Optimized design of reactor for preparing acrylonitrile by propylene ammoxidation method

Abstract

The invention relates to the technical field of acrylonitrile preparation through a propylene ammoxidation method, and discloses an optimized design of a reactor for acrylonitrile preparation through a propylene ammoxidation method, the reactor is a fluidized bed reactor, and the optimized design on the reactor comprises the structural design of enabling reaction gas to directly enter a catalyst bed layer of the reactor before mixing, in this way, unexpected homogeneous combustion is reduced, more importantly, possible explosion is prevented, and production is safer. As a large amount of heat is released in the main reaction during production, a large amount of coil pipes are designed in the reactor and are mainly divided into vaporization coil pipes and overheating coil pipes through optimal design, so that the temperature of the reactor is prevented from runaway, the temperature of a bed layer of the reactor is uniform, the reutilization of reaction heat is realized, the fluidization degree of a catalyst is improved, and the yield of the catalyst is increased. Continuous reactions that produce undesired products are minimized by reducing catalyst backmixing. According to the acrylonitrile production reactor with the optimized design, production is safer, possible explosion of mixed gas is prevented, the temperature of the reactor is stable, temperature runaway of the reactor is prevented, reaction heat in the production process is recycled, backmixing of the catalyst is reduced, unexpected continuous side reactions are minimized, and the acrylonitrile yield is higher.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of propylene ammoxidation process for preparing acrylonitrile, in particular to the optimization design of the reactor for preparing acrylonitrile by propylene ammoxidation process. BACKGROUND

[0002] Acrylonitrile, also known as cyanoethylene, is an important mass-produced chemical and an important organic synthesis raw material, which has many uses, mainly used for manufacturing polyacrylonitrile fiber (acrylic), ABS resin, AS resin, etc. At the same time, it is also used for manufacturing nitrile rubber, polyacrylamide, acrylate, adiponitrile, water-resistant agent, antioxidant, dye and adhesive, etc. In addition, acrylonitrile is also a kind of aprotic polar solvent, which is helpful for fumigation of cereals. There are mainly three processes for preparing acrylonitrile, including cyanoethanol method, acetylene method and propylene ammoxidation method. The cyanoethanol method has been eliminated because of the high toxicity and cost of raw material hydrocyanic acid. Although the acetylene method has simple production and good yield, it still has the problem of high toxicity of raw material hydrocyanic acid, and has many side reactions, product refining is difficult, and the price of raw material acetylene is higher than that of propylene, which is inferior to the propylene ammoxidation method in terms of technology and economy. At present, the propylene ammoxidation method is the main production method, which uses propylene, ammonia and air as raw materials, has the characteristics of cheap raw materials, low production cost and simple process, but it also has some shortcomings that can be improved, including the problem of possible explosion of mixed gas after mixing of raw gas, which is not safe enough. Although there are many ways to prevent the explosion of mixed gas in the reactor at present, the cost is high and the process is complex. At present, the fluidized bed reactor is commonly used in the propylene ammoxidation process, but the large amount of heat released in the main reaction leads to the difficulty in controlling the temperature of the reactor, the temperature of the whole reaction bed is not uniform, and even the temperature is too high. The large amount of heat released in the reaction is not reused, and the fluidization degree of the catalyst is not enough, which causes the continuous side reaction due to the back mixing of the catalyst, the generation of the undesired by-products, and the low yield of acrylonitrile. The raw gas entering the reactor line and the coil in the optimization design in the present application and the related operation can solve the above problems in the production of the fluidized bed reactor to the greatest extent, realize the stable temperature in the reactor, and realize the safe and stable production, and the yield of acrylonitrile is higher. SUMMARY

[0003] The present application aims to provide the optimization design of the reactor for preparing acrylonitrile by propylene ammoxidation process, in order to solve the problems of explosion of the existing reactor, difficulty in stable control of the temperature due to large amount of heat released in the reaction, insufficient fluidization degree of the fluidized bed bed layer, continuous side reaction due to back mixing, low yield, and unstable and continuous production.

