WO2010063584A1

WO2010063584A1 - Method for generating a coarse-grained ammonium sulfate product

Info

Publication number: WO2010063584A1
Application number: PCT/EP2009/065454
Authority: WO
Inventors: Reinhard Scholz; Günter Hofmann; Johannes Widua
Original assignee: Gea Messo Gmbh
Priority date: 2008-12-01
Filing date: 2009-11-19
Publication date: 2010-06-10
Also published as: DE102008059754A1

Abstract

The invention relates to the continuous production of a coarse-grained ammonium sulfate product (22) by crystallizing an ammonium sulfate solution in a DTB crystallization stage (1) in which a suspension of mother liquor and ammonium sulfate crystals is continuously circulated in an internal circuit (flow conduit 6, circulation pump 5) during evaporation of water and from the top region (3) of which a clarified substream of solution is continuously taken off in an external circuit (7), heated (9) for dissolution of the solids contained therein and, as a clear solution, recirculated back to the bottom region (2) of the crystallization stage (1), wherein vapors (11) are regularly taken off from the top of the crystallization stage, fresh solution (10) is introduced from outside into the crystallization stage and a substream of the suspension together with the product crystals contained therein is taken off (18) from the bottom region (2) of the crystallization stage. For avoidance of cyclic variations of the grain size spectrum of the crystals contained in the suspension taken off it is proposed to operate the crystallization stage at a suspension density in the bottom region of greater than 40 M-%, in particular greater than 42 M-%.

Description

Process for producing a coarse ammonium sulphate product

description

The invention relates to a method for the continuous production of a coarse-grained crystalline ammonium sulfate product according to the preamble of patent claim 1.

Ammonium sulphate ((NhU) ₂ SO ₄ ) is a large scale product used primarily as a fertilizer in agriculture for the supply of nitrogen and sulfur. In industrial terms, ammonium sulfate is by-produced in some chemical processes, particularly in the production of caprolactam. To meet the fertilizer requirements, the ammonium sulphate must be available as a coarse-grained product (RRSB) in the range of 1 to 4 mm). This is important in order to ensure a good throwing power and, in the case of mixing with other fertilizers, to suppress the tendency for segregation, which would be intensified by fines in a grain mixture.

In principle, coarse-grained crystallizates can be prepared from solutions in crystallizers of the type DTB (Draft Tube Baffled) or of the Oslo type. In both cases, the problem arises that the average grain size of the product crystals produced in the crystallizer is subject to periodic fluctuations, i. Phases with a high coarse grain fraction alternate with phases in which predominantly fine-grained crystals (for example, grain size less than 1 mm) are obtained. This is the result of a strong spontaneous primary nucleation with increased supersaturation in the crystallizer. In operating phases with pronounced coarse grain formation, all nuclei formed are dissolved by the strong fine crystal dissolution due to the temperature increase in the heat exchanger of the external circuit of a DTB crystallizer or in the circulation of an Oslo crystallizer.

EP 0632738 B1 discloses a continuous crystallization process in which a coarse-grained ammonium sulfate crystallizate can be prepared from a supersaturated aqueous ammonium sulfate solution in a DTB crystallizer. In this process, a suspension of supersaturated ammonium sulfate solution and already formed crystals within the DTB crystallizer in an internal Circulation constantly circulated. Evaporation of the solvent (water) constantly creates new supersaturation, which is degraded again as a result of the crystallization taking place. The vapors produced during evaporation are removed at the head of the DTB crystal transistor. From a separated by Strömungsleitwände in the upper region of the crystallization space of the DTB crystallizer from the internal circulation of the suspension part (clarification), in which there is a clarified solution with a substantially consisting of crystallization nuclei and fine crystals solid fraction, in contrast to the bottom portion of the crystallizer, a partial stream stripped of clarified solution and returned to the dissolution of the solid content contained therein back into the bottom region of the crystallization space. To dissolve the solids content, a heat exchanger is activated in the external circuit, which raises the temperature of the clarified solution and thus the dissolving power of the ammonium sulfate solvent. In addition, in the external circuit before the heat exchanger and the feed line opens, can be fed by the new concentrated ammonium sulfate solution in the crystallizer. From the bottom area, a partial stream of suspension with the proportion of solid contained in the desired particle size of the product crystallizate is continuously withdrawn. The product crystals are separated from the mother liquor in a thickener and then centrifuged, and the mother liquor is returned to the DTB crystallizer. In order to increase the production of a sufficiently coarse-grained crystal and to improve the production in terms of cyclic variations in grain size, in addition to the feeding of saturated ammonium sulfate solution in this process, a crystallizate suspension of ammonium sulfate is fed into the crystallizer at a constant flow rate from an external source , There are no indications in EP 0632738 B1 of the manner of preparation of the suspension, that is to say whether it was prepared, for example, by dissolution of comminuted product crystallizate or in a separate crystallizer. It is merely given that this suspension must satisfy certain conditions:

The temperature of the fed suspension must not exceed the operating temperature in the crystallizer. In addition, the suspension must contain 6 to 24% by volume of crystallizate, with at least 35% of the crystals being greater than 1.2 mm, and the feed of the suspension should be such that the weight of the crystals in the feed suspension is in the range of 4 - 25% of the weight of the crystals in the withdrawn from the bottom of the crystallizer suspension with the product crystals. This targeted feeding of crystal suspension in a crystallizer to influence the grain size is also referred to as seeding.

