Method and system for separating alpha-olefins from Fischer-Tropsch oil

Abstract

The present disclosure relates to a method and system for separating alpha-olefins from Fischer-Tropsch oil, and the method includes step a: Fischer-Tropsch oil enters a first extraction tower for extraction; step b: after step a extraction treatment, the first mixture enters an extractant regeneration tower through the bottom of the first extraction tower for rectification; step c: after step b rectification treatment, the second mixture enters a second extraction tower through the top of the extractant regeneration tower for extraction; step d: after step c extraction treatment, the third mixture flows out through the top of the second extraction tower; step e: after step a extraction treatment, the fourth mixture enters an alkane-olefin separation unit through the top of the first extraction tower for separation to obtain alpha-olefins. This method can remove more than 90% of oxygen-containing compounds, and the yield of oil products can be maintained at more than 97%, with high separation efficiency and large processing capacity, which can meet the requirements of fine chemical processes and can be applied to industrial production.

Description

Technical Field

[0001] The present disclosure relates to the field of chemical industry, and in particular to a method for separating α-olefins from Fischer-Tropsch oil; the present disclosure also relates to a system for separating α-olefins from Fischer-Tropsch oil. Background Art

[0002] α-olefin refers to a monoolefin with double bonds at the end of the molecular chain. It is an important chemical raw material and organic synthesis intermediate, and is widely used in the chemical industry. It can be used as a comonomer for high-performance polyolefins, a synthetic intermediate for surfactants, a plasticizer, and an additive for synthetic hydrocarbon lubricants and oils. α-olefins are also important in the fragrance, paper, daily chemical and other industries.

[0003] At present, α-olefins in the market are mainly derived from ethylene polymerization, paraffin cracking, Fischer-Tropsch synthesis and other methods. The α-olefins produced by the Fischer-Tropsch method have continuous carbon numbers and contain a large number of high-carbon α-olefins, which have obvious advantages over the ethylene polymerization method that can only produce even-carbon numbers. However, the α-olefins produced by the Fischer-Tropsch method are also produced along with alkanes. Separating alkanes and olefins and obtaining high-purity single-carbon α-olefin products are of great significance to increasing the added value of Fischer-Tropsch synthesis products. During the Fischer-Tropsch reaction, about 1~5%wt of oxygenated compounds will be generated, which are mainly divided into five categories: aldehydes, acids, alcohols, ketones, and esters. This type of impurity will affect the alkane-olefin separation process and the application of downstream products, so measures need to be taken to remove oxygenated compounds.

[0004] CN109054886A uses methanol, ethanol, isopropanol and organic base as solvents to extract and remove oxygenated compounds. These solvents will dissolve in small amounts in Fischer-Tropsch oil; US4686317 mentions the use of heavy organic solvents for extraction, and then uses water to recover the organic solvent; CN100413824C uses methanol and water as extractants. These prior arts all introduce new impurities, and the boiling points of methanol, ethanol and isopropanol are included in the distillation range of Fischer-Tropsch oil and there is the possibility of azeotropy, so they cannot be removed by distillation, causing difficulties in separation. In addition, these prior arts only propose the use effect after extraction, and do not clearly propose the removal rate of oxygenated compounds. It can be seen that if this method is applied to Fischer-Tropsch oil, it will be difficult to achieve the desired effect.

[0005] CN108822885A uses dimethyl sulfoxide, cyclopentane, pyridine, butyl acetate, glycerol, cyclohexane, methylcyclohexane, etc. as composite extractants to extract and remove oxygenated compounds, with a removal rate of up to 99%. The composite extractant has a complex composition and is difficult to regenerate and reuse in industrial implementation; CN01817099 uses water-acetonitrile for extraction, and the alcohol removal rate is 82.6%. The above method has some improvements in the extractant and clearly proposes the removal rate, but there are also problems such as the introduction of new sulfide-containing impurities and nitrogen-containing impurities, and the above solvents are toxic. CN114479919A discloses a method for removing oxygenated compounds in high carbon number Fischer-Tropsch oil. Although this method can effectively reduce the content of oxygenated compounds in oil products to below 60 ppm, the oil yield is relatively low.

[0006] In addition, the prior art uses polar solvents to remove oxygenated compounds from Fischer-Tropsch oil by extraction processes. Since the polarity of olefins in Fischer-Tropsch oil is higher than that of alkanes, it is inevitable that relatively more olefins enter the extraction phase during the process of extracting and removing oxygenated compounds, and are lost together with oxygenated compound impurities, thereby reducing the yield of oil products, especially the yield of olefins.

[0007] In summary, the existing technology for removing oxygenated compounds from Fischer-Tropsch oil has technical problems such as incomplete process system, high difficulty in industrialization, complex components or process, high energy consumption, toxic solvents and introduction of new heteroatom substances such as sulfur and nitrogen, low product yield, and low economy. Summary of the invention

[0008] In order to solve the problems existing in the prior art, the present disclosure provides a method and system for separating α-olefins from Fischer-Tropsch oil.

[0009] According to a first aspect of the present disclosure, there is provided a method for separating α-olefins from Fischer-Tropsch oil, comprising the following steps:

[0010] Step a: Fischer-Tropsch oil enters the first extraction tower for extraction;

[0011] Step b: After the extraction treatment in step a, the first mixture comprising the fraction, the first extractant and the oxygen-containing compound flows out from the bottom of the first extraction tower and enters the extractant regeneration tower for rectification;

[0012] Step c: After the distillation treatment in step b, the second mixture containing the fraction and the oxygen-containing compound flows out from the top of the extractant regeneration tower and enters the second extraction tower for extraction;

[0013] Step d: After the extraction treatment in step c, a third mixture containing the fraction and oxygen-containing compounds flows out from the top of the second extraction tower;

[0014] Step e: After the extraction treatment in step a, the fourth mixture containing the fraction and the oxygen-containing compound flows out from the top of the first extraction tower and enters the alkane-olefin separation unit for separation to obtain α-olefin.

[0015] In one embodiment of the present disclosure, the step a further comprises: providing a first extractant from a first extractant tank to the first extraction column.

[0016] In one embodiment of the present disclosure, the step c further comprises: after the distillation treatment in the step b, the first extractant flows out from the bottom of the extractant regeneration tower and is recovered to the first extractant tank.

[0017] In one embodiment of the present disclosure, the first extractant used in the first extraction tower includes at least one of carbonates, sulfones, NMP, DMF, and polyols.

[0018] In one embodiment of the present disclosure, the first extractant includes at least one of ethylene carbonate, propylene carbonate, dimethyl sulfoxide, cyclopentane, polyethylene glycol 300, and polyethylene glycol 500.

[0019] In one embodiment of the present disclosure, the second extractant used in the second extraction tower includes at least one of ethylene carbonate, propylene carbonate, butylene carbonate, and dimethyl carbonate.

[0020] In one embodiment of the present disclosure, the volume ratio of the first extractant used in the first extraction tower to the fraction entering the first extraction tower is 1:1-10:1; and the number of extraction stages of the first extraction tower is 1-5.

[0021] In one embodiment of the present disclosure, the volume ratio of the second extractant used in the second extraction tower to the fraction entering the second extraction tower is 0.1:1-2:1; and the extraction stages of the second extraction tower are 1-5.

[0022] In one embodiment of the present disclosure, before step a, step f is further included: the Fischer-Tropsch oil enters a first distillation tower for distillation, and a fraction with a carbon number greater than or equal to n flows out from the bottom of the first distillation tower and enters a second distillation tower for distillation, and a fraction with a carbon number less than or equal to m flows out from the top of the second distillation tower; wherein n and m are positive integers between 5 and 12 and n≤m;

[0023] Alternatively, step f is further included between step a and step e: the fourth mixture enters a first distillation tower for distillation, a fraction with a carbon number greater than or equal to n flows out from the bottom of the first distillation tower, and enters a second distillation tower for distillation, and a fraction with a carbon number less than or equal to m flows out from the top of the second distillation tower; wherein n and m are positive integers between 5 and 12 and n≤m;

[0024] Alternatively, after step e, step f is further included: the α-olefin mixture enters a first distillation tower for distillation, the α-olefins with a carbon number greater than or equal to n flow out from the bottom of the first distillation tower and enter a second distillation tower for distillation, and the α-olefins with a carbon number less than or equal to m flow out from the top of the second distillation tower; wherein n and m are positive integers between 5 and 12 and n≤m.

[0025] In one embodiment of the present disclosure, the number of plates of the first distillation tower is greater than or equal to 50, and the reflux ratio is greater than or equal to 4:1.

[0026] In one embodiment of the present disclosure, the number of plates of the second distillation tower is greater than or equal to 50, and the reflux ratio is greater than or equal to 4:1.

[0027] In one embodiment of the present disclosure, when the step f is before the step a, the step d comprises: the third mixture flows out from the top of the second extraction tower and enters the first distillation tower and / or the second distillation tower for distillation;

[0028] When step f is between step a and step e, or after step e, step d comprises: the third mixture flows out from the top of the second extraction tower and enters the first extraction tower for extraction.

[0029] In one embodiment of the present disclosure, the number of plates of the extractant regeneration tower is 6-20, the bottom temperature is 120-200°C, the top temperature is 10-100°C, the top pressure is 5-101Kpa, the reflux ratio is 0.2:1-5:1, and the temperature of the first mixture when entering the extractant regeneration tower is 20-150°C.

[0030] According to a second aspect of the present disclosure, a system for separating α-olefins from Fischer-Tropsch oil is also provided, the system being used to perform the method described in the first aspect of the present disclosure, the system comprising:

[0031] Connecting pipelines;

[0032] a first extraction tower, connected to the connecting pipeline, wherein the Fischer-Tropsch oil is configured to enter the first extraction tower through the connecting pipeline for primary extraction;

[0033] An extractant regeneration tower connected to the connecting pipeline at a downstream position of the bottom output of the first extraction tower to rectify the extract phase obtained by the primary extraction;

[0034] A second extraction tower is connected to a connecting pipeline located at a position downstream of the top output of the extractant regeneration tower to perform secondary extraction on the fraction obtained by distillation;

[0035] The alkane-olefin separation unit is connected to the connecting pipeline at the top output downstream position of the first extraction tower to separate the alkane-olefin from the raffinate phase obtained by the primary extraction.

