Gas-Sparged Liquids

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 

The ratio Pq/Pq for flat-blade turbine impeller systems can also be estimated by Equation 7.35 [8]. 

Calculate the power requirements, with and without aeration, of a 1.5 m-diameter stirred tank, containing water 1.5 m deep, equipped with a six-blade Rushton turbine that is 0.5 m in diameter d, with blades 0.2.5 d long and 0.2 d wide, operating at a rotational speed of 180 rpm. Air is supplied from the tank bottom at a rate of 0.6 m min C Operation is at room temperature. Values of water viscosity = 0.001 kg m and water density p = 1000 kg m 

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

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. 

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]. 

Internal-loop airlift reactors (ALRs) are widely used for their self-induced circulation, improved mixing, and excellent heat transfer [1], This work reports on the design of an ALR with a novel gas-liquid separator and novel gas distributor. In this ALR, the gas was sparged into the annulus. The special designed gas-liquid separator, at the head of the reactor, can almost completely separate the gas and liquid even at high gas velocities. 

A chemical sensor array (consisting of eight conducting polymer sensors) derived from an electronic nose [62], for the characterization of headspace gas from a sparged liquid sample... 

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. 

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. 

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. 

Bubbling column reactor BCR Also called "gas sparged reactor", it is little used in hydrogenations. Gas is fed, with partial recycling to increase turbulence, at the bottom of a virtually stationary L phase. Mixing is by far less efficient than in STR or JLR. BCR is preferred only when the overall reaction is slow it is an alternative for TBR (S 2.2.6) with better temperature control as a result of higher liquid holdup. 

Gas Sparging with Mechanical Agitation Calderbank (1958) correlated gas hold-up for the gas-liquid dispersion agitated by a flat-blade disk turbine impeller as... 

The power required by an impeller in a gas sparged system Pm is usually less than the power required by the impeller operating at the same speed in a gas-free liquids Pmo. The Pm for the flat-blade disk turbine can be calculated from Pmo (Nagata, 1975), as follows ... 

Liquid mixing time decreases sharply for an initial increase in the gas sparging rate and approach an asymptotic value that is determined by the height and diameter of the downcomer and the liquid properties [5]. A higher liquid velocity shortens the gas residence time and results in a decrease of gas holdup and interfacial area. The radial profile of the liquid is parabolic. These are disadvantageous for mass transfer. The mounting of internals in a fixed bed is often used to improve the radial profile of the liquid velocity. This motivates us to mount internals in an EL-ALRs to improve the radial profile of the gas holdup and the liquid velocity and to intensify turbulence. 

Industrial fluid-fluid reactors may broadly be divided into gas-liquid and liquid-liquid reactors. Gas-liquid reactors typically may be used for the manufacture of pure products (such as sulfuric acid, nitric acid, nitrates, phosphates, adipic acid, and other chemicals) where all the gas and liquid react. They are also used in processes where gas-phase reactants are sparged into the reactor and the reaction takes place in the liquid phase (such as hydrogenation, halogenation, oxidation, nitration, alkylation, fermentation, oxidation of sludges, production of proteins, biochemical oxidations, and so on). Gas purification (in which relatively small amounts of impurities such as C02, CO, COS, S02, H2S, NO, and... 

Slurry Bubble Column Reactors As in the case of gas-liquid slurry agitated reactors, bubble column reactors may also be used when solids are present. Most issues associated with multiphase bubble columns are analogous to the gas-liquid bubble columns. In addition, the gas flow and/or the liquid flow have to be sufficient to maintain the solid phase suspended. In the case of a bubble column fermenter, the sparged oxygen is partly used to grow biomass that serves as the catalyst in the system. Many bubble columns operate in semibatch mode with gas sparged continuously and liquid and catalyst in batch mode. 

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