CN1711125A

CN1711125A - Safe removal of volatile, oxidizable compounds from particles, in particular polymer particles

Info

Publication number: CN1711125A
Application number: CN 200380103443
Authority: CN
Inventors: F·-O·梅林; E·诺伊曼; A·-A·菲内特; U·尼肯
Original assignee: Basell Polyolefine GmbH
Current assignee: Basell Polyolefine GmbH
Priority date: 2002-11-22
Filing date: 2003-11-21
Publication date: 2005-12-21
Anticipated expiration: 2023-11-21
Also published as: DE10254807A1; CN100522322C

Abstract

In a method of removing volatile oxidizable compounds from particles present in a container, a gas stream is continuously introduced into the container, the gas stream takes up the oxidizable compound from the particles in the container and a gas stream laden with the oxidizable compound is discharged from the container. In the method of the present invention, oxygen is added to the gas stream which has been discharged and the oxidizable compound present in the discharged gas stream is subsequently at least partly catalytically oxidized by means of the oxygen and this oxidized gas stream forms at least part of the gas stream introduced, so that the gas stream is circulated. This makes safe and inexpensive removal of the oxidizable compounds from the particles possible.

Description

Method for safely removing volatile oxidizable compounds from particles, in particular polymer particles

The invention relates to a method for removing one or more volatile oxidizable compounds present in particles in a container by means of a gas stream, said compounds forming an explosive mixture with oxygen, wherein an inlet gas stream is introduced into the container, the gas stream entrains the oxidizable compounds on the particles, and an outlet gas stream carrying the oxidizable compounds is discharged from the container. The invention also relates to a device for carrying out said method.

The polymerization product formed by the polymerization is usually pelletized or granulated and then temporarily stored in a silo for further processing. Depending on the polymerization process used, the pelletized material contains more or less residual monomer, which is released from the pellets upon storage in the silo.

In order to obtain a particulate material with as low residual monomer as possible, the silo is usually vented to drive the residual monomer from the polymerization product in a diffusion-controlled manner. The expelled monomer is then oxidized to CO, usually by catalytic oxidation₂And water or otherwise rendered harmless.

Venting of the silo can lead to the formation of explosive monomer/air mixtures: the explosion limits of Low Density Polyethylene (LDPE) are 2.7 and 36% by volume of ethylene. To this end, Beret al in "purification criteria for LDPE patches" chem.Ing.Progr.73, 44-49 developed a method of ensuring safe operation of ventilated silos. For this purpose, they calculated an air flow sufficient to keep the ethylene content below the explosion limit of 2.7% by volume.

The disadvantage of this method is the need for complex regulation and monitoring of the air flow and the necessity to take safety measures for the introduction of large quantities of inert gas or water when the explosion limit is exceeded, which in undesirable cases can lead to a shut-down of the entire plant. Furthermore, there is always a risk of locally forming monomer/air mixtures above the explosion limit due to an unreasonable distribution of the particulate material in the silo.

Alternatively, the monomers may be stripped from the polymerization product with an inert gas such as nitrogen to prevent formation of an explosive mixture. However, nitrogen is more expensive than air, and air can be supplied in unlimited amounts, which is particularly noticeable especially in high production and polymers with high residual monomer content.

It is an object of the present invention to overcome the above-mentioned disadvantages of the prior art and to provide a process and an apparatus which allow safe and inexpensive removal of residual monomers from polymer particles, in particular pelletized polymers.

We have found that this object is achieved by a method having the features of claim 1 and by an apparatus having the features of claim 13. Claims 2 to 12 and 14 to 18 define preferred embodiments of the invention.

In a method for removing volatile oxidizable compounds from particles present in a container, a gas stream is continuously introduced into the container, the gas stream entrains the oxidizable compounds from the particles in the container, and the gas stream with the oxidizable compounds is subsequently discharged from the container. In the process of the invention, oxygen is added to the exhaust gas stream and the oxidizable compound present in the exhaust gas stream is subsequently at least partially catalytically oxidized by the oxygen, this oxidized gas stream constituting at least part of the inlet gas stream, so that the gas stream is recycled.

