Countercurrent gas-liquid contacting

The SDRs reported so far are mainly applicable to liquid reaction systems and their main limitations are short residence times and low production capacities. These problems are addressed by the so-called rotor-stator SDR, which allows a countercurrent gas-liquid contacting at higher throughputs and longer residence times [104]. 

Acidic gaseous streams from the detergent alkylation units usually are combined and sent to a relief gas scrubber where HF acid is removed by countercurrent gas-liquid contacting using circulating limewater solution ... 

This is a special case of countercurrent gas-liquid contact in which the liquid enters the equipment at the adiabatic-saturation temperature of the entering gas. This can be achieved by continuously reintroducing the exit liquid to the contactor without removal or addition of heat, as shown in Figure 8.11. For this case, equation (8-25) becomes... 

Plate Towers Plate (tray) towers are countercurrent gas-atomized spray scrubbers using one or more plates for gas-liquid contacting. They are essentially the same as, if not identical to, the devices used for gas absorption and are frequently employed in apphcations in which gases are to be absorbed simultaneously with the removal of dust. Except possibly in cases in which condensation effects are involved, countercurrent operation is not significantly beneficial in dust collection. 

Absorption is usually carried out in a countercurrent tower, through which liquid descends and gas ascends. The tower may be fitted with trays, filled with packing, or fitted with sprays or other internals. These internals provide the surface area required for gas-liquid contact. 

In this operation a soluble gas is absorbed from a gas mixture with a liquid. Generally the gas mixture and the liquid are brought in contact with each other in a packed column. Here the liquid flows from the top and the gas mixture rises from the bottom (countercurrent). The absorbed gas or vapor can be recovered later by desorption. The packing used in a gas-liquid absorption column is the same as the packing used in packed distillation columns and aids in good gas-liquid contact. This operation is also used to strip toxic vapors from exit gases. Such absorption columns are known as scrubbers. 

In the third type of gas-liquid-solid reaction, only two of the three phases take part into the reaction, the third phase being an inert phase. This type of reaction can be further subdivided into three catagories. Some reactions are strictly gas liquid reactions, but they are often carried out in packed-bed reactors operating under countercurrent-flow conditions. Here, the solid imparts momentum transfer and allows better gas-liquid contact and gas-liquid interfacial mass... 

EQUATIONS FOR GAS-LIQUID CONTACTS. Consider the countercurrent gas-liquid contactor shown diagrammatically in Fig. 23.9. Gas at humidity and temperature T j, enters the bottom of the contactor and leaves at the top with a humidity and temperature Liquid enters the top at temperature and leaves at the bottom at temperature The mass velocity of the gas is Gy, the mass of vapor-free gas per unit area of tower cross section per hour. The mass velocities of the liquid at inlet and outlet are, respectively, G a and G t,. Let dZ be the height of a small section of the tower at distance Z from the bottom of the contact zone. Let the mass velocity of the liquid at height Z be G,., the temperatures of gas and liquid be Ty and T, respectively, and the humidity be j/f. At the interface between the gas and the liquid phases, let the temperature be 7j and the humidity be 3 . The cross section of the tower is S, and the height of the contact section is Zj-. Assume that the liquid is warmer than the gas, so the conditions at height Z are those shown in Fig. 23.8a. The following equations can be written over the small volume S dZ. 

Combine material and energy balances to develop the equation for the operating line for a countercurrent adiabatic gas-liquid contact operation. 

Adiabatic gas-liquid contact is usually carried out in some sort of packed tower, frequently with countercurrent flow of gas and liquid. Figure 8.5 is a schematic diagram of such a tower of cross-sectional area S. A mass balance for substance A over the lower part of the tower is... 

Figure 8.5 Continuous countercurrent adiabatic gas-liquid contact.

Catalysis in internal nanospaces of liquid catalysis. A catalysis of this sort probably occurs in molten salts carried in pores of inert porous carriers, to increase gas-liquid contact surface areas — vanadium pentoxide catalysts for the oxidation of SO2 to SO3 [5]. Reactants diffuse into the nanolayer below the catalysts surface and the product countercurrently diffuses out. 

