Catalytic oxidation of hydrogen sulphide on activated carbons

Activated carbon is a suitable adsorbent for removal of hydrogen sulphide from natural, synthesis or other product gases. The process depends predominantly on physical adsorption, though catalytic oxidation is also involved. During catalytic oxidation the H₂S is converted in the presence of oxygen to elemental sulphur, which is adsorbed onto the internal surface of the activated carbon, thus leading to a sulphur load of up to 120% by weight. The oxidation rate depends on the partial pressure of both reactants, H₂S and O₂ and is largely controlled by the characteristics of the activated carbon. The activity of the catalyst can be improved by impregnating the activated carbon with promoters such as iron and iodine. The regeneration of spent carbon is currently carried out using hot gas desorption methods at temperatures around 450 °C.     

Sulfonated latex particle as acid catalysts for the continuous inversion of sucrose

Sulfonated polystyrene latex particles were used as acid catalysts in the continuous inversion of sucrose at 50–70°C. The particles (surface charge 168 μeq/g) were confined in a stirred reactor with a semipermeable membrane; sucrose solution was pumped in, and product solution was pumped out. The catalytic activity of the particles was unchanged after 20 days continuous use. Variation of particle size (0.13 and 0.42 μm) and stirring rate showed that internal and external mass transfer was not a controlling factor. The kinetics were pseudo-first-order; the rate constant at 70°C was 2.30/N min as compared with 0.07/N min for macroporous sulfonated ion-exchange resin; the apparent activation energy was 111 kJ/mol as compared with 121 kJ/mol for the homogeneous acid-catalyzed reaction. The faster rate was attributed to the very great surface area, high charge density, and lack of internal diffusional resistance of the latex particles. The proposed mechanism comprised adsorption of sucrose on the particle surface, followed by inversion and desorption of product.     

A kinetic investigation of coal liquefaction at short reaction times

Coal liquefaction kinetics have been studied at very short reaction times (less than 250 seconds) in order to emphasize the initial underlying physical and chemical processes involved. These studies were made possible by the use of a continuous flow stirred tank reactor (CSTR) which avoids the problems of slow heat up and cool down associated with the massive equipment required for running high-temperature and high-pressure liquefaction reactions. Preliminary physical (NMR and ESR) and chemical analytical results are presented on the coal liquids and reaction residues from Illinois No. 6 hv bituminous and Wyodak Black Thunder subbituminous coals.

ESR results showed that radical concentration in the solid residue changed during coal liquefaction. These changes were accompanied by changes in the NMR-derived aromaticity. The rate of decrease of organic-based radicals was different for Wyodak Black Thunder and Illinois No. 6 coals, perhaps indicating a different mechanism for the quenching of radicals in these bituminous and subbituminous coals. NMR spectra of the liquid products indicated that the initially produced material was relatively aromatic, and that subsequent products had lower aromatic content. This is consistent with secondary hydrogenation of the primary liquefaction products. Finally, the total oxygen contents of the coal residues decreased gradually during the first three minutes of coal liquefaction at 390°C. A corresponding decrease in the hydroxyl content of these residues was also noted.     

Zur optimierung der Polymerfraktionierung auf der Grundlage von Lösungsgleichgewichten in Lösungsmittel-Fällungsmittel-Systemen

The phase separation behavior of dilute polydisperse polymer solutions primarily depends on the partial concentration of each polymer component. So the equilibrium data obtained by turbidimetric titration with “calibration” fractions and their mixtures can be used for the construction of a solubility diagram (Claesson-diagram) which generally allows to describe fractionation processes. It can be shown that the characteristic features of precipitation fractionations can well be derived from the solubility diagram. This diagram then offers the possibility for a complete discussion of parameter variation (concentration, step of γ-variation, process-scheme, temperature, solvent/nonsolvent-system) on the efficiency of a fractionation step. On one hand these results correspond – as for the concentration variation – to well known experimentell experiences. On the other hand it is possible for the first time to give definite predictions for the influence of temperature and the solvent/nonsolvent system. The temperature selection can be shown to be inconsiderable for the separation effect. The usual characterization of solvents and nonsolvents as “good” or “poor” is absolutely insufficient, if not misleading, for there is no connection between these properties and the shape of the solubility diagram. Based on the analysis of the single parameters the optimum conditions for the preparation of narrow preparation of narrow precipitation fractions can be derived.     

High-Throughput Experimentation in Oxidation Catalysis

High-throughput experimentation in catalysis comprises the following components: (i) automated high-throughput synthesis, (ii) testing in Stage I and Stage II, for which to some extent novel assays are necessary, (iii) data handling and experimental design tools, and (iv) robotics. This contribution covers these topics, using examples from the research of the authors, but also from the literature, in order to illustrate the problems and opportunities associated with high-throughput experimentation in catalysis, focusing particularly on heterogeneous catalysis.     

