Wide Range Of Conversions Research Articles

A Kinetic Study of the SCWO of a Sulfonated Lignin Waste Stream

The kinetics of the supercritical water oxidation (SCWO) of a sulfonated lignin waste from an industrial source was investigated using a semi-batch, continuous-flow tube reactor. Experiments were carried out at reduced pressures from 0.9 to 1.4 and reduced temperatures from 0.9 to 1.2. In addition, the feed concentration, excess oxidant, flow rate in the reactor, and reactor length were varied to obtain a wide range of experimental residence times and conversions. The residence time and conversion data were used to calculate the reaction orders and Arrhenius parameters. The kinetic model was validated by comparison of the predicted to the experimental conversions. Overall, the results of the kinetic analysis were found to compare very favorably with those from model compound studies in the literature. Based on these results, it was concluded that the overall rate-limiting step was the oxidation of the sufonated lignin ring C−C bonds.

Stereochemical configuration of 2-hydroxyethyl methacrylate/styrene copolymers obtained in N,N′-dimethylformamide solution over a whole range of conversion

High resolution 1H-NMR spectra of 2-hydroxyethyl methacrylate (HEMA)-styrene (S) copolymers prepared at low conversion by free radical copolymerization in N,N-dimethylformamide (DMF) solution at 50°C, have been analyzed in terms of sequence distribution and stereoregularity. The Bootstrap effect for the HEMA/S/DMF system was confirmed by comparison of the data obtained with those previously reported for the HEMA/S/bulk system. Reactivity ratios, statistical parameters, and the coisotacticity parameter σHS = 0.40, estimated at low conversion, were used to describe the observed experimental changes in copolymer composition and triad fraction intensities over a wide range of conversions.

A new kinetic model for the NO–carbon reaction

The kinetics of the NO–carbon reaction was studied by thermogravimetric analysis (TGA). The reactivity as a function of carbon conversion was systematically investigated through characterization of the pore structure and carbon active site evolution during reaction. Total surface area measured by CO 2 adsorption at 273 K gives a relatively constant normalized reactivity over a wide range of carbon conversions. The carbon active surface area (ASA) and unoccupied active surface area (UASA) were estimated from low-temperature NO chemisorption capacities. Large amounts of stable C–O complexes are found to form during the reaction. Results show that ASA or UASA measured in this way do not represent the reactive surface area under gasification conditions. A new kinetic model is introduced by taking into account the contributions from different carbon–oxygen surface complexes during reaction, based on the recently proposed unified kinetic scheme of Yang and coworkers.

The reaction between limestone and SO 2 under periodically changing oxidizing and reducing conditions – effect of temperature and limestone type

The reaction between limestone and SO 2 was investigated under both oxidizing and periodically changing oxidizing and reducing conditions at atmospheric pressure in a fixed-bed quartz reactor. Three limestones of different reactivities were sulfated at temperatures between 800° and 875°C with a gas mixture of 1500 ppm SO 2, 10% CO 2 and alternating O 2 and CO (0% or 4%). The experimental data found that periodically changing oxidizing and reducing conditions could have either a positive or a negative effect on the degree of conversion of CaO to CaSO 4 compared to limestone samples sulfated under constant oxidizing conditions. In the temperature range 825–875°C the conversion was relatively constant for all three limestones under oxidizing conditions. However, when the limestone samples were sulfated with periodically changing oxidizing and reducing gas mixtures a wide range of conversions were found. The highest degree of conversion was found at 825°C for all three limestones and the lowest degree of conversion was found at 875°C. The rapid decrease in conversion at high temperatures was due to the rapid release of SO 2 due to both the decomposition of CaSO 4 as well as a high rate of CaS oxidation. A comparison of the sulfation rates of several experiments performed with Köping limestone under alternating conditions showed a wide range of rates, indicating the difficulties in using laboratory rate data in sulfur capture modelling for fluidized-bed boilers.

