Process for producing ammonium sulfate crystals

Abstract

The present invention provides a continuous process for producing ammonium sulfate crystals, wherein said process comprises: i) feeding to a series of crystallization sections, which crystallization sections are heat integrated in series, a solution of ammonium sulfate; ii) crystallizing ammonium sulfate crystals from said solution of ammonium sulfate; iii) purging a fraction of the solution of ammonium sulfate from each of said crystallization sections; and iv) discharging ammonium sulfate crystals from each crystallization section, characterized in that: a fraction of said solution of ammonium sulfate is purged from at least one crystallization section to at least one other crystallization section; and an apparatus suitable for producing ammonium sulfate crystals.

Claims

1. A continuous process for producing ammonium sulfate crystals, wherein the process comprises: i) feeding a solution of ammonium sulfate comprising impurities to a heat-integrated series of crystallization sections; ii) crystallizing ammonium sulfate crystals from the solution of ammonium sulfate; iii) purging a fraction of the solution of ammonium sulfate from each of the crystallization sections; and iv) discharging ammonium sulfate crystals from each of the crystallization sections, wherein the ammonium sulfate crystals discharged from each of the crystallization sections are not charged to another crystallization section, and wherein the process further comprises: heat-integrating the crystallization sections by steam, and purging a fraction of the solution of ammonium sulfate from each crystallization section in the series to the next crystallization unit in the series, as defined by descending temperature of steam supply, with the exception that the fraction of the solution of ammonium sulfate purged from the final crystallization section in the series is discharged from the series.

2. The process according to claim 1, wherein each crystallization section has substantially equal production capacity of ammonium sulfate crystals.

3. The process according to claim 1, wherein the series comprises from two to four, crystallization sections.

4. The process according to claim 1, wherein each crystallization section comprises an Oslo-type crystallizer.

5. The process according to claim 1, wherein the temperature of steam entering the first crystallization section in the series is from 80 C. to 160 C.

6. The process according to claim 1, wherein the temperature of steam exiting the last crystallization section in the series is from 40 C. to 60 C.

7. The process according to claim 1, wherein each of the crystallization sections has a production capacity which is from 30 kta to 150 kta.

8. The process according to claim 1, wherein the ammonium sulfate crystals discharged from the crystallization sections have a mean median diameter which is from 1.0 mm to 4.0 mm.

9. The process according to claim 1, wherein the solution of ammonium sulfate is produced from a process for producing -caprolactam or acrylonitrile.

10. An apparatus suitable for producing ammonium sulfate crystals according to the process of claim 1, comprising: i) a series of crystallization sections of substantially equal production capacity of ammonium sulfate crystals which are configured to be heat integrated with respect to steam; ii) a steam supply system integrating the crystallization sections in series; iii) a feed of ammonium sulfate solution; and iv) a system of removal of ammonium sulfate crystals; wherein the ammonium sulfate crystals discharged from each of the crystallization sections are not charged to another crystallization section, and wherein each crystallization section is connected by a purge line to the next crystallization section in the series, as defined by descending temperature of steam supply, with the exception that a purge from the final crystallization section is discharged from the series.

11. The apparatus according to claim 10, wherein each crystallization section comprises an evaporative crystallizer and solid-liquid separation equipment.

12. The apparatus according to claim 10, wherein each of the crystallization sections is sized so as to have a production capacity from 30 kta to 150 kta.

Description

(1) The present invention will be more fully explained with reference to the following drawings.

(2) FIG. 1 describes an embodiment of the prior art, wherein four crystallization sections are arranged in parallel in view of the feed of solution of ammonium sulfate.

(3) FIG. 2 describes an embodiment of the present invention, wherein the purge lines are adapted to discharge a fraction of solution of ammonium sulfate from the crystallization sections.

