Method and device for preparing low-carbon olefin from plastic and application

Abstract

The invention relates to the field of plastics, and discloses a method and device for preparing low-carbon olefin from plastics and application, and the method comprises the following steps: continuously pressurizing and mixing water and plastics, and sequentially carrying out supercritical water degradation and steam cracking to obtain a low-carbon olefin material. Through the method, the low-carbon olefin material can be successfully prepared from the plastic, and the yield of the low-carbon olefin is relatively high.

Description

Technical Field

[0001] The invention relates to the field of plastics, and in particular to a method, a device and an application of preparing low-carbon olefins from plastics. Background Art

[0002] Plastic is one of the most widely used synthetic materials. Its production process is to convert fossil raw materials into small molecular monomers such as low-carbon olefins by chemical methods such as steam cracking, then polymerize the monomers into synthetic resins, and finally add additives to make plastics. The emergence of plastics has greatly facilitated people's lives, but the "white pollution" caused by discarded plastics has become a major problem facing mankind. It not only wastes resources, but also damages the environment and even threatens human health. Tiny plastic particles have been found in drinking water and in the bodies of marine animals. As of 2015, humans have produced approximately 6.3 billion tons of discarded plastics, of which only 9% have been recycled, 12% have been incinerated, and the remaining 79% have all been landfilled or flowed into the natural environment.

[0003] Low-carbon olefins usually refer to the general term for unsaturated hydrocarbons with carbon content of C4 or less, mainly including ethylene, propylene, butadiene and other organic chemical raw materials with high economic value. Steam cracking is the main industrial production method of low-carbon olefins. With the development of my country's economy, the demand for these organic chemical raw materials has increased year by year. Although the production scale of low-carbon olefins has also increased year by year, it still cannot meet the growing demand.

[0004] Waste plastics are misplaced resources. Turning waste into resources and changing "burden" into economic benefits is the only way to comprehensively utilize waste plastics. There are two major methods of plastic recycling: physical recycling and chemical recycling. Physical recycling generally involves simply cleaning and granulating waste plastics and then downgrading them to recycled plastics. As the number of recycling increases, the quality of recycled plastics gradually decreases until they are unusable. Chemical recycling is the process of converting waste plastics into valuable chemicals through chemical methods.

[0005] Direct pyrolysis of plastics causes serious coking, and the oil yield is generally less than 80%. In addition, the pyrolysis plastic oil obtained has a high aromatic content and a low alkane content, and can only be used as fuel oil. It cannot produce low-carbon olefins through the most important existing industrial production method of low-carbon olefins - steam cracking, and cannot achieve the recycling of waste plastics. Summary of the invention

[0006] The purpose of the present invention is to overcome the problems existing in the prior art and to provide a method, device and application for preparing low-carbon olefins from plastics.

[0007] In order to achieve the above objectives, the first aspect of the present invention provides a method for preparing low-carbon olefins from plastics, the method comprising: continuously pressurizing and mixing water and plastics, and then sequentially subjecting them to supercritical water degradation and steam cracking to obtain low-carbon olefin materials.

[0008] The second aspect of the present invention provides a device for preparing low-carbon olefins from plastics, wherein the device includes a pressurized mixing system, a reactor and a separation system connected in sequence, and the reactor is divided into a reactor supercritical water degradation section and a reactor steam cracking section connected in sequence.

[0009] The third aspect of the present invention provides the use of the above method and / or device in improving the yield of light olefins in the process of producing light olefins from plastics.

[0010] Through the above technical scheme, low-carbon olefin materials can be successfully prepared from plastics, and the yield of low-carbon olefins is relatively high, which can reach 20-50% by weight, while the yield of ethylene can reach 15-30% by weight, the yield of propylene can reach 5-15% by weight, and the yield of butene can reach 2-5% by weight. In addition, the operation cycle is long and the energy consumption is low. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a schematic diagram of the structure of the system of the present invention.

