Counter-current Flow Mode Research Articles

Gas-liquid countercurrent flow characteristics in a microbubble column reactor

Countercurrent flow mode is widely utilized in many chemical unit operations due to its effective enhancement of mass transfer driving force. While the microbubble column reactor has garnered attention for its substantial gas–liquid specific surface area and fast mass transfer rate under the concurrent flow condition, countercurrent flow microbubble column reactors remain relatively underexplored. Consequently, in this study a novel microbubble generator device featuring gas–liquid co-crossing a microfiltration membrane was developed. This device could generate microbubbles with narrow size distribution, and proved suitable for integration into the countercurrent microbubble column reactor. Further investigation ensued to elucidate the operational principles of this dispersion method. Finally, the hydrodynamic behavior of the countercurrent microbubble column reactor was examined. Remarkably, a gas–liquid-specific surface area 2300 m2/m3 was reached, surpassing that of conventional bubble column reactors by a significant margin. New gas–liquid chemical processes and process intensification technologies could be developed accordingly.

Utilization of unrejected ammonium for fouling prevention in a forward osmosis process tested with real primary wastewater from Hong Kong

Forward osmosis (FO) process is known to have high rejection towards foulants and ions. However, the polyamide surfaces of FO membranes are usually designed to be negatively charged, which results in low ammonium rejection. This study proposes to utilize the unrejected ammonium in the draw solution (DS) of FO processes by adding OCl− to generate monochloramine, which can diffuse back towards the feed side and prevent. Tests were performed to identify the optimal FO operating conditions, such as DS concentration, crossflow velocity, and flow direction. Optimal conditions were determined to be a DS concentration of 1.0 M, a crossflow velocity of 5.4 cm/s and counter-current flow mode. A 72 h fouling tests were also conducted with real wastewater from the Siu Ho Wan Sewage Treatment Works in Hong Kong as the feed. The existence of monochloramine alleviated membrane fouling. The novelty of this study lies on the fact that the chloramine is generated using the unrejected ammonia, and consequently utilizing the generated chloramine that is reversely diffusing towards the feed to prevent the fouling formation. Findings from this study suggest that the generation of monochloramine from unrejected ammonium ions can lengthen membrane life while endowing self-cleaning abilities that prevent severe fouling.

The effect of flow modes on the capture of the energy between concentrated brine and seawater by reverse electrodialysis

The discharge of concentrated brine from the desalination unit waste amounts of salinity gradient energy, which could be harvested and converted to electric energy by reverse electrodialysis (RED). However, the poor power and restricted conversion efficiency obstacle its development. A suitable flow mode of feed solutions in a RED stack can improve energy conversion efficiency. Here, the influences of co– and counter-current flow modes on the performances of a single-stage RED stack and multi-stage stacks harvesting the energy are studied in the work. Based on this, the models of flow mode affecting the RED performances are built and experimentally verified. Results show that the counter-current in a single-stage stack is better than the co-current, and the advantage is more obvious with the concentration increase of the concentrated solution, the concentration decrease of the diluted solution, and the decrease of flow rate. Besides feed parameters, the stack sizes also influence the performances of the single-stage stack in both modes. The counter-current in the multi-stage stacks is also more excellent than the co-current. However, the difference decreases with the increase of stage number. This work provides some support for utilizing the salinity gradient energy between concentrated brine and seawater.

Gas-liquid microdispersion and countercurrent flow in a miniaturized annular rotating device

The major challenge for the existing gas–liquid microdispersion devices is that they cannot achieve efficient dispersion performance at high volume ratio of gas to liquid. In this context, a miniaturized annular rotating device (m-ARD) with a high handling capacity, large specific surface area, relatively narrow bubble size distribution, and operating in a countercurrent flow mode had been proposed to achieve efficient gas–liquid microdispersion performance at the high phase ratio of gas to liquid. Variables were investigated to determine the gas hold-up, bubble size and its distribution, and specific surface area. The gas–liquid microdispersion mechanism in the m-ARD was also discussed. Two typical bubble generation routes, namely, churn and dripping flow were observed and a transition map was constructed. Furthermore, the handling capacity, average bubble size, and specific surface area could be up to 30 mL/min, 0.59 mm, and 1000 m2/m3, respectively. More gas inlets could be beneficial for much higher handling capacity. In addition, for predicting the gas hold-up and average bubble size, two dimensionless equations were proposed.

