Pressure Drop. Mass and Heat Transfer

Pressure Drop, Mass and Heat Transfer Pressure drop is more important in reactor design than in analysis or simulation. The size of the compressor is dictated by pressure drop across the reactor, especially in the case of gas recycle. Compressor costs can be significant and can influence the aspect ratio of a packed or trickle bed reactor. Pressure drop correlations often may depend on the geometry, the scale, and the fluids used in data generation. Prior to using literature correlations, it often is advisable to validate the correlation with measurements on a similar system at a relevant scale. 

There are many more data available for pressure-drop friction and heat transfer than for mass transfer. The similarity of Eqs. (3.46) to (3.48) for the transfer processes of momentum, mass, and heat, and the identical solutions, Eq. (3.52), lead to the possibility of deducing the mass-transfer characteristics for other situations from knowledge of the other two. 

The simplest kind of a fixed catalyst bed is a random packing of catalyst particles in a tube. Different particle shapes are in use like spheres, cylinders, rings, flat disc pellets or crushed material of a certain sieve fraction. Mean particle diameters range from 2 to 10 mm, the minimum diameter is limited primarily by pressure drop considerations, the maximum diameter by the specific outer surface area for mass and heat transfer. 

Reducing the bed length while keeping the space velocity the same will reduce the fluid velocity proportionally. This will affect the fluid dynamics and its related aspects such as pressure drop, hold-ups in case of multiphase flow, interphase mass and heat transfer and dispersion. Table II shows the large variation in fluid velocity and Reynolds number in reactors of different size. The dimensionless Reynolds number (Re = u dp p /rj, where u is the superficial fluid velocity, dp the particle diameter, p the fluid density and t] the dynamic viscosity) generally characterizes the hydrodynamic situation. 

From this comparison it is concluded that the BSR is indeed comparable to the monolithic reactors with respect to the pressure drop behavior, at least in the case of laminar flow (which normally prevails in monolithic reactors). A direct implication is that the mass and heat transfer characteristics of the two reactors will also be approximately the same in the case of laminar flow. 

The discussion of mass transfer and heat transfer in rod bundles pertains to the same geometrical and mathematical domains as the discussion of momentum transfer see Fig. 10. In the previous section it was stated that in a real-size BSR, the pressure drop and flow distribution are influenced only negligibly by the reactor wall the same holds true for the mass transfer and heat transfer characteristics. Consequently, only mass (and heat) transfer in the central subchannel will be discussed here. 

Irandoust and Andersson [134] described the modeling of three major flow patterns single-phase flow, cross flow, and two-phase flow. Work on monoliths has been performed with single-phase flow and the main issues for this type of flow are pressure drop, entrance effects, axial dispersion, and mass and heat transfer. The catalyst effectiveness is generally mass-transfer limited. 

The next step is to check if the fluid velocity is acceptable for the mlnimum/maximum pressure drop, as well as for good mass and heat transfer. For fixed bed reactors a practical particle diameter is between 3 and 5 mm. The pressure drop across a bed of particles can be calculated by means of the general equation ... 

Reactions between components of a gas and a liquid, the kinetics of which were discussed in Chapter 6, are carried out in a variet> of equipment, often having confusing names. The variety stems from a number of conditions that have to be fulfilled simultaneously efficient contact between gas and liquid—and eventually a solid catalyst, limitation of pressure drop, ease of removal of heat, low cost of construction and operation. Depending on whether the main mass transfer resistance is located in the gas or in the liquid, multiphase reactors or absorbers are operated either with a distributed gas phase and continuous liquid phase or vice versa. Whether co- or countercurrent flow of gas and liquid is used depends on the availability of driving forces for mass and heat transfer and reaction. 

The mathematical description considered in Section 10.3.3 was used as a modeling basis for the specially developed completely rate-based simulator [80]. This tool consists of several blocks including model libraries for physical properties, mass and heat transfer, reaction kinetics and equilibrium as well as specific hybrid solver and thermodynamic package. It also contains different hydrodynamic models (e.g., completely mixed liquid - completely mixed vapor, completely mixed liquid - vapor plug flow, mixed pool model, eddy diffusion model [80]) and a model library of hydrodynamic correlations for the mass-transfer coefficients, interfacial area, pressure drop, holdup, weeping and entrainment that cover a number of different column internals and flow conditions. 

Negligible pressure drop no internal mass and heat transfer limitations because of the small particle sizes that can be employed... 

Thus, a diffuser-confusor reactor has a higher pressure drop than a cylindrical one (Ap is up to 25 times higher), which is caused by a higher energy loss from the flow of a reaction mixture through local hydrodynamic resistance. As the mass and heat transfer processes are similar, the increase of hydraulic resistance (turbulisation of flows) should be accompanied by an intensification of the heat transfer through a wall. 

---