Base-catalyzed hydroxymethylation of phenol by aqueous formaldehyde. Kinetics and mechanism

The effects of formaldehyde and hemiformal equilibria on the rates of the base-catalyzed and hydroxymethylations of phenol by aqueous formaldehyde have been treated quantitatively. A kinetic model treating all seven simultaneous, parallel, consecutive, and competing reactions, as well as the equilibria, has been shown to apply to a wide variety of the conditions and to data from other works. Concentration-induced solvent effects on the rates and on the ortho/para-substitution patters have been studied. The temperature dependence of the rate constants, both for dilute systems and systems composed primarily of phenol and aqueous formaldehyde, has been determined.     

A theoretical investigation of the production of branched copolymers in continuous stirred tank reactors

A comprehensive mathematical model for free-radical copolymerization reactions has been developed for a homogeneous continuous stirred tank reactor. The present model is based on a fairly general copolymerization scheme accounting for the formation of linear and branched copolymer chains. Both chain transfer to polymer and terminal double bond reactions are considered in order to predict the long chain branching frequency. Changes in molecular weight, composition and degree of branching occurring during the copolymerization reaction are modelled using the method of moments. To break-down the dependence of the moment equations on higher order moments two different closure methods are considered. The predictive capabilities of the model are examined in relation to the solution copolymerization of methyl methacrylate with vinyl acetate. It is shown that both chain transfer to polymer and terminal double bond reactions significantly contribute to the broadening of the molecular weight and degree of branching distributions. Furthermore, the terminal double bond reaction effects significantly the copolymer number-average molecular weight and the concentration of terminal double bonds.     

Regioselective reactions of methyl pyropheophorbide a with formaldehyde based on hydroxymethylation

Regioselective reactions of methyl pyropheophorbide a (MPPa) with formaldehyde based on hydroxymethylation have been studied. It was found that MPPa can react regioselectively with formaldehyde under different conditions to produce a series of 3-dioxane, 12-, 13²- or 20-hydroxymethyl and 12-/13²-alkenyl-substituted chlorins via Prins reaction, Blanc chloromethylation and aldol reaction, respectively. The first examples of direct C–C bond formation at 12-position of chlorophyll derivatives were also reported to give a series of 12-vinyl-substituted chlorins. These chlorins showed extend Qy absorptions and efficient singlet oxygen generation property, indicating their potential as photosensitizers for application in photodynamic therapy.     

Radical hydroxymethylation of alkyl iodides using formaldehyde as a C1 synthon

Radical hydroxymethylation using formaldehyde as a C1 synthon is challenging due to the reversible and endothermic nature of the addition process. Here we report a strategy that couples alkyl iodide building blocks with formaldehyde through the use of photocatalysis and a phosphine additive. Halogen-atom transfer (XAT) from α-aminoalkyl radicals is leveraged to convert the iodide into the corresponding open-shell species, while its following addition to formaldehyde is rendered irreversible by trapping the transient O-radical with PPh₃. This event delivers a phosphoranyl radical that re-generates the alkyl radical and provides the hydroxymethylated product.

Halogen-atom transfer (XAT) based on phosphoranyl radical chemistry enables the hydroxymethylation of alkyl iodides with formaldehyde.     

CFD simulation of floating particles suspension in a stirred tank

Computational fluid dynamics (CFD) simulations were performed to predict the floating particles suspension in a baffled tank stirred by a standard Rushton turbine. An Eulerian multiphase model and a standard k-ε turbulence model with mixture properties were used in the CFD simulation. The impeller rotation was solved using a moving reference frame method. Flow pattern, power number and solid holdup distribution were obtained and compared with the results in literature. The effects of operating condition on floating particles suspension characteristics were studied. It indicated that the influences of impeller speed and solid loading on particle suspension varied with particle sizes. For small particles, the impeller speed and solid loading have no obvious effects on solid holdup distribution and suspension quality. For large particles, particle suspension quality becomes better first, and then keeps almost unchanged with enhancing of the impeller speed. Suspension quality is better for higher solid loading of large particles. Within the scope of the present study, solid loading has no great effect on suspension quality. Suspension quality becomes worse with increasing of the particle size. Large particles are easy to accumulate in the centres of the liquid free surface and the upper circular loop, and the vicinity of the shaft.     

