Handling Two Liquid Phases in ED Towers

One of the challenges of ED technology for aromatics purification is the handling of the possible formation of two liquid phases in the upper portion of the ED tower where nonaromatics are concentrated. The occurrence of a second liquid phase is due to the fact that the nonaromatics, such as paraffins, naphthenes and olefins, have significantly lower solubility in the polar solvent than aromatics.

One way to solve the problem of two liquid phases in the ED tower is to select a polar solvent that has

Table 1 Tray efficiency of ED tower for toulene purification

Section of

Theoretical

Actual

Overall

tower trays

trays

trays

efficiency

(%)

Below phenol feed tray

1-3

1.8

3

60.0

4-7

2.7

4

67.5

8-11

2.1

4

52.5

12-15

1.5

4

37.5

15-18

1.8

3

60.0

23-27

2.1

4

52.5

27-30

2.5

4

62.5

31 -34

2.65

4

66.0

35-39

2.35

5

47.0

Above phenol feed tray

43-65

10.8

23

47.0

49-65

8.6

17

50.7

57-65

4.5

9

50.0

61-65

2.8

5

56.0

enough solvency to dissolve both aromatics and nonaromatics in the mixture under process condition. In general, however, solvents with a high selectivity for compounds to be separated will have a reduced solvency (capacity), and vice versa. The selectivity versus solvency of the common commercial solvents for aromatic extraction is shown in Figure 7. Therefore, in order to eliminate two liquid phases, one may have to compromise the solvent selectivity, sometimes to a great extent.

A better way is to cope with two liquid phases in the ED tower, without sacrificing the solvent selectivity, for the following reasons:

1. Two liquid phases normally reduce the solvent selectivity in the three-phase equilibrium (vapour-liquid-liquid) condition in the ED tower. However, this can be compensated by intrinsic selectivity of a highly selective solvent. For example, the performance of sulfolane was

Figure 7 Selectivity versus solvency (solubility) of the common commercial solvents. DEG, diethylene glycol; TEG, triethylene glycol; DMS, dimethyl sulfoxide; NMP, N-methyl pyrrolidone.

compared with those of N-formyl morpholine (NFM) and N-methyl pyrrolidone (NMP). The ability of these solvents to enhance the relative volatility of «-heptane over benzene (an aromatic and nonaromatic separation) in a one-stage equilibrium cell was determined. Table 2 shows that, although two liquid phases were observed using sulfolane as the solvent, sulfolane still gave a better performance than the other solvents where a single liquid phase existed in the mixture. 2. Two liquid phases have no ill effects on the efficiency of small tray or packed towers with diameter from 0.08 m to 0.46 m. However, in larger towers, the heavy liquid phase tends to accumulate on the tray if the liquid phases are not well mixed. This problem can be eliminated by tray designs promoting gas agitation, forcing the two liquid phases to behave as a homogeneous liquid. For larger packed columns, liquid-liquid redistributors are specially designed to allow separate distribution of the two liquid phases.

Computer simulations have been developed which are capable of accurately predicting the development of two liquid phases in the ED tower. In one approach, the simulation algorithm starts from linearized pressure, temperature and concentration profiles and feed conditions given by the program operator. New estimates of composition are solved, using the material balance and equilibrium relationship for each tray. Then the equilibrium constants are re-estimated and a new temperature gradient is established to calculate a tray-by-tray energy balance. Accumulated errors are calculated for the energy, material and equilibrium balances. Appropriate column operation restraints are factored in at this point. A correction factor is found for the temperature, rate profiles, and liquid composition profile by inverting the accumulated error matrix. These correction factors are used to form new iterative estimates of composition to start the process again until the correction factors are small enough to call the components converged.

Table 2 ED solvent screening for aromatics recovery

Solvent

Relative volatility

Number of

(n-heptane/benzene)

liquid phases

Sulfolane

3.9

2

DMSO

3.6

1

NFM

3.1

1

NMP

2.6

1

Feed: 20% n-heptane and 80% benzene; pressure 1 atm; DMSO, dimethyl sulfoxide; NFM, N-formyl morpholine; NMP, N-methyl pyrrolidone.

Feed: 20% n-heptane and 80% benzene; pressure 1 atm; DMSO, dimethyl sulfoxide; NFM, N-formyl morpholine; NMP, N-methyl pyrrolidone.

Multicomponent vapour-liquid and liquid-liquid equilibria solutions are required for the algorithm. Two activity coefficient models, NRTL (nonrandom two liquids) and UNIQUAC (universal quasichemi-cal), are readily extendable to multicomponent systems and capable of such solutions. Experimental activity coefficients, y, at infinite dilution are used for calculating binary parameters for the NRTL equation. These parameters are then tested using experimental liquid-liquid ternary data, experimental vapour-liquid equilibrium data, and data from pilot plant or commercial plant. The NRTL equation is used in the algorithm to calculate activity coefficients and is given in the following equations:

and where Gij, yij, Sij and are empirical constants, yi is activity coefficient, R is the gas universal constant, T is absolute temperature, and Xi is liquid phase mole fraction of component i.

The simulation uses a Newton-Raphson-based flash algorithm that checks for two liquid phases by checking Gibbs free energies for components the program operator lists as possible second liquid phase formers. If two liquid phases are indeed present, regular solution theory provides a method of combining the liquid-phase activity coefficients.

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