NZ501438A

NZ501438A - Device for mixing and dissolving solid granules in a liquid, in particular for producing nitrophosphate fertilisers

Info

Publication number: NZ501438A
Application number: NZ501438A
Authority: NZ
Inventors: Laurent Limousin; Jean-Bernard Peudpiece; Mat Armand Le
Original assignee: Grande Paroisse S
Priority date: 1997-06-03
Filing date: 1998-05-22
Publication date: 2001-08-31
Also published as: FR2763867B1; EG21281A; ID23672A; EE04300B1; BG103888A; AU7775698A; CZ432799A3; JP2002502310A; GEP20012492B; OA11220A; FR2763867A1; ZA984529B; WO1998055213A1; PL337321A1; EP0988105A1; MD20000001A; EE9900554A; HUP0001895A2; IL132871A0; YU61299A

Abstract

A device for preparing a sulpho-ureic reactant for the digestion of a natural phosphate, in a plant for the production of phosphonitrogenous fertilizers, is provided. The device is a vessel of cylindrical shape with a vertical axis and a rounded bottom, an entry for the liquid, an entry for the granules, an exit for the solution, and a means of agitation. The entry for the granules is situated above the level of mixture in the vessel. The agitation is provided by two agitation means, one being a turbine with straight blades that creates entrainment of the solid granules towards the bottom of the vessel, the other being a radial turbine with straight blades located below the fist agitation means, creating a mixing to dissolve the granules. This combination of agitation allows the dissolution of granules that have a lower density than that of the solution.

