A continuous deep desulfurization method and system for liquefied petroleum gas

Abstract

The present application relates to a kind of liquefied petroleum gas continuous deep desulfurization method, system, the method includes: (1) make liquefied petroleum gas and extract lye contact and carry out alkali extraction, obtain first liquefied petroleum gas product and sulfidic alkali liquor;(2) make sulfidic alkali liquor and oxidizing gas contact, obtain to be separated material;(3) make to be separated material separation, obtain remaining oxidizing gas and contain disulfide alkali liquor;(4) make contain disulfide alkali liquor and carry out back extraction, obtain contain disulfide selective solvent and regenerated lye;(5) make at least part first liquefied petroleum gas product and carry out fractional distillation, make mixed carbon four above fraction and carry out decarburization five fraction distillation and obtain carbon five fraction;Selective solvent in step (4) includes carbon five fraction and a part of contain disulfide selective solvent.The present application significantly reduces the sulfur content of liquefied petroleum gas, substantially improves the effective utilization rate of alkali for extraction, converts carbon five fraction existing in liquefied petroleum gas into gasoline fraction and utilizes.

Description

Technical Field

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

[0002] Liquefied petroleum gas (LPG) is an important petroleum refining product. It is primarily derived from catalytic cracking, delayed coking, atmospheric and vacuum catalysis, hydrocracking, and continuous reforming. Its primary components are C3 and C4 hydrocarbons, with small amounts of C2 and C5 hydrocarbons. Its sulfides are primarily hydrogen sulfide and mercaptan sulfur, with small amounts of other low-boiling sulfides. LPG is used as a civilian and automotive fuel, as well as a chemical feedstock. For example, it can be used to extract commercial propane and commercial propylene, as a raw material for methyl tert-butyl ether (MTBE), a gasoline antiknock agent, and as a raw material for the production of composite gasoline and alkylated gasoline. LPG typically undergoes desulfurization and refining before it can be used as a target product. In the national, industry, group and enterprise standards involving liquefied petroleum gas and liquefied petroleum gas derivatives, the specific 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 shall not exceed 343mg / m 3 , the content of C5 and above components (volume fraction) shall not exceed 3.0% or not exceed 2.0%; 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%; national standard GB / T7716-2014 requires that the sulfur content of high-quality 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 the alcoholamine extraction method, and mercaptan extraction mostly adopts the alkali solution extraction method. The alcoholamine extraction is to contact the liquefied petroleum gas with a solution of an alcoholamine compound (such as methyldiethanolamine, MDEA) to absorb the hydrogen sulfide in the liquefied gas. The alcoholamine solution after absorbing hydrogen sulfide is continuously used after desorption treatment. The alkali solution extraction is to contact the liquefied petroleum gas after the hydrogen sulfide is removed by alcoholamine extraction with an inorganic alkali solution to absorb the mercaptans in the liquefied gas. The alkali solution after absorbing the 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 produce light hydrocarbon products with varying carbon numbers and even different isomers. To obtain low-carbon olefins that meet sulfur requirements, synthesize low-sulfur methyl tert-butyl ether, and produce composite gasoline or alkylate gasoline, refineries generally require the sulfur content of refined LPG to be no greater 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 containing acidic sulfides, such as natural gas, liquefied petroleum gas, light gasoline, naphtha, alkanes, and olefins. The method is to contact the hydrocarbon fluid with alkali solution, and the acidic sulfides in the hydrocarbon stream, mainly mercaptans, react with the alkali solution to form mercaptides and are removed from the hydrocarbon stream.

