System and process for efficient recovery of acetonitrile by-product acetonitrile from acrylonitrile production

Abstract

The present application relates to a kind of acrylonitrile by-product acetonitrile high-efficiency recovery preparation system and process, belong to acetonitrile recovery technical field.The system includes HCN removal unit, extraction rectification dehydration unit, solvent recovery unit and finished product refining unit, in the feed line upstream of HCN removal unit, additional pre-separation unit.The pre-separation unit is preferably cyclone flash tank, for pre-separation after preheating coarse acetonitrile raw material liquid, obtain gas phase material rich in hydrogen cyanide and liquid phase material with reduced hydrogen cyanide content.The liquid phase material is sent to HCN removal tower, and the gas phase material is directly incorporated into the overhead vapor system of HCN removal tower.The cyclone flash tank is provided with a rotating inner cylinder driven by fluid, with helical blade, and a conical reverse pushing component and water removal component, to realize efficient gas-liquid cyclone separation.The present application solves the technical problems of high energy consumption, limited system processing capacity and poor stability of HCN removal unit in existing acetonitrile refining process.

Description

TECHNICAL FIELD

[0001] The present application relates to a system and process for efficient recovery of acetonitrile as a byproduct of acrylonitrile production, belonging to the technical field of acetonitrile recovery. BACKGROUND

[0002] Acetonitrile is an important aprotic polar solvent, widely used in high performance liquid chromatography analysis, drug synthesis, DNA synthesis and chemical production. In industry, crude acetonitrile is usually obtained as a byproduct of acrylonitrile production, which inevitably contains hydrogen cyanide (HCN), water and other trace organic impurities. In order to obtain high-purity products that meet market demand, crude acetonitrile must be refined.

[0003] In the existing conventional process, a widely used high-purity acetonitrile refining process mainly includes four core units of HCN removal, extractive distillation dehydration, solvent recovery and product refining. The specific process is as follows: crude acetonitrile containing hydrogen cyanide (HCN) and water is directly and completely sent to the removal tower after preheating by the feed preheater. In the tower, the low-boiling-point HCN is separated from the top, and the overhead vapor is condensed by the condenser to return part of the flow. The uncondensed HCN waste gas is sent to the safety disposal unit; the bottom of the tower gets acetonitrile aqueous solution basically free of HCN. Subsequently, the acetonitrile aqueous solution enters the extractive distillation tower, and by adding a circulating solvent, the acetonitrile-water azeotrope is broken, and the acetonitrile is distilled from the top to obtain high-purity crude acetonitrile; the solvent-water mixture at the bottom of the tower is sent to the solvent recovery tower for separation, and the recovered solvent is cooled by the cooler and then recycled. Finally, the high-purity crude acetonitrile from the top of the extractive distillation tower enters the product tower for final refining, and high-purity acetonitrile product is obtained by side-drawing.

[0004] However, the above-mentioned prior art has exposed its inherent technical defects in long-term industrial practice:

[0005] The HCN removal unit has high energy consumption and large equipment load: since all the materials in the crude acetonitrile raw material liquid, including the highly volatile hydrogen cyanide, need to be separated by distillation in the HCN removal tower, the reboiler and overhead condenser of the tower must bear a huge heat load to provide sufficient gas phase and reflux to ensure the separation effect. This not only causes a large amount of energy consumption, making the operating cost high, but also requires larger size of tower body, heat exchanger and pump, etc. equipment, increasing the fixed asset investment.

[0006] System processing capacity is limited, there is a process bottleneck: in the whole process, HCN removal tower often become the "bottleneck" of the whole system processing capacity. When the need to increase production, the tower must deal with more raw materials, vaporization and condensation of more HCN, its separation capacity soon reach the upper limit (i.e. liquid limit). Therefore, the design capacity of the whole production line directly determines the maximum capacity, lack of operating flexibility and expansion potential.

[0007] System stability is poor, the operation control is difficult: the content of HCN in the crude acetonitrile raw material will fluctuate due to the upstream production conditions. Since the raw material is directly into the HCN removal tower, a precise multi-stage distillation equipment, any fluctuation of the raw material components will directly impact the stable operation of the tower, resulting in the need for operators to frequently adjust the reflux ratio, reboiler heat load and other parameters, increasing the operation difficulty, and may cause temporary unqualified product quality during the adjustment process, affecting the stable operation of the system. SUMMARY

[0008] The technical problem to be solved by the present application is to overcome the shortcomings of the prior art and provide a propylene nitrile byproduct acetonitrile efficient recovery preparation system and process, which effectively reduces the energy consumption of the HCN removal unit, reduces the equipment load, and improves the overall processing capacity and stability of the system.