[0012] Description of Reference Numerals

[0013] 1. Pressurized mixing system; 2. Supercritical water degradation section of reactor; 3. Steam cracking section of reactor; 4. Separation system DETAILED DESCRIPTION

[0014] 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.

[0015] The first aspect of the present invention provides a method for preparing low-carbon olefins from plastics, the method comprising: continuously pressurizing and mixing water and plastics, and then sequentially performing supercritical water degradation and steam cracking to obtain low-carbon olefin materials.

[0016] In the present invention, the supercritical water degradation refers to the degradation of plastics under conditions that can make water reach a supercritical state (form supercritical water). The cracking refers to the cracking under conditions that can make water form steam.

[0017] In the present invention, there is no special requirement for the conditions of supercritical water degradation, as long as the critical conditions of water are exceeded, the supercritical water degradation conditions include: the temperature is higher than 374°C, and can be 374.1°C, 380°C, 390°C, 400°C, 410°C, 420°C, 430°C, 440°C, 450°C, 460°C, 470°C, 480°C, 490°C, 500°C or the range formed by any two of the above values ​​and the value within the range, the pressure is higher than 22.1MPa, and can be 22.2MPa, 23MPa, 24MPa, 25MPa, 26MPa, 27MPa, 28MPa, 29MPa, 30MPa, 31MPa, 32MPa, 35MPa, 40MPa, 45MPa, 50MPa or the range formed by any two of the above values ​​and the value within the range. In the absence of any contrary description, "pressure" in the present invention refers to gauge pressure.

[0018] In the present invention, there is no particular requirement for the conditions of steam cracking, as long as water can be converted into steam. However, in a preferred embodiment of the present invention, the steam cracking conditions include: a temperature of not less than 700°C, which may be 700°C, 720°C, 740°C, 760°C, 780°C, 800°C, 810°C, 812°C, 814°C, 816°C, 818°C, 820°C, 830°C, 840°C, 850°C, 860°C, 880°C, 900°C or a range formed by any two of the above values ​​and a value within the range; a pressure of not more than 0.5 MPa, which may be 0.005 MPa, 0.01 MPa, 0.15 MPa, 0.2 MPa, 0.25 MPa, 0.3 MPa, 0.35 MPa, 0.4 MPa, 0.45 MPa, 0.5 MPa or a range formed by any two of the above values ​​and a value within the range.

[0019] In the present invention, the conditions of supercritical water degradation are controlled within the above range, which can not only make the plastic successfully dissolved in water and oiled, but also enable subsequent steam cracking to obtain higher light olefins. By controlling the steam cracking within the above range, higher light olefins can be obtained.

[0020] In the present invention, in order to obtain more low-carbon olefins, the weight ratio of water to plastic is 0.3-5:1, and can be 0.3:1, 0.5:1, 0.7:1, 1:1, 2:1, 3:1, 4:1, 5:1 or a range formed by any two of the above values ​​and a value within the range.

[0021] In the present invention, the weight average molecular weight and / or number average molecular weight of the plastic is not limited, and low carbon olefins can be obtained after supercritical water degradation and steam cracking. The method of the present invention is applicable to heat treatment of various common plastics to obtain low carbon olefins. In actual operation, the smaller the weight average molecular weight and / or number average molecular weight, the longer the treatment time, and the larger the weight average molecular weight and / or number average molecular weight, the longer the treatment time. Preferably, the weight average molecular weight of the polyolefin is 1000-1000000. 0, can be 1000, 10000, 100000, 200000, 300000, 400000, 500000, 600000, 700000, 800000, 900000, 1000000, 2000000, 4000000, 6000000, 8000000, 10000000 or a range formed by any two of the above values, and a value within the range, preferably 50000-300000.

[0022] In the present invention, in the actual preparation process, the higher the weight average molecular weight and number average molecular weight, the more light olefins can be obtained by using higher temperature and time.

