Riser-downcomer loop

The circulation time for airlift columns is defined as the time required for a fluid element to travel once around the riser-downcomer loop, and it characterizes the... 

Another type of instability can occur when an upflow riser is directly coupled with a downcomer that returns entrained particles to the bottom of the riser. A pressure balance between the riser and the downcomer is required to maintain steady operation. If the gas velocity is decreased at a given solids circulation rate, a critical state may be reached at which steady operation at a given solids flux is impossible instability occurs because solids cannot be fed to the riser at the prescribed rate. Such an instability, referred to as standpipe-induced (Bi et al., 1993), occurs at a lower critical velocity for a higher solids holdup in the riser. The point of instability can be predicted based on an analysis of the pressure balance in the riser-downcomer loop (Bi and Zhu, 1993). To circumvent stand-pipe-induced instability, the solids inventory in the standpipe needs to be sufficiently high or, alternatively, the riser needs to be uncoupled from the downcomer, e.g., by employing screw feeders. 

A schematic diagram of cyclohexane oxidation airlift loop reactor is illustrated in Fig.l. This reactor consists of outer vessel (riser), coneentric draft-tube(downcomer) and gas... 

Eulerian two-fluid model coupled with dispersed itequations was applied to predict gas-liquid two-phase flow in cyclohexane oxidation airlift loop reactor. Simulation results have presented typical hydrodynamic characteristics, distribution of liquid velocity and gas hold-up in the riser and downcomer were presented. The draft-tube geometry not only affects the magnitude of liquid superficial velocity and gas hold-up, but also the detailed liquid velocity and gas hold-up distribution in the reactor, the final construction of the reactor lies on the industrial technical requirement. The investigation indicates that CFD of airlift reactors can be used to model, design and scale up airlift loop reactors efficiently. 

The air-lift consists of two pipes, intercoimected at top and bottom. In one of the pipes (the riser) air is sparged at the bottom. The air rises and escapes at the top. Therefore, nnder most circnmstances there is no air present in the other pipe (the downcomer). The density difference between riser and downcomer canses an intensive liquid circulation. Two designs can be used, i.e., the internal (Figure 11.8A) and the external loop reactor (Figure 11.8B). 

Figure 7.11b shows an EL airlift reactor, in schematic form. Here, the downcomer is a separate vertical tube that is usually smaller in diameter than the riser, and is connected to the riser by pipes at the top and bottom, thus forming a circuit for liquid circulation. The liquid entering the downcomer tube is almost completely degassed at the top. The liquid circulation rate can be controlled by a valve on the connecting pipe at the bottom. One advantage of the EL airlift reactor is that an efficient heat exchanger can easily be installed on the hquid loop hne. 

The riser, gas-solid separator, downcomer, and solids flow control device are the fdur integral parts of a CFB loop. The riser is the main component of the system. In the riser, gas and solids commonly flow cocurrently upward, although they can also flow cocurrently downward. This chapter covers only the cocurrent gas and solid upward operation. In this operation, as shown in Fig. 10.1, the fluidizing gas is introduced at the bottom of the riser, where solid particles from the downcomer are fed via a control device and carried upward in the riser. Particles exit at the top of the riser into gas-solid separators. Separated particles then flow to the downcomer and return to the riser. 

The riser cannot be considered as an isolated entity in the CFB loop. When no particles are introduced into or discharged from the loop, the solids mass flow rate in the riser is equal to that in the downcomer. For a given quantity of solids in the loop, the presence of fewer solids in the riser implies the presence of more solids in the downcomer. Likewise, the pressure drop across the riser must be balanced by that imposed by the flow through its accompanying components such as the downcomer and the recirculation device. Furthermore, the flow characteristics of the riser can be significantly affected by the behavior of the accompanying components in the loop. 

The constraint of the pressure drop across the downcomer is graphically illustrated in Fig. 10.8. For a given solids inventory in the downcomer and given gas and solids flow rates, the pressures at the bottom of the riser and the downcomer can be determined at steady state. Under normal operating conditions (point A in the figure), the pressure drop across the riser is balanced by the pressure drop across the recirculation loop. If a small reduction in gas velocity takes place, the flow in the riser responds by moving upward along the pressure drop curve of the riser to point B. On point B of line AB, the decrease in the gas velocity causes the pressure drop across the riser to rise by SPt, which has to be balanced by the... 

The pressure drops in Eq. (10.16) can be obtained by assuming that the particles in the downcomer are in the incipient fluidization state. Neglecting the solids holdup in the connecting tube between the riser and the cyclone and in the cyclone, the mass balance of solid particles in the CFB loop can be expressed as... 

Figure 5 shows typical test facilities used by different investigators. In configuration, they all consist of a fast column or riser, a gas-solids separator, a downcomer for solids recycle, and a loop seal valve and/or an additional controlling device for adjusting solids circulation rate, mounted in positions appropriate with the inlet and outlet geometry. These facilities can be grouped into three types. 

