Concentration processes distillation

BOSAC [Bofors Sulfuric Acid Concentrator] A process for recovering sulfuric acid from the production of nitro-compounds. Spent acid is concentrated by distillation, using a heat exchanger with externally heated silica tubes. Developed by Bofors Nobel Chemikur, Sweden. Douren, L., Making the Most of Sulfuric Acid, More, A. I., Ed., British Sulphur, London, 1982, 317. 

The process is as described in Section 3.3.3.2 and consists of a distillation column containing seven theoretical plates, reboiler and reflux drum. Distillation is carried out initially at total reflux in order to first establish the column concentration profile. Distillate removal then commences at the required distillate composition under proportional control of reflux ratio. This model is based on that of Luyben (1973, 1990). 

Residua produced by distillation, which is a concentration process, contains significantly fewer hydrocarbon constituents than those in original crude oil. The constituents of residua may, depending on the crude oil, be molecular entities of which the majority contain at least one heteroatom. 

Freeze concentration involves the concentration of an aqueous solution by partial freezing and subsequent separation of the resulting ice crystals. It is considered to be one of the most advantageous concentration processes because of the many positive characteristics related with its application. Concentration processes such as evaporation or distillation usually result in removal of volatiles responsible for arom in addition the heat addition in these processes causes a breakdown in the chemical structure that affects flavor characteristics and nutritive properties. In contrast freeze concentration is capable of concentrating various comestible liquids without appreciable change in flavor, aroma, color or nutritive value (1.2.3) The concentrate contains almost all the original amounts of solutes present in the liquid food. 

Extractive distillation was the basis of a process introduced commercially by the Shell Development Co. and put into operation in 1940 at the Houston refinery of the Shell Oil Co., Inc., for separating toluene from virgin stocks (6) subsequently it was used also on hydroformates and cracked naphthas. This process, shown diagrammatically in Figure 3, involves the production of a toluene concentrate by distillation to remove low and high boiling contaminants, which then is extractively distilled with phenol to separate the aromatics from the paraffin (5). The extract is obtained as a bottoms stream from the extractive distillation tower, and is further fractionated in a distillation tower to separate raw toluene from the phenol, after which the toluene is acid treated and redis-... 

Extractive distillation processes are still widely used for nitric acid concentration. Because the operational and maintenance problems associated with sulfuric acid concentration plants are considerable, and their capital cost substantial, attention has been directed periodically to the use of extractive agents other than sulfuric acid. Phosphoric acid (I) acts like sulfuric acid but poses similar problems of reconcentration. Solutions of certain metallic salts, in particular metallic nitrates, permit similar enhancement of relative volatility and are readily reconcentrated in straightforward evaporation equipment, offering the possibility of a compact integrated concentration process. 

Solutions must be concentrated or the constituents must be isolated before trace amounts of the various organics present as complex mixtures in environmental water samples can be chemically analyzed or tested for toxicity. A major objective is to concentrate or isolate the constituents with minimum chemical alteration to optimize the generation of useful information. Factors to be considered in selecting a concentration technique include the nature of the constituents (e.g., volatile, nonvolatile), volume of the sample, and analytical or test system to be used. The principal methods currently in use involve (1) concentration processes to remove water from the samples (e.g., lyophilization, vacuum distillation, and passage through a membrane) and (2) isolation processes to separate the chemicals from the water (e.g., solvent extraction and resin adsorption). Selected methods are reviewed and evaluated. 

This compound finds a ready market principally for the production of phthalic anhydride. There is a great variety of processes to isolate napthalene from the acid-free or neutral oils. Frequently, the naphthalene is first concentrated by distillation, and the enriched oil then is worked up by crystallization. This process is prevalent in Europe. It is also possible to isolate the naphthalene by careful fractionation. Depending on the purity desired, additional chemical treatments may be required. Naphthalene usually is traded with freezing point as a measure of purity (80.3°C) for pure naphthalene. A good quality commonly used is 78°C naphthalene, which is about 96% pure. 

