A Sample Distillation Process

For our example which deals with the azeotropic mixture formed between benzene and cyclohexane, we have chosen extractive distillation (one of the homogeneous azeotropic distillation methods). The reason of choosing this method is due to the availability of information regarding this separation technique and its tendency to operate more efficiently, i.e. in separating and recycling the separating agent. A brief discussion of the process is given below.

After the mixture exited as the bottom product of the flash unit, it contains mostly our desire product of cyclohexane and also a significant amount of unreacted benzene, which is to be recycled back to the reactor for further conversion. Our main goal now is to further separate the remaining components in the mixture. As cyclohexane and benzene have been encounter most of the remaining composition with the mole % of 44.86 and 54.848 respectively (Table 1), we will consider this to be a binary mixture in our further discussion.

From the process flowsheeting, we would like to operate the distillation column at the pressure of 150 kPa. At this condition, cyclohexane and benzene will have boiling points of 94.34 ^°C and 93.49 ^°C respectively (Figure 3). This is a typical case where conventional distillation would struggle to perform the separation of this type of close boiling mixture. Thus, a special type of distillation technique, i.e. extractive distillation has been chosen in order to purify the desire product, i.e. cyclohexane to our desired purity of 99.3%.

As can be shown from Figure 3, this binary composition will form a minimum boiling, homogeneous azeotrope at the temperature of 91^oC and the corresponding composition at this point will be 45.5 mole % for cyclohexane and 55.5 mole % for benzene (Figure 4).

Temperature	5.4279 °C
Pressure	1376.6 kPa
Molar Flow Rate (kg mol/h)	Â
Hydrogen	0.914
Cyclohexane	159.447
Benzene	194.944
n-Hexane	0.127
Total	355.430
Mole % Composition	Â
Hydrogen	0.256
Cyclohexane	44.861
Benzene	54.846
n-Hexane	0.036
Total	1.000

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Figure 3: T-xy Plot for Benzene and Cyclohexane	Figure 4: x-y Plot for Benzene and Cyclohexane at 150 kPa

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Solvent Selection for the Benzene-Cyclohexane Binary Mixture

In order to perform a successive extractive distillation, a solvent needs to be chosen to "break" the azeotrope that forms at the operating pressure of the distillation column. Recommended solvent for the benzene-cyclohaxane mixture from the literature^,,, is aniline, with a solvent to feed ratio (S/F) of 4, which will shift the azeotropic point toward the corner of the high-boiling component cyclohexane, and the equilibrium curve of the original components fall below the diagonal (Figure 5).

Figure 5: Elimination of Azeotropic

Point with the Addition of a Separating Agent

As was stated in the above section, the primary goal of solvent selection is to identify a group of feasible solvents to perform a good separation. The desired product, i.e. cyclohexane should have a purity of above 99% to meet the market standard. Aniline was the first solvent that had been put to the simulator to be tried out, as it is of the same homologous group as benzene. As can be shown from the result in Table 2, this solvent will produce the desire production rate of 150 with the solvent flow rate of 3500, i.e. a S/F ratio of 9.85. However, the product purity can only reach 70.08% and this does not meet our product specification. As a result, other solvent may have to be researched to perform the desire separation.

We will have to perform the solvent selection criteria as stated in the preceding section. At the column pressure of 150 kPa, cyclohexane and benzene boil at 94.34 ^°C and 93.49 ^°C respectively and form a minimum-boiling azeotrope at 91 ^°C. The natural volatility of the system is benzene > cyclohexane, so the favored solvents most likely will be those that cause the benzene to be recovered in the distillate. However, in order to get a better quality of product, we would like to recover cyclohexane as the distillate rather than from the bottom stream. Thus, solvent to be chosen should give positive deviations from Raoult's law for cyclohexane and negative (or zero) deviation for benzene.

Feed Stream Molar Flow Rate (kg mol/h)	Â
Hydrogen	 0.914
Cyclohexane	 159.447
Benzene	 194.944
n-Hexane	 0.127
Total	 355.43
Solvent Stream (Aniline) molar flow rate (kg mol/h)	Â 3500
Solvent/Feed (S/F) ratio	9.85
Distillate Product Molar Flow Rate (kg mol/hr)	Â
Hydrogen	 0.914
Cyclohexane	 150
Benzene	 62.956
n-Hexane	 0.127
Aninline	 0.058
Total	 214.05
Mole % Composition in the Distillate Product	Â
Hydrogen	 0.427
Cyclohexane	 70.08
Benzene	 29.41
n-Hexane	 0.059
Aniline	 0.027
 Total	 1.000

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Â	Â	Solvent Class
Solute Class	Group	1	2	3	4	5	6	7	8	9	10	11	12
11	Aromatic, olefin, halogen aromatic, multihola paraffine without active H, monohalo paraffine	+	+	+	0	+	0	0	-	0	+	0	0
12	Paraffin, carbon disulfide	+	+	+	+	+	0	+	+	+	+	0	+
Solvent/solute class	Group	Solvent/solute class						Group
4	Active -H in multihalo paraffin	8						Primanry amine, ammonia, amide with 2H or N Ether, oxide, sulfoxide
7	Secamine	9

Turning to the Robbins Chart (Table 3), we note that solvents that may cause the positive deviation for cyclohexane (Class 12) and negative (or zero) to benzene (Class 11) came from the groups of 4, 7, 8 and 9, which consist of polyol, amine and ether. We further consider the solubility, the hydrogen bonding effect, and also the homologous characteristic of the solvent with the corresponding components in the feed mixture. As few candidate solvents that had been put to the computer simulation, included phenol (homologous to benzene), 1,2-benzenediol (homologous to benzene, with -OH group that will produce hydrogen bonding), 1,3-butanediol (with -OH group that will produce hydrogen bonding), and also 1,2-propanediol (same characteristic as with 1,3-butanediol). 1,2-propanediol (often known as propylene glycol), seem to give the most promising results compared to the other solvents. This result may be caused from the high solubility of benzene in this solvent and the hydrogen bonding that were formed between the two constituents. Simulation result of this solvent can be view in Table 4.