[0006] Directly discharging mercaptide-containing alkali liquor is neither economical nor environmentally friendly. Typically, an oxidation process is used to oxidize mercaptides into disulfides, thereby regenerating the mercaptide-containing alkali liquor (see U.S. Patent No. 2,853,432). Specifically, this process involves injecting air and an oxidation catalyst into the used alkali liquor to oxidize the mercaptides dissolved in the alkali liquor into disulfides, thereby regenerating the alkali liquor. However, the alkali liquor after oxidation and regeneration contains disulfides. If recycled for extraction of hydrocarbon streams, the hydrocarbon streams will absorb the disulfides, making it difficult to reduce the sulfur content of the alkali liquor and even increasing it. Therefore, it is necessary to separate the regenerated alkali liquor from the disulfides therein by sedimentation separation and / or back extraction with an organic hydrocarbon solvent. This allows the regenerated alkali liquor to continue to be used to remove mercaptans from the hydrocarbon stream, while effectively reducing the sulfur content of the hydrocarbon stream, thereby significantly 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 containing mercaptans, including liquefied petroleum gas, 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 an injected oxidizing gas and an oxidation catalyst to oxidize the mercaptides in the alkaline solution to disulfides, thereby regenerating the alkaline solution. The oxidation catalyst is a well-known metal phthalocyanine, such as sulfonated cobalt phthalocyanine or polyphthalocyanine cobalt. Typically, a metal phthalocyanine catalyst is intermittently or continuously injected into the oxidation step along with fresh alkali solution (commercially available metal phthalocyanine catalysts are typically present as a powdered solid or liquid mixture). In the phase separation step, the regenerated alkali solution from the oxidation step is allowed to settle with a mixture of disulfides and oxidizing gases, releasing excess oxidizing gases while separating the alkali solution from the aggregated disulfides, thereby removing the disulfides. Due to the small density difference between disulfides and the alkali solution and the relatively short settling time, it is often difficult to completely separate and remove the aggregated disulfides during the phase separation step. Therefore, a stripping step is employed. In the stripping step, the alkali solution from the phase separation step is mixed with an organic hydrocarbon solvent, and the remaining disulfides in the alkali solution are stripped into the organic hydrocarbon solvent to separate the regenerated alkali solution from the disulfide-containing solvent. The stripped regenerated alkali solution is then 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 liquor carried, especially for stripping solvents containing aromatics and easily emulsified, it is difficult to wash away the trace amount of alkali liquor carried even by water washing, which can easily lead to poisoning and deactivation of the hydrogenation catalyst during hydrogenation treatment. There is also a method of sending hydrocarbon stripping solvents back to the catalytic cracking unit for treatment and high-temperature cracking in the catalytic cracking unit. However, the trace amount of alkali liquor carried by the stripping solvent can 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 alkali solution oxidation regeneration 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] Furthermore, before the regenerated alkali liquor returns to the extraction system and comes into contact with the mercaptan-containing hydrocarbon stream, trace amounts of oxidizing gases often remain. This causes some mercaptans to be oxidized to disulfides in the extraction system, remaining in the extracted hydrocarbon stream. To address this issue, patent CN 200510132299.7 employs nitrogen displacement to remove oxidizing gases carried by the regenerated alkali liquor prior to stripping. However, this displacement is often incomplete, particularly when the stripping solvent contains oxidizing gases (e.g., naphtha and gasoline stored in refinery tank farms, which are susceptible to air exposure), potentially compromising the deep desulfurization of hydrocarbon streams, including liquefied petroleum gas.

[0011] In particular, the boiling point of liquefied petroleum gas (LPG) is below 0°C. Besides hydrogen sulfide, the sulfides it contains are primarily methyl mercaptan (boiling point 6.2°C), with other higher molecular weight sulfides being less common. After hydrogen sulfide is removed from the LPG via amine extraction, mercaptan sulfides are removed via alkali extraction. If the non-hydrogen sulfide, non-mercaptan, higher molecular weight sulfides in the LPG exceed the target sulfur content, alkali extraction will not achieve the desired desulfurization goal. Furthermore, if the content of C5+ components exceeds the standard, the resulting LPG will not meet quality standards, regardless of how the desulfurization is performed.

[0012] In summary, how to continuously and deeply desulfurize liquefied petroleum gas to significantly reduce the sulfur content of liquefied petroleum gas products, 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, the purpose of which is to reduce the sulfur content of liquefied petroleum gas products, convert and utilize the C5 fraction in the liquefied petroleum gas products and improve the effective utilization rate of alkali liquor.

[0014] In a first aspect, the present application relates to a continuous deep desulfurization method of liquefied petroleum gas, comprising the following steps: (1) contacting the liquefied petroleum gas with an extractive alkali solution for alkali extraction to obtain a first liquefied petroleum gas product and a sulfidic alkali solution; (2) contacting the sulfidic alkali solution from step (1) with an oxidizing gas for oxidation to obtain a separation material comprising residual oxidizing gas and a disulfidic alkali solution; (3) separating the separation material from step (2) to obtain residual oxidizing gas and a disulfidic alkali solution; (4) contacting the disulfidic alkali solution from step (3) with a selective solvent for back-extraction to obtain a disulfidic selective solvent and a regenerated alkali solution; (5) fractionally distilling at least part of the first liquefied petroleum gas product from step (1) to obtain a mixed carbon three and below fraction and a mixed carbon four and above fraction; and distilling the mixed carbon four and above fraction to remove a carbon five fraction to obtain a carbon four fraction and a carbon five fraction; wherein the extractive alkali solution in step (1) comprises the regenerated alkali solution from step (4); and the selective solvent in step (4) comprises the carbon five fraction from step (5) and part of the disulfidic selective solvent from step (4).

