High Feed Temperature Research Articles

The influence of feed temperature and composition on the conversion of KCl into KHSO4 in a membrane reactor combined with direct contact membrane distillation

The studies of a conversion of KCl with H2SO4 into KHSO4 were performed using a chemical reactor coupled with direct contact membrane distillation. The feed temperature at the inlet of the membrane distillation (MD) module amounted to 333 or 343K and the permeate temperature was 293K. The concentration of solutions with various initial molar ratios of KCl to H2SO4, in the MD membrane reactor resulted in the formation and periodical precipitation of KHSO4 in the feed tank. The application of the membrane reactor enabled a continuous removal of HCl from the reaction mixture through the polypropylene membrane to shift the equilibrium of the conversion. Pure hydrochloric acid reaching a concentration of 43g/dm3 was obtained.It was found that higher feed temperature and excess of sulphuric acid in relation to KCl was profitable. In the case of the conversion at 343K with the reaction mixture containing KCl and H2SO4 in the molar ratio of 1:2, the efficiency of KHSO4 production was high. The potassium conversion coefficient was about 93%. Pure KHSO4 was precipitated from the concentrated solution.

Investigation of coupling dehydrogenation and hydrogenation reactions in a fixed bed catalytic reactor with well-mixed catalyst pattern

The enhancement of ethylbenzene conversion by further displacement of the thermodynamic equilibrium via the influence of the dual-functionality of a well-mixed catalyst pattern has been investigated. A rigorous steady state mathematical model based on the dusty gas model is implemented for the simulation. The simulation results reveal that the introduction of the concept of the reaction coupling has significant effect on the displacement of the thermodynamic equilibrium and considerable enhancement of simultaneous production of styrene and cyclohexane. Almost 100% conversion of the ethylbenzene and benzene is achieved through the application of this approach. It is also found that considerable decrease in the reactor length is achieved by employing a reactor catalyst bed with different bed compositions. Effective operating regions with optimal conditions are observed. An effective reactor length criterion is used to evaluate the performance of the reactor under these optimal conditions. The effective reactor length is found to be sensitive and favored by high feed temperature and pressure. The sensitivity analysis shows that the key parameters of feed temperature, pressure, and the bed composition play an important role on the reactor performance. The results also show that almost 100% conversion of ethylbenzene and benzene at low temperature and shorter reactor length can be achieved by maintaining the reactor beds at different temperatures. This temperature switching policy may result in appreciable energy saving. Moreover, operating the reactor at low temperature protect the catalyst from the excessive temperatures which have destructive effects on the catalysts and the mechanical stability of the reactors. Also, the low temperature operation has significant contribution to the reduction of the operating cost.

Vacuum membrane distillation of seawater reverse osmosis brines

Seawater desalination by Reverse Osmosis (RO) is an interesting solution for drinking water production. However, because of limitation by the osmotic pressure, a high recovery factor is not attainable. Consequently, large volumes of brines are discharged into the sea and the flow rate produced (permeate) is limited. In this paper, Vacuum Membrane Distillation (VMD) is considered as a complementary process to RO to further concentrate RO brines and increase the global recovery of the process. VMD is an evaporative technology that uses a membrane to support the liquid–vapour interface and enhance the contact area between liquid and vapour in comparison with conventional distillation. This study focuses on VMD for the treatment of RO brines. Simulations were performed to optimise the operating conditions and were completed by bench-scale experiments using actual RO brines and synthetic solutions up to a salt concentration of 300 g L −1. Operating conditions such as a highly permeable membrane, high feed temperature, low permeate pressure and a turbulent fluid regime allowed high permeate fluxes to be obtained even for a very high salt concentration (300 g L −1). For the membrane studied, temperature and concentration polarisation were shown to have little effect on permeate flux. After 6 to 8 h, no organic fouling or biofouling was observed for RO brines. At high salt concentrations, scaling occurred (mainly due to calcium precipitation) but had only a limited impact on the permeate flux (24% decrease for a permeate specific volume of 43L m −2 for the highest concentration of salt). Calcium carbonate and calcium sulphate precipitated first due to their low solubility and formed mixed crystal deposits on the membrane surface. These phenomena only occurred on the membrane surface and did not totally cover the pores. The crystals were easily removed simply by washing the membrane with water. A global recovery factor of 89% can be obtained by coupling RO and VMD.

