Experimental validation of hybrid distillation‐vapor permeation process for energy efficient ethanol–water separation

BACKGROUND: The energy demand of distillation‐based systems for ethanol recovery and dehydration can be significant, particularly for dilute solutions. An alternative separation process integrating vapor stripping with a vapor compression step and a vapor permeation membrane separation step, termed membrane assisted vapor stripping (MAVS), has been proposed. The hydrophilic membrane separates the ethanol–water vapor into water‐rich permeate and ethanol‐enriched retentate vapor streams from which latent and sensible heat can be recovered. The objective of this work was to demonstrate experimentally the performance of a MAVS system and to compare the observed performance with chemical process simulation results using a 5 wt% ethanol aqueous feed stream as the benchmark. RESULTS: Performance of the steam stripping column alone was consistent with chemical process simulations of a stripping tower with six stages of vapor liquid equilibria (VLE). The overhead vapor from the stripper contained about 40 wt% ethanol and required 6.0 MJ of fuel‐equivalent energy per kg of ethanol recovered in the concentrate. Introduction of the vapor compressor and membrane separation unit and recovery of heat from both membrane permeate and retentate streams resulted in a retentate ethanol concentrate containing ca 80 wt% ethanol, but requiring only 2.2 MJ fuel kg^?1 ethanol, significantly less than steam stripping alone. CONCLUSION: Performance of the experimental unit with a 5 wt% ethanol feed liquid corroborated chemical process simulation predictions for the energy requirement of the MAVS system, demonstrating a 63% reduction in the fuel‐equivalent energy requirement for MAVS compared with conventional steam stripping or distillation. Published 2009 by John Wiley & Sons, Ltd.     

Separations of Hazardous Organics from Gas and Liquid Feedstreams Using Phosphazene Polymer Membranes

Abstract

In this paper the liquid-liquid and gas separation properties for the separation of hazardous organic feed streams using pervaporation and gas separation methods with poly[bis(phenoxy)phosphazene] based membranes are reported. Liquid transport behavior was determined using pervaporation techniques. The preliminary gas separations were studied using a mixed gas separation method which we have described previously. Using the membrane pervaporation technique, separation factors of 10,000 have been routinely achieved for the separation of methylene chloride from water. Other tests have shown similar results for the removal of hydrocarbon vapors from air. Membranes were prepared using solution casting techniques. Solvent evaporation rates during the casting and subsequent curing processes were controlled to provide a consistent membrane microstructure. These results suggest that polyphosphazene membrane technology could effectively be used in cleaning up air and ground water that has been contaminated with chlorinated hydrocarbons.     

Economics of Treating Waste Gases from an Air Stripping Tower Using Photochemically Generated Ozone

Air stripping towers have been recommended for the removal of volatile organic compounds (VOCs) in drinking water supply and industrial waste treatment systems. This technique removes VOCs economically in the liquid phase. It can, however, create adverse secondary environmental impacts by removing VOCs from the water and discharging them to the air.

A commonly proposed method for controlling .VOC emissions is filtration of the off-gas through adsorption of the stripped organics in the off-gas by granular activated carbon. The high incremental cost of this alternative has produced an interest in alternative control technologies.

One alternative currently available is based on short wavelength ultraviolet (UV) radiation. This technique combines the effects of ozone generation, free radical formation and photolysis of the contaminants to effectively control the VOC emissions. This technique is known as Advanced Photo Oxidation (APO)^R.

The cost for APO is $0.27/m³ for a 3.8 m³/hr contaminated water system. A system of this size is adequate for a groundwater decontamination project where a moderate length of time is available for restoration of the site. The cost of a conventional air stripping tower with Granular Activated Carbon (GAC) adsorption emissions control in this size range would be $0.40 to $0.45/m³ (J.M. Montgomery, 1986).

Additional testing will be required to fully develop design guidelines for different contaminants and larger systems. Another area for additional technical documentation is the application of this technique to the liquid phase oxidation of VOCs.     

Groundwater Cleanup by In-Situ Sparging. XIII. Random Air Channels for Sparging of Dissolved and Nonaqueous Phase Volatiles

Abstract

A mathematical model is developed to simulate the sparging of dissolved volatile organic compounds (VOCs) and nonaqueous phase liquid (NAPL) from contaminated aquifers. The sparging air moves through the aquifer in persistent, random channels, to which VOC must move by diffusion/dispersion to be removed. The dependence of the rate of remediation on the various model parameters is investigated and some practical conclusions are reached regarding the operation of air sparging wells for aquifer remediation. VOCs of low water solubility (such as alkanes) and present as NAPL are found to be removed by air sparging much more slowly than VOCs of higher water solubility (such as benzene, toluene, ethylbenzene and xylenes) and present as NAPL, due to the very small maximum concentration gradients which can be maintained around droplets of the former. These small concentration gradients result in very slow rates of NAPL solution.     

