High Feed Temperature Research Articles

Bifurcation analysis of methane oxidative coupling without catalyst

We present a detailed bifurcation analysis of methane oxidative coupling in the gas phase using a global kinetic model for the various oxidation, reforming and dehydrogenation reactions. The kinetic model satisfies the thermodynamic constraints and is validated with literature data as well as new data obtained under near isothermal conditions. It is used to determine the methane conversion and C2-products selectivity under various feed and operating conditions in large scale ideal adiabatic reactors. It is found that at higher CH4/O2 ratios (e.g. >4), ignition and extinction points exist only at either high feed temperatures and/or space times, which may not be of practical interest. Autothermal operation on the ignited branch with feed at near ambient conditions (∼300K and 1 bar) is feasible for practical range of space times (1 ms–1 s) only for low CH4/O2 ratios (e.g. 1.7–2.5), which includes the flammability range. Further, the best ethylene yields are obtained on the ignited branch close to the extinction point while best C2 yields may be obtained at higher space times or feed temperatures. Feeds with a high CH4/O2 ratio lead to higher selectivity of C2H4 but lower methane conversion and require higher inlet temperatures. The ratio C2H4/C2H2 decreases as the methane conversion increases or CH4/O2 ratio decreases. While oxidation (exothermic) chemistry dominates on the ignited branch near the extinction point, the dehydrogenation and reforming (endothermic) chemistries dominate as the space time or feed temperature is increased. It is shown that the highest yield of intermediate products and largest region of autothermal operation is obtained for an ideal reactor with perfect thermal back-mixing and zero species back-mixing (lumped thermal reactor model).

Detailed modeling and simulation of an out-in configuration vacuum membrane distillation process.

In this study, a detailed rigorous theoretical model was developed to predict the transmembrane flux of a shell-and-tube type vacuum membrane distillation (VMD) module for seawater desalination. Two modes of operation are used for performing the VMD, namely lumen-side feed (in-out) configuration and shell-side feed (out-in) configuration. In this study, detailed mathematical formulations are derived for an out-in configuration that is commonly used in seawater desalination applications. Experimental results and model predictions for mean permeate flux are compared and shown to be in good agreement. The results indicate that although the simple VMD model that maintains a constant permeate pressure is easy to use, it is likely to significantly overestimate the mean permeate flux when compared to the detailed model that considers the pressure build-up in the fiber lumen. The pressure build-up of water vapor in the fiber lumen is identified as the crucial factor that significantly affects the VMD performance because it directly reduces the driving force for vapor permeation through the membrane pores. Additionally, its effect is more pronounced at longer fiber lengths and higher permeate fluxes, and this is achieved at higher feed temperatures and velocities and at lower feed salinities. In conclusion, the results of the study are extremely important in module design for the practical applications of VMD processes.

Optimizing ethanol recovery in a spinning cone column

Abstract Spinning cone column (SCC) is a separation technology used in the food industry to recover natural aromas. It outperforms plate and packed columns given its enhanced mass transfer and capability to process slurries. Limited SCC design and operating guidelines are available, since theoretical studies characterized the SCC hydrodynamics with single components, while experimental studies with complex mixtures do not consider process modeling and optimization. Distillation of a water–ethanol mixture of 14.8% (v/v) in a lab scale SCC was optimized, varying the stripping rate, feed rate and temperature. A first principles model was developed to explain the results in terms of internal variables. Three operating regions were identified. At low stripping rates, the operation was unstable and low performing. At high stripping rates, ethanol recovery and concentration decreased due to a sharp increase in the heat loss that overweighed the mass transfer enhancement. To maximize recoveries, SCC should operate at intermediate-high stripping rates, high feed and intermediate temperatures. A high-performance region was found with ethanol recoveries higher than 94% and ethanol concentrations in the distillate over 70%. Our proposed guidelines improve the competitiveness of wine and beer dealcoholization using SCC technology, increasing the yield and marketability of both dealcoholized product and surplus distillate.

