Combined heat and power (CHP) retrofit for supplemental on-site power generation at Engen oil refinery.

Masters Degree. University of KwaZulu-Natal, Durban.This thesis is a critical evaluation of an opportunity project for on-site generation of electricity at the ENGEN refinery in Durban, South Africa. The key equipment discussed is a 2.5MW special purpose backpressure turbine which (prior to July of 2014), operated in continuous service as a compressor prime mover. The availability of the turbine since the plant decommissioning, has drawn business interest in a retrofit service application as a turbo-generator capable of electrical power production if re-engineered with a an optimal gearbox and electrical generator configuration. The assessment method employed for the data extraction and calculations in this thesis is the “Plant performance triangle”. Historical and current process data are filtered for meaningful calculations and engineering analysis. Data segmentation methods are used to analyse the refinery operation at varying boiler loads where High Pressure (HP) steam at 40 barg is routed to the turbine and let down to 10 barg Medium Pressure (MP) header. The thesis evaluates the profitability of the devaluation of this steam by the isentropic steam expansion from thermal to mechanical to finally electrical energy, as opposed to isenthalpic (adiabatic) steam “let-down” (throttling) or pressure relief. The design basis for the turbine operation is 42 tons/hr high-pressure (HP) steam to the turbine casing inlet. Calculations show that between 2.0 MW to 2.5 MW of electrical energy generation is possible with minimal additional consumption of HP steam from the refinery HP header. This is due to the steam load balancing of five onsite boilers between the high and medium pressure steam header mains. In essence, additional MP steam for power generation is “let-down” into the MP header resulting in the back-up of HP to MP “let-down” from parallel boilers into the MP header. By this, the refinery demand for steam at varying pressure headers is adjusted by automated boiler advanced control. The resultant economic value of electricity cost savings is approximately (conservatively – based on 2016 electricity prices) R9.9m per year. Two key parameters in the techno-economic assessment are fuel gas (combustible energy) and treated feed water cost. The cost of boiler feed water is assumed a fixed cost to the operation, however since the refinery steam headers require a mere 2.37 additional tons of HP steam to support the new turbine operation, added water costs do not pose a significant operating expense. Sensitivities are performed on varying water costs (R/kL), as this is a factor of the project profitability given the scarce water availability challenges in South Africa

The metathesis activity and deactivation of heterogeneous metal oxide catalytic systems

Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2004.During this investigation it was found that the 8 wt% W03/SiO2 catalyst was effective for the metathetical conversion of 1-octene or 1-heptene to longer chain internal olefins in the detergent range. Various factors were investigated and the following conclusions were drawn: (a) Characterisation of catalysts with different metal loadings showed that crystalline W03 is the predominant species On catalyst with higher loadings. The W03 is well dispersed at lower loadings and is present a surface species. The activity of the catalyst increase with loading up to 6 wt% and is independent of any further increase in metal loading. This gives an indication that crystalline material does not contribute significantly to the metathesis activity. The stability of the catalysts is greater at loadings of 8 wt% and higher which suggests that crystalline material may play a stabilising role by preventing over-reduction of the active species. (b) Alkali metal ion doping can be used to significantly reduce the formation of branched metathesis products and is thus useful for curbing skeletal isomerization activity. Excessive doping (> 0.5%) results in loss of Bronsted acidity and hence a dramatic loss of metathesis activity. (c) The 8 wt% WO3/Si02 catalyst has a long lifetime (700 h) when operating in the recycle mode using the optimised conditions used (460°C, 16 h-I and 1:5.6 feed: recycle ratio). Regeneration of the catalyst results in a longer lifetime (1200 h) suggesting a decrease in acidity or better dispersion. Coke formation seems to be the cause of deactivation. The catalyst seems to coke from inside the pores and these act as reservoirs for the deposits. (d) Carbon maps (EFTEM) of the coked catalyst showed that carbon was located around the W03 clusters and did not cover them. This explains why the catalyst is still active even after the accumulation of excessive amounts of coke (e) Coke formation is dependent on a number of factors including temperature, time online, LHSV and amount of olefin. Trace quantities of oxygenates (100 ppm) can CONCLUSIONS 117 be used as coke retarding additives. These may act by blocking acid sites that result in reactions that lead to coke formation. (f) The catalyst is sensitive to the typical oxygenates (300 ppm in the recycle mode) present in an FT-derived feed stream however the effect of these poisons is reversible upon reintroduction of a pure feed stream. The oxygenates lower the intensity of the yellow colour of the product which is believed to be caused by polyaromatics. This effect may prove to be beneficial in terms of the quality of the final product.Master