Groundwater Cleanup by In-Situ Sparging. VI. A Solution/Distributed Diffusion Model for Nonaqueous Phase Liquid Removal

Abstract

A microcomputer model for the sparging of aquifers contaminated with volatile organic compounds (VOCs) is presented which includes the kinetics of solution of nonaqueous phase liquid (NAPL) droplets and of diffusion from low-permeability porous layers. The well configuration modeled is a horizontal slotted pipe. Modeling results lead to the following conclusions. 1) The presence of low-permeability porous lenses of clay, till, silt, etc. results in marked increases in cleanup times. The extent of the increases depends strongly on the thickness of the structures. NAPL droplets of large size also result in marked increases in cleanup time. 2) Increases in air flow rate do not yield corresponding decreases in cleanup time if the system is limited by solution/diffusion kinetics. 3) The rate of induced water circulation plays a minor role in determining cleanup time. 4) Wells should be sufficiently deep and operated at an air flow rate such that air is delivered to the entire zone of contamination. 5) The spatial distribution of the VOC has little effect on the cleanup time as long as air is delivered to the entire contaminated zone. 6) Cleanup times increase roughly proportionally to increasing initial VOC concentration. 7) The terminal phase of cleanup typically shows substantial tailing as water containing VOC must circulate into the zone of aeration for the VOC to be stripped. 8) VOCs having Henry's constants of 0.05 or larger can readily be removed by sparging.     

Groundwater Cleanup by In-Situ Sparging. X. Air Channeling Model for Biosparging of Nonaqueous Phase Liquid

Abstract

A column model is developed to simulate the removal and biodegradation of dissolved and nonaqueous phase liquid (NAPL) volatile organic compounds (VOCs) from contaminated aquifers by biosparging. The model assumes that the injected air moves through the aquifer in persistent channels and that NAPL must dissolve and move to these channels by diffusion and dispersion. The dependence of model results on several of the parameters of the model is investigated, and suggestions for optimizing biosparging system operations are made. The removal of NAPL VOCs of quite low solubility (such as alkanes) from smear zones below the water is modeled, and is found to be an extremely slow process. Drawing down the water table to below the smear zone by pumping is suggested as a possible solution.     

Membrane‐assisted vapor stripping: energy efficient hybrid distillation–vapor permeation process for alcohol–water separation

BACKGROUND: Energy efficient alternatives to distillation for alcohol recovery from dilute solution are needed to improve biofuel sustainability. A process integrating steam stripping with a vapor compression step and a vapor permeation membrane separation step is proposed. The objective of this work is to estimate the energy and process costs required to make a fuel grade ethanol (0.5 wt% water) from 1 and 5 wt% ethanol aqueous streams using the proposed process. RESULTS: Using process simulation and spreadsheeting software, the proposed membrane‐assisted vapor stripping process was estimated to require as little as 8.9 MJ of fuel‐equivalent energy per kg of fuel grade ethanol recovered from a 1 wt% ethanol feed stream, 2.5 MJ kg^?1 for a 5 wt% ethanol solution. This represents an energy saving of at least 43% relative to standard distillation producing azeotropic ethanol (6 wt% water). Process costs were also found to be lower than for distillation at the 3.0 × 10⁶ kg‐ethanol year^?1 scale modeled. CONCLUSION: In this hybrid system, the stripping column provides high ethanol recoveries and low effluent concentrations while the vapor compression‐membrane component enables the efficient recovery of latent and sensible heat from both the retentate and permeate streams from the membrane system. Published in 2008 by John Wiley & Sons, Ltd.     

Separation of VOCs from N2 using poly(ether block amide) membranes

This work deals with the separation of volatile organic compounds (VOCs) from nitrogen streams for organic vapour emission control by poly(ether block amide) membranes. As representative air pollutant VOCs, n‐pentane, n‐hexane, cyclohexane, n‐heptane, methanol, ethanol, n‐propanol, n‐butanol, acetone, dimethyl carbonate, and methyl tert‐butyl ether were used in this study. The separation of both binary VOC/N₂ and multicomponent VOCs/N₂ gas mixtures was carried out, and the membranes exhibited good separation performance. A VOC concentration of more than 90 mol% was achieved at a feed VOC concentration of 5 mol%. It was found that the permeances of the VOCs were mainly dominated by their solubilities in the membrane, whereas the permeance of N₂ was affected by the presence of the VOCs. The permeance of N₂ in the VOC/N₂ mixtures was shown to be higher than pure N₂ permeance due to membrane swelling induced by the VOCs dissolved in the membrane. Nevertheless, theVOC/N₂ selectivity increased with an increase in the feed VOC concentration. Among the VOCs studied, the membrane showed a higher permeance to alcohol VOCs than paraffin VOCs. The effects of feed VOC concentration, temperature, stage cut, and permeate pressure on the separation performance were investigated.     

