Separation of N-methylmorpholine rich in water from mixtures containing N-methylmorpholine, water, methanol and high boiling residues

Abstract

A process for separating a mixture containing at least methanol, N-methylmorpholine (NMM), water and high boilers by distillation in a first distillation column and a second distillation column, the process comprising the steps of: (i) feeding the mixture containing at least methanol, NMM, water and high boilers to the first distillation column, wherein the water / (NMM + water) weight ratio is lower than the weight ratio in the azeotropic NMM / water mixture at the operating pressure of the second distillation column, (ii) separating methanol as an overhead stream of the first distillation column wherein a mixture containing NMM and water is obtained as a bottom draw stream of the first distillation column, wherein the water / (NMM + water) weight ratio is lower than the weight ratio in the azeotropic NMM / water mixture at the operating pressure of the second distillation column, (iii) adding water to the bottoms draw stream of the first distillation column in an amount, the amount is such that the water / (NMM + water) weight ratio is still lower than, but closer to, the water / (NMM + water) ratio in the azeotropic NMM / water mixture at the operating pressure of the second distillation column, or the water / (NMM + water) weight ratio corresponds to or higher than the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture, and feeding the resulting mixture to the second distillation column, (iv) separating an NMM / water mixture as an overhead stream of the second distillation column, (v) separating a mixture containing water and high boilers as a bottoms stream of the second distillation column.

Description

[0001] This invention relates to an economical method for separating water-rich N-methylmorpholine from a mixture containing at least methanol, N-methylmorpholine (NMM), water, and high-boiling substances by distillation. In particular, the invention relates to a method in which an NMM / water mixture having a near-azeotropic composition is separated, and this mixture can be fed as a segregating stream into a subsequent NMM oxidation to N-methylmorpholine oxide (NMMO).

[0002] N-Methylmorpholine (4-methylmorpholine; 4-methyltetrahydro-1,4-oxazine; NMM) is used, for example, as a catalyst in the production of polyurethanes. N-Methylmorpholine is also a starting material for the production of N-methylmorpholine oxide (which is an important solvent for cellulose).

[0003] NMM can be prepared by reducing aminated morpholine (MO) with formaldehyde in the presence of hydrogen.

[0004] Preferred methods for preparing NMM include continuously feeding morpholine, formaldehyde solution and hydrogen into a reactor equipped with a heterogeneous hydrogenation catalyst, and subjecting morpholine and formaldehyde to reductive amination to obtain NMM, wherein the formaldehyde solution is a methanol-formaldehyde solution.

[0005] The key feature of this method is that the continuously fed formaldehyde solution is a methanol-formaldehyde solution. The term "methanol-formaldehyde solution" means that formaldehyde is used in a solution containing methanol and preferably also water.

[0006] N-Methylmorpholine oxide (N-methylmorpholine N-oxide; NMMO) is used, for example, as a solvent for cellulose. Aqueous solutions of N-methylmorpholine N-oxide are technically used as solubilizers in the production of Lyocell fibers, in which pulp is dissolved in a solution of N-methylmorpholine N-oxide and water, and spinning is performed from this solution.

[0007] NMMO can be obtained by the oxidation of NMM. The oxidation step involves reacting NMM with a less than stoichiometric amount of aqueous hydrogen peroxide solution in an aqueous medium in the presence of carbon dioxide as a promoter.

[0008] In a preferred embodiment, the oxidation step includes reacting NMM with a less than stoichiometric amount of aqueous hydrogen peroxide solution in an aqueous medium while applying a vapor space to the aqueous medium having a carbon dioxide partial pressure p(CO2) of less than 0.75 bar absolute pressure, preferably less than 0.20 bar absolute pressure.

[0009] NMM forms an azeotrope with water. At approximately 1.5 bar, the NMM / water azeotrope contains 72 wt.% NMM and 28 wt.% water. At approximately 1.0 bar, the NMM / water azeotrope contains 74 wt.% NMM and 26 wt.% water. At approximately 10 bar, the NMM / water azeotrope contains 57 wt.% NMM and 43 wt.% water.

[0010] For the oxidation of NMM to NMMO, a certain amount of water in the NMM-containing mixture is advantageous. The oxidation reaction begins more rapidly in the presence of water. Furthermore, the heat capacity of water buffers the heat generated by the exothermic oxidation reaction. However, excessive water in the NMM-containing mixture will reduce the space-time yield of the oxidation reaction.

[0011] The object of this invention is to provide an economical method for separating a mixture containing at least methanol, NMM, water, and high-boiling substances into substantially pure methanol, NMM / water mixtures and a stream containing high-boiling substances by distillation. In particular, the invention relates to a method in which an NMM / water mixture having a near-azeotropic composition is obtained can be fed as a segregated stream into a subsequent NMM oxidation to NMMO.

[0012] This problem is solved by a method for separating a mixture containing at least methanol, N-methylmorpholine (NMM), water, and high-boiling substances by distillation in a first distillation column and a second distillation column, the method comprising the following steps:

[0013] (i) A mixture containing at least methanol, NMM, water, and a high-boiling-point substance is fed into a first distillation column, wherein the water / (NMM + water) weight ratio is lower than that weight ratio in the azeotropic NMM / water mixture at the operating pressure of a second distillation column.

[0014] (ii) Methanol is separated as the overhead stream of the first distillation column, and a mixture containing NMM and water is obtained as the bottom stream of the first distillation column, wherein the water / (NMM + water) weight ratio is lower than that weight ratio in the azeotropic NMM / water mixture at the operating pressure of the second distillation column.

[0015] (iii) Water is added to the bottom feed stream of the first distillation column in an amount such that the water / (NMM + water) weight ratio remains lower than, but is closer to, the water / (NMM + water) ratio in the azeotropic NMM / water mixture at the operating pressure of the second distillation column, or the water / (NMM + water) weight ratio corresponds to or is higher than the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture, and the resulting mixture is fed into the second distillation column.

