Ammonium Bicarbonate Production Method Using Anaerobic Digester Outputs

Abstract

A method of recovering ammonia bicarbonate from a biogas and digestate liquor uses an arrangement of equipment in an operational arrangement that requires relatively modest capital investment and that operates with simplicity to provide solid ammonia bicarbonate. In a specific form the method achieves this by upgrading of biogas to produce a rejected stream having a high mole fraction of CO.sub.2 at an elevated pressure that eliminates the need for further compression when using the CO.sub.2 for recovery of ammonia bicarbonate. The method operates can operate at a high pH produced inherently in a digestate stripper by concurrently stripping dissolved CO.sub.2 from solution to improve the simplicity of solid AB recovery.

Claims

1. A method for recovering ammonium bicarbonate from an ammonia rich digestate liquor containing dissolved CO.sub.2 and a biogas comprising methane and carbon dioxide produced by an anaerobic digester, the method comprising: a.) separating at least a portion of the biogas into a methane rich gas and a CO.sub.2 rich gas; b.) stripping at least a portion of the digestate liquor in a digestate stripper with a stripping gas comprising air while maintaining at least a portion of the digestate stripper at a pH of at least 8.5 wherein the pH level is achieved at least in part by the stripping of dissolved CO.sub.2 in the digestate and the stripper produces an ammonia lean solution that passes out of the digestate stripper and produces a stripper gas stream comprising ammonia and CO.sub.2; c.) passing at least a portion of the stripper gas stream to a stripping gas scrubber and contacting the stripping gas stream with a CO.sub.2 rich scrubbing solution comprising dissolved CO.sub.2; d.) recovering a liquid scrubber effluent from the stripping gas scrubber wherein the liquid scrubber effluent comprises ammonium bicarbonate and a dissolved CO.sub.2; e.) cooling at least a portion of the liquid scrubber effluent to at least initiate precipitation of ammonium bicarbonate from the liquid scrubber effluent and produce a cooled scrubber effluent; f.) passing the cooled liquid scrubber effluent to a precipitation reactor that precipitates at least a portion of the ammonium bicarbonate from the liquid scrubber effluent to produce a precipitate containing slurry comprising ammonium bicarbonate and to produce a lean scrubbing solution having a lower concentration of CO.sub.2 than the CO.sub.2 rich scrubbing solution; g.) heating a least a portion of the lean scrubbing solution to produce a heated scrubbing solution and contacting the heated scrubbing solution with at least a portion of the CO.sub.2 rich gas to produce the CO.sub.2 rich scrubbing solution; h.) adding a make-up stream comprising water to at least one of the lean scrubbing solution, the heated scrubbing solution, the CO.sub.2 rich solution, and/or directly into a CO.sub.2 saturation vessel; and, i.) recovering ammonia bicarbonate from the precipitate slurry.

2. The method of claim 1 wherein an ammonia bicarbonate product is produced from the recovered ammonium bicarbonate and a renewable natural gas is produced from the methane rich gas.

3. The method of claim 1 wherein the make-up stream contains volatile organic and/or inorganic acids to promote ammonia capture.

4. The method of claim 3 wherein the make-up stream contains volatile fatty acids derived from organic sources.

5. The method of claim 4 wherein the organic sources comprise animal waste.

6. The method of claim 1 wherein the CO.sub.2 rich scrubbing solution contains at least a portion of the CO.sub.2 rich gas and is derived, at least in part, from the heated scrubbing solution.

7. The method of claim 1 wherein digestate liquid enters the digestate stripper at a temperature no greater than 50° C.

8. The method of claim 1 wherein the precipitate containing slurry additionally comprises ammonium carbonate, ammonium, and ammonium organic acid complexes.

9. The method of claim 1 wherein a precipitation reactor precipitates at least a portion of the ammonium bicarbonate from the liquid scrubber effluent.

10. The method of claim 1 wherein an anaerobic digester converts a biomass material containing ammonia and organic nitrogen containing compounds to produce the digestate liquor and the biogas.