Gas Mixture Fractionation to Produce Two High Purity Products by Pressure Swing Adsorption

Abstract

Pressure swing adsorption processes have been traditionally used to produce one high purity gas stream from a gas mixture. One of the most common uses of this technology is in the production of ultrahigh purity hydrogen from various gas streams such as steam methane reformer (SMR) off-gas. However, many of these gas streams contain a second gas in sufficiently high concentrations, e.g., carbon dioxide in SMR off-gas, that the recovery of this secondary gas stream along with the primary product is extremely desirable. A new pressure swing adsorption (PSA) process, GEMINI-8, has been developed at Air Products and Chemicals, Inc., to achieve this goal. Process cycle steps for the GEMINI-8 PSA process are illustrated by SMR off-gas fractionation for the production of hydrogen and carbon dioxide. Capital and power savings of this process as well as other advantages compared with the previous technology are discussed.     

Microbial removal of alkanes from dilute gaseous waste streams: Mathematical modeling of advanced bioreactor systems

Many industries generate volatile organic compounds (VOCs) in dilute streams which must be removed before being released into the environment. Mathematical models for biological filters which can remediate waste streams are useful both as predictive tools and as a means to better understand the fundamental processes involved. Optimization of the system also necessitates a better understanding of the mechanisms by which biofilters work and can be approached through modeling and maximizing appropriate conditions for removal. In a trickle-bed bioreactor, VOCs (n-pentane and isobutane) were passed over a biofilm-coated packing which degraded the VOCs. Bacterial growth was controlled via liquid nutrient-limited media trickled through the reactor. Results from this trickle-bed system were analyzed by applying a simple mathematical model to accurately describe the processes which are believed to play important roles. The model was based on a two-step process: mass transfer in which the VOCs diffuse into the liquid biofilm, and kinetics by which VOCs are degraded by the biofilm. Modeling results revealed that both kinetic and mass transfer resistances were significant under typical operating conditions.     

硼同位素分离过程中络合剂的气提干燥模型

硼同位素分离富集的过程中,为了减少操作系统的副反应,需要原料液苯甲醚的含水质量分数在30×10-6以下.采用干燥剂氮气脱除苯甲醚的微量水分.应用解吸因子法设计了气提塔的理论模型,计算了最小和操作气液比、组分的气提率、塔釜液的组成及气提气用量,用Aspen Plus软件对气提塔进行了模拟,最后对整个过程进行了实验验证.实...     

Opportunities for Membrane Technologies in the Treatment of Mining and Mineral Process Streams and Effluents∗

Abstract

The membrane separation technologies of microfiltration, ultrafiltration, nanofiltration, and reverse osmosis are suitable for treating many dilute streams and effluents generated in mining and mineral processing. Membrane technologies are capable of treating these dilute streams in order to produce clean permeate water for recycle and a concentrate that can potentially be used for valuable metals recovery. Membrane technologies can be utilized alone, or in combination with other techniques as a polishing step, in these separation processes. A review of potential applications of membranes for the treatment of different process streams and effluents for water recycling and pollution control is given here. Although membranes may not be optimum in all applications, these technologies are recognized in the mining sector for the many potential advantages they can provide.

     

一氧化碳变换冷凝液汽提工艺技术改进探讨

介绍单塔汽提和双塔汽提两种冷凝液汽提工艺,建议采用单塔汽提侧线抽氨工艺处理含氨量较高的水煤浆变换冷凝液,利用一座汽提塔即可完成变换冷凝液的净化以及将酸性气体与氨分离的任务,侧线抽出的高浓度氨气可以制成氨水或液氨产品。     

Removal of trichloroethylene from air stripping off-gas by adsorption on activated carbon fibre

Recently commercialized activated carbon fibres (ACFs) have several advantages over conventional granular activated carbons (GACs) such as rapid adsorption kinetics. For removing trichloroethylene (TCE) in the air stripping off-gas from the treatment plant of contaminated ground water, the co-adsorption effect of water and TCE must be determined. Fundamental data of adsorption of TCE on ACF with respect to the coexistent humidity were obtained and the pilot-plant operation was investigated with and without temperature control of the gas influent to the adsorption unit. Heating the influent gas to 318 K results in lowering the relative humidity below 0.3, which is sufficient for utilizing the adsorption capacity of the ACF bed.     

