Harry Verelst

Pore-filling-dependent selectivity effects in the vapor-phase separation of xylene isomers on the metal-organic framework MIL-47.

Vapor-phase adsorption and separation of the C8 alkylaromatic components p-xylene, m-xylene, o-xylene, and ethylbenzene on the metal-organic framework MIL-47 have been studied. Low coverage Henry adsorption constants and adsorption enthalpies were determined using the pulse chromatographic technique at temperatures between 230 and 290 degrees C. The four C8 alkylaromatic components have comparable Henry constants and adsorption enthalpies. Adsorption isotherms of the pure components were determined using the gravimetric technique at 70, 110, and 150 degrees C. The adsorption capacity and steepness of the isotherms differs among the components and are strongly temperature dependent. Breakthrough experiments with several binary mixtures were performed at 70-150 degrees C and varying total hydrocarbon pressure from 0.0004 to 0.05 bar. Separation of the different isomers could be achieved. In general, it was found that the adsorption selectivity increases with increasing partial pressure or degree of pore filling. The separation at a high degree of pore filling in the vapor phase is a result of differences in packing modes of the C8 alkylaromatic components in the pores of MIL-47.

Local and global dispersion effects in Couette-Taylor flow—I. Description and modeling of the dispersion effects

The Couette-Taylor flow (CTF) is a periodic flow with zero mean axial velocity. Its local and global dispersion behaviour has been investigated by recording the rate of tracer dispersion at the injection point. This allowed the demonstration of the existence of a strong inter-vortex flux, thereby refuting the generally accepted concept of non-intermixing vortex units. The tracer responses depended strongly upon which specific part of the vortex was marked with tracer. This shows that the axial dispersion cannot be described by the single-parameter models presently used in the literature. A first principles two-parameter model, based upon the newly obtained insights, is proposed. It offers a unifying framework for all observations and resolves the contradictions encountered in the literature. The response dynamics of the model have been investigated to identify the possible parameter estimation strategies and in order to predict the long time limit behaviour.

Local and global dispersion effects in Couette-Taylor flow—II. Quantitative measurements and discussion of the reactor performance

Abstract For the first time, the extent to which laminar and turbulent Couette-Taylor flow (CTF) combine a limited axial dispersion with a high local mixing intensity has been quantified by simultaneously measuring the inter-vortex flux and the tangential dispersion. The correlations for the tangential dispersion indicate that the local mixing intensity of CTF is of the same order of magnitude as in coiled pipe flow. The axial dispersion was measured in such a way that the dispersion enhancing or reducing effects which originate from the slow intra-vortex transport were eliminated. This allowed the comparison of the measurements in this study with recent theoretical calculations of the inter-vortex flux in 2D-periodic flows. The variation of the inter-vortex flux with D mol 1 2 shows that in the laminar regime the inter-vortex flux can be described by the penetration model. A mechanism for the axial dispersion in the turbulent regime is also proposed. By comparing the ratios of the local and global dispersion coefficients, it is quantitatively shown that for CTF the local mixing intensity can be increased with a relatively smaller increase in axial dispersion than for conventional flow types.

Transient and stationary axial dispersion in vortex array flows—I. Axial scan measurements and modeling of transient dispersion effects

Abstract Whereas conventional RTD experiments yield incomplete and potentially misleading information, the axial scan method is shown to be a powerful technique to analyze the transient dispersion effects in vortex array flows (VAFs). Applying different initial tracer distributions to a given vortex, and making axial scans of the spreading tracer distribution, allowed, for the first time, to quantify and classify the complete set of strongly different transient dispersion modes. As a model system, the laminar Couette-Taylor flow has been selected. By working under high viscosity conditions, the time scale of the different acting phenomena has been enlarged to such an extent that even the fastest dispersion events could be extensively studied and quantified. It is shown that in laminar VAFs effective axial dispersion coefficients can be obtained which vary over orders of magnitude, just by applying different initial tracer distributions to a given vortex. A first principles two-dimensional model (valid when the mixing along the streamlines occurs fast) with which all observed transient dispersion effects can be accurately represented is proposed. The insights obtained in the present study are especially useful for the development of VAF reactors for the treatment of strongly viscous fluids.

Transient and stationary axial dispersion in vortex array flows-II. Decoupling of inter- and intra-vortex transport phenomena

Abstract In flows consisting of arrays of vortex units, the transient dispersion regime is of a very complex nature: the effective dispersion coefficient varies strongly with time, comprises both inter- and intra-vortex phenomena and depends heavily upon the injection condition. However, in an infinite array of identical vortices, it is found that when the cross-section of a vortex is injected homogeneously, the effective dispersion coefficient remains strictly constant and is independent of all intra-vortex transport phenomena. In other words, regardless of the presence of strong intra-vortex concentration gradients, the second-order moment of the tracer distribution varies with the time as if all the vortices are perfectly mixed. A mathematical proof for this phenomenon is given. A measurement method based upon this feature allows the separate and direct determination of the inter-vortex exchange coefficient in laminar Couette-Taylor flow, without the need to quantify the intra-vortex transport phenomena, or without the need to wait until all intra-vortex concentration gradients have disappeared. The obtained experimental correlation validates recently performed theoretical calculations and shows that the mass transfer occurs by means of a continuous, strictly ordered surface renewal mechanism. For the transient dispersion effects which are obtained when nonhomogeneous injections are applied, an approximate expression for the variation of the effective dispersion coefficient with the time is established. Within the (very good) accuracy of this expression, the effect of the inter- and intra-vortex transport upon the tracer dispersion rates could also be decoupled.

