Chemistry of inorganic compounds – Hydrogen or compound thereof – Elemental hydrogen
Reexamination Certificate
2001-11-26
2004-09-28
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Hydrogen or compound thereof
Elemental hydrogen
C423S653000, C423S654000, C252S373000
Reexamination Certificate
active
06797253
ABSTRACT:
The present invention relates to a novel and cost effective process for converting sour natural gas which may also be static and poor, i.e. natural gas which containing hydrogen sulfide and which may also be remotely located and contain substantial amounts of nitrogen and/or carbon dioxide, to useable fuels and chemicals, such as hydrogen, methanol and high octane diesel fuel. More particularly, the present invention relates to a method and apparatus for treating “sour natural gas, i.e., gas having a ratio of H
2
S to CH
4
of at least 0.1 moles H
2
S per mole CH
4
and preferably of at least 0.5 moles/mole, using a reforming catalyst and a sulfur capture agent. Preferably, the process according to the invention can be carried out using two reactors that repeatedly cycle reactants between three basic process steps—reforming, air regeneration and fuel reduction.
BACKGROUND OF THE INVENTION
A large fraction of the world's total natural gas reserves has the problem of being “sour” in that they contain substantial amounts of H
2
S, which is both highly toxic and tends to embrittle steel pipelines, making the transport of gases by pipeline highly dangerous and unreliable due to the possibility of leakage in the gas lines and transport equipment. Much of world's total natural gas reserves also has the problem of being “static,” i.e., the gas is located in remote geographic regions that make it uneconomical to transport the gas via pipeline or to refine and/or condense the gas on site and ship it to market in liquid form. The world's total natural gas reserves also include much that is poor in quality because the methane and other combustible gas components are diluted with non-combustible CO
2
and N
2
, making the unrefined gas a relatively low Btu fuel source.
Thus, for many years, the need has existed to convert sour natural gas which may also be static and/or poor into a more valuable commercial product which could then be transported in large quantities by inexpensive means (preferably by ship). The current state of industrial practice with sour natural gas that is also static and poor is illustrated by Exxon's development of the Natuna gas fields located in the middle of the South China Sea. Because the natural gas deposits contain high percentages of CO
2
and H
2
S, the gas is considered both poor and sour. In this project the CO
2
and H
2
S are removed by liquefying and fractionally distilling the gas. This approach, while technically feasible, is very expensive. The static gas problem was resolved by developing a local use for the gas on site, namely as a fuel for use in producing steam for secondary oil recovery in the same remote geographic location. The Exxon approach made good economic sense because it began with two low value natural resources (a static, poor quality sour gas field and a depleted oil field) and finished with a relatively high quality crude oil end product using secondary oil recovery techniques.
In principle, it is possible to use fractional distillation processes to purify poor sour gas, followed by a conventional steam reforming process to convert the purified gas into a mixture of CO and hydrogen (commonly known as “syngas”). The syngas can then be converted into liquid hydrocarbons via the well-known Fischer-Tropsch process or to other commercially useful products such as methanol. This conventional gas reforming approach, however, has a number of significant disadvantages.
For example, liquefaction and fractional distillation of natural gas consumes a great deal of energy as part of the process, and thus requires a high capital expense for the equipment necessary to carry out such techniques. In addition, since steam reforming is economical only in large scale applications, a given gas resource may not be large enough to sustain the cost of a steam reformer. Furthermore, steam reforming is an endothermic reaction and in the conventional steam reforming process the heat consumed by the reaction is supplied by heating the outside of the reactor. This requires that the walls of the reactor be at a temperature equal to or greater than the temperature at which the steam reforming reaction occurs. This temperature is substantially above the temperature at which conventional steels begin losing their mechanical strength and resistance to corrosion.
As a result, conventional steam reforming normally requires processing equipment formed from expensive superalloy metals. The operating pressure at which the steam reforming process takes place must also be reduced. That is, even though the natural gas issuing from a well head may be at a pressure high enough for use in the Fischer-Tropsch process, the chemical composition of such gases does not allow for their direct use with Fischer-Tropsch. Thus, although steam reforming can produce gas of the appropriate composition, the reforming process requires first depressurizing the gas. Compressing the syngas back up to the pressure needed for the Fischer-Tropsch process can be prohibitively expensive.
In addition to these specific disadvantages, a general problem exists with conventional steam reforming processes in that many sour gas resources are found in regions of the world which lack the infrastructure necessary to support complex industrial processes. By necessity, the only practical industrial operations in such regions are those which are relatively simple to install, operate and maintain. Further, if a natural gas resource is poor in quality because it contains substantial amounts of nitrogen but not a significant amount of CO
2
, no economically viable process exists within the present state-of-the-art to easily purify the gas on site. The only known approach is to treat the entire gas stream and remove the nitrogen using conventional (but very expensive) processing means.
On the other hand, if the natural gas is poor in quality because of a high rather than low CO
2
content, then in some situations the CO
2
can be put to advantageous use. It is known that nickel-based catalysts used in steam reforming can also be used with CO
2
in the reforming of methane. An article by Tomishge et al,
Catalysis Today
45, 35-39, (1988) discusses the CO
2
reforming of CH
4
and notes the advantage of utilizing both the CO
2
and CH
4
components of the natural gas. While nickel-based catalysts are the most widely used reforming catalysts, the literature reports a number of other noble metal catalysts that can serve as active reforming catalysts (see, for example, the article by Craciun et al,
Catalysis Letters
51, 149-151, 1998).
Prior art U.S. Pat. No. 5,827,496 teaches a method of supplying heat to packed bed reactors which are used to carry out endothermic reactions such as steam reforming. In this method, heat is generated inside the reactor by alternately reducing and oxidizing a material which in the reduced state is readily oxidized and in the oxidized state is readily reduced. This method is called “Autothermal Cyclic Reforming (ACR) or “Unmixed Reforming” (UMR).
The examples in the '496 patent show the production of hydrogen by steam reforming with a nickel catalyst in the presence of CaO. The CaO captures CO
2
by forming CaCO
3
in an exothermic process. While the CaCO
3
formation supplies the heat consumed by the steam reforming process, CaCO
3
must be regenerated in an endothermic process. In order to supply the heat necessary to regenerate the CaCO
3
, air is passed through the bed, oxidizing the nickel catalyst to NiO in a strongly exothermic process. The NiO is then reduced back to nickel and the production of hydrogen is resumed. Example 8 of the '496 patent describes a process for steam reforming diesel fuel to which thiophene has been added to a level of 2000 ppm sulfur by weight, producing an output hydrogen gas containing only 5 ppm H
2
S. Thus, the '496 patent teaches that the process could produce hydrogen that is significantly lower in sulfur content than the fuel being reformed.
The process described in the '496 patent has two significant limita
General Electric Co.
Medina Maribel
Nixon & Vanderhye P.C.
Silverman Stanley S.
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