Effect of Potassium Promotion on Iron-Based Catalysts for Fischer–Tropsch Synthesis

The effect of potassium on Fischer–Tropsch catalyst activity, kinetic parameters, and selectivity has been investigated for a precipitated iron catalyst that was employed with low H2/CO ratio synthesis gas. A wide range of synthesis gas conversions have been obtained by varying space velocities over catalysts with various potassium loadings. Differing trends in catalyst activity with potassium loading were observed depending on the space velocity of synthesis gas conversion. As potassium loading increased, the catalyst activity either decreased (low conversion), passed through a maximum (intermediate conversion), or increased (high conversion). This is shown to be a result of the increasing dependency of the Fischer–Tropsch synthesis on the hydrogen formed by the water-gas shift reaction with increasing synthesis gas conversions. Both the rate constant and the adsorption parameter in a common two-parameter Fischer–Tropsch rate expression decreased with potassium loading; therefore, observed maxima in Fischer–Tropsch rate with potassium loading can be due to the opposing influences of these parameters. The effect of potassium on alkene selectivity was dependent on the number of carbon atoms of the hydrocarbons as well as the carbon monoxide conversion level. The extent of isomerization of 1-alkene product decreased with potassium loading, while the selectivity to methane decreased only slightly with increasing potassium content at CO conversions about 50% and higher.

Hardness and Salt Effects on Catalytic Hydrogenation of Aqueous Nitrate Solutions

Liquid-phase hydrogenation using a solid Pd–Cu bimetallic catalyst provides a potential technique for the removal of nitrates from waters. In this study, hardness and salt effects on both the nitrate disappearance rate and the reaction selectivity were quantitatively evaluated. Reduction experiments were performed for a wide range of nitrate conversions in an isothermal semibatch slurry reactor. It was discovered in a series of runs using various nitrate salts as a source of nitrate ions that the apparent surface reaction rate constant increases in the order K+<Na+<Ca2+<Mg2+<Al3+and changes proportionally with the ionization potential of a cation present in the aqueous solution. Permanent hardness of drinking water exhibits no inhibitive impact either on the extent of nitrate removal or on reaction selectivity. On the other hand, the nitrate disappearance rate as well as nitrogen production yield decrease appreciably in the presence of hydrogencarbonates, which is attributed to identical structures of nitrate and hydrogencarbonate ions resulting in competitive adsorption of these species to Pd–Cu active sites. A detailed analysis of experimental data confirmed that efficiency of the Pd–Cu bimetallic catalyst in catalytic nitrate reduction is influenced by the migration of produced hydroxide ions from the Helmholtz layer.

Kinetics of buta‐1,3‐diene hydrogenation over 0.5% Pd/γ‐Al2O3 catalyst

AbstractThe gas‐phase hydrogenation of buta‐1,3‐diene to but‐1‐ene, trans‐ and cis‐but‐2‐ene and butane has been performed over 0.5% Pd/γ‐Al2O3 catalyst. The reaction has been investigated at atmospheric pressure and 0°C over a wide range of conversions. The kinetic model based on the present day knowledge of reaction mechanism has been advanced. The parameters of the kinetic model have been estimated through statistical data fitting. The kinetic model provides a fairly good description of selectivity in the whole range of conversions.

Kinetics of buta-1,3-diene hydrogenation over palladium catalysts

The gas-phase hydrogenation of buta-1,3-diene to, but-1-ene, trans and cis-but-2-ene and butane over several monometallic Pd and bimetallic PdPb catalysts was studied. The reaction was investigated at atmospheric pressure and 0–20°C over a wide range of conversions. Selectivity and stereoselectivity towards reaction products was constant up to high conversions, when (around 85% conversion) selectivity in but-1-ene decreased dramatically. The reaction mechanism was advanced, which also took into account deactivation phenomenon. The parameters of the kinetic model were estimated through statistical data fitting. Kinetic model provides fairly good description of selectivity in the whole range of conversions. The values of estimated parameters are presented and discussed.

Kinetic Aspects of Selectivity and Stereoselectivity for the Hydrogenation of Buta-1,3-diene over a Palladium Catalyst

The gas-phase hydrogenation of buta-1,3-diene to but-1-ene, trans- and cis-but-2-ene, and butane over Pd catalysts was studied. The reaction was investigated in a reactor operating at atmospheric pressure at 0−20 °C over a wide range of conversions. Special attention was focused on the selectivity as a function of conversion. Up to high conversions (low values of buta-1,3-diene mole fractions) selectivity and stereoselectivity toward reaction products were constant. At approximately 85% conversion the selectivity in but-1-ene was seen to decrease. The selectivity and stereoselectivity were described within a framework of the advanced reaction mechanism. Deactivation influenced the activity and selectivity of the reaction. Deactivation kinetics was taken into account, assuming the formation of higher molecular weight oligomerized olefins. The parameters of the kinetic model were estimated with nonlinear regression analysis. Simulations showed that the kinetic model is able to describe selectivity and stereoselectivity behavior in buta-1,3-diene hydrogenation over the whole range of conversions.

