Title of Invention

METHOD FOR INCREASING THE MOLECULAR WEIGHT OF A POLYMER GRANULATE

Abstract

The invention relates to a method for increasing the molecular weight of a polymer granulate, which comes from a polycondensation plant, is at least partially crystallised and brought into direct contact in a post-condensation stage with treatment gas containing nitrogen and passed in a circulating system, whereby the temperature of the polymer granulate in the post-condensation is increased to between 175 and 250°C with respect to the granulate coming out of the polycondensation plant. An inlet gas or unburnt gas, extracted from the air by physical methods, high in nitrogen and with a residual oxygen content of 0.1 - 5% vol., is added along with hydrocarbons to the treatment gas before it is passed via an oxidation stage and then to the post-condensation.

Full Text

Zimmer AG Borsigallee 1 60388 Frankfurt am Main Method for increasing the molecular weight of a polymer granulate The invention relates to a method for increasing the molecular weight of a polymer granulate from a polycondensation plant, is at least partially crystallised and, in a post-condensation stage, is brought into direct contact with a treatment gas which contains nitrogen and is passed in a circuit, whereby the temperature of the polymer granulate is increased in the post-condensation to between 175 and 250°C with respect to the granulate from the polycondensation plant. In the post-condensation, the treatment gas ensures uniformity or homogeneity in the temperature and the removal of the secondary products formed by the chemical reactions. As polymers, polyesters such as, for example, polyethylene terephthalate (PETP), polybutylene terephthalate (PBTP), polypropylene terephthalate (PTTP) or copolyesters and polyamides (PA) can be considered. These polymers are used pure or doped preferably for the manufacture of packaging materials, such as for example, films and bottles or, for example, for the manufacture of highly viscous yarns and fibres. The polycondensation of the most varied monomers and the increase in the molecular weight by post-condensation is already known. Details are described, for example, in EP 0 685 502 B1, in Rompp, "Chemie-Lexikon" (Chemical Dictionary), 10th edition, page 1316, and in Houben-Weyl, "Methoden der Organischen Chemie" (Methods in Organic Chemistry), 4th edition, Vol. E20, Part 1, "Festphasen-Polykondensation (Nachreaktion in fester Phase)" (Solid-phase polycondensation (secondary reaction in the solid phase)). Polycondensation plants are described in Ullmann's "Encyclopadie der technischen Chemie" (Encyclopaedia of technical chemistry), 4th edition, Vol. 19, pages 117-119. From DE 100 43 277, a method is known in which waste gas from the polycondensation plant containing nitrogen and optionally hydrocarbons is added to the treatment gas in the volume ratio of 1:1 to 1:1000 before it is passed to the post-condensation. The hydrocarbons are removed in an oxidation stage preceding the post-condensation, whereby, if required, additional oxygen is fed in. Using this method, the addition of pure, oxygen-free nitrogen from the outside to compensate for leakage can be reduced or completely omitted. This method can however only be used when the post-condensation plant is located in the vicinity of the polycondensation plant in order to facilitate the supply with its waste gas containing hydrocarbons. A further disadvantage is that the process control in the post-condensation plant depends on the operational state of the polycondensation plant. Operational disturbances or malfunctions in the polycondensation plant therefore lead similarly to operational disturbances or malfunctions in the following post-condensation plant. Retrofitting existing plant complexes with a method according to DE 100 43 277 is expensive and in some cases not viable economically. For the production of nitrogen from air, various methods are used. To obtain technically pure nitrogen, the pressure swing absorption process is normally used by applying molecular sieves. The "Linde" method of air liquefaction followed by fractionation is used preferably when highly pure nitrogen and simultaneously the noble gases contained in the air are to be extracted. Also nitrogen can be produced from air using membrane separation methods. It is also known that oxygen can be removed from technical nitrogen in that combustible gases such as hydrogen, ammonia, etc. are added and the gas mixture is catalytically oxidised. All methods are for example described in "Ullmann's Encyclopaedia of Industrial Chemistry", Sixth Edition. The object of the invention is therefore to further improve the polycondensation method mentioned in the introduction and thus to find an economical and at the same time operationally reliable solution. According to the invention, this is fulfilled in that an inlet gas high in nitrogen, extracted from air by physical or chemical methods and has a residual oxygen content of 0.1 to 5.0% vol., preferably 1.0 to 3.0% vol. of oxygen, and hydrocarbons, are admixed to the treatment gas, before the treatment gas is passed through