Use of a single-stage or multistage stirrer to prepare polymers

Details Use of a single-stage or multistage stirrer to prepare polymers Use of a single-stage or multistage stirrer to prepare polymers

1998-03-16

2001-06-26

Buttner, David J. (Department: 1712)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C523S318000, C523S324000, C422S135000

Reexamination Certificate

active

06252018

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to the use of a single-stage or multistage stirrer.
DISCUSSION OF RELATED ART
The preparation of polymers by liquid phase polymerization is customarily subdivided into bulk, solution, precipitation, suspension and emulsion polymerization, the latter two polymerization techniques differing from the others in that they start from an already two-phase initial mixture. The polymerization reaction is in this case conducted in a batch reactor, in a continuous flow pipe, in a cascade of stirred vessels or in a continuous stirred-vessel reactor. In this context it is the vessel reactors which have acquired the most importance in the chemical industry, since they permit very great flexibility in terms of operating conditions and mode of operation and can be adapted to virtually all process requirements. Stirred-vessel reactors are suitable for continuous and batchwise operation and possess broad scope for use, ranging from laboratory vessel to large-scale reactor. Stirred-vessel reactors are obtainable in standardized construction for numerous applications, in a wide variety of materials and combinations of materials. Stirred-vessel reactors are easy to access and clean and permit relatively simple changeover to alternative polymerization reactions (see for example Ullmann, Volume 3, 4th ed., pp. 505-510).
In addition to the customary cooling and heating devices, feed lines and discharge lines for starting materials and reaction products, the reactor vessels feature stirring devices which consist in most cases of a stirrer, driven by a stirring shaft, and for certain applications also have stators which act as flow disrupters for more complete mixing. The stirrers themselves are fastened to in most cases vertical stirrer shafts which project either from above or from below into the—generally cylindrical—reactor vessel. Central installation from above into the reactor vessel is generally preferred, since the stirrer shaft can be sealed off with relative ease. A disadvantage is that the stirrer shaft must have a relatively large diameter because of the bending moments which occur. Introducing the stirrer from below into the reactor, on the other hand, reduces the mass of the stirrer but at the same time requires more complex sealing of the stirrer shaft as it penetrates the reactor floor.
Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Volume B2, Chapter 25 discloses a very wide variety of stirrers, for example propeller stirrers, disk stirrers, anchor stirrers, impeller stirrers, blade stirrers, MIG stirrers, etc.
In comparison with customary stirring techniques, such as the preparation and homogenization of solutions, two-phase liquid phase polymerization is accompanied by a number of particular problems that are particularly significant for the design of the reactor and for the mode of operation. Virtually all polymerization reactions are exothermic. The reaction enthalpy released can reach very high levels of up to about 4000 kJ/kg. The mean molar masses of the polymers are usually in the range from 10
4
to 10
7
. This means that, as with polymerization in bulk or solutions, the viscosity can rise by up to six powers of 10 during the reaction. This leads to greater difficulty in mass transfer, heat dissipation, mixing and stirring owing to the increasing viscosity of the reaction mixture. A further problem is the decrease in jacket cooling area in relation to volume as the size of the reactor vessel increases.
Consequently, there are particular requirements for stirrers in connection with liquid phase polymerization in stirred-vessel reactors. These requirements include reliable homogenization and uniform stirring; in other words, differences in temperature and concentration must be compensated and the formation of dead zones must be avoided. For example, in the case where a monomer emulsion is supplied from above or below into the reactor, it must be mixed with the polymerizing reaction mixture in order to supply fresh monomer to all volume elements in the reactor, so that the formation of off-specification polymer is avoided. It is additionally known that, deposition of polymer on the walls and internals tends to occur particularly in dead zones in the reactor chamber. This is essentially an axial conveyance function.
The stirrer is also required, depending on the reaction system, to disperse, suspend or emulsify. For example, although it is possible in the case of emulsion polymerization first to produce an emulsion of water, surfactant solution and monomer mixture in a stock vessel and then to supply said emulsion to the stirred-vessel reactor, it is known that the monomer droplets need to be made smaller still by the stirrer. In the case of reaction regimes where pure monomer is added, moreover, it must be emulsified, in other words reduced in size to microdroplets and homogenized. It is preferred to supply a monomer mixture, an emulsion or a monomer/water mixture formed shortly before entry into the reactor.
The stirrer must not, furthermore, form any zones of high shear, so as to prevent the formation firstly of coagulum and secondly of foam. It is known, indeed, that in zones of particularly high shear there is formation of coagulum structures which likewise lead to off-specification product and contaminate the remaining product (gel specks, fine coagulum). In extreme cases the entire contents of a reactor may coagulate. A good overview of the problem is given by J. B. P. Soares and A. E. Hamilec “Overview of Emulsion Polym. Reactors” (paper given at the NATO Conference “Recent Advances in Polymeric Dispersions” in Elizonto, Navarra, Spain, in June 1996).
German Patent DE-C1-44 21 949 discloses a modified blade stirrer for emulsion polymerization which is preferably used to prepare homopolymers, copolymers or graft copolymers of vinyl chloride. To avoid excessive shearing of the polymer latex and resulting formation of coagulum, a blade stirrer is proposed which avoids sharp edges and angular profiles, featuring instead progressively curved outer edges which have a droplet-shaped profile when viewed from above, and rounded-off top and bottom edges.
Japanese Patent Application JP-A-07, 292,002 discloses a stirred-vessel reactor which has a stirrer of multistage construction for the emulsion polymerization of polymer latices, little microcoagulum being produced. The stirrers presented are essentially single-stage, two- or three-stage modified blade stirrers.
WO-A-93 22 350 likewise relates to different types of stirrer for polymerizations, where little microcoagulum is to be produced. This patent application too essentially describes variants of multiple blade stirrers.
Both JP-A-07, 292,002 and WO-A-93 23 350 describe stirrer systems which in each case have flow disrupters and angled stirrer blades and lead to dispersions of narrow distribution.
A factor of particular importance in the implementation of polymerization reactions in stirred-vessel reactors is that of heat dissipation. If it is desired to avoid the cooling internals in the interior of the reactor that are known from the prior art, an important requirement of the stirring system is that it should ensure high heat transfer with the reactor walls. Usually there are then suitable cooling devices provided on the outside of the reactor walls. The high reaction enthalpy released in the course of polymerization reactions is intensified in the case, for example, of emulsion polymerization in that a high monomer concentration is required in order to attain a high degree of polymerization with narrow molecular weight distribution and to suppress competing side reactions. A high concentration of monomer, however, is also associated with a high polymerization rate and, correspondingly, a high level of release of heat which must be conducted away from the system. If heat removal is inadequate the result, especially in the case of systems which have not been homogeneously mixed, is local temperature fields which have an adverse effect on product quality, especiall

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