Hence, distillation, insofar as the sulfur-containing constituents of crude oils are concerned, is a concentration process by which the majority of the sulfur-containing compounds (which are usually of high molecular weight) are concentrated into the higher-boiling fractions (Table 7-3 Figures 7-3,7-4,7-5, and 7-6). 

In short, distillation is, at best, looked upon as a means by which the lower-boiling fractions can be separated from a feedstock prior to being subjected to a suitable conversion (or refining) method. It is, in fact, the means by which the undesirable higher molecular weight materials are removed from the feedstock as atmospheric or vacuum residua. It would, indeed, be a very rare occasion if the distillation process actually served as an efficient means of desulfurization rather than a concentration process. 

The driving force, that is, a partial pressure gradient, is obtained by a temperature gradient between the feed and the distillate side. In the osmotic distillation (OD), a partial pressure gradient is activated by a difference in concentration. As a consequence, OD can proceed at ambient temperature and flavor and fragrance compounds can be more conveniently preserved, than in thermally activated concentration processes. 

There are continuing efforts to develop cost-effective processes for fuel alcohol production, although the economics are often dependent on the availability of subsidized feedstocks to compete with traditional fuels derived from oil. The pretreatment and fermentation of such feedstocks, derived from corn, sugar cane, and even municipal waste, yields a dilute aqueous solution of ethanol which must be separated from a complex mixture of waste materials and then concentrated by distillation to remove water. Both batch and continuous production processes have been developed, with the requirement for effective bioseparations during both the pretreatment and ethanol recovery parts of the process. 

Citrus essences are distilled aqueous solutions of the more volatile components from the corresponding citms juices, as defined by Shaw (10). Commercially, they are added to concentrated citms juices to impart fresh fmit flavor that may be lost during the concentration process. Essence may be collected from fresh juice either by partial distillation prior to juice evaporation or by condensation of volatiles from the early stages of evaporation (11). Two phases, namely, aqueous essence and essence oil, are obtained during recovery. 

Membrane processes are versatile and flexible they can be combined with other methods in hybrid processes. Adapted to actual needs they can treat various process streams of different compositions and concentrations. Membrane distillation coupled with evaporation or reverse osmosis may improve the purification efficiency and increase decontamination factors. The flow chart of such hybrid processes is presented in Figure 30.21. In Figure 30.21a, the combination of MD unit with evaporator is shown. 

The subsequent oxidation proceeds with air at 30 to 80°C and pressures up to 5 bar, if necessary after catalyst separation and a precautionary filtration. It can be carried out in CO- or countercurrent mode, in a single step or multistep process. The hydrogen peroxide formed during the oxidation is extracted from the reaction mixture with water e.g. in pulsating packed towers. The extraction yield is ca. 98%. The hydrogen pieroxide solutions obtained are 15 to 35% by weight and must be freed from residual organic compounds before they can be concentrated by distillation. 

To replace a lower-boiling solvent with a higher-boiling solvent on scale, usually the process stream is concentrated and the higher-boiling solvent is added to the concentrate. Then distillation is continued, with the second solvent chasing the first. 

Smoke condensates are obtained by condensing smoke in water or another solvent. They may be further fractionated, purified or concentrated. The fractionation steps have two purposes to obtain products of interesting olfactory properties and to reduce the concentration of undesirable by-products from the smoke. Only the water-soluble fraction is used. The organic phase will be abandoned because a work up of the tar fraction is too expensive. The smoke solution will be filtered in order to remove polycyclic aromatic hydrocarbons (PAHs). According to a Russian patent [18] it is also possible to use 2% chitin and 0.5% chitosan for removing PAHs almost quantitatively. Afterwards the components of the smoke solution may be concentrated by distillation. The resulting product will be processed into smoke flavouring preparations. 

Osmotic distillation is unique among concentration processes in that feeds containing thermally labile or shear-sensitive components, and those with appreciable volatility, can be concentrated with little or no loss of product integrity. This characteristic is particularly important for the concentration of fruit juices and other liquid foodstuffs, such as vegetable juices, and tea and coffee extracts. These feeds contain a complex mixture of essential volatile hydrophilic flavor, and fragrance components in very low concentrations. Substantial depletion of these components by direct destruction or evaporation may render the product... 

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