Download Meeting future SO2 emission challenges with Topsøe`s new

From nano-scale studies of working sulfuric acid catalysts to improved
industrial-scale sulfuric acid production
Kurt Christensen, Filippo Cavalca, Pablo Beato, Jens Hyldtoft and Stig Helveg
Haldor Topsoe A/S
Nymøllevej 55, DK-2800 Kgs.Lyngby, Denmark
Prepared for
39th International Phosphate Fertilizer &
Sulfuric Acid Technology Conference
Clearwater Beach, Florida
June 5th & 6th, 2015
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TABLE OF CONTENTS
INTRODUCTION ...................................................................................................3
BACKGROUND ...................................................................................................... 3
DYNAMICS OF THE ACTIVE PHASE............................................................... 5
CHEMISTRY AND COLORS ............................................................................. 10
CONCLUSIONS .................................................................................................... 14
REFERENCES....................................................................................................... 15
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INTRODUCTION
The main purpose of a catalyst is to increase the rate of chemical reactions without
being consumed. This is also the main function of the V2O5-based sulfuric acid
catalyst, which has been used for more than 50 years for SO2 oxidation in sulfuric
acid production:
SO2 + ½ O2 ⇌ SO3 + heat
(1)
In general, however, a catalyst is also a dynamic system for which the detailed
chemical composition and nano-scale structure depend on the composition,
temperature and pressure of the surrounding gas. As a consequence, the catalyst
adopts different conformations in the individual beds of a sulfuric acid converter,
even if the same catalyst type were loaded in all beds. One effect of the dynamic
nature of the catalyst is that after a change of feed gas composition, plant load or
bed temperatures, the catalyst will adjust to the new local conditions on a time
scale from minutes to a couple of days depending primarily on the temperature.
Another implication of the dynamic behavior is the different colors of the operating
catalyst that may also sometimes be observed after a shutdown depending on the
shutdown procedure.
As a catalyst producer, Topsoe continuously aims to improve the fundamental
understanding of catalysis in the industrial environments and to apply this
knowledge to rationally develop better catalysts for improved efficiency of
industrial sulfuric acid production. In the current paper, we present results from
novel advanced in situ techniques recently introduced in our laboratories and
combine the findings with bench-scale investigations and large-scale observations
from industrial plants to obtain a more complete picture of the working catalyst.
BACKGROUND
Commercial sulfuric acid catalysts are based on V2O5 mixed with alkali-metal
sulfates on an inactive porous silica support. The carrier is usually made from
diatomaceous earth, and the catalyst typically contains 2-5 wt% vanadium and 2-5
moles of alkali metal promoter per mole of vanadium. The alkali promoters are
typically potassium, sodium and cesium, and the catalysts are available as
cylindrical pellets, cylindrical rings or finned rings resembling a Daisy as shown in
Figure 1.
Figure 1. Sulfuric acid catalyst from Topsoe in the Daisy shape.
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Upon start-up, the catalyst takes up sulfur oxides from the gas environment to form
molten alkali pyrosulfates:
M2SO4 + SO3(g) ⇌ M2S2O7
M2S2O7 + SO3(g) ⇌ M2S3O10
(M=Na, K, Cs)
(2)
In this type of supported liquid phase (SLP) catalysts, the oxidation of SO2 takes
place as a homogeneous reaction in a liquid film covering the internal surface of
the support material [1, 2]. The promoting role of the alkali metals is commonly
attributed to their ability to form these relatively low-temperature-melting
pyrosulfates, which dissolve the vanadium oxides. The amount of sulfur oxides
bound as alkali metal pyrosulfates depends on the actual reaction conditions. Under
circumstances for which the catalyst is blown with hot air for a sufficiently long
time, up to 10% of the catalyst weight is desorbed as SO2/SO3 leaving the alkali
metals in the form of sulfates, M2SO4. The catalytic activity of the melt was
demonstrated conclusively in 1948 by Topsøe and Nielsen [3], who obtained a high
degree of conversion to SO3 by bubbling SO2 and oxygen through a column packed
with raschig rings and containing molten potassium pyrosulfate in which 14% wt%
V2O5 was dissolved. Since then, numerous investigations on the coordination
chemistry of the catalytic melt and the reaction mechanism have been published,
but in spite of this the reaction mechanism is not understood in detail yet.
