USE OF SUPERCRITICAL CO2 AS SOLVENT FOR ORGANIC POLYMERS IN A METHOD FOR COATING UREA-CONTAINING GRANULES

Abstract

A process may be utilized to coat urea-containing granules with organic polymers. The process may involve compressing gaseous carbon dioxide and condensing the carbon dioxide to obtain liquid carbon dioxide, increasing the pressure and/or the temperature above the critical point of carbon dioxide and obtaining supercritical carbon dioxide, dissolving an organic polymer in the supercritical carbon dioxide to obtain a polymer-containing solution, and mixing the polymer-containing solution with urea-containing granules and lowering the temperature and/or the pressure below the critical point of carbon dioxide and obtaining coated urea-containing granules and gaseous carbon dioxide. In some cases the organic polymer may include biodegradable polymers, and the polymer-containing solution may contain between 20 to 70% by weight biodegradable polymers.

Claims

1.-20. (canceled)

21. A process for coating urea-containing granules with organic polymers, the process comprising: compressing and condensing gaseous carbon dioxide to obtain liquid carbon dioxide; increasing at least one of pressure or temperature above a critical point of carbon dioxide to obtain supercritical carbon dioxide; dissolving an organic polymer in the supercritical carbon dioxide to obtain a polymer-containing solution; and mixing the polymer-containing solution by at least one of spraying the polymer-containing solution onto or bringing the polymer-containing solution into contact with urea-containing granules, in which a simultaneous or subsequent lowering of temperature and/or pressure below the critical point of carbon dioxide occurs, and coated urea-containing granules and gaseous carbon dioxide are obtained.

22. The process of claim 21 wherein the organic polymer includes biodegradable polymers.

23. The process of claim 22 wherein the biodegradable polymer includes polylactides (PLA), polyglycols, polycaprolactones, poly(hydroxybutyric acid), poly(hydroxyvaleric acid), polyalkyl terephthalates, polyanhydrides, poly(1,4-dioxane-2,5-dione), polyamino acids, polysaccharides, cellulose esters, cellulose hydrate, cellulose acetate, carboxymethyl cellulose, lignin, polyhydroxy fatty acids, starch, biodegradable polyesters, biodegradable polyamides, biodegradable polyimides, polyhydroxyalkanoates and polybutylene succinates (PBS), amylose, amylopectin, and/or mixtures, oligomers, derivatives, and copolymers thereof.

24. The process of claim 21 wherein the polymer-containing solution contains between 20 to 70% by weight biodegradable polymers.

25. The process of claim 21 comprising returning the gaseous carbon dioxide to the compressing and condensing step and converting the gaseous carbon dioxide to supercritical carbon dioxide.

26. The process of claim 21 comprising providing the gaseous carbon dioxide and the supercritical carbon dioxide in a connected industrial plant complex.

27. The process of claim 21 wherein the condensation occurs with the aid of ammonia from a connected ammonia refrigerating system.

28. The process of claim 21 comprising subjecting the liquid carbon dioxide, after the compressing and condensing, to a flash process for at least one of removal or outgassing of inert, non-condensable gases.

29. The process of claim 21 wherein the compressing of the gaseous carbon dioxide occurs in a first step in a low-pressure compression step and in a second step in a high-pressure compression step.

30. The process of claim 29 wherein a cold/heat recovery and/or cooler are provided at least one of before the first step, between the first and second steps, or after the second step.

31. The process of claim 21 wherein the urea-containing granules and/or the coated urea-containing granules contain at least one of ammonium salts, nitrates, phosphates, sulfur, potassium, calcium, ammonium sulfate, ammonium nitrate, phosphates, or sulfur.

32. The process of claim 21 wherein the urea-containing granules have a mean particle size of 0.5 to 8 mm.

33. The process of claim 21 comprising mixing the polymer-containing solution with the urea-containing granules in one or more coaters with tangential spraying.