Fig. 10.5. A schematic representation of a circulating fluidized bed. The CFB loop consists of a riser, gas-solid cyclone separators, standpipe type of downcomer, and a non-mechanical solids flow control device. Reprinted from [82] with permission from...

The polymer s loop circulation is set up and defined by the pressure balance between the two polymerization zones. As it flows dovm under gravity, the downcomer polymer bed pumps the gas downwards and recovers the head losses developed in the riser, the gas/polymer separator and all other sections of the... 

FIGURE 11.23 External-loop air-lift reactor (a) riser sparging, (b) downcomer sparging. 

In addition to low pressure drop, external-loop air-lift reactors have several advantages over bubble-column reactors. The former offer much flexibility in design in terms of height-to-diameter ratio, area ratio of downcomer to riser, sparger locations, and so on. Conditions can be manipulated... 

Figure 8.1 Internal-loop airlift bioreactor with (a) a baffle separating the riser and downcomer, (b) a continuous draught tube separating the riser and downcomer, and (c) a sectioned draught tube separating the riser and downcomer.

The ELALR can be used for these processes because gas disengagement is very efficient. The bnbbles have a relatively fast rise velocity and slow radial velocity. Hence, bubble-bnbble interactions are diminished in the external-loop variant relative to the bnbble colnmn or stirred-tank bioreactor, which, in turn, leads to higher gas holdnp sensitivity to liquid property variations in bubble columns than in ELALRs (Chisti, 1989 Joshi et al., 1990 Shariati et al., 2007). In other words, the bnbble-bnbble collision frequency is lower in ELALRs, which makes coalescence-adjnsting liqnid properties, such as viscosity, surface tension, or ionic strength, less important. So, while bubble column and internal-loop airlift bioreactor gas holdnp are nsnally similar, the downcomer gas holdup in an external-loop airlift bioreactor is only 0-50% of the riser gas holdup (Bello et al., 1984), which leads to much lower global gas holdup in ELALRs. 

Researchers have also combined reactor types. Guo et al. (1997), for example, designed an external loop airlift reactor that incorporates a fluidized bed within the downcomer section, shown in Figure 10.1. The fluidized bed section is used to immobilize microorganisms on carrier particles in order to protect them from damage. The design is meant for the production of enzymes, biofluidization, and wastewater treatment. Although shear rates were minimized, bubbles were not entrained within the downcomer. Furthermore, the gas-liquid mass transfer coefficient was observed to increase with gas holdup. The result was that the gas-liquid mass transfer was limited due to the fact that global gas holdup for the reactor was strictly defined by the riser gas holdup without any addition by the downcomer. 

The BCDT system gave higher gas holdups than a corresponding external loop (EL) system. This was attributed to a longer connector between the riser and downcomer in the EL system, which allowed escape of snbstantial amount of gas. For rheologi-cally complex hqnids, Chisti et al. (1986) obtained Eqnation 10.24 ... 

To estimate the superficial liquid velocity in the riser, this correlation must be combined with the Fanning friction factors in the riser and downcomer, f and f, the frictional loss coefficients at the top and bottom of the airlift, and and the gas hold-up in the riser, This approach predicted their experimental data in a pilot-plant scale external-loop airlift column for a carboxymethyl cellulose solution, with an error of 20%. 

A typical steam-generation thermosiphon circuit is shown in Exhibit 7-48. Briefly, this circuit is the difference of the water-steam mixture and the static head in the downcomer that maintains fluid circulation. Locating the steam drum at the top of the furnace provides the static head and collects the steam being generated Steam returns to the top portion of the drum, while the water lines come off die bottom. Loops and pockets must be avoided when layipg out die downcomer, and riser piping and any horizontal lines must slope toward the vwiste heat boiler or convection coil. Wh pumps are used in this circuit, it is called forced circulation. 

In airlift bioreactors the fluid volume of the vessel is divided into two interconnected zones by means of a baffle or draft-tube (Fig. 5). Only one of these zones is sparged with air or other gas. The sparged zone is known as the riser the zone that receives no gas is the downcomer (Fig. 5a-c). The bulk density of the gas-liquid dispersion in the gas-sparged riser tends to be less than the bulk density in the downcomer consequently, the dispersion flows up in the riser zone and downflow occurs in the downcomer. Sometimes the riser and the downcomer are two separate vertical pipes that are interconnected at the top and the bottom to form an external circulation loop (Fig. 5c). External-loop airlift reactors are less common in commercial processes compared to the internal-loop designs (Fig. 5a, b). The internal-loop configuration may be either a concentric draft-tube device or an split-cylinder (Fig. 5a, b). Airlift reactors have been successfully employed in nearly every kind of bioprocess—bacterial and yeast culture, fermentations of mycelial fungi, animal and plant cell culture, immobilized enzyme and cell biocatalysis, culture of microalgae, and wastewater treatment. 

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