Influence of Feed Composition on Distillate Flux and Membrane Fouling in Direct Contact Membrane Distillation

Two polymeric membranes, PTFE and PVDF, were employed in a direct contact membrane distillation system to investigate the effects of NaCl concentration, feed temperature, and flow rate on distillate flux and NaCl rejection. The distillate flux increases with increasing the feed flow rate or temperature, PTFE membrane has a higher distillate flux than that of PVDF, and PTFE membrane also has a larger solute rejection than that of PVDF membrane. The decrease of solute rejection by PVDF membrane is owing to the membrane pore wetting, especially at high feed temperature and flow rate. The membrane fouling tests under various feed compositions show that increasing NaCl concentration from 4.5 wt% to 10 wt% or the addition of CaSO4 or BSA into NaCl solution does not cause severe fouling on the PTFE membrane. The distilled water flux recovery (FR) is about 0.96–0.97. The addition of MgCl2 or MgSO4 has a little significant fouling tendency, the distilled water FR is about 0.87–0.88.

Numerical Simulation of p-Xylene Synthesis via Selective Toluene Disproportionation over Modified ZSM-5 Catalyst

Numerical modeling of selective toluene disproportionation reaction over modified ZSM-5 zeolites to produce p-Xylene was developed and validated. The results of parametric analysis using the authenticated model provide an optimum guideline for the selection of appropriate operating conditions to achieve maximum toluene conversion and p-Xylene selectivity and yield. The predicted data indicate that increasing feed temperature from 450 to 600°C at constant (WHSV)-1 enhances the toluene conversion by 150%. Low weight hourly space velocities ((WHSV)-1=100 g.h/mol) and feed temperature (450°C) provides 99% p-Xylene selectivity. Moreover, at constant feed temperatures of 450, 500, 550 and 600°C, the maximum p-Xylene yield (? 0.087) occurs at 1300, 650, 330, and 175 g.h/mol, respectively, which clearly demonstrates that operation at higher feed temperature reduces the needed reactor residence time. Despite of imposed adiabatic condition, the model prediction demonstrates that an almost isothermal behavior prevails inside the reactor (2°C decrease in reactor temperature) due to the low heat of reaction (<1kJ/mol).

Effect of flow deflector on the flux improvement in direct contact membrane distillation

This study experimentally investigated the influence of flow deflector on the performance of direct contact membrane distillation. A total of seven modules, including a smooth membrane module and six flow deflector modules were fabricated and tested. Effects of the number of flow deflector, deflector height, and the placement of deflectors are examined. The smooth module reveals a comparatively abrupt change of water flux when the inlet velocity is around 0.25 m s − 1 , and a plateau is seen for the water flux ratio vs. the Reynolds number for modules containing flow deflectors. Depending on the arrangement of flow deflectors, the maxima occur at a Reynolds number of 500 for D1 and 650 for the others. The plateau for module having more flow deflectors has shifted toward a lower Reynolds number while the pressure drop ratio does not reveal such a plateau. For the same pumping power, the test results show that the water flux for smooth module are smaller than that of all flow deflector modules, and it is applicable to all temperatures. It is also found that modules with more deflectors become more and more competitive at a higher feed temperature but are less effective when the velocity is high.