Removal of Volatile Organic Compounds from Water by Pervaporation Using Polyetherimide-Polyethersulfone Blend Hollow Fiber Membranes

Abstract

Removal of volatile organic compounds (VOCs) such as 1,2-dichloroethane, trichloroethylene, chlorobenzene and toluene from water solutions through polyetherimide (PEI)-polyethersulfone (PES) blend hollow fiber membranes was investigated by pervaporation (PV) in this work. The separation performances of the membranes were researched by varying the spinning conditions (such as coagulation temperature and air gap distance) for the preparation of the hollow fibers and the operation conditions (such as velocity, concentration, and temperature of feed liquids). For the PEI-PES blend hollow fiber membrane prepared when the air gap was 7 cm and the temperature of coagulation bath was 45°C, it possessed high selectivity to the aqueous solutions containing 0.04 wt.% of VOCs at 20°C. The separation factors to 1,2-dichloroethane, trichloroethylene, chlorobenzene and toluene were 7069, 5759, 3952, and 3205, respectively. It was found that the pervaporation performance of the blend hollow fiber membrane was strongly related to the molecular size of the VOCs. The order of the selectivities was 1,2-dichloroethane > trichloroethylene > chlorobenzene > toluene.     

Simulated economics assessment of hollow fiber CO<Subscript>2</Subscript> separation modules

Various conditions under which the hollow fiber membrane separation system would be the optimal selection are investigated in terms of cost effectiveness. Numerical simulation is carried out to examine the effects of different configurations such as single-stage, two-stage and three-stage CO₂ separation processes. In particular, the hollow fiber membrane processes for CO₂ separation with vacuum pumps, heat exchangers, coolers and compressors to provide pressurized feed streams are analyzed. Operating costs are evaluated and compared numerically for the processes with and without recycle streams to compare feasibility for commercial implementation while maintaining the purity and recovery ratio as high as possible.     

Adsorbent Regeneration by Non‐thermal Plasma for Elimination of Odorous Compounds from Indoor Air

The non‐thermal plasma (NTP) technique was shown to be a method to improve indoor air quality. In particular in kitchens, odorous emissions can be removed by NTP. A combined concept of adsorption of volatile organic compounds (VOCs) and plasma regeneration of the adsorber was tested in adsorption‐regeneration‐adsorption cycles. As reference VOCs, 2‐methylthiophene, 2‐methylpyrazine, 2‐acetylthiazole, nonanal, and trans‐2‐nonenal were selected in humid air streams. These odorous compounds are emitted during cooking and frying processes. The adsorption‐regeneration concept was also tested during a simulated frying process with garlic in rape oil. A hydrophobic zeolite was chosen as adsorber material and placed directly into the discharge zone of a plasma reactor.     

Emission of Volatile Organic Compounds to the Atmosphere in the Solvent Sublation Process. II. Volatile Chlorinated Organic Compounds

Abstract

The mass of trichloroethylene, chlorobenzene, and 1,3-dichlorobenzene removed from an aqueous solution and emitted to the atmosphere during solvent sublation was determined experimentally. It was shown that the emission of these compounds in solvent sublation was reduced by 30 to 85% over air stripping under the same experimental conditions. The efficiency of removal of these compounds from water was also studied. The reduction of emissions over air stripping was more effective for the more hydrophobic and less volatile compounds. Emissions are reduced as the thickness of organic layer on the top of the column is increased. The use of decyl alcohol as the layer compound decreases emissions to a greater extent than does paraffin oil. Removal of these chlorinated volatile organic compounds from water by solvent sublation at an elevated temperature of 45°C is significantly faster than at room temperature. However, the emissions to the atmosphere are also increased.     

Soil Cleanup by In-Situ Aeration. XX. Mass Transport of Volatile Organics in Wet Activated Carbon

Abstract

A mathematical model for the adsorption of volatile organic compounds (VOCs) from wet gas streams by granular activated carbon columns is developed, and modeling results are discussed. Capillary condensation of water in the carbon pores is found to have a very damaging effect on the rate of VOC adsorption, since diffusion rates of VOCs into the pores are greatly reduced. Implications of the results for the operation of soil vapor extraction systems for the remediation of hazardous waste sites are discussed, and suggestions are made on how to deal with the problem.     

STRIPPING RATE OF PROPIONIC ACID FROM STYRENE-DIVINYLBENZENE COPOLYMERIC MICROCAPSULES WITH TRI-W-OCTYL AMINE AS CORE MATERIAL

ABSTRACT

The stripping rate of propionic acid from microcapsules containing tri-n-octyl amine was investigated using distilled water and aqueous NaOH as stripping solutions. The experiments were conducted at 303K. The stripping rate was found to be controlled by diffusion through porous microcapsule membrane and increased with an increase in the concentration of propionic acid in the microcapsules for each solution system. It was found that the stripping rates in aqueous NaOH solution system was higher than in distilled water at the same concentration of propionic acid in microcapsules. The experimental results in the NaOH solution system could be analyzed using a permeation model considering mass transfer accompanied by irreversible instantaneous neutralization reaction between NaOH and the propionic acid/tri-n-octyl amine complex in the microcapsule membrane.