SE541158C2

SE541158C2 - Oxidation column for hydrogen peroxide production

Info

Publication number: SE541158C2
Application number: SE1750459A
Authority: SE
Inventors: Erik Bengtsson
Original assignee: Eb Nuberg Ab
Priority date: 2017-04-19
Filing date: 2017-04-19
Publication date: 2019-04-16
Also published as: SE1750459A1

Abstract

The present disclosure relates to a method for producing hydrogen peroxide in accordance with the anthraquinone process. The method comprises entering a hydrogenated liquid Working Solution (WS) 4 comprising hydroquinones and solvent into an oxidation column 100 via a WS inlet at an upper end of said column, the WS flowing downward through the column. The method also comprises entering a gas phase 2 comprising oxygen into the oxidation column via a gas inlet at a lower end of said column, the gas phase flowing upward through the column, counter currently with the WS, as bubbles formed by means of a plurality of hole trays 1 positioned within the column. The method also comprises allowing the hydroquinones of the WS to react with the oxygen of the gas phase in an exothermic reaction to form an oxidized WS 5 comprising quinones 5a and hydrogen peroxide 5b exiting the column. The method also comprises cooling the oxidation column to remove heat formed by the exothermic reaction.

Description

OXIDATION COLUMN FOR HYDROGEN PEROXIDE PRODUCTION TECHNICAL FIELD The present disclosure relates to the production of hydrogen peroxide according to the per se well known anthraquinone process. The method is described extensively in the literature, for example in Kirk-Othmer “Encyclopedia of Chemical Technology”, 4th edition, 1993, Vol.13, pp 961-995. More particularly the present disclosure relates to the production of hydrogen peroxide according to the anthraquinone method with use of an oxidation column.

BACKGROUND In the anthraquinone process, solvent solutions of alkylated anthraquinones and/or alkylated tetrahydroanthraquinones are first hydrogenated to the corresponding hydroquinones. This hydroquinone solution is then oxidized with air or oxygen in an oxidation column. Hydrogen peroxide (H2O2), is split off when the hydroquinones are oxidized to quinones.

US 3,880,596 to Degussa discloses a co-current oxidation column for H2O2production.

US 6,375,921 discloses a countercurrent bubble column including perforated trays. The space-time yield of gas-liquid reactions can according to the document be considerably increased if the perforated trays have a substantially uniform distribution of holes, if the cross-sectional area of the individual holes is 0.003 to 3 mm<2>, particularly 0.01 to 0.5 mm<2>, and if the open area of the trays is 3 to 20%, particularly 3 to 10%, and if the bubble column comprises passages for liquid between adjacent liquid zones.

However, the use of such small holes gives rise to problems with foaming and with separation of the off gas, resulting in a hold-up of gas in the gas-liquid mixture of 50%.

SUMMARY The present invention relates to an improvement of the oxidation column used for oxidizing the hydroquinone solution of the anthraquinone process for H2O2production.

According to an aspect of the present invention, there is provided a method for producing hydrogen peroxide in accordance with the anthraquinone process. The method comprises entering a hydrogenated liquid Working Solution (WS) comprising hydroquinones and solvents into an oxidation column via a WS inlet at an upper end of said column, the WS flowing downward through the column. The method also comprises entering a gas phase comprising oxygen into the oxidation column via a gas inlet at a lower end of said column, the gas phase flowing upward through the column, counter currently with the WS, as bubbles formed by means of a plurality of hole trays positioned within the column. The method also comprises allowing the hydroquinones of the WS to react with the oxygen of the gas phase in an exothermic reaction to form an oxidized WS comprising quinones and hydrogen peroxide exiting the column. The method also comprises cooling the oxidation column to remove heat formed by the exothermic reaction. The diameter of the holes in each of the hole trays is within the range of 2-7 mm.

According to another aspect of the present invention, there is provided an oxidation column configured for producing hydrogen peroxide in accordance with the anthraquinone process. The column comprises a WS inlet, at an upper end of the column, for entering a hydrogenated liquid WS comprising hydroquinones and a solvent. The column also comprises a gas inlet, at a lower end of the column, for entering a gas phase comprising oxygen. The column also comprises a WS outlet, at the lower end of the column, for exiting an oxidized WS comprising quinones and hydrogen peroxides in the solvent. The column also comprises a plurality of hole trays arranged on different levels along a longitudinal axis of the column, each extending in a plane substantially perpendicular to said longitudinal axis. The column also comprises a cooling system arranged on the outside or inside of the column for a liquid coolant. The diameter of the holes in each of the hole trays is within the range of 2-7 mm.

