Low-energy-consumption phthalic anhydride continuous recovery method and device

Abstract

The invention relates to the field of phthalic anhydride production, and discloses a low-energy-consumption phthalic anhydride continuous recovery method and device. Comprising the following steps: (1) introducing a crude phthalic anhydride raw material from which light components are removed into a phthalic anhydride tower, obtaining a phthalic anhydride product at the tower top, obtaining a mixture I containing phthalic anhydride and heavy components at the tower kettle, and pumping the mixture I as a continuous extraction stream into a residual liquid tower through the tower kettle of the phthalic anhydride tower for concentration; and (2) feeding the mixture I from the top in a residual liquid tower, carrying out evaporation separation by adopting a forced external circulation evaporation mode to obtain the recovered phthalic anhydride at the tower top and a mixture II at the tower kettle, enabling the tower kettle material to pass through a tower kettle pump of the residual liquid tower, feeding one part of the material to a reboiler of the residual liquid tower, heating and returning to the residual liquid tower, and continuously extracting one part of concentrated slag liquid. And (3) phthalic anhydride separated from the top of the residual liquid tower is returned to the bottom of the phthalic anhydride tower in a gas phase form to be recycled. According to the invention, the continuous operation of residue concentration can be realized, and phthalic anhydride is recycled to the greatest extent.

Description

Technical Field

[0001] The present invention relates to the field of phthalic anhydride production, and particularly to a method and device for continuous recovery of phthalic anhydride with low energy consumption. Background Art

[0002] Phthalic anhydride is the abbreviation of phthalic anhydride, which is an important organic chemical raw material, mainly used for manufacturing plasticizers, alkyd resins and polyester resins. In addition, it can also be used in the production of coatings, fuels, medicines and pesticides, etc.

[0003] The main existing industrial process for producing phthalic anhydride is as follows: reaction raw materials undergo catalytic oxidation reaction through a fixed bed reactor to be converted into phthalic anhydride, and at the same time, there are also by-products such as maleic anhydride, benzoic acid, naphthoquinone, citraconic acid, etc. The syngas coming out of the fixed bed is cooled to obtain crude phthalic anhydride, which enters the distillation system for further separation and purification. Currently, the conventional two-column distillation process is adopted in the phthalic anhydride distillation device. The first light removal column is used to remove light component impurities such as maleic anhydride and benzoic acid. In the second phthalic anhydride column, the phthalic anhydride product is obtained from the top of the column, and the stream containing heavy component impurities such as naphthoquinone is withdrawn from the bottom of the column. However, there is still a relatively high content of phthalic anhydride (greater than 80%) in the stream withdrawn from the bottom of the column that needs to be recovered. In addition, with the gradual enrichment of some high-boiling heavy components at the bottom of the column, problems such as a decrease in the purity of the phthalic anhydride product and blockage of the reboiler and tower internals will occur. To prevent this phenomenon, it is necessary to regularly discharge the residue.

[0004] The existing residue evaporation process in industrial devices is as follows: the stream withdrawn from the bottom of the phthalic anhydride column enters a residue collection tank, and steam is equipped outside the residue collection tank for heating. After a part of the phthalic anhydride material obtains heat and becomes a gas phase, it enters the condenser and is condensed into a liquid phase, and the liquid-phase phthalic anhydride is withdrawn from the lower section of the condenser and enters the phthalic anhydride column for recovery. The residue is intermittently discharged from the bottom of the residue collection tank.

[0005] CN 211921386 U discloses a crude toluene recovery device for the phthalic anhydride residue evaporation process, which uses a residue evaporation kettle with an internal residue evaporation reboiler for phthalic anhydride recovery. The residue evaporation kettle is connected with a residue evaporation column and a condenser. When a certain amount of phthalic anhydride evaporates, it is necessary to stop the heating heat source of the reboiler, the water supply of the condenser, and the vacuum system, and pressurize the residue evaporation kettle with nitrogen for slag discharge. The recovered phthalic anhydride is condensed into liquid phthalic anhydride by the condenser and enters the refining section.