Effect of Feed Liquid Temperature on the Structural Morphologies of d‐Limonene Microencapsulated Powder and Its Preservation

The influence of the feed liquid temperature of the spray drying was investigated on the particle morphology (vacuole size and shell thickness of hollow particle) and the stability of encapsulated d-limonene. The feed liquid was the mixture of d-limonene emulsified in a wall material of blending gum arabic (GA) and maltodextrin (MD) at the mass ratio of 1 : 2, and the liquid temperature was varied from 40 to 80 degrees C. The percentage of the hollow particle in the spray-dried powder was over 40% and slightly increased with the rising of the feed temperature. Hollow particles have shells of different thickness, which was estimated from the image with a confocal laser scanning microscope (CLSM). The shell thickness increased with the increase in the feed temperature. This finding was qualitatively supported by scanning electron microscopy (SEM) images of the fractured particles. In addition, release and oxidation stability of encapsulated d-limonene were measured at constant temperature and humidity. The powder at high temperature of feed has higher stability for release and oxidation of d-limonene than that at a lower feed temperature. The result may partly be attributed to the increase of the shell thickness of the particle at a higher feed temperature. The thicker shell wall possibly served as the barrier of the flavor release and the diffusion of oxygen and moisture from the surrounding environment.

Pervaporation study for the dehydration of tetrahydrofuran-water mixtures by polymeric and ceramic membranes

Pervaporation technology can effectively separate a tetrahydrofuran (THF) solvent-water waste stream at an azeotropic concentration. The performance of a Sulzer® 2210 polyvinyl alcohol (PVA) membrane and a Pervatech BV® silica membrane were studied, as the operating variables feed temperature and permeate pressure, were varied. The silica membrane was found to exhibit a flux of almost double that of the PVA membrane, but both membranes had comparable separation ability in purifying the solvent-water mixture. At benchmark feed conditions of 96 wt% THF and 4 wt% water, 50°C and 10 torr permeate pressure, the silica membrane flux was 0.276 kg/m2hr and selectivity was 365. For both membranes, flux was found to increase at an exponential rate as the feed temperature increased from 20 to 60°C. The flux through the silica membrane increases at a 6% faster rate than the PVA membrane. Flux decreased as permeate pressure was increased from 5 to 25 torr for both membranes. The amount of water in the permeate decreased exponentially as the permeate pressure was increased, but increased linearly with increasing temperature. Optimum conditions for flux and selectivity are at low permeate pressure and high feed temperature. When a small amount of salt is added to the feed solution, an increase in flux is observed. Overall models for flux and permeate concentration were created from the experimental data. The models were used to predict scale-up performance in separating an azeotropic feed waste to produce dehydrated THF solvent for reuse and a permeate stream with a dilute THF concentration.

Effective scale control for seawater RO operating with high feed water pH and temperature

An effective scale control program for seawater RO is widely recognized as an important factor in ensuring trouble free and cost effective operation. This is especially important under the high feed water pH and temperature conditions found in Southern Europe and Middle East.In addition, regulatory issues concerning maximum acceptable boron levels in drinking and irrigation water necessitate the growing need to operate plants at higher feed water pH. Operating at increased pH increases boron rejection but also increases the scaling tendencies of the water. This can lead to calcium carbonate and magnesium hydroxide precipitation in the membranes.A complete scale control program needs to control calcite and brucite saturation at the maximum operating reject pH and temperature. Antiscalant chemistry, optimum dose rate as well as accurate monitoring and control of the scale inhibitor are key factors in long-term cost effective “scale free” operation.This paper details the operating strategies and scale control issues related to both single and two pass seawater RO plants operating at elevated feed water pH and temperature.

Investigation of different hollow fiber module designs for flux enhancement in the membrane distillation process

In this paper, the flux enhancement effects of different hollow fiber membrane module designs with baffles, spacers and modified hollow fiber geometries were investigated through the direct contact membrane distillation process applied for desalination. By modeling the transport phenomena involved in the process, feed-side heat-transfer coefficient for modules with different baffles were estimated. It was found that the application of baffles can increase the feed-side heat-transfer coefficient from 2600 W/(m 2 K) for the unbaffled module to 3150 and 3750 W/(m 2 K) for the window and helical baffles, which correspond to 20% and 28% flux enhancements, respectively. In addition, it was observed that using spacers among the fibers may increase the effective membrane area 18–33%. Lastly, the application of different hollow fiber configurations with wavy geometries (twisted and braided) led to flux enhancements as high as 36% without inserting any external turbulent promoter. In overall, greater flux enhancements with modified hollow fiber membranes modules were achieved at higher feed temperatures. The proposed approaches in this study may be efficiently and easily applied to enhance the flux in industrial applications involving membrane distillation with hollow fiber membranes.