For many years, the mechanism was assumed to include reduction of vanadium to
V(IV) and re-oxidation to V(V) by oxygen as proposed by among others Mars and
Maessen [4]. However, in 1968-1973 Boreskov and coworkers [5, 6] found from
transient kinetic studies a number of facts contradicting previously suggested redox
mechanisms, for example that the rate of SO2 oxidation is much higher than the
rate of V reduction. Numerous subsequent studies have confirmed that an
associative cycle including only V(V) complexes is the governing mechanism [7]
as illustrated in Figure 2.
V
SO3
-4
(V O)2O(SO4)4
4
-4
SO2
IV
-2
2V O(SO4)2
O2
2VIVOSO4 (s)
1
(VVO)2O(SO4)4SO3
3
-2
2SO4
SO3
SO2
(VVO)2O(SO4)4O2-4
2
(VVO)2O(SO4)4O-4
SO2
SO3
Figure 2. The catalytic cycle of a sulfuric acid catalyst. Adapted from [7].
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However, V(IV) compounds still play an important role for the activity of the
catalyst, because an equilibrium exists between V(V) and V(IV) compounds in the
melt. The degree of reduction to inactive V(IV) increases at low temperature and
high SO2 partial pressure. Furthermore, at temperatures below 500°C, part of the
V(IV) compounds precipitate and cause depletion of V(V) in the melt, and this
partial solidification eventually causes the activity to drop to practically zero at
minimum operating temperatures of 300-350°C. The reduction to V(IV) shown in
Figure 2 is not a deactivation but rather a dynamic and reversible response of the
catalyst to the gas environment. That is, increasing temperature or lowering SO2
partial pressure results in a recovery of the catalyst activity.
Understanding the complex composition of the catalytic melt is a scientific
challenge that requires advanced characterization techniques in itself. However, the
dynamic response of the catalyst to the gas environment is an additional severe
complication, as ex situ studies of the cold catalyst in air, inert gas or vacuum may
lead to erroneous conclusions. Hence, new advanced in situ techniques are needed
to make further progress in the understanding of the dynamics and mechanism of
the sulfuric acid catalyst. We have met this challenge by introducing novel
experimental methods. In the following sections, examples and learnings from our
current studies at Topsoe are presented.
DYNAMICS OF THE ACTIVE PHASE
The catalytic melt of a sulfuric acid catalyst typically accounts for 20-40% of the
catalyst mass. The melt is considered mobile under operating conditions, although
the dynamic behavior is difficult to address in detail at room temperature, where
the active phase appears as solid lumps of oxides and sulfates of vanadium and
alkali metals on the internal surface of the carrier as shown in Figure 3. The
cylindrical structures seen in the figure are diatom skeletal fractions, which
originate from the diatomaceous earth typically used as carrier.
Figure 3. Scanning Electron Micrograph (SEM) of the inner surface of a
cleaved sulfuric acid catalyst.
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When heated and put on stream in SO2/O2/SO3 gas, the sulfates take up SO3 and
form pyrosulfates according to (2). This transformation causes the active phase to
melt and redistribute on the carrier. Due to capillary forces, however, the liquid
stays in the pores of the carrier. This redistribution is illustrated in Figure 4. In this
lab experiment, a pelletized porous silica carrier was partially impregnated by
submerging one end of the pellets for 30 seconds into an impregnation liquor
containing appropriate salts of V, K and Na. After calcination in air at 400°C for 15
min, visual inspection and scanning electron microscope (SEM) energy-dispersive
X-ray spectrometry (EDX) show how one end is fully impregnated while V, K and
Na are absent in the other end, cf. Figure 4a. Subsequently, the pellets were put on
stream in a glass reactor in a feed gas with 10% SO2 and 10% O2 at 500°C for three
days. After this treatment, the same analyses demonstrate a uniform color and
distribution of V, K and Na over the pellet, revealing a marked migration of the
active phase to the unimpregnated end of the pellets, as shown in Figure 4b.