11. The method of claim 10 wherein the waste material comprises agricultural waste.

12. The method of claim 2 wherein the stripping gas stream consists essentially of air and water vapor.

13. The method of claim 1 wherein the liquid scrubber effluent contains acidified ammonium.

14. The method of claim 1 wherein the digestate liquor contains dissolved CO.sub.2 that when stripped from solution raises the pH of the digestate stripper.

15. A method for recovering ammonium bicarbonate from a wastewater stream comprising water and suspended solids, the method comprising: a.) separating at least a portion of the suspend solids from the wastewater to produce a suspended solids stream and an ammonia rich stream; b.) passing at least a portion of the ammonia rich stream to an anaerobic digester to produce a biogas comprising methane and CO.sub.2 and to produce a digestate liquor comprising additional ammonia; c.) separating at least a portion of the biogas into a methane rich stream and a CO.sub.2 rich gas; d.) stripping the digestate liquor in a digestate stripper by contact with a stripping gas comprising air and water vapor wherein at least a portion of the digestate stripper retains liquid therein at a pH of at least 8.5; wherein the digestate stripper produces an ammonia lean solution and a stripper gas stream containing ammonia; e.) passing at least a portion of the stripper gas stream to a stripping gas scrubber and contacting the stripping gas stream with a CO.sub.2 rich scrubbing solution; f.) recovering a liquid scrubber effluent from the stripping gas scrubber, the liquid scrubber effluent comprising ammonium bicarbonate and ammonium carbonate wherein the liquid scrubber effluent has a low concentration of dissolved CO.sub.2 relative to the CO.sub.2 rich solution. g.) cooling at least a portion of the liquid scrubber effluent to produce a cooled scrubber effluent and passing the cooled scrubber effluent to a precipitation reactor that precipitates at least a portion of the ammonium bicarbonate and ammonium carbonate from the liquid scrubber effluent to produce a precipitated slurry containing ammonium bicarbonate and ammonium carbonate and to produce a lean scrubbing solution having a lower concentration of ammonium bicarbonate than the feed to the precipitation reactor. h.) heating at least a portion of the lean scrubbing solution to produce a heated scrubbing solution; i.) passing the heated scrubbing solution and the CO.sub.2 rich gas to a CO.sub.2 saturator to produce the CO.sub.2 rich scrubbing solution and discharging a CO.sub.2 off gas from the CO.sub.2 saturator; j.) adding a make-up stream comprising water to at least one of the heated scrubbing solution or the CO.sub.2 saturator; k.) recovering ammonium bicarbonate and ammonium carbonate from the precipitated slurry and, recovering the methane rich stream for production of a refined natural gas

16. The method of claim 15 wherein at least a portion of the suspended solid stream is passed to an acidification reactor that hydrolyzes the suspended solids to produce volatile fatty acids and at least a portion of the volatile fatty acids are introduced into the stripping gas scrubber;

17. The method of claim 16 wherein the volatile fatty acids are introduced into the stripping gas scrubber by mixing at least a portion of the volatile fatty acids with the CO.sub.2 rich scrubbing solution.

18. The method of claim 15 wherein the wastewater comprises waste effluent from farming operations.

19. The method of claim 16 wherein the biogas contains H2S; wherein the biogas is treated to oxidize the H2S and to produce a liquid oxidation stream rich in sulfuric acid and/or dissolved sulfates; and wherein the make-up stream comprises at least a portion of the liquid oxidation stream.