[0023] In the present invention, in order to obtain more low-carbon olefins, the plastic includes polyolefins, preferably, the plastic is selected from at least one of polyethylene, polypropylene and polybutylene; or, the polyethylene content in the plastic is not less than 80% by weight, and can be 80% by weight, 85% by weight, 90% by weight, 95% by weight, 100% by weight or a range formed by any two of the above values ​​and a value within the range. The method of the present invention is particularly suitable for polyethylene.

[0024] In the present invention, whether the plastic is newly prepared or recycled waste plastic that has been used for a period of time, more low-carbon olefins can be obtained through the method of the present application.

[0025] In the present invention, in order to obtain more light olefins by subsequent steam cracking, the pressure of the supercritical water degradation is preferably 23-40 MPa.

[0026] In the present invention, in order to obtain more light olefins by subsequent steam cracking, the supercritical water degradation temperature is preferably 380-480°C.

[0027] In the present invention, in order to obtain more light olefins by subsequent steam cracking, the supercritical water degradation time is 1-600 min, which can be 1 min, 3 min, 5 min, 6 min, 7 min, 8 min, 9 min, 10 min, 11 min, 12 min, 13 min, 14 min, 15 min, 16 min, 17 min, 20 min, 30 min, 40 min, 50 min, 51 min, 52 min, 53 min, 54 min, 55 min, 56 min , 57min, 58min, 59min, 60min, min, 61min, 62min, 63min, 64min, 65min, 70min, 75min, 80min, 90min, 100min, 150min, 200min, 250min, 300min, 350min, 400min, 450min, 500min, 550min, 600min or the range formed by any two of the above values, and the value within the range, is preferably 7-250min.

[0028] In the present invention, in order to obtain more light olefins, the steam cracking pressure is 0.01-0.4 MPa.

[0029] In the present invention, in order to obtain more low-carbon olefins and prevent coking so as to shorten the instrument operation cycle, the steam cracking temperature is 750-870°C.

[0030] In the present invention, in order to obtain more light olefins and prevent coking so that the instrument operation cycle is shorter, the steam cracking time is 0.05-0.5s, which can be 0.05s, 0.1s, 0.15s, 0.2s, 0.21s, 0.22s, 0.23s, 0.24s, 0.25s, 0.26s, 0.27s, 0.28s, 0.29s, 0.3s, 0.35s, 0.4s, 0.45s, 0.5s or the range formed by any two of the above values ​​and the value within the range, preferably 0.1-0.3s.

[0031] In the present invention, in order to prevent coking during supercritical water degradation and steam cracking, the supercritical water degradation and / or steam cracking is carried out in the presence of an anti-coking agent.

[0032] In the present invention, in order to better prevent coking, the anti-coking agent accounts for 0.005-0.1 weight % of the weight of the plastic, and can be 0.005 weight %, 0.025 weight %, 0.05 weight %, 0.075 weight %, 0.1 weight % or the range formed by any two of the above values ​​and the value within the range.

[0033] In the present invention, the anti-coking agent is not limited, as long as it can prevent coking during steam cracking. It is preferably a sulfur-containing compound, and more preferably, the sulfur-containing compound includes carbon disulfide and / or dimethyl disulfide.

[0034] Preferably, the method of continuously pressurizing and mixing water and plastic includes: when the plastic is pressurized to 5-50% of the supercritical water degradation pressure, mixing it with water that is 5-50% by weight of the plastic, and then continuing to pressurize the mixture to no less than the supercritical water degradation pressure and mixing it with the remaining water.

[0035] In the present invention, the above-mentioned method is used to mix water and plastic, which can reduce energy consumption by at least 50% compared with directly pressurizing polyolefin to heat treatment pressure and then mixing with all water.

[0036] Preferably, the method further comprises: separating light olefins from the light olefin material. The light olefins in the present invention mainly refer to at least one of unsaturated hydrocarbons with C4 or less. More preferably, the light olefins include at least one of ethylene, propylene and butene.

[0037] In the present invention, there is no limitation on the method for separating the light olefins from the light olefin material, as long as the light olefins can be successfully separated, for example, a cryogenic separation method can be used.