[0016] (iv) The NMM / water mixture is separated from the overhead stream of the second distillation column.

[0017] (v) The bottom stream of the second distillation column is used to separate a mixture containing water and high-boiling substances.

[0018] A typical engineer would expect that adding water before distillation in the second distillation column would significantly increase the energy demand in the second distillation column, since additional water must be evaporated during rectification. However, it has been surprisingly found that adding water before distillation in the second distillation column results in less overall process energy consumption compared to distilling an NMM / water mixture containing significantly less water than an NMM / water azeotrope, and adding water only after the (non-azeotropic) NMM / water mixture has been separated in the second distillation column.

[0019] In step (i), a mixture containing at least methanol, NMM, water, and a high-boiling-point substance is fed into the first distillation column, wherein the water / (NMM + water) weight ratio is lower than that in the azeotropic NMM / water mixture at the operating pressure of the second distillation column. A typical product mixture of formaldehyde reduction of amination morpholine using a methanol-formaldehyde solution contains...

[0020] 50 to 80 wt.-%, preferably 66 to 70 wt.-%, NMM

[0021] 8 to 24 wt.-%, preferably 14 to 18 wt.-%, of water.

[0022] 6 to 22 wt.-%, preferably 12 to 16 wt.-%, of methanol.

[0023] 0.01 to 5.0 wt.-%, preferably 0.2 to 1.0 wt.-%, of high-boiling-point substances.

[0024] High-boiling substances can include morpholine (MO), tetramethylenediamine (TMEDA), 2,2-bis(dimethylaminoethyl) ether (BDMAEE), 2,4-dimethylmorpholine (DMMO), and other high-boiling substances.

[0025] In step (ii), methanol is separated as the overhead stream in the first distillation column. Preferably, the water content of the methanol obtained as the overhead stream in the first distillation column is 0.02 wt.-% (200 wt.-ppm) or lower.

[0026] A mixture containing NMM and water is obtained as the bottom product stream in the first distillation column. Preferably, the methanol content of the bottom product stream of the first distillation column is 0.1 wt.-% (1000 wt.-ppm) or lower.

[0027] Typically, the first distillation column has 17 to 44 theoretical stages, with a reflux ratio correspondingly ranging from 2.0 mol / mol (44 stages) to 10 mol / mol (17 stages). If the first distillation column has 17 theoretical stages, the feed stage can be stage 7, or if the first distillation column has 44 theoretical stages, the feed stage can be stage 25. For example, the first distillation column can have 30 theoretical stages, with the feed stage being stage 15 (bottom stage 1) and a reflux ratio of 2.7 mol / mol.

[0028] The first distillation column is typically operated at a pressure of 1.0 to 10 bar, preferably 1.0 to 2 bar.

[0029] Preferably, the first distillation column is operated at a pressure as close to atmospheric pressure as possible, but high enough to compensate for pressure loss, because higher operating pressures require higher temperatures, which is energy-disadvantageous. Operating pressures above 10 bar are unfavorable because the reboiler temperature can rise to and exceed 200°C, which may cause discoloration of the NMM.

[0030] The water / (NMM + water) weight ratio is essentially the same as that in the mixture fed into the first distillation column, and significantly lower than that in the azeotropic NMM / water mixture at the operating pressure of the second distillation column. In the cases given above for the product mixture, this ratio is 12 to 26 wt.% water based on the total amount of water and NMM.

[0031] In step (iii), water is added to the bottom feed stream of the first distillation column in an amount such that the water / (NMM + water) weight ratio is still lower than, but closer to, the water / (NMM + water) ratio in the azeotropic NMM / water mixture at the operating pressure of the second distillation column, or the water / (NMM + water) weight ratio corresponds to or is higher than the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture.

[0032] The water / (NMM + water) weight ratio can be lower than that in the original (product) mixture, but closer to the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture at the operating pressure of the second distillation column. This condition is also known as "azeotrope left-hand" or "LAZ". Preferably, the water / (NMM + water) weight ratio is as close as possible to the corresponding ratio of the azeotrope. Preferably, it is less than 2 wt.% water lower than the corresponding ratio of the azeotrope.

[0033] The water / (NMM + water) weight ratio can correspond to or may be higher than the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture at the operating pressure of the second distillation column. The latter case is also referred to as "azeotropic right-hand side" or "RAZ". Preferably, the water / (NMM + water) weight ratio is as close as possible to the corresponding ratio of the azeotrope. Preferably, it is up to 5 wt.% higher than the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture, more preferably 1 to 3 wt.% higher.

[0034] The "right-side azeotrope" case is superior to the "left-side azeotrope" case. It has been found that the "right-side azeotrope" (RAZ) is always more economical than the "left-side azeotrope" (LAZ). Furthermore, NMM losses can only be minimized in the "right-side azeotrope" case.

[0035] The addition of water in step (iii) is carried out in a mixing device. Examples of suitable mixing devices are containers with pump circulation and residence time, static mixers, and pipes in which pipe mixing occurs due to turbulence.

[0036] The resulting mixture is fed into a second distillation column.

[0037] In step (iv), the NMM / water mixture is separated as the overhead stream in the second distillation column. Preferably, the content of high-boiling substances obtained as the overhead stream of the second distillation column is 0.1 wt.-% (1000 wt.-ppm) or less. More preferably, the morpholine content of the NMM / water mixture obtained as the overhead stream in the second distillation column is 0.1 wt.-% (1000 wt.-ppm) or less.

[0038] In step (v), the mixture containing water and high-boiling-point substances is separated as the bottom product of the second distillation column. Preferably, the NMM loss (defined as the mass of NMM in the sum of the top product of the first distillation column and the bottom product of the second distillation column divided by the mass of NMM fed into the first distillation column) is 0.002 kg / kg or less. The NMM loss can be substantially zero in the "azeotropic right-hand" case.

