External recirculation

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). 

External recirculation is the movement of the heated air within the bay to an external duc t, where this air mixes with inlet air, and the mixture serves as the cooling fluid within the bay. Inlet air does not have direct access to the tube bundle an adequate mixing chamber is essential. Recirculation over the end of the exchanger is illustrated in Fig. 11-48. Over-the-side recirculation also is used. External recirculation systems maintain the desired low temperature of the air crossing the tube bundle. 

FIG. 11-48 External recirculation with adequate mixing chamber. [Hydrocar-hon Vrocess, 59, 148-149 (October 1980).]... 

A modified type of airlift system widi gas and liquid flow patterns in which a pump transports the ah and liquid through die vessel. Here, an external loop is used, with a mechanical pump to remove the liquid. Gas and circulated liquid are injected into the tower through a nozzle. Figure 6.2 shows an airlift bioreactor diat operates widi an external recirculation pump. 

Fig. 6.2. Airlift bioreactor with external recirculation pump.

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. 

The operation of an LC-Finer is best described by means of a process flow schematic (Figure 2). The LC-Finer reactor maintains the catalyst (typically American Cyanamid 1442B cobalt molybdenum 1/32 inch extrudate or Shell 324 nickel molybdenum 1/32 inch extrudate) in constant motion, suspended by the recirculation of copious volumes of liquid. This recirculation results in a 35-50% bed expansion and the reactor operates at a uniform temperature with essentially no pressure drop. In a commercial unit there is a recycle of hydrogen rich gas along with a distillate liquid stream which is combined with the fresh SRC. The PDU differs from the commercial unit design in that there is no recycle gas or liquid streams. The bed expansion is maintained with an external recirculation loop. It should be noted that the PDU fractionator separates the liquid product into a light oil (L.O.) and a heavy oil (H.O.). The combination of these two oil streams is designated as total liquid product (TLP). 

Experimental results are presented for high pressure phase equilibria in the binary systems carbon dioxide - acetone and carbon dioxide - ethanol and the ternary system carbon dioxide - acetone - water at 313 and 333 K and pressures between 20 and 150 bar. A high pressure optical cell with external recirculation and sampling of all phases was used for the experimental measurements. The ternary system exhibits an extensive three-phase equilibrium region with an upper and lower critical solution pressure at both temperatures. A modified cubic equation of a state with a non-quadratic mixing rule was successfully used to model the experimental data. The phase equilibrium behavior of the system is favorable for extraction of acetone from dilute aqueous solutions using supercritical carbon dioxide. 

Fixed-bed bioreactor (A) schematic representation including the external recirculation loop with an aeration tank (B) photograph of a small-scale bioreactor. 

A detailed model for a gas-liquid column with external recirculation loop has been published by Orejas [11]. The model takes into account the axial dispersion and mass transfer from bubbles. An important conclusion is that the mass-transfer rate is fast compared with the chemical reaction. As a result, a pseudohomoge-neous model for liquid-phase reaction may be applied for design purposes. 

PFR differential fixed bed PFR integral fixed bed PFR solids transport (riser) CSTR external recirculation CSTR internal fluid recirculation CSTR spinning catalyst basket Batch internal fluid recirculation... 

Whereas for bubbling fluidized beds the solids holdup in the upper part of the reactor and the entrainment of catalyst are often negligible, these features become most important in the case of circulating fluidized beds These systems are operated at gas velocities above the terminal settling velocity ux of a major fraction or even all of the catalyst particles used (% 1 m s 1 < umass flow rales to be externally recirculated are high, up to figures of more than 1000 kg m 2s-1... 

Surface aeration is usually employed for slow reactions or for batch processes. It can be used in semicontinuous systems when it is desirable to recirculate the gas from the headspace. This is frequently the case in hydrogenation and is referred to as dead-end hydrogenation. In this system, gas is fed continuously to the reactor at the rate at which hydrogen is being consumed no compression costs to overcome the static head of liquid or external recirculation is needed. Feeding gas from the headspace may be preferred when there is a possibility of plugging sparger holes with reaction products. Surface aerators are also extensively used for waste-water treatment. There are two types of surface aerators the brush aerator, and the most commonly used turbine aerator. 

External recirculation, also represented schematically in Fig. 7.18, is an alternative to achieve perfect mixing, which is convenient when only gas streams are involved. The minimum external recirculation ratio depends on the reaction... 

Fig. 7.18. CSTR with fixed bed and internal (Berty reactor) or external recirculation.

Picture 6. Pilot Unit for continuous HP- reactions in or with supercritical fluids using different fixed bed reactors with external recirculation. 

Uniform temperature in the whole FAB-gasifier through internal and external recirculation of solids... 

In monolith reactor modeling, the surface reactions, the mass transfer within the channels, the residence-time distribution, and the external recirculation of gas and liquid must be considered. 

External recirculation, also represented schematically in Fig. 8.27, is an alternative to achieve perfect mixing, which is convenient when only gas streams are involved. The minimum external recirculation ratio depends on the reaction kinetics, but typically ratios higher than 10 are required to achieve perfect mixing [9]. Experiments performed with a CSTR give direct access to the net production rates of the species involved by solving the corresponding mass balances ... 

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