[0036] In one embodiment of the present disclosure, it also includes:

[0037] a first distillation tower connected to a connecting pipeline located upstream of the first extraction tower, and the Fischer-Tropsch oil is configured to enter the first distillation tower through the connecting pipeline for primary distillation;

[0038] Alternatively, the first distillation tower is disposed between the first extraction tower and the alkane-olefin separation unit, and the raffinate phase obtained from the primary extraction is configured to enter the first distillation tower for primary distillation;

[0039] Alternatively, the first distillation tower is connected to a connecting pipeline located downstream of the alkane-olefin separation unit, and the separated α-olefin mixture is configured to enter the first distillation tower for primary distillation;

[0040] The second distillation tower is connected with the connecting pipeline at the bottom output downstream position of the first distillation tower to perform secondary distillation on the heavy hydrocarbon fraction obtained by the primary distillation.

[0041] In one embodiment of the present disclosure, when the first distillation tower is connected to a connecting pipeline located at an upstream position of the first extraction tower, the top output port of the second extraction tower is configured to be connected to the connecting pipeline located at an upstream position of the first distillation tower and / or the second distillation tower;

[0042] When the first distillation tower is arranged between the first extraction tower and the alkane-olefin separation unit, or is connected to the connecting pipeline at a downstream position of the bottom output of the first distillation tower, the top output port of the second extraction tower is constructed to be connected to the connecting pipeline at an upstream position of the first extraction tower.

[0043] One beneficial effect of the present disclosure is that it provides a method for separating α-olefins of any carbon number in the C5-C12 fraction of Fischer-Tropsch oil products. The extraction process used in the method can remove more than 90% of oxygen-containing compounds, and the yield of the oil product can be maintained at more than 97%. The present disclosure uses high-boiling-point, slightly toxic, and easily separable extractants such as carbonates and sulfones in the extraction step, and the product does not affect the subsequent further separation process of the Fischer-Tropsch oil and various downstream application requirements. The method disclosed in the present disclosure has high separation efficiency and large processing capacity, can meet the requirements of fine chemical processes, and can be applied to industrial production.

[0044] Other features and advantages of the present disclosure will become apparent from the following detailed description of exemplary embodiments of the present disclosure with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.

[0046] Figure 1 is a structural diagram of a system for separating α-olefins from Fischer-Tropsch oil provided in one embodiment of the present disclosure;

[0047] Figure 2 is another system structure diagram for separating α-olefins from Fischer-Tropsch oil provided in one embodiment of the present disclosure;

[0048] Figure 3 Another system structure diagram for separating α-olefins from Fischer-Tropsch oil provided in one embodiment of the present disclosure.

[0049] Figures 1 to 3 The one-to-one correspondence between the component names and the reference numerals is as follows:

[0050] 1. The first distillation tower; 2. The second distillation tower; 3. The first extraction tower; 4. The extractant regeneration tower; 5. The second extraction tower; 6. The first extractant tank; 7. The second extractant tank; 8. The alkane-alkene separation unit. DETAILED DESCRIPTION

[0051] Various exemplary embodiments of the present disclosure will now be described in detail with reference to the accompanying drawings. It should be noted that the relative arrangement of components and steps, numerical expressions and numerical values ​​set forth in these embodiments do not limit the scope of the present disclosure unless otherwise specifically stated.

[0052] The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way intended to limit the present disclosure, its application, or uses.

[0053] Technologies, methods, and equipment known to ordinary technicians in the relevant art may not be discussed in detail, but where appropriate, the technologies, methods, and equipment should be considered as part of the specification.

[0054] It should be noted that like reference numerals and letters refer to similar items in the following figures, and therefore, once an item is defined in one figure, it need not be further discussed in subsequent figures.

[0055] In this document, “upper”, “lower”, “front”, “back”, “left”, “right”, etc. are only used to indicate the relative position relationship between related parts, rather than to limit the absolute positions of these related parts.

[0056] In this article, "first", "second", etc. are only used to distinguish each other, and do not indicate the importance and order, or the premise of each other's existence.

[0057] In this document, “equal”, “same”, etc. are not strictly limited in a mathematical and / or geometric sense, but also include errors that can be understood by those skilled in the art and are allowed in manufacturing or use.

[0058] The present invention provides a method for separating α-olefins from Fischer-Tropsch oil, which can be used as follows: Figure 1 , Figure 2 or Figure 3 The method is implemented by the system shown. The method comprises the following steps:

[0059] Step a: Fischer-Tropsch oil enters the first extraction tower 3 for extraction;

[0060] In one embodiment of the present disclosure, Figure 1 As shown, before step a, step f is also included: the Fischer-Tropsch oil enters the first distillation tower 1 for distillation, the fraction with a carbon number greater than or equal to n flows out from the bottom of the first distillation tower 1, and enters the second distillation tower 2 for distillation, and the fraction with a carbon number less than or equal to m flows out from the top of the second distillation tower 2; wherein n and m are positive integers between 5 and 12 and n≤m.

[0061] Specifically, Fischer-Tropsch oil is an oil produced by Fischer-Tropsch reaction using synthesis gas as raw material. Figure 1 As shown, in this embodiment, the Fischer-Tropsch oil can be first sent to the first distillation tower 1 for distillation, and the fraction with a carbon number less than n has a lower boiling point, so it can be separated to the top of the first distillation tower 1; the fraction with a carbon number greater than or equal to n has a higher boiling point, so it can be separated to the bottom of the first distillation tower 1. In the first distillation tower 1, the oil fraction with a carbon number less than the target is cut in the Fischer-Tropsch oil.

[0062] In one embodiment of the present disclosure, the number of plates of the first distillation tower 1 is 10-120, and the number of plates is selected from a positive integer between 10-120 (for example: 13, 24, 35, 46, 57, 68, 79). Preferably, the number of plates is greater than or equal to 50. In general, the more the number of distillation plates, the better the separation effect, but too many plates will increase energy consumption. The number of plates of the first distillation tower 1 of the present disclosure can be designed according to the actual target carbon number. When the oil product with a carbon number less than the target is easier to separate, a smaller number of plates can be used to reduce energy consumption; when the oil product with a carbon number less than the target is difficult to separate, a larger number of plates can be used to ensure the separation effect.

[0063] The bottom temperature of the first distillation tower 1 is 80-240°C (for example: 65.5°C, 76.6°C, 83.7°C, 97.8°C); the top pressure is 5-500Kpa (for example: 16.90Kpa, 30.95Kpa, 50.00Kpa, 80.05Kpa); the reflux ratio is 0.5:1-12:1 (for example: 7:1, 8:1, 9:1), preferably, the reflux ratio is greater than or equal to 4:1; the temperature of the Fischer-Tropsch oil when it enters the first distillation tower 1, that is, the feed temperature of the first distillation tower 1 is 20-150°C (for example: 31°C, 42°C, 53°C, 64°C, 75°C, 86°C, 97°C, 128°C, 139°C). The above parameters can be designed according to the actual target carbon number to accurately cut the oil fraction less than the target carbon number in the Fischer-Tropsch oil.

[0064] In one embodiment of the present disclosure, step f further comprises: a portion of isoolefins and isoparaffins having a carbon number of n flows out from the top of the first distillation tower 1. For hydrocarbon substances, when the carbon number is the same, isoolefins and isoparaffins have a lower boiling point than α-olefins. Therefore, in addition to the fraction having a carbon number less than n, a portion of isoolefins and isoparaffins having a carbon number of n flows out from the top of the first distillation tower 1.

[0065] The fraction with a carbon number greater than or equal to n can flow out from the bottom of the first distillation tower 1 and enter the second distillation tower 2 for further distillation. The second distillation tower 2 is mainly used to cut the fraction with a carbon number greater than the target carbon number, wherein the fraction with a carbon number greater than m can be separated to the bottom of the second distillation tower 2 due to its higher boiling point; the fraction with a carbon number less than or equal to m has a lower boiling point, so it can be separated to the top of the second distillation tower 2 and flow out from the top of the second distillation tower 2 to enter the downstream first extraction tower 3 for extraction.

[0066] n and m are both positive integers between 5 and 12 and n≤m. After two distillations, the fraction with a carbon number between n and m can be separated to the top of the second distillation tower 2. For example, when n=8 and m=10, fractions with carbon numbers of 8, 9, and 10 can be separated; when n=m=8, a single carbon fraction with a carbon number of 8 can be separated. The present disclosure can effectively separate the fractions in the target carbon number range or single carbon number by ultra-precision distillation, thereby obtaining α-olefins in the target carbon number range or single carbon number.

[0067] In one embodiment of the present disclosure, the number of plates of the second distillation tower 2 is 10-120, and the number of plates is selected from a positive integer between 10-120 (for example: 23, 34, 45, 56, 67, 78, 89, 100, 110, 120), preferably, the number of plates is greater than or equal to 50. The number of plates of the second distillation tower 2 can be designed according to the actual target carbon number. When the oil product with a carbon number greater than the target carbon number is easier to separate, a smaller number of plates can be used to reduce energy consumption; when the oil product with a carbon number greater than the target carbon number is difficult to separate, a larger number of plates can be used to ensure the separation effect.

[0068] The bottom temperature of the second distillation tower 2 is 80-240°C (e.g. 61.5°C, 72°C, 82.5°C, 93°C); the top pressure is 5-500Kpa (e.g. 16.80Kpa, 36.98Kpa, 67.08Kpa, 87.32Kpa); the reflux ratio is 0.5:1-15:1 (e.g. 4:1, 5:1, 6:1), preferably, the reflux ratio is greater than or equal to 4:1; the temperature of the fraction with a carbon number less than n when entering the second distillation tower 2, that is, the feed temperature of the second distillation tower 2 is 20-150°C (e.g. 31°C, 42°C, 52°C, 64°C, 76°C, 86°C, 97°C, 128°C, 139°C). The above parameters can all be designed according to the actual target carbon number to accurately cut the oil fraction with a carbon number greater than the target carbon number in the Fischer-Tropsch oil.