[0006] CN 219291415 U discloses a residue evaporation device for a phthalic anhydride production system, including a reaction kettle, and a distillation column and a condenser are arranged above the reaction kettle. Crude phthalic anhydride is intermittently transported to the reaction kettle, and the reaction kettle is used to ensure the liquid phase state of the material. When the reaction kettle is heated to a predetermined time, the conveying pipe connecting the reaction kettle to the distillation column is cut off, and nitrogen is introduced into the reaction kettle for slag discharge. The evaporated phthalic anhydride forms a liquid state after heat exchange through the condenser and a heat exchanger and is sent to the phthalic anhydride storage tank.

[0007] It can be seen that in the above-mentioned existing industrial installations for phthalic anhydride production and the publicly reported ones, the recovery of phthalic anhydride in the residue evaporation process all adopts batch operation, which cannot achieve continuous operation of the installation, has low efficiency, and is also prone to energy loss. The evaporation of phthalic anhydride is all based on the static evaporation method in tanks or kettles. The residence time is too long, which is prone to further reaction of high-boiling substances at high temperatures, generating impurities with higher boiling points, resulting in an increase in the viscosity of the residue liquid and increasing the difficulty of discharging the residue. The recovered phthalic anhydride is all liquid phthalic anhydride returned to the refining system after being condensed by a condenser.

[0008] Therefore, it is necessary to systematically design the phthalic anhydride recovery process in the phthalic anhydride production device, so as to achieve continuous discharge of the residue and efficient recovery of phthalic anhydride, and achieve the purpose of improving production efficiency and reducing energy consumption. Summary of the Invention

[0009] The purpose of the present invention is to overcome the defects of low efficiency, high energy consumption, difficult operation and inability to achieve continuous operation in the phthalic anhydride recovery process in the existing phthalic anhydride production device, and propose a method and device for continuous recovery of phthalic anhydride with low energy consumption.

[0010] In order to achieve the above purpose, the first technical solution of the present invention is a method for continuous recovery of phthalic anhydride with low energy consumption, which includes:

[0011] (1) Introduce the crude phthalic anhydride raw material into the phthalic anhydride tower. Phthalic anhydride product is obtained at the top of the tower, and a mixture Ⅰ containing phthalic anhydride and heavy components is obtained at the bottom of the tower. It is used as a continuous extraction stream and is pumped to the residue tower through the phthalic anhydride tower bottom pump for concentration.

[0012] (2) In the residue tower, the mixture Ⅰ material is fed from the top or middle. The bottom of the tower adopts continuous discharge of the residue and forced external circulation evaporation. After evaporation separation, the recovered phthalic anhydride is obtained at the top of the tower, and a mixture Ⅱ is obtained at the bottom of the tower. The bottom material of the tower passes through the residue tower bottom pump. Part of it is sent to the residue tower reboiler, heated and then returned to the residue tower. Part of the concentrated residue liquid is used as an extraction stream and is continuously sent to the downstream for treatment;

[0013] (3) No condenser is installed at the top of the residue tower. The separated phthalic anhydride is returned to the bottom of the phthalic anhydride tower in gaseous form for recovery. Introducing gaseous phthalic anhydride into the phthalic anhydride tower can replace part of the heat load of the phthalic anhydride tower reboiler;

[0014] Furthermore, before step (1), the crude phthalic anhydride raw material generated by the reaction is first removed of light component impurities and then sent to the phthalic anhydride tower.

[0015] Furthermore, in step (3), the phthalic anhydride distilled from the top of the residue tower is condensed by a condenser, introduced into the residue tower reflux tank, and then a part is drawn from the reflux tank as reflux and returned to the residue tower, and a part is used as liquid extraction and sent to the phthalic anhydride tower.

[0016] Furthermore, the top pressure of the phthalic anhydride tower is -0.098 to -0.07 MPa (gauge pressure), and the top temperature is 153 to 234 °C.

[0017] Furthermore, the top pressure of the phthalic anhydride tower is preferably -0.095 to -0.08 MPa (gauge pressure), and the top temperature is preferably 175 to 219 °C.

[0018] Furthermore, the top pressure of the residue tower is -0.095 to -0.06 MPa (gauge pressure), and the top temperature is 176 to 245 °C.

