Apparatus for producing pulverulent poly(meth)acrylate

Abstract

An apparatus for producing pulverulent poly(meth)acrylate in a reactor for droplet polymerization having an apparatus for dropletization of a monomer solution for the production of the poly(meth)acrylate having holes through which the monomer solution is introduced, an addition point for a gas above the apparatus for dropletization, at least one gas withdrawal point on the circumference of the reactor and a fluidized bed, and above the gas withdrawal point the reactor has a region having a constant hydraulic internal diameter and below the gas withdrawal point the reactor has a hydraulic internal diameter that steadily decreases. The reactor has a heating means in the region having a steadily decreasing hydraulic internal diameter.

Claims

1. An apparatus for producing pulverulent poly(meth)acrylate, comprising a reactor (1) for droplet polymerization comprising an apparatus (5) for dropletization of a monomer solution for the production of the poly(meth)acrylate comprising holes through which the monomer solution is introduced, an addition point (13) for a gas above the apparatus (5) for dropletization, at least one gas withdrawal point (19) on the circumference of the reactor (1) and a fluidized bed (11), wherein above the gas withdrawal point (19) the reactor (1) comprises a region having a constant hydraulic internal diameter and below the gas withdrawal point (19) the reactor has a hydraulic internal diameter that steadily decreases, wherein the reactor (1) comprises a heater (31) in the region having a steadily decreasing hydraulic internal diameter and the heater (31) supplies a heat output in the range of from 20 to 3000 W/m.sup.2.

2. The apparatus according to claim 1, wherein the heater (31) is an electric heater.

3. The apparatus according to claim 1, wherein the heater (31) is a double shell or takes the form of heating coils (31) applied to the outside of the reactor shell, wherein the double shell or the heating coils (31) have a heating medium flowing therethrough.

4. The apparatus according to claim 1, wherein the region having a steadily decreasing hydraulic internal diameter is conical.

5. The apparatus according to claim 1, wherein in the region having a steadily decreasing hydraulic internal diameter there are mechanical or pneumatic cleaning apparatuses (35) affixed to the exterior of the reactor (1).

6. The apparatus according to claim 5, wherein the mechanical or pneumatic cleaning apparatuses are tappers (35).

7. The apparatus according to claim 1, wherein there are mechanical or pneumatic cleaning apparatuses (35) affixed to the exterior of the reactor in the lower third of the region having a constant hydraulic interior diameter.

8. The apparatus according to claim 7, wherein the mechanical or pneumatic cleaning apparatuses are tappers (35).

9. The apparatus according to claim 5, wherein there are mechanical or pneumatic cleaning apparatuses (35) affixed to the exterior of the reactor in the lower third of the region of the shell having a constant hydraulic interior diameter.

10. The apparatus according to claim 9, wherein the mechanical or pneumatic cleaning apparatuses are tappers (35).

Description

(1) Working examples of the invention are shown in the figures and are more particularly described in the description which follows.

(2) FIG. 1 is a longitudinal section through a reactor for droplet polymerization.

(3) FIG. 2 is a schematic diagram of the region having a steadily decreasing hydraulic internal diameter with heating coils and tappers.

(4) FIG. 1 shows a longitudinal section through a reactor configured according to the invention.

(5) A reactor 1 for droplet polymerization comprises a reactor head 3 in which an apparatus for dropletization 5 is accommodated, a middle region 7 in which the polymerization reaction is performed, and a lower region 9 comprising a fluidized bed 11 in which the reaction is concluded.

(6) The polymerization reaction for producing the poly(meth)acrylate is carried out by supplying the apparatus for dropletization 5 with a monomer solution via a monomer feed 12. When the apparatus for dropletization 5 has two or more channels, it is preferable to supply each channel with the monomer solution via a dedicated monomer feed 12. The monomer solution exits through holes, not shown in FIG. 1, in the apparatus for dropletization 5 and breaks up into individual droplets which fall downward in the reactor. A gas, for example nitrogen or air, is introduced into the reactor 1 via a first addition point for a gas 13 above the apparatus for dropletization 5. This gas flow assists the breakup into individual droplets of the monomer solution exiting from the holes in the apparatus for dropletization 5. In addition, the gas flow helps to prevent the individual droplets from touching and coalescing to form larger droplets.