Hemicelluloses separation from caustic-containing process stream by ultrafiltration

ABSTRACTThe present work aimed to study the performance of ultrafiltration (UF) membranes for hemicelluloses separation, from a caustic-containing highly alkaline, cellulose purification solvent stream of viscose rayon production process, and to understand the membrane behaviour, effect of multiple use cycle, identify potential performance drawbacks and propose possible solutions. Screening experiments showed that an UF membrane with a nominal molecular weight cut-off value of 3 kDa can be used to separate hemicelluloses from alkaline process stream. UF membrane was found to be prone to performance deterioration with time due to fouling. Higher feed temperature helped in improving the membrane flux values while maintaining hemicelluloses retention.

Enhanced Freshwater Production Using Finned-Plate Air Gap Membrane Distillation (AGMD)

Air Gap membrane distillation (AGMD), a special type of energy efficient membrane distillation process, is a technology for producing freshwater from waste water. Having some benefits over other traditional processes, this method has been able to draw attention of researchers working in the field of freshwater production technologies. In this study, a basic AGMD system with flat coolant plate has been modified using a specially designed channelled coolant plate of portable size to observe its effect over the production rate and performance of the system. Attempt has been made to increase the amount of distillate flux by using the “fin effect” of the channelled coolant plate. A finned plate have been used instead of a flat coolant plate and experiments were conducted to compare the effect. Coolant temperature and feed temperature of the system have been varied from 10°C to 25°C and 40°C to 70°C respectively. Comparing the data, around 50% to 58% distillate enhancement has been observed for channelled coolant plate. Also, it was seen that the enhancement was higher for higher feed temperatures and coolant temperatures. With these findings, a better performing AGMD module has been introduced to mitigate the scarcity of freshwater.

Monolithic reactor for VOCs abatement: Influence of non-uniformity in the coating

This article presents a theoretical study of the catalytic mineralisation of ethanol via acetaldehyde over manganese – copper mixed oxide catalyst deposited on a monolith reactor. The steady-state operation of the monolith reactor is represented by means a heterogeneous, one-dimensional, adiabatic model, which accounts for external (gas-solid) resistances to mass and heat transfer and internal resistances to mass transfer inside the catalytic coating (washcoat).The effect of catalyst accumulation at the corners of a square section channel is analysed. Under the operating conditions considered, the diffusional resistances have a significant effect on the effective rate of removal of Volatile Organic Compounds and cannot be neglected. Non-uniform coating causes a considerable deterioration of the averaged effectiveness factors of the two reactions under consideration and lower temperatures along the reactor are found. These lower temperatures, together with lower temperature drops over the external film cause additional negative impacts on the Volatile Organic Compounds conversion. The decrease in the reactor performance, caused by the catalyst accumulation, is magnified as the feed concentration rises. Consequently, higher feed temperatures and/or lower space velocities are necessary to ensure the complete destruction of the volatile organic compounds involved (ethanol and acetaldehyde).

Occurrence of salt breakthrough and air-vapor pocket in a direct-contact membrane distillation

A polyvinylidene fluoride (PVDF) hollow fiber module with a low packing density and the permeate orifices on one side was used in a direct-contact membrane distillation process (DCMD). Comparative tests were also performed using a highly hydrophobic polytetrafluoroethylene (PTFE) membrane and a low hydrophobic PVDF membrane with different packing densities. Under varying operating conditions such as the feed and permeate flow rates as well as temperature, the performances of the DCMD were comprehensively investigated. It was rarely found that salt breakthrough in an idle mode and air-vapor pocket in an operative mode could occur. Partial wetting was for the first time found tending to happen in the PVDF membrane in idle mode, which led to salt breakthrough. High feed temperature and flow rate accelerated partial wetting. On the contrary, the PTFE membrane prevented partial wetting and blocked salt permeation. The air-vapor pocket formation at the shell side was attributed to a high vapor flux and incomplete condensation. An increase in packing density and the inversion of permeate orifices could help the mixture of hot vapor and cold permeate eliminate the air-vapor pocket, which was for the first time observed and illustrated.

