Process and plant for separating off and/or recovering nitrogen compounds from a liquid or sludge substrate

Abstract

The invention relates to a process for separating off and/or recovering nitrogen compounds, in particular for separating off ammonia and/or recovering or producing nitrogen fertilizer, from a liquid or sludge substrate, in which a liquid or sludge substrate is introduced into a degassing vessel to which subatmospheric pressure is applied and ammonia gas formed is introduced by means of a vacuum pump into at least one scrubber which is located downstream of the degassing vessel and to which subatmospheric pressure is applied and into which acid is introduced, wherein the acid or an ammonium salt-containing liquid obtained in the at least one scrubber is taken off from the at least one scrubber, cooled and subsequently sprayed back into the at least one scrubber. The invention further relates to a plant for carrying out such a process.

Claims

1. A process for separating off and/or recovering nitrogen compounds, in particular for separating off ammonia and/or recovering or producing nitrogen fertilizer, from a liquid or sludge substrate, in which a liquid or sludge substrate is introduced into a degassing vessel to which subatmospheric pressure is applied and ammonia gas formed is introduced by means of a vacuum pump into at least one scrubber which is located downstream of the degassing vessel and to which subatmospheric pressure is applied and into which acid is introduced, wherein the acid or an ammonium salt-containing liquid obtained in the at least one scrubber is taken off from the at least one scrubber, cooled and subsequently sprayed back into the at least one scrubber.

2. The process according to claim 1, wherein the ammonium gas is produced without use of a degassing fluid, for example air or steam.

3. The process according to claim 1, wherein the subatmospheric pressure is continuously applied to the degassing vessel and the at least one scrubber.

4. The process according to claim 1, wherein the subatmospheric pressure in the degassing vessel and the subatmospheric pressure in the at least one scrubber are produced by the vacuum pump, with the vacuum pump being located downstream of the at least one scrubber and residual gas obtained in the at least one scrubber being discharged by the vacuum pump.

5. The process according to claim 1, wherein the liquid or sludge substrate is heated, in particular by means of a heat exchanger or by means of steam injection, before entry into the degassing vessel.

6. The process according to claim 1, wherein a chemical for increasing the pH, in particular a caustic alkali, preferably sodium hydroxide, is added to the liquid or sludge substrate before entry into the degassing vessel, in particular before or after the step of heating.

7. The process according to claim 1, wherein liquid or sludge substrate which has been freed of ammonia is continuously discharged from the degassing vessel.

8. The process according to claim 1, wherein the ammonia gas is cooled after exit from the degassing vessel and water present in the ammonia gas is recirculated in the condensed state to the degassing vessel.

9. The process according to claim 1, wherein the ammonia gas is freed of solid particles and/or liquid droplets before entry into the at least one scrubber.

10. The process according to claim 1, wherein the acid or the ammonium salt-containing liquid is cooled by means of a cooling circuit before being sprayed back in.

11. The process according to claim 10, wherein the acid or ammonia salt-containing liquid is conveyed by means of a circulation pump through a liquid cooling apparatus.

12. The process according to claim 1, wherein the pH of the acid or ammonium salt-containing liquid is monitored.

13. The process according to claim 1, wherein the pH is monitored until the acid has been completely or essentially completely neutralized.

14. The process according to claim 1, wherein an ammonium salt-containing liquid present after neutralization of the acid is discharged from the at least one scrubber and is, in particular, passed to further processing, preferably the production of nitrogen fertilizer.

15. The process according to claim 1, wherein the ammonia gas is introduced by means of a vacuum pump into a first scrubber to which subatmospheric pressure is applied and into which acid is introduced and a residual gas obtained in the first scrubber is introduced by means of the vacuum pump into a second scrubber to which subatmospheric pressure is applied and into which acid is introduced and which is located downstream of the first scrubber.

16. The process according to claim 15, wherein a residual gas obtained in the second scrubber is introduced by means of the vacuum pump into a third scrubber to which subatmospheric pressure is applied and into which acid is introduced and which is located downstream of the second scrubber.

17. The process according to claim 15, wherein the first scrubber is exchanged with the second scrubber and optionally the second scrubber is replaced by the third scrubber when a residual gas obtained in the first scrubber has an ammonia content above a defined threshold value, preferably above 10 ppm.

18. The process according to claim 17, wherein the first scrubber after venting, emptying and renewed introduction of acid is connected downstream of the last scrubber connected in series, in particular the second or third scrubber.

