The Impact of Operating Parameters on the Gas-Phase Sulfur Concentration after High Temperature Sulfur Sorption on a Supported Mo-Mn Sorbent

The impact of operating parameters on H2S capture from a syngas mixture by a Mo-promoted Mn-based high-temperature sorbent was investigated. The parameters investigated included temperature, space velocity, H2S concentration in the feed gas, and steam content. The H2S and SO2 concentrations in the gas after passing over a bed of the sorbent were analyzed and compared with thermodynamic calculations. The results confirmed that low temperature, low space velocity, low H2S concentration, and a dry feed were favorable for achieving a low residual concentration of sulfur compounds in the effluent gas. The sorbent was able to reduce the residual H2S concentration to below 1 ppm under all tested conditions. However, the unavoidable steam content in the gas phase had a significant adverse effect on sulfur removal from the gas. An empirical model, containing the three variables, i.e., temperature, space velocity, and H2S concentration in the feed, was developed and can be used to predict the effluent H2S residual concentration after treatment by the 15Mn8Mo sorbent.publishedVersio

Investigations of molybdenum-promoted manganese-based solid sorbents for H2S capture

The promotion effect of Mo-addition to alumina-supported Mn-based sorbents for high-temperature desulphurization was explored. A series of Mn-based sorbents with fixed Mn-loading and different Mo-loadings were prepared by the wet impregnation method and both fresh and used sorbents were characterized with respect to their physical and chemical properties. The sorbents were active for H2S removal at 600 °C and could be regenerated by oxidation in diluted air. The sorbents were subjected to 10 repeated sorption/regeneration cycles, and some loss of capacity occurred during the cycles. The results show that Mo-addition promotes the Mn-based sulfur sorbent performance both in terms of capacity and stability. Over the range investigated (0–8 wt% Mo added to a 15 wt% Mn sorbent), the improvement increased with an increasing amount of Mo added. The sample with the highest Mo-addition (15Mn8Mo) also retained the capacity best, as over 90% of the capacity remained after 10 sorption-regeneration cycles, in spite of suffering the most from sintering (observed as loss in surface area, increased pore size, and growth in Mn particle size). Characterization of the fresh and used samples using XRD and Raman spectroscopy indicates that a mixed Mn–Mo oxide suggested to be MnMoO4 plays a role in the promotion mechanism. The sorbent, 15Mn8Mo, is suggested to be promising for high-temperature desulphurization of bio-syngas.publishedVersio

Phosphorus deactivation on co-based catalysts for Fischer-Tropsch

The effect of phosphorus on a cobalt-based catalyst for Fischer-Tropsch Synthesis (FTS) has been investigated. Phosphorus is an impurity present in biomass and, in this work, its deposition on the catalyst during biomass to liquid (BTL) operation, based on gasification and FTS, has been mimicked. For this purpose, four different cobalt-manganese-rhenium catalysts supported on alumina were prepared by incipient wetness impregnation with different phosphorus loadings. The results showed that below 800 ppm of phosphorus, the catalysts performance was not significantly affected, possibly because the interaction of phosphorus mainly was with the alumina support. However, above this threshold, the effect of phosphorus was noticeable with a decrease in intrinsic activity. The reduced performance can be attributed to a physical blocking of cobalt sites. But, in all poisoned catalysts, the product distribution was affected and shifted towards less valuable products as methane and light paraffins. The electronegativity of phosphorus might be the cause for this effect, as the effective H2/CO ratio on the catalyst surface may be increased due to a weakened metal-CO bonding and consequently, the selectivity of the hydrogenated products increased.publishedVersio

Sorption-enhanced Fischer-Tropsch synthesis – Effect of water removal

publishedVersio

Modeling Fischer–Tropsch kinetics and product distribution over a cobalt catalyst

A detailed kinetic model describing the consumption of key components and productdistribution in the Fischer–Tropsch synthesis (FTS) over a 20%Co/0.5Re γ-Al2O3commercial catalyst is developed. The developed model incorporates the H2O-assisted CO dissociation mechanism developed by Rytter and Holmen and a novelapproach to product distribution modeling. The model parameters are optimizedagainst an experimental dataset comprising a range of process conditions: total pres-sure 2.0–2.2 MPa, temperature 210–230C, CO conversion range of 10%–75% andfeed with and without added water. The quality of the model fit measured in termsof mean absolute relative residuals (MARR) value is 23.1%, which is comparable to lit-erature reported values. The developed model can accurately describe both positiveand negative effects of water on the rate kinetics, the positive effect of water on thegrowth factor, temperature and syngas composition on the kinetics and product dis-tribution over a wide range of process conditions, which is critical for the design andoptimization of the Fisher–Tropsch reactors.publishedVersio