Importantly, this redistribution is associated with a 50% increase of catalytic
activity from its initial value until the test termination.
Figure 4. Redistribution of the active phase in a sulfuric acid catalyst. Photo
and SEM EDX line scan of the sample loaded in glass reactors a)
before test and b) after three days on stream.
In situ Transmission Electron Microscopy (TEM)
The lab experiment described above clearly shows the mobility of the catalytic
melt in the sulfuric acid catalyst, but in order to directly unlock the state of the melt
in the working catalyst and its dynamic behavior in the heterogeneous pore system,
direct observations at nano-scale resolution are needed. We have therefore recently
introduced transmission electron microscopy (TEM) as a novel means for directly
monitoring catalyst samples reacting with an SO2/O2 gas mixture at temperatures
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up to 600°C and at nanometer resolution. These time-resolved observations provide
unprecedented new insight into e.g. a simulated converter startup.
As the industrial catalyst is quite heterogeneous across the micro- and nano-scales
(cf. Figure 3), a more well-defined model system is used for the initial studies. The
model catalyst was prepared by impregnation of 100 nm monodispersed silica
spheres with a mixture of vanadium oxide and alkali pyrosulfate to a composition
of 4.2 wt% V content and molar ratios of K/V=3.5 and Cs/V=0.4. The as-prepared
sample was investigated in a Philips CM300 transmission electron microscope in
vacuum (at about 10-9 bar) using a TIETZ F-114 charged-coupled device camera
for TEM imaging and a Gatan Imaging Filter (GIF) for electron energy loss
spectroscopy (EELS) and energy-filtered TEM (EFTEM). Figure 5 shows
combined TEM and V L2,3 EFTEM images revealing that the active phase of the
fresh catalyst is present in more concentrated lumps of material (marked by red
circles). The combined use of these techniques demonstrates that the vanadia phase
can be mapped out at nanometer resolution in the as-prepared catalyst.
Figure 5. A cluster of silica spheres with the vanadia phase resolved by TEM
(left) and V L2,3 ETEM (right). The red circles indicate regions of
relative higher vanadium concentration.
Whereas a TEM normally operates at ambient temperature and very low pressure
in the order of 10-9 bar, the CM300 transmission electron microscope is equipped
with a differentially pumped vacuum system that allows solid samples to be
exposed to reactive gas environments up to 1-10 mbar in pressure and up to 600°C
while still maintaining atomic resolution [8], cf. Figure 6. Furthermore, as a unique
capability, this particular microscope is entirely dedicated to studies involving
sulfur-containing gases, in relation to e.g. hydrodesulfurization and sulfuric acid
catalysts, in order to avoid that cross-contamination and sulfur-corrosion of the
microscope’s interior parts interfere with other experiments [9].
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In the present study, crushed catalyst powder was loaded on a micro-electromechanical-system (MEMS) heater and placed in the microscope. After an initial
inspection in vacuum, the samples were exposed to a gas mixture of 50% SO2 and
50% O2 at a total pressure of 10 mbar. Although this total pressure is low compared
to that in industrial reactors operating at 1.1-1.4 bar, it is much closer to industrial
values than a typical TEM operating pressure, and a partial pressure of SO2 of 5
mbar is still sufficient to keep the catalyst in its relevant pyrosulfate form.
Furthermore, the effects of the electron beam on the observations have been
investigated to establish experimental protocols in which the electron microscopy
observations are negligibly affected by the electron beam.
Figure 6. A photo of Topsoe’s in situ CM300 transmission electron
microscope dedicated to S-rich experiments. The reactive gas
environments are confined to the sample region by means of a
differential pumping system (sketch adapted from [8]). The gas
composition in the experiment is monitored by the ion current from a
mass-spectrometer attached to the second differential pumping stage
(graph).
