A process for separating normal paraffins from distillate oils using simulated moving beds

Abstract

A method for separating n-alkanes from distillate oil using a simulated moving bed, the simulated moving bed comprising multiple adsorbent beds packed with adsorbent, the multiple adsorbent beds being interconnected in a closed loop and divided into a desorption zone, a purification zone, an adsorption zone, and a buffer zone, the distillate oil comprising n-alkanes and aromatics, and optionally isoalkanes and cycloalkanes, wherein the method comprises the following steps: 1) injecting the distillate oil upstream of the adsorption zone, wherein the n-alkanes in the distillate oil are selectively adsorbed by the adsorbent in the adsorption zone; 2) injecting a desorbent upstream of the desorption zone, wherein the desorbent is used to desorb the adsorbed n-alkanes in the desorption zone, wherein the desorbent comprises n-alkanes and aromatics, and their carbon numbers are respectively greater than the maximum carbon number of the component in the distillate oil or less than the minimum carbon number of the component in the distillate oil; 3) removing the raffinate containing the desorbed n-alkanes downstream of the desorption zone; and 4) removing the raffinate containing the unadsorbed component downstream of the adsorption zone. This method can effectively separate n-alkanes from distillate oils, especially C10-C25 distillate oils, while also appropriately reducing the difficulty of system operation and lowering energy consumption and process costs.

