Two-Phase Flow in Fixed-Bed Reactors

The next level of complexity in implementing MR to study fixed-bed processes is to study two-phase flow phenomena. Initial studies focussed less on fully 

X-—The fraction of (inter-particle) void space pixels containing some liquid provides an upper estimate of liquid saturation, from which values of liquid holdup are obtained. By extrapolation of the data to zero liquid superficial 

—The wetting efficiency is obtained by calculating the fraction of the pixels identifying the surface of the packing that are in contact with liquid during gas-liquid flow. Liquid-containing voxels adjacent to the wall of the column, and the internal surface of the porous packing elements are not considered in the analysis. 

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M. D. Mantle, A. J. Sederman, L. F. Gladden 2001, (Single- and two-phase flow in fixed-bed reactors MRI flow visualisation and lattice-Boltzmann simulations), Chem. Eng. Sci. 56, 523. 

Mantle, M.D. Sederman, A.J. Gladden, L.F. 60. Single and two-phase flow in fixed-bed reactors ... 

Voidage. Single- and two-phase flows in fixed-bed reactors were visualized by three-dimensional NMR imaging and MRI velocimetry. The fluid velocity vector is determined at a pore-scale resolution of 156 pm. Characteristics of... 

The examples just presented show that axial dispersion has only a small effect on reactor performance for single-phase flow through a packed bed of particles. With two-phase flow through a packed bed or with gas flow in a fluidized bed, dispersion effects can be quite important because of the complex flow patterns these cases are discussed in later chapters. Complex flow patterns can also occur for various reasons with single-phase flow in fixed beds, and then reactor performance may be worse than predicted allowing for normal axial dispersion. When abnormal flow patterns are suspected because of poor reactor performance, the residence time distribution should be investigated. 

There are many different reactor designs but the two most commonly used are fixed bed and batch slurry phase. For a fixed bed reactor a given volume of solid particulate or monolith supported catalyst is fixed in a heated tube located within a furnace and liquid and/or gaseous reactants flow through the bed. This type of process is commonly used for large continuous-volume production where the reactor is dedicated to making only one product such as a bulk chemical or petroleum product. 

In a fixed bed reactor, gas phase reactions are generally carried out using a stationary bed of solid catalyst. In a typical reactor, suitable screens support the bed of catalyst particles, through which the gas phase flows. Gaseous reactants adsorb on the catalyst surface, reactions occur on this surface and reaction products desorb back to the gas phase. Two major types of fixed bed reactor are the conventional axial flow fixed bed reactor and the radial flow fixed bed reactor. These types are shown... 

When the selectivity of a catalytic reaction is liable to be bad because of transport limitations, fixed-bed catalysts cannot be used with liquid-phase reactants. When selectivity is less important, fixed-bed catalysts have some advantages. Firstly, the catalyst need not be separated from the reaction products-a flow of reactants can simply be passed through the reactor. Furthermore, the catalyst can be readily thermally pretreated in a gas flow. Hence, deactivated catalysts can be regenerated in situ in the reactor. Exchange of the catalyst of a fixed-catalyst bed by another catalyst is, however, usually a tedious procedure. Fixed-catalyst beds are therefore used only within dedicated reactors in which only one or a limited number of products is produced. Also the lifetime of catalysts employed in fixed-bed reactors must usually be long, viz., two to five years. 

Two-phase flow in three-phase fixed-bed reactors makes the reactor design problem complex [12], Interphase mass transfer can be important between gas and liquid as also between liquid and catalyst particle. Also, in the case of trickle-bed reactors, the rivulet-type flow of the liquid falling through the fixed bed may result (particularly at low liquid flow rates) in only part of the catalyst particle surface being covered with the liquid phase. This introduces a third mass transfer process from gas to the so-called gas-covered surface. Also, the reaction rates in three-phase fixed-bed catalytic reactors are highly affected by the heat transfer resistances resistance to radial heat transfer and resistance to fluid-to-particle heat transfer. As a result of these and other factors, predicting the local (global) rate of reaction for a catalyst particle in three-phase fixed-bed reactors requires not only... 

Nonequilibrium thermomechanical models for two-phase flow in three-phase fixed-bed reactors... 

