CA1109231A

CA1109231A - Method of stripping ammonia from ammoniacal solutions

Info

Publication number: CA1109231A
Application number: CA296,748A
Authority: CA
Inventors: Egon Haese; Rolf Willms
Original assignee: Sidmar SA; Dr C Otto and Co GmbH
Current assignee: Sidmar SA; Dr C Otto and Co GmbH
Priority date: 1977-02-09
Filing date: 1978-02-09
Publication date: 1981-09-22
Also published as: NL7801228A; GB1558659A; FR2380223A1; JPS53102899A; BR7800689A; FR2380223B1

Abstract

ABSTRACT OF THE DISCLOSURE

The disclosure describes a method of stripping ammonia from ammoniacal solutions, which are treated with milk of lime in a stripping column and from which ammonia is expelled with hot gas, vapour or an evaporating liquid whereas an effluent at the bottom of said column, containing lime sludge, calcium sulphate, unburnt hydroxide, calcium carbonate and other calcium compounds, is withdrawn from the column and conveyed to a separator in which sludge is separated from liquid. It is characterised in that a pumpable sludge is withdrawn from the separator and is burnt un-der reducing conditions in a combustion chamber under a supply of fuel gas and air at a temperature of 600 to 1200°C.

Description

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The inven-tion relates to a method of strippiny ammonia from ammoniacal solutions, which are treated with milk of lime in a stripping column and from which the ammonia is expelled with hot gas, vapour or evaporating liquid whereas the effluent from the bottom, containing lime sludge, calcium sul~hate, un-burnt hydroxide, calcium carbonate and other calciurn compounds, is withdrawn from the column and conveyed to a separator in which the sludge is separated from the liquid.
Coke oven liquor contains various substances, including free and bonded ammonia. For years, the prior art method of com-pletely recovering the ammonia has been to treat the liquor in two stages. Usually the method is as follows: In a first strip-ping stage the pre-heated coke oven liquor is treated with va-pour or evaporating liquid from the second column. The liquor enters the top of a column and travels through a number of ex-change trays, whereas the vapour or evaporating liquid is intro-; duced into the second column from beneath. In this manner, the free ammonia and any non-bonded components are expelled. In or-der to strip the bonded ammonia, milk of lime or caustic soda so-lution is usually added to the effluent from the first stripping column. The ammonia is liberated, by substituting calcium for ammonia, and can be expelled with water vapour as in the first column. The effluent from the bottom of the lime column contains the lime sludge, calcium sulphate, unburnt hydroxide, calcium car-bonate and other calcium compounds. The sludge is separated in downstream settling containers and, after drying, is stored in dumps without being re-used.
Waste water containing bonded but no free amrnonia can be processed in a single stage, in which case lime is added at the top of the column. The resulting lime sludge is processed as previously described.
As a result of recent environmental regulations, the 9~3~ -'' dumping of lime sludge has become progressively more difficult and expensive. It is no answer to replace calcium by sodium, since this greatly increases the salt content of the waste water.
An object of the invention is to avoid dumping lime sludge or additional burdening of the waste water.
According to the invention, in the case of a method of the above type, a pumpable sludge is withdrawn from the separator and is thereafter, burnt under reducing conditions in a combustion chamber in a supply of fuel gas and air at a temperature of 600 to 120Q~C. According to the invention, the resulting combustion gases are introduced into an i~er-sion chamber in which the calcium compounds produced by com-bustion are converted to calcium hydroxide by reaction with an aqueous fluid, whereas the evaporating liquids produced are conveyed away for further treatment~
More specifically, according to the invention, there is provided a method for stripping ammonia from ammoniacal solutions of coke oven liquor, said method including the steps of treating said ammoniacal solutions with calcium hydroxide in a stripping column to expel ammonia and other fluids, with-drawing effluent containing calcium compounds from said stripp-ing column, using a separator to separate sludge from the major part of the liquid in the withdra~n effluent, transferring said sludge to a combustion chamber, and generating a reducing atmosphere in said combustion chamber at a temperature in the range of about 600C to 1200C to reduce said calcium compounds.
According to another embodiment, the reducing com-bustion is brought about at a temperature of preferably 800 to 1000C and the resulting calcium compounds consist mainly ~of calcium oxide. According to another emboaiment, the reduc-ing combustion is carried out at a temperature of preferably

