A method for pre-oxidizing a cyclic hydrocarbon to promote cracking

Abstract

The present invention discloses a method for pre-oxidizing cyclic hydrocarbons to promote cracking. In the present invention, oxygen or air is used to partially pre-oxidize and convert cyclic hydrocarbons at a temperature lower than the boiling point of the cyclic hydrocarbons under pressurized conditions, and the conversion rate of the cyclic hydrocarbon raw material after the pre-oxidation operation does not exceed 15%; the liquid-phase product obtained after the pre-oxidation operation is used for cracking. The present invention adopts a method of pre-oxidizing cyclic hydrocarbons to convert part of the cyclic hydrocarbons into cyclic hydrocarbon derivatives with oxygen-containing functional groups. The cyclic hydrocarbon derivatives with oxygen-containing functional groups can be cracked first to generate active free radicals during the cracking process due to their lower bond energy C-O or O-O chemical bonds, thereby promoting the ring-opening cracking of the remaining cyclic hydrocarbons and greatly improving the conversion rate of cyclic hydrocarbons; the present invention has the advantages of strong production capacity, low use cost, long operation cycle, low energy consumption, and less carbon deposition.

Description

Technical Field

[0001] The invention belongs to the field of petroleum hydrocarbon steam cracking, relates to high value-added utilization of cyclic hydrocarbons, and specifically provides a method for pre-oxidation of cyclic hydrocarbons to promote cracking. Background Art

[0002] Oil has always been the main resource for human survival, known as the blood of modern industry, and plays an irreplaceable role in the national economy. However, recoverable oil is developing in the direction of heavy and inferior quality. The scientific utilization of heavy oil resources has become an urgent task facing the world's oil industry.

[0003] Heavy oil refers to components in conventional crude oil with a boiling point greater than 500°C or a relative density greater than 0.934. In a broad sense, it is also used to refer to the non-volatile oil products remaining after gasoline and diesel are extracted during crude oil processing, which have the characteristics of large molecular weight and high viscosity. According to the molecular configuration, heavy oil can be divided into aromatic heavy oil and alkane heavy oil; aromatic heavy oil is mainly composed of aromatic hydrocarbons and cycloalkanes, and alkane heavy oil is mainly composed of aliphatic hydrocarbons; among them, the density and viscosity of aromatic heavy oil are usually higher than those of alkane heavy oil.

[0004] Due to its relatively stable molecular structure, heavy aromatic hydrocarbon oil usually requires higher temperature / energy during the cracking process, thus showing high cracking inertness. Therefore, it needs to go through a series of processing steps during the refining process to convert it into more valuable products or improve its quality. Several common processing methods are introduced as follows: (1) Cracking: decomposing it into low-carbon olefins and light oil products (such as gasoline, fuel oil, etc.) through catalytic cracking, hydrocracking, etc.; (2) Coking: sending it to the coking furnace to produce high-value-added products such as coke; (3) Solvent extraction: utilizing the specific solubility of certain components to directly separate and extract them through solvent extraction; (4) Deep processing: through various chemical reactions and processing steps, such as alkylation, light hydrocarbonization, ring opening, etc., it is converted into specific chemicals or high-value-added products, such as lubricating oil, fragrance, fuel, etc. Among them, cracking is the most commonly used processing method in refineries. In response to different oil products and different product demands, the industry has successively developed catalytic cracking technology (DCC-I) for producing more propylene, catalytic cracking technology (DCC-II) for producing more propylene and high-quality gasoline, catalytic thermal cracking (CPP) technology for maximizing the production of ethylene and propylene, and selective catalytic cracking (MCP) technology.

[0005] Chinese Patent CN02144644.X proposes a method for producing light olefins by gas-phase oxidative cracking of hydrocarbons and co-producing carbon monoxide; in this method, the raw material hydrocarbons are vaporized and mixed with oxygen or air under atmospheric pressure, and the oxidative cracking reaction of hydrocarbons is carried out at a temperature of 600-950 °C. With the help of the action of oxygen, not only can paraffins be cracked to produce light olefins at a lower temperature, but also naphthenes can be cracked by ring opening to produce light olefins, thereby improving the cracking conversion rate of the raw material hydrocarbons and the yield of products such as olefins. This method has a significantly higher cracking conversion rate compared to the thermal cracking process and avoids a series of disadvantages in the use of catalysts compared to catalytic cracking.

