Multi-tubular reactors with monolithic catalysts

Abstract

Multi-tubular reactors for fluid processing incorporate reactor tubes containing thermally conductive monolithic catalyst structures with relative dimensions and thermal expansion characteristics effective to establish both a non-interfering or slidably interfering fit between the monolith structures and the reactor tubes at selected monolith mounting temperatures, and geometries at reactor operating temperatures such that the operating gaps between tubes and monoliths under the conditions of reactor operation do not exceed about 250 μm over tube sections where high heat flux to or from the monoliths is required.

Description

BACKGROUND OF THE INVENTION [0001] The present invention relates to the use of monolithic catalysts in multi-tubular catalytic reactors. More particularly, the invention relates to catalyst designs and methods for inserting, securing, and maintaining monolithic catalysts in the tubes of such reactors, or in shell-and-tube heat exchangers, for use in the chemical processing and / or energy conversion industries. [0002] Tubular catalytic reactors wherein a reactant stream is passed through a tube containing a bed of catalyst pellets, rings, spheres, or the like are presently used for the industrial production of chemicals in a variety of processes. These include processes involving highly exothermic or endothermic reactions wherein the management of the heat of reaction is required for process control. Examples of highly exothermic reactions include the selective catalytic oxidation of organic compounds e.g., the oxidation of benzene or n-butane to maleic anhydride, o-xylene to phthalic...

AI Technical Summary

Benefits of technology

[0011] The reactor tubes are also formed of a heat-conductive material such as a metal, that metal again having a known average linear coefficient of thermal expansion sometimes referred to hereinafter as the second coefficient of thermal expansion. The reactor tubes will also have a wall thickness and material density effective to insure that the heat conductivity of the tubes will not be a barrier to heat transfer to and from the heat-exchange fluid at reactor operating temperatures.

[0012] Finally, to meet the more stringent heat transfer requirements needed for efficiently carrying out highly exothermic or endothermic reactions in these reactors, the dimensions and coefficients of expansion of the monolith segments and reactor tubes are selected to reduce or substantially eliminate monolith-segment/reactor-tube gaps that might otherwise interfere with heat transfer between the segments and the tube walls of the reactor. For purposes of the present invention the monolith/reactor tube gap of interest is referred to as the operating monolith/tube gap. This gap depends on the dimensions of the monolith/reactor tube system, the thermal expansion characteristics of the monolith and tube, and the actual operating temperatures of the monolith and tube while mounted in the reactor and under actual use. To meet the heat transfer requirements of the invention the operating gap between the reactor tubes and the monolithic catalyst or catalyst support structure is reduced to a value not exceeding about 250 μm under the targeted operating conditions of the reactor for the reaction and catalyst involved in the selected process.

[0013] In a second aspect the invention includes a method for assembling a multi-tubular reactor incorporating an array of reactor tubes filled w

Problems solved by technology

Tubular reactors are relatively efficient but can be difficult to control.

In the case of exothermic reactions for example, hot spots can occur which can adversely affect reactor performance.

Due to effects such as process stream flow channeling and the fact that the effective thermal conductivity of the reaction system (catalyst pellets plus gaseous reactants) is quite low, localized heating that increases exothermic reaction rates can produce thermal runaways.

Uncontrolled, these can eventually lead to catalyst sintering or melting, damage to metal reactor envelopes, and even to reactor explosions.

Approaches to deal with these concerns have included processing strategies such as staging the catalysts or diluting the reactants, the latter through means such as reaction moderators, product recycling or the use of inert diluents, but such strategies invariably reduce process efficiencies.

However, problems relating to overall reactor temperature control remain.

One difficulty with any of the multi-tubular reactor designs so far considered is that the heat generated or required by the reaction must still be supplied or removed through the tube walls via the heat exchange medium present in the space in between the reactor tubes.

Thus a major contributor to the problem of catalyst superheating in exothermic reactions is the physical limitation on internal heat transfer performance that can be achieved in these reactors.

This physical limitation, expressed commonly as the heat transfer coefficient or the effective radial thermal conductivity of the reactor, is frequently still too low in comparison with the amount of heat evolved inside the reactor tube to enable the level of reactor temperature control needed to realize theoretical reactor efficiencies.

Monolithic catalysts to be used in multi-tubular reactors themselves present additional practical difficulties, specifically problems relating to the efficient loading and fitting of the catalysts into commercial reactor tubes.

Neither the reactor tubes nor the catalysts themselves are ideal in shape, and therefore gaps between the catalysts and the tube walls inevitably remain.

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