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600 to 900~C, so that the resulting calcium compounds consist mainly of calcium sulphide, in which case, advantageously the H2 : H20 ratio or the C0 : C02 ratio is greater than 2 : 1.
According to another embodiment, an aqueous fluid, i.e. part of the clear liquid which is separated in the separator, is sent to the immersion container. In a preferred embodiment of the invention, the evaporating liquids withdrawn from the immersion container and obtained during quenching of the com-bustion gases are used for pre-heating the aqueous starting solution in an indirect heat exchanger. According to another embodiment, the gas condensate formed during cooling is re-cycled to the immersion chamber.
According to another embodiment, in order to treat the liquor, such as coke oven liquor, containing both bonded and free ;

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ammonia, the free ammonia is firs-t stripped from the aqueou~
fluid in known manner in the top part of the column and the so-lution freed from free ammonia is introduced into the immersion chamber, after reacting with the calcium oxide produced by com-bustion, the solution from the immersion chamber is sent to -the top of the bottom part of the stripping column, whereas the eva-porating liquids from the immersion chamber are supplied to the column as stripping water-vapour.
When fuel gas and air burn under reducing conditions in a burner connected to the combustion chamber, the production of carbon dioxide cannot be avoided, even with an air coefficient of 0.6 to 0.9. The low temperature in the combustion chamber, into which lime sludge containing considerable quantities of water is sprayed, may further increase the formation of C02. ~-~owever, it has been found that, in the case of the short-period combustion-chamber used, the water/gas equilibrium, which is critical for C2 formation, remains constant at well above 1000C. The reac-tion gas in the immersion chamber contains only 2 - 4 vol.% C02, depending on the air coefficient and the fuel.
It has been found that about 7 to 12% of the resulting carbon dioxide reacts with the calcium hydroxide-containing solu-tion to form calcium carbonate. The calcium carbonate travels through the entire circuit, is separated in the separator, and is again sent to the combustion chamber, during which process the entire circuit remains in equilibrium. The resulting blanh value of the circulation can be up to 20% of the amount required in cir-culation in any case. The amount of fuel gas must be increased in proportion to the circulation blank value, and the resulting amount of liberated energy cannot be efEiciently used in all applications.
According to the invention, in order to obviate this disadvantage and irnprove the efficiency of the method, the hot Z3~

combustion gases produced during combustion under reduciny condi-tions are cooled in a waste-heat boiler to a temperature above the dew point, and the dust consisting of calcium compounds is deposited in a depositing device, conveyed to a reaction contain-er and brought into contact with an aqueous fluid to form a new reaction fluid.
In one embodiment, the aqueous fluid is the fluid for treatment and the new fluid formed in the reac-tion container is sent to the top of the stripping column. In -that case, according to another embodiment, the hot reaction gases leaving -the sepa-rator are used in an indirect heat exchanger for pre-heating the aqueous starting solution.
Another embodiment of the invention consists in strip-ping ammonia from solutions, more particularly coke oven liquor, containing both bonded and free ammonia. For this purpose, the free ammonia is first stripped from the aqueous fluid in the top part of the column. According to the last-mentioned embodiment, the solution freed from free ammonia is introduced into the re-; action container and the new reaction fluid formed in the reac-tion chamber is introduced at the top of -the bottom part of the stripping column, whereas the reaction gases leaving the separa-tor are introduced below the bot-tom tray into the top part of the column, which is not operated with lime. In that case, the a-queous fluid is a partly treated liquid. In the last-mentioned embodiment, according to the invention, the hot reaction gases from the dust separator are used in an indirect heat exchanger for pre-heatir.g the boiler feed water.
Advantageously, according to the invention, the evapo-rating liquids produced in the reaction container are directly in-troduced into the evaporating-liquid line extending from the co-lumn, and the gas condensate produced during cooling is recycled to the reaction container.