Description

A method for separating normal alkanes from distillate oil using a simulated moving bed CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the priority of patent application number 202311331416.7 and title “A method for adsorptive separation of normal alkanes in C10-C25 fraction oil” filed by the applicant with the China Patent Office on October 13, 2023. The contents of the above patent application are hereby incorporated herein by reference in their entirety. Technical Field The present application relates to a method for separating normal alkanes from distillate oil by using a simulated moving bed. In particular, the present application relates to a method for separating C10-C25 normal alkanes from C10-C25 distillate oil by using a simulated moving bed. Background Art C10-C13 normal alkanes are mainly found in kerosene fractions, and C14-C25 normal alkanes are mainly found in diesel fractions. C10-C13 normal alkanes are the main components of light liquid wax, and are mainly used to produce linear alkylbenzenes and fatty alcohols. C14-C20 normal alkanes are the main components of heavy liquid wax, and can be used to produce high value-added products such as petroleum grease plasticizers, chlorinated paraffins, higher fatty alcohols, cosmetics, and synthetic petroleum proteins. Normal alkanes above C20 are the main components of solid paraffin wax. Paraffin wax with high purity normal alkanes can be used as latent heat storage materials and are widely used in various fields such as aviation, aerospace, microelectronics and other high-tech systems, as well as housing energy conservation. Adsorption-desorption is very effective for the separation of isomers with small differences in boiling points or for the separation of components with different structural characteristics. For example, separation by adsorption-desorption can effectively separate normal alkanes and non-normal alkanes with the same or similar carbon numbers, including isoalkanes, aromatics, and cycloalkanes. The simulated moving bed adsorption-desorption separation system is a complex and efficient separation system. It can effectively improve the separation efficiency by using the countercurrent contact of liquid and solid phases, and thus can be widely used in many fields such as petrochemicals, food, and medicine. Specifically, in the field of petrochemicals, by selecting appropriate adsorbents and desorbents, the simulated moving bed separation system can achieve effective separation of normal alkanes and non-normal alkanes, especially C10-C25 normal alkanes and C10-C25 and non-normal alkanes. US4036745A discloses a method for separating normal alkanes from a feed stream containing a mixture of normal alkanes, isoalkanes and aromatics. The content of pollutant aromatic hydrocarbons, US4036745A adopts a two-stage desorption step. In the first stage, a first desorbent is contacted with an adsorbent adsorbed with normal alkanes and aromatic hydrocarbons in a purification zone for a first desorption. In the second stage, a second desorbent is contacted with the adsorbent that has been desorbed by the first desorption in a desorption zone for a second desorption to obtain an extract containing normal alkanes and the second desorbent. Since the first desorbent and the second desorbent are used in two stages respectively, it is necessary to set up feeding equipment related to the first desorbent and the second desorbent respectively. In addition, since the first desorbent and the second desorbent are usually recycled, it is necessary to further separate the mixture of the first desorbent and the second desorbent separated from the extract and the residual liquid to obtain the separated first desorbent and the second desorbent. In this case, it is necessary to set up another device for separating the first desorbent and the second desorbent. Such a separation system related to the simulated moving bed has a complex structure, is difficult to operate, and has high energy consumption and process costs. Therefore, there is still a need for a method and system for separating normal alkanes from distillate oil, especially C10-C25 distillate oil, by adsorption-desorption using a simulated moving bed, wherein the method and system can not only effectively separate the normal alkanes but also appropriately reduce the difficulty of system operation and reduce energy consumption and process costs. Summary of the invention The object of the present invention is to provide a method for separating normal paraffins from distillate oil by adsorption-desorption using a simulated moving bed, and a system for separating normal paraffins from distillate oil by adsorption-desorption. In a first aspect, the present invention relates to a method for separating normal alkanes from distillate oil using a simulated moving bed, wherein the simulated moving bed comprises a plurality of adsorbent beds filled with adsorbents, wherein the plurality of adsorbent beds are interconnected in a closed loop manner and divided into a desorption zone, a purification zone, an adsorption zone and a buffer zone, wherein the distillate oil comprises normal alkanes and aromatic hydrocarbons and optionally isoalkanes and cycloalkanes, wherein the method comprises the following steps: 1) injecting distillate oil from the upstream of the adsorption zone, wherein normal alkanes in the distillate oil are selectively adsorbed by the adsorbent in the adsorption zone; 2) injecting a desorbent from the upstream of the desorption zone, and desorbing the adsorbed normal alkanes by using the desorbent in the desorption zone, wherein the desorbent comprises normal alkanes and aromatic hydrocarbons, and the carbon numbers of the desorbents are respectively greater than the maximum carbon number of the components in the distillate oil or less than the minimum carbon number of the components in the distillate oil; 3) removing an extract containing desorbed normal paraffins from the downstream of the desorption zone; and 4) The raffinate containing the non-adsorbed components is withdrawn from the downstream of the adsorption zone. In a second aspect, the present invention relates to a system for separating normal paraffins from distillate oil by adsorption-desorption, wherein the system comprises a simulated moving bed, the simulated moving bed comprises a plurality of adsorbent beds filled with adsorbent, the plurality of adsorbent beds are interconnected in a closed loop manner and are divided into: (i) a desorption zone located between upstream desorbent injection and downstream extract withdrawal, (ii) a purification zone located between the upstream withdrawal of the extract and the downstream injection of the distillate, (iii) an adsorption zone located between upstream distillate injection and downstream raffinate withdrawal, and (iv) a buffer zone located between upstream raffinate removal and downstream desorbent injection, wherein at least one, preferably one, desorbent injection port is provided upstream of the desorption zone; The distillate oil contains normal alkanes and aromatic hydrocarbons and optionally isoalkanes and cycloalkanes; the desorbent contains normal alkanes and aromatic hydrocarbons, and their carbon numbers are respectively greater than the maximum carbon number of the components in the distillate oil or less than the minimum carbon number of the components in the distillate oil. By utilizing a simulated moving bed and relying on adsorption-desorption effects and by selecting a suitable composite desorbent, the method and system of the present invention can effectively separate normal alkanes from distillate oil, especially C10-C25 distillate oil, while also appropriately reducing the difficulty of system operation and reducing energy consumption and process costs. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings are used to provide further understanding of the present application and constitute a part of the specification. Together with the following specific embodiments, they are used to explain the present application, but do not constitute a limitation of the present invention. FIG1 is a schematic diagram of a process for separating normal alkanes from C10-C25 fraction oil by adsorption-desorption using a composite desorbent using a simulated moving bed according to an embodiment of the present invention. 2 is a schematic diagram of a process for separating normal alkanes from C10-C25 fraction oil by adsorption-desorption using a first desorbent and a second desorbent respectively using a simulated moving bed according to an embodiment of the prior art. Figure 3 is a concentration distribution diagram of heavy aromatic hydrocarbons (from raw materials) and light aromatic hydrocarbons (from desorbents or flushing agents) flows in each adsorption column during the process of carrying out the method for separating normal alkanes using a composite desorbent according to Example 1 of the present invention and the method for separating normal alkanes using a first desorbent and a second desorbent respectively according to Comparative Example 3 which is not of the present invention. Description of Reference Numerals 101-112 adsorption column DETAILED DESCRIPTION The present application will be further described in detail below through specific implementation methods. It should be understood that the specific implementation methods described herein are only used to illustrate and explain the present application, but do not limit the present invention in any way. Any specific numerical value disclosed herein (including the endpoint of the numerical range) is not limited to the exact value of the numerical value, but should be understood to also cover values ​​close to the exact value, such as all possible numerical values ​​within the range of ±5% of the exact value. In addition, for the disclosed numerical range, the endpoint values ​​of the range, the endpoint values ​​and the specific point values ​​in the range, and the specific point values ​​can be arbitrarily combined to obtain one or more new numerical ranges, and these new numerical ranges should also be regarded as specifically disclosed herein. Unless otherwise specified, the terms used herein have the same meaning as commonly understood by those skilled in the art. If a term is defined herein and its definition is different from the commonly understood meaning in the art, the definition herein shall prevail. The expressions "comprising" or "including" herein should be interpreted as including all the specifically mentioned features as well as optional, additional, unspecified features. As used herein, the use of the term "comprising" also discloses solutions in which there are no further features than the specifically mentioned features, such as the expressions "consisting essentially of" and "consisting of". In the context of the present application, the terms "one or more" and "at least one" have the same meaning and can be used interchangeably; the terms "one or several" and "at least one" have the same meaning and can be used interchangeably. In the context according to the present application, the term "plurality" refers to two or more, such as three or more, four or more. In the context of the present application, the terms "desorption" and "desorption" have the same meaning and can be used interchangeably. I. Simulated moving bed The method for separating normal alkanes from distillate oil by means of adsorption-desorption of the present invention is carried out using a simulated moving bed. The simulated moving bed may include a plurality of adsorbents filled with Adsorbent bed. In a simulated moving bed, the adsorbent may be loaded in a column container in the form of a solid bed, for example, the adsorbent may be loaded into a plurality of adsorption columns connected in series and end to end; or the adsorbent may be loaded into a plurality of adsorbent beds separated by screens or grids, in which case the plurality of adsorbent beds are placed in one or more adsorption towers, for example, one or two adsorption towers. In the context of the present application, the term "adsorbent bed" includes both (1) the case where the adsorbent is loaded into a plurality of adsorption columns connected in series and end to end, and (2) the case where the adsorbent is loaded into a plurality of adsorption beds separated by screens or grids in one or more adsorption towers. In the first case, one "adsorbent bed" generally corresponds to one adsorption column. The function of the grid is to redistribute the liquid flow from the upper bed to the lower bed, mix the liquid flow introduced from the outside with the liquid flow from the upper bed, and lead part of the liquid flow from the upper bed out of the adsorption tower. The grid allows the liquid flow to pass through and intercepts the adsorbent particles from escaping the adsorbent bed. The upper and lower surfaces are generally made of metal wire mesh, metal sintered mesh or Johnson Screen. Each adsorbent bed is connected to at least one pipeline, through which liquid streams are introduced into the adsorbent bed from the outside and liquid materials are led out of the adsorbent bed. The individual liquid streams entering and leaving the same adsorbent bed can be controlled by switch valves, for example, individually controlled by switch valves arranged in parallel. If the simulated moving bed includes n liquid streams entering or leaving the adsorbent bed and m adsorbent beds, n×m switch valves can be set. In some cases, at a certain moment, n of the switch valves connecting each liquid stream to a different adsorbent bed are in an open state, and the remaining switch valves are in a closed state. Under normal circumstances, the flow direction of the liquid stream between the adsorbent beds is opposite to the movement direction of the adsorbent beds, thereby forming an effect of relative countercurrent simulated movement of the liquid and solid phases. During the operation of the simulated moving bed, each pipeline passes through at least four liquid streams entering or leaving it in sequence, including desorbent, extract, raw material and raffinate, and the liquid phase circulates in the adsorption tower or between the adsorption columns to form a closed loop connected end to end. The four liquid streams flowing into and out of the adsorbent bed separate the adsorbent bed into four functionally different areas, including desorption area, purification area, adsorption area and buffer area. The desorption zone is located between the upstream desorbent injection and the downstream extraction of the extract. The desorbent enters from the upstream of the desorption zone and is used to displace the normal alkanes in the adsorbent pores. Contains normal alkanes and desorbent, and they flow out of the adsorbent bed as an extract. The purification zone is located