Apparatus and method for removing ammonia from an ammonia-containing liquid

Abstract

Apparatus for removing ammonia from an ammonia-containing liquid comprises a stripping column (10) comprising an ammonia-containing liquid inlet (12), one or more air inlets (14), an ammonia-containing gas outlet (16) and a waste liquid outlet (18); a pump (20) connected to the stripping column for reducing the pressure within the stripping column to below atmospheric pressure; heating means for internally heating the stripping column; and a condenser (22) downstream of the ammonia-containing gas outlet for removing liquid vapour from the ammonia-containing gas. Also disclosed is a method for removing ammonia from an ammonia-containing liquid.

Description

APPARATUS AND METHOD FOR REMOVING AMMONIA FROM AN AMMONIA- CONTAINING LIQUID

[0001] The present invention relates to an apparatus and method for removing ammonia from an ammonia-containing liquid, and in particular to apparatus comprising a vacuum stripper for removing ammonia from wastewater .

[0002] Ammonia-containing liquids such as for example landfill leachate and ef fluent from anaerobic digesters must be made safe for discharge , and it is necessary to remove ammonia to requisite discharge standards . Ammonia is also a valuable material for use in many di f ferent applications and industries .

[0003] There are two principal known methods of stripping ammonia as a gas from liquid : the first is a chemical process that involves increasing the pH of the liquid, and the second is a thermal method that uses heat only . Both methods break the ionic bond that holds ammonia as NH4+and converts the ammonia to NH3, the free form . In large installations the chemical method is often used because the energy requirement for the thermal stripping of ammonia is prohibitive , and the cost of producing the necessary heat generally makes it economically unviable .

[0004] International patent publication WO 2012 / 110760 discloses a system for removing ammonia from landfill leachate which comprises an ammonia stripper for removing the ammonia from the leachate as an ammonia-containing gas , followed by decomposition of the ammonia in a thermal destructor . The thermal destructor can use heat from burning landfill gas , such as methane .

[0005] A disadvantage of the thermal decomposition of ammonia in a thermal destructor is that it requires combustion, which may not be possible in certain locations or jurisdictions. A further disadvantage of thermal decomposition of ammonia is the amount of heat energy which is required.

[0006] Ammonia may be recovered from the stripped ammonia- containing gas by scrubbing the gas. For example, the ammonia in the ammonia-containing gas discharged from the ammonia stripper may be scrubbed with acid to form an ammonium salt. Alternatively, the ammonia-containing gas may be scrubbed using a cold-water scrubber. A cold-water scrubber typically removes the ammonia from the ammonia-containing gas as a solution of ammonia and water. Ammonia may then be removed from the ammonia-water solution, and the water recovered and recycled for reuse in the cold-water scrubber.

[0007] Vacuum stripping of ammonia from wastewater is a process which involves stripping the ammonia under reduced pressure. Reducing the pressure in the stripper lowers the boiling point and increases the vapour pressure of the wastewater at a given temperature, which facilitates removal of ammonia. For example, at 500 mbar absolute pressure the boiling point of water is approximately 81.5°C, and at 250 mbar absolute pressure it is approximately 65°C.

[0008] However, vacuum stripping of ammonia from wastewater has drawbacks, in that it requires a long retention time of wastewater within the stripper, for example up to 90 minutes, to sufficiently allow the volatiles to boil, and also requires the addition of a base (e.g. sodium hydroxide) to raise the pH of the wastewater. Raising the pH is necessary to break the ionic bond of the ammonium ion and allow the ammonia to be present in the free form, as a dissolved gas.For example , US patent number 4093544 discloses a vacuum stripping method in which the pH of the wastewater is increased to a highly basic condition ( e . g . pH 11- 12 ) .

[0009] The present invention seeks to provide an apparatus and method which overcomes the disadvantages of the known methods .

[0010] Thus , according to the present invention in a first aspect there is provided apparatus for removing ammonia from an ammonia-containing liquid, the apparatus comprising : a stripping column comprising an ammonia-containing liquid inlet , one or more air inlets , an ammonia-containing gas outlet and a waste liquid outlet ; a pump connected to the stripping column for reducing the pressure within the stripping column to below atmospheric pressure ; heating means for internally heating the stripping column; a condenser downstream of the ammonia-containing gas outlet for removing liquid vapour from the ammonia-containing gas .

[0011] The apparatus of the first aspect of the present invention may potentially be used to remove ammonia from any ammonia-containing liquid, such as landfill leachate , anaerobic digesters and in petrochemical applications . The ammonia-containing liquid is preferably ammonia-containing water .

