Note: Descriptions are shown in the official language in which they were submitted.
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10
PRODUCTION OF HYDROGEN-CONTAINING GAS STREAMS
This application claims the priority of United States Provisional
Application 60/107,127, filed November 5, 199$.
This invention relates to the production of a hydrogen-containing gas by
catalytic reforming of a hydrocarbon feedstock.
In the catalytic reforming of a hydrocarbon feedstock to produce a
hydrogen-containing gas, in general, such reforming is accomplished by the use
of
a reforming gas such as steam and/or carbon dioxide; in particular, steam, in
the
presence of a suitable rei;orming catalyst. In general, such a feedstock
comprises
methane and the hydrogen-containing gas contains hydrogen and carbon
monoxide, with such gas often being referred to in the art as a "synthesis
gas."
The steam-reforming reaction is endothermic and is operated at high
temperature in order that the equilibrium favors production of hydrogen. The
heat
required for the endothermic reforming reaction is supplied by preheating the
feed
and by heating during thc: reforming process; in particular, the reforming
reaction
is accomplished in a reactor often referred to in the art as a steam reformer.
Generally the steam reforming reactor is a tubular reactor where the tubes are
heated in a fired furnace. However, other reactor configurations are also
possible,
e.g. non-fired tubular reactors or pre heated adiabatic packed beds.
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The present invention is directed to improving the steam-reforming
process for conversion of a hydrocarbon feedstock to a synthesis gas.
In accordance with an aspect of the present invention, a synthesis gas (a
gas that contains hydrogen, and also generally contains carbon monoxide), is
produced by reacting steam and a hydrocarbon, with the hydrocarbon preferably
being methane, in the presence of a steam-reforming catalyst, wherein the
steam-
refarming catalyst is supported on a mesh or mesh-like material, or the
catalyst is
in the form of a mesh i.e;., the mesh is formed from a catalytic material.
'Che term "supported on the mesh" includes coating the catalyst on the
10 mesh as well as entrapping the catalyst in the interstices of the mesh. The
catalyst
that is supported on the mesh, in a preferred embodiment, is comprised of a
steam-
reforming catalyst supported on a particulate support with the supported steam-
reforming catalyst being supported on the mesh.
More particularly, the mesh like material is comprised of fibers or wires,
15 such as a wire or fiber mesh, a metal felt or gauze, metal fiber filter or
the like.
The mesh like structure. may be comprised of a single layer, e.g. a knitted
wire
structure or a woven wire structure, or may include more than one layer of
wires;
and is preferably comprised of a plurality of layers of wires or fibers to
form a
three dimensional network of materials. In a preferred embodiment, the support
20 structure is comprised of a plurality of layers of fibers that are randomly
oriented
in the layers. One or more metals may be used in producing a metal mesh.
Alternatively the mesh fibers may be formed from or include materials other
than
metals alone or in camibination with metals; e.g. carbon or metal oxides or a
ceramic. In a preferred embodiment, the mesh includes a metal. In the case
25 where the mesh supports the catalyst the material forming the mesh is
preferably
non-catalytic with respect to steam reforming. As hereinabove indicated, in
one
embodiment, the material forming the mesh is a steam-reforming catalyst.
In a preferred embodiment wherein the mesh like structure is comprised of
a plurality of layers of fibers to form the three dimensional network of
materials
30 the thickness of such support is at least five microns, and generally does
not
exceed ten millimeters. In accordance with a preferred embodiment, the
thickness
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of the network is at least 50 microns and more preferably at least I00 microns
and
generally does not exceed 2 millimeters.
In general, the thickness or diameter of the fibers which form the plurality
of layers of fibers is less than about 500 microns, preferably less than about
150
S microns and more prE;ferably less than abaut 30 microns. In a preferred
embodiment the thickness or diameter of the fibers is from about 8 to about 25
microns.
The three dimensional mesh like structure may be produced as described
in U.S. Patent Number'.>,304,330; 5,080,962; 5,102,745 or 5, 096,663. It is to
be
understood, however, that such mesh like structure may be formed by procedures
other than as described in the aforementioned patents.
