(267a) Developing a Quantitative Spatial Resolution of Deactivation Effects on FT Synthesis in a Microchannel Reactor When Operating with Ultra-Low Sulfur Levels

Developing a quantitative spatial resolution of deactivation
effects on FT synthesis in a microchannel reactor when operating with ultra-low
sulfur levels

S. Deshmukh¹, H. Becker², S. Kampfe¹,
K. Cowen¹, D. Leonarduzzi², L. Barrio², J.
Pritchard²,

H.J. Robota¹

Velocys

Plain City, OH¹ and Milton Park, UK²

Introduction

Fischer-Tropsch (FT) is receiving new attention for its ability to
generate conventional fuel from stranded, or unconventional natural gas, and
alternative renewable biomass sources. Being a catalytic process, the catalyst
is subject to deactivation via multiple mechanisms [1-2]. Sulfur is the
foremost among the poisons causing permanent catalyst deactivation. Despite the
impact of “S”, the number of published quantitative investigations related to
practical conditions is quite modest. 
While the recommendations for industrial syngas are typically <15 ppb
S, [3-5] other publications would suggest unobservable impact at much higher
exposures. [6,7]

In this paper, we present results of a detailed investigation of
sulfur poisoning in a fixed bed microchannel reactor, with an emphasis on
quantitative verification of delivered H₂S concentrations. The
poisoned catalyst was carefully extracted and analyzed to determine the spatial
distribution of sulfur along the catalyst bed and within catalyst particles.
This information is used to develop a quantitative model able to predict the
impact in a commercially operating microchannel FT reactor.

Feeding
H₂S augmented syngas

The
impact of contaminants, e.g. sulfur, on active Cobalt sites does not change
with catalyst operating productivity. However, high catalyst productivity
implies that the catalyst is exposed to a higher flux of these contaminants.
With catalyst productivities exceeding 2000 v/v/h, the Velocys reactor
passes up to 10-times the number of contaminant molecules per unit time
compared to a conventional fixed bed reactor.

Detection and accurate quantification of syngas contaminants at
low ppbv levels is challenging, as this approaches available equipment’s limit
of detection. Nonetheless, it is critical to understanding the impact of poison
concentration on catalyst deactivation rates. To accurately confirm the levels
of sulfur delivered to our reactors, measurements were made using a Proton
Transfer Reaction Mass Spectrometer (PTR-MS) at the point of delivery to the
catalyst. [8] Figure 1 shows calibration of the instrument for the detection of
ultra-low level H₂S in syngas feed with the PTR-MS, indicating
reliable measurements at <15 ppb detection levels.

A clean syngas feed was simulated by flowing desired amounts of
pure CO, H₂ and N₂ streams from cylinders using mass flow
controllers and blending to the target composition. Sulfur was introduced into the
clean syngas by replacing part of the N₂ feed with H₂S-doped
N₂ feed.  All tubing and
connections used in the contaminant flow path were treated with sulfur-resistant
inert coatings to avoid transmission losses to the surfaces.

Figure  SEQ Figure \* ARABIC
1. PTR_MS
calibration and response to ultra-low ppb level H₂S

Microchannel fixed bed testing

The microchannel testing apparatus and methodology used to
evaluate the catalyst and/or process performance has previously been described.
[9,10] A microchannel reactor with 1 process channel and 2 adjacent coolant
channels was loaded with a commercially manufactured, undiluted, particulate
catalyst in the process channel as a packed bed. FT synthesis was performed by
feeding clean and doped syngas as described earlier.

Figure
2 shows
the impact of ultra-low levels of H₂S in the syngas feed, 13 ppb in
this case, on a catalyst operating under representative commercial conditions
(H₂:CO = 1.79, 28% inerts, 310 ms contact time). The deactivation
rate, as measured by the decrease in CO conversion with time on stream, is
higher even with such ultra-low levels of H₂S present in the reactor
feed.

Figure  SEQ Figure \* ARABIC
2. Decline in CO
conversion with time during and after low level H₂S poisoning.

In order to understand the quantitative impact of sulfur poisoning
under practical extended operating conditions, an extended duration exposure of
a catalyst to low level H₂S in the feed was performed. As a
trade-off between readily quantifiable impact and the required operating
duration, the catalyst was exposed to 100 ppb H₂S for a period of
300 hours, which is equivalent to 1 year exposure to H₂S at a level
of ~4 ppb.

Catalyst deactivation commensurate with expectations from the
tests using lower levels of feed H₂S was observed. Upon completion
of the test, the reactor and catalyst were decommissioned to render the
catalyst safe for handling and analysis.

Figure  SEQ Figure \* ARABIC
3. simulated long
term exposure of catalyst to syngas operation in a laboratory single channel
reactor.  The loss of CO conversion with
time is readily visible during the 300 hour H₂S exposure.

Spent catalyst analysis

Spent catalyst from the “1 year exposure” test was collected in
aliquots of ~50 mg starting at the reactor inlet, with a spatial resolution of
1 cm. The samples were analyzed by ICP to quantify the sulfur accumulated in
each section. Based on our prior experience with more heavily S-exposed catalyst
charges, the top of the bed appears to be nearly saturated with sulfur (~5500
ppm)  with a concentration of S at
approximately 4500 ppm. The sulfur content then decreases linearly to ~500 ppm
at about 15% of the bed length. The subsequent axial sulfur profile is
relatively, flat but there is still quantifiable sulfur content at 35% of
the bed.

