GB2034684A

GB2034684A - Wet Oxidation Treatment

Info

Publication number: GB2034684A
Application number: GB7939268A
Authority: GB
Original assignee: GALASSI G
Current assignee: GALASSI G
Priority date: 1978-11-13
Filing date: 1979-11-13
Publication date: 1980-06-11
Also published as: FR2440918B1; SE7909344L; CH649071A5; DE2944190A1; DE2944190C2; ATA712679A; ES485723A1; NL7908096A; AT392260B; FI793303A; GB2034684B; SE450953B; FR2440918A1

Abstract

The wet oxidation process e.g. for the purification of waste waters, is carried out with an oxidizing agent containing at least 60% oxygen. The employment of liquid oxygen eliminates the need to install expensive compression units. The addition of ozone improves the reactivity of the oxidation reaction. The addition of organic substances to the waste waters can give autogenous heating.

Description

SPECIFICATION Wet Oxidation Treatment The present invention relates to the treatment of waste water and other liquids containing oxidizable material.

In general, the currently adopted techniques for the treatment of waste waters comprise essentially two operating methods; a) separation of polluting substances from water: decantation, precipitation, adsorption, reverse osmosis, etc.

b) destruction of polluting agents present in water: biological oxidation with activated sludge, incineration with chemical oxidation, anaerobic digestion, etc.

Among the processes which entail the destruction of polluting agents, oxidation is one of the most important, as shown by the indexes universally adopted in order to point out the degree of pollution of waters, i.e. BOD, COD, TOC.

Oxygen is the least expensive oxidizing agent known, but is hardly reactive at room temperature. It is possible to manipulate three factors in order to increase the oxidation velocity: a) Increase temperature; b) Use a catalyst; c) Employ activated sludge, which is a sort of natural catalyst of oxidation processes.

High temperature as a means to destroy polluting substances is employed in incineration furnaces where waste waters are heated up to 1 000-1 2000C. At such temperature pollutants reach full oxidation within less than 1.5 seconds.

It is not necessary, however, to reach such high temperatures; normally, a temperature of 200 30000 is sufficient to obtain an approximately 90% reduction in the COD. A process which consists of the oxidation of matter at temperatures above 1050C and pressures above 2 bars has been called "wet oxidation" and such is the definition of it adopted in this Specification.

The known wet oxidation processes have been carried out only with the employment of air as the oxidizing agent. There are drawbacks with the employment of air in this process, ranging from the need to install expensive air compression units, to the large size of the reactors due to the presence of nitrogen and to the low reactivity of air.

It has now been found that the use of pure oxygen or oxygen-rich mixtures instead of air in wet oxidation processes leads to considerable advantages. A further advantage is achieved in the case of employment of liquid oxygen since the latter may be stored in a tank and pumped, in the liquid phase, into the oxidizing plant by means of a high pressure pump; the oxygen is then vaporized in an evaporator, thus reaching, in its gaseous phase, a pressure corresponding to the pump head: in this way one can achieve a considerable saving of compression energy and a much lower investment.

The subject of the invention therefore, is a wet oxidation process of the pollutants contained in waste waters (or of other substances in other liquids), characterised by the employment, as the oxidizing agent, of liquid oxygen or of a gaseous mixture containing oxygen at a concentration above 60% in volume.

A further aspect of the invention concerns the heating process for heating waste water/oxygen mixtures which are to be subjected to wet oxidation. Such heating is suitably carried out in one of four different ways: 1-Autogenous Heating In the wet oxidation process it is usually indispensable to reach a minimum temperature of 17000 in respect of the water to be purified in order to achieve good purification, but in most cases it is necessary to operate at a temperature of 25000 (and more still if one wants to reduce contact time): in fact, by increasing oxidation temperature one obtains an increase in oxidation yield and a decrease in contact time.

If on the other hand one adds to the water being treated a significant amount of an organic substance such as ethyl alcohol or some other organic compound capable of producing an exothermic oxidation reaction in the presence of oxygen, further heat develops as one reaches a temperature of 220#25O0C. Such heat causes the water temperature to rise to values which may even exceed those of the heating means employed.

It may appear paradoxical that, in order to lower the COD and destroy oxidizable polluting substances contained in the water to be purified, one should add to the water an organic substance which thus involves a considerable rise in the initial COD. However one should keep in mind that thanks to the increase in temperature produced by this addition in the wet oxidation process, one achieves a reduction of the COD and of the other pollutants by far superior, both in percentage and in absolute terms, to that obtainable when treating the waste waters subjected to purification in their natural state.