In most two-phase processes, one phase has to be dispersed in the other, preferably in such a way that there can be sufficient mass transfer between the phases, and that afterwards the phases can be separated. When a solid has to be used in a dispersed state, the solid particles are usually brought into the desired shape outside the reactor. If the solid is a raw material that is to be converted, it often has to be broken and ground till a sufficiently small particle size. If, on the other hand, the solid reactant is a catalyst, the particles are carefully built up, so that the size and the structure is best suited to the catalytic process (see section 12.1). In fixed bed reactors, the catalyst particles are resting on a grid and the fluid phases (gas and/or liquid) are passed through them. In fluidized beds and in slurry reactors the particles are suspended in a fluid phase and the relative flow velocity of the particles is determined primarily by gravity. The suspension is maintained by an upward fluid flow in both cases. Examples of reactors with solid/fluid dispersions are given in Chapters 10 - 12. 

Mixed phase downflow fixed bed reactors can operate in several flow regimes. These bear resemblance to flow regimes in two phase flow in pipes. These flow regimes are the gas continuous ("trickle bed"), bubble flow, pulse flow and gas continuous blurring regime or spray flow. Many maps have been proposed in the... 

Fixed-bed reactors employed for lipase-catalyzed hydrolysis and interesterification reactions are highly efficient and have been used on a large scale (Table 5). The two phases may flow through the reactor in the opposite or same directions. If no solvents are used, the effect of viscosity of some substrates (i.e., oil) may be minimized by employing high temperatures which lead to faster rates of inactivation of lipases. 

The presence of two phases in the reaction mixture may seem to be a mass-transfer engineering problem, but even moderate stirring of the mixture produces an emulsion, which greatly facilitates the phase transfer steps of the reaction mechanism. In our fixed-bed reactor, the turbulence resulting from the flow rates used seemed to suffice to eliminate external mass transfer hmitations. At MeOH SA of 20 and identical LHSV values, similar acid conversions were observed for two linear flow velocities differing by a factor of two. 

There are reports of numerous examples of dendritic transition metal catalysts incorporating various dendritic backbones functionalized at various locations. Dendritic effects in catalysis include increased or decreased activity, selectivity, and stability. It is clear from the contributions of many research groups that dendrimers are suitable supports for recyclable transition metal catalysts. Separation and/or recycle of the catalysts are possible with these functionalized dendrimers for example, separation results from precipitation of the dendrimer from the product liquid two-phase catalysis allows separation and recycle of the catalyst when the products and catalyst are concentrated in two immiscible liquid phases and immobilization of the dendrimer in an insoluble support (such as crosslinked polystyrene or silica) allows use of a fixed-bed reactor holding the catalyst and excluding it from the product stream. Furthermore, the large size and the globular structure of the dendrimers enable efficient separation by nanofiltration techniques. Nanofiltration can be performed either batch wise or in a continuous-flow membrane reactor (CFMR). 

Lapidus, L., Flow distribution and diffusion in fixed-bed two-phase reactors. Ind. Eng. Chem. 49, 1000 (1957). 

Fixed- or packed-bed reactors refer to two-phase systems in which the reacting fluid flows through a tube filled with stationary catalyst particles or pellets (Smith, 1981). As in the case of ion-exchange and adsorption processes, fixed bed is the most frequently used operation for catalysis (Froment and Bischoff, 1990 Schmidt, 2005). Some examples in the chemical industry are steam reforming, the synthesis of sulfuric acid, ammonia, and methanol, and petroleum refining processes such as catalytic reforming, isomerization, and hydrocracking (Froment and Bischoff, 1990). 

Trickle-bed reactors usually consist of a fixed bed of catalyst particles, contacted by a gas liquid two-phase flow, with co-current downflow as the most common mode of operation. Such reactors are particularly important in the petroleum industry, where they are used primarily for hydrocracking, hydrodesulfurization, and hydrodenitrogenation other commercial applications are found in the petrochemical industry, involving mainly hydrogenation and oxidation of organic compounds. Two important quantities used to characterize a trickle-bed reactor are... 

The second section presents a review of studies concerning counter-currently and co-currently down-flow conditions in fixed bed gas-liquid-solid reactors operating at elevated pressures. The various consequences induced by the presence of elevated pressures are detailed for Trickle Bed Reactors (TBR). Hydrodynamic parameters including flow regimes, two-phase pressure drop and liquid hold-up are examined. The scarce mass transfer data such gas-liquid interfacial area, liquid-side and gas-side mass transfer coefficients are reported. 

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