Process for initiating an ammonium sulfate crystallization process

Abstract

A continuous process for producing crystalline ammonium sulfate, said process comprising a start-up operation followed by a steady-state operation, wherein the start-up operation comprises: i) in a crystallizer, evaporating solvent from an approximately saturated ammonium sulfate solution; ii) replacing evaporated solvent with further approximately proximately saturated ammonium sulfate solution; iii) introducing to the crystallizer seed crystals of ammonium sulfate; iv) continuing to evaporate solvent, until a desired degree of supersaturation is reached; and v) recovering crystalline ammonium sulfate from ammonium sulfate solution in a recovery unit, and the steady-state operation comprises: vi) continuously feeding approximately saturated ammonium sulfate solution into the crystallizer and continuously withdrawing ammonium sulfate crystals from the crystallizer, such that the total combined volume of ammonium sulfate solution and ammonium sulfate crystals within the crystallizer remains constant; and vii) recovering crystalline ammonium sulfate from ammonium sulfate solution in a recovery unit, characterized in that the degree of supersaturation in the crystallizer during the start-up operation is maintained between 1.2% and the point at which primary nucleation occurs; and apparatus suitable for carrying out the process.

Claims

1. A continuous process for producing crystalline ammonium sulfate, wherein the process comprises a start-up operation followed by a steady-state operation, and wherein the start-up operation comprises: i) in a crystallizer, evaporating solvent from an approximately saturated ammonium sulfate solution; ii) replacing evaporated solvent with further approximately saturated ammonium sulfate solution; iii) introducing seed crystals of ammonium sulfate to the crystallizer; iv) continuing to evaporate solvent, until a desired degree of supersaturation is reached; and v) recovering crystalline ammonium sulfate from ammonium sulfate solution in a recovery unit, and wherein the steady-state operation comprises: vi) continuously feeding approximately saturated ammonium sulfate solution into the crystallizer and continuously withdrawing ammonium sulfate crystals from the crystallizer, such that the total combined volume of ammonium sulfate solution and ammonium sulfate crystals within the crystallizer remains constant; and vii) recovering crystalline ammonium sulfate from ammonium sulfate solution in a recovery unit, wherein the degree of supersaturation in the crystallizer during the start-up operation is maintained between 1.2% and the point at which primary nucleation occurs, and wherein the seed crystals of ammonium sulfate are introduced to the crystallizer only during the start-up operation and not during the steady state operation of the crystallizer.

2. A process according to claim 1, wherein the supersaturation in the crystallizer during the start-up operation is maintained between 1.2% and 2.5%.

3. A process according to claim 1, wherein the supersaturation in the crystallizer during the start-up operation is maintained between 1.3% and 2.2%.

4. A process according to claim 1, wherein the average size of seed crystals is below the average size of crystalline ammonium sulfate.

5. A process according to claim 4, wherein the average size of seed crystals fed to the crystallizer is below 1.4 mm.

6. A process according to claim 5, wherein the average size of seed crystals fed to the crystallizer is from 0.8 to 1.2 mm.

7. A process according to claim 1, wherein the seed crystals of ammonium sulfate have an average crystal surface area of from 3 to 4 m.sup.2 kg.sup.1.

8. A process according to claim 1, wherein in step iii) the seed crystals of ammonium sulfate are fed for a time period of up to 6 hours.

9. A process according to claim 1, wherein step iii) is practiced by introducing a mixture of the seed crystals of ammonium sulfate with the solution of ammonium sulfate in a form of a slurry having a density at the start of step iii) which is from 1240 to 1260 kgm.sup.3.

10. A process according to claim 1, wherein the average size of crystals leaving the crystallizer is at least 1.4 mm.

11. A process according to claim 1, which further comprises a crystal size classification step, wherein a fine crystal fraction and a coarse crystal fraction are produced.

12. A process according to claim 11, wherein at least a portion of the fine crystal fraction is fed from the size classification step to step iii).

13. A process according to claim 1, wherein the temperature in the crystallizer is above 40 C.

Description

(1) FIG. 1 depicts a typical set-up according to the prior art. A fresh solution of ammonium sulfate enters, through line (a), a mixing unit (1), where it is mixed with input from line (e) to form a feed solution of ammonium sulfate. The feed solution of ammonium sulfate then passes through line (b) into crystallizer (2), where it is subjected to crystallization, such that a suspension of ammonium sulfate crystals is produced. The suspension of ammonium sulfate crystals passes through line (c) into recovery unit (3), where ammonium sulfate solution is separated from crystalline ammonium sulfate; removed through line (e); and recycled back into mixing unit (1). The crystalline ammonium sulfate is recovered through line (d).

(2) FIG. 2 depicts a process according to the present invention. Lines (a) to (e), and units (1) to (3) are as defined for FIG. 1. During the start-up operation, a suspension of ammonium sulfate crystals is fed from seed crystal preparation unit (4) through line (f) to crystallizer (2).

(3) FIG. 3 depicts a process according to the present invention. Lines (a) to (e), and units (1) to (3) are as defined for FIG. 1. During the start-up operation, a suspension of ammonium sulfate crystals is fed from seed crystal preparation unit (4) through line (f) to crystallizer (2). The suspension of ammonium sulfate crystals passing through line (c) enter size classification unit (5), where a fine crystal fraction is separated and removed through line (i) to seed crystal preparation unit (4). The coarse crystal fraction is passed through line (h) to recovery unit (3). During steady-state operation, ammonium sulfate crystals are fed through line (g) into mixing unit (1). Any excess of seed crystals is passed through line (j) for further processing.

(4) The present invention is illustrated by but not limited to the following examples.