[0009] The propylene nitrile byproduct acetonitrile efficient recovery preparation system provided by the present application comprises:

[0010] The HCN removal unit comprises an HCN removal tower, a tower top condenser connected to the top of the HCN removal tower, and a reflux tank;

[0011] The extractive distillation dehydration unit comprises an extractive distillation tower, the feed inlet of which is in fluid communication with the bottom outlet of the HCN removal tower;

[0012] The solvent recovery unit comprises a solvent recovery tower, the feed inlet of which is in fluid communication with the bottom outlet of the extractive distillation tower, and the bottom outlet thereof is in fluid communication with the upper part of the extractive distillation tower through a circulating solvent pipeline;

[0013] The product refining unit comprises a product tower, the feed inlet of which is in fluid communication with the top outlet of the extractive distillation tower;

[0014] The system further comprises a pre-separation unit arranged upstream of the feed pipeline of the HCN removal unit,

[0015] The feed inlet of the pre-separation unit is used to receive the preheated crude acetonitrile raw material liquid;

[0016] The pre-separation unit comprises at least one gas phase outlet for discharging hydrogen cyanide gas phase material rich in hydrogen cyanide, and a liquid phase outlet for discharging liquid phase material with reduced hydrogen cyanide content.

[0017] The liquid phase outlet of the pre-separation unit is in fluid communication with the feed inlet of the HCN removal tower;

[0018] The gas phase outlet of the pre-separation unit is in fluid communication with the vapor line of the overhead of the HCN removal tower to the overhead condenser;

[0019] The pre-separation unit is a cyclone flash tank, comprising:

[0020] A cyclone assembly for generating a downward outer vortex along the inner wall of the tank body;

[0021] A reverse push assembly capable of converting the downward outer vortex into an upward inner vortex;

[0022] A water removal assembly for adsorbing water droplets thrown out by the outer vortex;

[0023] A jet inlet port is provided on the tank wall corresponding to the position of the cyclone assembly, and the jet inlet port is arranged at a tangential position along the tank wall.

[0024] The cyclone assembly comprises a support frame arranged inside the tank body, and an inner cylinder is rotatably connected to the support frame, and helical blades are arranged on the outer wall of the inner cylinder, and at least part of the material entering the tank body through the jet inlet port can be sprayed onto the helical blades, so that the inner cylinder rotates.

[0025] A rotary sealing mechanism is arranged on the support frame, which can isolate the rotary connection between the inner cylinder and the support frame from the inside of the tank body.

[0026] The inner cylinder and the tank body are coaxially arranged.

[0027] The reverse push assembly comprises a reverse cone-shaped flow guide cover, and a flow guide opening is formed in the bottom of the flow guide cover.

[0028] The flow guide cover is arranged below the cyclone assembly.

[0029] A flow guide frame is arranged on the outer circle of the flow guide cover, and the flow guide cover is connected to the inside of the tank body through the flow guide frame.

[0030] A flow guide groove is formed in the inner surface of the flow guide cover, and a flow guide gap is left between the outermost circle of the flow guide cover and the inner wall of the tank wall.

[0031] The water removal assembly comprises a support cylinder mounted on the inner wall of the tank body, and a water absorption part is arranged on the inner wall of the support cylinder, the water absorption part is composed of strips, and the root position of the strip connecting the inner wall is lower than the head position.

[0032] A mist eliminator is arranged in the upper region inside the tank body, and a flow guide assembly is arranged below the mist eliminator, the flow guide assembly comprises a flow guide cylinder, and a flow guide plate is arranged inside the flow guide cylinder.

[0033] The preparation process of the acrylonitrile by-product acetonitrile high-efficiency recovery preparation system comprises the following steps:

[0034] The de-cyanation step: the crude acetonitrile raw material liquid is subjected to rectification in the HCN removal tower to obtain hydrogen cyanide vapor containing tower top and de-cyanation acetonitrile aqueous solution at the tower bottom; the hydrogen cyanide vapor containing tower top is condensed by the tower top condenser and part of the condensate is refluxed, and the uncondensed gas is sent to a safety disposal unit;

[0035] The dehydration step: the de-cyanation acetonitrile aqueous solution is sent into the extractive rectification tower together with the circulating solvent to perform extractive rectification, so as to obtain high-purity crude acetonitrile at the tower top and solvent-water mixed liquid at the tower bottom;

[0036] The solvent recovery step: the solvent-water mixed liquid is subjected to rectification in the solvent recovery tower to obtain pure recovered solvent at the tower bottom and the pure recovered solvent is recycled to the dehydration step as the circulating solvent;

[0037] The product refining step: the high-purity crude acetonitrile is subjected to rectification in the product tower to obtain high-purity acetonitrile product;

[0038] Before the de-cyanation step, the following pre-separation step is further included:

[0039] Step one: the preheated crude acetonitrile raw material liquid is subjected to reduced pressure flash evaporation to obtain gas phase material rich in hydrogen cyanide and liquid phase material with reduced hydrogen cyanide content;

[0040] Step two: the liquid phase material obtained in step one is sent into the HCN removal tower as feed;

[0041] Step three: the gas phase material obtained in step one is combined with the hydrogen cyanide containing vapor at the tower top generated in the de-cyanation step and is sent into the tower top condenser together.

[0042] The pre-separation step is performed under inert gas protection and positive pressure conditions.

