EP4669629A1

EP4669629A1 - REDUCED CARBON DIOXIDE EMESON OXIDATION PROCESS

Info

Publication number: EP4669629A1
Application number: EP24759853.5A
Authority: EP
Inventors: Staffan Torssell; Henrik LENDRUP
Original assignee: Stora Enso Oyj
Current assignee: Stora Enso Oyj
Priority date: 2023-02-22
Filing date: 2024-02-19
Publication date: 2025-12-31
Also published as: WO2024176084A1; SE546350C2; SE2330097A1

Abstract

The present document relates to a process and system for continuous oxidation of a substance comprising one or more of a primary hydroxy group, an aldehyde group and/or a carbonyl group in which process carbon dioxide formed in the oxidation reaction is separated from nitrogen-containing off-gases of the reaction The nitrogen-containing off-gases are then recycled.

Description

OXIDATION METHOD WITH REDUCED CARBON DIOXIDE EMISSON

Technical field

[0001] The present document relates to a continuous process and system for oxidizing a substance comprising one or more of a primary hydroxy group, an aldehyde group and/or a carbonyl group, in which process carbon dioxide is separated from off-gases from the reaction and captured while nitrogen in the offgases is recycled.

Background art

[0002] Oxygen is a very attractive oxidant for redox reductions as is it abundant, has low molecular weight and often only generates water as a non- hazardous by-product. In so called Type I aerobic oxidation reactions oxygen is not incorporated in the product but acts to regenerate the active catalyst.

[0003] However, aerobic oxidation reactions are highly exothermic. In particular, aerobic oxidation reactions where oxygen is combined with flammable materials, such as organic solvents, under high temperature and pressure requires strict safety measures to be taken to avoid fatal consequences. Organic solvents can ignite in the presence of oxygen. This can be avoided by keeping the oxygen concentration below the concentration of oxygen where explosions can occur. Further, heat released in the reaction has to be managed to ensure safe scale-up of the oxidation processes. For this reason, it is often advantageous to perform aerobic oxidation reactions under continuous flow. In this way, the internal volume of the reaction system is limited which allows for better heat dissipation and safer operation.

[0004] The synthesis of bio-based chemicals and polymers from renewable biomass has been developed as a sustainable alternative to reduce dependence on fossil-fuel resources. FDCA can e.g. be used for the production of polyethylene furanoate (PEF), which is a substitute for PET. The common route for synthesizing FDCA is by oxidation of 5-hydroxymethylfurfural (5-HMF). Oxidation of 5-HMF to produce FDCA is a complex reaction involving several intermediate molecules such as 5-hydroxymethyl-2-furancarboxylic acid (HMFCA), 2,5-diformylfuran (DFF) and 5-formyl-2-furancarboxylic acid (FFCA).

[0005] FDCA can be produced in batch mode. However, continuous systems for production of FDCA can decrease reaction time and increase yield and thereby improve the production process.

[0006] WO2014/163500 discloses a continuous process for oxidizing 5- methoxymethyl furfural to produce FDCA, wherein the feedstock, an acetic acidcontaining solvent and oxygen-containing gas is continuously feed into the reactor. A vaporous stream containing methyl acetate is recovered from the system, condensed, and recycled, which increased the yield of FDCA obtained in the process. The vaporous stream also contained volatile compounds, such as oxygen, nitrogen and carbon dioxide, which were separated from the heavier compounds (such as methyl acetate, acetic acid, and water) by e.g., flash distillation, before discharging the volatile compounds.

[0007] WO 2017/123763 discloses a process for producing 2,5-furandicarboxylic acid pathway products, such as HMFCA, DFF, FFCA and FDCA. The process involves contacting an oxidation feedstock comprising a furanic oxidation substrate and an oxidation solvent with oxygen in the presence of a heterogenous oxidation catalyst comprising a solid support and a noble metal. During this process, carbon dioxide is unavoidably formed due to over-oxidation of the feedstock. For small- scale pilot plants and lab setups this is not a big problem since the amounts are small and the gas streams are not recycled. However, for large industrial plants the amount of carbon dioxide formed will be significant and the release into the atmosphere will have a negative impact on the sustainability of the process.

Further, the build-up of carbon dioxide in case the off-gases from the oxidation reaction are re-cycled will negatively impact the oxidation process. [0008] There is thus a need for a continuous oxidation process that reduces the emission of carbon dioxide to the atmosphere and/or that allows the off-gases to be re-cycled without build-up of carbon dioxide in recycled off-gases.