between the upstream extraction of the extract and the downstream injection of the raw material. The liquid flow flowing from the upstream to the downstream flushes the non-normal alkanes outside the adsorbent pores to the downstream. The adsorption zone is located between the upstream raw material injection and the downstream raffinate removal. The raw material flow to be separated containing normal alkanes enters the adsorbent bed from the upstream of the adsorption zone and contacts the adsorbent, wherein at least a portion of the normal alkanes are adsorbed into the adsorbent pores, while the non-adsorbed components and the desorbent flow out of the adsorbent bed from the downstream of the adsorption zone as the raffinate. The buffer zone is located between the upstream extraction of residual liquid and the downstream injection of desorbent. The net flow of the buffer zone is a negative value (the net flow refers to the actual flow of circulating liquid to the downstream minus the flow of liquid carried by the adsorbent bed to the upstream), the purpose of which is to prevent the non-normal alkane components at the bottom of the adsorption zone from entering the desorption zone through the buffer zone to contaminate the extracted liquid. Under normal circumstances, each liquid stream moves to the next adsorbent bed at a specific interval, i.e., a step time. The intervals between each liquid stream entering or leaving the adsorbent bed are different, depending on the number of adsorbent beds between each adjacent liquid stream, i.e., the time between two adjacent liquid streams in the same adsorbent bed is equal to the number of adsorbent beds between the corresponding two adjacent liquid streams in the simulated moving bed. For example, if the number of adsorbent beds in the desorption zone is 5, then for a certain adsorbent bed, after the desorbent is introduced, the extract is taken out from the adsorbent bed after an interval of 5 step times. II. Method for separating normal alkanes from distillate oil In a first aspect, the present invention relates to a method for separating normal alkanes from distillate oil using a simulated moving bed, wherein the simulated moving bed comprises a plurality of adsorbent beds filled with adsorbents, wherein the plurality of adsorbent beds are interconnected in a closed loop manner and divided into a desorption zone, a purification zone, an adsorption zone and a buffer zone, wherein the distillate oil comprises normal alkanes and aromatic hydrocarbons and optionally isoalkanes and cycloalkanes, wherein the method comprises the following steps: 1) injecting distillate oil from the upstream of the adsorption zone, wherein normal alkanes in the distillate oil are selectively adsorbed by the adsorbent in the adsorption zone; 2) injecting a desorbent from the upstream of the desorption zone, and desorbing the adsorbed normal alkanes by using the desorbent in the desorption zone, wherein the desorbent comprises normal alkanes and aromatic hydrocarbons, and the carbon numbers of the desorbents are respectively greater than the maximum carbon number of the components in the distillate oil or less than the minimum carbon number of the components in the distillate oil; 3) removing an extract containing desorbed normal paraffins from the downstream of the desorption zone; and 4) The raffinate containing the non-adsorbed components is withdrawn from the downstream of the adsorption zone. For the sake of convenience and clarity of expression, the above-mentioned separation method according to the present invention is divided into steps 1)-4). However, the numbers 1)-4) do not represent the order of implementation. For a simulated moving bed, steps 1), step 2), step 3) and step 4) can be implemented in any order or simultaneously. For example, step 1) and step 2) can be carried out simultaneously, or step 1) can be implemented first and then step 2), or step 2) can be implemented first and then step 1). For example, step 3) and step 4) can be carried out simultaneously, or step 3) can be implemented first and then step 4), or step 4) can be implemented first and then step 3). Preferably, according to the present invention, during the operation of the simulated moving bed, the injection of the distillate oil, the injection of the desorbent, the removal of the extract and the removal of the residual liquid are carried out simultaneously and continuously. Preferably, the structure and function of the simulated moving bed are as defined in Part I above. The term "selective adsorption" refers to the different adsorption capacities of different components in the mixture to be separated according to the different structures of the adsorbent. According to the present invention, the "selective adsorption" preferably refers to a higher adsorption capacity for normal alkanes in the distillate oil than for other components in the distillate oil, including isoalkanes, aromatics and cycloalkanes. Optionally, before feeding the distillate oil into the adsorbent bed, the distillate oil is hydrotreated to saturate the olefins that may be contained therein to form alkanes and to remove the sulfur-containing compounds and nitrogen-containing compounds that may be contained therein to avoid poisoning the adsorbent. The term "distillate" refers to a component with a certain distillation range (boiling point range) separated during the fractionation of crude oil. According to the present invention, the distillate preferably contains normal alkanes with a carbon number of more than 10 and aromatics with a carbon number of more than 10. More preferably, the distillate contains normal alkanes of C10-C30 and aromatics of C10-C30. More preferably, the distillate contains normal alkanes of C10-C25 and aromatics of C10-C25. Still more preferably, the distillate contains normal alkanes of C14-C25 and aromatics of C14-C25. Optionally, in addition to the normal alkanes and aromatics, the distillate oil further comprises isoalkanes with a carbon number of 10 or more and / or cycloalkanes with a carbon number of 10 or more, preferably further comprises C10-C30 isoalkanes and / or C10-C30 cycloalkanes, preferably further comprises C10-C25 isoalkanes and / or C10-C30 cycloalkanes, more preferably further comprises C14-C25 isoalkanes and / or C14-C25 cycloalkanes. According to the context of the present application, the term "desorbent" refers to an agent which displaces the adsorbed normal alkanes from the adsorbent and preferably enables the purpose of reusing the adsorbent. Preferably, the maximum carbon number of each component in the desorbent is less than the maximum carbon number of each component in the distillate oil. Small carbon number. More preferably, the maximum carbon number of each component in the desorbent is at least one carbon atom less than the minimum carbon number of each component in the distillate, preferably at least two carbon atoms less. Preferably, the desorbent comprises normal alkanes of C5-C8 and aromatics of C6-C8, and optionally isoalkanes of C5-C8. Preferably, the desorbent consists of normal alkanes of C5-C8 and aromatics of C6-C8, and optionally isoalkanes of C5-C8. According to the present invention, the C5-C8 normal alkane can be selected from n-pentane, n-hexane, n-heptane, n-octane, or a mixture thereof. According to the present invention, the C6-C8 aromatic hydrocarbons can be selected from benzene, toluene, ethylbenzene, xylene, or a mixture thereof, wherein the xylene can be o-xylene, m-xylene, p-xylene, or a mixture thereof. Preferably, based on the total volume of the desorbent, the desorbent comprises 30-98% by volume, preferably 40-96% by volume, more preferably 50-95% by volume, still more preferably 60-90% by volume, or 60-80% by volume of normal alkanes of C5-C8. For example, based on the total volume of the desorbent, the desorbent may comprise 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 96%, 97%, 98% by volume, or a range of C5-C8 normal alkanes consisting of any two of the above values ​​or one of the above values ​​and one of the above ranges, or one of the above ranges and another of the above ranges. Preferably, based on the total volume of the desorbent, the desorbent comprises 1-70 volume %, preferably 2-60 volume %, or 2-50 volume %, more preferably 3-40 volume %, or 3-35 volume %, even more preferably 4-30 volume %, or 4-25 volume %, still more preferably 5-20 volume %, or 10-20 volume % of C6-C8 aromatic hydrocarbons. For example, based on the total volume of the desorbent, the desorbent may comprise 1 volume %, 2 volume %, 3 volume %, 4 volume %, 5 volume %, 6 volume %, 8 volume %, 10 volume %, 12 volume %, 15 volume %, 18 volume %, 20 volume %, 25 volume %, or a range of C6-C8 aromatic hydrocarbons consisting of any two of the above values ​​or one of the above values ​​and one of the above ranges, or one of the above ranges and another of the above ranges. Preferably, based on the total volume of the desorbent, the desorbent comprises 0-50% by volume, preferably 0-40% by volume, more preferably 0-30% by volume, and still more preferably 0-20% by volume of C5-C8 isoparaffins. For example, based on the total volume of the desorbent, the desorbent may comprise 0%, 2%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 50%, 60%, 70%, 80%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, 100%, 101%, 102%, 103%, 104%, 105%, 106%, 107%, 108%, 109%, 110%, 111%, 112%, 113%, 114%, 115%, 116%, 117%, 118%, 119%, 120%, 121%, 122%, 123%, 124%, 125%, 126%, 127%, 128%, 129%, 130%, 131%, 132%, 133%, 134%, 135%, 136%, 137%, 138%, 139%, 140%, 141%, 142%, 143%, 144%, 145%, 146%, 147%, 148%, 149%, 150%, 151%, 152%, 153%, 154%, 155%, 156%, 157%, 158%, 159%, 160%, 161%, 162%, 16 Or C5-C8 isomeric alkanes in a range consisting of any two of the above values, or one of the above values ​​and one endpoint of one of the above ranges, or one endpoint of one of the above ranges and one endpoint of another of the above ranges. In some embodiments, the desorbent comprises 50-95 vol. % or 60-90 vol. % or 60-80 vol. % of C5-C8 normal alkanes, 5-20 vol. % or 10-20 vol. % of C6-C8 aromatics, and 0-30 vol. % or 0-20 vol. % of C5-C8 isoalkanes, based on the total volume of the desorbent. In some embodiments, based on the total volume of the desorbent, the desorbent is composed of 50-95 volume % or 60-90 volume % or 60-80 volume % of C5-C8 normal alkanes, 5-20 volume % or 10-20 volume % of C6-C8 aromatics and 0-30 volume % or 0-20 volume % of C5-C8 isoalkanes. In one embodiment, based on the total volume of the desorbent, the desorbent is composed of 60 volume % of normal hexane, 20 volume % of benzene and 20% volume % of isooctane. In one embodiment, based on the total volume of the desorbent, the desorbent is composed of 60 volume % of normal pentane, 20 volume % of benzene and 20% volume % of isooctane. In some embodiments, the desorbent is composed of 70-95 volume %, preferably 70-90 volume % or 70-80 volume % of C5-C8 normal alkanes and 5-30 volume %, preferably 10-30 volume % or 20-30 volume % of C6-C8 aromatics, based on the total volume of the desorbent. In some embodiments, based on the total volume of the desorbent, the desorbent is composed of 70-95% by volume, preferably 70-90% by volume or 70-80% by volume of C7-C8 normal alkanes and 5-30% by volume, preferably 10-30% by volume or 20-30% by volume of C7-C8 aromatics. In one embodiment, based on the total volume of the desorbent, the desorbent is composed of 80% by volume of n-heptane and 20% by volume of toluene. In one embodiment, based on the total volume of the desorbent, the desorbent is composed of 95% by volume of n-heptane and 5% by volume of toluene. The feed amount of the desorbent is positively correlated with the feed amount of the distillate feedstock and the content of normal paraffins in the distillate feedstock. In other words, the feed amount of the desorbent increases with the increase in the feed amount of the distillate feedstock and / or the increase in the content of normal paraffins in the distillate feedstock. Preferably, the separation process according to the invention does not comprise a step of injecting a flushing agent or other desorbents, other than the step of injecting a desorbent upstream from the desorption zone. According to the present invention, the flushing agent or other desorbent refers to normal alkanes, isoalkanes, aromatics and / or cycloalkanes with a carbon number greater than the maximum carbon number of the components in the distillate oil or less than the minimum carbon number of the components in the distillate oil, especially refers to normal alkanes, isoalkanes, aromatics and / or cycloalkanes with a carbon number greater than the maximum carbon number of the components in the distillate oil or less than the minimum carbon number of the components in the distillate oil. The aromatic and / or isoparaffinic hydrocarbons with the smallest carbon number among the components in the oil. According to the present invention, the "flushing agent or other desorbent" refers to a reagent that can displace or flush out the aromatics and optional isoparaffins and cycloparaffins from the distillate outside the adsorbent pores (including the void volume of the adsorbent bed). According to the present invention, during the operation of the simulated moving bed, the liquid phase and the solid phase move in the opposite direction, so that the adsorbent is washed by a composite desorbent containing specific normal alkanes and aromatics when passing through the desorption zone. During this process, a certain amount of desorbent containing aromatics will be adsorbed outside the adsorbent pores (including within the void volume of the adsorbent bed); when such an adsorbent re-enters the adsorption zone, it can inhibit the adsorption of aromatics in the distillate oil outside the adsorbent pores (including within the void volume of the adsorbent bed) to a certain extent. On the other hand, since the composite desorbent containing both specific normal alkanes and aromatic hydrocarbons is injected from the upstream of the desorption zone and flows from the upstream to the downstream, it enters the purification zone after leaving the desorption zone. Therefore, it also has a flushing effect on the aromatic hydrocarbons from the distillate oil adsorbed outside the adsorbent pores (including the void volume of the adsorbent bed) in the purification zone. Compared with the method of separately feeding a flushing agent containing aromatic hydrocarbons and a desorbent containing normal alkanes, the composite desorbent used in the method of the present invention not only plays the role of flushing the aromatic hydrocarbons from the distillate oil adsorbed outside the adsorbent pores (including the void volume of the adsorbent bed), but also can inhibit the adsorption of aromatic hydrocarbons in the distillate