Page 249 - Modeling of Chemical Kinetics and Reactor Design

Industrial and Laboratory Reactors 219

economics of separating the reaction mixture and the costs of returning
the unconverted reactant back into the reaction.
In chemical laboratories, small flasks and beakers are used for liquid
phase reactions. Here, a charge of reactants is added and brought to
reaction temperature. The reaction may be held at this condition for a
predetermined time before the product is discharged. This batch reactor
is characterized by the varying extent of reaction and properties of
the reaction mixture with time. In contrast to the flasks are large
cylindrical tubes used in the petrochemical industry for the cracking
of hydrocarbons. This process is continuous with reactants in the tubes
and the products obtained from the exit. The extent of reaction and
properties, such as composition and temperature, depend on the posi-
tion along the tube and does not depend on the time.
Another classification refers to the shape of the vessel. In the case
of the laboratory vessel installed with a stirrer, the composition and
temperature of the reaction is homogeneous in all parts of the vessel.
This type of vessel is classified as a stirred tank or well mixed reactor.
Where there is no mixing in the direction of flow as in the cylindrical
vessel, it is classified as a plug flow or tubular flow reactor.
Knowledge of the composition and temperature at each point of the
reactor enables the designer to describe the behavior of a chemical
reactor. Concentrations of species at any point may change either
because the species are consumed by chemical reaction or they reach
this position via mass transfer. Correspondingly, the temperature at any
point may change because the heat is being absorbed or released by
chemical reaction or heat transfer. The rate of the chemical reaction
and the rate of mass and heat transfer affect the concentration and
temperature of a given section of the system. Concentration, tempera-
ture, and molecular properties determine the reaction rate. This process
occurs through microkinetic properties that are identical in any chemical
reactor (i.e., if the temperature and composition in two reactors are
the same, then the reaction rate is also the same). Macrokinetic
properties of the reactor affect the outcome of the process through the
kinetics of heat and mass transfer. The rates of mass and heat transfer
depend on the properties relative to the reactor, such as size of the
reactor, size and speed of the impeller, and the area of heat exchang-
ing surfaces.
Reactors without the effect of macrokinetic properties are composed
of elements that are either perfectly insulated from the viewpoint of