Multiple reactor system and method for fischer-tropsch synthesis

A Fischer-Tropsch process is provided for synthesizing hydrocarbons, involving multiple Fischer-Tropsch reactor stages (110) arranged in series, and characterized by very low carbon monoxide conversion per Fischer-Tropsch reactor stage (110) and intermediate removal of water between reactor stages (110). In one embodiment, the system utilizes an iron-based catalyst and balances the molar H.sub.2 /CO feed ratio in the synthesis gas (108) with the overall H.sub.2 /CO consumption ratio across all of the Fischer-Tropsch reactor stages (110). In a preferred embodiment, carbon dioxide is recycled from the last in series of the Fischer-Tropsch reactor stages (110) to the synthesis gas generator (106). The system may advantageously utilize a gaseous hydrocarbon feed (102), such as obtained from natural gas, as feed for producing the synthesis gas (108).

FIELD OF THE INVENTION 
The present invention relates in general to Fischer-Tropsch synthesis of 
hydrocarbons from synthesis gas and, in particular, to Fischer-Tropsch 
synthesis involving a plurality of staged Fischer-Tropsch reactors which 
may be operated in a manner that is especially advantageous when using a 
gaseous hydrocarbon feed, such as natural gas, that tends to produce 
hydrogen-rich synthesis gas. 
BACKGROUND OF THE INVENTION 
Fischer-Tropsch processing is a well known technique for synthesizing 
hydrocarbon products. In general, Fischer-Tropsch synthesis processes 
involves converting a synthesis gas including hydrogen and carbon monoxide 
to hydrocarbon products in the presence of a Fischer-Tropsch catalyst. The 
most commonly used Fischer-Tropsch catalysts are iron-based and 
cobalt-based. 
A Fischer-Tropsch process is generally thought to involve a complex 
combination of reactions. Some important reactions include the following: 
I. 2 H.sub.2 +CO.fwdarw.--CH.sub.2 --+H.sub.2 O 
II. H.sub.2 O+3CO.fwdarw.--CH.sub.2 --+2CO.sub.2 
III. H.sub.2 O+CO.fwdarw.CO.sub.2 +H.sub.2 
Reactions I and II produce hydrocarbon products. Reaction III, referred to 
as the water-gas shift reaction, does not produce hydrocarbon products. In 
Reaction I, hydrogen and carbon monoxide are consumed in a molar ratio of 
hydrogen to carbon monoxide (H.sub.2 /CO consumption ratio) of 2 to 
produce hydrocarbon products. Therefore, if Reaction I were the only 
reaction occurring during the Fischer-Tropsch synthesis, the H.sub.2 /CO 
consumption ratio in the process would be 2. The effect of Reactions II 
and III, however, is to reduce the H.sub.2 /CO consumption ratio. 
Early Fischer-Tropsch work involved gasification of coal to form synthesis 
gas. Synthesis gas produced in this manner is typically lean in hydrogen, 
often having a molar H.sub.2 /CO ratio of only about 0.6 to 0.7. In this 
situation, because the synthesis gas includes such a low H.sub.2 /CO 
ratio, reduction of the H.sub.2 /CO consumption ratio caused by Reactions 
II and III was not detrimental. Rather, Reaction III was generally 
considered to be beneficial because it produced additional hydrogen. 
Consumption of carbon monoxide in Reaction III was not a problem due to 
the relative surplus of that component in the system relative to hydrogen. 
More recently, there has been significant interest in the use of gaseous 
hydrocarbon feeds, such as natural gas and petroleum gas, as the feed 
material for producing synthesis gas. Synthesis gas produced from natural 
gas tends to be rich in hydrogen and lean in carbon monoxide, with a 
H.sub.2 /CO ratio that is typically 2 or greater. If only Reaction I were 
present during the Fischer-Tropsch synthesis, a H.sub.2 /CO ratio of 2 in 
the synthesis gas would be optimal because it would match the H.sub.2 /CO 
consumption ratio in Reaction I. Unlike the situation with synthesis gas 
produced by coal gasification, Reactions II and III are detrimental when 
operating with such a hydrogen-rich synthesis gas, because Reactions II 
and III consume disproportionately large quantities of carbon monoxide. 
Therefore, when operating with a hydrogen-rich synthesis gas, it would 
generally be desirable to promote Reaction I and suppress Reactions II and 
III. 
Cobalt-based catalysts, which tend to promote Reaction I and suppress 
Reactions II and III, have been proposed as preferred catalysts for 
Fischer-Tropsch synthesis when operating with a hydrogen-rich synthesis 
gas. With cobalt-based catalysts, H.sub.2 /CO consumption ratios that 
approach 2 are readily achievable. One problem with cobalt-based 
catalysts, however, is that they are expensive. Another problem with 
cobalt catalysts is that during the Fischer-Tropsch synthesis they tend to 
produce substantial amounts of undesirable methane and other light 
hydrocarbons, as opposed to more desirable higher molecular weight 
hydrocarbon products. 
Iron-based catalysts have also been proposed for use in Fischer-Tropsch 
processes operating with a hydrogen-rich synthesis gas. Iron catalysts are 
typically substantially less expensive than cobalt catalysts. Also, iron 
catalysts tend to promote production of the more desirable higher 
molecular weight hydrocarbon products. A significant problem with 
iron-based catalysts, however, is that they tend to operate at a low 
H.sub.2 /CO consumption ratio, due to the higher activity of iron 
catalysts for promoting Reactions II and III. Consumption ratios of less 
than 1.2 are typical. The result is that significant carbon in the system 
is lost as a carbon dioxide waste product, and there is a significant 
excess of unreacted hydrogen, which is also wasted. This requires 
additional methane and oxygen for synthesis gas generation to produce a 
given quantity of hydrocarbon products. The low H.sub.2 /CO consumption 
ratio has largely discouraged the use of iron-based catalysts in 
Fischer-Tropsch operations using hydrogen-rich feed, such as natural gas, 
to produce the synthesis gas. 
