Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymerizing in tubular or loop reactor
Reexamination Certificate
1999-09-08
2001-06-26
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymerizing in tubular or loop reactor
C526S088000, C526S918000, C422S138000, C422S134000
Reexamination Certificate
active
06252016
ABSTRACT:
This invention relates to a process for preparing polymers, in particular to a continuous process for preparing polymers.
Polymers are typically commercially prepared in batch processes. Batch processes require several hours, in some cases greater than eight hours, to feed the reactants, including monomer or monomers into the reactor, conduct the polymerization reaction, cool the resulting polymer, remove the polymer, and clean the reactor. The equipment required for batch processes typically includes reactors which can hold up to 75,000 liters and may cost more than $1,000,000 per reactor. Despite these disadvantages, batch processes do have some advantages, such as providing polymer products with narrow particle size distributions. By narrow particle size distribution is meant that the polydispersity index as measured by an instrument, such as the Matec Applied Sciences CHDF-1100 Eluant Delivery System is less than 2.0. Generally, polymer products with polydispersity indexes greater than 2.0 are not desired due to negative effects on rheology and application performance.
To improve the deficiencies of the batch processes, continuous polymerization processes have been developed. Continuous polymerization processes are potentially more efficient than a batch process. In a continuous process, monomer and other reactants are continuously fed into and through the reactor while, at the same time, polymer is continuously removed from the reactor. A continuous process may produce more product per day, utilizing smaller, less expensive reactors. Continuous processes utilizing continuous stirred tank reactors or tubular reactors are two types of continuous processes.
A continuous stirred tank reactor is a single stirred tank type reactor in which monomers and other reactants are continuously fed into the reactor while polymer is continuously removed from the reactor. The tank type reactor used in this continuous process is similar to the reactors used in a batch process except that because the process is continuous, the capacity of the reactor can be much less. A continuous stirred tank reactor train process is made up of two or more stirred tank reactors connected in series. Monomer and other reactants can be continuously fed into the first reactor and partially reacted. The contents of the first reactor are continuously fed to a second reactor where they may be further reacted. Additional monomer and reactants may be continuously fed to the second reactor. The contents of the second reactor may be continuously fed to a third reactor and so on. Continuous stirred tank reactors have an advantage over the batch process of utilizing less costly equipment because of the ability to use reactors with less capacity, yet still obtain the same output in the same or shorter reaction times. However, because some of the contents of the reactors are back-mixed during the continuous feeding of materials from one reactor to the next, leading to extended retention time of some of the reactants in the process, continuous stirred tank reactor processes tend to yield polymers with particle size distributions having polydispersity indexes of greater than 2.0. This is due to the wide retention time distribution of some of the reactants in the process.
Other attempts have been made to continuously polymerize monomers through the use of tubular reactors. Tubular reactors consist of a cylindrical channel immersed in a temperature control medium. Reactant is fed in one end of the tubular reactor and polymerized inside the tubular reactor, and polymer is removed from the other end of the tubular reactor. In order to facilitate good heat exchange between the temperature control medium and the reactant, the cylindrical channel must be narrow, typically from 1 to 15 cm in diameter. In addition, the flow rate must be balanced to eliminate plugging, yet yield sufficient heat exchange for the polymerization to occur.
Tubular reactors have been used in polymerization processes to yield polymers with particle size distributions having polydispersity indexes of less than 2.0. However, because of the narrow cylindrical channel and the need to expose the reaction mixture to the heated walls for a specified residence time to facilitate complete polymerization, tubular reactors, when used to make polymers, need to be long. In some cases, the length of the tubular reactor has been extended to greater than 200 meters. The long, narrow tubular reactors have consistent problems with plugging of the tubes if the flow rate is not right. There have been attempts to solve this problem by increasing the flow rate of the reaction mixture, but the increased flow rate further increases the need to extend the length of the tubular reactor for sufficient residence time and heat transfer.
U.S. Pat. No. 4,713,434 ('834) attempts to solve the plugging problem of tubular reactors. This patent discloses a tubular reactor for continuous emulsion polymerization where the internal surface of the tube is lined with a saturated polyolefin. The saturated polyolefin may be a fluorinated saturated polyolefin. Although this patent resolves the issue of plugging, it does not solve the problem of inefficient heat transfer. This tubular reactor is still required to be on the order of 70 meters long.
Therefore, there is a need for a continuous process utilizing a reactor that has better heat transfer efficiency than a tubular reactor and that is non-plugging. In addition, as an added benefit there is a need for a continuous process which provides polymers with particle size distributions of 2.0 or less.
We have found a continuous process for preparing polymers that meets this need. The present invention is a continuous process for preparing polymers including continuously feeding a reaction mixture containing at least one monomer to at least one non-cylindrical channel, continuously controlling the temperature of the non-cylindrical channel by exposing the surface of the non-cylindrical channel not exposed to the reaction mixture containing at least one monomer to a temperature control medium, polymerizing the monomer in at least one non-cylindrical channel and continuously removing polymer from at least one non-cylindrical channel. 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 flat.
The surface of the one or more non-cylindrical channels not exposed to the reaction mixture containing at least one monomer can be exposed to a temperature control medium. The temperature control medium may be a solid, gas or liquid. A typical gas medium may be applied by simply exposing the non-cylindrical channel to air. Liquid medium may be for example, water, brine, or glycol solvents such as ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, and the like. Solid medium may be for example 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.
The non-cylindrical channels can be immersed in the temperature control medium by methods known in the art such as simply exposing to air, placing them in a forced air oven or placing them in a bath containing liquid or solid temperature control medium. However, it is preferable that the temperature control medium flows through separate, alternating channels to the non-cylindrical channels in which the reaction mixture containing at least one monomer flows. By alternating, it is meant that the channel next to a non-cylindrical channel in which the reaction mixture containing at least one monomer flows, contains temperature contr
Kempner James Szkolnick
Wu Richard Shu-Hua
Wudarski Jill Ann
Egwim Kelechi
Greenblatt Gary D.
Rohm and Haas Company
Wu David W.
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