Process for producing water-absorbent polymer particles by polymerizing droplets of a monomer solution

Abstract

The present invention relates to a process for producing water-absorbent polymer particles by polymerizing droplets of a monomer solution comprising less than 0.3% by weight of persulfate and at least 0.05% by weight of azo initiator and thermal aftertreatment of the formed polymer particles at less than 100° C. in a fluidized bed for 60 to 300 minutes.

Claims

1. A process for producing water-absorbent polymer particles by polymerizing droplets of a) at least one ethylenically unsaturated monomer which bears an acid group and optionally is at least partly neutralized, (b) at least one crosslinker, c.sub.l) from 0.01 to less than 0.3% by weight, based on monomer a), of at least one persulfate, c.sub.2) at least 0.05% by weight, based on monomer a), of at least one azo initiator, d) optionally one or more ethylenically unsaturated monomer copolymerizable with the monomer mentioned under a), e) optionally one or more water-soluble polymer, and f) water, in a surrounding heated gas phase in a reaction zone and thermal posttreatment in a fluidized bed, wherein a temperature of a gas leaving the reaction zone is less than 150° C., a temperature of the water-absorbent polymer particles during the thermal posttreatment is less than 100 ° C., and a residence time of the water-absorbent polymer particles in the fluidized bed is from 60 to 300 minutes.

2. A process according to claim 1, wherein the heated gas phase is flowing cocurrent to the droplets through the reaction zone.

3. A process according to claim 1, wherein the monomer solution comprises from 0.05 to 0.15% by weight, based on monomer a), of the at least one persulfate.

4. A process according to claim 1, wherein the monomer solution comprises from 0.2 to 0.5% by weight, based on monomer a), of the at least one azo initiator.

5. A process according to claim 1, wherein the temperature of the gas leaving the reaction zone is from 110 to 120° C.

6. A process according to claim 1, wherein the temperature of the gas entering the reaction zone is from 160 to 200° C.

7. A process according to claim 1, wherein a gas velocity inside the reaction zone is from 0.1 to 2.5 m/s.

8. A process according to claim 1, wherein the temperature of the water-absorbent polymer particles during the thermal posttreatment is from 60 to 80° C.

9. A process according to claim 1, wherein the residence time of the water-absorbent polymer particles in the fluidized bed is from 120 to 240 minutes.

10. A process according to claim 1, wherein a gas velocity inside the fluidized bed is from 0.3 to 2.5 m/s.

11. A process according to claim 1, wherein the gas entering the fluidized bed comprises from 0.02 to 0.15 kg steam per kg of dry gas.

12. A process according to claim 1, wherein the ethylenically unsaturated monomer which bears an acid group is an ethylenically unsaturated carboxylic acid.

13. A process according to claim 1, wherein the ethylenically unsaturated monomer which bears an acid group is acrylic acid.

14. A process according to claim 1, wherein the monomer solution comprises from 0.1 to 0.15% by weight, based on monomer a) of the persulfate.

Description

(1) Preferred embodiments are depicted in FIGS. 1 to 12.

(2) FIG. 1: Process scheme (without external fluidized bed)

(3) FIG. 2: Process scheme (with external fluidized bed)

(4) FIG. 3: Arrangement of the T_outlet measurement

(5) FIG. 4: Arrangement of the dropletizer units

(6) FIG. 5: Dropletizer unit (longitudinal cut)

(7) FIG. 6: Dropletizer unit (cross sectional view)

(8) FIG. 7: Bottom of the internal fluidized bed (top view)

(9) FIG. 8: openings in the bottom of the internal fluidized bed

(10) FIG. 9: Rake stirrer for the intern fluidized bed (top view)

(11) FIG. 10: Rake stirrer for the intern fluidized bed (cross sectional view)

(12) FIG. 11: Process scheme (surface-postcrosslinking)

(13) FIG. 12: Process scheme (surface-postcrosslinking and coating)

(14) The reference numerals have the following meanings: 1 Drying gas inlet pipe 2 Drying gas amount measurement 3 Gas distributor 4 Dropletizer units 5 Cocurrent spray dryer, cylindrical part 6 Cone 7 T_outlet measurement 8 Tower offgas pipe 9 Baghouse filter 10 Ventilator 11 Quench nozzles 12 Condenser column, counter current cooling 13 Heat exchanger 14 Pump 15 Pump 16 Water outlet 17 Ventilator 18 Offgas outlet 19 Nitrogen inlet 20 Heat exchanger 21 Ventilator 22 Heat exchanger 23 Steam injection via nozzles 24 Water loading measurement 25 Conditioned internal fluidized bed gas 26 Internal fluidized bed product temperature measurement 27 Internal fluidized bed 28 Rotary valve 29 Sieve 30 End product 31 Static mixer 32 Static mixer 33 Initiator feed 34 Initiator feed 35 Monomer feed 36 Fine particle fraction outlet to rework 37 External fluidized bed 38 Ventilator 39 External fluidized bed offgas outlet to baghouse filter 40 Rotary valve 41 Filtered air inlet 42 Ventilator 43 Heat exchanger 44 Steam injection via nozzle 45 Water loading measurement 46 Conditioned external fluidized bed gas 47 T_outlet measurement (average temperature out of 3 measurements around tower circumference) 48 Dropletizer unit 49 Monomer premixed with initiator feed 50 Spray dryer tower wall 51 Dropletizer unit outer pipe 52 Dropletizer unit inner pipe 53 Dropletizer cassette 54 Teflon block 55 Valve 56 Monomer premixed with initiator feed inlet pipe connector 57 Droplet plate 58 Counter plate 59 Flow channels for temperature control water 60 Dead volume free flow channel for monomer solution 61 Dropletizer cassette stainless steel block 62 Bottom of the internal fluidized bed with four segments 63 Split openings of the segments 64 Rake stirrer 65 Prongs of the rake stirrer 66 Mixer 67 Optional coating feed 68 Postcrosslinker feed 69 Thermal dryer (surface-postcrosslinking) 70 Cooler 71 Optional coating/water feed 72 Coater 73 Coating/water feed

(15) The drying gas is fed via a gas distributor (3) at the top of the spray dryer as shown in FIG. 1. The drying gas is partly recycled (drying gas loop) via a baghouse filter (9) and a condenser column (12). The pressure inside the spray dryer is below ambient pressure.

(16) The spray dryer outlet temperature is preferably measured at three points around the circumference at the end of the cylindrical part as shown in FIG. 3. The single measurements (47) are used to calculate the average cylindrical spray dryer outlet temperature.

(17) The product accumulated in the internal fluidized bed (27). Conditioned internal fluidized bed gas is fed to the internal fluidized bed (27) via line (25). The relative humidity of the internal fluidized bed gas is preferably controlled by adding steam via line (23).