Accordingly, there is a need for an improved Fischer-Tropsch process in 
which the inherent advantages of iron-containing catalysts for promoting 
higher molecular weight products can be realized without the excessive 
waste of carbon and hydrogen, especially when using a hydrogen-rich 
synthesis gas, such as is produced from a natural gas feed. 
SUMMARY OF THE INVENTION 
The present invention provides a Fischer-Tropsch method of hydrocarbon 
synthesis that permits much higher H.sub.2 /CO consumption ratios to be 
attained when using iron catalysts and that avoids excessive losses of 
carbon to waste products. The method of the present invention is, 
therefore, very desirable when processing a hydrogen-rich synthesis gas, 
such as may be produced from natural gas or other gaseous hydrocarbon 
feeds. In one aspect, the method of the present invention involves 
conducting the Fischer-Tropsch reaction as a staged process with multiple 
Fischer-Tropsch reactor stages arranged in series and with a very low 
conversion of carbon monoxide in at least one, and preferably in all, of 
the Fischer-Tropsch reactor stages. Carbon monoxide conversions are 
typically less than 70% for each reactor stage, and more preferably in a 
range of from about 40 to about 60% for each reactor stage. Each 
Fischer-Tropsch reactor vessel is preferably a slurry reactor, in which 
particles of the catalyst are in a slurry with high molecular weight 
hydrocarbon products of the Fischer-Tropsch synthesis. 
A significant advantage of the present invention is that it significantly 
increases the H.sub.2 /CO consumption ratio for iron-based catalysts and, 
accordingly, also reduces the loss of carbon to the production of carbon 
dioxide, resulting in a larger quantity of hydrocarbon product from a 
given quantity of gaseous hydrocarbon feed. With the present invention, it 
has been found that when the carbon monoxide conversion in a 
Fischer-Tropsch reactor stage begins to significantly exceed 50%, the 
water-gas shift reaction (Reaction III) becomes increasingly more active. 
Therefore, when the carbon monoxide conversion is maintained at a low 
level in each reactor stage, the water-gas shift activity may be 
significantly reduced and it is possible to convert a higher percentage of 
carbon monoxide in the synthesis gas to hydrocarbon product. The invention 
requires at least two reactor stages in series, but because the invention 
involves such a low conversion per reactor stage, it is typically 
preferred to have at least three reactor stages in series to achieve an 
acceptable yield of the desired hydrocarbon products. 
In a further refinement, the process of the present invention involves 
intermediate removal of water between Fischer-Tropsch reactor stages, so 
that the partial pressure of water in the reactor vessels is maintained at 
a relatively low level, further suppressing Reactions II and III and 
tending to further increase H.sub.2 /CO consumption ratios. The 
intermediate water removal, in combination with the low carbon monoxide 
conversion per reactor stage, can result in a significant improvement in 
the overall H.sub.2 /CO consumption ratio and accompanying reduction in 
the loss of carbon to carbon dioxide waste product. With the process of 
the present invention, a H.sub.2 /CO consumption ratio of at least 1.5, 
and often significantly higher, can typically be attained when using an 
iron-based catalyst in a slurry reactor. 
In one preferred embodiment of the process of the invention, further 
operational enhancement is available in which the process is controlled so 
that the H.sub.2 /CO ratio in the synthesis gas feed and the overall 
H.sub.2 /CO consumption ratio across all of the series of Fischer-Tropsch 
reactor stages will have approximately the same values, typically within 
about 0.2 of each other and preferably even closer. This control is 
typically accomplished by recycling carbon dioxide from the last reactor 
stage in the series to the synthesis gas production step. Recycling of the 
carbon dioxide to balance H.sub.2 /CO feed and consumption ratios, in 
combination with other features of the process of the invention, conserves 
carbon in the system for ultimate conversion to hydrocarbon product and 
tends to lower the H.sub.2 /CO feed ratio so that it is approximately 
equal to the overall H.sub.2 /CO consumption ratio. The result is that the 
present invention can provide significant operating advantages through 
increased hydrocarbon product yield and increased energy conversion 
efficiency. When the process of the invention is operated in this manner, 
it is typically possible to produce C.sub.5 + hydrocarbon products that 
include at least about 90% of the carbon originally contained in the 
gaseous hydrocarbon feed, and to operate with a high energy conversion 
efficiency, so that the C.sub.5 + hydrocarbon products have a total 
heating value of at least about 65% of the total heating value of the 
gaseous hydrocarbon feed. 
While the process of the invention may be operated using any 
Fischer-Tropsch catalyst, the invention is contemplated for use primarily 
with iron-based catalysts, due to the significant advantages attainable 
when using iron-based catalysts with the present invention. Furthermore, 
certain iron-based catalysts are particularly preferred. One preferred 
iron-based catalyst is a precipitated iron catalyst of a very fine 
particle size, typically no larger than about 60 microns. Not to be bound 
by theory, these very fine precipitated iron catalysts are believed to be 
advantageous over iron catalysts of a larger particle size, because it is 
believed that larger iron particles tend to trap and effectively 
immobilize water in interior pore spaces of the particles, which tends to 
promote Reaction II and consequently lowers the overall H.sub.2 /CO 
consumption ratio of the reacting synthesis gas. In contrast, it is 
believed that with the use of the very fine precipitated iron catalysts, 
less water tends to become trapped in interior pore spaces of the catalyst 
particles, significantly suppressing Reaction II. 
Other preferred iron-based catalysts for use with the process of the 
invention are a supported iron catalysts, comprised of a thin layer of 
iron deposited on a support material, such as silica, alumina or carbon. 
