Method and device for continuous production of polymers

Details Method and device for continuous production of polymers Method and device for continuous production of polymers

2000-11-14

2003-04-29

Teskin, Fred (Department: 1713)

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

Synthetic resins

Polymerizing in tubular or loop reactor

C526S088000, C526S328000, C526S329200, C422S131000

Reexamination Certificate

active

06555629

ABSTRACT:

Process and apparatus for the continuous preparation of polymers
The invention relates to a process and an apparatus for the continuous preparation of polymers in which at least two reaction partners (starting materials) are conducted through a micromixer and combined and mixed with one another.
The reaction partners are, for example, one or more liquid or dissolved monomers and one or more initiators.
DE-A 19 41 266 discloses a process for carrying out chemical reactions between gaseous and/or liquid reaction partners, in which at least two reaction partners are divided into spatially separate microstreams by a multitude of microchannels assigned to each of them, which microstreams then exit into a shared mixing and reaction compartment. The microstreams exit, in the shape of fluid lamellae of the starting materials, from the microchannels into the mixing/reaction compartment at equal flow rates for each starting material. Each fluid lamella of a starting material is conducted into the mixing and reaction compartment immediately adjacent to a fluid lamella of another starting material, the adjacent fluid lamellae mixing with one another by diffusion and/or turbulence.
For the mixing, use is made of what is termed a microstructure lamella mixture having at least one mixing chamber and an upstream guide component for feeding the fluids to be mixed to the mixing chamber. The guide component can be composed of a plurality of plate-like elements which are layered one above the other and are pierced by channels which run at an incline or transversely to the longitudinal axis of the micromixer. The channels of adjacent elements cross without contact and open out into the mixing chamber. Another constructional possibility of a micromixer is a parallel arrangement of channels. The fluids to be mixed flow from opposite directions into the mixer and exit mixed and perpendicularly thereto into the mixing chamber. The mixer brings the reaction partners intimately into contact with one another, i.e. mixes them well with one another. As is implied by the cited document, the quality of the mixing and the effect of the mixing means on the yield of the desired product is greatly dependent on the ratio of the chemical reaction rates given by the reaction kinetics to the mixing rate. In the case of chemically slow reactions, these generally proceed considerably slower than the mixing. If the chemical reaction rates and the mixing rate are of the same order of magnitude, complex interactions occur between the reaction kinetics and the local, turbulence-specific mixing behavior in the reactor used and in the mixing means which is generally a micromixer. If the chemical reaction rates are considerably faster than the mixing rate, the reaction rates and the yields are essentially determined by the mixing, i.e. by the local time-dependent rate and concentration field of the reaction partners.
In the prior art, it is customary to use a number of mixing means or mixers to carry out fast reactions. In this case, a distinction is made between dynamic mixers, such as agitators, turbines or rotor-stator systems and static mixers such as Kenics mixers, SMV mixers and jet mixers.
In the field of solution polymerization of acrylate-based monomers, with or without additives, batch or semibatch technology is usually used, which cannot ensure uniform product quality, owing to the batchwise preparation of polymers. The changeover from the known batch or semibatch technology to a continuous production of solution polymers is accompanied by problems, to the extent that polymers of this type can become insoluble in the solvent at high molecular weights. In the case of a polymer defined as a solution addition polymer, it is a small, high-molecular-weight fraction of the polymer in the molar mass distribution. This high-molecular-weight fraction can, inter alia, be formed by an initially poor mixing of monomers and initiator, since a local deficiency of initiator can lead to the formation of macromolecules having a very high degree of polymerization, which are known to form, in the case of free-radical polymerization, within a time of less than one second. These high-molecular-weight fractions lead to a considerable broadening of the molar mass distribution up to the formation of bimodal molar mass distributions. This causes unwanted deposits to form in the reactor system. Precipitation of the insoluble molecules out of the solution is known to be favored by solid surfaces, such as reactor walls, internals, etc. In tube reactors, which are frequently equipped with static mixers to intensify the mixing operations and heat transfer, there is a high and thus unfavorable surface/volume ratio. This means that, in comparison with a stirred-tank batch reactor of comparable capacity, a higher probability of deposit formation in the reactor system must be assumed, which in the case of the continuously operated tube reactor can lead to a blockage of the system and excludes long-term operation. The generally low amounts of polymer having high molar masses in the product mixture can be sufficient to block a tube reactor, since the process is operated over very long periods. If in the case of the tube reactor there is, at the start of the reactor during metering, a poor homogenization of the reaction mixture over a system-dependent mixing section, intensive deposit formation can occur, particularly in this area.
With the acrylate-based monomers, these can be, for example, copolymers, as are described in DE-A 40 27 594. These addition copolymers are potentially based on alkyl esters and functionalized alkyl esters of &agr;,&bgr;-ethylenically unsaturated carboxylic acids with or without copolymerizable vinyl monomers. A further monomer is, for example, styrene.
EP 0 749 987 A1 discloses a process for continuous anionic polymerization. In this case the monomer system consists of at least one (meth)acrylic acid monomer. The initiator consists of organometallics, as are used for the anionic polymerization. As reactions of this type are very fast reactions which lead to complete conversion within 0.2 to 0.3 seconds, an adiabatic tube reactor having an upstream micromixer was developed for a continuous reaction procedure. The micromixer is a turbulently mixing tangential flow mixer. The residence time in the mixer is approximately 0.05 seconds. The starting materials (monomers, solvent and initiator) are, before being fed into the mixer, cooled to from −14 to −40° C. to prevent the reaction from starting in the mixer. The reaction occurs in the tube reactor. Owing to the adiabatic reaction procedure, depending on the monomerinitiator system, a final temperature of from 44° C. to 91° C. is achieved.
Starting from the above-described prior art, the object of the invention is to provide a process and an apparatus for continuous production of free-radical solution polymers, in which blockage or plugging of the reactor system is substantially avoided and the apparatus can be operated for relatively long periods without interruption.
This object is achieved according to the invention by a process of the type described at the outset in such a manner that the starting materials are, prior to their entry into the micromixer, preheated to the extent that they reach a required reaction temperature after entry into the micromixer, in which they are mixed by diffusion and/or turbulence in such a manner that formation of bimodal molar mass distributions or high-molecular-weight fractions is suppressed and that a polymerization of the monomeric reaction partners takes place in a tube reactor downstream of the micromixer. For this purpose it is necessary that mixing takes place immediately, i.e. that the mixing time is less than the reaction time to form an individual polymer chain. Preferred mixing times are, depending on reaction time, in the range from one second to instantaneous mixing. Typical reaction times are familiar to those skilled in the art and are, depending on reaction type and temperature, in the range from milliseconds to

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