an oxidation stage and then into the post-condensation. The inlet or unburnt gas high in nitrogen can be produced economically from the air, for example, by the pressure swing absorption method mentioned above. The inlet gas high in nitrogen and the hydrocarbons can be added to the used treatment gas separately or premixed. For use in the post-condensation, the treatment gas may have a maximum of 100 ppm of oxygen. Therefore, the quantity of hydrocarbons admixed to the treatment gas must be sufficient to completely remove, where possible, the residual oxygen passed with the unburnt inlet gas high in nitrogen. The hydrocarbons should be selected from the group of alkanes, alkenes and alkines or mixtures of them. Propane, butane, petroleum ether, LPG or mixtures of them are preferred. Here, LPG is taken to mean the liquid gas usually used to heat dwellings. The quantity of admixed hydrocarbons is controlled based on the measurement of the carbon monoxide content, the hydrocarbon content, the oxygen content or the so-called "lambda value" (the ratio of the oxygen content to the hydrocarbon content) of the treatment gas after the oxidation stage. The control based on the measurement of the carbon monoxide content is prefered, because this measurement does not need any residual oxygen in the treatment gas. Methods and devices for carrying out these measurements are known to the specialist. The treatment gas then passed to the post-condensation is almost free of oxygen and is suitably dried before it enters the post-condensation. Ways of implementing the method are explained with the aid of the drawing. It shows a possible flow chart for the method without limiting the invention to this way of implementation. From a known polycondensation plant (1) polymer granulate is obtained which is passed in the line (2) to a crystallisation in order to avoid sticking together. This crystallisation may be single or multi-stage. Methods relating to this are known to the person skilled in the art. In the following a two-stage crystallisation system is described. In the first stage (3) the polymer granulate is fluidised in the fluidised bed using fluidising gas high in nitrogen from the line (4) the said granulate being brought to an increased temperature. The temperature in the fluidised bed lies in the region of 100 to 250°C and is preferably at least 150°C. The polymer granulate then passes through the line (5) to the second crystallisation stage (6), which is formed, for example, as a paddle mixer, and where the granulate is indirectly heated. The two-stage crystallisation, mostly with a temperature increase in each stage, already provides an increase in the level of crystallisation, but not a sufficient increase in the molecular weight of the polymer. For a further increase in the molecular weight and therefore also in the viscosity, the post-condensation reactor (8) is provided, to which the granulate is passed through the line (7). To the reactor (8), dry, practically oxygen-free treatment gas is passed through the line (9a), the principal component of the said treatment gas being nitrogen. The treatment gas is passed upwards through the packed bed located in the reactor, whereby the temperature is evened out and reaction products are removed. The dwell time of the granulate in the reactor (8) is usually in the range from 8 to 22 hours. Polymer granulate with increased molecular weight is removed from the reactor (8) in the line (10) and is passed effectively to cooling and dust removal stages which are not shown. Used treatment gas is drawn out of the post-condensation reactor (8) in the line (11) and is mixed with the gas in the line (12). The gas mixture formed is passed through the line (13) for dust removal by a cyclone (14) and gas free of dust is drawn through the line (15) with the aid of the blower (16) and distributed on the lines (17) and (18). The gas in line (17) is passed back into the first crystallisation stage (3) via the heater (19) and the line (4). The gas flow in the line (18) is passed back to the reactor (8) as treatment gas after fine purification. Firstly, hydrocarbons are added to the gas in the line (18) through the line (30a). To compensate for gas losses, the gas high in nitrogen, which has been extracted from the air by physical methods, is added through the line 30b. The order for addition can also be reversed. It is also possible to admix the gas containing nitrogen together with the hydrocarbons, optionally after prior premixing, through the line (30a). The first heating takes place in the indirect heat exchanger (20). Then the gas is passed through the line (21) to a heater (22) in order to achieve the desired inlet temperature for the oxidation reactor (23) in the line (22a). The oxidation reactor (23) contains, for example, a packed bed of a grainy oxidation catalyst (e.g. based on platinum or palladium) in order to eliminate hydrocarbons oxidatively. If required, oxygen, e.g. in the form of air, is fed in through the line (24). The gas leaving the reactor (23) through the line (25) has an increased temperature which may be close to 400°C. The gas releases part of its