A method for separating dimethyl ether in a methanol to propylene process

Abstract

The present invention discloses a method for separating dimethyl ether in a methanol-to-propylene process, comprising: (1) passing a raw material containing DME into an MTP reactor for a propylene synthesis reaction, wherein the obtained product mixture contains unconverted DME; (2) passing the product mixture into a quenching absorption stabilization device to separate and obtain liquefied gas, wherein the liquefied gas contains hydrocarbons with a carbon number of 1-6 and unconverted DME; (3) passing the liquefied gas through a depropanizer, a deethanizer, and a propylene tower for rectification and separation, wherein: hydrocarbons with a carbon number of 3 or less are separated at the top of the depropanizer, and then further separated to obtain a propylene product and a propane product; hydrocarbons with a carbon number of 4 or more are separated at the bottom of the depropanizer; and a DME component mixture is extracted at a position below the liquefied gas feed port of the depropanizer and above the bottom of the tower. This method can efficiently separate DME and propane.

Description

Technical Field

[0001] The present invention relates to the technical field of methanol to propylene (MTP) process, and in particular to a method for separating dimethyl ether (DME) in the methanol to propylene (MTP) process. Background Art

[0002] The methanol-to-propylene process (MTP process) was successfully developed by Lurgi in Germany in the 1990s. The process involves the following: First, refined methanol output from the methanol plant is fed to a dimethyl ether (DME) pre-reactor, where it is converted into DME and water. The methanol, water, DME vapor, and recycled olefins and steam are then fed to the MTP reactor, where the majority of the methanol and DME are converted into a product mixture of hydrocarbons, primarily light olefins (C2-C5 hydrocarbons, such as ethylene, propylene, ethane, propane, butane, and butene), and more primarily propylene. The product mixture exiting the MTP reactor undergoes cooling, separation, compression, drying, distillation, and impurity removal, ultimately yielding various high-purity hydrocarbon products. This process, an important supplemental method for producing propylene, produces relatively few by-products, resulting in a simpler separation and purification process and higher propylene yields.

[0003] The by-product propane produced in the MTP process can be used as a feedstock for propane dehydrogenation or light hydrocarbon cracking units, or as liquefied petroleum gas (mainly composed of hydrocarbons, with the main components being propane, butane and other alkanes).

[0004] However, the propane product produced by this process often contains a large amount of DME. This is because the DME formed as an intermediate product in the MTP process is not fully converted in the MTP reactor, and because there is no dedicated removal site for oxygenated DME in the light hydrocarbon stream during subsequent separation, the propane stream often contains DME. However, propane dehydrogenation or light hydrocarbon cracking units have strict restrictions on the dimethyl ether content in the feed propane, such as requiring the total oxygenate content (including dimethyl ether, methanol, and acetone) in the feed propane to be ≤30 mg / kg. Furthermore, the General Administration of Quality Supervision, Inspection and Quarantine explicitly prohibits the blending of dimethyl ether with civilian liquefied petroleum gas (Quality Inspection Special Letter (2008) No. 17, "Notice on Issues Related to Gas Cylinder Filling"). Therefore, it is necessary to find a suitable method to remove DME from propane and improve the purity of the propane product. Therefore, how to optimize the MTP process to obtain a higher-purity propane product and achieve higher DME utilization has been a long-standing concern for researchers.

[0005] Chinese patent CN203904242U discloses a method for removing and recycling DME in an MTP process. In this method, the bottom discharge port of the depropanizer is connected in series with an extraction separation tank, a methanol recovery tower, a DME reactor, and an MTP reactor. The depropanizer is also provided with a methanol feed port. Methanol is directly injected into the depropanizer, which simply and efficiently dissolves the DME product at the top of the depropanizer and separates it from propylene and C3. DME / methanol accumulates at the bottom of the tower and is separated, ensuring the purity of the propylene product. DME is recovered through a simple extraction and separation process and returned to the reactor for reuse. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to simply and efficiently separate dimethyl ether from the product stream of a methanol to propylene (MTP) reactor in a MTP process.

