with extraction and / or oxidation-assisting effects, such as low-carbon alcohols, may be added to the alkali solution.

[0008] The stripping solvents used in the literature are mostly naphtha fractions, gasoline fractions, etc. The stripping solvent is usually sent to the hydrogenation unit for desulfurization treatment after being used to absorb disulfides. However, due to the influence of the trace amount of alkali solution carried, especially for the stripping solvent containing aromatics and easily emulsified, it is difficult to wash away the trace amount of alkali solution carried even with water washing, which can easily lead to poisoning and deactivation of the hydrogenation catalyst during hydrogenation treatment. There are also methods that send hydrocarbon stripping solvents back to the catalytic cracking unit for treatment and high-temperature cracking in the catalytic cracking unit. However, the stripping solvent carrying trace amounts of alkali solution will also be seriously harmful to the catalytic cracking catalyst. This makes the treatment of the waste stripping solvent a problem that is not easy to solve in refineries.

[0009] In addition, in the back-extraction step, the disulfides produced after the oxidation regeneration of the alkali solution are usually not completely removed from the regenerated alkali solution, and there is often a large amount of residual content. This makes it easy for the regenerated alkali solution to transfer disulfides into the hydrocarbon stream when it is returned to the extraction system for use, resulting in a decrease in the extraction desulfurization rate of hydrocarbon streams including liquefied petroleum gas, and even the abnormal phenomenon of increased sulfur content.

[0010] In addition, before the regenerated alkali solution returns to the extraction system and contacts the hydrocarbon stream containing mercaptans, there are often trace amounts of oxidizing gases remaining, which causes some mercaptans to be oxidized to disulfides in the extraction system and remain in the hydrocarbon stream after extraction. For this reason, patent CN 200510132299.7 uses nitrogen replacement to remove the oxidizing gases carried in the regenerated alkali solution before back extraction, but the replacement is often not complete, especially when the back extraction solvent carries oxidizing gases (for example, naphtha and gasoline stored in the refinery tank area are used as back extraction solvents and are easily exposed to air), which is likely to affect the deep desulfurization of hydrocarbon streams including liquefied petroleum gas.

[0011] In particular, the boiling point of liquefied petroleum gas is below 0°C, and the sulfides it contains, in addition to hydrogen sulfide, are mainly methyl mercaptan (boiling point 6.2°C), and other sulfides with higher molecular weight are less. After the liquefied petroleum gas is extracted with alcohol amine to remove hydrogen sulfide, it is then extracted with alkaline solution to remove mercaptan sulfides. Once the non-hydrogen sulfide and non-mercaptan sulfides with higher molecular weight contained in the liquefied petroleum gas are higher than the target sulfur content requirement, the required desulfurization target cannot be achieved by alkaline solution extraction anyway. In addition, if the content of components above C5 exceeds the standard, no matter how the desulfurization is carried out, the obtained liquefied petroleum gas cannot become a qualified product.

[0012] In summary, how to continuously and deeply desulfurize liquefied petroleum gas to significantly reduce the sulfur content of liquefied petroleum gas products, and to convert and utilize the C5 fraction in liquefied petroleum gas products and improve the effective utilization rate of alkali solution is a technical problem that needs to be solved urgently. Summary of the invention

[0013] The present invention provides a method and system for continuous deep desulfurization of liquefied petroleum gas, aiming at reducing the sulfur content of liquefied petroleum gas products, converting and utilizing the C5 fraction in the liquefied petroleum gas products and improving the effective utilization rate of alkali liquor.

[0014] In a first aspect, the present invention relates to a method for continuous deep desulfurization of liquefied petroleum gas, which comprises the following steps: (1) contacting liquefied petroleum gas with an extraction alkali solution to perform alkali extraction, thereby obtaining a first liquefied petroleum gas product and a mercaptan-containing alkali solution; (2) contacting the mercaptan-containing alkali solution from step (1) with an oxidizing gas to perform oxidation, thereby obtaining a material to be separated comprising residual oxidizing gas and a disulfide-containing alkali solution; (3) separating the material to be separated from step (2), thereby obtaining residual oxidizing gas and a disulfide-containing alkali solution; (4) contacting the disulfide-containing alkali solution from step (3) with an oxidizing gas to perform oxidation. (5) subjecting at least a portion of the first liquefied petroleum gas product obtained in step (1) to fractional distillation to obtain a mixed C3 or less fraction and a mixed C4 or greater fraction; subjecting the mixed C4 or greater fraction to decarbonization and distillation to obtain a C4 fraction and a C5 fraction; wherein the extraction alkali liquor in step (1) comprises the regenerated alkali liquor obtained in step (4); and the selective solvent in step (4) comprises the C5 fraction obtained in step (5) and a portion of the disulfide-containing selective solvent obtained in step (4).

[0015] In a second aspect, the present invention relates to a continuous deep desulfurization system for liquefied petroleum gas, the continuous deep desulfurization system comprising:

[0016] Alkali extraction unit, oxidation unit, separation unit, back extraction unit and fractional distillation decarbonization five distillation unit;

[0017] The alkali extraction unit is provided with a liquefied petroleum gas inlet pipeline, an extraction alkali solution inlet, a first liquefied petroleum gas product outlet and a mercaptan-containing alkali solution outlet pipeline; the mercaptan-containing alkali solution outlet pipeline is connected to the mercaptan-containing alkali solution inlet of the oxidation unit; the mercaptan-containing alkali solution outlet pipeline is provided with an oxidizing gas inlet;

[0018] The oxidation unit is provided with a material to be separated outlet pipeline connected to the material to be separated inlet of the separation unit;

[0019] The separation unit is provided with a disulfide-containing alkali solution outlet pipeline and a residual oxidizing gas outlet; the disulfide-containing alkali solution outlet pipeline is connected to the disulfide-containing alkali solution inlet of the stripping unit;

[0020] The stripping unit is provided with a selective solvent inlet, a regeneration alkali solution outlet and a disulfide-containing selective solvent outlet;

[0021] The regeneration alkali solution outlet is connected to the extraction alkali solution inlet of the alkali extraction unit;

[0022] The fractional distillation decarbonization five distillation unit comprises a first fractional distillation unit and a decarbonization five distillation tower, the inlet of the material to be distilled of the first fractional distillation unit is connected with the first liquefied petroleum gas product outlet of the alkali extraction unit, the first fractional distillation unit is provided with a mixed carbon three or less fraction outlet and a mixed carbon four or more fraction outlet, the mixed carbon four or more fraction outlet is connected with the inlet of the decarbonization five distillation tower, and the decarbonization five distillation tower is provided with a carbon five fraction outlet pipeline and a carbon four fraction outlet;

[0023] The C5 fraction outlet pipeline is communicated with the selective solvent inlet of the back-extraction unit, and the selective solvent inlet of the back-extraction unit is communicated with the disulfide-containing selective solvent outlet.