20. The method of claim 15 wherein at least a portion of the CO.sub.2 off gas is passed to the stripping gas scrubber.

21. A method for recovering ammonium bicarbonate from a wastewater stream comprising water and suspended solids, the method comprising; a.) separating at least a portion of the suspend solids from the wastewater to produce a suspended solids stream and an anaerobic feed stream; b.) passing the anaerobic feed stream to an anaerobic digester to produce biogas comprising methane and CO.sub.2 and to produce a digestate liquor comprising ammonia; c.) separating at least a portion of the biogas into a methane rich stream and a CO.sub.2 rich gas; d.) stripping at least a portion of the digestate liquor in a digestate stripper by contact with a stripping gas comprising air and water vapor in the digestate stripper at a temperature of from 35 to 50° C. wherein the digestate stripper retains liquids therein at a pH of at least 8.5; wherein stripping the dissolved CO.sub.2 from the digestate liquor raises the pH of the digestate stripper; and wherein the digestate stripper produces an ammonia lean solution and a stripper gas stream containing ammonia; e.) passing at least a portion of the stripper gas stream to a stripping gas scrubber and contacting the stripping gas stream with a CO.sub.2 rich scrubbing solution; f.) passing at least a portion of the suspended solids stream to an acidification reactor that hydrolyzes the suspended solids to produce volatile fatty acids and a recovered water stream and passing at least a portion of the volatile fatty acids into the stripping gas scrubber; g.) recovering a liquid scrubber effluent from the stripping gas scrubber, the liquid scrubber effluent comprising ammonium bicarbonate and ammonium carbonate; h.) cooling at least a portion of the liquid scrubber effluent to produce a cooled scrubber effluent; i.) passing the cooled scrubber effluent to a precipitation reactor that precipitates at least a portion of the ammonium bicarbonate. ammonium carbonate and ammonium organic acid compounds to produce a lean scrubbing solution having a lower concentration of CO.sub.2 than the CO.sub.2 rich scrubbing solution; j.) heating at least a portion of the lean scrubbing solution to produce a heated scrubbing solution and passing the heated scrubbing solution and the CO.sub.2 rich gas to a CO.sub.2 saturator to produce the CO.sub.2 rich scrubbing solution and discharging a CO.sub.2 off gas from the CO.sub.2 saturator; k.) adding a make-up stream comprising water to at least one of the heated scrubbing solution or the CO.sub.2 saturator wherein the make-up stream comprises at least a portion of recovered water stream; l.) recovering ammonium bicarbonate and ammonium carbonate from the precipitated slurry; recovering the methane rich stream for production of a refined natural gas; and recovering volatile fatty acids from the acidification reactor.

22. The method of claim 21 wherein the acidification reactor provides a solids and water to the anaerobic digester.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1 is a flow diagram for the practice of the invention showing the addition of a precipitation reactor and a CO.sub.2 separator.

[0030] FIG. 2 is a flow diagram for the practice of this invention showing a full flow arrangement for starting with a water waste stream and producing an AB product and a methane rich gas.

DETAILED DESCRIPTION OF THE INVENTION

[0031] This invention can recover AB from any aqueous stream that contains large amounts of ammonia or ammonium. Most streams that are used in the invention will have recoverable ammonia or ammonium of less than 1.0 wt. %, less than about 0.5 wt. % and more typically in a range of 0.1 to 0.2 wt. %.

[0032] A more complete description of the invention is given in conjunction with the following explanation of the Figures. For clarity the figures omit additional process equipment that is well known to those skilled in the art and readily incorporated without explanation by those skilled in the art. Such equipment includes pumps, compressors, temperature sensors, pressure sensors, control loops and other such items.

[0033] FIG. 1 shows a block diagram of the basic embodiment of the invention. The primary vessels of the method include a digestate stripper 20, a stripping gas scrubber 30, a precipitation vessel 40 and a CO.sub.2 saturator 50.

[0034] Stripper 20 receives a digestate stream 12 comprising a digestate liquor from an anaerobic digester (as shown in FIG. 2) that primarily contains dissolved ammonia and CO.sub.2. The digestate stream 12 may also contain lesser amounts of residual solids, dissolved salts and organics. Depending on the remaining particulate material in digestate stream 20, trace suspended solid treatment (not shown) may provide a benefit to the operation of the scrubber The digest stream may optionally receive heat from an optional heating zone 14 that heats the digestate stream to an input temperature suitable for the desired operation of the digestate stripper 20. Any conventional heater or heat exchanger can provide the desired heating in zone 14. The arrangement of FIG. 1 shows an optionally heated digestate stream 12′ entering digestate stripper 20.

[0035] Digestate stripper 20 strips ammonia from the digestate by contact therein with a stripping gas 16. The stripping gas will preferably consist essentially of air but also may include minor amounts of H2S. The pressure in digestate stripper 20 is normally at or near atmospheric pressure but in certain cases can be operated under a vacuum that will usually range from 0.5 to 0.9 atmospheres.