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

[0016] an alkali extraction unit, an oxidation unit, a separation unit, a back-extraction unit, and a fractional distillation and carbon five distillation unit;

[0017] the alkali extraction unit is provided with a liquefied petroleum gas inlet pipeline, an extractive alkali solution inlet, a first liquefied petroleum gas product outlet, and a sulfidic alkali solution outlet pipeline; the sulfidic alkali solution outlet pipeline is in communication with a sulfidic alkali solution inlet of the oxidation unit; and the sulfidic alkali solution outlet pipeline is provided with an oxidizing gas inlet;

[0018] the oxidation unit is provided with a separation material outlet pipeline in communication with a separation material inlet of the separation unit;

[0019] the separation unit is provided with a disulfidic alkali solution outlet pipeline and a residual oxidizing gas outlet; the disulfidic alkali solution outlet pipeline is in communication with a disulfidic alkali solution inlet of the back-extraction unit;

[0020] the back-extraction unit is provided with a selective solvent inlet, a regenerated alkali solution outlet, and a disulfidic selective solvent outlet;

[0021] the regenerated alkali solution outlet is in communication with an extractive alkali solution inlet of the alkali extraction unit;

[0022] The fractional distillation decarbonization pentahydrate distillation unit comprises a first fractional distillation unit and a decarbonization pentahydrate distillation tower, wherein the inlet of the material to be distilled of the first fractional distillation unit is connected to the first liquefied petroleum gas product outlet of the alkali extraction unit, the first fractional distillation unit is provided with an outlet for a mixed C3 or less fraction and an outlet for a mixed C4 or greater fraction, the outlet for the mixed C4 or greater fraction is connected to the inlet of the decarbonization pentahydrate distillation tower, and the decarbonization pentahydrate distillation tower is provided with an outlet pipeline for a C5 fraction and an outlet for a C4 fraction;

[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 significantly reduces 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 present in the liquefied petroleum gas can be converted into a gasoline fraction after absorbing disulfide and utilized, thereby obtaining 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 embodiment of a continuous deep desulfurization system for liquefied petroleum gas according to 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 liquefied petroleum gas 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 Disulfide-containing selective solvent outlet 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 will be further described in detail below through the accompanying drawings and examples, through which the features and advantages of the present application will become more clear and distinct.

[0040] The word "exemplary" is used exclusively herein to mean "serving as an example, example, 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 indicated.

[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 back extraction 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 lower fraction and a mixed C4 or higher fraction; subjecting the mixed C4 or higher fraction to decarbonization of the C5 fraction to 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 mercaptans. In step (1), the mercaptans contained in the liquefied petroleum gas react with the alkali in the extracted alkali liquor to form mercaptans, thereby obtaining a low-sulfur first liquefied petroleum gas product and an alkali liquor containing mercaptans; in step (2), the mercaptans in the alkali liquor containing mercaptans are oxidized to disulfides by an oxidizing gas under the catalysis of an oxidation catalyst, thereby obtaining an alkali liquor containing disulfides. The alkali liquor containing disulfides and the remaining oxidizing gas are used as materials to be separated and then subjected to step (3); in step (3), the alkali liquor containing disulfides is separated from the remaining oxidizing gas through a separation process such as sedimentation separation; in step (4), the alkali liquor containing disulfides is contacted with a selective solvent, and the disulfides in the alkali liquor containing disulfides are absorbed by the selective solvent, thereby obtaining a regenerated alkali liquor and a selective solvent containing disulfides.

[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 from step (1) as a selective solvent for the back extraction of step (4), the disulfides are transferred into the selective solvent, and the resulting regenerated alkaline solution is recycled for the alkaline extraction of step (1), which can significantly reduce the sulfur content of the first liquefied petroleum gas product and obtain 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, the relatively high-boiling-point sulfides remaining in the liquefied petroleum gas product are gradually enriched into 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 is 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 it is desired to obtain a liquefied petroleum gas product with lower sulfur, 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, namely, the 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 embodiment of the continuous deep desulfurization method according to 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 from step (1) can be adjusted according to specific circumstances and is not limited. For example, it can be selected within the range of 1% to 99%.

