The Catalyst Review May 2017 - 8

SPECIAL FEATURE

By Calvin H. Bartholomew, PhD

Introduction
Catalyst deactivation, the loss over time of catalytic activity and/or selectivity, is a problem of great and continuing concern in the
practice of industrial catalytic processes. Whether rapid (e.g., order of seconds in fluid catalytic cracking) or slow (e.g., order of years
in ammonia synthesis) catalyst mortality is inevitable and potentially very costly. Indeed, costs to industry for catalyst replacement
and process shutdowns total more than $10 billion per year worldwide. Accordingly, there is clearly considerable motivation to
understand and minimize catalyst decay.
While loss of activity is likely to occur relatively slowly in most well-designed, well-controlled processes, in a poorly-designed, poorlycontrolled process, operational upsets can bring about catastrophic catalyst and equipment failures. For example, in steam reforming
of methane, great care must be taken to avoid reactor operation at excessively high temperatures or at steam-to-carbon ratios below
a critical value. Indeed, these undesirable conditions can cause rapid formation of large quantities of carbon filaments that plug
catalyst pores and voids, pulverize catalyst pellets, burst catalyst tubes, and force process shutdown, all within a few hours and at a
cost of millions of dollars.
While catalyst deactivation is inevitable for most processes, some of its immediate, drastic consequences may be avoided,
postponed, or even reversed if mechanisms of deactivation and reactivation and effects of catalyst, reactor, and process designs on
catalyst mortality are well-understood. In addition, modeling of catalyst deactivation based on this understanding can enable design
and process variables to be optimized and scaled. The critically necessary understanding and expertise for minimizing deactivation
impacts and costs for a company are best realized through wise investments in a combination of (1) in-house R&D expertise; (2)
studies of methods, principles and data available in books, reviews, proceedings of international symposia, and tens of thousands of
scientific papers and patents; and (3) subcontracted consulting and training.
Given the potentially large returns in profits and productivity from the study of catalyst deactivation, one might suppose that
companies selling or licensing catalysts and catalytic processes or operating catalytic processes to produce chemicals, fuels,
pharmaceuticals, etc., would move quickly to make these investments. Instead, we have observed an unfortunate trend over
the past two-to-three decades for many companies to emphasize short-term profits and to underinvest in R&D, consultants, and
employee training. Many of these companies have ultimately suffered declining profits due to inefficient, poorly-optimized catalysts
and catalytic processes; frequent process upsets and shut-downs; and undertrained employees who do not understand how to
minimize catalyst deactivation and maximize profits in catalytic processes.
This article focuses on basic mechanisms and management of catalyst decay. Principles of management are illustrated by case
studies which show, for example, how: (1) inadequate modeling and a lack of understanding of effects of catalyst design on
deactivation resulted in huge financial losses during a plant startup; (2) how design of a more stable catalyst saved the day; and (3)
how development of a comprehensive deactivation model has saved one company millions of dollars.

Six Mechanisms & Three Causes of Catalyst Deactivation
Mechanisms and Causes of Deactivation
There are many paths to catalyst decay. For example, a solid catalyst may be poisoned by strong adsorption of contaminants present
in a feed on catalytically-active sites of contaminants present in a feed. Its surface, pores, and voids may be poisoned and fouled by
carbon produced by dissociation of CO or CH4 on metals or by coke produced by cracking/condensation reactions of hydrocarbon
reactants, intermediates, and/or products on acid sites of the catalyst. Catalysts for NOx removal from a power plant flue gas can be
eroded by and/or plugged with fly ash. Catalytic converters used to reduce emissions from automotive gasoline or diesel engines
may be poisoned or fouled by fuel or lubricant additives and/or engine corrosion products. If the catalytic reaction is conducted at
high temperatures, thermal degradation may occur in the form of active phase crystallite growth, collapse of the carrier (support)
pore structure, and/or solid-state reactions of the active phase with the carrier or promoters. In addition, CO, oxygen, and/or
chlorine in the feed gas can cause formation of volatile carbonyls, oxides, or chlorides of the active phase, followed by gas-phase
transport from the reactor. Similarly, changes in the oxidation state of the active catalytic phase can be induced in the presence of
reducing or oxidizing gases in the feed. Finally, catalyst powders can undergo attrition in a fluidized- or slurrry-bed reactor, while
catalyst pellets can be mechanically crushed by the weight of tons of catalyst in large fixed-bed reactor.
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The Catalyst Review									

	

May 2017
The Catalyst Review May 2017

Table of Contents for the Digital Edition of The Catalyst Review May 2017
