Mass transfer and selectivity of ozone reactions

The present research concerns the behavior of mass transport in absorption of ozone accompanied by decomposition and ozonation reactions in aqueous solutions. On the basis of the film theory, a mathematical model has been formulated for the transport process taking into account molecular diffusion and the simultaneous decomposition and ozonation reactions of different orders. Analytical approximate and finite difference methods have been employed to predict concentration profiles, enhancement factor and selectivity of the chemical mass transfer in gas-liquid systems. The rate of mass transfer is enhanced by the chemical reactions as well as the availability of liquid reactant in the aqueous phase. When the ozonation reaction is much faster than the decomposition reaction, a large fraction of the absorbed ozone is utilized effectively in reacting with the liquid constituent and a high selectivity of the gas-liquid reactions can be obtained. Calculated results indicate that removal of certain organic pollutants by the oxidation process may be effective only in acidic or neutral solutions where a high mass transfer rate and selectivity is achieved.     

Drop mass transfer in a microfluidic chip compared to a centrifugal contactor

A model system was developed for enabling a multiscale understanding of centrifugal‐contactor liquid–liquid extraction. The system consisted of Nd(III) + xylenol orange in the aqueous phase buffered to pH = 5.5 by KHP, and dodecane + thenoyltrifluroroacetone (HTTA) + tributyphosphate (TBP) in the organic phase. Diffusion constants were measured for neodymium in both the organic and aqueous phases, and the Nd(III) partition coefficients were measured at various HTTA and TBP concentrations. A microfluidic channel was used as a high‐shear model environment to observe mass transfer on a droplet scale with xylenol orange as the aqueous‐phase metal indicator; mass‐transfer rates were measured quantitatively in both diffusion and reaction limited regimes on the droplet scale. The microfluidic results were comparable to observations made for the same system in a laboratory scale liquid–liquid centrifugal contactor, indicating that single drop microfluidic experiments can provide information on mass transfer in complicated flows and geometries. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3071–3078, 2014     

相转移催化元素硫歧化反应的动力学

在45—70℃、氢氧离子浓度为0.02—0.14mol/l范围内研究了四氯乙烯/水体系中氢氧化四丁铵相转移催化元素硫歧化反应初始阶段的动力学.实验得到表观速度表达式:R_(?)a=k~(?)[Q~+]_(aq)[OH~-]_(aq)[S]_(org)~(1/2),并导得表观活化能为79kJ/mol.用液液体系伴有快速化学反应的传质模型成功地解释了本体系动力学.推测了元素硫的歧化反应机理,从动力学上提出了本体系相转移催化的证据.     

Population balance modeling and simulation of liquid–liquid–liquid phase transfer catalyzed synthesis of mandelic acid from benzaldehyde

Mandelic acid has cosmetic, pharmaceutical, and antibacterial activities and is used in urinary antiseptic medicines. An attractive process for the production of mandelic acid is through reaction between benzaldehyde, sodium hydroxide, and chloroform in the presence of polyethylene glycol 4000 as a phase transfer catalyst. The liquid–liquid phase transfer catalyzed (L–L PTC) reaction can be intensified by converting it into three‐liquid phases (L–L–L PTC). We address the modeling of a well‐stirred reactor for the foregoing process, in which organic droplets surrounded by a thin film of catalyst‐rich phase are suspended in the aqueous phase. A population balance model is formulated for the L–L–L PTC reaction and solved by Monte Carlo simulation using interval of quiescence technique. Transport processes and intrinsic reaction kinetics are extracted from the experiments. This population balance model serves to assess and interpret the relative roles of various processes in L–L–L PTC reaction, such as diffusive transport, reaction, and interaction between dispersed phase droplets. The model is expected to be an effective tool for reactor design and scale up. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

Effect of third-phase properties on benzyl-n-butyl ether synthesis in phase transfer catalytic system

The production of benzyl-n-butyl ether from benzyl chloride and n-butanol is studied in phase transfer catalytic system. A third phase is formed when polyethylene glycol (PEG) and dodecane are used as the phase transfer catalyst (PTC) and an organic solvent, respectively. The production rate at the three-phase system is higher by seven times than that at the two-phase system. The ether production rate and its selectivity are dependent on the initial concentration of n-butanol. These are affected by the properties of the third phase, especially the concentrations of n-butanol and water in the third phase.