Because the catalytically active iron layer is thin, it is believed to 
have a low tendency to trap water in interior pore spaces and to thereby 
suppress Reaction II, in a manner similar to that noted above with respect 
to the very fine precipitated iron catalysts. A further advantage of using 
supported iron catalysts, however, is that a larger particle size can be 
used relative to precipitated iron catalysts. This larger particle size 
significantly simplifies separation of the catalyst particles from liquid 
hydrocarbon products produced during the Fischer-Tropsch synthesis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the following description, the present invention is set forth in the 
context of a staged reaction system oriented towards Fischer-Tropsch 
synthesis of hydrocarbons from synthesis gas generated from a natural gas 
or other gaseous hydrocarbon feed. While the process must include at least 
two Fischer-Tropsch reactor stages, the description herein is made 
primarily to the use of three Fischer-Tropsch reactor stages, but the same 
principles apply equally to the use of smaller and larger numbers of 
stages. 
Referring to FIG. 1, one embodiment of the Fischer-Tropsch process is shown 
in which a gaseous hydrocarbon feed 102 and oxygen gas 104 are fed to a 
synthesis gas generator 106, where synthesis gas 108 comprising hydrogen 
and carbon monoxide is generated. Although it is possible to add the 
oxygen gas 104 in the form of air, the use of a gas stream that is 
enriched in oxygen gas relative to air, such as may be produced by 
membrane separation of air, is preferred. Use of a gas stream enriched in 
oxygen gas reduces the need to process the large quantity of nitrogen 
present in air. The synthesis gas 108 is fed to the first in a series of 
three Fischer-Tropsch reactors 110 A, B and C. The Fischer-Tropsch 
reactors 110 A, B and C each have an internal reactor volume, containing a 
Fischer-Tropsch catalyst, where the Fischer Tropsch reaction occurs to 
produce Fischer-Tropsch reaction products, including the desired 
hydrocarbon products. The Fischer-Tropsch reactors 110 A, B and C, as 
shown in FIG. 1, are slurry reactors, meaning that catalyst particles are 
slurried in a liquid of high molecular weight hydrocarbons generated 
during the Fischer-Tropsch reaction. As synthesis gas passes through the 
slurry, hydrogen gas and carbon monoxide react to form hydrocarbon 
products. 
The method of the invention is described with reference to the use of a 
gaseous hydrocarbon feed, such as natural gas, because such feeds are 
preferred and are particularly advantageous for use with the present 
invention, but it should be recognized that in its broadest scope the 
invention is not limited to such feeds. The method could also be 
advantageously used with other feed materials (such as coal, tars, 
bitumen, soot, and petroleum coke) from which synthesis gas could be 
produced. Furthermore, a combination of different types of feed could be 
used. 
Each of the Fischer-Tropsch reactors 110 A, B and C shown in FIG. 1 
represent a different Fischer-Tropsch reactor stage of the staged 
Fischer-Tropsch reaction. It should be recognized that each 
Fischer-Tropsch reactor stage could include one or more than one reactor 
vessel. When a stage includes more than one reactor vessel, the reactor 
vessels within the stage would be arranged in parallel, with each 
processing a different portion of feed to that stage. In the case of 
multiple reactor vessels in a stage, the total reactor volume for the 
stage would be the sum of the internal reactor volumes of all of the 
reactor vessels in the stage. 
It should be noted that, as used herein, the term "Fischer-Tropsch 
reaction" does not refer to a single reaction, but to all reactions in a 
complex reaction system occurring in the reactors 110 A, B and C. In 
addition, it should be noted that the term "Fischer-Tropsch reaction 
products" includes all compounds resulting from the Fischer-Tropsch 
reaction taking place in the reactors 110 A, B and C. Fischer-Tropsch 
reaction products include, for example, hydrocarbon products, water and 
carbon dioxide. Furthermore, the "hydrocarbon products" referred to herein 
may include some quantities of oxygenated hydrocarbons, e.g., alcohols, 
ketones, aldehydes, etc. These oxygenated components, however, typically 
comprise only a small portion of the total hydrocarbon products, and 
preferably less than about 10 weight percent of the total hydrocarbon 
products. 
Exiting each Fischer-Tropsch reactor 110 A, B and C are a gaseous effluent 
112 A, B and C and a liquid effluent 114 A, B and C. Each of the liquid 
effluents 114 A, B and C includes heavier hydrocarbons, such as those of 
about C.sub.11 and larger. Typically, a large portion of the each of the 
liquid effluents 114 A, B and C will be in the wax range. Each of the 
gaseous effluents 112 A, B and C includes lighter hydrocarbons, carbon 
dioxide, water and unconverted hydrogen and carbon monoxide. Each of the 
gaseous effluents 112 A, B and C is cooled and then is sent to a separator 
116 A, B and C where condensed liquids 118 A, B and C are removed. 
Noncondensible gases 120 A and B from the first two reactors 110 A and B 
are fed to the next succeeding reactor in series, where unconverted 
hydrogen and carbon monoxide are reacted to form additional hydrocarbon 
products. The condensed liquids 118 A, B and C generally include water and 
liquid hydrocarbons, which are typically in a range of about C.sub.4 to 
C.sub.11. Because liquid hydrocarbons are a valuable product, they 
typically would be further separated from the water and recovered. The 
noncondensible gases 120 A, B and C generally include carbon dioxide, 
light hydrocarbons (typically about C.sub.4 and smaller) and unconverted 
hydrogen and carbon monoxide. 
As shown in FIG. 1, the noncondensible gases 120 A from the first 
Fischer-Tropsch reactor 110 A in series are fed to the second 
Fischer-Tropsch reactor 110 B in series. The noncondensible gases 120 B 
from the second Fischer-Tropsch reactor 110 B in series are fed to the 
third Fischer-Tropsch reactor 110 C in series. Accordingly, except for the 
last reactor in series, the unconverted hydrogen and carbon monoxide from 
each previous Fischer-Tropsch reactor is sent to the next succeeding 
Fischer-Tropsch reactor in the series. 