[0007] In order to achieve the above object, the present invention provides a method for separating dimethyl ether (DME) in a methanol to propylene (MTP) process, characterized in that it comprises the following steps:

[0008] (1) introducing a raw material containing DME into an MTP reactor to carry out a propylene synthesis reaction to obtain a product mixture containing unconverted DME;

[0009] (2) passing the product mixture into a quenching absorption stabilization device to separate and obtain liquefied gas, wherein the liquefied gas contains hydrocarbons with a carbon number of 1 to 6 and unconverted DME;

[0010] (3) The liquefied gas is distilled and separated through a depropanizer, a deethanizer, and a propylene tower, wherein:

[0011] Separating hydrocarbons with a carbon number of 3 or less at the top of the depropanizer, and then further separating to obtain propylene products and propane products;

[0012] Separating hydrocarbons having 4 or more carbon atoms at the bottom of the depropanizer;

[0013] The DME component mixture is withdrawn at a position below the liquefied gas feed port of the depropanizer and above the bottom of the depropanizer.

[0014] In the present invention, hydrocarbons with 3 or less carbon atoms are abbreviated as "C3-"; hydrocarbons with 3 carbon atoms are abbreviated as "C3"; hydrocarbons with 4 or more carbon atoms are abbreviated as "C4+"; hydrocarbons with 4 carbon atoms are abbreviated as "C4"; "C1", "C2", "C5", and "C5+" have similar meanings, where "C" represents a carbon atom; and the numbers 1, 2, 5, and 5+ represent the number of carbon atoms.

[0015] In the present invention, "methyl tert-butyl ether" is abbreviated as MTBE.

[0016] In the present invention, the fraction 3 / 10 to 5 / 10 in “the liquefied gas feed port is located at any tray within the region 3 / 10 to 5 / 10 from top to bottom” can be understood as, trays are installed inside the depropanizer tower body, and the total height of the depropanizer tower body is regarded as 10 parts, and the liquefied gas feed port is located at any tray within the region 3 / 10 to 5 / 10 from top to bottom of the tower body.

[0017] In the present invention, other fractions used to describe some positions on the tower body also have similar meanings.

[0018] In the method of the present invention, in step (1), the raw material contains water vapor in addition to DME.

[0019] In the method of the present invention, in step (2), the product mixture is passed through a quench absorption stabilization device to separate, in addition to obtaining liquefied gas, water, gasoline (primarily C5+ components), and dry gas (primarily C1 components). The C2-C4 components and unconverted DME are enriched in the liquefied gas and are further separated in subsequent steps.

[0020] In the method of the present invention, in step (3), the liquefied gas can be sequentially passed through a depropanizer, a deethanizer, and a propylene tower for distillation separation. Specifically, the liquefied gas can be first passed through a depropanizer for distillation; then the first mixture obtained from the top of the depropanizer is sequentially passed through a deethanizer and a propylene tower to separate C2-, propylene, and propane products; and the second mixture obtained from the bottom of the depropanizer is sequentially passed through an MTBE reactor and an azeotropic column to obtain MTBE and C4 products.

[0021] In the method of the present invention, in step (3), the liquefied gas can also be sequentially passed through a deethanizer, a depropanizer, and a propylene tower for distillation separation. Specifically, the liquefied gas can be first passed through a deethanizer for separation, and a C2- stream can be obtained at the top of the deethanizer, and a C3+ stream can be obtained at the bottom of the deethanizer; then the C3+ stream obtained from the bottom of the deethanizer is passed through a depropanizer for distillation separation, and the first mixture obtained from the top of the depropanizer is passed through the propylene tower to separate and obtain a propylene product and a propane product. The second mixture obtained from the bottom of the depropanizer is sequentially passed through an MTBE reactor and an azeotropic tower to obtain MTBE and a C4 product.

[0022] In the method of the present invention, in step (3), the DME component mixture is extracted at a position below the liquefied gas feed inlet of the depropanizer and above the bottom of the tower. In the method of the present invention, the DME component mixture extracted in step (3) can be a gaseous material or a liquid material, provided that the extraction position is below the liquefied gas feed inlet of the depropanizer and above the bottom of the tower. Preferably, the gaseous DME component mixture is extracted in step (3) to obtain a better separation effect.

[0023] In the method of the present invention, in step (3), the DME component mixture extracted contains C3- and C4+ in addition to DME, and the total content of C3- and C4+ is less than 55V%.

