Stock material or miscellaneous articles – Composite – Of metal
Reexamination Certificate
2001-06-26
2003-01-21
Barr, Michael (Department: 1762)
Stock material or miscellaneous articles
Composite
Of metal
C428S624000, C428S680000, C427S437000, C427S438000, C427S443100, C427S239000, C427S404000, C427S405000, C427S419100
Reexamination Certificate
active
06509103
ABSTRACT:
The present invention relates to a process for coating reactors for the high-pressure polymerization of 1-olefins. This invention furthermore relates to reactors and high-pressure reactor plants for the polymerization or copolymerization of 1-olefins, in particular ethylene, which include the reactors coated in accordance with the invention, and to a process for the preparation of ethylene homopolymers and copolymers in the reactors according to the invention.
The preparation of homopolymers and copolymers of ethylene at high pressure is a process which is carried out industrially on a large scale. In these processes, pressures above 500 bar and temperatures of 150° C. or above are used. The process is generally carried out in high-pressure autoclaves or in tubular reactors. High-pressure autoclaves are known in compact or elongate embodiments. The known tubular reactors (Ullmanns Encyclopädie der technischen Chemie, Volume 19, p. 169 and p. 173 ff (1980), Verlag Chemie, Weinheim, Deerfield Beach, Basle) are distinguished by simple handling and low maintenance and are advantageous compared with stirred autoclaves. The conversions which can be achieved in the above-mentioned apparatuses are limited.
In order to increase the capacity of the available apparatuses, the aim is to achieve the highest possible conversions. However, limiting factors are the polymerization temperature and polymerization pressure, which have a specific upper limit depending on the product type. For low-density LDPE waxes and LDPE polymers, this upper limit is about 330° C.; above this, spontaneous decomposition of ethylene may occur. Below a temperature of 150° C., heat dissipation problems may occur. Further, the pressure loss which occurs is a limiting factor; this pressure loss increases with falling temperature.
A crucial factor for the operation of a tubular reactor for the polymerization of ethylene is good heat dissipation. This heat dissipation is preferably achieved by jacket cooling, where a cooling medium, generally water, is passed through a cooling circuit. The temperature of the cooling medium is of great importance. At cooling medium temperatures below 150° C., a laminar layer of polyethylene, which can act as insulator and drastically reduce the heat dissipation, can form. If the temperature of the cooling medium is selected to be too high, the temperature difference between the reaction medium and the cooling medium is too low, which likewise results in unsatisfactory heat transfer coefficients (cf., for example, E. Fitzer, W. Fritz, Chemische Reaktionstechnik, 2
nd
Edition, page 152 ff., Springer Verlag Heidelberg, 1982).
In practice, however, a slow-flowing layer of polyethylene is also observed at temperatures above 150° C., resulting in a reduction in heat dissipation. One method of preventing the formation of this layer is so-called “stimulation” (EP-B 0 567 818, p. 3, line 6 ff.) Through periodic pressure reduction, the flow rate is drastically increased and the laminar layers briefly eliminated. However, the periodic reduction in pressure means that the average pressure during operation is reduced, which reduces the density of the ethylene and thus reduces the conversion and molecular weight of the product. In addition, the periodic reduction in pressure causes a considerable mechanical load in the apparatus, which results in increased repair costs and thus produces economic disadvantages.
The formation of laminar interfacial layers in tubular reactors or even stirred autoclaves for the polymerization of ethylene also has adverse consequences for the quality of the ethylene polymers. The material having a significantly longer residence time in the reactors is usually of high molecular weight, which is evident in macroscopic terms from the formation of so-called fisheyes. However, the material containing fisheyes has less good mechanical properties since nominal breaking points, where material failure occurs, form in the material, and the optical impression is also disadvantageous.
Attempts to coat the tubes with PTFE (polytetrafluoroethylene) have not resulted in success. Although PTFE is the obvious choice as a heat-resistant, polyethylene-incompatible material, it does, however, act as insulator, even in thin layers, and impairs heat transfer. Similar problems are also observed in processes which include the application of silane monolayers to the surface to be protected (Polymer Mater. Sci. and Engineering, proceedings of the ACS Division of Polymeric Materials Science and Engineering (1990), Volume 62, pages 259 to 263).
It is an object of the present invention
to provide a process which enables the conversion in reactors, in particular for the high-pressure polymerization of ethylene, to be improved, where this process should be based on coating of the reactors;
to provide correspondingly treated reactors,
to utilize these reactors for the construction of high-pressure reactors, and
to prepare polymers of 1-olefins in the reactors according to the invention.
We have found that this object is achieved by a process for coating a reactor for the high-pressure polymerization of 1-olefins, which comprises depositing a metal layer or a metal/polymer dispersion layer on the internal surface of a reactor in an electroless manner by bringing the surfaces into contact with a metal electrolyte solution which, besides the metal electrolyte, comprises a reducing agent and optionally a halogenated polymer to be deposited in dispersed form, by reactors coated in accordance with the invention for the high-pressure polymerization of ethylene, by the use of the reactors according to the invention for the high-pressure polymerization of ethylene, and by a process for the high-pressure polymerization of ethylene.
The reactors coated with an anti-adhesive metal coating or metal/polymer dispersion layer enable significantly improved conversion compared with uncoated reactors.
This solution according to the invention is based on a process for the electroless chemical deposition of metal layers or metal/polymer dispersion phases which is known per se (W. Riedel: Funktionelle Vernickelung, Verlag Eugen Leize, Saulgau, 1989, pages 231 to 236, ISBN 3-750480-044-x). The deposition of the metal layer or the metal/polymer dispersion phases serves for coating of the inside walls of the high-pressure reactor, which is known per se. The metal layer to be deposited by the process according to the invention comprises an alloy or alloy-like mixed phase comprising a metal and at least one further element. The metal/polymer dispersion phases according to the invention additionally comprise a polymer, for the purposes of the present invention a halogenated polymer, which is dispersed in the metal layer. The metal alloy is preferably a metal/boron alloy or a metal/phosphorus alloy having a boron or phosphorus content of from 0.5 to 15% by weight.
A particularly preferred embodiment of the coatings according to the invention involves so-called “chemical nickel systems”, i.e. phosphorus-containing nickel alloys having a phosphorus content of from 0.5 to 15% by weight; very particular preference is given to nickel alloys having a high phosphorus content of from 5 to 12% by weight.
In contrast to electrodeposition, the requisite electrons in chemical or autocatalytic deposition of the metal/phosphorus or metal/boron are not provided by an external current source, but instead are generated by chemical reaction in the electrolyte itself (oxidation of a reducing agent). The coating is carried out, for example, by dipping the workpiece into a metal electrolyte solution which has been mixed in advance with a stabilized polymer dispersion.
The metal electrolyte solutions used are usually commercially available or freshly prepared metal electrolyte solutions to which, in addition to the electrolyte, the following components are also added: a reducing agent, such as a hypophosphite or borohydride (for example NaBH
4
), a buffer mixture for setting the pH, an alkali metal fluoride, for example NaF, KF or LiF, carboxylic acid
Deckers Andreas
Hüffer Stephan
Klimesch Roger
Lerch Götz
Littmann Dieter
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