Description

<div class="application article clearfix" id="description"> 10 11 WO 98/55213 PCT/FR98/01030 DEVICE FOR MIXING AND DISSOLVING SOLID GRANULES IN A LIQUID, IN PARTICULAR FOR THE PRODUCTION OF PHOSPHONITROGENOUS FERTILIZERS. The invention relates to a device for mixing and dissolving in a liquid granules or solid particles of a substance soluble in this liquid but whose density is lower than that of the liquid. The device concerned is of the kind of those which include a vessel comprising means of agitation and provided with at least one entry for the liquid, an entry for the solid granules and an exit for the solution. The invention relates more particularly, but not exclusively, to such a device for the production of 15 phosphonitrogenous fertilizers, especially in accordance with the process of EP-A-0 560 882 (WO 92 10 443). This process makes use of the digestion of a natural phosphate, such as tricalcium phosphate, with a 2 0 sulpho-ureic reactant obtained by mixing and dissolving solid granules, or prills, of urea, in a solution of sulphuric acid. This reactant for sulpho-ureic digestion is a well-defined eutectic composition whose solution, in determined proportions, it is important to 25 prepare continuously. The problem becomes difficult because solid urea in granular form has a relative density of 0.6, jr.tc-iiuc'voat r-roporty OiTiCC' of W.2 ? p c P n./ p n while the sulphuric solution in which it is appropriate to dissolve the urea granules has a relative density of 1.4. The solid granules have therefore a strong tendency to float, which does not promote either dissolving or homogenizing. It is appropriate, in addition, that this dissolving of the solid granules should be performed as quickly as possible, especially so that the volume of the vessel should not be too large for a given solution flow rate. The device of the invention aims to meet these contradictory requirements created by a density of the solid to be dissolved which is clearly lower than, in particular lower than half, the density of the liquid. According to the invention a device for mixing and dissolving in a liquid granules or solid particles of a substance soluble in this liquid but having a density lower than that of the liquid, of the kind defined above, is characterized in that the entry of the granules into the vessel is situated in a high portion, above the level of the reaction mixture in this vessel, and that the means of agitation include a first means of agitation capable of creating an entrainment of the solid granules towards the lower portion of the vessel in the reaction mixture, and a second means of agitation, situated in the vessel at a level lower than that of the first means of agitation and capable of creating in the reaction mixture a motion promoting the dissolving of the granules, in particular a shearing motion. The first and second means of agitation are preferably means of mechanical agitation. The first means of agitation advantageously consists of a turbine with inclined straight blades, also known by the name of a "sabre turbine", in particular with four blades. The second means of agitation advantageously consists of a radial turbine with straight blades, also known by the name of a "Rushton turbine", in particular with six blades. The Rushton turbine is situated below the sabre turbine, at an appropriate distance. The vessel is generally of a cylindrical shape with a vertical axis and the first and second means of agitation are advantageously mounted on the same vertical shaft, preferably coaxial with the vessel. Such a device is particularly suitable for preparing a sulpho-ureic reactant for digestion of a natural phosphate in a plant for the production of phosphonitrogenous fertilizers, according to the process of EP-A-0 560 882, in which case the granules are solid urea granules or prills and the liquid is a sulphuric solution. Apart from the arrangements set out above, the invention consists of a certain number of other arrangements which will be discussed more explicitly below in relation to a preferred example of embodiment described with reference to the appended drawings, but which does not imply any limitation. Figure 1 of these drawings is a diagram of a device in accordance with the invention. Figure 2 is a vertical axial section, with external portion, on a larger scale, of the vessel of the device of Figure 1. Figure 3 is a larger-scale plan view of the sabre turbine. Figure 4 is a larger-scale view in elevation of the Rushton turbine with six blades. Figure 5, finally, is a top view in relation to Figure 4. When reference is made to Figure 1 of the drawings, there can be seen a device 1 for mixing and dissolving in a liquid L solid granules G of a substance soluble in the liquid but having a density lower than that of the liquid. In the example considered the solid granules G are granules of urea of relative density 0.6, while the liquid L consists of a sulphuric solution of relative density 1.4. The device 1 includes a vessel 2, of cylindrical general shape, with a vertical axis and whose bottom is curved, convex outwards. The vessel 2 is closed by a removable cover 3, in the upper portion, on which there are provided: an entry 4 for the sulphuric solution L, an entry 5 for the urea granules G, with water, and an entry 6 for recycling a fraction of the reaction mixture M produced in the vessel 2. Since the dissolution of urea in sulphuric acid is exothermic, two concentric cooling coils 7a, 7b are provided in the vessel 2 (which forms a reactor operated continuously). As can be seen in Fig. 2, the coils comprise turns extending substantially over half the height of the vessel 2 and surround means of agitation A. The turns of the coils are situated radially within counterblades or baffles 8 consisting of four vertical blades, spaced at 90°, situated in planes passing through the vertical axis of the vessel 2. The external vertical ridge of the counterblades 8 is close to the inner surface of the vessel 2. The cooling provided with the aid of the coils 7a, 7b is