19. The process according to claim 1, wherein an inorganic acid, in particular a mineral acid, preferably hydrochloric acid, sulphuric acid, nitric acid or a mixture of at least two of the mineral acids mentioned, is used as acid.

20. A plant for separating off and/or recovering nitrogen compounds from a liquid or sludge substrate, in particular for separating off ammonia and/or recovering or producing nitrogen fertilizer, preferably for carrying out a process according to any of the preceding claims, comprising a degassing vessel for liberating ammonia gas from a liquid or sludge substrate, at least one scrubber for separating off or scrubbing out ammonia from a gas phase located downstream of the degassing vessel, where the at least one scrubber has a spray device for spraying acid and/or an ammonium salt-containing liquid, and a vacuum pump for applying subatmospheric pressure to the degassing vessel and to the at least one scrubber.

21. The plant according to claim 20, wherein the plant further comprises a heating device, preferably a heat exchanger or steam injection device, located upstream of the degassing vessel.

22. The plant according to claim 20, wherein the plant further comprises a gas cooling apparatus, in particular a heat exchanger, located downstream of the degassing vessel.

23. The plant according to claim 20, wherein the at least one scrubber has a cooling circuit, in particular with a liquid cooling apparatus and a pump located upstream of the liquid cooling apparatus.

24. The plant according to claim 20, in particular according to claim 23, wherein the plant further comprises a pH measuring apparatus, in particular a pH measuring apparatus located upstream of the liquid cooling apparatus.

25. The plant according to claim 20, wherein the plant has three scrubbers, in particular a first scrubber, a second scrubber located downstream of the first scrubber and a third scrubber located downstream of the second scrubber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0127] The drawing shows:

[0128] FIG. 1: a block diagram of a plant for separating off and/or recovering nitrogen compounds from a liquid or sludge substrate with the process sequence for separating off and/or recovering nitrogen contents from a liquid or sludge substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0129] While the invention will be described in connection with one or more preferred embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.

[0130] The plant 1 shown schematically in FIG. 1 has a degassing vessel 7, three scrubbing stages 16a, 16b, 16c located downstream of the degassing vessel 7 and also a vacuum pump 8.

[0131] Each scrubbing stage 16a, 16b, 16c has a scrubber 17a, 17b, 17c. The scrubbers 17a, 17b, 17c are connected in series, i.e. after one another.

[0132] The scrubbers 17a, 17b, 17c are preferably exchangeable with one another, in particular in respect of their function and/or position.

[0133] Each of the scrubbers 17a, 17b, 17c has a spraying device 18a, 18b, 18c, in particular in the form of a nozzle.

[0134] In the plant 1 depicted, the second scrubber 17b is located downstream of the first scrubber 17a and the third scrubber 17c is located downstream of the second scrubber 17b.

[0135] The vacuum pump 8 is located essentially downstream of the last scrubber connected in series.

[0136] In the plant 1 depicted, the vacuum pump 8 is located downstream of the scrubber 17c.

[0137] The plant 1 preferably also has a heating device 3, in particular in the form of a heat exchanger, located upstream of the degassing vessel 7.

[0138] Furthermore, the plant 1 preferably has a gas cooling device 13 located preferably directly downstream of the degassing vessel 7 and, in particular, a filter 15 located between the gas cooling apparatus 13 and the first scrubber 17a.

[0139] Each scrubber 17a, 17b, 17c is preferably connected via a cooling circuit to a liquid cooling apparatus 23a, 23b, 23c, with a circulation pump 22a, 22b, 22c being located upstream of each liquid cooling apparatus 23a, 23b, 23c.

[0140] To measure the pH, each scrubbing stage 16a, 16b, 16c has a pH measuring apparatus 26a, 26b, 26c. The pH measuring apparatus 26a, 26b, 26c can be located essentially upstream of the circulation pump 22a, 22b, 22c or downstream of the liquid cooling apparatus 23a, 23b, 23c. As an alternative, each scrubber 17a, 17b, 17c can have a pH measuring apparatus 26a, 26b, 26c. However, the pH measuring apparatus 26a, 26b, 26c is preferably, as shown, located upstream of the liquid cooling apparatus 23a, 23b, 23c since the pressure conditions for a pH measurement are most favourable upstream of the liquid cooling apparatus 23a, 23b, 23c.

[0141] A process according to the invention during start-up of the plant 1 depicted schematically in FIG. 1 will be described below.

[0142] A subatmospheric pressure is applied preferably continuously to, in particular, the degassing vessel 7 and also the three scrubbers 17a, 17b, 17c by means of the vacuum pump 8.