It is to be noted that any feature of any of the aspects may be applied to any other aspect, wherever appropriate. Likewise, any advantage of any of the aspects may apply to any of the other aspects. Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. The use of “first”, “second” etc. for different features/components of the present disclosure are only intended to distinguish the features/components from other similar features/components and not to impart any order or hierarchy to the features/components.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments will be described, by way of example, with reference to the accompanying drawings, in which: Fig 1 is a schematic view of an embodiment of the inventive oxidation column in longitudinal section.

DETAILED DESCRIPTION Embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments are shown.

However, other embodiments in many different forms are possible within the scope of the present disclosure. Rather, the following embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers refer to like elements throughout the description.

Reference is made to figure 1, illustrating a cooled oxidation column 100 with hole trays 1, in accordance with an embodiment of the present invention. The column has an upper end (or top) and a lower end (or bottom) and may be positioned such that the longitudinal axis of the column is substantially vertical. The column operates in counter current mode with oxygen, here in compressed air 2, entering near to the bottom of the column and the off-gas 3 leaving at the top of the column 100. The hydroquinone solution, i.e. the hydrogenated “Working Solution” (WS) 4 comprising hydroquinone and a solvent or combination of solvents, enters near to the top of the column and the oxidized WS 5 comprising oxidized hydroquinones (i.e. quinones) 5a a together with dissolved hydrogen peroxide 5b in the solvent leaves via a WS outlet at the bottom of the column.

Counter Current Mode of Operation The counter current mode of the inventive column 100 is more efficient and the column may be made smaller then co-current mode columns where both oxygen and WS enters near to the bottom of the column and leaves the column near to the top as disclosed in US 3,880,596 to Degussa.

With co-current flow in the column of Degussa, the reaction rate (velocity) in the upper part of a co-current column is low because the concentration of both the reacting components, hydroquinone and Oxygen, are both low. The reaction conditions in the bottom of the co-current column are better because the concentration of both reacting components are high, but in practice the reaction in the bottom part is restricted by limited diffusion rates of gaseous oxygen to the liquid phase in which the reaction takes place.

In the counter current column 100 of the present invention, the reaction conditions are generally more favourable in the whole column thanks to the counter current mode of operation allowing the concentrations of both hydroquinone and oxygen to be relatively high along the whole longitudinal axis of the column, from the air/oxygen 2 inlet at the bottom of the column to the inlet of the hydrogenated WS 4 at the top of the column.

Hole Trays The inventive column is equipped with 3 to 10, preferably 8, hole trays 1, typically each extending in a plane perpendicular to the longitudinal axis of the column, thus when in use positioned substantially horizontally, at different levels along the longitudinal (i.e. vertical, when the column is in use) axis of the column 100, which redistribute the air (or oxygen) 2 bubbles and prevent axial back mixing of unoxidized hydroquinones to the WS 5a outlet. The flow pattern in columns with trays is improved compared with the flow pattern in columns without trays (empty columns).

The air bubbles in an empty column are increasing in size when they travel upwards in the column, because the static pressure is dropping and the bubbles gradually coalesce into fewer but larger bubbles. The interface area between the gas in the bubbles and the liquid WS is thus reduced and the oxidation reaction rate is limited by reduced diffusion of oxygen into the liquid phase of the WS. The bubble zone also contract towards the centre in an empty column, leaving the periphery in the upper part of the column free from gas and no oxidation reaction can take place. An axial downward flow of WS will establish in the periphery and the bubbles will carry the WS upwards in the centre. Unoxidized hydroquinones from the WS 4 inlet in the top of the column will than short-cut to the WS outlet in the bottom of the column so the reaction will not be complete. This phenomena is called “back-mixing” and is avoided by the use of trays as per the present disclosure.

These negative phenomena of reduced gas-liquid interface and back-mixing which reduce the performance of an oxidation column are prevented by means of the hole trays used in embodiments of the oxidation column 100 of the present invention.

Hole Tray Design Many small bubbles give a large interphase area between the gas phase in the bubbles and the liquid phase of the WS, and thus a high diffusion rate of oxygen from the bubbles into the WS. The small bubbles have a low raising velocity compared with larger bubbles and may thus form a foam, which may be undesirable.