(18) The spray dryer offgas is filtered in baghouse filter (9) and sent to a condenser column (12) for quenching/cooling. After the baghouse filter (9) a recuperation heat exchanger system for preheating the gas after the condenser column (12) can be used. The baghouse filter (9) may be trace-heated on a temperature of preferably from 80 to 180° C., more preferably from 90 to 150° C., most preferably from 100 to 140° C. Excess water is pumped out of the condenser column (12) by controlling the (constant) filling level inside the condenser column (12). The water inside the condenser column (12) is cooled by a heat exchanger (13) and pumped counter-current to the gas via quench nozzles (11) so that the temperature inside the condenser column (12) is preferably from 20 to 100° C., more preferably from 25 to 80° C., most preferably from 30 to 60° C. The water inside the condenser column (12) is set to an alkaline pH by dosing a neutralizing agent to wash out vapors of monomer a). Aqueous solution from the condenser column (12) can be sent back for preparation of the monomer solution.

(19) The condenser column offgas is split to the drying gas inlet pipe (1) and the conditioned internal fluidized bed gas (25). The gas temperatures are controlled via heat exchangers (20) and (22). The hot drying gas is fed to the cocurrent spray dryer via gas distributor (3). The gas distributor (3) consists preferably of a set of plates providing a pressure drop of preferably 1 to 100 mbar, more preferably 2 to 30 mbar, most preferably 4 to 20 mbar, depending on the drying gas amount. Turbulences and/or a centrifugal velocity can also be introduced into the drying gas if desired by using gas nozzles or baffle plates.

(20) Conditioned internal fluidized bed gas is fed to the internal fluidized bed (27) via line (25). The relative humidity of the external fluidized bed gas is preferably controlled by adding steam via line (23). To prevent any condensation the steam is added together with the internal fluidized bed into the heat exchanger (22). The product holdup in the internal fluidized bed (27) can be controlled via rotational speed of the rotary valve (28).

(21) The product is discharged from the internal fluidized bed (27) via rotary valve (28). The product holdup in the internal fluidized bed (27) can be controlled via rotational speed of the rotary valve (28). The sieve (29) is used for sieving off overs/lumps.

(22) The monomer solution is preferably prepared by mixing first monomer a) with a neutralization agent and secondly with crosslinker b). The temperature during neutralization is controlled to preferably from 5 to 60° C., more preferably from 8 to 40° C., most preferably from 10 to 30° C., by using a heat exchanger and pumping in a loop. A filter unit is preferably used in the loop after the pump. The initiators are metered into the monomer solution upstream of the dropletizer by means of static mixers (31) and (32) via lines (33) and (34) as shown in FIG. 1. Preferably a peroxide solution having a temperature of preferably from 5 to 60° C., more preferably from 10 to 50° C., most preferably from 15 to 40° C., is added via line (33) and preferably an azo initiator solution having a temperature of preferably from 2 to 30° C., more preferably from 3 to 15° C., most preferably from 4 to 8° C., is added via line (34). Each initiator is preferably pumped in a loop and dosed via control valves to each dropletizer unit. A second filter unit is preferably used after the static mixer (32). The mean residence time of the monomer solution admixed with the full initiator package in the piping before the droplet plates (57) is preferably less than 60 s, more preferably less than 30 s, most preferably less than 10 s.

(23) For dosing the monomer solution into the top of the spray dryer preferably three dropletizer units are used as shown in FIG. 4. However, any number of dropletizers can be used that is required to optimize the throughput of the process and the quality of the product. Hence, in the present invention at least one dropletizer is employed, and as many dropletizers as geometrically allowed may be used.

(24) A dropletizer unit consists of an outer pipe (51) having an opening for the dropletizer cassette (53) as shown in FIG. 5. The dropletizer cassette (53) is connected with an inner pipe (52). The inner pipe (53) having a PTFE block (54) at the end as sealing can be pushed in and out of the outer pipe (51) during operation of the process for maintenance purposes.

(25) The temperature of the dropletizer cassette (61) is controlled to preferably 5 to 80° C., more preferably 10 to 70° C., most preferably 30 to 60° C., by water in flow channels (59) as shown in FIG. 6.

(26) The dropletizer cassette has preferably from 10 to 1500, more preferably from 50 to 1000, most preferably from 100 to 500, bores having a diameter of preferably from 50 to 500 μm, more preferably from 100 to 300 μm, most preferably from 150 to 250 μm. The bores can be of circular, rectangular, triangular or any other shape. Circular bores are preferred. The ratio of bore length to bore diameter is preferably from 0.5 to 10, more preferably from 0.8 to 5, most preferably from 1 to 3. The droplet plate (57) can have a greater thickness than the bore length when using an inlet bore channel. The droplet plate (57) is preferably long and narrow as disclosed in WO 2008/086976 A1. Multiple rows of bores per droplet plate can be used, preferably from 1 to 20 rows, more preferably from 2 to 5 rows.

(27) The dropletizer cassette (61) consists of a flow channel (60) having essential no stagnant volume for homogeneous distribution of the premixed monomer and initiator solutions and two droplet plates (57). The droplet plates (57) have an angled configuration with an angle of preferably from 1 to 90°, more preferably from 3 to 45°, most preferably from 5 to 20°. Each droplet plate (57) is preferably made of a heat and/or chemically resistant material, such as stainless steel, polyether ether ketone, polycarbonate, polyarylsulfone, such as polysulfone, or polyphenylsulfone, or fluorous polymers, such as perfluoroalkoxyethylene, polytetrafluoroethylene, polyvinylidenfluorid, ethylene-chlorotrifluoroethylene copolymers, ethylene-tetrafluoroethylene copolymers and fluorinated polyethylene. Coated droplet plates as disclosed in WO 2007/031441 A1 can also be used. The choice of material for the droplet plate is not limited except that droplet formation must work and it is preferable to use materials which do not catalyze the start of polymerization on its surface.

(28) The throughput of monomer including initiator solutions per dropletizer unit is preferably from 150 to 2500 kg/h, more preferably from 200 to 1000 kg/h, most preferably from 300 to 600 kg/h. The throughput per bore is preferably from 0.1 to 10 kg/h, more preferably from 0.5 to 5 kg/h, most preferably from 0.7 to 2 kg/h.

(29) The start-up of the cocurrent spray dryer (5) can be done in the following sequence: starting the condenser column (12), starting the ventilators (10) and (17), starting the heat exchanger (20), heating up the drying gas loop up to 95° C., starting the nitrogen feed via the nitrogen inlet (19), waiting until the residual oxygen is below 4% by weight, heating up the drying gas loop, at a temperature of 105° C. starting the water feed (not shown) and at target temperature stopping the water feed and starting the monomer feed via dropletizer unit (4)

(30) The shut-down of the cocurrent spray dryer (5) can be done in the following sequence: stopping the monomer feed and starting the water feed (not shown), shut-down of the heat exchanger (20), cooling the drying gas loop via heat exchanger (13), at a temperature of 105° C. stopping the water feed, at a temperature of 60° C. stopping the nitrogen feed via the nitrogen inlet (19) and feeding air into the drying gas loop (not shown)

(31) To prevent damages the cocurrent spray dryer (5) must be heated up and cooled down very carefully. Any quick temperature change must be avoided.