perceivable heat in the heat exchanger (20) and then flows through the line (26) to a further heat exchanger (27), before it is fed to a dryer (29) through the line (28). The dryer can, for example, operate absorptively in a known way. The dried gas flows through the line (9) to the heat exchanger (27) and is passed through the line (9a) as treatment gas into the reactor (8). Preferably, this treatment gas exhibits a carbon monoxide content of 10 to 500 mg/Nm3. It is also possible that in addition waste gas from the polycondensation plant and containing nitrogen is admixed to the treatment gas in the volume ratio from 1:1 to 1:1000, before it is passed to the post-condensation. A method of admixing nitrogen-containing waste gas from the polycondensation plant to the treatment gas is known from DE 100 43 277. In this case, the waste gas containing nitrogen is introduced into the line (18) before the blower (30). Example In a conventional polycondensation plant (1) polyethylene terephthalate (PETP) is produced from terephthalic acid, isophthalic acid and ethylene glycol, the said polyethylene terephthalate being treated further according to the drawing. Table I shows the change in the polycondensation state of the PETP granulate: Processing takes place with a fluidised bed crystalliser (3) at 160°C and a paddle crystalliser (6) at 205°C; the dwell period in the fluidised bed crystalliser is 15 minutes and 60 minutes in the paddle crystalliser. In the reactor (8), the granulate is located in the packed bed. At a temperature of 205°C, the dwell time in the reactor (8) is 11 hours. Furthermore, an electric heater (22) and absorption drying (29) using a molecular sieve is applied. To the packed-bed crystalliser (3), 7532 kg/h of PETP granulate are passed at 60°C and air is fed in through the line (24). The gas quantities and temperatures are shown in Table II. Gas compositions are given in Table III; here, OC = organic component. Patent Claims Method for increasing the molecular weight of a polymer granulate, which comes from a polycondensation plant, is at least partially crystallised and, in a post-condensation stage, is brought into direct contact, with a treatment gas containing nitrogen, whereby the temperature of the polymer granulate in the post-condensation is increased with respect to the granulate coming from the polycondensation plant to between 175 and 250°C, characterised in that an inlet gas high in nitrogen, extracted from air using physical methods and with a residual oxygen content of 0.1 - 5% vol. and hydrocarbons are admixed to the treatment gas, before it is passed via an oxidation stage and then to the post-condensation. Method according to Claim 1, characterised in that the quantity of the admixed hydrocarbons is sufficient to remove, in the oxidation stage, the oxygen which was fed in by the inlet gas high in nitrogen, the said inlet gas being extracted from the air. Method according to Claim 1 or 2, characterised in that the hydrocarbons are selected from the group of the alkanes, alkenes and alkines or mixtures of them. Method according to Claim 3, characterised in that the hydrocarbons are selected from the group containing propane, butane, LPG or mixtures of them. Method according to Claim 1 to 4, characterised in that the quantity of the admixed hydrocarbons is controlled based on the measurement of the carbon monoxide content, the hydrocarbon content, the oxygen content or the ratio of the oxygen content to the hydrocarbon content of the treatment gas after the oxidation stage. Method according to Claim 1 to 5, characterised in that in addition nitrogen-containing waste gas from the polycondensation plant is admixed to the treatment gas in the volume ratio of 1:1 to 1:1000, before it is passed to the post-condensation. Method for the production of a gas high in nitrogen for use in increasing the molecular weight of a polymer granulate which comes from a polycondensation plant, wherein inert gas containing nitrogen is introduced into at least one reactor, the said inert gas is at least partially crystallised and the polymer granulate is brought into direct contact with a treatment gas containing nitrogen in a post-condensation, whereby the temperature of the polymer granulate in the post-condensation is increased to between 175 and 250°C with respect to the granulate coming from the polycondensation plant, characterised in that inlet gas containing nitrogen, extracted from air using physical methods and with a residual oxygen content of 0.1 - 5% vol. is added to the treatment gas along with hydrocarbons before it is passed via an oxidation stage and then into the post-condensation.

Documents:

2840-chenp-2005 abstract duplicate.pdf

2840-chenp-2005 claims duplicate.pdf

2840-chenp-2005 description (complete) duplicate.pdf

2840-chenp-2005 drawings duplicate.pdf

2840-chenp-2005-abstract.pdf

2840-chenp-2005-claims.pdf

2840-chenp-2005-correspondnece-others.pdf

2840-chenp-2005-description(complete).pdf

2840-chenp-2005-drawings.pdf

2840-chenp-2005-form 1.pdf

2840-chenp-2005-form 18.pdf

2840-chenp-2005-form 3.pdf

2840-chenp-2005-form 5.pdf

2840-chenp-2005-pct.pdf