A process for the production of naphtha from polyolefins by hydrocracking

Abstract

The present application belongs to the technical field of polyolefin catalytic conversion, and relates to a process for preparing naphtha by polyolefin hydrocracking. In a closed state, hydrogen is dispersed into micro-bubbles by a membrane tube and then enters polyolefin in a molten state, and a hydrocracking reaction is carried out under the action of a catalyst. A Ni / Beta catalyst is used, and the polyolefin and the Ni / Beta catalyst are filled in the reaction kettle in the form of mixed fillers. The reaction product is continuously produced and separated into gas and liquid, and the liquid product is subjected to fractionation to obtain liquefied petroleum gas and naphtha.

Description

Technical Field

[0001] This invention belongs to the field of polyolefin catalytic conversion technology, and relates to a process for preparing naphtha by hydrocracking of polyolefins, and more particularly to a process for preparing naphtha by hydrocracking of polyolefins using metal membrane microbubbles to enhance hydrogen mass transfer, thereby realizing the high-value utilization of waste plastics. Background Technology

[0002] With the continuous growth of plastic consumption, large amounts of waste polyolefins (such as polyethylene and polypropylene) are putting serious pressure on the environment. To achieve high-value utilization of these resources, polyolefin catalytic cracking and hydrocracking technologies have gradually become research hotspots. In existing technologies, most polyolefin hydrocracking reactions are carried out in batch reactors or fixed-bed reactors. Batch reactors are simple to operate and offer good controllability, but the low mass transfer efficiency of hydrogen in high-viscosity polyolefin melts leads to insufficient catalyst activity, and unsatisfactory naphtha yield and selectivity. While fixed-bed reactors can achieve continuous operation, feeding is difficult for high-viscosity polyolefin systems, resulting in poor material mass transfer and flowability, easily causing increased bed pressure drop and uneven local heat transfer. This necessitates larger equipment sizes and higher operating energy consumption, leading to large-scale plants and increased investment and energy consumption.

[0003] Currently, polyolefin hydrocracking suffers from low hydrogen mass transfer efficiency and high energy consumption. Furthermore, the continuous generation of reaction products mixes with the unreacted, high-viscosity polymer system, leading to further deep cracking or coking of the products. This reduces the selectivity of the target naphtha fraction. Additionally, the catalyst is prone to deactivation and exhibits low stability in this reaction system, and no ideal production process has yet been reported. Moreover, waste plastic feedstocks often contain moisture, which may affect the acidity of the catalyst and the active metal sites during the reaction, promoting side reactions or accelerating deactivation, further exacerbating the instability of the system. Summary of the Invention

[0004] This invention addresses the problems of low hydrogen mass transfer efficiency, easy deep cracking of products, and easy deactivation of catalysts in traditional polyolefin hydrocracking by proposing a novel process for the preparation of naphtha by polyolefin hydrocracking.

[0005] To achieve the above objectives, the present invention is implemented using the following technical solution: A process for producing naphtha via polyolefin hydrocracking involves, in a closed system, dispersing hydrogen gas into microbubbles through a membrane tube and introducing it into the molten polyolefin. The hydrocracking reaction then proceeds under the action of a catalyst. The polyolefin and catalyst are packed in a reactor as a mixed packing material. A back pressure valve is installed at the reactor outlet. The reaction products are continuously collected through the back pressure valve and separated into liquid and liquid phases. The liquid phase product is then fractionated to obtain liquefied petroleum gas (LPG) and naphtha. A typical process is as follows: a stainless steel reactor is used as the reactor. A metal membrane hydrogen supply assembly is installed inside the reactor. The metal membrane is fixedly installed by the stainless steel assembly. One end of the assembly is open, serving as a gas phase inlet connected to the hydrogen supply system, while the other end is sealed and submerged below the liquid surface in the reactor. A heating and insulation assembly is installed at the reactor outlet to ensure that the product is in a gaseous state before being collected from the reactor, preventing outlet blockage. A back pressure valve is installed so that the continuously introduced hydrogen gas can act as a carrier gas after reaction, continuously carrying out the gaseous products. After being cooled by a condenser, the high-boiling-point naphtha will condense and be stored. The gaseous products that cannot be condensed will be separated, realizing the continuous extraction, separation and collection of products, preventing deep cracking of products. At the same time, the back pressure valve can also play a role in stabilizing pressure, ensuring the stability of product quality.