Predicting the effect of screw wear on the performance of plasticating extruders

The effect of screw wear on the performance of a 2.5 in. diameter extruder is studied with the aid of computer simulations. The effect of progressively increasing flight clearance on the extrusion of low density polyethylene, polypropylene and nylon 6/6 is presented. The remedial effect of increased screw speed and its side effects on melting behavior, solids content, extrudate temperature and power consumption are also described.     

Nelder-Mead Simplex Optimization Routine for Large-Scale Problems: A Distributed Memory Implementation

The Nelder-Mead simplex method is an optimization routine that works well with irregular objective functions. For a function of $n$ parameters, it compares the objective function at the $n+1$ vertices of a simplex and updates the worst vertex through simplex search steps. However, a standard serial implementation can be prohibitively expensive for optimizations over a large number of parameters. We describe an implementation of the Nelder-Mead method in parallel using a distributed memory. For $p$ processors, each processor is assigned $(n+1)/p$ vertices at each iteration. Each processor then updates its worst local vertices, communicates the results, and a new simplex is formed with the vertices from all processors. We also describe how the algorithm can be implemented with only two MPI commands. In simulations, our implementation exhibits large speedups and is scalable to large problem sizes.     

Thermal degradation of aliphatic–aromatic polyamides: Kinetics of N,N′-Dihexylisophthalamide neat and in presence of copper lodide

An experimental study to determine the effect of copper (I) iodide (Cul) on the rate and product distribution of degradation of a model of an aliphatic–aromatic polyamide was carried out. N,N′-Dihexylisophthalamide (DHI) was reacted in both an inert argon atmosphere and a pure oxygen environment at 350°C with CuI added in amounts ranging from 0 to 20% by weight. The rate of disappearance of DHI was enhanced by an order of magnitude when 0.5% by weight of CuI was added and was an increasing function of increasing CuI loading. Reaction in pure O₂ increased the rate of DHI degradation by two orders of magnitude over that for neat DHI pyrolysis. The rate of disappearance of DHI in O₂ was relatively unchanged when 5% CuI by weight was added. The transformations of DHI and its products are organized in terms of a set of reaction rules. This “reaction operator” formalism allowed computer generation of the reaction network and facilitated estimation of kinetic parameters. © 1995 John Wiley & Sons, Inc.     

Crystallization of calcium sulphate hemihydrate in concentrated phosphoric acid solutions

We aim to show the existence of agglomeration by measuring and modelling secondary nucleation and crystal growth rates of calcium sulphate hemihydrate, CaSO₄-0.5 H₂O, in concentrated phosphoric acid solutions. Using a batch crystallizer we measured the evolution of the population density as a function of supersaturation, H₂SO₄ excess and stirring rates. All experiments were carried out at 90 °C in solutions at 40 wt.% of P₂O₅, simulating the usual conditions for crystallizing hemihydrate in the industrial processes of phosphoric acid production. Nucleation and growth rates were calculated from the population number densities, using the moments analysis method. A model is presented for describing the crystallization process of hemihydrate. It is shown that secondary nucleation and growth rates are quadratic functions of supersaturation. H₂SO₄ concentrations affect supersaturation but at the same supersaturation the growth rates are not significantly different. Nucleation is independent of the stirring rate, whereas growth rates are slightly affected for stirring rates up to 500 rpm. Taking agglomeration into account, the moments method fits very well the experimental data.     

Preparation, characterization, solution properties and rheological behaviour of polyacrylamide

In this report a survey is given on structure and properties of polyacrylamide homopolymers (PAAm) in solution. However the review is restricted to all those papers, where a molecular characterization of the polymers has been achieved as a basis to correlate this fundamental information with applicational properties.

Different polymerization methods are summarised in brief, the preparation and solution structure of long chain branched polyacrylamides as well as chemical modification reactions of linear PAAm are also mentioned. A number of experimental characterization methods (GPC, ultracentrifugation, intrinsic viscosity, and light scattering measurements) are described with special emphasis on the difficulties of the different procedures including some proposals for properly designed experimental techniques. The state of solution is discussed in view of experimental data obtained with different solvents. Moreover viscosity constant ø is calculated for aqueous solution and the unperturbed dimensions are estimated. All available data on cross correlations (e.g. [η] - M, S₀ - M, 2>^1/2 - M) are collected with the intention to give a survey of established relations and, comparing the given relationships, to suggest the reliable ones of them.

The phenomenon of long-term viscosity decrease of aqueous PAAm solutions has been investigated and discussed with regard to its molecular origin.

The viscoelastic properties are discussed in dependence on molecular weight, concentration, solvent quality, and shear rate (10⁶ · s⁻¹). Based on these data a simple equation was developed oped for the η₀-c-M relationship, which can be applied to other polymer systems as well. It is further described that the elastic nature (first normal stress difference) may overwhelm the viscous nature (shear stress) at relatively low shear rates. This high elasticity can cause deviation from laminar flow conditions. Moreover, it can be demonstrated — based on instationary measurements as well as the comparison of steady shear flow with dynamic rheology — that energetic interactions (H-bonds) strongly influence the rheological behaviour.