WO1999064541A1

WO1999064541A1 - Thermal cracking process and chamber

Info

Publication number: WO1999064541A1
Application number: PCT/US1999/012933
Authority: WO
Inventors: James W. Alpha; Donald M. Trotter
Original assignee: Corning Incorporated
Priority date: 1998-06-10
Filing date: 1999-06-09
Publication date: 1999-12-16

Abstract

An extruded, elongated body (10) has a continuous, refractory, metal or ceramic network encasing a plurality of parallel reactor tubes (16) that extend the length of the network, the network being capable of withstanding temperatures above 1050 degrees Celsius. The reactor tubes (16) have a cross-sectional dimension not over about 7.5 cm (3 inches). Hydrocarbons are thermally cracked or reformed by utilizing a reactor chamber that contains the body (10).

Description

THERMAL CRACKING PROCESS AND CHAMBER

FIELD OF THE INVENTION

A method of thermal cracking a stream containing hydrocarbons, a reactor chamber for use in such method, and elements of such reactor chamber.

BACKGROUND OF THE INVENTION

The invention is concerned with an improved reactor chamber and process for thermal cracking of hydrocarbons, such as ethane, propane, butane, naphtha or gas oil to form olefins, such as ethylene, propylene, or butenes. It is particularly concerned with a thermal cracking reactor chamber that avoids, or at least lessens, the formation of carbon deposits. Such deposits are commonly referred to as coke, and form on a reactor element wall during a thermal cracking process.

At the heart of a thermal cracking process is the pyrolysis furnace. Currently, such a furnace comprises a fire box through which a single, large, reactor tube runs in a serpentine manner. This reactor tube is composed of individual metal tubes, commonly referred to as tubulars, and metal fittings. The tubulars may be 6-8 feet in length, welded together and provided with fittings to form the single, large, metal, reactor tube. The reactor tube may total several hundred meters in length. It is heated by the fire box to an elevated temperature capable of maintaining the hydrocarbon stream at a maximum temperature of 750-850° C. A stream of feedstock is forced through the heated reactor tube under pressure and at a high velocity, and the product is quenched as it exits. For olefin production, the feedstock is frequently diluted with steam. During this operation, a carboniferous residue forms and deposits on the reactor tube walls. Initially, carbon residue appears in a fibrous form on the walls. It is thought that this results from a catalytic action, primarily due to nickel and iron in the metal alloy that the reactor tube is formed from. The carbon fibers on the wall appear to form a mat that traps pyrolitic carbon particles formed in the gas stream. This leads to buildup of a dense, coke deposit on the wall.

The problem of carbon deposits forming during the thermal cracking of hydrocarbons is one of long standing. It results in restricted flow of the gaseous stream of reaction material. It also reduces heat transfer through the tube walls to the gaseous stream. The temperature to which the reactor tube must be heated is then raised to maintain a constant temperature in the stream. This not only reduces process efficiency, but ultimately requires a temperature too high for equipment viability, as well as meeting safety requirements. A shutdown then becomes necessary to remove the carbon formation, a process known as decoking.

Numerous solutions to the problem of coking have been proposed. One such solution involves producing the reactor tube from metal alloys having special compositions. Other proposed solutions involve coating the interior wall of the reactor tube with a material that isolates the wall from the hydrocarbon stream. In still another proposal, the interior wall of the reactor tube is treated with an aluminum compound. This process involves aluminum surface conversion, as well as diffusion into the metal. It has also been proposed to introduce additives, such as sulfides, to the feedstock stream.

Despite these numerous proposals, the problem still remains. It is then a basic purpose of the present invention to provide an effective solution to this problem. Recently, a trend has developed toward cracking processes and furnaces that employ tubulars having smaller diameters. This trend is based on the desire to increase velocity of the hydrocarbon stream, it being recognized that a smaller diameter tubular permits greater velocity. The trend encounters the fact that coke buildup in a small diameter tube will tend to require an earlier shutdown for decoking.