[0043] Compared with the prior art, the present application has the following beneficial effects:

[0044] The cyclone flash tank of the present application combines passive flash evaporation with active centrifugal separation through a series of delicate structural designs such as tangential inlet, internal rotating components, conical reverse components and water removal components, thereby achieving extremely high gas-liquid separation efficiency. Compared with the traditional simple flash tank, the design of the present application can more thoroughly separate HCN and effectively reduce the entrainment of liquid foam in the gas phase, thereby further optimizing the load reduction effect on the downstream HCN removal tower.

[0045] The present application adds a pre-separation unit before the HCN removal tower, and separates most of the high-volatility HCN in the raw material in the form of gas in advance by using the simple flash principle. This greatly reduces the HCN content of the material entering the subsequent HCN removal tower, thereby significantly reducing the rectification separation load of the tower. Therefore, the reboiler heat load and the overhead condenser cold load required to maintain the same separation effect are greatly reduced, effectively saving the consumption of utilities such as steam and cooling water, and reducing the production operation cost.

[0046] In the existing process, the HCN removal tower is the capacity bottleneck of the whole system. The present application shares most of the HCN separation tasks of the HCN removal tower through the pre-separation unit, so that the load margin of the tower is greatly increased. Under the condition that the original equipment scale remains unchanged, the whole system can handle more crude acetonitrile raw material, thereby effectively improving the overall production capacity of the device and providing a simple and economical solution for expansion.

[0047] The introduction of the pre-separation unit plays an efficient "buffering" and "pretreatment" role for the fluctuation of the HCN content in the upstream raw material. Regardless of the fluctuation of the raw material, after being treated by the pre-separation unit, the liquid phase material components entering the HCN removal tower become more stable. This makes the operating conditions of the HCN removal tower stable, avoids the problem of frequent adjustment of operating parameters due to raw material fluctuation, simplifies the operation control, and ensures the long-term stability of the product quality. BRIEF DESCRIPTION OF DRAWINGS

[0048] Figure 1 is a system structure schematic diagram of an embodiment of the present application;

[0049] Figure 2 is one of the external structure schematic diagrams of a cyclone flash tank of an embodiment of the present application;

[0050] Figure 3 is the second external structure schematic diagram of a cyclone flash tank of an embodiment of the present application;

[0051] Figure 4 is the internal structure schematic diagram of a cyclone flash tank of an embodiment of the present application;

[0052] Figure 5 is the internal structure schematic diagram of a cyclone flash tank of an embodiment of the present application, omitting the water component structure;

[0053] Figure 6 is the internal structure schematic diagram of a cyclone flash tank of an embodiment of the present application, omitting the water component structure;

[0054] Figure 7 is Figure 3 is the left view of a cyclone flash tank in

[0055] Figure 8 isFigure 7 Full sectional view at point AA;

[0056] Figure 9 yes Figure 8 Enlarged view of a section at point C;

[0057] Figure 10 yes Figure 7 Full sectional view at point BB;

[0058] In the picture:

[0059] 1. Cyclone flash tank; 11. Reverse push assembly; 111. Flow guide frame; 112. Flow guide hood; 1122. Flow guide port; 1123. Flow guide gap; 12. Water removal assembly; 121. Support cylinder; 122. Water suction part; 13. Cyclone assembly; 131. Inner cylinder; 1311. Annular boss; 132. Spiral blade; 133. Support frame; 1331. Fixing plate; 1332. Pressure plate; 134. Rotary sealing mechanism; 1341. Ball bearing; 1342. Dynamic sealing ring; 14. Flow guide assembly; 141. Flow guide cylinder; 142. Flow guide plate; 143. Water outlet channel; 144. Water flow chamber; 15. Demister; 101. Gas phase outlet; 102. Pressure relief outlet; 103. Inertia inlet; 104. Return pipe; 105. Injection feed inlet; 106. Liquid level detector; 107. Liquid phase outlet; 108. Pressure measuring instrument;

[0060] 2. HCN removal tower;

[0061] 3. Extractive distillation column;

[0062] 4. Solvent recovery tower;

[0063] 5. Finished tower;

[0064] 6. Product cooler;

[0065] 7. Feed preheater. Detailed Implementation

[0066] Example

[0067] like Figures 1-10 As shown, the high-efficiency recovery and preparation system for acrylonitrile by-product acetonitrile according to the present invention includes:

[0068] The HCN removal unit includes an HCN removal tower 2, a tower top condenser and a reflux tank connected to the top of the HCN removal tower 2.

[0069] The extractive distillation and dehydration unit includes an extractive distillation column 3, whose inlet is in fluid communication with the bottom outlet of the HCN removal column 2.