[0024] In one embodiment of the method of the present invention, in step (3), the operating temperature of the top of the depropanizer is 30-70°C and the operating pressure is 1.35-1.7 MPa, and the operating temperature of the bottom is 85-95°C and the operating pressure is 1.35-1.8 MPa; in step (3), under the condition that the total number of trays of the depropanizer is 54-84 and the liquefied gas feed port is located at any tray in the 3 / 10 to 5 / 10 area from top to bottom, the DME component mixture is extracted from any tray in the 6 / 10 to 8 / 10 area from top to bottom. In the method of the present invention, in step (3), the extraction position of the DME component mixture is affected by the following factors: the composition, content and other properties of the feed, the operating parameters of the depropanizer, the total number of trays of the depropanizer, the position of the liquefied gas feed port tray, etc.

[0025] In a further embodiment of the method of the present invention, in step (3), the DME component mixture is extracted from any tray within the region of 7 / 10 to 8 / 10 from top to bottom, while the liquefied gas feed port is located at any tray within the region of 4 / 10 to 5 / 10 from top to bottom. The mixture extracted from this tray position has a higher DME content, enabling more efficient separation of DME from C3 and C4.

[0026] In a further embodiment of the present invention, the method further comprises the following step: (4a) directly recycling the DME component mixture back to the MTP reactor via a pipeline. Because the DME concentration in the withdrawn mixture is significantly high, recycling the DME component mixture back to the MTP reactor in step (4a) can reuse the incompletely reacted DME, thereby improving the feedstock conversion rate.

[0027] In a further embodiment of the method of the present invention, in step (3), the flow rate of the DME component mixture withdrawn at a position below the liquefied gas feed inlet and above the bottom of the depropanizer does not exceed 1 / 3 of the flow rate of the DME feedstock entering the MTP reactor. In a further embodiment of the method of the present invention, for example, the flow rate of the DME feedstock and the water vapor feedstock is 20-30 tons / hour; in step (3), the flow rate of the DME component mixture withdrawn at a position below the liquefied gas feed inlet and above the bottom of the depropanizer is 0-10 tons / hour.

[0028] In other words, the DME component mixture extraction rate can be adjusted over a wide range. For example, if the MTP reactor contains fresh catalyst and the DME reaction is relatively complete, with no unreacted DME, extraction can be omitted and the extraction rate can be set to zero. However, if the catalyst in the MTP reactor is significantly deactivated, the catalytic conversion rate is low, and a large amount of DME remains unreacted, the extraction rate of the DME component mixture can be increased until the propane product obtained from the bottom of the propylene column passes testing.

[0029] In a further embodiment of the method of the present invention, the following steps may also be included: (4b) passing the DME component mixture into a coupling tower, distilling and separating it again, and obtaining high-purity DME in the coupling tower; (6) recycling the high-purity DME back to the MTP reactor. In the method of the present invention, the depropanizer and the coupling tower are used for secondary distillation and separation, and high-purity DME is obtained in the coupling tower. In the method of the present invention, high-purity DME refers to DME with a concentration greater than 99% by volume. After distillation and separation in the coupling tower, the non-oxide hydrocarbon components are recycled back to the depropanizer, and the high-purity DME is recycled back to the MTP reactor. In this way, the adverse effects that may be caused by the recycling of components other than DME into the MTP reactor can be avoided.

[0030] In a further embodiment of the method of the present invention, in step (4b), the liquid phase of any one of the 18th to 22nd trays of the depropanizer flows into the first tray at the top of the coupling tower, and the gas phase of any one of the 40th to 56th trays of the depropanizer flows into the first tray at the bottom of the coupling tower; the gas phase at the top of the coupling tower enters the tray from which the liquid phase flows out of the depropanizer, and the liquid phase at the bottom of the coupling tower enters the tray from which the gas phase flows out of the depropanizer; in step (6), high-purity DME is extracted from the middle and lower part of the coupling tower side line. In the method of the present invention, there is no need to set up a condensing tower (top) and a reboiler (bottom) in the coupling tower. The coupling tower is connected in parallel with the depropanizer, and the temperature difference, pressure difference and other conditions at different tray positions of the depropanizer itself are used to drive the operation of the coupling tower to further distill and separate the DME component mixture.