It is important that oxygen is added only after the vessel and that the oxidizable compound is partially consumed in the catalytic oxidation. In this way at least a part of the oxidizable compound becomes harmless and the gas stream returned to the silo contains only inert gas or at most harmless amounts of oxygen. As a result, the amount of gas passing through the vessel depends only on how much oxidizable compound needs to be removed from the particulate at a given time, without the need to adhere to the ethylene content limitation, while avoiding the formation of explosive mixtures in the vessel.

First, the need for explosion protection can be eliminated or at least reduced, as long as it is ensured that the oxidation catalyst fails or the device shuts down immediately upon other failure. In particular, the step of flooding the entire container with nitrogen or water to render it inert in the event of a failure may be omitted, since an inert or at least non-explosive atmosphere is already present during all periods. Second, air can be used as an inexpensive purge gas because oxygen present therein is removed by reaction with the oxidizable compound prior to entering the container.

The volatile oxidizable compound can be any organic compound having a vapor pressure sufficiently high to be removed by the purge gas. The method is preferablyapplied to a compound having a sufficient vapor pressure even at room temperature, but the temperature of the container or the gas stream may be increased to ensure a sufficient vapor pressure, which should preferably be not less than 10⁴Pa (Pa). In principle, each of these is also conceivableThe oxidizable compounds are removed simultaneously, provided that the vapor pressures of the compounds are of the same order, or the removal is carried out in a two-stage process in which the more volatile components are removed first and the other components are subsequently removed at a higher temperature. In carrying out the process, the preferred temperatures are from room temperature to temperatures significantly below the softening point of the particles, otherwise there is a risk of agglomeration of the particles. The lower limit of the temperature only needs to take into account the need for the oxidizable compound to have a sufficient vapor pressure. For the process, a preferred lower limit is from 30 to 100 ℃ and in particular from 40 to 80 ℃.

Preferred organic compounds are residual monomers and/or solvents remaining in the polymer particles during the preparation of the polymer. Particularly important compounds of this type are olefins such as ethylene, propylene and 1-butene, 1-pentene or 1-hexene which firstly have a particularly high vapor pressure and can therefore be removed easily from the corresponding polymer particles, and secondly can be oxidized easily to carbon dioxide and water. Mixtures of these olefins are also preferably removed. The process is also particularly useful for removing materials required to carry out the polymerization reaction. Examples include the particularly preferred aliphatic hydrocarbons methane, ethane, propane, butane, pentane and hexane as well as other volatile solvents, auxiliaries and the like, without the process being restricted thereto. Other easily oxidizable compounds which are particularly preferably removed are styrene and other aromatic hydrocarbons. Partially oxidized hydrocarbons such as alcohols, aldehydes, carboxylic acids and ethers are also possible, provided that they have a sufficiently high vapor pressure. Preferably, partially oxidized compounds corresponding to the above hydrocarbons are removed.

Among the inorganic substances, ammonia is particularly important, but in this case selective oxidation to nitrogen or reduction of the nitrogen oxides formed, for example, by means of a three-way catalyst, must be ensured.

For the purposes of the present invention, the term "particle" includes all aggregates in the condensed state. They may be in particular solids which are preferably present as granules, powders, coarse powders or lumps, or droplets or wax droplets which are formed, for example, in a spray tower. The process is preferably used for degassing of granules of granulated material having a diameter of 1 to 10mm, particularly preferably 2 to 6mm, very particularly preferably 3 to 5mm, since in this case the pressure drop experienced by the gas flowing through the container is low, while having sufficient surface area for removing the oxidizable compound.

In another preferred embodiment of the invention, the particulate material is a polymer particle. It is particularly preferred that the polymer particles are polymer granules. Also particularly preferred is the application of a method of spraying liquid or waxy polymer particles, for example in a wax spraying apparatus, where a large amount of nitrogen is required to spray wax from a nozzle and a large amount of air is required to cool the wax particles. During the cooling of the wax particles, air carries away residual monomers from the wax particles which must be disposed of to be environmentally friendly. With the process of the invention, the air in the spray tower is replaced by an oxygen-free or oxygen-reducing gas mixture, thus preventing the formation of explosive mixtures.

