Counter-current Mode Research Articles

A Comparison of Co-Current and Counter-Current Modes of Operation in Urea Prilling Tower

In this paper, a model for urea prilling tower with co-current flow of cooling air and urea prills (particles) is presented. The process is modelled by simultaneous solution of the differential equations for hydrodynamics, heat and mass transfer between the air and prills. The process variables such as temperature, absolute and relative humidity of air along the height of the tower were obtained from this model. Temperature and moisture distribution of urea prills and their radial and vertical velocities were also calculated. The results of the present model were compared with the counter-current operation model available in the literature. The simulation results show that heat transfer performance for co-current operation is significantly less than that of the counter-current scenario. This is more pronounced for small prills, i.e. 1.0 mm than that of the large prills. The advantage of the model is that it can be used to investigate influence of operating parameters on efficiency of the co-current process. This also helps us to set the process control strategies for design and quality control purposes of the process.

CO2 methanation combined with NH3 decomposition by in situ H2 separation using a Pd membrane reactor

Combination of the reactions by means of membrane separation techniques are of interest. The CO2 methanation was combined with NH3 decomposition by in situ H2 separation through a Pd membrane. The CO2 methanation reaction in the permeate side was found to significantly enhance the H2 removal rate of Pd membrane compared to the use of sweep gas. The reaction rate of CO2 methanation was not influenced by H2 supply through the Pd membrane in contrast to NH3 decomposition in the retentate side. However, the CH4 selectivity could be improved by using a membrane separation technique. This would be caused by the active dissociated H species which might immediately react with adsorbed CO species on the catalysts to CH4 before those CO species desorbed. From the reactor configuration tests, the countercurrent mode showed higher H2 removal rate in the combined reaction at 673 K compared to the cocurrent mode but the reaction rate in CO2 methanation should be improved to maximize the perfomance of membrane reactor.

Continuous ion exchange for hydrometallurgy: Purification of Ag(I)–NaCl from divalent metals with aminomethylphosphonic resin using counter-current and cross-current operation

Continuous simulated moving bed (SMB) type ion exchange processes are usually operated in cross-current mode for metal separations whereas counter-current mode is commonly used for fine chemicals. The use of both process types was investigated for hydrometallurgical purification of Ag–NaCl solution from divalent impurity metals (Ca, Mg, Pb, Zn). An amphoteric aminomethylphosphonic functional ion exchange resin (Lewatit TP-260) was used as stationary phase. Also the influence of controlled partial neutralization of the resin on process performance was investigated. Successful separation was obtained with both counter-current and cross-current configurations. Ag product was collected in over 99% purity with practically 100% yield. All of the impurity metals were collected at the extract, and none of them migrated to the controlled partial neutralization zone. Column efficiency and the liquid to solid flow rate ratio were found to have strong influences on process performance. It was thus shown that counter-current simulated moving-bed process can be utilized for purification of precious metal solutions, especially when high yield is important. Remarkably improved performance is obtained by implementing a controlled partial neutralization procedure as an additional step within the continuous ion exchange unit.

Membrane-free electrodeionization for high purity water production

High purity water can be produced either by ion exchange or through electrodeionization. The latter deionization technology is superior to the former in many aspects, including no chemicals required, environmental friendliness, and easy operation. However, it usually needs the assistance of selective ion exchange membrane, causing a series of problems, such as high cost, difficulty in repairing, and membrane fouling. In this work, a new process called as membrane-free electrodeionization has been proposed and tested for high purity water production. It was operated in a batch countercurrent mode, alternating with a long service step and a short regeneration step. A total of 20 operational cycles were examined. The effluent conductivity was 0.056–0.059μS/cm only. The Na+ and Ca2+ concentrations, conductivity and pH value of the concentrate, as well as the voltage required were examined systematically during regeneration. The conductivity of the concentrate varied from 217μS/cm to 150μS/cm, the average pH value of the concentrate was 7.9, and the average voltage was 730V. Effective regeneration was achieved, and the resins after repeated regeneration still remained their good efficacy for deionization.