2-Flame Heating Instead of adding organic substances as described in order to increase water temperature, direct heating can be carried out by means of a flame in pipe-steel furnaces, thanks to which temperatures of 30000 and even 40000 may be reached easily.

3-Diathermic-fluid Heating Another technique which can be exploited consists of the employment of molten salt mixtures, eg, sodium nitrate, potassium nitrate and sodium nitrite which are solid up to about 1 5000 and liquify at higher temperatures, and may be used without problems up to temperatures beyond 5000 C: it is therefore possible to employ these salts to heat the water to be purified in a heat exchanger, carrying out the heating process of the above salts in separate equipment.

4-Direct Injection of Steam in to the Waste Water The oxidation temperature of the waste water can also be reached by direct injection of steam in to the reactor. After the oxidation, the outlet effluent can be cooled by means of direct injection of cool water.

A preferred feature of the invention is the addition of ozone to the oxygen employed for oxidation. It is a known fact that, in order to cause the oxidation temperature to drop, one makes use of catalysts. However, while this phenomenon has been verified for the vapour phase, the adoption of catalysts for the liquid phase has given poor results: there is also the problem of recovering and recycling the catalyst which in most cases is solid.

It has now been found out that if one employs ozone, instead of the catalyst, as an additive to oxygen, one achieves not only greater advantages in respect of the employment of catalysts, but since ozone produces oxygen in the course of the oxidation reaction, one no longer faces the problem of its recovery or pollution of the waste water.

A further subject of the invention is wet oxidation plant for removal of polluting substances from waste waters, characterised by the fact that provision is made for the use of liquid oxygen as source of oxidizing agent, which is to be injected into the reactor by means of a high pressure pump after vaporization by means of an evaporator.

Such a plant is illustrated by way of nonlimiting example in the attached drawing.

A suitable organic substance is added through a pipe 2 to polluted waste water from pipe 1 and thus mixed in pipe 3. The mixture undergoes a first heating treatment below 950C in exchanger G recovering some heat from the outlet effluent; it is then pumped through high pressure pump A through pipe 5 into mixer B into which oxygen is injected through pipe 17. The oxygen is drawn as a liquid phase from container 1 pumped by high pressure pump H, gasified in evaporator L where the heating liquid is water, and supplemented with ozone by means of ozonizer P.

The oxygen-enriched waste water is brought to process temperature in exchanger C employing a suitable diathermic fluid.

The diathermic fluid is heated continuously by flame furnace 0 and caused to circulate through pump N, while the required starting liquefaction takes place in container M.

The waste water, heated in the above manner, is conveyed to reactor D, where it stays for the time necessary to achieve oxidation and is then refrigerated in exchanger E, where heat is recovered in the form of vapour. At outlet 9 the waste water is conveyed to separator F and through pipe 10 to exchanger G and finally discharged through outlet 11.

The process of the invention is illustrated by the following non-limiting Examples each performed using plant as described with reference to the drawing.

Example 1 A waste water from pulp and paper industry called "black iiquor" having a COD of 347,000 mg/l was treated in the wet oxidation plant per the attached drawing. The oxidation temperature was 280-3800C and the outlet effluent had a COD of less than 2,000 mg/l.

Example 2 A waste water having an ammonia content of 16,150 mg/l was treated as above. At an oxidation temperature of 2600C, the ammonia content was reduced to a residue value of 75% and, at an oxidation temperature of3100C, to a residue value of 14%.

Example 3 A waste water called "ammonia liquor" having a COD of 35,250 mg/l was treated as above. The COD was reduced to 1,250 mg/l at a temperature of2500C.

Example 4 A waste water containing ethyl alcohol and other pollutants and having a COD of 66,662 mg/l was treated as above. The COD was reduced to 4,330 mg/l at a temperature of 235-2450C.

Example 5 A waste water called "sprung water" having a COD of 80,500 mg/í and containing 7,500 mg/í phenol was treated as above at an oxidation temperature of 2500C: the outlet effluent had a COD of 4,830 mg/l and contained 113 mg/l phenol when treated without agitation in the reactor, and 10 mg/l with agitation in the reactor.

Example 6 Some waste water derived from organic synthesis processes having a COD of 81,000 mg/l was treated in the wet oxidation plant shown in the attached drawing using a temperature of 1 90-2000C and with a contact time of 20 minutes. An outlet effluent having a COD of 22,350 mg/l was obtained. The above mentioned waste water whose COD had been made to rise to 163,000 mg/l by the addition of the same pollutants contained in the waste water was treated in the wet oxidation plant illustrated in the attached drawing. The temperature of reaction Rose in an auto genous manner from 2200C to 2800C exceeding, for some time, even this value: the outlet effluent had a COD of only 138 mg/l.