Separation and concentration of ionic liquid aqueous solution by vacuum membrane distillation

Due to the high cost of ionic liquids (ILs), how to economically recycle them from aqueous solutions is one of the critical issues for their large-scale applications. In this work, vacuum membrane distillation (VMD) technique was proposed to separate and concentrate the 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) aqueous solutions at high concentration (e.g. ≥20wt%), where a polyacrylonitrile-based (PAN-based) hydrophobic membrane was prepared and used. The chemical resistance and VMD separation performance of the PAN-based membrane were estimated. The results showed that the homemade membrane possessed good chemical resistance to several kinds of media and favorable VMD flux and rejection. Four different groups of VMD concentration processes with intermittent operation mode were then carried out to separately concentrate a 20wt% [Bmim]Cl solution. The concentration process, membrane fouling and wetting, as well as cleaning strategy were compared and discussed. It was found that for the hydrophobic PAN-based membrane, surface hydrophilization resulted from surface chemistry rearrangement happened especially at high feed temperature, which might aggravate the adsorption of [Bmim]Cl on membrane surface. Membrane top layer wetting would occur due to the synergistic effect of surface hydrophilization and deposition of [Bmim]Cl, which not only induced the reductions of the membrane flux and rejection, but also further caused internal pore wetting. Therefore, the fouling mechanism during the VMD concentration might be divided into three stages, surface deposition, top layer wetting and internal pore wetting. Timely cleaning (i.e. before surface wetting) could effectively prevent the fouling deterioration (i.e. from surface deposition to surface wetting) and improve the VMD concentration process, resulting in a high flux recovery over 95%. Under the optimal conditions, a final [Bmim]Cl concentration as high as 65.5wt% with a total [Bmim]Cl recovery over 99.5% was achieved, indicating that VMD was a promising candidate for concentrating ILs aqueous solutions.

Numerical simulation of 3D hollow-fiber vacuum membrane distillation by computational fluid dynamics

Computational fluid dynamics (CFD) was employed for modeling and simulation of the heat and mass transfer processes in hollow-fiber vacuum membrane distillation (VMD) under laminar flow conditions. A three-dimensional VMD model was first developed and validated by coupling the latent heat with the energy conservation equations and experimental data. Then it was used to analyze the effects of operating conditions and module dimensions on local temperatures, heat transfer coefficients, temperature polarization coefficients, heat and mass fluxes and total thermal efficiency in an operation of the feed flowing in the lumen of the fibers and the shell in vacuum. Thermal efficiency varied with feed temperature and feed velocity. Temperature polarization became more significant at high feed temperature and low feed velocity. Mass transfer was controlled by the heat transfer in the feed boundary. Local heat and mass fluxes decreased along the fiber length because of high local heat transfer coefficient in the inlet region and thin thermal boundary. More importantly, total thermal efficiency decreased with increasing module length, thus a short module was better used for high efficiency of VMD.

Degradation of Polypropylene Membranes Applied in Membrane Distillation Crystallizer

The studies on the resistance to degradation of capillary polypropylene membranes assembled in a membrane crystallizer were performed. The supersaturation state of salt was achieved by evaporation of water from the NaCl saturated solutions using membrane distillation process. A high feed temperature (363 K) was used in order to enhance the degradation effects and to shorten the test times. Salt crystallization was carried out by the application of batch or fluidized bed crystallizer. A significant membrane scaling was observed regardless of the method of realized crystallization. The SEM-EDS, DSC, and FTIR methods were used for investigations of polypropylene degradation. The salt crystallization onto the membrane surface accelerated polypropylene degradation. Due to a polymer degradation, the presence of carbonyl groups on the membranes’ surface was identified. Besides the changes in the chemical structure a significant mechanical damage of the membranes, mainly caused by the internal scaling, was also found. As a result, the membranes were severely damaged after 150 h of process operation. A high level of salt rejection was maintained despite damage to the external membrane surface.

Removal of volatile organic compounds from aqueous solutions applying thermally driven membrane processes. 2. Air gap membrane distillation

