Energy requirements for distillation

The "energy ratio in/out" column is the ratio of energy required for distillation to energy produced by burning the product. 

We win illustrate first the thermodynamic minimum energy requirement according to equation (10.1.4) for an example of distillation of a binary mixture of styrene (46.5 mol%) and ethylbenzene (53.5 mol%) producing two pure products at 110 °C (Humphrey and Keller, 1997). Then we will calculate the minimum energy requirement for distillation based on the minimum heat required. We consider evaporation briefly at the end of this section. 

Separation of alcohols, such as ethanol and butanol, from the fermentation broth is traditionally done by distillation. The higher the alcohol concentration in the fermentation broth, the lower the energy required for distillation. For ethanol fermentation, the broth usually contains 10-15% (w/w) ethanol. After distillation, ethanol concentration in the distillate is about 90% (w/w). The distillation process will not yield more than 93% (w/w) ethanol, which is the azeotropic mixture of ethanol-water. Azeotropic mixtures cannot be separated by distillation because the compositions of ethanol in the vapor and liquid phases are the same. Therefore, azeotropic distillation with benzene or dehydration with molecular sieves is usually used to remove the remaining water and produce fuel grade ethanol (99.9 wt-%). 

Most crude contains appreciable levels of salts (20-500 ppm) [9]. It is critical to remove these salts to prevent fouling and scaling of heat transfer surfaces. Loss of heat transfer efficiency can significantly increase the energy required for distillation. 

The suitabiHty and economics of a distillation separation depend on such factors as favorable vapor—Hquid equiHbria, feed composition, number of components to be separated, product purity requirements, the absolute pressure of the distillation, heat sensitivity, corrosivity, and continuous vs batch requirements. Distillation is somewhat energy-inefficient because in the usual case heat added at the base of the column is largely rejected overhead to an ambient sink. However, the source of energy for distillations is often low pressure steam which characteristically is in long supply and thus relatively inexpensive. Also, schemes have been devised for lowering the energy requirements of distillation and are described in many pubHcations (87). 

Reducing reflux saves reboiler duty. Also, the lower pressure will reduce the tower-bottom temperature, and this also cuts the reboiler energy requirement. For most distillation towers, the energy cost of the reboiler duty is the main component of the total operating cost to run the tower. 

A thermodynamic analysis of the energy requirements of desalting processes is presented, to clarify the conditions under which such calculations are valid. The effects of departure from isothermal operation, finite product recovery, differential as opposed to single-stage operation, and salt concentration in the feed are examined. A comparison shows that there is essentially no difference between the energy requirements for a distillation and a freezing process. The minimum heat consumption and maximum number of efFects for a multiple-effect evaporation plant are calculated. The above analysis leads to the conclusion that efficiencies in the range 10 to 20% will be very difficult to achieve. 

The remaining liquid flowing from the condenser (about 1/3 to 1/4 of the total) is the finished product, ready for whatever use is intended. The ratio of amount of alcohol returned to the column to amount collected as product is called the "reflux ratio." This ratio controls both product purity and amount of energy required for the distillation. The higher the reflux ratio, the purer the alcohol product and the more energy that is required for distillation. 

The energy required for the removal of the four valence electrons of titanium is so high that the Ti ion does not exist as such nor do the Zr and ions. Tltanium(IV) compounds are, in general, covalent and bear some resemblance to the corresponding compounds of Si, Ge, Sn and Pb, especially Sn (partly because of the similarities between the ionic and covalent radii of Ti and Sn). For instance Ti02 (rutile) is isomorphous with Sn02 (cassiterite), and both become yellow on heating both TiCU and SnCU are colourless, distillable liquids which are... 

The distillation and rectification is conducted in columns with a variable number of trays, the construction and design of which vary significantly. The setup of the distillation unit is directly linked to the quality requirements for the ethanol produced. The distillation yields a raw alcohol with an alcohol content of approximately 85-87%. The rectification is necessary to increase the alcohol content and to remove so-called fusel oils, which are C3-C5 alcohols. In modern distillation units, these two basic steps are integrated to optimize the energy requirement for the process. 

---