[0024] Beneficial effects:

[0025] The continuous deep desulfurization method or system of the present invention can significantly reduce the sulfur content in liquefied petroleum gas, while greatly improving the effective utilization rate of alkali solution used for extraction. At the same time, the C5 fraction in the liquefied petroleum gas can be converted into a gasoline fraction after absorbing disulfide and utilized, so as to obtain a variety of low-sulfur and low-carbon hydrocarbon products, etc., which is scientific and ingenious. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a structural schematic diagram of a specific implementation mode of a continuous deep desulfurization system for liquefied petroleum gas of the present invention;

[0027] Description of Reference Numerals

[0028] 1 Pre-alkali washing unit 2 Alkali extraction unit 3 Oxidation unit

[0029] 4 Separation unit 5 Back extraction unit 6 Fractional distillation decarbonization

[0030] Five distillation units

[0031] 1a Original LPG inlet 1b Fresh alkali solution inlet pipeline 1c Circulating alkali solution outlet

[0032] 1d LPG inlet pipeline 2a Extraction alkali liquid inlet

[0033] 2b First liquefied petroleum gas product outlet 2c Mercaptan salt lye outlet pipeline

[0034] 3a Oxidizing gas inlet 3b Outlet pipeline for materials to be separated

[0035] 4a Disulfide-containing alkali solution outlet pipeline 4b Residual oxidizing gas outlet

[0036] 5b outlet for disulfide-containing selective solvent 5c gasoline pipeline

[0037] 5d Regeneration alkali solution outlet 6a Inlet for material to be distilled

[0038] 6b C4 fraction outlet 6c C5 fraction outlet pipeline DETAILED DESCRIPTION

[0039] The present application is further described in detail below through the accompanying drawings and embodiments. Through these descriptions, the characteristics and advantages of the present application will become clearer and more specific.

[0040] The word "exemplary" is used exclusively herein to mean "serving as an example, embodiment, or illustration." Any embodiment described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments. Although various aspects of the embodiments are shown in the drawings, the drawings are not necessarily drawn to scale unless otherwise noted.

[0041] In addition, the technical features involved in different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0042] In a first aspect, the present invention relates to a method for continuous deep desulfurization of liquefied petroleum gas, the method comprising the following steps:

[0043] (1) contacting the liquefied petroleum gas with an extraction alkali solution to perform alkali extraction to obtain a first liquefied petroleum gas product and a mercaptan-containing alkali solution;

[0044] (2) contacting the mercaptan-containing alkali solution from step (1) with an oxidizing gas for oxidation to obtain a material to be separated comprising residual oxidizing gas and disulfide-containing alkali solution;

[0045] (3) separating the material to be separated from step (2) to obtain residual oxidizing gas and disulfide-containing alkaline solution;

[0046] (4) contacting the disulfide-containing alkali solution from step (3) with a selective solvent for stripping to obtain a disulfide-containing selective solvent and a regenerated alkali solution;

[0047] (5) subjecting at least a portion of the first liquefied petroleum gas product obtained from step (1) to fractional distillation to obtain a mixed C3 or less fraction and a mixed C4 or greater fraction; subjecting the mixed C4 or greater fraction to decarbonization of the C5 fraction distillation to obtain a C4 fraction and a C5 fraction;

[0048] Wherein, the extraction alkali solution in step (1) comprises the regenerated alkali solution obtained from step (4); and the selective solvent in step (4) comprises the C5 fraction obtained from step (5) and a portion of the disulfide-containing selective solvent obtained from step (4).

[0049] It should be noted that the liquefied petroleum gas subjected to continuous deep desulfurization contains mercaptan. After step (1), the mercaptan contained in the liquefied petroleum gas reacts with the alkali in the extraction alkali solution to form mercaptide salts, thereby obtaining a first liquefied petroleum gas product with low sulfur content and an alkali solution containing mercaptide salts; after step (2), the mercaptide salts in the alkali solution containing mercaptide salts are oxidized to disulfide by an oxidizing gas under the catalysis of an oxidation catalyst, thereby obtaining an alkali solution containing disulfide salts, and the alkali solution containing disulfide salts and the remaining oxidizing gas are used as materials to be separated to carry out step (3); after step (3), the alkali solution containing disulfide salts is separated from the remaining oxidizing gas by a separation process such as sedimentation separation; after step (4), the alkali solution containing disulfide salts is contacted with a selective solvent, and the disulfide salts in the alkali solution containing disulfide salts are absorbed by the selective solvent, thereby obtaining a regenerated alkali solution and a selective solvent containing disulfide salts.

[0050] It should be noted that the inventors of the present application have innovatively discovered during the long-term desulfurization research and development process that by using the C5 fraction in the first liquefied petroleum gas product obtained in step (1) as a selective solvent for the back extraction in step (4), the disulfide is transferred into the selective solvent, and the resulting regenerated alkali solution is recycled for the alkali extraction in step (1), the sulfur content of the first liquefied petroleum gas product can be significantly reduced, thereby obtaining a liquefied petroleum gas product with lower sulfur.

[0051] Furthermore, in step (5), part of the first liquefied petroleum gas product obtained from step (1) is fractionally distilled to obtain a mixed C3 or less fraction and a mixed C4 or greater fraction, and then the mixed C4 or greater fraction is distilled to obtain a C5 fraction. During the distillation process, relatively high-boiling-point sulfides remaining in the liquefied petroleum gas product are gradually enriched in the C5 fraction. The obtained C5 fraction does not contain oxidizing gases, hydrogen sulfide, or mercaptans. By using such a selective solvent for back extraction, the obtained regenerated alkali solution does not only contain sulfur but also does not contain oxidizing gases, and can be recycled for alkali extraction, thereby obtaining a liquefied gas product with even lower sulfur content.

[0052] It is easy to understand that if the sulfur content of the first liquefied petroleum gas product obtained after alkali extraction is relatively high, for example, exceeding 10 μg / g, and at the same time a liquefied petroleum gas product with lower sulfur is desired, the present method can also directly carry out decarbonization distillation of the first liquefied petroleum gas product, thereby obtaining a liquefied petroleum gas product with a lower sulfur content and a lower carbon five fraction, i.e., a second liquefied petroleum gas product, and at the same time obtain a carbon five fraction enriched in relatively high-boiling point sulfides as the selective solvent used in the reverse extraction of the present method, but the present method preferably adopts a fractional distillation method to obtain a carbon five fraction enriched in relatively high-boiling point sulfides as the selective solvent described in the present method.

[0053] According to a first specific implementation of the continuous deep desulfurization method of the first aspect of the present invention, in step (5), the first liquefied petroleum gas product subjected to fractional distillation accounts for 1% to 99% of the total volume of the first liquefied petroleum gas product obtained from step (1).

[0054] It should be noted that in step (5), the volume proportion of the first liquefied petroleum gas product subjected to fractional distillation in the total amount of the first liquefied petroleum gas product obtained in step (1) can be adjusted according to specific circumstances without limitation, for example, it can be selected within the range of 1% to 99%.

[0055] According to a first specific implementation of the continuous deep desulfurization method according to the first aspect of the present invention, the continuous deep desulfurization method further comprises the following steps between step (4) and step (5):

[0056] Another part of the disulfide-containing selective solvent obtained in step (4) is intermittently or continuously mixed with the feedstock of the absorption stabilization system of the catalytic cracking unit and then separated, and the disulfide-containing selective solvent is absorbed into the stabilized gasoline, which is then desulfurized and refined to obtain low-sulfur gasoline;

[0057] The sulfur content of the low-sulfur gasoline is not more than 10 μg / g.

