Process for preparing polymers

Details Process for preparing polymers Process for preparing polymers

2000-04-05

2001-11-20

Wu, David W. (Department: 1713)

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

Synthetic resins

Removing and recycling removed material from an ongoing...

Reexamination Certificate

active

06320000

ABSTRACT:

This invention relates to a process for preparing polymers resulting in reduced volatile organic compound (VOC) emissions and minimal gel formation. The process for preparing polymers utilizes the displacement of air in a polymerization reactor with water vapor, a closed system, and a pump at high operating pressure to pump the polymer through a heat exchanger in an external loop.
Polymerizations are the reactions of monomers to form polymers. There are several problems associated with polymerizations. One problem is that most polymerizations are exothermic. For exothermic polymerizations, the heat evolved during the polymerization must be removed to control the reactor temperature. Typically, reactors for polymerizations are jacketed and coolant flows through the jacket to control the temperature of the polymerization. This method of cooling is not very efficient because the cooling area is limited to the reactor jacket, and therefore the time it takes to run the polymerization is extended to as long as 10 hours.
A second problem associated with polymerizations is that pressure may build up in the reactor as a result of inert gasses in the space above the reaction mixture. Typically, reactors for polymerizations are vented so that when the pressure reaches a certain point, the vent opens and releases the gasses. The gasses vented during the polymerization process include air, water, and VOCs including unreacted monomer. Due to environmental concerns over monomers and other VOCs, when the gasses are vented they must be sent to pollution control equipment. This is known as abatement. Abatement results in increased cost to operate the polymerization reaction.
A third problem associated with polymerizations is the formation of gel. Gel is an agglomeration of polymer particles. Gel can have adverse effects on the end use applications for the polymers. Therefore, gel is undesirable.
Therefore, there is a need for a process of preparing polymers which provides better efficiency through increased cooling area, minimizes VOC emissions, and does not form a significant amount of gel.
One approach to controlling the temperature of a polymerization is disclosed in EP 834518. The disclosure teaches removing polymer from the reactor and feeding it through an external loop to a heat exchanger, then returning the polymer to the reactor. The disclosure teaches that plate and frame heat exchangers are not suitable for these purposes and that spiral heat exchangers are preferred. The pump was limited to a low shear pump and was placed on the inlet side of the heat exchanger. The disclosure does not teach how to avoid the fomation of gel or minimize VOC emissions.
The use of plate and frame heat exchangers in a system similar to that described above is disclosed in Industrial Polymerization Apparatus (IV), Emulsion Polymerization Rectors (Part 1), China Synthetic Rubber Industry, 17 (5), 299-303 (1994), L. Feng and Y. Li. The disclosure does not teach how to avoid the formation of gel or how to minimize VOC emissions.
Despite these disclosures, there is a continuing need for a process of preparing polymers which provides better efficiency through increased cooling area, minimizes VOC emissions, and does not form a significant amount of gel.
We have found that replacing air in a polymerization reactor with water vapor and using a closed system reactor with a pump run at high operating pressure meets this need. By closed system reactor is meant that inert gasses do not escape during the polymerization. By high operating pressure is meant from 25 psig to 100 psig.
The present invention provides a process for preparing polymers including: providing an open system reactor; replacing air in the reactor with water vapor; closing the system; feeding at least one monomer to the reactor; reacting the at least one monomer in the reactor to form a polymer; and pumping the polymer with a pump at high operating pressure in an external loop through a heat exchanger back to the reactor.
In a second aspect, the present invention provides a method for reducing VOC emissions during a polymerization process including: a) providing an open system reactor; b) replacing air in the reactor with water vapor; c) closing the system; d) feeding at least one monomer to the reactor; e) reacting the at least one monomer in the reactor to form a polymer; and f) pumping the polymer with a pump at high operating pressure in an external loop through a heat exchanger back to the reactor.
The reactor utilized in this invention must be able to be run in both an open system and a closed system mode. By open system is meant that inert gasses may exit the system without pressure build up. One method of providing an open system is to utilize a vented reactor where the vent is kept open.
By closed system is meant that gasses and pressure may build up in the system. One method of providing a closed system is to utilize a vented reactor where the vent is kept closed during the polymerization. Typically, in a low operating pressure system, the vent in a closed system is closed and set to open at a low pressure such as 10 psig.
The reactor may be jacketed. When the reactor is jacketed, coolant such as water flows through the jacket. The flow rate of the coolant may be constant or may vary. Generally, when jacketed reactors are utilized, 25% of the total thermal energy generated by the polymerization reaction (BTUs per minute) will be removed by the coolant in the jacket.
The process of this invention requires that the system be open while air in the reactor is replaced by water vapor. This may be accomplished by a method selected from feeding cold water to the reactor and heating the water, feeding hot water to the reactor, sparging the reactor with steam, and combinations thereof. The temperature of the hot water may range from 50° C. to 99° C., preferably 80° C. to 95° C. The amount of hot water fed to the reactor will depend upon the recipe for the polymer to be prepared and the reactor size, but in a 2500 liter reactor, the amount of water typically ranges from 90 kg to 700 kg, preferably 270 kg to 550 kg. The reactor may be sparged with steam either from the top or the bottom of the reactor.
It is known in the art that oxygen in the air within a polymerization reactor inhibits polymerization. To overcome this problem, those skilled in the art have traditionally utilized a helium or nitrogen sweep to replace the air in the reactor. The replacement of air with water vapor in the process of this invention therefore provides additional benefits by making the helium or nitrogen sweep optional, which may reduce the cost to run the process.
After air in the reactor has been replaced by water vapor, the system must be closed. In a vented system, the system may be closed by closing the vent valve. In normal operations, the closed vent valve may be set at 10 psig, meaning that if pressure builds up in the reactor, the vent valve will open when the pressure reaches 10 psig. In the process of this invention, the vent valve remains closed during the polymerization because the vent valve controller is set at a high pressure. The vent valve setting ranges from 25 psig to 100 psig, preferably from 25 psig to 40 psig.
After the system is closed, at least one monomer is fed to the reactor. Among the monomers that may be useful are ethylenically unsaturated monomers which include, but are not limited to (meth)acrylic ester monomers including methyl acrylate, ethyl acrylate, butyl acrylate, 2-ethylhexyl acrylate, decyl acrylate, methyl methacrylate, butyl methacrylate, hydroxyethyl methacrylate, and hydroxypropyl acrylate; acrylamide or substituted acrylamides; styrene or substituted styrene; vinyl acetate or other vinyl esters; vinyl monomers such as vinyl chloride, vinylidene chloride, N-vinyl pyrolidone; and acrylonitrile or methacrylonitrile. Butyl acrylate, methyl methacrylate, and styrene are preferred. By (meth)acrylic is meant both acrylic and methacrylic.
Ethylenically unsaturated acid containing monomers or salts thereof may also be useful. Suita

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