The formation and influence of carbon on cobalt-based Fischer-Tropsch synthesis catalysts: An integrated review

The existing open and patent literature on the topic of carbon deposition was integrated with current work to provide a clearer understanding on the role of carbon as a deactivation mechanism of cobalt catalysts in the Fischer-Tropsch synthesis (FTS). Although the FTS did occur in the presence of active surface carbon and various hydrocarbon products, there may be a gradual transformation into inactive more stable forms of carbon in extended runs. This carbon could be possibly detrimental for catalyst activity. A deactivating polymeric carbon build-up may occur on Co/Pt/Al2O3 FTS catalysts, taken from a 100-bpd slurry bubble column reactor operated at commercially relevant FTS conditions. This polymeric carbon is located both on the support and on cobalt. This is an abstract of a paper presented at the 236th ACS National Meeting (Philadelphia, PA 8/17-21/2008)

Fundamental issues on practical Fischer–Tropsch catalysts : how surface science can help

The present article highlights the contribution of surface science and molecular modeling to the understanding of Fischer–Tropsch catalysis, in particular related to carbon-induced Co Fischer–Tropsch catalyst deactivation. The role of atomic and graphitic carbon in surface restructuring is discussed. Both forms of surface carbon stabilize surface roughness, while molecular CO promotes mobility of Co surface atoms. In a proposed chain growth mechanism on Co(0 0 0 1) chain elongation proceeds via alkylidyne + CH. The resulting acetylenic species is hydrogenated to alkylidyne, the route to further growth. (Cyclo-)polymerization of acetylenic species produces (aromatic) forms of polymeric surface carbon, a slow side reaction

The Effect of CO Partial Pressure on Important Kinetic Parameters of Methanation Reaction on Co-Based FTS Catalyst Studied by SSITKA-MS and Operando DRIFTS-MS Techniques

A 20 wt% Co-0.05 wt% Pt/γ-Al2O3 catalyst was investigated to obtain a fundamental understanding of the effect of CO partial pressure (constant H2 partial pressure) on important kinetic parameters of the methanation reaction (x vol% CO/25 vol% H2, x = 3, 5 and 7) by performing advanced transient isotopic and operando diffuse reflectance infrared Fourier transform spectroscopy–mass spectrometry (DRIFTS-MS) experiments. Steady State Isotopic Transient Kinetic Analysis (SSITKA) experiments conducted at 1.2 bar, 230 °C after 5 h in CO/H2 revealed that the surface coverages, θCO and θCHx and the mean residence times, τCO, and τCHx (s) of the reversibly adsorbed CO-s and active CHx-s (Cα) intermediates leading to CH4, respectively, increased with increasing CO partial pressure. On the contrary, the apparent activity (keff, s−1) of CHx-s intermediates, turnover frequency (TOF, s−1) of methanation reaction, and the CH4-selectivity (SCH4, %) were found to decrease. Transient isothermal hydrogenation (TIH) following the SSITKA step-gas switch provided important information regarding the reactivity and concentration of active (Cα) and inactive -CxHy (Cβ) carbonaceous species formed after 5 h in the CO/H2 reaction. The latter Cβ species were readily hydrogenated at 230 °C in 50%H2/Ar. The surface coverage of Cβ was found to vary only slightly with increasing CO partial pressure. Temperature-programmed hydrogenation (TPH) following SSITKA and TIH revealed that other types of inactive carbonaceous species (Cγ) were formed during Fischer-Tropsch Synthesis (FTS) and hydrogenated at elevated temperatures (250–550 °C). The amount of Cγ was found to significantly increase with increasing CO partial pressure. All carbonaceous species hydrogenated during TIH and TPH revealed large differences in their kinetics of hydrogenation with respect to the CO partial pressure in the CO/H2 reaction mixture. Operando DRIFTS-MS transient isothermal hydrogenation of adsorbed CO-s formed after 2 h in 5 vol% CO/25 vol% H2/Ar at 200 °C coupled with kinetic modeling (H-assisted CO hydrogenation) provided information regarding the relative reactivity (keff) for CH4 formation of the two kinds of linear-type adsorbed CO-s on the cobalt surface

The impact of cobalt aluminate formation on the deactivation of cobalt-based Fischer–Tropsch synthesis catalysts

