Cycling distillation columns

The process engineer identifies heat exchange equipment in a process by the operation or function it serves at a particular location in the flow cycle. For example, the bottom vaporizer on a product finishing distillation column is usually termed Finishing Column ReboUer E-16, or Reboiler E-16 the overhead vapor condenser on this column is termed Condenser E-17 etc. The usual operations involved in developing a process flowsheet are described in Table 10-11, or Chapter 1, Volume 1. 

In practice, the successive boil/condense cycles occur naturally in the distillation column, and there is no need to isolate liquid mixtures at intermediate stages of purification. Fractional distillation is therefore relatively simple to carry out and is routinely used on a daily basis in chemical plants and laboratories throughout the world (Figure 11.19). 

FIGURE 11.19 A simple fractional distillation column used in a chemistry laboratory. The vapors from a boiling mixture of liquids rise inside the column, where they condense on contact with the cool column walls, drip back, and are reboiled by contact with more hot vapor. Numerous boil/condense cycles occur before vapors finally pass out the top of the column, reach the water-cooled condenser, and drip into the receiver. 

The experimental study presented in this paper provides new data on liquid-vapour equilibrium of HI-I2-H20 mixtures at high temperature and pressure (Doizi, 2009 Larousse, 2009). These results are important to improve the modelling of the distillation column used in the SI cycle. 

EBMax is a liquid phase ethylbenzene process that uses Mobil s proprietary MCM-22 zeolite as the catalyst. This process was first commercialized at the Chiba Styrene Monomer Co. in Chiba, Japan in 1995 (16-18). The MCM-22-based catalyst is very stable. Cycle lengths in excess of three years have been achieved commercially. The MCM-22 zeolite catalyst is more monoalkylate selective than large pore zeolites including zeolites beta and Y. This allows the process to use low feed ratios of benzene to ethylene. Typical benzene to ethylene ratios are in the range of 3 to 5. The lower benzene to ethylene ratios reduce the benzene circulation rate which, in turn, improves the efficiency and reduces the throughput of the benzene recovery column. Because the process operates with a reduced benzene circulation rate, plant capacity can be improved without adding distillation capacity. This is an important consideration, since distillation column capacity is a bottleneck in most ethylbenzene process units. The EBMax process operates at low temperatures, and therefore the level of xylenes in the ethylbenzene product is very low, typically less than 10 ppm. 

Solvent is recovered in the second distillation column, which is a vacuum steam-distillation column, with reflux of the organic phase to get high-purity aromatics. The final column is a multistage extraction column, where water is used to wash the solvent from the raffinate. There is a closed cycle for water to minimize solvent loss, and a small amount of water can be tolerated in the extraction solvent. 

Another solution is given in Fig. 7.3 (Dimian, 1996). This time the heat integration considers a more global viewpoint based on site integration . Excess heat available at high temperature is exported to the utility system. The heat needed to drive the distillation columns is imported from the steam network, at a temperature level compatible with the site policy. Exported energy as high-pressure steam is more valuable, and can be used to produce electricity in a combined heat and power cycle. Therefore, heat recovery is more efficient if treated as a plantwide problem. 

The difficulty of removing acetaldehyde completely by distillation is indicated by the fact that a sample, presumably purified by Nozaki and Bartlett s [22] best techniques still contained about 3 X 10 A/acetaldehyde as estimated by UV spectroscopy at 290 nm (the absorption maximum for acetaldehyde) [13]. Using as a criterion the conversion after 4 hr when the purified monomer is warmed at 45°C with benzoyl peroxide (cone 8.27 X 10 moles/ liter), after two degassing cycles at oil pump pressure of 30 min each, in sealed ampoules, the most satisfactory procedure used by them consisted of fractional distillation through a distillation column of at least 50 theoretical plates (cf. Table III). 

The condenser of a distillation column uses chilled water instead of cooling water. The chilled water, at a flow rate of 120 gpm, enters at 50°F and exits at 60°F. This corresponds to 50 tons of refrigeration (1 ton of refrigeration = 200 Btu/min). The chilled water is obtained from a refrigeration cycle similar to that shown in Figure 9.13 However, instead of a propane refrigerant, a Freon, R-12 (dichlorodifluoromethane), has been used. In the refrigeration cycle, the chilled water enters the... 

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