Continuous process for preparing polymers

Details Continuous process for preparing polymers Continuous process for preparing polymers

2001-03-19

2002-04-30

Teskin, Fred (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S072000, C526S918000

Reexamination Certificate

active

06380326

ABSTRACT:

This invention relates to a continuous process for preparing polymers and a method for the reduction of polymer fouling on reactor surfaces, especially in a continuous process for preparing emulsion polymers.
Polymers are typically prepared in batch, semi-continuous, or continuous processes. Such processes are susceptible to various degrees to polymer build-up or fouling on the reactor surfaces. Polymer fouling results in the need to shut the reactors down and clean the reactor surfaces which reduces available production time and may, for certain reactor geometries such as tubular reactors, be highly inconvenient.
European Patent Application 926 161 A discloses a continuous process for preparing polymers in a reactor having a non-cylindrical channel which provides low levels of fouling. Even lower levels of fouling are desired.
“Effects of Dissolved Gas on Emulsions, Emulsion Polymerization, and Surfactant Aggregation” by M. E. Karaman, et al. J. Phys. Chem, 100, 15503-15507(1996) discloses that the presence of dissolved gas in a batch polymerization has a role in emulsion stability and emulsion polymerization.
We have now discovered that minimizing, preferably eliminating, a separate gas phase in a continuous polymerization reactor reduces polymer fouling on the reactor surfaces.
STATEMENT OF THE INVENTION
According to a first aspect of the present invention there is provided a continuous process for preparing a polymer including continuously feeding at least one reaction mixture containing at least one monomer to a reactor wherein the reactor does not contain a gas phase; polymerizing the monomer in the reactor; and continuously removing the polymer from the reactor.
According to a second aspect of the present invention there is provided a method for reducing polymer fouling during a continuous process for preparing a polymer including continuously feeding at least one reaction mixture containing at least one monomer to a reactor wherein the reactor does not contain a gas phase; polymerizing the monomer in the reactor; and continuously removing the polymer from said reactor.
DETAILED DESCRIPTION
The present invention is directed to a process for preparing a polymer including feeding at least one reaction mixture containing at least one monomer to a reactor wherein the reactor does not contain a gas phase; and polymerizing the monomer in the reactor. Preferably, the present invention is directed to a continuous process for preparing a polymer including continuously feeding at least one reaction mixture containing at least one monomer to a reactor wherein the reactor does not contain a gas phase; polymerizing the monomer in the reactor; and continuously removing the polymer from the reactor. In the case of a continuous process the reactor may be a tubular reactor or channel whether, cyclindrical or non-cylindrical in cross-section. By “non-cylindrical” it is meant any shape whereby the reactant is exposed to a greater surface area for a given length than a cylindrical shape. Suitable non-cylindrical shapes of the channel are for example, oval, ellipse, square, triangular, and rectangular.
In a one embodiment a continuous process for preparing polymers includes continuously feeding at least one reaction mixture containing at least one monomer to at least one channel; optionally, continuously controlling the temperature of the channel by exposing the surface of the channel not exposed to the reactant to a temperature control medium; polymerizing the monomer in at least one channel; and continuously removing the polymer from at least channel; desirably the rate at which the at least one reaction mixture containing at least one monomer is fed to at least one channel containing polymer is controlled, such that the amount of monomer in the at least one channel does not exceed the amount that may be swollen into the polymer in the at least one channel.
The surface of the one or more channels not exposed to the reaction mixture containing at least one monomer may be exposed to a temperature control medium for the purpose of heating or cooling the reaction misture. The temperature control medium may be a solid, gas or liquid. A typical gas medium may be applied by simply exposing the channel to air. A liquid medium may be for example, water, brine, or glycol solvents such as ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, and the like. A solid medium may be for example a refrigerated or an electrically heated metal plate. It is preferable that the temperature control medium be a liquid.
The process may be operated at any temperature. The temperature typically ranges from 0 to 350° C., preferably 1 to 200° C., more preferably 3 to 100° C. The process may be operated under vacuum as low as 25 mm Hg, or at pressures up to 5,000 psi. The flow rate through the channel for the process may range from 50 ml/min to 750 L/min.
Pockets of gas in the reactor have been observed as increased fouling regions in the reactor. Therefore, a distinct gas phase is prevented, in part, through reactor design so that pockets of gas are not maintained during the polymerization process; further, the amount of gas present in the reaction mixture desirably ranges from 0 to 100% of saturation at reaction conditions (i.e., no free gas phase), preferably 0 to 50% of saturation. Preheating feed streams, substituting gases, and reduced pressures (vacuum) may be used to remove gas from the reaction mixture containing at least one monomer being fed to one or more non-cylindrical channels. With pre-heating of the feed streams in a vented environment, the solubility of gas in the feed streams decreases, thereby lowering the amount of gas entering the reaction mixture. Sparging feeds with a gas such as Helium will displace the air normally present; Helium has an advantage in that its solubility varies only slightly with temperature.
The use of reduced pressure on the reaction mixture may be effected in various ways. In one embodiment vacuum may be maintained on the head space of a feed tank containing the reaction mixture. In a second embodiment gas permeable tubing may be used to deliver the reaction mixture to the reactor. This tubing may be placed in a reduced pressure enviroment such as a vacuum chamber,which allows gas present in the reaction mixture to be removed from the feed stream, through the tubing wall into the vacuum chamber. In a third embodiment a commercially available device for degassing may be used such as a Versator manufactured by The Cornell Machine Company. The Versator contains a degassing chamber which uses centrifugal force to increase the gas liquid interface which leads to increased degassing efficiency.
One advantage to using the Versator to degas is that it may also be used as a premixer to emulsify the feed streams when an emulsion polymerization is being carried out. The monomer emulsion formed by the degassed aqueous and monomer streams is very stable.
The channels may be constructed of any material suitable for forming into the desired shape and capable of providing sufficient heat transfer when exposed to a temperature control medium. Such materials are for example plastics such as polycarbonate and polypropylene, stainless-steel types 304 and 316; titanium, Monel, Incoloy 825, Hastelloy C, phosphor bronze, and cupronickel. In addition, the portion of the channel exposed to the reaction mixture containing at least one monomer may be coated with materials such as graphite or polytetrafluoroethylene to aid in flow.
When more than one channel is used, the channels may be the same or different length and may be run in series or in parallel. Each channel may also be run at different reaction conditions, such as at different temperature and pressure conditions.
At least one reaction mixture containing at least one monomer is fed to the channels and flows through the channels, preferably alternating with the temperature control medium. When the polymer is “grown out”, the rate at which the reaction mixture is fed is critical. By “grown out” is meant that a polymer chai

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