WO2012072178A1

WO2012072178A1 - Method for cleaning a reactor and/or equipment thereof

Info

Publication number: WO2012072178A1
Application number: PCT/EP2011/005608
Authority: WO
Inventors: Heinz BÖLT; Anton Wellenhofer; Wolfgang Müller; Andreas Meiswinkel; Anina Wöhl; Christian Thaller; Marco Harff
Original assignee: Linde Aktiengesellschaft
Priority date: 2010-11-30
Filing date: 2011-11-08
Publication date: 2012-06-07
Also published as: TW201229234A

Abstract

The present invention relates to a method for cleaning a reactor and/or equipment thereof used in oligomerization or polymerization of ethylene and/or alpha-olefins, comprising the step of purging the reactor and/or equipment with a purge stream containing solvent selected from the group consisting of n-decane, decaline and mixtures thereof, and/or by products) of the oligomerization or polymerization process having ≥ 10 carbon atoms, in the presence of supercritical ethylene.

Description

Method for cleaning a reactor and/or equipment thereof

The present invention relates to a method for cleaning a reactor and/or equipment thereof used in the oligomerization or polymerization of ethylene and/or alpha-olefins.

Especially in technologies related to oligomer and/or polymer production, such as the polymerization of ethylene, or the oligomerization of ethylene, such as the selective ethylene di-, tri- and tetramerization for producing comonomer-grade linear alpha-olefins, equipment fouling is a problem and is frequently encountered. In such processes waxes and polymers are inherent side-products which cannot be avoided completely. These waxes and polymers may lead to fouling of the reactor itself, as well as of peripheral and downstream equipment. In the worst case, part of the equipment, such as pipes, in the subsequent process sections are plugged with the consequence that the process has to be shut down.

In the polymerization of ethylene and/or other alpha-olefins, deposition of polymeric solids may as well occur in reactor and/or equipment.

To control the polymer and wax deposits, the reactor has to be cleaned periodically. To avoid any long shut down periods, a spare reactor to prevent any production loss is required.

For cleaning a reactor bearing solid deposits (e.g. high molecular weight oligo- mers/polymers), the reactor has so far to be opened and usually plant personnel has to enter the reactor for mechanical cleaning. After cleaning, the reactor has to be rendered inert again. The reason is that any traces of moisture and oxygen will poison the highly sensitive organo- metallic catalyst used for oligomerization and polymerization. This procedure is very time- consuming. In conclusion, a typical reactor downtime is about 1 week or longer, given rise to a considerable overall productivity loss.

To overcome these disadvantages, a procedure to avoid any mechanical cleaning was already disclosed in WO 2007/016995 Al . It was proposed that a hot solvent having a temperature of at least about 75°C is introduced into the reactor to dissolve the polymer deposits. The pre- ferred solvents are toluene, xylenes, benzene and mixtures thereof. After cleaning, the solvent may be recovered in a solvent recovery unit, for example by distillation, crystallization, thin- film evaporation, wiped-film evaporation and/or falling film evaporation.

As long as the polymeric solid material features a sufficiently short carbon chain length, the process of WO 2007/016995 Al works in a satisfying manner. This is especially true for the use of zirconium-based catalyst systems which usually provide only relatively short chain length polyethylene and waxes. However, the situation changes if trace-levels of impurities are inherently (via feedstock impurities) or inadvertently (caused by upset process conditions) introduced into the oligomerization reactor. Especially traces of moisture, air and/or corrosion products may induce the formation of long chain and potentially branched polyethylene with relatively poor solubility in aromatic solvents.

This situation aggravates considerably with the use of other catalyst systems, for example chromium-based catalyst systems for the selective tri- or tetramerization of ethylene as disclosed in WO 2009/006979 A2. Although only traces of polyethylene are formed under on- spec conditions, Cr-induced polymer tends to feature a longer chain length and a morphology that slows down the dissolution process considerably.

It is therefore an object of the present invention to provide a method for cleaning a reactor and/or equipment thereof used in oligomerization or polymerization which method overcomes the disadvantages of the prior art. Especially a method shall be provided which improves and simplifies cleaning of the reactor and or equipment.

This object is achieved by a method for cleaning a reactor and/or equipment thereof used in oligomerization or polymerization of ethylene and/or alpha-olefins, comprising the step of purging the reactor and/or equipment with a purge stream containing solvent selected from the group consisting of n-decane, decaline and mixtures thereof, and/or by-product(s) of the oligomerization or polymerization process having > 10 carbon atoms, in the presence of supercritical ethylene. Surprisingly, it was found that with the method of the present invention a better cleaning effect can be achieved which is assumed to rely especially in the presence of supercritical ethylene. Even long-chain and branched polymer deposits can be mobilized with the inventive method. Further, solubility of the polymers is enhanced by the usage of long-chain alkenes, probably due to the similarity in molecular structure of the olefins and polymers. Due to the higher solubility, the time required for the purging procedure can be reduced significantly. In a preferred embodiment, the purge stream consists of solvent and/or by-product(s). In a most preferred embodiment by-product(s) of the oligomerization/polymerization are utilized only, and no additional solvent is necessary, which has to be purchased externally. Therefore, process costs can be reduced.