Summary

[0009] An object of the present document is to overcome or at least mitigate the above identified problems.

[0010] The present document thus provides a continuous oxidation process for oxidising a substance comprising one or more of a primary hydroxy group, an aldehyde group and/or a carbonyl group, wherein the oxidation process comprises the steps of: a) contacting an oxidation feedstock comprising said substance with an atmosphere comprising oxygen and nitrogen and an oxidation catalyst to form an oxidized product; b) separating said oxidized product from off-gases from the oxidation reaction of step a), said off-gases comprising carbon dioxide and nitrogen; c) passing the off-gases through a condenser under conditions condensing at least part of the carbon dioxide while maintaining the nitrogen in gaseous form; d) separating at least part of said carbon dioxide from said nitrogen; and e) recycling at least part of said nitrogen-containing off-gases to step a) of said continuous oxidation process; wherein steps a)-e) are performed at a pressure at 30 bar or more.

[0011 ] The substance comprising one or more of a primary hydroxy group, an aldehyde group or a carbonyl group may be a furanic oxidation substrate such as diformylfuran (DFF), hydroxymethylfurancarboxylic acid (HMFCA), formylfurancarboxylic acid (FFCA), and/or 5-(hydroxymethyl(furfural) (HMF). [0012] The oxidized product formed in the continuous oxidation process may be diformylfuran (DFF), hydroxymethylfurancarboxylic acid (HMFCA), formylfurancarboxylic acid (FFCA), and/or 2, 5-furandicarboxylic acid (FDCA).

[0013] The continuous oxidation may be performed at a pressure of from about 50 bar to about 90 bar.

[0014] Steps a) and b) in the continuous oxidation process may be performed at a temperature of from about 110 °C to about 140 °C. Steps c) and d) may be performed at a temperature of from about 0 °C to about 30 °C. In step d) the carbon dioxide may be collected as a liquid in a high-pressure tank.

[0015] The oxidation feedstock may further comprise an oxidation solvent, such as an oxidation solvent comprising water and a water-miscible organic solvent.

The oxidation catalyst may comprise a solid support and a noble metal, such as platinum and/or gold.

[0016] The oxidation catalyst may further comprise a promoter metal, such as Bi.

[0017] The present document also discloses a system for a continuous oxidation process, such as the continuous oxidation process of any one of claims 1 to 10, for oxidising a substance comprising one or more of a primary hydroxy group, an aldehyde group or a carbonyl group, said continuous oxidation system comprising: a) an oxidation reactor for contacting an oxidation feedstock comprising said substance with an atmosphere comprising oxygen and nitrogen and an oxidation catalyst to form an oxidized product; b) a first high-pressure gas-liquid separator for separating said oxidized product from off-gases produced in the oxidation reactor of step a), said offgases comprising at least nitrogen and carbon dioxide; c) a condenser for condensing at least part of said carbon dioxide from said off-gases while maintaining nitrogen in gaseous form; d) a second high-pressure gas-liquid separator for separating the condensed carbon dioxide from said nitrogen in gaseous form; and e) means for recycling at least part of said nitrogen from said second high- pressure gas-liquid separator to the oxidation reactor of said continuous oxidation process at high pressure; and f) a compressor for compressing said recycled gas stream to the operating pressure of the oxidation reactor.

[0018] The system may further comprise means to increase the oxygen concentration in the recycled gas stream to the desired operating concentration by addition of air or oxygen in nitrogen through an additional high-pressure gas feeding system.

[0019] The system may further comprise a container for collecting the oxidized product, a high-pressure tank for collecting said separated carbon dioxide, and/or means for feeding said oxidation feedstock into said oxidation reactor at high pressure together with said atmosphere comprising oxygen and nitrogen.

Brief description of drawings

[0020] Figure 1 shows a system for implementing a continuous oxidation process of the present document.

Detailed description

[0021 ] In the process according to WO2017/123763 or WO2019/014382 for production of 2,5-furan dicarboxylic acid (FDCA), 5-hydroxylmethyl furfural (HMF) is oxidized using a heterogenous oxidation catalyst in a continuous fixed-bed reactor together with oxygen in nitrogen as oxidant. During this process carbon dioxide is unavoidably formed due to over-oxidation of the feedstock. As mentioned above, for small-scale pilot plants and lab setups this is not a big problem since the amounts are small and the gas streams are not recycled.