[0070] a solvent recovery unit, the solvent recovery unit comprising a solvent recovery column 4 having a feed inlet in fluid communication with the bottom outlet of the extractive distillation column 3, and a bottom outlet in fluid communication with the upper part of the extractive distillation column 3 via a circulating solvent line;

[0071] a product finishing unit, the product finishing unit comprising a product column 5 having a feed inlet in fluid communication with the top outlet of the extractive distillation column 3;

[0072] the system further comprises a pre-separation unit arranged upstream of the HCN removal unit feed line,

[0073] the pre-separation unit having a feed inlet for receiving the preheated crude acetonitrile feedstock liquid;

[0074] the pre-separation unit comprising at least a gas phase outlet for discharging a hydrogen cyanide rich gaseous phase material, and a liquid phase outlet for discharging a liquid phase material having a reduced hydrogen cyanide content;

[0075] the liquid phase outlet of the pre-separation unit being in fluid communication with the feed inlet of the HCN removal column 2;

[0076] the gas phase outlet of the pre-separation unit being in fluid communication with the vapour line from the top of the HCN removal column 2 to the top condenser;

[0077] the pre-separation unit being a cyclone flash tank 1, comprising:

[0078] a cyclone assembly 13 for generating a downward outer vortex flow along the inner wall of the tank body;

[0079] a reverse push assembly 11 capable of converting the downward outer vortex flow into an upward inner vortex flow;

[0080] a water removal assembly 12 for adsorbing water droplets thrown off by the outer vortex flow;

[0081] a jet inlet 105 is arranged on the tank wall at a position corresponding to the cyclone assembly 13, and the jet inlet 105 is arranged at a tangential position on the tank wall.

[0082] The cyclone assembly 13 comprises a support frame 133 arranged inside the tank body, and an inner cylinder 131 is rotatably connected to the support frame 133. The outer wall of the inner cylinder 131 is provided with helical blades 132. At least part of the material entering the tank body through the jet inlet 105 can be sprayed onto the helical blades 132, so that the inner cylinder 131 rotates.

[0083] The support frame 133 is provided with a rotating sealing mechanism 134, which can isolate the rotating connection between the inner cylinder 131 and the support frame 133 from the inside of the tank body.

[0084] As Figure 9As shown, the upper end of the inner cylinder 131 is provided with an annular boss 1311 extending inwardly, the support frame 133 is divided into an upper fixed plate 1331 and a detachable pressing plate 1332, the pressing plate 1332 is connected by bolts to press the annular boss 1311 on the fixed plate 1331, a sealing gasket is placed or sealing glue is applied at the contact position of the pressing plate 1332 and the fixed plate 1331, annular grooves are formed on the fixed plate 1331 and the pressing plate 1332, and annular grooves are also formed on the annular boss 1311, the rotating sealing mechanism 134 includes ball bearings 1341 installed in the annular grooves, a plurality of dynamic sealing rings 1342 are installed at the outer side of the ball bearings 1341, and perfluoropolyether lubricating oil is injected at the position of the ball bearings 1341. Such oil not only has excellent lubricating properties, but more importantly, it must be absolutely inert to HCN chemically and cannot produce dangerous reactions under any possible failure conditions (such as high temperature, mixing).

[0085] The inner cylinder 131 and the tank body are coaxially arranged, which can make the inner cylinder 131 more stable when moving.

[0086] The reverse pushing assembly 11 includes a reverse cone-shaped flow guide cover 112, and a flow guide opening 1122 is formed in the bottom of the flow guide cover 112.

[0087] The flow guide cover 112 is arranged below the cyclone assembly 13.

[0088] A flow guide frame 111 is arranged on the outer circle of the flow guide cover 112, and the flow guide cover 112 is connected to the tank body through the flow guide frame 111.

[0089] A flow guide groove is formed in the inner surface of the flow guide cover 112, and a flow guide gap 1123 is left between the outermost circle of the flow guide cover 112 and the inner wall of the tank wall.

[0090] The water removal assembly 12 includes a support cylinder 121 which is attached to the inner wall of the tank body, and a water absorption part 122 is arranged on the inner wall of the support cylinder 121. The water absorption part 122 is composed of strips, which are distributed on the inner wall of the support cylinder 121, and the root position of the strip connecting the inner wall is lower than the head position.

[0091] A demister 15 is arranged in the upper area inside the tank body, and a flow guide assembly 14 is arranged below the demister 15, the flow guide assembly 14 includes a flow guide cylinder 141, and a flow guide plate 142 is arranged inside the flow guide cylinder 141.

[0092] The flow guide plate 142 is arranged obliquely, the flow guide cylinder 141 is provided with a water outlet passage 143 at the lowest end of the flow guide plate 142, a water flow cavity 144 is reserved between the flow guide cylinder 141 and the inner wall of the tank body, the water flow cavity 144 is provided with a reflux pipe 104 which is in communication with the outside, and the reflux pipe 104 is connected to the feed end of the feed preheater 7.

[0093] Demister 15 is a passive physical "filter" designed to capture and intercept tiny droplets entrained in the gas stream and recombine them into larger droplets that rely on gravity to fall back into the tank.

[0094] Demister 15 is usually a mesh pad woven from a large number of fine metal wires or fibers.