Fewer and larger bubbles will give the opposite effect; low diffusion rate and high raising velocity.

It has now been found that the diameter of the holes in the trays 1 should be within the range of 2 to 7 mm, preferable 5 to 6 mm. The reason for this is that the bubble raising velocity should be high enough to travel upwards in the column 100 at the same time as the WS is travelling downwards, and at the same time the interphase area between gas and liquid should be large enough to allow enough diffusion of gaseous oxygen of the bubbles into the liquid phase of the WS to allow for high kinetic reaction velocity. The WS is preferably saturated with dissolved oxygen, implying that the diffusion rate of oxygen into the WS is not limiting to the oxidizing reaction rate.

The diameter of the trays 1 (as well as of the column 100) is given by the suitable cross-sectional gas load in the column, which is preferably 1000 to 1800 Nm<3>/m<2>,h., more preferably 1200 to 1700 Nm<3>/m<2>,h. At these conditions, the gas/liquid hold-up ratio in the column (or in the reaction zone of the column, defined as between the inlet of gas phase 2 and the inlet or level of hydrogenated WS 4) is quite stable within the range of 30/70 to 50/50, e.g. at around 40/60, volume by volume, especially when the gas is air. Gas/liquid separation may then be good at both ends of the column 100 and no, or a lower amount of, foam is produced.

The number of holes in each of the trays 1 is given by the total gas (air) flow and the preferred gas velocity in the holes which is 1 to 4 Nm<3>/hole,h, preferably about 2.5 Nm<3>/hole,h. At this hole velocity, uniform bubbles are formed, at the same time as the pressure drop when the gas is passing through the holes is establishing a gas head 6 below each of the trays 1 with a height of 30 to 100 mm. The gas head 6 act to evenly distribute the gas to the holes in the tray and stops WS to pass down through said holes.

The WS is led downwards in “downcommers” 7, substantially an opening through each of the trays 1 (much larger than the holes of the trays) at the periphery of the tray, providing a connection via which the WS can pass to bypass the gas head 6 under the tray 1. The velocity in the downcommers should preferably be maximum 0.2 m/s to prevent small gas bubbles with low raising velocity to travel downwards with the WS.

The gas head 6 under each tray 1 is re-distributing the bubbles evenly over substantially the whole cross section of the column and re-creates small bubbles in case of coalescence. The gas head is also preventing back-mixing between the tray sections.

Cooling of the Column The oxidation is an exothermic reaction. If the hydroquinone concentration in the WS 4 inlet is for example 80 g/l, the temperature of the oxidized WS 5a at the outlet may be about 13°C higher than at the inlet, provided that the reaction is complete and the column is not cooled.

There is a need to specify a maximum allowed reaction temperature in the column, because if it is too hot, there will be unwanted side reactions and there might also be a risk of fire if the flash point of the WS solvents is exceeded. There may be a maximum allowed temperature to 62°C, wherein the most preferred maximum temperature may be 55°C.

If we want 55°C at the WS 5 outlet, the temperature of the WS 4 inlet may need to be at most 42°C if the column 100 is not cooled. A problem is that the reaction rate/velocity at 42°C is less than half of the reaction velocity at 55°C. For this reason, the column 100 is cooled so that the temperature at both the inlet and the outlet of the WS 4/5 is for example in the range of 50°C to 6o°C, e.g. about 55°C. The difference in temperature between the hydrogenated WS 4 entering the column and the oxidized WS 5 exiting the column is preferably less than 5°C, such as less than 2°C. The reaction velocity is then high in the whole column, without having any unwanted side reactions or risk of fire.

Preferably the cooling system may e.g. be arranged by running cooling water 9 in “limpet coils” or “half pipes” 8 welded on the column shell, on the outside of the column 100, as indicated in figure 1. Other types of cooling systems may additionally or alternatively be arranged, e.g. any type of helixshaped cooling duct arranged on the outside or inside of the column 100 for any liquid coolant. To arrange the cooling system outside of the column may be preferred to reduce the risk of leakage of coolant within the column.