[0039] Typically, the second distillation column has 5 to 20 theoretical stages, with a reflux ratio correspondingly ranging from 0.02 mol / mol (20 stages) to 0.5 mol / mol (5 stages). If the second distillation column has 5 theoretical stages, the feed stage can be stage 4, or if the first distillation column has 20 theoretical stages, the feed stage can be stage 18. For example, the second distillation column can have 10 theoretical stages, with the feed stage being stage 8 (bottom stage 1) and a reflux ratio of 0.03 mol / mol.

[0040] Typically, investment costs are associated with an increase in the number of theoretical stages, and operating costs (including evaporator energy consumption (evaporator heat load)) are associated with an increase in the reflux ratio.

[0041] The second distillation column is typically operated at a pressure of 1.0 to 10 bar, preferably 1.0 to 2 bar.

[0042] Preferably, the first distillation column is operated at a pressure close to atmospheric pressure but high enough to compensate for pressure loss, because higher operating pressures require higher temperatures, which is energy-disadvantageous. Operating pressures above 10 bar are unfavorable because the reboiler temperature can rise to and exceed 200°C, which may cause discoloration of the NMM.

[0043] This problem is also addressed by an alternative method for separating a mixture containing at least methanol, N-methylmorpholine (NMM), water, and high-boiling substances by distillation in a single distillation column with a side-out, the method comprising the following steps:

[0044] (i) Provide a mixture containing at least methanol, N-methylmorpholine (NMM), water and a high-boiling-point substance, wherein the water / (NMM + water) weight ratio is lower than that weight ratio in an azeotropic NMM / water mixture at the operating pressure of the distillation column.

[0045] (ii) Water is added to the mixture in an amount such that the water / (NMM + water) weight ratio remains lower than, but is closer to, the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture at the operating pressure of the distillation column, or the water / (NMM + water) weight ratio corresponds to or is higher than the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture, and the resulting mixture is fed into the distillation column.

[0046] (iii) Methanol is separated as the overhead stream of a distillation column.

[0047] (iv) The NMM / water mixture is separated as a side-outflow from the distillation column.

[0048] (v) The mixture containing high-boiling substances is separated from the bottom stream of the distillation column.

[0049] A typical engineer would expect that adding water before distillation in a single distillation column would significantly increase the energy demand in the column, since additional water must be evaporated during rectification. However, it has been surprisingly found that adding water before distillation in a single distillation column results in less overall process energy consumption compared to distilling an NMM / water mixture containing significantly less water than an NMM / water azeotrope, and adding water only after separating the (non-azeotropic) NMM / water mixture in a single distillation column.

[0050] In step (i), a mixture containing at least methanol, NMM, water, and a high-boiling-point substance is provided, wherein the water / (NMM + water) weight ratio is lower than that in an azeotropic NMM / water mixture at the operating pressure of a single distillation column. The composition of the provided mixture is typical of a mixture of products of formaldehyde reduction of aminated morpholine using a methanol-formaldehyde solution, as indicated above.

[0051] In step (ii), water is added to the mixture in an amount such that the water / (NMM + water) weight ratio is still lower than, but closer to, the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture at the operating pressure of the distillation column, or the water / (NMM + water) weight ratio corresponds to or is higher than the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture.

[0052] The water / (NMM + water) weight ratio can be lower than that in the original (product) mixture, but closer to the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture at the operating pressure of a single distillation column. This condition is also known as "azeotrope left-hand" or "LAZ". Preferably, the water / (NMM + water) weight ratio is as close as possible to the corresponding weight ratio of the azeotrope. Preferably, it is less than 2 wt.% water lower than the corresponding ratio of the azeotrope.

[0053] The water / (NMM + water) weight ratio can correspond to or may be higher than the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture at the operating pressure of a single distillation column. The latter case is also referred to as "azeotropic right-hand side" or "RAZ". Preferably, the water / (NMM + water) weight ratio is as close as possible to the corresponding ratio of the azeotrope. Preferably, it is up to 5 wt.% higher than the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture, more preferably 1 to 3 wt.% higher.

[0054] The “azeotropic right side” case is superior to the “azeotropic left side” case. It has been found that, in the case of a single column with lateral removal, the “azeotropic right side” (RAZ) is also consistently more economical than the “azeotropic left side” (LAZ).

[0055] The addition of water in step (iii) can be carried out in a mixing apparatus as described above for a dual-tower setup. Alternatively, water can be added in a feed tank containing a mixture of methanol, NMM, water, and high-boiling-point substances.

[0056] The resulting mixture is fed into a single distillation column with side-extraction.

[0057] In step (iii), methanol is separated as the overhead stream of a distillation column. Preferably, the water content of the methanol obtained as the overhead stream is 0.02 wt.-% (200 wt.-ppm) or lower.

[0058] In step (iv), the NMM / water mixture is separated as a side-flow from the distillation column. Preferably, the content of high-boiling substances obtained as the overhead stream of the second distillation column is 0.1 wt.-% (1000 wt.-ppm) or lower. More preferably, the morpholine content of the NMM / water mixture obtained as the overhead stream of the second distillation column is 0.1 wt.-% (1000 wt.-ppm) or lower.

[0059] In step (v), the mixture containing high-boiling-point substances is separated as the bottom product of the distillation column. Preferably, the NMM loss (defined as the mass of NMM in the top product of the first distillation column plus the mass of NMM in the bottom product of the second distillation column divided by the mass of NMM fed into the first distillation column) is 0.0020 kg / kg or less. The NMM loss can be substantially zero in the "azeotropic right-hand" case.

[0060] Typically, a single distillation column has 24 to 55 theoretical stages, with reflux ratios correspondingly ranging from 2.6 mol / mol (55 stages) to 9.1 mol / mol (24 stages). If the distillation column has 24 theoretical stages, the feed stage can be stage 13, or if the distillation column has 55 theoretical stages, the feed stage can be stage 37. If the distillation column has 24 theoretical stages, the side draw can be stage 5, or if the distillation column has 55 theoretical stages, the side draw can be stage 9. For example, a distillation column can have 40 theoretical stages, with the feed stage at stage 25 (bottom stage 1), the side draw at stage 7, and a reflux ratio of 3.1 mol / mol.