Plate columns for gas/liquid contacting are not treated here, since their contacting principle is similar to that of bubble columns. They are mostly used for countercurrent flow under conditions where the mass transfer rate is not a limiting factor, so that the mass exchange is more determined by the countercurrent-effect, see section 7.3. A review on mass transfer data is given by Trambouze et al.(1984). 

Packed columns are often used for gas/liquid contacting when a high volumetric mass transfer rate has to be combined with countercurrent gas and liquid flows. They are particularly suitable for chemically enhanced absorption. This is the phenomenon that the absorbed component is completely converted by the rapid chemical reaction within the diffusion layer, close to the gasAiquid interface (see section 5.4.2.1), In such processes there is no need for a large liquid volume, the reaction rate per unit volume is then directly proportional to the interfacial area. 

Liquid extraction, the only operation in this category, is basically very similar to the operations of gas-liquid contact described in the previous part. The creation of a new insoluble liquid phase by addition of a solvent to a mixture accomplishes in many respects the same result as the creation of a new phase by the addition of heat in distillation operations, for example, or by addition of gas in desorption operations. The separations produced by single stages, the use of countercurrent cascades and reflux to enhance the extent of separation—all have their liquid-extraction counterparts. The similarity between them will be exploited in explaining what liquid extraction can accomplish. 

These operations are all similar in that the mixture to be separated is brou t into contact with another insoluble phase, the adsorbent solid, and the unequal distribution of the ori al constituents between the adsorbed phase on the solid surface and the bulk of the fluid then permits a s aration to be made. All the techniques previously found valuable in the contact of insoluble fluids are useful in adsorption. Thus we have batchwise single-stage and continuous multistage separations and separations analogous to countercurrent absorption and stripping in the field of gas-liquid contact and to rectification and extraction with the use of reflux. In addition, the rigidity and immobility of a bed of solid... 

Sieve tray columns have also fotmd an application in staged countercurrent liquid extraction operations. The perforated plates, arranged in much the same way as in gas-liquid contact, act to break up accumulations of the dispersed phase and provide fresh surfaces for renewed mass transfer. Each of the plates, or rather the space between them, is a potential equilibrium stage, but the efficiency E is generally quite low. 

Figure 8.1.14. (a) Schematic of a countercurrent continuous contact gas-liquid absorber, (b) Operating line and equilibrium curve for a countercurrent gas-liquid absorber, (c) Operating line and equilibrium curve for a countercurrent gas-liquid stripper. 

Fixed-bed reactors in the form of gas absorption equipment are used commonly for noncatalytic gas-liquid reactions. Here the packed bed serves only to give good contact between the gas and liquid. Both cocurrent and countercurrent operations are used. Countercurrent operation gives the highest reaction rates. Cocurrent operation is preferred if a short liquid residence time is required. 

The term three-phase fluidization requires some explanation, as it can be used to describe a variety of rather different operations. The three phases are gas, liquid and particulate solids, although other variations such as two immiscible liquids and particulate solids may exist in special applications. As in the case of a fixed-bed operation, both co-current and counter- current gas-liquid flow are permissible and, for each of these, both bubble flow, in which the liquid is the continuous phase and the gas dispersed, and trickle flow, in which the gas forms a continuous phase and the liquid is more or less dispersed, takes place. A well established device for countercurrent trickle flow, in which low-density solid spheres are fluidized by an upward current of gas and irrigated by a downward flow of liquid, is variously known as the turbulent bed, mobile bed and fluidized packing contactor, or the turbulent contact absorber when it is specifically used for gas absorption and/or dust removal. Still another variation is a three-phase spouted bed contactor. 

The Contacting Scheme. In gas-liquid systems semibatch and countercurrent contacting schemes predominate. In liquid-liquid systems mixed flow (mixer-... 

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