It has been reported that cobalt aluminate formation is a cause of deactivation during Fischer–Tropsch synthesis (FTS), as it forms at the expense of active cobalt and is irreducible during FTS. To study this quantitatively, wax-coated Co/Pt/Al2O3 catalyst samples were removed periodically from an extended demonstration reactor FTS run operated at commercially relevant conditions and analysed with X-ray Absorption Near Edge Spectroscopy (XANES). With XANES, wax protected spent samples could be analysed in a pseudo in-situ mode. Under commercially relevant FTS conditions the catalyst undergoes reduction and minimal amounts of cobalt aluminate were found. It is proposed that the cobalt aluminate is formed from the residual CoO present in the catalyst after reduction. Additionally, the formation of aluminate was investigated with XANES and X-ray photoelectron spectroscopy (XPS) and TPR-MS on catalysts taken from laboratory continuous stirred tank reactor (CSTR) runs with varying water partial pressure (1–10 bar). Even at high water partial pressures (PH2O=10 bar, PH2O/PH2=2.2) only around 10% cobalt aluminate is formed while the metallic fraction of cobalt still increased compared to the fresh catalyst. The work shows that cobalt aluminate formation during FTS at realistic conditions is not a major deactivation mechanism

Providing fundamental and applied insights into Fischer-Tropsch catalysis: Sasol-Eindhoven University of Technology collaboration

Although Fischer-Tropsch synthesis (FTS) was discovered more than 90 years ago, it remains a fascinating topic, having relevance from both an industrial and academic perspective. FTS based on cobalt and iron catalysts was studied in depth during an extensive 15-year collaboration between Eindhoven University of Technology, The Netherlands, and Sasol, South Africa. The primary objective of the collaboration was to obtain fundamental information that could assist in understanding practical issues in FTS over iron and cobalt catalysts. For iron-based catalysts, industrial slurry reactor work was combined with SSITKA and DFT modeling, resulting in improved clarity, with respect to the kinetics and mechanisms of FTS. This knowledge is important, with respect to designing large-scale industrial processes. In the case of cobalt-based FTS research, the combination of commercially relevant supported cobalt catalysts with sophisticated characterization tools, as well as the application of flat model catalyst systems, has led to significantly improved knowledge of deactivation mechanisms. This improved knowledge has assisted in the understanding of new catalysts systems and regeneration processes. Finally, the success of the collaboration has been due to many factors. It has been beneficial to both parties to have had a long-term collaboration, in which important fundamental catalysis topics were investigated that often took a substantial period of time. The access to high-quality modeling and characterization tools and fundamental understanding, as well as industrially relevant supported catalysts operated under realistic conditions, has proved vital in our contribution toward the advancement of the science and technology of FTS

Role of Transient Co-Subcarbonyls in Ostwald Ripening Sintering of Cobalt Supported on γ‑Alumina Surfaces

The stability and mobility of atomic cobalt and of cobalt subcarbonyl species on γ-Al₂O₃ surfaces have been investigated using density functional theory (DFT) with a view to elucidate possible mobile species on these surfaces, which can act as agents in the Ostwald ripening process. The two most stable alumina surfaces γ-Al₂O₃(100) and γ-Al₂O₃(110) were probed at different levels of hydration. The stability of cobalt subcarbonyl species on γ-Al₂O₃(100) at high partial pressure of CO (10 bar) increases with increasing number of CO ligands attached to the central cobalt atom up to Co­(CO)₃ but exhibits a more complex behavior on γ-Al₂O₃(110). The effect of the hydration level on the stability of cobalt subcarbonyls was investigated. The interpretation of the DFT results in a thermodynamic model shows that at equilibrium the main cobalt subcarbonyl species present on the alumina surface at ca. 500K in the presence of CO are Co­(CO)₃ and Co­(CO)₄, with Co­(CO)₃ being the dominant species on dry γ-Al₂O₃(100) and wet γ-Al₂O₃(110). The fractional coverage of these species on a wetted alumina surface is lower than that on a dry alumina surface. The mobility of surface species was probed by exploring the potential energy surface of the adsorbed species on γ-Al₂O₃(100) and γ-Al₂O₃(110) at different hydration levels (Θ_OH = 8.5 and 17.7 OH/nm², respectively). Cobalt subcarbonyl species have a high mobility with activation barriers as low as 0.5 eV. It is argued that these species may contribute to the sintering process