Caracterizar los factores y condiciones actuales de la gestión de los residuos

1.2.1 In trod u ction

Airlift fermenters can be divided into two groups depending on the type of liquid recirculation employed (figure 1.3). External loop vessels have the riser and downcomer separated from each other by connecting horizontal sections near the top and bottom (figure 1.3f). These reactors usually have longer residence times for gas-liquid separation than the internal loop reactors (Choi, 1990). The internal loop reactors such as the concentric tube reactor contain the riser and downcomer in the same vessel. A cylindrical column contains an inner draft tube which separates the riser from the downcomer (figure 1.3c). Air is sparged at the base of the reactor which can travel up the draft tube and down the annulus, the downcomer or annulus can be sparged and hence the flow can be reversed. Split cylinder reactors have the draft tube divided into sections to increase comm unication between the riser and the downcomer (figure 1.3d) which increases oxygen transfer (Chisti and Moo-Young, 1987).

1.2.2 A irlift reactor geom etry

An important section of the concentric and split reactors is the top section or gas separator which connects the riser to the downcomer. The region largely dictates the extent o f gas disengagement and subsequently gas recirculation, downcomer gas holdup and the pressure difference between the riser and downcomer. Therefore, the design of the top section has a major influence on the liquid velocity of the airlift reactor. The top section also has an important function in the mixing of the liquid within the vessel. The mixing performance has been shown to increase with increasing liquid volume in the gas liquid separator up to a certain size, after which no further change in performance occurs (Chisti, 1989, W eiland, 1984, and Russell et al., 1994). Siegel and M erchuk (1991) investigated the influence of gas-liquid separator configurations in airlift reactors. They were able to manipulate the disengagement properties and hence the fluid residence time and gas recirculation rate of an airlift reactor while keeping all other m ajor design parameters constant. The top section had been designed so that movable baffles were controlled to change the reactor configuration from an external to an internal loop. They dem onstrated external loop reactor characteristics by operating the reactor w ith an enlarged top section with a clear zone o f horizontal two phase flow. H igh gas disengagement in the separator, low gas holdup in the downcomer, and relatively high liquid velocities were observed. In the concentric airlift, changing the liquid level had no significant effect on the riser or downcomer gas holdup. Thus liquid levels above the draft tube were not sufficient to cause additional turbulence in the top section which

o e o 0 o 0 o o o _ o d)

I

Figure 1.3 Types o f bioreactor (idealised) : a) stirred tank, b) bubble colum n, c)concentric draft tube airlift, d) concentric split draft tube airlift, e) Split cylinder internal loop reactor, f) external loop.

w ould cause increased gas entrainm ent into the downcomer. Russell et al. (1994) reported that liquid velocity and sectional gas holdups were found to be independent of the top section height in a concentric airlift reactor. This led to the conclusion that the downcom er is essentially independent o f the size o f the top section. Other important parameters in the design of the concentric airlift reactor are the ratio of height to diameter (H/D) and ratio of the cross sectional area of the riser to that of the downcomer (A q/A r). The large ratios o f H/D give airlift reactors there slim design and industrial reactors can

have ratios up to 10 (Choi, 1990). These high ratios allow high utilisation o f oxygen, efficient mass transfer, mixing and circulation properties (Onken and Weiland, 1983).

1.2.3 M echanical agitation in air sparged reactors

The power required for mixing and mass transfer in bubble columns and airlift reactors is supplied by the circulation of air from spargers, but some reactor designers have studied increased reactor performance by the addition of impellers. Possible increases in performance of a system will then be dictated by the type o f impeller used and its positioning within the vessel. The characteristics of different impellers have been studied in stirred tanks. The introduction of power into a stirred tank can be made by a variety o f impellers o f which the most common are the disc turbine, pitch blade, and the marine propeller (figure 1.4). The disc turbine has four or six blades mounted on a disc. The disc limits the short circuiting of gas along the drive shaft. The disc stirrer is a radial flow impeller whereby the radial and tangential velocity components are nearly equal in magnitude at the impeller tip. The tangential component decreases more rapidly than the radial distance. The average velocity, which is taken as the radial and the tangential flow together, is maximum at a certain distance from the impeller tip (Sachs and Rushton, 1954). The pitched blade turbine produces axial flow with the radial flow proportional to the projected blade width (figure 1.4b).

The propeller produces an axial flow regime (figure 1.4c). Joshi et al. (1982) found that the position at which maximum velocity was realised depended on the pitch of the propeller blade. The cross sectional area of the flowing stream from the impeller did not depend on the impeller speed and is only a function of the vertical distance from the impeller plane. Hence, Joshi concluded that the extent of entrainment is not a function of the impeller speed, but at higher impeller speeds the discharge velocity only increases, increasing the discharge flow rates.

The traditional power curves, power number (Np) versus stirrer based Reynolds number for impellers in a single phase have been used to characterise an impeller with a power number. The turbine impeller has Np values between 5.5 - 6.5 for a fully turbulent system (Re > 10,000) which are higher than for a pitched blade ( 1 - 3 ) and propeller (0.1 - 1) (Van't Riet and Tramper, 1991). Therefore, relatively more energy is supplied to the liquid and consumed energy is proportional to the pumping capacity of the stirrer. As mixing and mass transfer are a function o f power consumption, the stirrer w ith the highest power number should be used for a stirred tank (Smith, 1985).

The discharge efficiency of impellers can be used to compare their performance. Rushton and Oldshue (1953) expressed pumping capacity as a function o f the agitator diameter:

r t -

r

U

ijyÉillI

I

I !

a) dise turbine-radial flow b) pitched turbine-axial and radial flow

- r )