[0006] Preferably, the membrane tube is a metal membrane; the pore size of the metal membrane is 0.05-100 μm. In high-pressure polyolefin hydrocracking systems, ceramic membranes have relatively poor mechanical strength and pressure differential resistance, making it difficult to operate stably under high-viscosity melt and three-phase flow conditions.

[0007] Preferably, the reaction temperature is 260-300℃, the hydrogen pressure is 2-5MPa, and the reaction time is 3-30h.

[0008] Preferably, the hydrogen pressure is 2-5 MPa and the stirring rate during the reaction is 200-600 rpm.

[0009] Preferably, the hydrogen gas is dispersed into the molten polyolefin through a membrane tube, participates in the reaction, and then acts as a carrier gas to carry out the product. The high-boiling-point liquid product and the low-boiling-point gaseous product are separated by condensation. By continuously introducing hydrogen gas, the product can be continuously extracted, separated, and collected.

[0010] Preferably, nitrogen is turned on and preheated before hydrogen is introduced to remove moisture from the polyolefin and catalyst. This avoids competitive adsorption of water on acid sites and metal active sites, as well as the impact of the hydrothermal process on the zeolite structure, thereby improving catalytic activity and reaction stability.

[0011] This invention proposes a process for producing naphtha via polyolefin hydrocracking with enhanced gas-liquid mass transfer. The core of this process lies in utilizing a metal membrane to uniformly disperse hydrogen into microbubbles within a high-viscosity polyolefin melt, thereby overcoming the diffusion resistance and gas-liquid distribution bottlenecks inherent in traditional reactors. Simultaneously, a back-pressure valve enables the dynamic and continuous removal of light gas-liquid products, ensuring system pressure stability and, more importantly, eliminating competitive adsorption and self-inhibition effects of accumulated products at the catalytic interface. This prevents deep cracking caused by prolonged product residence, thus improving the selectivity of the target product, naphtha. Furthermore, continuous removal of low-boiling products helps maintain a stable reaction driving force and concentration gradient, avoiding over-cracking or limited mass transfer caused by the accumulation of light hydrocarbons. This promotes the reaction towards high-value-added liquid products, improving overall conversion efficiency and process intensification. The proposed reaction system utilizes a bifunctional catalyst of nickel and Beta molecular sieves. It efficiently catalyzes macromolecular chain scission at acidic sites and rapidly hydrogenates and saturates newly generated active olefin intermediates at metal sites, thus inhibiting secondary polymerization of olefins and catalyst coking at the source. The high degree of stability of the entire reaction system depends on the deep synergy of multiple parameters: the metal membrane pore size needs to be appropriate to balance the transmembrane pressure difference and the specific surface area of ​​the bubbles, preventing blockage caused by excessively small pores or bubble coalescence and deactivation caused by excessively large pores; the reaction pressure and hydrogen flow rate need to be closely matched to provide sufficient hydrogen dissolution and avoid excessively high local gas density; the stirring speed needs to ensure sufficient collisions between bubbles, melt, and catalyst particles without inducing high-energy-consuming eddies. The coupled process of microbubble mass transfer enhancement and dynamic product stripping proposed in this invention improves the utilization efficiency of hydrogen and catalyst, significantly shortens the reaction cycle, and provides a new approach for the continuous industrial production of high-yield naphtha.

[0012] Compared with the prior art, the advantages and positive effects of the present invention are as follows: 1. This invention uses a metal film to uniformly disperse hydrogen gas in the form of micron-sized bubbles in a high-viscosity polyolefin melt, which significantly increases the gas-liquid contact surface area and prolongs the gas residence time. This solves the problems of uneven gas-liquid distribution and mass transfer resistance in high-viscosity systems, enabling the reaction process to be controlled by kinetics rather than mass transfer, thereby improving the effective utilization rate of hydrogen gas and the overall reaction rate.

