Robert G. Rinker

Atmospheric Nitrogen Fixation by Lightning

Abstract The production Of nitrogen oxides (NO and NO2) by lightning flashes has been computed from a model of gaseous molecular reactions occurring as heated lightning-channel air cools by mixing with surrounding ambient air. The effect of ozone (O3) on the production of nitrogen oxides has also been investigated in this model and it has been found that the O3 oxidizes NO to NO2 mainly at the end of the cooling process. The maximum total global production rate of nitrogen oxides by lightning is estimated to be ∼6×1027 molecules per second, or 14.4×106 tonnes of NO2, per year.

Kinetics and products of reactions of MTBE with ozone and ozone/hydrogen peroxide in water

Methyl-t-butyl-ether (MTBE) has become a prevalent groundwater pollutant due to its high volume use as a nationwide gasoline additive. Given its physicochemical properties, it requires new treatment approaches. Both aqueous O(3) and a combination of O(3)/H(2)O(2), which gives *OH, can remove MTBE from water, making use of O(3) a viable technology for remediation of groundwater from fuel contaminated sites. Rate constants and temperature dependencies for reactions of MTBE with O(3) or with *OH at pH 7.2, in a range of 21-45 degrees C (294-318K) were measured. The second-order rate constant for reaction of MTBE with O(3) is 1.4 x 10(18)exp(-95.4/RT) (M(-1)s(-1)), and for reaction of MTBE with *OH produced by the combination of O(3)/H(2)O(2) is 8.0 x 10(9)exp(-4.6/RT) (M(-1)s(-1)), with the activation energy (kJ mol(-1)) in both cases. At 25 degrees C, this corresponds to a rate constant of 27 M(-1)s(-1) for ozone alone, and 1.2 x 10(9) M(-1)s(-1) for O(3)/H(2)O(2). The concentration of *OH was determined using benzene trapping. Products of reactions of O(3) and O(3)/H(2)O(2) with MTBE, including t-butyl-formate (TBF), t-butyl alcohol (TBA), methyl acetate, and acetone, were determined after oxidant depletion. A reaction pathway for mineralization of MTBE was also explored. Under continuously stirred flow reactor (CSTR) conditions, addition of H(2)O(2) markedly increases the rate and degree of degradation of MTBE by O(3).

Concentration forcing in ammonia synthesis—I Controlled cyclic operation

Abstract Qualitative and quantitative arguments are made, based on accepted reaction mechanisms, that the Haber synthesis of ammonia should provide enhanced production, compared to steady state operation, when operated in a controlled cyclic mode (concentration forcing). In the study reported here, the potential for such an enhancement was examined experimentally at several temperatures and pressures. The observed trends are explained in terms of phenomenological steps. For conditions of low temperature and pressure, at which most detailed kinetic studies have been conducted, appreciable production enhancement is observed. However, for conditions of current industrial operation using conventional ammonia catalysts, steady state should provide greater production than concentration forcing operation. When using cyclic operation, the relaxed steady state approximation provides a convenient basis for comparing experimental production of ammonia with that predicted by phenomenological models. This comparison indicates a failure of the models of Temkin, Brill, and Ozaki, Taylor, and Boudart to describe the observed kinetics of ammonia synthesis during high-frequency, concentration-cycling operation.

Methanol synthesis from hydrogen, carbon monoxide and carbon dioxide over a CuO/ZnO/Al2O3 catalyst

The steady-state synthesis of methanol over a commercial low-pressure CuO/ZnO/Al2O3 catalyst was studied in a fixed-bed, steady-flow reactor with a well mixed fluid phase at pressures of 2.89 MPa and 4.38 MPa, temperatures between 483 K and 513 K, and varying CO/H2/CO2 compositions. Additional experiments were conducted by replacing carbon monoxide with helium in the feed gas to determine the effect of carbon dioxide on the methanol formation rate. Carbon dioxide was found to contribute significantly to the total production rate of methanol. It was found that the optimum carbon dioxide concentration occurred near 2 mole percent carbon dioxide in the feed. Higher carbon dioxide concentrations resulted in decreasing methanol production rate. In the limited number of experiments in which carbon monoxide was replaced with helium in the synthesis gas, no optimum carbon dioxide concentration, in the range of 2–8 mole percent carbon dioxide in the feed, was found beyond which the methanol production rate decreased. Thus it appears that carbon dioxide inhibits carbon monoxide hydrogenation when above 2 mole percent in the feed but not its own hydrogenation to methanol. Significantly more water was produced during the helium experiments than during those experiments in which carbon monoxide was present in the synthesis gas mixture. Thus, the water—gas shift reaction appeared to proceed in the reverse direction in the helium experiments and in the forward direction when carbon monoxide was present in the feed.