However, for large industrial plants the amount of carbon dioxide formed will be significant and the release into the atmosphere will have a negative impact on the sustainability of the process. Also, in order to minimize the need of make-up carrier gas (nitrogen) and energy demand from high-pressure compressors, it is preferred that the high-pressure off-gases can be recycled as carrier gas without pressure reduction from the back end of the oxidation reactor to the front where oxidant (air) is added to reach the target oxygen levels in the gas feed.

[0022] However, due to the formation of carbon dioxide in the process, the carbon dioxide levels in the system will increase when the off-gases are recycled which can have a negative impact on the oxidation process. Thus, an off-gas purge stream is needed to vent carbon dioxide from the process into the atmosphere. This will have a negative environmental impact and reduce the amount of off-gases that can be recycled.

[0023] The present inventors have now surprisingly found a way to purify the offgases from carbon dioxide in a continuous oxidation process thus being able to recycle the carrier nitrogen gas without a successive build-up of carbon dioxide in it. Further, the present process allows for capture and collection of the carbon dioxide thus avoiding release of this into the atmosphere.

[0024] The present document is directed to a continuous oxidation process for oxidising a substance comprising one or more of a primary hydroxy group, an aldehyde group and/or a carbonyl group which reduces the amount of carbon dioxide that is released into the atmosphere. This continuous oxidation process comprises the steps of: a) contacting an oxidation feedstock comprising the substance comprising one or more of a primary hydroxy group, an aldehyde group and/or a carbonyl group with an atmosphere comprising oxygen and nitrogen and an oxidation catalyst to form an oxidized product; b) separating said oxidized product from off-gases from the oxidation reaction of step a), said off-gases comprising carbon dioxide and nitrogen; c) passing the off-gases through a condenser under conditions condensing at least part of the carbon dioxide while maintaining the nitrogen in gaseous form; d) separating at least part of said carbon dioxide from said nitrogen; and e) recycling at least part of said nitrogen-containing off-gases to step a) of said continuous oxidation process; wherein steps a)-e) are performed at a pressure of 30 bar or more.

[0025] The present continuous oxidation process is performed at a pressure of at least 30 bar, such as from about 30 bar to about 90 bar, such as from about 50 bar to about 90 bar, from about 50 bar to about 80 bar, from about 60 bar to about 80 bar, from about 65 bar to about 75 bar, or at about 70 bar. As explained elsewhere herein, it may be preferred to perform all of steps a)-e) at substantially the same pressure.

[0026] In step a) of the continuous oxidation process, an oxidation feedstock comprising a substance comprising one or more of a primary hydroxy group, an aldehyde group and a carbonyl group is fed into the oxidation reactor. Oxygen and nitrogen are also fed into the reactor to provide an atmosphere comprising oxygen and nitrogen. The nitrogen may be newly introduced nitrogen and/or nitrogen recycled in the process (see below). Oxygen has to be continuously supplied to the oxidation reactor as oxygen is consumed in the oxidation reaction and may be supplied as air or pure oxygen. The air or oxygen in nitrogen may be supplied through a high-pressure gas feeding system. Typically, in the continuous oxidation process of the present document, the oxygen level in the atmosphere comprising oxygen and nitrogen is below that of air (i.e. less than 21 vol%). The air or pure oxygen is therefore mixed with nitrogen to achieve the desired level of oxygen in the atmosphere comprising oxygen and nitrogen. An oxidation reaction is then carried out in the oxidation reactor in the presence of an oxidation catalyst to form an oxidized product. Step a) is typically carried out at a temperature of from about 100 °C to about 140 °C. The oxidation reaction may e.g. be an oxidation reaction as disclosed in WO2017/123763 or WO2019/014382. Increasing the temperature to more than 140 °C will increase the amount of side products and it is thus preferred to keep the temperature in step a) at a maximum of 140 °C.

[0027] The substance comprising one or more of a primary hydroxy group, an aldehyde group and/or a carbonyl group is for example a furanic oxidation substrate. Examples of furanic oxidation substrates include diformylfuran (DFF), hydroxymethylfurancarboxylic acid (HMFCA), formylfurancarboxylic acid (FFCA), and 5-(hydroxymethyl(furfural) (HMF). HMF can be oxidized to FDCA either via DFF and then FFCA as intermediates or via HMFCA and then FFCA as intermediates. Thus, for example, any of these substances, or a combination thereof, may be used as the starting material for the continuous oxidation reaction of the present document.

[0028] The oxidized product formed in the continuous oxidation process of the present document comprises diformylfuran (DFF), hydroxymethylfurancarboxylic acid (HMFCA), formylfurancarboxylic acid (FFCA), and/or 2, 5-furandicarboxylic acid (FDCA).