[0013] 2. By employing a backpressure valve to achieve continuous removal of products during the reaction process, the traditional batch hydrocracking process exhibits "quasi-continuous reaction" characteristics. This method can effectively reduce product residence and secondary reactions on the catalyst surface, reduce competitive adsorption and self-inhibition effects, thereby improving the selectivity of the target product.

[0014] 3. The uniform and stable distribution of microbubbles in the reaction system ensures a continuous supply of hydrogen to the catalytic interface, maintains the stable activity of the catalyst, effectively reduces coking and deactivation, and extends the catalyst's lifespan and replacement cycle. Simultaneously, it effectively mitigates the risks of pressure and temperature fluctuations during the reaction process, improving the overall safety of the system.

[0015] 4. The dynamic product removal mode of this invention breaks the limitation of long-term product retention in traditional batch reactors and provides a technical basis for the smooth transition of the process to continuous or semi-continuous production, and has excellent prospects for industrial application. Attached Figure Description

[0016] Figure 1 Schematic diagram of the reaction apparatus.

[0017] Figure 2 The reaction results of membrane tubes with different pore sizes.

[0018] Figure 3 The reaction results at different hydrogen flow rates.

[0019] The attached figures are labeled as follows: 1 Hydrogen cylinder, 2 Nitrogen cylinder, 3 Rupture disc, 4 Reactor, 5 Membrane module, 6 Thermocouple, 7 Controller, 8 Condensation device, 9 Back pressure valve, 10 Storage tank, 11 Gas outlet, 12 Liquid outlet. Detailed Implementation

[0020] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described below with reference to specific embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in these embodiments can be combined with each other.

[0021] Numerous specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways than those described herein, and therefore the invention is not limited to the specific embodiments disclosed in the following specification.

[0022] The following embodiment uses a stainless steel reactor 4 equipped with a heating jacket and a stirrer as the reactor. The reactor has a hydrogen inlet and an inert gas inlet, which are connected to a hydrogen cylinder 1 and a nitrogen cylinder 2, respectively. Valves and flow meters are installed on the inlet pipelines to achieve precise flow regulation and prevent reactants from flowing back into the gas supply system. Inside the reactor are a membrane module 5, a stirrer, and a thermocouple 6. The upper end of the membrane module 5 is threadedly connected to the hydrogen pipeline outlet, and the lower end is welded to a metal membrane tube, which is sealed at the bottom. The signal output of the controller 7 is connected to the switches of the stirrer and thermocouple 6 to adjust reaction parameters and ensure stable reaction. Other valves or flow meters can be controlled individually or integrated into the controller, depending on the actual process requirements; these details are not elaborated here. The reactor has a gas outlet at the top, with a needle valve, heating belt, condenser 8, back pressure valve 9, gas-liquid separator 10 (storage tank), and liquid outlet 12 connected sequentially to the gas outlet pipeline. The pressure of the reaction system is stabilized by adjusting the opening of the back pressure valve. The gas outlet is connected to a gas collection bag 11 (the collected gas is analyzed by chromatography to calculate the gaseous products; in actual production, a tail gas treatment device can be installed according to the specific process). The reactor is also equipped with a rupture disc 3 and a pressure gauge to monitor the reaction pressure in real time and automatically release pressure when it rises abnormally, ensuring safe operation of the equipment.

[0023] After nitrogen purging, the system is heated to the preheating temperature and stirred to remove moisture adsorbed on the catalyst and polyolefin surface. Then, the temperature is raised to the reaction temperature, hydrogen is introduced and pressurized, and the hydrogen flow rate is controlled by a mass flow meter, allowing hydrogen to permeate through the metal membrane to form microbubbles that are highly dispersed into the polyolefin melt. Once the system stabilizes, the reaction temperature and pressure are adjusted, and the pressure inside the reactor is stabilized by a back pressure valve, enabling continuous removal of gaseous and liquid products for hydrocracking under the action of a Ni / Beta catalyst. After the reaction, hydrogen supply is stopped, and stirring is turned off when the system temperature drops to 200 °C. The reaction products are separated in a gas-liquid separator. The gaseous products can be collected or discharged, while the liquid products are fractionated to obtain liquefied petroleum gas and naphtha fractions, with the naphtha fraction being C5–C12 alkanes. Through this process, hydrogen microbubbles are uniformly distributed in the polyolefin melt, allowing the catalyst activity to be fully utilized, achieving highly efficient hydrocracking and a significant increase in naphtha yield, while maintaining controllable process conditions and stable operation. Example 1

