Extraction and Extractive Distillation

Extraction and Extractive Distillation. The choice of an extraction or extractive distillation solvent depends upon its boiling point, polarity, thermal stabiUty, selectivity, aromatics capacity, and upon the feed aromatic content (see Extraction). Capacity, defined as the quantity of material that is extracted from the feed by a given quantity of solvent, must be balanced against selectivity, defined as the degree to which the solvent extracts the aromatics in the feed in preference to paraffins and other materials. Most high capacity solvents have low selectivity. The ultimate choice of solvent is deterrnined by economics. The most important extraction processes use either sulfolane or glycols as the polar extraction solvent. 

Products from catalytic reformers (the reformate) is a mixture of aromatics, paraffins and cycloparaffins ranging from Ce-Cg. The mixture has a high octane rating due to presence of a high percentage of aromatics and branched paraffins. Extraction of the mixture with a suitable solvent produces an aromatic-rich extract, which is further fractionated to separate the BTX components. Extraction and extractive distillation of reformate have been reviewed by Gentray and Kumar. 

For example, the search for potential solvents for liquid extraction and extractive distillation is carried out through a group contribution molecular design of solvents (MOLDES) approach (Pretel et ak, 1994). 

We now consider a more complex example where liquid/liquid extraction and extractive distillation are among the processes to appear in the solution. 

We wish to devise a separation process based on distillation, liquid/liquid extraction, and extractive distillation for the mixture of solvents shown in Fig. 

For extractive distillation, extraction and absorption processes, highly selective solvents are required. The economic importance of extraction and extractive distillation processes can be recognized from the fact that the worldwide production of the BTX aromatics (benzene, toluene, xylenes, ethylbenzene) as important primary petrochemical products for the industrial manufacturing of many chemical products... 

Extraction and extractive distillation can also be accomplished using N-Forylmorpholin (NFM) as the solvent. Krupp Uhde offers an extractive distillation process for license that employs NFM as the solvent. N-Forylmorpholin is employed in the extractor and in the distillation column to lower the vapor pressure of the aromatics relative to nonaromatics of a similar boiling point. In this way aromatics can be separated from nonaromatics in a very small and simple distillation column. N-Forylmorpholin has properties that make it an excellent solvent for this application. It is highly selective, thermally stable and has a boiling point in the range of the products to be separated. A flow diagram for this extractive distillation process is given in Fig. 3. 

FIGURE 7.8-4 Liquid-liquid extraction and extractive distillation recovery using a less volatile solvent. 

Reversible complexatlon reactions have long been used to improve the speed and selectivity of separation processes, especially those Involving the separation or purification of dilute solutes (j ). Such reactions are the basis of a multitude of separation unit operations Including gas absorption, solvent extraction, and extractive distillation. When a reversible complexatlon reaction (carrier) Is Incorporated into a membrane, the performance of the membrane can be improved through a process known as facilitated transport. In this process, shown schematically In Figure 1, there are two pathways available for the transport of the solute through the membrane. The solute can permeate through the membrane by a solution-diffusion mechanism and by the diffusion of the solute-carrier complex. Other solutes are not bound by the carrier due to the specificity of the complexatlon reaction this Increases the selectivity of the process. 

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. 

An effective assessment of the use of ionic liquids as solvents in commercial liquid-liquid extraction and extractive distillation processes requires consideration of numerous factors associated with the thermo-physical and chemical properties of ionic liquids as well as the economical and environmental impact of the use of ionic liquids in the chemical and petrochemical industry. 

For the use of Ionic Liquids as solvents it is very important to know about their interaction with different solutes. Activity coefficients at infinite dilution of a solute i(yi ) can be used to quantify the volatihty of the solute as well as to provide information on the intermolecular energy between solvent and solute. Values of yv" are also important for evaluating the potential uses of ILs in hquid-hquid extraction and extractive distillation. Since ILs have a negligible vapor paessure, the gas-hquid chromatography (GLC) using the ionic hquid as stationary phase, is the most suitable method for measuring activity coefficients at infinite dilution y, . 

---