(4) FIG. 1 describes an embodiment of the prior art. Four crystallization sections, (1), (2), (3), (4), each comprising a crystallizer of equal size are arranged in parallel with respect to the feed of ammonium sulfate solution. A solution of ammonium sulfate passes through feed line (5) into each crystallization section, where crystallization occurs to form a slurry of ammonium sulfate crystals in an ammonium sulfate solution. Steam is fed to the crystallization section (1), via line (6), where it is used to evaporate solvent from the ammonium sulfate solution, thereby aiding crystallization. The steam does not directly contact the ammonium sulfate solution, but transfers heat indirectly thereto via a heat exchange unit. A solvent-comprising vapor stream is formed in crystallization section (1), and is discharged through line (7) to crystallization section (2), where it is used to evaporate solvent, analogous to the process in crystallization section (1). The solvent-comprising vapor stream formed in crystallization section (2) is discharged through line (8) to crystallization section (3) where it is used to evaporate solvent analogous to the process in crystallization section (1). The solvent-comprising vapor stream formed in crystallization section (3) is discharged through line (9) to crystallization section (4) where it is used to evaporate solvent analogous to the process in crystallization section (1). The solvent-comprising vapor stream formed in crystallization section (4) is discharged via line (10). Ammonium sulfate crystals are discharged from crystallization section (1) though line (11) for further processing. A fraction of ammonium sulfate solution comprising impurities is purged through line (12). Ammonium sulfate crystals are discharged from crystallization section (2) though line (13) for further processing. A fraction of solution of ammonium sulfate comprising impurities is purged through line (14). Ammonium sulfate crystals are discharged from crystallization section (3) though line (15) for further processing. A fraction of solution of ammonium sulfate comprising impurities is purged through line (16). Ammonium sulfate crystals are discharged from crystallization section (4) though line (17) for further processing. A fraction of solution of ammonium sulfate comprising impurities is purged through line (18). Optionally, the ammonium sulfate crystals from lines (11), (13), (15) and (17) are combined, either before or after any further processing step. The solutions of ammonium sulfate purged through lines (12), (14), (16) and (18) are treated as waste, and undergo further processing. Optionally, these solutions of ammonium sulfate are combined.

(5) FIG. 2 describes an embodiment of the present invention. The system is essentially the same as that of FIG. 1. Specifically, crystallization section (1); the feed of solution of ammonium sulfate (5); the steam system (6), (7), (8), (9), (10); the four lines through which ammonium sulfate crystals are discharged from the crystallization sections (11), (13), (15), (17); and purge line (18) are identical to those of FIG. 1. The purge lines (12), (14) and (16), are adapted to discharge a fraction of the solution of ammonium sulfate from crystallization sections (1), (2) and (3), respectively to crystallization sections (2), (3) and (4), respectively. Crystallization sections (2), (3) and (4) are adapted to receive a purge of solution of ammonium sulfate. Accordingly, a fraction of the solution of ammonium sulfate is purged from crystallization section (1) via line (12) into crystallization section (2); a fraction of the solution of ammonium sulfate is purged from crystallization section (2) via line (14) into crystallization section (3); and a fraction of the solution of ammonium sulfate is purged from crystallization section (3) via line (16) into crystallization section (4).

(6) The invention is illustrated by but not intended to be limited to the following Examples.

(7) The Examples are based on a simple calculation using approximate parameters. Input parameters are representative of operating ammonium sulfate crystallization plant data. These parameters are the total production capacity of the system, the target yield of the crystallization, and the separation factor (a feature of the crystal system). The separation factor and the total production rate of ammonium sulfate crystals are each respectively equal for each of the following examples, as discussed in detail below. In the examples all fresh feeds and all purge streams of solutions of ammonium sulfate, including any impurities, do not contain solids.

COMPARATIVE EXAMPLE

(8) In this comparative example a parallel ammonium sulfate crystallization system substantially as described in FIG. 1 is used. An aqueous ammonium sulfate lye containing on dry base 0.05 wt. % of impurities (=(weight of impurities in lye)/((weight of impurities in lye)+(weight of pure ammonium sulfate in lye)) is crystallized by evaporative crystallization in a set of four crystallization sections of equal capacity, arranged in series. The total production rate of ammonium sulfate crystals is 100 kta (on dry base: so, impurities and pure ammonium sulfate). Accordingly, 25 kta of ammonium sulfate crystals are produced per crystallization section. The total purge comprising streams (12), (14), (16) and (18) is 2 kta (on dry basis: so, impurities and pure ammonium sulfate). This implies that the total fresh feed comprising stream (5) is 102 kta (on dry base: so, impurities and pure ammonium sulfate)).

(9) Thus, the total yield of ammonium sulfate crystals is about 98.0 wt. % (100 kta ammonium sulfate crystals divided by 102 kta fresh feed).