CO2 capture in a miniaturized annular rotating device with countercurrent flow

Microdevices are becoming a hot technology for process intensification of CO2 capture. However, most microdevices are limited to a small handling capacity, low volume gas to liquid ratio, and cocurrent flow mode. In this context, we proposed a miniaturized annular rotating device (m-ARD), which has a high handling capacity, can be employed for high volume ratios of gas to liquid, and utilizes a countercurrent flow mode. Variables were investigated to determine the flow characteristic, absorption efficiency of CO2, and volume mass transfer coefficient. The gas hold-up and specific surface area in the m-ARD were up to 43.8 % and 1000 m2/m3, and low pressure drop was observed. Notably, the optimal volume gas to liquid phase ratio, absorption efficiency of CO2, and overall volumetric mass transfer coefficient were close to 200 %, 100 %, and 0.53 s−1, respectively. Furthermore, two dimensionless equations were built to predict the volumetric mass transfer coefficient and gas hold-up. In addition, the contributions of the specific surface area and mass transfer coefficient for enhancing CO2 absorption were discussed, respectively. This work provides a prospective device for enhancing CO2 capture and other gas–liquid separation and reaction processes.

Implementation of a Pilot-Scale Biotrickling Filtration Process for Biogas Desulfurization under Anoxic Conditions Using Agricultural Digestate as Trickling Liquid.

A pilot-scale biotrickling filter (BTF) was operated in counter-current flow mode under anoxic conditions, using diluted agricultural digestate as inoculum and as the recirculation medium for the nutrient source. The process was tested on-site at an agricultural fermentation plant, where real biogas was used. The pilot plant was therefore exposed to real process-related fluctuations. The purpose of this research was to attest the validity of the filtration process for use at an industrial-scale by operating the pilot plant under realistic conditions. Neither the use of agricultural digestate as trickling liquid and nor a BTF of this scale have previously been reported in the literature. The pilot plant was operated for 149 days. The highest inlet load was 8.5 gS-H2Sm-3h-1 with a corresponding removal efficiency of 99.2%. The pH remained between 7.5 and 4.6 without any regulation throughout the complete experimental phase. The analysis of the microbial community showed that both anaerobic and anoxic bacteria can adapt to the fluctuating operating conditions and coexist simultaneously, thus contributing to the robustness of the process. The operation of an anoxic BTF with agricultural digestate as the trickling liquid proved to be viable for industrial-scale use.

CFD simulation and optimization of an energy-efficient direct contact membrane distillation (DCMD) desalination system

In this paper, the CFD simulation and optimization of an energy-efficient direct contact membrane distillation (DCMD) desalination system in a counter-current flow mode are performed. Two-dimensional modeling was applied and the governing equations for momentum, heat, and mass transfer in the three regions i.e., feed channel, membrane, and permeate channel were solved and corresponding velocity, temperature, concentration fields, and the permeate water flux through the membrane was obtained. A two-step approach was considered to optimize the desalination system through the MD process. First, the model results were validated with the reported experimental data and good agreements were observed. The maximum deviation for the outlet concentration and hot stream outlet temperature with different feed velocities and temperatures was 0.33%. Then, the process was optimized for maximizing the permeate flux. Since the number of parameters was high, the response surface methodology (RSM) was used to find the optimum condition in order to obtain the maximum water flux. It was determined that the highest permeate water flux would be gained at the permeate temperature, the hot feed flow rate, the cold-water flow rate, the length and height of the channels of 35 °C, 20 lit/min, 10.25 lit/min, 20 cm, and 3 cm, respectively. At this optimum condition, a permeate flux of 53.5 kg/m2.h and a TPC of 0.82 are expected.