In the preferred embodiment where the mesh-like structure supports a
steam-reforming catalyst on a particulate support, the particulate catalyst
support
is a porous support and in a preferred embodiment, has a surface area that is
greater than 1 mz /g , and preferably a surface area greater than Sm2/g. In
most
cases, the surface area does not exceed 100m2/g. The surface area is measured
by
the Brunauer Emmett and Teller (BET) method. The support is a porous support
that is heat resistant, andf as representative examples of such supports there
may be
mentioned alumina, silicon carbide, silica, zirconia, titanic, calcium
aluminate,
calcium aluminum titan2~te, a siiica/alumina support etc.
The catalyst support on which the steam-reforming catalyst is supported is
a support that is in particulate form (with such supported catalyst being
supported
on the mesh-like structure). The term particulate as used herein includes and
encompasses spherical particles, elongated particles, fibers, etc. In general,
the
particulate support has an average particle size of at least 0.5 micron and no
greater then 20 microns although larger particles may be employed. In some
cases, the particle size may be as low as 0.002 micron. In the case where the
support particles are entrapped, the particulates are no greater than 3010 pm.
preferably no greater than 200 pm and mast preferably no greater than 100 pm.
When coating the catalyst on the mesh, the particulate support in the majority
of
cases does not exceed 10 microns.
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The steam catalyst reforming may be of a type known in the art. In
general, such catalyst includes nickel, ruthenium or rhodium, with or without
promoters such as alkali metals.
In accordance with an aspect of the present invention, the steam-reforming
catalyst (with or without a support) is supported on the mesh like structure
in an
amount of at least 5%, and preferably at least l0%, with the amount of
catalyst
generally not exceeding 60% and more generally not exceeding 50%, all by
weight, based on mesh, catalyst, and, if present, particulate support.
In an embodiment of the invention, the mesh like structure that is formed
from a steam reforming catalyst and/or functions as a support for the steam-
reforming catalyst (the mesh-like structure preferably supports a steam-
reforming
catalyst supported on a particulate support) is in the form of a shaped
structured
packing to provide far turbulence of the gas phase flowing over the catalyst
in the
steam-reforming tubes. The mesh structure may be provided with suitable
15 corrugations in order to provide for increased turbulence. Alternatively,
the mesh
like structure may include tabs or vortex generators to provide for
turbulence. The
presence of turbulence generators permits mixing in the radial (and
longitudinal)
direction and permits improved heat transfer at the wall compared to the
processes
know in the art. This can be effected by adding turbulence generators to the
20 structure that contacts the wall. For example, the structural packing can
be in the
form of a module such <~s a roll of one or more sheets that is placed into the
tubes
of the reactor such that the channels in the module follow the longitudinal
direction of the tube. 'The roll can consist of sheets that are flat,
corrugated or
wavy or a combination 'whereof and the sheets can contain fins or holes to
promote
25 mixing. The sheets can also be shaped into corrugated strips that are
separated
from each other by a flat sheet that exactly fit the size of the tube and are
held
together by welds, wire:., a cylindrical flat sheet or combinations thereof.
It is to be understood that the mesh that is formed from a steam reforming
catalyst or that supports the steam-reforming catalyst (which steam-reforming
30 catalyst may or may not be supported on a particulate support) may be
employed
in a form other than as a structured sheet. For example, the mesh may be
formed
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WO OOI26137 PCTNS99/26148
as rings, particles, ribbons, etc. and employed in the tubes as a packed bed.
In one
embodiment the particle: dimecisions are smaller than those of packed bed
particles
that are known in the prior art. Thus, the supported catalyst on the mesh
(whether
or not used as a structured packing is preferably employed as a packed bed.
The steam-reforming catalyst which is supported on the mesh like
structure may be present on the mesh like support as a coating on the wires or
fibers that form the mesh like structure andlor may be present and retained in
the
interstices of the mesh liike structure.
In one embodiment, wherein the steam-reforming catalyst supported on a
10 particulate support is present as a coating on the mesh, the mesh may be
initially
coated with the particulate support, followed by addition of the steam-
reforming
catalyst to the particulate support present as a coating on the mesh.
Alternatively,
the catalyst supported on a particulate support may be coated onto the mesh.
The
particulate support with or without catalyst may be coated on the mesh by a
variety of techniques, e.;g., dipping or spraying.
