U.S. patent application number 10/778891 was filed with the patent office on 2004-12-30 for catalyst support and steam reforming catalyst.
Invention is credited to Alba, Enrique Aceves de, Chin, Ya-Huei, Daly, Francis P., Fitzgerald, Sean P., Li, XiaoHong, Perry, Steven T., Rozmiarek, Robert, Silva, Laura J., Taha, Rachid, Tonkovich, Anna Lee, Wang, Yong, Watson, Junko M..
Application Number | 20040266615 10/778891 |
Document ID | / |
Family ID | 36845320 |
Filed Date | 2004-12-30 |
United States Patent
Application |
20040266615 |
Kind Code |
A1 |
Watson, Junko M. ; et
al. |
December 30, 2004 |
Catalyst support and steam reforming catalyst
Abstract
The present invention provides a Mg-stabilized alumina catalyst
support and a catalyst comprising Rh on a Mg-stabilized alumina
support. The catalyst is characterized by surprisingly superior
results in catalyzing methane steam reforming.
Inventors: |
Watson, Junko M.; (Columbus,
OH) ; Daly, Francis P.; (Delaware, OH) ; Wang,
Yong; (Richland, WA) ; Tonkovich, Anna Lee;
(Marysville, OH) ; Fitzgerald, Sean P.; (Columbus,
OH) ; Perry, Steven T.; (Galloway, OH) ;
Silva, Laura J.; (Dublin, OH) ; Taha, Rachid;
(Dublin, OH) ; Alba, Enrique Aceves de; (Columbus,
OH) ; Chin, Ya-Huei; (Richland, WA) ;
Rozmiarek, Robert; (Richland, WA) ; Li, XiaoHong;
(Richland, WA) |
Correspondence
Address: |
FRANK ROSENBERG
18 ECHO HILL LANE
MORAGA
CA
94556
US
|
Family ID: |
36845320 |
Appl. No.: |
10/778891 |
Filed: |
February 13, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60482351 |
Jun 25, 2003 |
|
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Current U.S.
Class: |
502/325 |
Current CPC
Class: |
B01J 2219/00835
20130101; C01B 2203/1041 20130101; H01M 8/0631 20130101; B01J
2219/0086 20130101; H01M 4/9075 20130101; Y02E 60/50 20130101; C01B
2203/1288 20130101; C01B 2203/1082 20130101; Y02P 20/52 20151101;
B01J 23/58 20130101; B01J 2219/00862 20130101; C01B 2203/0833
20130101; C01B 2203/04 20130101; C01B 2203/1011 20130101; C01B
2203/1052 20130101; B01J 2219/00873 20130101; B01J 23/464 20130101;
B01J 35/1014 20130101; B01J 2219/00783 20130101; B01J 2219/00822
20130101; H01M 8/0618 20130101; C01B 3/40 20130101; B01J 21/005
20130101; C01B 2203/065 20130101; C01B 2203/1064 20130101; B01J
2219/00824 20130101; C01B 2203/1241 20130101; C01B 2203/1619
20130101; B01J 19/0093 20130101; C01B 2203/00 20130101; C01B
2203/0233 20130101; C01B 2203/0283 20130101; C01B 2203/066
20130101 |
Class at
Publication: |
502/325 |
International
Class: |
B01J 023/46 |
Claims
We claim:
1. A composition comprising: Mg and Al containing spinel material;
and having a hydrothermal stability such that, when tested by
treating the composition at 925.degree. C. and 15 atm H.sub.2O, for
100 hours, the composition has a surface area of at least 10
m.sup.2/g.
2. The composition of claim 1 having a hydrothermal stability such
that, when tested by treating the composition at 925.degree. C. and
15 atm H.sub.2O, for 100 hours, the composition has a surface area
of at least 15 m.sup.2/g.
3. The composition of claim 1 having a hydrothermal stability such
that, when tested by treating the composition at 925.degree. C. and
15 atm H.sub.2O, for 100 hours, the composition has a surface area
of 10 to about 20 m.sup.2/g.
4. The composition of claim 1 further comprising Rh.
5. The composition of claim 1 consisting essentially of: Mg and Al
containing spinel material and optionally a catalytically active
material; and having a hydrothermal stability such that, when
tested by treating the composition at 925.degree. C. and 15 atm
H.sub.2O, for 100 hours, the support has a surface area of at least
10 m.sup.2/g.
6. A steam reforming catalyst, comprising: a Mg and Al containing
spinel support; and Rh on the surface of the support; and the
catalyst being characterizable by a stability and reactivity such
that, when tested in a flowing stream of methane and water in a
water:methane ratio of 3 at 15 atm, a catalyst temperature of
880.degree. C., and a contact time of 5 ms or less that is adjusted
to obtain a methane conversion (after 100 hours TOS) of 70%, and
maintaining the same conditions for 1000 hours, there is a
continuous period of at least 400 hours in which the methane
conversion changes by 3% or less.
7. The steam reforming catalyst of claim 6 being characterizable by
a stability and reactivity such that, when tested in a flowing
stream of methane and water in a water:methane ratio of 3 at 15
atm, a catalyst temperature of 880.degree. C., and a contact time
of 5 ms or less that is adjusted to obtain a methane conversion
(after 100 hours TOS) of 70%, and maintaining the same conditions
for 1000 hours, there is a continuous period of at least 600 hours
in which the methane conversion changes by 3% or less.
8. The steam reforming catalyst of claim 7 wherein the catalyst
consists essentially of Rh and a Mg and Al containing spinel.
9. A microchannel reactor comprising a reaction microchannel,
wherein the reaction microchannel comprises the steam reforming
catalyst of claim 6.
10. The microchannel reactor of claim 9 further comprising a
microchannel heat exchanger adjacent the reaction microchannel.
Description
RELATED APPLICATIONS
[0001] In accordance with 35 U.S.C. sect. 119(e), this application
claims priority to U.S. Provisional Application No. 60/482,351,
filed Jun. 25, 2003.
FIELD OF THE INVENTION
[0002] The invention relates to catalysts, systems and methods of
steam reforming; and methods of making steam reforming
catalysts.
BACKGROUND OF THE INVENTION
[0003] Steam reforming is a chemical process in which a hydrocarbon
is reacted with steam (H.sub.2O) to form hydrogen (H.sub.2), CO and
CO.sub.2. For decades, steam reforming has been the principal
industrial process for making hydrogen. More recently, steam
reforming has attracted great interest as a possible means to
supply hydrogen for fuel cells. There have been intensive research
efforts over many years to improve the steam reforming process.
Despite these efforts, problems continue to exist with catalyst
performance and/or cost, and the need for catalyst replacement or
regeneration due to the rather harsh conditions (high temperature
and steam) in which steam reforming is typically conducted.
[0004] Known steam reforming catalyst support materials include
gamma alumina doped with a stabilizing element such as magnesium,
lanthanum, and barium. Incorporation of a stabilizer could delay
the irreversible transformation of gamma alumina into alpha
alumina. However, under severe steam reforming conditions the
supports continually sinter, resulting in a permanent loss of
active sites.
[0005] The effect of rare earth elements and alkaline earth
elements as dopants for alumina has been investigated. See Church
et al., Appl., Catal., A, 101 (1993) 105. Wachowski, et al., Mater.
Chem. and Phys., 37 (1994) 29. Some studies have indicated that
there is a correlation between final surface area of alumina
treated at high temperatures (1200.degree. C.) and ionic radius of
the dopant. Mizukami et al., in A. Cruca (Ed.), Studies in surface
science and catalysis, Vol 71, Elsevier Amsterdam, 1991, 557,
reported that La.sup.3+ and Ba.sup.2+ are the most effective
dopants among rare earths and alkaline earths, which are large ions
with high ionic charges. Yeung et al., J. Membrane Sci., 131 (1997)
9, described procedures for making mesoporous alumina membranes by
slip casting a 1 M alumina sol (6 wt %) containing 3 atom % of a
nitrate of Y, Ba, La, and Ce. Das et al. in Appl. Catal. A: 207,
95-102 (2001), reported that a more stable La-doped support could
be obtained by processing in ethanol rather than water. Martin et
al. in Appl. Catal. A: 131, 297-307 (1995) described experiments in
which Rh was sintered on alumina under H.sub.2 at temperatures of
700 to 900.degree. C.; under these conditions, the mean Rh particle
size varied from 1.1 to 2.1 nm. Temperature Programmed Reduction
(TPR) data was also reported. Schaper et al., in Sintering--Theory
and Practice, Proceedings of the 5.sup.th International Round Table
Conference on Sintering, Material Science Monographs, vol. 14,
pp173-176, reported on the influence of Mg, Cr, La and Zr on the
stability of gamma alumina catalyst supports and concluded that
only lanthanum resulted in a significant improvement in stability
against sintering.
[0006] Treatment in 10% flowing H.sub.2O has been used to pre-age a
support for a catalytic convertor. McCabe et al. in J. Catal. 151,
385-393 (1995), reported that presteaming alumina at 1223 K with
10% H.sub.2O in flowing air for 24 hours prior to Rh impregnation
eliminated irreversible occlusion of Rh during subsequent
hydrothermal aging.
[0007] Wang et al. in US published patent application 2003/00317105
described methods and catalysts for steam reforming. In one
preferred embodiment, the catalyst includes Rh on a Mg--Al spinel.
This reference does not describe pretreatment of the support at
elevated steam pressure.
[0008] Despite extensive efforts over many years, there remains a
need for catalytic systems and steam reforming methods that have
high performance and stability under the hydrothermal conditions
that are typical of steam reforming. Also, despite extensive
efforts, there remains a need for better methods of making steam
reforming catalysts.
