The present invention relates to a method for removing dust from polymer particles in a polymer conveying system using ionized gas and the use of said method in an in- plant polymer conveying system.

Background art

The transport of polymer particles such as polymer pellets is a necessary part of production, shipment and customer’s handling before processing. Several different methods for conveying polymer particles are generally used such as hydraulic conveying methods or conveying methods using a conveying gas.

When using conveying methods with conveying gas such as pneumatic conveying the polymer particles collide against the walls of the conveying lines so that abrasion in form of dust or angel hair occurs.

For cleaning the polymer particles from dust and angel hair gas conveying systems usually include dust removal devices such as elutriators. Elutriators usually work with gas counter flow principle. The gas counter flow, e.g. air counter flow conveys the dust and angel hair vertically upwards whereby gravity forces the heavier polymer particles downwards. However, static electricity forces are strong enough to keep big amount of dust on the pellet surface in spite of the air counter flow. Air counter flow cannot be increased above the limit where pellets are not any more falling downwards due to gravity.

Polymer pellets and powder are known to form static electricity during the pneumatic transport. Static electric forces keep the fine dust firmly on the pellet surface so that dust is only insufficiently removed in the dust removal devices.

Thus, there is a need for an improved method for conveying polymer particles in a gas conveying system, which improves dust removal from the conveyed polymer particles. It has surprisingly been found that when subjecting conveyed polymer particles with ionized gas static forces during conveying the polymer particles can be significantly reduced which improves dust removal. Summary of the invention

The present invention relates to a method for transporting polymer particles in a polymer conveying system, comprising the steps of

a) Transporting polymer particles through at least one conveying line;

b) Subjecting the conveyed polymer particles to ionized gas; and

c) Separating dust from the polymer particles.

Further, the present invention relates to the use of the method according to the present invention as described above or below in an in-plant polymer conveying system, suitably situated in post-production.

Still further, the present invention relates to the use of the according to the present invention as described above or below for improving dust and/or angel hair separation from polymer particles in polymer conveying systems.

Definitions

Ionized gas is a gas, which includes positive charges by stripping away electrons orbiting the atomic nuclei or negative charges by adding electrons or both by separating electrons and atomic nuclei.

A polyolefin is a polymer produced from olefin monomers having the general formula C _n H _2n . Suitably the olefin monomers are selected from alpha-olefin monomers having from 2 to 12 carbon atoms.

Conveying gas is a gas used to transport polyolefin particles in a polymer conveying system. The polymer conveying system can be a pneumatic conveying system or a vacuum conveying system. Relative humidity of the conveying gas (RHCG) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature.

Relative humidity of the storage gas (RH _SG ) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature of the storage gas under storing conditions.

Storage gas is the gas surrounding the polyolefin particles during storage. Usually, the polyolefin particles are stored in a storage bin such as a storage hopper or a storage silo. The storage bin can be intermediate storage bin in which the polyolefin particles are stored temporarily before further processing or a distribution storage bin in which the polyolefin particles are stored before distribution.

An in-plant polymer conveying system is a polymer conveying system, which is situated at a polymer production site, usable in any kind of post-production transport such as transporting the polymer powder from the polymerization reactor or transporting the polymer pellets after extrusion and pelletization.

Figures

Fig 1 shows a schematic overview of a pneumatic conveying and dust removal system without subjecting the polyolefin pellets to ionized air (Reference).

Fig 2 shows a schematic overview of a pneumatic conveying and dust removal system with subjecting the polyolefin pellets to ionized air (Example 1).

Fig 3 shows a schematic overview of a pneumatic conveying and dust removal system with subjecting the polyolefin pellets to ionized air and adding water into the conveying air (Example 2).

Fig 4 shows the amount of dust measured for the polyolefin pellets of examples 1 , 2 and Reference Detailed description of the invention

Method

The present invention relates to a method for transporting polymer particles in a polymer conveying system, comprising the steps of

a) Transporting polymer particles through at least one conveying line;

b) Subjecting the conveyed polymer particles to ionized gas; and

c) Separating dust from the polymer particles.

The polymer conveying system can be any polymer conveying system suitable for transporting polymer particles by means of a conveying gas. Suitably the polymer conveying system is a pneumatic polymer conveying system or a vacuum polymer conveying system, preferably a pneumatic polymer conveying system.

In a pneumatic polymer conveying system polymer particles are transported through conveying line(s) by means of conveying gas pressure.

