Pressure distillation column capacity

The absolute pressure may have a significant effect on the vapor—Hquid equiHbrium. Generally, the lower the absolute pressure the more favorable the equiHbrium. This effect has been discussed for the styrene—ethylbenzene system (30). In a given column, increasing the pressure can increase the column capacity by increasing the capacity parameter (see eqs. 42 and 43). Selection of the economic pressure can be faciHtated by guidelines (89) that take into consideration the pressure effects on capacity and relative volatiHty. Low pressures are required for distillation involving heat-sensitive material. 

D.P. Kurtz, K.J. McNulty and R.D. Morgan, Stretch the capacity of high pressure distillation columns, Chem. Engng. Progress, 87(2) (1991) 43-49. 

Kurtz. D.P.. KJ. McNutty. and R.D. Morgan "Stretch ihe Capacity of High-Pressure Distillation Columns." Client. Eng. Progress, 43 (February 1991). 

An increase in the selectivity and sensitivity of the detection of carbonyl compounds may be achieved by their conversion into special derivatives. Johnson and Hammond [58] condensed carbonyl compounds with 2,4,6-trichlorophenylhydrazine and, prior to the analysis, separated the products by means of thin-layer chromatography. Using an ECD, they were able to determine by GC 10-7—10-10 g of the substance. They prepared the derivatives in a reaction column as follows. A 0.40-g amount of 2,4,6-trichlorophenylhydrazine was dissolved in 40 ml of 1 N HC1 with heating and mixed with 40 g of Celite 545. w-Hexane was added to the wet mixture until a paste consistency was obtained, and the column (30 X 2 cm I.D.) was filled with the paste. In order to prepare the derivatives, the carbonyl compound was applied to the column in an amount corresponding to half of the theoretical column capacity and the column was eluted with 75-100 ml of -hexane. The n-hexane was distilled off at decreased pressure and the viscous derivatives were stored in 10 ml of w-hexane at —27°C. However, these derivatives are sometimes not separated satisfactorily on silicone phases. 

Downcomer pressure drop. If the clearance under the downcomer is too low, it may add substantially to the downcomer backup and consequently reduce downcomer capacity. Cases have been reported (61) where column capacity was increased by simply cutting 1 in off the bottom of the downcomer. Methods of estimating the backup caused by hydraulic losses through the opening under the downcomer are available in most distillation texts (48, 319, 371, 409). 

Floating pressure control. While tight pressure control is mandatory in distillation columns, it may be desirable to slowly manipulate the set point of the pressure controller. This is the essence of Shinskey s "floating pressure control (362). The method minimizes column pressure without violating constraints such as maximum condenser or column capacity, or the ability to send column products to downstream units. 

High capacity vacuum distillation columns typically have very large diameter overhead lines from the top of the column to the condenser because the available pressure drop is so small. Also, because the system pressures are so low the piping wall can be quite thin. Therefore, if the line is inadvertently fiUed with liquid (either process liquids during a column upset or water during hydrotesting) the lines or their supports could collapse. 

Once packing heights are determined in other sections from HETP (distillation) or Koa (absorption), the height allowances for the internals (from Figure 1) can be added to determine the overall column height. Column diameter is determined in sections on capacity and pressure drop for the selected packing (random dumped or structured). 

By virtue of its chemical and thermal resistances, borosilicate glass has superior resistance to thermal stresses and shocks, and is used in the manufacture of a variety of items for process plants. Examples are pipe up to 60 cm in diameter and 300 cm long with wall tliicknesses of 2-10 mm, pipe fittings, valves, distillation column sections, spherical and cylindrical vessels up 400-liter capacity, centrifugal pumps with capacities up to 20,000 liters/hr, tubular heat exchangers with heat transfer areas up to 8 m, maximum working pressure up to 275 kN/m, and heat transfer coefficients of 270 kcal/hz/m C [48,49]. 

B. 1,4-Cyclohexanedione. The purified 2,5-dicarbethoxy-l,4-cyclohexanedione (170 g., 0.66 mole) (Note 5) and 170 ml. of water are placed in a glass liner (vented) of a steel pressure vessel of 1.5-1. capacity (fitted with a pressure-release valve). The vessel is sealed, heated as rapidly as possible to 185-195°, and kept at this temperature for 10-15 minutes (Note 6). The reaction vessel is immediately removed from the heater, placed in a large tub of ice water, and cooled to room temperature. The gas pressure then is carefully released. The resulting yellow to orange liquid is transferred to a distillation flask with the aid of a minimum volume of ethanol, and most of the water and ethanol is removed under reduced pressure by means of a rotary evaporator. The flask is attached to a short heated column fitted with a short air condenser. The remainder of the water and ethanol is removed under reduced pressure, and the 1,4-cyclohexanedione is distilled, b.p, 130-133° (20 mm.). The product solidifies to a white to pale-yellow solid, m.p. 77-79°, deld 60-66 g. (81-89% yield from 2,5-dicarbethoxy-l,4-cyclohexanedione). The compound may be conveniently recrystallized from carbon tetrachloride (7 ml. per gram of dione) the purified product is obtained as white plates, m.p. 77-79° (90% recovery). 

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