Monolith Reactors in Three-Phase Processes
Doctoral thesis, 1995
The use of monoliths in chemical processes has been studied with the emphasis on liquid-phase hydrogenations. A monolithic catalyst consists of many narrow, parallel channels with catalytically active species either incorporated into the wall itself or into a layer of a porous oxide deposited on the wall (a washcoat).
The hydrodesulfurization of dibenzothiophene (DBT), which was taken as a model for sulphur containing molecules in heavy petroleum fractions, was studied on a CoMoS catalyst supported on a .gamma.-Al2O3 monolith at 6-8 MPa and 543-573 K. The experimental data were found to be consistent with a Langmuir-Hinshelwood type of rate model assuming two different active sites. In another set of experiments, the selective catalytic hydrogenation of acetylene in the presence of a liquid solvent was studied. Industrially a gas phase process with a Pd/.alfa.-Al2O3 catalyst is used. The introduction of a liquid solvent serves the dual purpose of continuously removing both heat and green oil. Green oil is a catalyst fouling byproduct formed by hydropolymerization of acetylene.
Mathematical models describing monolith reactors were developed and used to evaluate different designs of monolith reactors. To better account for the non-uniform distribution of active material, typical of washcoated monoliths, a method for estimating effectiveness factors was developed. Computer simulations were then used to make comparative studies of monolith and conventional trickle-bed reactors. The liquid-phase methanol synthesis was taken as a case study. A more general comparison was also carried out, indicating that a monolith can be advantageous for relatively fast reactions, especially when high pressure drops must be avoided.
The cocurrent flow of gas and liquid in a vertical channel has been analyzed on a more detailed level by using Computational Fluid Dynamics (CFD). A moving boundary formulation of the Finite Element Method (FEM) was used to analyze the deformation of large gas bubbles. The influence of gravitational, capillary, viscosity and inertial forces on the bubble shape and the thickness of the liquid film surrounding the gas bubble were analyzed. In addition, it was shown that it is possible to form a stable, toroidal gas bubble if the liquid viscosity is very high; this unusual flow pattern is termed stalactite flow.
computational fluid dynamics