Ozonation reactor, bubble column

Bubble columns (BCs) and stirred tank reactors (STRs) are the most frequently used types of reactors in laboratory ozonation experiments. Bubble columns can be roughly assumed to behave like perfectly mixed reactors with respect to the liquid phase, provided the ratio of height (h) to diameter (d) is small hid < 10). 

The system mostly applied in practice for supply of ozone is the bubble column and the stirred tank reactor. With these reactor systems it is always possible to set up the complete reactor modification as a plug flow reactor, a continuous flow single stirred tank reactor or a cascade of stirred tank reactors. 

To transfer ozone to a water phase very often fine bubble columns or stirred or agitated tank reactors are applied. The bubbles in these reactors are small. The value of m for ozone is relatively high. Furthermore the diffusion coefficient of ozone in the gas phase is very high compared with the diffusion coefficient of ozone in the water phase. These three reasons lead to the conclusion that in bubble columns and stirred tank reactors the resistance to mass transfer in the gas phase can be neglected. Equation (20) can than be simplified to ... 

The most frequently used contactors in full-scale waste water ozonation systems are bubble column reactors equipped with diffusers or venturi injectors, mostly operated in a reactor-in-series counter-current continuous mode. Many full-scale ozone reactors are operated at elevated pressure (2-6 barabs) in order to achieve a high ozone mass transfer rate, which in turn increases the process efficiency. 

PVC is a very inexpensive material that can be used for lab-scale ozone reactors, however, it is slowly but progressively attacked by ozone. Bubble columns or tube reactors can easily be constructed from PVC tubes. Generally, gas tightness is best achieved by welding, but it can only be operated at ambient pressure (Pabs =100 kPa). Its use in full-scale applications has seldom been reported (see Table A 3-5). 

Bubble columns and various modifications such as airlift reactors, impinging-jet-reactors, downflow bubble columns are frequently used in lab-scale ozonation experiments. Moderate /qa-values in the range of 0.005-0.01 s l can be achieved in simple bubble columns (Martin et al. 1994 Table 2-4 ). Due to the ease of operation they are mostly operated in a cocurrent mode. Countercurrent mode of operation, up-flow gas and down-flow liquid, has seldom been reported for lab-scale studies, but can easily be achieved by means of applying an internal recycle-flow of the liquid, pumping it from the bottom to the top of the reactor. The advantage is an increased level of the dissolved ozone concentration cL in the reactor (effluent), which is especially important in the case of low contaminant concentrations (c(M)) and/or low reaction rate constants, i. e. typical drinking water applications... 

Laboratory-scale bubble columns for ozonation preferably have a reactor liquid phase volume of VL = 2-10 L, with a height-to-diameter-ratio of hid = 5-10. The ozone/oxygen (ozone/air) gas mixture is supplied through a ceramic or stainless steel porous plate fine pore diffuser (porosity 3,10-40 pm hole diameter). PTFE-membranes are a comparatively new alternative for the ozone gas-to-water transfer (Gottschalk et al., 1998). 

Stirred tank reactors (STR) are the most frequently used reactors in lab-scale ozonation, partially due to the ease in modeling completely mixed phases, but they are very seldom used in full-scale applications. There are various modifications with regard to the types of gas diffusers or the construction of the stirrers possible. Normally lab-scale reactors are equipped with coarse diffusers, such as a ring pipe with holes of 0,1-1.0 m3 diameter. The k/ a-values are in the range of 0.02 to 2.0 s (see Table 2-4 ), which are considerably higher than those of bubble columns. From the viewpoint of mass transfer, the main advantage of STRs is that the stirrer speed can be varied, and thus also the ozone mass transfer coefficient, independently of the gas flow rate. 

Similar factors have been developed for bubble columns, which includes the concept of gas hold-up eG, the fraction of the reactor liquid volume occupied by the gas dispersed in the liquid phase. The number of such factors can be reduced when comparing the mass transfer of just one compound in the same liquid/gas system, e. g. for oxygen or ozone transfer in clean water/air systems the above relationship reduces to the first three terms. 

Some useful correlations which can be used for a first approximation of the kLa s or c s in laboratory-scale ozone reactors can be found in Dudley (1995) for bubble columns, and Libra (1993) for STRs. Various correlations found in the literature, empirical as well as those based on theoretical or dimensional analysis, have been compared to results from their own experiments. Dudley concluded that correlations based on theoretical support performed better than those developed by curve-fitting. 

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