Lothar Riekert

The efficiency of energy-utilization in chemical processes

Abstract Chemical processes may be regarded as a transfer of chemical energy from the starting material into the products. The work equivalent of any material is the maximum amount of shaft work or electrical energy which can possibly be obtained from it when heat and matter are exchanged with the environment. It depends on the materials temperature, pressure and chemical composition as well as on the respective properties of the environment. All contributions to the work equivalent of any substance can be expressed in the same consistent units and evaluated quantitatively, including the contribution due to chemical composition. A measure of the efficiency of energy utilization in a chemical process—based on the first and second laws of thermodynamics—can be obtained, if one compares the total output of work equivalent in all outgoing streams to the total input. This concept is developed in a formal way and applied to the analysis of two industrial processes.

Intracrystalline self-diffusion of H2O and CH4 in ZSM-5 zeolites

Abstract Intracrystalline self-diffusion coefficients of methane and water in pentasil zeolites have been determined by the n.m.r. pulsed field gradient technique; zeolite samples synthesized in different laboratories were investigated. The self-diffusivity at room temperature was found to be in the order of magnitude of 5 · 10−10 m2s−1 for water, 7 · 10−9 m2s−1 for methane, independent of origin and crystal size of the zeolite. The validity of the experimental method is verified through the consistency of results obtained with crystals of different size as well as before and after external coating with silica.

Comparison of nuclear magnetic resonance tracer exchange and molecular uptake of benzene on pentasils

Conventional NMR spectroscopy is applied to study tracer exchange in adsorbate–adsorbent systems. In the proposed experimental procedure, heat effects are completely excluded and possible transport resistances due to gas-phase diffusion and intercrystalline diffusion are reduced to a minimum. The influence of deviations from the spherical shape of the adsorbent particles as well as possible anisotropy of intracrystalline diffusion on tracer exchange curves is estimated. The obtained relations are likewise applicable to uptake studies. For benzene on different ZSM-5 specimens, the tracer exchange data are compared with the results of uptake measurements. Though carried out independently from each other with different techniques and different specimens, the experiments provide coinciding results. In particular, it appears that the ratio between the self-diffusion coefficient obtained in the tracer experiments and the diffusion coefficient resulting from uptake studies is indeed given by the logarithmic derivative of the adsorbate concentration versus the adsorbate pressure as conventionally assumed on comparing self-diffusion and uptake measurements.

Kinetics of sorption of benzene and n-paraffins in large crystals of MFI zeolites

Abstract The kinetics of sorption of benzene and C 6 ÷ C 9 n -paraffins into fairly large crystals of HZSM-5 and silicalite at 298 K were studied by gravimetry. The diffusivities characterizing the rates of sorption are listed. Diffusion of n -nonane is at least 10 2 times faster than that of benzene. The rate of sorption increases with decreasing length of n -paraffins; for n -hexane, it was already too high to be observed with precision.

Deactivation of dealuminated zeolite Y in the catalytic cracking of n-hexane

Zeolites Y with SiAl ratio from 2 to 300 were obtained by two methods of dealumination. The catalytic activity of these zeolites in the H form for the cracking of n-hexane and their pattern of deactivation due to coking was investigated at 773 K. HY with SiAl = 2.4 deactivates completely, whereas a steady state with residual activity is reached with dealuminated zeolites Y. The ratio between residual activity and initial activity increases with the SiAl ratio. HZSM-5 is much more resistant to deactivation than is HY with the same SiAl ratio.

Partial Oxidation of o-Xylene to Phthalic Anhydride in a Structured Fixed Bed Containing a Sequence of Catalysts

It is shown that an elevated yield of phthalic anhydride (PAA) can be obtained in a fixed bed consisting of a sequence of different catalysts. A suitable sequence of catalysts can be determined by a computerized search, following the strategy of biological evolution on the basis of experimentally obtained kinetic data.

Phase Transformations and Changes in Lattice Parameters of ZSM-5 As A Function of Al Content and Temperature

Abstract Very high quality ZSM-5 with Si/Al ratios from 35 to 12000 were synthesized and converted to the H-form. Their lattice parameters were determined from high resolution X-ray data as a function of temperature up to 585°C. The monoclinic phase with β angle reduced from 90.6° for ZSM-5 with a Si/Al ratio of 12000 to 90.09° is evident in ZSM-5 with a Si/Al ratio of 35 at room temperature (25°C). The monoclinic-orthorhombic phase transition is dependent on Si/Al and temperature. For highly silicious ZSM-5 the temperature range for this phase transition was found to be greater than that determined by MASNMR. Lowering the temperature of the monoclinic phase tends to increase the β angle.

The Effect of Temperature and Sorption of P-Xylene and Benzene on The Structure of ZSM-5

Abstract The effect of p-xylene and benzene adsorption on the structure of ZSM-5 with varying Si/Al ratios, temperature and pressure has been determined. High quality ZSM-5 with Si/Al ratios of 35, 100 and 800 were synthesized and converted to the hydrogen form. The lattice parameters were accurately determined from data obtained with a Guinier 4 circle camera. Room temperature sorption of p-xylene and benzene promoted the monoclinic-to-orthorhombic phase transition with some changes in the a, b and c parameters especially in the case of benzene sorption; where the a and b parameters become equal on full loading. Low pressure p-xylene and benzene flow over ZSM-5 samples maintained at various temperatures caused the monoclinic to orthorhombic phase transition with changes mainly in the a and b parameters and unit cell volume as a function of temperature.

Sorption and Diffusion of Benzene and Xylenes in Zeolite ZSM-5: Diffusion Anomalies and Hysteresis Effects

Abstract Equilibria and kinetics of sorption of benzene, p-xylene and o-xylene in zeolite ZSM-5 were investigated at 298 K for two samples of the zeolite, having the same composition (Si:Al = 35) but differing in crystal size. Sorption of benzene into ZSM-5 follows the pattern expected on the basis of the diffusion equation with constant D. The mobility of p-xylene in the zeolite decreases substantially when the sorbed amount exceeds 4 molecules/ unit cell, where the equilibrium isotherm exhibits a hysteresis loop. The relatively slow sorption of o-xylene is not controlled by diffusion.

Deposition Of Ternary Oxides As Active Components By Impregnation Of Porous Carriers

The ternary oxide coppermanganate is generated in highly dispersed form in the pores of inert α-Al 2 O 3 carriers by impregnation with a viscous solution containing both metals, drying and calcination. Kinetic data for this catalyst in the total oxidation of CO are compared with similar catalysts containing only CuO, MnO or CuO together with MnO as separate crystallites. In the case of ironmolybdate it is also possible to obtain crystallites of the ternary phase in the pores of α-alumina by way of impregnation, either with solutions containing ammoniummolybdate and ironnitrate or with the solution of ironmolybdate in a mixture of monoethanolamine and water. Properties of both types of catalysts and of catalysts consisting of pure ironmolybdate are compared, focusing on the selectivity in the partial oxidation of methanol to formaldehyde.