Co-current Mode Research Articles

Toluene removal from nonionic surfactant coacervate phase solutions by vacuum stripping

Cloud point extraction (CPE) has been demonstrated to remove volatile aromatic pollutants from wastewater by using a nonionic surfactant as a separating agent. To make this process economically feasible, the surfactant in the concentrated or coacervate phase containing most of the original pollutant must be recycled and reused. This work proposes a vacuum stripping process to strip the volatile pollutant from the coacervate, leaving the concentrated surfactant solution to be recycled to the CPE process. An acrylic column was packed with glass Raschig rings and operated under rough vacuum in co-current mode to remove toluene from the coacervate phase containing the t-octylphenolpolyethoxylate (OP(EO 7)) nonionic surfactant. Co-current operation is necessary to avoid excessive foaming. In this study, although the apparent Henry's law constant of toluene was drastically reduced due to toluene solubilization in surfactant micelles, up to 90% of toluene was removed from 300 mM OP(EO) 7 surfactant solution using a 30.5-cm long column. The packed column can be operated without flooding and plugging, despite the viscous nature of the coacervate. The toluene removal increases with decreasing liquid loading rate, and column pressure, but with increasing number of distributor holes. The overall liquid phase volumetric mass transfer coefficient is reported for this system.

Catalytic decomposition of H2S in a double-pipe packed bed membrane reactor: Numerical simulation studies

Abstract H2 recovery from large quantities of toxic H2S, the by-product of various minerals, coal and petroleum processing operations is an attractive proposition for efficient resource and energy utilization. This study has examined the analysis of a double-pipe membrane reactor internally packed with a Ru-Mo catalyst for the catalytic desulfurization of H2S and H2 generation. Due to the endothermic nature of the reaction, a non-isothermal model was employed. The effect of tube and shell-side residence times, sweep gas (argon) flow direction within the annular space between shell and inner membrane tube (relative to the H2S feed) and the shell-to-tube side pressure ratio on H2S conversion were investigated. The counter-current mode of reactor operation generally gave better H2S conversions than the co-current mode (40–60%). Irrespective of shell-to-tube side pressure ratio, conversion in both modes of operation exceeded the thermodynamically-imposed ceiling by between two- and five-fold increase. The niobium membrane proposed was internally-coated with platinum to improve resistance to H2S attack. The analysis showed that an optimum Pt thickness of about 1 μm is attainable although the possible formation of hairline cracks may necessitate the use of a higher coating. The model also accounted for the temperature-dependency of the membrane H2 permeability in order to have a more realistic assessment of the reactor performance.

A comparison of co-current and counter-current modes of operation for a dual-type industrial methanol reactor

In this paper, the dynamic behavior of a dual-type methanol reactor is investigated. For simulation purposed, a heterogeneous one-dimensional model has been developed. In the dual-type reactor system, a system with two-catalyst bed instead of one single catalyst bed is developed for methanol synthesis. In the first catalytic reactor, the synthesis gas is partly converted to methanol in a conventional water-cooled type reactor. In the second reactor, the reaction heat is used to preheat the feed gas to the first reactor. The continuously reduced temperature in this reactor provides increasing thermodynamic equilibrium potential. The proposed model was validated against measured daily process data of a methanol plant recorded for a period of 4 years and a good agreement was achieved. The validity of this model is also shown to be satisfactory for the investigation of the effect of co-current and counter-current flow pattern on performance of reactor. In the co-current mode, feed gas is entered into the tubes of the second reactor in the same direction with the reacting gas stream in shell side while in the counter-current mode the gas streams are in the opposite direction. Simulation results for both co-current mode and counter-current mode have been compared in terms of temperature, activity and methanol production rate. The results of this work show that the reactor in co-current mode operates with lower conversion but with longer catalyst lifetime in comparison with counter-current mode.

Mathematical modeling and simulation of hydrotreating reactors: Cocurrent versus countercurrent operations

This paper describes a model to predict the behavior of trickle-bed reactors used for catalytic hydrotreating of oil fractions with cocurrent and countercurrent operation modes. A dynamic plug-flow heterogeneous one-dimensional model, which has been previously validated with experimental data obtained in an isothermal pilot plant reactor with cocurrent operation, was employed to compare both modes of operation. The reactor model considers the main reactions present in the hydrotreating process: hydrodesulfurization, hydrodenitrogenation, and hydrodearomatization. Simulations were performed for both pilot and commercial trickle-bed reactors, and the results are discussed in terms of variations with time and axial position of partial pressure, temperature and concentrations in liquid phase. A superior performance of countercurrent operation mode was found over cocurrent mode. It was recognized that countercurrent mode can have great potential to be used for deep hydrodesulfurization of oil fractions since it minimizes the inhibiting effect of some products (e.g. H2S) in reactor zones where these species tend to concentrate in cocurrent operation, i.e. at the bottom of the catalytic bed.

