Column diameter atmospheric distillation

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. 

Example 18.6. A sieve-plate column operating at atmospheric pressure is to produce nearly pure methanol from an aqueous feed containing 40 mole percent methanol. The distillate product rate is 5800 kg/h. (a) For a reflux ratio of 3.5 and a plate spacing of 18 in., calculate the allowable vapor velocity and the column diameter. b) Calculate the pressure drop per plate if each sieve tray is in, thick with j-in, holes on a -in. triangular spacing and a weir height of 2 in. (c) What is the froth height in the downcomer ... 

Further details of a Destinorm apparatus for continuous distillation at reduced pressures (Fig. 162) will be discussed in chap. 5.2.2.5. A continuous plate column is exemplified in Fig. 167 bj a Labodest apparatus which can operate from atmospheric pressure down to about 20torr. It is provided with vapour-coUision bubble-cap plates of glass (chap. 7.3.3) having an efficiency between 80 and 100%. Their inode of operation is identical with that of industrial plates so that the test results can be transferred directly. Besides, they are well suited to demonstrations since the gradual exchange process can be observed. This type of column is built with lengths between 600 and 1300 mm and column diameters between 30 and 120 mm and has between 5 and 28 plates depending on the distance between the plates (35 to 150 mm). 

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. 

Packed Tower Example. Equation (17-17) will be used to estimate the H.E.T.P. values for the atmospheric distillation of a benzene-toluene mixture in a packed tower 5 ft. in diameter. The liquid and vapor rates are 480 and 400 lb. mols per hr., respectively. The calculation will be made for the section near the bottom of the column where the liquid and vapor are essentially toluene. 

Bromine (128 g., 0.80 mole) is added dropwise to the well-stirred mixture over a period of 40 minutes (Note 4). After all the bromine has been added, the molten mixture is stirred at 80-85° on a steam bath for 1 hour, or until it solidifies if that happens first (Note 5). The complex is added in portions to a well-stirred mixture of 1.3 1. of cracked ice and 100 ml. of concentrated hydrochloric acid in a 2-1. beaker (Note 6). Part of the cold aqueous layer is added to the reaction flask to decompose whatever part of the reaction mixture remains there, and the resulting mixture is added to the beaker. The dark oil that settles out is extracted from the mixture with four 150-ml. portions of ether. The extracts are combined, washed consecutively with 100 ml. of water and 100 ml. of 5% aqueous sodium bicarbonate solution, dried with anhydrous sodium sulfate, and transferred to a short-necked distillation flask. The ether is removed by distillation at atmospheric pressure, and crude 3-bromo-acetophenone is stripped from a few grams of heavy dark residue by distillation at reduced pressure. The colorless distillate is carefully fractionated in a column 20 cm. long and 1.5 cm. in diameter that is filled with Carborundum or Heli-Pak filling. 4 hc combined middle fractions of constant refractive index are taken as 3-l)romoaccto])lu iu)nc weight, 94 -100 g. (70-75%) l).p. 75 76°/0.5 mm. tif 1.57,38 1.5742 m.]). 7 8° (Notes 7 and 8). 

A process liquid is pumped from a storage tank to a distillation column, using a centrifugal pump. The pipeline is 80 mm internal diameter commercial steel pipe, 100 m long. Miscellaneous losses are equivalent to 600 pipe diameters. The storage tank operates at atmospheric pressure and the column at 1.7 bara. The lowest liquid level in the tank will be 1.5 m above the pump inlet, and the feed point to the column is 3 m above the pump inlet. 

Acetone and methanol are impossible to separate by simple distillation due to the presence of an azeotrope. However, the addition of water near the top of a column allows these two components to be separated. Five sets of steady-state operating data for the extractive distillation of an acetone-methanol azeotrope in a laboratory scale column have been provided by Kumar et al. (1984). A schematic diagram of the column is provided in Figure 14.19. The column had a diameter of 15 cm and was fitted with 13 bubble cap trays, a total condenser and a thermosiphon (equilibrium) reboiler. Unlike many experimental distillation studies, these experiments were not carried out at total reflux the acetone-methanol feed entered the column on the eleventh stage from the top (the condenser counts as the first stage) and the water was introduced on stage six. The column was operated at atmospheric pressure for all five runs. Additional details of the column, operational specifications, and computed product compositions for one of these experiments can be found in Table 14.9. 

For distillation at atmospheric pressure in a column 0.3 m (1 ft) in diameter the Peclet number is about 10, based on empirical correlations for dispersion on bubble-cap and sieve trays. This is in the range where significant enhancement of the efficiency should result because of gradients on the plate. For a column 1 m or larger in diameter, the Peclet number would be expected to be greater than 20,... 

Its higher thermal stability allows for stripping the extract phase at atmospheric pressure (rather than vacuum) resulting in lower capital cost for smaller diameter, thinner walled distillation columns, as well as lower power consumption. 

Another column excellent for fractionating small quantities of liquids at atmospheric pressure is the concentric tube column. This column is constructed from two precision bore tubes mounted concentrically with a very small annular space between them. During the distillation, the vapors and liquid move through this annular space and fractionation occurs. These columns are characterized by low throughput rate and a very low HETP and holdup. Some typical data for a 30.5-cm column, 6.5 mm in diameter, operated at a throughput rate of 1.2 ml per min, are an HETP of 0.36 cm (85 plates per column) with a holdup of only 0.018 ml per plate. These characteristics make this column ideal for the fractional distillation of a few milliliters of a liquid or for the purification of an analytical sample. 

Lately, a popular addition to a crude distillation system has been a preflash column ahead of the two stages shown in Fig. 15.10. The preflash tower strips out the lighter portions of a crude oil before the remainder enters the atmospheric column. It is the lighter portions that set the vapor loading in the atmospheric column, which, in turn, determines the diameter of the upper section of the column. 

From the economic point of view it is also desirable to operate distillation systems at atmospheric pressure. (Vacuum costs money and low operating pressure in the column results in large vapor volume and, consequently, large diameter). Assuming ... 

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