[0006] However, the following problems exist in the above method, which limit its possibility of large-scale industrial application:

[0007] Active oxygen is introduced into the cracking system in the form of oxygen molecules, resulting in the promotion of dehydrogenation of hydrocarbon molecules to form alkyl radicals while experiencing the combustion reaction of oxygen and raw material hydrocarbons and finally generating small molecule substances such as CO and CO2. This indicates that the enhanced conversion of some raw materials is essentially ineffective; in addition, oxygen will directly react with the cracking products (especially light olefins such as ethylene, propylene, and methane), thereby reducing the selectivity of these products; the patent shows that the selectivity of CO obtained by cracking using this method reaches 15%, and that of CO2 reaches 1%. Summary of the Invention

[0008] The present invention aims to solve the problems of the prior art and provides a method for pre-oxidizing cyclic hydrocarbons to promote cracking. The present invention has the advantages of strong production capacity, low use cost, long operation cycle, low energy consumption, and less carbon deposition.

[0009] The present invention provides a method for pre-oxidizing cyclic hydrocarbons to promote cracking, which includes:

[0010] In a high-pressure reactor, at a temperature below the boiling point of the cyclic hydrocarbon, the cyclic hydrocarbon is subjected to a liquid-phase pre-oxidation operation using an oxidizing gas to partially convert it into a cyclic hydrocarbon derivative with oxygen-containing functional groups;

[0011] The liquid-phase product obtained after the pre-oxidation operation is cracked.

[0012] Furthermore, the cyclic hydrocarbon used for liquid-phase pre-oxidation refers to one or more of monocyclic, bicyclic, polycyclic hydrocarbons and corresponding monocyclic, bicyclic, polycyclic hydrocarbons with alkyl side chains.

[0013] Furthermore, the oxidizing gas used is oxygen or air.

[0014] According to the embodiments of the present invention, the starting pressure of the pre-oxidation operation shall not be lower than 3 bar, preferably between 10-30 bar.

[0015] As a preferred embodiment of the present invention, the pre-oxidation temperature needs to be controlled to be more than 20 °C lower than the boiling point of the raw material hydrocarbon under the reaction pressure; if the raw material consists of two or more cyclic hydrocarbons, the lowest boiling point shall be taken as the standard.

[0016] As a preferred embodiment of the present invention, the conversion rate of the cyclic hydrocarbon raw material after the pre-oxidation operation does not exceed 15%, preferably 5-12%.

[0017] Furthermore, the stirring speed of the reaction kettle is 100-800 rpm, preferably 200-600 rpm.

[0018] As a preferred embodiment of the present invention, the liquid-phase product obtained after the pre-oxidation operation does not need further treatment and can be directly used for the cracking reaction.

[0019] As a preferred embodiment of the present invention, the cracking of the present invention can be steam cracking, catalytic cracking or cracking based on other process routes. It should be noted that due to the liquid-phase pre-oxidation operation of the present invention, the product is given better cracking activity, and thus more high-value chemicals including trienes (ethylene, propylene, 1,3-butadiene) and tribenzene (benzene, toluene, xylene) can be generated during the cracking process. Here, steam cracking can be directly adopted in the cracking process without adding a cracking catalyst. Of course, the initiation promotion mechanism and the catalytic promotion mechanism of the present invention are not contradictory, so catalytic cracking is also applicable to the present invention.