[0024] This embodiment uses a single-channel 316L stainless steel membrane tube (Hebei Baitai Filtration Technology Co., Ltd.) with an average pore size of 1 μm, a membrane tube length of 12.5 mm, an outer diameter of 17.3 mm, and a reactor volume of 2 L (Beijing Dongfang Shenglongda Technology Co., Ltd.). First, 20 g of Ni / Beta catalyst (Beta zeolite, Shanghai Xinnian Petrochemical Additives Co., Ltd.) was uniformly mixed with 600 g of polyethylene powder (PE, Honeywell International, Mw~8500). The reactor was then closed and its seal checked. The nitrogen inlet valve was opened, and nitrogen gas was introduced at a flow rate of 100 mL / min. Simultaneously, the gas outlet valve was opened for venting, and the process was continuously purged for 20 min to fully replace the air inside the reactor and create an inert atmosphere. The temperature was then increased to 120 °C at a rate of 5 °C / min, and stirring was started at 200 rpm. This temperature was maintained for 10 min to remove adsorbed moisture and volatile impurities from the catalyst and polyolefin surface. Nitrogen gas was then stopped, and hydrogen gas was introduced at a rate of 800 mL / min to 4.0 MPa. This allowed hydrogen to permeate the metal membrane, forming micron-sized microbubbles that were highly dispersed into the high-viscosity polyolefin melt, significantly enhancing the gas-liquid mass transfer process. The temperature was further increased to the reaction temperature of 250 °C, while the stirring speed was increased to 400 rpm to ensure complete melting of the polyethylene and thorough mixing with the catalyst, forming a homogeneous melt system. After the system stabilized, the temperature inside the reactor was controlled at 280 °C, and the hydrogen flow rate was adjusted to 400 mL / min. After the system had been running stably for 20 min, the pressure inside the reactor was stabilized at 5.0 ± 0.1 MPa by adjusting the back pressure valve opening. The light hydrocarbon gas and some liquid products generated during the reaction are continuously removed from the gas outlet under the action of hydrogen. After being cooled by the condenser, they enter the gas-liquid separation device, realizing a dynamic product removal process similar to a continuous reaction system.

[0025] Hydrocracking was carried out under Ni / Beta catalyst for 7 hours. After the reaction, hydrogen supply was stopped, and the system was allowed to cool naturally. Stirring was stopped when the system temperature reached 200 °C, and the system was further cooled to room temperature. Gas products were collected through the pipeline containing the back pressure valve, and liquid products were collected by opening the reactor. The liquid products collected in the gas-liquid separation unit were analyzed by online gas chromatography. The gas products collected at the back pressure valve outlet were fractionated at atmospheric pressure to obtain liquefied petroleum gas (LPG) fraction and trace amounts of naphtha fraction, with the naphtha mainly consisting of C5-12 alkanes. Gas chromatography analysis of the liquid products, based on the PE feed rate, yielded: 108.6 g of LPG (18.1%), 318 g of naphtha (53.0%), and 162.6 g of solid residue (27.1%). Example 2

[0026] Unless otherwise specified, this embodiment and the following embodiments are consistent with Embodiment 1.