Bubble size analysis in a two-phase countercurrent flow in the narrow rectangular column

The flow of bubbles in a two-phase system has great implications in chemical, petrochemical, and biochemical applications. This work enunciates the measurement of bubble size distribution and bubble aspect ratio in three-different axial zones in the countercurrent flow mode with a gas and liquid velocity range of 0.044–0.321 and 0.019–0.058 m/s, respectively. Bubble size is measured by the photographic technique. The bubble aspect ratio and bubble size distribution results reveal that the impact of gas and liquid velocities is significant on the Sauter mean bubble size. The Sauter mean bubble size increases as the gas velocity increases, whereas it decreases with the liquid velocity. The Sauter mean bubble diameter ranges from 2.65 to 6.16 mm. The distribution of bubble sizes follows the LogLogistic probability density function. In addition, a correlation is also proposed for the interpretation of bubble diameter in terms of Reynolds number and Froude number. The bubble aspect ratio changes with axial zones and gas and liquid velocities. Experiments reveal that the bubble aspect increases with liquid velocity while decreasing with gas velocity and axial zones. The bubble aspect ratio correlations are developed in terms of Eötvös and Reynolds numbers. The present results will be helpful for the process intensification of bubble-aided two-phase flow applications.

Assessment of pilot direct contact membrane distillation regeneration of lithium chloride solution in liquid desiccant air-conditioning systems using computer simulation.

Membrane distillation (MD) has been increasingly explored for treatment of various hyper saline waters, including lithium chloride (LiCl) solutions used in liquid desiccant air-conditioning (LDAC) systems. In this study, the regeneration of liquid desiccant LiCl solution by a pilot direct contact membrane distillation (DCMD) process is assessed using computer simulation. Unlike previous experimental investigations, the simulation allows to incorporate both temperature and concentration polarisation effects in the analysis of heat and mass transfer through the membrane, thus enabling the systematic assessment of the pilot DCMD regeneration of the LiCl solution. The simulation results demonstrate distinctive profiles of water flux, thermal efficiency, and LiCl concentration along the membrane under cocurrent and counter-current flow modes, and the pilot DCMD process under counter-current flow is superior to that under cocurrent flow regarding the process thermal efficiency and LiCl concentration enrichment. Moreover, for the pilot DCMD regeneration of LiCl solution under the counter-current flow, the feed inlet temperature, LiCl concentration, and especially the membrane leaf length exert profound impacts on the process performance: the process water flux halves from 12 to 6 L/(m2·h) whilst thermal efficiency decreases by 20% from 0.46 to 0.37 when the membrane leaf length increases from 0.5 to 1.5 m.

Reliable fluid-mechanical characterization of haemofilters: Addressing the deficiencies of current standards and practices.

Facile methods for accurate fluid-mechanical characterization of haemofilters (HF) are indispensable for haemofiltration process improvements, equipment design/optimization, and reliable module specifications. Currently employed methods, implemented through specific experimental in vitro protocols, are assessed herein in detail, considering the conditions prevailing during haemofiltration. Minimum number of key parameters required to fully describe the common countercurrent flow field, in the HF active section, include membrane permeance K and friction coefficients in lumen and shell side (ff and fs ). It is shown that the countercurrent flow mode itself is incapable of yielding these parameters, based on externally measured flow rates and pressures. Similarly, the relevant ISO protocol is deficient as it can only provide rough underpredictions of permeance K. The causes of such inherent deficiencies of current standards and practices are analyzed. In contrast, a recently developed methodology, accounting for the (heretofore ignored) pressure drop in module headers and combining a mechanistic theoretical model with experimental data from 2 special haemofilter operating modes, yields an accurate determination of the key parameters (K, ff , fs ). Additionally, it permits a full description of flow field for Newtonian liquids, for both constant and axially varying viscosity in fiber-lumen due to the transmembrane flux. Development of new reliable standards is suggested, facilitated by the insights gained in this work.

Development of a reactive extraction process for enhancing acetalizations of ethylene glycol and 1,2-butanediol with propyl aldehyde