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The invention relates to a method of stripping ammonia from ammoniacal solutions, which are treated with milk of lime in a stripping column and from which ammonia is expelled with hot gas, vapour or an evaporating liquid whereas an effluent at the bo-ttom of said column, containing lime sludge, calcium sulphate, unburnt hydroxide, calcium carbonate and other calcium compounds, is withdrawn from the column and conveyed to a separator in which sludge is separated from liquid, characterized in that a pumpable sludge is withdrawn from the separator and is burnt under reducing conditions in a combustion chamber under a supply of fuel gas and air at a temperature of G00 to 1200C.

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According to another feature,the effective mean yas velocity in the waste-heat boiler is 20-60 m/sec, preferably 30-50 m/sec.
Two embodiments of the invention are shown in the draw-ings, in which:
Fig. 1 is a block diagram of the method of treating coke oven liquor according to the invention, Fig. 2 shows a method of treating liquor containing only bonded ammonia, e.g. ammonium sulphate, Fig. 3 shows another embodiment of the method of treat-ing coke-oven liquor according to the invention, and Fig. 4 shows another embodiment of the method according to the invention, used for treating liquor containing only bond-ed ammonia, e.g. ammonium sulphate.
As shown in Figs. 1 and 3, the coke oven liquor to be treated is conveyed through a line 1 to the head of the top part 2 of a stripping column. The evaporating liquid 4 rising from the bottom part 3 of the same column and also containing ammonia which is stripped in the bottom part 3 is brought into intimate contact with the coke oven liquor, via exchange trays. A-t the top of the column, the resulting ammonia is withdrawn through line 5, together with the liquid which evaporates. The pre-trea-ted water travels along pipe 6 to be introduced into an immersion chamber 7.
The waste water, after being freed from ammonia and o-ther components, is conveyed along line 12 to a separator 13.
Clear liquid, which runs off at the top, is withdrawn through a pipe 14 and sent for further treatment if required, whereas a pump 1~ conveys the lime sludge in pumpable concentration along line 15 and via line 17 to combustion chamber 18. The required fuel gas for the combustion is supplied -through line 19 and the combustion air is supplied through line 20. Burner 21 produces a 23~

high flue gas into which the sludge is sprayed in finely-divided form. The reaction occurs at between about 600 and 1200"C, pre-ferably about 800 and 1000C. During the reaction, the calcium components are converted into calcium oxide and the sulphur com-ponents, into hydrogen sulphide. A-t the same time, according to the method of this invention, the organic constituents absorbed in the lime, e.g. tar, phenol and the like, are gasified or burnt.
According to the embodiment shown in Fig. 1, the resul-ting hot combustion gases are brought in chamber 7 into contact with the effluent from the top part 2 of the stripping column.
The irnmersion chamber also constitutes a reaction container in which ammonia is substituted with calcium. The liquid reaction mixture is conveyed from chamber 7 through pipe ~3, pump 9 and line 10 to the top of the bottom part 3 of the stripping column.
The evaporating liquids in chamber 7 are withdrawn through line 11 and are introduced through the bottom into the lower part 3 of the stripping column, said lower part 3 being also equipped with exchange trays or other corresponding elements. As a re-sult of the intimate contact between the two media, i.e. the evaporating liquid and the reaction mixture, the remaining li-berated ammonia is driven off from the reaction mixture and rea-ches the top part 2 of the stripper column, through an aperture between the top part and the bottom part of the column.
- In the embodiment shown in Fig. 3, the resulting hot combustion gases are conveyed through a line 34 to a waste-heat boiler 35 and cooled to a temperature above the dew point. The resulting saturated vapour travels through a line 29 to the bot-tom part 3 of the stripping column. The cooled gas flows through line 3~ into a dust separator 37. After being separated, the dust travels through a cut-off device (not shown) and line 42 to the reaction container 7, where the dust comes in contact with 1~)923~

the pre-treated liquor, so that the ammonia is replaced by cal-cium. The liquid reaction mixture is conveyed from container 7 through line 8, pump 9 and line 10 to the top of the bottom part