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

[0056] The other part of the disulfide-selective solvent obtained from 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-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 no 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 and stabilization system of the catalytic cracking unit, mixed with the raw materials of the absorption and 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 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 present in the refinery liquefied petroleum gas can be converted into a gasoline fraction after absorbing disulfide and utilized, and the sulfur content of the obtained desulfurized gasoline 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 and the selective solvent in contact in step (4) is ensured to be 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) continuously increases. Once it increases to above the required upper limit, the excess selective solvent (containing disulfide-selective solvent) needs to be discharged from step (4); once the discharge of the selective solvent (containing disulfide-selective solvent) in step (4) causes the volume ratio of the selective solvent in contact with the disulfide-containing alkali solution in step (4) to decrease to below the required lower limit, the discharge of the selective solvent (containing disulfide-selective solvent) from step (4) needs to be stopped; 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 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 embodiment 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 C3 and lower fractions are subjected to fractional 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 greater fraction. 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 greater 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 sulfur-enriched C5 fraction is substantially free of hydrogen sulfide and mercaptans and can be used as a selective solvent to strip disulfide-containing alkali liquor. The term "substantially free of hydrogen sulfide and mercaptans" means that hydrogen sulfide and mercaptans cannot be detected according to known detection methods, or the content of hydrogen sulfide and mercaptans (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 sulfides contained in the first liquefied petroleum gas product obtained are mainly relatively heavy non-mercaptan sulfur. Through fractional distillation or graded distillation operations, the relatively heavy sulfides contained can basically be enriched in the C5 fraction.

[0065] According to a specific embodiment 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 greater than 10 μg / g.

[0066] In summary, through the continuous deep desulfurization method of the present invention, the sulfur content of the first liquefied petroleum gas product obtained in step (1) is no 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 content, 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 includes a pre-alkali washing step before step (1):

[0068] Contacting the original liquefied petroleum gas 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-alkaline 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 several types of 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 alkaline 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 for oxidizing the mercaptide contained in the mercaptan-containing alkali solution to form disulfide, preferably 2 to 10 times the theoretical chemical demand.

[0076] It should be noted that the oxidation pressure in step (2) is more preferably not greater than 0.8 MPa. The theoretical chemical demand for the oxidizing gas is calculated based on the fact that 0.25 moles of oxygen are required for the oxidation of each mole of mercaptide to form disulfide. The amount of the oxidizing gas used is more preferably 2 to 4 times the amount of the oxidizing gas. 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 or polycobalt phthalocyanine 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 during use can be 5 μg / g to 1000 μg / g, preferably 10 μg / g to 400 μg / g. The cobalt phthalocyanine catalyst can also be used by being loaded on a porous material such as activated carbon in the form of a loaded fixed bed, and the content of the cobalt phthalocyanine catalyst loaded on the activated carbon carrier during use can be 0.01% to 10%, preferably 0.05% to 1.0%, and the loaded catalyst can use nitrile compounds containing nitrogen, phosphorus, oxygen, sulfur, arsenic, and antimony, as well as various basic nitrides as catalytic promoters. Preferably, a loaded fixed bed catalyst is used. In step (2), controlling the conditions of the oxidation process as above and selecting the above catalyst can better oxidize the mercaptide 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 alkaline solution obtained from 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 a subsequent liquid separator and a light hydrocarbon recovery device, or sent to the flue gas desulfurization system of a catalytic cracking unit for incineration, or can be recycled after being pressurized. 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 alkaline 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 unit 6;

[0086] The alkali extraction unit 2 is provided with a liquefied petroleum gas inlet pipeline 1d, an extracted 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 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 back-extraction 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 pentahydrate distillation unit 6 includes a first fractional distillation unit and a decarbonization pentahydrate distillation tower. The inlet 6a of the material to be distilled of the first fractional distillation unit 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 an outlet for a mixed C3 or less fraction and an outlet for a mixed C4 or greater fraction. The outlet for the mixed C4 or greater fraction is connected to the inlet of the decarbonization pentahydrate distillation tower. The decarbonization pentahydrate 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 according to 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 the 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, and an alkali solution containing mercaptide salts and a first liquefied petroleum gas product are obtained; the obtained first liquefied petroleum gas product is output through the first liquefied petroleum gas product outlet 2b, and 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 mixture of the two forms a material to be separated and flows into the separation unit 4 through the material to be separated outlet pipeline 3b. 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 residual oxidizing gas outlet 4b and then reused or treated. The obtained alkaline solution containing disulfide flows out through the alkaline solution containing disulfide outlet pipeline 4a, 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 regeneration alkali solution outlet 5d and enters the alkali extraction unit 2 through the extraction alkali solution inlet 2a. After the obtained disulfide-containing selective solvent flows out through the disulfide-containing selective solvent outlet 5b, a portion 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 portion 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 then desulfurized to obtain low-sulfur gasoline with a sulfur content of no more than 10 μg / g.