oil outside the adsorbent pores (including the void volume of the adsorbent bed). Therefore, in theory, it can reduce the aromatic hydrocarbon content from the distillate oil in the extract to a certain extent, and at the same time does not affect the yield of normal alkanes from the distillate oil in the extract. Due to the use of a composite desorbent containing both specific normal alkanes and aromatic hydrocarbons, compared with the method of separately feeding an aromatic-containing flushing agent and a normal-alkane-containing desorbent, the separation method of the present invention can omit the step of additionally feeding an aromatic desorbent or a flushing agent, and can also omit the related equipment and operations for additionally feeding an aromatic-containing desorbent or a flushing agent, and can also omit the energy consumption and process costs associated with the related equipment and operations. Preferably, the adsorption process in the adsorption zone and the desorption process in the desorption zone are performed independently of each other at a temperature of 150-250° C. and a pressure of 1.0-5.0 MPa. By means of the temperature and pressure, it is possible to ensure that the components of the distillate oil are liquid during the adsorption process, and it is beneficial to the mass transfer and diffusion of the normal alkanes in the distillate oil. Preferably, the adsorbent used in the method according to the present invention comprises 5A molecular sieve. The unit cell composition of the 5A molecular sieve is preferably Ca4Na4[Al 12 Si 12 O 48 ]·20H2O. The pores in the 5A molecular sieve can selectively adsorb normal alkanes, especially those with more than 10 carbon atoms. Preferably, the particle size of the adsorbent is 0.1-1.0 mm, more preferably 0.2-0.8 mm, and even more preferably 0.3-0.6 mm. According to the present invention, the particle size distribution of the adsorbent is determined using a Taylor standard sieve. Preferably, the crystal size of the 5A molecular sieve is 0.1-1.0 μm, preferably 0.2-0.8 μm, or 0.2-0.7 μm. According to the present invention, the crystal size is measured as follows: first, the molecular sieve is imaged using a scanning electron microscope (SEM), and then the size of the molecular sieve crystals imaged by the SEM is counted using SMILEVIEW software. Optionally, the water content of the adsorbent is 5 wt% or less, preferably 3 wt% or less, more preferably 1 wt% to 3 wt%.According to the present invention, the water content refers to the percentage of weight loss after calcination to constant weight at 600°C to the original weight. According to the present invention, the "water content of the adsorbent" refers to the water content relative to the total weight of the adsorbent. The specific adsorbent particle size, 5A molecular sieve crystal size and / or adsorbent water content are beneficial to increasing the content of normal alkanes with carbon numbers above 10, especially C10-C25, and especially C14-C25, in the extract flowing out of the simulated moving bed. The adsorbent containing 5A molecular sieve according to the present invention can be formed by molding the molecular sieve raw material, the binder and the auxiliary agent, and then heat treating (such as calcining) the molded body, transforming the binder, Ca 2+The method of the steps of exchange and activation is obtained. Preferably, the adsorbent can be prepared by a method comprising the following steps: (1) mixing a molecular sieve raw material, a binder and an auxiliary agent and performing a molding process to obtain a first product; (2) heat treating the first product to obtain a second product; (3) contacting the second product with an alkaline solution to obtain a third product; (4) The third product is reacted with a mixture containing Ca 2+ ion solution to perform an ion exchange reaction to obtain a fourth product; and (5) Activating the fourth product. According to the present invention, the molecular sieve raw material may be 4A molecular sieve raw powder, such as NaA molecular sieve raw powder. Preferably, the binder is selected from clay binders, silica sol, alumina sol or mixtures thereof. Preferably, the clay binder is selected from kaolin, halloysite, attapulgite or mixtures thereof. Preferably, the auxiliary agent is selected from lignin, sesbania powder, corn starch, bayberry tannin, cellulose or a mixture thereof. Preferably, on a dry basis, the weight ratio of the molecular sieve raw material to the binder is (70-95):(5-30), preferably (80-95):(5-20), and more preferably (85-95):(5-15). When the binder content is too high, the adsorbent bulk density will be high and / or the active component content will decrease. When the binder content is too low, the adsorbent strength will deteriorate. Preferably, the amount of the auxiliary agent used is 1-6 wt %, preferably 1.5-4 wt %, relative to the total dry weight of the molecular sieve raw material and the binder. According to the present invention, the molding process can be carried out according to any technology known to the technicians in the field of molecular sieve. Specifically, the molding process can be carried out by rolling ball molding, extrusion molding or tablet molding. The molding process can be carried out in a high-speed granulator, a sugar coating machine, a disc granulator, an extruder and / or a tablet press. For example, rolling ball molding includes placing a mixture of molecular sieve raw materials, a binder and an auxiliary agent into a rolling ball device, adding water to the mixture in the form of a spray while rolling, and adding a small amount of additional dry mixture thereto in the form of a scattering, so that the mixture agglomerates, rounds, forms small balls, and then throws to obtain spherical particles. For example, extrusion molding includes placing a mixture of molecular sieve raw materials, a binder and an auxiliary agent into an extrusion device, first kneading by adding a certain amount of water, and then obtaining a strip through extrusion, and the strip is dried, crushed and sieved to obtain particles. For example, tablet molding includes placing a mixture of molecular sieve raw materials, a binder and an auxiliary agent into a tableting device for compression molding, and then obtaining particles through crushing and sieving. The particles with the above-mentioned particle size of 0.1-1.0 mm, more preferably 0.2-0.8 mm, and even more preferably 0.3-0.6 mm are obtained by molding. The particles with the above-mentioned particle size can be spherical particles, which have good fluidity, are easy to be filled evenly, and are conducive to fluid distribution (in the present invention, they are conducive to the distribution of liquid streams in the simulated moving bed). Preferably, the method according to the present invention further comprises a step (1') of performing a first drying treatment on the shaped particles between steps (1) and (2) to obtain a dried first product. The first drying treatment can be performed by any known method in the field of molecular sieves. Preferably, the first drying treatment is performed under the following conditions: a temperature of 80-200° C., and / or a time of 1-8 hours. The first drying treatment can remove water in the macropores inside the particles. According to the present invention, the heat treatment in step (2) is preferably carried out under the following conditions: a temperature of 400-800°C, and / or a time of 1-8 hours. Preferably, the heat treatment is carried out by roasting. The term "roasting" refers to a heat treatment carried out at a temperature below the melting point of the material. In order to increase the adsorption capacity of the adsorbent, the binder can be subjected to a crystal transformation process to obtain a non-adhesive The term "transcrystal" refers to the process in which a material changes from one crystal structure to another. According to the present invention, the crystal transformation treatment of the binder is preferably carried out using an alkaline solution. Preferably, the contact in step (3) is carried out under the following conditions: temperature of 80-99°C; time of 1-8h; the alkaline solution includes but is not limited to sodium hydroxide and / or potassium hydroxide aqueous solution, wherein the concentration of the alkaline solution is preferably 0.5-5 mol / L; and / or the volume ratio of the second product to the alkaline solution is 1:(1-5). The method according to the present invention may further comprise between steps (3) and (4): washing the third product until the pH value of the washing liquid is below 10.0, wherein the washing liquid is preferably water. According to the present invention, the pH value is measured under room temperature (15-30° C., preferably 20-25° C.) and standard atmospheric pressure. Preferably, the ion exchange reaction is carried out under the following conditions: time is 1-8h; temperature is 10-99°C, preferably 50-99°C, more preferably 70-99°C; the Ca 2+ The solution of ions is an aqueous solution of calcium chloride and / or calcium nitrate; and / or the third product and the solution containing Ca 2+ ions in a solution having a volume ratio of 1:(2-8), wherein in the solution, the Ca 2+ The concentration of ions is 0.1-2.0 mol / L. Preferably, the method according to the present invention further comprises between steps (4) and (5): subjecting the fourth product to a second drying treatment to obtain a dried fourth product. The second drying treatment can be carried out by any known method in the field of molecular sieves. Preferably, the second drying treatment is carried out under the following conditions: a time of 1-8 hours and a temperature of 80-300°C. According to the present invention, the ion exchange reaction can be carried out by immersing the third product in a solution containing Ca 2+ in a solution of ions. The term "activation" refers to the process of treating a molecular sieve to enhance its adsorption and / or catalytic properties. According to the present invention, the activation treatment is preferably carried out under the following conditions: temperature of 250-350°C, preferably 270-320°C; and / or time of 1-4h, preferably 2-3h. In order to reduce the degree of hydrothermal damage of the molecular sieve at a higher temperature, the activation treatment can be carried out in a dry air atmosphere, or in a vacuum condition, or in a nitrogen atmosphere. By means of the above method for preparing the adsorbent, up to 98% of the binder raw material can be converted into molecular sieves, that is, the product adsorbent contains almost no binder. Preferably, in the 5A molecular sieve of the present invention, Ca 2+ The exchange degree is 67-97%, preferably 75-85%, for example 70%, 72%, 75%, 78%, 80%, 82%, 85%, 88%, 90%, 92%, 95%, or a range consisting of any two of the above values, or one of the above values ​​and one endpoint of the above range, or one endpoint of the above range and one endpoint of another range. 2+The exchange degree is the percentage of the exchangeable cation sites in the molecular sieve. The exchange degree is measured by using X-ray fluorescence spectroscopy (XRF) to analyze the weight percentage of Na2O and CaO in the adsorbent, which are recorded as m N and m C , then Ca 2+ Degree of ion exchange = m C / M C / (m C / M C +m N / M N ), where M N and M C are the molar masses of Na2O and CaO respectively. Preferably, the separation method according to the present invention further comprises the step of separating the extract to obtain normal alkanes from the distillate oil and the desorbent. Preferably, the separation method according to the present invention further comprises the step of separating the raffinate to obtain non-normal alkanes from the distillate oil and the desorbent. In the context of the present application, the term "non-normal alkanes" refers to hydrocarbons other than normal alkanes, including aromatic hydrocarbons, isoalkanes and cycloalkanes. Preferably, the separated desorbent is injected into the desorption zone of the simulated moving bed again for recycling. According to the present invention, the separation of the extract and the raffinate can be carried out by any commonly used separation method in the chemical industry. Since the desorbent and the normal alkanes and non-normal alkanes in the distillate oil have different boiling points, the separation of the extract and the raffinate can be carried out by a fractionation or rectification process. In the case of fractionation or rectification to separate the extract and the raffinate, since the aromatics and normal alkanes in the desorbent have similar boiling points, a mixture of the two will be obtained after fractionation or rectification. For the method of feeding the flushing agent containing aromatics and the desorbent containing normal alkanes separately, it is necessary to further separate the mixture so that the flushing agent containing aromatics and the desorbent containing normal alkanes can be respectively injected into the upstream of the desorption zone and the purification zone for recycling. According to the separation method using the composite desorbent containing both specific normal alkanes and aromatics of the present invention, the composite desorbent obtained by separating the raffinate and / or the extract can be directly injected into the upstream of the desorption zone again without further separating the mixture of normal alkanes and aromatics in the desorbent. In this case, the method according to the present invention can save the step of further separating the mixture, and can also save the related equipment and operation of separating the mixture and circulating the flushing agent containing aromatics, and can also save the energy consumption and process costs associated therewith. In order to further reduce the aromatic content in the n-alkane final product, the separation method according to the present invention may further include separating the normal alkane obtained by the simulated moving bed adsorption-desorption separation of the distillate oil. The alkanes are injected into an additional adsorption tower for additional separation. Preferably, the separation in the additional adsorption tower is carried out at a temperature of 80-120° C., a pressure of 0.05-1.5 MPa and / or a temperature of 0.5-8.0 h. -1 The experiment was carried out at a volume space velocity of . Preferably, the adsorbent in the additional adsorption tower is an X-type molecular sieve adsorbent and / or a Y-type molecular sieve adsorbent. More preferably, the water content of the X-type molecular sieve adsorbent and the Y-type molecular sieve adsorbent is 5 wt % or less, preferably 3 wt % or less, more preferably 1 wt % to 3 wt %, respectively. The separation method according to the present invention may include at least two additional