[0019] Furthermore, the top pressure of the residue tower is preferably -0.093 to -0.07 MPa (gauge pressure), and the top temperature is preferably 186 to 235 °C.

[0020] Furthermore, the mass ratio of the recycle amount at the bottom of the residue tower to the feed amount of the residue tower is: 10 to 20, preferably 15 to 18.

[0021] Furthermore, before performing step (2), to ensure the purity of the phthalic anhydride product at the top of the tower, the phthalic anhydride content in mixture I is higher than 80%.

[0022] Furthermore, the content of the residue in the concentrated residue liquid obtained at the bottom of the residue tower is 30 to 70%.

[0023] Another technical solution of the present invention is a device for implementing the method for continuously recovering phthalic anhydride with low energy consumption, including a phthalic anhydride tower, a phthalic anhydride tower reboiler, a phthalic anhydride tower bottom pump, a residue tower, a residue tower bottom pump, and a residue tower reboiler;

[0024] The phthalic anhydride tower is provided with a raw material inlet, a phthalic anhydride product outlet, a tower bottom discharge port, a tower bottom material circulation port, and a recycled phthalic anhydride feed port;

[0025] The residue tower is provided with a crude phthalic anhydride feed port, a gaseous phthalic anhydride outlet, a residue tower bottom discharge port, and a tower bottom material circulation port;

[0026] The phthalic anhydride tower bottom discharge port, the phthalic anhydride tower bottom pump, and the residue tower crude phthalic anhydride feed port are connected in sequence;

[0027] The residue tower bottom discharge port, the residue tower bottom pump, the residue tower reboiler, a control valve, and the residue tower bottom material circulation port are connected in sequence;

[0028] The gaseous phthalic anhydride outlet of the residue tower is connected to the recycled phthalic anhydride feed port of the phthalic anhydride tower.

[0029] Compared with the phthalic anhydride recovery process in the existing phthalic anhydride plant, the method of the present invention has the following advantages: The low - energy - consumption continuous phthalic anhydride recovery method of the present invention shortens the residence time of the bottom materials in the tower, greatly reduces the generation probability of high - viscosity and high - boiling - point impurities, reduces the viscosity of the residue after concentration, thereby reducing the treatment difficulty of slag discharge and improving the efficiency of residue discharge. At the same time, the method of gas - phase recovery of phthalic anhydride greatly reduces the energy consumption compared with the existing liquid - phase phthalic anhydride recovery process. In addition, the continuous slag - discharging operation of the present invention is also beneficial to ensuring the stable operation of the phthalic anhydride plant, as follows:

[0030] 1. The present invention can achieve continuous operation. Compared with the existing batch concentration, it can handle a large amount of materials, greatly improving the production efficiency.

[0031] 2. The present invention can reduce the viscosity of the materials at the bottom of the residue tower. The mixture Ⅱ obtained at the bottom of the tower has good fluidity, while the materials in the residue collection tank in the existing industry are relatively viscous. The present invention reduces the treatment difficulty of the residue compared with the existing process.

[0032] 3. In the present invention, phthalic anhydride is returned to the phthalic anhydride tower in gas phase. Compared with the existing process where the evaporated phthalic anhydride needs to be cooled and then enter the phthalic anhydride tower in liquid phase, it has significant energy - saving and consumption - reducing advantages.

[0033] 4. The method of the present invention concentrates the residue by setting up a residue tower and is equipped with a forced external - circulation evaporation system, so as to realize the continuous operation of residue concentration and also maximize the recovery and utilization of phthalic anhydride. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1 is a schematic diagram of a method and device for continuous low - energy - consumption phthalic anhydride recovery according to the present invention.

[0035] Reference Numerals:

[0036] 101 Phthalic anhydride tower, 102 Reboiler of phthalic anhydride tower, 103 Bottom pump of phthalic anhydride tower, 104 Residue tower, 105 Bottom pump of residue tower, 106 Reboiler of residue tower, 107 Control valve. DETAILED DESCRIPTION OF THE INVENTION

[0037] In the ranges disclosed herein, the endpoints and any value are not limited to the exact range or value. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and a single point value, and between single point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0038] A method for continuous low - energy - consumption phthalic anhydride recovery provided by the present invention includes:

[0039] The crude phthalic anhydride raw material after removing light components is introduced into the phthalic anhydride tower 101. Phthalic anhydride product is obtained at the top of the tower, and a mixture I containing phthalic anhydride and heavy components is obtained at the bottom of the tower. As a continuous extraction stream, it is sent to the residue tower 104 through the phthalic anhydride tower bottom pump 103 for concentration.