34. A plant complex for coating urea-containing granules with biodegradable polymers including a urea synthesis plant and a plant for coating the urea-containing granules, wherein the plant for coating the urea-containing granules comprises: a feed line for supercritical carbon dioxide from the urea synthesis plant; a feed line for a biodegradable polymer; a mixing device for the supercritical carbon dioxide and the biodegradable polymer, the mixing device being connected to the feed lines; a coater connected to the mixing device; and a feed line for granules, wherein the coater has a first outlet for gaseous carbon dioxide and a second outlet for a coated product.

35. The plant of claim 34 wherein the urea synthesis plant is connected to at least one of an ammonia synthesis unit as ammonia-urea complex or a urea granulation plant.

36. The plant of claim 34 wherein the first outlet for gaseous carbon dioxide is connected to at least one of a dust scrubber, a dust separator, or a cyclone.

37. The plant of claim 34 comprising for carbon dioxide compression: a CO.sub.2 feed line connected to a first compression zone; a second compression zone connected to the first compression zone; a third compression zone connected to the second compression zone; a fourth compression zone connected to the third compression zone; a first connecting line to a urea synthesis unit connected to the fourth compression zone; a branch between the second compression zone and the third compression zone; a second connecting line from the branch to a degassing system, with the second connecting line running through a refrigerating system; a pump connected to the degassing system; and a third connecting line connected to the pump and to the feed line for supercritical carbon dioxide.

38. The plant of claim 37 wherein elements for cold/heat recovery and/or coolers are included within the urea synthesis plant or a ammonia-urea complex in one or more of the following: the CO.sub.2 feed line; between the first compression zone and the second compression zone; between the second compression zone and the third compression zone; between the third compression zone and the fourth compression zone; or the first connecting line.

39. The plant of claim 37 comprising a passivation air supply line disposed between the second and third compression zones.

Description

[0058] In the drawings:

[0059] FIG. 1 shows a diagrammatic cross section of the plant complex according to the invention for the coating of urea-containing granules and

[0060] FIG. 2 shows a further diagrammatic cross section of the plant complex according to the invention for the coating of urea-containing granules and a sector from the ammonia-urea complex.

[0061] FIG. 1 shows a diagrammatic cross section of the plant complex according to the invention for the coating of urea-containing granules at least including an ammonia-urea complex (43) and a plant for the coating of urea-containing granules (26). The supercritical carbon dioxide (2a) obtained in the ammonia-urea complex (43) arrives via the feed line for supercritical carbon dioxide (14) from the ammonia-urea complex (43) together with a feed line (15) for a biodegradable polymer (1b) in a mixing device (8). A feed line for urea-containing granules (13) and a feed line from the mixing device (8) with the biodegradable polymer (1b) dissolved in the supercritical carbon dioxide are connected to a coater (9), for example a drum coater, fluidized bed coater, and/or fluidized bed coater with tangential spraying. The coater includes a first outlet (10) for gaseous carbon dioxide (2b) and a second outlet for the coated product (4b). The first outlet (10) for gaseous carbon dioxide (2b) is connected to a dust scrubber, dust separator and/or cyclone (39). The collected dust particles (27) can alternatively be disposed of as waste or returned to the process. Most of the dust (probably) consists of polymer particles. These are preferably (not shown) washed with water, then evaporated and returned to the mixing device (8). This evaporation can take place in the urea synthesis plant. This flow tends to be rather small and can simply be added to the main evaporation. It is more likely that the polymer will not dissolve, so that no evaporation, but possibly drying, is necessary.