SUMMARY OF THE INVENTION
[0009] In a first aspect, the invention provides a steam reforming
catalyst, comprising: a Mg and Al containing spinel support; and Rh
on the surface of the support; and being characterizable by a
stability and reactivity such that, when tested in a flowing stream
of methane and water in a water:methane ratio of 3 at 15 atm, a
test reactor wall temperature of 880.degree. C. (this temperature
is measured by a thermocouple attached to a 0.020 inch (0.051 cm)
thick metal wall (Inconel.RTM. 617 if available) opposite a point
where the catalyst (or finned catalyst support) is in direct
contact with the wall; this temperature is believed to be within
10.degree. C. of the actual catalyst skin temperature), and a
contact time of 5 ms or less that is adjusted to obtain a methane
conversion (after 100 hours time-on-stream (TOS)) of 70% at a
pressure of 15 atm (total combined pressure of methane and steam),
and then maintaining the same conditions for 1000 hours, there is a
continuous period of at least 400 hours (during the 1000 hours) in
which the methane conversion changes by 3% or less. Methane
conversion may be conveniently measured by gas chromatography. To
measure this property, it is important to provide sufficient heat
to maintain temperature. This should be done by placing the
catalyst inside a half-flattened metal rod with a rectangular
channel that is 5 cm long, 0.3 inch (7.6 mm) wide and 0.02 inch
(0.5 mm) high, and loading with a length of 0.7 inch (1.8 cm) of
catalyst. One side of the metal rod has been removed to place a
thermocouple adjacent to the catalyst chamber (thus, the
"flattened" form). If the form of the catalyst makes this placement
impossible, catalyst can be tested in a packed bed in a more
conventional tube, if necessary with dilution of the catalyst with
alumina in order to prevent cold spots. The invention further
includes a catalyst system that includes this catalyst, preferably
in the presence of steam and a hydrocarbon.
[0010] For purposes of this invention, the protocol for analyzing
catalyst properties uses the technique for temperature measurement
that is described in the aspect above. The test reactor for
measuring catalyst properties should be the half-flattened metal
rod that is described in the Examples section. This-test procedure
is chosen because it best conforms with the data; in a less
preferred embodiment, the catalyst properties can be alternatively
defined as being measured at a catalyst skin temperature of
870.degree. C.
[0011] Unless stated otherwise, "conversion percent" refers to
absolute conversion percent throughout these descriptions. "Contact
time" is defined as the total catalyst chamber volume (including
the catalyst substrate volume) divided by the total volumetric
inlet flowrate of reactants at standard temperature and pressure
(STP: 273K and 1.013 bar absolute). Catalyst chamber volume
includes any volume between a catalyst coating (or other flow-by
catalyst arrangement) and the opposite wall of a reaction
channel.
[0012] In a further aspect, the invention provides a composition
comprising a Mg and Al containing spinel material. This composition
has a hydrothermal stability such that, when tested by treating the
composition at 925.degree. C. and 15 atm H.sub.2O, for 100 hours,
the support has a surface area of at least 10 m.sup.2/g. This
composition is intended for use as a catalyst support or a
precursor for a catalyst support. In a preferred embodiment, the
composition is a catalyst composition comprising Rh on the
surface.
[0013] For purposes of this invention, the temperature used to
describe a method is defined as the peak temperature (that is, the
highest temperature) in the catalyst. "Average approach to
equilibrium conversion" is based on this peak temperature. Also,
other variables such as contact time and productivity, when used to
describe a method in conjunction with a temperature, are based on
only those parts of the catalyst that are within 20.degree. C. of
the peak temperature. When used to describe the inventive methods,
temperatures refer to catalyst skin temperatures that can be
measured directly (this is preferred) or calculated by techniques
known in the art. In the tests described in the Examples section,
the temperature at any time in the catalyst chamber was essentially
isothermal (that is, the temperature at any instant is believed to
have varied by less than 20.degree. C.).
[0014] In some preferred embodiments, the maximum catalyst
temperature is 1050.degree. C., more preferably 1000.degree. C. or
less, more preferably 950.degree. C. or less, and in some
embodiments, 900.degree. C. or less. Additional, or narrower
parameters, that may be used in preferred embodiments are described
in the Detailed Description section.
[0015] In some embodiments, the temperature along the length of a
catalytic reforming channel (preferably a microchannel) may be
nearly isothermal, as defined within 20.degree. C. from inlet to
outlet. In some alternate embodiments, the temperature may vary
along the length of the microchannel. For example, the temperature
at the inlet of the catalyst zone of a microchannel may be
considerably cooler than the outlet of the microchannel. In one
example for methane steam reforming, the inlet temperature of the
catalyst zone may be around 650.degree. C. and the end of the
catalyst section may be around 850.degree. C. The temperature may
rise monotonically in a linear fashion or may rise more quickly
near either the front or end of the catalyst bed. Thus, in some
examples, the section of the catalyst containing microchannel that
exceeds 800.degree. C. may only include the final 75%, or 50%, or
25%, or 10% of the catalyst bed, or any value therewithin. For the
example of a temperature profile ranging from 650.degree. C. to
850.degree. C., the reaction may equilibrate near 840.degree. C.
and demonstrate an approach to equilibrium greater than 80% as
defined by the peak temperature. The equivalent contact time spent
in the reaction zone that exceeds 800.degree. C. may be
considerably less than the overall reaction contact time as defined
by the entire reaction channel volume (i.e., the volume of the
channel containing catalyst). As an example, the contact time
within the entire reaction channel volume may be 5 ms, but only 1
ms in the reactor section that exceeds 800.degree. C. It is
envisioned that the contact time spent in the reaction zone
exceeding 800.degree. C. required to achieve at least 80% approach
to equilibrium defined at peak temperature will exceed 0.1 ms. In
some embodiments, the temperature of the catalyst-containing
microchannel may be highest near the end of the reaction zone, or,
alternatively, may be higher at the front or middle of the reactor
rather than near the end of the reaction zone.
[0016] A "stabilized alumina support" comprises a stabilizing
element that enhances stability in steam reforming conditions. As
is conventional patent terminology, "comprising" means including
and when this term is used the invention can, in some narrower
preferred embodiments, be described as "consisting essentially of"
or in the narrowest embodiments as "consisting of." Aspects of the
invention described as "comprising a" are not intended to be
limited to a single component, but may contain additional
components.
[0017] The present invention includes catalysts or catalyst systems
containing stabilized alumina and Rh that are characterized by
surprisingly good stability and conversion or selectivity. It may
be subsequently discovered that other supports or catalyst metals
may perform equivalently if substituted for the Mg stabilized
alumina and/or Rh in these catalysts or catalyst systems; however,
the present inventors are not presently aware of any such
equivalent materials.
[0018] Various embodiments of the invention can provide numerous
advantages such as one or more of the following: improved catalyst
performance, high stability under steam reforming conditions
(steam, high temperature and high pressure), high conversions at
relatively short contact times, selectivity control, lower cost,
ease of manufacturing, and low temperature operation.
BRIEF DESCRIPTION OF THE FIGURES
[0019] FIG. 1 is a representational, schematic, cross-sectional
view of reactor apparatus that can be utilized in some embodiments
of the present invention.
[0020] FIG. 2 shows of X-ray diffraction patterns of 6%
Mg/Al.sub.2O.sub.3 treated under (a) thermal and (b) hydrothermal
conditions.
[0021] FIG. 3 is a plot of BET surface area vs. thermal aging time
for sol-based 3% La/Al.sub.2O.sub.3, 10% Mg/Al.sub.2O.sub.3; and 7%
Ba/Al.sub.2O.sub.3, 5% La/Al.sub.2O.sub.3 using
.gamma.-Al.sub.2O.sub.3 as starting material. The sol-based
material was thermally treated at 1050.degree. C. while others were
treated at 1075.degree. C. in air at atmospheric pressure.
[0022] FIG. 4a shows steam reforming activity of a finned, coated
catalyst system whose support has been hydrothermally treated at
near ambient pressure (about 0.7 atm H.sub.2O). Methane conversion
(.diamond.) is indicated in the range of 80 to 90% equilibrium
selectivity, CO selectivity (.diamond-solid.) in the range of 60 to
70% and carbon balance (A) near zero.
[0023] FIG. 4b shows steam reforming activity including methane
conversion (.diamond.), CO selectivity (.diamond-solid.), and
carbon balance (.DELTA.) over 600 hours time on stream of a
Rh/La/Al.sub.2O.sub.3 catalyst whose support was thermally treated
at 1050.degree. C. for 50 hours. The vertical lines indicate system
upsets during testing.
[0024] FIG. 5 shows the results including methane conversion
(.diamond.), CO selectivity (.circle-solid.), and carbon balance
(.DELTA.) of testing a catalyst of Rh/MgAl.sub.2O.sub.4 that was
prepared by sol coating a finned substrate followed by steam
pre-aging at 25 atm, 925.degree. C. for 100 hours under
H.sub.2O/He=2/1. Methane steam reforming was conducted for 2800
hours of operation at 15 atm, 3 to 1 steam-to-carbon ratio, 4.5-3.0
ms contact time, and reactor wall temperature of 869.degree. C. A
step change in contact time (vertical line) was introduced at 2150
hours to move further away from the 840.degree. C. equilibrium
conversion.