[0040] In the residue tower 104, the mixture I material is fed from the top or the middle. In order to reduce the residence time of heavy components at the bottom of the tower and avoid the formation of highly viscous impurities by high-boiling substances at high temperatures, a method of continuously discharging residues is adopted at the bottom of the tower. In order to reduce coking and avoid blockage of the heat exchanger, a forced external circulation evaporation method is adopted at the bottom of the tower, and a control valve is set on the circulation pipeline (at the tower inlet) to ensure that the tower bottom circulation is a liquid-phase external circulation process. After evaporation separation, recovered phthalic anhydride is obtained at the top of the tower, and a mixture II is obtained at the bottom of the tower. The bottom material of the tower passes through the residue tower bottom pump 105. Part of it is sent to the residue tower reboiler 106, and after heating, it returns to the residue tower 104. Part of the concentrated slag liquid is used as an extraction stream and is continuously sent to the downstream for treatment.

[0041] No condenser is installed at the top of the residue tower 104. The separated phthalic anhydride returns to the bottom of the phthalic anhydride tower 101 in gaseous form for recovery. Introducing gaseous phthalic anhydride into the phthalic anhydride tower 101 can replace part of the heat load of the phthalic anhydride tower reboiler 102, thereby reducing energy consumption.

[0042] Preferably, the method of the present invention further includes: before performing step (1), first removing light component impurities from the crude phthalic anhydride raw material generated by the reaction, and then sending it to the phthalic anhydride tower 101.

[0043] More preferably, the method of the present invention further includes: before performing step (2), in order to ensure the purity of the phthalic anhydride product at the top of the tower, the phthalic anhydride content in the mixture I should be higher than 80%.

[0044] Preferably, the top pressure of the phthalic anhydride tower 101 is -0.098 to -0.07 MPa (gauge pressure), preferably -0.095 to -0.08 MPa (gauge pressure), and the top temperature is 153 to 234 °C, preferably 175 to 219 °C.

[0045] Preferably, the top pressure of the residue tower 104 is -0.095 to -0.06 MPa (gauge pressure), preferably -0.093 to -0.07 MPa (gauge pressure), and the top temperature is 176 to 245 °C, preferably 186 to 235 °C.

[0046] Preferably, the mass ratio of the circulation amount at the bottom of the residue tower 104 to the feed amount of the residue tower 104 is 10 to 20.

[0047] According to a preferred specific embodiment, the method of the present invention further includes: the phthalic anhydride distilled from the top of the residue tower 104 in step (3) can also be condensed by a condenser and then introduced into the residue tower reflux drum, and then a part is drawn out from the reflux drum as reflux and returned to the residue tower, and a part is taken out as a liquid phase and sent to the phthalic anhydride tower 101.

[0048] In step (1), the phthalic anhydride tower bottom pump 103 is a centrifugal pump, a magnetic pump or a canned motor pump, preferably a magnetic pump.

[0049] The residue tower reboiler 106 is a falling film evaporator, a wiped film evaporator, a shell and tube heat exchanger or a U-tube heat exchanger, preferably a U-tube heat exchanger.

[0050] The content of the residue in the concentrated residue liquid obtained at the bottom of the residue tower 104 is 30-70%, preferably 40-60%.

[0051] In the case where there is no special description, in a method for continuous concentration of residues of the present invention, the specific operating conditions in each operating tower are not particularly limited, and various conditions commonly used in the art can be adopted. The specific operating conditions of the operating towers applied are exemplarily provided in the embodiment part of the present invention, and those skilled in the art should not understand it as a limitation to the present invention.

