Distillation heteroazeotrope

One of the key thermodynamic parameter in heterogeneous batch distillation is the decanter split ratio liquid-liquid tie line at the decanter temperature or alternatively by the mole ratio of the entrainer-rich phase ZR to the overall liquid phase Z° into the decanter as follows ... 

Nitromethane shows the simplest residue curve map with one unstable curved separatrix dividing the triangle in two basic distillation regions. Methanol and acetonitrile give rise two binary azeotropic mixtures and three distillation regions that are bounded by two unstable curved separatrices. Water shows the most complicated residue curve maps, due to the presence of a ternary azeotrope and a miscibility gap with both the n-hexane and the ethyl acetate component. In all four cases, the heteroazeotrope (binary or ternary) has the lowest boiling temperature of the system. As it can be seen in Table 3, all entrainers except water provide the n-hexane-rich phase Zw as distillate product with a purity better than 0.91. Water is not a desirable entrainer because of the existence of ternary azeotrope whose n-hexane-rich phase has a water purity much lower (0.70). Considering in Table 3 the split... 

Simulation of the separation of the mixture fi-hexane-ethyl acetate by heteroazeotropic batch distillation using acetonitrile... 

Operating conditions of heteroazeotropic batch distillation with acetonitrile... 

S. Skouras, S. Skogestad, Separation of ternary heteroazeotropic mixtures in a closed multivessel batch distillation-decanter hybrid, Chem. Eng. Proc. 43 (2004) 291-304. 

Benzene dning, either by azeotropic or preferably heteroazeotropic distiHation, or on molecular sieves. To obtain a residual water content less than 30 ppm, the distillation column must have at least 15 trays. 

For heterogeneous batch distillation a new double column configuration operating in closed system is suggested. This configuration is investigated by feasibility studies based on the assumption of maximal separation and is compared with the traditional batch rectifier. The calculations are performed for a binary (n-butanol - water) and for a ternary heteroazeotropic mixture (isopropanol - water + benzene as entrainer). Keywords heteroazeotrope, batch distillation, feasibility studies. 

If components of a mixture form a heteroazeotrope or by the addition of an entrainer (E) a heteroazeotrope can be formed, the azeotropic composition can be crossed by decantation. In the pharmaceutical and fine chemical industries batch processes including the batch heteroazeotropic distillation (BHD) are widely applied. As far as we know the BHD was exclusively applied in the industry in batch rectifiers (equipped with a decanter) in open operation mode (with continuous top product withdrawal). The batch rectifier (BR) was investigated with variable decanter holdup by Rodriguez-Donis et al. (2002) and with continuous entrainer feeding by Modla et al. (2001, 2003) and Rodriguez-Donis et al. (2003), respectively. Recently the BHD was extensively studied for the BR and multivessel columns by Skouras et al. (2005a,b). 

Step 1 Production of 4 The 4-rich phase (x z a heteroazeotrope (xaz,a) is refluxed and the 5-rich one (a withdrawn as distillate. The bottoms is product 4. 

Product separation and purification the first distillation column is designed to produce a cut enriched with acetic acid by the removal of the lighter and heavier components (methyl iodide, methyl acetate, etc.. This-cut is then dehydrated by heteroazeotropic distillation. The aqueous fraction recovered at the top is refractionated to remove excess water. The heavy stream is treated in a finishing column which produces glads acetic add in the distillate, while the residual acetic add at the bottom is also recovered in a complementary fractionation that separates the heavy products such as propionic add. These high-alloy steel columns each have between 35 and 45 actual trays. 

The dehydrator is a column with-about 30 trays, designed to separate the water from the phenol by heteroazeotropic distillation in the presence of benzene and feed-and make-up toluene. The benzene/water azeotrope (bplJ>13 69 C, water content 8.8 per cent weight) and toluene/water azeotrope (bpl013 = 84 Q water content 13.5 per cent weight) leaving at the top are cooled rod condensed to yiddjwo phases ... 

Purification. The crude ethylene dichloride is then purified. It is first dehydrated by heteroazeotropic distillation (10 trays). With water, ethylene dichloride in fact forms an azeotrope (bp1013 = 72.3°C, water content 92 per cent weight) which settles to yield an organic phase that serves as a reflux, and an aqueous phase added to that produced-by the hot quench. It is then rid of the light components (ethylene dichloride, dichloro-ethyiene, trichloroethylene, etc.) and heavy components (1,1,2-trichloroethane, penta-chloroethane, perchloroethyiene, etc.) in two distillation columns with 45 and 55 actual trays respectively. This operation also produces a small amount of ethylene dichloride, which forms an azeotrope with trichloroethylene (bp1>0u = 82.1 C, ethylene dichloride content 56.5 per cent weight). 