[0020] As a preferred embodiment of the present invention, if the cracking is steam cracking, the cracking temperature is 600-800 °C, the residence time is 0.1-1.0 s, and the water-oil ratio is 0.2-2.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] 1. In the existing gas-phase oxidative cracking, gaseous oxygen molecules may directly react with hydrocarbon molecules at high temperatures. This part of the reaction is inevitable and coexists with the gas-phase oxidative cracking reaction, resulting in the loss of raw materials; in addition, in gas-phase oxidative cracking, oxygen will react with the cracked small molecules in a similar way to generate gases such as CO and CO2, consuming part of the products and converting them into waste gases; the present invention is a liquid-phase pre-oxidation operation, and active oxygen is attached to the raw material molecule to be treated in the form of a functional group (O2+R-H = R-O-O-H), which greatly avoids the loss of raw materials and high-value products while promoting cracking conversion, and reduces the emissions of CO and CO2.

[0023] 2. Compared with the existing steam cracking technology, the oxygen-containing functional groups generated by the liquid-phase pre-oxidation in the present invention can combine with the carbon atoms on the tube wall, which can remove carbon deposits to a certain extent, thereby reducing the coking degree of the furnace tubes, extending the operation cycle of the device, and reducing the maintenance cost.

[0024] 3. The pre-oxidation operation has low cost, less additional investment and is easy to implement. Brief Description of the Drawings

[0025] Figure 1 is a high-pressure stirring reactor used for pre-oxidation.

[0026] Figure 2 is a jacket heating type cracking evaluation device; in the figure: 1, heavy oil feed storage tank; 2, water feed storage tank; 3, heavy oil feed pump; 4, water feed pump; 5, heating jacket; 6, preheating furnace; 7, cracking furnace; 8, gas-liquid separation tank; 9, coil type water bath cooling tank; 10, water washing tank; 11, shell and tube heat exchanger; 12, aerosol adsorption tube; 13, low-temperature circulating water cooling. Specific Implementation Method

[0027] Next, in combination with the embodiments of the present invention, the technical solutions of the present invention will be clearly and completely described. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0028] Embodiment 1

[0029] Pre-oxidation process: Pour 1L of decalin (cis-trans ratio is 4:6) into a 2L high-pressure stirring autoclave ( Figure 1 ), then close the autoclave and displace the air in the autoclave with nitrogen (99.999%) three times and then evacuate the nitrogen. Fill the autoclave with pure oxygen (99.999%) until the pressure in the autoclave shows 10 bar and then stop filling with oxygen. Turn on the stirring paddle to make the oxygen in the autoclave reach dissolution equilibrium, and at this time the pressure in the autoclave will decrease. Repeat the same operation to replenish oxygen 2 - 3 times until the pressure in the autoclave stabilizes at 10 bar. Then start heating to 130 °C, adjust the stirring speed to 500 rpm, and wait for the pre-oxidation reaction to start. When the pressure in the autoclave is continuously lower than 9.8 bar for 30 s, it is regarded as the formal start of the pre-oxidation reaction. Since this reaction is an exothermic reaction, it is necessary to continuously monitor and control the temperature in the autoclave to keep it stable between 130 ± 1 °C after the reaction officially starts. When the pressure in the autoclave drops to 2 bar, start the cooling program and reduce the stirring speed to 300 rpm. When the temperature in the autoclave drops to at least 80 °C or below, evacuate the gas in the autoclave and open the autoclave to take samples. The obtained pre-oxidation product is named pre-oxidized decalin - 10 bar. By this method, only by adjusting the initial reaction pressure of the autoclave, pre-oxidized decalin - 10 bar, pre-oxidized decalin - 20 bar, pre-oxidized decalin - 30 bar, and pre-oxidized decalin - 40 bar can be obtained respectively.

[0030] Product conversion rate analysis: Use gas chromatography to analyze the product conversion rates of several pre-oxidation products in this embodiment, and the analysis results are shown in Table 1 for reference. It can be seen that as the degree of pre-oxidation (the increase of pre-oxidation pressure) deepens, the conversion rate of decalin gradually increases. When the initial pre-oxidation pressure is 40 bar, the conversion rate of decalin reaches 17.42%, exceeding the preset conversion rate threshold (15%).