Furthermore, WO 00/56416 a method for controlling the grain size in the continuous mass crystallization is known, which is also intended for the production of coarse ammonium sulfate crystals in an Oslo crystallizer or DTB crystallizer and in which similar to the method according to EP 0632738 B1 an inoculation is carried out with externally supplied crystal suspension. The inoculum is a crystallizate, which is manufactured in its parameters independent of the current crystallization process and which has a mean grain diameter of 0.1 - 1, 0 mm. The temperature of the seed product during the addition must not be in this process above the operating temperature in the crystallizer, but must be up to 40 ⁰ C, preferably 10 - 30 ⁰ C, lower. All other infeeds and returns are solids-free. This means in detail that the feed for the production of the ammonium sulfate product crystallizate is supplied as a preheated solids-free ammonium sulfate feed solution, and that the external circuit of solid solution withdrawn from the crystallizer out first leads to a heat exchanger by increasing the temperature of the solution causes a redissolution of the solids content, before the solids-free solution is then returned to the crystallizer. The heat energy for heating the heat exchanger is provided by means of the withdrawn from the crystallizer steam, which is first brought by a vapor compression to a higher temperature level provided. From the bottom of the crystallizer, a suspension with a solids content in the desired particle size is continuously withdrawn and separated by centrifugation in product and mother liquor, wherein the mother liquor is conveyed to an intermediate container and fed back from there into the circulation line of the external circuit of the crystallizer. The seed product is preferably added in an amount whose solids content is 5 to 30% by weight of the solid discharged from the crystallizer in each case. The solids content of the seed product can be generated, for example, by mechanically comminuting a portion of the product crystals and / or by a separate crystallization stage.

Finally, from JP 2005-194153 A a designed as a DTB crystallizer plant for the production of ammonium sulfate crystallizate is known in which a to a Clarifying zone of the crystallizer connected external circuit is provided for clarified solution in which either a heat exchanger or a supply line for solvent (eg water or undersaturated ammonium sulfate solution) is optionally installed to dissolve the solid content contained. In addition, the clarification zone is connected directly or indirectly via the external circuit to a further discharge line, with which a suspension containing as solid only fine crystals, can be completely discharged from the process, for example, to obtain fine-grained ammonium sulfate product. In addition, at the top of the clarification zone, a third discharge line is connected, can be deducted in the case of need, an excess of crystallization nuclei and ultrafine crystals and fed into a collection container. Solvent is added to the receiver to dissolve the solids. From there, the resulting solution is passed into a neutralization tank in which it is treated with sulfuric acid and ammonia and heated by the associated neutralization reaction. The heated solution is then fed to the crystallizer.

Object of the present invention is to improve a generic method to the effect that the production of coarse-grained ammonium sulfate can be done with the least possible investment costs at the lowest possible periodic fluctuations in the mean grain size.

This object is achieved by a process for the continuous production of a coarse ammonium sulfate product (particle size d 'at least 2.4 mm) by crystallization of an ammonium sulfate solution in a operated according to the DTB principle crystallization stage in which a suspension of mother liquor and

Ammonium sulfate crystals is constantly circulated in an internal circuit upon evaporation of water and continuously withdrawn from the upper part of a clarified substream of solution in an external circuit, heated to dissolve the solids contained therein and returned as a clear solution back to the lower part of the crystallization stage is continuously withdrawn from the top of the crystallization stage, fresh solution is introduced from the outside into the crystallization stage and a partial stream of the suspension with the product crystal contained therein is withdrawn from the bottom of the crystallization stage and wherein according to the invention it is provided that the crystallization stage with a Suspensi - Onsdichte in the soil area of over 40 M%, in particular over 42 M% is operated. The suspension density is defined as the quotient of solid mass in the suspension and total mass of the suspension. The suspension density in the bottom area, which is equivalent to the density of the suspension withdrawn from the bottom area of the crystallization stage, is usually always in the range of, for example, about 25% by weight, depending on the crystallization stage, sometimes also in the range from 30 to at most 40 M%.