Characterizing the interaction between enantiomers of eight psychoactive drugs and highly sulfated‐β‐cyclodextrin by counter‐current capillary electrophoresis

The estimation of apparent binding constants and limit mobilities of the complexes of the enantiomers that characterize the interaction of enantiomers with chiral selectors, in this case highly sulfated β-cyclodextrin, was approached using a simple and economic electrophoretic modality, the complete filling technique (CFT) in counter-current mode. The enantiomers of eight psychoactive drugs, four antihistamines (dimethindene, promethazine, orphenadrine and terfenadine) and four antidepressants (bupropion, fluoxetine, nomifensine and viloxazine) were separated for the first time for this cyclodextrin (CD). Estimations of thermodynamic and electrophoretic enantioselectivies were also performed. Results indicate that, in general, thermodynamic enantioselectivity is the main component explaining the high resolution found, but also one case suggests that electrophoretic enantioselectivity itself is enough to obtain a satisfactory resolution. CFT results advantageous compared with conventional capillary electrophoresis (CE) and partial filling technique (PFT) for the study of the interaction between drugs and chiral selectors. It combines the use of a simple fitting model (as in CE), when the enantiomers do not exit the chiral selector plug during the separation (i.e. mobility of electroosmotic flow larger than mobility of CD), and drastic reduction of the consumption (and cost; ~99.7%) of the CD reagent (as in PFT) compared with the conventional CE.

A comparison of co-current and counter-current modes for Fischer–Tropsch synthesis in two consecutive reactors of oxidative coupling of methane and Fischer–Tropsch

The results of three cases of two consecutive reactors are studied in two types of co-current and counter-current flow in second reactor where two consecutive reactors are oxidative coupling of methane (OCM) and Fischer–Tropsch (FT) reactors. FT reactor can be fixed bed or membrane fixed bed reactor with a hydrogen perm-selective membrane. Effect of CH4/O2 ratio, contact time, inlet temperature, and amount of N2 in OCM feed on C2 to C5+ hydrocarbons produced in FT reactor were studied. Results show that use of counter-current hydrogen-perm selective membrane FT reactor that sequenced after OCM reactor improves the C5+ yield as a desirable product and reduces the amount of CH4 and CO2 as byproducts of FT reactor in comparison to co-current and conventional reactor. This phenomenon can be explained with more H2 diffusion through the membrane and more CO conversion and more hydrocarbons productions, briefly. As compared to the conventional reactor, in co-current and counter-current reactors, yield of C5+ has increased almost 3% and 4% alternatively when using these systems.

Spiral-Wound Water-Gas-Shift Membrane Reactor for Hydrogen Purification

Water-gas-shift (WGS) membrane reactor is a promising approach to purify hydrogen produced by hydrocarbon or biomass reforming to fuel cell grade (<10 ppm CO). CO2-selective polymer membranes suitable for the above operation can potentially be fabricated into spiral-wound modules with catalyst packed on the feed side and sweep gas flowing on the permeate side. This paper presents a detailed model based on both mass and enthalpy balances as well as pressure drop to study this intricate reactive separation system. The kinetics of the WGS reaction was incorporated using a published rate expression for the commercial CuO/ZnO/Al2O3 catalyst. The resulting 1-D (cocurrent or countercurrent flow of feed and sweep streams) or 2-D (crossflow) system of differential equations was solved using COMSOL Multiphysics. The model was validated by comparing the predicted CO results with previously published experimental data for a lab-scale flat rectangular membrane reactor. The effects of the type of flow mode on concentration and temperature profiles within the membrane reactor were predicted. Also, a sensitivity study was carried out to quantify the effects of operating parameters like feed pressure, sweep to feed flow rate ratio, and steam/CO ratio on membrane area and hydrogen recovery. Results show that although the countercurrent flow mode is the most efficient in terms of CO reduction, the crossflow mode might provide a better trade-off between CO reduction and heat management. For the countercurrent mode, it was also shown that it is important to enhance both the CO2 permeance as well as the catalyst activity to reduce the membrane reactor size.

Modeling of facilitated transport of Cr(III) using (RNH3+HSO4−) ionic liquid and pseudo-emulsion hollow fiber strip dispersion (PEHFSD) technology

The permeation of chromium (III) using PEHFSD technology and the ionic liquid (RNH3+HSO4−), formed by reaction of the primary amine PRIMENE JMT and sulphuric acid, dissolved in n-decane as mobile carrier has been investigated. The alkaline feed solution containing Cr(III) was passed through the tube side, and pseudo-emulsions of the ionic liquid+n-decane+n-decanol and sulphuric acid were passed through the shell side in counter-current mode and using a single hollow fiber module for extraction and stripping. In this advanced membrane technology, the aqueous acidic strip solution is dispersed in the organic membrane solution in a tank with an impeller stirrer to form a strip dispersion. The pseudo-emulsion phase is circulated from the tank to the membrane module to provide a constant supply of the organic solution to the membrane micropores. Factors affecting chromium permeability, such as hydrodynamic conditions, carrier concentration in the organic phase, metal and NaOH concentrations in the feed phase, have been analyzed. A model is reported describing the transport mechanism, whereas the experimental data are quantitatively explained by mathematical equations describing the rate of transport. Different rate-controlling processes take place as long as the metal transport occurs.