(32) The openings in the bottom of the internal fluidized bed may be arranged in a way that the water-absorbent polymer particles flow in a cycle as shown in FIG. 7. The bottom shown in FIG. 7 comprises of four segments (62). The openings (63) in the segments (62) are in the shape of slits that guides the passing gas stream into the direction of the next segment (62). FIG. 8 shows an enlarged view of the openings (63).

(33) The opening may have the shape of holes or slits. The diameter of the holes is preferred from 0.1 to 10 mm, more preferred from 0.2 to 5 mm, most preferred from 0.5 to 2 mm. The slits have a length of preferred from 1 to 100 mm, more preferred from 2 to 20 mm, most preferred from 5 to 10 mm, and a width of preferred from 0.5 to 20 mm, more preferred from 1 to 10 mm, most preferred from 2 to 5 mm.

(34) FIG. 9 and FIG. 10 show a rake stirrer (64) that may be used in the internal fluidized bed. The prongs (65) of the rake have a staggered arrangement. The speed of rake stirrer is preferably from 0.5 to 20 rpm, more preferably from 1 to 10 rpm most preferably from 2 to 5 rpm.

(35) For start-up the internal fluidized bed may be filled with a layer of water-absorbent polymer particles, preferably 5 to 50 cm, more preferably from 10 to 40 cm, most preferably from 15 to 30 cm.

(36) Water-absorbent Polymer Particles

(37) The present invention further provides water-absorbent polymer particles obtainable by the process according to the invention.

(38) The inventive water-absorbent polymer particles have a mean sphericity from preferably from 0.80 to 0.95, more preferably from 0.82 to 0.93, even more preferably from 0.84 to 0.91, most preferably from 0.85 to 0.90. The sphericity (SPHT) is defined as

(39) $SPHT=\frac{4\pi A}{U^2},$
where A is the cross-sectional area and U is the cross-sectional circumference of the polymer particles. The mean sphericity is the volume-average sphericity.

(40) The mean sphericity can be determined, for example, with the Camsizer® image analysis system (Retsch Technology GmbH; Haan; Germany):

(41) For the measurement, the product is introduced through a funnel and conveyed to the falling shaft with a metering channel. While the particles fall past a light wall, they are recorded selectively by a camera. The images recorded are evaluated by the software in accordance with the parameters selected.

(42) To characterize the roundness, the parameters designated as sphericity in the program are employed. The parameters reported are the mean volume-weighted sphericities, the volume of the particles being determined via the equivalent diameter xc.sub.min. To determine the equivalent diameter xc.sub.min, the longest chord diameter for a total of 32 different spatial directions is measured in each case. The equivalent diameter xc.sub.min is the shortest of these 32 chord diameters. To record the particles, the so-called CCD-zoom camera (CAM-Z) is used. To control the metering channel, a surface coverage fraction in the detection window of the camera (transmission) of 0.5% is predefined.

(43) Water-absorbent polymer particles with relatively low sphericity are obtained by reverse suspension polymerization when the polymer beads are agglomerated during or after the polymerization.

(44) The water-absorbent polymer particles prepared by customary solution polymerization (gel polymerization) are ground and classified after drying to obtain irregular polymer particles. The mean sphericity of these polymer particles is between approx. 0.72 and approx. 0.78.

(45) The inventive water-absorbent polymer particles have a content of hydrophobic solvent of preferably less than 0.005% by weight, more preferably less than 0.002% by weight and most preferably less than 0.001% by weight. The content of hydrophobic solvent can be determined by gas chromatography, for example by means of the headspace technique. A hydrophobic solvent within the scope of the present invention is either immiscible in water or only sparingly miscible. Typical examples of hydrophobic solvents are pentane, hexane, cyclohexane, toluene.

(46) Water-absorbent polymer particles which have been obtained by reverse suspension polymerization still comprise typically approx. 0.01% by weight of the hydrophobic solvent used as the reaction medium.

(47) The inventive water-absorbent polymer have a dispersant content of preferably less than 0.5% by weight, more preferably less than 0.1% by weight and most preferably less than 0.05% by weight.

(48) Water-absorbent polymer particles which have been obtained by reverse suspension polymerization still comprise typically at least 1% by weight of the dispersant, i.e. ethylcellulose, used to stabilize the suspension.

(49) The inventive water-absorbent polymer particles have a bulk density preferably from 0.6 to 1 g/cm.sup.3, more preferably from 0.65 to 0.9 g/cm.sup.3, most preferably from 0.68 to 0.8 g/cm.sup.3.

(50) The average particle diameter (APD) of the inventive water-absorbent polymer particles is preferably from 200 to 550 μm, more preferably from 250 to 500 μm, most preferably from 350 to 450 μm.

(51) The particle diameter distribution (PDD) of the inventive water-absorbent polymer particles is preferably less than 0.7, more preferably less than 0.65, more preferably less than 0.6.

(52) The inventive water-absorbent polymer particles have a centrifuge retention capacity (CRC) of preferably from 35 to 100 g/g, more preferably from 40 to 80 g/g, most preferably from 45 to 60 g/g.

(53) The inventive water-absorbent polymer particles have a HC 60 value of preferably at least 80, more preferably of at least 85, most preferably of at least 90.

(54) The inventive water-absorbent polymer particles have an absorbency under a load of 21.0 g/cm.sup.2 (AUL) of preferably from 15 to 60 g/g, more preferably from 20 to 50 g/g, most preferably from 25 to 40 g/g.

(55) The level of extractable constituents of the inventive water-absorbent polymer particles is preferably from 0.1 to 30% by weight, more preferably from 0.5 to 25% by weight, most preferably from 1 to 20% by weight.

(56) The inventive water-absorbent polymer particles can be mixed with other water-absorbent polymer particles prepared by other processes, i.e. solution polymerization.

(57) Fluid-absorbent Articles

(58) The present invention further provides fluid-absorbent articles. The fluid-absorbent articles comprise of (A) an upper liquid-pervious layer (B) a lower liquid-impervious layer (C) a fluid-absorbent core between (A) and (B) comprising from 5 to 90% by weight fibrous material and from 10 to 95% by weight water-absorbent polymer particles of the present invention; preferably from 20 to 80% by weight fibrous material and from 20 to 80% by weight water-absorbent polymer particles of the present invention; more preferably from 30 to 75% by weight fibrous material and from 25 to 70% by weight water-absorbent polymer particles of the present invention; most preferably from 40 to 70% by weight fibrous material and from 30 to 60% by weight water-absorbent polymer particles of the present invention; (D) an optional acquisition-distribution layer between (A) and (C), comprising from 80 to 100% by weight fibrous material and from 0 to 20% by weight water-absorbent polymer particles of the present invention; preferably from 85 to 99.9% by weight fibrous material and from 0.01 to 15% by weight water-absorbent polymer particles of the present invention; more preferably from 90 to 99.5% by weight fibrous material and from 0.5 to 10% by weight water-absorbent polymer particles of the present invention; most preferably from 95 to 99% by weight fibrous material and from 1 to 5% by weight water-absorbent polymer particles of the present invention; (E) an optional tissue layer disposed immediately above and/or below (C); and (F) other optional components.

