Wetting rates, column packings

Low liquid rates lead to incomplete wetting of column packings, thus decreasing contacting efficiency. 

Chlorine is to be removed from a vent stream by scrubbing with a 5 per cent w/w aqueous solution of sodium hydroxide. The vent stream is essential nitrogen, with a maximum concentration of 5.5 per cent w/w chlorine. The concentration of chlorine leaving the scrubber must be less than 50 ppm by weight. The maximum flow-rate of the vent stream to the scrubber will be 4500 kg/h. Design a suitable packed column for this duty. The column will operate at 1.1 bar and ambient temperature. If necessary, the aqueous stream may be recirculated to maintain a suitable wetting rate. 

Similar effects occur in a packed column, although the flow patterns and arrangement of the surfaces are then obviously much more complex. Morris and Jackson(57) have recommended minimum wetting rates of 2 x 10-5 m3/s m for rings 25-75 mm in diameter and grids of pitch less than 50 mm, and 3.3 x 10-5 m3/s m for larger packings. 

In an attempt to test the surface renewal theory of gas absorption, Danckwerts and Kennedy measured the transient rate of absorption of carbon dioxide into various solutions by means of a rotating drum which carried a film of liquid through the gas. Results so obtained were compared with those for absorption in a packed column and it was shown that exposure times of at least one second were required to give a strict comparison this was longer than could be obtained with the rotating drum. Roberts and Danckwerts therefore used a wetted-wall column to extend the times of contact up to 1.3 s. The column was carefully designed to eliminate entry and exit effects and the formation of ripples. The experimental results and conclusions are reported by Danckwerts, Kennedy, and Roberts110 who showed that they could be used, on the basis of the penetration theory model, to predict the performance of a packed column to within about 10 per cent. 

Zhavoronkov et al. (Z2), 1951 Mass transfer studies (CO into water film) in two diameters of wetted-wall column and on wetted-plate packing (liquid mixed at intervals). Gas velocity had little effect on transfer rates. 

The amount of resin to pack in a column, column geometry, flow rates, pressure, column hardware, and wetted materials of construction should all be evaluated in development. Chromatography columns must be properly packed prior to validating the purification process. From a business perspective there should be some criteria other than purification of the product by which the quality of the packed column can be assessed prior to applying the feedstream, which by this time in the process is quite expensive. Height equivalent to a theoretical plate (HETP) and asymmetry determinations can be used to evaluate the quality of column packing, but may have limited value for some types of packed columns... 

To a chromatographic column, packed with 6.67 g of charcoal ("Nuchar C") with layers of sea sand at either end, 75 ml of acetone was added to wet the carbon. The column was heated to 40°C, and 25 ml of acetone was drained off. A solution of 20 g of dry crude dehydrocholic acid in 500 ml of acetone was poured into a reservoir atop the column and maintained in this reservoir at 40°C. This solution was then allowed to drop through the column at a constant rate over a 3-hour period. The column was then washed with 250 ml of acetone flowing through the column at a constant rate over a 1-hour-period at 40°C. The column effluent and wash acetone were combined and concentrated to a residual volume of about 100 ml which resulted in the formation of a thick slurry. The slurry was cooled with stirring at 0° to 5°C and aged for 30 min at this temperature. The slurry was filtered and the filter cake washed with cold acetone. The filter cake of U.S.P. dehydrocholic acid was sucked partially dry on the filter and then dried at 110°C for 3 hours. 

Similar to tray columns, packed columns operated at high gas velocities causes backmixing, and low gas velocities reduce the mass transfer rate. If the gas velocity is too high, the column will flood. In addition, at low liquid flow rates the packing will not wet completely, resulting in a reduction in mass-transfer. Another problem is the tendency for the liquid to channel. To minimize this effect, redistributors have to be installed every 5 to 10 m (16.4 to 30.5 ft) [23] to even out the liquid flow. 

A set of ternary mass transfer experiments was carried out by Toor and Sebulsky (1961b) and Modine (1963) in a wetted-wall column and also in a packed column. These authors measured the simultaneous rates of transfer between a vapor-gas mixture containing acetone, benzene, and nitrogen or helium, and a binary liquid mixture of acetone and benzene. Vapor and liquid streams were in cocurrent flow in the wetted-wall column and in countercurrent flow in the packed column. Their experimental results show that diffusional interaction effects were significant in the vapor phase, especially for the experiments with helium in the wetted wall column. 

Liquid and vapor flow countercurrently through openings between and within packing elements. At low vapor rates, there is relatively little disturbance of liquid by the vapor, and mass transfer proceeds in a fashion similar to that in a wetted wall column. At higher rates, there is considerable interaction between the phases, with vapor flow causing increases in liquid turbulence and holdup. In the so-called loading zone, there is an enhancement of mass transfer but, as rates are increased further, flooding occurs. 

The dynamic holdup depends mainly on the particle size and the flow rate and physical properties of the liquid. For laminar flow, the average film thickness is predicted to vary with, as in flow down a wetted-wall column or an inclined plane. In experiments with water in a string-of-spheres column, where the entire surface was wetted, the holdup did agree with theory [28]. For randomly packed beds, the dynamic holdup usually varies with a fractional power of the flow rate, but the reported exponents range from 0.3 to 0.8, and occasionally agreement with the 1/3 power predicted by theory may be fortuitous. 

Both stripping and rectification may be combined, as shown in Figure 7.2, to produce a sharper separation. The overall operation then corresponds to a distillation column, such as a packed or wetted-wall column, which can be described in terms of a continuum. In distillation and in absorption and stripping, the calculation methodology is to assume constant internal flow rates, which in general does not apply here but can be utilized as a workable simplification. 

An interesting situation arises in processes where the reaction product P evaporates and is taken out of the reactor with the gas phase (the supply phase). Let us assume that there are no chemical reactions in the gas phase, e.g., l ause the liquid phase reaction is catalysed. We consider the case of rapid reactions, so that all the desired product P is formed in the diffusion layer in the liquid phase, close to the interface. When P can undergo undesired reactions in the liquid phase it is essential to remove P as effectively as we can, e.g., by creating a large surface area and very high gas-phase mass transfer coefficients. At the same time it is essential that the volume of the liquid phase is minimized, since decomposition of P will occur just there. The obvious choice would then be a configuration where the liquid is the dispersed phase, such as in a spray tower or a spray cyclone, provided the heat removal rate is sufficient. Another suitable arrangement could be a gas/liquid packed bed or a wetted wall column. The latter reactor type is very suitable for heat removal (section 4.6.3.1)... 

The solvent circulation rate is selected so as to keep the vapor pressure of the solute above the rich solvent below the partial pressure of that solute in the entering gas stream. Obviously, this liquid rate should be sufficient to provide good wetting of the packing based on the column cross-sectional area required to handle the gas flow. The concentration of solute in the lean solvent will control the loss of solute in the exit gas stream from the absorber. The solute concentration in the lean solvent is determined by an equilibrium flash in the stripper therefore, the stripper pressure should be fixed to provide the desired solute recovery efficiency. 

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