[0052] The following combines Figure 1 A preferred specific embodiment of a low-energy consumption phthalic anhydride continuous recovery method and device of the present invention will be described in detail:

[0053] The device of the present invention for realizing the continuous recovery of low-energy consumption phthalic anhydride mainly includes a phthalic anhydride tower 101, a phthalic anhydride tower reboiler 102, a phthalic anhydride tower bottom pump 103, a residue tower 104, a residue tower bottom pump 105, a residue tower reboiler 106 and a control valve 107. The phthalic anhydride tower 101 is provided with a raw material inlet, a phthalic anhydride product outlet, a tower bottom discharge port, a tower bottom material circulation port and a recovered phthalic anhydride feed port. The residue tower 104 is provided with a crude phthalic anhydride feed port, a gaseous phthalic anhydride outlet, a residue tower bottom discharge port and a tower bottom material circulation port. The phthalic anhydride tower bottom discharge port, the phthalic anhydride tower bottom pump 103 and the residue tower crude phthalic anhydride feed port are connected in sequence. The residue tower bottom discharge port, the residue tower bottom pump 105, the residue tower reboiler 106, the control valve 107 and the residue tower bottom material circulation port are connected in sequence. The gaseous phthalic anhydride outlet of the residue tower 104 is connected to the recovered phthalic anhydride feed port of the phthalic anhydride tower 101.

[0054] The crude phthalic anhydride raw material after removing light component impurities is sent to the bottom of the phthalic anhydride tower 101 or the inlet of the phthalic anhydride tower reboiler 102. The rectification separation of phthalic anhydride and heavy components is carried out in the phthalic anhydride tower 101, and the phthalic anhydride product is obtained at the top of the tower. To ensure the purity of the phthalic anhydride product at the top of the tower, a part of the phthalic anhydride needs to be sacrificed and left in the tower bottom. Therefore, the mixture I drawn from the tower bottom contains a large amount of phthalic anhydride and heavy component impurities. The mixture drawn from the tower bottom is continuously sent to the top or middle part of the residue tower 104 through the phthalic anhydride tower bottom pump 103, and the concentration of the residue is carried out in the residue tower 104. A part of the mixture II at the tower bottom of the residue tower is sent to the residue tower reboiler 106 through the residue tower bottom pump 105 for heating, and the heated residue is then returned to the bottom of the residue tower. A part of the concentrated slag liquid is sent as a drawn stream to the downstream for treatment continuously. The phthalic anhydride in the gas phase drawn from the top of the residue tower 104 is introduced to the bottom of the phthalic anhydride tower 101 for recovery.

[0055] However, the phthalic anhydride residue concentration process in existing industrial plants mostly adopts batch operation. The existing process is as follows: The mixture I drawn from the bottom of the phthalic anhydride tower is introduced into the residue collection tank. A high-pressure steam pipeline is arranged outside the collection tank, and a certain evaporation amount is obtained through steam heating. The phthalic anhydride evaporated is cooled and then returned to the phthalic anhydride tower in liquid form. After evaporating for a certain time, the feeding of the mixture I to the residue collection tank is stopped, and the material at the bottom of the residue collection tank is intermittently discharged downstream. The recovered phthalic anhydride is condensed into liquid phthalic anhydride by the condenser and enters the refining section.

[0056] Example 1

[0057] This example is carried out using the Figure 1 shown process flow.

[0058] The top pressure of the phthalic anhydride tower is -0.098 MPa, the top temperature is 153 °C, and the phthalic anhydride content in the mixture I is 80%.

[0059] The top pressure of the residue tower is -0.095 MPa, and the top temperature is 176 °C.

[0060] The mass ratio of the circulation amount at the bottom of the residue tower to the feeding amount of the residue tower is 10.

[0061] The residue content in the concentrated slag liquid obtained at the bottom of the residue tower is 70%.

[0062] Example 2

[0063] This example is carried out using the Figure 1 shown process flow.

[0064] The top pressure of the phthalic anhydride tower is -0.07 MPa, the top temperature is 234 °C, and the phthalic anhydride content in the mixture I is 98%.

[0065] The top pressure of the residue tower is -0.06 MPa, and the top temperature is 245 °C.

[0066] The mass ratio of the recycle flow rate at the bottom of the residue tower to the feed flow rate of the residue tower is 20.

[0067] The content of the residue in the concentrated slag liquid obtained at the bottom of the residue tower is 30%.

[0068] Example 3

[0069] This example uses Figure 1 the process flow shown.

