Liquid-solid agitation

In eertain eases, the primary proeess objeetive is to keep solid partieles in suspension. Areas of applieation involve eatalytie reaetions, erystallization, preeipitation, ion exehange, and adsorption. Axial flow and pitehed-blade turbines are best suited in providing the essential flow patterns in a tank to keep the solids in suspension. The suspended solid is eharaeterized by two parameters  

Various eonelations are provided for ealeulating the minimum speed of the agitator to keep a given solid in suspension. Zwietering [40] developed the following equation  

Wg = weight ratio of solid to liquid in pereentage Pl = liquid density Pl = liquid viseosity 

For agitator types of propellers, turbines with flat blades and paddles, 4 and a are 1.5 and 1.4, respeetively. The eriterion for Equation 7-92 is the absenee of any immobile solid on the bottom of the tank. 

Weisman and Efferding [41] in constrast, related the degree of agitation to the height Hg oeeupied by the solid suspension, whieh is expressed as  

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Reeently, Corpstein et al. [43] found that high-effieieney impellers provide the same levels of solids suspension at redueed eapital and operating eosts. They introdueed the term just-suspended for the most eommonly eneountered level of liquid solid agitation. This oeeurs when none of the solid partieles remains stationary on the bottom of the vessel for longer than 1-2 see. They developed a eorrelation of the speed required to aehieve just-suspended eonditions as ... 

Hixson, A. W. and Baum, S. J. Ind. Eng. Chem. 33 (1941) 478, 1433. Agitation mass transfer coefficients in liquid-solid agitated systems. Agitation heat and mass transfer coefficients in liquid-solid systems. 

The energy of dissipation per unit mass of liquid in a liquid-solid agitated vessel is related to the power consumption per unit volume of liquid (Ps) as follows (Kato et al., 2001) ... 

R.R. Corpstein, J.B. Fasano, and K.J. Myers, The high efficiency road to liquid-solid agitation, Chem. Eng., 138-144 (October 1994). 

Hixon A.W., Baum S.J., Mass Transfer Coefficients in Liquid-Solid Agitation Systems, Ind. Engng. Chem. 33 (1941) 4, p. 478-485... 

Crystallization and dissolution data obtained from agitated vessel studies may be analysed by the methods discussed above, but a survey of the literature related to the subject of solid-liquid mass transfer in agitated vessels shows that there is an extremely wide divergence of results, correlations and theories. The difficulty is the extremely large number of variables that can affect transfer rates, the physical properties and geometry of the system and the complex liquid-solid-agitator interactions. 

Myers, K. M., A. Bakker and J. Fasano, Simulation and Experimental Verification of Liquid-Solid Agitation Performance, paper 188a, A.l.Ch.E. Annual Meeting, San Francisco, November 13-18, 1994. 

PHASE CONTACTING AND LIQUID-SOLID PROCESSING AGITATION OF LOW-VISCOSITY PARTICLE SUSPENSIONS... 

Zweiteriiig, T.N., 1958. Suspension of solid particles in liquids by agitators. Chemical Engineering Science, 8, 244. 

Extraction (sometimes called leaching) encompasses liquid-liquid as well as liquid-solid systems. Liquid-liquid extraction involves the transfer of solutes from one liquid phase into another liquid solvent it is normally conducted in mixer settlers, plate and agitated-tower contacting equipment, or packed or spray towers. Liquid-solid extraction, in which a liquid solvent is passed over a solid phase to remove some solute, is carried out in fixed-bed, moving-bed, or agitated-solid columns. 

Mass transfer across the liquid-solid interface in mechanically agitated liquids containing suspended solid particles has been the subject of much research, and the data obtained for these systems are probably to some extent applicable to systems containing, in addition, a dispersed gas phase. Liquid-solid mass transfer in such systems has apparently not been studied separately. Recently published studies include papers by Calderbank and Jones (C3), Barker and Treybal (B5), Harriott (H4), and Marangozis and Johnson (M3, M4). Satterfield and Sherwood (S2) have reviewed this subject with specific reference to applications in slurry-reactor analysis and design. 

In liquid-solid extraction (LSE) the analyte is extracted from the solid by a liquid, which is separated by filtration. Numerous extraction processes, representing various types and levels of energy, have been described steam distillation, simultaneous steam distillation-solvent extraction (SDE), passive hot solvent extraction, forced-flow leaching, (automated) Soxh-let extraction, shake-flask method, mechanically agitated reflux extraction, ultrasound-assisted extraction, y -ray-assisted extraction, microwave-assisted extraction (MAE), microwave-enhanced extraction (Soxwave ), microwave-assisted process (MAP ), gas-phase MAE, enhanced fluidity extraction, hot (subcritical) water extraction, supercritical fluid extraction (SFE), supercritical assisted liquid extraction, pressurised hot water extraction, enhanced solvent extraction (ESE ), solu-tion/precipitation, etc. The most successful systems are described in Sections 3.3.3-3.4.6. Other, less frequently... 

In addition, there are a few examples of heterogeneous nonaqueous sonochemistry, in both liquid-liquid and liquid-solid systems. Two recent reports have utilized ultrasonic agitation in place of or along with phase transfer catalysis for the preparation of dichlorocarbene from aqueous NaOH/CHCl3 (166), and for N-alkylation of amines with alkyl halides (167). Along the same lines, several papers have appeared in which... 

Hartmann, H., Detailed simulations of liquid and liquid-solid mixing—turbulent agitated flow and mass transfer , Ph.D. Thesis, Delft University of Technology, Delft, Netherlands (2005). 

