Fractional distillation column design

Fig. 9.22. Low-temperature fractional-distillation column, ( ) Norman s modification of a design by Dobson and Schaeffer, (b) An alternate column design (I. Dobson, Ph.D. thesis. Indiana University, 1967).

Since few liquid mixtures are ideal, vapor-liquid equilibrium calculations are somewhat more complicated than for the cases in the previous section, and the phase diagrams for nonideal systems can be more structured than Figs. 10.1-1 to 10.1-6. These complications arise from the (nonlinear) composition dependence of the species activity coefficients. For example, as a result of the composition dependence of yt, the vapor-liquid equilibrium pressure in a fixed-temperature experiment will no longer be a linear function of mole fraction, so that no.nideal solutions exhibit deviations from Raoult s law. However, all the calculational methods discussed in the previous section for ideal mixtures, including distillation column design, can be used for nonideal mix-, tures, as long as the composition dependence of the activity coefficients is taken into account. 

A commercial simulation program, HYSYS, was used for simulation of the fractional distillation column. The flow diagram for acetic acid extraction process is shown in Figure 18.7. The acetic acid concentration used in our design are 20 and 80 wt%. By removing water from the product flow, the acetic acid concentration the top of the distillation column is 38.6 wt%. The distillation column was optimized at 8 plates with feed entering at plate 4. 

Example 12-1 Design a continuous fractionating distillation column to separate 3.78 kg/sec of 40 percent benzene and 60 percent toluene into an overhead product containing 97 mass percent benzene and a bottom product containing 98 mass percent toluene. Use a reflux ratio of 3.5 mole to 1 mole of product. Latent molal heat of both benzene and toluene is about 357.1 kJ/kg. 

The simplest unit employing vacuum fractionation is that designed by Canadian Badger for Dominion Tar and Chemical Company (now Rttgers VFT Inc.) at Hamilton, Ontario (13). In this plant, the tar is dehydrated in the usual manner by heat exchange and injection into a dehydrator. The dry tar is then heated under pressure in an oil-fired hehcal-tube heater and injected directly into the vacuum fractionating column from which a benzole fraction, overhead fraction, various oil fractions as side streams, and a pitch base product are taken. Some alterations were made to the plant in 1991, which allows some pitch properties to be controlled because pitch is the only product the distillate oils are used as fuel. 

The design methods given in this section can be used for reboilers and vaporisers. Reboilers are used with distillation columns to vaporise a fraction of the bottom product whereas in a vaporiser essentially all the feed is vaporised. 

While the limiting phenomenon of upper limit flooding in a vertical pipe is similar to ultimate capacity in distillation, there is a distinct difference. Upper limit in a vertical pipe applies to a design where a conscious effort should be made to minimize gas-liquid contact. Carried to extremes, it would involve separate tubes for liquid flowing down and vapor going up. In contrast, ultimate capacity in a distillation column corresponds to the condition where effective mass transfer disappears due to high entrainment. One could force more vapor up through the contactor, but fractionation would be poor. 

Accumulators are not separators. In one application, an acciunulator placed after a total condenser provides reflux to a fractionator and prevents column fluctuations in flow rate from affecting downstream equipment. In this application the accumulator is called a reflux drum. A reflux drum is shown in Figure 6.3. Liquid from a condenser accumulates in the drum before being split into reflux and product streams. At the top of the drum is a vent to exhaust noncondensable gases that may enter the distillation column. The liquid flows out of the drum into a pump. To prevent gases from entering the pump, the drum is designed with a vortex breaker at the exit line. 

If one makes this choice, it is necessary to resort to cracking at a high temperature with the recycling of the fractions insufficiently cracked after a first process, and to accept increasing formation of coke that may reach 5% or more. However, the required equipment will never require the fractionation of the vapours and the condensation in more or less heavy fractions. It will therefore be possible to get rid of a costly distillation column in the design. The gas obtained must be sent to a gasometer of several cubic meters volume from where it is taken up by a compressor to be stored. 

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