Page 44 - Introduction to chemical reaction engineering and kinetics

26 Chapter 2: Kinetics and Ideal Reactor Models

there is a spread or distribution of residence times. Residence-time distribution
(RTD) is described in Chapter 13 for ideal-flow patterns, and its experimental
measurement and use for nonideal flow are discussed in Chapter 19.
(2) Mean residence time (t) is the average residence time of all elements of fluid in
a vessel.
(3) Space time (T) is usually applied only to flow situations, and is the time required
to process one reactor volume of inlet material (feed) measured at inlet condi-
tions. That is, r is the time required for a volume of feed equal to the volume
of the vessel (V) to flow through the vessel. The volume V is the volume of the
vessel accessible to the fluid. r can be used as a scaling quantity for reactor per-
formance, but the reaction conditions must be the same, point-by-point, in the
scaling.
(4) Space velocity (S,,) is the reciprocal of space time, and as such is a frequency
(time-l): the number of reactor volumes of feed, measured at inlet conditions,
processed per unit time.

2.2 BATCH REACTOR (BR)

2.2.1 General Features
A batch reactor (BR) is sometimes used for investigation of the kinetics of a chem-
ical reaction in the laboratory, and also for larger-scale (commercial) operations in
which a number of different products are made by different reactions on an intermittent
basis.
A batch reactor, shown schematically in Figure 2.1, has the following characteristics:

(1) Each batch is a closed system.
(2) The total mass of each batch is fixed.
(3) The volume or density of each batch may vary (as reaction proceeds).
(4) The energy of each batch may vary (as reaction proceeds); for example, a heat
exchanger may be provided to control temperature, as indicated in Figure 2.1.
(5) The reaction (residence) time t for all elements of fluid is the same.
(6) The operation of the reactor is inherently unsteady-state; for example, batch
composition changes with respect to time.
(7) Point (6) notwithstanding, it is assumed that, at any time, the batch is uniform
(e.g., in composition, temperature, etc.), because of efficient stirring.
As an elaboration of point (3), if a batch reactor is used for a liquid-phase reaction, as
indicated in Figure 2.1, we may usually assume that the volume per unit mass of material
is constant (i.e., constant density), but if it is used for a gas-phase reaction, this may not
be the case.

Liquid surface
Stirrer
Liquid contents
(volume = V)
Vessel (tank)
Heat exchanger
(if needed)
Figure 2.1 Batch reactor (schematic, liquid-phase reaction)