[0012] The apparatus of the first aspect of the present invention thus comprises a stripping column for vacuum stripping of ammonia from ammonia-containing liquid, i . e . the ammonia is stripped from the ammonia-containing liquid withinthe stripping column under reduced pressure, i.e. pressure lower than atmospheric pressure (1 atm) . The stripping column comprises an air inlet for allowing air to flow into the stripping column. Air flow within the stripping column assists with transport of ammonia gas from the liquid phase to the gaseous phase, and reduces the retention time of the ammonia within the liquid. For example, in the absence of induced air flow within the stripping column the retention time of ammonia within the liquid phase may be 1-2 hours, whereas with induced air flow the retention time is reduced considerably, for example to less than 15 minutes, e.g. 5-10 minutes. The apparatus of the first aspect of the present invention also comprises heating means for internally heating the stripping column, which helps to maintain the temperature within the column as the carrier liquid and ammonia are vapourised: as liquid is removed the latent heat of vapourisation causes the liquid temperature to drop, which reduces the effectiveness of the stripping column. The apparatus of the first aspect of the present invention is preferably configured so that ammonia may be removed from the ammonia-containing liquid without the addition of a base to the ammonia-containing liquid to raise its pH.

[0013] Thus, the apparatus of the first aspect of the present invention comprises a stripping column comprising an ammonia-containing liquid inlet, an air inlet, an ammonia- containing gas outlet and a waste liquid outlet.

[0014] The ammonia-containing liquid inlet is typically positioned at the upper end of the stripping column through which ammonia-containing liquid is introduced into the stripping column. The ammonia-containing liquid preferably enters the stripping column at a temperature of from for example 70 to 90°C, e.g. 72 to 85°C (preferably 80 °C) . Theapparatus of the present invention may thus further comprise means for heating the ammonia-containing liquid prior to it entering the stripping column, such as a heat exchanger

[0015] The air inlet is typically positioned at the lower end of the stripping column . The stripping column may comprise a plurality of air inlets at di f ferent positions within the column . For example , i f the stripping column comprises a plurality of trays positioned at di f ferent heights within the column an air inlet can be positioned adj acent one or more of , or each, tray . Due to the pressure di f ferential between the interior and exterior of the stripping column ( the stripping column interior is at lower than atmospheric pressure ) , air is sucked into the stripping column through the air inlet by the partial vacuum within the stripping column, causing an airstream within the stripping column . As discussed herein, airflow within the stripping column assists with the transport of ammonia gas from the liquid to gaseous phase : the partial pressure of ammonia within the air is zero , whereas the partial pressure of ammonia within the carrier liquid is not zero , and hence the ammonia trans fers from the liquid to the air .

[0016] The stripping column may comprise a plurality of heated trays for use in stripping ammonia from the carrier liquid . The trays are heated by the heating means as described herein . The ammonia is released from the liquid under reduced pressure and heating as ammonia-containing gas ( for example a mixture of ammonia and steam) and passes out of the stripping column through the ammonia-containing gas outlet .

[0017] The waste liquid outlet is typically at the bottom of the stripping column, through which the waste liquid exits the stripping column .

[0018] The apparatus of the first aspect of the present invention also comprises a pump connected to the stripping column for reducing the pressure within the stripping column. The pump may be any suitable pump as is known in the art, and is preferably connected to the stripping column towards the top of the column, for example near or adjacent the ammonia- containing liquid inlet. In preferred embodiments the pressure within the stripping column adjacent the ammonia- containing liquid inlet is lower than atmospheric pressure, for example 250-500mbar absolute. The reduced pressure within the stripping column assists with vapourisation of ammonia and carrier liquid. For example, at 500 mbar absolute pressure the boiling point of water is approximately 81.5°C, and at 250 mbar absolute pressure it is approximately 65°C.

[0019] As noted above, the apparatus of the first aspect of the present invention comprises one or more air inlets to cause an airstream to flow within the stripping column. The reduced pressure within the stripping column causes air to be sucked into the stripping column through the one or more air inlets. There will thus be a pressure differential within the stripping column between the one or more air inlets and the connection to the pump.

[0020] The apparatus of the first aspect of the present invention also comprises heating means for internally heating the stripping column, which helps to maintain the temperature within the column as the carrier liquid and ammonia are vapourised: as liquid is removed the latent heat of vapourisation causes the liquid temperature to drop, which reduces the effectiveness of the stripping column. As noted herein, the ammonia-containing liquid preferably enters the stripping column at a temperature of from for example 70 to 90°C, e.g. 72 to 85°C (preferably 80°C) . The heating means preferably internally heats the stripping column to providea temperature gradient within the column which increases down the column. Thus, the temperature at the top of the column, where, in preferred embodiments, the ammonia-containing liquid enters the column may be at approximately 80°C increasing to a temperature of approximately 90°C towards the bottom of the column.

