Extractive distillation costs

In the chemical processing industry, extraction is used when distillation is impractical or too costly. Extraction may be more practical than distillation when the relative volatilities of two components are close. In other cases, the components to be separated may be heat sensitive like antibiotics or relatively nonvolatile like mineral salts. When unfortunate azeotropes form, distillation may be ineffective. Several examples of cost-effective liquid-liquid extraction processes include the recovery of acetic acid from water using ethyl ether or ethyl acetate and the recovery of phenolics from water with butyl acetate. 

Iron and chloride catalysts are basically disposable because they are considered to be rather cheap and difficult to recover from residual products, while Ni-Mo and Co-Mo catalysts are too expensive to be considered disposable (82). Recovery of very fine particles of MoS2 by hydroclone separation has been shown to be promising (83). Disposable catalysts added at levels similar to that of ash mineral contents significantly reduce the potential recovery of oil in both distillation and extraction. This is problematic because equal volumes of oil adhere to solid particles after separation. Slurry transportation of residues suffers from the same problem. Even if the cost of the disposable catalysts is affordable, adding 1 to 5% of the catalyst to the... 

The transfer of mass from one phase to another is involved in the operations of distillation, absorption, extraction, humidification, adsorption, drying, and crystallization. The principal function of the equipment used for these operations is to permit efficient contact between the phases. Many special types of equipment have been developed that are particularly applicable for use with a given operation, but finite-stage contactors and continuous contactors are the types most commonly encountered. A major part of this chapter, therefore, is devoted to the design aspects and costs of stagewise plate contactors and continuous packed contactors. 

Cost calculations were made for this modified extractive distillation process they are shown by the dashed curves on Figures 5 and 7. The proposed heat exchange system provides considerable reduction in the annual costs of the extractive distillation process. However, the extractive distillation costs are still greater than those for a binary propane-propylene distillation process as indicated on Figures 5 and 7. 

Because of high energy costs and increasing regulation of solvents used in conventional distillation, alternative extraction and crystallization methods are now more frequently used. Furthermore, supercritical extraction combines the characteristics of both distillation and extraction it is efficient, clean, and is able to extract substances which are difficult to separate by other unit operations. Thus it has recently become an attractive possibility for many separations. 

Electrochemical separations have not found wide commercial application. The cost per unit mass cannot, in general, compete with conventional techniques such as distillation or extraction. However, for high-specific-value components the selectivity available with electrophoresis or enhanced membrane transport often makes these the processes of choice. 

In liquid phase reactions the choice of the type of solvent is often decided by the reaction mechanism. In the case of organic solvents one may have a choice between homologues. In many cases the solvent will be separated from the main product by distillation, or by extraction followed by distillation. A higher volatility of the solvent will generally reduce the distillation cost, but it will raise the reactor pressure (at a given temperature), which also implies expenses. A solvent with optimum volatility may be found for any given set of reaction conditions. 

Some aromatic oil substitutes called mild extract solvates (MES) are being evaluated as substitutes for aromatic oils that are banned. Another class of substitutions for aromatic oils in rubber is treated distillate aromatic extracts (T-DAE), which are also being considered. Relative costs of these possible substitutes will determine to what extent each type of oil will be used, compared to naphthenic oil. 

The extraction of titanium is still relatively costly first the dioxide Ti02 is converted to the tetrachloride TiCl4 by heating with carbon in a stream of chlorine the tetrachloride is a volatile liquid which can be rendered pure by fractional distillation. The next stage is costly the reduction of the tetrachloride to the metal, with magnesium. must be carried out in a molybdenum-coated iron crucible in an atmospheric of argon at about 1100 K ... 

Anhydrous Acetic Acid. In the manufacture of acetic acid by direct oxidation of a petroleum-based feedstock, solvent extraction has been used to separate acetic acid [64-19-7] from the aqueous reaction Hquor containing significant quantities of formic and propionic acids. Isoamyl acetate [123-92-2] is used as solvent to extract nearly all the acetic acid, and some water, from the aqueous feed (236). The extract is then dehydrated by azeotropic distillation using isoamyl acetate as water entrainer (see DISTILLATION, AZEOTROPIC AND EXTRACTIVE). It is claimed that the extraction step in this process affords substantial savings in plant capital investment and operating cost (see Acetic acid and derivatives). A detailed description of various extraction processes is available (237). 

Absorption. As a separation technique, absorption (qv), also called extractive distillation, starts with an energy deficit because the process mixes in a pure material (solvent) and then separates it again. This process is nevertheless quite common because it shares most of the advantages of distillation. Additionally, because it separates by molecular type, it can be tailored to obtain a high a. The following ratios are suggested for equal costs (7) ... 

Wood is the raw material of the naval stores iadustry (77). Naval stores, so named because of their importance to the wooden ships of past centuries, consist of rosin (diterpene resin acids), turpentine (monoterpene hydrocarbons), and associated chemicals derived from pine (see Terpenoids). These were obtained by wounding the tree to yield pine gum, but the high labor costs have substantially reduced this production in the United States. Another source of rosin and turpentine is through extraction of old pine stumps, but this is a nonrenewable resource and this iadustry is in decline. The most important source of naval stores is spent sulfate pulpiag Hquors from kraft pulpiag of pine. In 1995, U.S. production of rosin from all sources was estimated at under 300,000 metric tons and of turpentine at 70,000 metric tons. Distillation of tall oil provides, in addition to rosin, nearly 128,000 metric tons of tall oil fatty acids annually (78). 

In the example, the minimum reflux ratio and minimum number of theoretical plates decreased 14- to 33-fold, respectively, when the relative volatiHty increased from 1.1 to 4. Other distillation systems would have different specific reflux ratios and numbers of theoretical plates, but the trend would be the same. As the relative volatiHty approaches unity, distillation separations rapidly become more cosdy in terms of both capital and operating costs. The relative volatiHty can sometimes be improved through the use of an extraneous solvent that modifies the VLE. Binary azeotropic systems are impossible to separate into pure components in a single column, but the azeotrope can often be broken by an extraneous entrainer (see Distillation, A7EOTROPTC AND EXTRACTIVE). 

Optimization. Optimi2ation of the design variables is an important yet often neglected step in the design of extractive distillation sequences. The cost of the solvent recovery (qv) step affects the optimi2ation and thus must also be included. Optimi2ation not only yields the most efficient extractive distillation design, it is also a prerequisite for vaUd comparisons with other separation sequences and methods. 

Fig. 9. Extractive distillation sequence cost as a function of the feed ratio for the production of anhydrous ethanol from azeotropic ethanol using ethylene glvcol at reflux ratios of A, 1.15 r O, 1.2 r and 1.3 r (39). Point A represents a previously pubhshed design for the same mixture (37).

Liquid-liquid extraction is used primarily when distillation is imprac-tic or too costly to use. It may be more practical than distillation when the relative volatility for two components falls between 1.0 and 1.2. Likewise, liquid-liquid extraction may be more economical than distillation or steam-stripping a dissolved impurity from wastewater when the relative volatility or the solute to water is less than 4. In one case discussed by Robbins [Chem. Eng. Prog., 76 (10), 58 (1980)], liquid-liquid extraction was economically more attractive than carbon-bed or resin-bed adsorption as a pretreatment process for wastewater detoxification before biotreatment. 

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