[0004] In order to achieve the above-mentioned purpose, the present application provides the following optimization design: Optimization design 1: By setting the ammonia and propylene feed spray above the air grid, the air grid and the feed spray directly enter the catalyst bed to reduce the free space of the gas, which reduces the unwanted homogeneous combustion and prevents the possible explosion.

[0005] Optimization design 2: By designing a considerable number of different purpose coils in the fluidized bed reactor, these coils are optimized, including steam coils and superheated coils, the steam coils absorb the reaction heat by vaporizing 15% water, and the generated steam is further heated in the superheated coils to a certain temperature, and then delivered to the steam header as the power of the turbine drivers of the compressors and pumps in the process, such a design realizes the efficient reuse of reaction heat. By adjusting the number of vaporization coils, we can adjust the reaction rate, the higher the production rate, the more vaporization coils are used to achieve stable temperature control and uniform bed temperature. Since the vaporization coils are prone to fouling, which will affect the heat transfer efficiency and product yield after fouling, we prevent and remove fouling by rotating the coils. The other function of the optimized coil is to improve the fluidization degree of the catalyst, minimize the back mixing of the catalyst, minimize the continuous reaction of undesirable by-products, improve the utilization rate of the catalyst, and improve the product yield.

[0006] Compared with the existing process, the beneficial effects of the present application are:

[0007] (1) By reducing the free space of the gas in the reactor, the explosion caused by the mixing of flammable gas and air is avoided, the explosion is prevented, the production is safer, and the cost of this optimization design is very low.

[0008] (2) By setting the coils in the reactor and optimizing the design, a large amount of reaction heat is efficiently utilized, the generated steam is used as the power of the compressors and pumps, and the number of coils used is adjusted to control the production rate of the reactor.

[0009] (3) Since the fouling of the coils will affect the heat transfer rate and reduce the yield and production rate, the coils are designed to rotate, which makes them less prone to fouling and removes the fouling, avoiding stopping the reactor to remove the fouling, and realizing continuous and stable production.

[0010] (4) The optimized coil increases the fluidization degree of the fluidized bed, maximizes the back mixing of the catalyst, minimizes the continuous reaction of undesirable by-products, reduces undesirable by-products, improves the acrylonitrile yield, and facilitates the subsequent separation process. DETAILED DESCRIPTION

[0011] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all the other embodiments obtained by those skilled in the art without creative work belong to the scope of protection of the present application.

[0012] Example 1: The process uses a fluidized bed reactor, where propylene, ammonia and air are contacted with a solid fluidized bed catalyst. The feed gases to the reactor enter through two separate inlets, which are spatially separated, with air entering from the bottom of the reactor through an air grid, and propylene and ammonia entering through feed spargers located slightly above the air grid. The air grid and feed spargers enter the catalyst bed directly, to minimize freeboard. This design reduces unwanted homogeneous combustion, and prevents explosions that can occur. Once the hydrocarbons and ammonia are in intimate contact with the catalyst, uncontrolled combustion does not occur.

[0013] Example 2: The acrylonitrile fluidized bed reactor is equipped with a large number of coils, which are designed to conduct the heat of reaction outside the reactor, maintain a uniform temperature throughout the reactor bed, increase the degree of fluidization of the catalyst, minimize backmixing of the catalyst, and create a pseudo plug flow condition in the fluidized bed reactor, resulting in the continuous reaction of undesirable byproducts being minimized. The vaporization coils are designed to evaporate 15% of the water fed to the vaporization coils, and the specific water vaporization ratio makes it more accurate and easy to control the reactor temperature through the coils. The preheated boiler feed water from the effluent cooler is delivered to the cooling tank along with the water exiting the coils. The flash steam is separated in the cooling tank. The liquid is returned to the reactor vaporization coils, and the steam is delivered to the superheating coils, which are used to superheat the steam, increasing the steam temperature from approximately 252°C to approximately 400°C. The superheated steam is delivered to the steam header, which powers the turbines driving the air compressors, chillers, cooling tower pumps, oiler feed water pumps, etc. The vaporization coils are primarily used to regulate the reactor rate, and as the production rate increases, more and more vaporization coils are put into use to eliminate the additional heat generated by the reaction. Because of the fouling of a large number of vaporization coils, which can cause heat transfer loss and thus limit the production rate, the vaporization coils are rotated every day to prevent fouling. These vaporization coils are also used alternately, and before some vaporization coils stop working, the remaining non-working coils are put into use, ensuring the absorption of the reaction heat, and when the fouled vaporization coils stop working, they will expand as they are heated to the reactor temperature, and the expansion causes a large amount of fouling deposits to break and peel off from the coils, which also makes the descaling efficiency of the coils higher. The fouling is removed by using a forced coil rotation design program, in which the vaporization coils are rotated every four hours, while the superheating coils are not rotated because their surface temperature is much higher and they do not significantly foul.