The proposed catalyst loading, that is the ratio by volume of catalyst to aniline, is to be 0.03. Under the conditions of agitation to be used, it is estimated that the gas volume fraction in the three-phase system will be 0.15 and that the volumetric gas-liquid mass transfer coefficient (also with respect to unit volume of the whole three-phase system) kLa, 0.20 s-1. The liquid-solid mass transfer coefficient is estimated to be 2.2 x 10-3 m/s and the Henry s law coefficient M = PA/CA for hydrogen in aniline at 403 K (130°C) = 2240 barm3/kmol where PA is the partial pressure in the gas phase and CA is the equilibrium concentration in the liquid. 

A turbine type agitator is commonly used for liquid-solid systems. Mixing rates depend on the forces required to suspend all solid particles. Minimum levels can be determined for (1) lifting the particles, and (2) for suspending them in an homogeneous manner [200]. Similar requirements apply to liquid-liquid systems. For cases where two poorly miscible fluids of about equal volume are used in the reaction, the mixer is placed at the interface. For a bench-scale experimental system of about 2 liters capacity, the minimum rotational speed to obtain well-dispersed system is 300 to 400 rpm [201], depending on the type of mixer. This rotational value decreases as the vessel volume increases. 

Reactions of cell growth or those using immobilized enzymes are instances of gas-liquid-solid reactions. In principle, accordingly, any of the types of reactors described in Section 8.3 could be employed as fermentors. Mostly, however, mechanically agitated tanks are the type adopted. Aeration supplies additional agitation as well as metabolic need, and moreover sweeps away C02 and noxious byproducts. 

For the semi-batch stirred tank reactor, the model was based on the following assumptions the reactor is well agitated, so no concentration differences appear in the bulk of the liquid gas-liquid and liquid-solid mass transfer resistances can prevail and finally, the liquid phase is in batch, while hydrogen is continuously fed into the reactor. The hydrogen pressure is maintained constant. The liquid and gas volumes inside the reactor vessel can be regarded as constant, since the changes of the fluid properties due to reaction are minor. The total pressure of the gas phase (P) as well as the reactor temperature were continuously monitored and stored on a PC. The partial pressure of hydrogen (pnz) was calculated from the vapour pressure of the solvent (pvp) obtained from Antoine s equation (pvpo) and Raoult s law ... 

Agitated tank reactors Batch agitated reactor This is a batch stirred tank reactor. For liquid-solid systems, the liquid is agitated by a mechanical apparatus (impeller) and the reactor is of tank shape. For gas-solid systems, the gas is agitated and rapidly circulated through a fixed-bed of solids. This reactor is basically an experimental one used for adsorption, ion exchange, and catalysis studies. 

Agitated slurry reactor (ASR) This is a mechanically agitated gas-liquid-solid reactor (Figure 3.13). The liquid is agitated by a mechanical apparatus (impeller). The fine solid particles are suspended in the liquid phase by means of agitation. Gas is sparged into the liquid phase, entering at the bottom of the tank, normally just under the impeller. This reactor can also be of continuous type or of semibatch type. This type is used only in catalysis. 

The analysis of this type of reactor requires a uniform composition of fluid phase throughout the volume. While this is easily achieved by standard agitation devices for liquid-solid systems, i.e. impellers, it requites special design to be achieved for gas-solid systems. This type of reactor is basically used for laboratory experimentation. 

The following equation of Zwietering is used for liquid-solid systems and agitated vessels (McCabe et al. 1993 Zwietering, 1958 Dohi et al., 2002) ... 

The double lines in Figure 3.44 represent the Sh number based on the mass transfer coefficient, in the case of a single-particle fall in water, for three different particle densities (Harriot, 1962). This value is considered to be the minimum mass-transfer coefficient in liquid-solid films in agitated vessels. Taking into account the fact that the actual Sh value in an agitated vessel is 1.5 -8 times its minimum value, it is apparent that the mass transfer coefficients are much higher in the case of agitated vessels. 

Agitated vessels (liquid-solid systems) Below the off-bottom particle suspension state, the total solid-liquid interfacial area is not completely or efficiently utilized. Thus, the mass transfer coefficient strongly depends on the rotational speed below the critical rotational speed needed for complete suspension, and weakly depends on rotational speed above the critical value. With respect to solid-liquid reactions, the rate of the reaction increases only slowly for rotational speed above the critical value for two-phase systems where the sohd-liquid mass transfer controls the whole rate. When the reaction is the ratecontrolling step, the overall rate does not increase at all beyond this critical speed, i.e. when all the surface area is available to reaction. The same holds for gas-liquid-solid systems and the corresponding critical rotational speed. 

Selection of agitator drive and speed reducer Mechanical assessment of shaft. Vibrations Mass transfer at bubble/liquid, and liquid/solid... 

Most, if not all, milks contain sufficient amounts of lipase to cause rancidity. However, in practice, lipolysis does not occur in milk because the substrate (triglycerides) and enzymes are well partitioned and a multiplicity of factors affect enzyme activity. Unlike most enzymatic reactions, lipolysis takes place at an oil-water interface. This rather unique situation gives rise to variables not ordinarily encountered in enzyme reactions. Factors such as the amount of surface area available, the permeability of the emulsion, the type of glyceride employed, the physical state of the substrate (complete solid, complete liquid, or liquid-solid), and the degree of agitation of the reaction medium must be taken into account for the results to be meaningful. Other variables common to all enzymatic reactions—such as pH, temperature, the presence of inhibitors and activators, the concentration of the enzyme and substrate, light, and the duration of the incubation period—will affect the activity and the subsequent interpretation of the results. 

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