The supported catalyst particles may be applied to the mesh-like structure
by contacting the mesh-like structure with a liquid coating composition
(preferably in the form of a coating bath) that includes the particles
dispersed in a
liquid under conditions such that the coating composition enters or wicks into
the
mesh-like structure and forms a porous coating on both the interior and
exterior
portions of the mesh-like structure.
Alternatively, the mesh-like; structure is coated with a particulate support
containing active catalyst or the mesh-like structure may be coated with
particles
of a catalyst precursor.
In a preferred embodiment, the liquid coating composition has a kinematic
viscosity of no greater than 175 centistokes and a surface tension of no
greater than
300 dynes/cm.
In one embodiment, the supported catalyst or catalyst support is coated onto
the mesh by dip-coating. In a preferred embodiment, the three-dimensional mesh-
like material is oxidized before coating; e.g., heating in air at a
temperature of from
300°C up to 700°C. In some cases, if the mesh-like material is
contaminated with
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organic material, the meah-like material is cleaned prior to oxidation; for
example,
by washing with an organic solvent such as acetone.
The coating bath is preferably a mixed solvent system of organic solvents
and water in which the particles are dispersed. The polarity of the solvent
system is
preferably lower than tl~~t of water in order to prevent high solubility of
the catalyst
and to obtain a good quaility slurry for coating. The solvent system may be a
mixture
of water, amides, esters, and alcohols. The kinematic viscosity of the coating
bath is
preferably less than 175 centistokes and the surface tension thereof is
preferably less
than 300 dynes/cm.
In a preferred ennbodiment of the invention, the mesh-like structure that is
coated includes metal wires or fibers and the metal wires or fibers that are
coated are
selected or treated in a manner such that the surface tension thereof is
higher than 50
dynes/cm, as determinecl by the method described in "Advances in Chemistry,
43,
Contact Angle. Wettabiiity and Adhesion. American Chemical Society 1964."
In coating a mesh-like structure that includes metal fibers, the liquid
coating
composition preferably has a surface tension from about SO to 300 dynes/cm,
and
more preferably from about 50 to 150 dynes/cm, as measured by the capillary
tube
method, as described in 'T.C. Patton, "Paint Flow and Pigment Dispersion", 2"a
Ed.,
Wiley-Interscience, 1975, p. 223. At the same time, the liquid coating
composition
has a kinematic viscosity of no greater than 175 centistokes, as measured by a
capillary viscometer anc9 described in P.C. Hiemenz, "Principles of colloid
and
Surface Chemistry", 2"d I?d., Marcel Dekker Inc., 1986, p. 182.
In such an embodiment, the surface tension of the metal being coated is
coordinated with the viscosity and surface tension of the liquid coating
composition
such that the Liquid coating composition is drawn into the interior of the
structure to
produce a particulate coating on the mesh-like structure. The metal to be
coated
preferably has a surface tension which is greater than 50 dynes/cm and
preferably is
higher than the surface tension of the liquid coating composition to obtain
spontaneous wetting and penetration of the liquid into the interior of the
mesh.
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In the case where the metal of the structure that is to be coated does not
have
the desired surface tension, the structure may be heat-treated to produce the
desired
surface tension.
'The liquid coating composition can be prepared without any binders or
adhesives for causing adherence of the particulate coating to the structure.
The surface of the structure to be coated may also be chemically or
physically modified to increase the attraction between the surface and the
particles
that form the coating; ~;., heat treatment or chemical modification of the
surface.
The solids content of the coating bath generally is from about 2 % to about
SO%, preferably from about 5% to about 30%.
'The bath may also contain additives such as surfactants, dispersants etc. In
general, the weight ratio of additives to particles in the coating bath is
from 0.0001 to
0.4 and more preferably from 0.001 to 0.1.
'Che mesh-like material preferably is coated by dipping the mesh-like
15 material into a coating bath one or more times while drying or calcining in
between
dippings. The temperature of the bath is preferably at room temperature, but
has to
be sufficiently below the boiling point of the liquid in the bath.
After coating, the mesh-like material that includes a porous coating
comprised of a plurality of particles is dried, preferably with the material
in a vertical
20 position. The drying is preferably accomplished by contact with a flowing
gas (such
as air) at a temperature of from 20°C to 150°C more preferably
from 100°C to 150°C.