[0061] Distillation columns are typically operated at pressures of 1.0 to 10 bar, preferably 1.0 to 2 bar.

[0062] Preferably, the first distillation column is operated at a pressure as close to atmospheric pressure as possible, but high enough to compensate for pressure loss, because higher operating pressures require higher temperatures, which is energy-disadvantageous. Operating pressures above 10 bar are unfavorable because the reboiler temperature can rise to and exceed 200°C, which may cause discoloration of the NMM.

[0063] It has been proven that the single-distillation tower process is more economical than the double-distillation tower process in terms of investment and operating costs.

[0064] In a preferred embodiment of the method of the present invention, the mixture provided in step (i) containing at least methanol, N-methylmorpholine (NMM), water and a high-boiling substance is prepared by reducing aminated morpholine with formaldehyde using a methanol-formaldehyde solution.

[0065] Preferably, the mixture provided in step (i) containing at least methanol, N-methylmorpholine (NMM), water, and a high-boiling-point substance has the following composition:

[0066] 50 to 80 wt.-%, preferably 66 to 70 wt.-%, NMM

[0067] 8 to 24 wt.-%, preferably 14 to 18 wt.-%, of water.

[0068] 6 to 22 wt.-%, preferably 12 to 16 wt.-%, of methanol.

[0069] 0.01 to 5.0 wt.-%, preferably 0.2 to 1.0 wt.-%, of high-boiling-point substances.

[0070] The present invention further relates to a method for preparing N-methylmorpholine oxide (NMMO), the method comprising the following steps:

[0071] (a) Separate the NMM / water mixture in step (iv) of the above method;

[0072] (b) Optionally, water may be added to the NMM / water mixture; and

[0073] (c) subject the NMM / water mixture obtained in step (a) or (b) to an oxidation reaction to obtain NMMO.

[0074] Oxidation step (c) includes reacting N-methylmorpholine with a less than stoichiometric amount of aqueous hydrogen peroxide solution in an aqueous medium in the presence of carbon dioxide as a promoter.

[0075] In a preferred embodiment, oxidation step (c) comprises reacting N-methylmorpholine with a less than stoichiometric amount of aqueous hydrogen peroxide solution in an aqueous medium while applying a vapor space to the aqueous medium having a carbon dioxide partial pressure p(CO2) of less than 0.75 bar absolute pressure, preferably less than 0.20 bar absolute pressure. Although carbon dioxide is an effective promoter for the conversion of tertiary amines to their amine oxides, the applicant has found that an amount of free carbon dioxide exceeding the promoting amount may cause undesirable discoloration.

[0076] In the embodiments, the initial concentration of the tertiary amine in the aqueous medium (i.e., the concentration of the tertiary amine before the addition of hydrogen peroxide) is in the range of 40 to 95 vol.-%, preferably 60 to 85 vol.-%. Any aqueous solution of hydrogen peroxide can be used. For practical considerations, the concentration of hydrogen peroxide in the aqueous solution added to the aqueous medium is in the range of 10 to 71 wt.-%, preferably 29 to 51 wt.-%.

[0077] The invention is further illustrated by the following examples. Example

[0078] All instances were computed in an equation-oriented internal process simulator that operates in the same way as commonly known commercial software packages, such as those from AspenTech or AVEVA.

[0079] Table 1 summarizes all components, along with their abbreviations, CAS numbers, and molar masses. During the examples section, theoretical levels were determined, and these levels were always counted from the bottom of the column.

[0080] Table 1: A table of all components considered in the simulation, along with their abbreviations, CAS numbers, and molar masses.

[0081]

[0082] Replacement components of all residual and unspecified high-boiling substances in the mixture

[0083] Simulation results of the dual-tower setup

[0084] For a dual-tower setup, this example section describes three scenarios. All scenarios include a single MEOH separation (see [link to example]). Figure 1 and Figure 2 : Tower 1), an NMM / H2O separator (see Figure 1 and Figure 2 Tower 2), and mixing steps (see Tower 2), and mixing steps (see Tower 2). Figure 1 and Figure 2 (“Mixed”). All these cases share the same feed from the synthesis section, represented as flow “1” in the diagram below.

[0085] Figure 1 : A "Water Before" setup with two towers. Synthesis: Reaction section. Tower 1: MEOH separation. Mixing: Mixing device for adding water. Tower 2: NMM / H2O separation.

[0086] Figure 2: "Water After" setup with two towers. Synthesis: Reaction section. Tower 1: MEOH separation. Tower 2: NMM / H2O separation. Mixing: Mixing device for adding water.

[0087] While NMM / H2O separation differs for these cases, MEOH separation remains consistent. Preferred setups include a column with 30 theoretical stages and a feed stage at stage 15. Furthermore, the head pressure is set to 1.50 bar absolute, and a pressure drop across the column is assumed to be 4 mbar absolute per theoretical stage. Additionally, a pressure drop across the evaporator is assumed to be 162 mbar absolute, and a pressure drop across the condenser is assumed to be 150 mbar absolute. The condenser is set to an outlet temperature of 53°C based on the cooling water conditions at the production site, but the condensation temperature is approximately 69°C. Complete condensation is not possible due to residual inert gases from synthesis, such as hydrogen and carbon monoxide. A further characteristic of the MEOH column is the requirement of 1000 wt-ppm MEOH in the bottoms outlet stream (stream "3") and 200 wt-ppm H2O in the distillate outlet stream (stream "2"). Therefore, the reflux ratio is calculated to be 2.69 mol / mol, and the bottoms outlet flow rate is calculated to be 6447 kg / h. The evaporator requires 1610 kW and the condenser requires 1267 kW.

[0088] Table 2 summarizes the composition of the MEOH separation stream used in a dual-tower setup.

[0089] Table 2: Flow table for "front water" and "rear water" configurations with dual towers. Flow numbering is based on... Figure 1 and Figure 2 .