n-Butanol reacts with benzyl chloride and potassium hydroxide simultaneously. The reaction between n-butanol and potassium hydroxide occurs in the aqueous phase. Then, the selectivity of ether on a basis of initial n-butanol is below 0.6 in a stirred tank batch reactor. The selectivity is much improved at 0.9 by using a static triphase batch reactor in which the organic and aqueous phases are separated by the third phase. The interphase mass transfer can be accelerated by ultrasonic device.     

Investigation of hydrodynamic and mass transfer of mercaptan extraction in pulsed and non-pulsed packed columns

We investigated the hydrodynamic behavior and mass transfer characteristics of a pilot-scale conventional packed bed extraction column of mercaptan removal from liquid propane. The extraction column was filled with pall rings structured packing where mercaptan was extracted from the continuous phase to the dispersed phase, accompanied by a chemical reaction in propane-mercaptan-caustic system. The pulsing was introduced into the column to enhance the mass transfer rate. Hydrodynamic parameters such as hold up, flooding velocity and mean drop size were studied together with the effect of chemical reaction on increasing mass transfer performance. Finally, the mass transfer and axial mixing coefficients were obtained from the optimization of data by ADM. It was found that at the pulsation intensity from 0.003 to 0.007 m/s, the maximum mass transfer and minimum axial mixing occurred and it can be concluded that pulsation improves the efficiency of mass transfer just at low intensities.     

Mass transfer of phenol through supported liquid membrane

The separation of phenol from the aqueous solution was carried out at 25°C in a supported liquid membrane of batch type using benzene or dibenzo-18-crown-6 as carrier in the phenol-NaOH system. The mass transfer of phenol was investigated with a theoretical model based on the mass transfer with or without chemical reaction in the stripping side. Pseudo-first-order reaction type was used to measure overall and individual mass transfer coefficients of phenol. The influence of initial concentration of carrier on overall mass transfer coefficient was found to be more significant than those of agitation speed and initial concentrations of phenol and NaOH solutions. The numerical analysis of facilitated transport of phenol through liquid membrane gave a result that the chemical reversible reaction between phenol and carrier in the liquid membrane side was fallen into the region between fast and slow reaction with the tendency to be much closer to the slow reaction region.     

Electron and ion transfer potentials of ferrocene and derivatives at a liquid-liquid interface

The electrochemistry of ferrocene, dimethyl ferrocene and decamethyl ferrocene at the interface of immiscible electrolytes has been investigated. The aqueous redox electrolyte was the hexacyanoferrate couple. It has been found that the study of the electron transfer reaction can be complicated by both the transfer of the ferricenium ion formed in the organic phase (1,2-dichloroethane) during oxidation, and by oxidation of the organic base electrolyte anion. However, by using a high concentration of aqueous electrolyte, the difference between ionic and electronic transfer potentials can be increased sufficiently to observe these two processes separately. Ionic transfer potentials of the ferricenium ion have been measured by preparing the corresponding FeCl⁻₄ salts. For decamethyl ferrocene, no electron transfer reaction could be observed at the liquid/liquid interface and it is proposed that this is due to distance effects, which become more marked for this more hydrophobic ferrocene derivative.     

微孔射流毛细管反应器内液-液两相传质和反应选择性

微反应技术在化工过程强化领域已得到广泛应用,尤其适用于快速复杂竞争反应体系。对于液-液两相快速竞争反应,反应过程受传质限制,显著影响反应转化率和收率。本文开发了一种新型的微孔射流毛细管反应器（MJCM）,采用微孔射流强化进口处液-液两相传质性能,分别采用水-苯甲酸-煤油体系和水-氢氧化钠-甲苯-苯甲酸-氯乙酸乙酯体系研究了不同操作参数（流量、流量比、表面活性剂浓度、温度）和结构参数（孔径、管长）下液-液两相传质特性和反应选择性,并获得了舍伍德数Sh的关联式。结果表明：随着两相流量的增加,传质效率E呈下降趋势,总体积传质系数k_La呈增加趋势,反应选择性指数X_s则先减小后增大;孔径的增大则会减弱液液传质和反应选择性;随着毛细管长的增加,E和X_s逐渐增大,k_La则逐渐减小;水相-有机相流量比的变化对E和k_La会产生不同影响,而温度的适当升高则可以提升反应选择性,表面活性剂的加入降低了传质和反应选择性。与其他液液传质设备对比,MJCM在液-液两相传质、反应选择性方面性能良好,可以用于工业生产进行液液传质与反应过程的强化。     

