GB2369310A - Removal of contaminants from gas stream using acidic scrubber and oxidation - Google Patents

Abstract

Gas containing organic contaminants is introduced into scrubbing tower 10 at inlet 14, and undergoes countercurrent contact with aqueous liquor introduced at inlet 18. Gas stripped of contaminants is vented at 16. Contaminant containing liquor collects in a sump from which it is drawn by pump 24 into reaction vessel 30. An acid is introduced into vessel 30 and a water stream containing an oxidising agent is also introduced into the vessel. Heating coil 32 accelerates the reaction. Breakdown products are removed through outlet 40 and returned to tower 10. Heavily contaminated liquor is drawn off through line 28 and is made up with water, acid and oxidising agent.

Description

ORGANIC CONTAMINANT REMOVAL

This invention relates to the removal of organic materials from gas streams.

Many chemical plants generate gaseous emissions containing volatile organic compounds (VOCs) and other odorous or non odorous organic compounds. Release of such emissions into the atmosphere may represent a pollution problem both in terms of legally permissible threshold limits and, even if such limits are not breached, in objections from people living or working in the vicinity of the plant.

Various methods have been employed to avoid these pollution problems by such means as thermal oxidation (combustion) and absorption so as to break down or remove the organic matter from the gas stream before it is vented into the atmosphere.

Conventional gas scrubbing apparatus has not been widely used for such absorption duties. It employs very large quantities of water, representing high costs in purchasing, handling and disposal. Recirculating the water in the scrubber has been proposed but requires removal of the contaminants in their absorbed form to avoid equilibrium conditions in which no absorption occurs. Hitherto, no suitable chemical reaction has been readily available for efficient removal of many individual organic components or multiple organic contaminants. Some reactions such as that between aqueous sodium hydroxide and ester contaminants are relatively slow compared with ionic reactions so that long residence times are necessary to reduce the ester content sufficiently. Furthermore in this ester reaction a principal product is ethanol which can be desorbed from the recirculated liquid and entrained in the vent stream, thereby defeating the purpose of the scrubbing.

According to the present invention there is provided a method for removing organic contaminants from a gas stream by contacting the contaminated stream with water in

a scrubbing unit to produce a purified gas stream and an aqueous liquor containing the contaminants, characterised in that the aqueous liquor in the scrubbing unit has a pH of not greater than 6.5 and includes one or more treatment agents selected from oxidants and complexing reagents, in that aqueous liquor from the scrubbing unit is passed to a reaction zone outside the scrubbing unit to enhance absorption and breakdown of the contaminants, and in that aqueous liquor from the reaction zone is recycled to the scrubbing unit.

The method of the invention achieves a considerable saving in water consumption and permits simultaneous reactions and chemical mechanisms in the aqueous liquor, especially in the reaction zone, to break down the contaminants.

The method of the invention is applicable to scrubbing of gaseous emissions and process streams contaminated with hydrolysable and/or oxidisable vapours and gases such as esters, alcohols, aldehydes, ketones, amides, amines, alkenes and nitriles.

The acidic conditions in the aqueous liquor facilitate hydrolysis and oxidation reactions to break down the organic contaminants. In general the pH should be not greater than 6.5 but for certain contaminants it should preferably be not greater than 4.0. Acidification to the required pH level can be achieved by dosing, batchwise or continuously, of acidic liquid or by the simultaneous absorption of acidic gas.

Suitable acids include hydrochloric, sulphuric and sulphurous acids, which may be added as aqueous liquids as such, or generated in the aqueous liquor by simultaneous absorption of their vapours or vaporous precursors such as SO2.

The claimed treatment agents not only assist breakdown of the contaminants but also provide treatment of the breakdown products. The presence in the aqueous liquor of one or more oxidants assists oxidation of alcohols, amides, aldehydes and ketones.

The alcohols may have been produced from acid hydrolysis of contaminant esters.

The hydrolysis products of amides are carboxylic acids which have low vapour pressure and are thus retained in the aqueous liquor rather than escaping with the vent stream.

Suitable oxidants include ozone (03), potassium dichromate (K2Cr2O,), sodium chromate (NaZCr04 or Na2CrO4. 10H20), sodium dichromate (Na2Cr207 or Na2Cr207 2HzO) and potassium permanganate (KMnO4).

The presence in the aqueous liquor of one or more complexing reagents assists in hydrolysis of esters into complex ions and in reducing the vapour pressure of the contaminant materials and breakdown products such as alcohols. The reduced vapour pressure again helps to ensure that unwanted materials do not re-enter the gas stream. Suitable complexing reagents include calcium chloride, magnesium chloride and transition metal ions. The use of complexing reagents in the absence of an oxidant provides a sufficient treatment of alcoholic and ester contaminants.

In a preferred embodiment of the invention, the aqueous liquor contains both one or more oxidants and one or more complexing reagents. In this embodiment it is important to achieve the required acidic conditions by the use of an acid which does not react with the complexing reagent to generate salts which would precipitate out of solution). The combined use of oxidant (s) and complexing reagent (s) not only achieves the benefits of their separate use but also reduces the time required for the oxidation of alcohols. Moreover, the combination improves the process economics to make gas scrubbing a highly attractive alternative to thermal oxidation and adsorption processes for cleaning gas streams.