[0069] In one embodiment of the present disclosure, step f also includes: a fraction with a carbon number greater than m, some alkanes with a carbon number of m, and internal olefins flow out from the bottom of the second distillation tower 2. For hydrocarbon substances, the higher the carbon number, the higher the boiling point, and when the carbon number is the same, substances such as alkanes and internal olefins have a higher boiling point than α-olefins. Therefore, in addition to the fraction with a carbon number greater than m, some alkanes and internal olefins with a carbon number of m will flow out from the bottom of the second distillation tower 2, thereby increasing the proportion of α-olefins with a target carbon number in the material flowing out from the top of the second distillation tower 2. Specifically, the olefin content in the fraction flowing out from the top of the second distillation tower 2 is 85%-97%.

[0070] It is understandable that in the industrial production process, it is difficult to achieve high-purity separation of alkanes and olefins with the same carbon number by relying solely on the difference in boiling points. Therefore, in order to obtain α-olefins with higher purity, multiple steps such as extraction, adsorption, and separation are required in addition to distillation.

[0071] like Figure 1 As shown, the fraction with the target carbon number flows out from the top of the second distillation tower 2 and enters the first extraction tower 3 for extraction. In some other embodiments, the distillation step may also be after step a, which will be described in detail later. Figure 2 and Figure 3Fischer-Tropsch oil can directly enter the first extraction tower 3 for extraction. The oxygen-containing compounds in Fischer-Tropsch oil are mainly aldehydes, acids, alcohols, ketones, and esters, and the total content is usually 1-5wt%. The first extraction tower 3 is used to remove the oxygen-containing compounds in the fraction, thereby improving the purity of the final product.

[0072] In one embodiment of the present disclosure, step a further comprises: providing a first extractant to the first extraction tower 3 from the first extractant tank 6. The first extractant used in the first extraction tower 3 comprises at least one of carbonates, sulfones, NMP, DMF, and polyols. Specifically, the first extractant comprises at least one of ethylene carbonate, propylene carbonate, dimethyl sulfoxide, cyclopentane sulfone, polyethylene glycol 300, and polyethylene glycol 500. The first extractant is environmentally friendly and non-toxic. The first extractant can be selected from a variety of substances at the same time, so as to perform extraction in combination, so as to remove oxygen-containing compounds in the incoming liquid to the greatest extent in the first extraction tower 3.

[0073] In one embodiment of the present disclosure, the volume ratio of the first extractant used in the first extraction tower 3 to the fraction entering the first extraction tower 3 is 1:1-10:1 (for example: 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1); the number of extraction stages of the first extraction tower 3 is 1-5 stages (for example: 2 stages, 3 stages, 4 stages). A relatively large amount of the first extractant can be used for extraction in the first extraction tower 3, thereby improving the effect of removing oxygen-containing compounds. The first extractant can be stored in the first extractant tank 6, and the first extractant tank 6 is connected to the first extraction tower 3 through a connecting pipeline, thereby realizing dynamic control of the feed amount of the first extractant according to the amount of liquid entering the first extraction tower 3.

[0074] Since the extraction intensity of the first extraction tower 3 in the extraction unit is relatively high, part of the hydrocarbon fraction (including the target α-olefin) will enter the extraction phase together with the oxygen-containing compound. In order not to waste this part of the oil and improve the yield of the final product, the extraction phase separated by the first extraction tower 3 can be further separated and recovered.

[0075] Step b: After the extraction treatment in step a, the first mixture comprising the fraction, the first extractant and the oxygen-containing compound flows out from the bottom of the first extraction tower 3 and enters the extractant regeneration tower 4 for rectification.

[0076] Specifically, the extract phase flowing out from the bottom of the first extraction tower 3 is the first mixture, wherein the distillate and the oxygen-containing compounds have a lower boiling point and can therefore be separated to the top of the extractant regeneration tower 4; the first extractant has a higher boiling point and can therefore be separated to the bottom of the extractant regeneration tower 4.

[0077] In one embodiment of the present disclosure, the number of plates of the extractant regeneration tower 4 is 6-20, and the number of plates is selected from a positive integer between 6 and 20 (e.g., 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20). The bottom temperature of the extractant regeneration tower 4 is 120-200°C (for example, 160°C, 170, 180°C, 190°C); the top temperature is 10-100°C (for example, 30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C); the top pressure is 5-101Kpa (for example, 5Kpa, 10Kpa, 15Kpa, 20Kpa, 25Kpa, 30Kpa); the reflux ratio is 0.2:1-5:1 (for example, 0.5:1, 1:1, 3:1); the temperature of the extract phase of the first extraction tower 3 when entering the extractant regeneration tower 4, that is, the feed temperature of the extractant regeneration tower 4 is 20-150°C (for example, 31°C, 42°C, 52°C, 64°C, 76°C, 86°C, 97°C, 128°C, 139°C). The above parameters can be designed according to the type of the first extractant to accurately separate the first extractant in the mixture.

[0078] Step c: After the distillation treatment in step b, the second mixture containing the distillate and the oxygen-containing compound flows out from the top of the extractant regeneration tower 4 and enters the second extraction tower 5 for extraction.

[0079] Specifically, the second extraction tower 5 is used to separate the fractions in the second mixture, so as to recycle this fraction to improve the yield of the final product. The second extraction agent used in the second extraction tower 5 can be a carbonate extractant, specifically including at least one of ethylene carbonate, propylene carbonate, butylene carbonate, and dimethyl carbonate. The second extraction agent is environmentally friendly, non-toxic, and will not introduce heteroatom substances, thereby ensuring the purity of the system. The second extraction agent can select a variety of carbonate substances at the same time, so as to jointly extract to recover the fractions in the second extraction tower 5 to the greatest extent.

[0080] In one embodiment of the present disclosure, the volume ratio of the second extractant used in the second extraction tower 5 to the fraction entering the second extraction tower 5 is 0.1:1-2:1 (e.g., 0.5:1, 1:1, 1.5:1, 2:1); the number of extraction stages of the second extraction tower 5 is 1-5 (e.g., 2, 3, 4). The second extractant can be stored in the second extractant tank 7, which is connected to the second extraction tower 5 via a connecting pipeline, thereby dynamically controlling the feed amount of the second extractant according to the amount of liquid entering the second extraction tower 5.

[0081] In one embodiment of the present disclosure, step c further includes: after the distillation treatment in step b, the first extractant flows out from the bottom of the extractant regeneration tower 4 and is recovered to the first extractant tank 6. Specifically, the first extractant is separated in the extractant regeneration tower 4, and the first extractant can be recovered to the first extractant tank 6, thereby realizing repeated recycling in the system, thereby saving production costs and improving economic efficiency.

[0082] Step d: After the extraction treatment in step c, the third mixture containing the fraction and the oxygen-containing compound flows out through the top of the second extraction tower 5 .

[0083] In one embodiment of the present disclosure, when step f is before step a, step d further includes: the third mixture flows out from the top of the second extraction tower 5 and enters the first distillation tower 1 and / or the second distillation tower 2 for distillation. Specifically, the second extractant can extract oxygen-containing compounds from the mixture, and the fraction is in the raffinate phase at the top of the second extraction tower 5. However, since the second extraction tower 5 cannot achieve 100% extraction, a small amount of oxygen-containing compounds will still remain in the raffinate phase, thereby constituting the third mixture. The third mixture can be transported to the first distillation tower 1 and / or the second distillation tower 2 for distillation again. A small amount of α-olefins with a target carbon number are mixed in the third mixture, so it needs to be recovered to the front end of the system, thereby improving the yield of α-olefins and reducing oil waste.

[0084] Step e: After the extraction treatment in step a, the fourth mixture containing the fraction and the oxygen-containing compound flows out from the top of the first extraction tower 3 and enters the alkane-olefin separation unit 8 for separation to obtain α-olefin.

[0085] It should be noted that there is no sequential relationship between step e and steps b, c, and d, and step e can be performed simultaneously with steps b, c, and d. Specifically, the alkane-ene separation unit 8 can perform multiple separation steps, and the alkane-ene separation unit 8 includes an adsorption unit. Although the first extraction tower 3 achieves the removal of most of the oxygen-containing compounds in the fraction, there are still some that are not completely extracted. The fourth mixture containing the fraction and the oxygen-containing compounds flows out from the top of the first extraction tower 3 and enters the adsorption unit for adsorption to further remove the oxygen-containing compounds in the fourth mixture, thereby improving the purity of the final product.

[0086] In one embodiment of the present disclosure, the adsorption unit is a fixed bed and / or an adsorption tower. Figure 1 As shown, the present disclosure adopts an adsorption tower to perform deoxygenation adsorption on the fourth mixture, and the adsorption tower can completely remove the oxygen-containing compounds in the fourth mixture, thereby further improving the purity of the final product.

[0087] The alkane-olefin separation unit 8 also includes an alkane-olefin separation simulated moving bed. After adsorption treatment by the adsorption unit, the fifth mixture containing alkanes and olefins enters the alkane-olefin separation simulated moving bed from the adsorption unit for separation. Specifically, the alkane-olefin separation simulated moving bed is a device for liquid separation using the adsorption principle. The alkane-olefin separation simulated moving bed selectively adsorbs the fraction in the mixed liquid using an adsorbent, thereby achieving the separation of alkanes and olefins. Through the gradual movement of the material in the alkane-olefin separation simulated moving bed, the adsorbent undergoes a series of adsorption, desorption and regeneration processes, and finally achieves effective separation of the fifth mixture, thereby obtaining α-olefins.

[0088] In a specific embodiment of the present disclosure, the concentration of oxygen-containing compounds in the fourth mixture entering the adsorption unit is less than or equal to 200 ppm, and the concentration of oxygen-containing compounds in the mixture after adsorption treatment is less than 1 ppm. It can be considered that the oxygen-containing compounds have been completely removed, which can meet the requirements of subsequent moving bed adsorption separation and α-olefin product quality.