Example 7 Some waste water with an initial COD of 67,000 mg/l was treated in a wet oxidation continuous plant, as shown in the attached drawing, and at a process temperature of 2450C; the COD reduced to a residual value of 5,158 mg/l The above mentioned waste water whose COD had been made to rise to 423,000 mg/l by the addition of the pollutants was treated in a wet oxidation plant illustrated in the attached drawing: the temperature rose from an initial value of 2450C to an operative value between 2900C and 3050C and the outlet effluent measured at the time of analysis exhibited a COD value of 125 mg/l only.

Equally positive results were achieved by adding organic substances soluble in water, such as ethanol, to the waste water to be treated.

Example 8 Some waste water derived from an acrylonitrile production process and with a COD of 75,000 mg/l and CN rated at 0,475% exhibited a final COD of 4,350 mg/í and absence of cyanide, when operating at 2600C, while the COD removal yield was 94%. With the addition of ozone to oxygen, one obtained a significant reduction in temperature and in contact time, which was halved; the oxidation temperature was reduced by over 500C.

Claims

1. A wet oxidation process for removal of polluting substances from waste waters, characterized by the fact that oxidation is carried out with an oxidizing agent containing at least 60% oxygen.

2. A process according to Claim 1, characterized by the fact that pure or practically pure oxygen is employed for the oxidation.

3. A process according to Claim 2, characterized by the fact that liquid oxygen is employed as source of the oxygen.

4. A process according to any of Claims 1 to 3, characterized by the fact that oxidizing agent contains ozone.

5. A process according to any of Claims 1 to 4, characterized by the fact that oxidation is carried out at a temperature above 1 700C.

6. A process according to any of Claims 1 to 5, characterized by the fact that the heat required in order to reach and maintain the operative temperature is obtained by adding an easily oxygen-oxidable substance to the waste wasters being subjected to oxidation treatment.

7. A process according to any of Claims 1 to 5, characterized by the fact that the heat required to reach and maintain the temperature is provided from without, to the waste water mixture, by direct flame heating of the reactor.

8. A process according to any of Claims 1 to 5, characterized by the fact that the heat required to reach and maintain the temperature is provided from without, to the waste water mixture, by means of a heated diathermic fluid.

9. A process according to Claim 8, characterized by the fact that the diathermic fluid is a molten salt mixture.

10. A wet oxidation plant for removal of polluting substances from waste waters, characterized by the fact that provision is made for the use of liquid oxygen as source of oxidizing agent, which is to be injected into the reactor by means of a high pressure pump after vaporization by means of an evaporator.

11. A plant according to Claim 10, characterized by an ozonizer fitted into the oxygen duct itself or on an offtake, so that ozone can be included in the evaporated oxygen.

12. A plant according to Claim 10 or 11, characterized by mixing means for mixing the oxygen with the waste water before reaching the oxidation reactor.

13. A plant according to any of Claims 10 to 12, characterized by heating means for heating the reactor, said heating means containing a diathermic fluid.

14. A plant according to any of Claims 10 to 13, characterized by the fact that the diathermic fluid is composed of molten salts.

1 5. A plant according to any of Claims 10 to 14, characterized by heating means for heating the reactor using a direct flame.

16. A plant according to any of Claims 10 to 15, characterized by a heat exchanger designed to heat the waste water, a reactor, and a heat exchanger designed to cool the waste water, which are made up of a single pipe heated externally in its initial part and cooled externally in its initial part and cooled externally in its final part.

17. A plant according to any of Claims 11 to 16, characterized by the fact that the reactor is equipped with a stirrer.

18. A plant according to any of Claims 10 to 1 7, characterized by means for the direct injection of high pressure vapour to bring the waste waters to the oxidation temperature.

1 9. A plant according to any of Claims 10 to 1 8, characterized by means for cooling of the oxidized waste water by the direct injection of water.

20. The use of liquid oxygen to pressureoxidize polluting substances contained in waste waters or any other substance contained in solutions, as well as sludge derived from treatment plants or industrial processing residues.

21. The use of oxygen with ozone in order to pressure-oxidize polluting substances contained in waste waters or any other substance contained in any other solutions, as well as sludge derived from treatment plants or industrial processing residues.

22. The use of gaseous mixtures containing oxygen in a more than 60% ratio in order to pressure-oxidize polluting substances contained in waste waters or any other substance contained in any other solutions, as well as sludge derived from treatment plants or industrial processing residues.