Parallel heat and mass transfer

For drying, the typical parallel heat and mass transfer process, the feature of impinging streams that significantly enhances transfer, should of course be fully utilized. 

The continuous process developed in the laboratory proved to be a scalable process in the pilot plant. The trickle bed oxidation reactor was scaled by numbering up. Laboratory operating parameters were replicated for each oxidation column operated in parallel. Heat and mass transfer characteristics were identical in each oxidation column. Steady-state operation was demonstrated in the pilot plant for over 50 h. Three continuous reactors were operated in succession for production of a stable quenched product solution. Over 100 kg of API was produced during the pilot plant campaign using this setup. Also, the quality of the product produced by the continuous process was comparable to that of the batch process. 

Mala GM, Li D, Werner C (1997b) Flow characteristics of water through a micro-channel between two parallel plates with electro kinetic effects. Int J Heat Fluid Flow 18 491 96 Male van P, Croon de MHJM, Tiggelaar RM, Derg van den A, Schouten JC (2004) Heat and mass transfer in a square micro-channel with asymmetric heating. Int J Heat Mass Transfer 47 87-99 Maranzana G, Perry I, Maillet D (2004) Mini- and micro-channels influence of axial conduction in the walls. Int J Heat Mass Transfer 47 3993 004 Maynes D, Webb BW (2003) Full developed electro-osmotic heat transfer in microchannels. Int J Heat Mass Transfer 46 1359-1369... 

The present book is devoted to both the experimentally tested micro reactors and micro reaction systems described in current scientific literature as well as the corresponding processes. It will become apparent that many micro reactors at first sight simply consist of a multitude of parallel channels. However, a closer look reveals that the details of fluid dynamics or heat and mass transfer often determine their performance. For this reason, besides the description of the equipment and processes referred to above, this book contains a separate chapter on modeling and simulation of transport phenomena in micro reactors. 

In this chapter, we extend the discussion of the previous chapter to nonspherical shapes. Only solid particles are considered and the discussion is limited to low Reynolds number flows. The flow pattern and heat and mass transfer for a nonspherical particle depend on its orientation. This introduces complications not present for spherical particles. For example, the net drag force is parallel to the direction of motion only if the particle has special shape properties or is aligned in specific orientations. 

Particle Size and Desorption Rates. Bench-scale reactor studies of the desorption of toluene from single, 2- to 6-mm porous clay partides (14) showed desorption times that increased with the square of the particle radius, suggesting that diffusion controls the rate desorption. Parallel experiments performed in a small, pilot-scale rotary kiln at 300°C showed no effect of day partide size for diameters ranging from 0.4 to 7 mm. Additional single-partide studies with temperature profiles controlled to match those in the pilot-scale kiln had desorption times that were a factor of 2—3 shorter for the range of sizes studied (15). Hence, at the conditions examined, intrapartide mass transfer controlled the rate of desorption when single particles were involved and interpartide mass transfer controlled in a bed of particles in a rotary kiln. These results apply to full-scale kilns. As particle size is increased, intraparticle resistances to heat and mass transfer eventually begin to dominate. 

There are many standard texts of fluid flow, e.g. Coulson Richardson,1 Kay and Neddermann2 and Massey.3 Perry4,5 is also a useful reference source of methods and data. Schaschke6 presents a large number of useful worked examples in fluid mechanics. In many recent texts, fluid mechanics or momentum transfer has been treated in parallel with the two other transport or transfer processes, heat and mass transfer. The classic text here is Bird, Stewart and Lightfoot.7... 

We begin Ijiis chapter by pointing out numerous analogies between heat and mass ira hsfer and draw several parallels between them. We then discuss boundaiy conditions associated with mass transfer and one-dimensional steady and transient mass diffusionf followed by a discussion of mass transfer in a moving medium. Finally, we consider convection mass transfer and slmulttiheous heat and mass transfer. 

Heat transfer between a solid wall and a fluid, e.g. in a heated tube with a cold gas flowing inside it, is of special technical interest. The fluid layer close to the wall has the greatest effect on the amount of heat transferred. It is known as the boundary layer and boundary layer theory founded by L. Prandtl2 in 1904 is the area of fluid dynamics that is most important for heat and mass transfer. In the boundary layer the velocity component parallel to the wall changes, over a small distance, from zero at the wall to almost the maximum value occurring in the core fluid, Fig. 1.6. The temperature in the boundary layer also changes from that at the wall w to at some distance from the wall. 

We will now focus on heat and mass transfer in the laminar boundary layer of parallel flow on a plate. The flow is steady, dissipation is negligible and we will... 

Al-Nimr, M.A., and Alkam, M.K. (1998) Unsteady non-darcian fluid flow in parallel-plates channels filled with porous materials, Heat and Mass Transfer 33,315-318. 

---