[0021] The apparatus of the first aspect of the present invention further comprises a condenser downstream of the ammonia-containing gas outlet for removing liquid vapour from the ammonia-containing gas. Thus, in preferred embodiments, the ammonia-containing gas comprises ammonia and steam, and the condenser condenses out the steam from the ammonia- containing gas to leave predominantly ammonia gas. The condenser may comprise, for example, a cold-water loop. Condensed liquid, e.g. water, may pass to a liquid discharge outlet, and the ammonia gas may pass to an ammonia recovery unit, as described herein. The temperature of the ammonia- containing gas entering the condenser may be, for example approximately 80°C, and the temperature of the condensed liquid and ammonia gas leaving the condenser may be, for example, 70°C.

[0022] The apparatus of the first aspect of the present invention preferably further comprises an ammonia recovery unit for recovering ammonia downstream of the condenser. Thus, for example, the ammonia may be recovered as ammonium hydroxide, by dissolving the ammonia gas from the condenser in water. For example, the ammonia gas may pass into a water bath downstream from the condenser. The water is preferably at ambient temperature.

[0023] According to the present invention in a second aspect there is provided a method for removing ammonia from an ammonia-containing liquid, the method comprising the stepsof : passing an ammonia-containing liquid into a stripping column via an ammonia-containing liquid inlet ; wherein the interior of the stripping column is at a pressure of less than atmospheric pressure and wherein the interior of the stripping column is heated by heating means ; passing air into the stripping column via one or more air inlets ; allowing ammonia-containing gas to pass from the stripping column via an ammonia-containing gas outlet and waste liquid to pass from the stripping column via a waste liquid outlet ; removing liquid vapour from the ammonia-containing gas by passing the ammonia-containing gas through a condenser downstream of the ammonia-containing gas outlet , to provide ammonia gas .

[0024] The method of the second aspect of the present invention is preferably performed using an apparatus according to the first aspect o f the present invention, and features of the apparatus of the first aspect of the present invention as described herein equally apply as features of the method of the second aspect of the present invention . Thus , for example , the ammonia gas preferably passes from the condenser to an ammonia recovery unit , where the ammonia is recovered, for example as ammonium hydroxide by dissolving the ammonia gas in water .

[0025] Thus , in preferred embodiments of the method of the second aspect of the present invention, air flows into the stripping column via one or more air inlets , to assist with transport of ammonia gas from the liquid phase to the gaseous phase , and reduce the retention time of the ammonia within the liquid . For example , in the absence of induced airflow within the stripping column the retention time of ammonia within the liquid phase may be 1-2 hours , whereas with induced air flow the retention time is reduced considerably, for example to less than 15 minutes , e . g . 5- 10 minutes .

[0026] The method of the second aspect of the present invention preferably does not include the step of adding a base to the ammonia-containing liquid to raise its pH .

[0027] The present invention will now be described in detail with reference to the accompanying drawing in which :

[0028] Figure 1 shows a process flow diagram for the apparatus and method of the first and second aspects of the present invention .

[0029] Thus , referring to Figure 1 , an embodiment of an apparatus for removing ammonia from an ammonia-containing liquid is shown . The apparatus and method of the present invention are discussed with reference to the embodiment shown in Figure 1 in connection with the removal of ammonia from water, but as discussed above the apparatus and method of the present invention may potentially be used to remove ammonia from any ammonia-containing liquid, including landfill leachate , ef fluent from anaerobic digesters , and in petrochemical applications .

[0030] The embodiment of the apparatus shown in Figure 1 comprises a stripping column 10 comprising an ammonia- containing liquid inlet 12 , one or more air inlets 14 , an ammonia-containing gas outlet 16 and a waste liquid outlet 18 . A pump 20 is connected to the stripping column 10 for reducing the pressure within the stripping column 10 to below atmospheric pressure . The apparatus further comprises heatingmeans (not shown in Figure 1) for internally heating the stripping column 10. A condenser 22 is positioned downstream of the ammonia-containing gas outlet 16 for removing liquid vapour from the ammonia-containing gas leaving the stripping column 10.

