Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor
Reexamination Certificate
2001-05-08
2004-09-28
Elve, M. Alexandra (Department: 1725)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
C422S240000
Reexamination Certificate
active
06797238
ABSTRACT:
The present invention relates to an apparatus and a process for vaporizing a heavy hydrocarbon feedstock with steam.
A well-known process for upgrading hydrocarbon feedstock to obtain valuable gaseous (mainly olefins) and liquid products therefrom is the so-called thermal cracking process. To reduce the hydrocarbon partial pressure during the cracking phase, the hydrocarbon feedstock is normally diluted with superheated steam in order to promote the vaporization of the hydrocarbon feedstock, prior to introducing the vaporized hydrocarbon feedstock into the cracking section of a furnace (the radiant section). Such a process is also called steam-cracking. In processing heavy hydrocarbon feedstocks, e.g. materials with a boiling range above 230° C., the vaporization of the liquid material is normally carried out in a plurality of stages. The hydrocarbon feedstock is first preheated, whereafter the still liquid feedstock is admixed with superheated steam to form a two-phase gas/liquid mixture and to simultaneously heat the liquid. The mixture of steam and liquid thus formed is further externally heated by exhaust from the radiant section to partially vaporize the liquid, whereafter the remaining liquid is fully vaporized by introducing a further amount of superheated steam into the flow of steam and partially vaporized liquid. This further amount of steam is, for example, added to the hydrocarbon feedstock by means of a nozzle wherein steam is introduced as an annulus around a core of the hydrocarbon feedstock. It is important that the feed to the radiant section of a cracking furnace—where the actual cracking reaction takes place—is fully vaporized, as the presence of liquid droplets may cause serious coke formation and fouling in the coils of the remaining high temperature part of the convection bank as well as in the radiant coils.
In the known mixing nozzles coke may be formed in the flowline, especially at the location where steam is introduced for the final vaporization step. This may eventually result in a diminished passage for liquid and steam in the mixing nozzle, resulting in an increase of the pressure drop over the mixing nozzle. This problem was also addressed in EP-A-95197 and a possible explanation for the coke formation was given therein. The solution to this problem as disclosed in EP-A-95197 involved a specific apparatus comprising a first (inner) tubular element and a second (outer) tubular element surrounding the first tubular element to form an annular space, wherein a first inlet means is provided for introducing a heavy hydrocarbon feedstock into the inner tubular element and a second inlet means is provided for introducing superheated steam into the annular space. The inner tubular element and the outer tubular element are each provided with an open end for the supply of the superheated steam as an annulus around a core of the heavy hydrocarbon feedstock, the open ends terminating in openings arranged in a plane, substantially perpendicular to the longitudinal axes. The apparatus also includes a frusto-conically-shaped element at one end connected to the open end of the second tubular element, provided with a longitudinal axis substantially coinciding with the longitudinal axes of the tubular elements and diverging in a direction away from the outer tubular element. The arrangement of a slightly diverging frusto-conically-shaped element behind the location where the superheated steam meets the heavy hydrocarbon feedstock prevents the contact of liquid droplets with the wall of the element thereby avoiding the risk of coke formation in the apparatus.
SUMMARY OF THE INVENTION
The present invention provides an improvement to the mixing nozzle disclosed in EP-A-95197, which improvement is particularly useful for large scale mixing nozzles as will be explained in more detail hereinafter.
After the first introduction of superheated steam, whereby a mixture of steam and liquid feedstock is formed, it is preferred that an annular flow regime is developed in the pipe to the mixing nozzle to fully wet its inner surface and consequently maximize the heat convection efficiency. The annular flow regime should have developed when the feedstock enters the mixing nozzle. The core of the annular flow is formed by vapor containing a mixture of hydrocarbons and steam. The annular flow of feedstock enters the mixing nozzle disclosed in EP-A-95197 in the inner tubular element thereof. In this mixing nozzle a second amount of superheated steam is added to the annular ring surrounding said inner tubular element. The large velocity difference between the steam in this annular ring-shaped confined space between inner and outer tube on the one hand and the liquid annular flow in the inner tube on the other hand creates shear forces and severe hydrocarbon flashing at the point where both flows meet to produce the fine droplet distribution required for mass and heat transfer. The fine droplet distribution will then ensure effective vaporization. The arrangement of a slightly diverging frusto-conically-shaped element behind the location where the superheated steam meets the liquid heavy hydrocarbon feedstock prevents the contact of liquid droplets with the wall of this element thereby avoiding the risk of coke formation in the mixing nozzle itself. Thus, it is important that the steam/liquid mixture entering the mixing nozzle's inner pipe has an annular flow pattern.
In practice, the mixing nozzle is often physically arranged vertically, adjacent to the convection section and on top of the roof of the radiant section. To accommodate such configuration in a space economic way, the pipe extending from the outlet of the convection section—normally arranged at the bottom part thereof—should, after a first short horizontal part, go up substantially vertically and should then be bent 180 degrees to go down vertically so that it can enter the vertically arranged mixing nozzle at the top. Typically the pipe extending from the convection section will make three angles of about 90° to accommodate for such configuration. However, these angles could destroy the annular flow pattern and change it into a slug flow pattern, which is undesired from a heat convection and vaporization point of view. The straight length after the third 90° bend (as seen from the convection section) should suitably be at least 5 times the annular width (i.e. diameter) to re-establish the fully developed annular flow pattern before entering the nozzle, as otherwise the fine droplet distribution would be deteriorated, which would, in return, lead to malfunctioning of the nozzle. Normally the length/diameter (L/D) ratio will not exceed 30. Given the dimensions of the nozzle itself and the required L/D ratio of at least 5 for a straight pipe after the third 90° bend to ensure an annular flow pattern, scaling up of a furnace to ensure a greater throughput of feed would require a larger diameter straight pipe and hence a much greater length of the pipe after the third 90° bend. As a consequence, the length of the pipe extending vertically from the convection section after the first 90° bend should increase equally. This would be very uneconomical, because of the higher amount of materials needed to construct the straight pipes and the additional support for these pipes. Moreover, the entire construction would require expensive plotspace.
The improved apparatus of the present invention aims to provide a solution to these problems associated with scaling up furnaces, while still ensuring that the steam/liquid mixture entering the mixing nozzle's inner tube has an annular flow pattern.
This is effected by using a specific device capable of inducing a gentle swirl pattern to a liquid-containing stream, which device is connected to the feed inlet pipe of the mixing nozzle. This swirl-inducing device will effect bending of the flow direction of the hydrocarbon feedstock with 90° while simultaneously effecting a swirl pattern of the liquid part thereof, thereby forcing the liquid against the wall of the feed inlet pipe ext
Chandrasekharan Krishnamoorthy
Kloth Antonius Gijsbertus Johannes
Van Westrenen Jeroen
Elve M. Alexandra
Shell Oil Company
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