Method and system for preparing propylene from light hydrocarbon

Abstract

The invention relates to the technical field of propylene preparation through catalytic cracking, in particular to a method and a system for preparing propylene from light hydrocarbon. The method for preparing propylene from light dydrocarbon comprises the following steps: (1) carrying out olefin catalytic cracking reaction on a light dydrocarbon raw material to obtain a cracking product; the light hydrocarbon raw material contains at least one of C4-C6 olefins; (2) separating the cracking product through absorption-desorption to obtain ethylene-containing absorption tail gas and desorption liquid; recycling at least part of the absorption tail gas containing ethylene to the step (1), and carrying out olefin catalytic cracking reaction; in the circulating ethylene-containing absorption tail gas, the mass content of ethylene is 20-73%; the mass flow rate of the circulating ethylene-containing absorption tail gas is 10-150% of the mass flow rate of the light hydrocarbon raw material. And the yield of propylene is improved through the circulation of ethylene.

Description

Technical Field

[0001] The invention relates to the technical field of preparing propylene by catalytic cracking, and in particular to a method and system for preparing propylene by light hydrocarbons. Background Art

[0002] my country's petrochemical industry is rich in C4 resources. Refineries, ethylene plants and methanol-to-olefins plants all produce a large amount of C4 rich in olefins as by-products. The rational use of by-product C4 to produce chemical products with higher added value has always been a hot topic of concern for petrochemical companies.

[0003] At present, the mature C4 utilization technologies in the industry mainly include olefin disproportionation to produce propylene technology, olefin catalytic cracking to produce propylene and ethylene technology, and alkylation to produce oil technology. Among them, olefin catalytic cracking to produce propylene and ethylene technology has the characteristics of simple process, strong adaptability of catalyst to impurities in raw materials, and high propylene and ethylene yield, which has attracted the attention of enterprises.

[0004] The catalytic cracking of olefins to produce propylene and ethylene technology uses C4 and C5 hydrocarbons as raw materials, and cracks the olefins in the raw materials into propylene and ethylene under the action of a selective molecular sieve catalyst. CN101092323A discloses a method for catalytic cracking of carbon-containing olefins to produce light olefins. The method uses carbon-containing olefins as raw materials and converts them into ethylene and propylene reaction products through ZSM-5 molecular sieve catalysts, and obtains ethylene and propylene-rich product streams, circulating C4 streams, and C5 and above effluent streams through compression and distillation separation. The mass content of ethylene in the reaction product is greater than 20%, and the mass content of propylene is greater than 40%.

[0005] CN113651669A discloses a device and method for producing propylene. The method uses an absorption and desorption method to separate ethylene-rich gas and crude propylene from the reaction product of olefin catalytic cracking, avoids the use of propylene refrigerant that is not available in general refineries, and can achieve preliminary separation of products through circulating cooling water, thereby reducing the severity of the process on public engineering conditions and promoting the application of olefin catalytic cracking technology in refineries.

[0006] In the existing technology of co-producing ethylene and propylene, in order to obtain polymerization-grade ethylene and polymerization-grade propylene, the crude product needs to be further separated. For example, the ethylene and propylene generated by the olefin catalytic cracking unit of the methanol to olefins unit are sent to the main unit for olefin separation to obtain polymerization-grade ethylene and propylene, without the need to build a separate olefin separation unit. However, most refineries do not have the ability to separate ethylene. Refineries are mainly aimed at producing oil products, and the by-product liquefied gas enters the gas separation unit to separate polymerization-grade propylene and C4 liquefied gas. Although the remaining refinery dry gas contains a small amount of ethylene, it is not economical to set up ethylene distillation due to insufficient scale, so ethylene is difficult to use effectively. Even if ethylene is converted into aromatics through technical means such as superposition, the benefits are not good. Therefore, although the existing olefin catalytic cracking technology has a high total yield of propylene and ethylene, the refinery cannot separate polymerization-grade ethylene, and there are problems with the destination of the product, which limits the application of this technology in refineries.

[0007] In summary, the existing olefin catalytic cracking technology has problems such as difficulty in utilizing and separating ethylene and limited application in refineries. The present invention solves the above problems in a targeted manner and can be applied to the industrial production of propylene in refineries. Summary of the invention

[0008] The purpose of the present invention is to overcome the problems of the prior art such as the difficulty in utilizing and separating ethylene and the limited application in refineries, and to provide a method and system for preparing propylene from light hydrocarbons. The method for preparing propylene has the advantages of high propylene yield and the ability to obtain polymerization-grade propylene products in the refinery by relying on the existing gas separation device.