[0097] Part of the first liquefied petroleum gas product flows out from the first liquefied petroleum gas product outlet 2b and is subjected to fractional distillation 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 C5 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 a variant embodiment, the continuous deep desulfurization system of the present invention can also be equipped with a C5 desulfurization distillation tower directly connected to the first LPG product outlet 2b. The first LPG product is directly distilled through the C5 desulfurization distillation tower to produce a lower-sulfur LPG product with a lower C5 fraction content, namely a second LPG product. A C5 fraction enriched in relatively high-boiling-point sulfides is also produced, serving as a selective solvent for back extraction. In this embodiment, the C5 desulfurization distillation tower can be equipped with both a second LPG product outlet and a C5 fraction outlet. The resulting lower-sulfur second LPG product can be output through the second LPG product outlet directly as a higher-quality product, or partially or entirely fed into the first fractional distillation unit for fractional distillation to produce dry gas, propane, propylene, and a C4 fraction. This embodiment can produce a lower-sulfur LPG product, as well as low-sulfur dry gas, propane, propylene, and C4 fraction products, making it more suitable for 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, which 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. The mercaptans in the original liquefied petroleum gas come into contact with the alkali solution, and part of the mercaptans react with the alkali to form mercaptides. 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 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 may also be 1.2 MPa, or preferably 0.85 MPa. By controlling the conditions for the distillation of the decarbonized five fraction in the decarbonized five fraction distillation tower as described above, the resulting C5 fraction can be used as a selective solvent for stripping, thereby better obtaining a regenerated alkaline solution free of sulfides and oxidizing gases, thereby obtaining a liquefied petroleum gas product with lower sulfur content and higher 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 (LPG), such as LPG that has been desulfurized but contains mercaptans, utilizes a C5 fraction substantially free of hydrogen sulfide, mercaptans, and oxidizing gases, separated from the LPG after alkali extraction desulfurization, as a selective solvent during stripping to absorb disulfides produced by alkali oxidation. Furthermore, the disulfide-containing selective solvent obtained after stripping is not directly sent to an additional hydrogenation unit for treatment, nor is it returned to a catalytic cracking unit to be mixed with the catalytic feed for catalytic cracking to undergo secondary cracking. Instead, it is returned to an absorption and stabilization system located after the catalytic cracking reactor of the catalytic cracking unit, where it is absorbed and becomes part of the stabilized gasoline (without secondary cracking). The sulfur-containing gasoline then undergoes well-known desulfurization and refining operations (e.g., selective hydrodesulfurization) to become a low-sulfur gasoline product. In this way, the carbon pentavalent 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 a stripping solvent (selective solvent) to absorb 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 greatly reducing the 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 trace amounts of hydrogen sulfide (and part of the mercaptan) in the liquefied petroleum gas are removed.

[0110] The LPG, free of hydrogen sulfide, flows into the alkali extraction unit 2 via the LPG inlet line 1d. There, it countercurrently contacts fresh alkali liquor from the extraction alkali liquor inlet 2a or regenerated alkali liquor from the regeneration alkali liquor outlet 5d (connected to the extraction alkali liquor inlet 2a). Mercaptans in the LPG are absorbed by the alkali liquor to form mercaptides. The LPG, free of hydrogen sulfide and mercaptans, flows out of the first LPG product outlet 2b at the top of the alkali extraction unit 2 as a low-sulfur first LPG product. The mercaptide-containing alkali liquor obtained after alkali extraction flows through the mercaptide-containing alkali liquor outlet line 2c to the oxidation unit 3.

[0111] A portion of the low-sulfur first liquefied petroleum gas product can enter a first fractional distillation unit (not shown), where it undergoes stepwise distillation to obtain a mixed C3 or less fraction and a mixed C4 or greater fraction. The mixed C3 or less fraction is further fractionally distilled in a second fractional distillation unit to obtain low-sulfur C1-C2 (dry gas), propane, and propylene. The mixed C4 or greater fraction can then enter a decarbonization distillation tower for distillation of the decarbonization fraction. The C4 fraction, after removing the C5 fraction, can be distilled from the C4 fraction outlet 6b at the top of the tower 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 ultimately flow out of the C5 fraction from the C5 fraction outlet pipeline 6c to obtain a C5 fraction relatively rich in heavy sulfides. The C5 fraction can then enter a stripping unit for stripping as a selective solvent.