[0089] In one embodiment of the present disclosure, Figure 2 As shown, step f is also included between step a and step e: the fourth mixture enters the first distillation tower 1 for distillation, the fraction with a carbon number greater than or equal to n flows out from the bottom of the first distillation tower 1, and enters the second distillation tower 2 for distillation, and the fraction with a carbon number less than or equal to m flows out from the top of the second distillation tower 2; wherein n and m are positive integers between 5 and 12 and n≤m.

[0090] The specific working principles of the first distillation tower 1 and the second distillation tower 2 refer to the above-mentioned embodiment. After the Fischer-Tropsch oil is extracted once, the fourth mixture containing the fraction and the oxygen-containing compound can first enter the first distillation tower 1 and the second distillation tower 2 to remove the light and heavy fractions in sequence before flowing into the alkane-olefin separation unit 8. The fraction with a carbon number less than n and the fraction with a carbon number greater than m will be removed in sequence during the distillation process. Isoolefins, isoparaffins, internal olefins, alkanes with a target carbon number, and some oxygen-containing compounds can also be removed during the two distillations, thereby increasing the concentration of the target carbon number α-olefins and improving the purity of the finished product.

[0091] In one embodiment of the present disclosure, Figure 3 As shown, after step e, step f is also included: the α-olefin mixture enters the first distillation tower 1 for distillation, the α-olefins with a carbon number greater than or equal to n flow out from the bottom of the first distillation tower 1 and enter the second distillation tower 2 for distillation, and the α-olefins with a carbon number less than or equal to m flow out from the top of the second distillation tower 2; wherein n and m are positive integers between 5 and 12 and n≤m.

[0092] The specific working principles of the first distillation tower 1 and the second distillation tower 2 refer to the above-mentioned embodiment. After step a and step e, an α-olefin mixture containing various carbon numbers can be obtained, and a small amount of isoolefins and internal olefin components will also be mixed in the mixture. The α-olefin mixture can enter the first distillation tower 1 and the second distillation tower 2 to remove light and heavy components in turn. α-olefins with a carbon number less than n and α-olefins with a carbon number greater than m will be removed in turn during the distillation process, and isoolefins and internal olefins can also be removed during the two distillations, thereby increasing the concentration of the target carbon number α-olefins and improving the purity of the finished product.

[0093] In one embodiment of the present disclosure, reference Figure 2 and Figure 3 , when step f is between step a and step e, or after step e, step d includes: the third mixture flows out from the top of the second extraction tower 5 and enters the first extraction tower 3 for extraction. As mentioned above, a small amount of α-olefins with a target carbon number are mixed in the third mixture, which needs to be recovered to the front end of the system for re-extraction. The front end of the system in this embodiment is the first extraction tower 3, so the third mixture can be transported to the first extraction tower 3 for re-extraction, thereby improving the yield of α-olefins and reducing oil waste.

[0094] The present disclosure provides a method for separating α-olefins of any carbon number in the C5-C12 fraction of Fischer-Tropsch oil products. The extraction process used in the method can remove more than 90% of oxygen-containing compounds, and the yield of the oil products can be maintained at more than 97%. The present disclosure uses high-boiling-point, slightly toxic, and easily separable extractants such as carbonates and sulfones in the extraction step, and the product does not affect the subsequent further separation process of the Fischer-Tropsch oil and various downstream application requirements. The method disclosed in the present disclosure has high separation efficiency and large processing capacity, can meet the requirements of fine chemical processes, and can be applied to industrial production.

[0095] The present disclosure also provides a system for separating α-olefins from Fischer-Tropsch oil, which can be used to perform the above method. The system comprises: a connecting pipeline, a first extraction tower 3, an extractant regeneration tower 4, a second extraction tower 5, and an alkane-olefin separation unit 8.

[0096] The first extraction tower 3 is connected to the connecting pipeline, and the Fischer-Tropsch oil is configured to enter the first extraction tower 3 through the connecting pipeline for primary extraction;

[0097] The extractant regeneration tower 4 is connected to the connecting pipeline located at the bottom output downstream position of the first extraction tower 3 to rectify the extract phase obtained by the primary extraction;

[0098] The second extraction tower 5 is connected to the connecting pipeline located at the top output downstream of the extractant regeneration tower 4 to perform secondary extraction on the fraction obtained by distillation;

[0099] The alkane-olefin separation unit 8 is connected to the connecting pipeline at the top output downstream position of the first extraction tower 3 to perform alkane-olefin separation on the raffinate phase obtained by the primary extraction.

[0100] In one embodiment of the present disclosure, the system further comprises: a first distillation tower 1 and a second distillation tower 2, wherein the first distillation tower 1 is connected to a connecting pipeline located at an upstream position of the first extraction tower 3, and the Fischer-Tropsch oil is configured to enter the first distillation tower 1 through the connecting pipeline for primary distillation;

[0101] Alternatively, the first distillation tower 1 is disposed between the first extraction tower 3 and the alkane-olefin separation unit 8, and the raffinate phase obtained from the primary extraction is configured to enter the first distillation tower 1 for primary distillation;

[0102] Alternatively, the first distillation tower 1 is connected to the connecting pipeline at a downstream position of the alkane-olefin separation unit 8, and the separated α-olefin mixture is configured to enter the first distillation tower 1 for a primary distillation.

[0103] The second distillation tower 2 is connected to the connecting pipeline at the bottom output downstream position of the first distillation tower 1 to perform secondary distillation on the heavy hydrocarbon fraction obtained by the primary distillation.

[0104] In one embodiment of the present disclosure, when the first distillation tower 1 is connected to the connecting pipeline at an upstream position of the first extraction tower 3, the top output port of the second extraction tower 5 is configured to be connected to the connecting pipeline at an upstream position of the first distillation tower 1 and / or the second distillation tower 2;

[0105] When the first distillation tower 1 is arranged between the first extraction tower 3 and the alkane-olefin separation unit 8, or is connected to the connecting pipeline at the bottom output downstream position of the first distillation tower 1, the top output port of the second extraction tower 5 is constructed to be connected to the connecting pipeline at the upstream position of the first extraction tower 3.

[0106] The structure and specific operation mode of a system for separating α-olefins from Fischer-Tropsch oil provided by the present disclosure have been described in detail in the method section above, and will not be repeated here.

[0107] Example 1: Separation of C8 α-olefins from Fischer-Tropsch oil

[0108] The Fischer-Tropsch oil enters the first distillation tower 1, the number of plates of the first distillation tower 1 is 120, the bottom temperature is 137°C, the top temperature is 49°C, the top pressure is 50Kpa, the reflux ratio is 10:1, and the feed temperature is 115°C. The first distillation tower 1 can cut off the fraction with a carbon number less than 8 in the Fischer-Tropsch oil, and the fraction with a carbon number greater than or equal to 8 is separated to the bottom of the first distillation tower 1. The fraction with a carbon number greater than or equal to 8 flows out from the bottom of the first distillation tower 1 and enters the second distillation tower 2 for distillation.

[0109] The component contents of the raw material Fischer-Tropsch oil are shown in the following table (Table 1):

[0110] Table 1

[0111]

[0112] The component contents of the first distillation tower 1 bottom are shown in the following table (Table 2):

[0113] Table 2

[0114]

[0115] Note: In the table, “<C8=” indicates components with carbon number less than 8 and isoolefins or alkanes with carbon number less than or equal to 8, “C8=” indicates olefins with carbon number equal to 8, “nC8” indicates alkanes with carbon number equal to 8, and >nC8” indicates components with carbon number greater than 8 and internal olefins with carbon number equal to 8.

[0116] Comparing Table 1 and Table 2, it can be seen that after distillation in the first distillation tower 1, the components with carbon numbers less than 8 in the fraction are greatly reduced, including most of the isoolefins and oxygen-containing compounds. This achieves accurate cutting of the fraction with carbon numbers less than 8 in the Fischer-Tropsch oil.

[0117] The fraction with a carbon number greater than or equal to 8 enters the second distillation tower 2 for distillation. The number of plates of the second distillation tower 2 is 120, the bottom temperature is 98°C, the top temperature is 54°C, the top pressure is 10Kpa, the reflux ratio is 10:1, and the feed temperature is 134°C. The second distillation tower 2 can cut off the fraction with a carbon number greater than 8, and the fraction with a carbon number equal to 8 is separated to the top of the second distillation tower 2. The fraction with a carbon number equal to 8 flows out from the top of the second distillation tower 2 and enters the first extraction tower 3 for extraction.

[0118] The contents of the top components of the second distillation tower 2 are shown in the following table (Table 3):

[0119] Table 3

[0120]

[0121] Comparing Table 2 and Table 3, it can be seen that after the distillation in the second distillation tower 2, the fraction with a carbon number greater than 8 in the fraction is greatly reduced, thereby achieving accurate cutting of the fraction with a carbon number greater than 8 in the Fischer-Tropsch oil. At the same time, alkanes with a carbon number of 8 are also partially cut during the distillation process of the second distillation tower 2, and substances such as internal olefins with a carbon number of 8 are also cut, thereby increasing the content of C8 α-olefins in the oil product.

[0122] The fraction with carbon number equal to 8 enters the first extraction tower 3 through the top of the second distillation tower 2 for extraction. In the first extraction tower 3, polyethylene glycol 500 is used as the first extractant to extract the fraction. The first extraction tower 3 is an extraction sieve plate tower with a theoretical extraction stage of 2. The volume ratio of the first extractant to the Fischer-Tropsch single C8 oil product is 2.5:1. The first extraction tower 3 is used to remove oxygenated compounds in the fraction, thereby improving the purity of the final product.

