Page 187 - Principles of Catalyst Development

CATALYST CHARACTERIZATION 175

those distinguishing features important for proper use. The correct reactor
should be selected for the task at hand. It is not sufficient to use an apparatus
merely because it is available. Will it yield necessary data without complicat-
ing the interpretation? Carefully assess the requirements of the experiment
and keep the reactor as simple as possible. Avoid the temptation to build
versatility, for experience has shown that the more functions a piece of
equipment performs, the greater the probability of malfunction. Limitations
must be recognized, however, and expectations clearly defined.
Most experimental reactors have one common feature-small size.
Catalyst beds are small, usually less than 1 g, perhaps mixed with inert
material to give reasonable volumes (1-2 cm'). This is a distinct advantage.
Catalyst costs are low because only small amounts need be prepared. Heat
transfer and control difficulties are minimal and temperature is accurately
measured. Auxiliary equipment such as tubing, valves, pumps, and gauges,
are low cost and easily fabricated. Reagent supplies are reasonable and
available from easy storage. Space is conserved. Microanalytical equipment
exists for most applications. There are, however, compensating disadvan-
tages. With small9atalyst beds, there are reproducibility problems, especially
with laboratory preparations. Small beds are much more sensitive to low
concentrations of poisons, so that reactants must be thoroughly cleaned to
ultrapure levels. The catalyst is often used as a powder, since reactor
diameters are low. Although pore diffusion problems are minimized, press-
ure drop difficulties arise. With low flow rates, the particle Reynold s
number, Rep, is small and the flow regime ill defined. Values of Rep greater
than 25-50 are necessary for turbulent flow; below that it is laminar. Many
laboratory reactors have values less than 1-10, where external diffusion
correlations are uncertain. However, if these objections are overcome,
laboratory reactors offer the most rapid, least expensive, and simplest way
to accumulate rate data.
There are four major types (1) tubular, (2) gradientIess, (3) batch, and
(4) pulse.

7.5.2.1. Tubular Reactors
A typical tubular reactor system is shown in Fig. 7.31. The reactor itself
is made of glass, quartz, or stainless steel, depending on the temperature
and pressure. The diameter and length should not be less than that required
to give tube to particle diameter ratios of 5-1 0 and length to particle diameter
ratios of 50-100. Auxiliary flow and measuring equipment is shown in Fig.
7.32.
The reactor is operated either in an integral or differential mode. As
an integral reactor, initial concentrations of reactants are adjusted and the