[0058] It should be noted that the disulfide-containing selective solvent obtained after back-extraction is divided into two parts, one part is returned for circulation for back-extraction, and the other part is returned to the absorption stabilization system of the catalytic cracking unit, mixed with the raw material of the absorption stabilization system, separated, and absorbed into the stabilized gasoline (relying on the boiling point difference, the disulfide and the C5 fraction are absorbed into the stabilized gasoline), and the subsequent desulfurization and refining operations are carried out together with the stabilized gasoline (the desulfurization and refining operations of the stabilized gasoline can be carried out according to conventional methods such as selective hydrogenation, etc.), so that the C5 fraction existing in the refinery liquefied petroleum gas can be converted into a gasoline fraction after absorbing the disulfide and utilized, and the sulfur content of the desulfurized gasoline obtained is not more than 10μg / g.

[0059] It is easy to understand that there is no specific limit on the amount of the disulfide-containing selective solvent separated into the absorption stabilization system of the catalytic cracking unit in step (4), as long as the volume ratio of the disulfide-containing alkali solution to the selective solvent in step (4) is within the required range. The C5 fraction from step (5) is continuously fed into step (4), so that the volume ratio of the selective solvent in contact with the disulfide-containing alkali solution in step (4) increases continuously. Once it increases to above the required upper limit, it is necessary to discharge the excess selective solvent (containing the disulfide selective solvent) from step (4); once the discharge of the selective solvent (containing the disulfide selective solvent) in step (4) reduces the volume ratio of the selective solvent in contact with the disulfide-containing alkali solution in step (4) to below the required lower limit, it is necessary to stop discharging the selective solvent (containing the disulfide selective solvent) from step (4); if the volume ratio of the selective solvent in contact with the disulfide-containing alkali solution in step (4) remains unchanged, the amount of a portion of the selective solvent (containing the disulfide selective solvent) discharged from step (4) to the absorption stabilization system of the catalytic cracking unit should be substantially equivalent to the amount of the C5 fraction fed from step (5) to step (4).

[0060] It can be seen that the continuous deep desulfurization method of the present invention not only significantly reduces the sulfur content in liquefied petroleum gas, but also greatly improves the effective utilization rate of the alkali solution used for extraction. At the same time, the C5 fraction present in the liquefied petroleum gas can be converted into a gasoline fraction after absorbing disulfide for utilization.

[0061] According to a first specific implementation of the continuous deep desulfurization method according to the first aspect of the present invention, step (5) further includes the following steps:

[0062] The mixed carbon three and below fractions are subjected to graded distillation to obtain dry gas, propane and propylene.

[0063] It should be noted that, in this embodiment, the first liquefied petroleum gas product is fractionally distilled to obtain a mixed C3 or less fraction and a sulfur-enriched mixed C4 or more fraction, and the mixed C3 or less fraction can be further fractionally distilled to obtain dry gas (C1-C2), propylene and propane; the sulfur-enriched mixed C4 or more fraction is decarbonized to obtain a low-sulfur C4 fraction and a sulfur-enriched C5 fraction; the low-sulfur C4 fraction can be further fractionally distilled to obtain various isomers such as C4 olefins and C4 alkanes with different boiling points; and the obtained sulfur-enriched C5 fraction is substantially free of hydrogen sulfide and mercaptan, and can be used as a selective solvent to strip the disulfide-containing alkali solution. The "substantially free of hydrogen sulfide and mercaptan" means that hydrogen sulfide and mercaptan cannot be detected according to known detection methods, or the content of hydrogen sulfide and mercaptan (in terms of sulfur) is below the lower limit of detection.

[0064] It should be noted that after the liquefied petroleum gas containing no hydrogen sulfide is subjected to alkaline extraction and desulfurization in step (1), the sulfide contained in the first liquefied petroleum gas product obtained is mainly relatively heavy non-mercaptan sulfur. Through fractional distillation or graded distillation operation, the relatively heavy sulfide contained can basically be enriched in the C5 fraction.

[0065] According to a specific implementation of the continuous deep desulfurization method according to the first aspect of the present invention, the sulfur content of the first liquefied petroleum gas product obtained from step (1) is not more than 10 μg / g.

[0066] In summary, after the continuous deep desulfurization method of the present invention, the sulfur content of the first liquefied petroleum gas product obtained in step (1) is not more than 10 μg / g. It is easy to understand that the first liquefied petroleum gas product obtained in step (1) can be subjected to fractional distillation to obtain dry gas (C1-C2) with lower sulfur, propylene, propane, mixed C4 or various C4 isomers, etc.

[0067] According to a specific embodiment of the continuous deep desulfurization method according to the first aspect of the present invention, the continuous deep desulfurization method further comprises a pre-alkali washing step before step (1):

[0068] The raw liquefied petroleum gas is contacted with a pre-alkaline washing alkali solution to perform pre-alkaline washing to obtain the liquefied petroleum gas described in step (1);

[0069] The raw liquefied petroleum gas is selected from one or more combinations of catalytic cracking liquefied petroleum gas, coking liquefied petroleum gas, pyrolysis liquefied petroleum gas and atmospheric distillation liquefied petroleum gas.

[0070] It should be noted that the sulfur content of the liquefied petroleum gas product can be further reduced through the pre-alkali washing step, and the continuous deep desulfurization method of the present invention has a wide range of applications, and is applicable to but not limited to the above-mentioned liquefied petroleum gases.

[0071] According to a specific embodiment of the continuous deep desulfurization method according to the first aspect of the present invention, the conditions for performing the alkali extraction in step (1) include:

[0072] The temperature is -5 to 100°C, preferably 25 to 50°C; the pressure is 0.5 MPa to 4.0 MPa, preferably 1.0 MPa to 2.0 MPa; the volume proportion of the extracted alkali solution in the liquefied petroleum gas is 1% to 50%, preferably 5% to 40%; and / or,

[0073] The conditions for carrying out the oxidation in step (2) include:

[0074] The temperature is -5°C to 100°C, preferably 25°C to 80°C; the pressure is 0.1MPa to 2.0MPa, preferably 0.1MPa to 1.0MPa;

[0075] The amount of the oxidizing gas used is greater than or equal to 1 to 20 times the theoretical chemical demand of the oxidizing gas required to oxidize the mercaptide contained in the mercaptide-containing alkali solution into disulfide, preferably 2 to 10 times the theoretical chemical demand.

[0076] It should be noted that the pressure of oxidation in step (2) is preferably not greater than 0.8 MPa. The theoretical chemical demand of the oxidizing gas is calculated based on the fact that 0.25 moles of oxygen are required for oxidation of each mole of mercaptide to form disulfide, and the amount of the oxidizing gas is more preferably 2 to 4 times. In order to completely oxidize the mercaptide in the alkali solution to disulfide, the alkali solution needs to be heated to a temperature that ensures catalytic oxidation. Therefore, the temperature for oxidation in step (2) should be higher than that in step (1), and the pressure should be lower than that in step (1).

[0077] In step (2), the oxidation reaction of the mercaptan-containing alkali solution and the oxidizing gas requires the catalytic action of an oxidation catalyst, which can be a metal phthalocyanine catalyst, preferably a cobalt phthalocyanine compound such as sulfonated cobalt phthalocyanine and cobalt polyphthalocyanine as an oxidation catalyst. The cobalt phthalocyanine catalyst can be used by dissolving in an alkali solution or forming a stable emulsion in an alkali solution, and the content in the alkali solution when used can be 5μg / g~1000μg / g, preferably 10μg / g~400μg / g. The cobalt phthalocyanine catalyst can also be used in the form of a supported fixed bed by being loaded on a porous material such as activated carbon, and the content of the cobalt phthalocyanine catalyst loaded on the activated carbon carrier can be 0.01%~10%, preferably 0.05%~1.0%, and the supported catalyst can use thiocyanine compounds containing nitrogen, phosphorus, oxygen, sulfur, arsenic, and antimony as well as various basic nitrides as catalytic promoters. Preferably, a supported fixed bed catalyst is used. In step (2), by controlling the conditions of the oxidation process as above and selecting the above catalyst, the mercaptide salt can be better oxidized to disulfide, thereby further reducing the sulfur content of the liquefied petroleum gas product.

