Organic vapor mixtures separation

Organic vapor mixtures Separation of organic mixtures in refineries and petrochemical plants Requires better membranes and modules. Potential size of application is large... 

Separation of an organic vapor mixture with an immiscible sweep gas (e.g. steam) [20,21]... 

Pervaporation is a relatively new process with elements in common with reverse osmosis and gas separation. In pervaporation, a liquid mixture contacts one side of a membrane, and the permeate is removed as a vapor from the other. Currendy, the only industrial application of pervaporation is the dehydration of organic solvents, in particular, the dehydration of 90—95% ethanol solutions, a difficult separation problem because an ethanol—water azeotrope forms at 95% ethanol. However, pervaporation processes are also being developed for the removal of dissolved organics from water and the separation of organic solvent mixtures. These applications are likely to become commercial after the year 2000. 

After leaving the reactor the reaction mixture is passed to a settling tank where the denser HF is deposited in the lower phase. The organic phase is mixed gently with HF the HF phase contains tar components and traces of benzene. From the HF phase a side stream is refined. This side stream is heated in a preheater, partially vaporized, and separated into two components in a distillation column HF and benzene are distilled over the top while tar components are taken away at bottom. The top product is condensed, cooled, and collected in a settle tank. The bottom product is neutralized using potassium... 

Recycle designs of this type are limited to applications in which the components of the gas mixture, if sufficiently concentrated, can be separated from the gas by some other technique. With organic vapors, condensation is often possible adsorption, chemical scrubbing or absorption can also be used. The process shown in Figure 8.20 is used to separate VOCs from nitrogen and air or to separate propane, butane, pentane and higher hydrocarbons from natural gas (methane). 

An electrochemical flow detector, based on four working electrodes coated with different permselective film types, for use in flow-injection measurements of multiple nitroaromatic explosives was recently developed [18]. The resulting array response (Fig. 4) offers unique fingerprints of such explosive compounds. Electrochemical devices are extremely attractive for designing such e-nose-type multi-electrode arrays that combine several partially selective electrodes and lead to a distinct response pattern (signature) for mixtures of organic vapors without prior separation [2],... 

Contact Condensers Spray condensers, jet condensers, and barometric condensers all utilize water or some other liquid in direct contact with the vapor to be condensed. The temperature approach between the liquid and the vapor is very small, so tne efficiency of the condenser is high, but large volumes of the liquid are necessary. If the vapor is soluble in the liquid, the system is essentially an absorptive one. If the vapor is not soluble, the system is a true condenser, in which case the temperature of the vapor must be below the dew point. Direct-contact condensers are seldom used for the removal of organic solvent vapors because the condensate will contain an organic-water mixture that must be separated or treated before disposal. They are, however, the most effective method of removing heat from hot gas streams when the recovery of organics is not a consideration. 

The specific properties of zeolites, coupled with the separation properties of membranes, open the field to many areas of research for the future. This explains why the preparation and application of zeolite membranes is the subject of intensive research. By combining their adsorption and molecular sieving properties, zeolite membranes have been used for the separation of mixtures containing nonadsorbing molecules, different organic compounds, permanent gas-vapor mixtures, or water-organic mixtures. 

The extrapolation is to what is called pervaporation, where the feed mixture is a liquid, but the permeate vaporizes during permeation, induced by the relatively low pressure maintained on the permeate side of the membrane. Accordingly, the reject or retentate remains a liquid, but the permeate is a vapor. Thus, there are features of gas permeation as well as hquid permeation. The process is eminently apphcable to the separation of organics and to the separation of organics and water (e.g., ethanol and water). In the latter case, either water vapor may be the permeate, as in dehydration, or the organic vapor may be the permeate. The obvious, potential application is to the separation of azeotropic mixtures and close-boiling mixtures—as an alternative or adjunct to distillation or liquid-liquid extraction methods. 

Industrial separation of hydrocarbons from their mixtures with various gases, including air, is a specific case of a more general objective, separation of organic vapors from various gas/vapor mixtures. Early commercial vapor/gas membrane separation plants installed by Nitto Denko, MTR, GKSS were put into operation in 1988-1990. During the last 20 years, sales... 

