Extractive Distillation, Process

The steady-state design of a two-column extractive distillation system was developed in Chapter 5. Now, we want to design an effective control structure for this system. The process has two distillation columns, and a plantwide control structure must be developed that accounts for the interaction between the columns and for the solvent recycle. 

The bottoms of the extractive column has a very small impurity of acetone (0.01 mol%), so it is essentially a binary mixture of methanol and DMSO. The bottoms stream is fed to the 17-stage solvent-recovery column on Stage 8. The pressure is 1 atm, and the reflux ratio is 0.5. The distillate product is high-purity (99.95 mol%) methanol. The bottoms is recycled back to the extractive column as high-purity (99.99 mol%) DMSO solvent. 

Distillation Design and Control Using Asperi Simulation, Second Edition. William L. Luyben. 2013 John Wiley Sons, Inc. Published 2013 by John Wiley Sons, Inc. 

Reflux-drum levels are controlled by distillate flow rates. Note that both columns have small RRs. 

Extractive distillation with DMSO 20% feed rate 

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The second class of distillation operation using an extraneous mass-separating agent is extractive distillation. Here, the extraneous mass-separating agent is relatively involatile and is known as a solvent. This operation is quite different from azeotropic distillation in that the solvent is withdrawn from the column bottoms and does not form an azeotrope with any of the components. A typical extractive distillation process is shown in Fig. 3.11. ... 

Extractive distillation, using similar solvents to those used in extraction, may be employed to recover aromatics from reformates which have been prefractionated to a narrow boiling range. Extractive distillation is also used to recover a mixed ben2ene—toluene stream from which high quaUty benzene can be produced by postfractionation in this case, the toluene product is less pure, but is stiU acceptable as a feedstock for dealkylation or gasoline blending. Extractive distillation processes for aromatics recovery include those Hsted in Table 4. 

Separation and Purification. Separation and purification of butadiene from other components is dominated commercially by the extractive distillation process. The most commonly used solvents are acetonitrile and dimethylformarnide. Dimethylacetamide, furfural, and... 

Natural gas liquids may contain significant amounts of cyclohexane, a precursor for nylon 6 (Chapter 10). Recovery of cyclohexane from NGL hy conventional distillation is difficult and not economical because heptane isomers are also present which hoil at temperatures nearly identical to that of cyclohexane. An extractive distillation process has been recently developed by Phillips Petroleum Co. to separate cyclohexane. ... 

CAA [Cuprous ammonium acetate] A general process for separating alkenes, di-alkenes, and alkynes from each other by extraction of their cuprous complexes from aqueous cuprous ammonium acetate into an organic solvent. Exxon used it for separating C4 fractions containing low concentrations of butadiene. The liquid-liquid extraction processes for butadiene have all been replaced by extractive distillation processes. 

Distex A family of extractive distillation processes used in the petroleum industry from 1940. In one such process, furfural is used as the extracting agent for separating butadiene from other C4 hydrocarbons. 

Propylane An extractive distillation process for removing aromatic hydrocarbons from hydrogenated crude benzene, using propylene carbonate. Developed by Koppers. 

Ryan-Holmes A cryogenic extractive distillation process using liquid carbon dioxide, in which a light hydrocarbon is added in order to suppress the freezing of the carbon dioxide. Licensed by Process Systems International nine licenses had been granted by 1992. 

Suida An extractive distillation process for concentrating the dilute acetic acid obtained from the manufacture of cellulose acetate. It was originally used for separating the products of wood pyrolysis. Invented in 1926 by H. Suida in Vienna and operated in the 1930s. 

The C4 stream is fed to the middle of a fractionator, and a high boiling point solvent is fed at the top. The solvent, as it works its way down, strips out the butadiene as the C4 vapor works its way up the column. The solvent and butadiene come out the bottom and can easily be split in a second column. Two popular high boiling point solvents are N-methylpyrrolidone (NMP) and Dimethylformamide (DMF). The chapter on benzene has more details on the extractive distillation process. 

The styrene concentrate is fed to a solvent recovery process or an extractive distillation process. The solvent selectively pulls the styrene out of the hydrocarbon mixture. The styrene raffinate, sans styrene, is sent back to be mixed with the pygas (although it can also be fractionated to pull out a high quality mixed xylene.)... 

K and then recondensed at lower temperatures [173]. It was also demonstrated that there is a possibility of separating two ILs by distillation without their thermal decomposition. On the other hand, for example, BASF used ILs in the extractive distillation process at temperatures of 448 K and pressure of 5 kPa for more than three months without loss of the IL. The assumed nonvolatility of ILs had been a basis of their common reputation as "green" solvents. 

Two important extractive distillation processes were placed in commercial operation during World War II the recovery of butadiene from a C4 fraction using furfural as the entrainer (7, 22) and the segregation of toluene from petroleum fractions by means of phenol (14-16). 

The topic covered in the 10 papers of the first section is commonly referred to as salt effect in vapor-liquid equilibrium and is potentially of great industrial importance. This salt effect leads to extractive distillation processes in which a dissolved salt replaces a liquid additive as the separating agent the replacement often results in a greatly improved separating ability and reduced energy requirements. Two papers in this volume, those by Sloan and by Vaillancourt, illustrate the use of such processing to concentrate nitric acid from its aqueous azeotrope. Nevertheless, the effect has not been exploited by industry to nearly the extent that would seem to be merited by its scientific promise. 

The Extractive Distillation Process for Nitric Acid Concentration Using Magnesium Nitrate... 

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. 

For a continuous extractive distillation process to be possible there must be adequate enhancement of the nitric acid-water relative volatility, and a system equilibrium which permits virtually complete separation of nitric acid from magnesium nitrate, the latter taking up the water content of the weak acid feedstock. This requires addition to the weak nitric acid of solutions of magnesium nitrate usually containing 60 wt% or more of Mg(NC>3)2. Under these conditions a nitric acid-water relative volatility of greater than 2.0 is obtained at the low end of the liquid phase concentration at a nitric acid mole fraction below 0.05 (4, 7). 

Since in an extractive distillation process based on this ternary system the extractive agent is nonvolatile and remains in the liquid phase, and since because of the similarity of the molar latent heats of nitric acid and water there is substantially constant molar liquid overflow, the mole fraction of magnesium nitrate remains almost constant throughout the process. It is appropriate to represent the equilibrium situation as a pseudo-binary system for each magnesium nitrate concentration, and Figure 7 shows vapor-liquid equilibria on a nitric acid-water basis at a series of magnesium nitrate concentrations from zero to 0.25 mole fraction in the liquid phase. 

ICFs Commercial Process. In 1960 ICI constructed a concentration plant using this extractive distillation process (18) with a capacity of 16,000 tonnes/ annum of product acid (99.5 wt% HNO3) which has subsequently been extended. A flowsheet is given in Figure 8, and the process description is as follows. 

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)],... 

TABLE 13.7. Examples of Extractive Distillation Processes for the Separation of Ideal, Nonideal, end Azeotropic Systems... 

A separation process is sought that can satisfy both our present economic and enviromental constraints. It would also provide an alternative to present practice that relies on expensive azeotropic or extractive distillation processes used in the recovery of products from low relative volatility streams. As an example, virtually all industrial butadiene recovery processes now rely on extractive distillation using acetonitrile or other equivalent agent to enhance the relative volatility of the C4 components. The use of supercritical or near critical separation of these streams may satisfy these requirements provided certain pressure, temperature and recompression criteria can be met. Such a process would also reduce the need for a complex train of distillation towers. 

Modelling and optimisation of batch reactive and extractive distillation processes... 

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