Gas spargers

In this section devices are discussed, which are utilized to intensify mass transfer in gas/liquid systems, but cannot be regarded as stirrers. They are  

Sintered glass or ceramic plates and perforated metal plates are classical gas spargers (dispersers) for bubble columns. Static mixers and nozzles became available with the emergence of biological waste water purification. They all realize gas 

There are numerous publications from the field of chemical reaction ingineering, see the references in [629], which deal with mass transfer in bubble columns. They consistently report, that the volume-related mass transfer coefficient on the liquid-side k] a at not too high superficial gas velocities (c 200 m/h) is directly proportional to the superficial velocity v  

This is an importantant information when one realizes that fei = —— and the VAc 

In contrast with expression (4.109), the proposition of expression (4.110) is intellectually understandable, the rate of absorption G being directly proportional to the gas throughput q (number of gas bubbles), the liquid height (residence time of the gas bubbles) and the concentration difference Ac  

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For small instaUafions, column foam separators are more suitable. Waste flows downward in the column whereas gas spargers, located at the bottom, give countercurrent flow. The foam generated is carried upward to a foam breaker and coUector. 

As gas flows with fixed volumetric flow rate through an orifice gas sparger, bubbles are formed with diameter Analysis of bubble formation is based on the balance of buoyant force, as the bubbles leave the orifice and rise through the media (irApgDl)/6 with rest of the forces resulting from the surface tension, Trad. 

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. 

Fig. 26. Reference floe diameter of floe particle system versus mass transfer coefficient kpa for bubble columns with different gas spargers H = 1.08 m D = 0.4 m...

Fig.8 illustrates the liquid velocity distribution at the bottom section of the reactor when the draft-tube diameter is 0.45m, 1.05m and 1.45m respectively. Results show that the liquid velocity at the outlet of the draft-tube lowers when the draft-tube diameter is raised, to subsequently influence the shape and size of the vortex at the bottom of the gas sparger. 

In Fig. 11, it can be found that the draft-tube height also influences the shape and size of the liquid vortex around the gas sparger. 

Fig. 7.1 Experimental set up for sonochemical carbonization experiments [24] (1) Ultrasound Probe, (2) C02 gas sparger, (3) Conductivity meter, (4) pH meter, (5) C02 gas cylinder, (6) C02 gas flow meter, (7) Air flow meter, ( ) Air compressor (9) Magnetic stirrer, (10) Ca(OH)2 slurry...

Gas sparger 8 mm o. d. PTFE-tube 2 mm i. d. bubbler fine none defined holes in... 

Gas sparger design Number of holes in sparger ring Minimum speeds of suspension, re--suspension and flooding... 

The estimation of the gas hold-up is an important practical variable, since it provides increase in the liquid level of the tank caused by the gas sparger, that is, the minimum foaming height that will form on top of the reactor. 

Intensified turbulence can destroy the stability of large bubbles, which leads to smaller bubbles, and increase the frequency of bubble coalescence and breakup, which improves the surface renewal frequency of bubbles [17,18]. Intensified turbulence can be achieved by changing the gas sparger to decrease the initial bubble size and improve its radial distribution, but the effective region of the gas sparger is only limited to a certain height above the distributor [19]. 

With a gas sparger and a radial turbine of the Rushton type, gas loads of up to 500 m3/(m2h) can be achieved with reasonable energy consumption. This method of dispersing the gas phase is usually employed for fast reactions or in situations where the hourly demand for the gaseous reactant is high, as in industrial fermenters. With such high gas-feed rates, the gas reactant may not react completely, so that eventually the unreacted gas may be recycled externally. 

Nagata (1975) showed that in aerated suspensions, a significantly higher stirrer speed and thus power consumption per unit volume is required to establish the state of complete suspension. Furthermore, the propeller normally requires a higher stirrer speed for complete suspension than the turbine. Arbiter et al. (1969) reported that drastic sedimentation of suspended particles occurs when the aeration number JVA = QJN d (here Qg is the volumetric gas flow rate) exceeds a critical value. This critical gas flow coincided with the point where the power drawn by the agitator decreased suddenly with a small increase in the gas sparger rate. Thus, an increase in gas... 

If physico-chemical properties of the system are changed, the above relation may not be completely valid (13). Future efforts should be concentrated in examining the validity of the existing correlations for large diameter (tall and short) bubble columns operated with a variety of gas spargers. 

The dispersion coefficients of the liquid phase are dependent on the gas velocity and on the column diameter. The liquid flow, the type of gas sparger and physico-chemical properties like viscosity do not influence the dispersion coefficient, El/ or,at best, these parameters are of very minor importnace. For instance, Hikita and Kikukawa (46) found only a slight viscosity influence, i.e. El0Ci ... 

Case 1. The pressure drop across the liquid sparger is very much less than that caused by its acceleration over the gas sparger region. This case... 

Pressure drop across the gas sparger is expressed as follows AP = /lv sud + = kyVQ + Pxl G-... 

The homogeneous flow regime can be maintained as long as the LHS of Eq. (183) is less than the RHS. For the aspect ratio (h = H/D) > 1 the RHS is a constant quantity. The LHS depends on the design of the gas sparger and the physical properties of the liquid. The role of these parameters will now be discussed systematically. 

Figure 15. Nitrogen-gas sparger and blanketing system for loading tank cars and trucks (a) inline sparger and (b) complete system.

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