[0090]

[0091] Mixing device (see Figure 1 and Figure 2 The mixing unit (previously set up for NMM / H2O separation in the "forward water" case) is also added after NMM / H2O separation in the "backward water" case for comparison purposes, to ensure that the same NMM / H2O concentration is compared in both cases. This is technically unnecessary if the resulting NMM / H2O concentration is sufficient. This does not change the energy requirements of column 2 and therefore does not affect the results in this example. However, it ensures that the product concentration in stream "6" is the same in both the "forward water" and "backward water" cases. The added water stream (stream "4") is calculated such that the H2O concentration in product stream "6" is 29 wt-% H2O / (NMM+H2O).

[0092] The second distillation column used for NMM / H2O separation varies in number of stages and energy requirements depending on the specific circumstances, but the following assumptions remain the same for all cases. The head pressure is set to 1.50 bar absolute, and the pressure drop across the column is assumed to be 4 mbar absolute per theoretical stage. Furthermore, a pressure loss of 162 mbar absolute is assumed on the evaporator, and a pressure loss of 150 mbar absolute is assumed on the condenser. The condenser is then set to a full condenser because there is no inert gas and the condensation temperature is significantly higher than 53°C (approximately 102°C). Therefore, cooling water or air can be used as the cooling medium. A further characteristic of the NMM / H2O column is the requirement of 1000 wt-ppm MO in the product draw (stream "6") and a loss of only 0.002 kg / kg NMM throughout the entire separation sequence. The NMM loss is calculated as the sum of the NMM mass flow rates in streams "2" and "7" divided by the NMM mass flow rate in stream "1". Therefore, the reflux ratio and the bottom draw are the variables in the calculation.

[0093] For the "front water" case according to the present invention, two scenarios are considered: "LAZ" and "RAZ". For the "back water" case, only the RAZ scenario is presented here for comparison.

[0094] Figure 3 : Optimization curves for the number of stages (N) versus energy demand (Q) in a dual-tower setup with "front water" and "back water" for the "RAZ" case. "Front water" (solid line): Water is added before tower 2; see [link to relevant documentation]. Figure 1 "No NMM loss before water addition" (thin dashed line): Water is added before tower 2. See [reference needed] Figure 1 Furthermore, NMM loss is not allowed in the flask of Tower 2. "After Water" (thick dashed line): Water is added after Tower 2; see [link to Tower 2]. Figure 2 “RAZ”: Add as much water as possible to the feed of column 2 so that the water concentration is close to the azeotropic concentration but slightly above the water-rich side of the azeotropic concentration.

[0095] Figure 4 : Optimized curves for the number of stages (N) versus reflux ratio (v) in a dual-tower setup with "forward water" and "rear water" for the "RAZ" case. "Forward water" (solid line): Water is added before tower 2; see [link to relevant documentation]. Figure 1 "No NMM loss before water addition" (thin dashed line): Water is added before tower 2. See [reference needed] Figure 1 Furthermore, NMM loss is not allowed in the flask of Tower 2. "After Water" (thick dashed line): Water is added after Tower 2; see [link to Tower 2]. Figure 2 “RAZ”: Add as much water as possible to the feed of column 2 so that the water concentration is close to the azeotropic concentration but slightly above the water-rich side of the azeotropic concentration.

[0096] Starting with the latter, Figure 3 and Figure 4 The design differences between "forward water-RAZ" and "backward water-RAZ" are highlighted. They illustrate the energy-optimal design at a given theoretical number of stages and specifications. In other words, for a given theoretical number of stages, the minimum reflux ratio and favorable feed stage, and therefore the minimum energy requirement, are calculated while maintaining specifications of 1000 wt-ppm MO in the product stream and 0.002 kg / kg NMM loss throughout the distillation process. Following this method for different stages, a plot of theoretical stages (N) versus energy (Q) or reflux ratio (v) is produced, showing the possible optimal design at different stages. This optimization method is frequently used in the design of distillation columns. For example, at 30 theoretical stages, the minimum energy requirement is 1932 kW in the "forward water" case, while it increases significantly (to 3465 kW) in the "backward water" case.

[0097] In short, Figure 3 and Figure 4 This demonstrates that, for any given number of stages and considering the same volume, the energy requirement for adding water before NMM / H2O distillation is lower than the energy requirement for adding water after NMM / H2O distillation. (Conclusion 1)

[0098] In addition, the specification of 0.002 kg / kg NMM loss is derived from factory testing and is used in both cases for comparative reasons. Another advantage of adding water before NMM / H2O distillation is that it is the only energy-rational way to achieve virtually no NMM loss during distillation. Reducing the NMM loss to zero in the "after-water" case will push energy demand to its limits. For example, at 30 stages, the energy demand for the "after-water" case will increase from 3465 kW to 10660 kW to reduce the NMM loss from 0.002 kg / kg to 0.0003 kg / kg NMM loss. Considering the same theoretical number of stages 30, the energy demand for the "before-water" case will increase slightly from 1932 kW to 1940 kW to reduce the NMM loss from 0.002 kg / kg to only 0.0003 kg / kg. Furthermore, the NMM loss in the "before-water" setting can even be reduced to 10. -8 kg / kg, which means there is essentially no NMM loss in the NMM / H2O column. For example... Figure 3 As shown, above 25 theoretical levels, the NMM loss is reduced to 10. -8 kg / kg (“no NMM loss”) may not significantly increase energy requirements, but below 25 theoretical levels, reducing NMM loss requires more energy.

[0099] In summary, only a "forward water" setup can reduce NMM losses during distillation to virtually zero within reasonable energy requirement limits. (Conclusion 2)

[0100] Finally, alternative to the mixing unit in the feed, there is another way to add water. Water can also be added directly to the column using a second feed stage. In this case, the added water will be used directly as the stripping medium within the column. This can also result in a column setup with no NMM losses. However, this "direct stripping" setup will be energy-disadvantageous compared to the "forward water-RAZ-no NMM loss" setup. For example, at the same volume for 30 stages, the "direct stripping" setup would require 2369 kW, while the "forward water-RAZ-no NMM loss" setup would require 1937 kW.

