Vol.2 No.1 2009

60/88

Research paper : Innovation in distillation processes (M. Nakaiwa et al.)−57−Synthesiology - English edition Vol.2 No.1 (2009) including alcohol and acid productions, and as an important technique during the Italian Renaissance for alchemy (not the occult kind, but utilizing the laws of nature). During the late 18th century to early 19th century, a discovery was made that heat is a form of energy, and the concept of evaporative latent heat was introduced, leading to an understanding that the thermal energy contained within a vapor can be used for heating liquids. In the late 19th century, the foundations of the continuous distillation process were established through various experimentations, stimulated by a need of refining crude oil for use as automobile fuel [7][8] . This development of the distillation process from simple to continuous can be regarded as a realization of PI, where “simplification” of the operation as well as “increased efficiency” with high performance and continuous mass processing were achieved. Evaporative latent heat, a major scientific discovery of that time, brought revolutionary changes to the distillation process.Fig. 2 shows a schematic diagram of a typical continuous distillation column. The parts of the column above and below the feed position are called the rectifying section and the stripping section, respectively. In the rectifying section, the concentration of low boiling point component is higher than in the feed material, while in the stripping section the concentration of the high BP component is higher than in the feed. The interior of the column is designed such that the gas-phase mixture rising from the lower part and the liquid-phase mixture descending from the upper level come into contact. This is accomplished through the shelf-like constructions known as plates or trays with or without the various shapes of packings. Due to this mutual contact, the phase transition occurs via the transfer of evaporative latent heat between the gas and liquid-phase mixtures. In the vapor phase thus generated, there exists a higher concentration of low BP component while in the liquid phase a higher concentration of high BP component. Therefore, the low and high BP components can be concentrated toward the both ends of the column, respectively. Based on this principle, the separation of liquid mixture in a distillation column inevitably requires that the liquid solution should be converted into vapor with the supply of heat from a reboiler at the bottom of the column. Also, to allow the contact between liquid and vapor for mass transfer throughout the column, the latent heat of condensation must be removed from the vapor to reconvert it back to the liquid in the condenser at the top of the column.In a continuous distillation column process, cooling at the top of column and heating at the bottom are carried out simultaneously to achieve gas-liquid contact throughout the column. According to thermodynamic analysis, the most efficient operation of distillation can be theoretically done when the continuous heating in the stripping section and the continuous cooling in the rectifying section are conducted along the bottom-to-top direction of column, as shown in Fig. 3[9][10]. This is called an operation of reversible distillation. Specifically, using a distillation column with infinite number of plates, infinitely small amount of heats are used for cooling in each plate of the rectifying section and for heating in each plate of the stripping section, respectively. This is the only thermodynamically ideal and the most efficient operation of continuous distillation, no matter what energy-saving strategies are applied to distillation columns, or whether new energy-saving technologies are developed. As the energy-saving technology becomes more advanced, the operation should approach the reversible distillation. In other words, the effort of energy saving in the distillation process is described as a “detuning” from the reversible distillation process with minimal performance deterioration while achieving economical viability. However, bringing the operation of forced heat transfer such as heating and cooling into mass transfer that is the essence of distillation, would cause significant alterations to the process, and therefore careful consideration of their effects is necessary in developing the desired process. The next step is to consider Product(high boiling-point component)ReboilerHeatingProduct (low boiling-point component)CoolingCondenserStripping sectionRectifying sectionSolution mixture (feed)HeatingCoolingStripping sectionProduct from bottomProduct from top (distillate)FeedRectifying sectionFig. 2 Conventional continuous distillation column.Fig. 3 Reversible distillation operation.

※このページを正しく表示するにはFlashPlayer9以上が必要です