An important aspect of the present invention is operation of the process 
with a low carbon monoxide conversion to Fischer-Tropsch reaction products 
in at least one, preferably two, and more preferably all three of the 
Fischer-Tropsch reactors 110 A, B and C. As used herein, the term "carbon 
monoxide conversion" means the molar percentage of carbon monoxide that 
reacts to form Fischer-Tropsch reaction products. The carbon monoxide 
conversion in a single reactor may be determined by comparing the quantity 
of carbon monoxide fed to the reactor with the quantity of carbon monoxide 
exiting the reactor, the difference representing the quantity of carbon 
monoxide converted. For example, if 100 moles of carbon monoxide is fed 
into a reactor and 40 moles of carbon monoxide exits the reactor, then the 
carbon monoxide conversion for that reactor would equal 60%. With the 
present invention, a low carbon monoxide conversion is considered to be a 
conversion percentage of no larger than 70%. Generally, however, enhanced 
performance with the process of the present invention is attained if the 
carbon monoxide conversion in one or more of the Fischer-Tropsch reactors 
is smaller than about 65%, with a carbon monoxide conversion of smaller 
than about 60% being more preferred. An especially preferred carbon 
monoxide conversion is in a range of from about 40% to about 60%, with 
around 50% being particularly preferred. In most instances, the carbon 
monoxide conversion will be at least about 40%. 
A low carbon monoxide conversion in a Fischer-Tropsch reactor may be 
obtained by controlling operating conditions accordingly. In particular, 
it is generally desirable to have a short residence time in the reactor to 
prevent carbon monoxide conversion in the reactor from becoming too high. 
With the present invention, surprisingly, it is found that water-gas shift 
activity appears to be significantly suppressed at low carbon monoxide 
conversion rates. In particular, the water-gas shift activity appears to 
increase dramatically as the carbon monoxide conversion becomes 
significantly larger than about 50%. Above 70% carbon monoxide conversion, 
the water-gas shift activity becomes too high to obtain the benefits of 
low water-gas shift activity according to the present invention. 
A further aspect of the present invention is that, although the carbon 
monoxide conversion may be low for each reactor, there is a high overall 
conversion of carbon monoxide across all of the reactors. This high 
overall carbon monoxide conversion is typically at least about 80% and 
preferably at least about 85%, with overall carbon monoxide conversion of 
at least about 90% often being achievable. For example, if the carbon 
monoxide conversion for each of the three reactors is at a level of 50%, 
then the overall carbon monoxide conversion rate will be 87.5%. 
Although it is preferred that each of the Fischer-Tropsch reactors operate 
at a low carbon monoxide conversion, as noted above, the invention is not 
so limited. For example, one or more reactors could be operated at a low 
carbon monoxide conversion, below 70%, and one or more other reactors 
could be operated at a higher carbon monoxide conversion, above 70%. In 
this situation, it is desirable that at least the first reactor in the 
series of reactors be operated at the low carbon monoxide conversion. This 
is because the detrimental effects of high water-gas shift activity will 
typically be more detrimental in the first reactor than succeeding 
reactors. For this reason, it is preferred that at least the first reactor 
is operated at a carbon monoxide conversion of smaller than about 65%, and 
more preferably smaller than about 60%. 
Another important aspect of the invention is the removal of water from each 
of the reactors, as is shown in FIG. 1. As noted above, water is removed 
from the gaseous effluent 112 A, B and C of each of the Fischer-Tropsch 
reactors in series 110 A, B and C in the separators 116 A, B and C. One 
reason for removing water is to reduce the degree to which water generated 
in a preceding reactor is available to participate in the water-gas shift 
reaction in a succeeding reactor. Intermediate removal of water between 
stages, therefore, suppresses Reactions II and III in succeeding reactors. 
The combined effect of low carbon monoxide conversion per reactor and 
intermediate water removal is a relatively high hydrogen to carbon 
monoxide consumption ratio in each of the Fischer-Tropsch reactors, and 
also overall across all of the Fischer-Tropsch reactors, even when an 
iron-based catalyst is used. The hydrogen to carbon monoxide consumption 
ratio (H.sub.2 /CO consumption ratio) for a single reactor is the ratio of 
moles of hydrogen consumed in the reactor to the moles of carbon monoxide 
consumed in the reactor. This may be determined by comparing quantities of 
hydrogen and carbon monoxide fed to the reactor with quantities of 
hydrogen and carbon monoxide exiting the reactor, with the difference 
representing the quantity of each consumed. An overall H.sub.2 /CO 
consumption ratio across all of the Fischer-Tropsch reactors in series may 
be determined in a like manner by comparing quantities of hydrogen and 
carbon monoxide fed to the first reactor in series with the quantities of 
hydrogen and carbon monoxide exiting the last reactor in series. 
Frequently, each reactor is operated at approximately the same consumption 
ratio, in which case the overall H.sub.2 /CO consumption ratio will be 
approximately equal to the H.sub.2 /CO consumption ratios of the 
individual reactors. When iron-based catalysts are used with the present 
invention, the overall H.sub.2 /CO consumption ratio is controlled to be 
in a range having a lower limit of about 1.3, preferably about 1.4, more 
preferably about 1.5 and most preferably about 1.6 and having an upper 
limit of about 1.8 and preferably about 1.9. Most typical for the present 
invention is an overall H.sub.2 /CO consumption ratio in a range of from 
about 1.4 to about 1.8, with from about 1.5 to about 1.7 being even more 
typical. 
The process of the present invention may be used with Fischer-Tropsch 
Periodic Table Group VIII catalysts other than iron-based catalysts, such 
as cobalt-based catalysts, although iron-based catalysts are preferred 
because they tend to produce a smaller percentage of light hydrocarbons 
and because iron-based catalysts are considerably less expensive. With the 
present invention, the tendency of iron-based catalysts to have high 
water-gas shift activity is significantly reduced with the process of the 
present invention. It is noted that, as used herein, an iron-based 
Fischer-Tropsch catalyst is any Fischer-Tropsch catalyst including iron as 
the predominant catalytic metal. The iron-based catalyst may, however, 
include lesser quantities of other materials, as are known in the art, 
such as copper, magnesium and potassium. In particular, a small quantity 
of potassium is preferred to promote higher molecular weight hydrocarbon 
products, as is known in the art. Preferred iron-based catalysts include 
from about 0.2 percent to about 20 percent copper, from about 0.2 percent 
to about 25 percent potassium, and from about 0 percent to about 30 
percent magnesium, by weight relative to iron. Furthermore, the iron-based 
catalyst may be supported or unsupported. 