It is then another purpose to enable use of smaller diameter tubular in a thermal cracking furnace in order to permit operation of the furnace at a greater stream velocity. It has been found that the output yield of an olefin thermal cracking process, for example, increases with temperature. Therefore, it is desirable to operate at as high a temperature as is feasible with the furnace being employed. The current furnaces, employing metal tubulars and fittings, have a maximum safe operating temperature of about 1050° C. This provides an operating temperature in the hydrocarbon stream of not over about 850° C.

It is then a further purpose of the present invention to provide a thermal cracking furnace and process that are capable of operating at a higher temperature than is feasible with current commercial furnaces that employ a single, large, reactor tube.

SUMMARY OF THE INVENTION

The invention resides in part in a reactor chamber, for thermally cracking or reforming hydrocarbons, that comprises a plurality of extruded, elongated, refractory, metal or ceramic bodies joined together end-to-end to form a continuous network encasing a plurality of parallel, reactor tubes within, and running the length of the network, the refractory network being capable of withstanding temperatures above 1050° C. and the reactor tubes having a cross-section with a maximum dimension not over 7.5 cms. (3 inches). The invention further contemplates the individual refractory body, as well as an improved method of thermally cracking, or reforming, hydrocarbons which comprises passing a stream of hydrocarbons into, and through, a plurality of parallel tubulars encased in a refractory metal or ceramic network, and running the length of the network, the tubulars having a cross-section with a maximum dimension not over about 7.5 cms. (3 inches) and having an interior surface that is resistant to carbon deposition.

BRIEF DESCRIPTION OF THE DRAWING

In the accompanying drawing,

FIGURE 1 is a side view of an elongated, extruded, ceramic body in accordance with the invention,

FIGURE 2 is a cross-sectional view taken along lines 2-2 in FIGURE 1 , and

FIGURE 3 is a cross-sectional view taken along lines 3-3 in FIGURE 1.

DESCRIPTION OF THE INVENTION

The present invention is based on the concept of a thermal cracking furnace employing a reactor chamber composed of a plurality of small diameter, parallel, reactor tubes. This would be in contrast to the current, commercial furnace which employs a single, large, metal reactor tube.

The reactor chamber would be composed of a material that could be heated to a temperature above 1050° C. without softening to permit sagging, or other deformation. Such materials include silicon compounds, such as the carbide (SiC) and the nitride (SiN). They also include certain high temperature glass-ceramics, such as cordierite, and alloys, such as Fe-Cr-AI alloys.

It has been found that the silicon compounds do not attract carbon deposits, and therefore resist coking. Preferably then, the inner surface of a tubular would be composed, at least in part, of a silicon compound.

This would be inherent in a reactor chamber composed entirely of SiC or SiN. It may also be provided by coating the inside of a tubular with a wash of such silicon compound. Where a high temperature, glass-ceramic tubular is to be employed, the glass may be mixed with a silicon material prior to ceramming. Alternatively, the glass-ceramic tubular could have a wash coat applied to its interior surface. Silicon carbide, being a susceptor, will be the material of choice under certain conditions. It would permit induction heating in the thermal cracking furnace, rather than the present gas-fired heating. Thus, a choice of heat sources would be offered.

An electrically heated, reactor chamber would be desirable. This would provide uniform heating in all the reactor tubes in a chamber. This concern would favor a chamber composed of extruded, refractory alloys, in particular,

Fe-Cr-AI alloys. These alloys have high-temperature capability, are amendable to electric heating, and form an alumina surface skin which would resist coking.

