Results for Temperature-Dependent Relative Volatilities

In the previous section, the optimum economic steady-state designs of reactive distillation columns were quantitatively compared with conventional multiunit systems for a wide range of chemical equilibrium constants. Relative volatilities (a = 2) were assumed constant. Reactive distillation was shown to be much less expensive than the conventional process. In this section we explore how temperature-dependent relative volatilities affect the designs of these two systems. 

A fundamental difference between the two flowsheets is the ability in the conventional process to adjust reactor temperature and distillation column temperamres completely independently, which is not possible in the reactive distillation process. In the conventional system, the reactor temperature can be set at an optimum value and distillation temperatures can be independently set at their optimum values by adjusting column pressures. In reactive distillation, these temperatures are not independent because only one pressure can be set in the vessel. Therefore, the design of a reactive distillation requires a tradeoff between temperatures conducive for reaction (kinetics and equilibrium constants) and temperatures favorable for vapor-liquid separation. The temperature dependency of the relative volatilities will illustrate this important difference between the two processes. 

ECONOMIC COMPARISON OF REACTIVE DISTILLATION WITH A CONVENTIONAL PROCESS 

Results presented below show that, when relative volatilities are temperature dependent and decrease significandy as temperatures approach those required for reasonable reaction rates, the reactive distillation process becomes more expensive than the conventional flowsheet. A single value of the chemical equUibrium constant is used in this section ( eq)366 = 2. 

For an ideal mixture, Raoult s law relates the partial pressure of a component in the vapor phase to its hquid-phase composition and vapor pressure. The vapor pressure of a component is a function of temperature and can be calculated from a two-parameter Antoine equation over a limited temperature range  

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Table 3.7 provides the optimum design results for the conventional process over a range of temperature-dependent relative volatilities. Because the column relative volatilities are only slightly lower than those of the constant relative volatility case, we assume that the three design optimization variables are the same as in the constant relative volatility case. The slightly lower relative volatilities produce small increases in the number of trays, the reflux ratios, and the vapor boilups in both columns. There is a small increase in the recycle flowrate (D2). 

Reactive Distillation. Figure 3.18 and Table 3.8 give optimum design results for the reactive distillation process for a range of temperature-dependent relative volatilities. As the a39o parameter decreases, the optimum pressure decreases. This occurs because lower pressure helps the vapor-liquid equilibrium because it lowers temperatures and hence increases relative volatilities. However, a lower temperature is unfavorable for reaction because the reaction rates are too small. The result is a rapid increase in the required number of reactive trays. 

Results show that reactive distillation is significantly less expensive (by a factor of up to 3) than the conventional process for all values of the chemical equilibrium constant. However, when the relative volatilities show strong temperature dependence, reactive distillation becomes more expensive because of the mismatch between the temperatures that are favorable for reaction and the temperatures that are favorable for vapor-liquid equilibiium. 

Heat treatment of the S-type fluoride in a fluorine atmosphere Based on the results above mentioned, Fujimoto et al. developed a new fluorination procedure in order to prepare the perfluorinated pitch, and obtained two types of other fluorinated pitches [23,24], The new process is by the heat treatment at 200-400°C of S-type of fluorinated pitch prepared at relatively low temperature in a fluorine atmosphere. They firstly fluorinated the mesophase pitch at 70°C for 10 h (first step for the preparation of S-type fluorinated pitch) and then heated up to a selected temperature between 200°C and 400°C, and maintained this temperature for 12 h (second step for the heat-treatment of fluorinated pitch). Thus, they obtained two kinds of fluorocarbons, a transparent resin (R-type) and a liquid (L-type). L-type is a viscous fluid containing some volatile materials and the viscosity gradually becomes higher when it is kept for a few weeks in an air atmosphere even at ambient temperature. They reported that the R-type was obtained in the nickel boat in the heating zone and L-type at the bottom of the vertical reaction vessel which was cooled down by the water. Therefore, it is likely that the liquid fluorocarbon is formed by the vaporization of some component contained in the S-type fluoride or decomposition reaction during the heat treatment of the S-type fluoride. The yields of these compounds depends on the heat treatment temperature. In Fig. 3, the yields of the R-type and L-type fluorocarbons are plotted as a function of the heat treatment temperature of the S-type fluoride. The yield of the former decreases with increase of the heat treatment temperature and finally, at 400 C, it can not be obtained at all. On the other hand, the yield of the latter increases with increase of temperature and it is selectively obtained at 400°C. 

The sample (ca. 100 pg) can either be extracted in solvent before presentation to the instrument or can be introduced without preparation. Samples are gradually heated up to around 750°C and the resulting volatile products are carried onto the gas chromatography column by an inert carrier gas. Separation occurs as the components partition themselves between the stationary phase on the inner wall of the column and the mobile phase (the carrier gas). The time taken for a given molecule to traverse the entire length of the column is known as the retention time. The retention time is a function of the chemical structure of the component, the column type and the temperature profile it has been subjected to. It depends on the relative affinity of the compound for the stationary and mobile phases. 

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