[0123] The contents of the top components of the first extraction tower 3 are shown in the following table (Table 4):

[0124] Table 4

[0125]

[0126] The contents of the components at the bottom of the first extraction tower 3 are shown in the following table (Table 5):

[0127] Table 5

[0128]

[0129] Comparing Table 3 and Table 4, it can be seen that after extraction in the first extraction tower 3, the carbonyl content in the fraction is greatly reduced, that is, the oxygen-containing compounds in the fraction are largely removed. The raffinate phase flows out from the top of the first extraction tower 3 and enters the adsorption unit in the alkane-olefin separation unit 8 for adsorption.

[0130] It can be seen from Table 5 that some olefins with a carbon number equal to 8 enter the extraction phase together with oxygen-containing compounds during the extraction process. In order not to waste this part of the oil and to improve the yield of the final product, the extraction phase separated by the first extraction tower 3 needs to be further separated and recovered.

[0131] The extract phase separated by the first extraction tower 3 flows out from the bottom of the first extraction tower 3 and enters the extractant regeneration tower 4 for rectification. The extractant regeneration tower 4 has 15 plates, 8 kPa tower top pressure, 165°C tower bottom temperature, 55°C tower top temperature, 2:1 reflux ratio, and 40°C feed temperature. The extractant regeneration tower 4 can separate the first extractant, which flows out from the bottom of the extractant regeneration tower 4 and is recovered to the first extractant tank 6.

[0132] The contents of the top components of the extractant regeneration tower 4 are shown in the following table (Table 6):

[0133] Table 6

[0134]

[0135] The components in the bottom of the extractant regeneration tower 4 are shown in the following table (Table 7):

[0136] Table 7

[0137]

[0138] Comparing Table 5 and Table 7, it can be seen that after being distilled in the extractant regeneration tower 4, the first extractant in the mixture is separated to the bottom of the extractant regeneration tower 4, and can be recycled to the first extractant tank 6. The first extractant can be recycled repeatedly in the system, thereby saving production costs and improving economic efficiency.

[0139] Comparing Table 5 and Table 6, it can be seen that after distillation in the extractant regeneration tower 4, the fractions and oxygen-containing compounds in the mixture are separated to the top of the extractant regeneration tower 4, and the top product can enter the second extraction tower 5 for secondary extraction.

[0140] The mixture of the fraction with carbon number 8 and oxygen-containing compounds enters the second extraction tower 5 through the top of the extractant regeneration tower 4 for extraction. In the second extraction tower 5, ethylene carbonate is used as the second extractant for extraction. The second extraction tower 5 is an extraction packing tower with a theoretical extraction stage of 2. The volume ratio of the second extractant to the Fischer-Tropsch C8 oil product is 0.5:1. The second extraction tower 5 is used to separate the fractions in the mixture, so as to recycle this part of the fractions to improve the yield of the final product.

[0141] The contents of the top components of the second extraction tower 5 are shown in the following table (Table 8):

[0142] Table 8

[0143]

[0144] The contents of the components at the bottom of the second extraction tower 5 are shown in the following table (Table 9):

[0145] Table 9

[0146]

[0147] As shown in Table 9, after extraction by the second extraction tower 5, the extract phase contains a large amount of carbonyl groups, that is, the oxygen-containing compounds in the mixture are largely extracted by the second extractant. At the same time, the C8 fraction content in the extract phase is low, and the system generates less waste.

[0148] Comparing Table 6 and Table 8, it can be seen that after extraction in the second extraction tower 5, the carbonyl content in the fraction is reduced, that is, the oxygen-containing compounds in the fraction are partially removed. In addition, the content of olefins with a carbon number of 8 in the raffinate phase is greatly increased, and this part of the fraction can be transported to the first distillation tower 1 and / or the second distillation tower 2 for further distillation, thereby improving the yield of C8 α-olefins.

[0149] Although the first extraction tower 3 achieves the removal of most of the oxygen-containing compounds in the fraction with a carbon number of 8, some of them are still not completely extracted. The raffinate phase obtained by the first extraction tower 3 flows out from the top of the first extraction tower 3 and enters the adsorption unit in the alkane-olefin separation unit 8 for adsorption to further remove the oxygen-containing compounds, thereby improving the purity of the final product.

[0150] The content of components at the outlet of the adsorption unit is shown in the following table (Table 10):

[0151] Table 10

[0152]

[0153] By comparing Table 4 and Table 10, it can be seen that after adsorption by the adsorption unit, the carbonyl content in the fraction is greatly reduced, and the final content is reduced to 0, that is, the oxygen-containing compounds in the fraction are completely removed.

[0154] After adsorption treatment in the adsorption unit, the mixture containing alkanes with a carbon number of 8 and olefins with a carbon number of 8 enters the alkane-olefin separation simulated moving bed in the alkane-olefin separation unit 8 for separation, thereby obtaining C8 α-olefins. The purity of the final product C8 α-olefins is greater than 99%. The yield of C8 α-olefins in the whole process is about 98%.

[0155] Example 2: Separation of C10 α-olefins from Fischer-Tropsch oil

[0156] The Fischer-Tropsch oil enters the first distillation tower 1, the number of plates of the first distillation tower 1 is 120, the bottom temperature is 119°C, the top temperature is 87°C, the top pressure is 18Kpa, the reflux ratio is 12:1, and the feed temperature is 115°C. The first distillation tower 1 can cut off the fraction with a carbon number less than 10 in the Fischer-Tropsch oil, and the fraction with a carbon number greater than or equal to 10 is separated to the bottom of the first distillation tower 1. The fraction with a carbon number greater than or equal to 10 flows out from the bottom of the first distillation tower 1 and enters the second distillation tower 2 for distillation.

[0157] The component contents of the raw material Fischer-Tropsch oil are shown in the following table (Table 11):

[0158] Table 11

[0159]

[0160] The component contents of the first distillation tower 1 bottom are shown in the following table (Table 12):

[0161] Table 12

[0162]

[0163] Note: In the table, “<C10=” indicates components with carbon number less than 10 and isoolefins or alkanes with carbon number less than or equal to 10, “C10=” indicates olefins with carbon number equal to 10, “nC10” indicates alkanes with carbon number equal to 10, and >nC10” indicates components with carbon number greater than 10 and internal olefins with carbon number equal to 10.

[0164] Comparing Table 11 and Table 12, it can be seen that after distillation in the first distillation tower 1, the components with carbon numbers less than 10 in the fraction are greatly reduced, including most of the isoolefins and oxygen-containing compounds. This achieves accurate cutting of the fraction with carbon numbers less than 10 in the Fischer-Tropsch oil.

[0165] The fraction with a carbon number greater than or equal to 10 enters the second distillation tower 2 for distillation. The number of plates of the second distillation tower 2 is 60, the bottom temperature is 132°C, the top temperature is 113°C, the top pressure is 18Kpa, the reflux ratio is 10:1, and the feed temperature is 119°C. The second distillation tower 2 can cut off the fraction with a carbon number greater than 10, and the fraction with a carbon number equal to 10 is separated to the top of the second distillation tower 2. The fraction with a carbon number equal to 10 flows out from the top of the second distillation tower 2 and enters the first extraction tower 3 for extraction.

[0166] The contents of the top components of the second distillation tower 2 are shown in the following table (Table 13):

[0167] Table 13

[0168]

[0169] Comparing Table 12 and Table 13, it can be seen that after the distillation in the second distillation tower 2, the components with carbon numbers greater than 10 in the fraction are greatly reduced, thereby achieving accurate cutting of the fraction with carbon numbers less than 10 in the Fischer-Tropsch oil. At the same time, alkanes with a carbon number of 10 are also partially cut during the distillation process of the second distillation tower 2, and substances such as internal olefins with a carbon number of 10 are also cut, thereby increasing the content of C10 α-olefins in the oil product.

[0170] The fraction with a carbon number of 10 enters the first extraction tower 3 through the top of the second distillation tower 2 for extraction. In the first extraction tower 3, dimethyl sulfoxide is used as the first extractant to extract the fraction. The first extraction tower 3 is an extraction sieve plate tower with a theoretical extraction stage of 3. The volume ratio of the first extractant to the Fischer-Tropsch C10 oil product is 2.3:1. The first extraction tower 3 is used to remove oxygenated compounds in the fraction, thereby improving the purity of the final product.

[0171] The contents of the top components of the first extraction tower 3 are shown in the following table (Table 14):

[0172] Table 14

[0173]

[0174] The contents of the components at the bottom of the first extraction tower 3 are shown in the following table (Table 15):

[0175] Table 15

[0176]

[0177] Comparing Table 13 and Table 14, it can be seen that after extraction in the first extraction tower 3, the carbonyl content in the fraction is greatly reduced, that is, the oxygen-containing compounds in the fraction are largely removed. The raffinate phase flows out from the top of the first extraction tower 3 and enters the adsorption unit in the alkane-olefin separation unit 8 for adsorption.

[0178] It can be seen from Table 15 that some olefins with a carbon number equal to 10 enter the extraction phase together with oxygen-containing compounds during the extraction process. In order not to waste this part of the oil and to improve the yield of the final product, it is necessary to further separate and recycle the extraction phase separated by the first extraction tower 3. The extraction phase separated by the first extraction tower 3 flows out from the bottom of the first extraction tower 3 and enters the extractant regeneration tower 4 for distillation.

[0179] The extract phase separated by the first extraction tower 3 flows out from the bottom of the first extraction tower 3 and enters the extractant regeneration tower 4 for rectification. The extractant regeneration tower 4 has 15 plates, 8 kPa tower top pressure, 160°C tower bottom temperature, 98°C tower top temperature, 2:1 reflux ratio, and 40°C feed temperature. The extractant regeneration tower 4 can separate the first extractant, which flows out from the bottom of the extractant regeneration tower 4 and is recovered to the first extractant tank 6.

[0180] The contents of the top components of the extractant regeneration tower 4 are shown in the following table (Table 16):

[0181] Table 16

[0182]

[0183] The contents of components in the bottom of the extractant regeneration tower 4 are shown in the following table (Table 17):

[0184] Table 17

[0185]

[0186] Comparing Table 15 and Table 17, it can be seen that after being distilled in the extractant regeneration tower 4, the first extractant in the mixture is separated to the bottom of the extractant regeneration tower 4, and can be recycled to the first extractant tank 6. The first extractant can be recycled repeatedly in the system, thereby saving production costs and improving economic efficiency.