[0009] In order to achieve the above object, the present invention provides a method for preparing propylene from light hydrocarbons, the method comprising:

[0010] (1) a light hydrocarbon feedstock undergoes olefin catalytic cracking reaction to obtain a cracking product; the light hydrocarbon feedstock contains at least one of C4-C6 olefins;

[0011] (2) separating the cracking products by absorption-desorption to obtain an absorption tail gas containing ethylene and a desorption liquid;

[0012] Recycling at least a portion of the absorbed tail gas containing ethylene back to step (1) to undergo catalytic olefin cracking reaction; the mass content of ethylene in the recycled absorbed tail gas containing ethylene is 20% to 73%;

[0013] The mass flow rate of the circulating ethylene-containing absorption tail gas is 10%-150% of the mass flow rate of the light hydrocarbon feedstock.

[0014] A second aspect of the present invention provides a system for preparing propylene from light hydrocarbons, the system comprising:

[0015] Olefin catalytic cracking reaction unit; used for catalytic cracking of olefins in light hydrocarbon feedstock to obtain cracking products;

[0016] Absorption and desorption unit, comprising:

[0017] A compression unit, used to pressurize the cracking product to obtain rich gas and condensate;

[0018] Absorption unit: used for rich gas to contact with absorbent to obtain absorption liquid and absorption tail gas containing ethylene.

[0019] Desorption unit: used for desorbing the absorption liquid and / or condensate to obtain desorption gas and desorption liquid;

[0020] The top of the absorption unit discharge is connected to the feed inlet of the olefin catalytic cracking reaction unit, and is used for the absorption tail gas containing ethylene to circulate and perform the olefin catalytic cracking reaction.

[0021] Through the above technical scheme, the tail gas containing ethylene is separated from the cracking product, and at least part of the absorbed tail gas containing ethylene is recycled to cause olefin catalytic cracking reaction. The ethylene in the cracking reaction product is converted and utilized, avoiding high energy consumption of ethylene separation; the recycling of ethylene increases the yield of propylene, and the propylene yield can be increased by more than 10%.

[0022] According to a preferred embodiment of the present invention, the cracking product is separated into an absorption tail gas containing ethylene by an absorption and desorption method, and the absorbent is the C6+ heavy component in the reaction product, and no new material needs to be introduced. The separation by the absorption and desorption method does not require the use of propylene refrigerant, and only circulating cooling water and low-pressure steam are required to meet the separation requirements.

[0023] By adopting the method of the present invention, the olefin catalytic cracking technology has the necessary conditions for application in refineries, and its advantages are:

[0024] 1) The ethylene in the cracking reaction product is converted and utilized, avoiding the high energy consumption of ethylene separation;

[0025] 2) The recycling of ethylene increases the yield of propylene, which can be increased by more than 10%;

[0026] 3) Propylene can be separated to polymerization grade by relying on the existing gas separation equipment in the refinery;

[0027] 4) The technology has relatively low requirements for shared engineering and can be met by general refineries.

[0028] In summary, the present invention solves the problems in the prior art of the difficulty in utilizing and separating ethylene and the limited application in refineries, and achieves better technical effects. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 A schematic flow chart of a method for preparing propylene from light hydrocarbons provided in a preferred embodiment of the present invention.

[0030] Description of Reference Numerals

[0031] 1 is light hydrocarbon feedstock; 2 is cracking product; 3 is rich gas; 4 is condensate; 5 is desorption liquid; 6 is absorption tail gas circulated to the reactor; 7 is crude propylene; 8 is C4+ logistics; 9 is circulating C4-C6 logistics; 10 is absorbent; 11 is effluent C4-C6 logistics; 12 is effluent C6+ logistics; 13 is liquefied gas; 14 is C5-C6 logistics; 15 is effluent absorption tail gas, 16 is propane logistics, and 17 is propylene logistics.

[0032] A is a reaction unit, which includes an olefin catalytic cracking reactor, B is a compression unit, C is an absorption and desorption unit, D is a depropanizer, E is a dehexanizer, F is a debutanizer, and G is a distillation unit. DETAILED DESCRIPTION

[0033] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range and the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0034] In the present invention, unless otherwise specified, directional words such as "up, down, left, right" generally refer to up, down, left, right as shown in the reference drawings; "inside, outside" refer to inside and outside relative to the outline of each component itself.