[0070] The top pressure of the phthalic anhydride tower is -0.095 MPa, the top temperature is 175 °C, and the phthalic anhydride content in mixture I is 80%.

[0071] The top pressure of the residue tower is -0.093 MPa, and the top temperature is 186 °C.

[0072] The mass ratio of the recycle flow rate at the bottom of the residue tower to the feed flow rate of the residue tower is 15.

[0073] The content of the residue in the concentrated slag liquid obtained at the bottom of the residue tower is 54%.

[0074] Example 4

[0075] This example uses Figure 1 the process flow shown.

[0076] The top pressure of the phthalic anhydride tower is -0.08 MPa, the top temperature is 219 °C, and the phthalic anhydride content in mixture I is 80%.

[0077] The top pressure of the residue tower is -0.07 MPa, and the top temperature is 235 °C.

[0078] The mass ratio of the recycle flow rate at the bottom of the residue tower to the feed flow rate of the residue tower is 18.

[0079] The content of the residue in the concentrated slag liquid obtained at the bottom of the residue tower is 43%.

[0080] Example 5

[0081] This example uses Figure 1 the process flow shown.

[0082] The top pressure of the phthalic anhydride tower is -0.087 MPa, the top temperature is 205 °C, and the phthalic anhydride content in mixture I is 80%.

[0083] The top pressure of the residue tower is -0.085 MPa, and the top temperature is 210 °C.

[0084] The mass ratio of the circulation volume of the bottom of the residue tower to the feed volume of the residue tower is 18.

[0085] The content of the residue in the concentrated residue liquid obtained at the bottom of the residue tower is 51%.

[0086] Comparative Example 1

[0087] Prior art: After removing the light component impurities, the crude phthalic anhydride raw material is sent to the bottom of the phthalic anhydride tower. After rectification separation in the phthalic anhydride tower, the phthalic anhydride product is obtained at the top of the tower. The stream extracted from the bottom of the phthalic anhydride tower enters a residue collection tank, and steam is equipped outside the residue collection tank for heating. After a part of the phthalic anhydride material obtains heat and becomes a gas phase, it enters the condenser and is condensed into a liquid phase, and then the liquid-phase phthalic anhydride is extracted from the lower section of the condenser and enters the phthalic anhydride tower for recovery. The residue is intermittently discharged from the bottom of the residue collection tank.

[0088] The top pressure of the phthalic anhydride tower is -0.087 MPa, and the temperature is 205 °C.

[0089] The pressure of the residue collection tank is -0.085 MPa, and the temperature is 210 °C.

[0090] The pressure of the condenser is -0.087 MPa, and the temperature is 160 °C.

[0091] Table 1 lists the comparison of the separation efficiency and energy consumption between the examples and Comparative Example 1 of the prior art

[0092]

[0093] It can be found from Table 1 that due to the intermittent slag discharge process adopted in the existing process, the residence time of the heavy components in the residue collection tank is too long (generally more than 10 days), resulting in an increase in the content of the residue. The mass of the residue in Comparative Example 1 reaches 1.15% of the phthalic anhydride product, while the ratio of the residue to the phthalic anhydride product in the examples is less than 0.4%, and the viscosity of the residue liquid in the examples is significantly less than that in Comparative Example 1. In addition, the steam consumption of the examples is significantly lower than that of Comparative Example 1. Especially when comparing Example 5 and Comparative Example 1 under the same pressure, the annual steam consumption of Example 5 is reduced by more than half. In addition, the residue liquid system of the examples does not require the consumption of a cooling medium, while the annual consumption of the cooling medium in the residue liquid system of Comparative Example 1 is 17,948 tons. It can be seen that compared with the existing process, the present invention has significant energy savings.