Quite surprisingly, it has now been found that a crystallization operation at a suspension density above 40 M%, which has hitherto always been avoided because of the increasingly higher viscosity of the suspension and thus required greater expenditure of drive energy for the internal suspension circulation, the regularly observed oscillations produced mean crystal grain size very much attenuated or even completely disappears, especially when the suspension density is maintained at least 42 M%. This is all the more surprising as, with increasing input of mechanical energy into the suspension by the circulation pump, the comminution of larger crystals also increases, which would actually be counterproductive for the coarse grain crystallization. Nevertheless, when exceeding the usual limits of the suspension density an opposite effect with a dramatically increasing lengthening of the cycle time of the production vibrations, and at the same time considerable reduction of the fluctuation range of the average grain size. Apparently, the increased suspension density significantly more fine crystals and crystallization nuclei are retained in the suspension and preserved from destruction in the external circuit, so that there is always enough crystallization available to avoid unwanted Spontankeimbildung due to excessive supersaturation despite the pronounced coarse grain growth.

Crystallization with a suspension density in the range of 44-48 M%, in particular with a value of about 45 M%, at which the average crystal grain size of the ammonium sulfate produced remains practically constant, appears to be particularly favorable. Conveniently, the mother liquor, which is separated from the product crystallizate from the withdrawn suspension, recycled back into the crystallization stage. Likewise, undersize, which is separated from the desired coarse grain by sieving after drying of the product crystal, should be redissolved in a portion of the condensed vapors of the crystallization stage and the resulting solution returned to the crystallization stage. The suspension density in the crystallization stage can be determined by known measuring instruments; but it is also possible to monitor them via the current consumption of the drive motor of the circulating pump for the internal circuit.

As an alternative to the use of an indirect heat exchanger in the external

Solution cycle can be advantageously provided to provide the heat for solids dissolution in the external circuit as heat of reaction by the fresh solution in the form of separately fed sulfuric acid and ammonia is supplied from the outside. As a result, can be completely dispensed with the separate supply of heat energy from the outside in the process.

Particularly advantageous in terms of manufacturing costs, the inventive method, when the fresh solution, for. can be supplied from a caprolactam process, so is available as a waste product.

In any case, the method according to the invention makes it possible, with extremely simple means, to ensure safe process management with at least largely or even completely constant mean grain size of the product crystals in the coarse grain region. Inoculation of the internally circulated suspension with fine crystals and the required process and equipment costs can be completely dispensed with. However, the instead required higher use of drive energy for the internal suspension circuit is overcompensated by the advantage of a very uniform coarse grain production with high yield by far. It is also one of the advantages of the invention that the process stages usually downstream of the crystallization stage for the preparation of the product crystallizate (solid / liquid separation, drying, sieving, fine grain dissolution) can be utilized very uniformly and thus dimensioned comparatively small, since the resulting Amount of undersize grain is no longer subject to the usual cyclical fluctuations.

The invention will be described in more detail below with reference to the plant diagram shown schematically in the single figure in an exemplary embodiment.

Core of the illustrated system is a DTB crystallizer 1, the head is denoted by 4 and the bottom portion with 2. Inside the crystallizer 1, a flow guide tube 6 is arranged coaxially with the substantially cylindrical jacket of the crystallizer housing. The liquid level not shown is enclosed in operation plant in the region of the upper end of the Strömungsleitrohrs 6, at the lower end of a circulating pump 5 is arranged for an internal suspension circuit. The circulation pump 5, the drive of which is flanged on the outside of the bottom of the crystallizer 1, is designed so that it can promote a suspension with a density of eg 45 M%. In an upper region of the crystallizer 1, a flow guide wall ending below the liquid level and forming an annular, downwardly open chamber is provided, which is decoupled from the internal suspension cycle in the sense of a clarification area 3 as a flow-calmed space. Of course, this annular clarification chamber can also be divided by radial dividing walls into a plurality of partial chambers. From the clarification area 3, a line 7 of an external solution circuit is provided, which is provided with a circulating pump 8 and returns to the bottom area 2 of the crystallizer 1 in the vicinity of the lower end of the flow conduit 6. In the flow direction behind the circulation pump 8, an indirect heat exchanger 9 for solid solution in the clear solution in the line 7 is turned on. The external supply of ammonium sulfate solution to the crystallizer 1 is designated 10 and leads directly into the bottom region 2; However, the solution feed line could also lead, for example, to the line 7 of the external solution circuit.

At the head 4 of the crystallizer 1, a vapor discharge line 1 1 is connected, which leads to an indirectly cooled condenser 12. The coolant (for example water) used for cooling the condenser 12 can be supplied to the condenser 12 from a coolant feed line 13 and can be discharged again via a coolant discharge line 14. The withdrawn from the crystallizer 1 vapors can be performed as condensate from the condenser 12 by a Kondensatableitung in a condensate collection tank 15 and collected there for subsequent reuse within (condensate line 17) or outside (condensate drain 16) of the plant.