[0035] A first aspect of the present invention provides a method for preparing propylene from light hydrocarbons, the method comprising:

[0036] (1) a light hydrocarbon feedstock undergoes olefin catalytic cracking reaction to obtain a cracking product; the light hydrocarbon feedstock contains at least one of C4-C6 olefins;

[0037] (2) separating the cracking products by absorption-desorption to obtain an absorption tail gas containing ethylene and a desorption liquid;

[0038] At least part of the absorbed tail gas containing ethylene is recycled back to step (1) to cause the olefin catalytic cracking reaction; the mass content of ethylene in the recycled absorbed tail gas containing ethylene is 20%-73%; the mass flow rate of the recycled absorbed tail gas containing ethylene is 10%-150% of the mass flow rate of the light hydrocarbon raw material. The tail gas containing ethylene is separated from the cracking product, and at least part of the absorbed tail gas containing ethylene is recycled to cause the olefin catalytic cracking reaction, and the ethylene in the cracking reaction product is converted and utilized, avoiding the high energy consumption of ethylene separation; the recycling of ethylene increases the yield of propylene, and the yield of propylene can be increased by more than 10%.

[0039] In the present invention, a mixed light hydrocarbon containing C4 to C6 olefins is used as a raw material. Under the conditions of olefin catalytic cracking reaction, a series of complex reactions occur, and finally ethylene and propylene are converted with high selectivity through a reaction network of cracking, cyclization, dehydrogenation, polymerization, etc., while a part of C6 and above heavy components are produced as by-products. In order to obtain high selectivity of propylene and ethylene, the catalyst usually only converts the olefins in the mixed light hydrocarbon raw material, and the conversion rate of the alkanes in the raw material is extremely low, and the alkanes can be considered as inert components.

[0040] By adopting the method of the present invention, the lower the ethylene concentration in the circulating tail gas, the higher the propylene yield; according to a preferred embodiment of the present invention, the mass content of ethylene in the circulating ethylene-containing absorption tail gas is 35%-57%, preferably 35%-44%.

[0041] According to a preferred embodiment of the present invention, the mass flow rate of the circulating ethylene-containing absorption tail gas is 50%-100% of the mass flow rate of the light hydrocarbon feedstock.

[0042] According to a preferred embodiment of the present invention, another part of the absorption tail gas containing ethylene is used as external exhaust gas.

[0043] According to a preferred embodiment of the present invention, the method for absorbing and desorbing the cracking products comprises:

[0044] The cracking product is cooled and pressurized to obtain rich gas and condensate; the rich gas contacts the absorbent to obtain the absorbent and the absorption tail gas containing ethylene, and the absorbent and / or the condensate are desorbed to obtain the desorbed gas and the desorbed liquid. By adopting the method of the present invention, the reaction product is separated into the circulating absorption tail gas containing ethylene by the absorption-desorption method, and the absorbent is the C6+ heavy component separated from the cracking product, and no new material needs to be introduced. By adopting the absorption-desorption method for separation, no propylene refrigerant is needed, and only circulating cooling water and low-pressure steam are needed to meet the separation requirements.

[0045] In the present invention, the absorption-desorption can be achieved by the following process:

[0046] The compressor boosts the pressure to obtain rich gas and condensate; the rich gas enters the bottom of the absorption tower and contacts with the absorbent in countercurrent, and ethylene-rich absorption tail gas is obtained from the top of the absorption tower, and absorption liquid is obtained from the bottom of the absorption tower; the absorption liquid and condensate enter from the top of the desorption tower to obtain desorbed gas, and the desorbed gas returns to the absorption tower from the top of the absorption tower via a pipeline.

[0047] According to a preferred embodiment of the present invention, the boost pressure of the cracking product after cooling is 1.0-2.0 MPaG.

[0048] According to a preferred embodiment of the present invention, the absorbent is cooled to 40-42° C. by a refrigerant and then enters the absorption tower.

[0049] According to a preferred embodiment of the present invention, the operating pressure of the absorption tower is 1.0-1.5 MPaG.

[0050] According to a preferred embodiment of the present invention, the method for preparing propylene from light hydrocarbons further comprises:

[0051] (I) removing propanation from the desorption liquid to separate a crude propylene stream and a C4+ stream;

[0052] (II) The C4+ logistics is dehexaneized to separate the C4-C6 logistics and the C6+ logistics.

[0053] In the present invention, the olefins in the light hydrocarbon feedstock cannot be completely converted during the olefin catalytic cracking reaction. In order to improve the utilization rate of the olefins, the unreacted C4-C6 olefins in the reaction products are separated and recycled to the reactor. According to a preferred embodiment of the present invention, at least part of the C4-C6 logistics is recycled to cause the olefin catalytic cracking reaction; another part of the C4-C6 logistics is debutanized to separate and obtain a liquefied gas logistics and a C5-C6 logistics.

[0054] According to a preferred embodiment of the present invention, the mass flow rate of the circulating C4-C6 logistics is 200%-345% of the mass flow rate of the light hydrocarbon feedstock.

[0055] According to a preferred embodiment of the present invention, the mass of olefins in the circulating C4-C6 logistics is 32%-50%.