This cost differential can be tolerated only in applications in which polymeric membranes completely fail in the separation [78]. Demanding separation applications, where zeolite membranes could be justified, due to the high temperatures involved or the added value of the components, and have been tested at laboratory scale, are the following separation of isomers (i.e., butane isomers, xylene isomers), organic vapor separations, carbon dioxide from methane, LNG (liquefied natural gas) removal, olefines/paraffins and H2 from mixtures. In most cases, the separation is based on selective diffusion, selective adsorption, pore-blocking effects, molecular sieving, or combinations thereof. The performance or efficiency of a membrane in a mixture is determined by two parameters the separation selectivity and the permeation flux through the membrane. 

Sorption and diffusion in polymers are of fundamental and practical concern. However, data acquisition by conventional methods is difficult and time consuming. Again, IGC represents an attractive alternative. Shiyao and co-workers, concerned with pervaporation processes, use IGC to study adsorption phenomena of single gases and binary mixtures of organic vapors on cellulosic and polyethersulfone membrane materials (13). Their work also notes certain limitations to IGC, which currently restrict its breadth of application. Notable is the upper limit to gas inlet pressure, currently in the vicinity of 100 kPa. Raising this limit would be beneficial to the pertinent use of IGC as an indicator of membrane-vapor interactions under conditions realistic for membrane separation processes. 

In gas absorption a soluble vapor is absorbed by means of a liquid in which the solute gas is more or less soluble, from its mixture with an inert gas. The washing of ammonia from a mixture of ammonia and air by means of liquid water is a typical example. The solute is subsequently recovered from the liquid by distillation, and the absorbing liquid can be either discarded or reused. When a solute is transferred from the solvent liquid to the gas phase, the operation is known as desorption or stripping. In dehumidification a pure liquid is partially removed from an inert or carrier gas by condensation. Usually the carrier gas is virtually insoluble in the liquid. Removal of water vapor from air by condensation on a cold surface and the condensation of an organic vapor such as carbon tetrachloride out of a stream of nitrogen are examples of dehumidification. In humidification operations the direction of transfer is from the liquid to the gas phase. In the drying of solids, a liquid, usually water, is separated by the use of hot, dry gas (usually air) and so is coupled with the humidification of the gas phase. 

For other systems separate lines must be used for psychrometric lines. With nearly all mixtures of air and organic vapors the psychrometric lines are steeper than the adiabatic-saturation lines, and the wet-bulb temperature of any mixture other than a saturated one is higher than the adiabatic-saturation temperature. 

Dibromothiophene 723 Bromine (950 g) is added to a mixture of thiophene (297 g) with an equal volume of benzene as quickly as is possible without escape of bromine vapor. When evolution of HBr has become slow, ethanol (700 ml) and NaOH (250 g) are added and the mixture is boiled under reflux for 16 h. Then, after dilution with water, the organic layer is separated and distilled through a column it gives 2-bromothiophene (152 g), b.p. 158-162°, and 2,5-dibromothiophene (425 g), b.p. 200-210°. 

Porous membranes can be made of polymers (polysulfones, polyacrylonitrile, polypropylene, silicones, perfluoropolymers, polyimides, polyamides, etc.), ceramics (alumina, silica, titania, zirconia, zeolites, etc.) or microporous carbons. Dense organic membranes are commonly used for molecular-scale separations involving gas and vapor mixtures, whereas the mean pore sizes of porous membranes is chosen considering the size of the species to be separated. Current membrane processes include microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), gas and vapor separation (GS), and pervaporation (PV). Figure 1 indicates the types and sizes of species typically separated by these different separation processes. 

Pervaporation is a contraction of the terms permeation and evaporation because the feed is a liquid, and vapor exits the membrane on the permeate side. Pervaporation is a membrane process for liquid separation, and today, it is considered as a basic unit operation for the separation of organic-organic liquid mixtures because of its efficiency in separating azeotropic and close-boiling mixtures, isomers, and heat-sensitive compounds. It allows separations of some mixtures that are difficult to separate by distillation, extraction, and sorption. Pervaporation is one such type of membrane separation process with a wide range of uses such as solvent dehydration and separation of organic mixtures. When a membrane is in contact with a liquid mixture, one of the components can be preferentially removed from the mixture due to its higher affinity and quicker diffusivity in the membrane. 

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