EP0907693B1 - Steam cracking installation with means for protection against erosion - Google Patents

Description

The invention relates to a conventional steam cracking installation comprising at least one oven with pyrolysis tubes connected downstream to at least one heat exchanger quenching of cracked gases of multitubular type with tubular inlet plate. Such conventional installations have means of introducing agents decoking which consist of lines for introducing decoking fluids from the group of air, steam, and air / steam mixtures.

It is known to those skilled in the art that these conventional steam cracking ovens are subject to erosion phenomena due to the circulation of flaking coke fragments pyrolysis tubes during decoking periods (air, steam or air / steam mixtures). These coke fragments are endogenous coke fragments (internal to the installation) and lead to severe erosion phenomena, while, in the context of the present invention, no solid particles are introduced erosives (exterior) in the installation.

The prior art is illustrated by patent applications WO 90 12851 and EP-A-036151. In addition, application WO 96 20259 describes a method for decoking steam cracking furnace tubes with controlled injection of erosive particles or with water vapor circulation, introduced upstream of a diffuser impactor the input of an indirect quenching exchanger. The broadcasting impactor includes solid surfaces arranged opposite a cracked gas transfer duct to the exchanger. This diffusing impactor, located in the inlet cone of the exchanger is opaque at least 70% seen from the transfer duct.

It is also known to those skilled in the art that the most common erosion problems severe are observed at the tube plate of the quench exchanger, very generally thin (about 10 mm), therefore fragile.

The object of the invention is to provide an installation having means of protection against erosion, in particular at the level of the tube entry plate of the exchanger.

Another object of the invention is to propose means of protection which do not have no risk of obstruction, during periods when there is circulation of quantities significant fragments of coke flaking from the upstream walls (pyrolysis tubes).

Finally, another object of the invention is to propose robust fixing means and reliable for the means of protection, inside a cone with low volume internal.

To this end, an installation is provided for steam cracking of hydrocarbons in pyrolysis tubes, comprising at least one oven which comprises at least one multitubular quench exchanger (3) with an inlet tubular plate, this installation having means of introduction of decoking agents, these means consisting essentially of a plurality of lines for introducing decoking fluids from the air group, water vapor and air / steam mixtures, the so-called quench exchanger (3 ) comprising an inlet cone (2) connected upstream by a transfer duct (1), this installation comprising an impactor comprising solid surfaces arranged opposite the transfer duct (1), said impactor being opaque at least 50% seen from said transfer duct (1) located upstream, the impactor being disposed inside the cone and in such a way that there is a free passage between the periphery of the impactor and the cone of at least 40 mm and of pr at least 80 mm, over the greater part of the periphery of the impactor, the installation being characterized in that the inlet cone (2) has an outer metal wall (10), an inner metal wall (11 ) preferably welded to the outer wall (10), the space between these two walls being filled by a refractory lining (12) relatively non-conductive of heat, for example refractory concrete, said impactor (6) being fixed to the inner wall (10) by means of at least two fixing lugs (13).
The free passage can be equal to at most half the diameter of the tube plate.

By inlet cone is meant a substantially sectional transition zone increasing, generally at least partly frustoconical in shape, connected in upstream of the cracked gas transfer pipe from the pyrolysis tubes and downstream to the tube plate of the quench exchanger. This cone can also be flared, "in trumpet" or evolutionary form.

Preferably, the impactor will be opaque at least 70% or even 100%, seen from the transfer duct upstream.
By opaque impactor at least at 70%, is meant an impactor for which at least 70% of the current lines of the transfer duct, extended parallel to the axis of the cone meet the impactor.
In other words, the projected surface of the various elements of the impactor, on the end section of the transfer duct, represents at least 70% of this section. (The section of the pipe is the area delimited by the circle corresponding to the inside diameter of the pipe, just upstream of the cone, the surface being projected parallel to the axis of the cone).

The gas passages can be non-communicating or communicating, for example at the ends of solid surfaces which constitute the impactor as will be described later.
According to a characteristic variant, the installation is characterized in that the impactor comprises at least one row of substantially parallel bars. These parallel bars can be in particular of circular, circular with cut sides, square or rectangular section.
The bars can be substantially perpendicular to the axis of the exchanger according to another embodiment.
According to another embodiment, the axis of the bars may have an angle different from 90 degrees with the axis of the exchanger. These bars may for example have a V shape.
According to a characteristic variant, the impactor comprises at least two rows of parallel bars, arranged according to at least two levels, the bars being separated by empty spaces, and offset so that the empty spaces at one of the levels are in screw screw with the bars arranged on the other level.

According to a preferred variant, the bars are fixed to a perpendicular spacer to these bars, and arranged substantially in the middle.

We now refer to the figures, which will illustrate the invention more clearly:
FIG. 1 represents the characteristic part of an installation according to the invention: it represents a quench exchanger (3), with a plurality of tubes (4), an inlet tubular plate (5) and a transfer duct ( 1) to the inlet cone (2) of the quench exchanger. The impactor (6) comprises a plurality of surfaces (A, B, C, F), as well as empty spaces (E) forming several gas passages. This impactor is located opposite the transfer duct (1), downstream from the current lines of this transfer duct, symbolized by arrows.