[0056] According to a preferred embodiment of the present invention, part of the C6+ logistics is recycled as an absorption-desorption absorbent for separating cracking products; and another part of the C6+ logistics is discharged.

[0057] According to a preferred embodiment of the present invention, the crude propylene stream is subjected to rectification and separation to obtain propylene and propane. The method of the present invention can be used to obtain a polymerization-grade polypropylene product.

[0058] In the present invention, the olefin content in the light hydrocarbon feedstock should be as high as possible to reduce the accumulation of alkanes in the circulating material, thereby achieving more efficient cracking. The content of monoolefins in the C4 hydrocarbons produced as a by-product of the methanol to olefins unit is usually above 85%, which is a high-quality catalytic cracking feedstock. The C4 hydrocarbons in the refinery contain more alkanes, especially isobutane, and the olefin content is below 50%. It usually needs to be pretreated to remove most of the isobutane and increase the olefin concentration in the C4 feedstock. In short, as a raw material for catalytic cracking of olefins, the olefin content should be above 80%.

[0059] According to a preferred embodiment of the present invention, the light hydrocarbon feedstock comprises at least one of C4 olefins, C5 olefins and C6 olefins.

[0060] In the present invention, the isomers of olefins in the light hydrocarbon feedstock are not distinguished in the catalytic cracking reaction, and the rest are inert components. According to a preferred embodiment of the present invention, the mass content of olefins in the light hydrocarbon feedstock is 80%-95%.

[0061] In the present invention, there is no particular limitation on the catalyst for the olefin catalytic cracking reaction, which may be a conventional olefin cracking catalyst in the art. According to a preferred embodiment of the present invention, the catalyst for the olefin catalytic cracking reaction includes ZSM-5 molecular sieve.

[0062] According to a preferred embodiment of the present invention, the silicon-aluminum molar ratio of the ZSM-5 molecular sieve is 300-500.

[0063] According to a preferred embodiment of the present invention, the crystallinity of the ZSM-5 molecular sieve is 95-99%.

[0064] In the present invention, there is no particular limitation on the conditions of the olefin catalytic cracking reaction, which can be conventional olefin catalytic cracking reaction conditions in the art. Using the method of the present invention, the reaction temperature of the olefin catalytic cracking reaction is 480-590°C, and the reaction pressure is 0-0.1MPaG. The reaction temperature mainly affects the conversion rate of the catalyst to olefins. The reaction pressure affects the selectivity of the catalyst to propylene and ethylene. Since the cracking reaction is a molecular multiplication reaction, a lower reaction pressure is conducive to the cracking reaction and the improvement of the selectivity of ethylene and propylene. According to a preferred embodiment of the present invention, the olefin catalytic cracking reaction conditions include: a temperature of 480-590°C; a pressure of 0-0.1MPaG.

[0065] In the present invention, there is no special requirement for the reactor for the olefin catalytic cracking reaction, which may be a conventional olefin catalytic cracking reactor in the art. According to a preferred embodiment of the present invention, the olefin catalytic cracking reaction is carried out in a fixed bed reactor.

[0066] According to a preferred embodiment of the present invention, the method comprises:

[0067] (1) The light hydrocarbon feedstock is vaporized and overheated to undergo olefin catalytic cracking reaction to obtain cracking products;

[0068] (2) the cracking products are cooled and then pressurized to separate rich gas and condensate;

[0069] (3) the rich gas is contacted with the absorbent to obtain an absorption liquid and an absorption tail gas containing ethylene, and the absorption liquid and / or condensate are desorbed to obtain a desorbed gas and a desorbed liquid; at least a portion of the absorption tail gas containing ethylene is recycled and mixed with a light hydrocarbon feedstock to cause the olefin catalytic cracking reaction;

[0070] (4) removing propanation from the desorption liquid to separate a crude propylene stream and a C4+ stream; and fractionating and separating the crude propylene stream to obtain propylene and propane;

[0071] (5) The C4+ stream is dehexaned to separate into a C4-C6 stream and a C6+ stream; and at least a portion of the C4-C6 stream is recycled back to step (1) to undergo the olefin catalytic cracking reaction.

[0072] According to the method of the present invention, in order to improve the yield of propylene, the unreacted C4-C6 olefins and the ethylene produced by the reaction are circulated. In theory, in order to avoid the ineffective circulation and accumulation of alkanes, the C4-C6 olefins and alkanes should be separated and then circulated, and the ethylene should be separated from the rich gas and then circulated. However, the energy consumption of C4-C6 alkane-olefin separation is high and the process is complicated. It is more difficult to separate ethylene from the rich gas, and deep cold separation is required. Therefore, a method of circulating alkanes and alkenes together is adopted. However, in order to avoid a large amount of circulation of alkanes, it is necessary to control the concentration of olefins in the circulating material to avoid too low a concentration. The general rule is that the larger the circulation amount, the lower the concentration of olefins in the circulating material.