[0101] In summary, mixing water into the feed instead of feeding water directly into the tower using a second feed stage is energy-efficient. (Conclusion 3)

[0102] Along Figure 3 and Figure 4 Any tower design based on the curve is possible. For the "post-water-RAZ", a preferred design could be 37 stages, with feed on stage 29 and a reflux ratio of 0.92 mol / mol, representing a 2422 kW evaporator heat load and a 2474 kW condenser heat load. Table 3 summarizes the flow for the "post-water-RAZ" case excluding MEOH separation in tower 1. Flow numbers are in... Figure 2 The information is provided in the text.

[0103] Table 3: Flow table for a "post-water-RAZ" setup with dual towers. Flow numbering is based on... Figure 2 .

[0104]

[0105] Although along Figure 3 and Figure 4 Any tower design based on the curve is possible for "forward water-RAZ", but for "forward water-RAZ", a preferred design could be 10 theoretical stages, with feed at stage 8 and a reflux ratio of 0.028 mol / mol, representing a 1933 kW evaporator heat load and a 1923 kW condenser heat load. Table 4 summarizes the flow for the "forward water-RAZ" case excluding MEOH separation in tower 1. Flow numbers are in... Figure 1 The information is provided in the text.

[0106] Table 4: Flow table for a "forewater-RAZ" setup with dual towers. Flow numbering is based on... Figure 1 .

[0107]

[0108] As discussed above, there is also the possibility of reducing NMM loss to zero. Although along... Figure 3 and Figure 4 Any tower design for the "forward water-RAZ-no NMM loss" configuration is possible, but for this setup, a preferred design would be 22 stages, with the feed on stage 20 and a reflux ratio of 0.029 mol / mol, representing a 1939 kW evaporator heat load and a 1929 kW condenser heat load. Table 5 summarizes the streams for the "forward water-RAZ-no NMM loss" configuration excluding MEOH separation in tower 1. Stream numbers are in... Figure 1 The information is provided in the text.

[0109] Table 5: Flow table for a "forewater-RAZ-no NMM loss" setup with dual towers. Flow numbers are based on... Figure 1 .

[0110]

[0111] It is known that adding water immediately before distillation is advantageous compared to adding water after distillation of NMM / H2O, but it is still necessary to estimate the amount of water that should be added. Typical engineers will add as little water as possible to reduce the additional energy cost during distillation. Subsequently, different amounts of water were added for the "pre-water" case, starting with no water added, followed by water added to near the azeotropic point ("LAZ" and "RAZ" cases), and finally, significant water addition to achieve 40 wt% H2O / (NMM+H2O) in the product. Thus, four cases were presented. Figure 5 A curve showing another number of stages (N) versus energy requirement (Q), as described in the previous paragraphs, is shown. Therefore, the same specifications were set, with a maximum MO of 1000 wt-ppm in the product stream and a NMM loss of 0.002 kg / kg throughout the distillation process. It can be seen that the energy-optimal design is improved from no water addition (“no H2O feed”), to adding water close to the azeotropic concentration on the NMM-rich side (“LAZ”), and finally to adding water slightly above the azeotropic concentration on the H2O-rich side of the azeotrope (“RAZ”). For example, at the same number of stages with 31 stages, the “no H2O feed” case requires 3605 kW, the “LAZ” case requires 2166 kW, and the “RAZ” case requires only 1932 kW. If significantly more water is added, i.e., 40 wt% H2O / (NMM+H2O) (“40 wt% H2O”), the theoretical number of stages does not change, but the energy requirement increases significantly. For example, at the same theoretical level number across 7 theoretical levels, the "40wt%_H2O" setting requires 3252 kW, while the "RAZ" setting requires only 1952 kW.

[0112] Figure 5 : A "front water" setup with twin towers (see Figure 1 The optimization curves for the theoretical series (N) versus energy requirement (Q) are shown below. "No H2O Feed" (thick dashed line): No additional water is added to the feed of column 2. "LAZ" (thin dashed line): As much water as possible is added to the feed of column 2, making the water concentration close to the azeotropic concentration but slightly above the NMM-rich side of the azeotropic concentration. "RAZ" (solid line): As much water as possible is added to the feed of column 2, making the water concentration close to the azeotropic concentration but slightly above the water-rich side of the azeotropic concentration. "40wt%_H2O" (dotted line): As much water as possible is added to the feed of column 2, making the water concentration significantly higher than the azeotropic concentration (40 wt% H2O / (NMM+H2O)).

[0113] In short, Figure 5 This demonstrates how, at any given theoretical level, energy requirements can be reduced by adding water in an amount that results in a water concentration slightly on the water-rich side of the azeotropic concentration (“RAZ”). (Conclusion 4)

[0114] Simulation results of single tower configuration

[0115] Following the discussion of the dual-tower setup, a single-tower setup with lateral take-off is also discussed in this example section. For the single-tower setup, all four conclusions (1) through (4) regarding the dual-tower setup remain valid:

[0116] (1) For any given fraction, considering the same fraction, the energy requirement to add water before NMM / H2O distillation is lower than the energy requirement to add water after NMM / H2O distillation.

[0117] (2) Only the “pre-water” setting can reduce NMM losses during distillation to virtually zero within reasonable energy demand limits.

[0118] (3) Mixing additional water into the feed stream instead of feeding water directly into the tower via a second feed stage is energy-efficient under any contellation.

[0119] (4) At any given level, energy requirements are reduced by adding water in an amount that makes the water concentration slightly on the water-rich side of the azeotropic concentration (“RAZ”).

[0120] Figure 6 The “front water” setup for a single-tower design with lateral take-off is shown.

[0121] Figure 6 : "Forward water" setting with a side-extraction tower. Synthesis: Reaction section. Mixing: Mixing device for adding water. Tower 1: Side-extraction tower.