After drying, the coated mesh-like material is preferably calcined, for
example, at a
temperature of from 250'°C to 800°C, preferably 300°C to
500°C, most preferably at
about 400°C. In a preferred embodiment, the temperature and air flow
are
25 coordinated in order to produce a drying rate that does not affect
adversely the
catalyst coating, ~, cranking, blocking of pores, ete. In many cases, a slower
rate
of drying is preferred.
The thickness of the formed coating may vary. In general, the thickness is at
least 1 micron and in general no greater than 100 microns. Typically, the
coating
30 thickness does not exceed 50 microns and more typically does not exceed 30
microns.
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The interior portion of the mesh material that is coated has a porosity which
is sufFcient to allow the particles which comprise the coating to penetrate or
migrate
into the three dimensional network. Thus, the pore size of the three
dimensional
material and the particle size of the particles comprising the coating, in
effect,
5 determine the amount and uniformity of the coating that can be deposited in
the
interior of the network of material and/or the coating thickness in the
network. The
larger the pore sizes the greater the thickness of the coating which can be
uniformly
coated in accordance with the invention.
In the case where the particles are in the form of a catalyst precursor, the
product, after the deposit of the particles, is treated to convert the
catalyst precursor
to an active catalyst. In the case where the particles which are deposited in
the three
dimensional network of material is a catalyst support, active catalyst or
catalyst
precursor may then be applied to such support, ~, by spraying, dipping, or
impregnation.
In using a coating bath, the coating bath in some cases may include additives.
These additives change the physical characteristics of the coating bath, in
particular
the viscosity and surface tension such that during dipping penetration of the
mesh
takes place and a coating can be obtained with a homogeneous distribution on
the
interior and exterior of the mesh. Sols not only change the physical
properties of the
20 coating bath, but also act as binders. After the deposition, the article is
dried and
calcined.
As representative stabilizing agents there may be mentioned: a polymer like
polyacrylic acid, acryia~mines, organic quaternary ammonium compounds, or
other
special mixes which are selected based on the particles. Alternatively an
organic
25 solvent can be used for t:he same purpose. Examples of such solvents are
alcohols or
liquid paraf~ins. Control of the pH of the slurry, for example, by addition of
HN03
is another method of changing the viscosity and surface tension of the coating
slurry.
In a preferred embodiment wherein the mesh is comprised of a plurality of
layers of metal fibers, the particulate support with or without catalyst may
be
30 coated onto the mesh by an electrophoretic coating procedure, as described
in U.S.
Application Serial Number 09/156,023, filed on September 17, 1998. In such a
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procedure, the wire mesh is employed as one of the electrodes, and the
particulate
support, such as an alumina support of the requisite particle size, with or
without
catalyst, (which preferably also includes alumina in the form of a sol to
promote
the adherence of larger particles to the wire mesh) is suspended in a coating
bath.
5 A potential is applied across the electrodes, one of which is the mesh
formed from
a plurality of layers of fibers, and the mesh is electrophoretically coated
with the
alumina support with o~r without catalyst. If the alumina support does not
include
a catalyst, the steam-reforming catalyst , which is preferably comprised of
nickel
particles with or without one or more promoters, is then added to the catalyst
10 structure by dipping tlhe structure {which contains the alumina coating)
into or
impregnating the structure with an appropriate solution that contains the
nickel
catalyst and preferablly tine or more promoters. The Example illustrates
preparation of a cataly:>t by electrophoretic coating.
As hereinabove; indicated, the steam-reforming catalyst (with or without a
15 particulate support) rr~ay be supported on the mesh material by entrapping
or
retaining the catalyst in the interstices of the mesh. For example, in
producing a
mesh comprised of a plurality of layers of randomly oriented fibers, a
particulate
support may be included in the mix that is used for producing the mesh whereby
the mesh is produced with the particulate support retained in the interstices
of the
20 mesh. For example, such mesh may be produced as described in the
aforementioned patents, and with an appropriate alumina support being added to
the mesh that contains the fibers and a binder, such as cellulose. The thus
produced mesh includes the alumina particles retained in the mesh. The
particulate support r<:tained in the mesh is then impregnated with nickel by
25 procedures known in t:he art.
