Patent Description:
Various types of ethylene-based polymers are known, such as low density polyethylene (LDPE). LDPE can be used alone, blended or co extruded for a variety of packaging, construction, agricultural, industrial and consumer applications. The largest application of LDPE is in films produced for example by the blown or cast extrusion process in both mono and co extrusions. Films made with LDPE exhibit good optical properties, strength, flexibility, seal ability and chemical inertness. The end-use applications include packaging of bakery items, snack foods, consumer durables, disposable diapers, textiles, agricultural film and shrink film.

Polyethylene such as LDPE can be prepared via free radical polymerization. For carrying out the polymerization in high pressure reactors with initiator mixtures, it is common practice to premix the initiators, optionally with additional solvents, and meter such a mixture to the reactor.

<CIT> describes a process for the preparation of polyethylene in a high pressure reactor with at least two spatially separated initiator injection points by polymerizing ethylene and optionally further monomers in the presence of at least two different mixtures of free-radical polymerization initiators. The process comprises a) providing at least two different initiators as solution in a suitable solvent or in liquid state, b) mixing the initiators and optionally additional solvent in at least two static mixers and c) feeding each of the mixtures to a different initiator injection point of the high pressure reactor.

<CIT> discloses a mixing device as per the preamble of claim <NUM>.

While known static mixers give a desirable degree of mixing, the structure of static mixers is rather complex, resulting in a higher cost for fabrication.

It is an object of the present invention to provide a mixing device in which the above-described and/or other problems are solved.

Accordingly, the present invention provides a mixing device comprising:.

Advantageously, the mixing device according to the invention is a mixing device for making a mixture of liquid substances, preferably a mixture of liquid substances comprising initiators, additives and/or solvents for the preparation of an ethylene-based polymer. The mixing device is particularly suitable for making such mixture in a flow having a Reynolds number of at most <NUM>. Thus, the mixing device according to the invention can make a mixture in a generally laminar low.

The device according to the invention provides a desired degree of mixing of liquid substances comparable to known static mixers, while having a simple structure mainly using commonly available pipes such as standard pipes.

The mixing device according to the invention may be designed to be more compact than known static mixers and achieve a comparable degree of mixing. Further, the mixing device according to the invention may achieve a comparable degree of pressure drop and residence time as known static mixers.

The mixing device according to the invention comprises a supply section and a pipe-in-pipe section. The pipe-in-pipe section comprises an outer pipe and an inner pipe. A flow of liquid substances is supplied from the supply section to the pipe-in-pipe section via the outer flow inlet provided on the outer pipe of the pipe-in-pipe section. The flow enters the annular space between the outer pipe and the inner pipe, and then enters the inner pipe via the inner flow inlets.

The outer flow inlet is provided at a lower vertical position than the inner flow inlets and is provided such that a tangential flow is generated in the annular space. In use, when the flow is supplied to the annular space via the outer flow inlet, an upward circulating flow is generated in the annular space. The degree of mixing in the flow is substantially increased by the tangential flow generated in the annular space. Further, the flows entering the inner flow inlets provided at the same vertical position result in impinging flows, further increasing the degree of mixing.

The inner flow inlets may be formed by two passages through a circumferential wall of the inner pipe.

The number of the inner flow inlets may be two, three, four or more.

Preferably, the at least two inner flow inlets comprise two inner flow inlets spaced apart in circumferential direction over an angle of between <NUM> and <NUM> degrees, preferably of <NUM> degrees, with respect to each other. When the angle is about <NUM> degrees, the two inner flow inlets may herein sometimes be referred as facing each other. Such positioning of the inner flow inlets increases the effect of the impinging flows on mixing. In some preferred embodiments, the at least two inner flow inlets consist of two inner flow inlets facing each other. In some preferred embodiments, the at least two inner flow inlets consist of four inner flow inlets consisting of two pairs of two inner flow inlets facing each other.

Preferably, the vertical distance between the bottom end of the annular space and the top end of the annular space (height of the annular space) is <NUM> to <NUM>. This ensures a sufficient distance for the upward circulating flow to induce good mixing while keeping the size of the mixing device to become too large. More preferably, the height of the annular space is <NUM> to <NUM>. This has a further advantage of decreasing the residence time.