[0112] The alkali liquor that has absorbed the mercaptans, i.e., the mercaptan-containing alkali liquor, leaves the alkali extraction unit 2 through the mercaptan-containing alkali liquor outlet line 2c. It mixes with the oxidizing gas from the oxidizing gas inlet 3a before flowing into the oxidation unit 3. Under the action of the fixed-bed oxidation catalyst, i.e., a metal phthalocyanine-supported catalyst, loaded into the oxidation unit 3, the mercaptides in the mercaptan-containing alkali liquor are oxidized by the oxidizing gas into disulfides. 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 excess air exhaust leave the oxidation unit 3 through the material to be separated outlet line 3b and flow into the separation unit 4 (which may be a sedimentation separation system) for separation.

[0113] In separation unit 4, excess air exhaust is released through residual oxidizing gas outlet 4b and subsequently treated in a separator tank and light hydrocarbon recovery unit before clean discharge or incineration in the flue gas desulfurization system of the catalytic cracking unit. The disulfide-containing alkaline liquor (lean alkaline liquor), from which residual oxidizing gases have been removed, flows from separation unit 4 through disulfide-containing alkaline liquor outlet pipeline 4a and into stripping unit 5.

[0114] Before entering the system, the disulfide-containing alkali liquor (lean alkali liquor) first contacts the C5 fraction, which serves as the organic solvent (selective solvent) for stripping, from the C5 fraction outlet line 6c. After contact, the disulfides in the disulfide-containing alkali liquor (lean alkali liquor) are almost entirely absorbed by the C5 fraction. The alkali liquor is then separated from the disulfide-containing C5 fraction, and the disulfide-containing C5 fraction (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 stripping unit 5 via the selective solvent inlet for recycling, while the other part flows out through the gasoline line 5c and returns to the absorption and stabilization system of the catalytic cracking unit, where it is absorbed into the catalytically cracked stabilized gasoline. It then undergoes subsequent desulfurization and refining operations (not shown) along with the stabilized gasoline. The absorption and stabilization system operates conventionally, utilizing the boiling point difference to absorb the disulfides and C5 fraction into the stabilized gasoline. The desulfurization and refining operations for the stabilized gasoline are conventionally performed. 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 regeneration 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 illustrate the process of a preferred embodiment of the present invention and are not intended to provide details regarding vessels, heaters, coolers, pumps, compressors, valves, process control equipment, etc., which are basic equipment to those skilled in the art. The examples are given for the purpose of illustrating the present invention and 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 (system) diagram shown here desulfurizes the liquefied petroleum gas (LPG) shown in Table 1, except that no pre-alkali washing is performed. A 15% (by weight) NaOH solution is used as the extraction alkali solution. In the alkali extraction unit, the volume ratio of extraction alkali solution to LPG is 1:9, the temperature is room temperature, and the pressure is 1.6 MPa. Alkaline extraction produces a first LPG product and a mercaptan-containing alkali solution. The mercaptan-containing alkali solution flows to the oxidation unit, where it contacts an oxidizing gas for oxidation.

[0122] The first liquefied petroleum gas product obtained after extraction, initially in its entirety, is sent to a fractional distillation column for stepwise distillation. Once the process is fully established and operation gradually stabilizes, the amount of the first liquefied petroleum gas product sent to the fractional distillation can be reduced as needed. For example, the volume fraction of the first liquefied petroleum gas product sent to the fractional distillation can be adjusted between 1% and 99% (compared to the total amount of the first liquefied petroleum gas product obtained after extraction). Fractional distillation of the first liquefied petroleum gas product yields a mixed C3 or lower fraction and a mixed C4 or higher fraction. The mixed C4 or higher fraction obtained through fractional distillation is distilled in a decarbonization distillation column. Distillation in the decarbonization distillation column yields a C5 fraction and a C4 fraction (non-C5 fraction). The resulting C5 fraction is fed to a stripping unit via C5 fraction outlet line 6c for use as a selective solvent. The resulting C4 fraction can be used as a lower 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 oxidation catalyst, using activated carbon (brand GH-8, supplied by Beijing Guanghua Jingke Activated Carbon Factory) as a carrier. Sulfonated cobalt phthalocyanine is the primary active ingredient, at a concentration of 0.6%, and dodecyldimethylbenzyl ammonium chloride (1%) and isopropanolamine (5%) as co-active ingredients. The catalyst is prepared as follows: a dilute ammonia solution of sulfonated cobalt phthalocyanine and a mixed solution of dodecyldimethylbenzyl ammonium chloride and isopropanolamine are prepared according to the target loading, and equal volumes of the mixture are adsorbed onto the activated carbon, each solution then dried on a rotary evaporator.

