Slurry reactor hydrodynamics

DOE, Slurry Reactor Hydrodynamics Studies Final Report, US Department of Commerce, National Technical Information Service, USA (DOE, 1985)... 

Attempts have been made to expand the technique to include the analysis of soil biotransformations f23.29V While the hydrodynamic nature and physical structure of soil systems vary widely and are difficult to establish with certainty, two limiting conditions may be specified. The first is where the soil particles are suspended and all phases are well-mixed. This case is not typically found in nature, but is found in various types of engineered soil-slurry reactors. The reactors currently used in our systems experiments include continuous stirred tank reactors (CSTRs) operated to minimize soil washout. 

In the third section an extensive writing on two types of slurry catalytic reactors is proposed Bubble Slurry Column Reactors (BSCR) and Mechanically Stirred Slurry Reactors (MSSR). All the variables relevant in the design and for the scale-up and the scale-down of slurry catalytic reactors are discussed particularly from the point of view of hydrodynamics and mass transfer. Two examples of application are included at the end of the section. 

Hydrodynamics of slurry reactors include the minimum gas velocity or power input to just suspend the particles (or to fully homogeneously suspend the particles), bubble dynamics and the holdup fractions of gas, solids and liquid phases. A complicating problem is the large variety in reactor types (sec Fig. I) and the fact that most correlations are of an empirical nature. We will therefore focus on sparged slurry columns and slurries in stirred vessels. 

E will be different from 1 only if R4 is small relative to / 2, resulting in a bulk concentration of c — 0 and in a real parallel mechanism of the enhancement. The advantage of the concept of the enhancement factor as defined by eq 33 is the separation of the influence of hydrodynamic effects on gas-liquid mass transfer (incorporated in Al) and of the effects induced by the presence of a solid surface (incorporated in E ), indeed in a similar way as is common in mass transfer with homogeneous reactions. The above analysis shows that an adequate description of mass transfer with chemical reaction in slurry reactors needs reliable data on ... 

Baldi, G., Hydrodynamics and Gas-Liquid Mass Transfer in Stirred Slurry Reactors, Ch. 14 in Multiphase Chemical Reactors Theory, Design, Scale-up (A. Gianetto and P. L. Silveston, eds.). Hemisphere Publ. Co., New York, 1986. 

In this chapter, we review the reported studies on the hydrodynamics, holdups, and RTD of the various phases (or axial dispersion in various phases), as well as the mass-transfer (gas-liquid, liquid-solid, and slurry-wall), and heat-transfer characteristics of these types of reactors. It should be noted that the three-phase slurry reactor is presently a subject of considerable research investigation. In some cases, the work performed in two-phase (either gas-liquid or liquid-solid) reactors is applicable to three-phase reactors however, this type of extrapolation is kept to a minimum. Details of the equivalent two-phase reactors are considered to be outside the scope of this chapter. 

Bubble columns are convenient for catalytic slurry reactions also (67). It is therefore important to know how the hydrodynamic properties of the gas-in-liquid dispersion is influenced by the presence of suspended solid particles. In the slurry reactor absorption enhancement due to chemical reaction cannot be expected. However, if particle sizes are very small, say less than 5 yum, and if, in addition, the catalytic reaction rate is high a small absorption enhancement can occur ( 8). Usually the reaction is in the slow reaction regime of mass transfer theory. Hence, it is sufficient to know the volumetric mass transfer coefficient, kj a, and there is no need to separate k a into the individual values. 

FIG. 19-35 Effect of aeration number and stirrer speed on the power number— N increases in order of Ni< Ni< N < N4. [Adapted from Baldi, Hydrodynamics and Mass Transfer in SUrred-Slurry Reactors," in Gianetto and Silveston (eds.). Multiphase Chemical Reactors, Hemisphere Publishing Corp., 1986, Fig. 14.8.]... 

Three-phase fluidized beds and slurry reactors (see Figs. 30g-l) in which the solid catalyst is suspended in the liquid usually operate under conditions of homogeneous bubbly flow or chum turbulent flow (see regime map in Fig. 33). The presence of solids alters the bubble hydrodynamics to a significant extent. In recent years there has been considerable research effort on the study of the hydrodynamics of such systems (see, e.g., Fan, 1989). However, the scale-up aspects of such reactors are still a mater of some uncertainty, especially for systems with high solids concentration and operations at increased pressures it is for this reason that the Shell Middle Distillate Synthesis process adopts the multi-tubular trickle bed reactor concept (cf. Fig. 30e). The even distribution of liquid to thousands of tubes packed with catalyst, however poses problems of a different engineering nature. 

The design, scaleup and performance prediction of slurry reactors require models which must consider not only the hydrodynamic and mixing behavior of the three phases, but also the mass transfer between the phases along with the intrinsic kinetics. In the DCL and FTS processes, an axial dispersion model is applicable, with the solid phase assumed to follow sedimentation or dispersed flow model. However, in the CCC, where the solid particles take part in the reaction, dispersion model is no longer applicable. 

Muroyama K, Mitani Y, Yasunishi A. (1985) Hydrodynamic characteristics and gas-hquid mass transfer in a draft mbe slurry reactor. Chem. Eng. Commun., 34 87-98. 

Zahradnik J, Drapal L, Kastanek E, Reznikova J. (1992) Hydrodynamic and mass transfer characteristics of sectionahzed aerated slurry reactors. Chem. Eng. Process., 31 263-272. 

However, from a hydrodynamics point of view, this is very difficult to run. Primary reasons for this difiiculty comes from the fact that countercurrent systems always have flooding limitations, and the window of flow rates for stable operation is relatively small. Thus, the typical choice for stable operation of slurry reactors is co-current systems (Figure 6.3b), in which the gas and liquid is arranged to flow concurrently and the... 

Ghasemi S, Sohrabi M, Rahmani M. A comparison between two kinds of hydrodynamic models in bubble column slurry reactor during Fischer-Tropsch synthesis single-bubble and two-bubble class. Chem. Eng. Res. Des. 2009 87 1582-1588. 

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