Ethylene glycol (EG) is a widely used industrial chemical material, and integrated chemical-looping separation technology (CLS) was verified effective to remove 1,2-butanediol (1,2-BDO) and recover EG produced via the syngas to EG route. Thermodynamic experiments were carried out and employed to analyze the vapor–liquid and liquid–liquid behaviors of the system. Propyl aldehyde, apart from as the reactant, also shows excellent performance in water removal and acetals extraction. Based on the results of the thermodynamic analysis, a reactive extraction process was proposed to intensify the acetalization of EG and 1,2-BDO mixture with propyl aldehyde. The reactive extraction model was established on Aspen Plus v11, and the effects of flow mode (co-current and counter-current), number of stages, the feed-in mole ratio of aldehyde to DIOL on the conversions of EG and 1,2-BDO, the recovery of 2-ethyl-1,3-dioxolane (2ED), and the purity of acetals were investigated. Results show that co-current flow mode performs better than counter-current flow mode and an aldehyde to DIOL feed-in mole ratio of 1.35 to 1 is sufficient. The whole flowsheet, which consists of a seven-stage reactive extraction column and two distillation columns, was designed and optimized. Remarkably, the conversions of EG and 1,2-BDO are over 0.95 and the recovery of 2ED is 0.999 with the aldehyde to DIOL stoichiometric coefficient, which indicates the effectiveness of this process and provides fundamental data and guidance for industrial application.

Heat Transfer Enhancement of Ethylene Glycol using Corrugated Plate Heat Exchanger

This paper presents an experimental study on heat transfer rate for ethylene glycol using a flat plate heat exchanger and various corrugation angles of corrugated plate heat exchanger. Experimental set up provided with thermocouples to measure the temperatures along the length of each plate at seven locations. Additionally, four thermocouples were used to measure the inlet as well as outlet temperature of test fluid and hot fluid. Water was used as a hot fluid at constant temperature of 75°C and Ethylene glycol was used as a test fluid in a counter-current flow mode. The fluids flow rates were varied from 0.5 lpm to 4.5 lpm and the corresponding temperatures are measured. From the experimental readings, the heat transfer coefficient and Nusselt numbers were calculated for flat plate and corrugated plate exchangers. The heat transfer coefficient values and Nusselt numbers were compared with the corrugation angles (300, 400) of corrugated plate and flat plate heat exchangers. The heat transfer coefficient and Nusselt number enhances for corrugated plate with increasing in Reynolds number. The improvement in values is due to the high heat transfer rate caused by turbulence at the corrugation angle. Furthermore, as the increase of mass flow rate, gradual decrement observed for the heating effectiveness in corrugated plate as well as flat plate heat exchanger. This drop of effectiveness is due to decrease of time contact between the two fluids.

Analytical solutions for forced and spontaneous imbibition accounting for viscous coupling

Fluid-fluid momentum transfer can cause higher flow resistance when fluids flow in opposite directions as compared to the same direction. Conventional modelling of flow in porous media using simple, saturation dependent relative permeabilities does not account for such variations.We consider a generalized theory for multiphase flow in porous media based on mixture theory, where fluid mobilities follow from water-rock, oil-rock and water-oil interaction terms defined in momentum equations. Under strictly co- or counter-current flow modes, the generalized model produces explicit relative permeability expressions dependent on the flow mode, saturations, viscosities and interaction parameters. New expressions for counter-current relative permeabilities are derived assuming zero net flux, representative of counter-current spontaneous imbibition. These functions are compared to previously derived co-current relative permeabilities (assuming equal phase pressure gradients). The functions are incorporated into analytical solutions for forced and spontaneous imbibition (FI and SI) using the theory by Buckley and Leverett (1942) and McWhorter and Sunada (1990), respectively.Our results show that when accounting for viscous coupling; Counter-current relative permeabilities are always lower than co-current ones, including the end points. Both phase curves are reduced by the same saturation dependent coefficient. Increased viscous coupling in the FI case led to a more effective displacement, seen as an increased front saturation and average water saturation behind the front. For counter-current SI, increased viscous coupling resulted in lower imbibition rate. Increased viscosities reduces both oil and water counter-current relative permeabilities, and predict greater reduction in imbibition rate than only modifying the viscosities. The analytical solutions for SI were in agreement with numerical solutions of both a conventional and generalized model. The solutions for SI could be scaled exactly to a square root of time curve for arbitrary input parameters in the generalized model, especially including the strength of viscous coupling.