[0031] In this embodiment, the ammonia-containing liquid inlet 12 is positioned at the upper end of the stripping column 10 through which ammonia-containing liquid is introduced into the stripping column 10. In preferred methods, the ammonia-containing liquid enters the stripping column 10 at a temperature of from for example 70 to 90°C, e.g. 72 to 85°C (preferably 80 °C) . As discussed herein, the apparatus of the present invention may thus further comprise means for heating the ammonia-containing liquid upstream of the ammonia-containing liquid inlet 12, such as a heat exchanger (not shown in Figure 1)

[0032] The air inlet 14 is positioned towards the lower end of the stripping column 10. Whilst only one air inlet 14 is shown in Figure 1, the stripping column 10 may comprise a plurality of air inlets 14 at different positions within the column. For example, the stripping column shown in Figure 1 comprises a number of trays 24a to 24h positioned at different heights within the column 10, and an air inlet 14 can be positioned adjacent one or more of, or each, tray 24a to 24h. Due to the pressure differential between the interior and exterior of the stripping column 10 (the stripping column 10 interior is at lower than atmospheric pressure) , air is sucked into the stripping column 10 through the air inlet 14 by the partial vacuum within the stripping column 10, causing an airstream to flow within the stripping column 10. As discussed herein, airflow within the stripping column 10 assists with the transport of ammonia gas from the liquid togaseous phase : the partial pressure of ammonia within the air is zero , whereas the partial pressure of ammonia within the carrier liquid is not zero , and hence the ammonia trans fers from the liquid to the air .

[0033] The stripping column 10 thus comprise a plurality of trays 24a to 24h for use in stripping ammonia from the carrier liquid . In use the trays 24a to 24h are heated by the heating means (not shown in Figure 1 ) . The ammonia is released from the liquid under reduced pressure and heating as ammonia-containing gas ( typically a mixture of ammonia and steam) and passes out of the stripping column 10 through the ammonia-containing gas outlet 16 .

[0034] In the embodiment shown in Figure 1 , the waste liquid outlet 18 is at the bottom of the stripping column 10 , through which the waste liquid exits the stripping column 10 . In the embodiment shown in Figure 1 , the waste liquid passes from the waste liquid outlet 18 to a tank 26 and then to discharge .

[0035] The apparatus shown in Figure 1 comprises a pump 20 connected to the stripping column 10 for reducing the pressure within the stripping column 10 . The pump 20 may be any suitable pump as is known in the art , and in the embodiment shown in Figure 1 is connected to the stripping column 10 towards the top of the stripping column 10 . In preferred embodiments the pressure within the stripping column 10 at the top is lower than atmospheric pressure , for example 250-500mbar absolute . The reduced pressure within the stripping column 10 assists with vapourisation of ammonia and carrier liquid . For example , at 500 mbar absolute pressure the boiling point of water is approximately 81 . 5 ° C, and at 250 mbar absolute pressure it is approximately 65 ° C .

[0036] The apparatus of the first aspect of the present invention also comprises heating means (not shown in Figure 1) for internally heating the stripping column 10, which helps to maintain the temperature within the column 10 as the carrier liquid and ammonia are vapourised: as liquid is removed the latent heat of vapourisation causes the liquid temperature to drop, which reduces the effectiveness of the stripping column 10. As noted herein, the ammonia-containing liquid preferably enters the stripping column 10 at a temperature of from for example 70 to 90°C, e.g. 72 to 85°C (preferably 80°C) . The heating means preferably internally heats the stripping column 10 to provide a temperature gradient within the column 10 which increases down the column 10. Thus, in the embodiment shown in Figure 1, in use the temperatures of the trays 24a to 24h may increase down the stripping column 10. For example, the trays 24a to 24h may in use be maintained at temperatures of approximately 80, 81, 82, 83, 84, 85, 86, and 90°C respectively.

[0037] In the embodiment shown in Figure 1, the condenser 22 is positioned downstream of the ammonia-containing gas outlet 16 for removing liquid vapour from the ammonia- containing gas. Thus, in preferred embodiments, the ammonia- containing gas comprises ammonia and steam, and the condenser 22 condenses out the steam from the ammonia-containing gas to leave predominantly ammonia gas. The condenser 22 may comprise, for example, a cold-water loop. In the embodiment shown in Figure 1, the condensed liquid, e.g. water, passes to a liquid discharge outlet 28 via a tank 30, and the remaining ammonia gas passes to an ammonia recovery unit 32. In preferred embodiments, the temperature of the ammonia- containing gas entering the condenser 22 is approximately 80°C, and the temperature of the condensed liquid and ammonia gas leaving the condenser is approximately 70°C.