Preferably, the vertical distance between the outer flow inlet and the bottom end of the annular space is <NUM>% of the vertical distance between the bottom end of the annular space and the top end of the annular space (height of the annular space). Preferably, the vertical distance between the inner flow inlets and the top end of the annular space is <NUM>% of the vertical distance between the bottom end of the annular space and the top end of the annular space.

The outer pipe may have an inner diameter ID1, for example of <NUM> to <NUM>.

The inner pipe may have an inner diameter ID2, for example of <NUM> to <NUM>.

The inner pipe may have an outer diameter OD2, for example of <NUM> to <NUM>.

ID1 is larger than OD2. Preferably, ID1-OD2 is <NUM> to <NUM>.

These dimensions of the outer pipe and the inner pipe allow the use of standard pipe schedule (e.g. <NUM>", ½", <NUM>/<NUM>", <NUM>/<NUM>" SCH40) and lowers the cost of fabrication.

The inner flow inlets may have a diameter IFD, for example of <NUM> to <NUM>.

The outer flow inlet may have a diameter of OFD, for example of <NUM> to <NUM>.

Such diameters of the inner flow inlets and the outer flow inlet provides desired velocity and momentum for mixing action while lowering the risk of plugging.

Preferably, at least one, preferably both, of the following conditions are satisfied:.

Preferably, the following condition is satisfied:
the ratio of the vertical distance between the bottom end of the annular space and the top end of the annular space to ID1 = <NUM> to <NUM>, preferably <NUM> to <NUM>.

Most preferably, all of the following conditions are satisfied:.

Preferably, [ID1-OD2]/OFD is more than <NUM>.

Preferably, [ID1-OD2]/IFD is more than <NUM>.

In some preferred embodiments, the closed top end of the annular space is configured to be removable, i.e. the closed top end of the annular space is provided with a structure or mechanism which allows easy attachment to and detachment from the outer pipe and inner pipe. This allows easy cleaning of the annular space.

Preferably, the supply section comprises a supply pipe having a first, vertically extending pipe section of a first inner diameter, followed, in a direction of the flow, by a second, vertically extending pipe section of a second, reduced inner diameter, which second pipe section in turn is followed by a third, horizontally extending pipe section of the second inner diameter, wherein the horizontally extending pipe connects to the outer flow inlet.

The transition from the first, vertically extending pipe section to the second, vertically extending pipe section may be achieved by a section in which the inner diameter gradually decreases (herein sometimes referred as a diameter reducer) from the first inner diameter to the second inner diameter. This reduction in the inner diameter increases the velocity of the flow supplied to the outer flow inlet and achieves a higher degree of mixing. The diameter reducer preferably has an inner wall in the shape of a truncated cone. The inner wall of the diameter reducer may be angled by e.g. <NUM> to <NUM>° such as <NUM>° with respect to the longitudinal axis of the first and the second pipe sections.

Preferably, the ratio of the first inner diameter to the second, reduced inner diameter is at least <NUM>, preferably at least <NUM>, more preferably <NUM> to <NUM>.

The transition from the second, vertically extending pipe section to the third, horizontally extending pipe section may be achieved by an elbow section, preferably a curved elbow section. The degree of mixing increases at this elbow section.

Preferably, the mixing device according to the invention further comprises a pre-mixing section arranged upstream of the supply section, wherein the pre-mixing section comprises a main supply pipe section connecting to the vertically extending pipe section and at least two initiator supply pipe sections connecting to the main supply pipe section along the main supply pipe section.

Preferably, the main supply pipe section comprises a horizontally extending part and the two or more initiator supply pipes extend vertically and connect to the horizontally extending part.

The invention further provides a process for making a mixture of liquid substances, preferably a mixture of liquid substances comprising initiators, additives and/or solvents for the preparation of an ethylene-based polymer, comprising feeding the liquid substances to the mixing device according to the invention. Preferably, the liquid substances comprises initiators and solvents of the initiators. The initiators may be fed to the mixing device as a solution of the initiators in a solvent.

In the process, the mixing device is preferably operated at a pressure of at most <NUM> bar, preferably <NUM> to <NUM> bar.