Thermal Analysis and Performance Evaluation of Triple Concentric Tube Heat Exchanger

Triple concentric-tube exchanger (TCTHE) is an improved version of double concentric tube heat exchanger (DCTHE). Introducing an intermediate tube to a DCTHE provides TCTHE and enhances the heat transfer performance. Recognizing the need of experimental results, extremely scarce in the literature and essential to validate theoretical analyses, the aim of this work is to investigate thermal behavior of TCTHE. The present study includes design, development and experimental analysis of TCTHE for oil (ISO VG 22) cooling application required for industrial purposes. It comprises of water (cooling fluid) flowing through innermost tube as well as outer annulus and oil (hot fluid) flows through inner annulus. The experimental studies of the temperature distribution for three fluids along the length and heat transfer characteristics for TCTHE under insulated condition for counter current flow mode are carried out and discussed. The effect of change in oil (hot fluid) temperatures is analyzed keeping water inlet temperatures same at various operating conditions. The experiments have been conducted by varying flow rate of one of the fluids at a time and keeping other two fluid flow rates constant. The results are expressed in terms of temperature variation for all three fluids along the length. The effect of change in hot fluid inlet temperature is expressed in terms of heat transfer rate variation with respect to Reynolds number. The variation of non-dimensional parameters as temperature effectiveness and thermal conductance with respect to Reynolds number is also presented in this paper. Theoretical studies are carried out for evaluation of heat transfer rate using empirical correlations. Experimental validation is carried out for degree of cooling at different Reynolds numbers with theoretical analysis.

Introducing sorption coefficient through extended UNIQAC and Flory-Huggins models for improved flux prediction in forward osmosis

A 2D computational fluid dynamic-based finite element model was developed to describe water flux and concentration polarization in forward osmosis (FO) under steady-state conditions. The extended UNIQAC equation in the feed and the Flory Huggins equation on the membrane side were solved simultaneously to calculate the activity coefficients of the diffusion components. The ratio of the feed component activity coefficient to that on the membrane surface was defined as the sorption coefficient. Employing the sorption coefficient led to a significantly improved accuracy of the developed model in estimating water flux. The membrane structural parameter determining internal concentration polarization (ICP) should be maintained at reasonable values. Otherwise, it may significantly increase the reverse salt flux and external concentration polarization (ECP) on the active layer. This suggests the tradeoffs between the membrane structural parameter and membrane selectivity, between ECP and ICP. Water fluxes were similar in counter-current and co-current flow modes. ICP had the most significant resistance to flow, reducing the water flux up to 75%. ECP on the feed side had a limited effect on FO water flux, particularly at high flow velocity. ECP on the draw side and reverse salt flux had negligible effects on FO water flux in our modeling. Our study provides important insights into developing high-performance FO membranes with reasonably low structural parameters and improving FO water flux by reducing the key limiting factor (e.g. ICP).

Comparison of co-current and counter-current flow in a bifunctional reactor containing ammonia synthesis and 2-butanol dehydrogenation to MEK

In this study, a multi-tubular thermally coupled packed bed reactor in which simultaneous production of ammonia and methyl ethyl ketone (MEK) takes place is simulated. The simulation results are presented in two co-current and counter-current flow modes. Based on this new configuration, the released heat from the ammonia synthesis reaction as an extremely exothermic reaction in the inner tube is employed to supply the required heat for the endothermic 2-butanol dehydrogenation reaction in the outer tube. On the other hand, MEK and hydrogen are produced by the dehydrogenation reaction of 2-butanol in the endothermic side, and the produced hydrogen is used to supply a part of the ammonia synthesis feed in the exothermic side. Thus, 30.72% and 31.88% of the required hydrogen for the ammonia synthesis are provided by the dehydrogenation reaction in the co-current and counter-current configurations, respectively. Also, according to the thermal coupling, the required cooler and furnace for the ammonia synthesis and 2-butanol dehydrogenation conventional plants are eliminated, respectively. As a result, operational costs, energy consumption and furnace emissions are considerably decreased. Finally, a sensitivity analysis and optimization are applied to study the effect of the main process parameters variation on the system performance and obtain the minimum hydrogen make-up flow rate, respectively.