On examining Figure 1, we see that if we extend the arrows in the transfer duct (1), these arrows meet at least one of the surfaces solids A, B, C, F. This impactor (6) is therefore 100% opaque when viewed from the transfer (1) to the inlet cone (2) of the quench exchanger. It is nevertheless gas permeable thanks to the empty spaces (E) forming a plurality of passages gas. Advantageously, an impactor comprising a larger one will be used number of solid surfaces (for example between 8 and 20), and gas passages (E), than it is shown in Figure 1.

As can also be seen, this impactor comprises two levels of solid surfaces: A and C on the one hand, and B and F on the other hand, the empty spaces (E) at one of the levels being substantially opposite solid surfaces on the other level.
A, B, C, F are rectilinear bars of rectangular section, made of refractory alloy, and can be made integral, and fixed to the cone (2) or to the tubular plate (5) by fixing lugs not shown.

FIG. 2 represents in particular another type of impactor comprising a row (6) of bars inclined transversely, (6a, 6b, 6c, 6d), arranged at a single level.
FIGS. 3A, 3B illustrate another variant of an impactor (6) comprising a plurality of bars (6a, 6b, 6c), rectilinear, of circular section, substantially parallel and arranged in two levels. These bars are supported by a single central spacer (7) and substantially perpendicular to the axis of the bars. The end of the bars is located at a distance d from the cone of at least 80 mm.

According to FIG. 3B, the projections of the bars are not contiguous, the free space projected being at most 30% of the total space seen from the duct.

FIG. 4 represents a characteristic variant of construction of a cone particular, according to the invention, with a very reliable attachment system of an impactor internal.

The inlet cone (2) has an external metal wall (10) resistant to pressure, and an inner metal wall (11), such as a deceleration cone cracked gases. The space between the two metal walls (10) and (11) is filled by a refractory lining (12) such as refractory concrete. The impactor (6) is fixed to the inner metal wall (11) by fixing lugs (13), the number of at least two and preferably three or four.

This cone and this method of attachment are particularly advantageous according to the invention:
Indeed, the large external metal wall (10) is relatively cold, due to the relatively insulating refractory concrete. Its cost is therefore moderate. By cons, it becomes very difficult to hang the impactor (6) on this wall: the impactor (6) is indeed at very high temperature, close to that of cracked gases. The integral attachment of the impactor to the outer casing (10) therefore faces problems of differential expansion.

Attaching the impactor to the relatively cold tubular plate (5) is difficult for the same reasons. Fixing to refractory concrete by anchors is also tricky due to the differences in coefficient of thermal expansion between the concrete and the metallic impactor.
The fixing of the impactor on the inner wall (11) is on the contrary very simple and reliable because these two elements are metallic and at very similar temperatures.

The steam cracking installation operates in a conventional manner, with alternating cracking and decoking phases with typical circulation of an air / steam mixture. Non-erosive chemical additives can also be injected to reduce coking.
During the decoking phases, a large quantity of coke fragments flake from the pyrolysis tubes and are transported at high speed (for example 170 m / s) to the inlet of the quench exchanger.

According to the invention, the impactor prevents the direct impact of most of these fragments with the tube plate and significantly extends the life of the quench exchanger, for example double this life.
Parallel bars represent a very important advantage according to a characteristic variant of the invention, compared to toric bars because there is no possibility of accumulation and blockage by pieces of coke, between two adjacent bars: unlike to the toric bars or fragments cannot slide radially, the coke fragments can easily slide outwards, parallel to the bars, and be discharged into the external annular space, which is free except occasionally at the level of the fixing lugs.

The invention therefore provides a relatively simple, effective technical solution, without risk of obstruction by accumulations of coke.

Claims (5)

1. Hydrocarbon steam-cracking unit in pyrolysis pipes that comprises at least one furnace that comprises at least one multitube quenching exchanger (3) with a tubular input plate, whereby this unit uses means for introducing decoking agents, whereby said means consist of a number of lines for introducing decoking fluids from the group of air, water vapor, and air/vapor mixtures, whereby said quenching exchanger (3) comprises an input cone (2) that is connected upstream by a transfer pipe (1), whereby said unit comprises an impact separator that comprises solid surfaces that are placed opposite transfer pipe (1), whereby said impact separator is at least 50% opaque when viewed from said transfer pipe (1) that is located upstream, whereby the impact separator is placed inside the cone in such a way that there is free passage of at least 40 mm and preferably at least 80 mm between the periphery of the impact separator and the cone over the bulk of the periphery of the impact separator, the unit being characterized in that the input cone (2) comprises an outside metal wall (10), an inside metal wall (11) that is preferably welded to the outside wall (10), whereby the space between these two walls is filled by a refractory packing (12) that is a relatively poor heat conductor and is made of, for example, refractory concrete, whereby said impact separator (6) is attached to inside wall (10) by at least two attachment lugs (13).
2. Unit according to claim 1, wherein the impact separator comprises at least one row of parallel bars.
3. Unit according to claim 2, wherein the impact separator comprises at least two rows of parallel bars that are placed according to two levels, whereby the bars are separated by empty spaces and are offset in such a way that the empty spaces at one of the levels are opposite the bars that are arranged at the other level.
4. Unit according to one of claims 2 and 3, wherein the bars are attached to a bracket that is perpendicular to these bars and is arranged approximately in their center.
5. Unit according to one of claims 1 to 4, wherein said free passage between the periphery of the impact separator and the cone is at most equal to half of the diameter of the tubular input plate.

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