In a vacuum polymer conveying system polymer particles are transported through conveying line(s) by means of a vacuum usually produced from a vacuum pump.

The conveying gas is generally any gas suitable for conveying polyolefin particles. Suitable conveying gases are selected from air, nitrogen and noble gases such as argon. Mostly preferred the conveying gas is selected from air.

Usually, the temperature of the conveying gas is maintained in a range of from 20°C to 30°C, preferably of from 22°C to 25°C. In some embodiments, the temperature of the conveying gas is maintained in a range of from 5°C to 20°C, preferably of from lO°C to l5°C.

In order to maintain the conveying gas in the preferred temperature range the conveying gas can be either heated or cooled preferably by means of a heat exchanger. For example for transporting hot polymer pellets from the extruder and pelletizer to a storage bin the conveying gas can be cooled to reach target temperature.

At the downstream end of the conveying line the conveyed polymer particles are subjected to ionized gas.

Ionized gas such as ionized air, ionized nitrogen or ionized noble gas like ionized argon, is preferably produced in an ionizer. Thereby, the working principle of ionizers is well known in the art.

Preferably the ionizer is a high voltage ionizer preferably with an operating voltage of from 1 to 10 kV, more preferably from 2 to 8 kV, still more preferably from 3 to 5 kV. The ionizer can be a negative ion generator, a positive ion generator or an electrostatic discharge (ESD) ionizer, preferably an electrostatic discharge (ESD) ionizer.

Ionized gas can be negatively charged gas, positively charged gas or neutrally charged gas, preferably neutrally charged gas.

Preferably, the ionized gas is selected from ionized air, ionized nitrogen or ionized noble gas like ionized argon and more preferably is ionized air. It is preferred that process step b) is conducted in conjunction with the dust separation step c). This means that the conveyed polymer particles are preferably contacted with ionized gas directly before the separation of the dust and/or angel hair in process step c). Preferably, dust and/or angel hair are separated from the conveyed polymer particles in a dust separation device.

The dust separation device is suitably located near the downstream end of the polymer conveying system, preferably upstream the final storage bin. When locating the dust separation device at the end of the polymer conveying system only one dust separation step needs to be present in the conveying system upstream of the final storage of the polymer particles. However, the polymer conveying system may also include more than one dust separation device. The devices are then located at different parts of the conveying system. Consequently, the method of the present invention may include more than one dust separating step c).

Since dust is usually produced as abrasion during the conveying step a) of the inventive method the dust separation step is suitably conducted on conveyed polymer particles.

Additional dust separation steps c) may be situated upstream of treatment steps of the polymer particles such as e.g. crosslinking steps, impregnating steps, compounding steps or the like.

Dust separation devices can be any devices suitable for separating dust and angel hair from the polymer particles such as cyclone dust separation devices, vacuum based dust separation devices or pneumatic dust separation devices.

A preferred dust separation device is a pneumatic dust separation device, more preferably a counter gas flow dust separation device. In a counter gas flow dust separation device the counter gas flow, e.g. counter air flow, conveys the dust and angel hair vertically upwards whereby gravity forces the heavier polymer particles downwards. Mostly preferred is an elutriator. One commercially available example for a counter gas flow dust separation device suitable for separating dust in process step c) is the Pelletron DeDuster ^® from Pelletron Corporation, Lancaster, USA. Preferably, the conveyed polymer particles are contacted with ionized gas directly before the separation of the dust and/or angel hair in process step c).

Thereby, the ionizer is preferably located in the polymer conveying system in close proximity with the dust separation device.

The ionizer can be located within the dust separating device e.g. as such that the gas in the dust separating device is ionized and the polyolefin particles are subjected to the ionized gas.

The ionizer can also be located outside but in close proximity to the dust separating device as such that e.g. the gas introduced into the dust separating device for separating the dust from the polyolefin particles can be ionized by the ionizer before entering the dust separating device.

Preferably the dust separating device is a counter gas flow dust separating device, such as an elutriator, and ionized gas, such as ionized air, produced in an ionizer outside of the dust separating device is introduced into the counter gas flow dust separating device as ionized counter gas flow, such as ionized counter air flow.

Preferably, the method according to the present invention further comprises the step of

d) Storing the de-dusted polymer particles. Suitably, the storing step d) such as storing the polymer particles in a storage bin such as a silo is conducted directly downstream of the dust separation step c).