Analysis of process intensification and performance assessment for fermentative continuous production of bioethanol in a multi-staged membrane-integrated bioreactor system

The work analyses process intensification in production of fermentative biofuel (bioethanol) in a multi-staged membrane integrated bioreactor system from sugarcane juice (SCJ) using Saccharomyces cerevisiae (NCIM 3205). Membrane based clarification, sterilization and concentration of sugar cane juice could replace several energy-intensive and polluting steps of conventional production schemes. Integration of traditional bioreactor with membrane-based devices for downstream purification of bioethanol enabled the system for recycling of cells and residual sugars to the fermentation unit through largely fouling free cross flow microfiltration and nanofiltration membrane modules. The new process design is characterized by high yield (0.48 g/g), productivity (14.6 g/L/h) and concentration capability (97.6 g/L) using concentrated SCJ of 22 wt% under high cell density in a flexible, compact, eco-friendly and modular plant. The final membrane distillation stage concentrated ∼98% pure ethanol in a solar-driven direct contact membrane distillation configuration using PTFE/PET hydrophobic membrane with enhanced flux with vis-à-vis the existing processes. A rectangular flat sheet cross flow membrane module in counter-current flow mode of hot and cold streams was employed. The membrane-based system achieves quite high levels of process intensification essential to sustainable operation at industrial scale. The achieved process intensification in the new system has been analysed vis-a-vis the existing conventional systems in terms of eco-friendliness, flexibility, cost of equipment and production, consumption of energy and material, profitability, E-factor, atom efficiency and business sustainability.

Numerical analysis of performance of ideal counter-current flow pressure retarded osmosis

Pressure retarded osmosis (PRO) operated in counter-current flow mode is more efficient than in co-current flow mode to extract salinity gradient energy. Knowledge of the performance of counter-current flow PRO under various operating conditions (on equilibrium or off equilibrium) is of paramount importance to understand the potential capacity of the technology and to optimize process design. In this study, a systemic and rigorous numerical procedure was developed for performance simulation of counter-current flow PRO. An optimization technique was used to accurately determine the originally unknown flow rate of the draw solution at the feed entrance of membrane channel so that the procedure could also be used for PRO systems not at equilibrium. With this numerical procedure, new interesting findings were made about the ideal counter-current flow PRO. A characteristic parameter of the PRO, the required membrane area to reach equilibrium for any given operating condition, was determined and reported for the first time. Another exciting finding was that the no-flux zone (dead region) occurs adjacent the draw entrance at the critical feed fraction when the membrane area is greater than the required equilibrium area. Power density and specific energy in PRO under various conditions were investigated with this numerical procedure.

A comparative study of pilot-scale bio-trickling filters with counter- and cross-current flow patterns in the treatment of emissions from chemical fibre wastewater treatment plant

Two pilot-scale bio-trickling filters (BTFs) with counter-current and cross-current flow modes were constructed, and their performance tested, for purifying chemical fibre waste gas containing H2S, NH3 and VOCs with a maximum gas flow rate of 1008m3h−1. The counter-current type of BTF presented with superior biodegradation results compared to the cross-current type: it could start up quickly, tolerated high transient shock loadings, and possessed an average contaminant removal efficiency higher than 90% with an empty bed residence time of 59s. The contaminant removal efficiency could be increased by 50% during winter due to the addition of pipeline insulation. The abundance and diversity from microorganism analysis showed that Dyella, Bacillus, Candidimonas, Pandoraea and Thiomonas were the main bacterial strains forming the community treating the pollutants. The counter-current type BTF functioned most effectively and is proposed for practical application.

A stepwise model of direct contact membrane distillation for application to large-scale systems: Experimental results and model predictions

A model of mass and heat transfer in direct contact membrane distillation (DCMD) is presented. The distributions of water flux, temperature, and concentration in membrane module channels are modeled to provide cumulative water flux and outlet temperatures and concentrations. The membrane distillation coefficient (MDC)—a membrane-specific mass transfer coefficient—for a well-characterized PTFE membrane was shown to be accurately modeled as a constant value. The MDC was used with a stepwise modeling approach, in which the membrane area is discretized into multiple steps, to provide the distribution of process variables parallel to the membrane surface. A new generalized spacer modeling method was used to account for the presence of complex woven spacers in the flow channels, addressing a significant gap in the DCMD modeling literature. Model predictions showed good agreement with experiments in co-current and counter-current flow modes for different operating conditions and membrane sizes. The stepwise modeling approach was shown to be necessary for providing accurate mass and heat transfer predictions for large-scale DCMD modules, providing a useful tool for the design of large-scale DCMD systems.