The invention further relates to a reactor system comprising the mixing device according to the invention and a polymerization reactor for high pressure polymerization of an ethylene-based polymer, the polymerization reactor comprising an ethylene inlet for ethylene and a liquid mixture inlet connected to the outlet of the mixing device. It will be appreciated that the polymerization reactor may comprise further various inlets for other components such as optional comonomer(s), and other elements necessary for high pressure polymerization. The polymerization reactor is per se well-known, e.g. from <CIT> and is not described herein in detail.

The invention further relates to a high pressure polymerization process for the preparation of an ethylene-based polymer using the reactor system according to the invention, comprising feeding the liquid substances to the mixing device to obtain the mixture and feeding the mixture to the liquid mixture inlet and feeding ethylene to the ethylene inlet.

Preferably, the process comprises polymerizing ethylene and optional comonomers in the presence of the mixture at temperatures of <NUM> to <NUM> and pressures of <NUM> to <NUM> MPa.

Preferably, the process comprises pressurizing the mixture obtained by the mixing device to a pressure of <NUM> to <NUM> MPa before feeding it to the liquid mixture inlet.

Examples of suitable comonomers include α,β-unsaturated carboxylic acids, in particular but not limited to maleic acid, fumaric acid, itaconic acid, acrylic acid, methacrylic acid and crotonic acid;.

The initiator used in the present invention is typically an organic peroxide. Suitable organic peroxides include for example peroxyester, peroxyketone, peroxyketal and peroxycarbonate such as for example di-<NUM>-ethylhexyl-peroxydicarbonate, diacetylperoxydicarbonate, dicyclohexyl-peroxydicarbonate, tert. -amylperpivalate, cumyl perneodecanoate, tert. - butylpemeodecanoate, tert. -butyl perpivalate, tert. - butylpermaleinate, tert. - butylperisononanoate, tert. -butylperbenzoate, tert,-butylperoxy-<NUM>-ethylhexanoate. - butyl-hydroperoxide, d-tert. butyl peroxide, di-isopropylbenzol hydroperoxide, di- isononanoyl peroxide, didecanoylperoxide, cumol hydroperoxide, methyl isobutyl ketone hydroperoxide, <NUM>,<NUM>-bis-(tert. -butylperoxy)-butane and/or <NUM>,<NUM>-dimethyl-<NUM>,<NUM>- diphenylhexane. Also difunctional or higher functional peroxides may be applied.

The suitable solvents used in the present invention are those for making a solution of the initiators. Examples of suitable solvents are alcohols, ketones and aliphatic hydrocarbons, in particular octane, decane and isododecane and also other saturated C7 to C30 hydrocarbons, in particular linear C7 to C30 alkanes. The solution may comprise the initiators in proportions of from <NUM> to <NUM>% by weight, preferably from <NUM> to <NUM>% by weight.

It is noted that the invention relates to all possible combinations of features described herein, preferred in particular are those combinations of features that are present in the claims. It will therefore be appreciated that all combinations of features relating to the composition according to the invention; all combinations of features relating to the process according to the invention and all combinations of features relating to the composition according to the invention and features relating to the process according to the invention are described herein.

It is further noted that the term 'comprising' does not exclude the presence of other elements. However, it is also to be understood that a description on a product/composition comprising certain components also discloses a product/composition consisting of these components. The product/composition consisting of these components may be advantageous in that it offers a simpler, more economical process for the preparation of the product/composition. Similarly, it is also to be understood that a description on a process comprising certain steps also discloses a process consisting of these steps. The process consisting of these steps may be advantageous in that it offers a simpler, more economical process.

When values are mentioned for a lower limit and an upper limit for a parameter, ranges made by the combinations of the values of the lower limit and the values of the upper limit are also understood to be disclosed.

The invention is now elucidated by way of the following embodiments and examples, without however being limited thereto.

<FIG> shows a front view of an embodiment of a mixing device according to the invention. The mixing device comprises a pre-mixing section <NUM>, a supply section <NUM> and a pipe-in-pipe section <NUM>.

The pre-mixing section <NUM> comprises a main supply pipe section consisting of a vertically extending part <NUM> and a horizontally extending part <NUM>. Sub-supply pipes <NUM>-<NUM> are provided along the length of the horizontally extending part <NUM> of the main supply pipe. The vertically extending part <NUM> of the main supply pipe section is connected to the supply section <NUM>.