[0073] According to a preferred embodiment of the present invention, in step (3), another part of the absorbed tail gas containing ethylene is used as external exhaust gas.

[0074] According to a preferred embodiment of the present invention, in step (5), another part of the C4-C6 logistics is debutanized to separate the liquefied gas logistics and the C5-C6 logistics.

[0075] According to a preferred embodiment of the present invention, in step (5), part of the C6+ logistics is recycled as an absorption-desorption absorbent for separating the cracking products; and another part of the C6+ logistics is discharged.

[0076] A second aspect of the present invention provides a system for preparing propylene from light hydrocarbons, the system comprising:

[0077] Olefin catalytic cracking reaction unit; used for catalytic cracking of olefins in light hydrocarbon feedstock to obtain cracking products;

[0078] Absorption and desorption unit, comprising:

[0079] The compression unit is used to pressurize the cracking products to obtain rich gas and condensate.

[0080] Absorption unit: used for rich gas to contact with absorbent to obtain absorption liquid and absorption tail gas containing ethylene.

[0081] Desorption unit: used for desorbing the absorption liquid and / or condensate to obtain desorption gas and desorption liquid;

[0082] The top of the absorption and desorption unit discharge is connected to the olefin catalytic cracking reaction unit for circulating the absorption tail gas containing ethylene and mixing it with the light hydrocarbon feedstock.

[0083] According to a preferred embodiment of the present invention, the system for preparing propylene from light hydrocarbons further comprises:

[0084] Depropanization unit, used for depropanization of the stripping liquid to separate the crude propylene stream and the C4+ stream;

[0085] Dehexane unit, used for dehexane of C4+ logistics to separate C4-C6 logistics and C6+ logistics;

[0086] The top of the dehexanization unit discharge is connected to the feed inlet of the olefin catalytic cracking reaction unit, so as to circulate at least part of the C4-C6 logistics for the olefin catalytic cracking reaction;

[0087] The bottom of the dehexane unit discharge is connected to the absorption and desorption unit for circulating the absorbent flow to the absorption and desorption unit.

[0088] According to a preferred embodiment of the present invention, the system for preparing propylene from light hydrocarbons further comprises:

[0089] A distillation unit is used for distilling the crude propylene stream to separate propylene and propane;

[0090] The debutanizer unit is used to debutanize another part of the C4-C6 stream to separate the liquefied gas stream and the C5-C6 stream.

[0091] According to a preferred embodiment of the present invention, the present invention provides a system for preparing propylene from light hydrocarbons, such as Figure 1 As shown; the system for preparing propylene from light hydrocarbons comprises:

[0092] Reaction unit A;

[0093] Compression unit B;

[0094] Absorption and desorption unit C; the top of the discharge of the absorption and desorption unit C is connected to the reaction unit A;

[0095] Depropanizer D;

[0096] The dehexanizer E; the top of the dehexanizer E is connected to the reaction unit A, and the bottom of the dehexanizer E is connected to the absorption and desorption unit;

[0097] Debutanizer F is used to debutanize another part of C4-C6 flow to separate the liquefied gas flow and C5-C6 flow

[0098] The distillation unit G is used for distilling the crude propylene stream to separate propylene and propane.

[0099] The present invention provides a method for preparing propylene from light hydrocarbons, the flow diagram of the method is as follows Figure 1 As shown, the method includes:

[0100] (1) A light hydrocarbon feedstock 1 undergoes olefin catalytic cracking reaction in a reaction unit A to obtain a cracking product 2;

[0101] (2) the cracking product 2 is cooled and pressurized by the compression unit B, and separated into a rich gas 3 and a condensate 4;

[0102] (3) The absorption and desorption unit C includes an absorption tower and a desorption tower. In the absorption tower of the absorption and desorption unit C, the rich gas 3 contacts the absorbent 10 to obtain an absorption liquid and an absorption tail gas containing ethylene;

[0103] The absorption liquid and the condensate 4 are mixed, and desorbed in the desorption tower of the absorption and desorption unit C to obtain desorption gas and desorption liquid 5, and the desorption gas is returned to the absorption tower through a pipeline from the top of the desorption tower; wherein, part of the absorption tail gas 6 containing ethylene is circulated and mixed with the light hydrocarbon feedstock 1 to cause the olefin catalytic cracking reaction, and another part of the absorption tail gas 15 containing ethylene is discharged;

[0104] (4) In the depropanizer D, the desorption liquid 5 is depropanized to obtain a crude propylene stream 7 and a C4+ stream 8; the crude propylene stream 7 is rectified and separated in the distillation unit G to obtain a propylene stream 17 and a propane stream 16;

[0105] (5) In the dehexanizer E, the C4+ stream 8 is dehexanized to obtain a C4-C6 stream and a C6+ stream; at least a portion of the C4-C6 stream 9 is recycled and mixed with the light hydrocarbon feedstock 1 to undergo the olefin catalytic cracking reaction; another portion of the C4-C6 stream 11 is transported to the debutanizer F; a portion of the C6+ stream 10 is recycled as an absorbent to separate the cracking products; another portion of the C6+ stream 12 is discharged.