sufficient to avoid the temperature of 90°C being exceeded in the vessel, a threshold above which the urea incorporated into the mixture M tends to decompose, as indicated in EP-A-0 560 882. The means of agitation A include a first means of agitation A1 capable of creating an entrainment of the granules G towards the lower portion of the vessel 2 in the reaction mixture M, and a second means of agitation A2 situated at a level lower than that of the first means of agitation A1. This second means A2 is capable of creating in the mixture M a motion which promotes the dissolving of the granules G, in particular a shearing motion. The first and second means of agitation Al, A2 are mechanical and consist of two turbines of different types, secured to the same drive shaft 9 coaxial with the vessel 2. The driving in rotation of the shaft 9 is ensured by driving means 10 provided above the cover 3, outside the vessel. The means of agitation Al consists of a turbine 11 with straight blades 12, the mean plane of which is inclined in relation to the vertical geometric axis of the shaft 9. As can be seen in Fig. 3, the width of the blades 12 can decrease progressively in the radial direction which moves away from the axis. The turbine 11 is situated appreciably half-way up the vessel 2. The normal level N of the reaction mixture in the vessel 2 is situated at a distance hi above the mean plane of the turbine 11. This distance hi is smaller than the distance HI from this same mean plane to the bottom of the vessel 2. The inclination of the blades 12 and the direction of rotation of the shaft 9 are such that an essentially axial and additionally radial flow is created in the reaction mixture M as shown diagrammatically by the arrows F in Fig. 2. The reaction mixture is propelled from the bottom upwards in the external radial zones and from the top downwards in the zones situated radially towards the interior. This motion makes it possible essentially to draw in the granules of solid urea arriving via the entry 5 at the top of the vessel and to entrain them into the reaction mixture. This motion also makes it possible to mix the added liquids and their reactant already present in the vessel. In the example considered the turbine 11, or sabre turbine, comprises four blades 12 spaced angularly at 90°. The root of these blades is close to the external surface of the shaft 9 and is secured to a hub 13 wedged on this shaft. The flow created by the turbine 11 can take place following a direction parallel to the shaft 9, in the vicinity of this shaft. The second means of agitation A2 includes a radial turbine 14 with straight blades which are secured to the lower end of the shaft 9 coaxially with the turbine 11. The radial turbine 14, also called a Rushton turbine, comprises six uniformly spaced blades 15 consisting of small plates situated in planes passing through the geometric axis of the shaft 9. These small plates are secured half-way up on a disc 16 which they overlap radially outwards appreciably over half of their length. A sleeve 17 is secured above the disc 16 in its centre for mounting the turbine 14 at the end of the shaft 9. The mean plane of the turbine 14, which corresponds to the mean plane of the disc 16, is nearer to the bottom of the vessel 2 than to the level N. The mean plane of the turbine 14 is preferably situated at a distance H2 from the bottom of the vessel which is between one third and one half of the total height H of the level N above the bottom of the vessel (1/3 H < H2 < H/2 ). The distance E at which the radial turbine 14 is situated below the turbine 11 is chosen so as to obtain the best dissolving and homogenizing efficiency. Owing to its rotation the turbine 14 creates a shearing motion which intersects the streams of the flow created by the upper turbine 11, streams which entrain the solid granules of urea. This shearing motion enables the urea granules to dissolve efficiently in the reactant. The motion created by the turbine 14 also allows the composition of the reactant to be maintained by intimately homogenizing the liquid and solid products added to the vessel 2. When the diameter of the disc 16 (Fig. 4) is denoted by D, a length 1 of the order of D/4 and a height b of the order of D/5 are advantageously chosen for the blades 15. The bottom of the vessel 2 comprises an exit orifice 18 provided with a strainer 19 (Fig. 2). Pumping means 2 0 are connected to the orifice 18, to extract from the vessel 2 the sulpho-ureic digesting reactant consisting of a solution of urea in a solution of sulphuric acid. A conduit 21 (Fig. 1) directs this reactant towards another unit of the mixer type (not shown), in which a natural phosphate is digested by the reactant, in accordance with the process of EP-A-0 560 882. A fraction of the reactant pumped by the means 20, a fraction whose flow rate is controlled by a valve 22, is recycled into the vessel 2, being returned via a pipe 23 to the entry 6. As illustrated in Fig. 2 the recycling entry 6 includes a pipe 24 equipped with a tipless dip-pipe. The entry 4 for the sulphuric solution L is also fitted with a dip-pipe. The vessel 2 is equipped with gauges or equivalent means (not shown) intended to control the process parameters such as temperature, composition of the reaction mixture and the like. The driving means 10 include a reduction gear whose external bulk can be seen in Fig. 2, while the drive by a motor and belts is shown symbolically at 10a. On the same Fig. 2 a vent V can be seen in the top part of the vessel. In order to be secured, the vessel 2 comprises supports 25 placed in the top part of its outer cylindrical wall, slightly below the cover 3 and spaced at regular intervals. The external diameter of the blades of the radial turbine 14 is smaller than the external diameter of the blades 12 of the sabre turbine 11, and in - 10 - particular is approximately equal to 2/3 of this diameter. By way of nonlimiting example of embodiment, the external diameter of the blades of the turbine 14 is approximately 1 m, while that of the blades