Therefore, the dust separation device is preferably located in direct proximity of the storage bin. By locating the dust separation device in direct proximity of the storage bin further conveying steps which could result in the additional formation of dust and angel hair can be avoided. It has surprisingly been found that by subjecting the conveyed polymer particles to ionized gas the static forces which keep the dust and angel hair attached to the polymer particles can be significantly reduced so that dust and angel hair separation can be improved.

In a further embodiment of the present invention the method according to the invention comprises the steps of

a-l) Subjecting the polymer particles to moist conveying gas thereby producing wetted polymer particles;

a-2) Transporting the wetted polymer particles through at least one conveying line; and

a-3) Controlling the amount of moisture in the conveying gas of the polymer

conveying system as such that at the downstream end of the at least one conveying line the relative humidity of the conveying gas (RH _CG ) is 40% to 100%.

The conveyed wetted polymer particles of this embodiment then are subjected to the ionized gas in process step b) as described above.

The moist conveying gas is preferably produced by dosing water, preferably demineralized water, into the conveying gas of the polymer conveying system at the upstream end of the at least one conveying line.

Thereby, the water can be dosed in form of liquid water or in form of water vapour.

Preferably the water is introduced into the at least one conveying line by means of at least one nozzle.

Suitably the water is dosed into the conveying gas in or near at least one compressor and/or blower of the polymer conveying system at the upstream end of the at least one conveying line.

Thereby, the compressor and/or blower is preferably used for providing the conveying gas pressure in a pneumatic polymer conveying system. The water is preferably dosed into the conveying gas only in one point at the upstream end of the at least one conveying line.

It has been found that dosing the water only once is usually sufficient to increase dust separation at any stage of the polymer conveying system.

The amount of water introduced into the conveying line is preferably controlled by means of at least one valve which preferably is connected to the nozzle.

In the polymer conveying system the polymer particles are preferably brought into direct contact to the moist conveying gas before transporting the polymer particles through at least one conveying line. Consequently, the polymer particles are wetted by the moist conveying gas.

Preferably, the relative humidity of the conveying gas (RHCG) is controlled in process step a-3) as such that at the downstream end of the at least one conveying line the relative humidity of the conveying gas (RH _CG ) is in the range of from 60 % to 95%, more preferably from 65% to 90%, most preferably from 70% to 85%.

Relative humidity of the conveying gas (RHCG) depends on the temperature of the conveying gas. At higher temperatures of the conveying gas higher amounts of water have to be dosed into the conveying gas in order to reach the target relative humidity than at lower temperatures of the conveying gas.

When cooling the conveying gas, water deposition can occur. Thus, the method of the present invention may additionally include a water separation step in order to separate free water from the polyolefin particles.

The water separation step usually is conducted in a water separating device.

Further, the method of the present invention according to said embodiment preferably further comprises the steps of: d-l) Reducing the moisture in the storage gas surroundingthe conveyed polymer particles to 20% to 80% relative humidity of the storage gas (RH _SG ) to produce polyolefin particles with reduced moisture content; and

d-2) Storing the polymer particles with reduced moisture content in the storage gas with reduced relative humidity.

Preferably the relative humidity of the storage gas (RH _SG ) is reduced to 25% to 75%, more preferably 30% to 70%, still more preferably 35% to 65 % and most preferably 40% to 60%.

By means of reducing the relative humidity of the storage gas (RH _SG ) the moisture content of the conveyed polyolefin particles surrounded by the storage gas is reduced at the same time. Process steps d-l) and d-2) preferably take place at the downstream end of the polymer conveying system adjacent to or in the proximity of the final storage of the polymer particles. The reason for reducing the humidity of the polymer particles is to avoid sticking of the wetted polymer particles in the final storage bin which may cause problems when removing the polymer particles from the final storage bin such as a silo.

Further, unwanted chemical reactions of the polymer particles in the storage bin catalysed by moisture under pressure of storage can be avoided.

However, process steps f) and g) can also take place for intermediate storage before the polyolefin particles are e.g. further processed.

The storing conditions of the polyolefin particles, such as pressure and temperature, preferably reflect the conditions usually used in the art for storing polyolefin particles. The polymer particles conveyed by the method of the present invention can be polymer powder or polymer pellets, preferably polymer pellets.

