SIMULATION OF AEROSOL FORMATION IN GAS-LIQUID CONTACT DEVICES

In gas-liquid contact devices like absorbers, quench coolers, or condensers, aerosols can be formed by spontaneous phase transitions, initiated by homogeneous or heterogeneous nucleation, if a supersaturated gas phase emerges due to simultaneous heat and mass transfer processes or chemical reactions. Typical examples are the absorption of acid gases, like HCl or SO₃, the condensation of solvents in the presence of inert gases, and the humidification of cold gases by hot liquids.

In this article the basic principles of aerosol formation in contact devices are briefly described. A strategy for modeling and simulation of aerosol formation and particle dynamics is discussed. Simulation results generated with the process tool AerCoDe for the countercurrent absorption of HCl and the humidification of air are presented.     

SIMULATION OF AEROSOL FORMATION IN GAS-LIQUID CONTACT DEVICES

In gas-liquid contact devices like absorbers, quench coolers, or condensers, aerosols can be formed by spontaneous phase transitions, initiated by homogeneous or heterogeneous nucleation, if a supersaturated gas phase emerges due to simultaneous heat and mass transfer processes or chemical reactions. Typical examples are the absorption of acid gases, like HCl or SO₃, the condensation of solvents in the presence of inert gases, and the humidification of cold gases by hot liquids.

In this article the basic principles of aerosol formation in contact devices are briefly described. A strategy for modeling and simulation of aerosol formation and particle dynamics is discussed. Simulation results generated with the process tool AerCoDe for the countercurrent absorption of HCl and the humidification of air are presented.     

FORMATION OF HYDROBROMIC AND HYDROCHLORIC ACID AEROSOLS IN WET FLUE GAS CLEANING PROCESSES

The formation and behavior of hydrobromic and hydrochloric acid aerosols in a wet flue gas cleaning pilot plant were investigated. The optical three-wavelength extinction (3-WE) method was used to determine mean aerosol droplet diameters and droplet number concentrations. The experimental data are compared with theoretical results of the simulation tool AerCoDe (aerosol formation in contact devices). Results are presented for a raw gas temperature of 200°C and raw gas concentrations up to 260 mg/m³(STP) for HBr, and 2500 mg/m³ (STP) for HCl. Under these conditions aerosol formation for both species is initiated by heterogeneous nucleation. It is shown that during absorption processes HBr is forming essentially higher supersaturated gas phases in comparison to HCl, resulting in higher droplet number concentrations and smaller droplet sizes. For both species the number concentration is a strong function of the maximum degree of saturation, which corresponds to the classical theory of heterogeneous nucleation. HBr aerosol droplets cannot be detected by the 3-WE method directly after the first stage of the flue gas cleaning plant (quench column) because they are smaller than the detection limit ( ≈ 500 nm) of the 3-WE method for nonabsorbing particles. In this case the droplets are enlarged in a second step, and the number concentration is determined after enlargement. Using the actual number concentration as an input parameter, the mean diameter after the first stage can be calculated with the simulation tool AerCoDe.     

Gas-phase residence time distribution in a falling-film microreactor

Industrial-scale performance of gas-liquid reactors can be difficult to optimise for very rapid or highly exothermic reactions. Microstructured reactors for laboratory measurements offer new opportunities for the study of these reactions by enabling precise heat management and fine control of reactor operating conditions. For accurate experimental study, characterisation of the flow conditions within these new reactor devices is essential.The present study examines experimental residence time distributions for the gas phase through a microstructured falling-film reactor, in order to develop an appropriate flow model for further study of gas-phase mass-transfer characteristics in the system. For the gas-phase residence time distribution experiments, the detection system involves a flow of oxygen containing ozone as a tracer gas with continuous monitoring of the concentration by UV-light absorption. The experimental results are used to model the flow behaviour in the gas volume over the gas-liquid contact zone as a series of continuous stirred tank reactors whose number is a simple function of the gas Reynolds number.The experimental results are compared with computational fluid dynamics calculations of the gas flow within the reactor. The comparison indicates a clear correlation of the flow model behaviour with the appearance of recirculation loops in the reaction chamber and the effect of the gas jet at the entrance of the gas-liquid contact zone.     