(59) Fluid-absorbent articles are understood to mean, for example, incontinence pads and incontinence briefs for adults or diapers for babies. Suitable fluid-absorbent articles including fluid-absorbent compositions comprising fibrous materials and optionally water-absorbent polymer particles to form fibrous webs or matrices for the substrates, layers, sheets and/or the fluid-absorbent core.

(60) Suitable fluid-absorbent articles are composed of several layers whose individual elements must show preferably definite functional parameter such as dryness for the upper liquid-pervious layer, vapor permeability without wetting through for the lower liquid-impervious layer, a flexible, vapor permeable and thin fluid-absorbent core, showing fast absorption rates and being able to retain highest quantities of body fluids, and an acquisition-distribution layer between the upper layer and the core, acting as transport and distribution layer of the discharged body fluids. These individual elements are combined such that the resultant fluid-absorbent article meets overall criteria such as flexibility, water vapor breathability, dryness, wearing comfort and protection on the one side, and concerning liquid retention, rewet and prevention of wet through on the other side. The specific combination of these layers provides a fluid-absorbent article delivering both high protection levels as well as high comfort to the consumer.

(61) The products as obtained by the present invention are also very suitable to be incorporated into low-fluff, low-fiber, fluff-less, or fiber-less hygiene article designs. Such designs and methods to make them are for example described in the following publications and literature cited therein and are expressly incorporated into the present invention: WO 2010/133529 A2, WO 2011/084981 A1, US 2011/0162989, US 2011/0270204, WO 2010/082373 A1, WO 2010/143635 A1, U.S. Pat. No. 6,972,011, WO 2012/048879 A1, WO 2012/052173 A1 and WO 2012/052172 A1.

(62) The present invention further provides fluid-absorbent articles, comprising water-absorbent polymer particles of the present invention and less than 15% by weight fibrous material and/or adhesives in the absorbent core.

(63) The water-absorbent polymer particles and the fluid-absorbent articles are tested by means of the test methods described below.

(64) Methods:

(65) The measurements should, unless stated otherwise, be carried out at an ambient temperature of 23±2° C. and a relative atmospheric humidity of 50±10%. The water-absorbent polymers are mixed thoroughly before the measurement.

(66) Free Swell Rate (FSR)

(67) 1.00 g (=W1) of the dry water-absorbent polymer particles is weighed into a 25 ml glass beaker and is uniformly distributed on the base of the glass beaker. 20 ml of a 0.9% by weight sodium chloride solution are then dispensed into a second glass beaker, the content of this beaker is rapidly added to the first beaker and a stopwatch is started. As soon as the last drop of salt solution is absorbed, confirmed by the disappearance of the reflection on the liquid surface, the stopwatch is stopped. The exact amount of liquid poured from the second beaker and absorbed by the polymer in the first beaker is accurately determined by weighing back the second beaker (=W2). The time needed for the absorption, which was measured with the stopwatch, is denoted t. The disappearance of the last drop of liquid on the surface is defined as time t.

(68) The free swell rate (FSR) is calculated as follows:
FSR [g/gs]=W2/(W1×t)

(69) When the moisture content of the hydrogel-forming polymer is more than 3% by weight, however, the weight W1 must be corrected for this moisture content.

(70) Vortex

(71) 50.0±1.0 ml of 0.9% NaCl solution are added into a 100 ml beaker. A cylindrical stirrer bar (30×6 mm) is added and the saline solution is stirred on a stir plate at 60 rpm. 2.000±0.010 g of water-absorbent polymer particles are added to the beaker as quickly as possible, starting a stop watch as addition begins. The stopwatch is stopped when the surface of the mixture becomes “still” that means the surface has no turbulence, and while the mixture may still turn, the entire surface of particles turns as a unit. The displayed time of the stopwatch is recorded as Vortex time.

(72) Residual Monomers

(73) The level of residual monomers in the water-absorbent polymer particles is determined by the EDANA recommended test method No. WSP 410.2-05 “Residual Monomers”.

(74) Particle Size Distribution

(75) The particle size distribution of the water-absorbent polymer particles is determined with the Camziser® image analysis system (Retsch Technology GmbH; Haan; Germany).

(76) For determination of the average particle diameter and the particle diameter distribution the proportions of the particle fractions by volume are plotted in cumulated form and the average particle diameter is determined graphically.

(77) The average particle diameter (APD) here is the value of the mesh size which gives rise to a cumulative 50% by weight.

(78) The particle diameter distribution (PDD) is calculated as follows:

(79) $PDD=\frac{x_2-x_1}{APD},$
wherein x.sub.1 is the value of the mesh size which gives rise to a cumulative 90% by weight and x.sub.2 is the value of the mesh size which gives rise to a cumulative 10% by weight.
Mean Sphericity

(80) The mean sphericity is determined with the Camziser® image analysis system (Retsch Technology GmbH; Haan; Germany) using the particle diameter fraction from 100 to 1,000 μm.

(81) Moisture Content

(82) The moisture content of the water-absorbent polymer particles is determined by the EDANA recommended test method No. WSP 430.2-05 “Moisture Content”.

(83) Centrifuge Retention Capacity (CRC)

(84) The centrifuge retention capacity of the water-absorbent polymer particles is determined by the EDANA recommended test method No. WSP 441.2-05 “Centrifuge Retention Capacity”, wherein for higher values of the centrifuge retention capacity larger tea bags have to be used.

(85) Absorbency Under No Load (AUNL)

(86) The absorbency under no load of the water-absorbent polymer particles is determined analogously to the EDANA recommended test method No. WSP 442.2-05 “Absorption Under Pressure”, except using a weight of 0.0 g/cm.sup.2 instead of a weight of 21.0 g/cm.sup.2.

(87) Absorbency Under Load (AUL)

(88) The absorbency under load of the water-absorbent polymer particles is determined by the EDANA recommended test method No. WSP 442.2-05 “Absorption Under Pressure”.

(89) Absorbency Under High Load (AUHL)

(90) The absorbency under high load of the water-absorbent polymer particles is determined analogously to the EDANA recommended test method No. WSP 442.2-05 “Absorption Under Pressure”, except using a weight of 49.2 g/cm.sup.2 instead of a weight of 21.0 g/cm.sup.2.