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Figure 7 shows results from a heating of the as-prepared catalyst sample in the gas
environment. Up to 350°C, the TEM images show that the nano-scale structure of
the catalyst resembles the as-prepared sample as illustrated in Figure 7a. Upon
heating to higher temperature, the images reveal significant dynamic changes in the
catalyst structure. For instance Figure 7b shows that a redistribution of mass has
occurred as the sample is heated to 600°C, corresponding to the high temperature
found in the lower part of the first bed of typical sulfuric acid converters.
Subsequent cooling to 350°C and a repeated heating to 600°C results in even more
pronounced changes (Fig. 7c). Several TEM pictures taken during the experiment
are shown consecutively as a movie providing the first-ever vivid impression of
liquid phase restribution in the catalyst. Such dynamic observations open up
possibilities to extract information on wetting properties and dynamics of melt
distribution. Moreover, the observations can also be coupled with chemical
composition and catalytic activity from e.g. operando Raman (see next section).
Figure 7. In situ TEM of dynamic changes of the model catalyst during
exposure to 10 mbar of 50% SO2 and 50% O2 (a) at 350°C (b) at
600°C and (c) after cooling to 350°C and reheating to 600°C. The
SiO2 spheres have a diameter of ca. 100 nm.
Liquid loading and dust protection
One learning from the dynamic studies of the active phase is that activation of the
catalyst distributes the melt on the carrier in order to reduce the surface free energy
of the combined carrier-melt system at the prevailing temperature and gas
composition. At steady-state, the distribution depends on the surface properties of
the silica support, e.g. the wettability, surface tension and carrier morphology.
However, in the context of catalyst design another related and very important
parameter of an SLP catalyst is the liquid loading defined as the fraction of the pore
volume of the support filled with melt. This is investigated in Figure 8, where we
have varied the melt load on a catalyst in our lab and measured activity in 10%
SO2, 10% O2 and 500°C. The activity of the catalyst increases proportionally to the
liquid loading (which is proportional to the vanadium content) at low loadings but
decreases at high loading, which is explained by gas phase pore diffusion
restriction and liquid diffusion restriction in the deeper melt pools. As a
consequence, an optimum melt loading exists for a given melt composition and
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operating condition. Another implication is that higher vanadium content is not
always better for activity.
The melt normally stays in the pores of the porous catalyst in the attempt to reduce
total surface energy. However, if the catalyst is covered in dust, especially fine dust
with high surface area, part of the melt will gradually migrate to the smaller pores
of the dust causing depletion of vanadium and alkali metals. Depending on the
initial melt loading, the consequence for activity may be modest in the beginning
but at high dust build-up and repeated screening the conversion will eventually
drop (see Figure 8) and call for catalyst replacement.
Figure 8. Activity vs. liquid loading of a sulfuric acid catalyst prepared in lab.
Another consequence of the molten active phase is the stickiness of the sulfuric
acid catalyst under operating conditions. As a result, dust present in feed gases in
sulfuric acid plants typically deposits and accumulates within the top 10-15 cm of
catalyst in the first bed of the SO2 converter. Depending on the dust concentration,
particle size, void fraction and catalyst stickiness, this leads to a pressure drop
build-up over time which eventually may cause a costly shutdown of the plant
necessary for screening of the catalyst. A simple way to prolong the time between
screenings is to install a 10-15-cm top layer of VK38 25-mm Daisy, which
distributes the dust further down the bed due to its larger size and high void
fraction [10]. The top layer should be changed after a few catalyst screenings as it
will lose some of its stickiness when the melt is depleted by the fine dust.