adsorption towers. For example, at least one additional adsorption tower is used to further remove aromatic hydrocarbons from the crude normal alkane product, while the other additional adsorption towers are regenerated, so that the at least two additional adsorption towers are used alternately. After the crude normal alkane product is treated in the additional adsorption tower, the C10-C25 aromatic content in the final normal alkane product can reach less than 0.01 wt%. III. System for separating normal alkanes from distillate oil In a second aspect, the present invention relates to a system for separating normal paraffins from distillate oil by adsorption-desorption, wherein the system comprises a simulated moving bed, the simulated moving bed comprises a plurality of adsorbent beds filled with adsorbent, the plurality of adsorbent beds are interconnected in a closed loop manner and are divided into: (i) a desorption zone located between upstream desorbent injection and downstream extract withdrawal, (ii) a purification zone located between the upstream withdrawal of the extract and the downstream injection of the distillate, (iii) an adsorption zone located between upstream distillate injection and downstream raffinate withdrawal, and (iv) a buffer zone located between upstream raffinate removal and downstream desorbent injection, wherein at least one, preferably one, desorbent injection port is provided upstream of the desorption zone; The distillate oil contains normal alkanes and aromatic hydrocarbons and optionally isoalkanes and cycloalkanes; the desorbent contains normal alkanes and aromatic hydrocarbons, and their carbon numbers are respectively greater than the maximum carbon number of the components in the distillate oil or less than the minimum carbon number of the components in the distillate oil. Preferably, the simulated moving bed does not include an injection port for a flushing agent or other desorbents except for an injection port for a desorbent disposed upstream of the desorption zone. The flushing agent or other desorbents refer to normal alkanes, isoalkanes, aromatics and / or cycloalkanes having a carbon number greater than the maximum carbon number of the components in the distillate oil or less than the minimum carbon number of the components in the distillate oil, especially hydrocarbons having a carbon number greater than Aromatic hydrocarbons and / or isoparaffins having the largest carbon number among the components in the distillate oil or having a carbon number less than the smallest carbon number among the components in the distillate oil. Preferably, the simulated moving bed in the separation system has the structure of the simulated moving bed described in Part I of this disclosure. According to the present invention, the simulated moving bed may include at least 4 adsorbent beds, preferably 4 to 40 adsorbent beds, for example 4 to 30 adsorbent beds, more preferably 6 to 24 adsorbent beds, and even more preferably 12 to 24 adsorbent beds. In some embodiments, for each simulated moving bed, the desorption zone may include 1 to 10, preferably 2 to 6 adsorbent beds, the purification zone may include 1 to 12, preferably 2 to 8 adsorbent beds, the adsorption zone may include 1 to 10, preferably 2 to 6 adsorbent beds, and / or the buffer zone may include 1 to 8, preferably 1 to 4 adsorbent beds. For example, the desorption zone has 3 adsorbent beds, the purification zone has 4 adsorbent beds, the adsorption zone has 3 adsorbent beds, and the buffer zone has 2 adsorbent beds. According to the present invention, each adsorbent bed is connected to at least one pipeline, through which a liquid stream is introduced from the outside into the adsorbent bed and a liquid stream is led out of the adsorbent bed. The individual liquid streams entering and leaving the same adsorbent bed can be controlled by switch valves, preferably individually controlled by switch valves arranged in parallel. Preferably, each adsorbent bed is connected to a first pipeline for introducing or withdrawing a liquid stream. Preferably, the desorbent, the extract, the distillate feedstock and the residual liquid are respectively connected to the first pipeline by a second pipeline (four in total), and a switch valve is provided on each second pipeline. At a certain moment, four of the switch valves connecting each liquid stream to different adsorbent beds are in an open state, and the remaining switch valves are in a closed state. According to the present invention, the step time for each liquid stream to move to the next adsorbent bed can be 50 seconds to 300 seconds, preferably 45 seconds to 200 seconds. Preferably, the above-mentioned “injection port” refers to an opening of a second pipeline connected to a first pipeline connected to the adsorbent bed and having a switching valve installed thereon. Preferably, the adsorbent bed layer of the simulated moving bed is filled with an adsorbent, and its structure, performance and preparation method are preferably as described in Part II of the present disclosure. In addition, the distillate oil and desorbent added to the simulated moving bed are also preferably as described in Part II of the present disclosure. Preferably, the system for separating normal alkanes further comprises a device for separating the extract to obtain normal alkanes from the distillate oil and the desorbent. Preferably, the raffinate is separated to obtain non-normal alkanes from the distillate oil and the The device for desorbing the agent. Preferably, the separation device is a fractionation tower or a rectification tower. In this case, the device for separating the extracted liquid is called an extracted liquid tower, and the device for separating the raffinate is called a raffinate tower. A certain number of trays can be set in the extracted liquid tower and the raffinate tower according to the boiling points of the raw materials and the desorbent. The desorbent separated by the extracted liquid tower and / or the raffinate tower is connected to the desorbent injection port upstream of the desorption zone through a pipeline. The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings, but the accompanying drawings do not constitute a limitation of the present invention. FIG1 is a schematic diagram of a process for separating normal alkanes from C10-C25 fraction oil by adsorption-desorption using a composite desorbent using a simulated moving bed according to an embodiment of the present invention. Use the small-scale simulated moving bed device of continuous countercurrent to verify the effect of the present invention.Described small-scale simulated moving bed comprises 12 adsorption columns connected in series, and each column is 195 millimeters long, and column inner diameter is 30 millimeters, and the total loading amount of adsorbent is 1650 milliliters.Connect with circulating pump at the head and tail ends of 12 columns connected in series to form a closed loop.Desorbent, extract, distillate raw material and raffinate that flow into or out of simulated moving bed divide 12 adsorption columns into four districts.Specifically, 3 adsorption columns 101-103 between desorbent inflow and extract outflow are desorption zone, 4 adsorption columns 104-107 between extract outflow and distillate raw material inflow are purification zone, 3 adsorption columns 108-110 between distillate raw material inflow and raffinate outflow are adsorption zone, and 2 adsorption columns 111-112 between raffinate outflow and desorbent inflow are buffer zone. In the embodiment shown in FIG. 1 , the desorbent simultaneously comprises C5-C8 normal alkanes, C6-C8 aromatic hydrocarbons, and optionally C5-C8 isoalkanes. 2 is a schematic diagram of a process for separating normal alkanes from C10-C25 fraction oil by adsorption-desorption using a first desorbent and a second desorbent respectively using a simulated moving bed according to an embodiment of the prior art. A small-scale simulated moving bed device with continuous countercurrent is used to verify the effect of the present invention. The small-scale simulated moving bed includes 12 adsorption columns connected in series, each column is 195 mm long, the inner diameter of the column is 30 mm, and the total loading amount of the adsorbent is 1650 ml. The 12 columns connected in series are connected at both ends by a circulating pump to form a closed loop. The desorbent, extract, distillate oil feedstock and residual liquid flowing into or out of the simulated moving bed divide the 12 adsorption columns into four zones. Specifically, the three adsorption columns 101-103 between the second desorbent inflow and the extract outflow are the desorption zone, the four adsorption columns 104-107 between the extract outflow and the distillate oil feedstock inflow are the purification zone, and the distillate oil feedstock is the purification zone. The three adsorption columns 108-110 between the oil feed inflow and the raffinate outflow are adsorption zones, and the two adsorption columns 111-112 between the raffinate outflow and the second desorbent inflow are buffer zones. In the embodiment shown in FIG2 , the first desorbent flows into the adsorption column 105 downstream of the extract outflow position, wherein the second desorbent comprises normal alkanes of C5-C8 and optionally isoalkanes of C5-C8, and the first desorbent comprises aromatics of C6-C8 and optionally isoalkanes of C5-C8. During operation, a micro pump is used to continuously inject the first desorbent, the second desorbent and the distillate raw material into the simulated moving bed, and a micro pump is used to continuously discharge the extracted liquid from the simulated moving bed, and the residual liquid flows out of the simulated moving bed by pressure control. The flow rate of each area is adjusted by setting the circulation pump flow rate and the liquid flow flowing into or out of the simulated moving bed in the simulated moving bed. An adsorption column is moved in the opposite direction of the liquid flow every step time to achieve continuous countercurrent of the adsorbent and the liquid flow. Figure 3 is a concentration distribution diagram of heavy aromatic hydrocarbons (from feed) and light aromatic hydrocarbons (from desorbent or flushing agent) flow in the adsorption column during the process of carrying out the method of separating normal alkanes using a composite desorbent according to Example 1 of the present invention and the method of separating normal alkanes using a first desorbent and a second desorbent respectively according to Comparative Example 3 which is not the present invention. This application involves the following technical solutions: [1] A method for separating normal alkanes from C10-C25 fraction oil by adsorptive separation, characterized in that the method comprises the following steps: S1, contacting the C10-C25 fraction oil with an adsorbent for the first time to obtain a material to be desorbed comprising C10-C25 normal alkanes and the adsorbent, and a raffinate comprising C10-C25 isoalkanes, C10-C25 cycloalkanes, C10-C25 aromatics and a desorbent; S2, bringing the desorbent into second contact with the material to be desorbed to obtain an extract containing C10-C25 normal alkanes and the desorbent; The C10-C25 distillate oil includes C10-C25 normal alkanes, C10-C25 isoalkanes, C10-C25 cycloalkanes and C10-C25 aromatics; The desorbent includes C5-C8 normal alkanes, C6-C8 aromatic hydrocarbons and optionally C5-C8 isoalkanes; The particle size of the adsorbent is 0.3-0.6 mm. The adsorbent includes 5A molecular sieve, and the grain size of the 5A molecular sieve is 0.2-0.8 μm. [2] The method according to item [1] above, wherein the crystal size of the 5A molecular sieve is 0.2-0.7 μm; Optionally, the water content of the adsorbent is less than 3 wt %. [3] The method according to item [1] above, wherein, relative to the total volume of the desorbent, the content of C5-C8 normal alkanes is 50-95% by volume, the content of C6-C8 aromatics is 5-20% by volume, and the content of C5-C8 isoalkanes is 0-30% by volume. [4] The method according to item [1] above, wherein the temperature of the first contact and the second contact are 185-230°C and the pressure are 1.0-2.5 MPa respectively. [5] The method according to item [1] above, wherein the normal alkanes in the C10-C25 fraction are separated by adsorption using a liquid phase simulated moving bed process, and the simulated moving bed comprises 12-24 adsorbent beds; Step S1 comprises: subjecting the C10-C25 fraction oil to the first contact with the adsorbent bed to obtain a raffinate containing the C10-C25 isoalkanes, the C10-C25 cycloalkanes, the C10-C25 aromatics and a desorbent, and the adsorbent bed adsorbing the C10-C25 normal alkanes as the material to be desorbed; Step S2 includes: bringing the desorbent into contact with the material to be desorbed for the second time to obtain the extract. [6] The method according to item [1] or [5] above, wherein the method further comprises: Separating the extracted liquid to obtain the C10-C25 normal alkanes and the desorbent; The raffinate is separated to obtain the C10-C25 isoalkanes, the C10-C25 cycloalkanes, the C10-C25 aromatics and the desorbent. [7] The method according to item [6] above, wherein the method further comprises: contacting the material containing the C10-C25 normal alkanes obtained by separation of the extract with an X-type molecular sieve adsorbent and / or a Y-type molecular sieve adsorbent for a third time; The temperature of the third contact is 100-120°C, the pressure is 0.5-1.5 MPa, and the volume space velocity is 0.5-5.0 h -1 ; The water contents of the X-type molecular sieve adsorbent and the Y-type molecular sieve adsorbent are respectively less than 5% by weight. [8] The method according to item [1] above, wherein the method for preparing the adsorbent comprises the following steps: (1) mixing a molecular sieve raw material, a binder and an auxiliary agent and performing a molding process to obtain a first product; (2) heat treating the first product to obtain a second product; (3) contacting the second product with an alkaline solution to obtain a third product; (4) contacting the third product with a solution containing a calcium source to perform an ion exchange reaction to obtain a fourth product; (5) subjecting the fourth product to an activation treatment; The crystal grain size of the molecular sieve raw material is 0.2-0.8 μm, and the molecular sieve raw material includes NaA molecular sieve raw powder. [9] The method according to item [8] above, wherein in step (1), the binder comprises one or more of a clay binder, silica sol and alumina sol; The clay binder includes one or more of kaolin, halloysite and attapulgite; The auxiliary agent comprises one or more of lignin, sesbania powder, corn starch, bayberry tannin and methyl cellulose.