In another embodiment of the invention the aqueous liquor is employed at an elevated temperature, typically in excess of 40oC. This has the advantage of increasing the rate of breakdown of many contaminants and thereby permitting a reduction of the residence time of the contaminated stream in the scrubbing unit and reaction zone.

In general each 10 C rise in the liquor temperature gives an approximate doubling of the breakdown rate.

Removal of the breakdown products of the organic contaminants from the aqueous liquor can be effected batchwise or continuously. In many instances either occasional batch removal or a constant low rate bleed from the aqueous liquor is

sufficient to ensure a sufficiently low level of residual breakdown products in the recycled stream. Desorption of organic products recycled to in the scrubbing unit is thus avoided by removing most organic components within an economical residence time in the recirculation system. Removal of contaminated aqueous liquor should be compensated by corresponding water make-up and dosing of treatment agents.

In batchwise operation it is preferred to employ concentrations of acid to give a pH of not greater than 4.0, and as much oxidant and complexing reagent as will dissolve in the liquor. These concentrations assist in maximising the treatment capacity of the liquor. It is however important not to make the initial concentration of complexing reagent so high as to precipitate salts out of the aqueous liquor. The length of time for which a given batch of aqueous liquor can be used depends on several factors, including the concentration of contaminants in the feed gas stream and of the absorption efficiency in the scrubbing unit, which in turn depends on the liquid to gas ratio and Henry's coefficients and the mass transfer performance of the scrubbing unit.

The residence time of aqueous liquor in the scrubbing unit, reaction zone and associated pipework is dependent on the concentration of acid and treatment agents and on the liquor temperature. It should be chosen to give time for the individual reactions with the contaminants and breakdown products to occur and to ensure that breakdown products of the contaminants are not desorbed from the liquor at levels detrimental to the overall efficiency required to maintain the required efficiency in the scrubbing unit.

The individual treatment reactions vary according to the contaminants and the choice of acid and treatment agent. For example, with sulphuric acid used alone, esters are hydrolysed to form alcohol and acid; amides and nitriles are hydrolysed to form ammonium ions and carboxylic acid; amines form hydrogen sulphate; but there is no reaction with alcohols, aldehydes, ketones, carboxylic acids or alkenes. If the sulphuric acid is used in combination with potassium dichromate as an oxidant according to the invention then reactions occur as follows:

alcohols, including alcohols produced in the acid hydrolysis of esters, are oxidised to aldehydes, which in turn are oxidised to carboxylic acids ; amides are hydrolysed, possibly because of the presence of the dichromate ion ; . nitriles are hydrolysed to form amides and carboxylic acids. If the sulphuric acid is used in combination with potassium permanganate as an oxidant according to the invention then helpful additional reactions occur as follows: . alcohols, including alcohols produced in the acid hydrolysis of esters, are oxidised to aldehydes or ketones, which in turn are oxidised to carboxylic acids and in the case of ketones the oxidation to carboxylic acid is accompanied by degradation; . alkenes are oxidised to a variety of products including alcohol, carbonyl compounds, carboxylic acid and in some cases-for example with ethene-to carbon dioxide and water.

If dilute hydrochloric acid is used in combination with calcium chloride as a complexing agent according to the invention then additional reactions occur as follows: . alcohols, including alcohols produced in the acid hydrolysis of esters, form complexes with the calcium ions, resulting in vapour pressure reduction;

* with some alcohols under certain conditions there is a possibility of esterification to form a halogenoalkane; amides and nitriles may be hydrolysed, followed by complexing of the resultant ammonia with the calcium ion; * amines may form a complex, especially if a transition metal ion is substituted for the calcium and the amine is from the lower end of the homologous series;

. with certain carboxylic acids there is a small possibility of calcium salt being precipitated.

The invention is further described with reference to the accompanying figure, which is a flow diagram of apparatus for use according to the invention. The apparatus includes a scrubbing tower 10 with internal contact packing 12, a gas inlet 14 and outlet 16 and a liquor inlet 18 and outlet 19. A liquor removal line 22 incorporating a control valve 23 leads from the outlet 19 to a pump 24 from which a further liquor line 26 incorporating a control valve 25 runs to a reaction vessel 30.

The line 22 has a drainage branch 20 with a control valve 21. The line 23 has a bleed branch 28 with a control valve 29.

The reaction vessel 30 has an internal heating coil 32 to receive hot water. It also has an inlet 34 with a control valve 35, an inlet 36 with a control valve 37 and an inlet 38 with a control valve 39 for the introduction of respectively acid, treating agent and water.

A liquor outlet 40 from the reaction vessel 30 connects with a recycle line 42 which leads via a control valve 44 to a circulation pump 50. A feed line 52 with a control valve 53 leads from the pump 50 to the liquor inlet 18 of the scrubbing tower 10.