The term "bedl void volume" as used herein means the open space in the
portion of the reaction zone (for example the tubes of a steam reforming
catalyst)
that is not occupied bay the mesh wherein the openings or pores in the mesh
and
the openings or pores in any catalyst or particulate support on the mesh is
30 considered as being occupied by the mesh. Thus, in determining "bed void
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volume" the mesh is considered to be a closed sheet and any catalyst and
particulate support on tlae mesh is considered to be free of pores.
The term "mesh catalyst void volume" means the total of open space in the
mesh and the open space in any particulate support and catalyst on the mesh.
S The "bed void volume percent" is the ratio of the bed void volume to the
total volume of the portion of the reaction zone in which the mesh is placed
multiplied by 100.
The "mesh catalyst void volume percent" is the ratio of open volume in the
mesh, particulate support and catalyst divided by the total volume of mesh,
10 particulate support and catalyst (including pores or openings) multiplied
by 100.
The "mesh void volume percent" is the ratio of void volume in the mesh
without catalyst or particulate support to the total volume of the mesh
structure
(openings and mesh material) multiplied by 100.
The mesh like structure that is employed in the present invention (without
15 catalyst and/or particulate support on the mesh) has a mesh void volume
percent
that is at least 45%, arid is preferably at least 55% and is more preferably
at least
65% and still more prc;ferably is at least about 85% or 90%. In general, the
mesh
void volume percent does not exceed about 98%. In general, the average void
opening is at least 10 rnicrons and preferably at least 20 microns.
20 The steam-reforming reaction is generally effected in a tubular furnace,
often referred to as a steam-reformer or a steam-reforming furnace, which
furnace
includes a plurality of tubes that are appropriately heated in the furnace. In
accordance with a preferred aspect of the present invention, the mesh catalyst
(in
the form of a wire mesh that is the steam-reforming catalyst or in the form of
a
25 mesh that supports tl~e steam-reforming catalyst with or without a
particulate
support) is employed in the reaction zone (for example, the tubes in the
furnace)
in an amount such that the bed void volume percent is at least 70%. In most
cases the bed void volume percent is no greater than 97%. For example, in the
case where the steam-reforming catalyst (with or without a particulate
support)
30 supported on the mesh is in the form of a packed bed, the bed void volume
percent is generally between 70% and 97%. In the case where the steam
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reforming catalyst, with or without a particulate support, is supported on a
mesh
in the form of a structured packing, as opposed to a packed bed, the bed is
generally from 70% to 97% void volume percent and. In general the mesh
catalyst void volume percent is at least 50% preferably at least 60% and
generally
does not exceed 90%. In the case where the steam-reforming catalyst is
supported
on a particulate support., the catalyst is generally present on the
particulate support
in an amount from 3 to 20%, by weight, based on catalyst and particulate
support.
The steam reforming is generally accomplished at outlet temperatures of at
least 700°C, with the outlet temperature in most cases not exceeding
900°C. The
inlet temperature to the steam-reforming is generally in the order of at least
500°C, and generally does not exceed 600°C. The outlet pressure
of the tubes that
contain the steam-refonning catalyst is generally in the order of from about
15 to
60 bar with the pressure drop through the tubes generally not exceeding 0.42
bar/meter of tube length, and preferably not exceeding 0.31 bar/m. The steam-
15 reforming feed is generally comprised of a hydrocarbon (preferably methane)
and
steam, with the steam-~to-hydrocarbon ratios generally being at least 1.5, and
generally not exceeding 6:1.
'fhe steam-reforming reaction may be accomplished in any one of a wide
variety of steam-reforming furnaces, and may be combined with other processes;
for example, a shift reaction to increase the content of hydrogen and to
reduce the
carbon monoxide content.
In a separate embodiment, steam reforming is effected in multiple stages
where the heat is generated in one of the stages is used to provide or
supplement
the heat required for other stages. This can be done either in tubular
reactors
heated by hot gases or by using hot gases to preheat the feed to an adiabatic
reactor.
The present invention will be further described with respect to the
following examples; however, it is to be understood that the scope of the
invention
is not to be limited thereby:
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EXAMPLE 1
Small-scale preparation of a Ni containing steam reforming catalyst on a
wire mesh.
