Coating processes – Immersion or partial immersion – Metal base
Reexamination Certificate
2002-09-26
2004-08-31
Barr, Michael (Department: 1762)
Coating processes
Immersion or partial immersion
Metal base
C427S438000, C427S443100, C427S307000, C427S309000, C428S565000, C428S680000
Reexamination Certificate
active
06783807
ABSTRACT:
The present invention relates to a process for coating apparatuses and apparatus parts for chemical plant construction, including, for example, apparatus, container and reactor walls, discharge apparatuses, fittings, pumps, filters, compressors, centrifuges, columns, heat exchangers, dryers, comminuting machines, internals, packings and mixing elements.
Deposits in apparatuses and apparatus parts for chemical plant construction constitute a serious problem in the chemical industry. Particularly affected are apparatus, container and reactor walls, discharge apparatuses, fittings, pumps, filters, compressors, centrifuges, columns, dryers, comminuting machines, internals, packings and mixing elements. These deposits are also referred to as fouling.
The deposits may be harmful or obstructive to the process in various ways and lead to the necessity of repeatedly shutting down and cleaning corresponding reactors or processing machines.
Measuring means encrusted with deposits can lead to incorrect and misleading results, through which operating errors can occur.
A further problem arising from the formation of deposits is that, in particular in the case of deposits in polymerization reactors, the molecular parameters such as molecular weight or degree of crosslinking deviate substantially from the product specifications. If deposits become detached during operation, they may contaminate the product (for example, specks in finishes, inclusions in suspension beads). In the case of reactor walls, packings or mixing elements, undesired deposits can furthermore lead to an undesired change in the resistance type profile of the apparatus or impair the efficiency of the internals or mixing elements as such. Coarse fragments breaking off from deposits can lead to blockage of discharge and working-up apparatuses, while small fragments can impair the product produced.
The deposits whose formation is to be prevented are deposits which may be caused, for example, by reactions with and on surfaces. Further reasons are adhesion to surfaces, which may be caused by van der Waals forces, polarization effects or electrostatic double layers. Other important effects are stagnation on the surface and possibly reactions in said stagnant layers. Finally, other examples are precipitates from solutions, evaporation residues, cracking on locally hot surfaces and microbiological activities.
The causes are dependent on the respective combinations of substances and may be effective alone or in combination. While the processes which give rise to the undesired deposit have been thoroughily investigated (for example, A. P. Watkinson und D. I. Wilson, Experimental Thermal Fluid Sci. 1997, 14, 361, and literature cited therein), there are only a few standard concepts for preventing the deposits described above. The methods known to date have technical disadvantages.
Mechanical solutions have the disadvantage that they can give rise to considerably higher costs. Additional reactor internals may furthermore substantially change the flow profile of fluids in the reactors and hence necessitate an expensive new development of the process. Chemical additives can lead to undesired contamination of the product and in some cases pollute the environment.
For these reasons, attempts are increasingly being made to find possibilities for directly reducing the tendency to fouling by modification of the chemical reactors, reactor parts and processing machines for chemical products.
WO 00/40774 and WO 00/40775, published on Jul. 13, 2000, describe a process for the coating of surfaces, especially surfaces of reactors for the high-pressure polymerization of 1-olefins, by currentless deposition of an NiP/PTFE layer or a CuP/PTFE layer, by means of which the relevant metal surfaces may be modified to impart antiadhesion properties. However, a careful investigation shows that, when such a layer is used, the walls in chemical apparatuses still have a certain wettability by fluids. This wettability means that the antiadhesion properties can be further improved.
WO 96/04123 discloses self-cleaning surfaces which can be coated with polytetrafluoroethylene and have particularly hydrophobic properties. The structuring is carried out by etching or embossing the surface, by physical methods, such as sandblasting, or by ion etching with, for example, oxygen. The surface is then coated with Teflon. However, the mechanical stability of layers rendered hydrophobic in this manner is much too low for use in chemical apparatus construction, in particular for polymerization reactors in which strong sheer forces act.
Other known structured surfaces having hydrophobic properties (EP-A 0 933 388) are those which are produced, for example, by etching the relevant surface, thus producing protuberances or grooves on the surface and then coating the latter with a layer of a hydrophobic polymer, for example, polyvinylidene fluoride. These layers may furthermore contain fluorinated waxes, for example, Hostaflone®. Although the surfaces modified in this manner are hydrophobic, they are not very mechanically resistant. JP 63-293169 describes a process for protecting heat exchangers from corrosion, in particular by HCl-containing gases, which comprises four successive steps:
1. electrolytic deposition of an Ni layer from an NiCl
2
-containing concentrated aqueous HCl solution; the electrolytic deposition is responsible for the good adhesion of the subsequent layers;
2. electrolytic deposition of a further Ni layer by the use of a Weisberg bath, consisting of NiSO
4
, NiCl
2
, boric acid, COS0
4
, nickel formate and formalin solution and water;
3. currentless deposition of an Ni—P layer comprising 90-95% of Ni and 5-10% of phosphorus;
4. currentless deposition of an Ni—B layer comprising 90-99% of Ni and 1-10% of boron.
This multistage process is technically very complicated. It uses HCl, which gives rise to corrosion problems in workshops in which such coating is carried out and furthermore gives heat exchangers on which deposits and caked material can still form.
CH 633586 describes a process for metallization, for example with Ni—P alloys. The metallized layers are used for providing protection against corrosion and for improving the hardness (page 2, column 2, lines 27 to 29). However, if apparatuses or apparatus parts for chemical plant construction are coated with an Ni—P alloy, a sufficient reduction in the tendency to form deposits and to cake is not observed.
Galvanotechnik 81(3) (1990), 842 et seq. likewise describes a process for coating apparatuses, for example extruder screws, with Ni—P (chemical nickel). Hard and very hard-wearing coatings are obtained (cf. especially page 844, 2nd paragraph). Numerous metals can be applied as firmly adhering coating (page 843, column 2, 2nd paragraph), which is to be understood as meaning that the coating does not flake off. The problem of the formation of deposits is not solved.
Transactions of the Institute of Metal Finishings 61 (1983), 147-9 and J. Mat. Sci. Lett. 17 (1998), 119 (Y. Z. Zhang et al.) describe Ni—P-PTFE coating for preventing caking. For numerous applications in plant construction, however, the coatings described are in most cases not sufficiently stable since they flake off or exhibit cracks after a short time.
EP-A 0 737 759 describes a coating which is intended for protecting against corrosion and comprises two coats: an Ni—P coat and an Ni—P-PTFE coat. Both drawings 1A and 1B and the photographs 2 to 4 show coarse structures and cracks and holes in the coating. Holes can be closed by adding extremely fine PTFE particles, fluorinated graphite, ceramic or the like during the 2nd coating step (column 9, lines 1-9). EP-A 0 737 759 does not state how fine these additional particles have to be and how they are produced. However, the addition of a further reagent is inconvenient, and moreover there is no indication as to how cracks can be filled. However, algal growth is possible in the cracks of the coating, for example, and may adversely affect the mode of action of the coating.
U.S. Pat. No. 3,617,3
Diebold Bernd
Dillmann Peter
Frechen Thomas
Hüffer Stephan
Hungenberg Klaus-Dieter
Barr Michael
BASF - Aktiengesellschaft
Keil & Weinkauf
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