Advantages and Principle of ED Technology

Although liquid-liquid extraction (LLE) technologies have dominated the industrial processes for purifying benzene, toluene, xylene (BTX) aromatics from refining and petrochemical streams, ED technologies have gained ground quickly since the 1980s for more recent grassroots plant installations. In comparison to LLE, ED has the following advantages:

1. Lower capital costs. ED requires two major process units (ED tower and solvent stripper), while

Figure 1 Configuration of an ED process.

the popular LLE, using sulfolane as the solvent, requires four major process units, including LLE tower, extractive stripper, solvent recovery column, and raffinate wash tower (see Figure 2).

2. Higher operational flexibility. LLE uses only solvent selectivity (polarity) for separation, while ED uses both solvent selectivity and boiling point for separation, so it has one extra dimension for operational flexibility.

3. Less physical property restrictions. Interfacial tension and density difference between the liquid phases are important concerns for LLE, but not for ED.

The principle of ED for aromatic purification was studied as early as 1944. One example was the recovery of toluene from paraffins using phenol as the selective solvent. The effect of phenol on a paraffin-toluene mixture is plotted in liquid-vapour

Figure 2 Configuration of liquid-liquid extraction using sulfolane for aromatic recovery.
Figure 3 Effect of phenol on the vapour-liquid equilibrium of paraffin and toulene. Numbers on curves refer to mol% solvent in liquid.

diagrams as shown in Figure 3, in which the paraffin is considered as a hypothetical octane having the same boiling point as toluene. In the absence of phenol, there exists an azeotrope of paraffin and toluene. However, at 50mol% phenol, the azeotrope is destroyed and the mixture is easily separated; at 100mol% phenol, the separation between paraffin and toluene becomes very easy. Figure 4 illustrates the effect of phenol on the change in relative volatility between paraffin and toluene. Phenol causes an increase of activity coefficient for both paraffin and toluene, but the activity coefficient of the paraffin increases to a greater extent than that of toluene. Therefore, the relative volatility of paraffin over toluene can be increased from 1.0 (no separation) to 3.7 (easy separation) at near zero hydrocarbon concentration in phenol (infinite dilution).

Figure 4 Effect of phenol on the activity coefficient of paraffin and toluene.

The vapour-liquid equilibria of the paraffin-toluene-phenol system were applied to test a commercial ED tower for toluene purification. As shown in Figure 5, the McCabe-Thiele diagram, drawn on a phenol-free basis, was used to carry out the theoretical calculations from tray to tray in the ED tower.

The calculated results were then compared with the actual results generated from a commercial ED tower with 2.1m diameter and 65 trays. The hydrocarbon feed tray and the solvent feed tray are located at trays 19 and 39 (counted from the bottom of the tower), respectively. The tower was operated at a solvent-to-feed ratio (S/F) of 2.5, a reflux-to-overhead ratio (R/D) of 2.75, and reboiler temperature at 170°C under 1.3 atm bottom pressure. On the basis of the charge, overhead and bottoms analyses, tray-to-tray calculations were made.

Figure 6 shows the calculated concentration profiles for each component plotted against theoretical tray number. It also shows the plot of the tray analyses against actual tray number. The overall efficiencies calculated over small sections of the tower are given in Table 1. The average of the overall tray efficiencies throughout the tower is about 50%.

Based on the above principle, much more rigorous algorithms for tray-to-tray calculation of ED towers for multicomponent systems have been developed in

Figure 5 McCabe-Thiele diagram for paraffin and toulene separation in the presence of phenol. Part (B) is an enlargement of part of the diagram in (A).

Figure 4 Effect of phenol on the activity coefficient of paraffin and toluene.

Figure 5 McCabe-Thiele diagram for paraffin and toulene separation in the presence of phenol. Part (B) is an enlargement of part of the diagram in (A).

Figure 6 Calculated versus actual concentration profile of the componenets in an ED tower. Key: ▲, toluene; x phenol;-, calculated values.

methylcyclohexane;

recent years, with the help of advanced vapour-liquid equilibrium theories and high-speed computers.

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