5 The support of S x 5 cm is a metal mesh with a thickness of 0.8mm.
stainless steel fibers of 12 ~m in diameter and a mesh void volume percent of
90%. the metal mesh is composed of a plurality of layers of metal fibers. The
mesh is placed vertically in a bath containing a slurry. The aqueous slurry
contains 10 wt% alumina catalyst support with a surface area of about 10 m2/g,
0.11 wt% Nyacol'1'~" 20'% alumina sol and 0.055 wt% of a commercial quaternary
ammonium chloride agent. The pH of the slurry was adjusted with diluted HN03
to 5.5. The mesh is connected to the negative pole of a power supply and is
placed between and parallel to two vertical metal electrodes, which are
connected
to the positive pole of a power supply. A potential of S V is applied for 2
minutes,
I S during which the alumina is deposited into the mesh. The sample is
caicined in air
at 500°C for 60 minutes. The amount of catalyst support that is
deposited on the
mesh is 25.1% by weight of the combined mesh support and catalyst. The coated
wire mesh is impregnated to the incipient wetness point with 1.10 g of a 20
wt%
Ni(N03) Z 6H20 containing aqueous solution. The impregnated sample is heated
in
20 air at 525°C for 60 minutes to convert the Ni(N03) Z to NiO. After
calcination the
alumina on the support contains about I S wt% of NiO.
EXAMPLE 2
Twenty weight percent of Nickel oxide on calcium aluminate in distilled
25 water slurry was further milled in an Eiger Mill to obtain a slurry with a
mean
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particle size of < 3 microns. One tenth of one percent of Stockhausen, a
dispersent , and 0.1 wt percent of 20 % alumina sol (NyacolT"") in water were
added and mixed well with a magnetic stirrer. This slurry was further diluted
with
distilled water to 10 wt percent for dip coating.
Three monolith structure packings (1" dia. x 1" length) made with Inconel
600 fiber mesh material from US Filter Inc. were washed with acetone and heat-
treated at 350° C for one hour. Each structure was dipped into the
prepared slurry,
followed by removing excess slurry by an air gun, air-drying for 15 min. and
oven-drying at 125° C for one hour. This operation was repeated four
more times.
The average weight gains after each dipping were 6.1, 11.4, 16.1, 21.1, and
24.7
wt %, respectively. These coated monolith were finally subjected to calcining
at
500° C for one hour prior to testing for syngas reaction. The average
loading of
catalyst was 21.6 wt %.
EXAMPLE 3
The same procedures for slurry and monolith preparation as Example 1
and 2 were used. Dip-coating procedures were also the same except that two
additional successive coatings were carned out for three monolith structures
to
achieved higher catalysvt loading. The weight gains after each dipping were
6.5,
12.2, 17.4, 22.3, 25.7, 2'9.1, and 31.8 wt %, respectively. The catalyst
loading was
determined to be 28.2 w~t % after calcining at 500° C for one hour.
The present invention is particularly advantageous in that mass transfer
limitations in the catalyst are reduced by applying a thin coating of catalyst
on the
surface of a highly porous fibrous metal mesh. Consequently a volumetric
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activity can be obtained that is higher than or similar to processes of the
prior art
with a reduced amount of steam reforming catalyst. As a result of the high
void
volume of the catalyst bed and of the mesh catalyst it is possible to effect
the
steam reforming at a lower pressure drop than in processes of the prior art.
In addition, the present invention permits the reaction to be effected with
an improved heat transfer at the wall, e.g. by employing the mesh as a
structured
packing, preferably with turbulence generators that contact the wall, or as a
packed bed with smaller dimensions compared to those applied in processes
known in the art. At a particular heat flux the improved heat transfer per
pressure
drop and improved mass. transfer allow the steam reforming process to be
operated
at a lower wall temperature which extends tube life. The lower temperature
reduces coke formation on the catalyst and consequently allows the use of
lower
steam/carbon ratios (typically lower than 3) than in processes of the prior
art.
This results in a reduction of the separation costs downstream. The higher
volumetric activity and improved heat transfer also allow a higher heat flux
through the wall, which is particularly useful if a high methane conversion is
required.
Numerous modifications and variations of the present invention are
possible in light of the. above techniques; therefore, within the scope of the
appended claims, the invention may be practiced otherwise than as particularly
described.
14