In one embodiment of the present invention, it is possible to further 
enhance performance by balancing the molar feed ratio of hydrogen to 
carbon monoxide in the synthesis gas (H.sub.2 /CO feed ratio) with the 
overall H.sub.2 /CO consumption ratio. The H.sub.2 /CO feed ratio is the 
molar ratio of hydrogen to carbon monoxide in the synthesis gas feed to 
the first Fischer-Tropsch reactor in series. The overall H.sub.2 /CO 
consumption ratio is the same as described previously. Obtaining a good 
balance of the H.sub.2 /CO feed ratio and the overall H.sub.2 /CO 
consumption ratio, according to the invention, generally involves 
maintaining a high overall H.sub.2 /CO consumption ratio, as discussed 
previously, and controlling synthesis gas generation, such as by adding an 
appropriate amount of CO.sub.2 and/or water during synthesis gas 
generation, to maintain a synthesis gas feed having a H.sub.2 /CO feed 
ratio approximately equal to the H.sub.2 /CO consumption ratio. 
Referring again to FIG. 1, the synthesis gas generator 106 may apply any 
suitable technique for generating the desired synthesis gas, for example, 
partial oxidation, steam reforming, autothermal reforming or any 
combinations thereof. In the present invention, partial oxidation is 
preferred. The gaseous hydrocarbon feed 102 predominantly comprises 
normally gaseous hydrocarbons. As used herein, "gaseous hydrocarbon feed" 
includes feeds including at least some, and preferably comprising 
predominantly, normally gaseous light hydrocarbons (i.e., C.sub.4 -). The 
gaseous hydrocarbon feed may include some heavier hydrocarbon components 
(i.e., C.sub.5 +) and may include non-hydrocarbon components (such as 
nitrogen, helium, water, carbon dioxide and other carbon-containing 
compounds) although these non-hydrocarbon components are typically present 
in only small quantities. Typically, the hydrocarbon components of gaseous 
hydrocarbon feed 102 are comprised of at least 95 mole % of C.sub.4 - 
hydrocarbons, and more typically at least 99 mole % of C.sub.4 - 
hydrocarbons. In most instances, methane will be the predominant 
hydrocarbon component of the gaseous hydrocarbon feed 102. This would be 
the case, for example, when the gaseous hydrocarbon feed 102 is natural 
gas, and especially if LPG components (i.e., C.sub.3 and C.sub.4 
components) are removed prior to synthesis gas generation. If the feed gas 
to the synthesis gas generator 106 comprises predominantly methane, then 
the H.sub.2 /CO ratio in the synthesis gas 108 produced by partial 
oxidation would typically be around 2. This is typically higher than a 
desired H.sub.2 /CO feed ratio for the present invention, because it tends 
to be higher than the H.sub.2 /CO consumption ratio of iron-based 
catalysts, even when operating at a high H.sub.2 /CO consumption ratio 
according to the present invention, as discussed previously. Therefore, 
control of the H.sub.2 /CO feed ratio generally involves reducing the 
H.sub.2 /CO ratio in the synthesis gas 108 below that which would normally 
result from partial oxidation of the gaseous hydrocarbon feed 102. As 
shown in FIG. 1, an optional addition of carbon dioxide 130 is shown to 
the synthesis gas generator 106. The effect of adding the carbon dioxide 
130 is to reduce the H.sub.2 /CO feed ratio in the synthesis gas 108 to 
better match the overall H.sub.2 /CO consumption ratio across the 
Fischer-Tropsch reactors 110 A, B and C. The quantity of the carbon 
dioxide 130 should preferably be in an amount to provide the desired 
H.sub.2 /CO feed ratio in the synthesis gas 108. 
With continued reference to FIG. 1, if the gaseous hydrocarbon feed 102 is 
from a natural gas that includes a naturally high carbon dioxide content, 
then part or all of the carbon dioxide 130 may be supplied as that 
naturally-occurring carbon dioxide in the gaseous hydrocarbon feed 102. 
Otherwise, the carbon dioxide 130 would have to be supplied separately, 
such as from flue gases or another source. FIG. 1 also shows optional 
addition of steam 132 to the synthesis gas generator 106, also for the 
purpose of adjusting the H.sub.2 /CO feed ratio in the synthesis gas 108. 
The effect of adding the steam 132 is to increase the H.sub.2 /CO ratio, 
such as may be desirable if the rate of addition of the carbon dioxide 130 
is more than required to attain the desired H.sub.2 /CO feed ratio. 
In any reaction of methane with oxygen to make synthesis gas, the reaction 
is highly exothermic. In conventional processes, typically less than 75% 
of the heat of combustion of the gaseous hydrocarbon feed goes to heat of 
combustion of the synthesis gas. The rest of the heat of combustion of the 
gaseous hydrocarbon feed becomes sensible heat in the synthesis gas. If an 
optimum amount of steam and/or CO.sub.2 is added to the reaction, the 
energy yield of synthesis gas from methane rises and can become as high as 
90%. If this addition is primarily steam, the synthesis gas will contain a 
high H.sub.2 /CO ratio. If, however, the additive is primarily carbon 
dioxide, the H.sub.2 /CO ratio in the synthesis gas can be suppressed, 
typically to as low as about 1.4. The fraction of energy lost to sensible 
heat in the synthesis gas becomes less as does the synthesis gas 
temperature. The higher the energy efficiency of synthesis gas generation, 
the lower the amount of gaseous hydrocarbon feed and oxygen required to 
make a given quantity of synthesis gas. 
