Design and scale-up of Multitubular reactors Mark Matzopoulos1, Process Systems Enterprise Limited, 5th Floor East, 26-28

Hammersmith Grove, London W6 7HA, UK

[correspondance: m.matzopoulos@psenterprise.com];

Bart de Groot1, Hassan Mumtaz1, Mayank Patel1

Multitubular reactors are widely used for fixed-bed catalytic reactions, but good

design and operation are challenging. A good design eliminates danger areas where

hot spots can occur. This is achieved by adjusting reactor specifications to provide

optimal heat control. Detailed modelling is the only reliable way to accurately predict

heat transfer at all points throughout the reactor.

The approach allows the effects of changes to design variables (such as the catalyst

characteristics, the catalyst/inert ratio, tube pitch, tube length, coolant velocity, feed

reactant mass fraction, number of baffles, cooling water inlet temperature as well as

the number of active reactors and numerous other quantities) on key performance

indicators (such as throughput, conversion and yield, tube-side temperature profiles

and catalyst lifetime) to be calculated to a very high degree of predictive accuracy.

Multitubular reactors are complicated units that have been very difficult to model in

the past because of the complex catalytic reactions taking place in the tubes, the

large number of tubes, and the interrelationship between exothermic reaction in the

tubes and the shell-side cooling medium.

The approach takes into account the close coupling between the tube-side

phenomena and the shell-side heat transfer and hydrodynamics. The main advance

of the techniques described here over existing simulation approaches are that (1)

tube models incorporate high-accuracy first-principles representation of catalytic

reaction, species diffusion and bed heat transfer, including intra-particle and surface

effects, (2) models are validated against companies own laboratory and pilot plant

data and (3) mathematical optimisation techniques are used to determine the optimal

values of multiple design variables simultaneously rather than by trial-and-error

simulation.

The final design is verified using a computational fluid dynamics (CFD) model of the

shell side to ensure that no mechanical constraints such as shell-side fluid velocities

are violated.

An integrated modelling/experimental design methodology is presented which uses

specially-designed experimental procedures to obtain accurate estimates of the key

kinetic and heat transfer parameters from a limited number of carefully targeted

experiments. Formal model-based parameter estimation techniques ensure that

parameter interaction is taken into account and provide parameter confidence

information for subsequent risk analysis.

The approach is illustrated using the design of a high-performance new reactor for

the manufacture of propylene oxide. Apart from reactor design, the techniques can

be used for a wide range of applications including minimisation of design risk, new

catalyst design and assessment, derivation of safe and effective start-up procedures,

control design, and maximisation of operational flexibility. The techniques described

can also be used for operational decision support and troubleshooting. They apply to

a variety of reactors, including those for the production of methanol, acrylic acid and

Fischer-Tropsch synthesis for gas-to-liquid applications.