[0095] When the gas containing tiny droplets (i.e. "mist") passes through this complex mesh structure at high speed, gas molecules can easily bypass the wire mesh and continue on their way. However, droplets, which are much larger in mass than gas molecules, cannot change direction as flexibly as gas due to their own inertia, and will directly hit the metal wire in front and stick to it. This process is like a high-speed flying insect hitting the windshield of a car. After one droplet is captured, more tiny droplets will continue to hit the same place. These captured small droplets will gather and merge on the surface of the metal wire, like dew on leaves in the morning, and coalesce into larger and larger droplets. When the coalesced droplets grow large enough and heavy enough, their own gravity will exceed the drag force of gas rising and the surface tension of liquid. At this time, the large droplet will fall from the lower end of the wire mesh, and the large droplet falling into the deflector 142.

[0096] The deflector 142 is provided with a plurality of positive taper holes, and the steam passes through the positive taper holes. Since the positive taper holes are used, the steam is more easily passed through the positive taper holes. Since the upper end of the positive taper hole is small, the steam is sprayed out through the small hole at the upper end of the positive taper hole, which hinders the large droplet falling from the demister 15. The large droplet flows along the deflector 142 and flows into the water flow cavity 144 from the water outlet channel 143, and then enters the feed end of the feed preheater 7 through the reflux pipe 104 connected to the water flow cavity 144.

[0097] Pre-separation load reduction principle:

[0098] There is a huge difference in volatility between hydrogen cyanide (HCN, boiling point 26°C) in the raw material and acetonitrile (boiling point 82°C) and water (boiling point 100°C). The prior art sends all these materials with different properties into a precise multi-stage rectification tower (HCN removal tower 2) for separation, which violates the principle of optimal energy utilization and causes huge energy waste and equipment load.

[0099] The working principle of the present application is that, by virtue of the easy volatilization of HCN, most of the HCN is "pre-released" from the liquid phase through a simple pressure-reducing flash process before entering a complex rectification system. This is equivalent to adding a high-efficiency "rough processing" procedure before the "finishing" procedure of rectification. This shift of the main task of HCN removal from the HCN removal tower 2 with high energy consumption and heavy load to the pre-separation unit (cyclone flash tank 1) with simple structure and extremely low energy consumption greatly reduces the load of the material, especially the most difficult to handle HCN load, entering the HCN removal tower 2, so that the subsequent rectification process can be carried out at lower energy consumption and higher efficiency, thereby fundamentally solving the bottleneck problem of the prior art.

[0100] Dynamic enhanced separation principle:

[0101] The pre-separation unit (cyclone flash tank 1) of the present application is not a simple flash tank, and the ingenious internal structure design contains a multi-stage, dynamic enhanced separation principle:

[0102] Primary centrifugal separation principle: the material enters along the tangential direction of the tank wall through injection into the material inlet 105, generating a vortex (downward outer vortex). Under the action of a strong centrifugal force field, liquid droplets with a higher density are thrown to the tank wall, achieving preliminary and efficient gas-liquid separation.

[0103] Fluid-driven secondary centrifugal separation principle: the high-speed feed fluid impacts the spiral blades 132 of the cyclone assembly 13, converting the kinetic energy carried by the spiral blades 132 into rotational mechanical energy of the inner cylinder 131. The tiny liquid droplets adhering to the blades are thus accelerated twice and subjected to a secondary centrifugal force generated by mechanical rotation, which is much larger than the primary centrifugal force. Under the action of the secondary centrifugal force, the liquid droplets are actively and rapidly "thrown" from the blades to the water removal assembly 12 on the outside, achieving enhanced capture of tiny mist.

[0104] Controlled flow field separation principle: the inverted conical flow guide cover 112 of the reverse pushing assembly 11 smoothly guides the reverse direction of the separated outer vortex by using fluid mechanics principles, forming an upward inner vortex.

[0105] Multi-stage series separation principle: the entire separation process is multi-stage series. From primary centrifugal separation, to secondary dynamic centrifugal separation, to final fine filtration separation by the mist eliminator 15, the purity of the gas phase and the recovery rate of the liquid phase are both ensured to reach the highest level.

[0106] Working process:

[0107] (1) Efficient gas-liquid separation in the cyclone flash tank 1:

[0108] Feed, Flashing and Vortex Formation: The raw acetonitrile feedstock from the upstream storage tank is pressurized by a pump and heated by a feed preheater 7, forming a superheated liquid at high temperature and pressure. The liquid is injected into the cyclone flash tank 1 through the injection port 105 at high speed and tangentially to the tank wall. At the moment of entering the tank, the liquid flashes violently due to the sudden pressure drop, and a large amount of volatile hydrogen cyanide (HCN) vaporizes, forming a high-speed rotating gas-liquid two-phase mixture, i.e., the "downward outer vortex".

[0109] Dynamic Intensified Centrifugal Separation:

[0110] Inner Cylinder 131 Drive: The high-speed outer vortex impacts the helical blades 132 of the cyclone assembly 13, converting the strong kinetic energy carried by the outer vortex into rotational mechanical energy of the inner cylinder 131. Under the support of the support frame 133 and the internal rotating seal mechanism 134, the inner cylinder 131 begins to rotate at high speed. The rotating seal mechanism is realized by low-friction rotation of the ball bearings 1341, and the dynamic seal ring 1342 provides sealing, and the perfluoropolyether lubricating oil provides long-term lubrication in a chemically inert environment.