EXAMPLES

Comparative Example 1

(5) This comparative example was carried out in equipment corresponding substantially to that of FIG. 1.

(6) A 300 m.sup.3 Oslo-type fluid bed crystallizer (2) was used with an external circulation circuit for circulating slurry from the crystallizer body via a Begemann impeller pump, capacity 5000 m.sup.3hr.sup.1, and a heat exchanger. During start-up 180 m.sup.3 of aqueous ammonium sulfate solution was present in the crystallizer. The crystallizer was operated by evaporation at a temperature of 90 C. Due to the evaporation of water and the feeding of the system with an approximately saturated ammonium sulfate solution originating from a plant producing caprolactam, the concentration of dissolved ammonium sulfate increased to such a level that spontaneous formation of ammonium sulfate crystals took place resulting in an ammonium sulfate slurry of mainly fine crystals (average diameter 0.4 mm).

(7) The normal operation point of such a crystallizer uses an ammonium sulfate slurry with a density of 1400 kgm.sup.3 with crystals having an average crystal size of 2.4 mm. Therefore, time was required in this start-up phase to let the crystals grow to a size of 2.4 mm. This took approximately 40 hours before the steady-state operation point was reached.

(8) The product of such a crystallization unit for ammonium sulfate was split into a high value product, comprising material with a particle size above 1.4 mm and a low value product, comprising material with a particle size below 1.4 mm.

(9) Results are shown in Table 1, below.

Comparative Example 2

(10) The performance of the above system was translated into a model which described the system using a combined mass, heat and population balance using the Borland Delphi 5.0 programming language. The population balance describing the crystal size distribution in the system was implemented according to a first order discretization scheme similar to the description given by M. J. Hounslow, R. L. Ryall, V. R. Marshall; A discretized population balance for nucleation, growth, and aggregation; AIChE J., 34 (1988) pp1821-1832. The description of primary crystal nucleation and crystal growth were obtained from lab scale experiments with reference to Jiang and ter Horst in Crystal Growth & Design (2011), Vol. 11, 256-261. The description of secondary crystal nucleation was calibrated on the basis of production data from an operating ammonium sulfate crystallizer. The model was used to simulate the performance of the described crystallizer system, consisting of a start-up period of approximately 30 hours and a steady-state production period of 90 hours. The performance of the crystallizer was characterized by the amount of material produced during the start-up and steady-state period that contains particles retained by a sieve of 1.4 mm as compared to the total amount of produced material in that period.

(11) The simulation model was used to predict the scale-up of Comparative Example 1 by increasing the feed rate of the system by 40%. The volume of the crystallizer was 65% larger. This was calculated by keeping the velocity of the fluidized bed constant, and the ratio of dimensions of the crystallizer constant.

(12) Results are shown in Table 1, below.

Example 1

(13) This example was carried out in equipment corresponding substantially to that of FIG. 2.

(14) In the embodiment according to the invention the simulation model developed for the Comparative Example was adapted for the equipment configuration as depicted in FIG. 2, by installing a seed crystal preparation unit (4) next to the crystallizer (2). The seed crystal preparation unit (4) is used to feed the crystallizer (2) during the start-up with 50 m.sup.3 of 40 wt-% slurry of ammonium sulfate containing ammonium sulfate crystals with an average crystal size of 0.87 mm and a relative standard deviation of 0.38. The slurry of seed crystals was fed just before the spontaneous formation of ammonium sulfate crystals would have taken place (calculated by supersaturation) and added within a time period of one hour at 90 C. The ammonium sulfate feed solution (a) is replaced by water during the feeding of the slurry. In order to accommodate the addition of the slurry, the volume of ammonium sulfate solution in the crystallizer is reduced before the start of the slurry feed.

(15) Results are shown in Table 1.

(16) TABLE-US-00001 TABLE 1 Product after 120 Comp. Ex. 1 Comp. Ex. 2 Example 1 hours operation [tons] [tons] [tons] <1.4 mm steady- state operation 330 279 279 >1.4 mm steady-state operation 1163 1794 1794 <1.4 mm including start-up 428 607 289 >1.4 mm including start-up 963 1366 1711

(17) Table 1 gives the results of the Comparative Examples 1 and 2 and the Example 1.

(18) Regarding Comparative Example 1, it can be seen that each start-up operation results in a decrease in product having size >1.4 mm of 200 tons; an increase in product having size <1.4 mm of 98 tons; and therefore a net reduction of 102 tons of product; compared with steady-state operation.

(19) Comparative Example 2 shows for each start-up operation, a decrease in product having size >1.4 mm of 428 tons; an increase in product having size <1.4 mm of 328 tons; and therefore a net reduction of 100 tons of product; compared with steady-state operation.

(20) Example 1 shows for each start-up operation, a decrease in product having size >1.4 mm of only 83 tons; an increase in product having size <1.4 mm of 10 tons; and therefore a net reduction of only 73 tons of product; compared with steady-state operation.

(21) Comparative Example 2 and Example 1 are each for a system of the same flow rate. By using the process of Example 1 instead of the process of Comparative Example 2, an increase of 27 tons of total product for each start-up operation is observed. Further, and more importantly, this also leads to an increase of 345 tons of product having size >1.4 mm. This product having size >1.4 mm is more valuable than the smaller product.

(22) Because the start-up operation of Example 1 is quicker than that of Comparative Example 2, the crystallization process as a whole (for a period of 120 hours, when start-up is considered) has a higher production capacity and increased throughput. Further, because a much smaller proportion of product crystals have size <1.4 mm, they do not need to be reprocessed to produce larger crystals, thereby saving energy. Accordingly, Example 1 demonstrates a great improvement over the known process.