Influence of Hydrogen and Catalyst on Distillate Yields and the Removal of Heteroatoms, Aromatics, and CCR during Cracking of Athabasca Bitumen Residuum over a Wide Range of Conversions

The percent conversion of sulfur, nitrogen, vanadium, nickel, aromatics, and CCR (Conradson carbon residue) has been analyzed as a function of residuum conversion for five series of reactions carried out in a batch reactor under nitrogen, hydrogen, or hydrogen in the presence of a residuum hydrotreating catalyst, over residuum conversions ranging from 38 to 86%. Sulfur, nitrogen, and CCR conversions could be divided into three categories, up to 50% residuum conversion, 50-70% residuum conversion, and greater than 70% residuum conversion. Sulfur conversion in the first category was from the distillable liquids and resulted from a combination of thermal, hydrogenation, and hydrogen atom addition reactions. Nitrogen removal in the first category was due to incorporation of nitrogen into reactor solids in the early stages of the reaction and was therefore suppressed by hydrogen in the absence of a catalyst.

Anionic polymerisation of propylene oxide. Investigation of double‐bond and head‐to‐head content by NMR spectroscopy

AbstractPropylene oxide was anionically polymerised in bulk at 80°C under a range of initial conditions, from zero hydroxyl content to a mole ratio of hydroxyl to potassium catalyst of 10. The double bond content of the polymers was measured by 1H NMR spectroscopy over a wide range of conversions, and values of the transfer constant were determined. The results were fitted to a reaction mechanism involving suppression of the transfer reaction by hydrogen bonding of hydroxyl groups to active ion pairs. The head‐to‐head content of the polymers was measured by 13C NMR spectroscopy. The overall proportion (approximately 4%) of ‘head‐to‐head’ reactions relative to head‐to‐tail addition was not greatly affected by suppression of the transfer reaction, since an increase in the number of head‐to‐head additions compensated the decrease in proton abstraction from methyl groups via the transfer reaction.

Calorimetric Method for the Investigation of Copolymerization Kinetics

Abstract A calorimetric method for measurement of the copolymerization rate over a wide range of conversions has been derived. The well-known methyl methacrylate-styrene system was used as the example to prove the efficiency of the approach proposed.

Sequence distribution and stereoregularity in methyl methacrylate-methyl acrylate copolymers at high conversions

Methyl methacrylate (MMA)-methyl acrylate (MA) copolymerizations have been analysed over a wide range of conversions with the purpose of predicting the changes in copolymer composition and comonomer sequence distribution as a function of conversion. High resolution 1H n.m.r. spectra (200 MHz) of MMA-MA copolymers, prepared at two overall comonomer concentrations by free radical copolymerization in benzene solution at 50°C at low conversion, have been analysed in terms of comonomer composition, sequence distribution and stereoregularity. Reactivity ratios for the terminal model, statistical parameters, P ij , and co-isotacticity parameters, σ 11 = 0.23 and σ = σ 12 = σ 21 = 0.50, estimated at low conversion were used to describe the changes in copolymer composition and triad fraction intensities with conversion. The experimental sequence distribution intensities were in agreement with the estimated values. From the good agreement found between experimental and calculated values, it can be concluded that the terminal model, through reactivity ratios and Bernoullian statistics, provides a very precise description of copolymerization over the whole range of conversion and composition and overall concentration of monomers.

Hydrolysis of triolein by lipase in a hollow fiber reactor

Hydrolysis of triolein by lipase was carried out in a hollow fiber membrane reactor, oleic acid and glycerol being formed as products. It was found that hydrolysis of triolein by lipase in the hollow fiber reactor can be quantitatively explained by a Michaelis-Menten mechanism at the interface, together with competitive inhibition by oleic acid, diffusion of all chemical species and adsorption of lipase at the interface between the aqueous and organic phases. The reaction model could be applied to a wide range of conversions in the membrane reactor.

The conversion of butanes in HZSM-5

The distribution of products from the conversion of iso- and normal butane on HZSM-5 has been determined from 300 to 550°C over a wide range of conversions. The results clearly indicate that the nature of the products changes significantly in altering both the temperature and conversion conditions. High temperatures and low conversions favor low molecular weight products expected from a protolytic or radical attack on the butane. Reverse conditions favor C 3 and higher hydrocarbons. These products are indicative of a classical carbenium ion mechanism. As conditions were changed the transition in mechanism was indicated by the amount of methane produced. For the first time H 2 was detected as a primary product in the conversion of normal butane indicating that a tertiary C-H bond is not required for its production.