[0025] FIG. 6 shows the results including methane conversion
(.diamond.), CO selectivity (.circle-solid.), and carbon balance
(A) of testing a catalyst of Rh/MgAl.sub.2O.sub.4 that was prepared
by sol coating a finned substrate followed by steam pre-aging at 25
atm, 925.degree. C. for 100 hours under H.sub.2O/He=2/1. The metal
loading was 20 mg/in.sup.2 (25 wt % Rh oxide). Methane steam
reforming was conducted at 27 atm, 3 to 1 steam-to-carbon ratio,
3.0-4.2 ms contact time, and reactor wall temperature of
870-873.degree. C.
[0026] FIG. 7 shows the results including methane conversion
(.diamond.), CO selectivity (.circle-solid.), and carbon balance
(.DELTA.) of testing a catalyst of Rh/MgAl.sub.2O.sub.4 that was
prepared by sol coating a finned substrate followed by thermal
pre-aging at 1050.degree. C. for 50 hours. Methane steam reforming
was conducted at 15 atm, 3 to 1 steam-to-carbon ratio, 3.0-4.5 ms
contact time, and reactor wall temperature of about 840 to
950.degree. C.
[0027] FIG. 8 illustrates testing results including methane
conversion (.diamond.), CO selectivity (.diamond-solid.), and
carbon balance (.DELTA.) of a Rh/MgAl.sub.2O.sub.4 catalyst that
was prepared by sol coating a finned substrate, and without thermal
or hydrothermal pretreatment of the support.
[0028] FIG. 9 shows a plot representing H.sub.2 consumption as a
function of temperature on fresh Rh/MgAl.sub.2O.sub.4 catalyst that
was oxidized 500.degree. C.
[0029] FIG. 10 illustrates the SMR activity of certain powder
catalysts measured as a function of reduction temperature.
[0030] FIG. 11 is a plot of feed rate per gram Rh (N) versus
equilibrium methane conversion approach temperature difference
(.DELTA.T.sub.approach).
DESCRIPTION OF PREFERRED EMBODIMENTS
[0031] A steam reforming process requires a hydrocarbon (or
hydrocarbons) and steam (H.sub.2O). A reactant mixture can include
other components such as CO or nonreactive diluents such as
nitrogen or other inert gases. In some preferred processes, the
reaction stream consists essentially of hydrocarbon and steam. In
some preferred embodiments, the steam to carbon ratio in a reactant
stream is 3 to 1 to 1 to 1, and in some embodiments 1.5 to 1 or
less.
[0032] Hydrocarbons according to the present invention include:
alkanes, alkenes, alkynes, alcohols, aromatics, and combinations
thereof including fuels such as gasoline, kerosene, diesel, JP-8.
Preferably, the hydrocarbon is an alkane or a fuel. Preferred
alkanes are C.sub.1-C.sub.10 alkanes, such as methane, ethane,
propane, butane, and isooctane.
[0033] In some preferred embodiments, the catalyst (including all
support materials) contains 0.5 to 10 weight percent Rh, more
preferably 1 to 3 wt % Rh. In some preferred embodiments, the sum
of stabilized alumina support plus catalytically active materials
contains 0.5 to 10 weight percent, more preferably 1 to 7 wt %, and
still more preferably 2 to 5 wt % Rh.
[0034] Preferably, the stabilized alumina support contains 1 to 10,
more preferably 3 to 7 weight percent Mg. Several workers have
reported that La is a better stabilizer than Mg; with Schaper et
al., reporting that while La stabilized the alumina, Mg had no
effect. We have surprisingly found that Mg works better than La in
stabilizing a support in hydrothermal conditions. Therefore Mg--Al
spinel is is selected as the support for steam reforming.
[0035] Preferably, the Rh is present in the form of small particles
on the surface of the stabilized alumina support. More preferably,
the particles of Rh have a mean particle size in the range of 2.0
to 4.0 nm, more preferably 2.5 to 3.5 nm, as measured by electron
microscopy, where the mean is based on the number (not mass) of
particles.
[0036] The catalyst can be in the form of particles, preferably
having diameters less than 4 mm, more preferably less than 1 mm.
More preferably, the stabilized alumina forms a layer (of
agglomerated particles or a continuous film) having a thickness
less than 4 mm, more preferably less than 1 mm, and still more
preferably a thickness of less than 40 .mu.m. This layer is
preferably disposed on a porous substrate. Preferably the catalyst
contains an alumina layer disposed on a thermally conductive
surface. The surface could be, for example, a porous substrate or
reaction chamber wall(s).
[0037] In some embodiments, a stabilized alumina layer is coated
over, and preferably in direct contact with, a high surface area
material such as alumina, preferably (gamma)-alumina. This
configuration provides high surface area for good metal dispersion
and/or high metal loadings and also provides a stabilized alumina
layer for excellent stability. The high surface area material is
porous; the meaning of a stabilized alumina "disposed over" or
"coated over" a high surface area material means that the
stabilized alumina may also coat crevices and cavities within a
high surface area material or within a large pore substrate.
[0038] In some preferred configurations, the catalyst includes an
underlying large pore substrate. Examples of preferred large pore
substrates include commercially available metal foams and, more
preferably, metal felts. Prior to depositing any coatings, a large
pore substrate has a porosity of at least 5%, more preferably 30 to
99%, and still more preferably 70 to 98%. In some preferred
embodiments, a large pore substrate has a volumetric average pore
size, as measured by BET, of 0.1 .mu.m or greater, more preferably
between 1 and 500 .mu.m. Preferred forms of porous substrates are
foams and felts and these are preferably made of a thermally stable
and conductive material, preferably a metal such as stainless steel
or FeCrAlY alloy. These porous substrates can be thin, such as
between 0.1 and 1 mm. Foams are continuous structures with
continuous walls defining pores throughout the structure. Felts are
nonwoven fibers with interstitial spaces between fibers and
includes tangled strands like steel wool. Felts are conventionally
defined as being made of nonwoven fibers. In one embodiment, the
large-pore substrate has a corrugated shape that could be placed in
a reaction chamber (preferably a small channel) of a steam
reformer. Various substrates and substrate configurations are
described in U.S. patent application Ser. No. 09/640,903 (filed
Aug. 16, 2000), U.S. Pat. No. ______ which is incorporated by
reference. Another preferred substrate is a finned substrate that
is characterized by the presence of fins (such as square-wave type
fins) on the substrate's surface. Alternatively, the catalyst may
take any conventional form such as a powder or pellet.
[0039] A catalyst with a large pores (and including the
alumina-supported catalytically active sites) preferably has a pore
volume of 5 to 98%, more preferably 30 to 95% of the total porous
material's volume. Preferably, at least 20% (more preferably at
least 50%) of the material's pore volume is composed of pores in
the size (diameter) range of 0.1 to 300 microns, more preferably
0.3 to 200 microns, and still more preferably 1 to 100 microns.
Pore volume and pore size distribution are measured by mercury
porisimetry (assuming cylindrical geometry of the pores) and
nitrogen adsorption. As is known; mercury porisimetry and nitrogen
adsorption are complementary techniques with mercury porisimetry
being more accurate for measuring large pore sizes (larger than 30
nm) and nitrogen adsorption more accurate for small pores (less
than 50 nm). Pore sizes in the range of about 0.1 to 300 microns
enable molecules to diffuse molecularly through the materials under
most gas phase catalysis conditions.
[0040] In some preferred embodiments, the catalyst comprises a
metal, ceramic or composite substrate having a layer or layers of a
catalyst material or materials deposited thereon. The porosity can
be geometrically regular as in a honeycomb or parallel pore
structure, or porosity may be geometrically tortuous or random.
Preferred porous support materials include felts (nonwoven fibers
or strands), foams (including a foam metal or foam ceramic), fins
and honeycombs. In embodiments employing a porous substrate, the
average pore size (volume average) of the catalyst layer(s) is
preferably smaller than the average pore size of the substrate.
[0041] In a particularly preferred embodiment, alumina is deposited
on a finned metal substrate and a catalytic metal is deposited on
the alumina. Preferably, this support is a thermally conductive
metal that is sized to fit within a microchannel. Alternatively,
the finned support could be fabricated directly within the
microchannel and be integral to the microchannel. One method of
fabrication within a microchannel comprises the use of a slitting
saw, partial etching using a photochemical process, or a laser EDM.
This type of support provides numerous advantages including: high
heat flux with short heat transfer distances, high surface area,
and low pressure drop. Preferably, the support has a height
(including fins) of less than 5 mm and preferably less than 2 mm
and a fin-to-fin separation of 1000 .mu.m or less, and in some
embodiments, a fin-to-fin separation of 150 to 500 .mu.m. The fin
structure can be integral with a reaction chamber (and thus coated
in situ), or as a separate insert that can be coated prior to being
inserted into a reaction chamber.
[0042] In some embodiments, the catalyst, including the presence of
catalytically active surface sites, as measured by BET, has a
volumetric average pore size of less than 0.1 micrometer (.mu.m).
The catalyst, including the presence of catalytically active
surface sites, as measured by BET, nitrogen physisorption,
preferably has a surface area of more than 10 m.sup.2/g, and in
some embodiments a surface area of 20 to 500 m.sup.2/g.
[0043] Conventional catalysts typically undergo large decreases in
surface area when exposed to hydrothermal conditions. Preferred
catalysts of this invention have a surface area, as measured by
N.sub.2 adsorption BET, of at least 5 m 2/g, more preferably at
least 10 m.sup.2/g, and in some embodiments 5 to about 20
m.sup.2/g; and preferably maintain these surface areas after
exposure to 25 atm steam for 100 hours.
[0044] Certain aspects of the invention can best be described in
terms of properties such as stability, conversion or selectivity.