The polymer particles used according to the invention include in particular particles of polyolefins having the following structure

Wherein R1 and R2 are each hydrogen, a straight or branched chain saturated aliphatic hydrocarbon group having 1 to 6 carbon atoms, or a cycloaliphatic group. They also include pelletized materials comprising polyolefin copolymers. Preferred polyolefins are Polyethylene (PE), polypropylene (PP), poly (1-butene) (PB), polyisobutylene and poly (4-methyl-1-pentene) and copolymers of ethylene and propylene (i.e. random copolymers and polyolefin rubbers), terpolymers of ethylene, propylene and hydrocarbons containing two or more non-conjugated double bonds (i.e. polyolefin elastomers) and blends of PP, rubbers and PE, especially in situ produced blends (C3/C3 reactor blends). Other useful copolymers are copolymers of ethylene with acrylates and methacrylates.

Furthermore, the radicals R1 and/or R2 may also comprise aryl and aralkyl groups. Particularly useful polymers of this type are polystyrene and copolymers of styrene with other monomers of the above-mentioned type. The process is also particularly useful for polyesters, polyethers and other oxygen-containing polymers, since the monomers that can be used can likewise be oxidized completely to carbon dioxide and water, and for other polymers which, after preparation, contain solvents and/or monomer residues that meet the above criteria. In the case of hydrolysis-sensitive polymers and polycondensates, the removal of the monomers and/or solvents can be carried out in combination with drying of the particles, as described, for example, in DE4436046A 1.

Depending on the intended purpose, the granulated polymer may also comprise additives, such as stabilizers, plasticizers, colorants, light stabilizers, flame retardants, antioxidants or nucleating agents and/or fillers.

In the case of polymer particles, the process of the invention can be used not only for removing monomers and/or solvents, but also for deodorizing the particles. For example, steam may be used in place of air to extract odoriferous substances.

The addition of a stoichiometric amount of oxygen, based on the complete oxidation of the oxidizable compound, is particularly advantageous. In this way, the oxidizable compound is first converted essentially completely into non-combustible and preferably also harmless oxidation products such as CO₂And water and achieve substantially complete degassing. Secondly, the added oxygen is also completely consumed so that no explosive mixture is formed. A slight excess of oxygen may also be used to achieve complete oxidation, provided that oxygen does not build up in the loop over time and the concentration of oxygen in the vessel does not exceed about 7% by volume of the explosionAnd (4) limiting. The proportion of oxygen in the container is preferably from 0.1 to 5% by volume, more preferably from 0.5 to 4% by volume, particularly preferably from 1 to 3% by volume.

The use of air as the source of oxygen is particularly inexpensive and simple and is therefore particularly preferred. In a further preferred variant of the invention, the amount of oxygen added is adjusted on the basis of the oxygen content and/or the oxidizable compound content measured in the oxidized gas emission stream (i.e. after passage through the catalyst).

Another prerequisite is that the catalytic oxidation of the oxidizable compound can be carried out, that is to say that a suitable oxidation catalyst is available. Useful oxidation catalysts are in particular noble metal catalysts and metal oxide catalysts which are suitable for a wide range of organic compounds. They may be present in the form of monolithic catalysts, for example beds or plates of catalyst particles, to which the process of the invention is not restricted. For noble metal catalysts, especially for the oxidation of hydrocarbons, preferred active ingredients include platinum, palladium or rhodium, either in pure form or in a mixture. If possible, the oxidizable compound should oxidize compounds that are not harmful to the environment. The catalyst system selected, or the use of a combination of catalysts, must be matched to the particular conditions. In addition, the exit gas stream must be free of catalyst poisons (e.g., sulfur compounds).

In a preferred variant, the particles are introduced and discharged continuously from the vessel, the gas flow being conveyed in the opposite direction to the particles.

After the catalytic oxidation of the cycle gas has been run for an extended period of time to completely free the vessels and lines from oxygen, the loop is preferably purged with an inert gas, particularly nitrogen, during the start-up phase if an oxygen-containing gas, particularly air, is present in the system. The degassing device according to the invention is thus free of explosive oxygen content from the beginning and remains there. In a more preferred variant, after the inerting of the apparatus, the oxygen content in the container is continuously increased during the start-up phase to a predetermined preferred level of 0.1 to 5% by volume and is then kept constant.

Another aspect of the invention is a degassing apparatus for carrying out the above process. Important components of the apparatus include a vessel, a catalyst unit. A gas loop and a gas metering device.