[0078] According to a specific embodiment of the continuous deep desulfurization method according to the first aspect of the present invention, the conditions for performing the separation in step (3) include:

[0079] The temperature is -5°C to 80°C, preferably 25°C to 50°C; the pressure is 0.1MPa to 2.0MPa, preferably 0.1MPa to 1.0MPa; and / or,

[0080] The conditions for performing the back extraction in step (4) include:

[0081] The temperature is -5°C to 80°C, preferably 25°C to 50°C; the pressure is 0.1MPa to 2.0MPa, preferably 0.1MPa to 1.0MPa;

[0082] The volume ratio of the selective solvent to the disulfide-containing alkali solution obtained in step (3) is 1:(0.01-100), preferably 1:(0.1-10).

[0083] It should be noted that the separation in step (3) can be carried out by sedimentation separation, etc. After the separation in step (3), the remaining oxidizing gas obtained can be discharged cleanly after being treated in the subsequent liquid separator and light hydrocarbon recovery device, or sent to the flue gas desulfurization system of the catalytic cracking unit for incineration treatment, or it can be recycled after pressurization. When performing back extraction in step (4), the pressure is more preferably 0.1MPa to 0.5MPa. By controlling the conditions of separation and back extraction as above, a regenerated alkali solution free of sulfides and oxidizing gases can be better obtained to prepare a liquefied petroleum gas product with a lower sulfur content and free of oxidizing gases.

[0084] In a second aspect, the present invention relates to a continuous deep desulfurization system for liquefied petroleum gas, such as Figure 1 As shown, the continuous deep desulfurization system includes:

[0085] Alkali extraction unit 2, oxidation unit 3, separation unit 4, back extraction unit 5 and fractional distillation decarbonization five distillation unit 6;

[0086] The alkali extraction unit 2 is provided with a liquefied petroleum gas inlet pipeline 1d, an extraction alkali solution inlet 2a, a first liquefied petroleum gas product outlet 2b and a mercaptan-containing alkali solution outlet pipeline 2c; the mercaptan-containing alkali solution outlet pipeline 2c is connected to the mercaptan-containing alkali solution inlet of the oxidation unit 3; the mercaptan-containing alkali solution outlet pipeline 2c is provided with an oxidizing gas inlet 3a;

[0087] The oxidation unit 3 is provided with a material to be separated outlet pipeline 3b which is connected to the material to be separated inlet of the separation unit 4;

[0088] The separation unit 4 is provided with a disulfide-containing alkali solution outlet pipeline 4a and a residual oxidizing gas outlet 4b; the disulfide-containing alkali solution outlet pipeline 4a is connected to the disulfide-containing alkali solution inlet of the stripping unit 5;

[0089] The stripping unit 5 is provided with a selective solvent inlet, a regeneration alkali solution outlet 5d and a disulfide-containing selective solvent outlet 5b;

[0090] The regeneration alkali solution outlet 5d is connected to the extraction alkali solution inlet 2a of the alkali extraction unit 2;

[0091] The fractional distillation decarbonization five distillation unit 6 comprises a first fractional distillation unit and a decarbonization five distillation tower, the inlet 6a of the material to be distilled of the first fractional distillation unit is connected with the first liquefied petroleum gas product outlet 2b of the alkali extraction unit 2, the first fractional distillation unit is provided with a mixed C3 or less fraction outlet and a mixed C4 or more fraction outlet, the mixed C4 or more fraction outlet is connected with the inlet of the decarbonization five distillation tower, and the decarbonization five distillation tower is provided with a C5 fraction outlet pipeline 6c and a C4 fraction outlet 6b;

[0092] The C5 fraction outlet pipeline 6c is communicated with the selective solvent inlet of the back-extraction unit 5, and the selective solvent inlet of the back-extraction unit 5 is communicated with the disulfide-containing selective solvent outlet 5b.

[0093] According to a specific embodiment of the continuous deep desulfurization system of the second aspect of the present invention, the disulfide-containing selective solvent outlet 5b is connected to the inlet of the absorption stabilization system of the catalytic cracking unit through a gasoline pipeline 5c.

[0094] It should be noted that the liquefied petroleum gas containing mercaptan enters the alkali extraction unit 2 through the liquefied petroleum gas inlet pipeline 1d, and the fresh alkali solution or regenerated alkali solution used for alkali extraction enters the alkali extraction unit 2 through the extraction alkali solution inlet 2a. In the alkali extraction unit 2, the mercaptan reacts with the alkali to form mercaptide salts to obtain alkali solution containing mercaptide salts and a first liquefied petroleum gas product; the obtained first liquefied petroleum gas product is output through the first liquefied petroleum gas product outlet 2b, the obtained alkali solution containing mercaptide salts enters the oxidation unit 3 through the mercaptide salt alkali solution outlet pipeline 2c, and the oxidizing gas enters the mercaptide salt alkali solution outlet pipeline 2c through the oxidizing gas inlet 3a and then flows into the oxidation unit 3.

[0095] In the oxidation unit 3, in the presence of an oxidation catalyst, the oxidizing gas oxidizes the mercaptide salt into disulfide to obtain an alkaline solution containing disulfide. At the same time, there is residual oxidizing gas. The mixed material to be separated flows into the separation unit 4 through the outlet pipeline 3b of the material to be separated. The residual oxidizing gas and the alkaline solution containing disulfide undergo a separation process such as sedimentation separation in the separation unit 4. The residual oxidizing gas can be discharged through the outlet 4b of the residual oxidizing gas and then reused or treated. The obtained alkaline solution containing disulfide flows out through the outlet pipeline 4a of the alkaline solution containing disulfide, and then contacts with the carbon five fraction as a selective solvent flowing out from the carbon five fraction outlet pipeline 6c from the bottom of the decarbonization five distillation tower and enters the back-extraction unit 5. The disulfide in the alkaline solution is absorbed by the carbon five fraction to obtain a regenerated alkaline solution and a selective solvent containing disulfide.

[0096] The obtained regenerated alkali solution flows out through the regenerated alkali solution outlet 5d and enters the alkali extraction unit 2 through the extracted alkali solution inlet 2a. After the obtained disulfide-containing selective solvent flows out through the disulfide-containing selective solvent outlet 5b, a part of it is returned to the back-extraction unit 5 through the selective solvent inlet of the back-extraction unit 5 (connected to the C5 fraction outlet pipeline 6c) for recycling, and the other part flows into the absorption stabilization system of the catalytic cracking unit through the gasoline pipeline 5c. The disulfide-containing selective solvent is absorbed into the stabilized gasoline, and the stabilized gasoline is further desulfurized to obtain a low-sulfur gasoline with a sulfur content of not more than 10 μg / g.

[0097] After a portion of the first liquefied petroleum gas product flows out from the first liquefied petroleum gas product outlet 2b, it is fractionally distilled in the first fractional distillation unit to obtain a mixed C3 or less fraction and a mixed C4 or greater fraction; the obtained mixed C4 or greater fraction enters the decarbonization distillation tower for distillation of the decarbonization fraction to obtain a C5 fraction and a C4 fraction, and the obtained C5 fraction is used as a selective solvent for back extraction.