Figure  SEQ Figure \* ARABIC 4. Axial distribution of sulfur along the catalyst bed.

The recovered catalyst particles were characterized on a JEOL
6480LV SEM using a back-scattering detector and a beam voltage of 15 keV at
Oxford Materials Characterization Service. A sample at 10% of the bed length
(1875 ppm S) was first epoxy-embedded, polished, and coated with a 15 nm carbon
thickness to see the internal topology and composition by EDX analyzer. At this
point in the reactor, the examined catalyst granules had sulfur concentrated
near the superficial surface and not detectable in their interiors. An example
is shown in Figure 5 with the S:Co ratio as quantified by EDX at the locations
noted in Figure 5 summarized in Table 1.

Model development

Quantifying the impact of spatially distributed poisons in the
Velocys microchannel catalyst requires the use of a model which properly
accounts for the effects of both reaction kinetics and mass transfer
impact.  Such a model was developed using
gPROMS. It accounts for the convective transport of mass and enthalpy along the
catalyst bed and accounts for the diffusion of reactants into

Figure  SEQ Figure \* ARABIC
5. SEM/EDX
analysis of catalyst particles, showing the areas of composition analysis.

Table 1.  EDX
analysis of sulfur on catalyst particles LINK Excel.Sheet.12
"\\\\vmplab-dc01.vmplab.lan\\Data\\FT\\Post-operative
analysis\\EXP-15-DK8435 - S poisoning (1 year
equiv)\\SEM\\S_SEM_quanti.xlsx" "Sheet1!R4C1:R13C9" \a \f 5
\h  \* MERGEFORMAT

	Molar ratio
Spectrum #	Co/Si	mmol S /mol Co
Spectrum 1	1.1	20.9
Spectrum 2	1.4	1.5
Spectrum 3	1.2	2.0
Spectrum 4	1.0	3.4
Spectrum 5	1.3	2.6
Spectrum 6	1.2	15.0
Spectrum 7	0.5	32.4
Spectrum 8	1.8	21.0

the pores of the catalyst particle. On top of this steady state
model for the main FT reaction, a dynamic model describing the effects of
deactivation and poisoning is superimposed. This allows for the prediction of
the time dependent integral performance of the reactor. In addition to
conversion and selectivities, it also enables one to quantitatively estimate
the distribution of reactants, products, temperature, and catalyst activity.
The reaction kinetics were modeled using a modified form of the Yates and
Satterfield equation, with a factor, “F”, to account for the higher activity of
the Velocys catalyst. [11-13] Deactivation was modelled as a result of the
oxidation of metallic cobalt to inert cobalt oxide promoted by the presence of
water as described in literature. [14] The poisoning was included by balancing
the non-steady convective transport of H₂S in the gas phase and
non-steady diffusive transport inside the catalyst, where the sulfur adsorbs on
the active cobalt sites. This leads to a combined deactivation due to oxidation
and poisoning. An initial estimation of the deactivation parameters predicts a
distribution of sulfur after poisoning with 100ppb for 300h consistent with the
experimental observation and can simulate the distribution at for arbitrary
entry concentrations and exposure times (Figure 6).

 

Figure  SEQ Figure \* ARABIC
6. Time evolution
of the effective mass fraction of sulfur along the reactor axis (left) and mass
fraction profiles in the intra-particle direction at 12% of the bed length
(right).  Feed composition: 100 ppb
H₂S, H₂:CO 1.79, 28% inert, 310 ms contact time, 205 °C.

The deactivation rate of 0.08% per day after sulfur has been
switched off (Figure 2) was used to estimate the deactivation parameters
assigned to cobalt oxidation. With this fitted parameter the loss in activity
of 0.12% per day at 15 ppb sulfur was remarkably well predicted by the
simulation with a value of 0.121% per day. This enables the predictive
assessment of the effect of different levels of sulfur in the feed gas. Figure 7a
shows how the CO conversion declines for different H₂S feed levels after
the same overall amount of sulfur has been fed to the catalyst bed. At the
lowest sulfur level, a longer exposure time allows for the oxidation
contribution to have the largest impact, yielding the lowest final conversion.
This can also be seen in the final axial catalyst activity distribution shown
in Figure 7b. Poisoning accounts for the loss of activity close to the inlet,
which is similar for all H₂S levels at equivalent exposure. Due to
the longer run time at lower feed H₂S levels, the downstream part of
the bed, where the highest water partial pressures appear, is affected to the
greatest extent by oxidation.

 

Figure  SEQ Figure \* ARABIC
7. Conversion as
a function of time for different feed sulfur levels with total delivered sulfur
equivalent to 100 ppb for 100 hours (left). 
Final axial distribution of the relative catalyst effectiveness for each
of the different exposure conditions.

Conclusions

Velocys has used a high sensitivity analytical method to
accurately quantify the delivery of low ppb S levels in syngas feed to charges
of FT synthesis catalyst in microchannel reactors. With this method,
experiments were performed using doped and undoped syngas feeds to quantify the
impact of S poisoning on the catalyst deactivation rates. Analysis of spent
catalyst samples revealed gradients in concentration of S along the axial bed
length and in the radial catalyst granule direction. Models were developed and
validated to enable predictive assessment of the S poisoning in a commercial
reactor.

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