Methanol synthesis from hydrogen, carbon monoxide and carbon dioxide over a CuO/ZnO/Al2O3 catalyst: II. Development of a phenomenological rate expression

Abstract New steady-state kinetics data for the synthesis of methanol from H2/CO/CO2 over a CuO/ZnO/Al2O3 catalyst were correlated on the basis of reported catalytic mechanisms to formulate a rate expression for methanol formation. In developing the mechanistic model, it was assumed that hydrogen adsorption occurs on ZnO in contrast to carbon monoxide adsorption on Cu1+ and carbon dioxide adsorption on Cu0. Also, the concentration of sites containing adsorbed hydrogen was assumed constant during reaction. These assumptions led to the formulation of a rate expression containing the sum of contributions from carbon monoxide and carbon dioxide hydrogenation.

Supported liquid-phase catalysis: I. A theoretical model for transport and reaction

Abstract A simplified theoretical model is proposed for the transport and chemical reaction of gaseous species in supported liquid-phase catalysts (SLPC) in which catalytic liquids with contact angles less than 90 ° are dispersed within inert porous supports. It is based on the use of the dusty-gas model for the flux in the residual gas pore space. The variation of the structural dusty-gas parameters with liquid loading is correlated by using results borrowed from the parallel-pore model and the random-pore model along with an interconnected near-spherical cell description of the porous medium. The proposed model is free of adjustable parameters and is therefore predictive. It corroborates the existence of a maximum in the reaction rate predicted previously for some systems and also exhibits other trends observed experimentally. In addition, other interesting charac teristics of SLPC are predicted.

Dynamic analysis and control of a tubular autothermal reactor at an unstable state

Abstract The dynamic behavior of an autothermal reactor with internal countercurrent heat exchange is represented by a relatively simple mathematical approximat which retains essentially all of the steady state and dynamic features of real reactors. The system of partial differential equations is discretized in space by the method of orthogonal collocation and the convergence of the eigenvalues of the linearized model is found to be extremely difficult to achieve in the unstable region near blow-off. Different model reduction techniques are investigated and compared. Modal control with state or output feedback and a single manipulated input is used an attempt to stabilize the system. A technique based on a low-order collocation model provides good control action, whereas most of the conventional, approximate methods fail to stabilize the unstable model of the reactor. The features of the stable, closed-loop system are confirmed through simulated reactor transient tests.

Homogeneous catalysis of the water-gas shift reaction by ruthenium carbonyl complexes: studies in acidic solutions

Homogeneous catalysis of the water-gas shift reaction has been demonstrated for a system based upon Ru3(CO)12 in aqueous acidic diglyme solutions. At low partial pressures of carbon monoxide (Pco < 1 atm) the catalysis rate shows a first-order dependence upon both Pco and the total ruthenium concentration. In situ spectroscopic studies on the working catalyst lead to the formulation of the principal ruthenium species as a cationic dinuclear ruthenium carbonyl hydride derivative. The catalyst system shows a marked decrease in activity when Pco exceeds 1 atm, and this is attributed to an equilibrium between the dinuclear ruthenium hydride and Ru3(CO)13 favouring the latter at high values of Pco. A cyclic mechanism is proposed for the shift reaction catalysis and the systems activity dependence on various solution parameters.

On controlling an autothermal fixed-bed reactor at an unstable state—III: Model reduction and control strategies which avoid sstate estimation

Abstract A simplified approach for the control of fixed-bed reactors is discussed. Key elements include: (i) careful modeling of the reactor to obtain a lumped approximation with good internal structure (i.e. appropriate coupling between inputs and state variables through the different response modes); (ii) a major reduction in model size to an accurate, low-order form; (iii) derivation of a control algorithm for the low-order model; and (iv) direct application to the full-order system without the use of state estimation. The success of this approach is rooted in the choice of modeling techniques and of a model reduction procedure. The powerful new reduction method of Litz is evaluated here. Applications of the technique to a simulated unstable autothermal reactor are demonstrated for the case of modal control with incomplete state feedback. Results point to a high probability of success in subsequent experimental tests.

The hydrogenation and hydroformylation of alkenes as catalyzed by polymer-anchored rhodium trichloride under water gas shift reaction conditions

Abstract The ‘heterogenized’ water gas shift catalyst Rh/P4VP, prepared from the reaction of RhCl 3 with poly(4-vinylpyridine), is also active for hydrogenation and hydroformylation of 1-hexene and cyclohexene in aqueous ethoxyethanol under mild shift reaction conditions (typically 0.9 atm. P CO at 100°C). The catalytic activities for these systems were studied as functions of several experimental variables. Hydroformylation rates increased with the P CO but exhibited saturation behavior in the 1.5 atm. range. Rates for cyclohexane and hexane production were inhibited by CO at higher pressures. Cyclohexene hydroformylation and hydrogenation turnover frequencies were independent of the polymer-loading (50–150 μM RhCl 3 /1.0 g P4VP) indicating that the active species are of the same nuclearity as the principal species present. The temperature dependence did not follow simple Arrhenius behavior, but appeared segmented. These data are discussed in terms of possible mechanisms.