3 of the column, whereas the evaporating liquid formed in the re-action container 7 and consisting of ammonia and water-vapour is conveyed through line 11 to the evaporating-liquid line 5.
The cooled gas leaving the dust separator 37 is convey-ed through lines 38 and 39, either as a stripping gas to the top part 2 of the stripping column, or travels through a line 40 into heat exchanger 22, where it is provisionally cooled, and through line 41 to be used for another purpose. The heat-exchanger 22 can be used either for pre-heating the liquid under treatment or for pre-heating the boiler feed water. Any gas condensate is withdrawn through line 33 into container 7.
Fig. 2 shows how the process is used for treating li-quor containing only bonded ammonia, e.g. ammonium sulphate, in which case the content of hydrogen sulphide will be higher than during combustion under reducing conditions. -;
The liquor for treatment is conveyed through line 1 in-to a heat-exchanger 22, where it is indirectly pre-heated by the hot evaporatlng liquids withdrawn from the immersion chamber, and travels through lines 23, 25 to the stripping column 2. The reaction fluid coming from chamber 7 is conveyed through line 8, pump 9 and line 24 to be mixed with the liquor to be treated and travels therewith to the top of column 2, where most of the ammo-nia is substituted with calcium.
Column 2 operates in one stage. The lower part of the column is supplied through line 29 with a sufficient quantity of stripping water vapour, which can be replaced by an inert gas.
The ammoniacal evaporating liquids are withdrawn at the top of the column through line 5, whereas the water freed from ammonia and containing the corresponding calcium compound is conveyed 9Z3~

through line 12 to the separating container 13. The clear fluid is withdrawn from the top of container 13 through line 14. A
partial stream of fluid is conveyed through line 30, pump 31 and line 32 back to the immersion chamber 7.
The sludge (e.g. calcium sulphate) withdrawn from co-lumn 2 through line 12 and which settles in container 13 is con-veyed in pumpable concentration, via line 15, pump 16 and line 17, to the combustion chamber 18. As in the preceding example, heat is supplied by burning fuel gas with air, which are supplied to the burner 21 via lines 19 and 20.
When sulphate compounds are processed, -the reaction temperature in combustion-chamber 18 is 800 to 1200, preferably 900 to 1000C. The reaction gases are introduced into the fluid which is present in chamber 7 and are quenched therein. The re-sulting evaporating liquids and flue gas, the latter containing hydrogen sulphide when calcium sulphate is calcined, travel through line 11 to heat-exchanger 22, in which most of the heat of condensation of the water is supplied to the water to be treat-ed, which flows through line 1. The resulting gas condensate is 20 returned through line 33 to chamber 7.
The cooled flue gases can be given further a treatment.
For example, they can be sent through a line 26 to a device 27, where they are washed under oxidîzing conditions and the hydro-gen sulphide is obtained in the form of elementary sulphur, or alternatively elementary sulphur can be obtained in known manner in a Claus process. Device 27 may also be an apparatus for ob-taining sulphuric acid. The harmless gases withdrawn from device 27 are sent through line 29 for further use.
According to the embodiment illustrated in Fig. 4, ~he 30 water or liquor to be treated travels through line 1 to a heat-exchanger 22, where it is indirectly heated by the hot flue gases from the dust-separator 37, and then travels through line 23 to 3~L

the reaction container 7, where the pre-heated fluid is mixed with calcium hydroxide dust supplied through line 42 and is caused to react with said calcium hydroxide. In this case, am-monia is replaced by calcium, the liberated ammonia can be con-veyed through line 11 into the evaporating-liquid line 5.
The reaction fluid from container 7 is sent through line 8, via pump 9 and line 24 to the top of the column.
Column 2 is of the single stage type. The stripping water-vapour, which is generated in waste-heat boiler 35, is sent to the bottom part of the column 2 through line 29. The ammoniacal evaporating liquids are withdrawn at the top of the column through line 5, whereas the water freed from ammonia and containing the corresponding calcium compounds is conveyed through line 12 to the separating container 13. The clear fluid is with-drawn through line 14 from the top part of container 13, where-as the sludge, e.g. calciurn sulphate, deposited in container 13 is conveyed in pumpable concentration through line 15, pump 16 and line 17 to the combustion-chamber 18. As in the preceding example, heat is supplied by means of burning fuel gas with air, which are supplied to burner 21 respectively through lines 19 and 20.
When processing the sulphate compounds, the reaction temperature in chamber 18 is 600 to 1200, preferably 800 to 1000 C. As ln the other embodiments, the reaction gases are sent through line 34 to the waste-heat boiler 35 and cooled to a temp-erature above the dew-point. The gas is sent through line 36 to the dust-separator 37 and through line 38 to the heat-exchanger 22. The lime dust produced in the separator 37 is conveyed through a cut-o~f device (not shown~ and line 42 to container 7.
The cooled flue gases can be further treated. For ex-ample, they may be sent via line 26 to a device 27 where they are further processed. For example, they may be washed under oxidiz-3~

ing conditions and the hydrogen sulphide obtained in the form ofelementary sulphur, or alternatively elementary sulphur can be obtained in known manner in a Claus process. Device 27 can also be an apparatus for obtaining sulphuric acid. The harmless gases withdrawn from device 27 are sent through line 28 for further useO