[0122] For comparative purposes, the single-tower case is specified as in the dual-tower case. First, the head pressure is set to 1.50 bar absolute pressure, and the pressure drop across the tower is assumed to be 4 mbar absolute pressure / theoretical level. Second, the pressure loss on the evaporator is assumed to be 162 mbar absolute pressure, and the pressure loss on the condenser is assumed to be 150 mbar absolute pressure. Third, the condenser is set to an outlet temperature of 53°C based on the cooling water conditions at the production site, but the condensation temperature is approximately 69°C. Complete condensation is impossible due to residual inert gases from synthesis, such as hydrogen and carbon monoxide. Fourth, the side condenser is set as a full condenser because there are no inert gases and the condensation temperature is significantly higher than 53°C (approximately 107°C). Therefore, cooling water or air can be used as the cooling medium. Fifth, the MEOH product stream (stream "2") is specified as 200 wt-ppm H2O, and the reflux ratio is calculated accordingly. Sixth, the NMM product stream (stream "6") is specified as 1000 wt-ppm MEOH, and the side withdrawal amount is calculated accordingly. Seventh, specify the NMM product stream (stream "6") as 1000 wt-ppm MO, and calculate the reboiler output accordingly. Eighth, specify the NMM product stream (stream "6") as 29 wt% H2O / (NMM+H2O), and calculate the mass flow rate of the added water (stream "4") accordingly. Ninth, the maximum NMM loss during distillation is still specified as 0.002 kg / kg, but this can be left unfixed in the simulation because there are no residual degrees of freedom in the equations.

[0123] Nevertheless, the optimal design can still be represented as a curve of the theoretical series (N) versus operating cost (C). This method is similar to optimizing the theoretical series (N) versus energy demand (Q), since energy demand is directly related to the operating cost of the heating means (such as steam). Now, operating cost ( / h is the steam consumption (kg / h) multiplied by the steam cost ( / kg) and NMM product loss (kg / h) multiplied by product price ( The sum of ( / kg). This yields a design that represents the minimum operating cost at a given theoretical order while maintaining the above specifications. In other words, the optimal design is always oriented towards the lowest NMM loss while maintaining reasonable energy requirements within the product specifications. All optimization cases that produce an NMM loss greater than 0.002 kg / kg are discarded because they do not meet the minimum specification where the maximum NMM loss is 0.002 kg / kg. All other cases where the NMM loss is less than 0.002 kg / kg are considered valid optimization results.

[0124] Figure 7 and Figure 8The optimized results for this single-tower setup with lateral take-off are shown (“Forewater - OneCol - RAZ”: solid line). It shows the energy-optimal design at a given theoretical number of stages, producing a curve of theoretical number of stages (N) versus energy demand (Q) or reflux ratio (v). In contrast, a dual-tower setup with the previously described 30-stage MEOH separation (“Forewater - TwoCol - RAZ - 30stMEOH”: thick dashed line), 44-stage MEOH separation (“Forewater - TwoCol - RAZ - 44stMEOH”: thin dashed line), or 20-stage MEOH separation (“Forewater - TwoCol - RAZ - 20stMEOH”: dotted line) is also shown. Therefore, the total number of theoretical stages in the dual-tower setup is equal to the theoretical number of stages in the NMM / H2O tower (which can be expressed as follows). Figure 3 and Figure 4 The energy requirements for a dual-tower setup are the sum of the energy requirements for MEOH separation (which is 2557 kW for 20 stages, 1610 kW for 30 stages, and 1434 kW for 44 stages) and the energy requirements for NMM / H2O separation (which can be as shown in the diagram). Figure 3 and Figure 4 The result is obtained by adding the changes shown.

[0125] Figure 7 : Optimization curves for the number of stages (N) relative to energy demand (Q) in "RAZ" scenarios and for different MEOH separation stages with single or double tower configurations in the "forewater" setting. "TwoCol_20stMEOH" (dotted line): For scenarios with 20 MEOH separation stages, such as... Figure 1 The dual-tower configuration in the image. "TwoCol_30stMEOH" (thick dashed line): This indicates a configuration with 30 stages of MEOH separation. Figure 1 The dual-tower configuration in the image. "TwoCol_44stMEOH" (thin dashed line): This indicates a configuration with 44 stages of MEOH separation. Figure 1 The dual-tower setup in the image. "OneCol" (solid line): as shown... Figure 6 The single-tower configuration features lateral extraction.

[0126] Figure 8 : Optimized curves for the stage (N) relative to the reflux ratio (v) in a single-tower setup with a "forward water" configuration for the "RAZ" case. "OneCol" (solid line): as shown... Figure 6 The single-tower configuration features lateral extraction.

[0127] If the dual-tower setup is configured for 20 or 30 theoretical stages for MEOH separation, the single-tower setup is energy-efficient compared to the dual-tower setup at any stage.

[0128] For example, with 50 theoretical stages, a single-tower setup requires 3477 kW, while a dual-tower setup requires 3560 kW (with 30 theoretical stages of MEOH separation) or 4498 kW (with 20 theoretical stages of MEOH separation). Only when the number of theoretical stages with MEOH separation significantly increases to 44 theoretical stages can a dual-tower setup achieve energy-efficient results. For example, with 73 theoretical stages, a single-tower setup requires 3445 kW, while a dual-tower setup requires 3370 kW (with 44 stages of MEOH separation).

[0129] Regardless, a single-tower setup can be achieved with fewer overall theoretical stages under similar energy requirements. The dual-tower setup, presenting the minimum energy requirement of 44 theoretical stages for MEOH separation, is firstly due to the high MEOH tower, secondly due to the increased investment cost required for the second unit, and thirdly, it may not be able to achieve separation in an overall range of fewer than 50-60 stages. In contrast, a single-tower design can achieve separation even with reasonable energy requirements, ranging from 30-50 stages. For example, for a single-tower setup, the energy requirement for 50 stages is 3477 kW, while for the roughly same energy requirement of 3442 kW, a dual-tower unit with 44 stages of MEOH separation requires approximately 62 stages. In other words, 24% more theoretical stages and thus more investment costs are required to achieve 1% less energy requirement and thus less energy cost. Furthermore, a dual-tower setup requires two towers with complete perimeters compared to a single-tower design, meaning the actual investment cost difference may exceed 24%.

