Column diameter vacuum distillation

A vacuum distillation column is to operate under a top pressure of 50 mmHg. The plates are supported on rings 75 mm wide, 10 mm deep. The column diameter is 1 m and the plate spacing 0.5 m. Check if the support rings will act as effective stiffening rings. The material of construction is carbon steel and the maximum operating temperature 50° C. If the vessel thickness is 10 mm, check if this is sufficient. 

Two heat-sensitive organic liquids of an average molecular mass of 155 kg/kmol are to be separated by vacuum distillation in a 100 mm diameter column packed with 6 mm stoneware Raschig rings. The number of theoretical plates required is 16 and it has been found that the HETP is 150 mm. If the product rate is 5 g/s at a reflux ratio of 8, calculate the pressure in the condenser so that the temperature in the still does not exceed 395 K (equivalent to a pressure of 8 kN/m2). It may be assumed that a = 800 m2/m3, /x = 0.02 mN s/m2, e = 0.72 and that the temperature changes and the correction for liquid flow may be neglected. 

A packed column, 1.2 m in diameter and 9 m tall, is packed with 25 mm Raschig rings, and used for the vacuum distillation of a mixture of isomers of molecular mass 155 kg/kmol. The mean temperature is 373 K, the pressure at the top of the column is maintained at 0.13 kN/m2 and the still pressure is 1.3-3.3 kN/m2. Obtain an expression for the pressure drop on the assumption that this is not appreciably affected by the liquid flow and may be calculated using a modified form of Carman s equation. Show that, over the range of operating pressures used, the pressure drop is approximately directly proportional to the mass rate of flow rate of vapour, and calculate the pressure drop at a vapour rate of 0.125 kg/m2. The specific surface of packing, S = 190 m2/m3, the mean voidage of bed, e = 0.71, the viscosity of vapour, // = 0.018 mN s/m2 and the molecular volume = 22.4 m3/kmol. 

A packed column, 1.22 m in diameter and 9 m high, and packed with 25 mm Raschig rings, is used for the vacuum distillation of a mixture of isomers of molecular mass 155 kg/kmol. The mean temperature is 373 K, the pressure at the top of the column is maintained at 0.13 kN/m2, and the still pressure is 1.3 kN/m2. Obtain an expression for... 

When trays similar to those used in the atmospheric column are used in vacuum distillation, the column diameter may be extremely high, up to 45 ft. To maintain low pressure drops across the trays, the liquid seal must be minimal. The low holdup and the relatively high viscosity of the liquid limits the tray efficiency, which tends to be much lower than in the atmospheric column. The vacuum is maintained in the column by removing the noncondensable gas that enters the column by way of the feed to the column or by leakage of air. 

This method proved valuable in the examination of a sample of acrylic sheet known from elemental analysis to contain phosphorus. Dry vacuum distillation of the polymer and solvent extraction both yielded a liquid, which, on direct infrared examination, proved to be an impure material containing phosphate or phosphite. The vacuum distillate at 200 °C was submitted to gas-liquid chromatographic examination on a 1.8 m (6.35 mm diameter) column packed with 30% w/w of silicone E301 on Celite maintained at 130 "C. A sample of the main component was taken by method 3 and the infrared spectrum obtained was readily identified as that of triethyl phosphate. 

The residue from an atmospheric distillation tower can be sent to a vacuum distillation tower, which recovers additional liquid at 0.7 to 1.5 psia (4.8 to 10.3 kPa). The vacuum, which is created by a vacuum pump or steam ejector, is pulled from the top of the tower. Relative to atmospheric columns, vacuum columns have larger diameters and their internals are simpler. Often, instead of trays, random packing and demister pads are used. 

In summary, it has been demonstrated that packed columns should be the first choice for vacuum distillation operations. Such a service normally involves a column design restricted by the allowable pressure drop for the number of theoretical stages necessary to produce the specified product. Inherently, modern packed columns always provide a lower pressure drop than a trayed column designed to perform the same function. Only when the trayed column diameter is greatly increased can its pressure drop equal that of a packed column in vacuum service. However, such a design usually is uneconomical. 

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. 

Columns for both absorption and distillation vary in diameter from about 25 mm for small laboratory purposes to over 4.5 m for large industrial operations these industrial columns may be 30 m or more in height. Columns may operate at pressures ranging from high vacuum to high pressure, the optimum pressure depending on both the chemical and the physical properties of the system. 

An early CO distillation pilot plant described by Johns (in London (1961), reading list), used a two section vacuum insulated column packed with wire gauze rings. The upper section was 5.2 m long with a diameter of 3.2cm, the lower 4.6 m x 1.9 cm. The column contained about 600 plates. An intermediate boiler re-evaporated part of the downward flowing liquid at the junction. This plant accumulates product in the liquid and enriches toward the bottom. Enriched CO (0.4 g day-1 of 65% 13C and 0.05 g day-1 of 5% lsO) was withdrawn from the bottom... 

A vacuum-jacketed Vigreux column of 1.5-cm. internal diameter and ca. 90-cm. length is satisfactory. With shorter or less efficient columns, redistillation of the distillate may be necessary to reduce losses in yield. 

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