[0031] In a further embodiment of the method of the present invention, in step (4b), the total number of trays in the coupling column is 25-35, and high-purity DME is extracted from any one of the 15th to 25th trays in the coupling column sideline. In a coupling column having 25-35 trays and connected in parallel to the depropanizer at the aforementioned tray positions, it is preferred to extract DME from any one of the 18th to 22nd trays in the coupling column sideline, as the purity of the DME extracted at this position is higher.

[0032] In a further embodiment of the method of the present invention, in step (4b), the flow rate of the liquid phase flowing from the depropanizer tray into the coupling column is 1 / 4 to 3 / 4 of the flow rate of the original liquid phase in the depropanizer tray; and the flow rate of the gas phase flowing from the depropanizer tray into the coupling column is 1 / 4 to 3 / 4 of the flow rate of the gas phase in the depropanizer tray. Thus, the requirement for efficient separation of DME from the depropanizer can be met.

[0033] In the method of the present invention, step (4a) and step (4b) are not performed simultaneously. In the method of the present invention, after step (3), either step (4a) or step (4b) is optionally performed.

[0034] Specifically, after step (3), step (4a) can be optionally implemented to circulate the DME back to the MTP reactor in the form of a DME component mixture for re-reaction; alternatively, after step (3), steps (4b) and (6) can be optionally implemented sequentially to circulate the DME back to the MTP reactor in the form of high-purity DME for re-reaction.

[0035] In the method of the present invention, the towers are all distillation towers. The distillation towers are, for example, packed towers, the number of packings being equivalent to the number of trays, and the positions of the trays remain unchanged.

[0036] In an embodiment of the method of the present invention, the operating temperature of the MTP reactor is 400° C. to 520° C., and the operating pressure (gauge pressure) is 0.01 MPa to 0.2 MPa.

[0037] In a further embodiment of the process of the present invention, the operating temperature of the MTP reactor is 450° C. to 520° C., and the operating pressure (gauge pressure) is 0.05 MPa to 0.08 MPa.

[0038] Compared with the prior art, the method for separating dimethyl ether in the methanol to propylene process (MTP) provided by the present invention has at least the following beneficial effects:

[0039] 1. The present invention's method involves separating three streams from a depropanizer, wherein the DME component mixture constitutes the third stream. Unexpectedly, the inventors discovered the presence of a mixture at a certain location within the depropanizer, with a DME concentration (greater than 45% by volume) significantly higher than that in liquefied petroleum gas (approximately 2% by volume) and C3 products (approximately 10% by volume). This mixture, referred to herein as the DME component mixture, is extracted to significantly reduce the DME concentration of the remaining stream, thereby obtaining substantially DME-free propane with a propane content greater than 98.5% by volume without further DME separation.

[0040] 2. The method of the present invention can efficiently separate DME mixed in C3- and / or C4+ simultaneously through a single extraction step by adjusting the temperature and pressure at the top and bottom of the depropanizer. For example, after adding the step of extracting the DME component mixture, if DME appears at the top, the top temperature of the depropanizer is appropriately lowered or the top pressure is appropriately increased; if DME appears at the bottom, the bottom temperature is appropriately increased or the bottom pressure is appropriately lowered; if DME appears at both the top and the bottom, the reflux ratio is increased and the amount of DME extracted is increased. In this way, the DME concentration in the first mixture (mainly containing C3-) at the top of the depropanizer and / or the second mixture (mainly containing C4+) at the bottom of the depropanizer can be regulated separately or simultaneously to reduce the DME concentration in the top material and the bottom material to a sufficiently low level (less than 0.009V%).

[0041] In addition, the method of the present invention can recover all unreacted DME to the greatest extent by simultaneously regulating the DME concentrations at the top and bottom of the depropanizer, with a high recovery rate and simple steps.

[0042] In addition, the method of the present invention can recycle the extracted DME for repeated reaction, and the utilization rate of the DME raw material is high.

[0043] 3. The method of the present invention can choose to directly recycle the extracted DME component mixture to the MTP reactor, thereby simplifying the process, reducing equipment investment costs, and facilitating maintenance. Alternatively, the extracted DME component mixture can be passed through a newly built coupling tower for further separation to obtain high-purity (greater than 99% by volume) DME, which is then recycled to the MTP reactor. This prevents the introduction of other hydrocarbon substances into the MTP reactor and does not affect the single-pass processing capacity of the MTP reactor. Moreover, the coupling tower does not have a condenser or reboiler, and energy consumption does not change.