Adaptive nonlinear control of a continuous stirred tank reactor

The paper deals with continuous-time nonlinear adaptive control of a continuous stirred tank reactor (CSTR). Control strategy is based on the application of a controller consisting of a linear and a nonlinear part. The static nonlinear part is derived as an inversion and exponential approximation of measured or simulated input-output data. Design of the dynamic linear part is based on an approximation of nonlinear elements in the control loop by a continuous-time external linear model with directly estimated parameters. In the control design procedure, polynomial approach with the pole assignment method was used. The nonlinear adaptive control was tested by simulations on a nonlinear model of a CSTR with a consecutive exothermic reaction.     

Kinetic behavior of immobilized aldolase in a continuously stirred tank reactor

Aldolase was bound covalently to CNBr-activated Sepharose. The immobilized enzyme was investigated in a continuously stirred tank reactor (CSTR). In spite of the complexity of the system, the stationary performance of the CSTR can be described by a very simple equation. There are essentially two control parameters,V _m andK _eq— that govern the stationary performance of the aldolase reactor.     

Effect of ultrasound on free-radical polymerization in a continuous stirred tank reactor

Methyl methacrylate has been continuously polymerized in toluene solution with AIBN as an initiator at 60°C in a jacketed steel vessel of 890 ml volume with holding times of several hours. Provision was made for good mixing and introduction of a steady stream of monomer, solvent, and initiator. The effluent stream was analyzed for polymer content and molecular weight distribution by gel-permeation chromatography. After the reactor had been operating for three holding times, an ultrasonic probe in the reactor was turned on, and the effect of the ultrasound on the steady-state operation of the reactor was observed. The ultrasound caused a rapid increase in the conversion level. A small, concomitant increase in the degree of polymerization was also noted. It is proposed that the ultrasound breaks polymer chains, giving polymeric free radicals which increase in size while increasing the rate of polymerization. Simulation of the polymerization in the stirred tank reactor has been achieved by using a digital computer, and a quantitative treatment of the effect of ultrasound has been developed.     

Numerical simulation of hydrodynamics in an uncovered unbaffled stirred tank

Numerical simulations were carried out to investigate the turbulent flow with free-surface vortex in an uncovered unbaffled stirred tank. An Eulerian–Eulerian multiphase flow model coupled with a volume-of fluid method was applied to capture the gas–liquid interface and describe the flow field in the tank. Turbulence is computed using a Reynolds stress model with seven equations. The reliability and accuracy of the simulations are verified by comparing the predicted free surface profiles and power numbers with experimental data in the literature. The overall agreements between the simulation results and experimental measurements are obtained. The simulation results show that vortex on the liquid free surface in the uncovered unbaffled tank can be divided into central zone and peripheral zone, and the critical radius of the two zones keeps almost unchanged with enhancing of impeller speeds. At the same impeller speed, vortex becomes deeper and larger with increasing of impeller diameter. However, vortex shape changes not very much with increasing of impeller diameter under the same power consumption. Impeller clearance has no significant influence on the vortex shape. Circumferential flow is dominant in the uncovered unbaffled tank, and the flow field can be divided into forced vortex region and free vortex region. Power numbers in uncovered unbaffled stirred tanks decrease slightly with increasing of the Reynolds number, and are far lower than that in baffled stirred tanks.     