[0098] According to a specific embodiment of the continuous deep desulfurization system according to the second aspect of the present invention, the fractional distillation decarbonization five distillation unit 6 also includes a second fractional distillation unit, the outlet of the mixed carbon three and below fraction of the first fractional distillation unit is connected to the inlet of the second fractional distillation unit, and the second fractional distillation unit is provided with a dry gas outlet, a propane outlet and a propylene outlet.

[0099] It should be noted that the mixed C3 and below fractions obtained from the first fractional distillation unit are fractionally distilled in the second fractional distillation unit to obtain various low-sulfur, low-carbon hydrocarbon products, including dry gas (C1-C2), propylene, and propane.

[0100] In addition, as a variation of the embodiment, the continuous deep desulfurization system of the present invention can also be provided with a decarbonization five distillation tower directly connected to the first liquefied petroleum gas product outlet 2b, and the first liquefied petroleum gas product is directly decarbonized five distilled through the decarbonization five distillation tower, so as to obtain a liquefied petroleum gas product with a lower sulfur content and a lower carbon five fraction content, i.e., a second liquefied petroleum gas product, and at the same time, a carbon five fraction enriched with relatively high-boiling sulfides is obtained as a selective solvent used in reverse extraction. In this embodiment, the decarbonization five distillation tower can be provided with a second liquefied petroleum gas product outlet and a carbon five fraction outlet. The obtained second liquefied petroleum gas product with a lower sulfur content can be output through the second liquefied petroleum gas product outlet, can be directly output as a higher quality product, or can be partially or completely entered into the first fractional distillation unit for fractional distillation to obtain dry gas, propane, propylene and carbon four fractions. This embodiment can obtain a liquefied petroleum gas product with a lower sulfur content, and can obtain low-sulfur dry gas, propane, propylene and carbon four fraction products, which can be better applied to different scenarios.

[0101] According to a specific embodiment of the continuous deep desulfurization system according to the second aspect of the present invention, the continuous deep desulfurization system also includes a pre-alkali washing unit 1, and the pre-alkali washing unit 1 is provided with an original liquefied petroleum gas inlet 1a, a fresh alkali solution inlet pipeline 1b, a circulating alkali solution outlet 1c and a liquefied petroleum gas outlet; the circulating alkali solution outlet 1c is connected to the fresh alkali solution inlet pipeline 1b, and the fresh alkali solution inlet pipeline 1b is connected to the original liquefied petroleum gas inlet 1a; the liquefied petroleum gas outlet is connected to the liquefied petroleum gas inlet pipeline 1d of the alkali extraction unit 2.

[0102] It should be noted that the original liquefied petroleum gas enters the pre-alkali washing unit 1 through the original liquefied petroleum gas inlet 1a, and the fresh alkali solution or the circulating alkali solution enters the pre-alkali washing unit 1 through the fresh alkali solution inlet pipeline 1b, and the mercaptans in the original liquefied petroleum gas contact with the alkali solution, and part of the mercaptans react with the alkali to form mercaptides, and the resulting liquefied petroleum gas enters the alkali extraction unit 2 for alkali extraction.

[0103] According to a specific embodiment of the continuous deep desulfurization system according to the second aspect of the present invention, the conditions of the decarbonization five distillation tower include:

[0104] The temperature at the top of the tower is 35°C to 80°C, preferably 40°C to 60°C; the pressure at the top of the tower is 0.12MPa to 1.5MPa, preferably 0.35MPa to 1.0MPa; the temperature at the bottom of the tower is 60°C to 150°C, preferably 80°C to 100°C.

[0105] It should be noted that the tower top pressure can also be 1.2 MPa, or preferably 0.85 MPa. By controlling the conditions of the decarbonization five fraction distillation process in the decarbonization five fraction distillation tower as above, the obtained carbon five fraction is used as a selective solvent for back extraction, and the regeneration alkali solution free of sulfide and oxidizing gas can be better obtained, so as to obtain a liquefied petroleum gas product with lower sulfur and better quality.

[0106] In summary, the continuous deep desulfurization method according to the first aspect of the present invention, or the continuous deep desulfurization system according to the second aspect of the present invention, when desulfurizing liquefied petroleum gas, such as liquefied petroleum gas that has been desulfurized but contains mercaptan, uses the C5 fraction separated from the liquefied petroleum gas after alkali extraction desulfurization and substantially free of hydrogen sulfide, mercaptan and oxidizing gas as the selective solvent for back extraction, for absorbing disulfide produced by alkali oxidation; and the disulfide-containing selective solvent obtained after back extraction is not directly sent to an additional hydrogenation unit for treatment, nor is it returned to the catalytic cracking unit to be mixed with the catalytic feed for catalytic cracking reaction to result in secondary cracking, but is returned to the absorption stabilization system configured after the catalytic cracking reactor of the catalytic cracking unit, and becomes part of the stabilized gasoline after absorption (no secondary cracking is required). The sulfur-containing gasoline becomes a low-sulfur gasoline product after the well-known desulfurization and refining operation (such as selective hydrodesulfurization). In this way, the carbon pentad components in the liquefied petroleum gas are effectively removed from the liquefied petroleum gas, used as a back-extraction solvent (selective solvent) and then transferred into gasoline to be utilized to the greatest extent.

[0107] When the C5 fraction rich in heavy sulfides separated from liquefied petroleum gas is used as the stripping solvent (selective solvent) to absorb the disulfides mixed in the alkali solution, the amount of sulfides accumulated in the stripping solvent is not limited, as long as it is ensured that the regenerated alkali solution after desulfurization basically does not contain disulfides and oxidizing gases, thereby greatly improving the effective utilization rate of the alkali solution and also greatly reducing the external discharge treatment of the stripping solvent.

[0108] The operation process of the continuous deep desulfurization system of liquefied petroleum gas of the present application is described below:

[0109] like Figure 1 As shown, the original liquefied petroleum gas passes through the original liquefied petroleum gas inlet 1a and is mixed with the fresh alkali liquid from the fresh alkali liquid inlet pipeline 1b or the circulating alkali liquid flowing out of the pre-alkali washing unit 1 from the circulating alkali liquid outlet 1c, and then enters the pre-alkali washing unit 1, and the trace amount of hydrogen sulfide (and part of the mercaptan) in the liquefied petroleum gas is removed.

[0110] The liquefied petroleum gas from which hydrogen sulfide is removed flows into the alkali extraction unit 2 through the liquefied petroleum gas inlet pipeline 1d, and is countercurrently contacted with the fresh alkali solution from the extraction alkali solution inlet 2a or the regenerated alkali solution from the regenerated alkali solution outlet 5d (connected to the extraction alkali solution inlet 2a) in the alkali extraction unit 2, and the mercaptans in the liquefied petroleum gas are absorbed by the alkali solution to form mercaptide salts, and the liquefied petroleum gas from which hydrogen sulfide and mercaptans are removed flows out from the first liquefied petroleum gas product outlet 2b at the top of the alkali extraction unit 2 to become the first liquefied petroleum gas product with low sulfur. The mercaptide salt alkali solution obtained by alkali extraction flows to the oxidation unit 3 through the mercaptide salt alkali solution outlet pipeline 2c.

