David W. Agar

The capillary-microreactor: a new reactor concept for the intensification of heat and mass transfer in liquid–liquid reactions

Abstract The capillary-microreactor was used for studying the nitration of a single ring aromatic in an exothermic liquid–liquid two-phase reaction. In the capillary-microreactor, isothermal behaviour can be assumed due to the high heat transfer rates. In addition, a liquid–liquid two-phase plug-flow with a well-defined flow pattern of alternating plugs of the two phases is formed, giving a constant, uniform specific surface area for mass transfer between the two phases. In the nitration reaction, not only the mononitrated main product is formed, but also various by-products via consecutive and parallel reactions. Mass transfer experiments with different flow velocities, but identical residence times, carried out in the capillary-microreactor, yielded differences in conversions and the amounts of by-products formed. Simulations using a mathematical model describing interphase mass transfer and homogeneous chemical reaction indicate increasing mass transfer coefficients at higher flow velocities, suggesting that the mass transfer between the two phases is enhanced by the flow velocity. The enhancement of the mass transfer can be interpreted in terms of an internal circulation flow within the plugs, a conclusion corroborated by CFD calculations. The capillary-microreactor has thus proved to be a useful instrument for the quantitative elucidation of the mechanisms of exothermic liquid–liquid two-phase reactions.

Multifunctional reactors : Old preconceptions and new dimensions

Abstract The deliberate incorporation of additional process functions to enhance reactor performance has proved to be a fertile approach in developing novel reactors. In addition, the concept of these so-called multifunctional reactors offers a useful framework for reactor selection and classification. Two earlier classification schemes based on the underlying heat and mass transfer processes and the combination of reaction with various unit operations are reviewed. The utility of these conceptualisations in reactor analysis is illustrated using the Claus process and hydrogen cyanide synthesis as examples. A new taxonomy obtained by considering the phases participating in the reactor operation is proposed. The shortcomings of previous work on multifunctional reactors, primarily the absence of technical and economic comparisons with alternative reactor designs, are discussed. The potential for further improvements, especially those exploiting the spatial distribution of functionalties within the reactor and by using structured ‘multifunctional’ catalysts, is examined.

The Claus process: teaching an old dog new tricks

Abstract The potential of integrating reaction with adsorption in a single piece of equipment to favourably displace chemical equilibria has attracted considerable attention in recent years. In contrast to the most such multifunctional reactor concepts investigated so far, the aim of our work is to study an industrially relevant chemical system with all its peculiarities. The feasibility of enhancing conversion in fixed-bed adsorptive reactors has been evaluated for the Claus reaction used in sulphur recovery, which is usually carried out in a multistage process, to counteract the severe equilibrium limitations at high conversions. Both experimental and simulation results indicate that the adsorption/reaction kinetics and the adsorbent capacity have to exhibit appropriate and compatible values in order to overcome the equilibrium limitation and attain high conversions in a single-step process. A crucial point is the selection of suitable reaction conditions, since the occurrence of undesirable side-reactions (e.g. suppression of COS elimination in the case of the Claus process) may be amplified by the distorted concentration profiles in adsorptive reactor operation. In industrial applications such by-products may be of particular importance and thus exert a decisive influence on the concentration profile modifications possible.

Extended reactor concept for dynamic DeNOx design

Abstract An analysis of chemical reactors according to their use of internal and external heat and mass transfer reveals a new reactor configuration - a chromatographic reactor with periodically reversing flow. The application of this configuration in the removal of nitrogen oxides from stack gases by selective catalytic reduction with ammonia is described. The new reactor offers high NOx-removal rates without ammonia slip emissions and ameliorates problems associated with the fluctuations and distribution of the gas. The detailed kinetic modelling and preliminary experiments used to establish the feasibility of the process are presented. The necessary criteria for the use of this new reactor type and its relationship to the catalytic heat regenerator-reactor are discussed.

Thermal N2O decomposition in regenerative heat exchanger reactors

Abstract The decomposition of nitrous oxide (N2O), which may be carried out either catalytically or thermally, is a reaction of considerable environmental significance. A novel non-catalytic process for the destruction of nitrous oxide emissions is presented. Experimental studies on a laboratory scale demonstrate that a selective removal of N2O can be achieved in a purely thermal conversion step within the temperature range of 800°C–1000°C without adversely affecting the NOx levels in the gas. The new process is also suitable for the treatment of lean waste gas streams, since the integrated heat recovery, for instance in a regenerative reverse flow reactor, permits simple, robust autothermal operation with the recovery of useful heat whilst circumventing the danger of additional NOx formation. Further studies deal with the simultaneous occurrence of both homogeneous and heterogeneous reactions in the N2O decomposition process and their interaction. The stability limits in the operation of a flow reversal reactor could well offer a useful practical means of discriminating between the relative contributions of the two types of reaction and of identifying coupling mechanisms, as the combined system exhibits parametric sensitivities and thermokinetic instabilities, such as ignition–extinction phenomena and multiple periodic steady states, which depend on the underlying structure of the reactive processes. The concepts involved are illustrated using the results of preliminary model simulations. The stability structure of the system can be analysed expediently using a combination of dynamic simulation and an asymptotic countercurrent reactor model.