Modeling of a heat-coupled catalytic reactor with co-current oxidation and conversion flows

Mathematical model and numerical method are developed to analyze the operation of a coupled methane steam reformer. The reformer operates in co-current mode of endothermic (steam reforming) and exothermic (fuel oxidation) departments. The exhaust anode gas of a fuel-cell battery containing hydrogen is used as fuel. Two-dimensional, two-temperature mathematical model of monolith is used to describe processes in exothermic and endothermic departments of the reformer. Calculation results were compared with experiments. The agreement between the modeling and the experimental results is rather good. The model analysis points out that the considered reactor design has high parametric sensitivity, there is a probability of arisen of a hysteresis with points of ignition and extinction.

Experimental and Theoretical Study of Reactive Stripping in Monolith Reactors

In this paper, reactive stripping in monolithic reactors is studied experimentally and theoretically. For the experiments, different monolith types are coated with a catalyst and operated in cocurrent and countercurrent mode. The esterification of 1-octanol and hexanoic acid is investigated in a batch-recycle mode; the liquid phase is recirculated, whereas the stripping gas nitrogen with the stripped water is vented after each pass through the column. The experimental results show that the conversion of equilibrium-limited esterifications can be increased using reactive stripping, the process performance being influenced by the monolith type and operating mode. The theoretical description is given by a rate-based model, which is successfully validated against the experimental data and used to conduct sensitivity studies, with regard to operating parameters and different reactive zone arrangements within the monoliths.

Co-current and counter-current modes for methanol steam reforming membrane reactor

In this simulation study, methanol steam reforming reaction to produce synthesis gas has been studied in a membrane reactor when shell side and lumen side streams are in co-current mode or in counter-current mode. The simulation results for both co-current and counter-current modes are presented in terms of methanol conversion and molar fraction versus temperature, pressure, H 2 O / CH 3 OH molar feed flow rate ratio and axial co-ordinate. Although the methanol conversion is sometimes higher in the membrane reactor operated in co-current mode than in the counter-current one, in the last case it is possible to extract more hydrogen from the reaction zone. With this theoretical analysis, we present a set of parameters which permit to have a CO-free hydrogen stream and a complete recovery of the hydrogen from the lumen side of the reactor.

Transport of uranium from nitric acid solution by non-dispersive solvent extraction using a hollow fibre contactor

Summary Membrane based non-dispersive solvent extraction (NDSX) of U(VI) from aqueous nitric acid medium was carried out using a commercial hollow fibre module containing microporous hydrophobic polypropylene capillaries. The NDSX operation was carried out by pumping U(VI) solutions at varying concentrations of nitric acid (1-5 M) through the tube side and organic solvent (di-n-hexyl octanamide (DHOA) in normal paraffin hydrocarbon (NPH)) through the shell side of the hollow fibre capillaries. Extraction studies were performed under different hydrodynamic conditions and the overall mass transfer was evaluated under counter-current flow conditions. Counter-current mode was found to be better as compared to the co-current mode. The U(VI) mass transfer rates were positively influenced by the nitric acid as well as DHOA concentrations.

Steam Reforming of Methane in a Membrane Reactor: An Industrial Case Study

The methane steam-reforming reaction has been investigated from a modeling point of view, considering the effect of different parameters on methane conversion. In particular, we point our attention mainly toward the influence of the different retentate and permeate flux configurations on both methane conversion and hydrogen recovery. Both methane conversion and hydrogen recovery are higher in the membrane reactor operated in the countercurrent mode than in the cocurrent mode. A comparison between a traditional steam-reforming process and a membrane reactor process has also been made. The use of a membrane reactor can simplify the process scheme, and the operating temperature and the fuel consumption are lower than those of a traditional system.

Enhancement of CO conversion in a novel Pd–Ag membrane reactor for methanol synthesis

The present work investigates enhancement of CO conversion in a palladium–silver membrane reactor for methanol synthesis. A novel reactor configuration with perm-selective Pd–Ag (23 wt.% Ag) wall to hydrogen has been proposed. The reacting material is flowing through the tube side while synthesis gas is flowing through the shell side in co-current mode. The perm-selective layer of palladium allows hydrogen in synthesis gas to penetrate to the reaction side. The impact of thickness of Pd–Ag layer, feed flow rate of reaction side, temperature and pressure of reaction side, pressure of shell side, the amount of carbon monoxide in feed (relative to total amount of CO and CO 2), and stoichiometric number on the carbon monoxide conversion have been examined. The results demonstrate that membrane reactor can raise carbon monoxide conversion beyond equilibrium value.