Example 1 A WS 4 based on Ethylantraquinone (60 g/l) and Tetrahydroethylantraquinone (100 g/l) was fed into an industrial cooled oxidation column 100, designed as the inventive column discussed in relation to figure 1, but with eight hole trays 1. The working solution flow through the column was 120 m<3>/h and contained 77 gram/litre hydroquinone, corresponding to 11.0 gram per litre H2O2, at the inlet of the column. The column diameter was 2.4 metres and the height of the reaction zone (distance between the inlet of air 2 and the inlet or level of hydrogenated WS 4) was 18 meters. The air flow was 5370 Nm<3>/h. The pressure in the top was 1.4 bar(g), and 2.5 bar(g) at the bottom of the column. The Oxygen content in the off-gas 3 leaving at the top was 6 %.The temperature of the WS was 54°C at the inlet 4 and 55°C at the outlet 5 of the WS. The content of hydrogen peroxide 5b in the outlet 5 was measured regularly and was found to be 10.85 g/l in average. So the oxidation yield was 10.85/11.0 = 98.6 %.

The total volume of the reaction zone was 81 m<3>and the gas/liquid ratio in the reaction zone was about 40/60, giving the volume of WS in the reaction zone as about 49 m<3>. Thus, hold up time for the WS was about 49/120 = 0.40 hours.

Example 2 Oxidation of the same WS as in Example 1 was made in the same column 100 as in Example 1 and at the same conditions except that the height of the reaction zone was 16 meters, measured from the air 2 distributor in the bottom of the column up to the WS level at the top of the column. The content of hydrogen peroxide 5b in the outlet 5 was measured regularly and was found to be 10.82 g/l in average. Thus, the oxidation yield was 10.82/11.0 = 98.4 %.

The total volume of the reaction zone was 72 m<3>and the gas/liquid ratio was about 40/60, giving the volume of WS in the reaction zone as about 43 m<3>. Thus, the hold up time for the WS was about 43/120 = 0.36 hours.

The present disclosure has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the present disclosure, as defined by the appended claims.

Claims

1. A method for producing hydrogen peroxide in accordance with the anthraquinone process, the method comprising: entering a hydrogenated liquid Working Solution, WS, (4) comprising hydroquinones and solvent into an oxidation column (100) via a WS inlet at an upper end of said column, the WS flowing downward through the column; entering a gas phase (2), e.g. air, comprising oxygen into the oxidation column via a gas inlet at a lower end of said column, the gas phase flowing upward through the column, counter currently with the WS, as bubbles formed by means of a plurality of hole trays (1) positioned within the column; allowing the hydroquinones of the WS to react with the oxygen of the gas phase in an exothermic reaction to form an oxidized WS (5) comprising quinones (5a) and hydrogen peroxide (5b) exiting the column; and cooling the oxidation column to remove heat formed by the exothermic reaction; wherein the diameter of the holes in each of the hole trays (1) is within the range of 2-7 mm.

2. The method of claim 1, wherein the plurality of hole trays consist of 3-10, preferably 8, hole trays.

3. The method of any preceding claim, wherein the diameter of the holes in each of the hole trays (1) is within the range of 5-6 mm.

4. The method of any preceding claim, wherein the gas velocity in each of the holes of the hole trays (1) is within the range of 1-4 Nm<3>/h, preferably about 2.5 Nm<3>/h.

5. The method of any preceding claim, wherein the cross-sectional gas load in the column (100) is within the range of 1000 to 1800 Nm<3>/m<2>,h, more preferably 1200 to 1700 Nm<3>/m<2>,h.

6. The method of any preceding claim, wherein each of the hole trays (1) comprises a downcommer (7) through which the WS flows downward at a velocity of less than 0.2 m/s.

7. The method of any preceding claim, wherein the difference in temperature between the hydrogenated WS (4) entering the column and the oxidized WS 5 exiting the column is preferably less than 5°C, such as less than 2°C.

8. An oxidation column (100) configured for producing hydrogen peroxide in accordance with the anthraquinone process, the column comprising: a WS inlet, at an upper end of the column, for entering a hydrogenated liquid Working Solution, WS, (4) comprising hydroquinones and solvent; a gas inlet, at a lower end of the column, for entering a gas phase comprising oxygen; a WS outlet, at the lower end of the column, for exiting an oxidized WS (5) comprising quinones (5a) and hydrogen peroxides (5b); a plurality of hole trays arranged on different levels along a longitudinal axis of the column, each extending in a plane substantially perpendicular to said longitudinal axis, wherein the diameter of the holes in each of the hole trays (1) is within the range of 2-7 mm; and a cooling system arranged on the outside of the column (100) for a liquid coolant.