[0111] A portion of the first low-sulfur liquefied petroleum gas product can enter the first fractional distillation unit (not shown), and obtain a mixed C3 or less fraction and a mixed C4 or more fraction through step-by-step distillation. The mixed C3 or less fraction can be further fractionally distilled in the second fractional distillation unit to obtain low-sulfur C1-C2 (dry gas), propane and propylene. The mixed C4 or more fraction can enter the decarbonization distillation tower for decarbonization distillation. The C4 fraction after removing the C5 fraction can be distilled from the C4 fraction outlet 6b at the top of the tower, which can be used as a product. Alternatively, the C4 fraction can be further fractionally distilled to obtain isomer products such as C4 olefins and C4 alkanes with different boiling points. The relatively heavy sulfides in the first liquefied petroleum gas product eventually flow out from the C5 fraction outlet pipeline 6c with the C5 fraction to obtain a C5 fraction relatively rich in heavy sulfides. The C5 fraction can enter the stripping unit as a selective solvent for stripping.

[0112] The alkali liquor that has absorbed mercaptan, i.e., the mercaptan-containing alkali liquor, leaves the alkali extraction unit 2 from the mercaptan-containing alkali liquor outlet pipeline 2c, and flows into the oxidation unit 3 after mixing with the oxidizing gas from the oxidizing gas inlet 3a. Under the action of the fixed-bed oxidation catalyst, i.e., the metal phthalocyanine-supported catalyst, loaded in the oxidation unit 3, the mercaptide in the mercaptan-containing alkali liquor is oxidized into disulfide by the oxidizing gas, and the alkali liquor is regenerated to a certain extent and becomes the disulfide-containing alkali liquor (lean alkali liquor). The disulfide-containing alkali liquor (lean alkali liquor) and the excess air tail gas leave the oxidation unit 3 from the material outlet pipeline 3b to be separated, and flow into the separation unit 4 (which may be a sedimentation separation system) for separation operation.

[0113] In the separation unit 4, the excess air tail gas is released through the residual oxidizing gas outlet 4b, and is discharged cleanly after being treated in the subsequent liquid separator and light hydrocarbon recovery device, or sent to the flue gas desulfurization system of the catalytic cracking unit for incineration. The disulfide-containing alkali liquid (lean alkali liquid) from which the residual oxidizing gas is removed flows out of the separation unit 4 and flows into the stripping unit 5 through the disulfide-containing alkali liquid outlet pipeline 4a.

[0114] Before flowing in, the disulfide-containing alkali liquor (lean alkali liquor) first contacts the C5 fraction used as the organic solvent (selective solvent) for back extraction from the C5 fraction outlet pipeline 6c. After contact, the disulfides in the disulfide-containing alkali liquor (lean alkali liquor) are almost all absorbed by the C5 fraction. Then the alkali liquor is separated from the C5 fraction containing disulfide, and the C5 fraction containing disulfide (containing the disulfide selective solvent) flows out through the disulfide selective solvent outlet 5b and is divided into two parts, one part is returned to the back extraction unit 5 through the selective solvent inlet of the back extraction unit 5 for recycling; the other part flows out through the gasoline pipeline 5c and returns to the absorption stabilization system of the catalytic cracking unit, and is absorbed into the catalytic cracking stable gasoline, and then performs subsequent desulfurization and refining operations together with the stable gasoline (not shown). The operation of the absorption stabilization system is carried out in a conventional manner, and the disulfide and the C5 fraction are absorbed into the stable gasoline by relying on the boiling point difference. The desulfurization and refining operation of the stable gasoline is carried out in a conventional manner. After desulfurization, the sulfur content of the stabilized gasoline is reduced to below 10 μg / g. The regenerated alkali solution obtained by back extraction flows out through the regenerated alkali solution outlet 5d and flows into the alkali extraction unit 2 through the extraction alkali solution inlet 2a for recycling.

[0115] The present invention is further described in detail below by way of examples, but the present invention is not limited thereto.

[0116] The examples are intended to generally represent the process of a preferred embodiment of the present invention, and are not intended to give details of the vessels, heaters, coolers, pumps, compressors, valves, process control equipment, etc., which are basic equipment for those skilled in the art. The examples given are intended to illustrate the present invention, but are not intended to limit the present invention in any way.

[0117] Example 1

[0118] The basic properties of the liquefied petroleum gas to be desulfurized are shown in Table 1.

[0119] Table 1

[0120]

[0121] use Figure 1 The flow chart (system) shown in Table 1 is used to desulfurize the liquefied petroleum gas, but no pre-alkali washing is performed. A NaOH solution with a concentration of 15% (by weight) is used as the extraction alkali solution. In the alkali extraction unit, the volume ratio of the extraction alkali solution to the liquefied petroleum gas is 1:9, the temperature is room temperature, and the pressure is 1.6MPa. After alkali extraction, the first liquefied petroleum gas product and the mercaptan-containing alkali solution are obtained. The mercaptan-containing alkali solution flows to the oxidation unit to contact with the oxidizing gas for oxidation.

[0122] The first liquefied petroleum gas product obtained after extraction is initially sent to a fractional distillation tower for step-by-step distillation. After the process is fully connected and the operation is gradually stabilized, the amount of the first liquefied petroleum gas product sent to the fractional distillation can be reduced according to actual needs. For example, the volume fraction of the first liquefied petroleum gas product sent to the fractional distillation (compared with the total amount of the first liquefied petroleum gas product obtained after extraction) can be adjusted in the range of 1% to 99%. The first liquefied petroleum gas product is fractionally distilled to obtain a mixed carbon three or less fraction and a mixed carbon four or more fraction. The mixed carbon four or more fraction obtained by fractional distillation is distilled in a decarbonization five distillation tower, and a carbon five fraction and a carbon four fraction of a non-carbon five fraction are obtained by distillation in the decarbonization five distillation tower. The obtained carbon five fraction enters the reverse extraction unit through the carbon five fraction outlet pipeline 6c and is used as a selective solvent. The obtained carbon four fraction can be used as a low-sulfur low-carbon hydrocarbon product. The operating conditions of the decarbonization five distillation tower are: tower top pressure 0.5 MPa, tower top temperature 51.5° C., tower bottom temperature 90° C., and reflux ratio 1.0.

[0123] The oxidation unit is loaded with a metal phthalocyanine type oxidation catalyst, with activated carbon (provided by Beijing Guanghua Jingke Activated Carbon Factory, brand GH-8) as a carrier, sulfonated cobalt phthalocyanine as the main active component, with a content of 0.6%, and dodecyl dimethyl benzyl ammonium chloride (content 1%) and isopropanolamine (content 5%) as auxiliary active components. The preparation process of the catalyst is as follows: according to the target loading, a dilute ammonia solution of sulfonated cobalt phthalocyanine, a mixed solution of dodecyl dimethyl benzyl ammonium chloride and isopropanolamine are respectively prepared, and equal volumes of activated carbon are successively adsorbed, and each time it is dried by a rotary evaporator.

[0124] In the oxidation unit, the amount of air injected is 4 times the theoretical chemical demand for oxidizing gas required to oxidize the mercaptide contained in the mercaptide alkali solution into disulfide, which is equivalent to a volume ratio of air injection to mercaptide alkali solution of about 3.1:1 under standard conditions, an alkali solution oxidation temperature of 50°C, and a pressure of 0.40MPa. After the oxidation process in the oxidation unit, a material to be separated containing residual oxidizing gas and disulfide alkali solution is obtained. The material to be separated flows into the separation unit.

