Liquid-solid countercurrent contact

Figure 8.2 depicts the four main zones and their immediate proximity to each other in the Molex process. As indicated earlier, the Sorbex process operates on a liquid-solid countercurrent contacting principle. Zone 1 is referred to as the... 

The easiest way to understand the SMB concept is to consider a true moving bed (TMB) as described in Eigure 10.1, in which a countercurrent contact is promoted between the solid and liquid phases. The solid phase moves down the column due to gravity and exits the system in Zone I. The liquid (eluent) stream follows exactly the opposite direction. It is recycled from Zone IV to Zone I. The feed, containing components A and B are injected at the middle of the column, and the fresh eluent is replenished in Zone I. 

Phase densities differ by a Phase densities differ by only about 10%. factor of 100-10,000 1. Viscosity in both phases is Liquid phase viscosity moderate, solid low. phase rigid. Phase separation is rapid Phase separation is slow surface-tension and complete. effects prevent completion. Countercurrent contacting is Countercurrent contacting is slow and quick and efficient. imperfect. ... 

If the contact time of the liquid/solids suspension with the gas phase is relatively short and also for reactors of high H/dc values, it may be useful to distinguish a co-current and countercurrent contacting pattern and, especially in relation to the gas holdup, up-flow and down-flow. 

The basic idea of a moving bed system is to promote a countercurrent contact between the solid and the liquid phases. The concept and principles of SMB are discussed at length in Chapter 13, and applications of protein purifications and other complex molecules are given. 

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... 

Another advantage of the fins is that a liquid that wets the solid surface (as is generally the case with oil and a porous catalyst) tends to occupy the grooves between the fins, as depicted in Fig. 6. Thus, an organized division of the available channel cross section for liquid flow and gas flow is obtained, with a relatively large part available for liquid flow in combination with a relatively large liquid/solid contact area. In the case of countercurrent flow, the fins may stabilize the relatively thick liquid layer against formation... 

Fig. 30. Contacting patterns and contactor types for gas-liquid-solid reactors, (a) Co-current downflow trickle bed. (b) Countercurrent flow trickle bed. (c) Co-current downflow of gas, liquid, and catalyst, (d) Downflow of catalyst and co-current upflow of gas and liquid, (e) Multi-tubular trickle bed with co-current flow of gas and liquid down tubes with catalyst packed inside them coolant on shell side, (f) Multi-tubular trickle bed with downflow of gas and liquid coolant inside the tubes, (g) Three-phase fluidized bed of solids with solids-free freeboard, (h) Three-phase slurry reactor with no solids-free freeboard, (i) Three-phase fluidized beds with horizontally disposed internals to achieve staging, (j) Three-phase slurry reactor with horizontally disposed internals to achieve staging, (k) Three-phase fluidized bed in which cooling tubes have been inserted coolant inside the tubes. (1) Three-phase slurry... 

The processes are basically the same with little variation in flow scheme. The sour gas containing HaS and/or COa enters the plant through a scrubber, which removes any free liquids and/or entrained solids. The sour gas then enters the bottom of the absorber and flows upward in countercurrent contact with the descending aqueous amine solution. Sweetened gas leaves the top of the absorber and flows to a dehydration unit, where saturation water from the aqueous amine solution is removed. 

The major use of methanol as an antifreeze is at present in desiccant-antifreeze applications for natural processing [7578], The methanol-containing desiccant-antifreeze is used in both -collecting areas and natural pipelines. The amount of methanol used in this application depends on temperatures and climatic conditions. Methanol consumption for this use was about 202217 thousand t for North America in 1990 [75]. The future consumption for methanol in this area will depend essentialfy on the development of new natural reserves and gas-processing plants. Usually, the stream is countercurrently contacted with the liquid desiccant-antifreeze agent in a bubble tower to remove water from the gas and to vaporize some desiccant into the to prevent subsequent solid formation at low temperatures [78]. 

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. 

Dehumidification of air can be effected by bringing it into contact with a cold surface, either liquid or solid. If the temperature of the surface is lower than the dew point of the gas, condensation takes place and the temperature of the gas falls. The temperature of the surface tends to rise because of the transfer of latent and sensible heat from the air. It would be expected that the air would cool at constant humidity until the dew point was reached, and that subsequent cooling would be accompanied by condensation. It is found, in practice, that this occurs only when the air is well mixed. Normally the temperature and humidity are reduced simultaneously throughout the whole of the process. The air in contact with the surface is cooled below its dew point, and condensation of vapour therefore occurs before the more distant air has time to cool. Where the gas stream is cooled by cold water, countercurrent flow should be employed because the temperature of the water and air are changing in opposite directions. 

Cooling. A gas stream may be dehumidified by bringing it into contact with a cold liquid or a cold solid surface. If the temperature of the surface is lower than the dew point of the gas, condensation will take place, and the temperature of the surface will tend to rise by virtue of the liberation of latent heat It is therefore necessary to remove heat constantly from the surface. Because a far larger interfacial surface can be produced with a liquid, it is usual to spray a liquid into the gas and then to cool it again before it is recycled. In many cases, countercurrent flow of the gas and liquid is obtained by introducing the liquid at the top of a column and allowing the gas to pass upwards. 

Practical separation techniques for liquid particles in gases are discussed. Since gas-borne particulates include both liquid and solid particles, many devices used for dry-dust collection (discussed in Sec. 17 under Gas-Solids Separation ) can be adapted to liquid-particle separation. Also, the basic subject of particle mechanics is covered in Sec. 6. Separation of liquid particulates is frequently desirable in chemical processes such as in countercurrent-stage contacting because liquid entrainment with the gas partially reduces true countercurrency. Sep-... 

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