Both the catalysts and methods can be characterized in terms of
hydrocarbon conversions and selectivities in steam reforming
processes. Hydrocarbon conversion is preferably at least 50%, more
preferably at least 80% and still more preferably at least 90%. The
foregoing conversion values can be either absolute or equilibrium
conversions. If not specified, it is conventional to consider
conversion values to be absolute conversions. Under conditions
where conversion approaches 100%, absolute and equilibrium
conversion is the same. "Equilibrium conversion" is defined in the
classical manner, where the maximum attainable conversion is a
function of the reactor temperature, pressure, and feed
composition. For the case of hydrocarbon steam reforming reactions,
the equilibrium conversion increases with increasing temperature
and decreases with increasing pressure. In some embodiments,
hydrocarbon equilibrium conversion is in the range of 70 to
100%.
[0045] Hydrogen selectivity, defined as moles H atoms in H.sub.2 in
the product gas divided by moles H in all product gases, is
preferably at least 50%, more preferably at least 60%, still more
preferably at least 85%, and yet still more preferably at least
95%. For some embodiments, carbon monoxide selectivity is
preferably less than 65%, more preferably less than 40%.
[0046] In some preferred embodiments, the catalyst is
characterizable by a stability and reactivity such that, when
tested in a flowing stream of methane and water in a water:methane
ratio of 3 at 15 atm, a catalyst temperature (actually a reactor
wall temperature as explained above) of 880.degree. C., a reactor
pressure of 15 atm (note that in the reactor design described in
the Examples section, pressure drop through the reactor is
negligible) and a contact time of 5 ms or less that is adjusted to
obtain a methane conversion (after 100 hours TOS) of 70%, and
maintaining the same conditions for 1000 hours, there is a
continuous period of at least 400 hours (more preferably at least
600 hours, and in some embodiments from 400 to about 2800 hours
TOS) in which the methane conversion changes by 3% or less. To
measure this property, it is important to provide sufficient heat
to maintain temperature. This should be done by placing the
catalyst inside a metal tube with a rectangular channel cut through
the long axis of the metal tube, the channel being 5 cm long, 0.3
inch (7.6 mm) wide and 0.02 inch (0.5 mm) high, and loading into
the channel a length of 0.7 inch (1.8 cm) of catalyst. If the form
of the catalyst makes this impossible, catalyst can be tested in a
more conventional tube, if necessary with dilution of the catalyst
with alumina in order to prevent cold spots. The invention further
includes a catalyst system that includes this catalyst, preferably
in the presence of steam and a hydrocarbon.
[0047] In some preferred embodiments, the catalysts, systems and
methods can be characterized by a stability such that after 595
hours of continuous operation in hydrocarbon steam reforming
conditions, the conversion of hydrocarbon is at least 70%, more
preferably at least 80%, still more preferably at least 85%, and in
some embodiments 80 to about 90% of the equilibrium conversion, and
where the conversion has diminished by less than 10%, more
preferably less than 5%, and still less preferably less than 3%
over the 595 hours of continuous operation. Selectively to CO is
preferably essentially unchanged over the course of the 595 hours
of continuous operation. "Systems" are formed by a catalyst in
conjunction with a reactor, preferably a microreactor.
[0048] Some preferred embodiments of the inventive catalysts and
methods may also be described in terms of their exceptionally high
activity. Preferably, the catalyst possesses a catalytic activity
such that, when exposed to a stream of CH.sub.4 and H.sub.2O at a
steam-to-carbon ratio of 3, at a catalyst temperature of
840.degree. C. and a pressure of 27 atm at a flow rate of 6000 SLPM
(standard liters per minute measured at 0.degree. C. and 1 atm)
total flow per gram of Rh, the catalyst converts methane to
products at a rate of at least 2 mol CH.sub.4/g Rh, more preferably
at least 10, still more preferably at least 25, and in some
embodiments 3 to about 50 mol CH.sub.4/g Rh. This activity is
preferably maintained for at least 400 hours, more preferably at
least 600 hours.
[0049] In another aspect, the invention provides a method of
methane steam reforming, comprising: passing a steam comprising
methane and water in a steam to carbon ratio of 3 or less through a
catalyst at a contact time of 10 ms or less and maintaining the
same conditions for at least 1000 hours, wherein there is a
continuous period of at least 500 hours in which the methane
conversion changes by 3% or less; and wherein at least 2 mol of
methane are converted per gram of Rh each minute.
[0050] In some embodiments, a catalyst's properties (such as
stability, conversion and selectivity) are defined by the following
test procedure (referred to as "Test Procedure I"). In this test
procedure, about 7 mg of catalyst is loaded into a reaction chamber
that is a 0.162 inch.times.0.021 inch.times.2.0 inch
(4.1.times.0.53.times.51 mm) slot in a 0.5 inch (1.3 cm) diameter
solid Inconel 617 rod. If necessary, the catalyst can be crushed,
cut or scraped from a larger piece of catalyst to obtain a catalyst
that will fit in the slot. The catalyst can be on a finned support
(as described in the examples) or in other forms such as pellets or
powders. The catalyst is reduced to an optimal extent, as can be
determined through routine experimentation. The reduction is under
flowing hydrogen, typically in the range of 300 to 500.degree. C.
Then the reactor is pressurized to 15 atm under an inert
atmosphere, and then heated in nitrogen to the reaction
temperature. In this test procedure, the steam reforming reaction
is carried out at 850.degree. C., a steam:methane ratio of 3:1, at
0.15 atm and a contact time of 4.5 milliseconds. Catalyst packing
can be varied to obtain this pressure and contact time; if the
exact conditions cannot practically be obtained, results can be
extrapolated to estimate results at the specified conditions. In
this procedure, pressure drop through the reaction chamber is
preferably 10 psig or less. In preferred embodiments of a catalyst
tested with this procedure there is at least 85% methane
conversion, and, in some embodiments 80 to about 90% methane
conversion. CO selectivity is preferably at least 50% and remains
constant.
[0051] The present invention also provides methods of steam
reforming in which a hydrocarbon is reacted with water vapor at
short residence times (or alternatively, described in contact
times) over the catalysts described herein. The residence time is
preferably less than 0.1 s. Short contact times are preferably
5-100 milliseconds (msec), in some embodiments, 10-25 msec.
[0052] The steam reforming reaction is preferably carried out at
more than 400.degree. C., more preferably 500-1000.degree. C., and
still more preferably 650-900.degree. C. The reaction can be run
over a broad pressure range from sub-ambient to very high, in some
embodiments the process is conducted at a pressure of from 10 atm
to 30 atm, more preferably 12 atm to 25 atm. The H.sub.2O partial
pressure is preferably at least 0.2 atm, in some embodiments at
least 2 atm, and in some embodiments in the range of 5 atm to 20
atm.
[0053] The catalyst support is made from a composition that
includes an alumina precursor. An "aluminum precursor" is any form
of aluminum (such as an alumina slurry) that can be used to form
solid alumina. The catalyst may be made starting from an alumina
sol or solid alumina. Suitable, commercially available materials
include colloidal alumina suspended in aqueous medium from Sasol,
or Engelhard alumina ground to a particle size of 70-100 mesh.
[0054] Starting from alumina particles, a slurry can be prepared by
milling in water or organic phase. Stabilizing elements can be
added as solids or in solution. It has been discovered that adding
acid to adjust the pH to 3 to 6, preferably 5 to 6, results in a
stable slurry even for solid contents greater than 20 wt %, and in
some cases solid content in the range of 15 to 25 wt %. Adding a
binding agent to the milled alumina can improve slurry
characteristics.
[0055] Once alumina has been stabilized, it can be dried before a
subsequent pre-aging treatment. Drying can be accomplished by
removing or evaporating a liquid phase of the slurry; alternatively
the slurry can be coated on a porous substrate and then dried prior
to a subsequent pre-aging treatment.
[0056] A stabilized alumina is preferably subjected to a pre-aging
treatment comprising a hydrothermal or thermal treatment, prior to
applying a catalytically active material. In a hydrothermal
treatment, the support material is exposed to steam at high
temperature. Based on the results for which we have comparative
data, we have discovered that thermal treatment of La-stabilized
alumina at ambient conditions produces superior results as compared
to hydrothermal treatments of La-stabilized alumina conducted at
atmospheric pressure. In thermal pre-aging, the support materials
are thermally treated at high temperatures in the absence of added
water. Preferred conditions for the thermal treatment include
heating to a temperature of from 1000 to 1150.degree. C. A
particularly preferred set of conditions for a nonhydrothermal
treatment is heating at 1050-1100.degree. C. for 24-50 hours in air
at atmospheric pressure.
[0057] On the other hand, hydrothermal treatment of Mg-stabilized
alumina at elevated pressure resulted in supports that are superior
to both ambient pressure hydrothermal treatments and thermal
treatments. Preferred hydrothermal pre-aging conditions include at
least 5 atm partial pressure of steam, and at least 900.degree. C.,
for a duration of at least 80 hours. More preferred conditions
include at least 10 atm steam partial pressure, and in some
embodiments 5 to 25 atm steam; temperatures of at least 900.degree.
C., more preferably at least 950.degree. C., and in some
embodiments 900 to 1100.degree. C.; and times of at least 100
hours, and in some embodiments 50 to 300 hours.