[0125] Afterwards, in the separation unit, the excess air tail gas is separated from the disulfide-containing alkali solution, and the obtained disulfide-containing alkali solution (lean alkali solution) flows into the back extraction unit. The conditions of the separation unit are: temperature of 50°C and pressure of 0.30MPa.

[0126] In the back-extraction operation, the inlet pressure of the back-extraction unit is 0.30MPa, and the C5 fraction including the residual heavy sulfide from the bottom of the decarbonization distillation tower is in contact with the disulfide-containing alkali solution (lean alkali solution) at room temperature. The total volume of the C5 fraction and a part of the recycled disulfide-rich C5 fraction (i.e., containing disulfide selective solvent) is in a volume ratio of 1:1 to the disulfide-containing alkali solution (lean alkali solution). The disulfide in the disulfide-containing alkali solution (lean alkali solution) is absorbed into the C5 fraction; then the disulfide-rich C5 fraction is separated from the alkali solution, and the disulfide-rich C5 fraction (containing disulfide selective solvent) after separation is ) will be divided into two parts, one part is returned to the back extraction unit and mixed with the C5 fraction including the residual heavy sulfide from the bottom of the decarbonization distillation tower 6 as a selective solvent for recycling, and the other part is returned to the absorption stabilization system arranged after the catalytic cracking reactor in the catalytic cracking unit, mixed with the raw material of the absorption stabilization system and separated by the absorption stabilization system, and the other part of the C5 fraction rich in disulfide (including the disulfide selective solvent) is absorbed into the gasoline and discharged together with the stabilized gasoline, and then the desulfurization and refining operation of the stabilized gasoline is carried out by selective hydrogenation to reduce its sulfur content to less than 10μg / g. Among them, the amount of the other part of the C5 fraction rich in disulfide (including the disulfide selective solvent) mixed with the raw material of the catalytic cracking absorption stabilization system is equal to the amount of the C5 fraction obtained from the bottom of the decarbonization distillation tower 6, and the ratio is 1:1. The regenerated alkali solution obtained by back extraction is returned to the alkali extraction unit as the extraction alkali solution for recycling.

[0127] The results show that not only can a low-sulfur first liquefied petroleum gas product be obtained, but also after treatment, the first liquefied petroleum gas product can be subjected to fractional distillation to obtain a variety of low-sulfur light hydrocarbon components, including low-sulfur C1-C2 (dry gas), propane, propylene and C4 fractions, as well as various isomers of C4. No sodium mercaptide can be detected in the alkali solution after regeneration (determined by potentiometric titration), indicating that the alkali solution containing mercaptans has been completely regenerated.

[0128] Comparative Example 1

[0129] use Figure 1 The flow chart shown in the figure is used to desulfurize the liquefied petroleum gas in Table 1. The operating conditions and catalysts used are the same as those in Example 1, except that the first liquefied petroleum gas product after alkali extraction is not subjected to fractional distillation and C5 removal, and the C5 fraction is not used as the selective solvent-organic solvent for disulfides in the back-extraction alkali solution. Instead, the refined gasoline product (sulfur content not more than 10 μg / g, distillation range 33°C to 200°C, aromatic volume fraction 28.1%) stored in the tank area of ​​the refinery is used as the selective solvent-organic solvent for disulfides in the back-extraction unit. The desulfurization results are shown in Table 2.

[0130] Table 2

[0131]

[0132] Table 3 gives the determination data of the residual disulfide sulfur content in the regenerated alkali solution after back extraction. The sulfur content is determined as follows: 90℃-120℃ petroleum ether (sulfur-free) is mixed with equal mass of the regenerated alkali solution after back extraction to transfer the residual disulfide in the regenerated alkali solution after back extraction into the petroleum ether, and then the sulfur content in the petroleum ether is measured after standing and separation. This sulfur content value is the residual disulfide sulfur content in the regenerated alkali solution after back extraction.

[0133] Table 3

[0134]

[0135] It can be seen from the data in Table 3 that when the C5 fraction in the first liquefied petroleum gas product after alkali extraction is used as the back-extraction solvent (selective solvent), the disulfide sulfur content in the regenerated alkali solution after back-extraction is much lower than when refined gasoline is used as the back-extraction solvent (selective solvent), indicating that the effect of using the C5 fraction in the first liquefied petroleum gas product after alkali extraction as the back-extraction solvent (selective solvent) is better. It was also found during operation that when the C5 fraction in the first liquefied petroleum gas product after alkali extraction is recycled as the back-extraction solvent (selective solvent), the disulfide content absorbed by it is several times higher than the disulfide content absorbed when refined gasoline is recycled as the back-extraction solvent (selective solvent) (~3000μg / g), and it will not affect the extraction and desulfurization of liquefied petroleum gas. In this way, the C5 fraction in the first liquefied petroleum gas product after alkali extraction can be discharged as little as possible (sent to the absorption stabilization system of the catalytic cracking unit), while when refined gasoline is used as the back-extraction solvent (selective solvent), the disulfide remaining in the regenerated alkali solution after back-extraction is sometimes quite large, which may be due to the emulsification of aromatic components and alkali solution, and air may still remain in the regenerated alkali solution after back-extraction, so the refined gasoline used as the back-extraction solvent (selective solvent) needs to be discharged relatively more.

[0136] It can be inferred that when the C5 fraction in the first liquefied petroleum gas product after alkali extraction is used as the selective solvent, the effective utilization rate of alkali solution will be higher and the alkali solution discharge will be less.

[0137] In the description of the present application, it should be noted that the terms "upper", "lower", "inside", "outside", "front", "back", "left", "right", etc. indicate directions or positional relationships based on the directions or positional relationships in the working state of the present application. They are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction. Therefore, they should not be understood as limitations on the present application.

[0138] In the description of this application, it should be noted that, unless otherwise clearly specified and limited, the terms "install", "connect", and "connect" should be understood in a broad sense. For ordinary technicians in this field, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0139] The present application has been described above in conjunction with preferred embodiments, but these embodiments are only exemplary and serve only as an illustration. On this basis, various replacements and improvements can be made to the present application, all of which fall within the scope of protection of the present application.

Claims

1. A method for continuous deep desulfurization of liquefied petroleum gas, characterized in that: The continuous deep desulfurization method comprises the following steps: (1) contacting the liquefied petroleum gas with an extraction alkali solution to perform alkali extraction to obtain a first liquefied petroleum gas product and a mercaptan-containing alkali solution; (2) contacting the mercaptan-containing alkali solution from step (1) with an oxidizing gas for oxidation to obtain a material to be separated comprising residual oxidizing gas and disulfide-containing alkali solution; (3) separating the material to be separated from step (2) to obtain residual oxidizing gas and disulfide-containing alkaline solution; (4) contacting the disulfide-containing alkali solution from step (3) with a selective solvent for stripping to obtain a disulfide-containing selective solvent and a regenerated alkali solution; (5) subjecting at least a portion of the first liquefied petroleum gas product obtained from step (1) to fractional distillation to obtain a mixed C3 or less fraction and a mixed C4 or greater fraction; subjecting the mixed C4 or greater fraction to decarbonization of the C5 fraction distillation to obtain a C4 fraction and a C5 fraction; Wherein, the extraction alkali solution in step (1) comprises the regenerated alkali solution obtained from step (4); and the selective solvent in step (4) comprises the C5 fraction obtained from step (5) and a portion of the disulfide-containing selective solvent obtained from step (4).

