Model Comparisons of Cooled Tubular Multi-Tube Fixed Bed Reactor - Analysis and Control of Highly Exothermal Fast Equilibrium Reactions

Many industrially important vapor phase reactions are equilibrium controlled and highly exothermal in nature. Very often a cascaded multi-staged adiabatic reaction system is adopted in the industry where the heat of reaction is transferred by inter-stage cooling and equilibrium limitation is relieved by partial removal of product through condensation. Re-initiation of the reaction for the leftover reactants requires re-heating, and total installation and operating costs to carry out these conversions are generally high, especially if high conversion is expected for concentrated inlet reactants. One possible improvement in operation mode is to design and build a thermally cooled multi-tube tubular reactor system where both heat of reaction and equilibrium limitation can be alleviated in a one piece stand-alone reactor.

However, this operation mode is more complex than the above stated one in terms of system design and control. We found that pronounced heat transfer effect in this cooled, highly exothermal reactor created a need to model detailed temperature and conversion profiles within each tube of the tubular reactor. Radial heat removal is strongly influenced by the effective radial thermal conductivity which decreases significantly near the tube wall. Considering the possibility of instability and runaway in this type of reactor, a two-dimensional model serves a much better purpose in design and control than a corresponding one-dimensional model. The type of equilibrium reactions considered is very fast and highly exothermic, therefore effective thermal conduction should be distinguished between the gas and solid catalysts because solid temperature profile is generally higher than that of gas, and temperatures predicted by the two-dimensional heterogeneous models are generally very different from the one-dimensional homogeneous models. The development of the modern-day computer allows efficient computation in a two-dimensional model for most steady-state and some dynamic calculations. The one-dimensional model, backed by its corresponding two-dimensional model, can be used for on-line process control applications more effectively.

A two-dimensional heterogeneous tubular reactor model is proposed. Validation of the model requires experimental work to determine key process parameters. Because most of the physical properties and reaction kinetics of the system are already known, conceptual modeling of the process option could allow early visualization of certain aspects of the proposed design and help develop necessary control strategies for the operation of such reactors. The model can be run dynamically and can also be used to study transient behaviors of startup and shut down operations.

References:

1. Lordanidis, A., A. Mathematical Modeling of Catalytic Fixed Bed Reactors. Ph.D. Thesis, University of Twente, 2002.
2. Froment, G., F. and Bischoff, K., B. Chemical Reactor Analysis and Design. 2^nd Edition, John Wiley & Sons, Inc. 1990.