[0038] The second aspect of the present invention provides a device for preparing low-carbon olefins from plastics, wherein the device includes a pressurized mixing system, a reactor and a separation system connected in sequence, and the reactor is divided into a reactor supercritical water degradation section and a reactor steam cracking section connected in sequence.

[0039] Preferably, the continuous pressurized mixing of water and plastic is carried out in a pressurized mixing system; more preferably, the supercritical water degradation is carried out in the supercritical water degradation section of the reactor; further preferably, the steam cracking is carried out in the steam cracking section of the reactor; further preferably, the separation of light olefins from the light olefin material is carried out in a separation system.

[0040] In the present invention, in actual operation, plastic and water are pressurized and mixed through a pressurized mixing system and then introduced into a reactor, successively passing through a supercritical water degradation section of the reactor and a steam cracking section of the reactor, and then passing through a separation system to obtain light olefins.

[0041] The second aspect of the present invention provides the use of the above method in improving the yield of light olefins in the process of producing light olefins from plastics.

[0042] In the present invention, the low-carbon olefins have been described above and will not be described again here.

[0043] The present invention will be described in detail below through examples. The plastics used in the following examples and comparative examples, waste polyethylene B was purchased from Dongguan Fangsen Plastic Chemical Co., Ltd., polyethylene A, polyethylene C, polypropylene D are new materials prepared by the inventor's laboratory, and their GPC test results are shown in Table 1 (Mw is weight average molecular weight):

[0044] Table 1

[0045] plastic M Polyethylene A 127793 Waste recycled polyethylene B 95607 Polyethylene C 4287543 Polypropylene D 397084

[0046] Gas yield test and calculation method: Cool the product to below 5°C, use a flow meter to measure the product gas volume, and use gas chromatography to analyze the product gas composition to obtain the gas density. Gas yield = gas volume × gas density / plastic feed weight;

[0047] Ethylene yield test and calculation method: gas chromatography analysis of ethylene content in product gas, ethylene yield = gas yield × ethylene content;

[0048] Propylene yield test and calculation method: gas chromatography analysis of propylene content in product gas, propylene yield = gas yield × propylene content;

[0049] Butadiene yield test and calculation method: gas chromatography analysis of butadiene content in product gas, butadiene yield = gas yield × butene content.

[0050] Operation cycle: the time from the start of operation to the time when it cannot operate due to blockage.

[0051] Examples 1-9 and Comparative Examples 1-2

[0052] like Figure 1 As shown, the reactor is tubular, sulfur-containing compounds are added to water and polyethylene A, a screw extruder heats the plastic to a molten state, the screw extruder (pressurized mixing system 1) increases the plastic pressure to 10% of the pressure of the supercritical water degradation section 2 of the reactor, and water 10% of the weight of the plastic is added from the side line of the screw. After the screw continues to increase the pressure to the pressure of the supercritical water degradation section 2 of the reactor, the above materials and the remaining water are sent to the supercritical water degradation section 2 of the reactor together, and reacted in the supercritical water degradation section 2 of the reactor and the steam cracking section 3 of the reactor. The obtained product is subjected to deep cold separation by the separation system 3 to obtain light olefins. The parameters and product components of the supercritical water degradation section of the reactor and the steam cracking section of the reactor are shown in Tables 2 and 3.

[0053] Table 2

[0054]

[0055]

[0056] Table 3

[0057]

[0058] " / " means not to be performed or added

[0059] Among them, in Example 4, polyethylene A is replaced with waste recycled polyethylene B.

[0060] In Example 5, polyethylene A was replaced with polyethylene C.

[0061] In Example 6, polyethylene A is replaced with polypropylene D.

[0062] Example 10

[0063] According to the method of Example 1, the difference is that, instead of adding 10% of the weight of water of polyethylene from the screw side line when the polyethylene pressure is increased to 10% of the pressure of the supercritical water degradation section of the reactor, all the water is added to the supercritical water degradation section of the reactor when the polyethylene pressure is increased to the pressure of the supercritical water degradation section of the reactor. The results show that the method of Example 1 reduces the energy consumption of the screw extruder by 50% compared with the embodiment of the present application, but the light olefin yields of the two are similar.