(91) Bulk Density

(92) The bulk density of the water-absorbent polymer particles is determined by the EDANA recommended test method No. WSP 460.2-05 “Density”.

(93) Extractables

(94) The level of extractable constituents in the water-absorbent polymer particles is determined by the EDANA recommended test method No. WSP 470.2-05 “Extractables”.

(95) Color Value (CIE Color Numbers [L, a, b])

(96) Measurement of the color value is done by means of a colorimeter model “LabScan XE S/N LX17309” (HunterLab; Reston; U.S.A.) according to the CIELAB procedure (Hunterlab, Volume 8, 1996, Issue 7, pages 1 to 4). Colors are described by the coordinates L, a, and b of a three-dimensional system. L characterizes the brightness, whereby L=0 is black and L=100 is white. The values for a and b describe the position of the color on the color axis red/green resp. yellow/blue, whereby positive a values stand for red colors, negative a values for green colors, positive b values for yellow colors, and negative b values for blue colors. The HC60 value is calculated according to the formula HC60=L−3b.

(97) The measurement of the color value is in agreement with the tristimulus method according to DIN 5033-6.

(98) The EDANA test methods are obtainable, for example, from the EDANA, Avenue Eugène Plasky 157, B-1030 Brussels, Belgium.

EXAMPLES

Preparation of the Base Polymer

Example 1

(99) The process was performed in a concurrent spray drying plant with an integrated fluidized bed (27) and an external fluidized bed (29) as shown in FIG. 1. The cylindrical part of the spray dryer (5) had a height of 22 m and a diameter of 3.4 m. The internal fluidized bed (IFB) had a diameter of 3 m and a weir height of 0.25 m.

(100) The drying gas was fed via a gas distributor (3) at the top of the spray dryer. The drying gas was partly recycled (drying gas loop) via a baghouse filter (9) and a condenser column (12). Instead of the baghouse filter (9) any other filter and/or cyclone can be used. The drying gas was nitrogen that comprises from 1% to 4% by volume of residual oxygen: Before start of polymerization the drying gas loop was filled with nitrogen until the residual oxygen was below 4% by volume. The gas velocity of the drying gas in the cylindrical part of the spray dryer (5) was 0.79 m/s. The pressure inside the spray dryer was 4 mbar below ambient pressure.

(101) The spray dryer outlet temperature was measured at three points around the circumference at the end of the cyclindrical part as shown in FIG. 3. Three single measurements (47) were used to calculate the average cylindrical spray dryer outlet temperature. The drying gas loop was heated up and the dosage of monomer solution is started up. From this time the spray dryer outlet temperature was controlled to 118° C. by adjusting the gas inlet temperature via the heat exchanger (20). The gas inlet temperature was 168° C. and the steam content of the drying gas was 0.058 kg steam per kg dry gas.

(102) The product accumulated in the internal fluidized bed (27) until the weir height was reached. Conditioned internal fluidized bed gas having a temperature of 120° C. and a steam content of 0.058 kg steam per kg dry gas was fed to the internal fluidized bed (27) via line (25). The gas velocity of the internal fluidized bed gas in the internal fluidized bed (27) was 0.65 m/s. The residence time of the product was 150 min. The temperature of the water-absorbent polymer particles in the internal fluidized bed was 80° C.

(103) The spray dryer offgas was filtered in baghouse filter (9) and sent to a condenser column (12) for quenching/cooling. Excess water was pumped out of the condenser column (12) by controlling the (constant) filling level inside the condenser column (12). The water inside the condenser column (12) was cooled by a heat exchanger (13) and pumped counter-current to the gas via quench nozzles (11) so that the temperature inside the condenser column (12) was 45° C. The water inside the condenser column (12) was set to an alkaline pH by dosing sodium hydroxide solution to wash out acrylic acid vapors.

(104) The condenser column offgas was split to the drying gas inlet pipe (1) and the conditioned internal fluidized bed gas (25). The gas temperatures were controlled via heat exchangers (20) and (22). The hot drying gas was fed to the concurrent spray dryer via gas distributor (3). The gas distributor (3) consists of a set of plates providing a pressure drop of 2 to 4 mbar depending on the drying gas amount.

(105) The product was discharged from the internal fluidized bed (27) via rotary valve (28) into sieve (29). The sieve (29) was used for sieving off overs/lumps having a particle diameter of more than 800 μm.

(106) The monomer solution was prepared by mixing first acrylic acid with 3-tuply ethoxylated glycerol triacrylate (internal crosslinker) and secondly with 37.3% by weight sodium acrylate solution. The temperature of the resulting monomer solution was controlled to 10° C. by using a heat exchanger and pumping in a loop. A filter unit having a mesh size of 250 μm was used in the loop after the pump. The initiators were metered into the monomer solution upstream of the dropletizer by means of static mixers (41) and (42) via lines (43) and (44) as shown in FIG. 1. Sodium peroxodisulfate solution having a temperature of 20° C. was added via line (43) and [2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride solution together with Bruggolite FF7 having a temperature of 5° C. was added via line (44). Each initiator was pumped in a loop and dosed via control valves to each dropletizer unit. A second filter unit having a mesh size of 140 μm was used after the static mixer (42). For dosing the monomer solution into the top of the spray dryer three dropletizer units were used as shown in FIG. 4.

(107) A dropletizer unit consisted of an outer pipe (51) having an opening for the dropletizer cassette (53) as shown in FIG. 5. The dropletizer cassette (53) was connected with an inner pipe (52). The inner pipe (53) having a PTFE block (54) at the end as sealing can be pushed in and out of the outer pipe (51) during operation of the process for maintenance purposes.

(108) The temperature of the dropletizer cassette (61) was controlled to 8° C. by water in flow channels (59) as shown in FIG. 6. The dropletizer cassette (61) had 256 bores having a diameter of 170 μm and a bore separation of 15 mm. The dropletizer cassette (61) consisted of a flow channel (60) having essential no stagnant volume for homogeneous distribution of the premixed monomer and initiator solutions and one droplet plate (57). The droplet plate (57) had an angled configuration with an angle of 3°. The droplet plate (57) was made of stainless steel and had a length of 630 mm, a width of 128 mm and a thickness of 1 mm.

(109) The feed to the spray dryer consisted of 12.45% by weight of acrylic acid, 33.40% by weight of sodium acrylate, 0.022% by weight of 3-tuply ethoxylated glycerol triacrylate, 0.036 to 0.072% by weight of [2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochlorid, 0.0029% by weight of Bruggolite FF7, 0.054% by weight of sodiumperoxodisulfate and water. The degree of neutralization was 71%. The feed per bore was 1.6 kg/h.

(110) The resulting water-absorbent polymer particles were analyzed. The results are summarized in Table 1.