[0062] FIG. 2 shows a further diagrammatic cross section of the plant complex according to the invention for the coating of urea-containing granules (26) and a sector from the ammonia-urea complex (43). Carbon dioxide, preferably carbon dioxide from the carbon dioxide scrubbing of the steam reforming process, is, in the carbon dioxide compression step shown within the ammonia-urea complex (43), introduced via at least one carbon dioxide CO.sub.2 feed line (28) into a (with the pressure increasing from the first to the fourth step) first compression step (33) (preferred pressure ranges from 5 bar to 10 bar), a second compression step (34) (preferred pressure ranges from 20 bar to 30 bar), a third compression step (35) (preferred pressure ranges from 70 bar to 90 bar) and a fourth compression step (36) (preferred pressure ranges from 140 bar to 180 bar). A first connecting line (42) is connected from the fourth compression step (36) to a urea synthesis unit. Within the meaning of the invention, the expression “urea synthesis unit” includes the reactors and feed lines provided within the urea synthesis plant for the actual synthesis of urea from CO.sub.2 and NH.sub.3. This connection makes it possible for the compressed carbon dioxide to be used in the synthesis of urea. The plant complex according to the invention includes a branch (44) between the second compression step (34) and the third compression step (35). Starting from the branch (44), a second connecting line leads to a degassing system (37), the second connecting line running through a refrigerating system (32) upstream of the degassing system (37) (being continuously connected). The refrigerating system (32) makes possible liquefaction of the gaseous carbon dioxide. The degassing system (37) makes possible the removal/reduction of inert gases, for example argon, noncondensable components of CO.sub.2 and nitrogen, via the exhaust air line (41). A pump (38) is connected to the degassing system and makes possible an increase in pressure of the liquid carbon dioxide to 150 bar to 200 bar. A third connecting line is connected to the pump (38) and the feed line (14) for supercritical carbon dioxide. Preferably, by increasing the temperature, the liquid carbon dioxide (2c) is converted into supercritical carbon dioxide (2a) in the third connecting line and in the associated areas. Elements for cold/heat recovery (29) and/or coolers (30) and/or heaters (31) are included in the following elements: the carbon dioxide CO.sub.2 feed line (28), between the first compression step (33) and the second compression step (34), between the second compression step (34) and the third compression step (35), between the third compression step (35) and the fourth compression step (36); and/or the first connecting line (42) to the urea synthesis plant. Preferably, the first cooler (30) in the flow direction of the CO.sub.2 feed line (28) belongs to the ammonia synthesis plant, and the remaining elements for generating the supercritical carbon dioxide belong to the urea synthesis plant. A passivation air feed line (40) is arranged between the second compression step (34) and the third compression step (35), which is required, for example, in a downstream hydrogen H.sub.2 removal reactor (not shown) and in the urea synthesis. The structure described in FIG. 1 is connected via the supply line for supercritical carbon dioxide (14).

LIST OF REFERENCE INDICATIONS

[0063] (1) organic polymer [0064] (1b) biodegradable polymer [0065] (2a) supercritical carbon dioxide [0066] (2b) gaseous carbon dioxide [0067] (3) polymer-containing solution [0068] (4a) urea-containing granules [0069] (4b) coated urea-containing granules [0070] (8) mixing device [0071] (9) coater [0072] (10) outlet for gaseous carbon dioxide [0073] (11) outlet for coated product [0074] (12) feed line for urea-containing granules [0075] (14) feed line for supercritical carbon dioxide [0076] (15) feed line (15) for the biodegradable polymer [0077] (23) carbon dioxide compression step [0078] (26) plant for the coating of urea-containing particles/granules [0079] (27) dust particles [0080] (28) CO.sub.2 feed line (28) connected to a first compression step [0081] (29) elements for cold/heat recovery (29) [0082] (30) cooler [0083] (31) heater/heating element [0084] (32) refrigerating system/cooler [0085] (33) first compression step [0086] (34) second compression step [0087] (35) third compression step [0088] (36) fourth compression step [0089] (37) degassing system [0090] (38) pump [0091] (39) dust scrubber, dust collector, acid scrubber and/or cyclone [0092] (40) passivation air feed line [0093] (41) exhaust air line [0094] (42) first connecting line to a urea synthesis unit [0095] (43) ammonia-urea complex