[0044] In actual implementation, staff can make choices according to their needs. The method has a wide range of industrial applications and is highly practical. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1The present invention is a flow chart of a method for producing propylene (MTP) from DME in the prior art.

[0046] Figure 2 Schematic diagram of a method for separating dimethyl ether (DME) in an MTP process according to one embodiment of the present invention.

[0047] Figure 3 FIG2 is a flow chart of a method for separating dimethyl ether (DME) in an MTP process according to another embodiment of the present invention.

[0048] Description of Figure Numbers:

[0049] MTP reactor-1; quench absorption stabilization equipment-2; depropanizer-3; deethanizer-4; propylene tower-5; methyl tert-butyl ether (MTBE) reactor-6; azeotropic tower-7; DME-8; water vapor-9; product mixture-10; dry gas-11; liquefied gas-12; gasoline-13; water-14; C3--15; C4+-16; C2- products-17; C3-18; C4 products-19; propylene products-20; propane products-21; MTBE products-22; methanol-23; DME component mixture-24; high-purity DME-25; coupling tower-26. DETAILED DESCRIPTION

[0050] The present invention will be further described below with reference to specific embodiments, and the advantages and features of the present invention will become clearer as the description proceeds. However, these embodiments are merely exemplary and do not constitute any limitation to the scope of the present invention. It should be understood by those skilled in the art that the details and forms of the technical solutions of the present invention may be modified or replaced without departing from the spirit and scope of the present invention, and such modifications and replacements fall within the scope of protection of the present invention.

[0051] Figure 1 The present invention is a flow chart of a method for producing propylene (MTP) from DME in the prior art.

[0052] like Figure 1 As shown, first, in step (1), DME 8 raw material and water vapor 9 are introduced into the MTP reactor 1 at a flow rate of 25 tons / hour, and a propylene synthesis reaction is carried out at a temperature of 460° C. and a pressure of 0.06 MPa to obtain a product mixture 10.

[0053] Then, in step (2), the product mixture 10 is passed into a quench absorption stabilization device 2. In the quench absorption stabilization device 2, the product mixture 10 is separated into dry gas 11, liquefied gas 12, gasoline 13 and water 14.

[0054] Then, in step (3), the liquefied gas 12 is introduced into the depropanizer 3 through the liquefied gas feed port on the side wall of the depropanizer 3 for distillation separation. The depropanizer 3 has a total of 69 trays, and the liquefied gas 12 enters the depropanizer 3 from the 30th tray. The operating temperature at the top of the depropanizer 3 is 50°C and the operating pressure is 1.5 MPa; the operating temperature at the bottom of the depropanizer 3 is 90°C and the operating pressure is 1.55 MPa.

[0055] In step (3), as the distillation separation continues, the first mixture C3- (C3 and lower hydrocarbons below C3) 15 obtained from the top of the depropanizer 3 is extracted and transported to the deethanizer 4 for distillation separation of the C2- and C3 components. A C2- product 17 is obtained at the top of the deethanizer 4, and C3 18 is obtained at the bottom of the tower. The C3 18 material is then passed into the propylene tower 5 for distillation separation to obtain a propylene product 20 and a propane product 21. A second mixture C4+ (C4 and lower hydrocarbons below C4) 16 is obtained from the bottom of the depropanizer 3. The C4+16 is extracted and transported to the MTBE reactor 6 for synthesis reaction with methanol 23. The resulting mixture containing the product MTBE is then passed into the azeotropic tower 7 for separation to obtain an MTBE product and a C4 product.

[0056] In step (1), as the operating time of the MTP reactor 1 increases and the catalyst activity decreases, unconverted DME begins to appear in the product mixture 10, and the amount may even increase. Accordingly, in addition to C3- and C4+, the liquefied gas 12 also contains unconverted DME. The content of unconverted DME changes dynamically over time. As shown in Table 1, the DME content in the liquefied gas 12 is 1.7969 V%.

