UREA GRANULATION PROCESS

Abstract

A process of granulation of a urea melt, comprising: adding a first additive containing carboxymethyl starch to one or more first stage(s) of the granulation process, to form a carcarboxymethyl starch containing inner layer of urea granules, and adding a second additive containing calcium lignosulfonate to one or more second stage(s) of the granulation process, downstream said first stages, to form granules with a coating containing calcium lignosulfonate.

Claims

1-16. (canceled)

17. A urea granular product comprising granules made of at least 96% urea with a layer containing carboxymethyl starch and a coating layer containing calcium lignosulfonate.

18. The urea product according to claim 17, wherein the carboxymethyl starch containing layer has a thickness of 0.6 to 1.05 mm and the coating layer has a thickness of 0.1 to 0.2 mm.

19. The urea product according to claim 17, wherein the carboxymethyl starch containing layer contains 0.1% to 0.8% in weight of carboxymethyl starch and the coating layer contains 0.3% to 1% in weight of calcium lignosulfonate.

20. The urea product according to claim 17, wherein the urea product is obtainable with a process of granulation of a liquid urea melt containing at least 96% wt urea, the granulation process comprising: adding a first additive containing carboxymethyl starch to one or more first stage(s) of the granulation process; performing one or more second stage(s) of the granulation process downstream said one or more first stage(s); wherein no amount of said first additive is added to said one or more second stage(s) of the granulation process.

Description

DESCRIPTION OF THE DRAWINGS

[0054] FIG. 1 is a scheme of an embodiment of the process according to the invention.

[0055] FIG. 2 is a scheme of a urea granule obtainable with the process of the invention.

[0056] FIG. 3 is a scheme of a preferred system for metering the first additive.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0057] FIG. 1 illustrates a longitudinal fluid-bed granulator 1. The granulator 1 has several sets of urea melt sprayers located at different sections of the granulator itself.

[0058] FIG. 1 illustrates first sets 2, 3 of urea melt sprayers and a second set 4 of urea melt sprayers. In a real case, the number of said sets of urea melt sprayers may be for example 20 to 30. Each of the sets 2 to 4 of urea melt sprayers may include several sprayers, for example forming an array of sprayers around a vessel of the granulator 1.

[0059] The granulation process follows a main flow direction G from an inlet section to an outlet section of the granulator 1. The granulation process results in a granular product 5 which is withdrawn from the outlet section of the granulator.

[0060] A urea melt is fed from an evaporation section 6 via a main urea melt header 7. The evaporation section 6 receives a urea aqueous solution produced in a urea plant and removes water to bring the urea melt to a target concentration, e.g. 96% or more.

[0061] The main urea melt header 7 splits into a first header 8 feeding the first sets of sprayers 2, 3 and a second header 9 feeding the second set 4 of sprayers.

[0062] A first additive 10 containing carboxymethyl starch is added to the urea melt of the first header 8 and a second additive 11 containing calcium lignosulfonate is added to the urea melt in the second header 9. The flow rate of the urea melt in the headers 8 and 9 is governed by suitable flow control valves, as illustrated in FIG. 1.

[0063] Therefore, the granulation stages in the portion 12 and portion 13 of the granulator 1 are performed with urea melt containing the first additive 10, while the granulation stage in the last portion 14 (last stage of granulation) is performed with urea melt containing the second additive 11. This last stage of granulation forms a coating layer containing a desired amount of the second additive, and essentially free of the first additive.

[0064] The additives 10 and 11 may also be added directly to the sprayers or to the urea melt feeding lines of the sprayers.

[0065] FIG. 1 illustrates also an input 15 of fluidization air and an input 16 of seeds for the granulation process.

[0066] FIG. 2 illustrates a granule 20 obtainable with the process. The granule includes an inner core 21, formed by a seed; a layer 22 formed in the stages 12 and 13 and containing carboxymethyl starch as a result of the addition of the first additive 10 to the urea melt 8; a coating 23 formed in the last stage 14 and containing calcium lignosulfonate as a result of the addition of the second additive 11 to the urea melt 9. The coating 23 is substantially free of carboxymethyl starch.

[0067] FIG. 3 illustrates a preferred system for metering the first additive 10.

[0068] The figure illustrates: a low-pressure recovery section 30 of a urea production plant; a urea tank 31; a stirred reactor 32.

[0069] The low-pressure recovery section 30 produces a urea aqueous solution 33. Downstream the urea tank 31, a part of this solution is fed to the stirred reactor 32 via line 34. The stirred reactor 32 receives also the carboxymethyl starch powder 35 and optionally an amount of urea-water recycle 36 and/or an amount of urea melt via line 37.