[0058] When an underlying substrate is used, an alumina slurry or
sol can be coated over the substrate at any stage in the
preparative process. For example, particles of a stabilized and
heat-treated alumina can be slurry coated onto the substrate
followed by depositing, drying and activating a metal via the
impregnation method. Alternatively, a vapor coat or soluble form of
alumina (or other high surface area material) could be applied onto
a substrate prior to addition of a catalytic metal. In another
embodiment, the substrate may be coated with a buffer layer formed
in situ using chemical vapor deposition. The buffer layer may not
have a high surface area, but may be used to create a layer with a
CTE (coefficient of thermal expansion) between the base metal
substrate and the higher surface area catalyst support to promote
good adhesion of the layers. The buffer layer may also be used to
inhibit corrosion of the base metal substrate by creating a near
dense and almost pin-hole free coating. Although solution (such as
spray coating) or slurry coating is typically less expensive, vapor
coating of the various materials could also be employed. The
hydrothermal or thermal pre-aging treatment can be conducted either
before or, preferably, after applying the alumina to a
substrate.
[0059] A catalytically active material can be deposited onto
alumina using known techniques such as the incipient wetness
technique. Preferably, a precursor of the catalytically active
material is added after the pre-aging treatment.
[0060] As described in the Examples section, the reduction
temperature range for an alumina/Rh catalyst was studied by a
temperature-programmed reduction technique. Although sintering
might be expected at higher temperatures, it was surprisingly
discovered that the activity of a catalyst reduced at 300.degree.
C. was better as compared to an identical catalyst reduced at
125.degree. C. In some preferred embodiments, catalysts are reduced
in a hydrogen atmosphere at temperatures in the range of
200-400.degree. C., more preferably 250-350.degree. C.; reduction
at these temperatures results in substantially improved activity.
It was found that the metal dispersion of a catalyst, as measured
using a hydrogen chemisorption technique, showed higher dispersion
on the catalyst reduced at 300.degree. C. as compared to an
identical catalyst reduced at 125.degree. C.
[0061] The present invention includes methods and systems in which
a steam reforming catalyst is disposed within a microchannel
reaction channel. The height and/or width of a reaction
microchannel (wall-to-wall, not counting catalyst) is preferably 5
mm or less, and more preferably 2 mm or less, and in some
embodiments 50 to 1000 .mu.m. Both height and width are
perpendicular to the direction of flow. The length of a reaction
channel is parallel to flow through the channel and is typically
longer than height and width. Preferably, the length of a reaction
chamber is greater than 1 cm, more preferably in the range of 1 to
100 cm. Typically, the sides of the reaction channel are defined by
reaction channel walls. These walls are preferably made of a hard
material such as a ceramic, an iron based alloy such as steel, or a
nickel-based alloy. In some preferred embodiments, the reaction
chamber walls are comprised of stainless steel or inconel which is
durable and has good thermal conductivity.
[0062] In addition to the reaction channel(s), additional features
such as microchannel or non-microchannel heat exchangers may be
present. Microchannel heat exchangers are preferred. Adjacent heat
transfer microchannels enable temperature in the reaction channel
to be controlled precisely to promote steam reforming and minimize
unselective reactions in the gas phase. The thickness of a wall
between adjacent process channels and heat exchange channels is
preferably 5 mm or less. Each of the process or heat exchange
channels may be further subdivided with parallel subchannels. The
heat exchange fluids can be gases or liquids and may include steam,
liquid metals, or any other known heat exchange fluids. Especially
preferred heat exchangers include combustors in which a fuel is
oxidized to produce heat for the steam reforming reaction. The
incorporation of a simultaneous exothermic reaction to provide an
improved heat source can provide a typical heat flux of roughly an
order of magnitude above the convective cooling heat flux. The flow
of hot fluid through a heat exchanger may be cross flow,
counter-flow or co-flow.
[0063] The reactors preferably include a plurality of microchannel
reaction channels and/or a plurality of adjacent heat exchange
microchannels. A plurality of microchannel reaction channels may
contain, for example, 2, 10, 100, 1000 or more channels. In some
preferred embodiments, multiple heat exchange layers are
interleaved with multiple reaction microchannels (for example, at
least 10 heat exchanger layers interleaved with at least 10 layers
of reaction microchannels. Typically, flow into and/or out of some
or all of a plurality of reaction channels passes through a
manifold or manifolds that combines the fluid flow. In preferred
embodiments, microchannels are arranged in parallel arrays of
planar microchannels.
[0064] Preferred reactors usable in the present invention include
those of the microcomponent sheet architecture variety (for
example, a laminate with microchannels). Examples of integrated
combustion reactors that could be used in the present invention are
described in U.S. patent application Ser. No. 10/222,196, filed
Aug. 15, 2002, which is incorporated herein by reference. Some
other suitable reactor designs and methods of making reactors are
disclosed in U.S. patent application Ser. No. 10/306,722, filed
Nov. 27, 2002, and Ser. No. 10/408,744, filed Apr. 7, 2003, which
are also incorporated herein, in full, by reference.
[0065] The catalyst can fill up a cross-section of the reaction
channel (a flow-through catalyst) or only occupy a portion of the
cross-section of a reaction channel (flow-by). The use of a flow-by
catalyst configuration can create an advantageous capacity/pressure
drop relationship. In a flow-by catalyst configuration, gas
preferably flows in a 0.1-2.0 mm gap adjacent to a porous insert or
a thin layer of catalyst that contacts the microchannel wall
(preferably the microchannel wall that contacts the catalyst is in
direct thermal contact with a heat exchanger, preferably a heated
fluid or exothermic reaction process stream contacts the opposite
side of the wall that contacts the catalyst).
[0066] The reaction channel contains a steam reforming catalyst.
Suitable catalyst structures include (but are not limited to)
porous catalyst materials, fins, washcoats, pellets, and powders.
In one preferred embodiment, a reaction channel contains a catalyst
material that defines at least a portion of at least one wall of a
bulk flow path. In this preferred embodiment, the surface of the
catalyst defines at least one wall of a bulk flow path through
which the mixture passes. During operation, the mixture flows
through a microchannel, past and in contact with the catalyst in
the microchannel. The term "bulk flow path" refers to an open path
(contiguous bulk flow region) within the reaction chamber. A
contiguous bulk flow region allows rapid gas flow through the
reaction chamber without large pressure drops. In preferred
embodiments there is laminar flow in the bulk flow region. Bulk
flow regions within each reaction channel preferably have a
cross-sectional area of 5.times.10.sup.-8 to 1.times.10.sup.-2
m.sup.2, more preferably 5.times.10.sup.-7 to 1.times.10.sup.-4
m.sup.2. The bulk flow regions preferably comprise at least 5%,
more preferably 30-80% of either 1) the internal volume of the
reaction chamber, or 2) the cross-section of the reaction channel.
One example of a bulk flow path is the space between fins in a
finned catalyst. When a combustion reaction is used to heat the
steam reforming reaction chamber in an integrated combustion
reactor, the combustion reaction preferably contains a bulk flow
path having the properties discussed above.
[0067] In some embodiments, the catalyst is provided as a porous
insert (such as a foam monolith) that can be inserted into (or
removed from) each channel in a single piece; preferably the porous
insert is sized to fit within a microchannel with a width of less
than 2 mm. In some embodiments, the porous catalyst occupies at
least 60%, in some embodiments at least 90%, of a cross-sectional
area of a microchannel. In an alternative preferred embodiment, the
catalyst is a coating (such as a washcoat) of material within a
microchannel reaction channel or channels.
[0068] One embodiment of a reactor 2 is shown in cross-section in
FIG. 1. The reaction chamber 4 contains catalyst 6 and has an inlet
8 and outlet 10. In FIG. 1, the catalyst is shown on the top and
bottom of the reaction chamber with an open channel from the
reactor inlet to the outlet--this configuration is called
"flow-by." Other configurations, such as "flow-through" where flow
is directed through a porous catalyst, are, of course, possible. To
improve heat transfer, a microchannel heat exchanger 12 can be
placed in contact with the reaction chamber. The microchannel heat
exchanger 12 has channels 14 for passage of a heat exchange fluid.
These channels 14 have at least one dimension that is less than 2
mm. The distance from the channels 14 to catalyst 6 is preferably
minimized in order to reduce the heat transport distance. In
preferred embodiments, a reaction chamber 4 is connected to fuel
tank 16 such that fuel from the tank can flow into the reaction
chamber. Although a fuel tank is shown in FIG. 1, it should be
recognized that any fuel source, such as a pipeline could be used.
A liquid fuel stream may flow through a separate vaporizer or be
vaporized within a section of the steam-reforming reactor. In some
preferred embodiments a fuel is vaporized in a microchannel
vaporizer and/or preheated in a microchannel preheater. The product
gases (including H.sub.2) then may either flow into hydrogen input
device 22, or the product of the reforming reactor may flow into a
water gas shift reactor to convert some of the carbon monoxide into
carbon dioxide and additional hydrogen. Additionally, the product
stream may flow into a secondary clean up process to further purify
hydrogen or reduce carbon monoxide. The secondary clean-up process
may include a preferential oxidation reactor, membrane separation
of either hydrogen or carbon monoxide, a sorption based separation
system for either hydrogen or carbon monoxide, and the like. These
elements form a highly simplified fuel processing system 30. In
practice, fuel processing systems will be significantly more
complex. Typically, heat from a combustor will be used to generate
heat for other processes such as generating steam (not shown) that
can be utilized for steam reformer and/or water gas shift reactor.
The hydrogen input device 22, can be, for example: a fuel cell, a
storage tank, a refueling station, a hydrocracker, hydrotreater, or
an additional hydrogen purifier. In a fuel cell, the H.sub.2 is
combined with O.sub.2 to generate electricity. Various fuel cells
are well-known and commercially available and need not be described
here.
