3. Choice of constructive materials

Packed column absorbers are downloaded from the nozzle bodies of various shape (ring wooden lattice particulate material, etc.). Gas-liquid contact occurs mainly on the wetted surface of the nozzle at which the scrubbing liquid flows. The surface of the nozzle unit to unit volume can be quite high and therefore a relatively small volume can create significant mass transfer surface. However, in some cases, surface active surface. Fluid flow in the nozzle is essentially the character of the film, resulting in packed absorbers can be regarded as a kind of film. At the same time, between the packings and film absorbers, absorbers including a nozzle sheet, there are differences. The film absorbers liquid film flow occurs over the entire height of the machine, whereas in the checkerwork - a packing element height. When the liquid overflow from one packing element to another liquid film is destroyed and a new film is formed on the underlying element. Some of the liquid thus falls as droplets through a nozzle located below the layers. Under certain conditions, the liquid film flow in packed absorbers broken, and the contact between gas and liquid is carried in the bubbling regime.

The packed absorber (. Figure 1) consists of a column, in which the supporting grate 1 is placed; this lattice is laid a layer of packing 2. scrubbing liquid is fed to the nozzle by means of the dispenser 3. In the illustrated in Fig. 5 and nozzle absorber laid continuous layer over the entire height. Sometimes the nozzle is laid several layers, each separate setting a lattice supporting layer (Fig. 1 b). In some cases, lay-nozzle between the individual layers of the device 4 is set to redistribute the liquid.

Fig.1 packed absorbers:

and - with a continuous nozzle loading; b - with primary store attachments;

1 - supporting lattice; 2 - head; 3 - liquid distribution device;

4 - redistributor; 5 - gutter; 6 pipe.

The movement of gas and liquid in the packed countercurrent absorbers typically performed as illustrated in Fig. 5. Co current regeneration (downlink) is used rarely. Recently, however, a lot of attention paid to the creation of direct-flow packed absorbers working with large gas rates (up to 10 m / s). At such speeds, which in the case of counterblow unattainable because of the occurrence of flooding, it intensifies the process and reduced size apparatus; concurrent with the flow resistance is considerably lower than in counterblow. The use of such absorbers is expedient in cases where the direction of motion of the phases does not affect significantly the driving force.

Lack of packed absorbers - the difficulty of heat dissipation using outrigger refrigerators. The proposed design absorbers with internal heat removal by means of a nozzle placed in the cooling members do not receive.

The packed column liquid flows through the nozzle element is mainly in the form of a thin film, so the phase contact surface is substantially wetted surface nozzles, and packed devices may be considered as a kind of film. However, in the past the liquid film flow occurs over the entire height of the machine, and packed absorbers - only packing element height. When the liquid overflow from one packing element to another liquid film is destroyed and a new film is formed on the underlying element. In this part of the fluid passes through the layers located below the nozzle in the form of jets, droplets and sprays. Part of the nozzle surface is wetted with a fixed (stagnant) liquid.

The basic characteristics of a nozzle and its specific surface area (m2 / m3) and the free volume of e (m3 / m3). The amount of free volume for non-porous nozzle is usually determined by water filling nozzle. The ratio of water volume to the volume occupied by the nozzle, the magnitude e gives. The equivalent diameter of the nozzle is according to the formula

Hydrodynamic modes. Packed absorbers can operate in different hydrodynamic regimes. The first mode - the film - there is at low densities, irrigation and low gas speeds. The amount of liquid holdup in the nozzle at the same time is practically independent of the gas velocity.

The second mode - hang mode. When countercurrent phases due to an increase of the liquid phase to the gas contact surface friction force occurs braking fluid gas stream. As a result, the fluid flow rate decreases and the thickness of the film and the amount of fluid retained in the nozzle increases. The hang time with an increase in the gas velocity increases the wetted surface of the nozzle and respectively - the intensity of the mass transfer process. In quiet mode, hang over the film is broken: there are swirls, splashes, ie create conditions for the transition to the bubbling. All this contributes to increase the mass transfer rate.

Bubble (Belleville) absorbers

Belleville absorbers are usually vertical column inside of which at a certain distance from each other placed horizontal partitions - plates. With trays carried directional movement phases and multiple interaction of liquid and gas. At present various constructions of disk devices used in industry. By way of overflow trays with bubble absorber column can be divided into: 1) to trays with drainage devices and 2) without the plates with dispensing devices (Appendix -C)..

Tray column with drain devices. In these columns, the liquid overflow from the tray to tray carried out by means of special devices - drain tubes, pockets, etc. The lower column tubes are immersed in a glass on a plate and form downstream hydraulic valves, excluding the possibility of the passage of gas through the drain device.

