Heat exchange – With purge – or drainage – cock or plug
Reexamination Certificate
2003-04-04
2004-09-28
McKinnon, Terrell (Department: 3743)
Heat exchange
With purge, or drainage, cock or plug
C422S198000, C422S202000, C422S204000, C252S372000, C252S377000
Reexamination Certificate
active
06796369
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the prevention of metal dusting corrosion, and in particular to preventing or inhibiting metal dusting corrosion of a tubesheet inside a shell-and-tube heat exchanger, such as a gas heated reformer.
The invention is discussed herein with respect to gas heated reformers. However, persons skilled in the art will recognize that the invention may be applied to any shell-and-tube heat exchanger wherein a shellside gas may cause metal dusting to occur on the back of a tubesheet.
Syngas (a mixture of hydrogen and carbon monoxide) is produced by steam reforming and/or partial oxidation of natural gas or other hydrocarbons. Syngas processes are being developed which teach the use of hot reformed gas as a heat source to provide heat for the endothermic reforming of more feedstock. Such processes often use a “gas heated reformer,” a shell-and-tube heat exchanger type device comprising tubes containing catalyst used for the reforming and a shell in which hot gases from a second reforming step provide the thermal energy required for the endothermic reaction.
There are two basic types of gas heated reformers. The first is a classical shell-and-tube heat exchanger in which the heating and cooling streams do not mix. The second is a “2 in 1 out” type of reformer in which the reacting stream mixes with the heating stream within the unit.
The selection of materials of construction for gas heated reformers is a concern because most metals are prone to “metal dusting,” a form of localized degradation or corrosion that occurs in environments containing carbon and hydrogen compounds but almost no oxygen. Metal dusting occurs when carbon monoxide gases are cooled such that the equilibrium of the reaction in equation 1 moves to the right hand side:
2 CO⇄C+CO2 Equation 1
The carbon formed by this reaction diffuses into metal surfaces forming metal carbides. The metal carbide separates from the parent metal and leaves the system. This process is collectively referred to as metal dusting.
At temperatures sufficiently high where the equilibrium of the reaction in equation 1 favors the left side, no carbon can form and thus metal dusting cannot take place. At low temperatures, the kinetics for the reaction in equation 1 are low and the reaction rate is extremely slow so that metal dusting does not occur, or if it does occur, it is at a rate so low as to cause no concern. At intermediate temperatures, generally between about 800° F. and 1,300° F., metal dusting is a concern. Most proposed gas heated reformers will operate within the range of temperatures where metal dusting does occur.
Some commonly used techniques or methods to reduce metal dusting provide dense layers of alloys or other materials on the metal surface which prevent the gas from contacting the base metal. Examples are sulfides or oxides coupled with alumina and silica. However, the temperatures in the process and the thermal expansions and contractions caused by the temperature changes bringing the unit from ambient to operating temperature can produce defects in the surface oxide and sulfide layers. Some alloys, such as Alloy 601 H, provide resistance to metal dusting. Another technique is to use a pack cementation process which produces an aluminum-rich layer on the treated metal.
The pack cementation process is a process that deposits an aluminum rich layer on a metallic surface. The metal piece(s) is (are) placed in a retort and covered with an aluminum containing powder. The powder also contains a halide which is used to move the aluminum (as an aluminum halide) to the base metal surface where it alloys with the base metal forming a metal aluminide. The process needs high temperatures to mobilize the aluminum, so it takes place in a furnace at temperatures or about 170-2000° F. depending on the base metal and the amount of aluminum to be deposited. One company that can apply such an aluminum rich coating is Alon Surface Technologies Inc.
Air Products and Chemicals, Inc. has developed a gas heated reformer in which the tubes are protected using such a pack cementation process. The shell walls are protected by refractory, but the back of the tubesheet has no metal dusting corrosion protection. This is due to the metallurgy of the tubesheet and the potential for thermal distortion of the tubesheet during the pack cementation process and the difficulty of attaching tubes to the tubesheet afterwards. Once the tubes have been attached to the tubesheet, the assembly is too large to be so protected. The problem is how to prevent the back of the tubesheet in this type of gas heated reformer from undergoing metal dusting attack.
A similar gas heated reformer is taught in U.S. Pat. No. 4,919,844 (Wang), which discloses an enhanced heat transfer reformer (EHTR). Metal dusting was not a concern in the operation of the first EHTR's, since the operating conditions were deliberately chosen such that the temperature of the syngas contacting the back of the tubesheet was not in the range where metal dusting occurs. However, to take advantage of the full potential of the EHTR, later versions and processes in which EHTR units have been incorporated operate at conditions at which the back of the tubesheet are exposed to gas that will result in metal dusting.
In the 2 in 1 out type EHTR, a first stream comprising a mixture of natural gas and steam is fed to the top of the tubesheet, and the mixture then passes through containing tubes. The gas is heated as it passes through the tubes and reacts to form a mixture (syngas) of hydrogen, carbon monoxide, and carbon dioxide according to the steam reforming and water gas shift reactions. The feed mixture may or may not be subjected to an adiabatic pre-reforming step prior to being fed into the EHTR. The feed mixture also may contain any hydrocarbon other than natural gas that is normally reformed to provide syngas.
A second stream of hot reformed gas from a conventional steam methane reformer, autothermal reformer, or other syngas generating device known in the industry. Since it has been reformed, this second stream is somewhat hotter than the first stream. The heat contained in the second stream is used to provide the energy for reforming the first stream. This second stream enters the EHTR at the end of the unit where the first stream is exiting from the tubes; it mixes with the gas exiting the tubes and then passes up the unit on the shellside giving up its heat as it goes. The heat transferred from the shellside to the tubeside of the unit is sufficient for the reaction occurring inside the tubes. Once the shellside gas reaches the top of the unit, it exits the unit for further processing.
As the EHTR has been incorporated into additional processes, the need for greater efficiency has resulted in modified operating conditions (e.g., temperatures and pressures) such that the exiting gas is within the range where metal dusting occurs. Therefore, there is a need to protect the back of the tubesheet from metal dusting. The prior art has not adequately addressed this need.
U.S. Pat. No. 5,935,517 (Röll et al.) discloses a method to protect a refractory lined transfer line from metal dusting. Gas tight chambers are formed within the refractory with a ring, and the chambers are purged with a CO-free gas (e.g., water vapor, H
2
, N
2
, CO
2
or mixtures thereof) that does not result in metal dusting, the purge gas diffusing through the generally porous refractory.
An article entitled “Mega-ammonia round-up” in
Nitrogen & Methanol
, No. 258, July-August, 2002, which discusses the KBR (Kellogg, Brown & Root) KRES reactor, states on page 45 at column 3, last paragraph, that; “By limiting the mixed feed inlet temperature to 580-610° C. and applying a refractory face to the shell side of the tubesheet . . . in a lower-grade material than the alloy 601 used in previous units . . . ” This seems to imply that KBR has chosen to use a material selection (alloy 601 or refractory lining) to avoid metal dusting on the back of the tubesheet
Air Products and Chemicals Inc.
Gourley Keith D.
McKinnon Terrell
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