EXAMPLES
[0069] To test stability and surface area, supports were prepared
from a slurry in a two-step process and from a sol in a one-step
process. The resulting supports were analyzed after various
pretreatments.
[0070] Slurry Coating
[0071] Support powder was wet-milled in water or organic phase to a
uniform particle size of 0.5 um. The pH was adjusted before the wet
milling to 3-6, more specifically, 5-6. At this pH, the viscosity
of slurry was lowered, allowing a higher solid concentration in the
slurry while maintaining an effective milling. At these conditions,
the catalyst particles are slightly charged and repelled from each
other. The resulting catalyst slurry is highly stable with minimum
particle agglomeration. Therefore, sedimentation rate was retarded,
and uniform suspension in the liquid was maintained for days
without the need of constantly agitating the slurry.
[0072] After the wet milling, binding agents such as alumina sol or
surfactant were added to the slurry to improve interparticle
adhesion. Addition of binders was done after the wet milling
process, since binder could increase the slurry viscosity, reducing
the overall effectiveness of wet milling. The viscosity of the
slurry is adjusted once more just prior to coating the substrates
by adding dilute nitric acid after the milling. This can improve
flow during the coating process to achieve uniform coverage of
catalyst slurry on the substrate with a high solid loading slurry.
With high solid loading slurry, the number of dip coating cycles
can be minimized to achieve the target catalyst loading on the
substrates.
[0073] Dip coating or spin coating was used for coating substrates.
Dip coating applies to foam or felt substrates with inner
interstitial spaces, while spin coating was used for flat surfaces
such as a ceramic plate or metal foil. In the dip coating process,
uniformity of coating was achieved by withdrawing the substrate
that was fully immersed in the slurry vertically at a fixed rate,
between 5 cm/min and 20 cm/min. The coated substrate was then dried
under controlled drying to achieve a crack-free coated catalyst
layer. The dip coating process was repeated until the target weight
gain was achieved. After drying the last coat, the substrate was
annealed at high temperature. This step was carried out to improve
the adhesion and to decompose the surfactant, if surfactant was
added as binding agent.
Example 1
[0074] A 17 wt %-22 wt % solid loading catalyst slurry was prepared
by mixing powder with deionized water. Two different powders were
used for the ball mill, standard MgO--Al.sub.2O.sub.3 spinel and
hydrothermally treated MgO--Al.sub.2O.sub.3 spinel. The
hydrothermally treated spinel has a higher density than the fresh
spinel due to the formation of alpha alumina during the
hydrothermal treatment. Solid/liquid mixture was milled in a 2"
ball mill bottle along with 3 mm alumina grinding balls for 24 or
62 h, depending on the type of the starting material. Standard
MgO--Al.sub.2O.sub.3 required 24 h of milling while the
hydrothermally treated MgO--Al.sub.2O.sub.3 required 62 h to
achieve an average particle size that is less than 1 .mu.m. After
the milling, particle size analysis showed average particle sizes
of 0.5 .mu.m and 0.8 .mu.m, for standard and hydrothermally treated
catalysts, respectively. Nitric acid was added to adjust the pH to
4, and 1 wt % of PQ Al.sub.2O.sub.3 is added as binding agent.
Changing the pH to 4.5-5 significantly reduced the slurry
viscosity. Therefore, by modifying the pH, the slurry can be
prepared with higher solid loadings, which reduces the number of
dipping or spin coating in the coating steps, without compromising
coating quality. In addition, it was found that the sedimentation
rate of the higher solid loading slurries (17 to 22 solids %) was
comparable to a pH-unmodified slurry with a much lower solid
loading of 11%.
[0075] A catalyst slurry with 13.8 wt % solid loading was
separately prepared by mixing slurry from Example 1 (above) with PQ
Al.sub.2O.sub.3, surfactant polyoxyethylene 10 lauryl ether
(Cl.sub.2EO.sub.10) and 2 wt % HNO.sub.3. The final weight ratio of
slurry was: support powder:PQ
Al.sub.2O.sub.3:Cl.sub.2EO.sub.10=100:5:8, pH=3.4.
[0076] Metal foils and metal felts were coated with the slurries.
FeCrAlY intermetallic alloy in the form of felt
(2".times.0.35".times.0.01") structure was fired at 900.degree. C.
for 2 h in a box furnace. At this temperature, Al migrates to the
surface and oxidizes to form a native alpha Al.sub.2O.sub.3 scale.
The formation of a uniform layer of metal oxide scale on the
intermetallic alloy substrates significantly improves the adhesion
between the oxide coating and the alloy substrate. It is believed
that the scale reduces the thermal expansion mismatch between the
ceramic coating and the metal substrate, which could lead to
spalling of catalyst layer. Upon cooling to room temperature, the
felt was coated with the slurry from Example 1. The felt was first
immersed vertically in the slurry, then withdrawn at a constant
rate of 5-10 cm/min, dried at room temperature for 10 min, then
dried at 110.degree. C. in vacuum oven. The dip coating step was
repeated 3 times to achieve the target weight loading density of
0.09 g cat/in.sup.2. After the final drying, the substrate was
calcined at 5.degree. C./min to 500.degree. C., and held at that
temperature for 2 h. A control piece was made using the slurry
without any modifications, dip coating without controlling the rate
of substrate withdrawal. By modification of slurry pH, the slurry
became less viscous, allowing a higher solid concentration without
affecting the flow characteristic during dip coating. As a result,
a uniform catalyst layer can be obtained in minimum number of dips.
The dip cycle was reduced from 5 to 3 with an increase in solid
loading of from 11 wt % to 22.20 wt %. Examining the coated
substrate under a microscope showed a uniform coating layer with
minimal cracks. In addition, adhesion was tested by sonicating the
samples in petroleum ether for 30 min showing significant
enhancement in adhesion for the slurry modified with 1 wt % of PQ
Al.sub.2O.sub.3. The weight loss at the end of the sonication test
was negligible.
[0077] FeCrAlY foil (0.02" thick) was cleaned in acetone and
isopropanol prior to firing to 900.degree. C. for 2 h. A layer of
native alpha Al.sub.2O.sub.3 formed due to diffusion of Al to the
surface. The substrate was then spin coated at 600 rpm for 30
seconds with slurry, dried at 200-300.degree. C. on a hot plate for
0.5-1 min, the spin coating was repeated for 3-6 times to achieve
the target weight loading, then calcined at 500.degree. C. for 2
hrs. A hydrothermal test was performed on this foil at 900.degree.
C. with 60% H.sub.2O for 100 h at 12 atm in a sealed Inconel tube.
After the hydrothermal test, the coated foil then underwent thermal
cycling from 100.degree. C. to 900.degree. C. at a heating and
cooling rate of 5.degree. C./min for 10 times. No spalling of the
catalyst powder was observed at the end of both tests. Catalysts
for comparing pre-aging treatments were prepared as described
below. The catalyst was reduced under a 10% H.sub.2 in N.sub.2
stream at 300.degree. C. for 1 hour.
[0078] A finned substrate was coated with both slurry (see above)
and sol (see below) coating techniques. The finned substrate, which
is more stable than felts in prolonged steam reforming conditions,
were a thin FeCrAlY plate with surface fins. Width at the fin base
is 0.16 inch (4.1 mm), length is 0.7 inch (1.8 mm), channel width
is 0.15 inch (0.38 mm) and depth is 0.14 inch (3.6 mm); the fins
are separated by 0.10 inch (2.5 mm) wide and 0.14 inch (3.6 mm)
tall walls; forming a total of 6 channels through the finned
support. These supports were cleaned and heat-treated to grow thin
aluminum oxide layer. Each substrate was cleaned ultrasonically in
2-propanol and then 20 wt % nitric acid for 20 minutes. After
rinsing with deionized water, the fins were dried at 100.degree. C.
for 30 minutes. The fins were then placed in a furnace in air and
heated to 100.degree. C. at a rate of 3.5.degree. C./min and held
at 1000.degree. C. for 8 hours, then allowed to cool to room
temperature slowly (.about.3.5.degree. C./min)
[0079] Solution (Sol) Coating Method:
[0080] A plate with surface fins was solution coated with alumina
sol (A100H) and a solution containing stabilizing ion as
follows:
[0081] 1. Alumina sol (SASOL 14N4-25) was coated onto the fin
surface by submerging the fin into the sol (ex-situ or in-situ).
Remove the excess sol.
[0082] 2. Dry the coated fin at 100.degree. C. at a rate of
3.5.degree. C./min for 1 hour.
[0083] 3. Calcine the fin by heating to 450.degree. C. at a rate of
3.5.degree. C./min and hold at 450.degree. C. for 4 hours, then
allowed to cool to room temperature (3.5.degree. C./min).
[0084] 4. Stabilizing ion such as lanthanum ion, barium ion or
magnesium ion was impregnated onto the alumina coated fin using 10
wt % aqueous solution of lanthanum nitrate, barium acetate, or
magnesium nitrate by submerging the fin into the solution. Excess
solution was removed.
[0085] 5. Dry the coated fins at 100.degree. C. at a rate of
3.5.degree. C./min for 1 hour.
[0086] 6. Calcine the fin by heating to 1000.degree. C. at a rate
of 3.5.degree. C./min and hold at 1000.degree. C. for 4 hours, then
cooled to room temperature (3.5.degree. C./min).
[0087] 7. Pre-age (if utilized) the fins using the procedures in
the following section.
