Xiaohong Ren

Deactivation and regeneration of a naphtha reforming catalyst

Abstract A series of naphtha reforming catalysts from different stages of the deactivation (coking) and the regeneration (decoking) processes were investigated by NMR and chemical engineering methods. The dependence of the tortuosity on the coke content was determined for both processes by NMR measurements of the intraparticle self-diffusion coefficients of adsorbed liquid n -heptane. The shrinkage of the accessible pore volume as a function of increasing coke content due to the deactivation process is compared to nitrogen adsorption (BET) measurements which show an equivalent behavior. A crude model was adapted to predict qualitatively the relationship between the tortuosity and the average pore diameter. Longitudinal ( T 1 ) and transverse ( T 2 ) NMR relaxation times measured for protons of adsorbed liquid n -heptane, provide information on the pore morphology changes which can be corroborated by the tortuosity measurements. The chemical composition of the coke layer, which was investigated by 1 H magic angle spinning (MAS) and 13 C cross polarization (CP)/MAS NMR spectroscopy, is shown to change during both deactivation and decoking processes. Moreover, the micro-structure of the fresh catalyst and the fully regenerated catalyst was investigated by scanning electron microscopy (SEM). The experimental results indicate that a full recovery of the activity of the clean catalyst is not achieved by the regeneration process, and that the quality of regeneration depends on the coke content reached during the deactivation/regeneration cycle.

Molecular mobility in fixed-bed reactors investigated by multiscale NMR techniques

The complex problem of a fixed-bed reactor consisting of catalytically active particles provides an exceptional opportunity of combining a wide range of NMR methods which have become available over time as tools to probe porous media. This work demonstrates the feasibility of different NMR techniques for the investigation of the intra- and interparticle pore space over length scales from nanometers up to centimeters. Many industrially relevant cracking reactions leave a coke residue on the inner surface of the porous catalyst particles so that the active sites become inaccessible to the reactants. Moreover, the pore space shrinks due to the formation of coke, thereby hindering molecular transport. The presence of the coke residue and its influence on the mobility of adsorbed fluid molecules are probed by 129Xe spectroscopy, NMR cryoporometry, relaxation dispersion measurements, and investigations of the reduced diffusivity in the intraporous space. The voids surrounding the random arrangement of catalyst pellets represent another pore space of much larger dimensions, the properties of which can be more directly investigated by mapping the fluid density and the velocity distribution from velocity-encoded imaging. Propagator representations averaged over large sample volumes are discussed and compared to velocity images obtained in selected axial slices of the reactor.

Investigation of reinforcement of the modified carbon black from wasted tires by nuclear magnetic resonance

Pyrolysis has the potential of transforming waste into recyclable products. Pyrolytie carbon black (PCB) is one of the most important products from the pyrolysis of used tires. Techniques for surface modifications of PCB have been developed. One of the most significant applications for modified PCB is to reinforce the rubber matrix to obtain high added values. The transverse relaxation and the chain dynamics of vulcanized rubber networks with PCB and modified PCB were studied and compared with those of the commercial carbon blacks using selective 1H transverse relaxation (T2) experiments and dipolar correlation effect (DCE) experiments on the stimulated echo. Demineralization and coupling agent modification not only intensified the interactions between the modified PCB and the neighboring polyisoprene chains, but also increased the chemical cross-link density of the vulcanized rubber with modified PCB. The mechanical testing of the rubbers with different kinds of carbon blacks showed that the maximum strain of the rubber with modified PCB was improved greatly. The mechanical testing results confirmed the conclusion obtained by nuclear magnetic resonance (NMR). PCB modified by the demineralization and NDZ-105 titanate coupling agent could be used to replace the commercial semi-reinforcing carbon black.

Surface Modification of Pyrolytic Carbon Black from Waste Tires and Its Use as Pigment for Offset Printing Ink

Abstract Pyrolysis has the potential of transforming waste into valuable products. Pyrolytic carbon black (PCB) is one of the most important products resulting from the pyrolysis of used tires. One of the most significant applications of modified pyrolytic carbon black is its use as pigment for offset printing ink to obtain high added values. Inverse gas chromatography (IGC) results show that a large quantity of inorganic matters and carbonaceous deposit are removed by treating the pyrolytic carbon black with nitric acid solution. Plenty of active sites originally occupied by inorganic ash and coke are recovered. The surface energy of pyrolytic carbon black (TWPC) modified by titanate-coupling agent-especially the specific interaction γ sp s , determined by the specific probe molecule, toluene-shows the strong interaction between the TWPC and the synthetic resins. The offset printing ink performance confirms the IGC prediction. And TWPC has the great potential of applications in printing ink industry as pigment.