[0106] (6) In the debutanizer F, the C4-C6 stream 11 is debutanized to separate the liquefied gas stream 13 and the C5-C6 stream 14.

[0107] The present invention will be further described below by way of examples, but are not limited thereto.

[0108] Example 1

[0109] Given a flow rate of 10 tons / hour and a monoolefin content of 87% C4 hydrocarbon material, Figure 1 The process shown in the figure, after vaporization, heat exchange of inlet and outlet materials and heating to a temperature of 550°C, enters the olefin catalytic cracking reactor, the reaction temperature is 545°C, and the reaction pressure is 0.08MPaG. The cracking reactor is a fixed bed, in which a layer of ZSM-5 molecular sieve catalyst is laid, the silicon-aluminum molar ratio is 400, and the crystallinity of the catalyst is 95%.

[0110] The reaction product enters the compression unit after heat exchange cooling and is pressurized to 1.50MPaG after secondary compression. The rich gas at the outlet of the compressor enters the bottom of the absorption tower of the absorption and desorption unit. After countercurrent contact with the absorbent, the C3 and above components are absorbed and flow out from the bottom of the tower. The absorption tail gas is C2 and lighter components and flows out from the top of the tower. Most of the absorption tail gas is circulated, and the circulation amount is 53% (mass) of the flow rate of C4 hydrocarbon materials, of which the mass content of ethylene is 56.7%. The absorption tower bottom liquid and the compression unit condensate enter the desorption tower together, and a small amount of C2 entrained is removed from the top of the tower and returned to the absorption tower for reabsorption. The bottom of the tower is a desorption liquid without C2 components.

[0111] The desorbent is separated into crude propylene in the depropanizer, wherein the flow rate of propylene obtained by distillation of the crude propylene flow is 5.3 tons / hour. The depropanizer bottom liquid is C4+ material, which enters the dehexanizer for separation, and the top gas phase is produced as circulating C4-C6 material, and the circulating flow rate is 345% (mass) of the C4 raw material flow rate, wherein the mass content of olefins is 32.0%. The top liquid phase is produced as an external discharge C4-C6 logistics, which is sent to the debutanizer to further separate liquefied gas and C5-C6 logistics. Most of the dehexanizer bottom liquid (97%) is returned to the absorption and desorption unit as an absorbent, and the rest is discharged. Based on the olefins in the C4 raw material, the propylene yield is 61.0%.

[0112] Example 2

[0113] The conditions and steps of Example 1 are followed, except that the circulation amount of the absorbed tail gas and the circulating C4-C6 logistics is changed, wherein the circulation amount of the absorbed tail gas is 150% (mass) of the C4 raw material flow rate, and the circulating C4-C6 logistics flow rate is 200% (mass) of the C4 raw material flow rate.

[0114] After the device was running stably, the mass content of ethylene in the circulating absorption tail gas was measured to be 20.8%, and the mass content of olefins in the circulating C4-C6 logistics was 50.0%. The flow rate of propylene in the crude propylene was 5.06 tons / hour. Based on the olefins in the C4 raw material, the propylene yield was 58.2%.

[0115] Example 3

[0116] The conditions and steps of Example 1 are followed, except that the circulation amount of the absorbed tail gas and the circulating C4-C6 logistics is changed, wherein the circulation amount of the absorbed tail gas is 92% (mass) of the C4 raw material flow rate, and the circulating C4-C6 logistics flow rate is 247% (mass) of the C4 raw material flow rate.

[0117] After the device was running stably, the mass content of ethylene in the circulating absorption tail gas was measured to be 34.3%, and the mass content of olefins in the circulating C4-C6 logistics was 42.7%. The flow rate of propylene in the crude propylene was 5.22 tons / hour. Based on the olefins in the C4 raw material, the propylene yield was 60.0%.

[0118] Example 4

[0119] The conditions and steps of Example 1 are the same, except that the circulation amount of the absorbed tail gas and the circulating C4-C6 logistics is changed, wherein the circulation amount of the absorbed tail gas is 34% (mass) of the C4 raw material flow rate, and the circulating C4-C6 logistics flow rate is 239% (mass) of the C4 raw material flow rate.

