Tower pressure relative volatility

While working in a plant, a troubleshooter read a pressure gauge daily for several weeks and only realized it was inaccurate when one day the blower was down. The gauge still read about normal operating pressure. Had this have been a distillation unit, it could have been more serious. In distillation service, pressure is a more important variable than in many other unit operations. Relative volatility is a function of pressure. Pressure, or more accurately delta-P, is the best indication of the tower hydraulics. 

The net effect of reducing the stripper pressure was to greatly reduce the amount of isobutane in the heavier normal butane bottoms product. Undoubtedly, most of the improvement in fractionation was due to enhanced tray efficiency, which resulted from suppressing tray deck leaking, or dumping. But there was a secondary benefit of reducing tower pressure increased relative volatility. 

The first two factors help make fractionation better, the last factor makes fractionation worse. How can an operator select the optimum tower pressure, to maximize the benefits of enhanced relative volatility, and reduced tray deck dumping, without unduly promoting jet flooding due to entrainment ... 

A separation process is sought that can satisfy both our present economic and enviromental constraints. It would also provide an alternative to present practice that relies on expensive azeotropic or extractive distillation processes used in the recovery of products from low relative volatility streams. As an example, virtually all industrial butadiene recovery processes now rely on extractive distillation using acetonitrile or other equivalent agent to enhance the relative volatility of the C4 components. The use of supercritical or near critical separation of these streams may satisfy these requirements provided certain pressure, temperature and recompression criteria can be met. Such a process would also reduce the need for a complex train of distillation towers. 

Operating the column at the minimum pressure minimizes the energy cost of separation. Towering this pressure increases the relative volatility of distillation components and thereby increases the capacity of the reboiler by reducing operating temperature, which also results in reduced fouling. Reducing pressure also affects other parameters, such as tray efficiencies and latent heats of vaporization. 

The bottoms pressure is usually selected to permit use of a readily available heating medium (steam or hot oil), as well as to stay below a temperature that could cause product degradation. In the ECH-EB system, degradation is not considered a problem, and column bottoms pressure is solely a function of the pressure drop across the tower internals. Because, as seen in step 1, relative volatility can vary appreciably with pressure, it is advantageous in this case to install low-pressure-drop, high-efficiency tower internals. 

As an example of such an operation, consider the process of Fig. 9.54, The separation of toluene (bp 110.8 C) from paraffin hydrocarbons of approximately the same molecular weight is either very difficult or impossible, due to low relative volatility or azeotropism, yet such a separation is necessary in the recovery of toluene from certain petroleum hydrocarbon mixtures. Using isooctane (bp = 99.3°C) as an example of a paraffin hydrocarbon, Fig. 9.54a shows that isooctane in this mixture is the more volatile, but the separation is obviously difficult. In the presence of phenol (bp = 181.4 C), however, the isooctane relative volatility increases, so that, with as much as 83 mole percent phenol in the liquid, the separation from toluene is relatively easy. A flowsheet for accomplishing this is shown in Fig. 9.546, where the binary mixture is introduced more or less centrally into the extractive distillation tower (1), and phenol as the solvent is introduced near the top so as to be present in high concentration upon most of the trays in the tower. Under these conditions isooctane is readily distilled as an overhead product, while toluene and phenol are removed as a residue. Although phenol is relatively high-boiling, its vapor pressure is nevertheless sufficient for its appearance in the overhead product to be prevented. The solvent-recovery section of the tower, which may be relatively short, serves to separate the phenol from the isooctane. The residue from the tower must be rectified in the auxiliary tower (2) to separate toluene from the phenol, which is recycled, but this is a relatively easy separation. In practice, the paraffin hydrocarbon is a mixture rather than the pure substance isooctane, but the principle of the operation remains the same. 

Relative volatility Divide the vapor pressure of a lighter material by the vapor pressure of a heavier material. The bigger the resulting number, the larger the relative volatility. It s easier to separate two components in a distillation tower if they have a larger relative volatility. 

Minimize tower pressure to maximize relative volatility and minimize reflux rates... 

At the beginning of the design, the engineer must first set the material balance for the tower. This is most often done on the basis of making a separation between two components whose volatilities, i.e., boiling point or vapor pressure characteristics relative to each other, make them adjacent in the listing of the components in the feed. These two components are called key components and are defined as follows. 

---