[0031] Summary Table of Analysis of Conversion Rate of Pre-oxidation Products

[0032]

[0033] Use Figure 2 The device is used to compare the cracking conversion rates and the yields of main cracking products of pure decalin and several pre-oxidized decalins in this example. The cracking conditions are as follows: Liquid feed: 2 g / min; Steam: 80 mL / min; Preheating section temperature settings (six zones): 200, 250, 300, 350, 400, 400 °C; Cracking section temperature settings (five zones): all 650 °C. The cracking conversion rates and the yields of main products are respectively shown in Table 2 and Table 3. It can be seen from the analysis of Table 2 that the net conversion rate of pre-oxidized decalin cracking (cracking conversion rate minus pre-oxidized decalin conversion rate) is higher than that of pure decalin. The reasons are as follows: During the pre-oxidation process, decalin molecules and oxygen molecules combine to form an R-O-O-H molecular structure. Since the bond energy of the peroxy bond is lower than that of the carbon-carbon bond and the carbon-hydrogen bond, it will break first during the cracking process to form R-O· and ·O-H. These two free radicals will further participate in the hydrogen abstraction reaction to promote cracking conversion. At the same time, it is found that with the further deepening of the pre-oxidation degree of the raw material, the promotion effect of cracking conversion will reverse. The possible reason for the analysis is that with the further deepening of the pre-oxidation degree, the R-O-O-H structure will be converted into an R=O structure, resulting in a decrease in the cracking initiation effect; at the same time, with the deepening of the reaction degree, it may lead to the combination of two or more single rings to form polycyclic compounds, and the formation of polycyclic compounds is not conducive to cracking conversion to produce high-value-added small molecule substances. The promotion effect of the pre-oxidation operation on cracking is not only reflected in the increase of the cracking conversion rate, but also reflected in the increase of the yield of cracking products. It can be seen from Table 3 that the yields of cracking products at three pre-oxidation degrees of 10 bar, 20 bar, and 30 bar are all higher than those of pure decalin cracking products to varying degrees, and the deeper the oxidation degree, the greater the increase in yield. Similarly, this promotion effect reverses in the cracking products of decalin pre-oxidation products at 40 bar. The reason is the same as the above analysis.

[0034] Summary Table of Decalin Cracking Conversion Rate

[0035]

[0036] Note: The decalin conversion rate value here includes the decalin converted after the pre-oxidation operation

[0037] Summary Table of Yields of Main Cracking Products

[0038]

[0039] Example 2

[0040] Pre-oxidation process: Add to a 2 L high-pressure stirring autoclave (Figure 1 ) Pour 1 L of cyclohexane into it, then immediately close the reaction kettle and displace the air in the kettle with nitrogen (99.999%) three times, and then evacuate the nitrogen. Fill the kettle with pure oxygen (99.999%) until the pressure in the kettle shows 10 bar, and then stop filling with oxygen. Turn on the stirrer to make the oxygen in the kettle reach the dissolution equilibrium, and at this time the pressure in the kettle will decrease. Repeat the oxygen supplementation operation 2 - 3 times until the pressure in the kettle stabilizes at 10 bar. Then start heating up to 75 °C, adjust the stirring speed to 500 rpm, and wait for the pre-oxidation reaction to start. When the pressure in the kettle remains below 9.8 bar for 30 s, it is regarded as the official start of the pre-oxidation reaction. Since this reaction is an exothermic reaction, it is necessary to continuously monitor and control the temperature in the kettle to keep it stable between 75 ± 1 °C after the reaction officially starts. When the pressure in the kettle drops to 2 bar, start the cooling procedure and reduce the stirring speed to 300 rpm. When the temperature in the kettle drops to at least below 50 °C, evacuate the gas in the kettle and open the kettle to take samples. The obtained pre-oxidation product is named pre-oxidized cyclohexane - 10 bar. According to this method, pre-oxidized cyclohexane - 20 bar is also obtained.

[0041] Analysis of product conversion rate: Use gas chromatography to analyze the product conversion rate of the two pre-oxidation products in this example. The conversion rates of the two pre-oxidized cyclohexanes are 4.92% and 8.72% respectively.