Optimal design of an industrial scale dual-type reactor for direct dimethyl ether (DME) production from syngas

Since in foreseeable future DME as a clean-burning fuel with versatile applications will play a significant role in the transportation sector, the least improvement in its production process which can increase the production capacity is economically favorable. In this study, an industrial dual-type reactor is designed and optimized by DE algorithm for increasing DME production and also overcoming the reaction equilibrium limitations of the direct DME synthesis. The proposed reactor configuration is composed of two fixed bed reactors: (1) the water-cooled reactor and (2) the gas-cooled reactor. The syngas feed is preheated with the hot exit product from the first reactor (water-cooled), and is entered into the first reactor. The reactors are simulated by means of a one-dimensional steady-state heterogeneous model. The simulation results indicate that the best flow pattern for the proposed configuration is the counter-current mode. Maximizing the outlet mole fraction of DME from the second reactor is the objective of the optimizer. Seven operating and geometrical parameters of the system are considered as the decision variables. The results show that the proposed configuration can enhance DME production capacity about 60ton/day in comparison to conventional industrial DME reactor which operates based on the indirect DME synthesis process.

A numerical approach of conjugate hydrogen permeation and polarization in a Pd membrane tube

A numerical method accounting for conjugate hydrogen permeation in a dense palladium (Pd) membrane tube is developed. In the method, hydrogen permeation across the membrane is treated by introducing a source–sink pair and a gas mixture produced from water gas shift reactions serves as the feed gas of the membrane tube. The influences of flow patterns of feed gas and sweep gas as well as their flow rates on hydrogen separation are investigated. A concentration polarization index (CPI) is also conducted to indicate the extent of polarization along the membrane surface. The predicted results suggest that counter-current modes are able to give the better performance of hydrogen separation compared to co-current modes, and hydrogen can be completely recovered if the flow rate of feed gas is low to a certain extent. However, lower flow rates of feed gas and sweep gas will trigger serious concentration polarization. With counter-current modes, the feed gas sent into the membrane tube from the lumen side or the shell side is flexible. The optimum Reynolds number of sweep gas in accordance with the Reynolds number of feed gas is correlated by an arctangent function. This provides a useful reference for the operation of hydrogen separation by controlling sweep gas.

Experimental study of liquid spreading in structured packings

Optimization of industrial gas–liquid columns dedicated to CO2 capture requires prediction of liquid distribution within packed beds. In this context, liquid hold-up as well as liquid spreading from a source point have been investigated for Mellapak 250.X structured packing. Local liquid hold-up measurements have been achieved in a 400mm diameter column by means of gamma-ray tomography with operation in the counter-current mode at different positions downstream the source point injection. Liquid hold-up and retention map measurements have been performed for two fluid systems: Air/Water and Air/MEA 30wt%. A correlation that relates global liquid hold-up and liquid load taking into account liquid viscosity is proposed. This correlation has been further used to determine spread factors using a simple dispersion model for all investigated operational conditions. Liquid dispersion model is found to well reproduce experimental data in the range of operational conditions that were tested which enables to determine spread factors for various operating conditions. The spread factor is observed not to vary with the liquid load, the gas capacity factor in the range of 20–80% of flooding or the liquid viscosity. This led us to stipulate that liquid dispersion is controlled by packing geometry only. Nevertheless, the effect of surface tension on liquid hold-up and dispersion is discussed since its effect is not fully understood and calls for further experiments if one wants to apply those results for hydrocarbons.

Modeling and Optimization of Membrane Reactors for Carbon Capture in Integrated Gasification Combined Cycle Units

This paper investigates the alternative of precombustion capture of carbon dioxide from integrated gasification combined cycle (IGCC) plants using membrane reactors equipped with H2-selective zeolite membranes for the water gas shift reaction. Specifically, a one-dimensional and isothermal membrane reactor model is developed. This model is used for simulation and optimization studies considering cocurrent and countercurrent modes of reactor operation. The simulation results indicate successful countercurrent cases that satisfy all of the specified targets and constraints. With use of this developed model, a novel optimization problem is formulated and solved to guide the selection of the optimal reactor design among typical scenarios of operation. The optimization results suggest as optimal solution a reactor design with a preshift followed by a membrane reactor. The obtained optimal design enables a more efficient membrane use by placing it in the optimal location. This design also results in savings of as high as 25% (in the range of 10–25%) in terms of membrane material when compared to the original membrane reactor design. For the price range of zeolite membranes considered on the order of $1000–10 000/m2 and for large-scale applications, in which the membrane surface areas are on the order of 2000 m2, 25% of savings implies cost reductions on the order of millions of dollars (as high as $5 000 000 in this case).