[0057] Table 1 Volume fraction of each stream (V%)

[0058]

[0059] (In Table 1 above, — means not detected)

[0060] In step (3), after such liquefied gas 12 is transferred to the depropanizer 3, under the conventional operating conditions of the depropanizer 3 described above, the DME component in the distillation tower is generally slightly heavier than propane and lighter than C4, and close to propane. Therefore, DME is easily mixed with propane and flows out of the top of the depropanizer 3, and is difficult to separate. In step (3), after distillation and separation in the deethanizer 4 and propylene tower 5, DME is further enriched in propane, reaching a higher concentration of approximately 10% by volume, resulting in substandard propane product 21. This is shown in Table 1 above and Tables 2, 3, and 4 below.

[0061] Table 2 Propane product composition test results (April 4, 2022, 13:10)

[0062] Ingredient name Content (V%) methane 0.0072 Ethane 0.1873 Ethylene 0.0144 Propane 68.5372 Propylene 1.4334 Isobutane 1.2317 n-butane 0.2305 n-Butene / trans-Butene 0.2593 Cis-butene 0.0792 Isobutylene 0.0432 Butadiene 0.0005 dimethyl ether 27.9700 C5 0.0061

[0063] Table 3 Propane product composition test results (April 11, 2022, 16:04)

[0064]

[0065]

[0066] (In Table 3 above, — indicates not detected)

[0067] Table 4 Propane product composition test results (April 12, 2022, 10:17)

[0068] Ingredient name Content (V%) methane 0 Ethane 0 Ethylene 0 Propane 90.77 Propylene 0.41 Isobutane 0.52 n-butane 0.29 n-Butene / trans-Butene 0.36 Cis-butene 0.004 Isobutylene 0.018 Butadiene 0.001 dimethyl ether 7.6 C5 0.004

[0069] The chromatographic analysis data of the propane products in Tables 2, 3 and 4 show that the DME content in the propane product 21 is relatively high, namely 27.97V%, 7.86V% and 7.6V%, respectively.

[0070] The data of upper table 1 are obtained by software calculation. Further observe the variation of DME content data in the prior art method shown in table 1, wherein, the DME content in the liquefied gas 12 that enters depropanizer 3 is 1.7969V%, and the DME content in the first strand of mixture that the tower top of depropanizer obtains is 3.8902V%, and the DME content in the second strand of mixture that the tower bottom of depropanizer obtains is 0V%.As can be seen, in the production process of the MTP process of prior art, DME is substantially enriched in tower top (to 3.8902V%) in the discharging of depropanizer 3.Based on such apparent data, people tend to select to extract DME from tower top and carry out aftertreatment, for example, separate DME.

[0071] Example 1

[0072] Figure 2 Schematic diagram of a method for separating dimethyl ether (DME) from an MTP process according to one embodiment of the present invention.

[0073] In Example 1, the method for producing propylene from DME (MTP) is similar to that described in the prior art, except that, in step (3), a third mixture, a gaseous DME component mixture 24, is withdrawn from the 50th tray of the depropanizer 3, and DME is separated through this operation. Then, in step (4a), the gaseous DME component mixture 24 is directly recycled back to the MTP reactor 1 via a pipeline, reusing the unreacted DME and improving the conversion rate of the feedstock.

[0074] In this embodiment 1, in step (3), the flow rate of the gaseous DME component mixture extracted can be dynamically adjusted, and generally does not exceed 1 / 3 of the flow rate of the DME raw material entering the MTP reactor 1. In other words, when the MTP system is running, when the feed flow rate of the DME raw material is 25 tons / hour, the extraction flow rate of the gaseous DME component mixture can be dynamically adjusted in real time within the range of 0-8 tons / hour. The starting point of the adjustment is to ensure that the DME content in the propane product 21 obtained from the bottom of the propylene tower 5 is detected to be qualified.

[0075] For example, if the MTP reactor contains fresh catalyst and the DME reaction is relatively complete, with no unreacted DME present, extraction can be omitted and the extraction rate can be set to 0 tons / hour. As the catalyst gradually deactivates, the catalytic conversion rate decreases, and the amount of unreacted DME increases within the MTP reactor, the extraction rate of the gaseous DME component mixture can be increased accordingly. The extraction rate of the gaseous DME component mixture can also be coordinated with the operating temperature and pressure at the top and bottom of the depropanizer 3. During the extraction process, if DME appears at the top, the top temperature of the depropanizer can be appropriately lowered or the top pressure increased. If DME appears at the bottom, the bottom temperature can be appropriately raised or the bottom pressure reduced. If DME appears at both the top and bottom, the DME extraction rate can be increased, and the reflux ratio can be increased, provided that the top operating temperature is maintained at 30-70°C and the pressure is maintained at 1.35-1.7 MPa, and the bottom operating temperature is maintained at 85-95°C and the pressure is maintained at 1.35-1.8 MPa.