The vessel for containing the polymer particles is provided with a gas inlet and a gas outlet. In the simplest case, the container is a silo for storing the granulated polymer. Such a silo is usually used for the temporary storage of freshly prepared granulated polymer before transfer or packaging. The use of silo chambers for degassing granulated polymers has been known per se for a relatively long time. Furthermore, other types of vessels, such as extraction columns, flow tubes, filter screens, stirred tanks or fluidized-bed reactors, can in principle also be used in the process according to the invention.

In a preferred arrangement, the polymer particle outlet and the gas inlet are located on one side of the vessel, particularly preferably on the lower side, and the polymer particle inlet together with the gas outlet is located on the opposite side of the vessel, particularly preferably on the upper side. When the degassing device is in operation, the gas stream flows counter-currently to the polymer particles, and the freshly introduced gas stream, which is free of oxidizable compound, is brought into contact with the particles which have been substantially degassed, so that a low residual content of the particulate oxidizable compound is maintained.

The catalyst device used according to the invention comprises an oxidation catalyst for oxidizing residual monomers carried by the outflowing gas stream from the container with oxygen. The oxidation catalyst preferably comprises a plurality of conventional monolithic three-way catalysts or oxidation catalysts for automotive exhaust purification. In a particularly preferred embodiment, the catalyst unit operates automatically. The gas outlet of the vessel is connected via a gas outlet line to a catalyst unit which in turn is connected via a return line to the gas inlet of the vessel, so that the gas flow can be circulated. The introduction of the oxygen required for the oxidation of the oxidizable compound in the outlet gas stream is effected by means of an air metering unit located in the gas outlet line.

A lambda probe for measuring the oxygen content is preferably arranged on the return line for measuring the oxygen content of the oxidized gas stream. The regulating unit regulates the amount of oxygen introduced via the metering unit as a function of the oxygen content measured by the lambda probe.

The method and apparatus of the present invention will be explained below for the removal of ethylene monomer from Low Density Polyethylene (LDPE) billet with the aid of the accompanying drawings, but the present invention is not limited to the described embodiments. It must be emphasized that the process of the invention is not limited to polyethylene nor to conventional polymers, but is generally suitable for removing volatile combustible substances.

The figure shows a flow diagram of an apparatus for removing ethylene from polymer particles in the form of particulate material 2. Here, the polymerization process and granulation technique to obtain the particulate material are not critical. The process of the present invention is applicable to all conventional polymerisation processes whether the polymerisation is carried out in the gas phase using a fluidised bed or in solution (bulk) and in dispersion (slurry), as all these processes produce a particulate material containing more or less ethylene or solvent residues which must be removed before further processing or transport. Also, the type of polymerization catalyst used (whether a Ziegler-Natta, chromium or metallocene catalyst, or an initiator in the case of free radical polymerization) has little or no effect on the process of the present invention.

The process of the invention can be applied to granular materials obtained by cold granulation or hot granulation. The shape of the particles also plays a minor role, having substantially no effect on the pressure drop and degassing kinetics of the vessel.

The apparatus comprises a bin 1 of conventional construction for storing particulate material 2. Particulate material 2 enters at the top of the silo 1 through a polymer inlet 7 and is conducted out of the bottom of the silo 1 through a polymer outlet 8. The particulate material 2 introduced into the silo has an ethylene content of from 0.1 to 1% by weight, but because of the inert atmosphere of the silo 1 the process is in principle not restricted to a specific ethylene content, and degassing of particulate materials having a higher or lower ethylene content is in principle also possible. The particulate material is typically introduced and degassed continuously at a rate at which the upstream polymerization apparatus provides particulate material, but batch operations may also be performed. Particulate material 2 is conveyed from the respective production unit via line 13 to the silo 1. The degassed particulate material is discharged from the silo 1 via line 14 and passed on to packaging, transport or storage facilities as required.

Furthermore, the silo 1 has a gas inlet 3 at the bottom and a gas outlet 4 at the top, through which the gas flow is conducted into and out of the silo 1. In operation, the gas stream flows through the silo 1 containing the particulate material 2 and entrains ethylene therethrough. The positions of the top particulate material inlet 7 and the gas outlet 4 of the silo 1 and the bottom particulate material outlet 8 and the gas inlet 3 of the silo 1 ensure countercurrent degassing and thus substantial removal of residual monomer from the particulate material 2.

