Internal recirculation

Flocculation is accelerated and higher overflow rates are achieved by external or internal recirculation of settled soflds into the feed which leads to the collection of fine particles by interception. Addition of conditioned fine sand to the feed induces separation by differential sedimentation, and sometimes increases overflow rates to 6—8 m/h. 

Another version of a fluidized-bed reactor has been introduced by Vogelbusch (Austria). This reactor has an internal recirculation loop set up by means of a high flow impeller. The system utilises porous glass beads for immobilizing the cells. However, glass beads may not work with all types of cells. 

For catalytic investigations, the rotating basket or fixed basket with internal recirciilation are the standard devices nowadays, usually more convenient and less expensive than equipment with external recirculation. In the fixed basket type, an internal recirculation rate of 10 to 15 or so times the feed rate effectively eliminates external diffusional resistance, and temperature gradients. A unit holding 50 cm (3.05 in ) of catalyst can operate up to 800 K (1440 R) and 50 bar (725 psi). 

Air Recirculation Recirculation of air which has been heated as it crosses the tube bundle provides the best means of preventing operating problems due to low-temperature inlet air. Internal recirculation is the movement of air within a bay so that the heated air which has crossed the bundle is directed by a fan with reverse flow across another part of the bundle. Wind skirts and louvers are generally provided to minimize the entiy of low-temperature air from the surroundings. Contained internal recirculation uses louvers within the bay to control the flow of warm air in the bay as illustrated in Fig. 11-47. Note that low-temperature inlet air has access to the tube bundle. 

FIG. 11-47 Contained internal recirculation (with internal louvers). [Hydrocarbon Process, 59, 148-149 (October 1980).]... 

Exothermic processes, with cooling through heat transfer surfaces or cold shots. In use are sheU-and-tube reactors with smaU-diameter tubes, or towers with internal recirculation of gases, or multiple stages with intercoohng. Chlorination of methane and other hydrocarbons results in a mixture of products whose relative amounts... 

Flue Gas Recirculation Flue gas recirculation, alone or in combination with other modifications, can significantly reduce thermal NO. Recirculated flue gas is a diluent that reduces flame temperatures. External and internal recirculation paths have been applied internal... 

One such laboratory flow reactor for a gas-phase reaction catalyzed by a solid (particles indicated) is shown schematically in Figure 1.2. In this device, the flowing gas mixture (inlet and outlet indicated) is well mixed by internal recirculation by the rotating impeller, so that, everywhere the gas contacting the exterior catalyst surface is at the same composition and temperature. In this way, a point rate of reaction is obtained. 

Reactions between liquids and gases of low solubility is conducted in stirred vessels. The liquid is charged first and the gas is fed continuously at the rate of solution or reaction, sometimes with internal recirculation in larger units. Miscible liquid reactions may be conducted similarly in order to moderate undesirable temperature play. 

Figure 13.24. The effect of variation of mixer internal recirculation on extraction efficiency ...

To reduce nuisance and hazards from dust inside the livestock building itself. Because the dust is practically all generated in the building, only filtration of internally recirculated air can effectively reduce the concentration of dust inside the building. 

The Control was for a standard room and filtered was when an internal recirculating filter was fitted. Typical outdoor values on the farms were 0.1-0.2 mg/m3. 

An SOFC is operating on 100 % methane (CH4) and a fuel utilization of 85%. (a) What is the composition of the effluent (spent) fuel in mol % Assume that the methane is completely reformed within the fuel cell, and the moisture required for reforming is supplied by internal recirculation. 

Flue gas recirculation Flue gas recirculation, alone or in combination with other modifications, can significantly reduce thermal NO,. Recirculated flue gas is a diluent that reduces flame temperatures. External and internal recirculation paths have been applied internal recirculation can be accomplished by jet entrainment using either combustion air or fuel jet energy external recirculation requires a fan or a jet pump (driven by the combustion air). When combined with staged-air or staged-fuel methods, NO emissions from gas-fired burners can be reduced by 50 to 90 percent. In some applications, external flue-gas recirculation can decrease thermal efficiency. Condensation in the recirculation loop can cause operating problems and increase maintenance requirements. 

Fuel-staged burners Use of fuel-staged burners is the preferred combustion approach for NO control because gaseous fuels typically contain httle or no fixed nitrogen. Figure 24-33 illustrates a fuel-staged natural draft refinery process heater burner. The fuel is spht into primary (30 to 40 percent) and secondary (60 to 70 percent) streams. Furnace gas may be internally recirculated by the primary gas jets for additional NO control. NO reductions of 80 to 90 percent nave been achieved by staging fuel combustion. 

INTEGRAL (INTERNAL) RECIRCULATION FROM PUMP DISCHARGE TO SEAL RECOMMENDED FOR... 

Multiple decaying peaks at regular intervals indicate strong internal recirculation... 

Thus far, we have considered the reaction engineering of molecules in containers that conserve mass. The same equations describe the populations of living systems in their environment, a container operating in batch or flow mode with inputs, outputs, disturbances, catalysts, internal recirculation patterns, and even diffusion (svvirtirtiirig, walking, flying) (Figure 8-19). 

Suction specifc speed is calculated for the pump s performance at best efficiency point with the maximum diameter impeller and provides an assessment of a pump s susceptibility to internal recirculation. It is expressed mathematically by the following equation ... 

Key Features of an Internal Recirculation Reactor [After Shah, 1979]... 

Internal recirculation would give better mixing in the catalyst baskets. 

This concept was demonstrated at the Institute of Process Engineering and Cryogenics at ETH (Switzerland) during the last two years with the FilmCooled Hydrothermal Burner (FCHB). The FCHB operated at pressures of 25 MPa and temperatures up to 2000 K, cf. [1], Experiments and detailed analysis led to the basic design approach for SCWO reactors discussed herein which is based on wall boundary layer control and internal recirculation. 

Other reactor concepts show similar features (see [1] and [4]), which include boundary layer control and internal recirculation. Reference [5] proposes boundary layer control utilizing transpiration cooling. 

The experimental results were promising The bed voidage of the second stage can be maintained at a reasonable level and the internal recirculation rate can be controlled smoothly by the fluidization air to the degassing chamber. Thus, the design criteria are satisfied for subsequent scale-up to commercial size. 

Several reactors are presently used for studying gas-solid reactions. These reactors should, in principle, be useful for studying gas-liquid-solid catalytic reactions. The reactors are the ball-mill reactor (Fig. 5-10), a fluidized-bed reactor with an agitator (Fig. 5-11), a stirred reactor with catalyst impregnated on the reactor walls or placed in an annular basket (Fig. 5-12), a reactor with catalyst placed in a stationary cylindrical basket (Fig. 5-13), an internal recirculation reactor (Fig. 5-14), microreactors (Fig. 5-16), a single-pellet pulse reactor (Fig. 5-17), and a chromatographic-column pulse reactor (Fig. 5-18). The key features of these reactors are listed in Tables 5-3 through 5-9. The pertinent references for these reactors are listed at the end of the chapter. 

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