New liquid membrane technology for simultaneous extraction and stripping of copper(II) from wastewater

In this paper, a new liquid membrane technique, hollow fiber renewal liquid membrane (HFRLM), is presented, which is based on the surface renewal theory, and integrates the advantages of fiber membrane extraction, liquid film permeation and other liquid membrane processes. The results from the system of CuSO₄+D2EHPA in kerosene+HCl show that the HFRLM process is very stable. The liquid membrane is renewed constantly during the process, the direct contact of organic droplets and aqueous phase provides large mass transfer area. These effects can significantly reduce the mass transfer resistance in the lumen side. Then the mixture of feed phase and organic phase flowing through the lumen side gives a higher mass transfer rate than that of stripping phase and organic phase, because the aqueous layer diffusion of feed phase is the rate-controlling step. The overall mass transfer coefficient increases with increasing flow rates and D2EHPA concentration in the organic phase, and with decreasing initial copper concentration in the feed phase. The overall mass transfer coefficient also increases with increasing pH in the feed phase, and reaches a maximum value at pH of 4.44, then decreases. Also, there is a favorable w/o volume ratio of 20:1 to 30:1 for this process. Compared with hollow fiber supported liquid membrane and hollow fiber membrane extraction processes, HFRLM process has a high mass transfer rate. Mathematical model for the HFRLM process based on the surface renewal theory is developed. The calculated results are in good agreement with experimental results under the conditions studied.     

The influence of the interfacial resistance on the liquid—liquid mass transfer of carboxylic acids

The instantaneous solute concentration in freely suspended single droplets were measured as function of the exchange time by means of a radionuclide technique with and without chemical reaction in liquid—liquid systems in several drop sizes covering the range of Reynolds numbers 114–443.The transfer rates of benzoic acid and/or caprylic acid from toluene drop into water as well as of caprylic acid into aq. NaOH solution were investigated. Benzoic and caprylic acids prevail in the toluene phase as dimers and in the water phase as monomers. In case of the mass transfer of the benzoic acid the chemical reaction of the dimer to the monomer causes an interfacial resistance which has the same order of magnitude as the mass transfer resistance in the continuous phase.In case of the mass transfer of the caprylic acid the solute forms a monolayer at the interface. The rate determining step is the desorption of the solute from this monolayer.In case of the mass transfer of the caprylic acid to aq. NaOH solution the mass transfer resistance of the continuous phase can be neglected due to the enhancement of the mass transfer by the instantaneous reaction in this phase. The mass transfer rate is very high at the beginning of the mass exchange due to the interfacial turbulence caused by the chemical reaction. Also at longer contact times the mass exchange rate is high probably due to the reduction of the free desorption energy (increase of the desorption constant) caused by electrically charged monolayer.     

Rhodium‐catalyzed hydroformylation of unsaturated fatty esters in aqueous media assisted by activated carbon

Concerns about the environmental impact of chemical transformations prompted chemists to develop clean chemical processes using water as a solvent. Although appropriate for small partially water‐soluble molecules, these processes do not allow for the transformation of hydrophobic substrates due to the mass transfer limitation between the aqueous and the organic phase. In this context, we show that activated carbons can be used as mass transfer additives to promote the rhodium‐catalyzed hydroformylation of methyl oleate and other unsaturated olefins. Due to its mesoporous and hydrophobic character, the Nuchar®WV‐B activated carbon proved to be especially effective as mass transfer promoter. Actually, a significant increase in the conversion was observed. Additionally, more than 90% aldehydes were formed during the course of the reaction. When compared to other mass transfer promoters such as co‐solvents or cyclodextrins, Nuchar®WV‐B was by far the most efficient. Thus, the use of activated carbons appeared to be a suitable solution for the aqueous rhodium‐catalyzed hydroformylation of hydrophobic bio‐sourced substrates. Practical applications: The easiness with which the FAME hydroformylation could be implemented in water using activated carbons as mass transfer promoters is a major advantage in a context of an industrial–environmental approach. This finding is of importance as the obtained oxo‐products can be used in many industrial areas such as surfactants, polymers, or lubricants.     