Separation ofthewater/ester azeotrope by heteroazeotropic distillation (bp,. 013 - 83°C water content 14 per cent weight), in the presence of make-up water and a polymerization inhibitor ( = 50 trays). The aqueous, phase obtained by settling is used as reflux. The organic phase, containing 97 per cent weight estet, is purified, after neutralization. The withdrawal, consisting of water and methanoL is redistilled (=35 trays), and the alcohol recovered at the top is recycled. 

Dehydration. The crude methacrylic acid is dehydrated by heteroazeotropic distillation of water in the presence of a solvent (ketoneX of which the traces dissolved in the aqueous phase are recovered by stripping. This treatment is also employed to separate and recycle the methacrolein present in the organic phase. 

Should it prove impossible to effect a satisfactory separation by changing the distillation pressure, the next step will be to find a suitable additive with which one of the constituents forms a.heteroazeotrope, or a homoazeotrope that is easily split up [34]. An approximate method for separating heteroazeotropic mixtures based on the mathematical model for the liquid-liquid-vapour equilibrium of two multicomponent systems was elaborated by BrU et al. [49a]. 

The mode of operation and the dimensioning of a heteroazeotropic distillation as exemplified by the separation of the system water-acetic acid has been described by Wolf et al. [61b]. Morozova and Platonov [61c] analyzed the structure of phase diagrams of multicomponent mixtures using a digital computer. They studied the requirements for the separation of azeotropic mixtures. In order to achieve optimum column combinations Serafimov et al. [58 c] studied the ternary mi.xture isopropanol/ benzene/water on the basis of a mathematical treatment of the separation of heteroazeotropic mixtures. In another paper [58 d] a procedure was presented for the separation into its components of the water-containing mixture with acetone, ethanol, benzene and butyl acetate by means of the thermodynamic and topological analysis of the phase diagram structure. 

In azeotropic distillation, mitures that are difficult to separate are separated by addition of a substance that forms an azeotrope with only one of the components of the mixture. This process is rarely used. A widely used special case is heteroazeotropic distillation, in which water is present as azeotrope component and forms a miscibility gap. 

Processes for completely fractionating binary mixtures with heteroazeotropes consist of two distillation columns and one decanter (Fig. 11.3-1). As the azeotrope lies within the miscibility gap of the liqnid the azeotrope can be broken by decantation. The two fractions from the decanter are at different sides of the azeotropic point. Purification of these two rather impnre fractions is performed by distillation. The pure products are recovered as bottoms from the distillation columns C-1 and C-2. 

The process shown in Fig. 11.3-1 utilizes the specific advantages of distillation and decantatioa The advantage of distillation is the ability to produce pure fractions. However, distillation cannot break azeotropes. On the other hand, decantation can break heteroazeotropes but cannot produce pure products. Thus, the combination of distillation and decantation is a very effective process for fractionating mixtures with heteroazeotropes. Such processes are extensively used in industiy. Table 11.3-1 lists some important binary mixtures fiactioiated by the process shown in Fig. 11.3-1. 

The combination of distillation and decantation can also be applied to multicomponent mixrnres. A process for the separation of the temaiy mixture acetone/water/1-butanol is depicted in Fig. 11.3-2. One organic compound (acetone) is miscible with water, the other one (1-butanol) is inmiscible. The mixture water/l-butanol exhibits a heteroazeotrope. A boundary distillation line runs from the heteroazeotrope to the low boiler acetone. 

Fig. 2-29. Azeotropic distillation unit for heteroazeotropes (a) with corresponding vapor-liquid equilibrium diagram (b).

In these units the solvent in die extract dnuning out of the extractor is distilled off, condensed, and recycled to die extractor. The disdllatkm provides both concentrated extract and fresh solvent fised. If the solid is moist and a nonaqueous solvent is used, water will codistill with the solvent. If the water-rich portions of the distillate are replaced with fresh solvent and not recycled to the bed, the solid will dry as leaching proceeds. The amount of solvent replacement required can be minimized if the solvoit and water form a heteroazeotrope. 

Further increase in nonideality and transition to heteroazeotropes makes it again possible to separate mixtures, not using just a distillation column, but a column with decanter complex. Cases e and / occur, but very seldom therefore, we will not consider them further. 

Thus, distillation is a two-phase (Uquid-vapor) multistage counterflow potentially equilibrium process (in some cases - in cases of heteroazeotropic distillation - three phases may occur on the trays two hquid phases and one vapor phase). 

Separation of Azeotropic Mixtures by Distillation Under Two Pressures or Heteroazeotropic and Extractive Distillation... 

On many occasions, general geometric theory of distillation allows development of flowsheets of multicomponent azeotropic mixture separation without using such special methods as distillation under two pressures or heteroazeotropic and extractive distillation with entrainers (i.e., with additional components injected into the unit). 

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