Relative volatility distillation process control

For control purposes, somewhat simplified mathematical models usually are adequate. In distillation, for instance, the Underwood-Fenske-Gilliland model with constant relative volatilities and a simplified enthalpy balance may be preferred to a full-fledged tray-by-tray calculation every time there is a perturbation. In control situations, the demand for speed of response may not be realizable with an overly elaborate mathematical system. Moreover, in practice not all disturbances are measurable, and the process characteristics are not known exactly. Accordingly feedforward control is supplemented in most instances with feedback. In a well-designed system (Shinskey, 1984, p. 186) typically 90%... 

Figure 6.6 shows the process for complete one-pass conversion. Pure component B is fed into the reactor on flow control. The concentration of B in the reactor, zb, is zero (or very small) because we assume complete one-pass conversion of B. A large recycle stream of component A is fed into the reactor. The reactor effluent is a mixture of unreacted component A and product C. This binary mixture is separated in a distillation column. We assume that the relative volatility of A is greater than that of C, so the distillate product is recycled back to the reactor. 

Most LWR fuel rod cladding is made of Zircaloy (and its derivatives), which is an alloy of primarily zirconium and tin. Other alloying elements include niobium, iron, chromium, and nickel. Zircaloy was chosen because it has a very low cross section for thermal neutrons. Naturally occurring zirconium contains about l%-5% hafnium. The hafnium must be removed because it has a very high thermal neutron cross section and is often used in making control rods for reactors. The separation process used in the United States is a liquid-liquid extraction process. It is based on the difference in solubility of the metal thiocyanates in methyl isobutyl ketone. In Europe, a process known as extractive distillation is used to purify zirconium. This method employs a separation solvent that interacts differently with the zirconium and hafnium, causing their relative volatilities to change. This enables them to be separated by a normal distillation process. The separated zirconium is then alloyed with the required constituents. 

---