The method of the present invention can be used for any kind of polymer particles. Thereby, the polymer particles can be thermoplastic polymer particles, elastomeric polymer particles or plastomeric polymer particles.

Preferably the polymer particles are polyolefin particles such as thermoplastic polyolefin particles, elastomeric polyolefin particles or plastomeric polyolefin particles.

It is preferred that the polyolefin of the polyolefin particles is selected from alpha- olefin homo-or copolymers including one or more alpha-olefin monomers having from 2 to 12 carbon atoms, preferably from 2 to 10 carbon atoms and mostly preferred having from 2 to 8 carbon atoms.

Preferably, the polyolefin is ethylene or propylene based polyolefins such as ethylene homo- or copolymers or propylene homo- or copolymers.

Ethylene copolymers are preferably polymers comprising ethylene monomer units as major component and one or more comonomer units selected from alpha-olefin monomers having from 3 to 12 carbon atoms, preferably alpha-olefin monomers having from 3 to 10 carbon atoms, more preferably alpha-olefin monomers having from 3 to 8 carbon atoms such as propylene, 1 -butene, 1 -hexene and l-octene.

The ethylene copolymers can be ethylene random copolymers, ethylene block copolymers, ethylene-based elastomers or ethylene-based plastomers.

Propylene copolymers are preferably polymers comprising propylene monomer units as major component and one or more comonomer units selected from alpha-olefin monomers having from 2 and 4 to 12 carbon atoms, preferably alpha-olefin monomers having from 2 and 4 to 10 carbon atoms, more preferably alpha-olefin monomers having from 2 and 4 to 8 carbon atoms such as ethylene, 1 -butene, 1- hexene and l-octene. The propylene copolymers can be propylene random copolymers, propylene block copolymers, propylene -based elastomers or propylene-based plastomers.

Use

The method of the present invention can be used in any kind of polymer conveying system suitable for transporting polymer particles by means of conveying gas such as in-plant polymer conveying system like post in post production polymer conveying systems or costumer polymer conveying systems. Thereby, the present invention relates to the use of the method according to the present invention as described above or below in an in-plant polymer conveying system, suitably situated in post-production.

As discussed above and shown below in the examples the method of the present invention surprisingly improves dust separation from polymer particles in a polymer conveying system suitable for transporting polymer particles by means of conveying gas.

Thus, the present invention also relates to the use of the method according to the present invention as described above or below for improving dust and/or angel hair separation from polymer particles in polymer conveying systems.

Examples

a) Dust removal from pellets of bimodal propylene copolymer resins

Reference

In a polymer conveying system as illustrated in Fig 1 , pellets of a bimodal propylene copolymer resin having a density of 905 kg/m ³ (ISO 1183) and a melt flow rate (230°C, 2.16 kg) of 0.25 g/lO min (ISO 1133) suitable for pipe applications were conveyed through a conveying line to a counter flow elutriator. The pellets used for the conveying experiment were taken from the storage silo. The relative humidity of the conveying air (RHCG) measured at the entry of the elutriator was 10 to 40 %. Amount of conveyed pellets: about 45 t/h

Conveying air flow: about 5 t/h

Conveying air temperature: about 35°C to 70°C In the elutriator counter flow air was introduced in order to separate dust and angel hair from the propylene copolymer pellets. The amount of dust still attached to the pellets removed from the elutriator was measured.

Example 1

The experiment of the reference as described above was repeated with the difference that ionized air was introduced into the elutriator as counter air flow. As ionizer in line ionizer Conveyostat ^® , commercially available from Simco-Ion, Hatfield, USA, and operating at an operating voltage of 4 kV AC, had been integrated into the air feeding line of the elutriator. All other parameters were kept constant.

The polymer conveying system used for example 1 is illustrated in Fig 2.

Example 2

The experiment of the example 1 as described above was repeated with the difference that demineralized water was fed into the conveying line directly downstream of the air blower with a water flow of 300 kg/h. All other parameters were kept constant. The relative humidity of the conveying air (RH _CG ) measured at the entry of the elutriator was 60 to 80 %.

The polymer conveying system used for example 2 is illustrated in Fig 3. The amounts of dust attached to the pellets removed from the elutriator of the reference example and examples 1 and 2 are shown in Fig. 4.

Thereby, it can be seen that dust removal has been increased in example 1 more than two-times compared to the reference example. The additional use of moist conveying air for transporting the pellets additionally improves dust removal.