Noble Metal Promoted CoMn Catalysts for Fischer-Tropsch Synthesis

Noble metal (Pt, Re and Ru) promoters were investigated for Fischer–Tropsch synthesis at light olefin favouring conditions over Co–Mn catalysts. Characterisation and testing found promotion to increase catalyst activity without compromising the selectivity promoting effects of Mn. The noble metal promoted catalysts showed olefin and C5+ selectivities that were slightly increased compared to the unpromoted catalysts indicating some degree of influence on the selectivity

The impact of sequential H2-CO-H2 activation treatment on the structure and performance of cobalt based catalysts for the Fischer-Tropsch synthesis

acceptedVersio

The effect of aerosol-deposited ash components on a cobalt-based Fischer–Tropsch catalyst

Postprint version of published articleThe effect of ash salts on Co-based Fisher–Tropsch catalysts was studied using an aerosol deposition technique. The major elements in the ash were found to be K, S and Cl. The ash was deposited on a calcined catalyst as dry particles with an average diameter of approx. 350 nm. The loading of ash particles was varied by varying the time of exposure to the particles in a gas stream. Catalyst characterization did not reveal significant differences in cobalt dispersion, reducibility, surface area, pore size, or pore volume between the reference and the catalysts with ash particles deposited. Activity measurements showed that following a short exposure to the mixed ash salts (30 min), there were no significant loss of activity, but a minor change in selectivity of the catalyst . Extended exposure (60 min) led to some activity loss and changes in selectivity. However, extending the exposure time and thus the amount deposited as evidenced by elemental analysis did not lead to a further drop in activity. This behavior is different from that observed with pure potassium salts, and is suggested to be related to the larger size of the aerosol particles deposited. The large aerosol particles used here were probably not penetrating the catalyst bed, and to some extent formed an external layer on the catalyst bed. The ash salts are therefore not able to penetrate to the pore structure and reach the Co active centers, but are mixed with the catalyst and detected in the elemental analysis.acceptedVersio

Fischer–Tropsch Synthesis at High Conversions on Al2O3-Supported Co Catalysts with Different H2/CO Levels

Fischer-Tropsch experiments with different H2/CO ratios at high CO conversions have been carried out over an alumina-supported 20wt%Co-0.5wt%Re catalyst in a stainless steel fixed-bed reactor at 210 oC and 20 bar and with H2/CO ratios between 1.04 and 2.56. The results indicate that for H2/CO ratios above 2.1, CO conversion levels up to at least 85% can be obtained without significant short-term deactivation or loss of selectivity towards heavier hydrocarbons. Except for very low conversion anomalies, the data collapse into a single trendline for selectivity to C5+ products versus average hydrogen partial pressure in the reactor irrespective of the H2/CO feed ratio. The present results are important for development of small scale biomass to liquids plant based on a once-through process concept in order to avoid recycle streams

The water assisted vinylene mechanism for cobalt Fischer-Tropsch synthesis assessed by multi-catalyst modelling of kinetics and deactivation

The paper describes development of a mechanism and a consistent rate expression for Fischer-Tropsch (FT) synthesis over cobalt-based catalysts. The developed mechanism relies on a two-step hydrogen assisted activation of CO. The carbon atom of CO is first hydrogenated by surface hydrogen to formyl; followed by the rate-limiting step whereby the oxygen atom is hydrogenated by adsorbed water. The produced CH* monomer is incorporated into the growing chain giving vinylene intermediate. The vinylene intermediate is either terminated to an olefin by adding hydrogen to the α-carbon atom or propagates by adding hydrogen to the β-carbon position. The resulting expression for CO consumption, the Fischer-Tropsch rate, can respond positively or negatively to the partial pressure of water, in agreement with experimental observations. A special feature is that the chain propagation probability does not depend on the partial pressure of hydrogen. The resulting kinetic model is tested on several cobalt catalysts supported on alumina; spanning from γ-alumina with average pore sizes ranging from 6.1 to 18.3 nm to α-alumina with a wide pore structure; and with cobalt particle sizes from 8 to 19 nm. Water was added sequentially to the syngas feed, causing enhanced deactivation, for testing the water response on activity and selectivity. A deactivation model comprising sintering and cobalt oxidation, and the FT-kinetics, describe the observed CO conversions with great precision for all catalysts. Selectivities are also well described, but with slight deviations at least partly due the effect of deactivation. Trends in some of the kinetic parameters are rationalized in terms of cobalt crystallite and pore sizes