Autothermal production of synthesis gas

A process and apparatus for the auto-thermal production of hydrogen rich synthesis gas wherein a mixture of steam and a hydrocarbon feed gas is reacted by passing through a catalyst counter-currently to the flow of the combustion reaction effluent of the process. Reaction tubes are mounted within a heat exchange chamber of the reactor and are adapted to contain catalyst to effect the reaction of the mixture. Oxygen or oxygen-enriched air is introduced into a combustion chamber within the reactor to effect combustion, and the combustion reaction effluent is passed through a second catalyst zone to provide additional reaction and is thereafter passed about the exterior of the reaction tubes to effect heat exchange with the mixture passing through the tubes. The exothermic heat of reaction from combustion thus provides the heat for the endothermic reaction occurring within the reaction tubes and within the second catalyst zone.

BACKGROUND OF THE INVENTION 
The present invention relates to a process and an apparatus for producing a 
hydrogen-rich synthesis gas, for example, an ammonia synthesis gas. 
The process for producing ammonia from a hydrocarbon feed stream, such as 
natural gas, is, of course, well known. Thus, a mixture of the hydrocarbon 
feed gas and water in the form of steam is subjected to an endothermic 
catalytic reaction to yield carbon monoxide and hydrogen. This reaction is 
commonly referred to as primary reforming. It is then necessary to 
introduce nitrogen, which is typically done in the form of air, to produce 
the requisite ammonia synthesis gas by what is referred to as secondary 
reforming. 
In prior commercial ammonia processes, the primary and secondary reforming 
steps have typically been carried out in separate reactors, and such 
process is quite suitable and satisfactory in plant situations where it is 
necessary or desirable to produce steam for other uses within the plant. 
Thus, in such processes, the hot reaction effluent from the secondary 
reforming operation is used to generate and/or to superheat steam, either 
for use otherwise within the ammonia process or for export. 
In situations where the production of steam is not necessary, it is 
accordingly advantageous to utilize the heat available from the secondary 
reforming step for other purposes within the synthesis gas production 
process. One such use of the available heat from secondary reforming is to 
provide the heat necessary for primary reforming. The provision of a 
process and apparatus to achieve such use at a high level of efficiency is 
accordingly a principal objective of the present invention. 
The production of ammonia, as well as other products such as methanol which 
are derived from hydrocarbons, has evolved in the last several years into 
a sophisticated state-of-the-art technology, in which cost effective 
improvements are essential but are exceptionally difficult to accomplish. 
In view of this, it is quite desirable to be able to achieve both primary 
reforming and secondary reforming in a single reactor, so that the overall 
cost of the production process can be reduced by elimination of expensive 
reactors and associated non-essential equipment. 
There have been prior efforts to provide satisfactorily such reactors, but 
certain significant shortcomings have been encountered. For example, U.S. 
Pat. No. 3,751,228 describes a reactor in which the hot reformed gaseous 
product is removed from the bottom of the reactor, rather than utilized to 
provide heat for the reforming reaction. Instead, hot gas is introduced 
from outside the reactor to provide the necessary heat for the reforming 
step. A similar reactor is described in U.S. Pat. No. 4,127,289. 
U.S. Pat. No. 4,071,330 describes a reactor which is positioned within a 
fired furnace and utilizes heat transfer from the furnace across the shell 
of the reactor, to provide the requisite heat for the endothermic 
reforming reaction. The shell is formed of a heat conducting material such 
as high nickel-chrome steel. 
In U.S. Pat. No. 3,549,335, an autothermal reactor is illustrated and 
described, which includes an outer shell with an inner shell spaced 
therefrom to provide an annular passageway through which the hydrocarbon 
and steam mixture passes, through openings in the inner shell at the lower 
section of the reactor and through the primary reforming catalyst bed 
positioned outside of the tubes. The gas is thereafter brought into 
contact with the combustion reaction product and ultimately removed from 
the reactor. Such reaction process, utilizing atmospheric air for the 
combustion step, does not provide efficient utilization of the exothermic 
heat of reaction, as is highly desirable in today's sophisticated and 
competitive state of the art. 
SUMMARY OF THE INVENTION 
As indicated by the foregoing, it is an objective of the present invention 
to provide an improved process and apparatus for autothermal production of 
a hydrogen-rich synthesis gas such as an ammonia synthesis gas in which 
efficient utilization is made of the exothermic heat of reaction within 
the synthesis gas production process. 