The maximum ethylene concentration reached at equilibrium can be calculated according to henry's law. Since diffusion from the particulate material also plays an important role, the ethylene concentration achieved in the silo 1 can be dependent on the conditionsWellbelow the equilibrium concentration. The design of the degasser, in particular the residence time of the particulate material and the required gas flow rate sufficient to remove the ethylene, is generally familiar to the person skilled in the art. Specific references that may be mentioned herein are Beret et al, "purification criterion for LDPE make bin" chem. Ing. progr.73, 44-49, which is also incorporated herein. The amount of air flow is selected so that the pressure drop in the silo is not more than 10⁴Pa, preferably less than 5X 10³Pa, to achieve as economical an operation as possible. The operating conditions of the catalyst must also be taken into account (see below). Preferred average degassing times are from a few hours to a few days.

The gas stream continuously returned to the silo 1 via the gas return line 10 consists essentially of only nitrogen and carbon dioxide, ideally of only recycled gas containing very little, if any, ethylene, as will be explained in more detail below. In particular, the gas flow of the silo contains no or only trace amounts of oxygen and therefore does not need to provide explosion protection, even in the event of an operational failure, eliminating the formation of explosive mixtures with oxygen in the silo 1.

The ethylene-rich gas stream discharged from the silo 1 is conveyed to the catalyst unit 5 via a gas outlet line 9. The catalyst unit 5 consists essentially of only an oxidation catalyst for oxidizing ethylene present in the exhaust gas to carbon dioxide and water according to the following formula

Conventional catalysts for the purification of automobile exhaust gases, which essentially consist of a honeycomb support coated with a noble metal, such as platinum, palladium or rhodium, are preferably used for this purpose. Suitable catalysts include pure oxidation catalysts, which typically have platinum and palladium as active components, and three-way catalysts based on platinum and rhodium. However, other catalyst systems for the purification of industrial exhaust gases by total oxidation (catalytic post-combustion) can also be used. The oxidation of ethylene and other hydrocarbyl monomers on noble metal surfaces and metal oxides is well known and is described, for example, in VDI Berichte1034(1993) 123-138. The operating range of this noble metal catalyst was about 180-. The lowest reaction temperature at which essentially 100% conversion is obtained depends on the substance to be oxidized, which in the case of ethylene is 280 ℃. In contrast, propylene only needs 210 ℃ and aliphatic hydrocarbons such as pentane need temperatures up to 350 ℃. Other examples of applications of the method of the invention can also be found in VDI Berichte1034(1993) 130-132.

The increase in catalyst temperature, assuming adiabatic temperature conditions, can be calculated from the following equation

Δ T_{ad} = \frac{Δ H_{r} \cdot}{ρ_{G} \cdot c_{p}^{G}} c (C_{2} H_{4})

In the formula,. DELTA.H_r50305kJ/kg (reaction enthalpy of oxidation reaction)

c_p ^G1kJ/kg K (heat capacity of gas),

ρ^G＝1.2kg/m³(density of gas) and

c(C₂H₄) Concentration of ethylene in the exhaust gas stream in kg/m³。

C as an approximation here_p ^GAnd ρ^GIt can be assumed to be independent of gas composition and temperature. For the total oxidation of low ethylene concentrations, about 41921 K.c (C) was obtained as a function of the ethylene load of the gas stream₂H₄)/(kg/m³) Is increased by a temperature increase value.

Since the maximum operating temperature of the noble metal catalyst of about 600 ℃ should not be exceeded for a long time, it must be ensured without further measures that the proportion of ethylene in the gas stream does not significantly exceed a value of about 1% by weight. In order to be able to handle also higher ethylene loads, the temperature increase can be reduced by arranging a further loop 15 to the catalyst unit 5, so that the oxidized gas stream can be recycled to reduce the ethylene concentration in the catalyst unit 5, if necessary. In any case, it must be avoided that the catalyst is heated to above 950 ℃ otherwise there is a risk of irreversible damage.

The inlet temperature of about 280 c required for the catalyst ethylene oxide operation can be obtained by using an air preheater and heat exchanger. This is suitable in case the gas flow load is high and results in an adiabatic temperature rise of more than 250 c. Since the oxidized gas stream has to be cooled to a temperature well below the softening point of the particulate material before being returned to the silo, it is advantageous to use the heat recovered during cooling to preheat the gas stream being sent to the catalyst unit 5. Alternatively, in the case of an adiabatic temperature rise below 200 ℃, the catalyst unit 5 can also be operated autothermally by means of a reversible flow-direction catalyst bed by heating the incoming gas stream using the exiting oxidized gas stream. The design of the autothermally operated catalyst unit 5 is also generally familiar to the person skilled in the art.