Monte Carlo description of A f homopolymerization: Diffusional effects

We have developed a new algorithm that simulates the structure buildup process during homopolymerization of Af monomers. The algorithm is an off lattice percolation solution on a cube with periodic boundary conditions. All monomers are treated as point particles. After choosing any two nodes at random, the probability of their reaction is computed and compared with a random number. We present results where the probability rule is a step function over the interunitary distance between the chosen pair. We reason that this simulates, in a simplistic but effective fashion, the importance of diffusion vs reaction times in the problem. Thus, the case where units react in immediate neighborhoods corresponds to a diffusion limited growth and where they react with equal probability with all other units to the mean field or kinetic limited growth. We present results for five step reactive potentials with careful analysis of finite size effects. We find that the gel conversion is delayed as a consequence of diffusion limitations. We find that cyclization is only partially responsible for this effect. We also find that small mobilities, such as might be available from rotational and translational degrees of freedom, give results realistically close to the mean-field results, thus explaining the experimental success of the mean-field theory. We find that the (effective) scaling exponent γ depends on the reactive radius and thus on diffusion. It is not entirely clear to us if this suggests nonuniversal behavior or is simply a crossover problem. We conclude that the dependence of the critical exponent on diffusion over a wide range of conversions suggests that some of the ‘‘nonuniversal’’ experimental observations might be due to diffusional effects.

Study of the properties of pentasil-containing catalysts in reactions of transformation of hydrocarbons 5. Kinetics and mechanism of aromatization of propane and propylene on Ga and H forms of pentasils

The kinetics of transformations of propane and propylene on Ga-modified pentasils and the H-form of zeolite were investigated in a wide range of conversions (20–95%). The selectivity of formation of aromatic hydrocarbons (ArH) on Ga-pentasils is significantly higher than on HTsVM. Quantitative data which demonstrate acceleration of aromatization of propane and propylene in modification of H-TsVM zeolite with gallium were obtained. An especially large increase in the reaction rate is observed in the case of the starting olefin: the rate constants on Ga/HTsVM and HTsVM are respectively equal to 9.21 and 1.44 sec−1. The significant effect of H2 on the selectivity of action of the Zn/HTsVM system in aromatization of propane was demonstrated. The effect of H2 was virtually not manifested on Ga/HTsVM in the conditions studied. The data obtained in the study permitted introducing important corrections in the scheme of the mechanism of aromatization of lower alkanes examined in the literature.

Motionless mixers for the design of multitubular polymerization reactors

AbstractExperimental investigations of thermal bulk polymerization of styrene in pilot plants of different sizes have been performed. Each pilot plant is composed of a tubular recycle reactor, connected in series with a tubular reactor, both completely filled with Sulzer motionless mixers. Kinetic, reactor and viscosity models have been verified in a wide range of styrene conversions (up to 96%) temperatures (up to 210 °C) and polystyrene molar masses (up to 360 000). Scale‐up studies were carried out which confirmed that multitubular reactors of special design can be applied for industrial polymerization process.

Carbon dioxide absorption into promoted carbonate solutions

AbstractCO2 absorption rates into diethanolamine (DEA)‐promoted carbonate solutions were measured over a wide range of carbonate conversions and temperatures. Analysis of the data showed that the CO2‐DEA reaction rate was controlled by the rate of formation of zwitterion intermediate for carbonate conversions below 30%, and was controlled by the rate of abstraction of proton from the zwitterion intermediate for carbonate conversions greater than 30%. Hydroxyl ion, free amine, and carbonate ion were identified to act as bases to abstract the proton from the zwitterion. The modeling of the CO2 absorption rates and the effects of carbonate conversion and amine concentration on the CO2‐DEA reaction kinetics were investigated.

Molecular weight-conversion curves in the diffusion-controlled free radical polymerization of styrene

To calculate molecular weights of polymer formed for a wide range of conversions in radical polymerization, we derive formulae based on termination rate being related to conversion through the free volume theory and reptation theory. The curves calculated for the relationship between molecular weight and conversion fit the experimental data obtained in the bulk polymerization of styrene initiated chemically.