A preferred source for carbon dioxide for use in synthesis gas generation 
is from recycle within the process, since carbon dioxide is produced as a 
Fischer-Tropsch reaction product. FIG. 2 shows one embodiment of the 
process of the present invention including carbon dioxide recycle. As 
shown in FIG. 2, a portion of noncondensible gases 120 C from the last 
Fischer-Tropsch reactor 110 C is recycled as a recycle stream 136 to the 
synthesis gas generator 106. As noted previously, the noncondensible gases 
120 C include a significant quantity of carbon dioxide, as well as some 
light hydrocarbons produced during the Fischer-Tropsch reaction and 
unreacted hydrogen and carbon monoxide. A small bleed stream 134 is, 
however, removed to prevent an undesirable buildup of inert materials, 
such as nitrogen, in the system. Typically the bleed stream 134 is nor 
more than about 10% as large as the recycle stream 136. FIG. 2 also shows 
the optional addition of the steam 132. 
Although FIG. 2 shows recycle of gases only from the last Fischer-Tropsch 
reactor 110 C, it should be recognized that recycle of a portion of one or 
more of the noncondensible gases 120 A and B from the first two 
Fischer-Tropsch reactors 110 A and B could be used instead of or in 
addition to recycle of the noncondensible gases 120 C. In a preferred 
embodiment, however, the recycle is substantially entirely from the last 
Fischer-Tropsch reactor 110 C, as is shown in FIG. 2. The noncondensible 
gases 120 A and B will necessarily include larger quantities of unreacted 
hydrogen and carbon monoxide than the noncondensible gases 120 C, and it 
is desirable to pass those unreacted components to the next reactor in 
series for further reaction to produce desired hydrocarbon products. 
With the present invention, control of the H.sub.2 /CO feed ratio in the 
synthesis gas through addition of carbon dioxide to the synthesis gas 
production step, combined with attainment of a high overall H.sub.2 /CO 
consumption ratio across the Fischer-Tropsch reactors as described above, 
permits the H.sub.2 /CO feed ratio and the overall H.sub.2 /CO consumption 
ratio to be approximately matched to each other. With this embodiment of 
the present invention, the overall H.sub.2 /CO consumption ratio can 
typically be controlled to be in a range of from about 0.2 smaller than to 
about 0.2 larger than the H.sub.2 /CO feed ratio, preferably in a range of 
from about 0.15 smaller than to about 0.15 larger than the H.sub.2 /CO 
feed ratio, more preferably in a range of from about 0.1 smaller than to 
about 0.1 larger than the H.sub.2 /CO feed ratio, and most preferably in a 
range of 0.05 smaller than to about 0.05 larger than the H.sub.2 /CO feed 
ratio. Typically, both the H.sub.2 /CO feed ratio and the overall H.sub.2 
/CO consumption ratio are larger than about 1.3, preferably larger than 
about 1.4 and more preferably larger than about 1.5. Typically both the 
feed ratio and the overall H.sub.2 /CO consumption ratio are smaller than 
about 1.9 and more typically smaller than about 1.8. A particularly 
preferred range for both the H.sub.2 /CO feed ratio and the overall 
H.sub.2 /CO consumption ratio is from about 1.4 to about 1.8, with a range 
of from about 1.5 to about 1.7 being even more preferred. 
One significant benefit of the carbon dioxide recycle, as shown in FIG. 2, 
is that efficiency of conversion of the gaseous hydrocarbon feed 102 into 
the desired higher molecular weight hydrocarbon products is significantly 
enhanced. By balancing the H.sub.2 /CO feed and consumption ratios, 
neither carbon monoxide nor hydrogen in the synthesis gas 108 are wasted 
in the large quantities that can result when an excess of one component is 
present. Furthermore, the synthesis gas 108 exiting the synthesis gas 
generator 106 will typically be an equilibrium gas composition including 
an equilibrium concentration of carbon dioxide. By recycling carbon 
dioxide, the equilibrium carbon dioxide concentration can be satisfied by 
the recycled carbon dioxide, rather than through consumption of fresh 
gaseous hydrocarbon feed 102. The result is that, with the high H.sub.2 
/CO consumption ratio of the present invention and the balancing of 
H.sub.2 /CO feed and consumption ratios according to the present 
invention, a larger proportion of carbon from the gaseous hydrocarbon feed 
is typically incorporated into desirable higher molecular weight 
hydrocarbon products. Typically, C.sub.5 + hydrocarbon products from the 
process of the invention will include at least about 85% of the carbon 
originally present in hydrocarbons of the gaseous hydrocarbon feed 102, 
and preferably at least about 90%. Furthermore, there is typically a high 
energy conversion efficiency in preparing hydrocarbon products from the 
gaseous hydrocarbon feed 102. Typically, with the process of the 
invention, the C.sub.5 + hydrocarbon products will have a total heating 
value of at least about 65%, and preferably at least about 70%, of the 
total heating value of hydrocarbons in the gaseous hydrocarbon feed. In 
this regard, total heating value is determined for the C.sub.5 + 
hydrocarbon products and the hydrocarbons of the gaseous hydrocarbon feed 
using the lower heats of combustion of the relevant components of each 
stream. The lower heat of combustion is a property of fuel materials 
determined by controlled combustion to form combustion products which 
include water, if at all, in the gaseous state. The lower heats of 
combustion for a number of materials are listed in Chemical Engineers' 
Handbook, 5th Ed. (McGraw-Hill 1973) at pages 3-145 through 3-147. To 
compare the total heating value of the C.sub.5 + hydrocarbon products to 
that of the hydrocarbons in the gaseous hydrocarbon feed, the total 
heating value of a quantity of the C.sub.5 + hydrocarbon products is 
compared to the total heating value of a corresponding quantity of gaseous 
hydrocarbon feed consumed to prepare the quantity of C.sub.5 + hydrocarbon 
products. 
As noted previously, the Fischer-Tropsch catalyst could be any 
Fischer-Tropsch catalyst effective for catalyzing the Fischer-Tropsch 
reaction. The preferred catalyst for use with the present invention, as 
discussed above, is an iron-based catalyst. Examples of suitable catalyst 
structures include precipitated catalysts, fused catalysts, sintered 
catalysts, cemented catalysts and supported catalysts. To further enhance 
performance of the process of the present invention, however, certain 
iron-based catalysts are preferred, and especially to promote a high 
H.sub.2 /CO consumption ratio in the Fischer-Tropsch reactors. 