[0076] The present inventors found that under normal operation of the entire MTP system, unexpectedly, a higher concentration of DME (45V%) was enriched in the gas phase near the 50th tray of the depropanizer 3 (rather than at a position above the feed inlet), as shown in Tables 5, 6, 7 and 8 below.

[0077] Table 5 Test results of gaseous DME component mixture (April 19, 2022, 16:01)

[0078]

[0079]

[0080] Table 6 Test results of gaseous DME component mixture (April 30, 2022, 12:00)

[0081] Ingredient name Content (V%) methane 0.0008 Ethane 0.0015 Ethylene 0.0021 Propane 17.77 Propylene 19 Isobutane 10.7 n-butane 1.29 n-Butene / trans-Butene 1.5 Cis-butene 0.5 Isobutylene 2.46 Butadiene 0.01 dimethyl ether 46.31 C5 0.51

[0082] Table 7 Test results of gaseous DME component mixture (October 26, 2022, 8:00)

[0083]

[0084]

[0085] Table 8 Test results of gaseous DME component mixture (October 26, 2022, 16:00)

[0086] Ingredient name Content (V%) methane 0.00 Ethane 0.00 Ethylene 0.00 Propane 0.97 Propylene 4.11 Isobutane 22.57 n-butane 3.49 n-Butene / trans-Butene 7.45 Cis-butene 2.12 Isobutylene 11.3917 Butadiene 0.042 dimethyl ether 48.01 C5 0.07

[0087] The chromatographic analysis data of the gaseous DME component mixtures in Tables 5, 6, 7 and 8 show that the DME contents in the gaseous DME component mixtures extracted in step (3) of the present invention are 79.25V%, 46.31V%, 45.03V% and 48.01V%, respectively, all being above 45V%, and the concentrations are much higher than that in the propane product 21 (10V%).

[0088] The method of the present invention reduces the DME content in the second mixture obtained at the bottom of the depropanizer 3 to below 0.009V% by extracting the gaseous DME component mixture in step (3), and reduces the DME content in the first mixture obtained at the top of the depropanizer 3 to below 0.009V%. Furthermore, the DME content in the propane product is reduced to below 0.4V%, thereby achieving the effect of highly efficient DME separation, as shown in Tables 9, 10, 11 and 12 below.

[0089] Table 9 Volume fraction of each stream (V%)

[0090]

[0091] (In Table 9 above, — indicates not detected)

[0092] Table 10 Propane product composition test results (April 19, 2022, 0:08)

[0093]

[0094]

[0095] Table 11 Propane product composition test results (October 26, 2022, 16:00)

[0096] Ingredient name Content (V%) methane 0.002 Ethane 0.0089 Ethylene 0.0039 Propane 97.11 Propylene 1.65 Isobutane 0.79 n-butane 0.0042 n-Butene / trans-Butene 0.01 Cis-butene 0.0009 Isobutylene 0.03 Butadiene 0.0015 dimethyl ether 0.4 C5 0.0031

[0097] Table 12 Propane product composition test results (October 26, 2022, 20:00)

[0098]

[0099]

[0100] The chromatographic analysis data results in Tables 9, 10, 11 and 12 show that after DME is separated by the method of the present invention, the DME content in the propane product 21 is relatively low, namely 0.0242V%, 0.05V%, 0.4V% and 0.34V%, respectively.

[0101] Thus, by extracting the gaseous DME component mixture at the 50th tray of the depropanizer 3, a good DME separation effect is achieved, and a propane product 21 with a DME content that meets the use requirements can be obtained without further separation processing.

[0102] Example 2

[0103] Figure 3 The present invention is a flow chart of a method for separating dimethyl ether (DME) from an MTP process according to another embodiment of the present invention.