Further, after purging the reactor, residual traces of solvent and/or by-product(s) in the reactor and/or equipment will not poison the very sensitive catalyst utilized for oligomerization/polymerization. This could occur if foreign or external purging media are used.

Using the internal purging procedure with by-product(s) only, the reactor does not have to be opened and, subsequently, rendered inert again. This safes a significant amount of time and prevents any problems caused by introducing traces of moisture and oxygen during reactor opening, which would poison the sensitive catalyst.

In the present invention, the term "equipment" is to be understood to comprise any necessary equipment besides the reactor to carry out the oligomerization or polymerization, such as pipes and valves.

The inventive method is suitable for cleaning reactors and/or equipment used in oligomerization or polymerization. Oligomerization shall comprise di-, tri- and tetramerization of olefins, especially ethylene to produce linear alpha-olefins. Polymerization shall comprise polymerization of ethylene and other alpha-olefins, as well as copolymerization thereof.

In the oligomerization, polymer residues are inherently formed as side-products which may form deposits in the reactor and/or equipment. In the polymerization process, the product as such (polymer) can form respective deposits. In both, oligomerization and polymerization process, reactor and/or equipment have to be cleaned to avoid plugging thereof.

The by-products of oligomerization and polymerization to be used in the inventive method for cleaning the reactor, are preferably by-products having from 10-20 carbon atoms. Preferably, the by-product(s) have been separated from the product stream and discharged from the reactor by suitable means well known in the art, for example distillation in the case of oligomerization. Of course, by-product(s) do not contain any active catalyst components.

By the term "purge/purging" it is to be understood that reactor and/or equipment may be flushed with solvent and/or by-product(s) continuously, or solvent and/or by-product(s) may be introduced into reactor and/or equipment as a batch charging.

The inventive method is preferably applied directly after the regular oligomeriza- tion/polymerization reaction cycle is finished.

In a preferred embodiment, the method is carried out at a temperature of above 9.25°C, preferably 100-300°C, more preferably 150-250°C.

More precisely, the temperature of the inventive method is adjusted to exceed the critical temperature of the mixture of ethylene and by-product(s) in the reactor's headspace. Typically, ethylene is the most abundant component in the reactor's headspace, therefore the temperature has to exceed the critical temperature of ethylene (9.25°C). However, to obtain maximum performance of the inventive method, the temperature is increased so as to at least approach or exceed the expected melting range of the polymeric deposits (especially polyethylene) to be dissolved.

In this regard, hot solvent and/or by-product(s) may be introduced into the reactor and/or the equipment. Alternatively, solvent and/or by-product(s) may be heated inside the reactor and/or equipment to the desired temperature. In a further preferred embodiment, the method is carried out at an ethylene pressure of above 5.04 MPa, preferably 5.5-12.5 MPa, more preferably 5.8-6.5 MPa.

The pressure is chosen to exceed the critical pressure of the mixture in the reactor's head- space. Again, as ethylene is typically the most abundant component in the reactor's head- space, it suffices to adjust the pressure so as to exceed the critical pressure of ethylene (5.04 MPa).

Even preferred, solvent and/or by-product(s) have a residence time in the reactor and/or equipment of 0.1-24 hours, preferably 0.5-2.5 hours.

After that certain period of time, the solid deposits are substantially completely dissolved in the purging (flushing) medium. Then, the polymer-by-product solution can be discharged to a separation unit.

In another preferred embodiment the purge stream is agitated.

An acceleration of the polymer dissolution can be achieved by some means of agitation whenever available. For example, in a stirred tank reactor, it would be the preferred method to keep the stirrer running while the conditions of the inventive method are maintained. Likewise, in a bubble column reactor, it would be the preferred method to keep the ethylene recirculation running, again maintaining the conditions of the inventive method in the reactor's headspace.

Moreover, it is preferred that the purge stream is in a subsequent step discharged to a separation unit.

In a further embodiment, the equipment used for discharging is heated, preferably at 100- 300°C, more preferably 150-250°C. For easier handling, it is advisable to heat the equipment connecting the reactor with the separation unit so as to avoid any polymer precipitation, which may lead to plugging. Finally, it is preferred that solvent, by-product(s) and/or ethylene is (are) recovered in the separation unit and/or polymeric residue is disposed off.

Solvent, by-product(s) and ethylene may be recovered in the separation unit, and the polymeric residue may be separated and disposed off. This, can, for example, be achieved by first flushing ethylene via pressure reduction and subsequently by cooling the polymer-by-product solution, whereby precipitating the polymers. If a purge-by-product recovery is desirable, the polymer precipitation can be filtered off or the olefin can be distilled off.

In the case of ethylene oligomerization, the products are distributed according to a Schulz- Flory or Poisson-distribution. Therefore, sufficient amounts of higher olefins are available for reactor flushing. Even after the use for flushing, the higher olefins (by-products) are not lost and can be recovered, for example by distillation, and can further be used for commercial purposes.

