Packed columns foaming

For distillation, E0 is typically in the range 0.5 to 0.9. The vertical spacing between trays ranges from 200 to 900 mm. In some trayed columns, an undesirable bubbly foam can form above the liquid-vapor mixture. Antifoam chemicals must be added to such columns or diameters or tray spacings must be increased. Packed columns foam less often than trayed columns. 

Foaming in fractionation and absorption columns can drastically lower capacity and lead to premature flooding, liquid carryover, and solvent losses. In packed columns, foaming can also lead to poor distributor and redistributor action. 

The mixture to be separated tends to form foam, which collapses more readily in a packed column. 

Selection of Equipment Packed columns usually are chosen for very corrosive materials, for liquids that foam badly, for either small-or large-diameter towers involving veiy low allowable pressure drops, and for small-scale operations requiring diameters of less than 0.6 m (2 ft). The type of packing is selected on the basis of resistance to corrosion, mechanical strength, capacity for handling the required flows, mass-transfer efficiency, and cost. Economic factors are discussed later in this sec tion. 

In a packed column, liquid and vapor flow counter-currently and separation between the liquid and vapor phases takes place continuously. In contrast, in a column with trays, separation occurs in stages. In a packed column, vapor does not bubble through the liquid as in the columns with trays. For this reason, and due to the absence of the vapor-flow orifices, packed columns operate at a much lower pressure drop. In addition, because liquid and vapor contact in a packed column is less agitated than in a trayed column, packed columns are less likely to foam. 

Packed columns are more suitable for handling foaming systems. 

In the first edition of this book, I forecast that the ultimate HPLC column would be a wall-bonded capillary column that would avoid the voiding and back-pressure problems seen with packed columns. A new type of column, the monolith silica column, recently emerging from research laboratories very closely fits this description. A monolith column has a honeycomb foam of silica, which is bonded with an organic separating phase, completely filling the inside of the column. 

The main factors favoring packed columns are (1) very corrosive applications, where plastic or ceramic packings are favored over trays, which are almost always constructed of metal (2) low pressure drop requirement, which is easier to achieve with packings than with trays (3) small-diameter columns, because trays require access for inspection and maintenance and (4) foaming systems, which are easier to handle in packed towers. 

Liquids tending to foam may be handled more readily in packed columns because of the relatively low degree of liquid agitation by the gas. 

Foams can be made by releasing a gas within a foamable liquid. For example, the opening and pouring of a bottle of soda releases pressurized CO2 gas, producing foam bubbles. Chemical reactions can also generate bubbles within the liquid. Other methods of foam production are to force both liquid and gas through a packed column (Khan et al. 1988), to spray foamable liquid onto a screen on which a fan is blowing (Aubert et al. 1986), or to use the foam-extrusion process (Han 1981). 

Packed columns are preferred for liquids with high foaming tendencies. 

When column diameters are less than 0.6 m (2.0 ft) packed towers can be considerably cheaper. However, if alloy metals are necessary, plate towers may result in less cost. Using ceramic or other similar resistant materials for packing and materials of construction, packed towers can serve to handle corrosive materials and acids. Because the gas flow in packed towers may offer less degree of agitation, packed tower operation may be better for liquids that tend to foam. When liquids are thermally sensitive, packed columns may offer less holdup and thus prevent changes taking place in the liquids due to thermal reaction. 

Countercurrent packed column target species Henry s law constant <10 kPa/mol fraction feed gas concentration <1 vol%. Efficiencies 95+%. Vulnerable to plugging. OK for foaming and corrosive. 

---