[0120] After the device was running stably, the mass content of ethylene in the circulating absorption tail gas was measured to be 72.3%, and the mass content of olefins in the circulating C4-C6 logistics was 41.5%. The flow rate of propylene in the crude propylene was 4.93 tons / hour. Based on the olefins in the C4 raw material, the propylene yield was 56.7%.

[0121] Example 5

[0122] According to the steps of Example 1, a C4 raw material with an olefin concentration of 81% is used, the reaction temperature is 580°C, the reaction pressure is 0.04MPaG, and the circulation amount of the absorbed tail gas and the circulating C4-C6 logistics is changed at the same time, wherein the circulation amount of the absorbed tail gas is 67% (mass) of the C4 raw material flow rate, and the circulating C4-C6 logistics flow rate is 273% (mass) of the C4 raw material flow rate.

[0123] After the device was running stably, the mass content of ethylene in the circulating absorption tail gas was measured to be 44.4%, and the mass content of olefins in the circulating C4-C6 logistics was 36.0%. The flow rate of propylene in the crude propylene was 4.80 tons / hour. Based on the olefins in the C4 raw material, the propylene yield was 59.4%.

[0124] Example 6

[0125] According to the steps of Example 1, a C4 raw material with an olefin concentration of 94% is used, the reaction temperature is 580°C, the reaction pressure is 0.07MPaG, and the circulation amount of the absorbed tail gas and the circulating C4-C6 logistics is changed at the same time, wherein the circulation amount of the absorbed tail gas is 79% (mass) of the C4 raw material flow rate, and the circulating C4-C6 logistics flow rate is 272% (mass) of the C4 raw material flow rate.

[0126] After the device was running stably, the mass content of ethylene in the circulating absorption tail gas was measured to be 44.0%, and the mass content of olefins in the circulating C4-C6 logistics was 45.2%. The flow rate of propylene in the crude propylene was 5.84 tons / hour. Based on the olefins in the C4 raw material, the propylene yield was 62.1%.

[0127] Comparative Example 1

[0128] According to the raw material specifications, operating conditions and steps described in Example 3, the circulation amount of the circulating C4-C6 logistics remains unchanged, that is, the circulating C4-C6 logistics flow rate is 247% (mass) of the C4 raw material flow rate, and only the following conditions are changed: the absorption tail gas is not circulated. After the device runs stably, the flow rate of propylene in the crude propylene is 4.73 tons / hour, and the propylene yield is 54.4% based on the olefins in the C4 raw material.

[0129] Comparative Example 2

[0130] The conditions and steps of Example 1 are the same, except that the reaction temperature is changed to 480°C, the reaction pressure is changed to 0.1 MPaG, and the circulation amounts of the absorbed tail gas and the circulating C4-C6 logistics are changed, wherein the circulation amount of the absorbed tail gas is 9% (mass) of the C4 raw material flow rate, and the circulating C4-C6 logistics flow rate is 221% (mass) of the C4 raw material flow rate.

[0131] After the device was running stably, the mass content of ethylene in the circulating absorption tail gas was measured to be 90.1%, and the mass content of olefins in the circulating C4-C6 logistics was 38.4%. The flow rate of propylene in the crude propylene was 4.21 tons / hour. Based on the olefins in the C4 raw material, the propylene yield was 48.3%.

[0132] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, the technical solution of the present invention can be subjected to a variety of simple modifications, including the combination of various technical features in any other suitable manner, and these simple modifications and combinations should also be regarded as the contents disclosed by the present invention and belong to the protection scope of the present invention.

Claims

1. A method for preparing propylene from light hydrocarbons, characterized in that: The method includes: (1) a light hydrocarbon feedstock undergoes olefin catalytic cracking reaction to obtain a cracking product; the light hydrocarbon feedstock contains at least one of C4-C6 olefins; (2) separating the cracking products by absorption-desorption to obtain an absorption tail gas containing ethylene and a desorption liquid; Recycling at least a portion of the absorbed tail gas containing ethylene back to step (1) to undergo the catalytic cracking reaction of olefins; The mass content of ethylene in the circulating ethylene-containing absorption tail gas is 20%-73%; The mass flow rate of the circulating ethylene-containing absorption tail gas is 10%-150% of the mass flow rate of the light hydrocarbon feedstock.

2. The method according to claim 1, wherein: The mass content of ethylene in the circulating ethylene-containing absorption tail gas is 35%-57%, preferably 35%-44%.

3. The method according to claim 1, wherein: The mass flow rate of the circulating ethylene-containing absorption tail gas is 50%-100% of the mass flow rate of the light hydrocarbon feedstock.