CHEMISTRY AND COLORS
Sulfuric acid catalysts are normally characterized by yellow, gold or orange colors
indicative of vanadium in +5 oxidation state V(V). Catalyst exhibiting a green, pale
green or pale blue color are considered to contain vanadium in the +4 oxidation
state V(IV). As the oxidation state couples to the reaction environment, it will
change and adapt to the surroundings. For instance, if the catalyst is exposed to
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moisture, the appearance and colors also change. When a catalyst is put on stream,
the catalyst composition adjusts to the gas composition and temperature, for
example with respect to the V(V)/V(IV) equilibrium illustrated in Figure 2. As a
consequence, the catalyst color also changes, although this is not visible in a steel
converter. Such changes will also happen during shutdown and depend on the
applied procedure. All in all, the color of a catalyst is not a valid method for
assessing how it will perform or have performed in a converter.
Operando Raman
In order to study the chemical transformations taking place in the operating sulfuric
acid catalyst, we are using an advanced operando Raman setup enabling both the
chemical composition to be studied by Raman spectroscopy and the sample color
to be observed by visual inspection [11]. Laser Raman spectroscopy provides a
vibrational spectrum, which contains information on the chemical bonding in the
sample. Operando Raman spectroscopy is a technique that is able to monitor the
structural changes in a sample on a molecular level during the reaction. For
operando Raman studies it is particularly important to avoid laser-induced heating,
which, in the worst case, can lead to thermal degradation of the sample. To
alleviate this problem, we have developed a novel technique that allows the sample
to be studied in a small fluidized bed reactor for securing uniform and well-defined
temperature, cf. Figure 9.
Figure 9. Detailed sketch of the Linkam CCR1000 reactor used for in situ
Raman (a) and schematic drawing of the sample holder with flow
indications by arrows to illustrate the fluidization principle (b) [11].
In order to simulate a converter startup, the Raman reactor was loaded with a 150300 µm sieve fraction of crushed VK38, and the fluidized bed of catalyst heated
first to 380°C in air at 1 atm. Under this condition, the catalyst is yellow/orange,
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and the Raman spectrum shown in Figure 10a resembles some characteristic
features of the dimeric V(V) complex, which are shown as the active species in
Figure 2.
In particular, the two bands at ∼860 and 770 cm−1, which are assigned to V-O-S
and V-O-V bridging bonds, have been claimed to be the signature of the active
catalyst [12]. The lack of V(V)-vanadyl bands (typically at ~1040 cm-1) indicates
the extended polymeric nature of the catalyst under this condition. After switching
the reactor feed gas to 10% SO2 and 10% O2 in nitrogen at 380°C in Figure 10b,
the catalyst color changes to greenish, and the Raman spectrum reveals structural
transformation from a polymeric vanadate to the well-defined molecular structure
of the (VO)3(SO4)54- ion implying a reduction of V(V) to V(IV). At this point, the
conversion of SO2 measured by infrared (IR) SO2 analysis of the effluent is very
low. As the temperature is raised to 480°C, a significant increase in conversion is
observed, and the color changes from greenish to ocher, as seen in Figure 10c. The
corresponding Raman spectrum shows a band centered at 1043 cm−1 characteristic
for V(V)-vanadyl bonds and a broad and structured band between 940 and 985
cm−1 together with two bands at 665 and 605 cm−1 assigned to the vibrational
modes of the SO4 ligands in different vanadium oxosulfate complexes.
From a detailed analysis of the spectra, it is possible to conclude that the active
phase of the industrial VK38 catalyst at 480°C corresponds to a mixture of
predominantly monomeric and to a minor extent dimeric vanadium(V) oxosulfate
species, which fits well with the proposed structures in Figure 2.
Figure 10. Operando Raman spectra while switching from synthetic air (a) to
SO2/O2 (b) at 380°C and in SO2/O2 at 480°C (c). The photographs to
the left of the spectra show the catalyst in the reactor under the
respective reaction conditions [11].
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Colors in industry
The studies presented above show that the composition and color of the operating
catalyst change with the temperature and gas composition. Catalyst containing
vanadium primarily in oxidation state +5 is yellow/orange, but if it is loaded in a
converter and operating at low temperature or high partial pressure of SO2, the
color will change to greenish/bluish because these conditions favor reversible
reduction of vanadium to oxidation state +4. Also the catalyst may change visual
appearance during a converter shutdown depending on the purging and cooling
procedure or during storage. When purged with air while the catalyst is still hot,
vanadium will be oxidized to V(V) and attain a yellow color even if it was green
when operating in the converter in e.g. the top of the first bed. As an example,
Figure 11 shows how a greenish catalyst sample from industry can change color to
yellow when heated in an air flow in a glass tube for 24 hours at 500°C.