(7) In order to make the cylindrical middle region 7 of the reactor as short as possible and also to avoid droplets hitting the wall of the reactor 1, the reactor head 3 preferably has a conical configuration as shown here, the apparatus for dropletization 5 being disposed within the conical reactor head 3 above the cylindrical region. However, it is also possible as an alternative to provide the reactor with a cylindrical configuration in the reactor head 3 as well, with a diameter the same as that of the middle region 7. However, a conical configuration of the reactor head 3 is preferred. The position of the apparatus for dropletization 5 is chosen such that there is still a sufficiently large distance between the outermost holes through which the monomer solution is supplied and the wall of the reactor to prevent the droplets from hitting the wall. To this end, the distance should be at least in the range of from 50 to 1500 mm, preferably in the range of from 100 to 1250 mm and more particularly in the range from 200 to 750 mm. It will be appreciated that a greater distance from the wall of the reactor is also possible. However, a corollary of greater distance is poorer utilization of the reactor cross section.

(8) The lower region 9 is capped off with a fluidized bed 11 and the polymer particles formed from the monomer droplets during the fall, fall into said fluidized bed. The post reaction to afford the desired product is performed in the fluidized bed. According to the invention the outermost holes through which the monomer solution is dropletized are positioned such that a droplet falling vertically downward falls into the fluidized bed 11. This can be achieved, for example, by virtue of the hydraulic diameter of the fluidized bed being at least as large as the hydraulic diameter of the area which is enclosed by a line connecting the outermost holes in the apparatus for dropletization 5, the cross-sectional area of the fluidized bed and the area formed by the line connecting the outermost holes having the same shape and the centers of the two areas being at the same position in a vertical projection of one onto the other. The outermost position of the outer holes relative to the position of the fluidized bed 11 is shown in FIG. 1 using a dotted line 15.

(9) In order furthermore to avoid droplets hitting the wall of the reactor in the middle region 7 as well, the hydraulic diameter at the height of the midpoint between the apparatus for dropletization and the gas withdrawal point is at least 10% larger than the hydraulic diameter of the fluidized bed.

(10) The reactor 1 may have any desired cross-sectional shape. However, the cross section of the reactor 1 is preferably circular. In this case, the hydraulic diameter is the same as the diameter of the reactor 1.

(11) Above the fluidized bed 11, the diameter of the reactor 1 increases in the embodiment shown here and the reactor 1 therefore widens conically from bottom to top in the lower region 9. This has the advantage that polymer particles that are formed in the reactor 1 and that hit the wall can slide downward along the wall and into the fluidized bed 11. To avoid encrustations, it is additionally possible to provide tappers, not shown here, on the outside of the conical section of the reactor, said tappers being used to set the wall of the reactor into vibration which causes adhering polymer particles to become detached and slide into the fluidized bed 11.

(12) To effect gas feeding for the operation of the fluidized bed 11, a gas distributor 17 below the fluidized bed 11 blows the gas into the fluidized bed 11.

(13) Since gas is introduced into the reactor 1 both from the top and from the bottom, it is necessary to withdraw gas from the reactor 1 at a suitable position. To this end, at least one gas withdrawal point 19 is disposed at the transition between the middle region 7 having a constant cross section and the lower region 9 which widens conically from the bottom upward. Here, the wall of the cylindrical middle region 7 projects into the lower region 9 which widens conically in an upward direction, the diameter of the conical lower region 9 at this position being larger than the diameter of the middle region 7. Thus an annular chamber 21, which encircles the wall of the middle region 7, is formed, into which the gas flows and can be drawn off through the at least one gas withdrawal point 19 connected to the annular chamber 21.

(14) The post reacted polymer particles of the fluidized bed 11 are withdrawn via a product withdrawal point 23 in the region of the fluidized bed.

(15) FIG. 2 is a schematic diagram of the region having a steadily decreasing hydraulic internal diameter with heating coils and tappers.

(16) In order to achieve heating of the conical lower region 9 it is possible, for example, to apply heating coils 31 to the outside of the conical lower region 9. In order to heat the reactor wall of the lower conical region 9, the heating coils 31 have a heat-transfer medium flowing therethrough, for example thermal oil, water or, preferably, steam. As an alternative to heating coils 31 applied to the conical lower region 9 which have a heat-transfer medium flowing therethrough, it is also possible to provide an electrical heating means for example.

(17) When heating coils 31 having a heat-transfer medium flowing therethrough are employed, the temperature and volume flow are set such that a heat output in the range of from 20 to 5000 W/m.sup.2 is supplied to the lower conical region 9 of the reactor 1.

(18) In order to stabilize the wall of the lower conical region 9 it is possible to apply reinforcing rings 33 to the wall. The arrangement of these reinforcing rings 33 and the heating coils 31 is such that the reinforcing rings 33 do not impede the supply of heat to the lower conical region 9 of the reactor 1.

(19) In order to prevent encrustations and in complement to the heating of the lower conical region 9, mechanical or pneumatic cleaning apparatuses, for example tappers 35, may additionally be affixed. Here, the tappers 35 are positioned between the heating coils 31 in order that they may act directly upon the wall of the lower conical region 9.

