Catalyst Deactivation
Deactivation in the form of coking is a major topic in contemporary catalyst research. Coking can be rapid and compensated by continuous regeneration, as in FCC catalysts, or slow and the subject of monthly or yearly maintenance, as in hydrotreating catalysts. The Pulse Mass Analyzer has both the short term mass resolution and long term stability to study these phenomena. Coking kinetics and the associated study of purging rates, conversion rates, regeneration rates, required temperatures, selectivity and yield can all benefit by knowing the mass changes of the catalyst bed very precisely in real time. The first five papers listed in the bibliography describe this application.

Poisons and diffusion blockers can have similar effects on catalyst activity. Assigning cause to one or the other is an important first step in poisoning studies. Poisons mask active sites or change the selectivity of the catalyst for particular reactions or reaction types. Diffusion blockers block access to particular activity centers or to entire passages of porous substrates. Poisons are usually metals or multiple bond light gas molecules. Diffusion blockers include carbon and the heavy products of the catalyst reaction itself.

The TEOM Series 1500 monitor can be used to determine the relative contributions of diffusion blockers and poisoning to a particular catalyst problem and explore possible solutions, or at least predict life based on feedstock composition.

To perform the measurements, a bed of catalyst is installed in the Reaction Kinetics Analyzer's sample holder, and then the catalyst is brought up to operating temperature and pressure in inert gas. Once conditions are established, a measured pulse of reactant gas is periodically inserted into the inert gas stream. When the bed is fresh, the entire pulse mass will be seen to be adsorbed on the catalyst and then some or all of it released as product. The first information from the Series 1500 monitor is how much of the pulse was absorbed and how much residue was left behind.

As the instrument continues to operate in this mode, residue will continue to accumulate. This residue could be poison, carbon, or heavy product. Observing the rate of residue accumulation as well as changing rates of adsorption and desorption gives the first clues as to what is happening in the bed. Poisoning would be characterized by very small residue and small effects on adsorption and desorption rates, even though large conversion losses may occur (monitored by a gas analyzer).

To differentiate between poisoning and diffusion blocking, one would first increase the temperature. Heavy product residue will evaporate off with heating, leaving elemental carbon or poisoning as the culprit. Substituting an oxidizing gas for the inert gas would remove carbon at high temperature, but not metallic poisons. Such a treatment and the resulting mass and conversion data would show the relative contributions of poisoning, coking, and heavy product retention.

Once the problem is known to be poisoning and the amount has been quantified, poison spiked reactant gas can be introduced. The ability of the Series 1500 sample bed holder to force 100% contact between reactant gas and catalyst makes quantitative work possible. The actual percentage of poison in feedstock retained can be calculated, as well as the effect of concentration on poisoning.

Studies of how temperature and pressure affect the retention of poison can be made, with real-time feedback of retention on a pulse-by-pulse basis. Long term tests with continuous reactant flow can take advantage of the monitor's automation and good long term stability to track slow accumulation of poisons out of actual process feedstock.

Catalyst Kinetics
Diffusion, reaction and adsorption kinetics are easily investigated using a TEOM Series 1500 Reaction Kinetics Analyzer. The basic steps in a catalytic reaction are well known but poorly understood. The first and last steps, diffusion into and out of the bed, are influenced only by the fluid dynamics of diffusion processes. These are physical processes, not chemical. The second and fourth steps, adsorption and desorption, deal with mass transfer at the surface. Only the middle step deals with an actual chemical reaction involving identifiable reactants and products. Up until now one could only study this five-step process by measuring the composition of the input and output of the reactor bed.

By tailoring the reaction conditions, each of these five steps can be investigated utilizing the TEOM Series 1500 monitor's microgram sensitivity and 0.1 second time resolution. Reaction Kinetics are discussed in the paper published by the University of Delaware.

Adsorption Properties

A major application of zeolites is in the area of adsorption and industrial separation processes. Zeolites, categorized as microporous materials,selectively adsorb molecules and compounds based upon differences in size and shape.

Inherent in the study of adsorption-based processes is the need for accurate equilibrium data that can be applied to system modeling work. This requires the application of experimental test conditions that closely approximate temperature, pressure and space velocities relevant in larger systems. Temperatures to 700 deg C and pressure to 60 atmospheres can be achieve simultaneously in the sample bed of the TEOM Series 1500 PMA system.

A number of studies point out the advantages of using the TEOM Series 1500 monitor for measurement of adsorption in microporous materials:

• A very well-defined gas-solid phase due to the high gas flow of reactant gas in carrier gas through the sample bed.

• Robust hardware design that preserves the mass resolution on the order of 1 microgram throughout the instrument's operating range.

• Experimental conditions that approximate those of practical conditions for both temperature, pressure and flow.