<FIG> and <FIG> show, in vertical cross-section, an enlarged view of part of the mixing device of <FIG>. The supply section <NUM> and the pipe-in-pipe section <NUM> are shown. <FIG> shows the pipe-in-pipe section <NUM> in more detail. <FIG> shows the supply section <NUM> in more detail.

The supply section <NUM> comprises a supply pipe having a first, vertically extending pipe section <NUM> of a first inner diameter. The first, vertically extending pipe section <NUM> is followed, in a direction of the flow, by a second, vertically extending pipe section <NUM> of a second, reduced inner diameter. A diameter reducer <NUM> is present between the first, vertically extending pipe section <NUM> and the second, vertically extending pipe section <NUM>, in which the inner diameter decreases from the first inner diameter to the second inner diameter. The diameter reducer <NUM> has an inner wall in the shape of a truncated cone.

The second, vertically extending pipe section <NUM> connects to a third, horizontally extending pipe section <NUM> of the second inner diameter. A curved elbow section <NUM> is present between the second, vertically extending pipe section <NUM> and the third, horizontally extending pipe section <NUM>. The third, horizontally extending pipe section <NUM> is connected to the pipe-in-pipe section <NUM>.

The pipe-in-pipe section <NUM> comprises an outer pipe <NUM> extending vertically and an inner pipe <NUM> having a closed top end and having an outlet for the mixture at a bottom end. The inner pipe <NUM> is arranged concentrically to the outer pipe <NUM> inside the outer pipe <NUM> such that an annular space <NUM> having a closed top end and a closed bottom end is formed between the outer pipe <NUM> and the inner pipe <NUM>.

The inner pipe <NUM> is provided with two inner flow inlets <NUM>, <NUM> for receiving a flow from the annular space <NUM> into the inner pipe <NUM>. The outer pipe <NUM> is provided with an outer flow inlet <NUM> for receiving a flow from the supply section <NUM> into the annular space <NUM>. The third, horizontally extending pipe section <NUM> of the supply section <NUM> connects to the outer flow inlet <NUM>. The outer flow inlet <NUM> is provided such that a tangential flow is generated in the annular space <NUM>. The two inner flow inlets <NUM>, <NUM> are provided higher than the outer flow inlet <NUM>, i.e. the vertical distance from the closed bottom end of the annual space <NUM> to two inner flow inlets <NUM>, <NUM> is larger than the vertical distance from the closed bottom end of the annual space <NUM> to the outer flow inlet <NUM>.

<FIG> is a top cross section of <FIG>. It can be understood that the outer flow inlet <NUM> is provided at such a position of the outer pipe <NUM> that a tangential flow is generated in the annular space <NUM>.

<FIG> illustrates a cross section of an example of a mixing device not according to the invention. The mixing device has the same pre-mixing section as the pre-mixing section <NUM> shown in <FIG>. Instead of the supply section <NUM> and the pipe-in-pipe section <NUM> as in the mixing device of <FIG>, the mixing device of <FIG> comprises a static mixer.

An exemplary process for making a mixture of initiators using the mixing device of <FIG> is now described in which different types of initiators are fed to the vertically extending sub-pipes <NUM> to <NUM>.

The different types of initiators fed to the vertically extending sub-pipes <NUM> to <NUM> contact each other at the junctions between and the vertically extending sub-pipes <NUM> to <NUM> and the horizontally extending part <NUM> of the main supply pipe section and a flow comprising the initiators flows down the vertically extending pipe part <NUM> of the main supply pipe section.

The flow comprising the initiators from the pre-mixing section <NUM> flows down to the supply pipe of the supply section <NUM>. The flow comprising the initiators enters the first, vertically extending pipe section <NUM> of a first inner diameter and continues to the diameter reducer <NUM> at which mixing occurs. The flow enters the second, vertically extending pipe section <NUM> and continues to the third, horizontally extending pipe section <NUM> via the curved elbow section <NUM> at which mixing occurs. The flow enters the outer flow inlet <NUM> of the outer pipe <NUM> from the third, horizontally extending pipe section <NUM>.