[0187] By comparing Table 15 and Table 16, it can be seen that after distillation in the extractant regeneration tower 4, the fractions and oxygen-containing compounds in the mixture are separated to the top of the extractant regeneration tower 4, and the top product can enter the second extraction tower 5 for secondary extraction.

[0188] The mixture of the fraction with carbon number 10 and oxygen-containing compounds enters the second extraction tower 5 through the top of the extractant regeneration tower 4 for extraction. In the second extraction tower 5, propylene carbonate is used as the second extractant for extraction. The second extraction tower 5 is a centrifugal extractor with a theoretical extraction stage of 3. The volume ratio of the second extractant to the Fischer-Tropsch C10 oil product is 0.5:1. The second extraction tower 5 is used to separate the fractions in the mixture, so as to recycle this part of the fractions to improve the yield of the final product.

[0189] The contents of the top components of the second extraction tower 5 are shown in the following table (Table 18):

[0190] Table 18

[0191]

[0192] The contents of the components at the bottom of the second extraction tower 5 are shown in the following table (Table 19):

[0193] Table 19

[0194]

[0195] As shown in Table 19, after extraction by the second extraction tower 5, the extract phase contains a large amount of carbonyl groups, that is, the oxygen-containing compounds in the mixture are largely extracted by the second extractant. At the same time, the C10 fraction content in the extract phase is low, and the system generates less waste.

[0196] Comparing Table 16 and Table 18, it can be seen that after extraction in the second extraction tower 5, the carbonyl content in the fraction is reduced, that is, the oxygen-containing compounds in the fraction are partially removed. In addition, the content of olefins with a carbon number of 10 in the raffinate phase is greatly increased, and this part of the fraction can be transported to the first distillation tower 1 and / or the second distillation tower 2 for further distillation, thereby improving the yield of C10 α-olefins.

[0197] Although the first extraction tower 3 achieves the removal of most of the oxygen-containing compounds in the fraction with a carbon number of 10, some of them are still not completely extracted. The raffinate phase obtained by the first extraction tower 3 flows out from the top of the first extraction tower 3 and enters the adsorption unit in the alkane-olefin separation unit 8 for adsorption to further remove the oxygen-containing compounds, thereby improving the purity of the final product.

[0198] The content of components at the outlet of the adsorption unit is shown in the following table (Table 20):

[0199] Table 20

[0200]

[0201] By comparing Table 14 and Table 20, it can be seen that after adsorption by the adsorption unit, the carbonyl content in the fraction is greatly reduced, and the final content is reduced to 0, that is, the oxygen-containing compounds in the fraction are completely removed.

[0202] After adsorption treatment in the adsorption unit, the mixture containing alkanes with a carbon number of 10 and olefins with a carbon number of 10 enters the alkane-olefin separation simulated moving bed in the alkane-olefin separation unit 8 for separation, thereby obtaining C10 α-olefins. The purity of the final product C10 α-olefins is greater than 99%. The yield of C10 α-olefins in the whole process is about 99%.

[0203] Example 3: Separation of C5 α-olefins from Fischer-Tropsch oil

[0204] The Fischer-Tropsch oil enters the first distillation tower 1, the first distillation tower 1 has 60 plates, a bottom temperature of 153°C, a top temperature of 45°C, a top pressure of 500Kpa, a reflux ratio of 4:1, and a feed temperature of 40°C. The first distillation tower 1 can cut off the fraction with a carbon number less than 5 in the Fischer-Tropsch oil, and the fraction with a carbon number greater than or equal to 5 is separated to the bottom of the first distillation tower 1. The fraction with a carbon number greater than or equal to 5 flows out from the bottom of the first distillation tower 1 and enters the second distillation tower 2 for distillation.

[0205] The second distillation tower 2 has 60 plates, a bottom temperature of 116°C, a top temperature of 42°C, a top pressure of 150Kpa, a reflux ratio of 4:1, and a feed temperature of 40°C. The second distillation tower 2 can cut off fractions with a carbon number greater than 5, and the fractions with a carbon number of 5 are separated to the top of the second distillation tower 2. The fractions with a carbon number of 5 flow out from the top of the second distillation tower 2 and enter the first extraction tower 3 for extraction.

[0206] According to the test, after ultra-precision distillation in the first distillation tower 1 and the second distillation tower 2, the content of components smaller than C5 in the Fischer-Tropsch single C5 oil product dropped from a small peak of about 6.3525% to 0.5380%, the content of components larger than C5 dropped from 6.0846% to 1.4350%, and the C5 olefin content increased from 72.6438% to 88.5380%.

[0207] In the first extraction tower 3, dimethyl sulfoxide is used as the first extractant to extract the fraction. The first extraction tower 3 is an extraction sieve plate tower, and its theoretical extraction stage is 3. The volume ratio of the first extractant to the Fischer-Tropsch single C5 oil product is 3:1. The first extraction tower 3 is used to remove oxygen-containing compounds in the fraction, thereby improving the purity of the final product.

[0208] After extraction in the first extraction tower 3, the oxygen-containing compounds in the fraction are largely removed, and the oxygen-containing compounds in the oil product are reduced from 1200ppm to about 90ppm. The raffinate phase flows out from the top of the first extraction tower 3 and enters the adsorption unit in the alkane-olefin separation unit 8 for adsorption. Some olefins with a carbon number equal to 5 enter the extraction phase together with the oxygen-containing compounds during the extraction process. In order not to waste this part of the oil product and to improve the yield of the final product, the extraction phase separated by the first extraction tower 3 needs to be further separated and recovered. The extraction phase separated by the first extraction tower 3 flows out from the bottom of the first extraction tower 3 and enters the extractant regeneration tower 4 for distillation.

[0209] The plate number of the extractant regeneration tower 4 is 15, the tower top pressure is 101 kPa, the tower bottom temperature is 167° C., the tower top temperature is 49° C., the reflux ratio is 0.5:1, and the feed temperature is 40° C. The extractant regeneration tower 4 can separate the first extractant, which flows out from the bottom of the extractant regeneration tower 4 and is recovered to the first extractant tank 6.

[0210] After distillation in the extractant regeneration tower 4, the fractions and oxygen-containing compounds in the mixture are separated to the top of the extractant regeneration tower 4, and the top product can enter the second extraction tower 5 for secondary extraction. In the second extraction tower 5, propylene carbonate is used as the second extractant for extraction. The second extraction tower 5 is an extraction packing tower with a theoretical extraction level of 1. The volume ratio of the second extractant to the Fischer-Tropsch single C5 oil is 1:1. The second extraction tower 5 is used to separate the fractions in the mixture, so as to recycle this part of the fraction to increase the yield of the final product. The second extraction tower 5 reduces the oxygen-containing compounds in the top material of the extractant regeneration tower 4 from 1800ppm to about 450ppm.

[0211] Although the first extraction tower 3 achieves the removal of most of the oxygen-containing compounds in the fraction with a carbon number of 5, some of them are still not completely extracted. The raffinate phase obtained by the first extraction tower 3 flows out from the top of the first extraction tower 3 and enters the adsorption unit in the alkane-olefin separation unit 8 for adsorption to further remove the oxygen-containing compounds, thereby improving the purity of the final product.

[0212] After adsorption treatment, the mixture containing alkanes with a carbon number of 5 and olefins with a carbon number of 5 enters the alkane-olefin separation simulated moving bed in the alkane-olefin separation unit 8 for separation, thereby obtaining C5 α-olefins. The yield of the whole process oil product is about 99%.

[0213] Example 4: Separation of C8 α-olefins from Fischer-Tropsch oil ( Figure 2 The process shown)

[0214] The Fischer-Tropsch oil enters the first extraction tower 3 for extraction. In the first extraction tower 3, polyethylene glycol 500 is used as the first extractant to extract the fraction. The first extraction tower 3 is an extraction sieve plate tower with a theoretical extraction stage of 2. The volume ratio of the first extractant to the Fischer-Tropsch single C8 oil is 2.5:1. The first extraction tower 3 is used to remove oxygenated compounds in the fraction, thereby improving the purity of the final product.

[0215] The component contents of the raw material Fischer-Tropsch oil are shown in the following table (Table 21):

[0216] Table 21

[0217]

[0218] The contents of the top components of the first extraction tower 3 are shown in the following table (Table 22):

[0219] Table 22

[0220]

[0221] The contents of the components at the bottom of the first extraction tower 3 are shown in the following table (Table 23):

[0222] Table 23

[0223]

[0224] Note: In the table, “<C8=” indicates components with carbon number less than 8 and isoolefins or alkanes with carbon number less than or equal to 8, “C8=” indicates olefins with carbon number equal to 8, “nC8” indicates alkanes with carbon number equal to 8, and >nC8” indicates components with carbon number greater than 8 and internal olefins with carbon number equal to 8.

[0225] Comparing Table 21 and Table 22, it can be seen that after extraction in the first extraction tower 3, the carbonyl content in the fraction is greatly reduced, that is, the oxygen-containing compounds in the fraction are largely removed. The raffinate phase flows out from the top of the first extraction tower 3 and enters the first distillation tower 1 for distillation.

[0226] It can be seen from Table 23 that some olefins with a carbon number equal to 8 enter the extraction phase together with oxygen-containing compounds during the extraction process. In order not to waste this part of the oil and to improve the yield of the final product, the extraction phase separated by the first extraction tower 3 needs to be further separated and recovered.

[0227] The extract phase separated by the first extraction tower 3 flows out from the bottom of the first extraction tower 3 and enters the extractant regeneration tower 4 for rectification. The extractant regeneration tower 4 has 15 plates, 8 kPa tower top pressure, 165°C tower bottom temperature, 55°C tower top temperature, 2:1 reflux ratio, and 40°C feed temperature. The extractant regeneration tower 4 can separate the first extractant, which flows out from the bottom of the extractant regeneration tower 4 and is recovered to the first extractant tank 6.