For flexibility of operation, the apparatus further includes a by-pass line 45 with a control valve 46 which enable liquor from the pump 24 to be recycled directly to the scrubbing tower 10 and an outlet line 47 with a control valve 48 which enable withdrawal of liquor from the reaction vessel 30.

In operation, gas with organic contaminants is introduced into the scrubbing tower 10 through the inlet 14 where it undergoes countercurrent contact with aqueous liquor introduced into the tower 10 through inlet 18. Gas stripped of contaminants is vented from the tower 10 through outlet 16. Liquor containing the absorbed contaminants collects in the sump at the base of the tower 10 from where it is drawn by pump 24 and passed through the lines 22 and 26 to the reaction vessel 30. In a typical treatment, an acid such as sulphuric acid is introduced into the vessel 30 through line 36 and a water stream containing an oxidising agent such as potassium permanganate is introduced into the vessel 30 through line 34. The initial water filling and any topup water for the reactor 30 is made through line 38 and valve 39.

Hot water is fed through the heating coil 32 to accelerate reaction between the contaminants and the oxidising agent in the vessel 30. Liquor containing the breakdown products of the said reaction is removed through outlet 40 and is returned under the action of pump 50 through the lines 42 and 52 to the tower 10. A treated batch of liquor can be removed through line 47 and valve 48.

For relatively light loadings of contaminants in the feed gas stream the aqueous liquor can be repeatedly recycled between the reactor 30 and the tower 10, subject to additions of acid and oxidising agent as required through inlets 34 and 36. For heavier loadings a portion of the liquor is continuously drawn off through the bleed line 28 and valve 29, with corresponding make up of water, acid and oxidising agent.

EXAMPLE In an example of contaminant removal according to the invention, apparatus of the above type, including a reaction vessel of 15 m3 internal volume and an associated scrubbing tower, was employed to remove propanal (C2HsCHO) vapour from air at ambient temperature. Such apparatus represents economical sizing for many absorption duties.

The total gas flowrate was 5,000 m3/hr with a propanal concentration of 250 mg/m3 at the inlet to the scrubbing tower.

Based on physical absorption a recirculation rate of 80 m3/hr was selected for the aqueous liquor to allow for 90% efficiency of propanal removal.

The aqueous liquor was dosed with sulphuric acid to give a pH of less than 2 and the resultant aqueous solution of propanal was recycled via the reactor vessel. Ozone was injected into the reactor vessel from an ozone generator at a rate sufficient to maintain a stoichiometric excess of ozone over propanal.

At 25 C the rate constant between ozone and propanal in the acidic conditions in the aqueous phase was 2.5 litres/mol. sec. The apparatus and procedure were found to allow a sufficiently large conversion of propanal in the scrubbing tower to ensure recirculation of a low enough concentration of propanal to maintain the desired 90% efficiency of propanal removal.

Claims (13)

1. A method for removing organic contaminants from a gas stream by contacting the contaminated stream with water in a scrubbing unit to produce a purified gas stream and an aqueous liquor containing the contaminants, characterised in that the aqueous liquor in the scrubbing unit has a pH of not greater than 6.5 and includes one or more treatment agents selected from oxidants and complexing reagents, in that aqueous liquor from the scrubbing unit is passed to a reaction zone outside the scrubbing unit to enhance absorption and breakdown of the contaminants, and in that aqueous liquor from the reaction zone is recycled to the scrubbing unit.

2. A method as claimed in claim 1, in which the contaminants are hydrolysable and/or oxidisable vapours and gases.

3. A method as claimed in claim 1 or claim 2, in which acidification to the required pH range is achieved by one or more acids selected from hydrochloric, sulphuric and sulphurous acids.

4. A method as claimed in any preceding claim, in which acidification to the required pH range is achieved by absorption of acidic vapours.

5. A method as claimed in any preceding claim, in which the aqueous liquor has a pH of not greater than 4.0.

6. A method as claimed in any preceding claim, in which the treatment agent comprises one or more oxidants selected from ozone, potassium dichromate, sodium chromate, sodium dichromate and potassium permanganate.

7. A method as claimed in any preceding claim, in which the treatment agent comprises one or more complexing reagents selected from calcium chloride, magnesium chloride and transition metal ions.

8. A method as claimed in any preceding claim, in which the aqueous liquor contains both one or more oxidants and one or more complexing reagents.

9. A method as claimed in any preceding claim, in which the aqueous liquor is employed at a temperature in excess of 40oc.

10. A method as claimed in any preceding claim, in which removal of breakdown products of the organic contaminants from the aqueous liquor is effected continuously.

11. A method as claimed in any of claims 1 to 9, in which removal of breakdown products of the organic contaminants from the aqueous liquor is effected batchwise.

12. A method as claimed in claim 11, which employs high concentrations of acid, oxidant and complexing reagent.

13. A method as claimed in any preceding claim, in which the residence time of the aqueous liquor in the scrubbing unit, reaction zone and associated pipework is chosen to ensure that breakdown products of the contaminants are not desorbed from the aqueous liquor.