[0143] Liquid or sludge substrate is introduced via a substrate feed conduit 2 into the degassing vessel 7. Owing to the subatmospheric pressure prevailing there, increased formation of ammonia gas 12 occurs in the degassing vessel 7. The ammonia gas 12 is preferably formed predominantly or exclusively in the degassing vessel 7 because of the subatmospheric pressure prevailing therein.

[0144] To increase exchange areas between the liquid or sludge substrate and the gas phase, the degassing vessel 7 preferably has appropriate internals 9. The transfer of gaseous ammonia from the liquid or sludge substrate into the gas phase can be promoted thereby.

[0145] The liquid or sludge substrate is preferably conveyed through the heating apparatus 3 before entry into the degassing vessel 7. The liquid or sludge substrate is heated thereby, so that the ratio of ammonium ions to ammonia is shifted in favour of ammonia even before entry into the degassing vessel 7.

[0146] Furthermore, preference is given to adding a caustic alkali 6 to the liquid or sludge substrate before entry into the degassing vessel 7. The associated pH increase likewise makes a shift in the ratio of ammonium ions to ammonia in favour of ammonia possible even before entry of the liquid or sludge substrate into the degassing vessel 7. Here, the caustic alkali 6 can be added to the liquid or sludge substrate via an alkali feed conduit 4 and/or via an alkali feed conduit 5. Depending on the introduction of caustic alkali, a pH increase in the liquid or sludge substrate before or after passage through the heating apparatus 3 can thus also be achieved.

[0147] Liquid or sludge substrate which is obtained in the degassing vessel 7 and has been freed of ammonia is preferably discharged continuously from the degassing vessel 7 via a discharge conduit 10 by means of a vacuum pump 11.

[0148] To effect further enrichment of ammonia in the gas phase, the ammonia gas 12 is passed through the gas cooling apparatus 13. Here, the ammonia gas 12 is cooled and water present in the ammonia gas 12 is recirculated as condensate 14 to the degassing vessel 7. After leaving the gas cooling apparatus 13, the ammonia gas 12 is preferably passed through the filter 15. Here, the ammonia gas 12 is purified of any solid particles and/or liquid droplets.

[0149] The purified ammonia gas 12 is then introduced by means of the vacuum pump 8 into the first scrubber 17a. An acid, preferably phosphoric acid, sulphuric acid or nitric acid, is introduced into this scrubber. The introduction of acid into the scrubber 17a can be effected by means of an acid feed conduit 20a.

[0150] In the scrubber 17a, ammonia is converted into a preferably water-soluble ammonium salt, for example ammonium phosphate, ammonium sulphate or ammonium nitrate, by reaction with the acid and thus separated off or scrubbed out from the gas phase.

[0151] To cool the heat of reaction or neutralization arising here, acid 21a or an ammonium salt-containing liquid 21a formed by reaction of acid with ammonia which is taken off from the scrubber 17a is conveyed by means of the circulation pump 22a through the liquid cooling apparatus 23a and sprayed as cooled acid or liquid 21a back into the scrubber 17a via the spraying device 18a.

[0152] Spraying of the acid or liquid 21a enables the reaction with ammonia in the scrubber 17a to be accelerated and in particular carried out more quantitatively.

[0153] The pH of the acid or liquid 21a in the scrubbing stage 16a is monitored by means of the pH measuring apparatus 26a.

[0154] When the acid which has originally been introduced into the scrubber 17a has been largely or completely neutralized, the scrubber 17a can no longer take up any further ammonia and an ammonium salt-containing liquid present then is preferably discharged from the scrubber 17a via an outlet line 27a and passed to a further use, in particular for producing nitrogen fertilizers.

[0155] According to the invention, it can in principle be provided for an optionally ammonia-containing residual gas 28a obtained by neutralization of the acid in the scrubber 17a to be introduced by means of the vacuum pump 8 into the downstream second scrubber 17b and an optionally ammonia-containing residual gas 28b obtained there can optionally be introduced into the third scrubber 17c, with an optionally ammonia-containing residual gas 28c obtained in the third scrubber 17c being discharged, preferably drawn off, by the vacuum pump 8.

[0156] However, preference is given to exchanging the first scrubber 17a with the second scrubber 17b and exchanging the second scrubber 17b with the third scrubber 17c when a residual gas 28a obtained in the first scrubber 17a has an ammonia content or an ammonia concentration above a defined threshold value, preferably above 10 ppm. The exchange of the scrubbers is preferably carried out with maintenance of the subatmospheric pressure prevailing in each case in the second scrubber 17b and in the third scrubber 17c.