(10) Therefore, the feed per crystallization section is on dry base 25.5 kta (pure ammonium sulfate and impurities) and the purge per crystallization section is on dry base 0.5 kta (pure ammonium sulfate and impurities). The concentration factor of the impurities in each crystallization section is 51 (25.5 kta fresh feed divided by 0.5 kta purge), and the concentration of impurities in the purge stream of each crystallization section is on dry base about 2.4 wt. % and hence the ammonium sulfate crystals in the streams (11), (13), (15) and (17) contain about 0.0024 wt. % impurities.

Example 1

(11) In Example 1 a coupled slurry system substantially as described in FIG. 2 is used. The total fresh feed comprising stream (5) is 102 kta (on dry base: so, impurities and pure ammonium sulfate), the total production rate of ammonium sulfate crystals in the streams (11), (13), (15) and (17) is 100 kta (on dry base: so, impurities and pure ammonium sulfate) and the total purge, stream (18) is 2 kta (on dry base: so, impurities and pure ammonium sulfate). The system is operated substantially as described for the Comparative Example, except that the amount of purge from one crystallization section to the next is controlled, as are the fresh feed rates to each crystallization section (described in Table 1, below). Here, in contrast to the Comparative Example, the feed rates for the different crystallization sections are not equal.

(12) TABLE-US-00001 TABLE 1 (all figures are on dry base) Crystallization section no. 1 2 3 4 Fresh feed (kta) 32 32 32 6 Impurity in fresh feed 0.05 0.05 0.05 0.05 (wt. %) Purge from previous 0 7 14 21 crystallization section (kta) Impurity in purge from 0 0.23 0.23 0.23 previous crystallization section (wt. %) Total feed (kta), 32 39 46 27 Impurity in total feed 0.05 0.082 0.104 0.188 (wt. %) Purge (kta) 7 14 21 2 Concentration factor 4.6 2.8 2.2 13.5 Impurity in purge 0.23 0.23 0.23 2.5 crystallization section (wt. %) Impurity in ammonium 0.00023 0.00023 0.00023 0.0025 sulfate crystals (wt. %)

(13) Total feed is defined as sum of fresh feed and purge from previous crystallization section.

(14) The third crystallization section is taken here as an example, the results for the other crystallization sections can be obtained in a similar manner. In the third crystallization section fresh feed comprising on dry base 32 kta (99.95 wt. % ammonium sulfate and 0.05 wt. % impurities) and a purge from the second crystallization section comprising on dry base 14 kta (about 99.77 wt. % ammonium sulfate and about 0.23 wt. % impurities) enter. The total amount of impurity charged to the third crystallizer is about ((32 kta times 0.05 wt. %+14 kta times 0.23 wt. %) 0.048 kta. The mean impurity concentration of the total feed is on dry base ((32 kta times 0.05 wt. %+14 kta times 0.23 wt. %)/(32 kta+14 kta)) 0.104 wt. %. The purge from the third crystallization section comprises on dry base 21 kta (ammonium sulfate and impurities). The concentration factor is (total feed being sum of fresh feed and purge from second crystallization section divided by purge of third crystallization section; so, 46 kta divided by 21 kta) about 2.2. The total production rate of the third crystallization section is 25 kta ammonium sulfate crystals (on dry base: so, impurities and pure ammonium sulfate).

(15) The concentrations of impurities in the purge from the third crystallization and in the produced ammonium sulfate crystals are simply obtained from the following mass balance equation:
imp.sup.3IN=purge.sup.3out*imp.sup.3purge+crystal.sup.3out*imp.sup.3crystal
where imp.sup.3IN=total amount of impurity charged to the third crystallizer (kta) purge.sup.3out=purge on dry base of third crystallizer (kta) imp.sup.3purge=impurity concentration in purge of third crystallizer (wt. %) crystal.sup.3out=production of ammonium sulfate crystals in third crystallizer (kta) imp.sup.3crystal=impurity concentration in ammonium sulfate crystals produced in third crystallizer (wt. %), which is ((imp.sup.3purge)/separation factor) is (imp.sup.3purge)/1000.

(16) After substitution it follows that the impurity concentration in purge of third crystallization section is on dry weight base about 0.23 wt. %)

(17) The separation factor is 1000, so the concentration of impurities in the ammonium sulfate crystals produced in the third crystallization section is (0.23 wt. %/1000) is 0.00023 wt. %.