Aerosol formation by heterogeneous nucleation in wet scrubbing processes

The phenomenon of aerosol formation is often watched in industrial wet scrubbing processes especially if strong acid gases are absorbed in aqueous solutions. Although the basic principles of aerosol formation are well known in general, there exist nearly no specific rules for the design of industrial processes in which aerosol formation can be expected. There are two reasons for the fragmentary knowledge concerning aerosol formation in wet scrubbing processes. Firstly, it is a very complex task to describe the formation and the growth of aerosols theoretically under typical conditions of industrial processes. Secondly, no reliable experimental data, especially for the validation of theoretical work, has been available to date. To bridge this gap, a semi-technical wet scrubbing plant has been developed and erected in which the formation and the growth of aerosols under realistic conditions can be studied and the characteristic aerosol parameters can be measured in situ. It is shown that HCl-aerosols are formed in a quench cooler for hot flue gases of about 200°C even at low HCl feed concentrations of about l000 mg m⁻³, but only in the presence of foreign nuclei. The aerosol droplets in the quench cooler grow up to 1–1.5 μm and can be enlarged in a direct-contact-cooling column up to about 2.5 μm. Furthermore, a strategy for aerosol evaporation is discussed.     

Review on gas–liquid separations in microchannel devices

Separations are indispensable to most chemical processes, regardless of operational scale. In the field of chemical microprocess engineering, separations have so far attracted rather limited attention compared to reactive processes, although increasingly more research is directed towards this area. Microstructured devices offer the opportunity of intensifying transport processes associated with separation operations by providing high ratios of contact areas to volume, short transport distances and high driving force gradients. These attributes have so far been exploited for various microseparations. In this review, we focus on separations where gas and liquid phases are present, and in particular on absorption, stripping and distillation. Two main approaches for contacting the two phases have been employed: continuous and dispersed phase contactors. Removal of components from gas mixtures by absorption and stripping of volatiles from liquid mixtures have been achieved in falling film devices, thin porous plate microcontactors, as well as in dispersed flow systems. Distillation has been performed in microchannel devices, utilizing capillary, centrifugal and gravity forces, vacuum or carrier gas.     

Aerosole und ihre technische Bedeutung

Aerosols and Their Technical Significance Aerosols occur in many technical processes. For example, aerosols are formed to generate products of highly disperse solids in gas phase processes. Particle formation and growth in the aerosol state decisively determine the product properties by the size, shape, and structure of the particles. Undesired aerosol formation can also occur in technical processes. These undesired aerosols pollut process gases or products or increase the pollutant content of exhaust gases. If undesired aerosol formation cannot be avoided, efficient separation techniques have to be used. Efficient separators are also required to recover fine product particles from the gas phase. Moreover, aerosols, whether desired or undesired, have to be measured and characterized. This requires high performing measuring techniques. The paper outlines the technical significance of aerosols.     

Hollow fiber membrane contactor as a gas-liquid model contactor

Microporous hollow fiber gas-liquid membrane contactors have a fixed and well-defined gas-liquid interfacial area. The liquid flow through the hollow fiber is laminar, thus the liquid side hydrodynamics are well known. This allows the accurate calculation of the fiber side physical mass transfer coefficient from first principles. Moreover, in the case of gas-liquid membrane contactor, the gas-liquid exposure time can be varied easily and independently without disturbing the gas-liquid interfacial area. These features of the hollow fiber membrane contactor make it very suitable as a gas-liquid model contactor and offer numerous advantages over the conventional model contactors. The applicability and the limitations of this novel model contactor for the determination of physico-chemical properties of non-reactive and reactive gas-liquid systems are investigated in the present work. Absorption of CO₂ into water and into aqueous NaOH solutions are chosen as model systems to determine the physico-chemical properties for non-reactive and reactive conditions, respectively. The experimental findings for these systems show that a hollow fiber membrane contactor can be used successfully as a model contactor for the determination of various gas-liquid physico-chemical properties. Moreover, since the membrane contactor facilitates indirect contact between the two phases, the application of hollow fiber model contactor can possibly be extended to liquid-liquid systems and/or heterogeneous catalyzed gas-liquid systems.     

A New Annular Centrifugal Contactor for Pharmaceutical Processes

A new annular centrifugal contactor capable of rapid disassembly for inspection and cleaning is presented. The unique design enhances the utility of this liquid-liquid centrifuge in processes requiring cleaning and inspection of all wetted parts. This new model differs from previous designs in that it can be manufactured to meet the requirements of the pharmaceutical industry. Annular centrifugal contactors are vertical, direct-drive centrifuges providing an efficient mixing and separation device in a single unit. They can be directly coupled for multiple-stage processes. Other features include small hold up volume, rapid start-up and shutdown, variable speed control, interchangeable heavy phase weirs, and very compact size. Mixing and disengaging residence times of 3 and 20 s, respectively, are typical. Single stage efficiencies for processes with rapid kinetics are greater than 90%. Annular centrifugal contactors can be used to convert processes from batch to continuous due to their ability to handle intermittent feed flows and 100% phase ratio changes.     