Air gap membrane distillation technique (AGMD) was applied for removal of ethanol, butanol and acetone–butanol–ethanol mixture from water. The influence of various parameters (feed temperature, cooling wall temperature) on pure water transport in AGMD was determined and the efficiency of organic solvents removal from water through two porous membranes (PTFE and PP) was investigated in the detail.Selectivity of organic components recovery did not change significantly with an increase of feed temperature from 41°C to 63°C in case of AGMD applied for ethanol and butanol recovery, however significantly higher fluxes were obtained at higher feed temperature. Slightly higher fluxes were obtained during experiments performed with PTFE membrane comparing with PP one.It was found that both PTFE and PP membranes are wetted during AGMD process of water–butanol feed mixture if organic concentration exceeds 2.5wt% (PTFE) and 1wt% (PP) at 63°C feed temperature. This fact limits the possibility of wider AGMD application in organic solvent recovery.The comparison of the efficiency of AGMD with the efficiency of thermopervaporation (TPV) was also performed. In case of 1wt% ethanol feed mixture it was found that AGMD seems to be more suitable due to higher process separation index value (PSI in the range of 6kgm−2h−1) than in the case of TPV (PSI in the range of 3kgm−2h−1). On the contrary, in the case of water–butanol or water–ABE systems TPV is much more efficient method for organics recovery from aqueous mixtures than AGMD. In contact with 3wt% ABE mixture PSI is equal to 28 and 9kgm−2h−1 for TPV and AGMD processes, respectively.

Very high flux MFI membranes for alcohol recovery via pervaporation at high temperature and pressure

Abstract Ultra-thin MFI membranes (0.5 μm) were evaluated for recovery of alcohols from dilute aqueous mixtures by pervaporation at high temperature and pressure for the first time. The feed pressure was sufficiently high to keep the feed in liquid state at high temperature, while the permeate pressure was kept at atmospheric and a low flow of sweep gas was used to reduce the partial pressure on the permeate side. Atmospheric pressure on the permeate side is more practical than vacuum. High feed temperature and pressure result in high fugacity in the liquid feed, which, in combination with lower permeate pressure, results in a large driving force. Consequently, the membrane exhibited very high fluxes for feeds comprised of 10 wt% ethanol/water and 5 wt% n-butanol/water mixtures at 110 °C and 140 °C, respectively. The flux observed for 10 wt% ethanol/water mixtures was as high as about 52 kg m−2 h−1, i.e., 6 times higher than the highest previously reported flux for this separation using zeolite membranes in pervaporation. For 5 wt% n-butanol/water mixtures, the flux was 40 kg m−2 h−1, which is 11 times higher than the highest previously reported flux for this separation by zeolite membranes. At these conditions, the membrane displayed separation factors for ethanol/water and n-butanol/water mixtures of 5 and 16, respectively. However, after about 6 h of operation, the separation factor decreased significantly and the flux increased due to formation of defects in the membrane when the feed was saturated with silica. Lower membrane stability was observed for silica free feeds. The work has shown that it is possible to obtain high flux in pervaporation by using ultra-thin membranes in combination with high feed temperature. The observed membrane selectivity was not excellent, due to pressure drop over the support as well as likely concentration polarisation on the feed side resulting from the high flux. Membrane stability was also an issue at these conditions, however it was shown that stability could be improved by saturating the feed with silica.

Economic and thermal feasibility of multi stage flash desalination plant with brine–feed mixing and cooling

Improving the performance of MSF (multi stage flash) desalination plants is a major challenge for desalination industry. High feed temperature in summer shortens the evaporation range of MSF plants and limits their yield. Installing a cooler at the feed intake expands the evaporation range of MSF plants and increases their yield. Adding a cooler and a mixing chamber increases the capital and operational costs of MSF plants. This paper presents thermal and economic analysis of installing a feed cooler at the plant intake. The profit of selling the additionally produced water must cover the cost of the cooling system. The selling prices for a reasonable breakeven depend on the selected cooling temperature. The cost of installing coolers capable of maintaining feed–brine mixture temperatures of 18–20 °C shows breakeven selling prices of 0.5–0.9 $/m3. These prices fall within the current range of potable water selling prices.

Membrane fouling and wetting in a DCMD process for RO brine concentration

Membrane wetting and fouling were studied using hollow-fiber polyvinylidene fluoride (PVDF) membranes to concentrate reverse osmosis (RO) brine in a direct contact membrane distillation (DCMD) process. The effect of the operating conditions, such as the feed temperature, the flow velocity and the feed solution concentration, was investigated. Membrane wetting was more significant at high feed temperatures, and the salts in the feed promoted membrane wetting; membrane wetting resulted in a decrease in the flux and salt rejection. A theoretical model was developed to simulate the DCMD process, and the results of the model were analyzed. The concentration factor significantly affected the flux and the electrical conductivity of the distillate. When the concentration factor (CF) was less than 3.5, CaSO4 did not crystallize because of high ionic strength and higher solubility of CaSO4 at the membrane surface than in the bulk solution; beyond CF of 3.5, membrane fouling was exacerbated because of CaSO4 crystallization. Membrane fouling was more significant at the higher of the two temperatures investigated for long-time DCMD operation: square CaSO4 crystals formed at 77°C, whereas snowflakes formed at 55°C. The permeate was lower in the divalent cations, Mg2+ and Ca2+, than in the monovalent cations, Na+ and K+.