第二液相对气液二相体系传质的影响

以CO2气体-K2CO3/KHCO3水溶液吸收过程为研究体系,用酸解法测量气体被吸收的速率,通过对比试验考察了加入第2液相(有机相)对体系传质速率的影响。经过试验研究证明,第2液相的加入对气液传质过程的影响程度与加入的物质有关,在试验条件下甲苯对体系强化作用高于异辛醇和庚烷对体系的强化作用。当第2液相加入量较小时,随加入量的增加,其对气液传质过程的促进作用增强,但当第2液相加入量较大时,这种作用则不明显。同时第2液相对传质作用的影响与流动场有关,增加流动场的搅动有助于强化气液传质作用。     

Mass transfer effects on hydroformylation catalyzed by a water soluble complex

The rate of hydroformylation of 1-octene catalyzed by a water soluble catalyst is measured in mechanically agitated batch reactor at various stirrer speeds and organic phase holdups. The data have been analyzed by coupling reaction kinetics to a pseudo-homogeneous gas–liquid–liquid model based on Higbie's penetration theory which takes into account the presence of the dispersed organic phase. A rapid liquid–liquid mass transfer of the reactants is assumed leading to an equilibrium between the continuous and the dispersed phases. The predicted values of the rate are in good agreement with the experimental one. The depletion of the organic substrate in the continuous phase is found negligible.     

Study of auto-accelerated reactions in heterogeneous liquid/liquid mixture: Reaction products effects on mass transfer kinetics

The alkaline hydrolysis of aromatic esters exhibits auto-accelerated kinetics when performed under two phase conditions. This auto-acceleration behavior is linked to reaction products effects (surfactant and co-solvent) on mass transfer kinetics. The co-solvent increases the equilibrium concentration of organic compounds in aqueous solution, while the surfactant modifies the mass transfer coefficient and interfacial area. In this research work, the selected reaction is the alkaline hydrolysis of methyl benzoate carried out in a calorimetric reactor Mettler RC1 in isothermal semi-batch mode. The parameters estimation of mass transfer and chemical kinetics has been conducted using thermal power profile. Two functions—giving the evolution during reaction of the mass transfer coefficient and the equilibrium concentration—are proposed accordingly to surfactant and co-solvent concentrations.     

Mass transfer with chemical reaction in liquid foam reactors

Mass transfer studies were conducted in a stable liquid foam reactor under various operating conditions to evaluate gas holdup, effective interfacial area, liquid-phase mass transfer coefficient and a modified interfacial mass transfer coefficient to include the surface-active agents employed. Gas holdup and effective interfacial area were evaluated experimentally. The interfacial mass transfer coefficient was evaluated semitheoretically, by considering the interfacial region as a separate phase and using the experimental data developed for mass transfer accompanied by a fast first-order chemical reaction. The liquid-phase mass transfer coefficient was also evaluated semitheoretically, using Danckwert's theory for the liquid phase and the experimental data on mass transfer accompanied by a slow pseudofirst-order chemical reaction. An experimental unit was set up to provide a stable flowing foam column, simulating the foam reactor. Mass transfer rates were studied for superfacial gas velocities in the range from 1.5 × 10⁻² m/s to 5 × 10⁻² m/s, giving gas residence times in the range from 20 to 55 seconds. A cationic and nonionic surface-active agent and three different wire mesh sizes, giving bubble size distributions in the range from 2.2 to 5.4 mm Sauter mean diameters, were employed. It is observed that gas holdup is insensitive to the type of surface-active agent; it is however, dependent on wire mesh size and gas velocity. The bubble diameter and, hence, the interfacial area are found to be insensitive to gas velocity in the range studied; they are, however, strong functions of wire mesh size. The liquid-phase mass transfer coefficient increases with increase in gas velocity. The surface-active agent introduces additional resistance to mass transfer in both reaction cases, this being the controlling one in the case of the fast reaction. A comparison with conventional packed bed contactors indicates the mass transfer rates to be about 8 times lower for the foam reactor, for the fast reaction case; for slow reactions, the foam reactor has mass transfer rates approximately 2-4 times higher than those for conventional packed bed contactors.     