4. The method according to any one of claims 1 to 3, wherein: The method of separating the cracking products by absorption-desorption comprises: The cracking product is cooled and pressurized to obtain rich gas and condensate; the rich gas is contacted with an absorbent to obtain an absorption liquid and the absorption tail gas containing ethylene, and the absorption liquid and / or condensate are desorbed to obtain desorbed gas and desorbed liquid.

5. The method according to any one of claims 1 to 4, wherein: The method further includes: (I) removing propanation from the desorption liquid to separate a crude propylene stream and a C4+ stream; (II) The C4+ logistics is dehexaneized to separate the C4-C6 logistics and the C6+ logistics.

6. The method according to claim 5, wherein: At least a portion of the C4-C6 logistics is recycled to cause the olefin catalytic cracking reaction; another portion of the C4-C6 logistics is subjected to debutanization to separate into a liquefied gas logistics and a C5-C6 logistics; Preferably, the mass flow rate of the circulating C4-C6 stream is 200%-345% of the mass flow rate of the light hydrocarbon feedstock; and / or Part of the C6+ stream is recycled as the absorbent used to separate the cracking products; the other part of the C6+ stream is discharged.

7. The method according to claim 6, wherein: The mass of olefins in the circulating C4-C6 logistics is 32%-50%.

8. The method according to claim 5 or 6, wherein: The crude propylene stream is distilled to separate propylene and propane.

9. The method according to any one of claims 1 to 8, wherein: The light hydrocarbon feedstock contains at least one of C4 olefins, C5 olefins and C6 olefins; preferably, the olefin mass content in the light hydrocarbon feedstock is 80%-95%; and / or The olefin catalytic cracking reaction catalyst comprises ZSM-5 molecular sieve; and / or The olefin catalytic cracking reaction conditions include: a temperature of 480-590°C; and / or a pressure of 0-0.1 MPaG; and / or The olefin catalytic cracking reaction is carried out in a fixed bed reactor.

10. The method according to claim 9, wherein: The silicon to aluminum molar ratio of the ZSM-5 molecular sieve is 300-500; and / or The crystallinity of the ZSM-5 molecular sieve is 95-99%.

11. The method according to any one of claims 1 to 10, wherein: The method includes: (1) a light hydrocarbon feedstock is vaporized and overheated to undergo catalytic olefin cracking reaction to obtain a cracking product; the light hydrocarbon feedstock contains at least one of C4-C6 olefins; (2) the cracking products are cooled and then pressurized to separate rich gas and condensate; (3) the rich gas is contacted with the absorbent to obtain an absorption liquid and an absorption tail gas containing ethylene, and the absorption liquid and / or condensate are desorbed to obtain a desorbed gas and a desorbed liquid; at least a portion of the absorption tail gas containing ethylene is recycled and mixed with a light hydrocarbon feedstock to cause the olefin catalytic cracking reaction; (4) removing propanation from the desorption liquid to separate a crude propylene stream and a C4+ stream; and fractionating and separating the crude propylene stream to obtain propylene and propane; (5) The C4+ stream is dehexaned to separate into a C4-C6 stream and a C6+ stream; and at least a portion of the C4-C6 stream is recycled back to step (1) to undergo the olefin catalytic cracking reaction.

12. A system for preparing propylene from light hydrocarbons, characterized in that: The system includes: Olefin catalytic cracking reaction unit; used for catalytic cracking of olefins in light hydrocarbon feedstock to obtain cracking products; Absorption and desorption unit, comprising: The compression unit is used to pressurize the cracking products to obtain rich gas and condensate. Absorption unit: used for rich gas to contact with absorbent to obtain absorption liquid and absorption tail gas containing ethylene. Desorption unit: used for desorbing the absorption liquid and / or condensate to obtain desorption gas and desorption liquid; The top of the absorption unit discharge is connected to the feed inlet of the olefin catalytic cracking reaction unit, and is used for the absorption tail gas containing ethylene to circulate and perform the olefin catalytic cracking reaction.

13. The system according to claim 12, wherein: The system further comprises: Depropanization unit, used for depropanization of the stripping liquid to separate the crude propylene stream and the C4+ stream; Dehexane unit, used for dehexane of C4+ logistics to separate C4-C6 logistics and C6+ logistics; The top of the dehexanization unit discharge is connected to the feed inlet of the olefin catalytic cracking reaction unit, so as to recycle at least part of the C4-C6 logistics back to the olefin catalytic cracking reaction unit; The bottom of the dehexanization unit discharge is connected to the absorption unit feed port, so as to circulate the C6+ logistics to the absorption unit for use as absorbent.

14. The system according to claim 12 or 13, wherein: The system further comprises: A distillation unit is used for distilling the crude propylene stream to separate propylene and propane; The debutanizer unit is used to debutanize another part of the C4-C6 stream to separate the liquefied gas stream and the C5-C6 stream.

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