[0029] In step b) of the method of the present document the oxidized product(s) and the oxidation solvent is separated from off-gases that are also formed in the oxidation reaction of step a). These off-gases mainly comprise carbon dioxide and nitrogen and may also comprise residual oxygen and/or gases present in the air if air is supplied at the beginning of the process. The separation of oxidized product and oxidation solvent from off-gases is performed under conditions where the oxidized product(s) is in the liquid solvent phase while the off-gases are maintained in gaseous phase. This separation is preferably performed by leading the oxidized product(s) and the oxidation solvent together with the off-gases into a first high-pressure gas-liquid separator. Typically, the oxidized product(s) are already in the liquid solvent phase in the oxidation reactor, but it is also possible to condense the oxidized product(s) in the first high-pressor gas-liquid separator should these be in gaseous form in the oxidation reactor. The temperature in step b) is typically from about 100 °C to about 140 °C, such as from about 120 °C to about 130 °C. It is preferred to not use a temperature lower than 100 °C for in this separation step as that may cause crystallization of the oxidation product(s). The oxidized product(s) are then collected in a separate tank (which does not have to be at high pressure) while the off-gases in gaseous phase are transferred to a condenser.

[0030] In step c), the off-gases are passed through a condenser under conditions condensing at least part of the carbon dioxide while maintaining the nitrogen in gaseous form. Typically at least 40 vol% or at least 50 vol%, such as from about 40 vol% to about 60 vol%, of the carbon dioxide in the off-gases is condensed. The condenser can be a shell and tube heat exchanger, or any other design known in the art. The carbon dioxide can be made to condense by lowering the temperature below the critical temperature and adjusting the pressure to be in the liquid region according to carbon dioxide’s pressure-temperature phase diagram. The condensation is typically achieved by maintaining the pressure but lowering the temperature in the condenser. The temperature in the condenser may e.g. be from about 0 °C to about 30 °C, such as from about 10 °C to about 20 °C. This will cause the carbon dioxide to condense, while the nitrogen is maintained in the gaseous phase. A lower temperature allows a higher amount of carbon dioxide to be condensed.

[0031] In step d) the liquid carbon dioxide is separated from the gaseous nitrogen. This is typically performed in a second high-pressure gas-liquid separator. The temperature in the second high-pressure gas-liquid separator is typically the same as in the condenser, e.g. from about 0 °C to about 30 °C, such as from about 10 °C to about 20 °C.

[0032] The first and second high-pressure gas-liquid separators are standard high-pressure gas-liquid separators well known to the person skilled in the art. In such a high-pressure gas-liquid separator gas is typically led out at the top of a tank while the liquid is lead out at the bottom of the tank. The liquid level in the gas-liquid separators is maintained by regulating the liquid flow out from the bottom of the gas-liquid separator.

[0033] Keeping the pressure during the condensing step c) and separation step d) at at least 30 bar ensures that not too low temperatures are required to condense the carbon dioxide to liquid phase.

[0034] The separated carbon dioxide may be collected as a liquid in a separate high-pressure receiving tank. The captured and collected carbon dioxide in liquid form may be collected as a carbon dioxide rich gas stream by releasing the pressure of the receiving tank. The captured and collected carbon dioxide in liquid or gas form can be used as an enriched feedstock for high-value chemicals, such as for example methanol or formic acid without the need of gas separation of flue gases using gas diffusion membranes or similar separation techniques.

[0035] Typically, about at least 40-60 vol%, such as about 50 vol% of the carbon dioxide present in the off-gases may be removed by the present continuous oxidation process.

[0036] One advantage of the present process is that it allows a higher amount of off-gases to be re-cycled without the levels of carbon dioxide in the system successively increasing. As mentioned above, build-up of carbon dioxide in recycled off-gases can have a negative impact on the oxidation process. Further, the present process avoids the need for an off-gas purge stream to vent carbon dioxide from the process into the atmosphere which would have a negative environmental impact and reduce the amount of gas that can be recycled. Also, due to the large amount of off-gases (nitrogen) that can be recycled with the current process, compressor dimensions may be reduced which may lower energy consumption.

[0037] The process also allows reduction of carbon dioxide emission to the atmosphere as the carbon dioxide may be captured in the process. The captured carbon dioxide may then be used as a high-quality feedstock for high-value chemicals.