2. The continuous deep desulfurization method according to claim 1, characterized in that: In step (5), the first liquefied petroleum gas product subjected to fractional distillation accounts for 1% to 99% of the total volume of the first liquefied petroleum gas product obtained from step (1).

3. The continuous deep desulfurization method according to claim 1, characterized in that: The continuous deep desulfurization method further includes the following steps between step (4) and step (5): Another part of the disulfide-containing selective solvent obtained in step (4) is intermittently or continuously mixed with the feedstock of the absorption stabilization system of the catalytic cracking unit and then separated, and the disulfide-containing selective solvent is absorbed into the stabilized gasoline, which is then desulfurized and refined to obtain low-sulfur gasoline; The sulfur content of the low-sulfur gasoline is not more than 10 μg / g.

4. The continuous deep desulfurization method according to claim 1, characterized in that: Step (5) also includes the following steps: The mixed carbon three fractions and below are subjected to graded distillation to obtain dry gas, propane and propylene.

5. The continuous deep desulfurization method according to claim 1, characterized in that: The continuous deep desulfurization method further comprises a pre-alkali washing step before step (1): The raw liquefied petroleum gas is contacted with a pre-alkaline washing alkali solution to perform pre-alkaline washing to obtain the liquefied petroleum gas described in step (1); The raw liquefied petroleum gas is selected from one or more combinations of catalytic cracking liquefied petroleum gas, coking liquefied petroleum gas, pyrolysis liquefied petroleum gas and atmospheric distillation liquefied petroleum gas.

6. The continuous deep desulfurization method according to claim 1, characterized in that: The conditions for performing the alkaline extraction in step (1) include: The temperature is -5 to 100°C, preferably 25 to 50°C; the pressure is 0.5 MPa to 4.0 MPa, preferably 1.0 MPa to 2.0 MPa; the volume proportion of the extracted alkali solution in the liquefied petroleum gas is 1% to 50%, preferably 5% to 40%; and / or, The conditions for carrying out the oxidation in step (2) include: The temperature is -5°C to 100°C, preferably 25°C to 80°C; the pressure is 0.1MPa to 2.0MPa, preferably 0.1MPa to 1.0MPa; The amount of the oxidizing gas used is greater than or equal to 1 to 20 times the theoretical chemical demand of the oxidizing gas required to oxidize the mercaptide contained in the mercaptide-containing alkali solution into disulfide, preferably 2 to 10 times the theoretical chemical demand.

7. The continuous deep desulfurization method according to claim 1, characterized in that: The conditions for performing the separation in step (3) include: The temperature is -5°C to 80°C, preferably 25°C to 50°C; the pressure is 0.1MPa to 2.0MPa, preferably 0.1MPa to 1.0MPa; and / or, The conditions for performing the back extraction in step (4) include: The temperature is -5°C to 80°C, preferably 25°C to 50°C; the pressure is 0.1MPa to 2.0MPa, preferably 0.1MPa to 1.0MPa; The volume ratio of the selective solvent to the disulfide-containing alkali solution obtained in step (3) is 1:(0.01-100), preferably 1:(0.1-10).

8. The continuous deep desulfurization method according to claim 1, characterized in that: The sulfur content of the first liquefied petroleum gas product obtained from step (1) is not more than 10 μg / g.

9. A continuous deep desulfurization system for liquefied petroleum gas, characterized in that: The continuous deep desulfurization system includes: Alkali extraction unit (2), oxidation unit (3), separation unit (4), back extraction unit (5) and fractional distillation decarbonization unit (6); The alkali extraction unit (2) is provided with a liquefied petroleum gas inlet pipeline (1d), an extraction alkali solution inlet (2a), a first liquefied petroleum gas product outlet (2b) and a mercaptan-containing alkali solution outlet pipeline (2c); the mercaptan-containing alkali solution outlet pipeline (2c) is connected to the mercaptan-containing alkali solution inlet of the oxidation unit (3); the mercaptan-containing alkali solution outlet pipeline (2c) is provided with an oxidizing gas inlet (3a); The oxidation unit (3) is provided with a material to be separated outlet pipeline (3b) which is connected to the material to be separated inlet of the separation unit (4); The separation unit (4) is provided with a disulfide-containing alkali solution outlet pipeline (4a) and a residual oxidizing gas outlet (4b); the disulfide-containing alkali solution outlet pipeline (4a) is connected to the disulfide-containing alkali solution inlet of the stripping unit (5); The stripping unit (5) is provided with a selective solvent inlet, a regeneration alkali solution outlet (5d) and a disulfide-containing selective solvent outlet (5b); The regeneration alkali solution outlet (5d) is connected to the extraction alkali solution inlet (2a) of the alkali extraction unit (2); The fractional distillation decarbonization five distillation unit (6) comprises a first fractional distillation unit and a decarbonization five distillation tower, the inlet (6a) of the first fractional distillation unit for the material to be distilled is connected to the first liquefied petroleum gas product outlet (2b) of the alkali extraction unit (2), the first fractional distillation unit is provided with a mixed C3 or less fraction outlet and a mixed C4 or more fraction outlet, the mixed C4 or more fraction outlet is connected to the inlet of the decarbonization five distillation tower, and the decarbonization five distillation tower is provided with a C5 fraction outlet pipeline (6c) and a C4 fraction outlet (6b); The C5 fraction outlet pipeline (6c) is connected to the selective solvent inlet of the back-extraction unit (5), and the selective solvent inlet of the back-extraction unit (5) is connected to the disulfide-containing selective solvent outlet (5b).

10. The continuous deep desulfurization system according to claim 9, characterized in that: The disulfide-containing selective solvent outlet (5b) is connected to the inlet of the absorption stabilization system of the catalytic cracking unit through a gasoline pipeline (5c).

11. The continuous deep desulfurization system according to claim 9, characterized in that: The fractionated distillation decarbonization five-element distillation unit (6) further comprises a second fractionated distillation unit, wherein the outlet of the mixed carbon three and below fraction of the first fractionated distillation unit is connected to the inlet of the second fractionated distillation unit, and the second fractionated distillation unit is provided with a dry gas outlet, a propane outlet and a propylene outlet.

12. The continuous deep desulfurization system according to claim 9, characterized in that: The continuous deep desulfurization system further comprises a pre-alkali washing unit (1), wherein the pre-alkali washing unit (1) is provided with an original liquefied petroleum gas inlet (1a), a fresh alkali liquid inlet pipeline (1b), a circulating alkali liquid outlet (1c) and a liquefied petroleum gas outlet; the circulating alkali liquid outlet (1c) is connected to the fresh alkali liquid inlet pipeline (1b), and the fresh alkali liquid inlet pipeline (1b) is connected to the original liquefied petroleum gas inlet (1a); and the liquefied petroleum gas outlet is connected to the liquefied petroleum gas inlet pipeline (1d) of the alkali extraction unit (2).

13. The continuous deep desulfurization system according to claim 9, characterized in that: The conditions of the decarbonization five distillation tower include: The temperature at the top of the tower is 35°C to 80°C, preferably 40°C to 60°C; the pressure at the top of the tower is 0.12MPa to 1.5MPa, preferably 0.35MPa to 1.0MPa; the temperature at the bottom of the tower is 60°C to 150°C, preferably 80°C to 100°C.

Images