Fast lean-rich cycling for enhanced NOx conversion on storage and reduction catalysts

Lean NOx (NO+NO2) reduction was carried out using rapid periodic injection of C3H6 over a NOx storage and reduction (NSR) monolith catalyst containing Pt/Rh/BaO/CeO2/Al2O3. The effects of injection rate, feed temperature, rich phase composition, lean phase duration, and feed concentration of CO2 were systematically varied to quantify their effects on the cycle-averaged NOx and propylene conversions, and product selectivities. A factor of 10 increase in the injection frequency from conventional NSR cycling frequency of 0.014 to 0.14Hz resulted in much higher NOx conversion at high feed temperatures (above 300°C). Both NOx and propylene conversions were higher over the entire range of feed temperatures (150–400°C). High frequency injection with propylene resulted in a nearly constant catalyst temperature, in contrast to large swings in the catalyst temperature during lower frequency injection. NOx conversion exceeding 90% was achieved for a feed having a cycle-averaged stoichiometric number (lean to rich ratio) of 6; slower injection required a stoichiometric ratio of 4 to achieve the same NOx conversion. The same high frequency operation using H2 as the reductant not only did not show any enhancement, but resulted in decreased NOx conversion. A prolonged approach to the cyclic steady state was observed during high frequency operation. Moreover, the detrimental effect of CO2 on NOx conversion was observed to decrease at higher frequencies. These observations collectively suggest that the generation of reactive intermediate species “HCxNyOz”, whose lifetime on the catalyst surface exceeds the cycle duration and which reacts with NOx to produce N2, are required to achieve conversion enhancement during high frequency operation. The degree of mixing of the rich and lean feeds upstream of the catalyst was found to be an important reactor design parameter that invites further study.

Separated performances of ammonium sulphate and ammonium chloride solutions treated by vacuum membrane distillation

Polytetrafluoroethylene (PTFE) flat‐sheet membrane with the mean pore size of 0.1 μm was used to treat the high concentration (NH4)2SO4 and NH4Cl solutions by vacuum membrane distillation (VMD). The permeate flux was varied in the range 8–48 L h−1 m−2 and was dependent on the operating conditions. The effects of permeate pressures and feed parameters in terms of pH values, feed temperature, flow rate and feed concentration on both the permeate fluxes and the permeate concentrations of total nitrogen (TN) were investigated. Experiment results showed the feed temperature has the most significant effect on the permeate flux. The feed pH value was observed to be the most dominant factor on permeate concentration of TN. Ammonium was rejected with low feed pH value, high feed temperature and low permeate pressure. When the feed pH value was 3, the permeate concentration of TN was kept in the range of 1–3 mg/L and the ammonia nitrogen rejection rate is 99.95%, despite that the feed concentration was increased.

Experimental and theoretical research on N-methyl-2-pyrrolidone concentration by vacuum membrane distillation using polypropylene hollow fiber membrane

In this work, N-methyl-2-pyrrolidone (NMP) aqueous solution was concentrated by means of vacuum membrane distillation (VMD) using polypropylene (PP) hollow fiber hydrophobic membrane. The effects of operating variables including feed concentration, feed temperature and vacuum degree on VMD performance were studied experimentally and theoretically. The results indicated that higher feed temperature and/or higher vacuum pressure could lead to an enhancement in H2O removal and no change in NMP rejection. The permeation flux of 9.5L/m2h and the rejection 98% can be achieved at the feed temperature of 80°C and vacuum degree of 0.09MPa. Higher feed concentration leads to a decrease in H2O flux and an increase in NMP flux and NMP molar fraction in permeate side. It proved that the Knudsen-viscous flow diffusion dominated the mass transform of H2O/NMP vapor in our experiment condition. Furthermore, “Dusty-Gas” model was used to predict membrane properties and Wilson equation method was used for predicting γ of the NMP aqueous solution. The theoretical data fitted well with the experimental data in our experiment conditions.