[0036] Contact between the digestate and the stripping gas 16 in the digestate stripper will cause the pH in the digestate stripper to attain a value of at least 8.5 and preferably more than 9. The stripper needs no chemical addition to control the pH in the digestate stripper. Preferably the digestate stripper operates without any chemical addition.

[0037] Stripping of dissolved CO.sub.2 in the digestate stripper along with the alkalinity associated with a high amount of bicarbonate and carbonate in the digestate was found to naturally raise the pH in digestate stripper 20 to a pH of 8.5 to 9 or about so that the digestate stripper requires no pH adjustment and can operate without the use of any additives. Temperature in the stripping vessel can range from room temperature to 50° C. or thereabout. Even at about room temperature the natural rise in the pH can enable a recover on the order of 50 to 70% of the ammonia in the digestate liquor.

[0038] An ammonia lean solution 18 exits a lower portion of digestate stripper 20 and a stripper gas 22 passes out of an upper portion of digestate stripper 20. The ammonia lean solution typically comprises a waste stream since it usually contains some solid material as well as other chemicals in trace amounts. Thus, the ammonia lean solution 18 may undergo waste treatment before disposal.

[0039] Stripper gas stream 22 will principally comprise gaseous ammonia along with water vapor, CO.sub.2 and other constituents typically present in trace amounts. Such trace constituents typically include sulfur compounds. Optionally (and not shown), a dilute aqueous ammonia solution could be generated by separating the CO.sub.2 from the ammonia in stripper gas 22 and scrubbing/condensing the ammonia from that CO.sub.2 depleted gas stream. An optional pressure differential control between the digestate stripper 20 and scrubber 30 can include placement of a pressure control valve (not shown) in the conduit for transferring stripper gas 22.

[0040] Stripping gas scrubber 30 receives stripper gas 22 and contacts the stripping gas stream with a source of CO.sub.2. The stripper gas contact with CO.sub.2 converts the gaseous ammonia to AB and as well as some AC. Absorption of the gaseous ammonia into the scrubbing solution converts it to predominantly a dissolved ammonium form. The presence of the CO.sub.2 produces a high concentration of AB in scrubber 30. In most cases the scrubber 30 as well as the CO.sub.2 rich scrubbing gas has a neutral or near neutral pH. The scrubbing vessel temperature ranges on the low side from 5° C. to 10° C. higher than the stripping vessel can go up to a temperature of 50° C. or thereabout.

[0041] A CO.sub.2 rich scrubbing solution 32 (as hereinafter described) enters scrubber 30 (in most cases an upper portion of scrubber 30) and supplies all of the necessary CO.sub.2 to substantially convert the gaseous ammonia to the AB and at times some AC. Alternatively or in addition to the CO.sub.2 scrubbing solution 32, CO.sub.2 may optionally enter scrubber 30 directly via an optional CO.sub.2 input stream 31. The CO.sub.2 input stream 31 can feed CO.sub.2 from any available CO.sub.2 source. In the case of manure management, the CO.sub.2 sources include, without limitation, CO.sub.2 from the biogas upgrade system and CO.sub.2 from an acid phase digestion system as long as it is generated within a manure management process. With regard to CO.sub.2 entering scrubber 50, unless an external source provides CO.sub.2 via optional input stream 31, all CO.sub.2 that enters saturator 30 is derived from the digester that produces the digestate. An ammonia depleted stripper gas stream 33 exits scrubber 30 and if necessary subject to further treatment before release.

[0042] Scrubber 30 produces a liquid scrubber effluent 34 that comprises AB and water and may include other ammonia derived compounds such as AC as well as trace chemicals such as the previously mentioned sulfur compounds. The AB concentration in liquid scrubber effluent 34 may range from 0.8% to 14% wt. % depending on the CO.sub.2 concentration in scrubber 30 as well as the solubility resulting from the temperature liquid in the scrubber.

[0043] To obtain the desired AB product, a relatively dilute solution and/or a precipitate can be generated by manipulating the temperate and pH of the scrubbing solution. Liquid scrubber solution effluent 34 typically passes to a cooler often in the form of a heat exchanger 36. Chilling of effluent 34 reduces the solubility of the AB and increases the mass of AB particles for removal from solution 34. Preferably the liquid scrubber effluent is cooled to a temperature of 20° C. to 25° C. Chilling may take place in any number of ways. For example, by a separate chilling step in a stand-alone chiller/heat exchanger. This invention particularly benefits from passing the liquid scrubber effluent 34 to chiller 36 and then passing a cooled scrubber effluent to a precipitation reactor 40 via a line 38.