Figure 11. Colors alone cannot be used for assessment of the catalyst
performance. Here reduced vanadium is reoxidized by heating of a
greenish sample in air.
Catalyst design for optimal performance
The reaction rate for a catalyst may be limited either by external mass and heat
transfer to the pellet surface, by internal mass transfer in the porous support or by
the intrinsic reaction rate itself. For sulfuric acid catalysts operating at a low
temperature, the main limitation is the intrinsic reaction rate, which depends on the
chemistry in the catalytic melt. In particular, as the molecular turnovers happens on
vanadia species in the melt, accessibility to the bulk of the melt by the gaseous
reactants is important to optimize activity and it is therefore crucial to control the
distribution of the catalytic melt on the support and gas solubility and transport
through the melt. The present in situ techniques provide valuable insight into the
mechanisms governing the intrinsic catalyst performance, and the learnings are
integrated in Topsoe’s continuous developments of improved industrial catalysts.
One example is the significant changes of the melt properties, upon replacement of
part of the potassium and sodium sulfates by cesium sulfates. The introduction and
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optimization of cesium-promoted catalysts, e.g. VK59 and VK69, for lowtemperature operation had a significant impact on possible inlet temperatures and
SO2 emissions in industry [13]. The fundamental studies also revealed how the
activity of normal commercial sulfuric acid catalysts is seriously surpressed by
deep pools of catalytic melt restricting gas transport in the active phase. These
limitations have been overcome in the new high-activity VK-701 catalyst based on
our novel LEAP5™ technology [14]. The improvements include a very high V(V)
content and were accomplished by changing the intrinsic morphology and surface
properties of the carrier and optimizing the active phase for high SO3
concentration.
CONCLUSIONS
Catalysts are dynamic systems that adapt to the local environment in which they
operate. As a consequence, in situ studies at relevant temperatures, pressure and
gas composition are necessary to get a true picture of the working catalysts. This is
particularly true for sulfuric acid catalysts for which the active phase is a liquid
making up about one third of the catalyst mass, i.e. a so-called supported liquid
phase (SLP) catalyst. In order to look directly at the state of the working catalyst,
we have recently introduced new advanced in situ techniques including highresolution transmission electron microscopy and Raman spectroscopy to directly
resolve the dynamic state of catalyst samples interacting with an SO2/O2/SO3 gas
mixture at temperatures up to 600°C. These techniques provide unprecedented
insight into sulfuric acid catalysis.
In situ high resolution TEM studies combined with lab-scale results show how the
active phase takes up SO3 and becomes mobile. From time-resolved series of TEM
images, the rate of the dynamic changes and the final wetting characteristics and
melt distribution can be derived. The operando Raman observations demonstrate
that color changes occurring during a simulated converter startup are directly
related to measured catalyst chemistry and observed SO2 conversion. The results
indicate that the vanadium changes from a dimeric V(V) complex in hot air to
primarily V(IV) compounds in SO2/O2 gas at 380°C. When the temperature is
increased to 480°C, vanadium is reoxidized, and the spectral information obtained
supports that the active phase of the industrial VK38 catalyst at 480°C corresponds
to an equilibrium between predominantly monomeric and to a minor extent dimeric
vanadium(V) oxosulfate species.
The studies presented here show that the melt distribution and chemistry interact
with the gas environment and that its physical and dynamical state is just as
important as the gross composition of a sulfuric acid catalyst for understanding and
controlling activity. Moreover, the detailed in situ observations also help
explaining macro-scale phenomena in the industrial converter such as catalyst
colors, oxidation states, melt transformation, melt extraction by dust, pressure drop
build-up and catalytic activity.
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