[0064] Comparative Example 3

[0065] The method of Example 1 was followed, except that the temperature of the supercritical water degradation section of the reactor was changed to 360° C. Result: No ethylene, propylene and butadiene were detected in the product.

[0066] Comparative Example 4

[0067] The method of Example 1 was followed, except that the supercritical water degradation was not performed, and steam cracking was performed directly. Result: No ethylene, propylene and butadiene were detected in the product.

[0068] It can be seen from the above results that the method of the present invention has a high yield of light olefins in the light olefin material prepared, and has a long operation cycle and low energy consumption.

[0069] 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 light olefins from plastics, characterized in that: The method comprises: continuously pressurizing and mixing water and plastic, and then sequentially performing supercritical water degradation and steam cracking to obtain a low-carbon olefin material.

2. The method according to claim 1, wherein: The supercritical water degradation conditions include: a temperature higher than 374°C and a pressure higher than 22.1MPa; And / or, the steam cracking conditions include: a temperature of not less than 700° C. and a pressure of not more than 0.5 MPa; And / or, the weight ratio of water to plastic is 0.3-5:1; And / or, the weight average molecular weight of the plastic is 1000-10000000; and / or, the plastic comprises polyolefin; Preferably, the plastic is selected from at least one of polyethylene, polypropylene and polybutylene; Alternatively, the content of polyethylene in the plastic is not less than 80% by weight.

3. The method according to claim 1 or 2, wherein: The supercritical water degradation pressure is 23-40MPa; And / or, the supercritical water degradation temperature is 380-480°C; And / or, the supercritical water degradation time is 1-600 min, preferably 7-250 min; And / or, the steam cracking pressure is 0.01-0.4 MPa; And / or, the steam cracking temperature is 750-870°C; And / or, the steam cracking time is 0.05-0.5 s, preferably 0.1-0.3 s.

4. The method according to claim 1 or 2, wherein: The supercritical water degradation and / or steam cracking is carried out in the presence of an anti-coking agent; Preferably, the anti-coking agent accounts for 0.005-0.1% by weight of the plastic; Preferably, the anti-coking agent is a sulfur-containing compound; More preferably, the sulfur-containing compound includes carbon disulfide and / or dimethyl disulfide.

5. The method according to claim 1 or 2, wherein: The method of continuously pressurizing and mixing water and plastic includes: when the plastic is pressurized to 5-50% of the supercritical water degradation pressure, mixing it with water that is 5-50% by weight of the plastic, and then continuing to pressurize the mixture to no less than the supercritical water degradation pressure and mixing it with the remaining water.

6. The method according to claim 1 or 2, wherein: The method further comprises: separating light olefins from the light olefin material; And / or, the low-carbon olefin includes at least one of ethylene, propylene and butene.

7. A device for preparing low-carbon olefins from plastics, wherein: The device comprises a pressurized mixing system, a reactor and a separation system which are connected in sequence. The reactor is divided into a reactor supercritical water degradation section and a reactor steam cracking section which are connected in sequence.

8. The device according to claim 7, wherein: The continuous pressurized mixing of water and plastic as described in any one of claims 1 to 6 is carried out in a pressurized mixing system; And / or, the supercritical water degradation in any one of claims 1 to 6 is carried out in a supercritical water degradation section of a reactor; and / or, the steam cracking in any one of claims 1 to 6 is carried out in a steam cracking section of a reactor; And / or, the separation of light olefins from the light olefin material in claim 6 is carried out in a separation system.

9. Use of the method described in any one of claims 1 to 6 and / or the device described in any one of claims 7 to 8 in improving the yield of light olefins in a process for producing light olefins from plastics.

10. The use according to claim 9, wherein: The low-carbon olefin includes at least one of ethylene, propylene and butene.

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