[0038] The oxidation feedstock may in addition to the substance comprising one or more of a primary hydroxy group, an aldehyde group and/or a carbonyl group further comprise an oxidation solvent, such as an organic solvent or a multicomponent solvent comprising water and a water-miscible organic solvent, such as a water-miscible aprotic organic solvent. For example, the oxidation solvent can be selected from one or more an oxidation solvent selected from the group consisting of tetrahydrofuran, a glyme, dioxane, methyl ethyl ketone (“MEK”), and gamma-valerolactone. The glyme can be selected from the group consisting of a monoglyme (1 ,2-dimethoxyethane), ethyl glyme, diglyme (diethylene glycol dimethyl ether), ethyl diglyme, triglyme, butyl diglyme, tetraglyme, and a polyglyme. The water-miscible organic solvent may be selected from the group consisting of a light water-miscible organic solvent and a heavy water-miscible organic solvent. The water and the water-miscible organic solvent may be present in a ratio of from or any number in between 1 :6 to 6:1 v/v water: water-miscible organic solvent. The water and the water-miscible organic solvent may be present in a ratio of 1 :1 v/v water: water-miscible organic solvent. The water-miscible organic solvent may be at least 10 vol % of the multicomponent solvent. The oxidation solvent can be a multi-component solvent comprising water and two different water-miscible organic solvents. The water- miscible organic solvents both can be water-miscible aprotic organic solvents. The concentration of the substrate in the oxidation feedstock is typically 5 wt% or more. The mole ratio of substrate to oxygen is typically 2:1 or more. The ratio between the amount of oxygen and nitrogen in the atmosphere comprising oxygen and nitrogen depends on the pressure, temperature and oxidation solvent used and has to be adjusted to prevent explosions in the reactor tank by keeping the oxygen concentration below the Limiting Oxygen Concentration (LOC) of the specific oxidation system. The skilled person knows how to adjust this to avoid explosions. [0039] The oxidation catalyst may comprise a solid support and a noble metal, such as platinum and/or gold. The solid support may e.g. comprise a material selected from the group consisting of a metal oxide, a carbonaceous material, a polymer, a metal silicate, a metal carbide, and any combination of two or more thereof. For example, the oxidation catalyst may comprise a material selected from the group consisting of carbon, zirconium dioxide, titanium dioxide, silicon carbide, silicon dioxide, AI2O3, and any combination of two or more thereof. The oxidation catalyst may further comprise a promoter metal, such as a promoter selected from the group consisting of Ti, Zr, Cr, Mo, W, Mn, Ru, Cu, Zn, Sb, Bi, Sn, Au, Ag, Pb, Te and any combination thereof, in particular Bi. The surface area of the catalyst may e.g. be less than 200 m²/g (but not zero), such as from about 20 to about 50 m²/g.

[0040] The present document also discloses a system 1 as exemplified in Fig. 1 for a continuous oxidation process, such as the continuous oxidation process disclosed herein, for oxidizing a substance comprising one or more of a primary hydroxy group, an aldehyde group and/or a carbonyl group, said continuous oxidation system comprising: a) a continuous oxidation reactor 2 for contacting an oxidation feedstock comprising said substance with an atmosphere comprising oxygen and nitrogen and an oxidation catalyst to form an oxidized product; b) a first high-pressure gas-liquid separator 3 for separating said oxidized product from off-gases produced in the oxidation reactor of step a), said offgases comprising at least nitrogen and carbon dioxide; c) a condenser 4 for condensing at least part of said carbon dioxide from said off-gases while maintaining nitrogen in gaseous form; d) a second high-pressure gas-liquid 5 separator for separating the condensed carbon dioxide from said nitrogen in gaseous form; and e) means 6 for recycling at least part of said nitrogen from said second high- pressure gas-liquid separator to the oxidation reactor of said continuous oxidation process at high pressure; and f) a compressor 10 for compressing said recycled gas stream to the operating pressure of the oxidation reactor.

[0041] The system may be specifically adapted to perform the continuous oxidation process of the present document.

[0042] The system may further comprise means to increase the oxygen concentration in the recycled gas stream to the desired operating concentration by addition of air or oxygen in nitrogen through an additional high-pressure gas feeding system 7.

[0043] The system may further comprise a container for collecting the oxidized product 8, a high-pressure tank 9 for collecting the separated carbon dioxide, and/or means 11 for feeding the oxidation feedstock into the oxidation reactor at high pressure together with the atmosphere comprising oxygen and nitrogen.