Experimental Investigation on the Separation of Bentonite using Ceramic Membranes: Effect of Turbulence Promoters

The static turbulence promoters presented in this work are designed to enhance filtration within tubular ceramic membranes of 0.5 micron pore size. Permeate flux enhancement still remains a topical problem during tangential crossflow filtration. The decline in flux with time is due to the usual phenomena of concentration polarization and membrane fouling, operating parameters including the system pressures, feed composition, membrane type and configuration, and the hydrodynamics within the membrane module. Solute accumulates on the membrane surface and forms a high concentration gel layer, thus increasing the effective membrane thickness and reduces its hydraulic permeability. Turbulence promoters of varying pitch lengths have been incorporated into the work to ultimately reduce the deposition of bentonite particles on the membrane surface during microfiltration. Yeast suspensions have previously been used as feed suspensions in order to compare the effectiveness of the turbulence promoters with an organic foulant. The objective of this work was to investigate the influence of static promoter geometry on flux sustainability enhancement during bentonite suspension filtration. All experiments have been conducted on a tubular ceramic membrane and the experimental membrane rig as shown in this paper. The effects of feed concentration, feed temperature, system pressures, and crossflow rates on the membrane flux sustainability were investigated. It was found that the promoters greatly improved flux sustainability and membrane efficiency over time and in some cases, a loss of 3% in membrane efficiency was realized with turbulence promoters at higher feed temperatures. The use of the turbulence promoter caused a large scouring of the membrane surface and membrane cleaning was significantly improved compared to the experiments without the promoters.

Effect of the Switch Strategy on the Stability of Reactor Networks

The dynamics of a three-catalytic-reactor network, where an exothermic irreversible reaction occurs, is studied. The network is operated by periodically switching feed and discharge positions (with the period being T). We analyzed the effect of two different switch strategies and observed different spatio-temporal patterns. Such differences result from the different spatio-temporal symmetry that is induced by the switch strategies. Their effect on the stability is studied by comparing the stability range of T-periodic regimes as the switch time is varied for different values of the feed temperature. This analysis is conducted by a numerical continuation technique based on the spatio-temporal symmetry of the forced system. For low (ambient) feed temperature, the stability ranges of a diabatic reactor conform to criteria previously proposed for adiabatic reactors [Sheintuch, M; Nekhamkina, O. AIChE J. 2005, 51, 224]. We observe that these criteria cannot be applied at higher feed temperatures, as a consequence of the external heat losses. One of the examined switch strategies is used to determine a significant widening of the operability conditions.

Application of vacuum membrane distillation for ammonia removal

This study aims to investigate the applicability of membrane distillation for ammonia removal from its aqueous solutions. Among the different recognized membrane distillation configurations, vacuum membrane distillation is applied in the present work for ammonia removal. The effects of different operating parameters on ammonia removal from aqueous solutions of different concentrations have been investigated. Experimental results showed that high feed temperatures, low downstream pressures and high initial feed concentrations and pH levels enhance ammonia removal efficiency. The pH is found to be the most dominant factor. Temperature and concentration polarizations within feed boundary layer proved to have a significant influence on mass transport. High feed flow velocity is an important tool diminishing temperature and concentration polarizations effects. The resistance to mass transfer is found to switch from being predominantly located in the feed side at low feed temperatures and flow velocities to be nearly located within the membrane pores at 55.7 °C and reasonably higher feed flow velocity of 0.84 m/s. Although high feed temperatures and low downstream pressures increase remarkably the total transmembrane flux and the ammonia removal rate, the corresponding ammonia separation factors were decreased. Ammonia removal efficiencies higher than 90% with separation factors of more than 8 were achieved in this study.