of the turbine 12 is approximately 1.5 m. The internal diameter of the vessel 2 is approximately 3 m. The counterblades 8 project inwards over a radial distance of approximately 0.3 m, and the two coils 7b, 7a have a coil turn diameter of approximately 1.8 m and 2 m. The height of the vessel from the bottom to the base of the cover is approximately 3.7m, the normal level N being approximately 3.25 m above the bottom. The operation of the device follows from the above exp1anat i on s. In normal running, the sulphuric acid is delivered via the entry 4 and the solid granules of urea, with water, are delivered via the entry 5 into the vessel 2, from the top, in specific proportions. A fraction of the reactant pumped at the exit 18 is reinjected via the entry 6 in the top portion of the vessel. The sabre turbine 11 driven in rotation by the shaft 9 creates the axial motion, already referred to, of the reactant, and this makes it possible to draw the solid granules into the reaction mixture and to prevent these granules from remaining at the surface because of their markedly lower relative density. This - 11 - motion also makes it possible to mix the added liquids and the reaction mixture already present. In the event of incident on this turbine 11 the granules of solid urea float at the surface of the 5 much denser reactant and there is no longer any production of the reactant with the desired composition. A first consequence of such an incident is a rise in the level N in the vessel with a stoppage of the production. 10 The lower turbine 14 with radial blades creates a shearing motion which makes it possible to dissolve efficiently in the reaction mixture the solid granules entrained downwards by the motion due to the turbine 11. This turbine 14 also allows the composition 15 of the reactant to be maintained by intimately homogenizing the liquid and solid products added to the vessel 2 . In the case of incident on this turbine 14 the solid granules of urea are no longer dissolved 2 0 completely and then form thick and viscous blocks which can plug the suction strainer 19 of the pump 22 and/or cause the level N in the vessel to rise, resulting in a stoppage of the production there as well. In both cases of incident on the turbine 11 25 and/or on the turbine 14, a loss of equilibrium of the heat balances is produced and a change in the activity of the reactant is also produced, with very serious consequences to the finished product (stoppage of the - 12 - production, product which does not meet the desired assay, etc). The example and the counterexamples which follow contribute to making the above comments more precise. EXAMPLE 1100 litres of 92% sulphuric acid are introduced into a reactor in accordance with the above description, of 10,000 litre capacity. The stirring is then started at 50 rev/min. 4.0685 tons of urea are introduced while the temperature of the mixture is maintained below 80°C by cooling (in order not to convert the urea to biuret). When the operation is finished the temperature control at 65°C (to within 2°C) is started. 178.3 litres of water are then introduced m order to obtain a "base stock" consisting of the reactant as defined above: 3.6 moles of urea -1 mole of sulphuric acid - 1 mole of water. Into the same reactor there are then introduced conjointly 6.7315 t/h of urea granules, 1820 1/h of sulphuric acid and 295.38373 1/h of water. As soon as a vessel filling level of 7500 litres is reached, 7.312 m3/h of reactant can be exported towards the phosphate digestion vessel. On an Israeli phosphate ("ZIN containing 31.1 % of P205") it is then possible to produce 15.35 tons/hour of phosphonitrogenous fertilizer (finished product also denoted by the acronym USP) by mixing 5 t/h of phosphate with 7.312 m3/h of reactant - 13 - (that is 10.35 t/h of reactant at 65°C) in accordance with the teaching of Patent Application EP-A-0 560 882, or WO 92 10443. COUNTEREXAMPLES 1. The above reactor is fitted with a single sabre blade placed at a distance HI from the bottom of the vessel equal to 1/3 (one third) of the height of the vessel. This blade ensures a motion of the liquid from the bottom upwards. This motion allows the granules (prills) of urea to circulate from the surface of the mixture towards the interior of the vessel. However, bearing in mind the kinetics of dissolving of these same granules in a sulpho-ureic reactant as defined above, it becomes necessary to increase the residence time of these granules in the vessel and hence to lower the production throughputs (and hence the introduction of the raw materials) of the finished product (USP). Pilot trials have shown that if this operation of reduction in rates is not performed, undissolved granules enter the feed liquid of the reactant/phosphate mixer and/or will block the pump suction. In all cases, since the reactant is no longer in the chemical equilibrium conditions as described in EP-A-0 560 882 (WO 92 10443), there is a change m the degree of digestion of the phosphate and hence a finished product not in accordance with the subject-matter of the patent application. - 14 - It should be noted that it is industrially not possible to increase the size of the vessel (heat balance, size of the pump, stirrer size, etc). 2. The above reactor is equipped with a single Rushton turbine fitted at a height H2 equal to half the height of the reactor. It does not create enough vertical motion to make the urea granules flow into the mixture; the operation therefore stops at the "base stock" stage without allowing the reactant to form. The urea becomes wet and then agglomerates into a ball which dissolves only after a very long time (when the pump suction is not blocked first). The above explanations show that the combination of the two turbines 11 and 14 mounted coaxially on the same shaft, according to the special placing of the invention, makes it possible to obtain results which are particularly advantageous for the dissolving of a solid product in a liquid of density which is markedly higher than that of the solid product. 15 </div>