[0010] , The method according to item [8] above, wherein in step (1), on a dry basis, the weight ratio of the molecular sieve raw material to the binder is (85-95):(5-15); On a dry basis, the amount of the auxiliary agent used is 1-6% by weight relative to the total weight of the molecular sieve raw material and the binder.

[0011] , The method according to item [8] above, wherein step (1) further comprises: forming the product of the molding process into particles with a particle size of 0.30-0.60 mm, and performing a first drying process on the particles to obtain the first product; The conditions of the first drying treatment include: temperature of 80-200° C. and time of 1-4 hours.

[0012] , according to the method described in item [8] above, wherein in step (2), the heat treatment conditions include: temperature of 500-740°C and time of 1-4h.

[0013] , The method according to item [8] above, wherein in step (3), the contacting conditions include: a temperature of 80-99°C and a time of 1-5h; The alkaline solution includes sodium hydroxide solution and / or potassium hydroxide solution; The concentration of the alkaline solution is 0.5-2 mol / L, and the volume ratio of the second product to the alkaline solution is 1:(1-5); Optionally, step (3) further comprises: washing the solid obtained by the contact until the pH value of the washing liquid is below 10.0, to obtain the third product.

[0014] , The method according to item [8] above, wherein in step (4), the conditions of the ion exchange reaction include: a time of 1-4 hours and a temperature of 10-99°C; The calcium source includes calcium chloride and / or calcium nitrate; The volume ratio of the third product to the solution is 1:(2-6); In the solution, the concentration of the calcium source is 0.1-2.0 mol / L; Step (4) further comprises: subjecting the solid product obtained by the ion exchange reaction to a second drying treatment to obtain the fourth product; The conditions of the second drying treatment include: time of 1-4 hours and temperature of 80-200°C.