[0228] The contents of the top components of the extractant regeneration tower 4 are shown in the following table (Table 24):

[0229] Table 24

[0230]

[0231] The contents of the components in the bottom of the extractant regeneration tower 4 are shown in the following table (Table 25):

[0232] Table 25

[0233]

[0234] Comparing Table 23 and Table 25, it can be seen that after distillation in the extractant regeneration tower 4, the first extractant in the mixture is separated to the bottom of the extractant regeneration tower 4, and can be recycled to the first extractant tank 6. The first extractant can be recycled repeatedly in the system, thereby saving production costs and improving economic efficiency.

[0235] By comparing Table 23 and Table 24, it can be seen that after distillation in the extractant regeneration tower 4, the fractions and oxygen-containing compounds in the mixture are separated to the top of the extractant regeneration tower 4, and the top product can enter the second extraction tower 5 for secondary extraction.

[0236] The mixture of the fraction with carbon number 8 and oxygen-containing compounds enters the second extraction tower 5 through the top of the extractant regeneration tower 4 for extraction. In the second extraction tower 5, ethylene carbonate is used as the second extractant for extraction. The second extraction tower 5 is an extraction packing tower with a theoretical extraction stage of 2. The volume ratio of the second extractant to the Fischer-Tropsch C8 oil product is 0.5:1. The second extraction tower 5 is used to separate the fractions in the mixture, so as to recycle this part of the fractions to improve the yield of the final product.

[0237] The contents of the top components of the second extraction tower 5 are shown in the following table (Table 26):

[0238] Table 26

[0239]

[0240] The contents of the components at the bottom of the second extraction tower 5 are shown in the following table (Table 27):

[0241] Table 27

[0242]

[0243] As shown in Table 27, after extraction by the second extraction tower 5, the extract phase contains a large amount of carbonyl groups, that is, the oxygen-containing compounds in the mixture are largely extracted by the second extractant. At the same time, the C8 fraction content in the extract phase is low, and the system generates less waste.

[0244] Comparing Table 24 and Table 26, it can be seen that after extraction in the second extraction tower 5, the carbonyl content in the fraction is reduced, that is, the oxygen-containing compounds in the fraction are partially removed. In addition, the content of olefins with a carbon number equal to 8 in the raffinate phase increases, and this part of the fraction can be transported to the first extraction tower 3 for extraction again, thereby improving the yield of C8 α-olefins.

[0245] As mentioned above, the raffinate phase flowing out of the top of the first extraction tower 3 can enter the first distillation tower 1 for distillation. The number of plates of the first distillation tower 1 is 120, the bottom temperature is 137°C, the top temperature is 49°C, the top pressure is 50Kpa, the reflux ratio is 10:1, and the feed temperature is 115°C. The first distillation tower 1 can cut off the fraction with a carbon number less than 8 in the Fischer-Tropsch oil, and the fraction with a carbon number greater than or equal to 8 is separated into the bottom of the first distillation tower 1. The fraction with a carbon number greater than or equal to 8 flows out from the bottom of the first distillation tower 1 and enters the second distillation tower 2 for distillation.

[0246] The component contents of the first distillation tower 1 bottom are shown in the following table (Table 28):

[0247] Table 28

[0248]

[0249] Comparing Table 22 and Table 28, it can be seen that after distillation in the first distillation tower 1, the components with carbon numbers less than 8 in the fraction are greatly reduced, including some isoolefins and oxygen-containing compounds. This achieves accurate cutting of the fraction with carbon numbers less than 8 in the Fischer-Tropsch oil.

[0250] The fraction with a carbon number greater than or equal to 8 enters the second distillation tower 2 for distillation. The number of plates of the second distillation tower 2 is 120, the bottom temperature is 98°C, the top temperature is 54°C, the top pressure is 10Kpa, the reflux ratio is 10:1, and the feed temperature is 134°C. The second distillation tower 2 can cut off the fraction with a carbon number greater than 8, and the fraction with a carbon number equal to 8 is separated to the top of the second distillation tower 2.

[0251] The contents of the top components of the second distillation tower 2 are shown in the following table (Table 29):

[0252] Table 29

[0253]

[0254] Comparing Table 28 and Table 29, it can be seen that after the distillation in the second distillation tower 2, the fraction with a carbon number greater than 8 in the fraction is greatly reduced, thereby achieving accurate cutting of the fraction with a carbon number greater than 8 in the Fischer-Tropsch oil. At the same time, alkanes with a carbon number of 8 are also partially cut during the distillation process of the second distillation tower 2, and substances such as internal olefins with a carbon number of 8 are also cut, thereby increasing the content of C8 α-olefins in the oil product.

[0255] The fraction with a carbon number of 8 flows out from the top of the second distillation tower 2 and enters the adsorption unit in the alkane-olefin separation unit 8 for adsorption to further remove oxygen-containing compounds, thereby improving the purity of the final product.

[0256] The content of components at the outlet of the adsorption unit is shown in the following table (Table 30):

[0257] Table 30

[0258]

[0259] By comparing Table 29 and Table 30, it can be seen that after adsorption by the adsorption unit, the carbonyl content in the fraction is greatly reduced, and the final content drops to 0, that is, the oxygen-containing compounds in the fraction are completely removed.

[0260] After adsorption treatment in the adsorption unit, the mixture containing alkanes with a carbon number of 8 and olefins with a carbon number of 8 enters the alkane-olefin separation simulated moving bed in the alkane-olefin separation unit 8 for separation, thereby obtaining C8 α-olefins. The purity of the final product C8 α-olefins is greater than 99%. The yield of C8 α-olefins in the whole process is about 98%.

[0261] Example 5: Separation of C10 α-olefins from Fischer-Tropsch oil ( Figure 3 The process shown)

[0262] The Fischer-Tropsch oil enters the first extraction tower 3 for extraction. In the first extraction tower 3, dimethyl sulfoxide is used as the first extractant to extract the fraction. The first extraction tower 3 is an extraction sieve plate tower with a theoretical extraction stage of 3. The volume ratio of the first extractant to the Fischer-Tropsch single C10 oil is 2.3:1. The first extraction tower 3 is used to remove oxygenated compounds in the fraction, thereby improving the purity of the final product.

[0263] The component contents of the raw material Fischer-Tropsch oil are shown in the following table (Table 31):

[0264] Table 31

[0265]

[0266] The contents of the top components of the first extraction tower 3 are shown in the following table (Table 32):

[0267] Table 32

[0268]

[0269] The contents of the components at the bottom of the first extraction tower 3 are shown in the following table (Table 33):

[0270] Table 33

[0271]

[0272] Note: In the table, “<C10=” indicates components with carbon number less than 10 and isoolefins or alkanes with carbon number less than or equal to 10, “C10=” indicates olefins with carbon number equal to 10, “nC10” indicates alkanes with carbon number equal to 10, and >nC10” indicates components with carbon number greater than 10 and internal olefins with carbon number equal to 10.

[0273] Comparing Table 31 and Table 32, it can be seen that after extraction in the first extraction tower 3, the carbonyl content in the fraction is greatly reduced, that is, the oxygen-containing compounds in the fraction are largely removed. The raffinate phase flows out from the top of the first extraction tower 3 and enters the adsorption unit of the alkane-olefin separation unit 8 for adsorption.

[0274] It can be seen from Table 33 that some olefins with a carbon number equal to 10 enter the extraction phase together with oxygen-containing compounds during the extraction process. In order not to waste this part of the oil and to improve the yield of the final product, the extraction phase separated by the first extraction tower 3 needs to be further separated and recovered.

[0275] The extract phase separated by the first extraction tower 3 flows out from the bottom of the first extraction tower 3 and enters the extractant regeneration tower 4 for rectification. The extractant regeneration tower 4 has 15 plates, 8 kPa tower top pressure, 160°C tower bottom temperature, 98°C tower top temperature, 2:1 reflux ratio, and 40°C feed temperature. The extractant regeneration tower 4 can separate the first extractant, which flows out from the bottom of the extractant regeneration tower 4 and is recovered to the first extractant tank 6.

[0276] The contents of the top components of the extractant regeneration tower 4 are shown in the following table (Table 34):

[0277] Table 34

[0278]

[0279] The contents of components in the bottom of the extractant regeneration tower 4 are shown in the following table (Table 35):

[0280] Table 35

[0281]

[0282] Comparing Table 33 and Table 35, it can be seen that after distillation in the extractant regeneration tower 4, the first extractant in the mixture is separated to the bottom of the extractant regeneration tower 4, and can be recycled to the first extractant tank 6. The first extractant can be recycled repeatedly in the system, thereby saving production costs and improving economic efficiency.

[0283] By comparing Table 33 and Table 34, it can be seen that after distillation in the extractant regeneration tower 4, the fractions and oxygen-containing compounds in the mixture are separated to the top of the extractant regeneration tower 4, and the top product can enter the second extraction tower 5 for secondary extraction.

[0284] The mixture of the fraction with carbon number 10 and oxygen-containing compounds enters the second extraction tower 5 through the top of the extractant regeneration tower 4 for extraction. In the second extraction tower 5, ethylene carbonate is used as the second extractant for extraction. The second extraction tower 5 is an extraction packing tower with a theoretical extraction stage of 2. The volume ratio of the second extractant to the Fischer-Tropsch C10 oil is 0.5:1. The second extraction tower 5 is used to separate the fractions in the mixture, so as to recycle this part of the fractions to improve the yield of the final product.

[0285] The contents of the top components of the second extraction tower 5 are shown in the following table (Table 36):

[0286] Table 36

[0287]

[0288] The contents of the components at the bottom of the second extraction tower 5 are shown in the following table (Table 37):

[0289] Table 37

[0290]

[0291] As shown in Table 37, after extraction by the second extraction tower 5, the extract phase contains a large amount of carbonyl groups, that is, the oxygen-containing compounds in the mixture are largely extracted by the second extractant. At the same time, the C10 fraction content in the extract phase is low, and the system generates less waste.