EXAMPLES

(20) The production of poly(meth)acrylate is carried out using a reactor for droplet polymerization of the type shown in FIG. 1. The region of the reactor having a constant diameter has a height of 22 m and a diameter of 3.4 m. The fluidized bed has a diameter of 3 m and a height of 0.25 m.

(21) Nitrogen having a residual oxygen fraction of from 1 to 4 vol % was supplied at the top of the reactor as drying gas. The amount of drying gas was set such that the gas velocity in the cylindrical section of the reactor was 0.8 m/s. The temperature was measured at the product outlet and maintained at 117 C. during operation of the reactor by adjusting the temperature of the drying gas.

(22) The supplied gas for generating the fluidized bed had a temperature of 122 C. and a relative humidity of 4%. The gas velocity in the fluidized bed was 0.8 m/s and the residence time of the product in the fluidized bed was 120 min. The product was withdrawn from the reactor via a cellular wheel lock and supplied to a moving bed of 3 m in length, 0.65 m in width and 0.5 m in height. The gas supplied to the moving bed had a temperature of 60 C. and the amount of gas was set such that the gas velocity in the moving bed was 0.8 m/s. The gas employed was air. The residence time of the product in the moving bed was 1 min. The product withdrawn from the moving bed was finally sieved to remove particles having a particle diameter of more than 800 m.

(23) To produce the monomer solution supplied to the reactor, acrylic acid was mixed initially with 3-tuply ethoxylated glyerol triacetate as crosslinker and subsequently with a 37.3 wt % sodium acrylate solution. The monomer solution was brought to a temperature of 10 C. Admixed therewith as initiators using a static mixer, prior to addition of the monomer solution into the reactor, were sodium persulfate solution at a temperature of 20 C. and 2,2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride along with Bruggolite FF7 at a temperature of 5 C. Addition into the reactor was effected via 3 channels with dropletizer casettes each sealed at the bottom with a dropletizer plate having 256 bores of 170 m in diameter and a distance between bores of 15 mm.

(24) The dropletizer casettes were brought to a temperature of 8 C. using water flowing through the channels encircling the dropletizer casettes.

(25) The dropletizer plates were angled about their central axis at an angle of 3 to the horizontal. The material used for the dropletizer plates was stainless steel. The dropletizer plates were of 630 mm in length, 128 mm in width and 1 mm in height.

(26) The monomer solution supplied to the reactor comprised 10.45% of acrylic acid, 33.40 wt % of sodium acrylate, 0.018 wt % of 3-tuply ethoxylated glycerol triacetate, 0.072 wt % of 2,2-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 0.0029 wt % of a 5 wt % solution of Bruggolite FF7 in water, 0.054 wt % of a 15 wt % solution of sodium persulfate in water, and water. The monomer solution was supplied to the reactor at a rate of 1.6 kg/h per bore.

(27) The lower conical region of the reactor had an area of 24.75 m.sup.2 and a wall thickness of 5 mm. The lower region was heated electrically. In addition, there were 6 Netter PKL 2100/5 pneumatic tappers mounted to the lower conical region. Each tapper was actuated such that it generated two impacts with a gap of 3 s at 50 s intervals.

(28) The product withdrawn from the reactor had a bulk density of 680 g/l and an average particle diameter of 407 m.

(29) Several experiments were performed where the lower conical region was heated with different heat outputs. The heat output and the results are shown in Table 1.

(30) TABLE-US-00001 TABLE 1 heat output and results Specific Exam- heat supplied Average operating duration before shutdown ple [W/m.sup.2] and results 1 0 shutdown and cleaning required after 4.5 hours 2 55 shutdown and cleaning required after 36 hours 3 240 shutdown and cleaning required after 9 hours 4 1200 operating time of more than 14 days, controlled fouling 5 3750 operating time of more than 14 days, controlled fouling, increased yellowing of product

(31) As is apparent from Table 1, operating duration increases with increasing heat output. Particularly when low heat outputs are used, fouling occurs which requires that the process be brought to an end to clean the reactor. As is apparent from the table, even a low heat output results in a marked increase in operating duration compared to operation without heating of the lower conical region.

(32) However, a heat output which is too high does not result in an increase in operating duration but rather results in a decrease in product quality which manifested itself as yellowing of the product in Example 5.

LIST OF REFERENCE NUMERALS

(33) 1 reactor 3 reactor head 5 apparatus for dropletization 7 middle region 9 lower region 11 fluidized bed 12 monomer feed 13 addition point for gas 15 position of the outermost holes in relation to the fluidized bed 11 17 gas distributor 19 gas withdrawal point 21 annular chamber 23 product withdrawal point 29 reactor axis 31 heating coil 33 reinforcing ring 35 tapper