The preferred iron-based catalysts have a structure that avoids the 
presence of significant deep pore spaces in the interior of the iron phase 
of the catalyst. This may be accomplished, for example, using either 
unsupported or supported iron-based catalysts. Unsupported iron catalysts 
will typically be made by precipitation as is known in the art. To avoid 
excessive deep pore volume in these unsupported catalysts, they should be 
limited to particles having a size of smaller than about 60 microns and 
preferably smaller than about 30 microns. Particle size may generally be 
determined by sieve analysis or, in the case of particles too small for 
sieve analysis, by other methods as are known for determining particle 
size. One problem with using such very fine catalyst particles is that 
they can be difficult to separate from hydrocarbon product due to the 
small particle size. Supported catalysts are typically made by 
precipitating a thin layer of catalytic iron material on small particles 
of a support material, as is known in the art. Typical support materials 
include inorganic oxides, and particularly ceramic materials, such as 
silica and alumina. Another support material is carbon. An advantage of 
using a supported catalyst is that larger catalyst particles may be used 
than for unsupported catalysts, which simplifies separation of catalyst 
particles from hydrocarbon product. When using a supported catalyst, the 
particles preferably have a size of larger than about 50 microns, and more 
preferably in a range of from about 50 microns to about 200 microns. 
Not to be bound by theory, but to aid in the understanding of the 
invention, iron-based catalysts that avoid the inclusion of a significant 
amount of deep pore volume in the catalyst particles are believed to 
suppress Reaction II. In pore volumes deep within the catalytic iron, 
hydrogen and carbon monoxide can diffuse in and react at the catalyst 
surface inside the pore, producing water as a by-product. It is difficult, 
however, for the water to diffuse out of these deep internal pore volumes, 
with a result being that a high concentration of water builds up in the 
pore volume. As water builds up in the pore volume, Reaction II is 
promoted. By contrast, water produced at the exterior surface of the 
catalyst, or in interior pore volumes that are shallow and near the 
exterior surface, can more easily diffuse out and away from the catalyst, 
so that Reaction II does represent a problem of the same magnitude as in 
the deep pore volumes where the water must diffuse out and will likely 
contact chemisorbed carbon monoxide on its way to the gas phase, causing 
Reaction II to occur. In the very fine unsupported catalysts noted above, 
significant deep pore volume is avoided due to the small particle size. In 
the supported catalysts, the catalytic iron is present in only a very thin 
layer on the support and an excessive amount of deep pore volume in the 
catalytic layer is thus avoided. 
Reactive conditions in the Fischer-Tropsch reactors may vary depending upon 
the specific operation, as is known generally in the art. When using 
iron-based catalysts, typical reactor temperatures are in a range of from 
about 210.degree. C. to about 280.degree. C., with lower temperatures 
generally promoting production of higher molecular weight hydrocarbon 
products. Typical operating pressures using iron-based catalysts in slurry 
reactors are from about 110 KPa to about 3500 KPa. With the present 
invention, operating pressures of from about 1000 KPa to about 4000 KPa 
are preferred. Furthermore, as is known in the art, catalyst compositions 
can be varied to promote different hydrocarbon products. For example, 
addition of a small amount of potassium to iron catalysts is known to 
promote production of higher molecular weight products. 
Although other reactors, such as fixed-bed reactors or fluid bed reactors, 
could be used with the present invention, slurry reactors are preferred. 
Slurry reactors are preferred primarily because it is relatively easy to 
control the temperature in the reactor and the reactors are not highly 
susceptible to the development of hot zones in the reactor. The relative 
ease of temperature control is important due to the highly exothermic 
nature of the Fischer-Tropsch reaction. Temperature control is typically 
accomplished through the use of cooling tubes extending into the internal 
reactor volume in which the catalyst slurry is disposed during hydrocarbon 
synthesis. 
One significant advantage of the present invention is the relative ease 
with which the reactors used to practice the process of the invention may 
typically be constructed and installed. Because the invention contemplates 
use of a plurality of staged reactors with relatively low conversion per 
reactor, it is often possible to keep the reactors small enough so that 
the reactors may be shop-fabricated and then transported to the field for 
installation. This is in contrast to the more conventional situation when 
a single reactor with a high conversion is used, often with the aid of 
large recycle volumes. Typically, such reactors are so large that 
transportation of the reactor is not feasible, necessitating expensive 
field fabrication. Also, with the present invention, the reactor volume 
required for each succeeding stage is typically less than the preceding 
stage, because throughput becomes smaller with each successive reactor 
stage. Preferably, each successive reactor stage has an internal reactor 
volume no larger than about 70% of the internal reactor volume of the 
immediately preceding reactor stage. This can result in a significant 
savings on reactor costs. The result is that even though the present 
invention uses multiple reactors instead of a single reactor, the multiple 
reactor system is typically cost competitive with single reactor designs. 
The process could be conducted using reactor vessels that have decreasing 
reactor volumes in successive Fischer-Tropsch reactor stages. Preferably, 
however, reactor vessels of a single size and design could be used in all 
stages to further reduce capital costs. For example, in a three stage 
process, the first Fischer-Tropsch reactor stage could include three of 
the reactor vessels arranged in parallel, the second Fischer-Tropsch 
reactor stage could include two of the reactor vessels arranged in 
parallel and the third Fischer-Tropsch reactor stage could include only a 
single reactor vessel. 
Furthermore, it should be recognized that the present invention has been 
described primarily with reference to the use of Fischer-Tropsch reactor 
stages that are each a physically separate unit, but the invention is not 
so limited. For example, the plurality of reactors could be provided as a 
single unit that is divided into separate reactor volumes, each acting as 
one of the plurality of reactor stages. 