[0044] Precipitation reactor 40 generates a precipitate containing slurry 42 and a lean scrubbing solution 44. Precipitate containing slurry 42 will comprise primarily AB. Precipitation reactor 40 operates at conditions to increase the amount precipitate in slurry 42.

[0045] Slurry 42 typically can be recovered as a final product or can be sent to further processing/concentrating as desired. Such processing typically can include physical separation via membrane separation, centrifugation, and vortex separators to mention a few that anyone trained in the art can envision.

[0046] Lean scrubbing solution 44 passes to a heating zone 46 to heat the lean scrubbing solution and provide a heated scrubbing solution 48 that enters a CO.sub.2 saturator 50. Heating zone 46 raises the temperature of lean scrubbing solution 44 to a range of from 35° C. to 50° C. and preferably from 40° C. to 50° C. Any conventional heater or heat exchanger can provide the desired heating in zone 46. A pump or pumps (not shown) can transport and adjust the pressure of lean scrubbing solution 44 and heated scrubbing solution 46 in its delivery to CO.sub.2 scrubber 50 via line 48.

[0047] Water, preferably fresh water, is added back to the scrubbing solution to make up for the water lost to precipitate containing slurry 42. Water is also lost by the presence of water in the CO.sub.2 off gas, in the scrubber off gas and/or in the liquid scrubber effluent> The water may be added at any location that such that it eventually reaches CO.sub.2 saturator 50. For example, the CO.sub.2 saturator may receive water by passing it into lean scrubbing solution 44, heated scrubbing solution 48 and directly into CO.sub.2 saturator 50, or any combination thereof. FIG. 1 shows these possible water entry locations as steams 43, 45, and 47, respectively.

[0048] Saturator 50 saturates the heated scrubbing solution with CO.sub.2 recovered from biogas produced by the anaerobic digestor that provides digestate stream 12 (shown in FIG. 2). This recovered CO.sub.2 passes to saturator 50 as a CO.sub.2 rich gas 52. Saturator 50 usually operates at or about ambient pressure. Saturator 50 produces the CO.sub.2 rich solution 32 and a CO.sub.2 off gas stream 54. The CO.sub.2 rich gas 52 typically has a CO.sub.2 concentration ranging from 90 to a 99% mole fraction. Off gas stream 54 can optionally provide an additional source of CO.sub.2 for scrubber 30. Thus, a CO.sub.2 stream 56 containing CO.sub.2 recovered from off gas stream 54 may enter scrubber 30 either with a stripper gas (not shown) or pass directly into scrubber 30.

[0049] FIG. 2 shows an embodiment of the invention that provides additional advantages, benefits, and products. In this embodiment the method starts with a raw waste stream as the feed. In FIG. 2 the reference numbers are the same for like elements as those shown in FIG. 1. The following description only describes the added method steps that don't appear in FIG. 1; that were not described in conjunction with the description of FIG. 1; and any method operations that vary between the method as described in FIG. 1 versus that described in FIG. 2.

[0050] The initial feed comprises a raw waste stream 62 that contains a high amount of solid material. In farm operations that rely on animals, raw waste stream 62 can comprise manure and other semi-solid waste and/or waste milk. Such streams are typically sourced from holding tanks or settling ponds. Accordingly, a raw waste stream 62 passes into a solids removal unit 60 that concentrates the removed solids into a suspended solids stream 64 and a wastewater steam 66. Wastewater stream 66 has a low enough concentration of total suspended solids to allow one or more moderate to high rate anaerobic treatment systems to be used including but not limited to anaerobic filters, anaerobic contact processes and high rate granular systems such as an upflow anaerobic sludge blanket, internal circulation reactor, expanded granular sludge blanket, external circulation sludge blanket, and similar processes. Any type of solids removal unit may be used that efficiently provide a high degree of suspended solids removal from raw waste stream 62. Common equipment for the separation of such solids include centrifugation, membrane filtration and/or simple screening or filtration.