Theoretical Investigation of a Pd‐membrane Reactor for Methanol Synthesis

AbstractA numerical study is performed in order to evaluate the performance and optimal operating conditions of a palladium membrane reactor for methanol synthesis. A novel reactor configuration with a Pd wall, which is perm‐selective to hydrogen, has been proposed. In this configuration the reactants are added to the tube side while pure hydrogen is added to the shell side, as a result, the hydrogen diffuses across the membrane from the shell side to the tube side. In this membrane reactor, hydrogen penetrates to the reaction side in order to maintain a suitable hydrogen level in the whole length of the reactor and shift the equilibrium reaction. The effects of different parameters on the methanol output mole fraction were investigated in the co‐current mode. These parameters were membrane thickness, reaction side flow rate, reaction side pressure, shell side pressure and H2/CO2 ratio in the feed.

Co-current and counter-current modes for water gas shift membrane reactor

In this study the performances of a membrane reactor (MR) are estimated when both shell side stream (sweep gas) and lumen side stream are continuously either in parallel flow configuration (co-current mode) or in counter-flow configuration (counter-current mode). Two mathematical models have been formulated and steady-state mass-balance gave two-dimensional differential equations, which were solved by using the orthogonal collocation technique. Simulation results for both co-current mode and counter-current mode have been compared in terms of hydrogen molar fraction (in the shell side) vs. axial co-ordinate at different hydrogen permeances, temperatures, and lumen pressures. At the operative conditions considered, a very similar CO conversion value has been obtained for both modes.

Millisecond catalytic wall reactors: dehydrogenation of ethane

Heat transfer between exothermic and endothermic reaction streams can take place at extremely high rates if one or both reactions take place catalytically on opposite sides of a thin wall. This eliminates the resistance to heat transfer in thermal boundary layers, potentially permitting much smaller reactors. A coupled catalytic wall reactor operating at temperatures of ∼1000°C was built which enabled efficient heat exchange between exothermic methane catalytic combustion and endothermic homogeneous ethane cracking. Multiple passes were added on the combustion side to reduce heat losses in the system. A two-pass system operating in a co-current mode gave 77% ethane conversion and 87% selectivity to ethylene with a residence time of ∼50 ms , with a residence time on the combustion side of ∼10 ms . This reactor has been operated for many hours without any deterioration in performance.

Designing Ozone Bubble Columns: A Spreadsheet Approach to Axial Dispersion Model

When designing ozone bubble columns, two major sources of uncertainties usually exist: (1) the measurement techniques and the estimation methods of the various operating parameters; and (2) the application of the pertinent design model. This paper presents a simple and easy-to-use, yet accurate and reliable design model for describing the performance of ozone bubble columns for water and wastewater treatment applications. This mode! is a modified non-isobaric steady-sate one-phase axial dispersion model (1P-ADM). The 1P-ADM is different from the complete axial dispersion model, or referred to as the two-phase axial dispersion model (2P-ADM), in its simple use for practical design and process control of full-scale contacting chambers. The 2P-ADM is represented by a system of two non-linear partial differential equations. In order to solve that system of equations, an elaborate numerical solving technique is needed. On the other hand, die 1P-ADM is composed of a single non-homogeneous linear second-order ordinary differential equation representing the liquid phase. Yet, this liquid-phase differential equation accounts for the countering effects of die gas bubbles' shrinkage and expansion caused by gas depletion and absorption and reduced liquid hydrostatic head. The differential equation was solved analytically by the method of variation of parameters. Expressing the 1P-ADM in terms of dimensionless operating parameters and with the available analytical solution of the differential equation, the model predictions of the dissolved and the gaseous ozone profiles along the column height were examined using a simple spreadsheet approach. Therefore, describing mat analytical solution in terms of a simple spreadsheet program facilitated obtaining the model predictions for any operating conditions represented by the model parameters entered into die spreadsheet program. Consequently, using die 1P-ADM for process design and/or on-line process control becomes very feasible. The 1P-ADM was initially tested to evaluate its predictions of the dissolved ozone profiles for water treatment conditions. The model provided excellent predictions of the dissolved ozone profiles along the bubble column for die counter-current and the co-current flow modes.

Membrane permeation of a binary compound for co-current and cross flow modes

This study reveals the behavior of air-sweep pervaporation in a mixture of two volatile organic compounds (VOCs) combination from a solution for two flow modes of operation, namely co-current and cross flow mode. Two models, single compound and binary compound permeation, were studied. Results indicated that the membrane shows a higher affinity for one of the compounds compared to the other. Higher affinity may have led to higher sorption and a higher permeability rate. The flux production in the cross flow mode is found to be higher than the co-current flow mode. However, selectivity is unaffected by the flow mode of operation. A study on the mutual coupling effect shows that, due to the presence of one compound, the flux of the other compound is reduced. The reduction in the flux production is found to be higher in the co-current flow mode than the cross flow mode, especially for a higher concentration of the interfering compound. The model study indicates that the transfer of the VOC through the bulk liquid phase to and sorption on the membrane surface is the rate-limiting step. A difference in the flux values from the experiments and that predicted by the binary compound permeation model is noted. The difference was found to be constant irrespective of the compound used. This may be mainly due to the flow mode of operation. To incorporate this effect and various other influencing effects, preliminary modification of the model is proposed in the form of an empirical constant.