The separation of immiscible liquid slugs within plastic microchannels using a metallic hydrophilic sidestream

This paper describes experiments and related modelling on a new method for separating aqueous phase slugs from the surrounding organic matrix phase in segmented two phase flow in a plastic microcapillary film (MCF). Kerosene or paraffin oil was metered through a plastic capillary of 630 microns diameter and aqueous phase slugs were generated within the capillary by the continuous sidestream injection of water. It was found that the resulting aqueous phase slugs formed in the MCF could be subsequently easily separated from the organic phase by piercing the downstream sidewall of the plastic capillary with a hydrophilic metal hypodermic needle to draw off an aqueous sidestream. Optical scrutiny of the phase separation process indicated that two distinct disengagement mechanisms are involved, in which the metal needle tip either remains submerged in the aqueous phase or becomes periodically exposed to the organic phase at certain stages of the segregation process. The separation efficiency, i.e. the degree of residual phase cross-contamination, was determined as a function of both the sidestream needle angle and the depth of needle penetration into the capillary for a given flow rate and phase ratio. It was established that the separation efficiency was very sensitive to the downstream pressure balance between the organic mainstream flow in the plastic capillary and the aqueous sidestream flow through the needle. A mathematical model for the pressure balance conditions was developed by making certain simplifying assumptions and taking the Laplace interfacial pressure into account. The model predictions agreed surprisingly well with the experimental findings, thus providing circumstantial evidence for the validity of the insights into the phase separation mechanism.

Synthesis and Application of Carbonated Fatty Acid Esters from Carbon Dioxide Including a Life Cycle Analysis

Carbon dioxide can be used in various ways as a cheap C1 source. However, the utilization of CO2 requires energy or energy-rich reagents, which leads to further emissions, and therefore, diminishes the CO2-saving potential. Therefore, life cycle assessment (LCA) is required for each process that uses CO2 to provide valid data for CO2 savings. Carbon dioxide can be incorporated into epoxidized fatty acid esters to provide the corresponding carbonates. A robust catalytic process was developed based on simple halide salts in combination with a phase-transfer catalyst. The CO2-saving potential was determined by comparing the carbonates as a plasticizer with an established phthalate-based plasticizer. Although CO2 savings of up to 80 % were achieved, most of the savings arose from indirect effects and not from CO2 utilization. Furthermore, other categories have been analyzed in the LCA. The use of biobased material has a variety of impacts on categories such as eutrophication and marine toxicity. Therefore, the benefits of biobased materials have to be evaluated carefully for each case. Finally, interesting properties as plasticizers were obtained with the carbonates. The volatility and water extraction could be improved relative to the epoxidized system.

Adsorptive reactors for enhancing equilibrium gas-phase reactions—two case studies

The conversion enhancement potential of fixed-bed adsorptive reactors has been evaluated for two heterogeneously catalysed gas-phase equilibrium reactions: the Claus reaction used in sulphur recovery and the direct synthesis of hydrogen cyanide from carbon monoxide and ammonia. It was found that kinetics of both reaction and adsorption as well as adsorbent capacity have to be very compatible to achieve high conversions. The specific parameters of the reaction/catalyst system, such as the deposition of solids (e.g. sulphur, coke) or the formation of undesirable by-products have to be taken into account for the successful application of adsorptive reactor concepts. A crucial point is the selection of the reaction conditions (i.e. temperature), since the occurrence of side reactions may be enhanced in adsorptive reactor operation due to the inherently distorted concentration profiles.

Enhancement of carbon dioxide absorption into aqueous methyldiethanolamine using immobilised activators

Blends of ‘activating’ primary or secondary amines (diethanolamine, DEA) with tertiary amines, (methyldiethanolamine, MDEA) are commonly used for the removal of CO2 from gas mixtures. To avoid undesirable side-effects from these activators, such as increased corrosion or higher energy requirements for regeneration, we propose using immobilised primary or secondary amine groups on solid supports. In this manner the activating additives can be localised to those parts of the absorption column where the high absorption rates achieved are truly beneficial and excluded elsewhere. The studies presented were carried out to provide an initial evaluation of the feasibility of this novel concept. Preliminary experiments carried out in a discontinuously operated stirred tank reactor reveal similar enhancement of the CO2 absorption into ‘activated’ MDEA solution, when the soluble DEA additive is replaced by a suspended solid adsorbent, containing the equivalent quantity of immobilised amine groups. Further experiments examined the CO2 absorption in a three phase fluidised bed column. They demonstrated that the immobilised activator can be employed in a continuously operated process too. All experimental results support the basic feasibility of using immobilised primary amines in place of homogeneous additives to enhance CO2 absorption in tertiary amine solutions.

Enzyme immobilisation in permselective microcapsules

The objective of this investigation was to study the permselective behaviour of calcium alginate membranes, including the modifying effects of silica additives, which were subsequently used as microcapsule shells. Diffusion experiments and HPLC were carried out to ascertain the size-exclusion property of the membranes for a mixed molecular-weight dextran solution. Hollow microcapsules containing the enzyme dextranase were prepared using double concentric nozzles and the encapsulation performance was evaluated based on an analysis of the enzyme reactivity and stability. To improve mass transport within the microcapsules, magnetic nanoparticles were introduced into the liquid core and agitated using an alternating external magnetic field. The modified membranes exhibited better size-exclusion behaviour than the unmodified membranes. The magnetic nanoparticles slightly improved mass transport inside the microcapsule. The encapsulated enzyme yielded nearly 80% of the free enzyme activity and retained about 80% of the initial catalytic activity even after being used for eight reaction cycles.