[0051] A suspended solids stream 64 passes suspended solids from solids removal unit 60 to an acidification reactor 70. Before entering or as part of an acidification reactor pretreatment, the solids in stream 64 may undergo treatment to enhance degradability via a variety of different mechanical or organically acceptable methods (not shown). These methods include grinding to reduce particle size, steam explosion, enzymatic hydrolysis and high temperature treatment.

[0052] Acidification reactor 70 will preferably operate with a short hydraulic retention time on the order of 0.5 to 2 days. Typically, acidification reactor 70 operates at ambient pressure in a temperature range of from ambient to 35° C. Acidification reactor 70 hydrolyzes the solids and ultimately converts them to soluble products, primarily volatile fatty acids (acetic, propionic, butyric and higher order acids) recovered via line 74. Gases produced in acidification reactor 70 exit via line 72 to be processed as required.

[0053] Acidification reactor 70 provides a recovered water stream 74 that supplies at least a portion of the make-up water to replace water lost with removal of precipitate containing slurry 42 as previously described. Make-up water from the acidification reactor can contain volatile organic and/or inorganic acids that will promote ammonia capture. Preferably, the volatile fatty acids are derived from organic sources. Such organic sources may include animal waste.

[0054] The recovered water 74 may enter CO.sub.2 saturator 50 via addition to heated scrubbing solution 48 as depicted in FIG. 2; by direct introduction into saturator 50; or by addition other streams entering saturator 50. Stream 74 may pass through a filter 76 to remove any residual solids and a stream 74′ with reduced residual solids exits filet 76. Recovered water may as an alternative or in addition thereto, pass ultimately to the CO.sub.2 rich solution at the location of water addition streams 43, 45 and/or 47.

[0055] The recovered water stream 74 provides water that ultimately enters the CO.sub.2 rich scrubber solution 32. The addition of this water improves ammonia capture in stripping gas scrubber 30 by capturing ammonia as organic acid-ammonia complexes such as ammonium acetate, and ammonium propionate for example. The improved ammonia capture results in higher concentrations of ammonium in liquid scrubber effluent 34 so that the resultant precipitate containing slurry 42 contains increased ammonia concentrations.

[0056] The solids processed in acidification reactor 70 can optionally receive high energy shearing via pumps or other means well known and understood, to significantly reduce the size of the solids to maximize the amount of hydrolysis that can occur. In this case, optionally, an acidification effluent 78 comprising mostly solids and water can pass to an anaerobic digester 80. Alternately, the effluent stream from acidification reactor 70 can be wasted via line 79. As its primary feed, anaerobic digester 80 receives wastewater stream 66 from suspended solids removal unit 60. Digester 80 produces two primary outputs; one is a biogas stream 84. The other output is digestate liquor stream 82 that that provides the essential input of digestate liquor stream 12 as described in conjunction with FIG. 1 Digestate liquor stream 82 may undergo optional filtration of fine particles in a filter 86 before it passes as digestate liquor stream 12 for processing as described in conjunction with FIG. 1.

[0057] Biogas stream 84 passes to a biogas purification section 90 that separates the biogas into a CO.sub.2 rich gas 52′ and a methane steam 92. CO.sub.2 rich gas 52′usually provides CO.sub.2 for saturator 50. Line 94 may also carry at least a portion of CO.sub.2 rich gas 52 to scrubber 30 via CO.sub.2 input stream 31. Line 94 may supply all the CO.sub.2 needed in scrubber 30 to produce the AB in liquid scrubber effluent 34.

[0058] Purification section 90 typically provides a high purity methane in methane stream 92. Thus, stream 92 can provide, with any additionally needed purification, an NRG product stream.

[0059] The pressure of purification section 92 is preferably sufficient for the operation of saturator 50. Because a similar pressure can be used in both purification section 92 and saturator 50, the need for any significant pressure adjustment between purification section 92 and saturator 50 is eliminated.

[0060] As described, the present invention provides numerous advantages, some of which have been described above and others which are inherent in the invention. Also, modifications may be proposed without departing from the teachings herein. Accordingly, the scope of the invention is only to be limited as necessitated by the accompanying claims.