Characterization of the start-up behavior and steady-state performance of biotrickling filter removing low concentration toluene waste gas

A biotrickling filter packed with the regular ceramic spheres was designed and fabricated to purify the low toluene-concentration waste gas. Particular attention was made to the study of the start-up behavior of the biotrickling filter. Moreover, the steady performances were investigated to establish the experimental correlation between the operation factors and removal efficiency. It was found that the start-up process of the biotrickling filter exhibited three stages including the biofilm formation, biofilm development, and biofilm stabilization. The OD600 nm of the circulated liquid, gas pressure drop, elimination capacity, and temperature rise maintained at a rather low level in the biofilm formation stage, then increased rapidly in the biofilm development stage, and finally reached a stable value in the biofilm stabilization stage. It was also found that the removal efficiency of the biotrickling filter in the stable period decreased with increase in the waste gas flow rate, circulated liquid flow rate, and diameter of the ceramic sphere. More interestingly, it was revealed the operation modes have a significant influence on the removal efficiency of the biotrickling filter, and the microorganism show a high activity under the operating temperature ranging from 30°C to 40°C. The experimental correlations for describing the effect of operation factors on the removal efficiency of the biotrickling filters under both the co-current and counter-current operation modes were obtained. The correlation results are in good agreement with the experimental data.

Bubble Diameter and Effective Interfacial Area in a Novel Hybrid Rotating and Reciprocating Perforated Plate Bubble Column

This investigation reports on the experimental and theoretical investigation carried out to evaluate the bubble diameter and effective interfacial area in a novel Hybrid Rotating and Reciprocating Perforated Plate Bubble Column. Air-water system is used in this investigation. Countercurrent mode is employed. The effects of agitation level, superficial gas velocity and superficial liquid velocity on the bubble size distribution are studied. The mean bubble diameter is predicted using photographic technique. A simple correlation is developed for the determination of mean bubble diameter. It is found that the mean bubble diameter values for hybrid column are 1.8 to 2.5 times smaller when compared with conventional reciprocating plate column. The interfacial area is calculated based on the experimental results of the gas holdup and bubble diameter. Effects of agitation level, superficial gas velocity, superficial liquid velocity and plate free area on the interfacial area have been investigated. Correlations are developed for the determination of interfacial area for both mixer-settler and emulsion regions. It could be noted that the interfacial area for the hybrid column is 3 to 6 times higher in both mixer-settler region and emulsion region than that of conventional reciprocating plate column which is quite large.

Transport of Au(CN)2 − by Mixtures of Amine Primene JMT and Phosphine Oxide Cyanex 923 Using the Pseudo-Emulsion Based Hollow-Fiber Strip Dispersion Technology

The transport of Au(CN)2 − between alkaline aqueous solutions and organic phases consisting of a mixture of the amine Primene JMT and the phosphine oxide Cyanex 923 in xylene was studied using the pseudo-emulsion based hollow-fiber strip dispersion (PEHFSD) technology. The feed phase was passed through the lumen side, and the pseudo-emulsions of the extractant mixtures and NaOH were passed through the shell side in a countercurrent mode using a single hollow-fiber contactor for extraction and stripping. In this membrane technology, the strippant (NaOH solution) is dispersed in the organic (Primene JMT + Cyanex 923 in xylene) membrane solution in a tank with an impeller stirrer adequate to form strip dispersion. The pseudo-emulsion phase is circulated from the reservoir tank to the membrane contactor to provide a constant supply of the organic solution to the membrane fibers. Furthermore, this technology allows a direct contact between the organic and strip solutions, providing a greater area for stripping and facilitating the metal recovery from the strip solution once both phases are separated. Various hydrodynamic and chemical parameters, such as flow of feed phase, extractant mixtures and gold concentrations, organic diluents, variation in feed pH, and the selectivity of the system with respect to the transport of different metal-cyano complexes, were investigated. Aqueous and membrane mass transfer coefficients were estimated for the present system.