[0111] Primary Centrifugal Separation: Under the action of the initial outer vortex centrifugal force, most of the liquid droplets with higher density are thrown to the inner wall of the tank.

[0112] Secondary Intensified Centrifugal Separation: A portion of the smaller liquid droplets adhere to the high-speed rotating helical blades 132, and are given a very high tangential velocity, thereby subjected to a strong secondary centrifugal force, and are actively and rapidly thrown to the tank wall.

[0113] Liquid Droplet Capture and Collection:

[0114] All liquid droplets thrown out by the primary and secondary centrifugal separation are captured by the water removal assembly 12 installed on the tank wall. The inner wall of the support cylinder 121 of the assembly is provided with a large number of special structure water absorption parts 122, and the design of the root lower than the head can effectively prevent the captured liquid from being rolled up by the gas flow, and guide the liquid droplets to flow stably along the wall to the bottom of the tank.

[0115] Gas Flow Turning and Final Purification:

[0116] The "downward outer vortex" that has basically completed gas-liquid separation is smoothly guided by the reverse push assembly 11 when it reaches the bottom of the tank. The gas flow cannot be discharged from the bottom of the flow guide 1122, but only enters the center low pressure area in the opposite direction, forming an "upward inner vortex".

[0117] This upward inner vortex is stabilized by the flow guide assembly 14, in which the flow guide cylinder 141 acts as a vortex stabilizer, and the flow guide plate 142 further regulates the flow field.

[0118] Finally, the inner vortex in the stock passes through the demister 15 installed at the upper end of the tank, and the remaining and the smallest mist is thoroughly captured after the final fine filtration.

[0119] Product diversion: After the above-mentioned multi-stage and dynamic enhanced separation, two streams of materials are obtained:

[0120] Gas phase product: The gas phase product with high purity but rich in HCN is discharged from the demister 15, and is discharged from the gas phase outlet 101 at the upper end of the tank into the overhead condenser connected to the top of the HCN removal tower 2.

[0121] Liquid phase product: The liquid phase product with significantly reduced HCN content is collected at the bottom of the tank, and is directly sent to the HCN removal tower 2 from the liquid phase outlet 107 at the lower end of the tank.

[0122] The whole pre-separation process is operated under the protection of the external nitrogen system and maintains stable positive pressure. Inert gas is input into the tank through the inert input port 103, the tank is connected with a pressure relief outlet 102 at the upper end, and pressure measuring instruments 108 are arranged above and below the demister 15 in the tank, and a liquid level detector 106 is arranged at the upper end of the tank below the reverse pushing assembly 11, which is used to control the amount of liquid in the tank.

[0123] As shown in Figure 1 , the preparation process of the acrylonitrile byproduct acetonitrile high-efficiency recovery preparation system of the present application comprises:

[0124] Pre-separation step:

[0125] Step 1.1 (preheating and pressure reduction flash evaporation): The crude acetonitrile raw material liquid from the storage tank is pressurized by a pump and then sent to the feed preheater 7, which is heated to a preset flash evaporation temperature by using the heat of the subsequent section; then, the high-temperature and high-pressure liquid after preheating is reduced in pressure by a throttle valve at the injection inlet 105, so that it is rapidly flashed when entering the cyclone flash tank 1, forming a gas phase material rich in hydrogen cyanide (HCN) and a liquid phase material with significantly reduced HCN content.

[0126] Step 1.2 (dynamic enhanced separation): In the cyclone flash tank 1, the gas-liquid two-phase mixture generated in step 1.1 is formed into a high-speed rotating “downward outer vortex” by tangential feeding; the kinetic energy of the vortex is used to drive the cyclone assembly 13 (including the inner cylinder 131 and the spiral blade 132) in the tank to rotate at high speed, so as to implement secondary centrifugal enhanced separation on the entrained droplets; the separated droplets are captured by the water removal assembly 12 and are collected into the liquid phase at the bottom of the tank; the gas flow forms an “upward inner vortex” under the guidance of the reverse pushing assembly 11, and is finally purified through the flow guide assembly 14 and the demister 15.

[0127] Step 1.3 (product split): The gaseous material enriched with HCN from step 1.2 is discharged from the top of the cyclone flash tank 1; the liquid material with reduced HCN content is discharged from the bottom of the cyclone flash tank 1.

[0128] Step 1.4 (safety control): The whole pre-separation step is carried out under the protection of an external nitrogen system, maintaining a slight positive pressure of 10 kPaG to 100 kPaG to prevent air from entering and ensure safe operation.