(111) TABLE-US-00001 TABLE 1 Effect of the azo initiator Bulk Density CRC AUNL AUL Residual Monomers Extractables Moisture FSR Example Azo initiator [g/cm.sup.3] [g/g] [g/g] [g/g] [ppm] [wt. %] [wt. %] [g/gs] L a b HC 60 1a 0.08*) 0.74 32.6 37.5 24.2 15100 4.0 6.1 0.28 95.0 2.2 1.3 91.1 1b 0.12*) 0.73 36.6 44.8 24.8 13600 4.2 5.6 0.22 95.2 1.5 0.9 92.5 1c 0.16*) 0.76 47.0 55.1 25.1 11700 3.8 6.3 0.14 96.1 1.7 1.2 92.5 *)% by weight based on monomer a)

Example 2

(112) The process was performed in a concurrent spray drying plant with an integrated fluidized bed (27) and an external fluidized bed (29) as shown in FIG. 1. The cylindrical part of the spray dryer (5) had a height of 22 m and a diameter of 3.4 m. The internal fluidized bed (IFB) had a diameter of 3 m and a weir height of 0.25 m.

(113) The drying gas was feed via a gas distributor (3) at the top of the spray dryer. The drying gas was partly recycled (drying gas loop) via a baghouse filter (9) and a condenser column (12). Instead of the baghouse filter (9) any other filter and/or cyclone can be used. The drying gas was nitrogen that comprises from 1% to 4% by volume of residual oxygen: Before start of polymerization the drying gas loop was filled with nitrogen until the residual oxygen was below 4% by volume. The gas velocity of the drying gas in the cylindrical part of the spray dryer (5) was 0.62 m/s. The pressure inside the spray dryer was 4 mbar below ambient pressure.

(114) The spray dryer outlet temperature was measured at three points around the circumference at the end of the cyclindrical part as shown in FIG. 3. Three single measurements (47) were used to calculate the average cylindrical spray dryer outlet temperature. The drying gas loop was heated up and the dosage of monomer solution is started up. From this time the spray dryer outlet temperature was controlled to 116° C. by adjusting the gas inlet temperature via the heat exchanger (20). The gas inlet temperature was 167° C. and the steam content of the drying gas was 0.058 kg steam per kg dry gas.

(115) The product accumulated in the internal fluidized bed (27) until the weir height was reached. Conditioned internal fluidized bed gas having a temperature of 123° C. and a steam content of 0.058 kg steam per kg dry gas was fed to the internal fluidized bed (27) via line (25). The gas velocity of the internal fluidized bed gas in the internal fluidized bed (27) was 0.65 m/s. The residence time of the product was 240 min. The temperature of the water-absorbent polymer particles in the internal fluidized bed was 84° C.

(116) The spray dryer offgas was filtered in baghouse filter (9) and sent to a condenser column (12) for quenching/cooling. Excess water was pumped out of the condenser column (12) by controlling the (constant) filling level inside the condenser column (12). The water inside the condenser column (12) was cooled by a heat exchanger (13) and pumped counter-current to the gas via quench nozzles (11) so that the temperature inside the condenser column (12) was 45° C. The water inside the condenser column (12) was set to an alkaline pH by dosing sodium hydroxide solution to wash out acrylic acid vapors.

(117) The condenser column offgas was split to the drying gas inlet pipe (1) and the conditioned internal fluidized bed gas (25). The gas temperatures were controlled via heat exchangers (20) and (22). The hot drying gas was fed to the concurrent spray dryer via gas distributor (3). The gas distributor (3) consists of a set of plates providing a pressure drop of 2 to 4 mbar depending on the drying gas amount.

(118) The product was discharged from the internal fluidized bed (27) via rotary valve (28) into sieve (29). The sieve (29) was used for sieving off overs/lumps having a particle diameter of more than 800 μm.

(119) The monomer solution was prepared by mixing first acrylic acid with 3-tuply ethoxylated glycerol triacrylate (internal crosslinker) and secondly with 37.3% by weight sodium acrylate solution. The temperature of the resulting monomer solution was controlled to 10° C. by using a heat exchanger and pumping in a loop. A filter unit having a mesh size of 250 μm was used in the loop after the pump. The initiators were metered into the monomer solution upstream of the dropletizer by means of static mixers (41) and (42) via lines (43) and (44) as shown in FIG. 1. Sodium peroxodisulfate solution having a temperature of 20° C. was added via line (43) and [2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride solution together with Bruggolite FF7 having a temperature of 5° C. was added via line (44). Each initiator was pumped in a loop and dosed via control valves to each dropletizer unit. A second filter unit having a mesh size of 140 μm was used after the static mixer (42). For dosing the monomer solution into the top of the spray dryer three dropletizer units were used as shown in FIG. 4.

(120) A dropletizer unit consisted of an outer pipe (51) having an opening for the dropletizer cassette (53) as shown in FIG. 5. The dropletizer cassette (53) was connected with an inner pipe (52). The inner pipe (53) having a PTFE block (54) at the end as sealing can be pushed in and out of the outer pipe (51) during operation of the process for maintenance purposes.

(121) The temperature of the dropletizer cassette (61) was controlled to 8° C. by water in flow channels (59) as shown in FIG. 6. The dropletizer cassette (61) had 256 bores having a diameter of 170 μm and a bore separation of 15 mm. The dropletizer cassette (61) consisted of a flow channel (60) having essential no stagnant volume for homogeneous distribution of the premixed monomer and initiator solutions and one droplet plate (57). The droplet plate (57) had an angled configuration with an angle of 3°. The droplet plate (57) was made of stainless steel and had a length of 630 mm, a width of 128 mm and a thickness of 1 mm.

(122) The feed to the spray dryer consisted of 10.45% by weight of acrylic acid, 33.40% by weight of sodium acrylate, 0.018% by weight of 3-tuply ethoxylated glycerol triacrylate, 0.025% by weight of [2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 0.0036% by weight of Bruggolite FF7, 0.054 to 0.270% by weight of sodiumperoxodisulfate and water. The degree of neutralization was 71%. The feed per bore was 1.6 kg/h.

(123) The resulting water-absorbent polymer particles were analyzed. The results are summarized in Table 2.

(124) TABLE-US-00002 TABLE 2 Effect of the persulfate Bulk Density CRC AUNL AUL Residual Monomers Extractables Moisture FSR Example Persulfate [g/cm.sup.3] [g/g] [g/g] [g/g] [ppm] [wt. %] [wt. %] [g/gs] L a b HC 60 2a 0.12*) 0.73 30.3 42.0 25.9 21400 4.7 5.5 0.28 94.9 2.0 1.4 90.7 2b 0.20*) 0.69 30.0 41.0 26.1 20100 4.8 5.5 0.28 94.3 2.3 1.8 88.9 2c 0.40*) 0.72 31.5 40.8 26.4 18100 4.8 4.5 0.29 94.2 2.1 3.0 85.2 2d 0.60*) 0.74 32.8 45.2 24.5 15200 4.9 4.8 0.30 94.5 1.6 3.7 83.4 *)% by weight based on monomer a)

Example 3

(125) The process was performed in a concurrent spray drying plant with an integrated fluidized bed (27) and an external fluidized bed (29) as shown in FIG. 1. The cylindrical part of the spray dryer (5) had a height of 22 m and a diameter of 3.4 m. The internal fluidized bed (IFB) had a diameter of 3 m and a weir height of 0.25 m.