Due to the manner in which the outer flow inlet <NUM> is provided at the outer pipe <NUM> as shown in <FIG>, a tangential flow is generated in the annular space <NUM> between the outer pipe <NUM> and the inner pipe <NUM>. An upward circulating flow is generated in the annular space <NUM>, in which mixing occurs. The upward flow reaches and enters the two inner flow inlets <NUM>, <NUM> positioned higher than the outer flow inlet <NUM>. The two flows entering the two inner flow inlets <NUM>, <NUM> facing each other result in impinging flows, further increasing the degree of mixing. The obtained mixture flows down the inner pipe <NUM> and leaves the mixing device via the outlet at the bottom end of the inner pipe <NUM>. A high degree of mixing has been achieved in the mixture of initiators leaving the outlet.

Computer simulation was performed on the mixing of peroxides fed to two examples of the mixing device illustrated in <FIG> (according to the invention) and an example of the mixing device illustrated in <FIG> (not according to the invention).

Following conditions were used in the computer simulation.

The mixing device of <FIG> has the same pre-mixing section as that in <FIG>. The static mixer of <FIG> has a total length of <NUM>, of which the total length of mixing elements is <NUM>.

Initiators: four different peroxides having varying densities in the range of <NUM> to <NUM>/m<NUM> (at <NUM>) and varying viscosities in the range of <NUM> to <NUM> mPas (at <NUM>). Solvent: density of <NUM>/m<NUM> at <NUM> and viscosity of <NUM> mPas at <NUM>.

In the simulation, the peroxides are fed to the vertically extending sub-pipes <NUM> to <NUM> as a <NUM>% concentration solution in the solvent. The solvent is fed to the vertically extending sub-pipe <NUM>. The feed velocities and Reynolds numbers of the flows are selected such that the flow in the pre-mixing section would be laminar. The values of Coefficient of variation (CoV) were calculated as a factor of the location in the mixing device.

At junctions of the vertically extending sub-pipes and the horizontally extending part of the main pipe section, CoV dropped significantly. Little drop was observed along the vertically extending part of the main pipe. CoV at the end of the pre-mixing section was far higher than a typical industrial standard CoV=<NUM> which is considered as 'good' mixing. Thus, both in the mixing device of <FIG> and <FIG>, the mixing achieved in the pre-mixing section is insufficient.

In the mixing device of <FIG>, the CoV significantly decreased in the static mixer and reached a value lower than <NUM>, showing the good mixing effect of the static mixer.

In the mixing device of <FIG>, decrease in the CoV occurred at various parts of the supply section and the mixing section. It was observed by the computer simulation that a substantial decrease in the CoV occurs at the diameter reducer, the elbow section, the part of the annular space just after the outer flow inlet and the part of the inner pipe just after the inner flow inlets. The CoV finally obtained was lower than <NUM>, showing the good mixing effect of the mixing device.

Accordingly, it can be concluded that the mixing device according to the invention having a pipe-in-pipe section with a much lower length (<NUM>) than the mixing device having a static mixer with a length (<NUM>) achieves comparable degree of mixing.

Claim 1:
A mixing device comprising:
- a supply section (<NUM>), and
- a pipe-in-pipe section (<NUM>) comprising:
- an outer pipe (<NUM>) extending vertically and
- an inner pipe (<NUM>) having a closed top end and having an outlet for the mixture at a bottom end,
wherein the inner pipe (<NUM>) is arranged concentrically to the outer pipe (<NUM>) inside the outer pipe (<NUM>) such that an annular space (<NUM>) is formed between the outer pipe (<NUM>) and the inner pipe (<NUM>), wherein the annular space has a closed top end and a closed bottom end,
wherein the outer pipe (<NUM>) is provided with an outer flow inlet (<NUM>) for receiving a flow from the supply section (<NUM>) into the annular space,
the outer flow inlet (<NUM>) being provided such that a tangential flow is generated in the annular space (<NUM>) during use, characterized in that the inner pipe (<NUM>) is provided with at least two inner flow inlets (<NUM>, <NUM>) for receiving a flow from the annular space (<NUM>) into the inner pipe, and
wherein the two or more inner flow inlets are provided closer to the top end of the annular space than the outer flow inlet.