Ola Olsvik

Pyrolysis of natural gas: chemistry and process concepts

Abstract Methane can be converted directly to C 2 hydrocarbons by pyrolysis or thermal coupling. The reaction is highly endothermic and the heat must be supplied at high temperatures. Ethene, ethyne, benzene and hydrogen are the main products. Excessive carbon formation can be avoided using short reaction times and low partial pressures of methane preferably by hydrogen dilution of the feed. More than 90% selectivity of C 2 hydrocarbons may be obtained from methane. High yields of ethyne (>85%) are obtainable at extreme conditions of temperatures (> 2000 K) and short reaction times ( −2 s). The primary reactions during the pyrolysis of methane are now clearly defined. However, details of the later stages especially the formation of carbon (coke) are not yet fully understood. The formation of the main gas products can be described by simulation models based on elementary reactions. Pyrolysis of methane has been carried out commercially for many years. The processes differ mainly by the way heat is supplied to the reactor. At present, this technology seems to be an economical way of converting methane only under special conditions where ethyne is the desired product.

Deactivation during carbon dioxide reforming of methane over Ni catalyst: microkinetic analysis

The kinetics of CO 2 reforming of methane was investigated experimentally over a Ni/CaO-Al 2 O 3 catalyst at temperatures of 500 and 650°C, total pressures of 0.1 and 0.5 MPa and a CO 2 /CH 4 ratio of 1. The kinetic data including carbon formation and deactivation were measured simultaneously by an oscillating microbalance reactor. A microkinetic model was developed to analyze the kinetic data and extended with reactions for the carbon formation (encapsulating and filamentous) and deactivation. The deactivation was described at a microkinetic level, and the changes in activity and selectivity were related to the changes in the surface site coverage of different species caused by an increase in the surface site coverage of encapsulating carbon with time on stream. The deactivation rate depends significantly on the ensemble size required for each reaction. Ensemble sizes of 3, 4 and 6 were determined for the reforming reaction, water-gas shift reaction and encapsulating carbon forming reactions, respectively. The carbon formation deactivated faster than the dry reforming as a result of the larger ensemble size required for the carbon formation. Higher pressure and lower space velocity increased the carbon formation. The cyclical treatment of carbon formation and gasification reduced the carbon formation rate.

Dynamic simulation and optimization of a catalytic steam reformer

Abstract The operational performance of a heated fixed-bed reactor has been studied. Previous work indicates that the distribution of heat transfer resistances is important. No generally accepted correlation exists for the wall heat transfer coefficient and the effective radial thermal conductivity, and correlations proposed in the literature have been tested and evaluated. It is found that the outer reactor tube wall temperature is very sensitive to the applied correlation for the wall heat transfer coefficient and none of the evaluated correlations match the real situation perfectly. However, the methane conversion is rather insensitive to the choice of correlation. The effective radial thermal conductivity can be determined from the assumption of equal radial heat- and momentum Peclet number. It is found that using spatially varying physical properties and gas velocities only has a minor effect on the temperature distribution. Thus, the inlet conditions can be used to determine the effective radial thermal conductivity. By applying the correlation by De Wasch and Froment (1972) for the wall heat transfer coefficient, the resulting model is demonstrated for two simulation scenarios: (1) stop in steam supply and (2) stop in gas feed supply (CH 4 , H 2 , CO and CO 2 ). Finally, the optimal methane conversion is obtained for changing feed flow with a limiting value for the outer reactor tube wall temperature applied as a constraint.

Modelling and simulation of a steam reforming tube with furnace

A two-dimensional model of catalytic fixed-bed reactor located in a furnace chamber is developed and implemented for steam reforming of methane into synthesis gas. Partial differential equations describing conservation laws for mass, energy and radiation are solved, and compared to experimental values. Two concepts of steam reforming with different configuration of the furnace chamber is evaluated.

