Geometric constraints, defined

It is not the catalytic activity itself that make zeolites particularly interesting, but the location of the active site within the well-defined geometry of a zeolite. Owing to the geometrical constraints of the zeolite, the selectivity of a chemical reaction can be increased by three mechanisms reactant selectivity, product selectivity, and transition state selectivity. In the case of reactant selectivity, bulky components in the feed do not enter the zeolite and will have no chance to react. When several products are formed within the zeolite, and only some are able to leave the zeolite, or some leave the zeolite more rapidly, we speak about product selectivity. When the geometrical constraints of the active site within the zeolite prohibit the formation of products or transition states leading to certain products, transition state selectivity applies. 

On the laboratory scale, because of geometric constraints, there is often very little opportunity to optimize the gun-to-bed distance, and thus this parameter takes on a fixed setting often defined by personal preference. Once a move is made to the production scale, there is more opportunity to reconsider gun positioning (although logic does not always prevail). 

Within the geometric constraints for self-wiping, the OD/ID ratio can be specified to impart a specific average shear rate, define a free volume, or determine allowable shaft diameter (power transmission). While a 2-lobe machine could be designed to have a low OD/ID ratio, as in the 3-lobe unit, this would not be very sensible. The purpose of creating a 2-lobe unit is to have a machine that would be less likely to be volume limited, but still have the power transmission... 

As defined in Chapter 3, constraints are limitations that restrict the position or relationship between features of the geometric shape. Some examples of constraints are making two lines parallel or perpendicular to each other. We call these geometric constraints. We call the process of dimensioning the length of a line or the diameter of circle dimensioning constraints (see Figure 7-4). 

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