Gas sparging

For bubble columns with height/diameter 5, a simple open pipe at the bottom of the column is often adequate. For height/diameter 5, a ring or finger-style perforated pipe sparger is desirable to obtain uniform radial distribution of the gas and to prevent excessive channeling of the gas up the center of the vessel. For heat transfer in bubble agitated columns, see Hart (1976) and Tamari and Nishikawa (1976). 

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At a given gas sparging rate, interfacial area a is constant at low mixer speeds. When mixer speed is increased above a critical speed a starts increasing and varies linearly with Ai For Rushton turbines this critical speed, as deterrnined in an sodium sulfate system, is given by the following ... 

Activate inert gas sparging into reactor liquid to effect mixing... 

Foaming of the bioreactor is a nuisance, reflects on the mass transfer process and must be prevented, for many reasons. The problems related to foaming are obvious if they are due to gas sparging. The problems are the loss of broth, clogging of the exhaust gas system... 

FIGURE 11.2 Mechanically agitated vessel with gas sparging. 

Example 11.18 Consider a gas-sparged CSTR with reaction occurring only in the liquid phase. Suppose a pilot-scale reactor gives a satisfactory product. Propose a scaleup to a larger vessel. 

As in boundary-layer flows, smaller reference floe diameters are found with gas sparging than with the same average power input in a baffled stirred tank 127] or [44,45]. This can be explained if it is assumed that the floes come into close contact with the gas phase and find their way into the zones of higher stress. 

In view of the importance of the particle/bubble contact, it may be assumed that the stress acting on the particles during gas sparging is determined by electrostatic interactions as well as by hydrophobic and hydrophilic interactions, which are determined by the nature of the liquid/solid system. The use of Pluronic as additive leads to the reduction of destruction process [44,47] possibly due to less bubble/floc contact which is also described by Meier et. al. [67]. 

It can be seen that for the same average power input, greater stresses are produced by gas sparging than by many impellers. Fig. 17. According to the comparison in Fig. 17, evidently zones exist in bubble columns in which the energy densities are 20 times higher than in a stirred tank. But the comparison on the basis of average power input in Fig. 16 shows that also impeller (for example small inclined blade impellers) exist which produce more shear than bubble columns. 

In contrast to this, the enzyme resin is stressed less by gas sparging than by stirring (see Fig. 18 and 20). The same activity losses were observed first with 1 to 8 times greater specific adiabatic compression power Pj/ V than the maximum power density necessary for stirring. As in the case of the smooth disc, the effects of power input are only weak. The type of gas sparger and therefore the gas exit velocity are of no recognisable importance. The behaviour of the enzyme resin particles is thus completely different from that of the clay min-eral/polymer floes and the oil/water/surfactant droplet system, which are particularly intensively stressed by gas sparging. 

This result makes it clear that particle stress is strongly dependent on the interaction between the particles and the interface, so that electrostatic and also hydrophobic and hydrophilic interactions with the phase boundary are particularly important. This means that the stress caused by gas sparging and also by boundary-layer flows, as opposed to reactors with free turbulent flow (reactors with impellers and baffles), may depend on the particle system and therefore applicability to other material systems is limited. 

The stress caused by gas sparging and also by boundary-layer flows, as opposed to reactors with free turbulent flow (reactors with impellers and bafQes), may depend on the particle system. 

Friesen, K.J., Fairchild, W.L., Loewen, M.D., Lawrence, S.G., Holoka, M.H., Muir, D.C.G. (1993) Evidence for particle-mediated transport of 2,3,7,8-tetrachlorodibenzofuran during gas sparging of natural water. Environ. Toxicol. Chem. 12, 2037-2044. 

For bubble columns the value of knq Am strongly depends on the superficial gas velocity. For the stirred tank reactor gas sparged is immediately sucked into the gas cavities behind the stirred blades. The value of k)iq depends strongly on both the superficial gas velocity, the pressure at the stirrer level and the liquid volume. 

Inorganic carbon first removed by acidification and inert gas sparging... 

Figure 4 1,4 - Dioxane degradation with various gas sparges. Frequency = 358 kHz and initial 1,4 - Dioxane concentration =Co = 1 mM. Q = 1,4 - Dioxane concentration at time, t.

Rapid shaking of a horizontal graduated cylinder containing a protein solution produces a foam that can be measured by its volume (2 16). Foaming capacity of sparged foams is measured by the ratio of the volume of gas in foam to the volume of gas sparged, or by the maximum volume of foam divided by the gas flow rate ], 10, 20, 21). 

A variety of gas-liquid contacting equipment with mechanical moving elements (e.g., stirred (agitated) tanks with gas sparging) are discussed in Chapters 7 and 12, including rotating-disk gas-liquid contactors and others. 

The ratio of the power requirement of gas-sparged (aerated) liquid in a stirred tank, Pq, to the power requirement of ungassed liquid in the same stirred tank, Pq, can be estimated using Equation 7.34 [7]. This is an empirical, dimensionless equation based on data for six-flat blade turbines, with a blade width that is one-fifth of the impeller diameter d, while the liquid depth Hp is equal to the tank diameter. Although these data were for tank diameters up to 0.6 m. Equation 7.34 would apply to larger tanks where the liquid depth-to-diameter ratio is typically... 

Gas-liquid mass transfer in fermentors is discussed in detail in Section 12.4. In dealing with in gas-sparged stirred tanks, it is more rational to separate and a, because both are affected by different factors. It is possible to measure a by using either a light scattering technique [9] or a chemical method [4]. Ihe average bubble size can be estimated by Equation 7.26 from measured values of a and the gas holdup e. Correlations for have been obtained in this way [10, 11], but in order to use them it is necessary that a and d are known. 

It would be more practical, if A in gas-sparged stirred tanks were to be directly correlated with operating variables and liquid properties. It should be noted that the definition of k a for a gas-sparged stirred tank (both in this text and in general) is based on the clear liquid volume, without aeration. 

It is worth remembering that the power requirement of gas-sparged stirred tanks per unit liquid volume at a given superficial gas velocity Uq is proportional to L where N is the rotational speed of the impeller (T ) and L is the tank size (L), such as the diameter. Usually, k a values vary in proportion to (Pq/V) " and Uq", where m = 0.4-0.7 and = 0.2-0.8, depending on operating conditions. 

In evaluating k a in gas-sparged stirred tanks, it can usually be assumed that the liquid concentration is uniform throughout the tank. This is especially true with small experimental apparatus, in which the rate of gas-liquid mass transfer at the free liquid surface might be a considerable portion of total mass transfer rate. This can be prevented by passing an inert gas (e.g., nitrogen) over the free liquid. 

The main reasons for mixing the hquids in the fermentors with a rotating stirrer and/or gas sparging are to... 

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