[0015] , according to the method described in item [8] above, wherein in step (5), the conditions of the activation reaction include: time of 1-4h and temperature of 250-350°C. Example The following examples will further illustrate the method of the present application, but are not intended to limit the present application. For Examples 1-9 and Comparative Examples 1-3, the dry basis weight refers to the weight after calcination at 600°C to constant weight, wherein the dry basis weight of the original NaA molecular sieve powder is 81% of the original weight; the dry basis weight of the halloysite is 73% of the original weight; and the dry basis weight of the kaolin is 79% of the original weight. Example 1 Adsorbent X-1 was prepared by the following steps and used to separate normal alkanes from C10-C25 fraction oil: 61.7kg (dry basis weight 50kg) of NaA molecular sieve raw powder, 4.1kg of halloysite (dry basis weight 3kg) and 1kg of corn starch were mixed evenly to form a mixed powder, wherein the grain size of the NaA molecular sieve raw powder was 0.8μm. The mixed powder was rolled into small balls with a diameter of 0.3-0.6mm in a sugar coating pan, and then dried at 85℃ for 4h. Roasted at 550℃ for 4h. 40L of the above-mentioned roasted small balls were soaked in 80L of 1.0mol / L NaOH aqueous solution at 97℃ for 4h, and then washed with deionized water until the pH value was below 10.0 to obtain NaA small balls. 40L of NaA small balls were soaked in 200L of 1.0mol / L CaCl2 aqueous solution for ion exchange, the exchange liquid temperature was kept at 95℃, the exchange time was 4h, and then washed with deionized water and dried (time was 4h, temperature was 150℃). Then, the temperature was raised to 280°C in a dry air atmosphere and activated for 2 h to obtain 5A pellet adsorbent X-1. The water content of the adsorbent was 3.0% by weight. The parameters are listed in Table 1. The test was conducted using a small simulated moving bed as shown in Figure 1. The above-mentioned adsorbent was loaded into 12 adsorption columns, wherein the adsorption zone, purification zone, desorption zone and buffer zone consisted of 3 adsorption columns, 4 adsorption columns, 3 adsorption columns and 2 adsorption columns, respectively. The composition of the C10-C25 distillate oil feedstock is shown in Table 2. The desorbent consists of 80 volume % of n-heptane (nC7) and 20 volume % of toluene. The separation was carried out under the following conditions: temperature 235°C, pressure 2.5MPa, distillate oil feedstock flow rate 1158mL / h, desorbent flow rate 1734mL / h, extract flow rate 1015mL / h, The flow rate of the raffinate was 1877 mL / h and the step time was 150 s. The composition of the extract was analyzed by gas chromatograph, wherein the instrument was Agilent 7890; the chromatographic column was HP-5, 50 m×0.32 mm×0.5 μm; the chromatographic analysis method was as follows: the injection volume was 0.2 μL; the injection port temperature was 290°C; the split ratio was 150:1; the column flow rate was 3.9 mL / min; the initial column temperature was 50°C, maintained for 3 min, and the temperature was increased to 300°C at 5°C / min and maintained for 5 min; the FID detector had a detector temperature of 320°C. Among the components of the extract except the desorbent, the content of normal alkanes was 99.6% by weight, the content of aromatics was 0.07% by weight, and the yield of nC10-nC25 normal alkanes was shown in Table 3. Example 2 Adsorbent X-2 was prepared by the steps of Example 1, except that the grain size of the NaA molecular sieve raw powder was 0.3 μm, the activation temperature was 300° C., and the water content of the adsorbent was 1.5% by weight. The parameters of adsorbent X-2 are listed in Table 1. A method similar to that of Example 1 was used to separate normal alkanes from the C10-C25 distillate oil. The composition of the C10-C25 distillate oil feedstock is shown in Table 2, except that the desorbent consisted of 60 volume % of normal hexane (nC6), 20 volume % of isooctane (iC8) and 20 volume % of benzene, and separation was carried out under the following conditions: temperature 200° C., pressure 2.5 MPa, distillate oil feedstock flow rate 1158 mL / h, desorbent flow rate 2000 mL / h, extract flow rate 1381 mL / h, raffinate flow rate 1777 mL / h, and step time 150 s. The composition of the extract was analyzed by gas chromatography, wherein the instrument was Agilent 7890; the chromatographic column was HP-5, 50 m×0.32 mm×0.5 μm; the chromatographic analysis method was as follows: the injection volume was 0.2 μL; the injection port temperature was 290°C; the split ratio was 150:1; the column flow rate was 3.9 mL / min; the initial column temperature was 50°C, maintained for 3 min, and the temperature was increased to 300°C at 5°C / min and maintained for 5 min; the FID detector had a detector temperature of 320°C. Among the components of the extract except the desorbent, the content of normal alkanes was 99.6% by weight, the content of aromatics was 0.05% by weight, and the yield of nC10-nC25 normal alkanes was shown in Table 3. Example 3 Adsorbent X-1 was used to separate normal alkanes from C10-C25 distillate using a method similar to Example 1. The composition of the C10-C25 distillate feedstock is shown in Table 2, except that the desorbent consisted of 60% by volume of normal pentane (nC5), 20% by volume of isooctane (iC8) and 20% by volume of benzene. Separation was performed under the following conditions: temperature 185°C, pressure 2.5 MPa, distillate feedstock flow rate 1158 mL / h, desorbent flow rate 1891 mL / h, extract flow rate 1172 mL / h, extract flow rate 1877 mL / h, and step time 150 s. The extracted liquid was analyzed by gas chromatograph. The composition of the liquid, wherein the instrument is Agilent7890; the chromatographic column is HP-5, 50m×0.32mm×0.5μm; the chromatographic analysis method is as follows: the injection volume is 0.2μL; the injection port temperature is 290℃; the split ratio is 150:1; the column flow rate is 3.9mL / min; the initial column temperature is 50℃, maintained for 3min, and the temperature is increased to 300℃ at 5℃ / min, maintained for 5min; the FID detector has a detector temperature of 320℃. In the components of the extract except the desorbent, the content of normal alkanes is 99.5% by weight, the content of aromatics is 0.06% by weight, and the yield of nC10-nC25 normal alkanes is shown in Table 3. Example 4 Adsorbent X-1 was used and normal alkanes were separated from the C10-C25 distillate oil by a method similar to that of Example 1. The composition of the C10-C25 distillate oil feedstock is shown in Table 2, except that the desorbent consisted of 95 volume % of normal heptane (nC7) and 5 volume % of toluene, and the separation was carried out under the following conditions: temperature 235°C, pressure 2.5 MPa, feed flow rate 1158 mL / h, desorbent flow rate 1734 mL / h, extract flow rate 1015 mL / h, raffinate flow rate 1877 mL / h, and step time 150 s. The composition of the extract was analyzed by gas chromatography, wherein the instrument was Agilent 7890; the chromatographic column was HP-5, 50 m×0.32 mm×0.5 μm; the chromatographic analysis method was as follows: the injection volume was 0.2 μL; the injection port temperature was 290°C; the split ratio was 150:1; the column flow rate was 3.9 mL / min; the initial column temperature was 50°C, maintained for 3 min, and the temperature was increased to 300°C at 5°C / min and maintained for 5 min; the FID detector was at a detector temperature of 320°C. Among the components of the extract except the desorbent, the content of normal alkanes was 99.5% by weight, the content of aromatics was 0.1% by weight, and the yield of nC10-nC25 normal alkanes was shown in Table 3. Example 5 Adsorbent X-1 was used and the method of Example 4 was used to separate normal alkanes from C10-C25 fraction oil. Then, a small distillation tower was used to remove the desorbent in the extract, wherein the distillation tower had 60 trays, a tower pressure of 15 kPa (absolute pressure), a bottom temperature of 196°C, a tower top temperature of 58°C, and a reflux ratio of 0.5, to obtain 13000 mL of crude nC10-nC25 normal alkanes. The obtained crude normal alkanes were then injected into an adsorption column filled with 1000 mL of NaX molecular sieve adsorbent (water content of 3 wt%). The separation was carried out under the following conditions: temperature 100°C, pressure normal pressure, volumetric space velocity 5h -1 The aromatic content detected at the outflow port was 0.01 wt%. Example 6 Adsorbent X-3 was prepared by the following steps and used to separate C10-C25 distillate oil. Ionized normal alkanes: 71.6kg (dry basis weight 58kg) of NaA molecular sieve powder, 12.7kg (dry basis weight 10kg) of kaolin and 2kg of sesbania powder were mixed evenly to form a mixed powder, wherein the grain size of the NaA molecular sieve powder was 0.2μm. The mixed powder was rolled into a small ball with a diameter of 0.3-0.6mm in a sugar coating pan, and then dried at 150℃ for 3h. Roasted at 600℃ for 4h. 40L of the above-mentioned roasted small balls were soaked in 60L of 2mol / L NaOH aqueous solution at 90℃ for 5h, and then washed with deionized water until the pH value was below 10.0 to obtain NaA small balls. 30L of NaA small balls were soaked in 150L of 1.5mol / L CaCl2 aqueous solution for ion exchange, the exchange liquid temperature was kept at 80℃, the exchange time was 3h, and then washed with deionized water and dried (time was 2h, temperature was 200℃). In a dry air atmosphere, the temperature was raised to 290°C and activated for 2 hours to obtain 5A pellet adsorbent X-3. The water content of the adsorbent was 2.1% by weight. The parameters are listed in Table 1. A small-scale simulated moving bed test was carried out according to the method of Example 1. The composition of the extract was analyzed by gas chromatograph, wherein the instrument: Agilent 7890; chromatographic column: HP-5, 50m×0.32mm×0.5μm; chromatographic analysis method: injection volume 0.2μL; injection port temperature 290℃; split ratio 150:1; column flow rate 3.9mL / min; initial column temperature 50℃, maintained for 3min, heated to 300℃ at 5℃ / min, maintained for 5min; FID detector, detector temperature 320℃. Among the components of the extract except the desorbent, the content of normal alkanes was 99.5% by weight, the content of aromatics was 0.08% by weight, and the yield of nC10-nC25 normal alkanes is shown in Table 3. Example 7 Adsorbent Y-1 was prepared by the following steps and used to separate normal alkanes from C10-C25 fraction oil: 61.7kg (dry basis weight 50kg) of NaA molecular sieve raw powder, 4.1kg of halloysite (dry basis weight 3kg) and 1kg of corn starch are evenly mixed to form a mixed powder, wherein the grain size of the NaA molecular sieve is 2.5μm. The mixed powder is rolled in a sugar coating pan to form small balls with a diameter of 0.3-0.6mm, and then dried at 85℃ for 8h. Roasted at 550℃ for 4h. 40L of the above-mentioned roasted small balls are soaked in 80L of 1.0mol / L NaOH aqueous solution at 97℃ for 4h, and then washed with deionized water until the pH value is below 10.0 to obtain NaA small balls. 40L of NaA small balls are soaked in 200L of 1.0mol / L CaCl2 aqueous solution for ion exchange, maintaining the exchange liquid temperature at 95℃, the exchange time is 4h, and then washed with deionized water and dried (time is 4h, temperature is 150℃). Process in a dry air atmosphere The temperature was raised to 500°C and activated for 2 hours to obtain 5A pellet adsorbent Y-1. The water content of the adsorbent was 0.2% by weight. The parameters are listed in Table 1. The test was conducted using a small simulated moving bed as shown in FIG1 , wherein the above-mentioned adsorbent was loaded in 12 adsorption columns, wherein the adsorption zone, purification zone, desorption zone and buffer zone were composed of 3 adsorption columns, 4 adsorption columns, 3 adsorption columns and 2 adsorption columns, respectively. The composition of the C10-C25 distillate oil feedstock is shown in Table 2. The desorbent was composed of 80 volume % of n-heptane (nC7) and 20 volume % of toluene. Separation was performed under the following conditions: temperature 235° C., pressure 2.5 MPa, distillate oil feedstock flow rate 1158 mL / h, desorbent flow rate 1734 mL / h, extract flow rate 1015 mL / h, residual liquid flow rate 1877 mL / h and step time 150 s. The composition of the extract was analyzed by gas chromatography, wherein the instrument was Agilent 7890; the chromatographic column was HP-5, 50 m×0.32 mm×0.5 μm; the chromatographic analysis method was as follows: the injection volume was 0.2 μL; the injection port temperature was 290°C; the split ratio was 150:1; the column flow rate was 3.9 mL / min; the initial column temperature was 50°C, maintained for 3 min, and the temperature was increased to 300°C at 5°C / min and maintained for 5 min; the FID detector was at a detector temperature of 320°C. Among the components of the extract except the desorbent, the content of normal alkanes was 99.6% by weight, the content of aromatics was 0.07% by weight, and the yield of nC10-nC25 normal alkanes was shown in Table 3. Example 8 Adsorbent Y-2 was prepared by the method of Example 1, except that the particle size of the adsorbent was 0.9-1.0 mm. The parameters of adsorbent Y-2 are listed in Table 1. The test was conducted using a small simulated moving bed as shown in Figure 1, in which the above-mentioned adsorbent was loaded in 12 adsorption columns, wherein the adsorption zone, purification zone, desorption zone and buffer zone were composed of 3 adsorption columns, 4 adsorption columns, 3 adsorption columns and 2 adsorption columns respectively. The composition of the C10-C25 distillate oil feedstock is shown in Table 2. The desorbent was composed of 80 volume % of n-heptane (nC7) and 20 volume % of toluene. Separation was carried out under the following conditions: temperature 235°C, pressure 2.5MPa, feed flow rate 1158mL / h, desorbent flow rate 1734mL / h, extract flow rate 1015mL / h, extract flow rate 1877mL / h, step time 150s. The composition of the extract was analyzed by gas chromatography, wherein the instrument was Agilent 7890; the chromatographic column was HP-5, 50 m×0.32 mm×0.5 μm; the chromatographic analysis method was as follows: the injection volume was 0.2 μL; the injection port temperature was 290°C; the split ratio was 150:1; the column flow rate was 3.9 mL / min; the initial column temperature was 50°C, maintained for 3 min, and the temperature was increased to 300°C at 5°C / min and maintained for 5 min; the FID detector was at a detector temperature of 320°C. Among the components of the extract except the desorbent, the content of normal alkanes was 99.6% by weight, the content of aromatics was 0.07% by weight, and the yield of nC10-nC25 normal alkanes was shown in Table 3. Example 9 Adsorbent Y-3 was prepared by the method of Example 1, except that the grain size of the NaA molecular sieve raw powder was 1.2 μm. The parameters of adsorbent Y-3 are listed in Table 1. A method similar to that of Example 1 was used to separate normal alkanes from the C10-C25 fraction. The composition of the C10-C25 fraction feedstock is shown in Table 2, except that the desorbent consisted of 80 volume % of normal heptane (nC7) and 20 volume % of toluene, and the separation was carried out under the following conditions: temperature 235° C., pressure 2.5 MPa, feed flow rate 1158 mL / h, desorbent flow rate 1734 mL / h, extract flow rate 1015 mL / h, raffinate flow rate 1877 mL / h, and step time 150 s. The composition of the extract was analyzed by gas chromatography, wherein the instrument was Agilent 7890; the chromatographic column was HP-5, 50 m×0.32 mm×0.5 μm; the chromatographic analysis method was as follows: the injection volume was 0.2 μL; the injection port temperature was 290°C; the split ratio was 150:1; the column flow rate was 3.9 mL / min; the initial column temperature was 50°C, maintained for 3 min, and the temperature was increased to 300°C at 5°C / min and maintained for 5 min; the FID detector was used, and the detector temperature was 320°C. Among the components of the extract except the desorbent, the content of normal alkanes was 99.5% by weight, the content of aromatics was 0.08% by weight, and the yield of nC10-nC25 normal alkanes was shown in Table 3. Comparative Example 1 Adsorbent X-1 was used and normal alkanes were separated from C10-C25 distillate oil by a method similar to Example 1. The composition of the C10-C25 distillate oil feedstock is shown in Table 2, except that the desorbent consisted of 80 volume % of n-heptane and 20 volume % of isooctane, and the separation was carried out under the following conditions: temperature 200°C, pressure 2.5 MPa, feed flow rate 1158 mL / h, desorbent flow rate 2000 mL / h, extract flow rate 1381 mL / h, raffinate flow rate 1777 mL / h, and step time 150 s. The composition of the extract was analyzed by gas chromatography, wherein the instrument was Agilent 7890; the chromatographic column was HP-5, 50 m×0.32 mm×0.5 μm; the chromatographic analysis method was as follows: the injection volume was 0.2 μL; the injection port temperature was 290°C; the split ratio was 150:1; the column flow rate was 3.9 mL / min; the initial column temperature was 50°C, maintained for 3 min, and the temperature was increased to 300°C at 5°C / min and maintained for 5 min; the FID detector was used, and the detector temperature was 320°C. Among the components of the extract except the desorbent, the content of normal alkanes was 99.2% by weight, and the content of aromatics was 0.5% by weight. Comparative Example 2 Adsorbent X-1 was used to separate normal alkanes from C10-C25 fraction oil in a similar manner as in Example 3. The composition of the C10-C25 fraction oil raw material is shown in Table 2, except that the desorbent was 60 volume % of n-pentane (nC5) and 40 volume % of isooctane (iC8), and separation was performed under the following conditions: temperature 185°C, pressure 2.5 MPa, feed flow rate 1158 mL / h, desorbent flow rate 1891 mL / h, extract flow rate 1172 mL / h, raffinate flow rate 1877 mL / h, and step time 150 s. The composition of the extract was analyzed by gas chromatograph, wherein the instrument: Agilent 7890; chromatographic column: HP-5, 50 m×0.32 mm×0.5 μm; chromatographic analysis method: injection volume 0.2 μL; injection port temperature 290°C; split ratio 150:1; column flow rate 3.9 mL / min; initial column temperature 50°C, maintained for 3 min, heated to 300°C at 5°C / min, maintained for 5 min; FID detector, detector temperature 320°C. Among the components of the extract excluding the desorbent, the content of normal alkanes was 99.1% by weight and the content of aromatic hydrocarbons was 0.5% by weight. Table 1 Table 2 Table 3 First, it can be seen from the data of Examples 1-9 and Comparative Examples 1-2 that the specific composite desorbent of the present invention can significantly reduce the aromatic content in the extract. Therefore, the specific composite desorbent of the present invention can effectively separate normal alkanes from distillate oil by using a simulated moving bed through adsorption-desorption. In addition, it can be seen from the data of Examples 1-9 that when a specific adsorbent is used, not only can the normal alkanes be effectively separated from the distillate oil, but the yield of C10-C25 normal alkanes, especially C14-C25 normal alkanes, in the extract can also be increased. Comparative Example 3 Adsorbent X-1 was used to separate normal alkanes from nC10-nC25 distillate oil by a method similar to that in Example 1. The composition of the C10-C25 distillate oil feedstock is shown in Table 2, except that the experiment was conducted using the small simulated moving bed shown in Figure 2. Specifically, 12 adsorption columns were filled with X-1 adsorbent, wherein the adsorption zone, purification zone, desorption zone and buffer zone consisted of 3 adsorption columns, 4 adsorption columns, 3 adsorption columns and 2 adsorption columns, respectively. The composition of the C10-C25 distillate oil is shown in Table 2. The first desorbent consisted of 70% toluene and 30% isooctane (iC8), and the second desorbent consisted of 80% by volume of n-heptane (nC7) and 20% by volume of isooctane (iC8). Separation was performed under the following conditions: temperature 185°C, pressure 2.5 MPa, fraction oil feed flow rate 1158 mL / h, first desorbent flow rate 210 mL / h, extract flow rate 1200 mL / h, second desorbent flow rate 1780 mL / h, extract flow rate 1948 mL / h, and step time 150 s. The composition of the extract was analyzed by gas chromatograph, wherein the instrument: Agilent 7890; chromatographic column: HP-5, 50 m×0.32 mm×0.5 μm; chromatographic analysis method: injection volume 0.2 μL; injection port temperature 290°C; split ratio 150:1; column flow rate 3.9 mL / min; initial column temperature 50°C, maintained for 3 min, heated to 300°C at 5°C / min, maintained for 5 min; FID detector, detector temperature 320°C. Among the components of the extract except the desorbent, the content of normal alkanes was 99.5% by weight, the content of aromatics was 0.12% by weight, and the yield of nC10-nC25 normal alkanes was shown in Table 4. Table 4 It can be seen from the data in Table 4 that, compared with the method of Comparative Example 3 in which a desorbent containing light normal alkanes and a flushing agent containing light aromatics are fed into the upstream of the desorption zone and the purification zone, respectively, the method of Example 1 of the present invention can further reduce the content of heavy aromatics in the extract. In addition, the method of Example 1 of the present invention can also obtain a yield of C10-C25 normal alkanes in the extract that is comparable to or even higher than that of the method of Comparative Example 3. In addition, the present disclosure provides inferred weight percentages of heavy aromatics and light aromatics in the liquid streams inside the adsorption columns 101-112 in Table 5 below. Table 5 Combined with Figure 3, it can be seen that when the flow rates of the extract, the residual liquid and the distillate oil feedstock entering or leaving the simulated moving bed are basically the same, the flow rates of the light normal alkanes entering the simulated moving bed upstream of the desorption zone are basically the same, and the flow rates of the light aromatics entering the purification zone are also basically the same, it is speculated that in the adsorption zone, the light aromatics content in the liquid stream of Example 1 is significantly higher than that in Comparative Example 3. This may be due to the fact that the desorbent containing aromatics is fed upstream of the desorption zone in Example 1, so that the adsorbent carries a certain amount of light aromatics desorbent. When the adsorbent that adsorbs light aromatics leaves the desorption zone and enters the adsorption zone, it can inhibit the adsorption of heavy aromatics by the adsorbent, resulting in a reduction in the amount of heavy aromatics adsorbed in the adsorption zone. In addition, in the purification zone, the light aromatics content in the liquid streams of Example 1 and Comparative Example 3 is comparable, so the elution effect on heavy aromatics is also comparable. It can be seen from the change curve of the weight percentage of light aromatics and heavy aromatics in the liquid phase flow in each adsorption column of the simulated moving bed in Figure 3 that the composite desorbent of the present invention fed upstream of the desorption zone not only has an eluting effect on heavy aromatics but also has an inhibitory effect on the adsorption of heavy aromatics because a certain amount of light aromatics are adsorbed on the surface of the adsorbent before entering the adsorption zone. Therefore, compared with the method of feeding a light normal alkane desorbent and a light aromatic flushing agent upstream of the desorption zone and in the purification zone respectively, the method of the present invention can in principle further reduce the content of heavy aromatics in the extract. Since the method of Example 1 of the present invention results in a relatively lower content of heavy aromatics in the extract and a comparable or even higher content of normal alkanes as compared to the method of Comparative Example 3, when producing the same weight of normal alkanes, the method of Example 1 of the present invention can omit the related equipment and operations for additionally feeding an aromatic-containing flushing agent and optionally omit the subsequent separation of the mixture of the aromatic-containing flushing agent and the normal-alkane-containing desorbent and the related equipment and operations for circulating the aromatic-containing flushing agent, and can also omit the energy consumption and process costs associated therewith.