[0124] In the oxidation unit, the air injection rate is four times the theoretical chemical demand for oxidizing gas to convert the mercaptides contained in the mercaptan-containing caustic solution into disulfides. This corresponds to a standard air injection volume to mercaptan-containing caustic solution ratio of approximately 3.1:1. The caustic solution oxidation temperature is 50°C and the pressure is 0.40 MPa. After the oxidation process in the oxidation unit, the material to be separated is obtained, consisting of residual oxidizing gas and disulfide-containing caustic solution. The material to be separated flows into the separation unit.

[0125] The excess air and disulfide-containing alkali liquor are then separated in the separation unit, and the resulting disulfide-containing alkali liquor (lean alkali liquor) flows into the back-extraction unit. The conditions in the separation unit are: temperature 50°C and pressure 0.30 MPa.

[0126] In the back-extraction operation, the inlet pressure of the back-extraction unit is 0.30 MPa, 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 portion 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 separated disulfide-rich C5 fraction (containing disulfide selective solvent) is ) is further divided into two parts. One part is returned to the stripping unit and mixed with the C5 fraction, including residual heavy sulfur compounds, from the bottoms of the decarbonization penta distillation tower 6 to be recycled as a selective solvent. The other part is returned to the absorption and stabilization system located after the catalytic cracking reactor in the catalytic cracking unit, mixed with the feedstock of the absorption and stabilization system, and then separated by the absorption and stabilization system. The other part of the disulfide-rich C5 fraction (including the disulfide-selective solvent) is absorbed into gasoline and discharged together with the stabilized gasoline. The stabilized gasoline is then desulfurized and refined using selective hydrogenation to reduce its sulfur content to below 10 μg / g. The amount of the disulfide-rich C5 fraction (including the disulfide-selective solvent) mixed with the feedstock of the catalytic cracking absorption and stabilization system is equal to the amount of the C5 fraction obtained from the bottoms of the decarbonization penta distillation tower 6, with a ratio of 1:1. The regenerated alkali liquor obtained after stripping is returned to the alkali extraction unit and recycled as extraction alkali liquor.

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

[0128] Comparative Example 1

[0129] use Figure 1 The flow chart shown in FIG. 1 is used to desulfurize the liquefied petroleum gas described in Table 1. The operating conditions and catalyst used were the same as those in Example 1, except that the first liquefied petroleum gas product after alkali extraction was not subjected to fractional distillation and C5 removal, nor was the C5 fraction used as the selective organic solvent for stripping disulfides from the alkali liquor. Instead, a refined gasoline product (sulfur content no greater than 10 μg / g, distillation range 33°C to 200°C, aromatics volume fraction 28.1%) stored in the refinery's tank farm was used as the selective organic solvent for stripping disulfides in the stripping unit. The desulfurization results are shown in Table 2.

[0130] Table 2

[0131]

[0132] Table 3 shows 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 fully 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. 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. During operation, it was also discovered that when the C5 fraction in the first liquefied petroleum gas product after alkali extraction was recycled as the back-extraction solvent (selective solvent), the disulfide content absorbed by it was several times higher than the disulfide content absorbed when refined gasoline was recycled as the back-extraction solvent (selective solvent) (~3000 μg / g), and it did not affect the extraction and desulfurization of the liquefied petroleum gas. Therefore, when the C5 fraction in the first liquefied petroleum gas product after alkali extraction was used as the back-extraction solvent (selective solvent), the amount of disulfide discharged to the outside (sent to the absorption and stabilization system of the catalytic cracking unit) was as small as possible. However, when refined gasoline was used as the back-extraction solvent (selective solvent), a considerable amount of disulfide sometimes remained in the regenerated alkali liquor after back-extraction. This may be due to the emulsification of aromatic components and the alkali liquor. In addition, air may still remain in the regenerated alkali liquor after back-extraction, so the refined gasoline used as the back-extraction solvent (selective solvent) needs to be discharged to the outside.

[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 needs to be explained that the terms "upper", "lower", "inner", "outer", "front", "back", "left", "right" and the like indicate the orientation or positional relationship based on the working state of the present application, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the devices or elements referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation on the present application.

[0138] In the description of the present application, it needs to be explained that the terms "mounting", "connecting", "connection" should be understood in a broad sense unless otherwise specifically defined and limited. For those skilled in the art, the specific meanings of the above terms in the present application can be understood according to the specific circumstances.

