Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Organic component
Patent
1995-03-10
1996-08-20
Langel, Wayne
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Organic component
423351, 528272, 528274, B01J 800
Patent
active
055476520
DESCRIPTION:
BRIEF SUMMARY
The present invention refers to a process for the purification of an inert gas containing impurities formed of organic compounds.
The invention relates particularly to a process for the purification of an inert gas coming from solid state polycondensation SSP reactors of polyester resins.
The impurities present in the inert gas streams (nitrogen) used as cleaning gas in the SSP reactors are generally formed of aldehydes and glycols (acetaldehyde and ethyleneglycol in the case of polyethyleneterephthalate) and glycol oligomers.
These impurities are stripped from polymer chips and accumulate in the inert gas stream. They are present in the gas to be purified, in quantities, defined as methane equivalent, up to about 2000-3000 ppm or more.
The method so far used for the purification of inert gases coming from SSP reactors includes: the first stage; water formed in the previous stages.
The oxidation stage is carried out with oxygen or with gas containing oxygen (generally air) by using an oxygen excess on the stoichiometric quantity as regards the impurities.
The oxidation stage performance is controlled so that the gaseous stream at the outlet contains an oxygen excess of 50-500 ppm.
The reaction is generally carried out at a temperature between 500.degree. C. and 600.degree. C. by circulating the gaseous stream on a catalyst bed formed of a support coated with platinum or platinum/palladium.
The high oxygen content present in the gaseous stream coming out of the oxidation section does not allow to recycle the same to the SSP reactor, previous drying-process, due to the possible oxidation reactions and/or polymer degradation.
It is necessary a deoxidation treatment with hydrogen in order to eliminate the present oxygen. The performance of the deoxidation section is monitored by controlling the stream hydrogen excess at the outlet and the oxygen content.
The last stage is a drying-treatment carried out by circulating the gas on a silica gel, molecular sieves or other beds of drying materials. In this stage the water stripped from polymer chips and generated in the oxidation and deoxidation stages, is eliminated.
After this stage the gas is recycled to the SSP reactor. It has been unexpectedly found, that it is possible to purify with satisfactory results the stream of an inert gas coming out of a SSP reactor, even using the stoichiometric quantity of oxygen necessary to the oxidation (combustion) of the impurities or a quantity slightly higher than the stoichiometric one, not so high however to have oxygen concentrations higher than about 10 ppm in the gas coming out of the reactor.
The thus purified gas is directly recycled to the SSP reactor, previous a drying-treatment, omitting in this way the deoxidation stage with hydrogen.
The possible oxygen traces present in the recycled gas do not cause oxidative effects and/or polymer degradation. Even if the invention process operates with the oxygen stoichiometric quantity or little higher, it allows to reduce the impurities to acceptable values, less than 10 ppm (defined as methane equivalent).
To realize the purification process in continuous it is necessary the stoichiometry continuous control of the oxidation reaction; it is necessary to monitor the oxygen quantity present in the gaseous stream at the outlet of the oxidation reactor with devices able to detect the O.sub.2 (10 ppm or less) with very short answer times (less than a few second), sampling included.
It has been found, and this is another aspect of the invention, that the zirconia sensors for oxygen are analysers suitable for working the invention process.
These sensors include a ceramic material made of zirconia doped with yttrium that, when it is heated at a temperature higher than 620.degree. C., is converted into electrolytic conductor for the oxygen ions and it is able to function as oxygen concentration cell.
The sensor is generally formed of a probe with porous platinum coating on the internal and external surface. Air or a gas with known oxygen concentration, which is used as reference gas,
REFERENCES:
patent: 2756121 (1956-07-01), Grimes
patent: 4919903 (1990-04-01), Gandhi et al.
Database WPI, Section Ch, Week 8711, Derwent Publications, Inc. (London, GB), AN 87-073107 & DD,A,240 672 (VEB Leuna-Werk Ulbricht).
Boveri Giuseppina
Ghisolfi Guido
Giordano Dario
Langel Wayne
Manzo Edward D.
Murphy Mark J.
Ringsred Ted K.
Sinco Engineering S.p.A.
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