Claims

1. A method for separating normal alkanes from distillate oil using a simulated moving bed, the simulated moving bed comprising a plurality of adsorbent beds filled with adsorbent, the plurality of adsorbent beds being interconnected in a closed loop manner and divided into a desorption zone, a purification zone, an adsorption zone and a buffer zone, the distillate oil comprising normal alkanes and aromatics and optionally isoalkanes and cycloalkanes, wherein the method comprises the following steps: 1) injecting distillate oil from the upstream of the adsorption zone, wherein normal alkanes in the distillate oil are selectively adsorbed by the adsorbent in the adsorption zone; 2) injecting a desorbent from the upstream of the desorption zone, and desorbing the adsorbed normal alkanes by using the desorbent in the desorption zone, wherein the desorbent comprises normal alkanes and aromatic hydrocarbons, and the carbon numbers of the desorbents are respectively greater than the maximum carbon number of the components in the distillate oil or less than the minimum carbon number of the components in the distillate oil; 3) removing an extract containing desorbed normal paraffins from the downstream of the desorption zone; and 4) The raffinate containing the non-adsorbed components is withdrawn from the downstream of the adsorption zone.

2. The method according to claim 1, wherein the distillate oil comprises normal alkanes with a carbon number of 10 or more and aromatics with a carbon number of 10 or more, preferably normal alkanes with a carbon number of C10-C30 and aromatics with a carbon number of C10-C30, preferably normal alkanes with a carbon number of C10-C25 and aromatics with a carbon number of C10-C25, and more preferably normal alkanes with a carbon number of C14-C25 and aromatics with a carbon number of C14-C25; Optionally, the distillate oil contains isoparaffins with a carbon number of 10 or more and / or cycloparaffins with a carbon number of 10 or more, preferably C10-C30 isoparaffins and / or C10-C30 cycloparaffins, preferably C10-C25 isoparaffins and / or C10-C25 cycloparaffins, more preferably C14-C25 isoparaffins and / or C14-C25 cycloparaffins.

3. The method according to claim 1 or 2, wherein the desorbent comprises or consists of C5-C8 normal alkanes and C6-C8 aromatic hydrocarbons, and optionally C5-C8 isoalkanes; Preferably, based on the total volume of the desorbent, the desorbent comprises or consists of 30-98% by volume, preferably 40-96% by volume, more preferably 50-95% by volume, still more preferably 60-90% by volume or 60-80% by volume of normal C5-C8 alkanes; 1-70% by volume, preferably 2-60% by volume, or 2-50% by volume, more preferably 3-40% by volume, or 3-35% by volume, even more preferably 4-30% by volume, or 4-25% by volume, still more preferably 5-20% by volume or 10-20% by volume of C6-C8 aromatics; and 0-50% by volume, preferably 0-40% by volume, more preferably 0-30% by volume, still more preferably 0-20% by volume of C5-C8 isoalkanes; Preferably, based on the total volume of the desorbent, the desorbent consists of 70-95% by volume, preferably 70-90% by volume or 70-80% by volume of C5-C8 normal alkanes, and 5-30% by volume, preferably 10-30% by volume or 20-30% by volume of C6-C8 aromatics; preferably consists of 70-95% by volume, preferably 70-90% by volume or 70-80% by volume of C7-C8 normal alkanes, and 5-30% by volume, preferably 10-30% by volume or 20-30% by volume of C7-C8 aromatics.

4. A method according to any one of the preceding claims, wherein: In addition to the step of injecting a desorbent from the upstream of the desorption zone, the method does not include the step of injecting a flushing agent or other desorbents, wherein the flushing agent or other desorbents refer to normal alkanes, isoalkanes, aromatic hydrocarbons and / or cycloalkanes having a carbon number greater than the maximum carbon number of the components in the distillate oil or less than the minimum carbon number of the components in the distillate oil, especially aromatic hydrocarbons and / or isoalkanes having a carbon number greater than the maximum carbon number of the components in the distillate oil or less than the minimum carbon number of the components in the distillate oil.

5. The method according to any one of the preceding claims, wherein the adsorption process in the adsorption zone and the desorption process in the desorption zone are carried out independently of each other at a temperature of 150-250°C and a pressure of 1.0-5.0 MPa.

6. The method according to any one of the preceding claims, further comprising the step of separating the extract to obtain normal alkanes from the distillate oil and the desorbent; and / or the step of separating the raffinate to obtain the desorbent and non-normal alkanes from the distillate oil; Preferably, the separation steps are respectively achieved by a distillation process.

7. The method according to any one of the preceding claims, wherein the adsorbent comprises 5A molecular sieve; Preferably, the particle size of the adsorbent is 0.1-1.0 mm, more preferably 0.2-0.8 mm, and even more preferably 0.3-0.6 mm, and / or the crystal size of the 5A molecular sieve is 0.1-1.0 μm, preferably 0.2-0.8 μm; Optionally, the adsorbent has a water content of 5 wt% or less, preferably 3 wt% or less, more preferably 1 wt% to 3 wt%.

8. A method according to any one of the preceding claims, wherein the adsorbent is prepared by a process comprising the steps of: (1) mixing a molecular sieve raw material, a binder and an auxiliary agent and performing a molding process to obtain a first product; (2) heat treating the first product to obtain a second product; (3) contacting the second product with an alkaline solution to obtain a third product; (4) The third product is reacted with a mixture containing Ca 2+ The ion solution contacts the exchange reaction to obtain a fourth product; and (5) subjecting the fourth product to an activation treatment, Preferably, Ca 2+ The ion exchange degree is 67-97%, preferably 75-85%, wherein the exchange degree indicates that Ca 2+ The percentage of exchangeable cation sites in the molecular sieve occupied by the ion.

9. The method according to claim 8, wherein the molecular sieve raw material is 4A molecular sieve raw powder, preferably NaA molecular sieve raw powder; and / or The binder is selected from clay binders, silica sol, alumina sol or mixtures thereof, wherein the clay binder is preferably selected from kaolin, halloysite, attapulgite or mixtures thereof; and / or The auxiliary agent is selected from lignin, sesbania powder, corn starch, bayberry tannin, methyl cellulose or a mixture thereof, Preferably, on a dry basis, the weight ratio of the molecular sieve raw material to the binder is (70-95):(5-30), preferably (80-95):(5-20), more preferably (85-95):(5-15), Preferably, the amount of the auxiliary agent used is 1-6 wt %, preferably 1.5-4 wt %, relative to the total dry weight of the molecular sieve raw material and the binder.

10. The method according to claim 8 or 9, wherein: The heat treatment in step (2) is roasting, and is carried out under the following conditions: a temperature of 400-800° C., and / or a time of 1-8 h; and / or The contacting in step (3) is carried out under the following conditions: temperature of 80-99° C.; time of 1-8 h; the alkaline solution includes but is not limited to sodium hydroxide and / or potassium hydroxide aqueous solution, wherein the concentration of the alkaline solution is preferably 0.5-5 mol / L; and / or the volume ratio of the second product to the alkaline solution is 1:(1-5); and / or The ion exchange reaction in step (4) is carried out under the following conditions: time is 1-8h; temperature is 10-99°C, preferably 50-99°C, more preferably 70-99°C; the Ca-containing 2+ The solution of ions is an aqueous solution of calcium chloride and / or calcium nitrate; and / or the third product and the solution containing Ca 2+ ions in a solution having a volume ratio of 1:(2-8), wherein in the solution, the Ca 2+ The concentration of ions is 0.1-2.0 mol / L; and / or The activation treatment in step (5) is carried out under the following conditions: temperature of 250-350° C., preferably 270-320° C.; and / or time of 1-4 h, preferably 2-3 h.

11. The method according to claim 6, wherein the separated normal alkanes from the distillate oil are injected into an additional adsorption tower for additional adsorption, Preferably, the separation in the additional adsorption tower is carried out at a temperature of 80-120°C, 0.05- 1.5MPa pressure and / or 0.5-8.0h -1 It is carried out at a volume space velocity of ; Preferably, the adsorbent in the additional adsorption tower is an X-type molecular sieve adsorbent and / or a Y-type molecular sieve adsorbent. More preferably, the water content of the X-type molecular sieve adsorbent and the Y-type molecular sieve adsorbent is 5 wt % or less, preferably 3 wt % or less, more preferably 1 wt % to 3 wt %, respectively.

12. A system for separating normal paraffins from distillate oil by adsorption-desorption, wherein the system comprises a simulated moving bed, the simulated moving bed comprising a plurality of adsorbent beds filled with adsorbent, the plurality of adsorbent beds being interconnected in a closed loop and divided into: (i) a desorption zone located between upstream desorbent injection and downstream extract withdrawal, (ii) a purification zone located between the upstream withdrawal of the extract and the downstream injection of the distillate, (iii) an adsorption zone located between upstream distillate injection and downstream raffinate withdrawal, and (iv) a buffer zone located between upstream raffinate removal and downstream desorbent injection, wherein at least one, preferably one, desorbent injection port is provided upstream of the desorption zone; The distillate oil contains normal alkanes and aromatic hydrocarbons and optionally isoalkanes and cycloalkanes; the desorbent contains normal alkanes and aromatic hydrocarbons, and their carbon numbers are respectively greater than the maximum carbon number of the components in the distillate oil or less than the minimum carbon number of the components in the distillate oil.

13. The system according to claim 12, wherein: The simulated moving bed does not include an injection port for a flushing agent or other desorbents except for an injection port for a desorbent arranged upstream of the desorption zone. The flushing agent or other desorbents refer to normal alkanes, isoalkanes, aromatics and / or cycloalkanes whose carbon number is greater than the maximum carbon number of components in the distillate oil or less than the minimum carbon number of components in the distillate oil, especially aromatics and / or isoalkanes whose carbon number is greater than the maximum carbon number of components in the distillate oil or less than the minimum carbon number of components in the distillate oil.

14. The system according to claim 12 or 13, wherein: The simulated moving bed comprises at least 4 adsorbent beds, preferably 4 to 40 adsorbent beds, such as 4 to 30 adsorbent beds, more preferably 6 to 24 adsorbent beds, still more preferably 12 to 24 adsorbent beds, wherein the distillate oil, adsorbent and desorbent are preferably as defined in any one of claims 1 to 11.

15. The system according to any one of claims 12 to 14, wherein: The system further comprises a device for separating the extracted liquid to obtain normal alkanes from the distillate oil and the desorbent, and / or a device for separating the raffinate to obtain the desorbent and non-normal alkanes from the distillate oil. Preferably, the separation devices are respectively distillation towers.

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