(126) The drying gas was feed via a gas distributor (3) at the top of the spray dryer. The drying gas was partly recycled (drying gas loop) via a baghouse filter (9) and a condenser column (12). Instead of the baghouse filter (9) any other filter and/or cyclone can be used. The drying gas was nitrogen that comprises from 1% to 4% by volume of residual oxygen: Before start of polymerization the drying gas loop was filled with nitrogen until the residual oxygen was below 4% by volume. The gas velocity of the drying gas in the cylindrical part of the spray dryer (5) was 0.56 m/s. The pressure inside the spray dryer was 4 mbar below ambient pressure.

(127) The spray dryer outlet temperature was measured at three points around the circumference at the end of the cyclindrical part as shown in FIG. 3. Three single measurements (47) were used to calculate the average cylindrical spray dryer outlet temperature. The drying gas loop was heated up and the dosage of monomer solution is started up. From this time the spray dryer outlet temperature was controlled to 115° C. by adjusting the gas inlet temperature via the heat exchanger (20). The gas inlet temperature was 162° C. and the steam content of the drying gas was 0.077 kg steam per kg dry gas.

(128) The product accumulated in the internal fluidized bed (27) until the weir height was reached. Conditioned internal fluidized bed gas having a temperature of 114 to 184° C. and a steam content of 0.077 kg steam per kg dry gas was fed to the internal fluidized bed (27) via line (25). The gas velocity of the internal fluidized bed gas in the internal fluidized bed (27) was 0.65 m/s. The residence time of the product was 45 to 300 min. The temperature of the water-absorbent polymer particles in the internal fluidized bed was 71 to 116° C.

(129) The spray dryer offgas was filtered in baghouse filter (9) and sent to a condenser column (12) for quenching/cooling. Excess water was pumped out of the condenser column (12) by controlling the (constant) filling level inside the condenser column (12). The water inside the condenser column (12) was cooled by a heat exchanger (13) and pumped counter-current to the gas via quench nozzles (11) so that the temperature inside the condenser column (12) was 50° C. The water inside the condenser column (12) was set to an alkaline pH by dosing sodium hydroxide solution to wash out acrylic acid vapors.

(130) The condenser column offgas was split to the drying gas inlet pipe (1) and the conditioned internal fluidized bed gas (25). The gas temperatures were controlled via heat exchangers (20) and (22). The hot drying gas was fed to the concurrent spray dryer via gas distributor (3). The gas distributor (3) consists of a set of plates providing a pressure drop of 2 to 4 mbar depending on the drying gas amount.

(131) The product was discharged from the internal fluidized bed (27) via rotary valve (28) into sieve (29). The sieve (29) was used for sieving off overs/lumps having a particle diameter of more than 800 μm.

(132) The monomer solution was prepared by mixing first acrylic acid with 3-tuply ethoxylated glycerol triacrylate (internal crosslinker) and secondly with 37.3% by weight sodium acrylate solution. The temperature of the resulting monomer solution was controlled to 10° C. by using a heat exchanger and pumping in a loop. A filter unit having a mesh size of 250 μm was used in the loop after the pump. The initiators were metered into the monomer solution upstream of the dropletizer by means of static mixers (41) and (42) via lines (43) and (44) as shown in FIG. 1. Sodium peroxodisulfate solution having a temperature of 20° C. was added via line (43) and [2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride solution together with Bruggolite FF7 having a temperature of 5° C. was added via line (44). Each initiator was pumped in a loop and dosed via control valves to each dropletizer unit. A second filter unit having a mesh size of 140 μm was used after the static mixer (42). For dosing the monomer solution into the top of the spray dryer three dropletizer units were used as shown in FIG. 4.

(133) A dropletizer unit consisted of an outer pipe (51) having an opening for the dropletizer cassette (53) as shown in FIG. 5. The dropletizer cassette (53) was connected with an inner pipe (52). The inner pipe (53) having a PTFE block (54) at the end as sealing can be pushed in and out of the outer pipe (51) during operation of the process for maintenance purposes.

(134) The temperature of the dropletizer cassette (61) was controlled to 8° C. by water in flow channels (59) as shown in FIG. 6. The dropletizer cassette (61) had 256 bores having a diameter of 170 μm and a bore separation of 15 mm. The dropletizer cassette (61) consisted of a flow channel (60) having essential no stagnant volume for homogeneous distribution of the premixed monomer and initiator solutions and one droplet plate (57). The droplet plate (57) had an angled configuration with an angle of 3°. The droplet plate (57) was made of stainless steel and had a length of 630 mm, a width of 128 mm and a thickness of 1 mm.

(135) The feed to the spray dryer consisted of 10.45% by weight of acrylic acid, 33.40% by weight of sodium acrylate, 0.036% by weight of 3-tuply ethoxylated glycerol triacrylate, 0.0043% by weight of Bruggolite FF7, 0.270% by weight of sodiumperoxodisulfate and water. The degree of neutralization was 71%. The feed per bore was 1.6 kg/h.

(136) The resulting water-absorbent polymer particles were analyzed. The results are summarized in Table 3.

(137) TABLE-US-00003 TABLE 3 Effect of temperature and residence time in the internal fluidized bed Temper- Residence Bulk Residual ature Time Density CRC AUNL AUL Monomers Extractables Moisture FSR Example [° C.] [min] [g/cm.sup.3] [g/g] [g/g] [g/g] [ppm] [wt. %] [wt. %] [g/gs] L a b HC 60 3a 71 45 0.82 30.9 35.7 25.3 25200 6.3 12.3 0.23 93.4 2.7 1.2 89.8 3b 90 45 0.73 45.1 50.3 20.4 16300 7.2 8.9 0.21 93.6 1.2 2.9 84.9 3c 116 45 0.68 48.3 53.4 16.2 13500 9.3 4.6 0.23 92.6 0.6 4.3 79.7 3d 71 150 0.79 31.9 36.6 24.5 15000 4.1 10.2 0.21 93.4 2.5 1.7 88.3 3e 90 150 0.72 48.0 53.1 19.2 13000 5.2 5.3 0.24 93.7 1.4 3.1 84.4 3f 116 150 0.64 52.3 59.8 16.2 8800 6.1 1.1 0.26 94.4 0.2 4.8 80.0 3g 71 300 0.75 45.5 50.2 23.3 10500 3.6 7.2 0.21 93.8 2.1 1.8 88.4 3h 90 300 0.71 48.7 52.5 17.3 8100 4.5 4.4 0.24 93.6 1.1 4.8 79.2 3i 115 300 0.60 53.3 58.3 12.2 7200 5.6 1.0 0.25 92.9 0.3 7.0 71.9 3j 75 300 0.73 46.2 51.2 21.3 9800 3.6 6.9 0.21 93.8 1.9 1.6 89.0 3l 75 240 0.72 45.9 51.8 18.9 10100 3.8 7.5 0.23 93.4 1.8 2.5 85.9 3k 77 300 0.70 46.9 52.9 19.7 9300 3.8 6.2 0.23 93.7 1.7 2.0 87.7