One consideration with the use of slurry reactors is separation of catalyst 
particles from high molecular weight hydrocarbon product. FIG. 3 shows one 
embodiment of the process of the present invention including recovery of 
crude hydrocarbon products. As shown in FIG. 3, liquid effluents 114 A, B 
and C are withdrawn from each of the Fischer-Tropsch reactors 110 A, B and 
C. The liquid effluents 114 A, B and C each consists of a hot slurry of 
high molecular weight hydrocarbons and catalyst particles. Liquid 
effluents 114 A, B and C are each transferred to separation unit 140, 
where catalyst particles are separated to produce a crude wax product 142. 
The separation unit 140 could include one or a combination of a variety of 
separation methods, such as filtration, hydrocycloning, centrifuging, 
magnetic separation, or simple gravity settling. In a preferred 
embodiment, the separation unit 140 is a settling tank in which the 
catalyst particles are allowed to settle to the bottom and a crude wax 
product 142 is recovered. The separation unit 140 could include more than 
one settling tank, if desired. The crude wax product 142 contains a 
substantial quantity of high molecular weight hydrocarbons, generally from 
about C.sub.11 to about C.sub.100, or larger. Dense slurries 144 A, B and 
C of settled catalyst are withdrawn from the bottom of the settling tank 
140 and returned to the Fischer-Tropsch reactors 110 A, B and C. A portion 
of the dense slurries 144 A, B and C may be removed and new catalyst added 
as required for periodic replacement of old catalyst with fresh catalyst. 
The dense slurries 144 A, B and C each have a high catalyst concentration, 
typically comprising from about 20 weight percent to about 60 weight 
percent of catalyst particles. This is in contrast to the catalyst 
concentration present in the Fischer-Tropsch reactors 110 A, B and C, in 
which the catalyst is in a slurry typically comprising from about 4 weight 
percent to about 30 weight percent of catalyst particles. 
FIG. 3 also shows liquid separators 146 A, B and C which are used to 
separate water from light hydrocarbon liquids in the condensed liquids 118 
A, B and C. The condensed liquids 118 A, B and C are fed to the 
corresponding liquid separators 146 A, B and C, where water 148 A, B and C 
is removed and light liquid hydrocarbons 150 A, B and C are recovered. The 
light liquid hydrocarbons 150 A, B and C typically include hydrocarbons 
from about C.sub.4 to about C.sub.11. The crude wax product 142 and the 
light hydrocarbon liquids 150 A, B and C could be sold individually or 
could be combined and sold as a single product. Alternatively, it is 
possible to upgrade these crude products into more valuable products. For 
example, crude wax product 142 could be purified to prepare a purified wax 
product. 
In a preferred embodiment of the invention, it is contemplated that at 
least part of the crude hydrocarbon products will be converted to at least 
one distillate product, typically in at least one of the naphtha, jet fuel 
and diesel ranges. Referring now to FIG. 4, one embodiment for crude 
product upgrading is shown for preparing distillate products. In FIG. 4, a 
crude hydrocarbon feed 160 is fed to a cracking unit 162 to prepare a 
synthetic crude oil 164. The synthetic crude oil 164 itself could be a 
saleable product, but in the embodiment shown in FIG. 4, it is sent to a 
distillation unit 166. In the distillation unit, the synthetic crude oil 
164 is distilled to make three distillate products, a naphtha range 
product 168, a jet fuel range product 170 and a diesel range product 172. 
Tower bottoms 174 are recycled back to the cracking unit 162. The 
distillate products 168, 170 and 172 are highly paraffinic and make 
excellent blending stocks for blending with conventional petroleum 
distillate products. As an example, the jet fuel range product 170 could 
be blended with conventional jet fuel refined from petroleum. As another 
example, the diesel range product 172 could be blended with conventional 
diesel fuel refined from petroleum. Also, the naphtha range product could 
be blended with conventional gasoline range products refined from 
petroleum. Other distillate products could also be produced, such as in 
the lubricating oil range. 
The crude hydrocarbon feed 160, as shown in FIG. 4, includes at least a 
portion of the crude wax product 142 (as shown in FIG. 3) and may also 
include some or all of the light hydrocarbon liquids 150 A, B and C (as 
shown in FIG. 3). In the cracking unit 162, higher molecular weight 
hydrocarbons are cracked back to lighter hydrocarbons to form the 
synthetic crude oil 164. The cracking unit 162 may include any suitable 
cracking operation. A preferred cracking method is hydrocracking, in which 
case hydrogen 174 is added to the cracking until 162 so that the synthetic 
crude oil 164 will be a highly saturated. The hydrogen for the 
hydrocracking process may be obtained by separating out a small amount of 
hydrogen from the synthesis gas 108 (as shown in FIGS. 1 through 3) prior 
to introduction of the synthesis gas 108 into the Fischer-Tropsch 
reactors. This could be done by simple membrane separation. When removing 
hydrogen from the synthesis gas, it is important that the synthesis gas 
108 be generated to be somewhat more rich in hydrogen than desired for 
feed to the Fischer-Tropsch reactors, so that after the hydrogen removal, 
the synthesis gas 108 will have the desired H.sub.2 /CO feed ratio to 
balance with the H.sub.2 /CO consumption ratio, as discussed above. An 
even more preferred embodiment would involve a cracking unit that consists 
of a combined hydrocracker/isomerization unit, so that a larger degree of 
hydrocarbon branching will be present in the synthetic crude oil 164. 
While various embodiments of the present invention have been described in 
detail, it is apparent that further combinations, modifications and 
adaptations of the invention will occur to those skilled in the art. For 
example, any of the features disclosed in or discussed in connection with 
any of FIGS. 1-3 may be combined in any compatible combination with any 
other features disclosed in or discussed in connection with any of FIGS. 
1-3. Furthermore, the product upgrading of FIG. 4 may be combined with any 
of the staged hydrocarbon synthesis flow diagrams shown in any one of 
FIGS. 1-3. It is to be expressly understood that such combinations, 
modifications and adaptations are within the spirit and scope of the 
present invention. It is further intended that the claims appended hereto 
be interpreted to extend to the maximum extent permitted by the prior art.