Evaluation of mass-transfer and kinetic parameters forRhodospirillum rubrum in a continuous stirred tank reactor

The photosynthetic bacteriumRhodospirillum rubrum has been evaluated for its ability to produce hydrogen from carbon monoxide and water in a continuous stirred tank reactor according to the watergas shift reaction. An assessment of mass-transfer parameters and reaction kinetics was made for this sparingly soluble substrate system. Experiments were conducted in a nonsteady-state fashion with continuous liquid and gas flow, which allowed for separation of the mass-transfer and kinetic-limited regions. Based on the data obtained, mass-transfer coefficients for the system were determined, and a mathematical expression for the reaction kinetics was formulated. The results showed that the hydrogen production was inhibited by elevated levels of dissolved carbon monoxide in the liquid.     

Catalytic asymmetric hydroxymethylation of silicon enolates using an aqueous solution of formaldehyde with a chiral scandium complex

Catalytic asymmetric hydroxymethylation of silicon enolates has been achieved. In this reaction, an aqueous solution of formaldehyde can be used to realize an easy and safe procedure, and high enantioselectivities have been obtained. This is the first example of catalytic asymmetric reactions in aqueous media with a chiral scandium complex.     

Temperature oscillation calorimetry by means of a Kalman-like observer: the joint estimation of Qr and UA in a stirred tank polymerization reactor

The present work is concerned with the joint estimation of the rate of heat produced by the emulsion terpolymerization of styrene, butyl acrylate and methyl methacrylate (Q_r), and the overall heat transfer coefficient (UA) from temperature measurements and reactor heat balance. By making specific assumptions on the dynamics of the parameters UA and Q_r, we designed a Kalman-like observer to carry out the estimation of these two time-varying parameters, without the need for neither additional measurements nor on-line samples. Temperature oscillations are induced in the cooling jacket in order to ensure the observability of the system. One further aspect of our approach is that it only requires the reactor energy balance to perform the estimation.     

Base-catalyzed cascade 1,6-sulfur-Michael/Henry reaction of trifluoromethyl-substituted styrylisoxazoles: Diastereoselective synthesis of tetrahydrothiophenes with a trifluoromethylated quaternary center

An effective diastereoselective [3+2] annulation of 1,4-dithiane-2,5-diol to trifluoromethyl-substituted styrylisoxazoles catalyzed by DABCO is described, giving highly functionalized tetrahydrothiophenes containing trifluoromethylated quaternary center in excellent yields and diastereoselectivities.     

Curing of a phenol–formaldehyde–tannin adhesive in the presence of wood

The curing of a phenol–formaldehyde–tannin (PFT) adhesive in the presence of pine or eucalyptus wood has been studied using differential scanning calorimetry. The influence of the adhesive/wood ratio on the activation energy (E_a), the temperature of the maximum of the exothermic peak (T_p) and the enthalpy of the curing process (ΔH) was analysed. E_a, T_p and ΔH of the curing reaction decreased when wood was added in the curing system. The adhesive/wood interaction did not depend significantly on wood species.     

Development of a continuous homogeneous process for denitration by treatment with formaldehyde

Removal of nitric acid from high level liquid wastes (HLLW) of nuclear fuel reprocessing plants is warranted for simplifying the procedure for waste fixing. Chemical denitration aims to reduce the waste volume by destroying the acidity and subsequent concentration by adding suitable reductants. Reduction of nitric acid to gaseous products is an attractive way to accomplish denitration. Nitric acid reduction with formaldehyde proceeds with the formation of CO₂, NO₂, NO or N₂O depending on the reaction conditions and all the reaction products except water can be eliminated from the system in gaseous form. The HNO₃–HCHO reaction is governed by a complex mechanism of exhibiting relatively long induction period, depending upon the temperature, concentration of reactants and nitrous acid reaction intermediate. In the present work, a homogeneous denitration process with formaldehyde which offers safety and is governed by controlled kinetics was demonstrated on a laboratory scale. The induction period before commencement of the reaction was eliminated by maintaining the reaction mixture at a pre determined temperature of 98 °C. Based on the results accrued from lab scale experiments, the equipment for pilot plant scale operation was designed, the reaction efficiency for continuous denitration was determined and the investigation of nitric acid destruction was extended to full-scale plant capacity. The role of organics in the waste in foaming up of the reaction mixture was also studied using a synthetic waste solution.