[0070] The stream 36 contains less urea than the stream 34. The stream 37 contains more urea than said stream 34. Therefore, the streams 36, 37 can be used to adjust the concentration of urea in the reactor 32.

[0071] The carboxymethyl starch powder 35 is fed by a hopper 38 with a suitable solid feeder.

[0072] The stirred reactor produces the first additive by dissolving the carboxymethyl starch powder 35 into the aqueous urea. The temperature in the reactor 32 is preferably 60° C. to 100 ° C. and the residence time is preferably 30 min to 5 h to provide complete dissolution without deterioration of starch.

EXAMPLES

[0073] Experimental tests were carried out in batch mode.

[0074] An amount of urea seeds (about 65 kg) having a mean diameter of 1.5 mm was manually loaded in a granulation chamber and a fluidized bed was generated by blowing fluidization air. Thereafter, urea melt was fed to sprayers (about 600 kg/h) and the granulation process was started. During the granulation test the hydrostatic height of the fluidized bed was kept constant by the regulation of the overflow product valve.

[0075] Experimental tests were divided in two phases based on the additive admixed with the urea melt: in the first one the additive was a solution of carboxymethyl starch, in the second one the additive was a solution of calcium lignosulfonate. The extent of each phase was calculated on the basis of the formula:

[00001]$d\left(t\right)=d_0.Math.\sqrt[3]{e^{\frac{{\stackrel{.}{m}}_{\mathrm{in}}.Math.t}{m_0}}}$

[0076] Wherein: d denotes the urea particles mean diameter, d(t) is the mean diameter at time t; d.sub.0 is the initial mean diameter (t=t.sub.0); m.sub.0 the amount of urea in the granulation chamber; m.sub.in the urea mass flow rate; t is the time.

[0077] This relationship was used to identify the shift time of additives matching the selected diameter of the granule.

[0078] Table 2 shows the crushing strength, the caking index, the abrasion degradation and the dust formation for urea without additive and with the couple of additives (carboxymethyl starch and calcium lignosulfonate).

TABLE-US-00001 TABLE 2 Crushing Caking Abrasion Dust strength index test formation [kg] [kg] [%] [%] 96% urea melt with 1.85 250 5 5 no additive 0.45% wt carboxymethyl 3.3 50 2 1 starch + 99.7% urea melt with no additive 0.45% (wt) carboxymethyl 3.5 30 1.5 negligible starch + 0.7% (wt) calcium lignosulfonate

[0079] Crushing strength was tested by subjecting individual particles of urea of 3.00 mm diameter to a measured force, applied by means of an automatic metal plunger with a fixed travel speed of 10 mm/min. The force at which the particle fractures was taken as a measure of strength. The average strength of 20 particles was reported.

[0080] The caking index was measured by the following procedure: an amount of particles was kept under a pressure of about 143 kgf (2 bar applied on a surface of 70 cm.sup.2) for 24 hours at room temperature. The lump of material was then taken out and broken, the force needed for breaking the lump material was taken as measure of the caking index.

[0081] Abrasion resistance is the resistance to the formation of dust and fines as result of granule to granule and granule to equipment contact. A 100 cm3 portion of screened urea sample (between 1 mm-3.5 mm) was weighed accurately and charged to a rotary drum together with 50 steel balls of 7.9 mm diameter. The drum was closed and rotated at 60 rpm for 2.5 minutes. Thereafter the content was removed, hand screened over a 4.45 mm screen to recover the steel balls, and finally screened on a 1.00 mm screen. The material retained on the 1.00 mm screen was finally weighed accurately and degradation was calculated as follow:

[00002]$\mathrm{Degradation},\%=100-100\times \frac{1\mathrm{mm}<\mathrm{Wt}\mathrm{of}\mathrm{fraction}\mathrm{recovered}<3.5\mathrm{mm}}{1\mathrm{mm}<\mathrm{Wt}\mathrm{of}\mathrm{fraction}\mathrm{charged}<3.5\mathrm{mm}}$

[0082] Dust formation during the granulation process was measured as ratio between the result of the material balance around the granulation and collection chamber and the sprayed urea melt.

[00003]$\mathrm{Dust}\left[\%\right]=\frac{\left({\stackrel{.}{m}}_{in}.Math.{\tau }_{\mathrm{test}}+m_o\right)-m_{\mathrm{discharged}}}{{\stackrel{.}{m}}_{in}.Math.{\tau }_{test}}$

[0083] where τ.sub.test denotes the test duration.