The key features of the reactor chamber material are (1 ) the ability to withstand temperatures above 1050° C. to permit higher temperature operation, and (2) the ability to not attract deposition of carbon. It has been estimated that an increase in operating temperature of about 50° C. in a current, commercial furnace could increase output yield by 10-15%. Likewise, the use of a smaller diameter tubular would enable operating at higher velocities. This in turn would increase throughput and ultimate production. Thermal cracking furnaces that employ smaller diameter, metal tubulars are in use. However, because of their smaller size, these furnaces must be decoked more frequently, e.g. every 7-20 days. The increased down time thus required becomes a serious problem that would be greatly alleviated by the extruded, reactor chamber of the present invention. Structurally, each tubular in the present multiple, tubular chamber would be smaller in diameter than the present metal tubulars which commonly have a 10 cm. (4 inch) diameter. A maximum interior diameter of 7.5 cm. (3 inches) is contemplated for each tubular in the present arrangement. An interior diameter of 1.3-3.8 cm. (0.5-1.5 inches) for each tubular is deemed preferable. One method of producing the desired plurality of tubulars would be to bundle a plurality of tubes into a compact structure. The bundle could be heated to provide incipient fusion between adjacent tubes. Alternatively, a bonding network surrounding the tubes could be provided. One such network could be a glass-ceramic bond, that is, a sintered, glass bond that crystallizes to form a high temperature glass-ceramic. The sintered, crystallized glass would provide a network bonding the plurality of tubes into a unitary body. Such materials have been used heretofore as coatings on metal bodies.

A preferable approach involves extruding an elongated, body having a plurality of elongated, parallel, open passages extending the length of the body. The geometry of the passages is dependent only on the ability to produce a suitable, extruding die. However, the simplest geometry is a circular, cylindrical passage of uniform cross-section throughout its length.

For example, an elongated, cylindrical, alloy or ceramic body corresponding to the currently used metal tubes might be employed. The tubulars within such body will, of course, be open at each end of the body. This will permit a steady flow of feedstock through each tubular in the furnace.

Extrusion of continuous, metal, or ceramic, cylindrical bodies is common practice in the production of substrates for automotive catalytic converters. There, the continuously extruded body is transversely sliced to produce bodies of desired length. These are commonly referred to as logs. It is envisioned that the present bodies would be produced in like manner, but sliced to provide longer lengths. The length would preferably be as long as convenient handling would permit. This would reduce the number of joints required in a complete furnace. Accordingly, lengths up to 6 meters (20 feet) are contemplated. The preferred embodiment of the invention is further described with reference to the accompanying drawings.

In the drawings, FIGURE 1 is a side view of an elongated, extruded, refractory metal or ceramic body 10 in accordance with the invention. Body 10 is shown as a cylindrical body having a circular cross-section. It will be appreciated that the body may take other shapes, such as hexagonal or triangular. The shapes will depend on what is considered most practical and efficient in a given application, in particular, a thermal cracking reaction chamber.

In a thermal cracking chamber, a large number of bodies 10, with multiple tubulars within each body, may be assembled end to end to provide a sufficient length for the cracking process. As indicated earlier, present furnaces, constructed with large, metal tubulars, involve serpentine arrays to provide the desired long path for the process. It is anticipated that the present extruded bodies will be assembled in a similar array.

Various means may be used to assemble ceramic bodies 10 into a long, reaction chamber length. The ends 12 and 14 of body 10 may be oppositely beveled to provide fitting of the bodies together. A sealing material may be applied between the beveled ends of adjacent lengths. Various clamping arrangements are known for holding lengths of tubing in place. In the current furnaces, metal tubulars are welded together. The present structure may employ a crystallizable glass that can be fusion sealed and crystallized to form a glass-ceramic bond. In any event, bodies 10 will be joined at their ends to form a continuous length for a furnace.

FIGURE 2 is a cross-sectional view taken along lines 2-2 in FIGURE 1. As shown in FIGURE 2, cylindrical, extruded body 10 has a plurality of circular tubulars 16. Tubulars 16 extend the entire length of body 10. Each tubular 16 is of uniform diameter over its length, and is shown of circular cross-section. While this is deemed desirable, it will be appreciated that tubulars are not required to be of uniform size, or to have a circular cross-section. Body 10 is shown with seven tubulars 16. However, the number of tubulars may be less, or may be more, depending on the size desired.

FIGURE 3 is a cross-sectional view taken along lines 3-3 in FIGURE 1. As shown in FIGURE 3, tubulars 16 are uniform in size, extend the full length of body 10, and are open at ends 12 and 14.