In the process and apparatus of the present invention, a mixture of steam 
and hydrocarbon feed gas is subjected to primary reforming by passing 
through a catalyst countercurrently to the flow of the combustion reaction 
effluent of the process. The mixture is passed through reaction tubes 
which contain primary reforming catalyst and is thereafter brought into 
contact with oxygen or oxygen enriched air to effect combustion. The 
combustion reaction effluent is passed through a second catalyst zone to 
provide additional reaction, that is, the secondary reforming reaction, 
and to produce the synthesis gas. The synthesis gas product is passed 
about the exterior of the reaction tubes, thus utilizing the exothermic 
heat of combustion to provide the heat for the endothermic primary and 
secondary reaction steps.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the FIGURE of Drawing, the autothermal reactor is 
designated generally by the numeral 1. The reactor comprises a heat 
exchange chamber 2 and a first inlet 3 for introduction of a mixture of 
steam and hydrocarbon feed gas, such as natural gas. A plurality of 
reaction tubes 4 (only two are illustrated for purposes of clarity) are 
mounted within the heat exchange chamber in tube plates 5 and 6. The 
reaction tubes are designed such that a fixed bed primary catalyst 7 may 
be positioned therein. The catalyst, of course, may be any suitable 
reforming catalyst, such as nickel, with the choice of a particular 
catalyst being well within the skill of the art. 
Means shown as a cone shaped collector 10 with a vertically extending tube 
11 are positioned in communication with the reaction tubes adjacent tube 
plate 6 to provide for passage of the reacted partially reformed gases 
from the reaction tubes to a combustion reaction chamber 12 which is 
provided at the bottom portion of the reactor 1. While the configuration 
of collector 10 is illustrated as a cone, it will be readily understood 
that other configurations may also be used. 
A second inlet 13 is provided at the bottom of the reactor to introduce 
oxygen or oxygen-enriched air to effect combustion within the combustion 
chamber. A partition 14 is provided adjacent the end of vertically 
extending tube 11 to separate combustion chamber 12 from heat exchange 
chamber 2. Means are provided in the form of a plurality of openings 15 in 
partition 14 so that the combustion reaction effluent may pass 
therethrough and enter a second catalyst zone, designated generally by 
numeral 16, whereby the effluent may pass through the catalyst zone and 
undergo additional or secondary reforming to produce the desired synthesis 
gas. Again, the reforming catalyst will be any of those typically used, 
and is a matter well within the ability of those skilled in the art to 
select. Also, for purposes of clarity of illustration, only a relatively 
small proportion of catalyst is shown, but it will be understood that 
sufficient catalyst will be provided to achieve an entire zone of 
catalyst. 
As the synthesis gas thus produced passes upwardly from the second catalyst 
zone, it is directed by means of flow baffles 20 about the exterior of the 
reaction tubes 4 to provide intimate contact between the reaction tubes 
and the hot effluent. This in turn permits efficient utilization of the 
exothermic heat of combustion to provide the heat for the endothermic 
reaction occurring within the reaction tubes 4. 
An outlet 21 is also provided approximately adjacent inlet 3, through which 
the synthesis gas is removed for purification and further processing to 
produce ammonia (or other product, depending upon the particular reaction 
process). The reactor is illustrated as including manways 22, as is 
conventional, to provide for servicing or other maintenance. The reactor 
could also be provided with additional inlets and outlets, if desire, for 
flow distribution or for the introduction of additional fuel gas or steam 
to the combustion chamber. 
In effecting the conversion process of this invention as applied to ammonia 
synthesis production, the mixture of steam and natural gas or other 
hydrocarbon feed gas is brought into reactor 1 through inlet 3 at a 
temperature of approximately 900.degree. to 1300.degree. F. The mixture 
passes through the openings in tube sheet 5 and through reaction tubes 4, 
exiting from the reaction tubes through the cone-shaped collector 10 and 
passing through tube 11 and exiting into the lower part of reactor 1 and 
into the combustion chamber 12 at a temperature of approximately 
1100.degree. to 1400.degree. F. Oxygen or oxygen enriched air at a 
temperature ranging from ambient to approximately 1000.degree. F. is 
introduced into the combustion chamber through inlet 13 to effect 
combustion. The resulting combustion reaction effluent is thus at a 
temperature of approximately 2500.degree.-3500.degree. F. and passes 
upwardly through openings 15 in partition 14, and through the second 
catalyst zone 16, whereby the secondary reforming operation occurs. 
The synthesis gas mixture thus produced by the secondary reforming is at a 
temperature of approximately 1500.degree.-2100.degree. F. and flows 
upwardly, as illustrated and described above, into intimate contact with 
the reaction tubes 4, whereby the desired heat exchange takes place to 
heat the steam and hydrocarbon feed gas mixture within tubes 4 and to cool 
the synthesis gas mixture. Upon exiting outlet 21, the temperature of the 
synthesis gas mixture is approximately 1000.degree.-1300.degree. F. 