The gas discharge pipe 9 is provided with a compressor or blower 11 for the transport gas stream and an automatically regulated air metering unit in the form of a regulating valve 6 for mixing air or another oxygen carrier into the ethylene-containing gas stream. The flow rate in continuous operation is chosen so that a stoichiometric amount or a slight excess of oxygen is added and substantially complete oxidation of the ethylene is achieved. In all cases a significant excess of oxygen should be avoided, so as to ensure that the oxygen concentration in the silo 1 is below the explosion limit of about 7% by volume. Small amounts of ethylene negatively affect the degassing equilibrium at the bottom of the silo but do not negatively affect the safety of the plant.

The amount of air added is regulated by measuring the oxygen content of the oxidized gas stream in gas return line 10 and opening regulating valve 6 sufficiently to allow a small amount of unconsumed oxygen to leave catalyst unit 5. The oxygen content can be measured using a conventional lambda probe, which is also used for automobile exhaust purification. Alternatively or simultaneously, the content of the oxidized compound (here ethylene) can be measured after passing through the catalyst unit 5. This can be done using continuous measurement methods, in particular spectroscopic measurements such as UV/visible, IR or raman spectroscopy, which are sufficiently sensitive and selective to operate, but is not limited thereto.

During the start-up phase of the degasser, a constant amount of ethylene is discharged from the silo 1, provided that the ethylene content of the particulate material 2 is constant. The gas oxidized by means of the catalyst unit 5 contains, apart from a small amount of oxygen, only inert nitrogen and the oxidation products carbon dioxide and water, which are likewise inert.

The oxidized gas is largely recycled back to the silo 1 via the gas return line 10 and is again charged with ethylene in the silo 1. In the ideal case, the gas fed to the silo 1 (gas feed stream) is composed entirely of recycle gas, so that no further inert gas is required. However, the addition of air for ethylene oxidation, i.e. oxygen, continuously increases the amount or pressure of the purge gas, so that an excess is removed from the circuit via the exhaust line 12. This is most simply provided by an overpressure valve or a line (not shown) immersed in the liquid, so that the pressure in the circuit is constant at preferably 10³-10⁵A low overpressure level of Pa. In addition, the oxidized gas stream may be separated from the water vapor by a water separator 16, and the water separator 16 may be combined with a cooler if desired. As a result, dried granulesThe material does not have to be wetted with water or the wet particulate material will be effectively dewatered. The process of the invention is therefore suitable for the removal of residual monomers in conjunction with the drying of the particulate material. Although a water separator may be located upstream of the catalyst, this is not necessary as water vapor accumulating in the gas does not interfere with the catalytic oxidation. In order to make efficient use of energy, the gas entering the catalyst may be thermally coupled to the exhaust gas.

When the degassing apparatus is started,the entire apparatus, i.e. the silo 1, the catalyst unit 5 and the lines 9, 10, are preferably first purged with nitrogen or another inert gas to remove oxygen. The compressor 11 is then opened to bring the catalyst unit 5 to operating temperature. The loading of the silo 1 and the degassing of the particulate material 2 then begin. It is preferred to provide excess oxygen to the gas exiting the silo so that a proportion of 0.1-5% by volume of oxygen is present in the total gas flow through the vessel 1 (i.e. after oxidation). It is particularly preferred to set the oxygen content to 0.5 to 4% by volume, in particular 1 to 3% by volume. With this amount of oxygen, the oxygen concentration is reliably kept below the explosion limit of about 7% by volume and complete oxidation of ethylene to carbon dioxide and water is ensured. Since the addition of excess oxygen is essentially for kinetic reasons, it is possible that a smaller amount of oxygen is already sufficient, provided that the reaction rate over the catalyst is sufficiently high to ensure that the conversion of the gas during the residence time of the catalyst unit 5 is sufficient for the ethylene content of the recycle gas downstream of the catalyst unit.