[0014] It should be noted that:

[0015] (1) In the present application, the feed spray of ammonia and propylene is directly arranged in the catalyst bed, and the feed spray is a common ordinary spray, which makes the manufacturing cost of the reactor lower and has no other complex design.

[0016] (2) In the present application, the coil of the reactor is a U-shaped curved multi-pass coil, which has higher heat exchange efficiency and also increases the fluidization degree of the fluidized bed, preventing back mixing.

[0017] (3) The main reaction in the process of preparing acrylonitrile by propylene ammoxidation is an exothermic reaction, which will release a large amount of heat, so the reaction temperature is controlled by setting the coil, which not only reaches the required reaction temperature, but also makes the main reaction proceed in the positive direction, protects the catalyst and improves the product yield.

[0018] The above shows and describes the basic principles, main features and advantages of the present application. It should be understood by those skilled in the art that the present application is not limited by the above examples, and the above examples and descriptions in the specification are only preferred examples of the present application and are not intended to limit the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

[0019] Although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments, or make equivalent replacements to some technical features, as long as they are within the spirit and principles of the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. An optimization of a reactor for the ammoxidation of propylene to produce acrylonitrile, characterized in that, Optimized reaction gas inlet pipelines and a catalyst bed design that allows the gas to enter the reactor directly before mixing prevent potential gas mixture explosions. The use of a reasonable number of coils with different functions within the reactor prevents excessive heat release during the reaction from causing reactor temperature rise, allowing for precise temperature control and higher catalyst fluidization. These optimized fluidized bed reactor designs result in safer production, more stable reactor temperatures, reuse of reaction heat, fewer continuous side reactions, and higher acrylonitrile yields.

2. According to the optimized design of claim 1, by designing separate inlets for the raw material gases ammonia and propylene in the reactor, and by having propylene and ammonia enter through a feed sprayer located slightly above the air grid, directly entering the catalyst bed after passing through the feed sprayer, the free space is reduced, while air is directly introduced from the bottom of the reactor. This optimized design saves costs and prevents explosions, thus making the reactor safer during the production process.

3. According to the optimized design described in claim 1, by installing a large number of optimized coils within the reactor, the reactor temperature is kept stable, preventing overheating or even runaway due to the heat of the production reaction, thus achieving heat recovery and reuse. Two types of coils are used: a vaporization coil absorbs a large amount of reaction heat by vaporizing a portion of the circulating water to generate steam; and a superheating coil overheats the generated steam, which is then transported to a steam manifold to power the turbine drives of compressors and pumps. Furthermore, by adjusting the number of heat exchange coils used, temperature control throughout the production process is achieved, ensuring reactor temperature stability. Scaling on the coils is removed through rotation and other operations, preventing the heat exchange efficiency from being affected. These optimized coil designs enable the reuse of reaction heat with higher efficiency, preventing excessive reactor overheating. The optimized coil design ensures the fluidization degree of the catalyst, reduces catalyst backmixing, and makes continuous reactions that generate undesirable products less likely to occur, ensuring stable and continuous production and achieving a high product yield.