[0104] The method for separating dimethyl ether (DME) from an MTP process in Example 2 is similar to the method in Example 1, except that: in step (4b), 1 / 2 of the liquid phase in the 20th tray of the depropanizer 3 is introduced into the first tray at the top of the coupling tower 27, and 1 / 2 of the gas phase in the 50th tray of the depropanizer 3 is introduced into the penultimate tray at the bottom of the coupling tower 27 for distillation separation; the gas phase at the top of the coupling tower 27 is directed back to the 20th tray of the depropanizer 3, and the liquid phase at the bottom of the coupling tower 27 is directed back to the 50th tray of the depropanizer 27. In step (6), high-purity DME 25 is extracted from the 20th tray of the coupling tower 27 and circulated back to the MTP reactor 1.

[0105] After being processed by the method of Example 2, the propane product 21 was sampled and analyzed. The test results are shown in Table 13 below.

[0106] Table 13 Volume fraction of each stream (V%)

[0107]

[0108] (In Table 13 above, — indicates not detected)

[0109] The data results in Table 13 show that the dimethyl ether content in the propane product 21 is significantly reduced to 0.0172V%, and the DME concentration in the high-purity DME 25 is 99.5V%.

Claims

1. A method for separating dimethyl ether (DME) in a methanol to propylene (MTP) process, characterized in that: The following steps are involved: (1) introducing a raw material containing DME into an MTP reactor to carry out a propylene synthesis reaction to obtain a product mixture containing unconverted DME; (2) passing the product mixture into a quenching absorption stabilization device to separate and obtain liquefied gas, wherein the liquefied gas contains hydrocarbons with a carbon number of 1 to 6 and unconverted DME; (3) The liquefied gas is distilled and separated through a depropanizer, a deethanizer, and a propylene tower, wherein: Separating hydrocarbons with a carbon number of 3 or less at the top of the depropanizer, and then further separating to obtain propylene products and propane products; Separating hydrocarbons having 4 or more carbon atoms at the bottom of the depropanizer; Extracting a DME component mixture at a position below the liquefied gas feed port and above the bottom of the depropanizer; Wherein, in step (3), the operating temperature of the top of the depropanizer is 30-70°C, the operating pressure is 1.35-1.7 MPa, and the operating temperature of the bottom of the depropanizer is 85-95°C, the operating pressure is 1.35-1.8 MPa; In step (3), under the condition that the total number of trays of the depropanizer is 54-84 and the liquefied gas feed port is located at any tray in the region of 3 / 10 to 5 / 10 from top to bottom, the DME component mixture is extracted from any tray in the region of 6 / 10 to 8 / 10 from top to bottom; The following steps are also included: (4b) passing the DME component mixture into a coupling tower, and performing further distillation and separation to obtain high-purity DME in the coupling tower; (6) recycling the high-purity DME back to the MTP reactor; Wherein, in step (4b), the liquid phase of any one of the 18th to 22nd trays of the depropanizer flows into the first tray at the top of the coupled distillation tower, and the gas phase of any one of the 40th to 56th trays of the depropanizer flows into the first tray at the bottom of the coupled tower; The gas phase at the top of the coupling tower enters the tray of the depropanizer and flows out of the liquid phase, and the liquid phase at the bottom of the coupling tower enters the tray of the depropanizer and flows out of the gas phase; In step (6), high-purity DME is extracted from the lower part of the coupling tower side line; There is no need to arrange a top condenser and a bottom reboiler in the coupling tower.

2. The method according to claim 1, characterized in that In step (3), under the condition that the liquefied gas feed port is located on any tray in the 4 / 10 to 5 / 10 area from top to bottom, the DME component mixture is extracted from any tray in the 7 / 10 to 8 / 10 area from top to bottom.

3. The method according to claim 1, characterized in that In step (3), the flow rate of the gaseous DME component mixture extracted from a position below the liquefied gas feed inlet of the depropanizer and above the bottom of the tower does not exceed 1 / 3 of the DME raw material flow rate entering the MTP reactor.

4. The method according to claim 1, wherein In step (4b), the total number of trays of the coupling tower is 25-35, and high-purity DME is extracted from any one of the 15th to 25th trays of the coupling tower side line.

5. The method according to claim 1, wherein In step (4b), the amount of the liquid phase flowing from the depropanizer tray into the coupling tower is 1 / 4 to 3 / 4 of the amount of the liquid phase in the tray in the depropanizer; the amount of the gas phase flowing from the depropanizer tray into the coupling tower is 1 / 4 to 3 / 4 of the amount of the gas phase in the tray in the depropanizer.

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