A New Annular Centrifugal Contactor for Pharmaceutical Processes

A new annular centrifugal contactor capable of rapid disassembly for inspection and cleaning is presented. The unique design enhances the utility of this liquid-liquid centrifuge in processes requiring cleaning and inspection of all wetted parts. This new model differs from previous designs in that it can be manufactured to meet the requirements of the pharmaceutical industry. Annular centrifugal contactors are vertical, direct-drive centrifuges providing an efficient mixing and separation device in a single unit. They can be directly coupled for multiple-stage processes. Other features include small hold up volume, rapid start-up and shutdown, variable speed control, interchangeable heavy phase weirs, and very compact size. Mixing and disengaging residence times of 3 and 20 s, respectively, are typical. Single stage efficiencies for processes with rapid kinetics are greater than 90%. Annular centrifugal contactors can be used to convert processes from batch to continuous due to their ability to handle intermittent feed flows and 100% phase ratio changes.     

Trace element evaporation during coal gasification based on a thermodynamic equilibrium calculation approach

Thermodynamic equilibrium calculations using the HSC-Chemistry program were performed to determine the distribution and mode of occurrence of potentially toxic and corrosive trace elements in gases from coal gasification processes. The influence of temperature, pressure and gas atmospheres on equilibrium composition was evaluated. In these reducing conditions, the behaviour of the trace elements is complex, but some form of organization can be attempted. Elements were classified into three groups. Group A includes those elements that, according to thermodynamic data at equilibrium, could probably be condensed in coal gasification. Mn is classified in this group. Group B contains those elements that could be totally or partially in gas phase in gas cleaning conditions, and can be divided into two subgroups, depending on whether the cleaning conditions are hot or cold. Co, Be, Sb, As, Cd, Pb, Zn, Ni, V, Cr are elements in this group. Group C contains those elements that could be totally in gas phase in all the possible conditions, including flue gas emissions. Se, Hg and B are the elements that make up this group.     

New criteria for application of the well-mixed model to gas-liquid mass transfer studies

The well-mixed model presents several advantages for the evaluation of interphase mass transfer rates in gas-liquid contactors: it simplifies the mathematical analysis and eliminates the need for measuring numerous mixing parameters. Although the well-mixed model corresponds to a theoretical concept, it is shown in this paper that, under certain conditions, it can be safely applied to the liquid phase of contactors for the purpose of determining the overall volumetric mass transfer coefficient K_La_L. Based on a computer simulation of absorption and mixing processes, simple criteria were developed that specify the conditions for which the well-mixed model is applicable in practice, for either a semi-batch gas-liquid contactor or a continuous flow absorber at steady-state. The dimensionless group K_La_Lτ_c, where τ_c is the average circulation time in the liquid phase of the contactor, was found to be a key parameter in establishing the validity of the well-mixed model.     

Effect of surface resistance on gas absorption accompanied by a chemical reaction in a gas—liquid contactor

An analysis of gas absorption accompanied by chemical reaction in the presence of interfacial resistance is presented. The analysis indicates that the effect of interfacial resistance on interphase mass transfer is significantly higher in presence of a reaction compared to the pure absorption case. For fixed values of surface resistance and contact time, the difference between the amount of gas transferred across the interface with and without surface resistance increases as the value of reaction velocity increases. For ranges of contact time and surface resistance of practical relevance, the influence of surface resistance is too high to be neglected while designing gas-liquid contactors.     

A simulation tool for aerosol formation during sulphuric acid absorption in a gas cleaning process

A simulation tool has been developed to predict sulphuric acid aerosol formation in typical industrial absorption processes for gas cleaning. The underlying model comprises homogeneous nucleation and the growth of a polydisperse droplet collective under the special circumstances of a gas–liquid contact device where heat and mass transfer processes between the bulk phases take place simultaneously. The model is applied to a hot flue gas (200 °C) with sulphuric acid concentrations between 5 and 100 mg m⁻³ (STP) (STP: standard temperature and pressure). The simulation yields high droplet number concentrations up to 10¹⁶ m⁻³ especially for low gas inlet concentrations of sulphuric acid (5 mg m⁻³ (STP)), and very small droplet sizes in the range 20–100 nm. The droplet number concentrations decrease and the droplet sizes increase with increasing sulphuric acid inlet concentrations. It is shown that small droplets (<20 nm) need relatively high supersaturation for growing. If the saturation in the absorption equipment is not high enough the droplets partially re-evaporate but do not vanish due to the extremely low vapor pressure of concentrated sulphuric acid. The resulting size distributions of the aerosol droplets are not very sensitive with respect to the nucleation model used. This is demonstrated by comparing nucleation models with and without hydrate formation. The new simulation tool allows an estimate of the true sulphuric acid removal efficiency of absorption processes which is often not more than 50% due to aerosol formation. In general, the simulation results enable a deeper insight in the mechanisms of aerosol formation and behavior in absorption processes.     

