Cracking processes

Details Cracking processes Cracking processes

1994-01-04

2001-07-10

Yildirim, Bekir L. (Department: 1764)

Mineral oils: processes and products

Chemical conversion of hydrocarbons

With prevention or removal of deleterious carbon...

C208S106000, C208S047000

Reexamination Certificate

active

06258256

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to processes for the cracking of hydrocarbons, particularly for the thermal cracking of a gaseous stream containing hydrocarbons.
In thermal cracking operations a diluent fluid such as steam is usually combined with a hydrocarbon feed such as ethane and/or propane and/or naphtha, and introduced into a cracking furnace. Within the furnace, the feed stream which has been combined with the diluent fluid is converted to a gaseous mixture which primarily contains hydrogen, methane, ethylene, propylene, butadiene, and small amounts of heavier gases. At the furnace exit this mixture is cooled to remove most of the heavier gases, and then compressed. The compressed mixture is routed through various distillation columns where the individual components such as ethylene are purified and separated.
One recognized problem in thermal cracking is the formation of coke. Because coke is a poor thermal conductor, as coke is deposited higher furnace temperatures are required to maintain the gas temperature in the cracking zone at necessary levels. Higher temperatures increase feed consumption and shorten tube life. Also, cracking operations are typically shut down periodically to burn off deposits of coke. This downtime adversely affects production.
Another problem in thermal cracking is the embrittlement of the steel walls in the reaction system. Such embrittlement is due to carburization of the system metallurgy, and ultimately leads to metallurgical failure.
A variety of solutions have been proposed for addressing the problem of coke formation in thermal cracking processes. U.S. Pat. No. 5,015,358 describes certain titanium antifoulants; U.S. Pat. Nos. 4,863,892 and 4,507,196 describe certain antimony and aluminum antifoulants; U.S. Pat. Nos. 4,686,201 and 4,545,893 describe certain antifoulants which are combinations of tin and aluminum, aluminum and antimony, and tin, antimony and aluminum; U.S. Pat. Nos. 4,613,372 and 4,5524,643 describe certain antifoulants which are combinations of tin and copper, antimony and copper, and tin, antimony and copper; U.S. Pat. Nos. 4,666,583 and 4,804,487 describe certain antifoulants which are combinations of gallium and tin, and gallium and antimony; U.S. Pat. No. 4,687,567 describes certain antifoulants which are combinations of indium and tin, and indium and antimony; U.S. Pat. No. 4,692,234 describes certain antifoulants which are combinations of tin and silicon, antimony and silicon, and tin, antimony and silicon; U.S. Pat. No. 4,551,227 describes certain antifoulants which are combinations of tin and phosphorus, phosphorus and antimony, and tin, antimony and phosphorus; U.S. Pat. No. 4,511,405 describes certain tin antifoulants, and antifoulants which are combinations of tin and antimony, germanium and antimony, tin and germanium, and tin, antimony and germanium; U.S. Pat. No. 4,404,087 describes certain tin antifoulants, and antifoulants which are combinations of tin and antimony, germanium and antimony, tin and germanium, and tin, antimony and germanium; and U.S. Pat. No. 4,507,196 describes certain chromium antifoulants, and antifoulants which are combinations of chromium and tin, antimony and chromium, and tin, antimony and chromium.
In King et al, “The Production of Ethylene by the Decomposition of n-Butane; the Prevention of Carbon Formation by the Use of Chromium Plating”, Trans. of the E.I.C., 3, #1, 1 (1959), there is described an application of a {fraction (3/1000)} inch thick chromium plate to a stainless steel reactor. This chromium plate is described as peeling-off the surfaces of the steel after a period of several months of operation, which was attributed to the high temperatures required for the reaction, and periodic heating and cooling.
SUMMARY OF THE INVENTION
Accordingly, one object of the invention is to provide an improved method for the cracking of hydrocarbons, where catalytic coking is minimized, and carburization in the reactor system is reduced.
Among other factors the invention is based on the discovery that a chromium protective layer effective for resisting carburization and coking, can be provided on a portion, or portions of the reactor system exposed to hydrocarbons, which, unlike prior art chromium layers, is resistant to peeling.
According to this invention there is used an intermediate bonding layer which anchors the chromium protective layer to the steel substrate to be protected. In this regard, the reactor system comprises a steel portion having provided thereon a chromium protective layer to isolate the steel portion from hydrocarbons, applied to a thickness effective for completely isolating the steel portion from the hydrocarbon environment. The protective layer is anchored to the steel substrate through an intermediate carbide-rich, bonding layer.
Simply providing a protective plating, cladding or other coating such as a paint to a reactor system will not be sufficient to completely address the aforementioned problems. Such a protective layer must be of sufficient thickness to provide a complete, uninterrupted coating of the underlying base metal, and it must remain complete over time.
Cracks have been observed to form in chromium protective layers, especially after the initial heating of an electroplated material. These cracks can allow steam (which is typically present) to attack the steel chromium interface and undermine the chromium protective layer. According to another embodiment of the invention there is provided a novel procedure which includes a step of treating a chromium coated surface with hydrocarbons in the absence of steam which produces a metal carbide filler of the cracks which effectively seals-off the chromium coating and carbide-rich bonding layer from H
2
O attack.
An effective protective layer must resist deleterious chemical alteration, as well as peeling. Additionally, the protective layer must maintain its integrity through operation. As such, the protective coating must be sufficiently abrasion resistant during start-up and operation. The chromium-based coatings according to the invention have these advantages.
Preferably, the chromium protective layer is applied as a reducible paint which upon curing in a H
2
-rich (or pure) environment, in the absence of hydrocarbon or steam, forms a continuous chromium metal layer of substantial thickness, indistinguishable from an electroplated material, except that it is virtually free of cracks and very finely and cleanly anchored to the underlying steel through a carbide-rich bonding layer. Chromium paint protection can be applied and cured in situ to an existing plant.
Moreover, a chromium paint such as that described above can be applied to a previously chromium-plated surface. The curing treatment for the paint causes chromium metal to fill cracks in the plate as they form, thereby producing a smooth, substantially crack-free chromium coating. The paint can also be used to repair damaged, previously chromium-plated steel.
The chromium paints are especially useful to treat welds and other hard to reach areas that are otherwise untreatable by plating.
With the foregoing, as well as other objects, advantages, features and aspects of the disclosure that will become hereinafter apparent, the nature of the disclosure may be more clearly understood by reference to the detailed description and the appended claims.


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