Example 4

(138) The process was performed in a concurrent spray drying plant with an integrated fluidized bed (27) and an external fluidized bed (29) as shown in FIG. 1. The cylindrical part of the spray dryer (5) had a height of 22 m and a diameter of 3.4 m. The internal fluidized bed (IFB) had a diameter of 3 m and a weir height of 0.25 m.

(139) The drying gas was feed via a gas distributor (3) at the top of the spray dryer. The drying gas was partly recycled (drying gas loop) via a baghouse filter (9) and a condenser column (12). Instead of the baghouse filter (9) any other filter and/or cyclone can be used. The drying gas was nitrogen that comprises from 1% to 4% by volume of residual oxygen: Before start of polymerization the drying gas loop was filled with nitrogen until the residual oxygen was below 4% by volume. The gas velocity of the drying gas in the cylindrical part of the spray dryer (5) was 0.79 m/s. The pressure inside the spray dryer was 4 mbar below ambient pressure.

(140) The spray dryer outlet temperature was measured at three points around the circumference at the end of the cyclindrical part as shown in FIG. 3. Three single measurements (47) were used to calculate the average cylindrical spray dryer outlet temperature. The drying gas loop was heated up and the dosage of monomer solution is started up. From this time the spray dryer outlet temperature was controlled to 116° C. by adjusting the gas inlet temperature via the heat exchanger (20). The gas inlet temperature was 169° C. and the steam content of the drying gas was 0.058 kg steam per kg dry gas.

(141) The product accumulated in the internal fluidized bed (27) until the weir height was reached. Conditioned internal fluidized bed gas having a temperature of 117° C. and a steam content of 0.058 to 0.225 kg steam per kg dry gas was fed to the internal fluidized bed (27) via line (25). The gas velocity of the internal fluidized bed gas in the internal fluidized bed (27) was 0.65 m/s. The residence time of the product was 150 min. The temperature of the water-absorbent polymer particles in the internal fluidized bed was 81° C.

(142) The spray dryer offgas was filtered in baghouse filter (9) and sent to a condenser column (12) for quenching/cooling. Excess water was pumped out of the condenser column (12) by controlling the (constant) filling level inside the condenser column (12). The water inside the condenser column (12) was cooled by a heat exchanger (13) and pumped counter-current to the gas via quench nozzles (11) so that the temperature inside the condenser column (12) was 45° C. The water inside the condenser column (12) was set to an alkaline pH by dosing sodium hydroxide solution to wash out acrylic acid vapors.

(143) The condenser column offgas was split to the drying gas inlet pipe (1) and the conditioned internal fluidized bed gas (25). The gas temperatures were controlled via heat exchangers (20) and (22). The hot drying gas was fed to the concurrent spray dryer via gas distributor (3). The gas distributor (3) consists of a set of plates providing a pressure drop of 2 to 4 mbar depending on the drying gas amount.

(144) The product was discharged from the internal fluidized bed (27) via rotary valve (28) into sieve (29). The sieve (29) was used for sieving off overs/lumps having a particle diameter of more than 800 μm.

(145) The monomer solution was prepared by mixing first acrylic acid with 3-tuply ethoxylated glycerol triacrylate (internal crosslinker) and secondly with 37.3% by weight sodium acrylate solution. The temperature of the resulting monomer solution was controlled to 10° C. by using a heat exchanger and pumping in a loop. A filter unit having a mesh size of 250 μm was used in the loop after the pump. The initiators were metered into the monomer solution upstream of the dropletizer by means of static mixers (41) and (42) via lines (43) and (44) as shown in FIG. 1. Sodium peroxodisulfate solution having a temperature of 20° C. was added via line (43) and [2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride solution together with Bruggolite FF7 having a temperature of 5° C. was added via line (44). Each initiator was pumped in a loop and dosed via control valves to each dropletizer unit. A second filter unit having a mesh size of 140 μm was used after the static mixer (42). For dosing the monomer solution into the top of the spray dryer three dropletizer units were used as shown in FIG. 4.

(146) A dropletizer unit consisted of an outer pipe (51) having an opening for the dropletizer cassette (53) as shown in FIG. 5. The dropletizer cassette (53) was connected with an inner pipe (52). The inner pipe (53) having a PTFE block (54) at the end as sealing can be pushed in and out of the outer pipe (51) during operation of the process for maintenance purposes.

(147) The temperature of the dropletizer cassette (61) was controlled to 8° C. by water in flow channels (59) as shown in FIG. 6. The dropletizer cassette (61) had 256 bores having a diameter of 170 μm and a bore separation of 15 mm. The dropletizer cassette (61) consisted of a flow channel (60) having essential no stagnant volume for homogeneous distribution of the premixed monomer and initiator solutions and one droplet plate (57). The droplet plate (57) had an angled configuration with an angle of 3°. The droplet plate (57) was made of stainless steel and had a length of 630 mm, a width of 128 mm and a thickness of 1 mm.

(148) The feed to the spray dryer consisted of 10.45% by weight of acrylic acid, 33.40% by weight of sodium acrylate, 0.018% by weight of 3-tuply ethoxylated glycerol triacrylate, 0.036% by weight of [2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 0.0029% by weight of Bruggolite FF7, 0.054% by weight of sodiumperoxodisulfate and water. The degree of neutralization was 71%. The feed per bore was 1.6 kg/h.

(149) The resulting water-absorbent polymer particles were analyzed. The results are summarized in Table 4.

(150) TABLE-US-00004 TABLE 4 Effect of the steam content in the internal fluidized bed Steam Content Bulk Density CRC AUNL AUL Residual Monomers Extractables Moisture FSR Example [kg/kg] [g/cm.sup.3] [g/g] [g/g] [g/g] [ppm] [wt. %] [wt. %] [g/gs] L a b HC 60 4a 0.077 0.75 40.0 48.5 26.5 14000 4.2 5.7 0.27 96.1 1.0 2.9 87.4 4b 0.172 0.70 36.6 44.7 25.1 9600 5.8 7.8 0.18 96.1 0.9 3.0 87.1 4c 0.225 0.71 37.4 45.0 25.4 1400 2.5 10.7 0.08 94.2 2.6 1.7 89.1