[0129] Decyanation step: The crude acetonitrile raw material liquid is subjected to rectification in the HCN removal column 2 to obtain hydrogen cyanide vapor at the top and decyanated acetonitrile aqueous solution at the bottom; the hydrogen cyanide vapor is condensed by the overhead condenser and part of it is returned as reflux, and the uncondensed gas is sent to a safety disposal unit;

[0130] The liquid material obtained in step 1.3 is sent as the main feed to the HCN removal column 2;

[0131] The gaseous material obtained in step 1.3 is combined with the HCN-containing vapor produced at the top of the HCN removal column 2 and is sent together to the overhead condenser for cooling;

[0132] In the HCN removal column 2, the residual HCN in the liquid material is further separated to the top by multi-stage rectification to obtain HCN-containing vapor at the top and decyanated acetonitrile aqueous solution at the bottom which is essentially free of HCN; the combined HCN-containing vapor is condensed by the overhead condenser, part of which is returned as reflux to the top, and the uncondensed gas is sent to a safety disposal unit for harmless treatment.

[0133] Dehydration step: The decyanated acetonitrile aqueous solution is sent together with the circulating solvent to the extractive distillation column 3 for extractive distillation to obtain high-purity crude acetonitrile at the top and solvent-water mixture at the bottom;

[0134] The decyanated acetonitrile aqueous solution obtained in the decyanation step is sent together with the circulating solvent (such as ethylene glycol) from the solvent recovery step to the extractive distillation column 3;

[0135] Through extractive distillation, the circulating solvent is used to break the acetonitrile-water azeotrope system, so that acetonitrile is distilled out as light components from the top to obtain high-purity crude acetonitrile at the top and solvent-water mixture at the bottom.

[0136] Solvent recovery step: The solvent-water mixture is subjected to rectification in the solvent recovery column 4 to obtain pure recovered solvent at the bottom and circulate to the dehydration step as the circulating solvent;

[0137] The solvent-water mixture obtained in the dehydration step is sent to the solvent recovery column 4;

[0138] The waste water is obtained at the top of the column and the pure recovered solvent is obtained at the bottom of the column by rectification using the boiling point difference between the solvent and water; the pure recovered solvent is cooled and then recycled to the dehydration step as the circulating solvent, so as to realize the closed loop circulation and reuse of the solvent.

[0139] Product refining step: the high-purity crude acetonitrile is subjected to rectification in a product column 5 to obtain high-purity acetonitrile product;

[0140] The high-purity crude acetonitrile obtained in the dehydration step is sent to the product column 5;

[0141] The high-purity acetonitrile product meeting the specification requirements is obtained by further removing trace amounts of heavy component impurities through precise rectification in the form of side-draw; the drawn product vapor is condensed and cooled by a product cooler 6 and then sent to a product storage tank.

[0142] Before the de-cyanation step, a pre-separation step is further included:

[0143] Step one: the preheated crude acetonitrile raw material liquid is subjected to vacuum flash evaporation to obtain gas-phase material rich in hydrogen cyanide and liquid-phase material with reduced hydrogen cyanide content;

[0144] Step two: the liquid-phase material obtained in step one is sent to the HCN removal column 2 as feed;

[0145] Step three: the gas-phase material obtained in step one is combined with the overhead hydrogen cyanide-containing vapor generated in the de-cyanation step and then sent to the overhead condenser.

[0146] The pre-separation step is carried out under inert gas protection and positive pressure conditions.

[0147] In the present application, the description of the structure direction and relative position relationship, such as front, back, left, right, up and down, does not constitute a limitation on the present application, but is only for convenience of description.

Claims

1. A system for preparing acrylonitrile byproduct acetonitrile with high efficiency recovery, comprising: an HCN removal unit, the HCN removal unit comprising an HCN removal tower (2), a tower top condenser and reflux tank connected to the tower top of the HCN removal tower (2); an extractive distillation dehydration unit, the extractive distillation dehydration unit comprising an extractive distillation tower (3), the feed inlet of which is in fluid communication with the tower bottom outlet of the HCN removal tower (2); a solvent recovery unit, the solvent recovery unit comprising a solvent recovery tower (4), the feed inlet of which is in fluid communication with the tower bottom outlet of the extractive distillation tower (3), and the tower bottom outlet of which is in fluid communication with the upper part of the extractive distillation tower (3) through a circulating solvent pipeline; a product refining unit, the product refining unit comprising a product tower (5), the feed inlet of which is in fluid communication with the tower top outlet of the extractive distillation tower (3); characterized in that the system further comprises a pre-separation unit arranged on the feed pipeline of the HCN removal unit upstream, the feed inlet of the pre-separation unit is used to receive the preheated crude acetonitrile raw material liquid, the pre-separation unit at least comprises a gas phase outlet used to discharge a gas phase material rich in hydrogen cyanide gas phase, and a liquid phase outlet used to discharge a liquid phase material with reduced hydrogen cyanide content, the liquid phase outlet of the pre-separation unit is in fluid communication with the feed inlet of the HCN removal tower (2), and the gas phase outlet of the pre-separation unit is in fluid communication with the vapor pipeline from the tower top of the HCN removal tower (2) to the tower top condenser.

