Mineral oils: processes and products – Chemical conversion of hydrocarbons – Cracking
Reexamination Certificate
2000-05-10
2002-12-03
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
Cracking
C585S648000, C585S650000, C585S652000, C585S925000
Reexamination Certificate
active
06488839
ABSTRACT:
The object of the invention is to carry out a steam-cracking of light feedstocks under very severe conditions and at a very high conversion level, in particular for feedstocks that are high in ethane or in a C
2
/C
3
mixture.
Typically, these feedstocks contain at least 20% by weight of ethane and/or propane and at least 80% by weight of hydrocarbons with 2 and/or 3 carbon atoms. Most generally, these are mixtures of ethane and propane, which can also comprise variable amounts of propylene, as well as small amounts of ethylene, methane and hydrocarbons with 4 carbon atoms or more.
The steam-cracking of light feedstocks is a process that is extensively described in technical literature, for example in the work that is well known in the steam-cracking industry: “ETHYLENE KEYSTONE TO THE PETROCHEMICAL INDUSTRY” Ludwig Kniel, Olaf Winter, Karl Stork, Editor MARCEL DEKKER, INC., New York, 1980, (Reference 1).
The technological background is described in, for example, patents U.S. Pat. No. 4,762,958, FR-A 2 032 437 and FR-A 2 760 465.
Ethane, and to a lesser extent propane, is a feedstock that is fairly refractory for which it is difficult to obtain high conversion levels. The high conversion levels require very rigorous operating conditions of furnaces that result in high coking speeds that increase the skin temperatures of pyrolysis tubes and reduce the cycle times. The work that is cited provides ethane steam-cracking yields, page 65, at 50% and 60% of conversion, value typically used in steam-cracking furnaces. It is indicated on page 112 of this work that a conversion of 70% is beyond industrial possibilities with the technologies that are known at this time. The cracking furnaces actually comprise tubes or coils for circulating a gas mixture that contains water vapor, ethane and products that are obtained from cracking. These tubes are made of refractory alloys that are high in chromium, nickel and iron, such as the “HK40” alloy that is well known to one skilled in the art and that consists mainly of 25% chromium, 20% nickel and a balance of iron (aside from minor additions).
These alloys are limited in their operating conditions by metallurgic constraints, which limits the conversion of ethane and/or propane.
This does not mean that it is impossible for a given furnace to exceed nominal conversion, for example 60%, but if the heating of the burners is advanced to increase this conversion, it will not be possible to avoid an accelerated aging of the tubes and ruptures of the tubes, in particular by creep and carburation. The cycle times are also reduced very significantly because of a drastic increase of coking under very severe conditions required for high conversion levels of ethane and/or propane.
The manufacturers of pyrolysis tubes have made significant progress, however, and have developed refractory alloys that are higher-performing than the HK40: In particular the 25/35 alloys (Cr, Ni) with a balance of Fe plus some additions (Si, Mn, Nb, Ti . . . ), for example the “HP MOD” alloys, are known. More recently, 35/45 alloys (Cr, Ni), and even alloys with 40 or 45% of chromium, which contain less than 15% of iron, with a balance of nickel, aside from minor additions including 1 to 2.5% of silicon, were developed.
A list of materials for pyrolysis tubes, with their recommended use-limit compositions and temperatures is provided in “Proceedings of the 10th Ethylene Producers Conference” (1998) published by AlChE (Reference 2) in the article “Coke Reduction and Coil Life Extension,” pp. 107-108 (Reference 2A).
With the highest-performing materials, the practicable conversion of ethane industrially has been brought to about 65%.
The thermal fluxes that are used in the cracking zone with radiation are variable according to the types of furnaces but between 50 and 120 KW/m
2
, if they are related to the outside surface of the tubes. It is possible in particular to refer to the work of Reference 1, page 131, which mentions fluxes between 50 and 80 KW/m
2
. It is also possible to refer to the work: “PROCEDES DE PETROCHIMIE [PROCESSES OF PETROCHEMISTRY],” Vol. 1, TECHNIP Editions, Paris, 1985 of A. Chauvel, G. Lefebvre, L. Castex (Reference 3), page 159, which mentions the mean fluxes between 75.5 and 104.5 KW/m
2
.
Flow values of 50 KW correspond to the use of old HK40-type alloys, which are no longer used for the hottest end portion of the pyrolysis coils. With modern alloys, the fluxes that are used are typically between 80 and 120 KW. A correspondence between the flux and the maximum skin temperatures of the tubes is given in Reference 3, page 160.
Obviously, the tendency of the steam-cracking industry is to increase the thermal fluxes, by taking advantage of the better alloys that are available, to increase productivity as well as the level of severity of the steam-cracking (i.e., the conversion in the case of ethane).
Whereby the advances of the alloys are not without limit, it is difficult today, however, to exceed approximately 65% of conversion for ethane.
The metallurgists have therefore turned, in the most recent state of the art, to new improvements, in other directions:
Thus, two publications:
“A Low Coking Environment for Pyrolysis Furnaces—CoatAlloy—, M. Bergeron, E. Makarajh, T. McCall,” and “Results of a Furnace Tube Surface Treatment in a Full Furnace Trial, D. Mullenix, A. Kurlekar”
were presented in the conference: “1999, AlChE Spring National Meeting, Eleven Annual Ethylene Producers Conference Mar. 16, 1999 HOUSTON, Tex.” (Reference 4).
In these publications, recent (1998) industrial results of ethane steam-cracking with pyrolysis tubes comprising anti-coking surface coatings are presented. According to these publications, the limitation of coking makes it possible to increase the thermal fluxes and the conversion and to operate in a satisfactory manner (cycle time, behavior of materials) at a conversion of 70%. It was also proposed to use tubes that comprise a welded inside fin, in particular a helicoidal one, which has the effect of increasing the thermal transfer, and therefore the performances of the furnaces that use this technique.
To exceed this maximum conversion limit, developments have also been undertaken to use ceramic pyrolysis tubes or pipes. The operating temperatures of the ceramics are extremely high, and these materials completely eliminate the catalytic coking. A plan for a furnace with ceramic pyrolysis tubes for high-conversion ethane cracking was thus presented by one of the major engineering firms that build steam-cracking devices (Reference 2, article “Coke-free Cracking—Is It Possible” by Khoi (Paul) X Pham, Dennis Duncan, Joseph M. Gondolfe (Reference 2B), pp. 127-150).
In this article, it is shown that cycle times of at least 7 days were obtained with a ceramic tube for an ethane conversion of 75% (p. 139). It is indicated that the metallic tube of the same geometry led to clogging within 3 hours. The dwell times used are very short (less than 50 milliseconds (ms)), which is in accordance with the philosophy and the evolution of the steam-cracking industry for several decades.
The ethane conversions at 77% and more thus are not currently being envisioned and considered as possible within the scope of an industrial operation except with very particular and complex furnace designs that use ceramic materials.
The orientations of the steam-cracking industry of light feedstocks for the preferred production of ethylene are thus:
Use of the highest-performing refractory alloys and, moreover, ceramic materials.
Use of the highest acceptable thermal fluxes.
Use of the shortest possible dwell times (in particular less than or equal to 100 milliseconds).
The invention has as its object a steam-cracking process that makes it possible to crack the ethane and light feedstocks with conversions that are greater than or equal to 77% and even 80% or even greater than 95%. This process makes it possible not only to be able to carry out these conversions on a pilot laboratory furnace or on an industrial furnace on a temporary basis, but on i
Busson Christian
Lenglet Eric
Nougier Luc
Griffin Walter D.
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
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