Development of simultaneous membrane distillation–crystallization (SMDC) technology for treatment of saturated brine

We have developed the simultaneous membrane distillation–crystallization (SMDC) hybrid desalination technology for the concurrent productions of pure water and salt crystal from the saturated brine solutions. The effects of feed temperature variation from 40°C to 70°C on the SMDC performance in terms of membrane flux and kinetics of NaCl crystallization have been investigated. Increasing feed temperature increases membrane flux but the flux declines rapidly with time at higher feed temperatures (60°C and 70°C) due to the occurrences of membrane scaling and wetting facilitated by salt oversaturation at the boundary layer. In order to prevent salt oversaturation, we have calculated the critical fluxes at different Reynolds numbers and crystallizer temperatures. For instance, the critical fluxes when the feed temperature is 70°C increase from 5kgm−2h−1 to 20kgm−2h−1 for the laminar and turbulent flows, respectively. By keeping the membrane flux lower than the critical flux, a stable membrane performance during a continuous SMDC operation over the period of 5000min has been achieved. Increasing feed temperature also increases the yield of NaCl crystals from 7.5kgperm3 solution to 34kgperm3 for feed temperatures of 40°C and 70°C after 200min operation, respectively. However, the average crystal sizes decrease from 87.40µm to 48.82µm with increasing feed temperatures from 40°C to 70°C due to a higher nucleation rate at a higher degree of supersaturation. Regardless of the feed temperature, the NaCl crystals are in a uniform cubical shape with the coefficient of variations which are in the range of 30–38% that implies a narrow dispersion.

Steam reforming of n-heptane for production of hydrogen and syngas

Simultaneous production of hydrogen and syngas from the catalytic reforming of n-heptane in circulating fast fluidized bed reactors (CFFBR) and circulating fast fluidized bed membrane reactors (CFFBMR) is investigated. This paper presents modeling and simulation approach for the analysis of these reformers. Complete conversion of heptane (100%) is attained at high steam to carbon feed ratios and shorter reactor lengths by both configurations. However, the CFFBMR is very efficient in hydrogen production and can produce exit hydrogen yield up to 473.14% higher than the CFFBR. It was found that operating the CFFBMR at the optimal conditions results in a minimum value of hydrogen to carbon monoxide ratio (H2/CO) within the recommended practical range for the syngas used as a feedstock for the gas to liquid processes (GTL). The results of the sensitivity analysis conducted for the CFFBMR has shown that the reaction side pressure and the feed temperature have significant effects on increasing the heptane conversion (up to 100%) and the temperature effect is stronger than the reaction side pressure effect. Considerable improvement in the hydrogen to carbon monoxide ratio (H2/CO) has been achieved by increasing the reaction side pressure, while the high feed temperature has negative effect on this ratio. It seems that the practical range of H2/CO ratio can be achieved by controlling the reformer length and the right combinations of the operating conditions.

Parametric sensitivity analysis to investigate heptane reforming in circulating fast fluidized bed membrane reactors

In this paper, we present mathematical modeling and numerical simulation tools in searching the high parameter space of steam reforming of heptane for the key design parameters, which have the potential to give high heptane conversion, high hydrogen yield and hydrogen to carbon monoxide ratio within the industrial limits for the syngas used as a feedstock for the gas to liquid processes (GTL). The system under consideration is the novel circulating fast fluidized bed membrane reactor (CFFBMR). The simulation results show that the hydrogen membrane has a significant role in the displacement of the thermodynamic equilibriums of the reversible reactions and production of ultraclean hydrogen, which can be used as a fuel for the fuel cells. Also the results of the sensitivity analysis show that the best performance of the CFFBMR can be obtained by a proper selection of combination of several parameters of high feed temperatures, high steam to carbon feed ratios, high reaction side pressures coupled with a large permeation area of a composite thin film membrane. These parameters are interacting in a very complex manner to give 100% conversion of heptane and 496.94% increase in hydrogen yield compared to the reformer without hydrogen membrane. It was found that under these selected operating conditions a low H2/CO ratio of 1.15 is achieved satisfying the practical recommended industrial range.