In a preferred variant, after the inerting, the plant is started up with a large excess of oxygen, but absolutely below 7% by volume. This initially causes oxygen to build up during start-up of the device until the desired oxygen level is reached. From this point on, only the stoichiometric amount of oxygen, i.e. the amount of oxygen consumed by oxidation, is added, so that the oxygen content is maintained at the desired level. The duration of the start-up phase depends on the engineering constraints of the process such as the ethylene content of the polyethylene, the vessel size, the flow rate through the vessel, the catalyst, etc., and can be limited to a few minutes, but also to hours.

Since the oxidized gas stream returned from the catalyst unit 5 to the silo 1 also contains only small amounts of oxygen, there is no danger of explosive mixtures forming during any operating phase. This also applies in the event of a plant failure, as long as the gas flow is immediately shut off when a considerable oxygen concentration downstream of the catalyst unit 5 occurs, for example, as a result of a malfunction or failure. It is therefore only when the plant is started up that a purge with the more expensive inert gas is required, while the purge gas used can be continuously regenerated in operation itself. The build-up of carbon dioxide in the purge gas does occur over time, but this does not negatively affect the degassing process.

The invention has been illustrated by means of a preferred embodiment, however, other variants can be derived therefrom, in particular, the process is equally applicable to the degassing of polymers and copolymers of polypropylene, polybutene-1 and other α -olefins.

Examples 1 to 3

The following examples relate to an apparatus for preparing polyethylene. The process uses what is known to the person skilled in the art as a gas-phase fluidized-bed process, as described in EP475603A, EP089691A and EP571826A, and has a production capacity of about 8 metric tons/hour. The monomer recycle degassing is placed downstream of the polymerization reactor and serves to remove most of the monomer from the polymer. Subsequent granulation in the extruder is also carried out with countercurrent degassing, so that the monomer content in the granular material is further reduced.

The degassing device of the invention is installed downstream of the polymerization plant and comprises a conventional silo containing the particulate material with a diameter of 4 meters and a height of 26 meters. The particulate polymer degassing apparatus used was constructed asdescribed above and comprised a conventional platinum/palladium oxidation catalyst with a pre-air preheater which ensured a temperature of about 300 ℃ entering the catalyst. Degassing was carried out in a silo at 60 ℃. This corresponds to 3.9X 10 at an ethylene loading of 0.1% of the polyethylene particles⁴Pa.t (PE)/kg (ethylene) of the Henry constant of ethylene and 3.9X 10⁴A saturated vapor pressure of Pa.

Determining by means of a headspace gas chromatograph B of the particulate material samples before introduction into the silo and after discharge from the siloThe content of alkene. In this method, volatile components are driven out of the polymer by heating and analyzed by chromatography. In addition, the ethylene content in the reflux line after oxidation was determined by gas chromatography. In all examples, the mean residence time of the particulate material in the silo was 20 hours. The pressure drop of the silo is less than 2 x 10³Pa。

Examples 1 and 2 were carried out at a lower ethylene loading of the pellet material, which is common in pelletized polyethylene after degassing in the extruder of the pelletizing plant. As can be seen from the table below, a very low residual monomer content of about 10ppm is obtained. Substantially complete oxidation of ethylene is achieved in the oxidation catalyst.

In example 3, no degassing was carried out in the extruder used for the granulation, resulting in a content of ethylene of 0.3% by weight in the granular material. In this case, 1000kg/h of the oxidized gas stream are circulated through the second circulation line for diluting the gas stream entering the catalyst, thereby reducing the temperature rise of the catalyst. Here, substantially complete degassing can be obtained.

As can be seen from the table below, a substantially complete oxidation of ethylene in the catalystunit is also achieved in this example. The residual ethylene content of the particulate material treated by the process of the present invention reaches a sufficiently low level. The explosion protection for the silo can be omitted.

Comparative examples

For comparison, the same polymerization apparatus and the same silo as in examples 1 to 3 were used and degassing was carried out in a conventional manner using air. However, air is directly blown into the silo without pretreatment, and ethylene carried over after being discharged from the silo is catalytically oxidized. No gas recycle was performed after ethylene oxidation.

The measured values obtained after reaching the degassing equilibrium for examples 1 to 3 and comparative example C1 are reported in the table below.