Phase inversion and mass transfer during liquid–liquid dispersed flow through mini-channel

The present study aims to identify means of process intensification during liquid–liquid flow through a mini-channel. During liquid–liquid flow, depending on the flow conditions either the organic or the aqueous phase can be dispersed and with increase in flow velocity the dispersed phase can spontaneously invert to form the continuous phase or vice-versa. The present study aims to investigate the phenomena of phase inversion and its influence on mass transfer during toluene/acetic acid-water flow in a 1.98 mm glass mini-channel. It is observed that for organic phase as dispersed regime, higher mass transfer efficiency is achieved when the liquid–liquid mixture is in the phase inversion zone which marks the transition from organic to aqueous phase dispersion. The mixture velocities as well as the inlet concentration of diffusing species influence mass transfer characteristics in this zone. The results have indicated some interesting observations which can be exploited for process intensification in monolith and micro-reactor.     

INFLUENCE OF LIQUID PROPERTIES ON THE DETERMINATION OF THE VOLUMETRIC MASS TRANSFER COEFFICIENT WITH THE HYDRAZINE METHOD

The oxidation of hydrazine in aqueous solution by atmospheric oxygen in presence of a homogeneous catalyst (copper tetrasulphophthalocyanine) is well suited for the determination of volumetric mass transfer coefficients. In contrast to other chemical methods the hydrazine oxidation permits to vary coalescence behaviour of the liquid phase by adding electrolytes or organic compounds, because without these solutes the reaction liquid does not inhibit bubble coalescence. With appropriate high molecular additives the hydrazine method can also be used for mass transfer measurements in liquids of high viscosity. Compared to the dynamic method the hydrazine method as a steady state method is far less influenced by systematic errors originating from the evaluation model. For this reason the application of the hydrazine method for testing semi-industrial and industrial gas-liquid contacting devices should be especially attractive.     

A refined model for ozone mass transfer in a semibatch stirred vessel

The mathematical model proposed by Anselmi et al. (1984) for a semibatch stirred gas‐liquid contactor is refined to describe the mass transfer of ozone absorption and decomposition in aqueous solution with the decomposition rate expression of general reaction orders (not necessarily integers). Three system equations are employed to describe the ozone concentrations in the bulk liquid (C_ALb), the hold‐up gas (C_AGi), and the outlet gas in the free volume above the liquid surface (C_AGe), respectively. The effect of ozone decomposition on the mass transfer, which is reflected by the enhancement factor (E_r) defined as the ratio of mass absorbed per unit area in time t with chemical reaction (r) to that without chemical reaction or of the purely physical absorption, is considered in the refined model. Furthermore, the refined model also takes into account the variation of E_r with C_ALb, which changes with time during the course of gas‐liquid contacting. Thus this analysis extends the applicability of the model of Anselmi et al. (1984) and is of special importance for ozone mass transfer in the cases of basic solutions and of low mass transfer coefficients, in which the effect of decomposition on absorption is significant, and in the system with variable liquid phase ozone concentration.     

Interfacial area and mass transfer in carbon dioxide absorption in TEA aqueous solutions in a bubble column reactor

The hydrodynamic behaviour and mass transfer of carbon dioxide removal process by aqueous solutions of triethanolamine (TEA) are analysed. The experiments were made in a bubble column reactor (BCR) as gas–liquid contactor. The interfacial area and mass transfer coefficient were calculated by using a photographic method based on the bubble diameter determination. The influence of operation conditions, liquid phase nature and chemical reaction on the mass transfer coefficient and gas–liquid interfacial area has been also analysed.