The pressure within the reactor may range from essentially atmospheric up 
to the synthesis gas conversion pressure, which with today's technology is 
approximately 1200 psig, depending upon the applicable process conditions. 
A typical pressure for the production of ammonia synthesis gas is about 
700 psig. 
It will be appreciated from the foregoing description that the process and 
reactor of this invention can be utilized for the production of synthesis 
gases to produce products other than ammonia, such as methanol, hydrogen, 
oxo-alcohol, or a hydrocarbon by Fischer-Tropsch. Inasmuch as the central 
process steps are the same as those described for ammonia synthesis gas 
production, the process of the present invention will not again be 
described with respect to such synthesis gases. 
It is significant to the successful operation of the process of this 
invention that oxygen or oxygen-enriched air be introduced into the 
combustion chamber, instead of atmospheric air, to effect combustion. By 
oxygen-enriched air, it is intended to define an air mixture containing an 
oxygen content of approximately 25% or greater by volume. The oxygen 
content may vary from such lower limit up to 100%, depending upon the 
specific reaction process. Thus, with ammonia synthesis gas production, 
the O.sub.2 content may vary from approximately 25% to about 40% or more, 
with approximately 35% by volume being optimum for most ammonia synthesis 
gas process conditions. In methanol production, on the other hand, 
essentially 100% oxygen will be used. In any event, those skilled in the 
art, given the disclosure here, will be able to determine appropriate 
proportions and whether oxygen or oxygen-enriched air should be used. 
The use of oxygen-enriched air instead of atmospheric air provides a number 
of important advantages. Thus, better control of the nitrogen content of 
the combustion effluent is achieved due to the ability to control the 
ratio of oxygen to nitrogen in the mixture. Control of the nitrogen 
content is extremely important to the process of this invention, because 
nitrogen tends to carry out heat from the reactor, thereby decreasing the 
high level heat that is otherwise available from the combustion reaction 
for process use. By controlling the nitrogen content, therefore, the 
present process avoids unnecessary loss of available heat and enables the 
highest level of available heat to be matched with the highest level of 
use, within the process. 
As those skilled in the art will appreciate, steam could also be introduced 
into the combustion chamber with the oxygen-enriched air. This would 
enable the introduction of additional steam reactant to compensate for the 
depletion resulting from the primary reforming reaction. It would also 
facilitate control of the combustion temperature and enhance operation of 
upstream oxygen-enriched air preheating equipment. 
It will also be apparent to those skilled in the art that the process and 
apparatus of the present invention have significant additional advantages 
over prior processes and reactors. Thus, the capital cost necessary for 
the present reactor is significantly lower than for standard fired 
reformers. Additionally, the present invention is readily susceptible to 
use with high pressure reforming, and is very suitable for modularization, 
which is of paramount importance in developing countries or in the 
utilization of off-shore associated gas. Furthermore, start-up time can be 
reduced, which results in turn in a savings in gas usage, and the reformer 
time on stand-by, with inefficient gas use when the synthesis unit is 
down, can also be reduced. The present invention is also more amenable to 
automatic start-up and control than multiple pass, multiple burner fired 
primary reformers currently in use. 
It should also be mentioned that although the autothermal reactor 
illustrated and described herein is a vertically disposed reactor with the 
heat exchange chamber positioned above the secondary reformer catalyst bed 
and combustion chamber, other physical arrangement for such reactor will 
become apparent to those skilled in the art, given a reading of the 
present disclosure. Similarly, although a preferred form of the invention 
utilizes reaction tubes with catalyst therein as illustrated and 
described, it would be within the skill of the art, given the disclosure 
herein, to modify the flow path within the reactor so that the gaseous 
product exiting the second or secondary reforming catalyst zone would pass 
through the tubes and the incoming steam-feed gas mixture would pass 
through a catalyst bed outside of the tubes. Such embodiments, of course, 
are intended to be included within the scope of the present invention, as 
long as the essential features and principles described above are present. 
It should also be mentioned that the shell or wall of reactor 1 is 
insulated internally, as shown at 8, with a material such as reinforcing 
ceramic to minimize heat transfer across the shell. This results in 
conservation of heat, the protection of personnel in the vicinity of the 
reactor, and also in a lower capital cost since a material such as carbon 
steel can be used for the shell. Additionally, the reaction tubes 4, 
within tubes plates 5 and 6, are hung or suspended within the reactor from 
the wall thereof as illustrated in 25. This allows the use of thin wall 
tubes, which are less expensive and have better heat transfer 
characteristics than thicker tubes; since thin wall tubes have more 
strength in tension than compression, they are mounted within the reactor 
vessel by suspension, as otherwise the tubes could deform or even 
collapse.