Claims

<div class="application article clearfix printTableText" id="claims"> CLAIMS

1. Device for preparing a sulpho-ureic reactant for digestion of a natural phosphate, m a plant for the production of phosphonitrogenous fertilizers, by mixing and dissolving in a liquid (L) formed by a sulphuric solution / of urea (G) solid granules whose density is lower than that of the liquid, including a vessel of cylindrical shape with a vertical axis and with a bottom, comprising means of agitation (A) and provided with at least one entry for the liquid (L), an entry for the solid granules, and an exit for the solution, wherein the entry of the granules into the vessel is situated in a high portion, above the level of the mixture in this vessel, and wherein the means of agitation (A) include a first means of agitation (Al) capable of creating an entrainment of the solid granules (G) towards the bottom of the vessel, and a second means of agitation (A2) situated m the vessel at a level lower than that of the first means of agitation (Al) and capable of creating in the mixture a motion promoting the dissolving of the granules.

2 . Device according to claim 1 wherein the motion promoting the dissolving of the granules is a shearing motion.

3. Device according to Claim 1, wherein the first and second means of agitation (A1,A2) are means of mechanical agitation.

4. Device according to Claim 3, wherein the first means of agitation (Al) consists of a turbine with inclined straight blades, also known by the name of a "sabre turbine". Qfrcc- c\ - 5 JUL 2Q0I L RECEIVED U ' ' - v b ' < - 16 -

5. Device according to Claim 4, wherein the turbine comprises four blades.

6. Device according to either Claim 4 or Claim 5, wherein the second means of agitation (A2) consists of a radial turbine with straight blades, also known by the name of a "Rushton turbine".

7. Device according to Claim 6, wherein the radial turbine with straight blades comprises six blades.

8. Device according to Claims 4 and 6, wherein the two turbines are mounted on the same vertical shaft, the radial turbine being situated below the sabre turbine, at an appropriate distance (E) .

9. Device according to Claim 8, wherein the external diameter of the blades of the radial turbine is smaller than the external diameter of the blades of the sabre turbine.

10. Device according to either Claim 8 or Claim 9 wherein the external diameter of the blades of the radial turbine is approximately 2/3 of the diameter of the blades of the sabre turbine.

11. Device according to Claim 8, wherein the mean plane of the radial turbine is situated at a distance (H2) from the bottom of the vessel of between one third and one half of the total height (H) of the level (N) above the bottom of the vessel (1/3 H ^ H2 -H/2).

12. Device according to one of the preceding claims, wherein an entry is provided in the vessel cover for recycling a fraction of the reaction mixture (M) produced m the vessel. Property QhiC'r of i-'LZ - 5 JUL 2001 j RECE1 V E D - 17 -

13. Device for preparing a sulpho-ureic reactant for digestion of a natural phosphate, substantially as herein described with reference to any embodiment thereof as shown m the accompanying drawings. </div>