Ammonia recovery from anaerobically digested cattle manure by steam stripping

Ammonia recovery from anaerobically digested cattle manure effluents through steam stripping was studied at a stripping tower temperature of 98-99 degrees C and a steam-water ratio approximately 56-72 g/L. The digested manure effluents were first treated by microfiltration and then the permeate was used as feed in steam stripping. The stripping performance was evaluated under different feed pH values, ammonia concentrations and temperatures. The increase of the initial feed pH does not significantly improve ammonia stripping efficiency due to the fact that the stripped effluent pH is increased during steam stripping. This suggests that steam stripping of anaerobically digested manure effluents for ammonia recovery may not need pre-raised pH. In contrast, the pH value of the synthetic ammonia wastewater containing NH4Cl dramatically decreases after steam stripping. Increasing the feed temperature slightly improves ammonia stripping efficiency, but reduces the concentration of the recovered ammonia in the condensate due to an increased condensate volume at a higher feed temperature. Therefore, the feed temperature should be controlled at an optimum point that can compromise the condensate ammonia concentration and the ammonia stripping efficiency. Experimental results indicate that recovery of ammonia from anaerobically digested cattle manure effluents as NH4OH is technically feasible.

Removal of 1,1,1-trichloroethane from water using a polyvinylidene fluoride hollow fiber membrane module: Vacuum membrane distillation operation

Polyvinylidene fluoride (PVDF) hollow fiber membranes with an asymmetric structure have been prepared by the phase inversion method using dimethylacetamide (DMAc) as solvent and LiCl–H 2O as additives and applied for volatile organic compounds (VOCs) removal from water. The membrane was characterized using scanning electron microscopy (SEM) for observing its microstructures and a gas permeation method for measuring its porosity, pore size and pore size distribution. The results have provided the conditions for tailor-made hollow fiber membranes with the desired morphology suitable for vacuum membrane distillation (VMD). Moreover, the structural parameters such as mean pore size and effective porosity determined are being used as parameters in the mathematical modeling of the VMD process. A hollow fiber membrane module was then fabricated using a selected PVDF hollow fiber membranes and applied for removal of 1,1,1-trichloroethane (TCA) from water. The effects of various operating parameters including downstream vacuum level, feed temperature, feed flow rate and TCA feed concentration on the performance of the module were investigated both theoretically and experimentally. A high feed temperature and/or a low downstream pressure favored the enhancement of TCA removal, but also yielded dramatically a high water permeation flux, resulting in an excessive dilution of the permeated VOC. Under optimal operating conditions, particularly the downstream vacuum level, feed temperature and feed flow rate, TCA removal efficiency up to 97% was achievable. A mathematical model, which takes into consideration of mass transfer resistances in both liquid and membrane phases, predicts well the TCA removal data in comparison with the experimentally observed results under the favorable operation ranges employed in vacuum membrane distillation.

Improvement in steam stripping of sour water through an industrial-scale simulation

Sour water containing hydrogen sulfide, carbon dioxide, hydrogen cyanide, and ammonia is mainly purified by steam stripping. Increased concern in recent times over effluent water quality and energy saving has caused sour water stripping to attract considerable attention. However, the development of the operation methodology and structure of this process has not been systemic, leading to inefficiencies in the systems commonly used. In this paper, the characteristics of a sour water stripping column are investigated by using an industrial-scale simulation. From these results, we propose several guidelines (high feed temperature, low composition of CO2 in the feed, need of rectifying section, low mass flow rate of the second recycle stream, high reflux ratio, and modified structure using a pump-around side cooler) for improving stripper performance through changes in the operating condition and process structure. The proposed structure and guidelines can be applied not only to reduce steam consumption and lower the ammonia concentration in the effluent water but also to operate the system stably.

