Distillation process performance

Volcanic and Other Surface Deposits. Sulfur is recovered from volcanic and other surface deposits by a number of different processes, including distillation, flotation, autoclaving, filtration, solvent extraction, or a combination of several of these processes. The Japanese sulfur deposits are reached by tunnel, and mining is done by the room-and-pillar, chamber-and-pillar with filling, and cut-and-fill systems. Sulfur was historically extracted from the ore by a distillation process performed in rows of cast-iron pots, each containing about 180 kg of ore. Each row of pots is connected to a condensation chamber outside the furnace. A short length of pipe connects each pot with a condenser. Brick flues connect combustion gases under the pots. Sulfur vapor flows from the pots to the condensation chamber where the liquid sulfur is collected. The Japanese ore contains 25—35 wt % sulfur. This method has been superseded by other sources of sulfur production. 

The cmde dimethyl terephthalate is recovered and purified by distillation in most processes. Although distillation (qv) is generally a powerful separation technique, the mode of production of the terephthaHc acid determines its impurity content, which in turn may make purification by distillation difficult. Processes resulting in the alteration of the impurities by catalytic treatment have been developed so that distillation can perform the necessary purification. 

The design of any of the distillation processes discussed requires choosing an operating pressure, bottoms temperature, reflux condenser temperature and number of trays. This is normally done using any one of several commercially available process simulation programs which can perform the iterative calculations discussed in Chapter 6. 

Plastic membrane This is done by the use of a water permeable plastic membrane held deep enough under the sea so that the hydrostatic pressure is greater than the osmotic pressure of the seawater. The water distills out of the solution through the membrane and is pumped to the surface. Large areas of the membranes, mechanically supported to withstand the very high pressures are essential to make the process perform rapidly for the most economical production. 

Polymaleic acid (PMA). The use of chemicals based on PMA and some derivatives has become standard practice for very brackish waters and seawater distillation processes around the world, where the TDS may reach 50,000 ppm TDS, or where total hardness levels exceed 500 to 1,000 ppm CaC03. Its use in RO systems is growing. However, PMA has limited dispersing properties and may need to be formulated with a dispersant chemical to provide satisfactory performance with some RO designs. It is claimed that PMA is also a successful silica deposit control agent and therefore may be incorporated into formulations where this is a problem. 

The distillation process separates HNO3 from the U-bearlng HNO3-H2SO4. Similarly, HF can be recovered with the HNO3 if it is present in the initial solution. Because nitrates are mobile in the environment, their presence in a final waste form requires that more stringent performance criteria be used to ensure their immobilization. It is desirable to reduce both the volume and quantity of nitrates in the final waste stream discharged from the reclamation process. 

An analysis of the solar distillation process shows that performance is remarkably insensitive to all variables except solar radiation rate. As atmospheric temperature changes, basin and cover temperatures move similarly, so that their difference remains... 

Figure 13.15 Flow scheme and performance data for a membrane distillation process for the production of water from salt solutions [31]. Feed salt solution is heated to 100 °C and passed counter-current to cool distillate that enters at 42 °C. The distillate product is almost salt-free as shown by its low conductivity. The distillate flux is almost constant up to salt concentrations as high as 20 % NaCI. Reprinted from J. Membr. Sci. 39, K. Schneider, W. Holz, R. Wollbeck and S. Ripperger, Membranes and Modules for Transmembrane Distillation, p. 25. Copyright 1988, with permission from Elsevier...

Figures 8-10 show the curves of tocopherol concentration in the residue (% w/w) vs the percentage of the distance on the evaporator (from the feed point) for feed flow rate ranging from 0.5 to 1.0 kg/h for the falling film molecular distillation unit. The initial tocopherol concentration was 8.50% (w/w). For a feed flow rate of 0.5 kg/h (Fig. 8), it can be observed that at the end of the distillation, the tocopherol concentration in the residue will be higher, at 150°C (about 15% [w/w]). At 160°C, at 80% of the distillation, the tocopherol concentration reaches a maximum and then decreases, because the tocopherols are already recovered in the vapor phase. Figures 8-10 show that by increasing the feed flow rate at the same temperature (160°C), the tocopherol concentration can increase until it doubles the initial concentration (for a feed flow rate of 0.6 kg/h). From this point, it decreases, requiring an increase in the temperature to concentrate more (for a feed flow rate of 1.0 kg/h at 170°C). For all feed flow rates (Figs. 8-10), at 180°C, practically all the tocopherols are found in the vapor phase. With this study, it is possible to observe which temperature is the best in order to recover the fatty acids (first step = 125°C) and, then, recover the tocopherols in the vapor phase (distillate) and the phytosterols in the liquid phase (residue) (second step = 170°C). At the lowest temperature (120°C) the tocopherol recovery was minimum (about 5%). By increasing the feed flow rate from 0.5 to 1.0 kg/h (100%), the quantity of tocopherol in the residue at 170°C, e.g., increases, which means that the process performance has decreased.

In the first case, product purities are controlled indirectly by controlling front positions. In distillation columns the front positions are easily controlled with cheap, reliable and fast online temperature measurements on sensitive trays inside the column [27]. A similar procedure was recently proposed for moving-bed chromatographic processes with UV rather than temperature measurement [37]. However, the performance of such an approach is usually limited. Exact product specifications cannot be guaranteed because of this indirect approach. Furthermore, in combined reaction separation processes the relationship between the measured variable and the variable to be controlled is often non-unique, which may lead to severe operational problems as shown for reactive distillation processes [23], It was concluded that these problems could be overcome if in addition some direct or indirect measure of conversion is taken into account. 

TABLE 13-5 Key Components and Typical Number of (Real) Stages Required to Perform the Separation for Distillation Processes of Industrial Importance... 

The fact that component efficiencies in multicomponent systems are unbounded means that the arithmetic average of the component Murphree efficiencies is useless as a measure of the performance of a multicomponent distillation process. Taylor, Baur, and Krishna [AIChE J., 50, 3134 (2004)] proposed the following efficiency for multicomponent systems ... 

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