In the case of selective ethylene tri- and/or tetramerization, around 4-5% of the products are higher olefins, in particular CIO olefins, including branched olefins. These olefins are common and inherent side-products of the otherwise highly selective reaction. These products may serve as suitable solvent (by-product) in the inventive method.

In a most preferred embodiment, the inventive method is utilized for cleaning a reactor and/or equipment employed in the oligomerization of ethylene.

Additional features and advantages of the present invention can be taken from the following examples which are not to be understood to limit the invention, the scope thereof is only defined by the accompanying claims.

Example 1, Effect of Solvents in the Presence of Supercritical Ethylene:

Various solubility tests with polymer samples, the latter formed during ethylene trimerization (catalyst as disclosed in WO 2009/006979), were performed. Different solvents like, e.g., tol- uene, p-xylene, n-decane, 1-decene and decaline were tested. In each case 0.5 g polymer was suspended in 100 ml solvent and heated to 150 -170°C under supercritical ethylene pressure (> 6.0 MPa). Interestingly, 1-decene and decane showed much better properties as a solvent than the aromatic compounds. The aromatics led more to a mere swelling of the polymer deposits, rather than really dissolving the material, as achieved by the CIO olefins or alkanes.

After cooling down, the polymer formed lumps or agglomerates in the aromatic solvents. In contrast, the polymer was smoothly dispersed in decene and decane as small flakes in the liquid. This has the advantage that the liquid with the smoothly dispersed polymer can easily be handled, transported and filtered, in contrast to swollen polymer in the aromatic solvents, which tends to stick to the walls of the pipes and the inner surfaces of the valves.

Example 2, Effect of Supercritical Ethylene:

In a 300 ml stirred pressure reactor, 0.5 g polymer was suspended in 100 ml 1-decene or decane, respectively. In a first experiment, the reactor-headspace was filled with 6.0 MPa of ethylene, whereas in a second experiment 6.0 MPa of nitrogen was used. The gas pressure was maintained at 6.0 MPa in both cases while the mixture was heated to 170°C under continuous stirring for 30 min. After cooling down to ambient temperature and depressurizing the reactor's headspace, the morphology of the precipitated polymer residue was investigated. It was found that the polymer was smoothly dispersed in a turbid liquid phase in the experiment using supercritical ethylene, indicating a fairly small particle size distribution. The latter finding was supported by the observation that the polymer particles showed no detectable settling behavior while resting in a glass cylinder for several hours. No significant difference was observed when the 1-decene solvent was replaced by decane in the presence of supercritical ethylene.

Experiments in the absence of supercritical ethylene (replaced by nitrogen) showed no such smooth dispersion in the liquid phase, but instead the pressure reactor's internals (stirrer, baffles, cooling coil) were heavily affected by sticky polymer threads and -deposits. Example 3, Effect of Sub- versus Supercritical Ethylene:

The experimental setup described in Experiment 2 was used again for an experimental series comparing the effect of ethylene at 3.0 MPa, 170°C and at 6.0 MPa, 170°C. In the 6.0 MPa - experiment, a pressure exceeding the critical pressure of ethylene (5.04 MPa), the smoothly dispersed polymer was observed again in the liquid phase. Lowering the ethylene pressure to 3.0 MPa resulted again in sticky polymer threads, attached to the reactor's internals.

In summary, the invention takes advantage from avoiding coagulation of the polymer particles during precipitation from suitable solvents, preferentially decane or decenes or higher C10+ alkanes or alkenes or mixtures thereof, brought about by the presence of supercritical ethylene.

The features disclosed in the foregoing description and in the claims may, both separately and in any combination thereof, be material for realizing the invention in diverse forms thereof.

Claims

1. Method for cleaning a reactor and/or equipment thereof used in oligomenzation or polymerization of ethylene and/or alpha-olefins, comprising the step of purging the reactor and/or equipment with a purge stream containing solvent selected from the group consisting of n-decane, decaline and mixtures thereof, and/or by-product(s) of the oligomenzation or polymerization process having > 10 carbon atoms, in the presence of supercritical ethylene.

2. Method according to claim 1, wherein the method is carried out at a temperature of above 9.25°C, preferably 100-300°C, more preferably 150-250°C.

3. Method according to claim 1 or 2, wherein the method is canied out at an ethylene pressure of above 5.04 MPa, preferably 5.5-12.5 MPa, more preferably 5.8-6.5 MPa.

4. Method according to any of the preceding claims, wherein solvent and/or byproducts) have a residence time in the reactor and/or equipment of 0.1-24 hours, preferably 0.5-2.5 hours.

5. Method according to any of the preceding claims, wherein the purge stream is agitated.

6. Method according to any of the preceding claims, wherein the purge stream is in subsequent step discharged to a separation unit.

7. Method according to claim 6, wherein equipment used for discharging is heated, preferably at 100-300°C, more preferably 150-250°C.

8. Method according to any of the claims 6 or 7, wherein solvent, by-product(s) and/or ethylene is (are) recovered in the separation unit and/or polymeric residue is disposed off.