[0042] Use Figure 2 The device is used to compare the cracking conversion rate and the yields of the main cracking products of pure cyclohexane and the two pre-oxidized cyclohexanes in this example. The cracking conditions are as follows: Liquid feed: 3.0 g / min; Steam: 100 mL / min; Preheating section temperature setting (six zones): 200, 250, 300, 350, 400, 450 °C; Cracking section temperature setting (five zones): all 700 °C. The cracking conversion rate and the yields of the main products are respectively shown in Tables 4 and 5. Summarizing the results of Tables 4 and 5, it can be seen that after the pre-oxidation operation, the cracking conversion rate and the yields of the cracking products of cyclohexane are improved to varying degrees. The reasons are attributed to (1) the partial conversion of cyclohexane caused by pre-oxidation; (2) the initiation and cracking effect of n-hexyl hydroperoxide.

[0043] Table 4 Summary table of cyclohexane cracking conversion rate

[0044]

[0045] Note: The cyclohexane conversion rate value here includes the cyclohexane converted after the pre-oxidation operation

[0046] Table 5 Summary table of yields of main cracking products

[0047]

[0048]

[0049] In addition, it should be understood that, after reading the above description of the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of the present application.

Claims

1. A method for pre-oxidizing a cyclic hydrocarbon to promote cracking, characterized in that, Comprising: In a high-pressure reactor, at a temperature lower than the boiling point of the cyclic hydrocarbon under the reaction pressure, the cyclic hydrocarbon is subjected to a liquid-phase pre-oxidation operation using an oxidizing gas to partially convert it into a cyclic hydrocarbon derivative with oxygen-containing functional groups; the liquid-phase product obtained after the pre-oxidation operation is used for pyrolysis; The oxidizing gas is oxygen or air; the pressure in the reactor is controlled by the addition amount of the oxidizing gas, and the starting pressure of the pre-oxidation operation is not less than 3 bar; the conversion rate of the cyclic hydrocarbon raw material after the pre-oxidation operation does not exceed 15%; The pre-oxidation temperature is controlled to be more than 20 °C lower than the boiling point of the raw material hydrocarbon under the pre-oxidation pressure condition; if the raw material consists of two or more cyclic hydrocarbons, it is more than 20 °C lower than the boiling point of the raw material hydrocarbon with the lowest boiling point under the pre-oxidation pressure condition.

2. The method for pre-oxidizing a cyclic hydrocarbon to promote cracking according to claim 1, characterized in that, The cyclic hydrocarbon used for liquid-phase pre-oxidation refers to one or more of monocyclic, bicyclic, polycyclic hydrocarbons and the corresponding monocyclic, bicyclic, polycyclic hydrocarbons with alkyl side chains.

3. The method for pre-oxidizing a cyclic hydrocarbon to promote cracking according to claim 1, characterized in that, The starting pressure of the pre-oxidation operation is between 10 - 30 bar.

4. The method for pre-oxidizing a cyclic hydrocarbon to promote cracking according to claim 1, wherein, The conversion rate of the cyclic hydrocarbon raw material after the pre-oxidation operation is 5 - 12%.

5. The method for pre-oxidizing a cyclic hydrocarbon to promote cracking according to claim 1, wherein The stirring speed of the reactor is 100 - 800 rpm.

6. The method for pre-oxidizing a cyclic hydrocarbon to promote cracking according to claim 1, characterized in that, The liquid-phase product obtained after the pre-oxidation operation is directly used for the pyrolysis reaction without further treatment.

7. The method for pre-oxidizing a cyclic hydrocarbon to promote cracking according to claim 1 or 6, characterized in that, It is characterized in that The pyrolysis reaction is steam pyrolysis or catalytic pyrolysis.

8. The method for pre-oxidizing a cyclic hydrocarbon to promote cracking according to claim 7, wherein, The pyrolysis is steam pyrolysis, the pyrolysis temperature is 600 - 800 °C, the residence time is 0.1 - 1.0 s, and the water-oil ratio is 0.2 - 2.

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