[0130] In summary, in most cases, the single-tower design is more economically advantageous than the dual-tower process in terms of both investment and operating costs.

[0131] Along Figure 7 and Figure 8 Any tower design with the given curve is possible for a single-tower setup. A preferred design could be 40 stages, with feed on stage 25, gaseous side-extraction on stage 7, and a reflux ratio of 3.09 mol / mol, representing a 3611 kW evaporator heat load and a 1405 kW condenser heat load. Table 6 summarizes the flow patterns for a “pre-water-RAZ” setup with a single-tower configuration. Flow numbers are in... Figure 6 The information is provided in the text.

[0132] Table 6: Flow table for "forewater-RAZ" setup with a single tower. Flow numbering is based on... Figure 6 .

[0133] .

Claims

1. A method for separating a mixture containing at least methanol, N-methylmorpholine (NMM), water, and a high-boiling substance by distillation in a first distillation column and a second distillation column, the method comprising the steps of: (i) The mixture containing at least methanol, NMM, water, and high-boiling-point substances is fed into the first distillation column, wherein the water / (NMM + water) weight ratio is lower than that weight ratio in the azeotropic NMM / water mixture at the operating pressure of the second distillation column. (ii) Methanol is separated as the overhead stream of the first distillation column, wherein the bottom stream of the first distillation column yields a mixture containing NMM and water, wherein the water / (NMM + water) weight ratio is lower than that weight ratio in the azeotropic NMM / water mixture at the operating pressure of the second distillation column. (iii) Water is added to the bottom feed stream of the first distillation column in an amount such that the water / (NMM + water) weight ratio remains lower than, but is closer to, the water / (NMM + water) ratio in the azeotropic NMM / water mixture at the operating pressure of the second distillation column, or the water / (NMM + water) weight ratio corresponds to or is higher than the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture, and the resulting mixture is fed into the second distillation column. (iv) The NMM / water mixture is separated as the overhead stream of the second distillation column. (v) The bottom stream of the second distillation column is used to separate a mixture containing water and high-boiling substances.

2. The method according to claim 1, wherein, The water / (NMM + water) weight ratio of the mixture obtained in step (iii) and fed into the second distillation column is higher than the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture.

3. The method according to claim 2, wherein, Compared to the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture, the water / (NMM + water) weight ratio of the mixture obtained in step (iii) and fed into the second distillation column is up to 5 wt.-% water, preferably 1 to 3 wt.-% water.

4. The method according to any one of claims 1 to 3, wherein, The first tower has 17 to 44 theoretical levels.

5. The method according to claim 4, wherein, The reflux ratio in the first column is 2 to 10 mol / mol.

6. The method according to any one of claims 1 to 5, wherein, The second tower has 5 to 20 theoretical levels.

7. The method according to claim 6, wherein, The reflux ratio in the second column is 0.02 to 0.5 mol / mol.

8. A method for separating a mixture containing at least methanol, N-methylmorpholine (NMM), water, and a high-boiling substance by distillation in a single distillation column, the method comprising the steps of: (i) Providing a mixture containing at least methanol, N-methylmorpholine (NMM), water, and a high-boiling-point substance, wherein the water / (NMM + water) weight ratio is lower than that weight ratio in the azeotropic NMM / water mixture at the operating pressure of the distillation column. (ii) Water is added to the mixture in an amount such that the water / (NMM + water) weight ratio remains lower than, but is closer to, the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture at the operating pressure of the distillation column, or the water / (NMM + water) weight ratio corresponds to or is higher than the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture, and the resulting mixture is fed into the distillation column. (iii) Methanol is separated as the overhead stream of the distillation column. (iv) The NMM / water mixture is separated as a side-outflow from the distillation column. (v) The mixture containing high-boiling substances is separated as the bottom stream of the distillation column.

9. The method according to claim 8, wherein, The water / (NMM + water) weight ratio of the mixture obtained in step (ii) and fed into the distillation column is higher than the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture.

10. The method according to claim 9, wherein, Compared to the water / (NMM + water) weight ratio in the azeotropic NMM / water mixture, the water / (NMM + water) weight ratio of the mixture obtained in step (iii) and fed into the second distillation column is up to 5 wt.-% water, preferably 1 to 3 wt.-% water.

11. The method according to any one of claims 8 to 10, wherein, The tower has 24 to 55 theoretical levels.

12. The method according to claim 11, wherein, The reflux ratio in this column is 2 to 10 mol / mol.

13. The method according to any one of claims 1 to 12, wherein, The mixture provided in step (i) containing at least methanol, N-methylmorpholine (NMM), water and high-boiling substances is prepared by reducing morpholine with formaldehyde using a methanol-formaldehyde solution.

14. The method according to any one of claims 1 to 13, wherein, The mixture provided in step (i) containing at least methanol, N-methylmorpholine (NMM), water, and a high-boiling-point substance has the following composition: 50 to 80 wt.-%, preferably 66 to 70 wt.-%, NMM 8 to 24 wt.-%, preferably 14 to 18 wt.-%, of water. 6 to 22 wt.-%, preferably 12 to 16 wt.-%, of methanol. Up to 5.0 wt.-%, preferably 0.2 to 1.0 wt.-%, of high-boiling-point substances.

15. A method for preparing N-methylmorpholine oxide (NMMO), the method comprising the following steps: (a) In step (iv) of the method according to any one of claims 1 to 14, the NMM / water mixture is separated; (b) Optionally, water may be added to the NMM / water mixture; as well as (c) subject the NMM / water mixture obtained in step (a) or (b) to an oxidation reaction to obtain NMMO.

Images