Energy distillation

The partial oxidation of natural gas has been considered as an alternative to steam reforming for many years. While natural gas partial oxidation was practiced commercially for ammonia manufacture over 30 years ago (ref. 2), it s use for methanol synthesis remains to be demonstrated commercially. Autothermic reforming (ref. 3), and various other improvements, such as low energy distillation (ref. 4), demonstrate that methanol production can be improved in the short term. 

The ORNL report highlighted the following separation problems as being High-Energy Distillation Processes with Potential for Replacement with Lower-Energy Alternatives ... 

Huang, Y., Baker, R. W., Vane, L. M. (2010). Low-energy distillation-membrane separation process. Industrial and Engineering Chemistry Research, 49, 3760—3768. 

Another variable that needs to be set for distillation is refiux ratio. For a stand-alone distillation column, there is a capital-energy tradeoff, as illustrated in Fig. 3.7. As the refiux ratio is increased from its minimum, the capital cost decreases initially as the number of plates reduces from infinity, but the utility costs increase as more reboiling and condensation are required (see Fig. 3.7). If the capital... 

Figure 3.7 The capital-energy tradeoff for stand-alone distillation columns.

Sucksmith, I., Extractive Distillation Saves Energy, Chem. Engg., 88 185, June 28, 91, 1982. 

Kaibel, G., Distillation Column Arrangements with Low Energy Consumption, IChemE Symp. Ser., 109 43, 1988. 

Let us now consider a few examples for the use of this simple representation. A grand composite curve is shown in Fig. 14.2. The distillation column reboiler and condenser duties are shown separately and are matched against it. Neither of the distillation columns in Fig. 14.2 fits. The column in Fig. 14.2a is clearly across the pinch. The distillation column in Fig. 14.26 does not fit, despite the fact that both reboiler and condenser temperatures are above the pinch. Strictly speaking, it is not appropriately placed, and yet some energy can be saved. By contrast, the distillation shown in Fig. 14.3a fits. The reboiler duty can be supplied by the hot utility. The condenser duty must be integrated with the rest of the process. Another example is shown in Fig. 14.36. This distillation also fits. The reboiler duty must be supplied by integration with the process. Part of the condenser duty must be integrated, but the remainder of the condenser duty can be rejected to the cold utility. 

Various heat pumping schemes have been proposed as methods for saving energy in distillation. Of these schemes, use of the column overhead vapor as the heat pumping fluid is usually the most economically attractive. This is the vapor recompression scheme shown in outline in Fig. 14.6. 

Distillation capital costs. The classic optimization in distillation is to tradeoff capital cost of the column against energy cost for the distillation, as shown in Fig. 3.7. This wpuld be carried out with distillation columns operating on utilities and not integrated with the rest of the process. Typically, the optimal ratio of actual to minimum reflux ratio lies in the range 1.05 to 1.1. Practical considerations often prevent a ratio of less than 1.1 being used, as discussed in Chap. 3. 

If complex distillation columns are being considered, then these also can bring about significant reductions in capital cost. The dividing-wall column shown in Fig. 5.17 not only requires typically 20 to 30 percent less energy than a conventional arrangement but also can be typically 30 percent lower in capital cost than a conventional two-column arrangement. ... 

Starting irom the original flowsheet, LinnhofT and Parker have shown that it is possible by a combination of distillation modifications and network improvements to reduce the energy consumption of this process by approximately 60 percent. 

Once the distillation is integrated, then driving forces between the composite curves become smaller. This in turn means the capital/energy tradeofiF for the heat exchanger network should be adjusted accordingly. 

Ethylbenzene Separation. Ethylbenzene [100-41-4] which is primarily used in the production of styrene, is difficult to separate from mixed Cg aromatics by fractionation. A column of about 350 trays operated at a refluxTeed ratio of 20 is required. No commercial adsorptive unit to accomplish this separation has yet been installed, but the operation has been performed successhiUy in pilot plants (see Table 5). About 99% of the ethylbenzene in the feed was recovered at a purity of 99.7%. This operation, the UOP Ebex process, requires about 40% of the energy that is required by fractional distillation. 

With increasing energy costs, maximum methanol conversion is desirable, eliminating the need for the energy-intensive distillation for methanol... 

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