Zeolite membrane reactor for isobutane dehydrogenation: Experimental results and theoretical modelling

Dehydrogenation of isobutane has been studied in a packed-bed zeolite membrane reactor using Pt–In catalyst. Enhanced dehydrogenation yields were obtained in the membrane reactor due to the separation of hydrogen from the reaction medium. Two sweeping modes were studied, namely the co-current and the countercurrent modes. Although the separation factor is higher in countercurrent than in co-current, the yield of reaction in these two sweeping modes is quite the same. The dehydrogenation reaction was limited by transport properties of the membrane in co-current mode while in countercurrent mode the limitation by kinetics was predominant. Reactor operation was described through theoretical modeling. A good agreement between the model and the experimental values was obtained with a co-current sweep gas. In countercurrent mode, the values predicted by the model for the yield of isobutene were slightly overestimated, due to hydrogen removal which induced catalyst local deactivation, or because the kinetic rate is inadequate when we are working far from the neighborhood of the equilibrium.

Counter-current vs co-current flow in carbon-in-pulp adsorption circuits

Carbon-in-pulp (CIP) and carbon-in-leach (CIL) technology is applied successfully to recover gold from cyanided pulps. These plants are operated on a counter-current basis allowing for small concentrations of gold to be recovered at the expense of inefficiencies such as back-mixing, additional carbon breakage by interstage pumps and difficulties in carbon inventory management. In this study, linear regression techniques were utilised in order to study the adsorptive behaviour of carbon in gold bearing solutions with the aim of determining whether a partial co-current mode of operation will be feasible in CIP operations. It has been found that these empirical techniques are able to predict the adsorption rate of gold onto activated carbon and that the point of discontinuity of the constant rate adsorption period can accurately be determined. Thus, the behaviour of a single activated carbon particle can be predicted making use of standard linear regression techniques and using inputs such as, slurry density, rate of agitation, carbon loading and carbon and gold concentration. This information can then be utilised to maximise the use of the constant rate adsorption period by using a partial co-current mode of operation and reducing inefficiencies such as back-mixing and gold loss due to the abrasive action of interstage pumping on the adsorbent.

Numerical approach for a study of the hydrogen isotopes separation by palladium alloy membranes

In this work, a simple and stable method is proposed to design a hydrogen permeator operating with Pd/Ag membranes in countercurrent or cocurrent mode. The method is based on an iterative calculation that requires as input data a tentative value of the outlet permeate composition. The isotopic competition effect in the permeation process has been included in the flux formula. The model supported by a computer code has been used to study the hydrogen isotopes extraction from the gas stream arising from a tritiated water gas shift reactor under conditions relevant to a solid breeding blanket of ITER (International Thermonuclear Experimental Reactor) size. The model and the code have also been tested with experimental data reported in the open literature.

Lumen mass transfer in hollow‐fiber membrane processes with constant external resistances

AbstractHollow‐fiber membrane processes with a constant external resistance having a constant or variable shell concentration resulting from an operational mode of cocurrent or countercurrent are studied. By solving numerically the continuity mass‐conservation equation with the corresponding boundary conditions, the lumen laminar mass‐transfer coefficients for both cases are correlated. The correlations greatly improve the calculating accuracy of the overall mass‐transfer coefficient and can be used to obtain the lumen mixed‐cup concentration by an algebraic equation substituting the partial differential equation. A separation factor m' is introduced to characterize the effect of the operational mode. Calculation results demonstrate that the lumen mass‐transfer coefficient is independent of the real lumen and shell concentrations, but it is greatly influenced by m'. The countercurrent mode, compared to the cocurrent mode, provides not only a higher mean driving force, but a higher lumen mass‐transfer coefficient. This conclusion is novel and valid for the tube‐shell heat or mass‐transfer processes and is supported by the experimental data in the literature and our gas membrane separation experiments.

Ethane dehydrogenation using a high-temperature catalytic membrane reactor

Experiments have been conducted for the dehydrogenation of ethane to ethylene using a high-temperature catalytic membrane reactor under isothermal conditions. Typically, conversions of up to 7 times (cocurrent mode) and 8 times (countercurrent mode) higher than the equilibrium value achievable in conventional fixed-bed reactors have been attained at high sweep flow rates. The membrane used in this study was a thin layer of Pd-23 wt% Ag on porous Vycor glass. The significant improvement in the ethane conversion is attributed to the exclusive and continuous permeation of hydrogen through the membrane.