[Luyben] Process Mod.. - Student subdomain for University of Bath

4 PROCESS MODELING, SIMULATION, AND CONTROL FOR CHEMICAL ENGINEERS
FIGURE 1.4
that only permits liquid to pass through it, thus preventing “blow through” of the
steam vapor). We want to control the temperature of the oil leaving the heat
exchanger. To do this, a thermocouple is inserted in a thermowell in the exit oil
pipe. The thermocouple wires are connected to a “temperature transmitter,” an electronic
device that converts the millivolt thermocouple output into a 4- to 20-
milliampere “control signal.” The current signal is sent into a temperature
controller, an electronic or digital or pneumatic device that compares the desired
temperature (the “setpoint”) with the actual temperature, and sends out a signal to
a control valve. The temperature controller opens the steam valve more if the temperature
is too low or closes it a little if the temperature is too high.
We will consider all the components of this temperature control loop in more
detail later in this book. For now we need’only appreciate the fact that the automatic
control of some variable in a process requires the installation of a sensor, a
transmitter, a controller, and a final control element (usually a control valve). Most
of this book is aimed at learning how to decide what type of controller should be
used and how it should be “tuned,” i.e., how should the adjustable tuning parameters
in the controller be set so that we do a good job of controlling temperature.
Example 1.3. Our third example illustrates a typical control scheme for an entire
simple chemical plant. Figure 1.5 gives a simple schematic sketch of the process
configuration and its control system. Two liquid feeds are pumped into a reactor in
which they react to form products. The reaction is exothermic, and therefore heat
must be removed from the reactor. This is accomplished by adding cooling water to
a jacket surrounding the reactor. Reactor emuent is pumped through a preheater
into a distillation column that splits it into two product streams.
‘Traditional steadystate design procedures would be used to specify the
various pieces of equipment in the plant:
Fluid mechanics. Pump heads, rates, and power; piping sizes; column tray
layout and sizing; heat-exchanger tube and shell side batlling and sizing
Heat transfer. Reactor heat removal; preheater, reboiler, and condenser heat
transfer areas; temperature levels of steam and cooling water
Chemical kinetics. Reactor size and operating conditions (temperature, pressure,
catalyst, etc.)