Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Organic component
Reexamination Certificate
2001-12-03
2004-06-15
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Organic component
C528S272000
Reexamination Certificate
active
06749821
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the purification of an inert gas containing impurities formed of organic compounds. The invention further relates to a process for the purification of an inert gas recycled from a polymerization reactor and particularly a solid-state polycondensation (SSP) reactor for aromatic polyester resins.
BACKGROUND OF THE INVENTION
Polymer resins are molded into a variety of useful products. One such polymer resin is polyethylene terephthalate (PET) resin. It is well known that aromatic polyester resins, particularly PET, copolymers of terephthalic acid with lower proportions of isophthalic acid and polybutylene terephthalate are used in the production of beverage containers, films, fibers, packages and tire cord. U.S. Pat. No. 4,064,112 B1 discloses a solid-state polycondensation or polymerization (SSP) process for the production of PET resins.
While for fibers and films the intrinsic viscosity of the resin must generally be between 0.6 to 0.75 dl/g, higher values are necessary for molding materials such as containers and tire cord. Higher intrinsic viscosity such as greater than 0.75 dl/g can only with difficulty be obtained directly through polycondensation of molten PET, commonly called the melt phase process. The SSP process pushes polymerization to a higher degree thereby increasing the molecular weight of the polymer by the heating and removal of reaction products. The polymer with a higher molecular weight has greater mechanical strength and other properties useful for production of containers, fibers and films, for example.
An SSP process starts with polymer chips that are in an amorphous state. U.S. Pat. No. 4,064,112 B1 teaches crystallizing and heating the chips in a crystallizer vessel under agitation to a density of 1.403 to 1.415 g/cm
3
and a temperature ranging between 230° and 245° C. (446° and 473° F.) before entering into the SSP reactor. Otherwise the tacky chips tend to stick together.
The SSP reactor may consist of a cylindrical reactive section containing a vertical mobile bed into which the polymer chips are introduced from above and a frusto-conical dispensing section at the base for dispensing the product chips. The polycondensation reactor typically operates at temperatures between 210° and 220° C. (410 and 428° F.).
Various reactions occur during polycondensation of PET. The main reaction that increases the molecular weight of PET is the elimination of the ethylene glycol group:
PET—COO—CH
2
—CH
2
—OH+HO—CH
2
—CH
2
—OOC—PET→PET—COO—CH
2
—CH
2
—OOC—PET+HO—CH
2
—CH
2
—OH
An inert gas such as nitrogen is run through the polymerization reactor to strip the developing polymer of the impurities. The impurities present in the inert gas stream used in the production of polyethylene terephthalate in an SSP process generally include water and organics such as aldehydes and glycols, typically acetaldehyde and ethylene glycol and glycol oligomers. Also, volatile impurities include low molecular weight PET oligomers, such as the cyclic trimer of PET. Water is removed from the inert gaseous stream before it is recycled to the SSP because it can precipitate a reversal of the polymerization process. The organic impurities are removed to strengthen the polymer product and to assure that the impurities do not taint the compatibility of the end product with its use. Especially important is the prevention of organic impurities from leaching out of a resin container into the beverage contents. These impurities are stripped from polymer chips and accumulate in the inert gaseous stream. The organic impurities are present in the inert gaseous stream to be purified, in quantities, defined as methane equivalent, of about 2000 to 3000 ppm or more. U.S. Pat. No. 5,708,124 B1 discloses maintaining the ratio of inert gas mass flow rate to PET polymer solids mass flow rate to below 0.6 in an SSP reactor.
It is also well known that polyamide resins, and among them particularly PA6, PA6,6, PA11, PA12 and their copolymers, find wide application both in the fiber and flexible packaging sectors, and in the manufactured articles production by blow and extrusion technology. While the resin relative viscosity for fibers is low at about 2.4 to 3.0, higher relative viscosities of 3.2 to 5.0 are needed for articles produced by blow and extrusion technologies. The relative viscosity is increased to above 3.0 by means of an SSP process operating at temperatures of between 140° and 230° C. (284° and 446° F.), depending on the polyamide types used. U.S. Pat. No. 4,460,762 B1 describes an SSP process for a polyamide and different methods to accelerate this reaction.
An SSP process for polyamide resins is also described in the article “Nylon 6 Polymerization in the Solid State,” R. J. Gaymans et al.,
Journal of Applied Polymer Science
, Vol. 27, 2515-2526 (1982) which points out the use of nitrogen as a heating and flushing aid. The reaction is carried out at 145° C. (293° F.).
It is also known that the molecular weight of polycarbonate can be increased through an SSP process. Developing polyamides and polycarbonates also emit organic impurities that must be purged by an inert gas stream that must then be purified.
EP 0 222 714 B1 discloses a method for making polyethylene terephthalate and polyethylene isophthalate with very low generation of acetaldehyde to reduce the amount of purification required of the inert gas.
The conventional method used for the purification of an inert gaseous stream recycled from an SSP process includes an oxidation step for converting the organic impurities to CO
2
and a drying step to eliminate the water formed in the polymerization process and the oxidation step. The oxidation step is carried out with oxygen or with gas containing oxygen, such as air, by using an oxygen concentration of no more than in slight excess of the stoichiometric quantity as regards the organic impurities. The oxidation step is controlled according to U.S. Pat. No. 5,612,011 B1 so that the inert gaseous stream at the outlet contains an oxygen concentration of not more than 250 ppm and preferably according to U.S. Pat. No. 5,547,652 B1 so that the inert gaseous stream at the outlet contains an oxygen concentration of not more than 10 ppm. These patents taught that a previously required deoxidation step of reducing the oxygen with hydrogen between the oxidation and drying steps was not required.
The oxidation reaction is conventionally carried out at a temperature between 250° and 600° C. (482° and 1112° F.) by circulating the inert gaseous stream over a catalyst bed formed of a support coated with platinum or platinum and palladium. The low oxygen content present in the inert gaseous stream exiting the oxidation section allows for recycling the same to the SSP process after the drying step. Moreover, higher oxygen concentrations in the recycled inert gaseous stream present the risk of oxidation reactions which degrade the polymer product, for example, by “yellowing” the product.
Japanese Publication 20885/71 discloses a method of reconstituting inert gas employed in the solid-state polycondensation or polymerization of linear polyesters comprising contacting the gas with one metal oxide at 150° to 300° C. (302° to 572° F.). The organic reaction products contained in the inert gas are oxidized to water and carbon dioxide. However, because the metal oxide loses its activity, it must be heated in the presence of air in a batch process. Accordingly, this publication does not pertain to a continuous catalytic gas purification process.
The last inert gas purifying step is a drying step carried out by circulating the gas over a silica gel, molecular sieves or other beds of drying materials. In this step, the water both stripped from polymer chips by the inert gas stream and generated in the oxidation step is eliminated. After this step, the inert gas is recycled to the SSP process. The small traces of oxygen, when present in the recycled inert gaseous stream, do not cause oxidation effects and/or polymer degradation. Eve
McGehee James F.
Nakano Yoshiki
Richmond John R.
Tegawa Kaoru
Medina Maribel
Paschall James C.
Tolomei John G.
UOP LLC
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