2. The system according to claim 1, wherein the pre-separation unit is a cyclone flash tank (1), comprising: a cyclone assembly (13) used to generate a downward outer vortex along the inner wall of the tank body; a reverse pushing assembly (11) capable of converting the downward outer vortex into an upward inner vortex; a water removal assembly (12) used to adsorb water droplets thrown out by the outer vortex; and a spray inlet (105) arranged at a tangential position of the tank wall corresponding to the position of the cyclone assembly (13).

3. The system according to claim 2, wherein the cyclone assembly (13) comprises a support frame (133) arranged inside the tank body, a inner cylinder (131) rotatably connected to the support frame (133), and helical blades (132) arranged on the outer wall of the inner cylinder (131), at least part of the material entering the tank body through the spray inlet (105) can be sprayed onto the helical blades (132), so that the inner cylinder (131) rotates.

4. The system according to claim 3, wherein the support frame (133) is provided with a rotating sealing mechanism (134) capable of isolating the rotating connection between the inner cylinder (131) and the support frame (133) from the inside of the tank body.

5. The system according to claim 4, wherein the inner cylinder (131) and the tank body are coaxially arranged.

6. The system according to claim 2, wherein the reverse pushing assembly (11) comprises an inverted conical designed flow guide cover (112) with a flow guide opening (1122) arranged at the bottom of the flow guide cover (112).

7. The system according to claim 6, wherein the flow guide cover (112) is arranged below the cyclone assembly (13).

8. The system according to claim 7, wherein the flow guide cover (112) is connected to the inside of the tank body through a flow guide frame (111) arranged on the outer circle of the flow guide cover (112).

9. The system according to claim 8, wherein the flow guide cover (112) is provided with a flow guide groove on the inner surface, and a flow guide gap (1123) is left between the outermost circle of the flow guide cover (112) and the inner wall of the tank wall. ​ ​ ​ ​ ​ ​ 2. The acrylonitrile byproduct acetonitrile high-efficiency recovery preparation system according to claim 1, characterized in that, ​ 3. The acrylonitrile byproduct acetonitrile high-efficiency recovery preparation system according to claim 2, characterized in that, ​ 4. The system for preparing acrylonitrile byproduct acetonitrile with high efficiency recovery according to claim 2, characterized in that, ​ 5. The acrylonitrile byproduct acetonitrile high-efficiency recovery preparation system according to claim 1, characterized in that, ​ ​ ​ 6. The acrylonitrile byproduct acetonitrile high-efficiency recovery preparation system according to claim 5, characterized in that, ​ 7. The acrylonitrile byproduct acetonitrile high-efficiency recovery preparation system according to claim 1, characterized in that, The water removal assembly (12) comprises a supporting cylinder (121) which is attached to the inner wall of the tank body, and a water absorbing part (122) is arranged on the inner wall of the supporting cylinder (121), the water absorbing part (122) is composed of strips, and the root position of the strip connecting the inner wall is lower than the head position.

8. The acrylonitrile byproduct acetonitrile high-efficiency recovery preparation system according to claim 1, characterized in that, The inner part of the tank body is provided with a demister (15) in the upper area, and a flow guide assembly (14) is arranged below the demister (15), the flow guide assembly (14) comprises a flow guide cylinder (141), and a flow guide plate (142) is arranged in the flow guide cylinder (141).

9. A process for efficiently recovering acetonitrile as a byproduct of acrylonitrile production, based on the system for efficiently recovering acetonitrile as a byproduct of acrylonitrile production according to claim 1, comprising: A de-cyanation step: crude acetonitrile raw material liquid is subjected to rectification in a HCN removal tower (2) to obtain hydrogen cyanide vapor containing tower top and de-cyanation acetonitrile aqueous solution at the tower bottom; the hydrogen cyanide vapor containing steam is condensed by the tower top condenser and part of it is refluxed, and the uncondensed gas is sent to a safety disposal unit; A dehydration step: the de-cyanation acetonitrile aqueous solution is sent into an extractive rectification tower (3) together with a circulating solvent to perform extractive rectification, obtaining high-purity crude acetonitrile at the tower top and solvent-water mixed liquid at the tower bottom; A solvent recovery step: the solvent-water mixed liquid is subjected to rectification in a solvent recovery tower (4) to obtain pure recovered solvent at the tower bottom and recycle it to the dehydration step as a circulating solvent; A product refining step: high-purity crude acetonitrile is subjected to rectification in a product tower (5) to obtain high-purity acetonitrile product; The process is characterized in that, before the de-cyanation step, a pre-separation step is further included: Step one: preheated crude acetonitrile raw material liquid is subjected to flash evaporation under reduced pressure to obtain gas phase material rich in hydrogen cyanide and liquid phase material with reduced hydrogen cyanide content; Step two: the liquid phase material obtained in step one is sent into the HCN removal tower (2) as feed; Step three: the gas phase material obtained in step one is combined with the hydrogen cyanide containing steam at the tower top generated in the de-cyanation step and sent into the tower top condenser together.

10. The process for the efficient recovery of acetonitrile as a byproduct from the production of acrylonitrile according to claim 9, characterized in that, The pre-separation step is performed under inert gas protection and positive pressure conditions throughout.

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