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A method for stripping ammonia from ammoniacal solu-tions of coke oven liquor, said method including the steps of:
treating said ammoniacal solutions with calcium hydroxide in a stripping column to expel ammonia and other fluids, withdrawing effluent containing calcium compounds from said stripping column, using a separator to separate sludge from the major part of the liquid in the withdrawn effluent, transferring said sludge to a combustion chamber, and generating a reducing atmosphere in said combustion chamber at a temperature in the range of about 600°C to 1200°C
to reduce said calcium compounds.

2. The method according to claim 1 wherein said calcium compounds, after reduction, are introduced into an immersion tank in which the reduced calcium compounds are converted to calcium hydroxide by an ammoniacal fluid.

3. The method according to claim 1 including the step of maintaining the temperature within said combustion chamber in the range of 800°C to 1000°C whereby the resulting reduced calcium compounds consist primarily of calcium oxide.

4. The method according to claim 1 wherein said ammonia-cal solutions include ammonium sulfate and the method includes the step of maintaining the temperature within said combustion chamber in the range of about 600°C to 900°C whereby the result-ing reduced calcium compounds consist primarily of calcium sulfide.

5. The method according to claim 4 wherein the hydrogen-to-water ratio and the carbon monoxide-to-carbon dioxide ratio in said combustion chamber are maintained greater than 2:1.

6. The method according to claim 2 including the step of withdrawing evaporated liquids from the liquid level in said immersion tank, and preheating said ammoniacal solutions with the heat of said evaporated liquids in an indirect heat ex-changer prior to their being introduced into said stripping column.

7. The method according to claim 6 wherein a condensate is formed in said indirect heat exchanger, and including the step of recycling said condensate to the immersion tank.

8. The method according to claim 2 including the step of stripping free ammonia from the ammoniacal solution in the top part of the stripping column, introducing the solution freed from ammonia in the top part of the stripping column into said immersion tank where it reacts with calcium oxide from said combustion chamber, conveying the thus-reacted solu-tion to the top of the bottom part of the stripping column, and introducing evaporated liquids produced in the immersion tank as a stripping vapor in said stripping column.

9. The method according to claim 1 including the step of cooling hot gases containing calcium compounds from said combustion chamber in a waste-heat boiler to a temperature below the dew-point temperature of said gases, separating the calcium compounds from said hot gases in a dust separator, and conveying said calcium compounds to a reaction chamber, and introducing an aqueous fluid into said reaction chamber to form calcium hydroxide.

10. The method according to claim 9 including the step of conveying the calcium hydroxide formed in the reaction chamber to the top of said stripping column.

11. The method according to claim 9 including the step of preheating said ammoniacal solution from which ammonia is to be stripped with hot reaction gases leaving said separator.

12. The method of claim 2 wherein free ammonia is first stripped from the ammoniacal solution in the top part of the stripping column, the effluent freed from free ammonia in said stripping column being introduced into said immersion tank, supplying fluid formed in the immersion tank to the top of the bottom part of the stripping column, and passing the re-action gases leaving said separator into the top part of the stripping column to strip ammonia from said ammoniacal solu-tion.

13. The method according to claim 9 including the step of passing hot gases from said dust separator to an indirect heat exchanger for preheating said ammoniacal solution.

14. The method according to claim 9 including the step of combining evaporating liquids formed in said combustion cham-ber with ammonia leaving said stripping column.

15. The method according to claim 9 wherein the effec-tive mean gas velocity in the waste-heat boiler is 20-60 meters per second.

16. The method according to claim 15 wherein the mean gas velocity in the waste-heat boiler is 30-50 meters per second.