[0044] As is illustrated in Fig. 1 , the oxidation reactor, the first and second high- pressure gas-liquid separators, and the condenser are all operating at high pressure. Further, the means for recycling at least part of the nitrogen may operate under high pressure via a second compressor 10. Examples of suitable pressures are given elsewhere herein. If a high-pressure tank for collecting the separated carbon dioxide is present in the system, also this is at high pressure.

[0045] In the process and system of the present document, it may be preferred to keep the same pressure for all the steps taking place within the dotted line in Fig. 1 . A smaller loss in pressure during the process may take place, but keeping substantially the same pressure is advantageous as it is easier to condense out more carbon dioxide at a higher pressure and because all pressure loss during the process has to be compensated for when the off-gases are recycled to the beginning of the process. Increasing the pressure of the off-gas will require larger compressors which will increase the cost for the process. Maintaining the pressure substantially the same during all process steps within the dotted line thus increases process efficiency and results in a more economical process.

Claims

1 . A continuous oxidation process for oxidising a substance comprising one or more of a primary hydroxy group, an aldehyde group and/or a carbonyl group, said oxidation process comprising the steps of: a) contacting an oxidation feedstock comprising said substance with an atmosphere comprising oxygen and nitrogen and an oxidation catalyst to form an oxidized product; b) separating said oxidized product from off-gases from the oxidation reaction of step a), said off-gases comprising carbon dioxide and nitrogen; c) passing the off-gases through a condenser under conditions condensing at least part of the carbon dioxide while maintaining the nitrogen in gaseous form; d) separating at least part of said carbon dioxide from said nitrogen; and e) recycling at least part of said nitrogen-containing off-gases to step a) of said continuous oxidation process; wherein steps a)-e) are performed at a pressure at 30 bar or more.

2. The continuous oxidation process according to claim 1 , wherein said substance comprising one or more of a primary hydroxy group, an aldehyde group or a carbonyl group is a furanic oxidation substrate.

3. The continuous oxidation process according to claim 1 or 2, wherein said furanic oxidation substrate is diformylfuran (DFF), hydroxymethylfurancarboxylic acid (HMFCA), formylfurancarboxylic acid (FFCA), and/or 5-(hydroxymethyl(furfural) (HMF).

4. The continuous oxidation process according to any one of the preceding claims, wherein said oxidized product is diformylfuran (DFF), hydroxymethylfurancarboxylic acid (HMFCA), formylfurancarboxylic acid (FFCA), and/or 2,5-furandicarboxylic acid (FDCA).

5. The continuous oxidation process according to any one of the preceding claims, wherein the process is performed at a pressure of from about 50 bar to about 90 bar.

6. The continuous oxidation process according to any one of the preceding claims, wherein steps a) and b) are performed at a temperature of from about 110 °C to about 140 °C.

7. The continuous oxidation process according to any one of the preceding claims, wherein steps c) and d) are performed at a temperature of from about 0 °C to about 30 °C.

8. The continuous oxidation process according to any one of the preceding claims, wherein in step d) said carbon dioxide is collected as a liquid in a high- pressure tank.

9. The continuous oxidation process according to any one of the preceding claims, wherein said oxidation feedstock further comprises an oxidation solvent, such as an oxidation solvent comprising water and a water-miscible organic solvent.

10. The continuous oxidation process according to any one of the preceding claims, wherein said oxidation catalyst comprises a solid support, a noble metal, such as platinum and/or gold, and optionally a promoter metal, such as Bi.

11. A system (1 ) for a continuous oxidation process, such as the continuous oxidation process of any one of claims 1 to 10, for oxidising a substance comprising one or more of a primary hydroxy group, an aldehyde group or a carbonyl group, said continuous oxidation system comprising: a) a continuous oxidation reactor (2) for contacting an oxidation feedstock comprising said substance with an atmosphere comprising oxygen and nitrogen and an oxidation catalyst to form an oxidized product; b) a first high-pressure gas-liquid separator (3) for separating said oxidized product from off-gases produced in the oxidation reactor (2) of step a), said off-gases comprising at least nitrogen and carbon dioxide; c) a condenser (4) for condensing at least part of said carbon dioxide from said off-gases while maintaining nitrogen in gaseous form; d) a second high-pressure gas-liquid separator (5) for separating the condensed carbon dioxide from said nitrogen in gaseous form; and e) means (6) for recycling at least part of said nitrogen from said second high- pressure gas-liquid separator (5) to the oxidation reactor (2) of said continuous oxidation process at high pressure; and f) a compressor (10) for compressing said recycled gas stream to the operating pressure of the oxidation reactor (2).