Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Including heat exchanger for reaction chamber or reactants...
Reexamination Certificate
1999-02-24
2002-06-04
Tran, Hien (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Including heat exchanger for reaction chamber or reactants...
C422S132000, C422S198000, C422S200000, C422S201000, C165S177000, C165S184000, C366S336000, C366S339000, C526S064000
Reexamination Certificate
active
06399031
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a curved, tubular flow reactor having an essentially circular or elliptical cross section, to the use of this apparatus for carrying out chemical reactions continuously and to a process for continuous polymerization using the novel apparatus.
2. Description of the Background
When chemical reactions are carried out on an industrial scale, continuous-reaction engineering often provides advantages over reactions being run discontinuously. This is particularly true in those cases where large volumes have to be coped with and discontinuous-reaction engineering is likely to result in uneven product quality from batch to batch. However, it is often difficult for continuous-reaction engineering to be put into practice. One of the fields to which this is applied is emulsion polymerization and suspension polymerization of ethylenically unsaturated monomers.
There has been no lack of attempts at the practical implementation of continuous-reaction engineering in this field. The first patents on continuous emulsion polymerization were filed as early as 1937 and 1938 by I. G. Farbenindustrie (GB 517,951 and FR 847,151). By now numerous approaches to continuous emulsion polymerization are known. Described most frequently is the use of a continuous stirred-tank reactor, an overview being provided, for example, by Encyclopedia of Polymer Science and Engineering, 1986, Vol. 6, pp. 11 to 18. Emulsion polymerization in such reactors results in a wide particle size distribution, and the establishment of a steady state takes a relatively long time or fails altogether. Moreover, owing to the shear-force action of the stirrer, there is an increased tendency for the polymer to coagulate and then all too readily to settle on the stirrer or on reactor internals.
A particular refinement of the abovementioned stirred reactors is described in DE-A-1,071,341. The reactor, a cylindrical tube, is equipped with disks seated on a shaft, which result in Taylor rings being formed when the stirrer is set in rotation. The main effect of this is to improve mixing perpendicular to the flow direction, thereby improving the space-time yield. Reactors of this type always comprise moving parts. This has the drawback that constructional measures are required for supporting and sealing the shaft. Moreover, accretions and deposits of polymer are formed owing to gas bubbles present or being formed in the system, which collect preferentially at the shaft and in low-flow zones and there trigger coagulation of the polymer. Finally, the rotating disks give rise to shear forces which likewise lead to coagulation of the polymer.
A further improvement is represented by reactors which comprise two concentric cylinders, the inner and/or outer cylinder being set in rotation. Depending on the speed of rotation and the flow velocity, various stable flow patterns are formed which can cover a range with laminar Couette low at one end, the formation of Taylor ring vortices in between, up to turbulent vortex flow. Polymer International 30 (1993), 203-206 describes a continuous Couette-Taylor vortex reactor and its use for continuous emulsion polymerization) resulting in latex particles having a relatively narrow particle size distribution. Chemical Engineering Science, Volume 50, pp. 1409-1416 (1995) describes the emulsion polymerization of styrene in a continuous Taylor vortex-flow reactor. Since reactors of this type do not employ a conventional agitator, they are particularly suitable for preparing polymer dispersions which have a particularly pronounced tendency to coagulate under the influence of shear forces. Moreover, similar drawbacks arise as in the reactor described in DE-A-1 071 341.
An alternative to the stirred reactors is the use of pulsed, packed columns as described in Chem. Eng. Sci. Vol. 47, 2603-2608 (1992) and EP-A-336 469. It is thus possible for styrene or vinyl acetate to be polymerized continuously. However, columns of this type will very readily foul and plug.
A further alternative to the stirred-tank reactors and stirred tubular reactors mentioned are, in general, simple “empty” tubular reactors, i.e tubular reactors without additional internals such as a static mixer or a stirrer. Owing to the large specific surface area of a tube they are particularly advantageous with strongly exothermal polymerizations. Thus, for example, continuous suspension polymerization is described in DE-A-2342788 and DE-A-880938. There are also numerous studies which address emulsion polymerization in a tubular reactor, whether it be with laminar or with turbulent flow. An overview of this work is found in Reactors, Kinetics and Catalysis, AIChE Journal, 1994, Vol. 40, 73-87.
One of the main problems in carrying out an emulsion polymerization in a tubular reactor is that of the tubes being plugged by coagulate, which is a particular problem in the turbulent flow domain. Attempts have therefore been made, by additional measures and special reaction engineering, to prevent coagulate formation, for example with the aid of pulsed tubular reactors, see the last-mentioned publication, or by optimizing the reactor dimensions and the material for the tubes, see ACS Symp. Ser. 104, 113 (1979). Patent publications which describe tubular reactors for emulsion polymerization are DE-A-26 17 570, DD-A-238 163, DD-A-234 163, DD-A-233 573, DD-A-233 572, DE-A-33 02 251 and CZ-A-151 220. FR-A-842 829 and EP-A-633 061 disclose a continuous-polymerization tubular reactor, in which curved tube segments are linked by means of long straight tube segments. Reactors of this type result in a broad residence time distribution of the unit volumes.
Tubular reactors in most cases are employed in the form of helically wound reactors, see the two last-mentioned publications. The flow conditions in wound tubular reactors have been particularly well studied. The centrifugal forces arising in helically wound tube produce a secondary flow perpendicular to the principal flow direction. Said secondary flow was first described by Dean and is therefore known as a Dean vortex. The Dean vortices in a helically wound tube favor a narrow residence time distribution of the unit volumes and, in the case of laminar flow, result in higher heat transfer coefficients and mass transfer coefficients, compared with a straight tube with corresponding laminar flow. A further improvement is achieved by an abrupt change in direction of the centrifugal forces, see Saxena in AIChE Journal Vol. 30, 1984, pp. 363-368 (also compare FIGS.
13
and
14
). The change in direction is effected by a 90° kink in the helical winding. By means of a total of 57 kinks, Saxena et al were able to achieve the narrowest known residence time distribution in an apparatus with laminar flow. As a result of the laminar flow, the reaction volume is kept small and only low shear forces arise. Even with laminar flow, a high heat transfer coefficient is achieved here.
Notwithstanding all the studies and insights with respect to reaction engineering in connection with disperse systems in tubular reactors, these have not so far been able to find general acceptance in practice. The reason for this is that, on the one hand, the plugging problem in tubular reactors has not yet been solved satisfactorily and that, on the other hand, the model described by Saxena et al, having kinked helical windings, has decisive drawbacks in practice, such as the difficult fabrication, bulkiness, and the fact that descending turns involving descending flow are present, which may have an adverse effect on productivity. In particular, the formation of gas bubbles of monomers or air can be observed, which results in greater coagulate formation and disruptions of the progress of the polymerization, and in uneven flow and a change in the residence time of the liquid phase. Additionally there is the risk that those fractions of the reaction mixture which have a higher specific gravity, e.g. the particles present in the dispersion, will settle to a greater extent
Herrmann Christof
Hübinger Wolfgang
Kremeskötter Jens
Minges Roland
Moritz Hans-Ulrich
BASF - Aktiengesellschaft
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Tran Hien
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