Sustainability and Stability - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 815

Fouling can cause catalyst pore blocking and catalyst particles agglomerate together. Pres-
sure drop increase or throughput decrease; Increase internal mass transfer effect, i.e. lower
effectiveness factor.
Catalyst activity can be modeled as an inverse function of foul concentration, which
increases with increasing catalyst residence time.

16.4.2. Poisoning
Chemisorption of otherwise inert compounds in the process stream can make the active
sites inaccessible to reactants. The poison may even modify the catalyst and/or active sites.
The toxicity depends on the energy well of the adsorbed state.
For reversible adsorption of poison, the active site modification can be characterized by the
addition of a competitive adsorbate. For example,

K P C P
1þK A C A þK P C P

q P ¼ (16.34)

where P is the poisonous compound that is not participating in the desired reaction. When K P
is small (bond is weak), equilibriummay be reached in the process and the poison is regarded
as reversible. If K P is very big (bond is very strong), equilibrium may not be reached in the
process and the poison is regarded as irreversible (as when equilibrium is reached, most
of the active sites would be covered by the poison). Poison can either be impurities in the
process stream or the product itself. There are three main types of poisons:

(1) Molecules with reactive heteroatoms (e.g. sulfur);
(2) Molecules with multiple bonds between atoms (e.g. unsaturated hydrocarbons);
(3) Metallic compounds or metal ions (e.g. Hg, Pd, Bi, Sn, Cu, Fe).

Poisons can be eliminated by physical separation, a dummy bed (for adsorption). Poison
types 1) and 2) can be converted to nontoxic compounds by chemical treatment. If a product
is poisonous, selectively remove it during the process can be exercised.

16.4.3. Sintering
Sintering is caused by growth or agglomeration of small crystals with make up the catalyst
or its support. Structural rearrangement (sintering) leads to a decrease in surface area and/or
effective number active sites. Sintering occurs when temperature exceeds
(1)

1 / 2 T m (i.e. half of its melting temperature) if “dry”;

1 / 3 T m if steam is present as steam facilitates reorganization of many metals, alumina, and
silica. Table 16.2 shows the sintering temperature for some common metals.

(2)

TABLE 16.2 Sintering Temperature for Common Metals

Metal Cu Fe Ni Pt Pd

1 / 3 T m , C

360 500 500 570 500