Closely boiling component

Differential temperature as well as differential pressure can be used as a primary control variable. In one instance, it was hard to meet purity on a product in a column having close boiling components. The differential temperature across several bottom section trays was found to be the key to maintaining purity control. So a column side draw flow higher in the column was put on control by the critical temperature differential. This controlled the liquid reflux running down to the critical zone by varying the liquid drawn off at the side draw. This novel scheme solved the control problem. 

The ideal concept is usually a good approximation for close boiling components of a system, wherein the components are all of the same family of hydrocarbons or chemicals for example paraffin hydrocarbons. When odd or non-family components are present, the possibility of deviations from non-ideality becomes greater, or if the system is a wide boiling range of components. 

Figure 5.2-2. Typical vapour-liquid equilibrium curves a) system obeying Henry s law b) regular system c) system of close-boiling components d) azeotrope.

With difficult separations, involving close boiling components, postpone the most difficult separation to late in the sequence. 

A distillalion column is used to separate two close-boiling components that have a relative volatility close to one. The reflux ratio is quite high (IS) and many trays are required (150). To control the compositions of both products the flow rates of the product streams (distillate D and bottoms B) an manipulated. Gas chromatographs are used to measure the product compositions. Base level is controlled by steam flow rate to the icboiler and reflux drum level is controlled by reflux flow rate. 

Among hybrid separations not involving membranes, adsorptive distillation (87) offers interesting advantages over conventional methods. In this technique a selective adsorbent is added to a distillation mixture. This increases separation ability and may present an attractive option in the separation of azeotropes or close-boiling components. Adsorptive distillation can be used, for instance, for the removal of trace impurities in the manufacturing of fine chemicals (it may allow for switching some fine chemical processes from batchwise to continuous operation). 

Description Extractive distillation is used to separate close-boiling components using a solvent that alters the volatility between the components. An ED Sulfolane unit consists of two primary columns they are the ED column and the solvent recovery column. Aromatic feed is preheated with lean solvent and enters a central stage of the ED column (1). The lean solvent is introduced near the top of the ED column. Nonaromatics are separated from the top of this column and sent to storage. The ED column bottoms contain solvent and highly purified aromatics that are sent to the solvent recovery column (2). In this column, aromatics are separated from solvent under vacuum with steam stripping. The overhead aromatics product is sent to the BT fractionation section. Lean solvent is separated from the bottom of the column and recirculated back to the ED column. 

Azeotropic distillation is accomplished by adding to the liquid phase a volatile third component which changes the volatility of one of the two components more than the other, so the components are separated by distillation. The two components to be separated often are close boiling components which do or do not azeotrope in the binary mixture, but sometimes they are components which do azeotrope although they are not close boiling components. 

Other systems that require the addition of a separating agent to enhance or effect their separation include azeotropes and mixtures of close boiling components. 

The separation of close boiling components by ordinary fractionation may require... 

P-xylene is the most valuable xylene isomer due to its importance for the production of terephthalic acid, for which there is a demand in the polymer industry. Because of the complexity of separating the close-boiling components in the Cg-aromatic fraction, it is of great interest to produce p-xylene selectively. It has been reported that over modified H-ZSM-5 catalysts p-xylene can be produced in great excess of its thermodynamic equilibrium value in various shape-selective reactions (ref. 1). We will show that enhanced para-selectivity can be achieved even over unmodified H-ZSM-5 catalysts in the methanol reaction ... 

In extractive distillation, an extractive agent is added to the mixture to be distilled for the purpose of modifying the relative volatility of the key components without forming an azeotrope. Extractive distillation is usually employed to improve the separability between close-boiling components for which ordinary distillation would not be economically feasible. 

The separation of close boiling components by ordinary fractionation may require too many stages or could be practically impossible because of the proximity of the distribution coefficients (7T-values). The compositions of equilibrium vapor and liquid phases in such systems are almost identical. In general, close boiling components are likely to have different chemical structures, and would therefore interact differently with a third component. In extractive distillation, a solvent that is less volatile than the feed components is added to the mixture for the purpose of preferentially depressing the volatility of one of the feed components. In azeotropic distillation, the added component, or entrainer, forms an azeotrope with one of the components to be separated. The azeotrope may have either a higher or lower boiling temperature than the other component and may leave the separation device either in the bottoms or overhead product. 

Extractive distillation is a technique used to break a homogeneous azeotrope or to facilitate separation of close-boiling components. A high-boiling solvent with a particular affinity for one of the components Is introduced to lower its vapor pressure. The other component is then readily distilled from the solution and a second column used to recover the solvent. The flow sheet, is given in Fig. 12.12. 

Another method for separating very close boiling components takes advantage of the chemical dissimilarity of the feed compounds. In extractive distillation, the other component (solvent) that is added to the column is a high-boiling liquid that alters the relative volatilities between feed components. While normally the lower boiling feed components... 

The separation of close boiling components in a mixture has been accomplished commercially by cascading several evaporations and condensations of such mixtures in a device known as a distillation or rectification column. There are two types of columns, namely, packed columns and plate columns. The former are vertical cylinders filled with a variety of packings that provide a large surface area per unit volume to promote maximum contact between the downward liquid flow and the upward vapor flow. Since this type of column can encounter poorer vapor-liquid contact than the plate column, it is only seldomly used in cryogenic separation systems. 

Adsorptive distillation is an integrated operation in, which adsorption is combined with distillation to separate the close boiling components or constant boiling liquid mixtures. It is a three-phase mass transfer operation in which the adsorbent, usually in the form of (fine) fluidized powder, is introduced into the eolumn along with an inert carrier gas. The adsorbent selectively adsorbs one of the eomponents and flows into the desorption column in, which the adsorbed eomponent is desorbed. Thus adsorptive distillation is successful in separation and in avoiding the formation of azeotropes. Most commonly used adsorbents in the industry are silica gel, activated carbon, zeolite and alumina. Though adsorptive distillation has been reported long back, its industrial and commercial applications are very limited. However, potential application fields for adsorptive distillation inelude separation of toluene/methyl eyclohexane, naphtha reformates, p-xylene/ m-xylene, etc. 

Uno, S. Kurihara, K. Ochi, K. Kojima, K. Determination and correlation of vapor-liquid equilibrium for binary systems consisting of close-boiling components. Fluid Phase Equilib. 2007, 257, 139-146. 

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