PLGA microdevices for retinoids sustained release produced by supercritical emulsion extraction: Continuous versus batch operation layouts

Retinyl acetate (RA) was selected as a model compound to be entrapped in poly(lactic‐co‐glycolic)acid (PLGA) microspheres using supercritical emulsion extraction (SEE). Several oil‐in‐water emulsions prepared using acetone and aqueous glycerol (80% glycerol, 20% water) were processed using supercritical carbon dioxide (SC‐CO2) to extract the oily phase and to induce microspheres formation. The characteristics of the microspheres obtained by conventional liquid emulsion extraction and SEE were also compared: SEE produced spherical and free flowing microspheres, whereas the conventional liquid–liquid extraction showed large intraparticles aggregation. Emulsion extraction by SC‐CO2 technology was tested using two different operation layouts: batch (SEE‐B) and continuous (SEE‐C). SEE‐C was performed using a packed tower to produce emulsion/SC‐CO2 contact in countercurrent mode, allowing higher microsphere recovery and process efficiencies. Operating at 80bar and 36°C, SEE‐C produced PLGA/RA microspheres with mean sizes between 3.3 and 4.5 μm with an excellent encapsulation efficiency of 80%–90%. Almost all the drug was released in about 6 days when charged at 2.7% (w/w), whereas only 40% and 10% of RA were released in the same period of time when the charge was 5.2% and 8.8% (w/w), respectively. Release kinetics constants calculated from the experimental data, using a mathematical model, were also proposed and discussed. © 2011 Wiley‐Liss, Inc. and the American Pharmacists Association J Pharm Sci 100:4357–4367, 2011

An improved tank in series model for PEMFC

This study presents an improved Tank in Series Reactor (TSR) Proton Exchange Membrane (PEM) fuel cell model with mass and energy balance equations accounting for evaporation and condensation in cathode channels. The TSR model includes the modified charge balance equation suitable for potentiostatic fuel cell operation mode. Polarization curves calculated with the improved model agree with experimental data from the literature. The developed TSR model is able to predict the limiting two-dimensional profiles in PEMFC. Simulation results illustrate the influence of co-current and counter current mode on PEM fuel cell performance.

Development of numerical simulation code of membrane reactor for detritiation

The plasma exhaust gas from a fusion machine needs to be processed with very high tritium removal efficiency. Such a high decontamination factor is achieved by multistage processes. The last stage of the three-step CAPER process developed at the Tritium Laboratory Karlsruhe (TLK) is based on a so-called permeator catalyst (PERMCAT) reactor. The PERMCAT principle is based on isotopic swamping in a counter current mode. To support experiments, dynamic process simulation is indispensable for process design, analysis, and operation. A first numerical calculation code has been developed for the process simulation of PERMCAT. The numerical model developed was based on counter current one-dimensional plug flows partitioned with a membrane. In this study, the axial dispersion was newly taken into consideration to the numerical simulation code and its effect was examined. Improvement of the numerical simulation code considering a 2-dimensional model was developed and test calculations were carried out.

Filtration combustion of solid fuel in countercurrent reactors

The works studying filtration combustion of solid fuels in the countercurrent mode are reviewed. The phenomenon of superadiabatic overheating is described. The data of experimental and theoretical studies and some results of the development of technological processes of production of thermal or electric energy from substandard raw materials and extraction of valuable components (for example, rare metals) from unconventional sources (industrial wastes, poor ores, burrows, etc.) are presented. The processes of safe treatment of problematic wastes are considered.

Cobalt(II) membrane-extraction by DP-8R/Exxsol D100 using pseudo-emulsion based hollow fiber strip dispersion (PEHFSD) processing

The transport of cobalt(II) from acidic sulphate solutions through PEHFSD was investigated using the extractant DP-8R in Exxsol D100. The acidic feed solution containing Co(II) was passed through the tube side, and pseudo-emulsions of DP-8R/Exxsol D100 and sulphuric acid was passed through the shell side in counter-current mode, using a single microporous hydrophobic polypropylene hollow fiber contactor for extraction and stripping. In PEHFSD the aqueous strip (sulphuric acid) solution was dispersed in the organic membrane solution in a reservoir tank with an impeller stirrer to form a strip dispersion. This pseudo-emulsion phase is circulated from the tank to the membrane module to provide a constant supply of the carrier solution to the membrane micropores. Several hydrodynamic and chemical parameters, such as variation in feed pH (3–7), cobalt concentration in feed (0.17–1.7 × 10 −3 M), carrier concentration (0.16–1.28 M), etc., were investigated. Mass transfer modelling was performed and the validity of the model was evaluated with experimental data. Moreover, the system was applied in the cobalt purification of synthetic lithium solutions derived from the processing of rechargeable batteries, with cobalt/lithium separation factor values around 25.