[0088] 8. Impregnate Rh metal onto the pre-aged fin by dipcoating
into a 10 wt % solution of rhodium nitrate.
[0089] 9. Dry the coated fins at 100.degree. C. at a rate of
3.5.degree. C./min for 1 hour.
[0090] 10. Calcine the fin by heating to 950.degree. C. at a rate
of 3.5.degree. C./min and hold at 950.degree. C. for 1 hour, then
allow them to cool to room temperature slowly (3.5.degree.
C./min).
[0091] An X-ray diffraction pattern of the thermally pre-aged
material was compared to that of hydrothermally treated (at 1 atm)
material in FIG. 2. FIG. 2(a) shows X-ray diffraction pattern of 6%
Mg/Al.sub.2O.sub.3 thermally treated at 1100.degree. C. for 24
hours whereas FIG. 2(b) shows the pattern for the same composition
material hydrothermally treated at 900.degree. C. and 12 atm for
100 hours in a flow of steam and nitrogen mixture at a ratio of 2
to 1. It was found that the major phases consist of alpha alumina
and spinel phases. The relative intensity of peaks indicating the
spinel phase was higher on the thermally treated sample (a) than
the hydrothermally treated sample (b). Crystallinity of both
samples was similar to each other, which is in agreement with the
BET surface area results.
[0092] Surface Area
[0093] In thermally pre-aged samples, La was found to be more
effective than Mg in stabilizing the surface area. See FIG. 3.
After aging at 1100.degree. C. for 24 h, XRD analysis of a La
stabilized Al.sub.2O.sub.3 sample showed a combination of delta,
theta, and gamma alumina phases. Gamma alumina has the highest
surface area among different phases of alumina.
[0094] Thermal treatment of sol-based lanthanum-, barium-, and
magnesium-doped alumina was carried out in a powder form and BET
surface area was measured. The thermally stabilized La-doped
alumina was found to be more stable than the sample that was
hydrothermally treated at ambient pressure. This trend was not
observed for the material that was hydrothermally treated at 25
atm--in the samples hydrothermally pre-aged under increased
pressure, the Mg-doped support was found to be the most stable. The
Table below shows surface area stability of variously prepared
alumina powders. The sol-derived powder exhibited the greatest
surface area after hydrothermal treatment. Among three formulations
(one-step MgAl.sub.2O.sub.4, two-step Mg/Al.sub.2O.sub.3, and
two-step La/Al.sub.2O.sub.3), the Mg-containing supports had
similar surface area around 44 m.sup.2/g but the La-containing
support had 59 m.sup.2/g. Steam aging at 25 atm decreased the
surface area by 86% and 94% on two-step 6% Mg/Al.sub.2O.sub.3 and
two-step 3% La/Al.sub.2O.sub.3, respectively. However, the degree
of surface area reduction was much smaller (54%) on one-step 6%
MgAl.sub.2O.sub.4. This could be possibly due to the formation of
more spinel (less isolated y-alumina which transforms to
.alpha.-alumina with increasing temperature) in the one-step method
because of the intimate contact of Mg nitrate with AlOOH that can
be achieved through this method. See the following table in which
the atmosphere comprises 60% water, so H.sub.2O partial pressure at
25 atm is 15 atm.
1TABLE Surface Area of Treated Supports S.A. One-step 6%
MgAl.sub.2O.sub.4 reduction Treatment m.sup.2/g (%) 1000.degree.
C., 4 hrs, air, 1 atm 92.01 925.degree. C., 20 hrs, steam, 1 atm
65.06 29.3 925.degree. C., 100 hrs, steam, 1 atm 44.17 52.0
925.degree. C., 100 hrs, steam, 25 atm 20.15 78.1 1050.degree. C.,
50 hrs 39.84 56.7 1050.degree. C., 100 hrs S.A. Two-step 6%
Mg/Al.sub.2O.sub.3 reduction m.sup.2/g (%) 1000.degree. C., 4 hrs,
air, 1 atm 81.97 925.degree. C., 100 hrs, steam, 1 atm 46.24 43.6
975.degree. C., 200 hrs, steam, 1 atm 46.29 43.5 925.degree. C.,
100 hrs, steam, 25 atm 6.37 92.2 1050.degree. C., 50 hrs 27.50 66.5
1050.degree. C., 100 hrs 12.20 85.1 Two-step S.A. 0.4 wt %
Mg/Al.sub.2O.sub.3/surf reduction m.sup.2/g (%) 1050.degree. C.,
100 hrs 48.84 S.A. Two-step 3% La/Al.sub.2O.sub.3 reduction
m.sup.2/g (%) 1000.degree. C., 4 hrs, air, 1 atm 105.30 925.degree.
C., 100 hrs, steam, 1 atm 59.30 43.7 925.degree. C., 100 hrs,
steam, 25 atm 3.28 96.9 1050.degree. C., 50 hrs 68.09 35.3
1050.degree. C., 100 hrs 79.50 24.5 Two-step S.A. 3%
La/Al.sub.2O.sub.3/surf reduction m.sup.2/g (%) 1000.degree. C., 4
hrs, air, 1 atm 65.33 1050.degree. C., 100 hrs 54.60 16.4
[0095] Performance and Stability in Methane Steam Reforming
Conditions
[0096] The FeCrAlY fins were used as support substrate for washcoat
materials. After heat-treatment of the fins to form a surface
oxide, they were coated with a layer of Dispal 14N4-25 alumina sol
obtained from SASOL and a layer of doping solution. The synthesis
method has been described earlier. Steam methane reforming (SMR)
reaction tests were carried out on fresh, hydrothermally-treated
and thermally-treated catalysts at 15 atm, 850.degree. C., S/C=3/1,
CT=4.5 ms. A fresh (not preaged) Rh/La/Al.sub.2O.sub.3 catalyst (25
mg/in.sup.2) tested over 750 hours of life testing, showed steady
methane conversion and carbon monoxide selectivity for
approximately 250 hours; however, activity of the catalyst showed a
continual decline over the next 500 hours. FIG. 4a shows activity
of catalyst whose support has been hydrothermally treated at 1 atm.
The support (La/Al.sub.2O.sub.3) was pre-treated under steam and
nitrogen mixture at 2 to 1 ratio at near ambient pressure (about
0.7 atm H.sub.2O) and heated (at a ramp up rate of 3.5.degree.
C./min) to 950.degree. C., 15 atm, and held at these conditions for
100 hours. Then, the support was cooled at about 3.5.degree.
C./min. The total loading of the catalyst was 30.3 mg/in.sup.2.
Over 500 hours on stream, methane conversion fell by 6%. FIG. 4b
shows the SMR activity over Rh/La/Al.sub.2O.sub.3 catalyst whose
support was thermally treated (at a ramp up rate of 3.5.degree.
C./min) to 1050.degree. C. and held at 1050.degree. C. for 50
hours. Then the support was cooled at about 3.5.degree. C./min. The
total catalyst loading was 31.4 mg/in.sup.2. Over 600 hours on
stream at the same conditions, the activity of the catalyst
decreased by less than 3% and did not experience the deactivation
that was observed in fresh or hydrothermally treated (at ambient
pressure) catalysts. This catalyst also survived system upsets
(inadvertent shut downs and start ups) without any loss of catalyst
performance.
[0097] A slurry of an La-doped alumina was coated onto a metal foil
and tested for 850 hours at 12 atm, 825.degree. C., S/C=2/1, CT=9
ms, showed a drop in methane conversion from 60% to 48% over the
first 600 hours on stream. Thus, the catalysts prepared from a sol
possess superior stability as compared with the slurry-derived
catalyst.
[0098] FIG. 5 shows the results of testing a catalyst of
Rh/MgAl.sub.2O.sub.4 whose support was prepared by sol coating a
finned substrate followed by steam pre-aging at 25 atm, 925.degree.
C. for 100 hours under H.sub.2O/He=2/1. The fin was then coated
with Rh nitrate solution (1% Rh) and calcined at 950.degree. C. for
1 hour. The metal loading was 13.6 mg/in.sup.2 (9.4 wt % Rh oxide
measured by weight gain after calcinations assuming all Rh in form
of Rh.sub.2O.sub.3). Methane steam reforming was conducted for 2800
hours of operation at 15 atm, 3 to 1 steam-to-carbon ratio, 4.5-3.0
ms contact time, and reactor wall temperature of 869.degree. C. A
step change in contact time was introduced at 2150 hours to move
further away from the 840.degree. C. equilibrium conversion. There
were no system upsets which contributed to a smooth operation for
2800 hours TOS. As can be seen, there was no change in activity for
over 2000 hours of continuous operation. Notably, methane
conversion remained at 70%.+-.3% for at least 600 hours (between
2200 and 2800 hours) at a catalyst operating temperature of less
than 900.degree. C. and a contact time of 5 ms or less.
[0099] FIG. 6 shows the results of testing a catalyst of
Rh/MgAl.sub.2O.sub.4 whose support was prepared by sol coating a
finned substrate followed by steam pre-aging at 25 atm, 925.degree.
C. for 100 hours under H.sub.2O/He=2/1. The metal loading was 20
mg/in.sup.2 (25 wt % Rh oxide). Methane steam reforming was
conducted at 27 atm, 3 to 1 steam-to-carbon ratio, 3.0-4.2 ms
contact time, and reactor wall temperature of 870-873.degree. C. At
650 hours time-on-stream (TOS), the steam to carbon ratio dropped
due to a water pump malfunction and resulted in a loss of activity;
however, after conditions were restored, the catalyst reached
another steady-state level at 64%. Thus, two periods of at least
400 hours of steady state operation were observed; one at 70%
conversion and one at 64% conversion. In FIG. 6, the horizontal
lines at 76 and 78% are equilibrium conversions at 840 and
850.degree. C., respectively. The vertical line at 90 hours
indicates the change to 3 ms contact time.
