Extractive distillation additive selection

If the molecular species in the liquid tend to form complexes, the system will have negative deviations and activity coefficients less than unity, eg, the system chloroform—ethyl acetate. In a2eotropic and extractive distillation (see Distillation, azeotropic and extractive) and in Hquid-Hquid extraction, nonideal Hquid behavior is used to enhance component separation (see Extraction, liquid—liquid). An extensive discussion on the selection of nonideal addition agents is available (17). 

Solvents used for extractive distillation vary considerably, but in almost all cases solvent selection presents a trade-off between its selectivity and solvency (194). The effectivity of the solvent can sometimes be improved by the addition of a salt (195). 

DMA and TMA. Product ratios can be varied to maximize MMA, DMA, or TMA production. The correct selection of the N/C ratio and recycling of amines produces the desired product mix. Most of the exothermic reaction heat is recovered in feed preheating (3). The reactor products are sent to a separation system where firstly ammonia (4) is separated and recycled to the reaction system. Water from the dehydration column (6) is used in extractive distillation (5) to break the TMA azeotropes and produce pure anhydrous TMA. The product column (7) separates the water-free amines into pure anhydrous MMA and DMA. Methanol recovery (8) improves efficiency and extends catalyst life by allowing greater methanol slip exit from the converter. Addition of a methanol-recovery column to existing plants can help to increase production rates. 

Additionally, in order to have little or no impurities in the distillate of the extractive distillation column and also to avoid separation problems in the regeneration column, selective solvents for extractive distillation should have a boiling point that is at least 40 °C higher than the boiling points of the components to be separated. In the case of extraction, of course, it is most important that the solvent shows a miscibility gap with the feed and the raffinate stream. At the same time, a high... 

The extractive distillation data show that in general, for satisfactory operation, furfural is limited to high solvent-to-C4 feed ratios. The data in Table IV indicate the limiting solvent ratio to be ca. 12 to 1. In addition to its poor selectivity compared with other solvents, furfural reacts with itself to form a polymer (4) and with butadiene to form codimers which have been characterized in detail by Hillyer (5). Solvent losses to polymer and in the associated solvent purification system amount to a considerable yearly operating expense. 

Extractive distillation is used for the separation of azeotropes and closeboiling mixtures. In this process, a solvent is added to the distillation column. This solvent is selected so that one of the components, B, is selectively attracted to it. Since the solvent is usually chosen to have a significantly higher boiling point than the component being separated, the attracted component, B, has its volatility reduced. Thus, the other component, A, becomes more volatile and is easy to remove in the distillate. An additional column is required to separate the solvent and component B (Wankat, 1988). 

A system for separating a binary mixture by extractive distillation, followed by ordinary distillation for recovery and recycle of the solvent, is shown on previous page. Are the design variables shown sufficient to completely specify the problem If not, what additional design variable(s) would you select ... 

Axens/Uhde Benzene, Toluene Reformate and/or hydrotreated Pyrolysis gasoline BT cuts In extractive distillation, the addition of a selective solvent modifies the vapor pressures of the hydrocarbons in the feed in such a way that paraffinic and naphthenic components can be separated from the aromatics by distillation. 65 2009... 

Kogan [66] has shown that upon addition of a third component to a binary mixture that component increases in relative volatility in which the third comjjonent is less soluble. Further principles applicable to the selection of additives have been discussed by Kafarov and Gordijewski [67] and Kogan [68]. The connections between gas-liquid chromatography and extractive distillation have been elucidated by Rock [69] and Porter and Johnson [70]. They have pointed out that the latter Tuethod provides a simple means of finding suitable additives for extractive distillation. 

A modified McCabe-Thiele method employed in extractive distillation has been described by Nagel and Sinn [78]. Kortiim and Faltusz [79] have dealt with a variety of problems involved in selective separating processes ranging from the design of an automatic apparatus of special steel for continuous operation to the calculation of the minimum reflux ratio and the required amount of additive. 

In the selective processes so far described an alteration in the phase equilibrium is attained either by the use of a carrier vapour or by the addition of other liquids (azeotropic and extractive distillation). 

Extractive distillation exploits the differing solubilities of the azeotrope-forming components in a higher boiling extractant. Aids to the selection of suitable extractants (e.g., expert systems) are available [Erdmann 1986a, Trum 1986]. Recovery of the extractant requires an additional downstream column. 

In most cases not binary but multicomponent systems have to be separated. Sometimes an additional component is needed as an entrainer e.g. for the separation by extractive distillation. As an example selected separation factors of 12 for the system benzene (l)-cyclohexane (2)-NMP (3) calculated using modified UNIFAC and default UNIQUAC parameters from a simulator are shown in Figures 11.8 and 11.9, respectively. As benzene and cyclohexane form an azeotrope, the main task of the entrainer NMP is to shift the separation factor between benzene and cyclohexane as far from unity as possible this means au 1 or au 1- In practice, typical entrainer concentrations of 50-80 mol% are employed to achieve satisfying separation factors. A higher entrainer concentration usually improves the separation factor. 

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