Furthermore, a suspension withdrawal line 18 with a suspension pump 30 is connected to the crystallizer 1 in the bottom region 2. The suspension withdrawal line 18 leads to a solid / liquid separation device 19, which may for example consist of a designed as a hydrocyclone thickener with downstream centrifuge.

The separated in the solid / liquid separator 19 mother liquor contained therein finely divided crystals can by a mother liquor discharge 27 in a release apparatus 24 are introduced while the separated solid is fed to a dryer 20 for drying. The dry crystallizate coming from the dryer 20 reaches a screening device 21, in which the fraction of fine crystals contained is separated off from an ammonium sulphate product 22 having the desired coarse grain size spectrum. The separated fine fraction is also conveyed via an undersize exhaust 23 as well as the liquid phase from the solid / liquid separator 19 into the dissolving apparatus 24. Since condensate can also be introduced via the condensate line 17 into the release apparatus 24, which is suitably equipped with a stirring apparatus, complete dissolution of the supplied solid to a reusable solution can be achieved. These

Solution can be achieved by a e.g. designed as an overflow line solution line 25 a solution tank 26, which is also suitably provided with a stirrer, fed and returned from this by means of a feed pump 28 through a dissolution line 29 into the crystallizer 1. The feed of the recirculated solution, for example, as shown, take place in the vicinity of the annular clarification area 3 of the crystallizer 1. However, a feed can also be particularly expedient, for example. be at the beginning of the line 7 of the external Lösungskreislaufs, in which case a heating in the heat exchanger 9 can then take place before the introduction into the crystallization space. To promote a full Feststoffauf solution in the external circuit can also be part of the

Condensate from the condensate tank 15 are fed into the line 7 (not shown).

Example: From a DTB crystallization stage operated according to the invention with a suspension density of 45 M%, an ammonium sulfate suspension could be withdrawn virtually without any cyclic change in the crystal size spectrum whose solids content after the separation of mother liquor and after drying a fine fraction (particle size less than 1, 8 mm) in an amount of about 20 M%. After separation of this fine fraction, the remaining 80% represented a coarse grain fraction whose mean grain size D50 was 2.4 mm. LIST OF REFERENCE NUMBERS

Crystallizer Bottom area Clarification area Head of crystallizer Internal circulating pump Flow tube External circuit supply External circulating pump Indirect heat exchanger External supply of ammonium sulfate solution Brine drain Condenser Coolant inlet Coolant drain Condensate drain Condensate drain Condensate line Suspension vent Solid / liquid separator Drying device Screening device Coarse-grained ammonium sulphate product Subgrain separator Dissolving apparatus Solution line Solution tank Mother liquor discharge Conveyor pump Solution line Suspension pump

Claims

claims

1. A process for the continuous production of a coarse ammonium sulfate product (particle size α "at least 2.4 mm) by crystallization of an ammonium sulfate solution in a operated according to the DTB principle Produktkristallisationsstufe in which a suspension of mother liquor and ammonium sulfate crystals in evaporation of Water is constantly circulated in an internal circuit and constantly withdrawn from the upper part of a clarified partial flow of solution in an external circuit, heated to dissolve the solids contained therein and returned as a clear solution back to the lower part of the crystallization stage, wherein continuously vapors from Withdrawn overhead of the crystallization stage, fresh solution introduced from the outside into the crystallization stage and a partial stream of the suspension with the product crystallizate contained therein is withdrawn from the bottom of the crystallization stage, characterized in that the crystallization nsstufe with a suspension density in the bottom region of over 40 M%, in particular over 42 M% is operated.

2. The method according to claim 1, characterized in that the suspension density is kept below 50 M%.

3. The method according to claim 1, characterized in that the suspension density in the range of 44 - 48 M%, in particular at 45 M% is maintained.

4. The method according to any one of claims 1 to 3, characterized in that the mother liquor separated from the withdrawn suspension of the product crystals, the mother liquor is returned to the crystallization stage and the product crystals are dried and sieved to remove undersize, wherein the separated undersize by means of a Part of the condensed vapors of the crystallization stage dissolved again and the solution thus obtained is also recycled to the crystallization stage.

5. The method according to any one of claims 1 to 4, characterized in that the suspension density is monitored based on the current consumption of the drive motor of the circulation pump for the internal circuit.

6. The method according to any one of claims 1 to 5, characterized in that the heat for dissolution of solids in the external circuit is provided as heat of reaction by the fresh solution in the form of separately fed sulfuric acid and ammonia is supplied from the outside.

7. The method according to any one of claims 1 to 5, characterized in that the fresh solution is fed from a caprolactam process.