[0139] The above describes the present application in combination with the preferred embodiments, but these embodiments are only exemplary and serve only to illustrate. On this basis, various substitutions and improvements can be made to the present application, which all fall within the protection scope 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, 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 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 back extraction 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 lower fraction and a mixed C4 or higher fraction; subjecting the mixed C4 or higher fraction to decarbonization of the C5 fraction distillation to obtain a C4 fraction and a C5 fraction; Wherein, the extraction alkali liquor in step (1) comprises the regenerated alkali liquor 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): The other part of the disulfide-containing selective solvent obtained from 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 no 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 C3 and lower fractions are subjected to fractional 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 includes a pre-alkali washing step before step (1): Contacting the original liquefied petroleum gas 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; the pressure is 0.5 MPa to 4.0 MPa; the volume proportion of the extracted alkali solution in the liquefied petroleum gas is 1% to 50%; and / or, The conditions for performing the oxidation in step (2) include: Temperature is -5℃ to 100℃; pressure is 0.1MPa to 2.0MPa; The amount of the oxidizing gas used is 1 to 20 times the theoretical chemical demand for oxidizing the mercaptide contained in the mercaptan-containing alkali solution to form disulfide, wherein the theoretical chemical demand for the oxidizing gas is calculated based on the requirement of 0.25 mole of oxygen for each mole of mercaptide oxidized to form disulfide.

7. The continuous deep desulfurization method according to claim 6, characterized in that: In step (1), the temperature is 25 to 50°C.

8. The continuous deep desulfurization method according to claim 6, characterized in that: In step (1), the pressure is 1.0 MPa to 2.0 MPa.

9. The continuous deep desulfurization method according to claim 6, characterized in that: In step (1), the volume proportion of the extracted alkali liquid in the liquefied petroleum gas is 5% to 40%.

10. The continuous deep desulfurization method according to claim 6, characterized in that: In step (2), the temperature is 25°C to 80°C.

11. The continuous deep desulfurization method according to claim 6, characterized in that: In step (2), the pressure is 0.1 MPa to 1.0 MPa.

12. The continuous deep desulfurization method according to claim 6, characterized in that: The amount of the oxidizing gas used is 2 to 10 times the theoretical chemical demand for oxidizing the mercaptide contained in the mercaptan-containing alkali solution to form disulfide.

13. 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; the pressure is 0.1 MPa to 2.0 MPa; and / or, The conditions for performing the back extraction in step (4) include: Temperature is -5℃ to 80℃; pressure is 0.1MPa to 2.0MPa; The volume ratio of the selective solvent to the disulfide-containing alkaline solution obtained from step (3) is 1:0.01 to 1:

100.

14. The continuous deep desulfurization method according to claim 13, characterized in that: In step (3), the temperature is 25°C to 50°C.

15. The continuous deep desulfurization method according to claim 13, characterized in that: In step (3), the pressure is 0.1 MPa to 1.0 MPa.

16. The continuous deep desulfurization method according to claim 13, characterized in that: In step (4), the temperature is 25°C to 50°C.

17. The continuous deep desulfurization method according to claim 13, characterized in that: In step (4), the pressure is 0.1 MPa to 1.0 MPa.

18. The continuous deep desulfurization method according to claim 13, characterized in that: The volume ratio of the selective solvent to the disulfide-containing alkaline solution obtained from step (3) is 1:0.1 to 1:

10.

19. 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 greater than 10 μg / g.

20. 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 extracted 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) 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 back-extraction 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 pentahydrate distillation unit (6) comprises a first fractional distillation unit and a decarbonization pentahydrate distillation tower, wherein the inlet (6a) of the material to be distilled of the first fractional distillation unit is in communication with the first liquefied petroleum gas product outlet (2b) of the alkali extraction unit (2), the first fractional distillation unit is provided with an outlet for a mixed C3 or lower fraction and an outlet for a mixed C4 or higher fraction, the outlet for the mixed C4 or higher fraction is in communication with the inlet of the decarbonization pentahydrate distillation tower, and the decarbonization pentahydrate 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).

21. The continuous deep desulfurization system according to claim 20, 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).

22. The continuous deep desulfurization system according to claim 20, characterized in that: The fractional distillation decarbonization pentahydrate distillation unit (6) further comprises a second fractional distillation unit, wherein 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.

23. The continuous deep desulfurization system according to claim 20, 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 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); and the liquefied petroleum gas outlet is connected to the liquefied petroleum gas inlet pipeline (1d) of the alkali extraction unit (2).

24. The continuous deep desulfurization system according to claim 20, 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; the pressure at the top of the tower is 0.12MPa to 1.5MPa; and the temperature at the bottom of the tower is 60°C to 150°C.

25. The continuous deep desulfurization system according to claim 24, characterized in that: The temperature of the tower top is 40°C to 60°C.

26. The continuous deep desulfurization system according to claim 24, characterized in that: The tower top pressure is 0.35 MPa to 1.0 MPa.

27. The continuous deep desulfurization system according to claim 24, characterized in that: The tower bottom temperature is 80°C to 100°C.

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