[0100] Performance of Thermal Pre-Aged Catalyst
[0101] FIG. 7 shows the results of testing a catalyst of
Rh/MgAl.sub.2O.sub.4 whose support was prepared by sol coating a
finned substrate followed by thermal pre-aging at 1050.degree. C.
for 50 hours in air. The metal loading was 31 mg/in.sup.2 (20 wt %
Rh oxide) (a thicker coating than the last example). Methane steam
reforming was conducted at 15 atm, 3 to 1 steam-to-carbon ratio,
3.04.5 ms contact time, and reactor wall temperature of about
840.degree. C. In FIG. 7, the two vertical lines at about 100 hours
and the vertical line at about 350 hours indicate system upsets.
After demonstrating a steady methane conversion at 88% for 600
hours, the conditions were changed to reflect a kinetically
controlled regime by lowering the contact time to 3.0 ms at 625
hours TOS without compensating for the drop in temperature due to
the higher flow rate used. The activity was steady for .about.200
hours. At 800 hrs on stream, furnace temperature was increased by
20.degree. C. (while maintaining the 3 ms contact time) to match
the temperature of the reactor at 4.5 ms contact time, and over the
next 500 hours, the catalyst steadily deactivated, dropping from
84% conversion to 78% conversion. After 1300 hours, the conditions
were changed back to the initial conditions (875.degree. C., 4 ms)
and it was found that the methane conversion level had dropped from
88% to 80%. In order to recover the initial conversion level of
methane (.about.88%), it was necessary to raise the furnace
temperature by 55.degree. C.
[0102] Performance of Non-Aged Catalyst
[0103] FIG. 8 shows the results of testing a catalyst of
Rh/LaAl.sub.2O.sub.4 whose support was prepared by sol coating a
finned substrate followed by drying. The metal loading was 25
mg/in.sup.2 Methane steam reforming was conducted for 750 hours. Up
to 250 hours, conditions were changed several times (vertical
lines) at 850 or 875.degree. C. and contact times between 2.5 and 6
ms with one system shut down at 25 hours. At 250 hours, conditions
were set to 15 atm, 3 to 1 steam-to-carbon ratio, 4.5 ms contact
time, and catalyst reactor wall temperature of about 875.degree. C.
By 750 hours TOS, methane conversion dropped 8%, probably due to a
phase change of .gamma.-Al.sub.2O.sub.3 to .alpha.-Al.sub.2O.sub.3
under the reaction conditions, which in turn reduced the surface
area significantly, thus, encapsulating some of Rh sites.
[0104] Effect of Reduction Temperature.
[0105] The temperature range over which Rh/MgO--Al.sub.2O.sub.3
catalysts (fresh and thermally stabilized) was reduced was studied
using temperature-programmed reduction (TPR) technique with 5%
H.sub.2 in Ar. FIG. 9 presents a plot of H.sub.2 consumption as a
function of temperature on the fresh catalyst that was oxidized
500.degree. C. There is a broad peak starting at 70.degree. C. and
tailing off to a baseline at about 280.degree. C. with multiple
shoulders, indicating hydrogen adsorption/absorption on the surface
and the bulk of the metal. The plot reveals that most of catalyst
reduction is completed at 200.degree. C. Similar results were
obtained on the pre-stabilized catalyst.
[0106] Metal dispersion of the fresh and pre-stabilized (PS) SMR
catalysts reduced at 125.degree. C. and 300.degree. C. were
measured using hydrogen chemisorption technique and presented in
Table 4.
2TABLE 4 Variation on dispersion % on catalysts supported on fresh
and pre-stabilized spinel with function of reduction temperature
Reduction T (.degree. C.) 10% Rh on fresh spinel 5% Rh on PS spinel
125 17.0 15.8 300 24.0 15.9
[0107] On 10% Rh on fresh spinel, the dispersion increased by 7%
while on the pre-stabilized support, there was only a small
improvement observed. This could be explained by the significant
difference in specific surface areas of the two supports (158
m.sup.2/g (fresh) vs 10 m.sup.2/g (pre-stabilized)).
[0108] The activity of the powder catalysts, which were diluted
with alpha alumina by 20 wt %, were measured as a function of
reduction temperature as shown in FIG. 10. Both catalysts (10%
Rh/fresh spinel and 5% Rh/pre-stabilized spinel) showed an
improvement in catalyst activity when they were reduced at
300.degree. C. rather than 125.degree. C. The relative increase in
methane conversion rate for fresh and pre-stabilized powder
catalysts were 22% and 13%, respectively.
[0109] Particle Size
[0110] On supports made of a hydrothermally-treated
magnesia-stabilized alumina support having a surface area about 10
m.sup.2/g, Rh nitrate solution was added using the incipient
wetness technique. The Rh was reduced at 300.degree. C. Hydrogen
chemisorption experiments showed the metal dispersion (i.e., the
percent of the total metal that is exposed on the surface) of 1%
Rh, 5% Rh, and 10% Rh on the pre-aged support to be 23%, 15%, and
10%, respectively. Transmission Electron Microscopic analysis of
the catalysts showed that the Rh particles on the 10% Rh/spinel
catalyst were about 1.5 nm in size. The Rh particle size on the 5%
Rh/preaged spinel catalyst ranged from 1.4 to 4.0 nm with an
average particle size of about 2.7 nm.
[0111] Catalyst Activity and Productivity
[0112] Catalyst activity and productivity were compared against the
best prior art literature (Wang et al., US published patent
application 2003/00317105). The approach temperature difference for
the catalyst and the gram moles of methane fed per gram Rh per
minute can be compared to the upper and lower limits of the
operating space described by the function below:
1.degree. C..times.N<.DELTA.T.sub.approach<50.degree.
C..times.N
[0113] where N is the gram moles of methane fed per minute per gram
Rh. For example, the values of N and .DELTA.T.sub.approach were
calculated for one condition of this study as follows: 0.122 SLPM
(or 0.00545 mol/min) of methane was fed with steam at a molar ratio
of 3:1 steam to carbon to an engineered catalyst loaded with
0.00024 grams Rh catalyst. Thus, for this example, N=(0.00545 mol
CH.sub.4/min)/(0.00024 g Rh)=23 mol CH.sub.4/min/g Rh. The average
of the measured inlet and outlet pressure across the engineered
catalyst at the time the data was taken was about 198 psig (14.7
bar). The calculated peak catalyst bed temperature at this same
condition was 856.degree. C. The measured methane conversion at
this condition (feeding 3:1 steam to carbon at 14.7 bar and
891.degree. C. at 23 mol CH.sub.4/min/g Rh) was 85.3%. Using the
NASA Lewis equilibrium code to calculate gas phase equilibrium for
a 3:1 steam to carbon mixture at 14.7 bar, it is found that the
equilibrium methane conversion to CO and CO.sub.2 is 85.3% at
825.degree. C. In other words, chemical equilibrium of a 3:1
steam:C mixture at 14.7 bar and 825.degree. C. represents a
conversion of 85.3% of the methane in the original mixture to
carbon monoxide and carbon dioxide. Therefore the
.DELTA.T.sub.approach for this example is 856.degree.
C.-825.degree. C.=31.degree. C.
[0114] The "approach temperature difference"
(.DELTA.T.sub.approach) is the difference between the "peak
catalyst temperature" (as defined in the Summary section for
methods of steam reforming) and the "apparent equilibrium
conversion temperature". The difference is a measure of catalyst
performance, with the difference increasing as the chemical
reaction kinetics of a given catalyst become slow enough that the
equilibrium composition is not reached at the average measured
reaction temperature.
[0115] The "apparent equilibrium conversion temperature" is the
apparent temperature based upon the conversion of the reactant in
question. It is the temperature required to produce an equilibrium
composition equivalent in reactant conversion to the measured
experimental conversion. The equilibrium composition is based upon
the measured average pressure of the inlet and outlet of the
reactor zone and the inlet molar composition. It is helpful to
obtain a curve of equilibrium conversion versus temperature to find
the apparent equilibrium conversion temperature. The equilibrium
distribution (or composition) for a given temperature, pressure,
and inlet mole fraction distribution can be calculated using Gibbs
free energies with programs such as the NASALEWIS equilibrium code
or FACTSAGE. The fractional conversion of a reactant is the
difference between the molar flow rate into and the molar flow rate
out of the reactor zone divided by the flow rate into the reactor
zone as shown below.
[0116] Fractional conversion=(moles methane in-moles methane
out)/(moles methane in)
[0117] Moles methane in=inlet flowrate of methane at STP/(22.4
L/mol)
[0118] Moles methane out=[outlet flowrate of total product dry
gas/(22.4 L/mol)].times.% methane in dry gas GC analysis
[0119] The data plotted in FIG. 11 shows lines corresponding to the
bounds set forth in equation 1.1 as well as data points from this
work (.circle-solid.) and the literature (.diamond-solid.). Some
preferred embodiments of the invention can be defined by the
.DELTA.T.sub.approach in the expression: 1.degree.
C..times.N<.DELTA.T.sub.approach<15.de- gree. C..times.N,
more preferably 1.degree. C..times.N<.DELTA.T.sub.app-
roach<5.degree. C..times.N and in some embodiments by the
expression:
1.5.degree. C..times.N<.DELTA.T.sub.approach<15.degree.
C..times.N
* * * * *