Counter current distillation

This UK resident filed a patent, published in 1986 in the US, on a rotating still (Moss, 1986 Figure 1.9a and b). Although specified as a separator for light and heavy fractions - multi-stage counter-current distillation - the so-called lamellar bodies normal to the radial flow path are perforated and are shown in the detail in Figure 9b. 

D. Absorption and and distillation, counter-current, gas and liquid individual coefficients and wetted surface area, Ouda et al. correlation for random pacldugs... 

E. Distillation and absorption, counter-current, random packings, modification of Onda correlation, Bravo and Fair correlation... 

Solids may be processed continuously or semicontinuously by pumping slurries or by using lock hoppers. An example is the separation of insoluble polymers by floatation with a variable-density SCF. For liquid feeds, multistage separation may be achieved by continuous counter-current extraction, much like conventional liquid-hquid extraction. The final produces may be recovered from the extract phase by a depressurization, a temperature change, or by conventional distillation. 

The separation of liquid mixtures by distillation depends on differences in volatility between the components. The greater the relative volatilities, the easier the separation. The basic equipment required for continuous distillation is shown in Figure 11.1. Vapour flows up the column and liquid counter-currently down the column. The vapour and liquid are brought into contact on plates, or packing. Part of the condensate from the condenser is returned to the top of the column to provide liquid flow above the feed point (reflux), and part of the liquid from the base of the column is vaporised in the reboiler and returned to provide the vapour flow. 

The biocatalyst may be supported on a Lewis acid. Elemental sulfur is removed from the liquid hydrocarbons and the recovered solvent is counter-currently washed with water in a separate unit. Prior to reuse, the solvent is distilled to decontaminate it from remaining water or sulfur slurry. The treated product not only has a reduced concentration of organic sulfur compounds, but also its viscosity is reduced. 

A liquid/liquid separation of product and catalyst is done in separate vessels after the reaction has taken place. The reaction mixture is sent to a gas separator and from there to a counter current washing tower (a simple phase separator is shown in the figure) in which the effluent is treated with aqueous Na2C03. The acidic HCo(CO)4 is transformed into the water soluble conjugate base NaCo(CO)4. The product is scrubbed with water to remove the traces of base. The oxo-crude goes to the distillation unit. 

The GS enriching process is a counter-current gas-liquid extraction done at a pressure of 2000 kPa in a sieve tray tower with the upper half operating at 30 C and the lower at 130 C. ( 5) In the top half of the tower, feedwater extracts deuterium from the upflowing cold H2S, reaching a maximum at the centre of the tower. The recycled lean H2S entering the lower hot half of the tower strips deuterium from the water, which then leaves the system depleted in deuterium. A cascade of several stages is used to reach the desired feed concentration for the final water distillation or finishing unit. Transfer between cascades can be either by gas or liquid from the centre of the tower. 

The Lurgi gasifiers used by Sasol operate at "low" temperatures and consequently phenols, and "tars" are "distilled" from the coal at the top of the gasifier, and carried out with the raw gas. On condensation two liquid phases are formed, "tar" and "gas liquor" (water). The "tar acids" (phenol, cresols etc) are dissolved in the "gas liquor" which is fed to the Phenosolvan unit where the acids are recovered by counter current extraction with butyl acetate or diisopropyl ether. The crude tar acids are fractionated to yield phenol, ortho, meta and para cresol and xylenols. The phenol is further refined to produce a high purity, colourless and stable product. Phenol is used mainly in the production of formaldehyde resins while the cresols are used as flotation frothers and in the manufacture of pesticides etc. 

Laboratory studies of the rearrangement process began with semi-continuous operation in a single, 200-mL, glass reactor, feeding 1 as a liquid and simultaneous distillation of 2,5-DHF, crotonaldehyde and unreacted 1. Catalyst recovery was performed as needed in a separatory funnel with n-octane as the extraction solvent. Further laboratory development was performed with one or more 1000-mL continuous reactors in series and catalyst recovery used a laboratory-scale, reciprocating-plate, counter-current, continuous extractor (Karr extractor). Final scale-up was to a semiworks plant (capacity ca. 4500 kg/day) using three, stainless steel, continuous stirred tank reactors (CSTR). 

Figure 5, which represents commercial extraction of a California wax-free light lubricating oil distillate, is used to review graphical techniques for studying counter-current multistage extractions. 