[0027] First, 20 g of Ni / Beta catalyst was uniformly mixed with 600 g of polyethylene powder. After purging with nitrogen for 20 min, the temperature was increased to 120 °C at a rate of 5 °C / min. Stirring was then started at 200 rpm and maintained at this temperature for 10 min. Nitrogen was then stopped, and hydrogen was introduced at a rate of 800 mL / min to a pressure of 4.0 MPa, allowing hydrogen to permeate through a 0.1 μm metal membrane. The temperature was further increased to 250 °C, while the stirring speed was increased to 400 rpm. The temperature inside the reactor was controlled at 280 °C, and the hydrogen flow rate was maintained at 400 mL / min. After 20 min, the pressure inside the reactor was stabilized at 5.0 ± 0.1 MPa by adjusting the back pressure valve opening. After 7 h of reaction, hydrogen supply and heating were stopped. The reactor was allowed to cool naturally to 200 °C, stirring was stopped, and the mixture was allowed to cool further to room temperature before the liquid product was collected. According to the test results, this embodiment yielded 139 g of liquefied petroleum gas (23.0% yield), 299.3 g of naphtha (49.9% yield), and 143.9 g of solid residue (24.0% yield). Example 3

[0028] First, 20 g of Ni / Beta catalyst was uniformly mixed with 600 g of polyethylene powder. After purging with nitrogen for 20 min, the temperature was increased to 120 °C at a rate of 5 °C / min, and stirring was started at 200 rpm. This temperature was maintained for 10 min, after which nitrogen purging was stopped, and hydrogen was introduced at 4.0 MPa, allowing hydrogen to permeate through a 1 μm metal membrane. The temperature was further increased to the reaction temperature of 250 °C, while the stirring speed was increased to 400 rpm. After the system stabilized, the temperature inside the reactor was controlled at 280 °C, and the hydrogen flow rate was 600 mL / min. After 20 min, the pressure inside the reactor was stabilized at 5.0 ± 0.1 MPa by adjusting the back pressure valve opening. After 7 h of reaction, hydrogen purging and heating were stopped, and the mixture was allowed to cool naturally to 200 °C. Stirring was then stopped, and the mixture was allowed to cool further to room temperature before the reactor was opened to collect the liquid product. The results of this embodiment show that the following were obtained: 119.6 g of liquefied petroleum gas (19.9% ​​yield), 280.3 g of naphtha (46.7% yield), and 162.2 g of solid residue (28.0% yield). Example 4

[0029] First, 20 g of Ni / Beta catalyst was uniformly mixed with 600 g of polyethylene powder. Nitrogen gas was introduced for 20 min, and the temperature was increased to 120 °C at a rate of 5 °C / min. Stirring was then started at 200 rpm and maintained at this temperature for 10 min. Nitrogen gas was then stopped, and hydrogen gas was introduced at 4.0 MPa, allowing it to permeate through a 1 μm metal membrane. The temperature was then increased to the reaction temperature of 250 °C, while simultaneously increasing the stirring speed to 600 rpm. After the system stabilized, the temperature inside the reactor was controlled at 280 °C, and the hydrogen flow rate was adjusted to 400 mL / min. After the system had been running stably for 20 min, the pressure inside the reactor was stabilized at 5.0 ± 0.1 MPa by adjusting the back pressure valve opening. After 7 hours of reaction, hydrogen gas was stopped, followed by heating. The mixture was allowed to cool naturally to 200 °C, at which point stirring was stopped. After further cooling to room temperature, the reactor was opened to collect the liquid product. Analysis showed that this example yielded 114.3 g of liquefied petroleum gas (19.1% yield), 335.0 g of naphtha (55.8% yield), and 145.7 g of solid residue (24.3% yield).

[0030] Comparative Example 1 Without installing a metal membrane tube, hydrogen gas was directly introduced into the reactor through a pipeline, and the pressure was controlled by a back pressure valve. All other conditions were the same as in Example 1. Testing showed that the liquefied petroleum gas yield was 7.9%, the naphtha yield was 29.1%, and the solid residue was 61.9%.

[0031] Comparative Example 2 No back pressure valve was installed at the outlet of the reaction system, and the reaction products were not dynamically removed during the reaction process. The remaining reaction conditions were the same as in Example 1. Testing showed that the liquefied petroleum gas yield was 5.3%, the naphtha yield was 16.3%, and the solid residue yield was 74.7%.

[0032] Comparative Example 3 The stirring speed was 200 rpm, and all other conditions were the same as in Example 1. Testing showed that the liquefied petroleum gas yield was 8.6%, the naphtha yield was 30.5%, and the solid residue was 59.5%.