A Model for Reforming on Ni Catalyst with Carbon Formation and Deactivation

The effect of crystal size on carbon formation and sintering was studied on nickel catalysts at steam reforming conditions. Different nickel supported catalysts were examined. As support three commercial hydrotalcites were used: HT30 (MgO/Al2O3 = 3/7), HT50 (MgO/Al2O3 = 5/5) and HT70 (MgO/Al2O3 = 7/3). These supports were compared with CaO-Al2O3 and α-Al2O3. For the sintering experiments an industrial Ni/CaAl2O4 catalyst was used for comparison. The hydrotalcite derived catalysts had different Mg/Al ratios and the lowest Mg/Al ratio gave the highest Ni dispersion. The hydrotalcite derived catalysts also had a higher dispersion than NiO/CaO-Al2O3, NiO/α-Al2O3 and Ni/CaAl2O4.Carbon formation studies were performed in the tapered element oscillating microbalance (TEOM) at 823K, total pressure of 20 bar and steam to carbon (S/C) ratios of 0.08 to 2.4. The TEOM is a powerful tool for in situ catalyst characterization. All the feed gases pass through the catalysts bed and the TEOM offers a high mass resolution and a short response time. With an on-line gas chromatograph or mass spectrometer, catalyst activity and selectivity can be determined as a function of time. From the TEOM experiments it seemed that the Ni crystal size had a large effect on the carbon threshold value (S/C ratio where the carbon gasification rate equals the carbon deposition rate). Increased crystal size gave an increased carbon threshold value. It was concluded that small nickel crystals resulted in a large saturation concentration of carbon giving a low driving force for carbon diffusion and hence a lower coking rate. TOF increased with increasing Ni crystal size. This could be explained by surface inhomogeneities on the large crystals. Sintering experiments were performed at 903K and 20 bar in a fixed-bed reactor system. For all the catalysts the sintering mechanism involving particle migration seemed to be dominating. Due to a higher degree of wetting of the substrate by the nickel particle, the catalysts with smallest nickel particles showed the highest resistance towards sintering.Hydrogenolysis of methane was used as a probe reaction for testing the catalysts activity. An increased TOF with increased Ni particle size was observed. This result coincides with results from the steam methane reforming experiments in the TEOM.The characteristics of the hydrotalcite derived catalysts prepared by impregnation of commercial hydrotalcite supports were compared with hydrotalcite derived catalysts prepared by the co-precipitation method. An improved dispersion with decreasing Mg/Al ratio in the hydrotalcite was found. The catalysts prepared by the co-precipitation method maintained a high dispersion at increased nickel loadings. Different techniques were used to determine the Ni particle size. The results showed an excellent correlation between the Ni particle size found by chemisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM).

High Pressure Autothermal Reforming (HP ATR)

The High Pressure Autothermal Reformer (HP ATR) for production of synthesis gas has been evaluated. The aim of using HP ATR is to lower the plant investment costs by converting more gas per unit volume and eliminating the synthesis gas compression step, bringing the synthesis gas from 20-40 up to 70-100 bars, needed for the methanol and DME synthesis. The energy efficiency of the process may also be improved. Thermodynamic calculations have been performed showing the possibility to obtain a low methane slip at high pressures. Fluid flow simulations and combustion kinetic calculations are carried out to study the performance of the ATR in the high pressure range (70-100 bar). Detailed reaction mechanisms were used to study the pressure effect on the tendency to form higher hydrocarbons in the combustion chamber.

Statoil's gas conversion technologies

Abstract Gas conversion technologies play an important role with respect to bring gas to the market as both fuel and/or petrochemicals. Statoil has during the last 15 years gained wide experience by developing and/or demonstrating different important gas conversion technologies, e.g. methanol, GTL, and MTE

Selecting optimum syngas technology and process design for large scale conversion of natural gas into Fischer-Tropsch products (GTL) and Methanol

Abstract Four different syngas technologies; Conventional Steam Reforming (CSR), Partial Oxidation (POX), Autothermal Reforming (ATR) and Gas Heated Reforming (GHR) have been evaluated for GTL (Fischer-Tropsch) production and compared to a new syngas concept (TGR). The conclusion is that the GHR based syngas route and the new TGR based concept proved to be both more energy efficient (30% lower CO 2 emissions) and have lower investments than the other alternatives. For methanol production, ATR based syngas production seems to be preferred above 5000 MTPD. However, continued development of steam reformers have increased the maximum capacity range for “Two Step Reforming”. A number of new “hybrid” flowsheets have been announced which combine GHR, CSR, and ATR. These flowsheets seem to have a potential for competing with ATR based syngas concepts even up to 10,000 MTPD.

Estimation of a deactivation model for the methanol synthesis catalyst from historic process data

Publisher Summary This chapter discusses the estimation of a deactivation model for the methanol synthesis catalyst that includes the effect of temperature and water, based on historic process data from a methanol plant. A model on the generalized power-law form was successfully fitted to process data from a limited period of time. The estimated model is of second order. No measurable effect of water was found, probably, because the variations in the feed compositions were too small. The model parameters are not valid for the total catalyst lifetime, because the deactivation process is fast in the beginning and slower after some time. Data from a larger period of time is needed to estimate a model that is valid over the total catalyst lifetime. The historic process data contains enough information to estimate a catalyst deactivation model that describes the effect of temperature but too little information to estimate the effect of the reaction-mixture composition.

Quenching of product gases from the prolysis of methane

Abstract Experiments in a tubular flow reactor with an indirect oil quencher were carried out to determine the temperatures at which the elementary reactions in thermal coupling of methane were terminated. The results indicated that it is not necessary to quench the product gas to room temperature. Simulations showed that very few reactions take place at temperatures below 800 °C in the residence time range from 0 to 1 s. This shows the possibility of recovering energy from the product gases. The yield of ethylene increased whereas the yield of acetylene decreased when the temperature in the lower part of the reactor system was decreased. The experiments were verified with simulations. The results support the idea that it may be possible to optimize the ethylene/acetylene ratio with respect to quenching time and temperature.