Claims

I claim:

1. An extruded, elongated body having a continuous, refractory metal or ceramic network encasing a plurality of parallel reactor tubes within, and extending the length of the network, the refractory network being capable of withstanding temperatures above 1050┬░ C, the reactor tubes having a cross- section with a maximum dimension not over about 7.5 cm. (3 inches).

2. An extruded, elongated body in accordance with claim 1 in which the reactor tubes have an interior surface comprising a silicon-containing compound that resists carbon deposition.

3. An extruded, elongated body in accordance with claim 1 in which the reactor tubes have an interior diameter of 1.3-3.8 cm. (0.5-1.5 inches).

4. An extruded, elongated body in accordance with claim 1 in which the network is composed, at least in part, of silicon carbide and/or silicon nitride.

5. An extruded, elongated body in accordance with claim 4 wherein the network is composed of a mixture of cordierite and silicon carbide and/or silicon nitride.

6. An extruded, elongated body in accordance with claim 1 wherein the reactor tubes have a silicon carbide and/or silicon nitride wash coat on their interior walls.

7. An extruded, elongated body in accordance with claim 1 wherein the network is composed of silicon carbide.

8. An extruded, elongated body in accordance with claim 1 wherein the network is composed of a refractory metal.

9. An extruded, elongated body in accordance with claim 8 wherein the network is composed of a Fe-Cr-AI alloy.

10. A reactor chamber, for thermally cracking or reforming hydrocarbons, that comprises a plurality of extruded, elongated, refractory, metal or ceramic bodies joined together end-to-end to form a continuous network encasing a plurality of parallel, reactor tubes within, and running the length of, the network, the refractory network being capable of withstanding temperatures above 1050┬░ C, and the reactor tubes having a cross-section with a maximum dimension not over 7.5 cm. (3 inches).

11. A reactor chamber in accordance with claim 10 wherein the reactor tubes have an interior surface that resists carbon deposition.

12. A reactor chamber in accordance with claim 11 wherein the interior surface of the reactor tubes is a silicon-containing compound.

13. A reactor chamber in accordance with claim 11 wherein the interior surface of the reactor tubes has a wash coat that resists carbon deposition.

14. A reactor chamber in accordance with claim 10 wherein the reactor tubes have a circular cross-section with a diameter of 1.3-3.8 cm. (0.5-1.5 inches).

15. A reactor chamber in accordance with claim 10 which comprises an inductively heated network consisting essentially of silicon carbide.

16. A reactor chamber in accordance with claim 10 which comprises an electrically heated network consisting essentially of a refractory alloy.

17. A reactor chamber in accordance with claim 16 wherein the alloy is an Fe-Cr-AI alloy and there is an alumina layer on the reactor tube surfaces.

18. A method of thermal cracking or reforming hydrocarbons which comprises passing a stream of hydrocarbons into, and through, a plurality of parallel tubulars encased within a refractory metal or ceramic network, the tubulars running the length of the network, having a cross-section with a maximum dimension not over 7.5 cm. (3 inches), and having an interior surface that resists carbon deposition.

19. A method in accordance with claim 18 which comprises passing a stream of hydrocarbons into, and through, a plurality of tubulars having interior surfaces that resist carbon deposition.

20. A method in accordance with claim 18 which comprises passing a stream of hydrocarbons into, and through, a plurality of tubulars having a maximum cross-section dimension in the range of 1.3-3.8 cm. (0.5-1.5 inches).

21. A method in accordance with claim 18 which further comprises heating the ceramic body to a temperature in excess of 1050┬░ C.

22. A method in accordance with claim 18 which comprises passing a stream of hydrocarbons into, and through, a plurality of tubulars encased in a network composed of silicon carbide and heating the network by induction heating.

23. A method in accordance with claim 18 which comprises passing a stream of hydrocarbons into, and through, a plurality of tubulars encased in a network composed of a refractory metal alloy and heating the network electrically.