Examples	PE flux [t/h]	Compressor product Amount [ m ]³/h]	Particulate material upstream of the storage chamber Ethylene content of the feed [ ppm]	Particles downstream of the storage chamber Ethylene of the granular material Content [ ppm]	Residue after oxidation Ethylene content [ppm]	Oxygen after oxidation In an amount of [% by weight Measurement of]
Examples	PE flux [t/h]	Compressor product Amount [ m ]³/h]			Residue after oxidation Ethylene content [ppm]		C1	8.3	1650	1750	10	-	-
1	8.3	1530	1570	8	17	1.9	C1	8.3	1650	1750	10	-	-
1	8.3	1530	1570	8	17	1.9	2	8.4	1550	1780	10	20	2.0
3	8.3	1550	3220	11	21	2.0	2	8.4	1550	1780	10	20	2.0

Claims

1. A method for the safe removal of one or more volatile oxidizable compounds which can form an explosive mixture with oxygen from particles (2) of a container (1), in which a gas stream is introduced into the container (1), the gas stream entrains the oxidizable compounds from the particles (2), and the gas stream with the oxidizable compounds is discharged from the container (1),

wherein:

adding oxygen to the discharged gas stream, and at least partially catalytically oxidizing the oxidizable compounds present in the discharged gas stream by the oxygen, and

-the oxidized gas stream constitutes at least a part of the gas stream introduced into the vessel (1), whereby the gas stream is recycled.

2. The method of claim 1 wherein the particulate material is polymer particles (2) and the volatile oxidizable compound is residual monomer and/or solvent remaining in the polymer particles (2) after production thereof.

3. The method of claim 2, wherein the polymer particles are solid polymer particles (2), in particular polyolefin particles.

4. The method of claim 2, wherein the particulate matter is spray liquid or waxy polymer particles.

5. The method of any preceding claim wherein oxygen is added to said oxidizable compound in a calculated amount required for substantially complete oxidation.

6. The method of any one of the preceding claims, wherein the oxygen is added as air.

7. A process as claimed in claim 6 or 7, wherein the amount of oxygen added is adjusted in dependence on the measured content of oxygen and/or oxidizable compound in the oxidized gas stream.

8. The process according to any one of the preceding claims, wherein the oxidation is carried out by means of a catalyst, the active component of which comprises at least one noble metal selected from the group consisting of platinum, palladium and rhodium.

9. The method of any of the preceding claims, wherein the particulate matter (2) is continuously introduced into the vessel (1) and discharged from the vessel (1).

10. The method of claim 8, wherein the gas stream is conveyed counter-current to the particulate matter (2).

11. The method according to any one of the preceding claims, comprising a pre-start-up phase in which the circuit is purged with an inert gas, in particular nitrogen.

12. The method of claim 11, wherein the oxygen content in the container (1) is continuously increased to a level of 0.5-5% by volume, in particular 1-4% by volume, during the start-up phase and then kept constant.

13. An apparatus for carrying out the method of any one of the preceding claims, the apparatus comprising:

-a vessel (1) containing polymer particles (2) having a gas inlet (3) and a gas outlet (4),

a catalyst unit (5) comprising an oxidation catalyst for the oxidation of residual monomers by oxygen,

-a gas circulation line comprising a gas outlet line (9) connecting the gas outlet (4) with the catalyst unit (5) and a return line (10) connecting the catalyst unit (5) with the gas inlet (3), and

-an air metering unit (6) connected to the gas outlet line (9) for introducing oxygen into the gas outlet line (9).

14. Apparatus according to claim 13, comprising a polymer particle inlet (6) and a polymer particle outlet (7), wherein the polymer particle inlet (6) and the gas inlet (3) are located on one side of the vessel (1) and the polymer particle outlet (7) and the gas outlet (4) are located on the opposite side of the vessel (1), thereby achieving countercurrent transport of the gas stream and the polymer particles (2).

15. The apparatus of claim 13 or 14, wherein the container is a silo (1) for storing the particulate polymer (2).

16. The apparatus of any one of claims 13 to 15, wherein the oxidation catalyst comprises a variety ofconventional monolithic three-way catalysts or oxidation catalysts for automotive exhaust purification.

17. The apparatus according to any of claims 13 to 16, wherein the catalyst unit (5) is autothermally operable.

18. The apparatus of any one of claims 13 to 17, further comprising:

-a lambda probe for determining the oxygen content in the return line (10), and

-an adjustment unit for adjusting the amount of oxygen passing through the metering unit (6) based on the oxygen content measured by the lambda probe.