The principle of operation of this type columns can be seen from ris.XI-16, where an example is shown in a perforated absorber plates. Fluid enters the upper plate 1 and merges with the plates on the plate 2 through the overflow device and removed from the bottom of the column. Gas enters the bottom of the device passes successively through each hole caps or plates. When this gas is distributed as bubbles in the layer and the jets of liquid on the tray, forming a layer of foam thereon, which is the main area of ​​mass transfer and heat transfer plate. Exhaust gas is removed from the top of the column.

A Overflow tubes on the plates so that the liquid flowed to adjacent plates in mutually antithetical directions. In recent years increasingly used as a drain device segments cut out in the plate and bounded threshold - overflow.

For plates with drainage devices include: sieve, bubble cap, valve and ballast, plate. Hydrodynamic modes plates. Efficacy plates of any design largely depends on the hydrodynamic conditions of their operation. Therefore, to describe the basic structures plates, consider these modes.

Depending on the velocity of the gas and water concentration are three main modes of hydrodynamic bubble plates: bubble, foam and a jet or injection. These modes differ in the structure of the bubble layer, which mainly determines the flow resistance and height, as well as the magnitude of the phase contact surface.

Bubble mode. This mode is observed at low gas velocities as it moves through the liquid layer in separate vials. Phase contact surface on the plate, working in the bubble mode is small.

Foam mode. With increasing gas flow emerging from the slit openings and separate bubbles coalesce into a continuous jet which at a certain distance from the place of the expiration destroyed due to the resistance of the bubbling layer to form a large amount of bubbles. At the same time on the plate there is a gas-liquid dispersion - foam, which is unstable and is destroyed immediately after the cessation of gas supply. In this gas-liquid contacting operation takes place on the surface of the bubbles and the gas jets and liquid drops on the surface, which in large quantities over a bubbling bed are formed at the exit of the gas bubbles from the bubble layer and fracture their shells. When the foam mode, the contact surface on the bubbling phase plates is maximized.

Inkjet (injection) mode. With further increase in the gas velocity increases the length of the gas jets, and they come out on the surface of the bubble layer, without breaking and form a large number of large splashes. The surface of contact between the phases in terms of the hydrodynamic regime is sharply reduced.

It should be noted that the transition from one mode to another takes place gradually. General methods of calculation of hydrodynamic regimes boundaries (critical points) for bubbling plates absent. Therefore, the design of disc apparatus is usually calculated by determining the gas velocity, the corresponding lower and upper limits of the work plate, and then select the operating speed of the gas.

The perforated plates. Column with sieve plates (Fig. XI-18) is a vertical cylindrical housing 1 with two horizontal trays in which the entire surface uniformly significant number of drilled holes with a diameter of 1-5 mm. To drain fluid and adjusting its level to serve as overflow pipe plate 3 whose lower ends are immersed in 4 beakers.

The gas flows through the holes in the plates and the liquid is distributed as fine bubbles and streams. With too small velocity of the gas fluid can leak (or "fall") through the holes onto the underlying plates, which should lead to significant reduction in the mass transfer rate. Therefore, the gas must travel at a certain speed, and having a pressure sufficient to overcome the pressure of the liquid layer on the tray and prevent dripping of liquid through orifice plate.

The perforated plates are simple devices, ease of installation, inspection and repair. Hydraulic resistance of these small plates. The perforated plates operate stably over a fairly wide range of gas velocities, and in a certain range of loads on the gas and liquid dish are highly effective. However, the perforated plates are sensitive to dirt and sediments that clog the hole plates. In the event of sudden termination of gas revenues or a significant reduction in its pressure with perforated plates merges all of the liquid, and to resume the process requires a re-run column. (Appendix -D). A variety of absorbers with perforated plates are so-called foam absorbers, plates which, as stated, differ from weir structure sieve device. With the same apparatus including the foam plates efficiency higher than that of absorbers with perforated plates. However, due to a great height in the foam plates hydraulic resistance foam absorbers considerably, which limits their field of application.

Cap trays. Less sensitive to dirt than a column with perforated plates, and have higher interval of the stable operation of the column with a cap trays (fig. H1-19). Gas is supplied to the plate 1 by two nozzles, breaking the cap 3 is then slotted into a large number of individual jets. Slots caps most often performed in the form of triangular teeth or rectangular. The gas passes through the layer of liquid flowing across the tray from one of the drain device 4 to the other. When moving through the bed a considerable part of fine jets and the gas dissolves in the liquid is distributed as bubbles. The intensity of the formation of foam and spray on the nozzle plate is dependent on the speed of motion of the gas cap and the depth of immersion in liquid.