Column diameter, countercurrent

Bubble columns are used very widely for reaction absorption applications. In bubble columns, the gas phase flows in the form of bubbles, either countercurrently or co-currently. Bubble columns provide significant liquid hold-up and sufficient liquid residence time. The column diameter sometimes exceeds 5 m, and its height reaches 10 m or more. 

Both solvent sublation and bubble fractionation are viable as continuous countercurrent processes for the removal of hydrophobic compounds from water. Both processes are primarily dependent on the size of air bubbles introduced into the column as well as the extent of axial dispersion in the aqueous phase. The fractional removal in solvent sublation is less dependent on the column diameter. 

The countercurrent contact zone height will depend primarily upon the number of stages required ( ) and the column characteristics. The effect of backmixing also increases the column diameter. A reasonable first approximation of extraction height (L) required for agitated columns is ... 

The influence of column diameter and surface tension on the HTU in packed columns in the countercurrent distillation of binary mixtures was studied by Gomez and StrumUlo [8a]. They found the relation... 

Internals, liquid, and gas distribution are practical aspects of column design discussed by Zenz [5]. Since in absorbers the liquid and the gas generally flow in countercurrent directions, there is a close interaction between the column diameter and the liquid and gas flow rates. If, for a given column diameter and liquid flow rate, the gas flow rate is too high, the liquid will be blown to the top of the column, which is said to be flooded. Zenz [5] derived the following relation for the maximum allowable gas and liquid flow rates above which flooding occurs ... 

Column diameter well below the minimum suggested. Akita and Yoshida s (1974) correlation has been modified for cocurrent and countercurrent mode of operation... 

The simplest form of a bubble column is a vertical tube in which a gas distributor is placed at the bottom packed or plate bubble columns are also used. The gas bubbles rise through the liquid phase, which may flow through the column either cocurrent or countercurrent to the gas. As a result of the short residence time of the gas bubbles in the liquid phase, bubble column reactors are preferred for reactions which require a short gas and a long liquid reaction time. Therefore the residence time distribution of the liquid phase is a characteristic factor for the design of the reactor. The dependence of the residence time distribution upon the column diameter has to be known for any scale-up of bubble columns. 

Piret et al. measured liquid holdup in a column of 2J-ft diameter and 6-ft packed height, packed with graded round gravel of lj-in. size, the total voidage of the bed being 38.8%. The fluid media, air and water, were in countercurrent flow. The liquid holdup was found to increase markedly with liquid flow rate, but was independent of gas flow rate below the loading point. Above the loading point, an increase of liquid hold-up with gas flow rate was observed. 

Kramers and Alberda (K20) have reported some data in graphical form for the residence-time distribution of water with countercurrent air flow in a column of 15-cm diameter and 66-cm height packed with 10-mm Raschig rings. It was concluded that axial mixing increased with increasing gas flow rate and decreasing liquid flow rate, and that the results were not adequately represented by the diffusion model. 

Hoogendoorn and Lips (H10) carried out residence-time distribution experiments for countercurrent trickle flow in a column of 1.33-ft diameter and 5- and 10-ft height packed with -in. porcelain Raschig rings. The fluid media were air and water, and ammonium chloride was used as tracer. The total liquid holdup was calculated from the mean residence time as found... 

Column reactors are the second most popular reactors in the fine chemistry sector. They are mainly dedicated reactors adjusted for a particular process although in many cases column reactors can easily be adapted for another process. Cocurrently operated bubble (possibly packed) columns with upflow of both phases and trickle-bed reactors with downflow are widely used. The diameter of column reactors varies from tens of centimetres to metres, while their height ranges from two metres up to twenty metres. Larger column reactors also have been designed and operated in bulk chemicals plants. The typical catalyst particle size ranges from 1.5 mm (in trickle-bed reactors) to 10 mm (in countercurrent columns) depending on the particular application. The temperature and pressure are limited only by the material of construction and corrosivity of the reaction mixture. 

In a bubble-column reactor for a gas-liquid reaction, Figure 24.1(e), gas enters the bottom of the vessel, is dispersed as bubbles, and flows upward, countercurrent to the flow of liquid. We assume the gas bubbles are in PF and the liquid is in BMF, although nonideal flow models (Chapter 19) may be used as required. The fluids are not mechanically agitated. The design of the reactor for a specified performance requires, among other things, determination of the height and diameter. 

In order to extract acetic acid from a dilute aqueous solution with isopropyl ether, the two immiscible phases are passed countercurrently through a packed column 3 m in length and 75 mm in diameter. It is found that if 0.5 kg/m2 of the pure ether is used to extract 0.25 kg/m2s of 4.0 per cent acid by mass, then the ether phase leaves the column with a concentration of 1.0 per cent acid by mass. Calculate ... 

In order to extract acetic acid from dilute aqueous solution with isopropyl ether, the two immiscible phases are passed countercurrently through a packed column 3 m in length and 75 mm in diameter. 

We consider a vertical cylindrical tube of length L and diameter Dq (radius 2Ro) with liquid admitted at the top such that it forms a falling film that coats the walls of the tube. We also add a gas into the top of the tube (cocurrent) or into the bottom (countercurrent). This is a standard unit in extraction processes called a wetted-wall column. 

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