Extractive distillation aliphatic

Some Available Data. A brief list of extractive distillation processes of actual or potential commercial value is in Table 13.7 the column of remarks explains why this mode of separation is adopted. The leading applications are to the separation of close-boiling aromatic, naphthenic, and aliphatic hydrocarbons and of olefins from diolefins such as butadiene and isoprene. Miscellaneous separations include propane from propylene with acrylonitrile as solvent (DuPont, U.S. Pat. 2,980,727) and ethanol from propanol with water as solvent [Fig. 13.24(b)],... 

Other separation processes can become advantageous, when separation problems such as unfavorable separation factors (0.95 < aj2 <1.05) or azeotropic points occur. In these cases, a special distillation process (extractive distillation) may be used. Extraction processes do not depend on a difference of vapor pressure between the compounds to he separated hut on the relative magnitudes of the activity coefficients of the compounds. As a result, extraction processes are particularly useful in separating the different aromatic compounds (Cg to C[2) from the different aliphatic compounds (Cg to C12). Absorption processes are ideally suited for the removal of undesired compounds from gas streams, e.g., sour gases (HjS, COj) from natural gas. 

Extractive distillation is a suitable distillation process for the separation of azeotropic systems or systems with separation factors tti2 close to unity. A typical extractive distillation process for the separation of aliphatics firom aromatics is shown in Figure 1. In extractive distillation processes, the high boiling selective solvent (entrainer), introduced not far from the top of the extractive distillation column, has to alter the volatilities in such a way that the separation factor attains a value very different from unity. Typical entrainers for the separation of aliphatics from aromatics are Ai-Methyl-pyrrolidone (NMP) or //-Formylmorpholine (NFM). In the presence of NMP or NFM,... 

Figure 1 Schematic diagram of an extractive distillation process (example separation of aliphatics from aromatics)...

In the 1940s extractive distillation was used to increase aliphatic-benzene relative volatilities, thereby increasing benzene recovery and... 

Extractive distillation uses a selective solvent (entrainer). Here the entrainer influences the ratio of the activity coefficients of the components in order to alter the separation factor far from unity. Often about 70% of the liquid phase inside the column consist of the entrainer. A typical extractive distillation process for separating aromatics (benzene) from aliphatics (cyclohexane) is show in Fig. 3.2-4. 

The recovery of pure aromatics from hydrocarbon mixtures is not possible using distillation process because the boiling points of many non-aromatics are very close to benzene, toluene, etc. Also, azeotropes are formed between aromatics and aliphatics. Three principle methods are used for separation azeotropic distillation, liquid-liquid extraction, and extractive distillation. Three major commercial processes have been developed for separation Udex, Sulpholane, and Arosolvan. Over 90% plants now use one of these processes. Each use an addition of solvent such as a mixture of glycols, tetramethylene sulfone, or N-methyl-2-pyrrolidone to aid in the extraction of aromatics. This occurs with high precision and efficiency. Pure benzene, toluene, and xylene are produced by these processes. 

The commercial production of styrene nowadays is carried out almost exclusively by catalytic dehydrogenation of ethylbenzene. Toray has developed a process for recovery from pyrolysis gasoline, which contains 3 to 5% styrene. The method involves hydrogenation of the aliphatic diene components of a close-cut pyrolysis gasoline (130 to 140 °C) followed by extractive distillation with dimethyl-acetamide. 

Examination of the Applicability of Extractive Distillation for the Separation of Aliphatics from Aromatics... 

Aromatic compounds such as benzene, toluene, or the different xylenes are mainly produced by the hydrogenated C5+-stream (pyrolysis gasoline) of a steam cracker. Besides the aromatics, this stream contains the different aliphatics and naphthenes. There is the question if all the aromatics (C6-C12) can be separated from the other C -Cu compounds by extractive distillation using for example sulfolane as entrainer. Simplifying, it is assumed that the aliphatics only consist of n-alkanes (n-hexane-M-dodecane). A temperature of 80 °C is chosen. The separation problem and the column configuration is shown in Figure 11.20. 

Figure 11.20 Column configuration for the separation of aliphatics from aromatics by extractive distillation using sulfolane as entrainer.

Another process that is sometimes used is extractive distillation [2,3]. The concept of extractive distillation is that a non-volatile polar solvent such as sulfolane is added, which has a different effect on the volatility of the components of the hydrocarbon mixture. Typically, the solvent decreases the volatility of the aromatic compounds, making them easier to separate from the aliphatics. The solvent is continuously added near the top of the extractive distillation column. Aliphatics are removed overhead and solvent and aromatics from the bottom of the column. The aromatics can be separated from the extractive solvent by distillation in a solvent recovery column. The boiling point of sulfolane (285 °C) is significantly higher than even o-xylene (144.4 C) enabling the isolation of the aromatic stream. Optionally, this can be further purified by a water wash. 

Such a process depends upon the difference in departure from ideally between the solvent and the components of the binary mixture to be separated. In the example given, both toluene and isooctane separately form nonideal liquid solutions with phenol, but the extent of the nonideality with isooctane is greater than that with toluene. When all three substances are present, therefore, the toluene and isooctane themselves behave as a nonideal mixture and then-relative volatility becomes high. Considerations of this sort form the basis for the choice of an extractive-distillation solvent. If, for example, a mixture of acetone (bp = 56.4 C) and methanol (bp = 64.7°Q, which form a binary azeotrope, were to be separated by extractive distillation, a suitable solvent could probably be chosen from the group of aliphatic alcohols. Butanol (bp = 117.8 Q, since it is a member of the same homologous series but not far removed, forms substantially ideal solutions with methanol, which are themselves readily separated. It will form solutions of positive deviation from ideality with acetone, however, and the acetone-methanol vapor-liquid equilibria will therefore be substantially altered in ternary mixtures. If butanol forms no azeotrope with acetone, and if it alters the vapor-liquid equilibrium of acetone-methanol sufficiently to destroy the azeotrope in this system, it will serve as an extractive-distillation solvent. When both substances of the binary mixture to be separated are themselves chemically very similar, a solvent of an entirely different chemical nature will be necessary. Acetone and furfural, for example, are useful as extractive-distillation solvents for separating the hydrocarbons butene-2 and a-butane. 

The conventional processes for the separation of these aromatic/aliphatic hydrocarbon mixtures are liquid extraction, when the aromatic range is 20-65 wt. %, extractive distillation for 65-90 wt. % of aromatics and azeotropic distillation for more than 90 wt. % of aromatic content. Typical solvents used for the extraction are polar components such as sulfolane (Choi et al., 2002), N-methyl pyrrolidone (NMP) (Krishna et al., 1987), ethylene glycols (Al-Sahhaf et al., 2003) and propylene carbonate (Ali et al., 2003). A step of distillation for separating the extraction solvent is required. 

Separation of classes of components. If a class of components is to be separated (e.g., a mixture of aromatics from a mixture of aliphatics), then distillation can only separate according to boiling points, irrespective of the class of component. In a complex mixture where classes of components need to be separated, this might mean isolating many components unnecessarily. Liquid-liquid extraction can be applied to the separation of classes of components. 

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