(18) As can be seen from the above results, the wt. % impurities in the ammonium sulfate crystals produced in crystallization sections (1), (2) and (3) of Example 1 are substantially lower than for the ammonium sulfate crystals produced in crystallization sections (1), (2) and (3) of the Comparative Example (0.00023 wt. % vs. 0.0024 wt. %). The wt. % impurity of ammonium sulfate crystals produced in crystallization section (4) of Example 1 is only a few percent higher than the wt. % impurity of ammonium sulfate crystals produced in crystallization section (4) of the Comparative Example (0.0025 wt. % vs. 0.0024 wt. %).

(19) Further, if the ammonium sulfate crystals produced in crystallization sections (1), (2), (3) and (4) in Example 1 are mixed, the mean wt. % impurities of the combined ammonium sulfate crystals is then about 0.0008 wt. % which is substantially lower than the mean wt. % impurities of the combined ammonium sulfate crystals in the Comparative Example (being 0.0024 wt. %).

(20) Comparison of the results of Example 1 with those of the Comparative Example clearly shows that the purge system of the present invention reduces mean impurity content in the crystalline ammonium sulfate produced, while the total yield of ammonium sulfate crystals remains unchanged.

Example 2

(21) In Example 2 a coupled slurry system substantially as described in FIG. 2 was used. The total production rate of ammonium sulfate crystals in the streams (11), (13), (15) and (17) is 100 kta (on dry base: so, impurities and pure ammonium sulfate). The mean impurity content of ammonium sulfate crystals produced in crystallization sections (1), (2), (3) and (4) is about 0.0024 wt. %, which is equal to the mean impurity content of ammonium sulfate crystals produced in crystallization sections (1), (2), (3) and (4) in the Comparative Example. The system is operated substantially as described for the Comparative Example, except that the amount of purge from one crystallization section to the next was controlled, as were the fresh feed rates to each crystallization section (described in Table 2, below). Here, in contrast to the Comparative Example, the feed rates for the different crystallization sections are not equal.

(22) The obtained results are given in Table 2.

(23) TABLE-US-00002 TABLE 2 Crystallization section no. 1 2 3 4 Fresh feed (kta) 33.5 33.5 33.5 0.02 Impurity in fresh feed 0.05 0.05 0.05 0.05 (wt. %) Purge from previous 0 8.5 17 25.5 crystallization section (kta) Impurity in purge 0 0.20 0.20 0.20 from previous crystallization section (wt. %) Total feed (kta), 33.5 42 50.5 25.52 Impurity in total feed 0.05 0.080 0.099 0.196 (wt. %) Purge (kta) 8.5 17 25.5 0.52 Concentration factor 3.9 2.5 2.0 49 Impurity in purge 0.20 0.20 0.20 9.1 crystallization section (wt. %) Impurity in ammonium 0.00020 0.00020 0.00020 0.0091 sulfate crystals (wt. %)

(24) Total feed is defined as sum of fresh feed and purge from previous crystallization section.

(25) As can be seen from the above results, the wt. % impurities in the ammonium sulfate crystals produced in crystallization sections (1), (2) and (3) of Example 2 are substantially lower than for the ammonium sulfate crystals produced in crystallization sections (1), (2) and (3) of the Comparative Example (0.00020 wt. % vs. 0.0024 wt. %). The wt. % impurity of ammonium sulfate crystals produced in crystallization section (4) of the Example 2 is higher than the wt. % impurity of ammonium sulfate crystals produced in crystallization section (4) of the Comparative Example (0.0091 wt. % vs. 0.0024 wt. %). The mean wt. % impurities of the combined ammonium sulfate crystals produced in crystallization sections (1), (2), (3) and (4) in Example 2 is about 0.0024 wt. %.

(26) In Example 2, the purge of crystallization section (4) is just 0.52 kta (on dry base: so, impurities and pure ammonium sulfate). This means that the total fresh feed on dry base to all crystallization sections is (sum of total amount of ammonium sulfate crystals produced on dry base and purge of crystallization section (4) on dry base) just 100.52 kta. So, the total yield of ammonium sulfate crystals is as high as (100 kta divided by 100.52 kta) 99.5 wt. %.

(27) Comparison of the results of Example 2 with those of the Comparative Example clearly shows that the purge system of the present invention increases the total yield of ammonium sulfate crystals, while the mean impurity content in the produced ammonium sulfate crystals remains unchanged.