[0033] Comparative Example 4 The reaction pressure was 2 MPa, and all other conditions were the same as in Example 1. Testing showed that the liquefied petroleum gas yield was 5.3%, the naphtha yield was 22.3%, and the solid residue was 70.4%.

[0034] Comparative Example 5 The hydrogen flow rate was 200 mL / min, and the other conditions were the same as in Example 3. The results showed a liquefied petroleum gas yield of 7.2%, a naphtha yield of 28.7%, and a solid residue of 60.3%.

[0035] Comparative Example 6 The catalyst / polyolefin mass ratio was 1:100 (600 g PE, 6 g catalyst), and the reaction time was extended to 22 h (to ensure complete reaction). All other conditions were the same as in Example 1. The results showed a liquefied petroleum gas yield of 16.3%, a naphtha yield of 51.3%, and a solid residue of 27.8%.

[0036] Comparative Example 7 The difference between this comparative example and Example 1 is that the catalyst and raw materials were not preheated for dehydration, and hydrogen was directly introduced after nitrogen was introduced. The rest of the operation process was the same as in Example 1. The results showed that the liquefied petroleum gas yield was 10.2%, the naphtha yield was 30.5%, and the solid residue was 57.9%.

[0037] The reaction results of Example 1 (1 μm), Example 2 (0.1 μm), Comparative Example 1 (without membrane tube), and 10 μm, 60 μm, and 1 / 4 inch membrane tubes were summarized. The remaining operating conditions for the 10 μm, 60 μm, and 1 / 4 inch membrane tubes were the same as in Example 1. The results are as follows: Figure 2 As shown, from Figure 2 As can be seen, the smaller the pore size of the membrane, the better the reaction promotion effect, but if the membrane pore size is too small, it will lead to an increase in the production of liquefied petroleum gas.

[0038] The reaction results of Example 1 (400 mL / min), Example 3 (600 mL / min), Comparative Example 5 (200 mL / min), and hydrogen flow rates of 300 mL / min and 500 mL / min were summarized. The operating parameters for 300 mL / min and 500 mL / min were the same as in Example 3. The results are as follows: Figure 3 As shown in the figure, the hydrogen rate is not linearly related to the reaction efficiency; there is a plateau in the hydrogen rate, and the conversion rate remains almost constant after the rate exceeds 400 mL / min.

[0039] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any person skilled in the art may make changes or modifications to the above-disclosed technical content to create equivalent embodiments for application in other fields. However, any simple modifications, equivalent changes, and modifications made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A process for preparing naphtha by hydrocracking of polyolefins, characterized in that, Hydrogen gas is dispersed into microbubbles through a membrane tube and enters the molten polyolefin, where it undergoes a hydrocracking reaction under the action of a catalyst. The polyolefin and the catalyst are packed in the reaction device in the form of a mixed packing. A back pressure valve is installed in the outlet pipeline of the reaction device. After gas-liquid separation, the gaseous products are continuously extracted through the back pressure valve, and the liquid products are fractionated to obtain liquefied petroleum gas and naphtha.

2. The process for preparing naphtha by hydrocracking of polyolefins according to claim 1, characterized in that, The membrane tube is a metal membrane; the pore size of the metal membrane is 0.05-100μm.

3. The process for preparing naphtha by hydrocracking of polyolefins according to claim 1, characterized in that, The reaction temperature is 260-300℃, the hydrogen pressure is 2-5MPa, and the reaction time is 3-30h.

4. The process for preparing naphtha by hydrocracking of polyolefins according to claim 3, characterized in that, The stirring rate during the reaction is 200-600 rpm.

5. The process for preparing naphtha by hydrocracking of polyolefins according to claim 1, characterized in that, After participating in the reaction, the hydrogen gas acts as a carrier gas to carry out the products. The high-boiling-point liquid products are separated from the low-boiling-point gaseous products by condensation. The products are continuously extracted by continuously introducing hydrogen gas.

6. The process for preparing naphtha by hydrocracking of polyolefins according to claim 1, characterized in that, Before introducing hydrogen, nitrogen is turned on and preheated to remove moisture from the polyolefin and catalyst.

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