[0038] In the embodiment shown in Figure 1, the ammonia recovery unit 32 recovers ammonia downstream of the condenser 22. In preferred embodiments, the ammonia is recovered as ammonium hydroxide, by dissolving the ammonia gas from the condenser 22 in water. In the embodiment shown in Figure 1, in use the ammonia gas passes into a water bath 34 downstream from the condenser 22. The water in the water bath 34 is preferably maintained at ambient temperature by cold jacket 36.

[0039] It will be appreciated that the specific embodiments described herein are for illustrative purposes only, and that further modifications and variations of the embodiments are possible without departing from the scope of the present invention as defined by the appended claims.

Claims

CLAIMS1 . Apparatus for removing ammonia from an ammonia-containing liquid, the apparatus comprising : a stripping column comprising an ammonia-containing liquid inlet , one or more air inlets , an ammonia-containing gas outlet and a waste liquid outlet ; a pump connected to the stripping column for reducing the pressure within the stripping column to below atmospheric pressure ; heating means for internally heating the stripping column; and a condenser downstream of the ammonia-containing gas outlet for removing liquid vapour from the ammonia-containing gas .2 . Apparatus according to claim 1 configured to remove ammonia from ammonia-containing water .3 . Apparatus according to claim 1 or 2 wherein the ammonia- containing liquid inlet is positioned at or towards the upper end of the stripping column .4 . Apparatus according to any preceding claim which further comprises means for heating the ammonia-containing liquid upstream of the stripping column .5 . Apparatus according to claim 4 wherein the upstream heating means comprises a heat exchanger .6 . Apparatus according to any preceding claim wherein an air inlet is positioned at or towards the lower end of the stripping column .7 . Apparatus according to any preceding claim which comprises a plurality of air inlets at di f ferent positions within the column .8 . Apparatus according to claim 7 wherein the stripping column comprises a plurality of trays positioned at di f ferent heights within the column and an air inlet positioned adj acent one or more of , or each, tray .9 . Apparatus according to any preceding claim wherein the stripping column comprises a plurality of heated trays which in use are heated by the heating means .10 . Apparatus according to any preceding claim wherein the heating means is configured to internally heat the stripping column to provide a temperature gradient within the column which increases down the column .11 . Apparatus according to any preceding claim wherein the pump is connected to the stripping column towards the top of the column .12 . Apparatus according to any preceding claim wherein the condenser comprises a cold-water loop .13 . Apparatus according to any preceding claim which comprises an ammonia recovery unit .14 . Apparatus according to claim 13 wherein the ammonia recovery unit is configured to recover ammonia as ammonium hydroxide , by dissolving the ammonia gas in water .15 . A method for removing ammonia from an ammonia-containing liquid, the method comprising the steps of :passing an ammonia-containing liquid into a stripping column via an ammonia-containing liquid inlet ; wherein the interior of the stripping column is at a pressure of less than atmospheric pressure and wherein the interior of the stripping column is heated by heating means ; passing air into the stripping column via one or more air inlets ; allowing ammonia-containing gas to pass from the stripping column via an ammonia-containing gas outlet and waste liquid to pass from the stripping column via a waste liquid outlet ; removing liquid vapour from the ammonia-containing gas by passing the ammonia-containing gas through a condenser downstream of the ammonia-containing gas outlet , to provide ammonia gas .16 . A method according to claim 15 which is performed using apparatus according to any one of claims 1 to 14 .17 . A method according to claim 15 or 16 wherein the retention time of ammonia within the liquid phase within the stripping column is less than 15 minutes .18 . A method according to claim 17 wherein the retention time of ammonia within the liquid phase within the stripping column is 5- 10 minutes .19 . A method according to any one of claims 15 to 18 wherein the ammonia-containing liquid enters the stripping column at a temperature of from for example 70 to 90 ° C .20 . A method according to claim 19 wherein the ammonia- containing liquid enters the stripping column at a temperature of approximately 80 ° C .

21. A method according to any one of claims 15 to 20 wherein the pressure within the stripping column adjacent the ammonia-containing liquid inlet is 250-500mbar absolute.

22. A method according to any one of claims 15 to 21 wherein the temperature within the stripping column adjacent the ammonia-containing liquid inlet is approximately 80°C increasing to a temperature of approximately 90°C towards the bottom of the column.

23. A method according to any one of claims 15 to 22 wherein temperature of the ammonia-containing gas entering the condenser is approximately 80°C, and the temperature of the condensed liquid and ammonia gas leaving the condenser is approximately 70°C.

24. A method according to any one of claims 15 to 23 wherein the ammonia gas from the condenser is dissolved in water to form ammonium hydroxide.

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