[0094] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A method for continuous recovery of phthalic anhydride with low energy consumption, characterized in that: The method comprises the following steps: (1) introducing a crude phthalic anhydride raw material into a phthalic anhydride tower, obtaining a phthalic anhydride product at the top of the tower, and obtaining a mixture I containing phthalic anhydride and heavy components at the bottom of the tower, which is used as a continuous extraction stream and is pumped through the bottom of the phthalic anhydride tower to a residual liquid tower for concentration; (2) In the residual liquid tower, the mixture I material is fed from the top or the middle, and the bottom of the tower adopts continuous discharge of residue and forced external circulation evaporation. After evaporation and separation, the top of the tower obtains the recovered phthalic anhydride, and the bottom of the tower obtains the mixture II. The bottom of the tower material passes through the residual liquid tower bottom pump, and a part of it is sent to the residual liquid tower reboiler, and then returns to the residual liquid tower after heating. A part of the concentrated slag liquid is used as the production stream and continuously sent to the downstream for treatment; (3) There is no condenser at the top of the residual liquid column, and the separated phthalic anhydride is returned to the bottom of the phthalic anhydride column in the form of gas phase for recovery. The introduction of gas phase phthalic anhydride into the phthalic anhydride column can replace part of the heat load of the reboiler of the phthalic anhydride column; The mass ratio of the circulating amount of the residual liquid tower kettle to the feed amount of the residual liquid tower is 10 to 20; The tower top pressure of the phthalic anhydride tower is -0.098 to -0.07 MPa (gauge pressure), and the tower top temperature is 153 to 234° C.; The top pressure of the residual liquid tower is -0.095 to -0.06 MPa (gauge pressure), and the top temperature is 176 to 245°C.

2. The method according to claim 1, characterized in that The phthalic anhydride evaporated from the top of the residual liquid tower in the step (3) is condensed by a condenser and introduced into the reflux tank of the residual liquid tower. A portion of the phthalic anhydride is then drawn out from the reflux tank and returned to the residual liquid tower as reflux, and a portion is extracted as a liquid phase and sent to the phthalic anhydride tower.

3. The method according to claim 1, characterized in that Before carrying out step (1), the crude phthalic anhydride raw material generated by the reaction is firstly freed from light component impurities and then sent to the phthalic anhydride tower.

4. The method according to claim 1, characterized in that: The top pressure of the phthalic anhydride tower is -0.095 to -0.08 MPa (gauge pressure), and the top temperature is 175 to 219°C.

5. The method according to claim 1, characterized in that The top pressure of the residual liquid tower is -0.093 to -0.07 MPa (gauge pressure), and the top temperature is 186 to 235°C.

6. The method according to claim 1 or 2, characterized in that The mass ratio of the circulation amount of the residual liquid tower kettle to the feed amount of the residual liquid tower is 15-18.

7. The method according to claim 1, characterized in that Before step (2), in order to ensure the purity of the phthalic anhydride product at the top of the tower, the phthalic anhydride content in the mixture I is higher than 80%.

8. The method according to claim 1 or 2, characterized in that The residue content in the concentrated residue liquid obtained from the bottom of the residual liquid tower is 30-70%.

9. A device for realizing the method for continuous recovery of low-energy phthalic anhydride according to claim 1, characterized in that: Including phthalic anhydride tower, phthalic anhydride tower reboiler, phthalic anhydride tower kettle pump, residual liquid tower, residual liquid tower kettle pump, residual liquid tower reboiler; The phthalic anhydride tower is provided with a raw material inlet, a phthalic anhydride product outlet, a tower bottom discharge port, a tower bottom material circulation port and a phthalic anhydride recovery feed port; The residual liquid tower is provided with a crude phthalic anhydride feed port, a gaseous phthalic anhydride outlet, a residual liquid tower kettle discharge port and a kettle material circulation port; The phthalic anhydride tower kettle discharge port, the phthalic anhydride tower kettle pump and the crude phthalic anhydride feed port of the raffinate tower are connected in sequence; The residual liquid tower kettle discharge port, the residual liquid tower kettle pump, the residual liquid tower reboiler, the control valve and the residual liquid tower kettle material circulation port are connected in sequence; The gas phase phthalic anhydride outlet of the residual liquid tower is connected to the phthalic anhydride recovery feed port of the phthalic anhydride tower.

10. The device according to claim 9, characterized in that The phthalic anhydride tower kettle pump is a centrifugal pump, a magnetic pump or a canned pump; The reboiler of the residual liquid tower is a falling film evaporator, a scraped film evaporator, a shell and tube heat exchanger or a U-tube heat exchanger.

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