Development of a Novel Reactor Concept for the Partial Oxidation of Methane to Syngas

The gas-to-liquid process, consisting of the partial oxidation of methane (POM) followed by the Fischer–Tropsch reaction, is a promising alternative to conventional oil processing for the production of liquid fuels. The cost of a conventional POM process is mainly determined by cryogenic air separation and could be greatly reduced by using oxygen permselective perovskite membranes instead. These membranes operate at temperatures similar to POM processes and can thus be integrated into a catalytic membrane reactor. To investigate the implications of this integration an adiabatic thermodynamic analysis has been carried out for both oxygen- and air-based POM processes, focusing on the influence of the feed temperature and composition on the syngas yield and quality. This analysis revealed that much higher feed temperatures are required for air-based POM processes to reach similarly high syngas yields. Because the Fischer–Tropsch reaction is carried out at much lower temperatures, recuperative heat exchange becomes essential for air-based POM processes. This is preferably carried out inside the reactor using the reverse flow concept, since external heat transfer at elevated temperatures is expensive. To combine the POM reaction, air separation and recuperative heat exchange into a single apparatus a reverse-flow catalytic membrane reactor (RFCMR) is proposed.

Hybrid RO MSF: a practical option for nuclear desalination

Hybrid RO MSF desalination combines the advantages of the high desalting performance of distillation processes and lower energy requirement of membrane processes. It allows a better match between power and water requirements and enables better utilisation of the power generated from MSF into the RO. Hybrid RO MSF can lead to an optimised feedwater temperature of the RO plant since is possible to use cooling seawater from the reject stage of the MSF plant as feed to the RO plant. Higher feed temperature is advantageous for the RO plant since water flux of the membrane is about 2.5% higher per degree temperature rise at a fixed pressure. Various RO MSF combinations coupled with a nuclear power plant were studied. The optimum hybrid RO MSF scheme will be reviewed in order to illustrate the considerable gain of this option. The potential advantages of RO MSF hybrid desalination systems with nuclear plant will be discussed. The appropriate combinations depend on the local conditions and power/water requirements. The required power to water ratio and product water quality is among the important factors determining the particular RO MSF schemes to be used.

Competing reactions in three-way catalytic converters: modelling of the NO x conversion maximum in the light-off curves under net oxidising conditions

Automobile exhaust gas conversion was simulated with a reactor model based on first principles. The monolithic reactor was modelled as adiabatically operating with a uniform flow distribution over the channels and with constant heat and mass transfer coefficients. The kinetic rate equations in the model were constructed from elementary step kinetics of the individual global reactions. The model predictions for light-off curves compare quite well with experimental data from the literature. Light-off is in the sequence hydrogen, carbon monoxide, propene, but the light-off temperatures do not differ very much. The nitric oxide conversion as function of the reactor feed temperature passes through a maximum at high hydrocarbon conversion, but does not reach the level of 50%, defined as light-off. Hydrogen is the major oxygen consumer in the front part of the reactor, while a slow reaction proceeds between CO and NO. At sufficiently high feed temperatures thermal reactor ignition occurs when the degree of CO surface coverage drops considerably. The corresponding increase of the O adatoms coverage causes increased reaction rates and a sudden temperature rise. Beyond the ignition point oxygen is mainly consumed by propene and by unconverted carbon monoxide. NO is reduced by unconverted hydrogen, while NO reduction by the hydrocarbon is not significant. Catalysts capable of increasing the NO surface coverage or the NO dissociation, or leading to a lower oxygen sticking coefficient would show a higher NO conversion maximum. The results indicate that well-known reaction mechanisms are capable to describe the behaviour of automotive exhaust gas converters, if mutual interactions of gaseous components and surface species are taken into account via elementary step kinetics.

The Application of Scanning Electron Microscopy to the Study of Temperatures and Temperature Distributions in M2 High Speed Steel Twist Drills

Temperatures developed in M2 High Speed Steel twist drills have been measured by studying micro-structural chancres in sections normal to the surface using scanning electron microscopy. Only in situations where drill temperatures exceeded 650°C could temperatures be measured. When drilling stainless steels under conditions of very high speed and feed temperatures of 650 C were exceeded. The factors which influence the temperature distribution in drills will be discussed. The advantages and disadvantages of this technique together with future applications will be fully discussed.