The principle Zr ore, zircon (Zr silicate) is processed by caustic fusion or by direct chlorination of milled coke and zircon mixts. Washing of the Na fusion cake leave an acid soluble hydrated Zr oxide, whereas chlorination yields mixed Si and Zr tetrachlorides which are separated by distillation. Removal of the Hf from the Zr takes place through counter current liq-liq extraction (Ref 33), For this purpose the oxide or the tetrachloride is dissolved in dil hydrochloric acid to which ammonium thiocyanate is added as a complexing agent. The organic extracting phase is methyl isobutylketone... 

The iron oxide used in the purification of coal gas gradually becomes richer in sulphur and can repeatedly be revivified by exposure to air until the sulphur amounts to about 40 per cent., of the whole the mixture is then of greater value as a source of sulphur. The spent oxide, as it is commonly termed, is frequently applied to the production of sulphur dioxide or sulphuric acid, but the extraction of its sulphur by carbon disulphide has been effected as a successful commercial process using the system of counter-currents the mass is exhausted of sulphur, whilst a saturated solution of sulphur is obtained, from which the solvent can be distilled and returned to the extraction process. 

Parastiilation. This relatively new approach to distillation, as proposed by A.F..O. Jenkins in 1983. is a method for multistage, counter-current contact between vapor and liquid and is reputed to provide 33 3 more ideal stages than traditional factional distillation at the same tray spacing. The... 

When the separation procedures described in detail above are unsatisfactory for the separation of a mixture of organic compounds, purely physical methods may be employed. Thus a mixture of volatile liquids may be fractionally distilled (compare Section 2.26) or a mixture of non-volatile solids may frequently be separated by making use of the differences in solubilities in inert solvents. The progress of such separations may be monitored by application of the various chromatographic techniques detailed in Section 2.31, or indeed these techniques may be employed on the preparative scale for effecting the separation itself (e.g. flash chromatography, p. 217). The techniques of counter current distribution, fractional crystallisation or fractional sublimation (Section 2.21) may also be employed where appropriate. 

Figure 13.15 Flow scheme and performance data for a membrane distillation process for the production of water from salt solutions [31]. Feed salt solution is heated to 100 °C and passed counter-current to cool distillate that enters at 42 °C. The distillate product is almost salt-free as shown by its low conductivity. The distillate flux is almost constant up to salt concentrations as high as 20 % NaCI. Reprinted from J. Membr. Sci. 39, K. Schneider, W. Holz, R. Wollbeck and S. Ripperger, Membranes and Modules for Transmembrane Distillation, p. 25. Copyright 1988, with permission from Elsevier...

With this assumption, and using a modified Neumann model for HI/I2/H20 mixtures description, CEA (Leybros, 2009) devised a flow sheet for the iodine section which decomposes almost all incoming HI and therefore returns relatively pure products (the iodine return flow contains only 4 molar% water and less than. 3 molar% HI) to the Bunsen section, an important feature for the counter-current reactor. Secondary helium heat is provided to the boiler of the column (235 kj/mol), whereas all other heat needs are fulfilled through internal heat recovery, with the help of a heat pump which transfers heat from the products of the distillation column to its feed. Mainly because of the presence of this heat pump, the iodine section uses 60 kj/mol of electric power on top of the helium heat. 

The processes used in industrial air-separation plants have changed very little in basic principle during the past 25 years. After cooling the compressed air to its dew point in a main heat exchanger by flowing counter current to the products of separation, the air feed, at an absolute pressure of about 6 MPa, is separated in a double distillation column. This unit is kept cold by refrigeration developed in a turbine, which expands a flow equivalent to between 8 and 15% of the air-feed stream down to approximately atmospheric pressure. 

Distillation is the most widely utilised industrial separation method. Distillation involves multiple contacts between counter-currently... 

Ind. Eng. Chem. Res., 41, 2735, 2002 Ellenberger, J Krishna, R., Counter-current operation of structured catalytically packed distillation columns, Chem. Eng. Sci., 54,1339-1345, 1999 Krishna, R Hardware selection and design aspects for reactive-distillation columns, Reactive Distillation, eds Sundmacher K. and Kienle, A., Wiley-VCH, Weinheim, Germany, 169-189,... 

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