[0292] Comparing Table 34 and Table 36, it can be seen that after extraction in the second extraction tower 5, the carbonyl content in the fraction is reduced, that is, the oxygen-containing compounds in the fraction are partially removed. In addition, the content of olefins with a carbon number equal to 10 in the raffinate phase increases, and this part of the fraction can be transported to the first extraction tower 3 for extraction again, thereby improving the yield of C10 α-olefins.

[0293] Although the first extraction tower 3 achieves the removal of most of the oxygen-containing compounds in the fraction with a carbon number of 10, some of them are still not completely extracted. The raffinate phase obtained by the first extraction tower 3 flows out from the top of the first extraction tower 3 and enters the adsorption unit in the alkane-olefin separation unit 8 for adsorption to further remove the oxygen-containing compounds, thereby improving the purity of the final product.

[0294] The content of components at the outlet of the adsorption unit is shown in the following table (Table 38):

[0295] Table 38

[0296]

[0297] By comparing Table 32 and Table 38, it can be seen that after adsorption by the adsorption unit, the carbonyl content in the fraction is greatly reduced, and the final content drops to 0, that is, the oxygen-containing compounds in the fraction are completely removed.

[0298] After adsorption treatment in the adsorption unit, the mixture containing alkanes and olefins enters the alkane-olefin separation simulated moving bed in the alkane-olefin separation unit 8 for separation, thereby obtaining an α-olefin mixture, wherein the C10 α-olefin content is about 89.3268%.

[0299] The α-olefin mixture will be transported to the first distillation tower 1 for distillation. The number of plates of the first distillation tower 1 is 120, the bottom temperature is 119°C, the top temperature is 87°C, the top pressure is 18Kpa, the reflux ratio is 12:1, and the feed temperature is 115°C. The first distillation tower 1 can remove α-olefins with a carbon number less than 10 in the Fischer-Tropsch oil, and α-olefins with a carbon number greater than or equal to 10 are separated to the bottom of the first distillation tower 1. α-olefins with a carbon number greater than or equal to 10 flow out from the bottom of the first distillation tower 1 and enter the second distillation tower 2 for distillation.

[0300] The component contents of the first distillation tower 1 bottom are shown in the following table (Table 39):

[0301] Table 39

[0302]

[0303] The first distillation tower 1 can accurately cut the fraction with a carbon number less than 10 in the α-olefin mixture, and the α-olefin with a carbon number greater than or equal to 10 can enter the second distillation tower 2 for distillation. The number of plates of the second distillation tower 2 is 60, the bottom temperature is 132°C, the top temperature is 113°C, the top pressure is 18Kpa, the reflux ratio is 10:1, and the feed temperature is 119°C. The second distillation tower 2 can cut off the α-olefin with a carbon number greater than 10, and the α-olefin with a carbon number equal to 10 is separated to the top of the second distillation tower 2. The α-olefin with a carbon number equal to 10 flows out through the top of the second distillation tower 2.

[0304] The contents of the components at the top of the second distillation tower 2 are shown in the following table (Table 40):

[0305] Table 40

[0306]

[0307] Comparing Table 39 and Table 40, it can be seen that after distillation in the second distillation tower 2, the α-olefins with a carbon number greater than 10 are greatly reduced, thereby achieving accurate cutting of the fractions with a carbon number greater than 10 in the Fischer-Tropsch oil. At the same time, substances such as internal olefins with a carbon number of 10 are also cut, thereby increasing the content of C10 α-olefins in the oil product. The purity of the final product C10 α-olefin is greater than 99%. The yield of C10 α-olefins in the whole process is about 99%.

[0308] The embodiments of the present disclosure have been described above, and the above description is exemplary, not exhaustive, and is not limited to the disclosed embodiments. Many modifications and changes will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The choice of terms used herein is intended to best explain the principles of the embodiments, practical applications, or technical improvements in the market, or to enable other persons of ordinary skill in the art to understand the embodiments disclosed herein. The scope of the present disclosure is defined by the appended claims.

Claims

1. A method for separating α-olefins from Fischer-Tropsch oil, characterized in that: The steps include: Step a: the Fischer-Tropsch oil enters a first extraction tower for extraction; the first extraction tower uses a first extracting agent selected from at least one of sulfones and polyols; Step b: After the extraction treatment in step a, the first mixture comprising the fraction, the first extractant and the oxygen-containing compound flows out from the bottom of the first extraction tower and enters the extractant regeneration tower for rectification; Step c: after the distillation treatment in step b, the second mixture containing the fraction and the oxygen-containing compound flows out from the top of the extractant regeneration tower and enters the second extraction tower for extraction; the second extraction tower uses a second extractant selected from at least one of ethylene carbonate, propylene carbonate, butylene carbonate and dimethyl carbonate; Step d: After the extraction treatment in step c, a third mixture containing the fraction and oxygen-containing compounds flows out from the top of the second extraction tower; Step e: After the extraction treatment in step a, the fourth mixture containing the fraction and the oxygen-containing compound flows out from the top of the first extraction tower and enters the alkane-olefin separation unit for separation to obtain α-olefin; Before step a, the method further includes step f: the Fischer-Tropsch oil enters a first distillation tower for distillation, and the fraction with a carbon number greater than or equal to n flows out from the bottom of the first distillation tower and enters a second distillation tower for distillation, and the fraction with a carbon number less than or equal to m flows out from the top of the second distillation tower; wherein n≤m; When the step f is before the step a, the step d comprises: the third mixture flows out from the top of the second extraction tower and enters the first distillation tower and / or the second distillation tower for distillation; Alternatively, step f is further included between step a and step e: the fourth mixture enters a first distillation tower for distillation, a fraction with a carbon number greater than or equal to n flows out from the bottom of the first distillation tower, and enters a second distillation tower for distillation, and a fraction with a carbon number less than or equal to m flows out from the top of the second distillation tower; wherein n≤m; Alternatively, after step e, the method further comprises step f: the α-olefin mixture enters a first distillation tower for distillation, the α-olefins with carbon numbers greater than or equal to n flow out from the bottom of the first distillation tower and enter a second distillation tower for distillation, and the α-olefins with carbon numbers less than or equal to m flow out from the top of the second distillation tower; wherein n≤m; When step f is between step a and step e, or after step e, step d comprises: the third mixture flows out from the top of the second extraction tower and enters the first extraction tower for extraction.

2. The method according to claim 1, characterized in that The step a further comprises: providing a first extractant from a first extractant tank to the first extraction tower.

3. The method according to claim 2, characterized in that The step c also includes: after the distillation treatment in the step b, the first extractant flows out from the bottom of the extractant regeneration tower and is recovered to the first extractant tank.

4. The method according to claim 1, characterized in that: The first extractant is selected from at least one of dimethyl sulfoxide, cyclopentane, polyethylene glycol 300 and polyethylene glycol 500.

5. The method according to claim 1, characterized in that The volume ratio of the first extractant used in the first extraction tower to the fraction entering the first extraction tower is 1:1-10:1; and the number of extraction stages of the first extraction tower is 1-5.

6. The method according to claim 1, characterized in that The volume ratio of the second extractant used in the second extraction tower to the fraction entering the second extraction tower is 0.1:1-2:1; and the extraction stages of the second extraction tower are 1-5.

7. The method according to claim 1, characterized in that n and m are positive integers between 5 and 12.

8. The method according to claim 7, characterized in that The number of plates of the first distillation tower is greater than or equal to 50, and the reflux ratio is greater than or equal to 4:

1.

9. The method according to claim 7, characterized in that: The number of plates of the second distillation tower is greater than or equal to 50, and the reflux ratio is greater than or equal to 4:

1.

10. The method according to claim 1, characterized in that The number of plates of the extractant regeneration tower is 6-20, the bottom temperature is 120-200°C, the top temperature is 10-100°C, the top pressure is 5-101Kpa, the reflux ratio is 0.2:1-5:1, and the temperature of the first mixture when entering the extractant regeneration tower is 20-150°C.

11. A system for separating α-olefins from Fischer-Tropsch oil, characterized in that: The system is used to perform the method according to any one of claims 1 to 10, and the system comprises: Connecting pipelines; a first extraction tower, connected to the connecting pipeline, wherein the Fischer-Tropsch oil is configured to enter the first extraction tower through the connecting pipeline for primary extraction; An extractant regeneration tower connected to the connecting pipeline at a downstream position of the bottom output of the first extraction tower to rectify the extract phase obtained by the primary extraction; A second extraction tower is connected to a connecting pipeline located at a position downstream of the top output of the extractant regeneration tower to perform secondary extraction on the fraction obtained by distillation; an alkane-olefin separation unit, connected to the connecting pipeline at a position downstream of the top output of the first extraction tower, so as to separate the alkane-olefin from the raffinate phase obtained by the primary extraction; a first distillation tower connected to a connecting pipeline located upstream of the first extraction tower, and the Fischer-Tropsch oil is configured to enter the first distillation tower through the connecting pipeline for primary distillation; Alternatively, the first distillation tower is disposed between the first extraction tower and the alkane-olefin separation unit, and the raffinate phase obtained from the primary extraction is configured to enter the first distillation tower for primary distillation; Alternatively, the first distillation tower is connected to a connecting pipeline located downstream of the alkane-olefin separation unit, and the separated α-olefin mixture is configured to enter the first distillation tower for primary distillation; A second distillation tower is connected to the connecting pipeline at a downstream position of the bottom output of the first distillation tower to perform a second distillation on the heavy hydrocarbon fraction obtained by the first distillation; When the first distillation tower is connected to the connecting pipeline at an upstream position of the first extraction tower, the top outlet of the second extraction tower is configured to be connected to the connecting pipeline at an upstream position of the first distillation tower and / or the second distillation tower; When the first distillation tower is arranged between the first extraction tower and the alkane-olefin separation unit, or is connected to the connecting pipeline at a downstream position of the bottom output of the first distillation tower, the top output port of the second extraction tower is constructed to be connected to the connecting pipeline at an upstream position of the first extraction tower.

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