[0157] The first scrubber 17a is preferably installed, after venting, emptying and renewed introduction of acid, downstream of the third scrubber 17c.

[0158] As a result, the second scrubber 17b now assumes the function and in particular position of the first scrubber 17a, the third scrubber 17c assumes the function and in particular position of the second scrubber 17b and the scrubber 17a into which acid has again been introduced assumes, after having a subatmospheric pressure applied to it by the vacuum pump 8, the function and in particular position of the third scrubber 17c.

[0159] As a result, an acid contained in the scrubber 17b is now converted into a preferably water-soluble ammonium salt, for example ammonium phosphate, ammonium sulphate or ammonium nitrate, by reaction with ammonia and thus separated off or scrubbed out from the gas phase.

[0160] To cool the heat of reaction or neutralization arising here, acid 21b or an ammonium salt-containing liquid 21b formed by reaction of acid with ammonia which has been taken off from the scrubber 17b is conveyed by the circulation pump 22b through the liquid cooling apparatus 23b and sprayed as cooled acid or liquid 21b back into the scrubber 17b via the spraying device 18b.

[0161] Spraying of the acid or liquid 21b enables the reaction with ammonia in the scrubber 17b to be accelerated and in particular carried out more quantitatively.

[0162] The pH of the acid or liquid 21b in the scrubbing stage 16b is monitored by means of the pH measuring apparatus 26b.

[0163] When the acid which was originally introduced into the scrubber 17b has been largely or completely neutralized, the scrubber 17b can no longer take up any further ammonia and an ammonium salt-containing liquid then present is preferably discharged from the scrubber 17b via an outlet conduit 27b and passed to a further use, in particular for producing nitrogen fertilizers.

[0164] Preference is given to exchanging the second scrubber 17b with the third scrubber 17c and the third scrubber 17c with the first scrubber 17a when a residual gas 28b obtained in the second scrubber 17b has an ammonia content or an ammonia concentration above a defined threshold value, preferably above 10 ppm. The exchange of the scrubbers is preferably carried out with maintenance of the subatmospheric pressure prevailing in each case in the third scrubber 17c and in the first scrubber 17a.

[0165] The second scrubber 17b is preferably installed, after venting, emptying and renewed introduction of acid, downstream of the first scrubber 17a.

[0166] As a result, the third scrubber 17c now assumes the function and in particular position of the second scrubber 17b, the first scrubber 17a assumes the function and in particular position of the third scrubber 17c and the second scrubber 17b assumes, after a subatmospheric pressure has been applied to it by the vacuum pump 8, the function and in particular position of the first scrubber 17a.

[0167] As a result, acid now present in the scrubber 17c is converted into a preferably water-soluble ammonium salt, for example ammonium phosphate, ammonium sulphate or ammonium nitrate, by reaction with ammonia and thus separated off or scrubbed out from the gas phase.

[0168] To cool the heat of reaction or neutralization arising here, acid 21c or an ammonium salt-containing liquid 21c formed by reaction of acid with ammonia which has been taken off from the scrubber 17c is conveyed by means of the circulation pump 22c through the liquid cooling apparatus 23c and sprayed as cooled acid or liquid 21c back into the scrubber 17c via the spraying device 18c.

[0169] Spraying of the acid or liquid 21c enables the reaction with ammonia in the scrubber 17c to be accelerated and in particular more quantitatively.

[0170] The pH of the acid or liquid 21c in the scrubbing stage 16c is monitored by the pH measuring apparatus 26c.

[0171] When the acid which was originally introduced into the scrubber 17c has been largely or completely neutralized, the scrubber 17c can no longer take up any further ammonia and an ammonium salt-containing liquid which is then present is preferably discharged from the scrubber 17c via an outlet line 27c and passed to a further use, in particular the production of nitrogen fertilizers.

[0172] The above-described exchange of the scrubbers 17a, 17b, 17c can in principle be repeated or continued as often as desired.

[0173] A residual gas 29 obtained in the last scrubber is preferably discharged, preferably drawn off, by the vacuum pump 8.

[0174] Particularly advantageously, continuous exchange of the scrubbers 17a, 17b and 17c makes it possible for ammonia to be removed continuously from the liquid or sludge substrate and for ammonia to be continuously converted into preferably water-soluble ammonium salts. As a result, a particularly high degree of outgassing of ammonia and a particularly high degree of precipitation of ammonia in the form of water-soluble ammonium salts can be achieved.

[0175] Thus it is apparent that there has been provided, in accordance with the invention, an invention that fully satisfies the objects, aims and advantages as set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims.