Ceramic dip pipe and tube reactor for ethylene production

Details Ceramic dip pipe and tube reactor for ethylene production Ceramic dip pipe and tube reactor for ethylene production

1997-09-19

2001-11-06

Knode, Marian C. (Department: 1764)

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

Including heat exchanger for reaction chamber or reactants...

C422S198000, C585S920000

Reexamination Certificate

active

06312652

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
BACKGROUND OF THE INVENTION
Steam cracking furnaces have long been used to crack a variety of hydrocarbon feedstocks to ethylene and other valuable olefinic gases. For the past 20 or 30 years cracking at short residence time and high temperature has been favored for its beneficial effect on selectivity to ethylene. Basic designs of such short residence time-high temperature steam cracking furnaces are illustrated by U.S. Pat. No. 2,671,198 (dated Jun. 20, 1972) and U.S. Pat. No. 4,342,642 (dated Aug. 3, 1982).
When thermally cracking a saturated hydrocarbon down to olefinic hydrocarbons—such as the cracking of ethane to predominantly ethylene or the cracking of heavier saturated hydrocarbons like those comprising a naphtha or gas oil feedstock down to ethylene and other higher olefins—in order to maximize the conversion and the selectivity of such cracking conversion of the saturated hydrocarbon feedstock into ethylene, it is desirable to input that quantity of heat (Q) needed to effect cracking of the saturated hydrocarbon feed very rapidly while minimizing the time that the initial cracking product—namely, ethylene—is exposed to this quantity of cracking heat. To fast crack the saturated hydrocarbon feed to ethylene and then quickly remove this so formed ethylene from this high heat environment maximizes the final yield of ethylene for the degree of conversion obtained. This then is the concept that underlies the millisecond residence time at a high temperature which is now the preferred mode for furnace cracking of saturated hydrocarbon feeds to olefin products.
A steam cracking furnace comprises a refractory lined firebox containing a multiplicity of high alloy metal cracking lines through the interior passage of which flows the hydrocarbon feedstock to be cracked, together with a suitable amount of diluting steam. The sensible heat and the heat of cracking are supplied by burners located on the floor and/or walls of the firebox and this heat transfers through the metallic materials of these reaction lines into the hydrocarbon feedstock that flows there-within. A metallic cracking line can be as long as 400 feet and coiled in a serpentine shape that runs vertically up and down in the firebox, or it may be as short as 40 feet in a straight single pass through the firebox, such as the design described in the U.S. Pat. No. 4,342,642 cited above.
Cracking furnaces, as constructed today, provide for millisecond residence time at high temperatures and are, with respect to their radiant heating cracking reaction lines, constructed of metallic materials. The fireboxes themselves, since these are lined with refractory materials, are capable of delivering a greater heat load than the metallic materials of the radiant cracking reaction lines located within the firebox can withstand. This maximum service temperature of the metallic materials of which the cracking reaction lines are constructed then dictates a long line in order to accomplish the desired quantity of heat (Q) input into the hydrocarbon mass flow therethrough for that short (milliseconds) time of residence of this hydrocarbon mass within the metallic cracking reaction line. Either this, or the time of residence of the hydrocarbon mass, including its ethylene content, within the metallic reaction cracking line must be increased.
Given the extreme conditions to which the materials of the cracking reaction lines are exposed in a thermal cracking operation—which involve thermal expansion and contractions of such materials as they are suspended within the firebox which radiantly heats them—to date, metallic materials have been regarded as the only materials practical for construction of such cracking lines. The strength and serviceability dictated by the dimensions required by a cracking line in order to achieve the needed rapid transfer of heat to accomplish the level and degree of cracking desired within the short residence times that are desired have, here to date, dictated the use of metallic materials for their construction.
SUMMARY OF THE INVENTION
The reaction lines of the cracking furnace of this invention comprise an inlet pipe coaxially located by suspension within a tube, both the inlet pipe and the outerly position coaxially located tube being constructed of a ceramic refractory material, wherein the lower open end of the ceramic inlet pipe is spaced apart from the lower closed end of the outerly position coaxially located ceramic tube. This reaction line construction is, by suspension means, positioned such that in part of its entire length a portion of the pipe and tube coaxial structure is located outside of the radiant heating volume of the firebox and the remaining length of the pipe and tube coaxial structure comprising this entire reaction line structure is located and suspended within the radiant heating volume of the furnace firebox.
A mixture of saturated hydrocarbon feedstock, as preheated to an appropriate temperature in a convection section of the furnace, and superheated dilution steam as formed in such convection section and brought into combination with the saturated hydrocarbon feedstock, is passed as a saturated hydrocarbon-steam diluted feedstock to the interior of said ceramic refractory inlet pipe and passes therethrough to the lower open end of such pipe at which point, because of the closed lower end of the outerly position coaxially located ceramic refractory tube, the flow of hydrocarbon-steam feed reverses in direction to flow upwardly within an annular space defined by the exterior surface of the inlet pipe and the interior surface of the outwardly coaxially position ceramic refractory tube. At that surface of the firebox structure which differentiates between the interior radiant heating volume of the firebox and the area exterior of this fire box, the hydrocarbon mass flow within the annular space between the exterior tube and interiorly located inlet pipe is at substantially its maximum heat content—hence temperature. As hydrocarbon flow continues upwardly in this annular space which is exterior of the firebox volume, heat is transferred from this reacted hydrocarbon mass to that preheated hydrocarbon-steam feed flowing downwardly through the interior passageway of the inlet pipe. This zone is the quenching zone of the pipe-tube structure, and in this quenching zone heat transfer from the annular space reacted hydrocarbon mass which at a high heat content/temperature level to the lower heat content/temperature level hydrocarbon mass flow within the inlet pipe increases the heat content/temperature level of the inputed preheated hydrocarbon-steam feed flow within the inlet pipe.
This further prepares the input hydrocarbon for and/or begins a partial cracking of this hydrocarbon-steam feed to better condition it for final cracking. Within the radiant heating volume of the furnace firebox of the furnace, that hydrocarbon-steam content flowing down and through the inlet pipe undergoes a further infusion of heat during its course of transit to the lower end of the inlet pipe and flow direction reversal to then transit upwardly through the annular space between the exterior tube and interior pipe. Within the flow length of this annular space the hydrocarbon mass flow undergoes its maximum heat input, this because the exterior of the outwardly posited coaxially located tube is exposed to the radiant and other heating produced by the floor and/or wall mounted burners operating within the radiant heating volume of the furnace firebox. It is within this annular space that cracking of the feed saturated hydrocarbon to olefin products, first being ethylene, predominantly occurs.
Since the tube and inlet pipe of this reaction line structure, both being of a ceramic refractory material, have much greater maximum service temperature than reaction lines of a metallic construction, the ceramic materials may be and are exposed to a much greater heat content/firebox temperature—by hundreds of BTUs and/or degrees. This provides for a significantly

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