Aerosolbildung bei der Absorption und Partialkondensation

Aerosol formation on absorption and partial condensation. The phenomenon of aerosol or mist formation on absorption and partial condensation has hardly been investigated but causes serious problems in the practice of industrial exhaust air purification, e.g. in the absorption of hydrogen chloride or on low-temperature condensation of solvent vapours. The mist droplets formed in these processes remain suspended in the gas phase and lead to unacceptably high pollutant concentrations in the exhaust air. This present study first surveys currently available knowledge and then goes on to explain under what thermodynamic boundary conditions aerosols are formed by certain substance systems. The author also applies known results relating to nucleation and growth mechanisms from aerosol physics to technical processes. It is seen that aerosols with very small droplet sizes, which are difficult to precipitate, are formed particularly during absorption processes.     

THE ANALYTICAL SOLUTION FOR A SEMI-BATCH GAS-LIQUID CONTACTOR MODEL

The time-domain analytical solution for a semi-batch gas-liquid contactor model is presented. The model assumes plug flow for the gas phase and a well mixed liquid for the liquid phase leading to a system of first-order partial differential-integral equations. It is shown that the method of characteristics combined with a transformation and the development of a special Cauchy problem leads to an analytical solution. The solution is useful for the dynamic study as well as for the experimental evaluation of mass transfer coefficients of gas-liquid contactors.     

Modeling and simulation of mass transfer in near-critical extraction using a hollow fiber membrane contactor

In this study is presented a general methodology to predict the performance of a continuous near-critical fluid extraction process to remove compounds from aqueous solutions using a hollow fiber membrane contactor. The stabilization of the gas-liquid interface in the membrane porosity and a high surface area to contact both phases represent some of the advantages that hollow fiber contactors offer over conventional contactor devices for the extraction of compounds from liquid feeds.A mathematical model has been developed integrating a resistances-in-series mass transfer system that takes into account boundary layers, membrane porosity and thermodynamic considerations with mass balances of the membrane contactor. Simulation algorithms were easily implemented with low calculation requirements.The system studied in this work is a membrane based extractor of ethanol and acetone from aqueous solutions using near-critical CO₂. Predictions of extraction percentages obtained by simulations have been compared to the experimental values reported by Bothun et al. [2003a. Compressed solvents for the extraction of fermentation products within a hollow fiber membrane contactor. Journal of Supercritical Fluids 25, 119-134]. Simulations of extraction percentage of ethanol and acetone show an average difference of 36.3% and 6.75% with the experimental data, respectively. More accurate predictions of the extraction of acetone could be explained by a better estimation of the transport properties in the aqueous phase that controls the extraction of this solute.When the model was validated, the effect of the configuration and the operating parameters was studied and local mass transfer resistances were evaluated. The proposed approach allows the evaluation of the relevance of membrane hydrophobicity for extraction in solutions under different thermodynamic conditions. This original methodology based on well-known phenomenological equations represents a general approach which could be applied in other processes using membrane contactors with different configurations.     

Effects of Orifice Orientation and Gas-Liquid Flow Pattern on Initial Bubble Size

In many gasliquid processes, the initial bubble size is determined by a series of operation parameters along with the sparger design and gasliquid flow pattern. Bubble formation models for variant gasliquid flow pat terns have been developed based on force balance. The effects of the orientation of gasliquid flow, gas velocity, liquid velocity and orifice diameter on the initial bubble size have been clarified. In ambient airwater system, thesultable gasllquid flow pattern is important to obtain smaller bubbles under the low velocity liquid crossflow con ditions with stainless steel spargers. Among the four types of gasliquid flow patterns discussed, the horizontal orifice in a vertically upward liquid flow produces the smallest initial bubbles. However the orientation effects of gas and liquid flow are found tobe insgnifican whenliq.uid velocity is.higher than. 3.2 m;sa or theorifice diameter is small enough.