Thermodynamic simulations of lignite-fired IGCC with in situ desulfurization and CO2 capture

Abstract IGCC cycles running on Greek lignite with in situ desulfurization and CO2 capture were simulated using the ASPEN PLUS process simulator and compared with other power cycles. Dolomite was chosen as the material for in situ desulfurization. Considering the power produced and residual sulfur content of the off-gas, an oxygen-blown gasifier operated at 1.8 MPa and 950°C, having a thermal efficiency of 38.9%, gives the best performance. The efficiency loss due to CO2 capture can be largely compensated using IGCC cycles.

Performance limits of isothermal packed bed and perforated monolithic bed reactors operated under laminar flow conditions. Part II: performance comparison and design considerations

Abstract The maximally attainable productivity of perforated monolithic bed reactors has been compared to that of the traditional packed bed of spheres for the case of laminar flow conditions and first-order isothermal reaction kinetics. Using the E number established in Part I, it could be shown in a very condensed, yet fully quantitative way that the maximal gain in total reactor productivity which can be obtained by switching from the tortuous pore system of the packed bed of spheres (large flow resistance) to the straight-running flow-through pores of a perforated monolithic bed (minimal flow resistance) typically is 25–40%. Much larger gains in total productivity can be obtained by using highly open-porous beds. Whether this high porosity is achieved using a perforated monolithic bed, a packed bed of hollow extrudates or a structured fibre-mesh bed is then only of secondary importance. The E number also allows to quantify the potential gain and the range of applicability of the more advanced reactor designs proposed in the past years (e.g., the parallel passage reactor). It could now be shown that the productivity gain of these advanced concepts can even amount up to a factor of 100. As these gains are to be realized in beds with ultra-small flow-through pores, the present study also provides a strong quantitative argument for the current research on (micro-)structured reactor beds.

Measurement of O2-N2 binary sorption on 5A zeolite by isotope tracer and perturbation chromatography

Isotope tracer chromatography allows to extract simply and quickly multi component adsorption data and is demonstrated for single component and binary adsorption equilibria for O2 and N2 on 5A zeolite as an example. In this modification of conventional tracer chromatography, a small pulse of an isotope tracer is injected in an adsorbable carrier gas (pure or multicomponent mixture) flowing through a column filled with adsorbent and is designed to operate at almost uniform pressure. Isotherm parameters are readily extracted by fitting measurements of residence times at various pressures and carrier composition. The isotherms were in excellent agreement with volumetric measurements. Isotope tracer chromatography is shown to be superior to perturbation chromatography since the influence of the injection volume on the carrier gas composition is substantially smaller for tracer experiments. Unfortunately, this new improved gas chromatographic technique requires rather expensive isotopes. The strength of this new approach lies in the advantage of working with small adsorbent samples (1 g) making a rapid screening of newly developed materials possible.

Performance limits of isothermal packed bed and perforated monolithic bed reactors operated under laminar flow conditions. I. General optimization analysis

Abstract A global optimization analysis of a general class of perforated monolithic bed reactors is presented for the case of an isothermal first-order reaction and for laminar flow conditions. The resulting design rules indicate how a given amount of catalyst material should best be perforated or distributed in space as a function of the available inlet pressure. It is shown that the influence of the different process variables ( k ,Δ P , V cata , C in / C out , D mol , S reac ) on the reactor productivity and on the optimal bed design can be grouped into a single dimensionless number E. This number also allows to discuss the sensitive relation between the total and the volumetric productivity of single bed reactors in a very general, compact manner. Two different perforated monolithic bed designs, a slit pore bed (SPB) and a cylindrical pore bed (CPB) are considered. It is found that, when there is an excess inlet pressure (i.e., for E ≪1), the optimal catalyst layer thickness is given by φ =0.3–0.5 and that the optimal pore diameter is in both cases 1.4–1.45 times smaller than the catalyst layer, independently of the internal catalyst diffusivity ( D int ) and the other process variables. When the available inlet pressure is limiting ( E >10 −4 ), and when the absolute reactor productivity is more important than the volumetric productivity, it is found that much more open structures, with much wider pores are needed, i.e., perforated beds show the same behavior as packed beds, where the occurrence of a pressure-drop limitation also induces a shift from the use of full particles to the use of hollow extrudates.

Diffusion in Zeolite Adsorbents: Measurement, Modelling and Structure-Performance Relation

Abstract Adsorption equilibria and micropore diffusivities of oxygen, nitrogen, argon and methane were determined for four molecular sieves of the LTA type with an improved chromatographic method. It was shown that obstructed sieves such as NaA have diffusivities, orders of magnitude lower than those in open channel types and that their main resistance for diffusion is located in the window of the zeolite cavity. It can be shown that the diffusivity in zeolites can be influenced by three different features: the zeolite structure, cation type and location, and the characteristics of the adsorbed molecules (size, mass, shape). The adsorption capacity and especially the relative separation constant are mainly determined by cations (number and specific charge), but also by stabilization effects of the surrounding framework.