INSTRUMENTATION AND PROCESS CONTROL
1) Need for control in the plant
During its operation, the plant must satisfy several requirements imposed by its designers and
the general technical, economic and social conditions in the presence of ever-changing external
influences or disturbances. Among such requirements are the following :i) Safety: The safe operation of a chemical process is a primary requirement for the well being
of the people in the plant and for its continued contribution to the economical development.
Thus, the operating pressure, temperature, concentration of chemical and so on should always
be within allowable limits.
ii) Production specifications: A plant should produced the desired amount and quality of the
final products. To maintain the final product composition desired, chemical process control is
essential.
iii) Environmental Regulations: Various central and state laws may specify the allowable limits
of temperature, concentration and flow rates of effluents from the plant.
iv) Operational constraints: The various types of equipments used in plants have constraint
inherent to their operation. Such constraints should be satisfied throughout the operation of
the plant. For example pumps must maintain a certain NPSH, tanks should not overflow or go
dry, distillation columns should not be flooded, the temperature in catalytic reactors should not
exceed the range of the catalyst. The control systems are needed to satisfy all these operational
constraints.
Instrumentation
Instrumentation is provided to monitor the key process variables during the plant operations.
They must be incorporated in automatic control loops or used for the manual monitoring of the
process operation. Instrument monitoring critical process variables will be fitted with automatic
alarms to alert the operators to critical and hazardous situations. It is desirable that the process
variable to be monitored and dependent variable (that is easier to measure) are monitored in
their places. For example in the control of distillation columns the continuous, on line analysis
of the overhead products is desirable but difficult and expensive to achieve reliably, so
temperature is often monitored as an indicator of compositions. The temperature instruments
may form part of a control loop controlling, say reflux ratio, with the composition of the
overhead checked frequently by sampling and lab analysis.
Instrumentation claims the following advantages:
1. It controls all those parameters, which control the process and do not allow them to cross
safe limit thereby enabling smooth and safe operation.
2. An efficiently controlled plant possesses high safety by carefully monitoring potential hazards
like fire, explosion, toxicity etc
3. By the monitoring of the process parameters like temperature, pressure and composition
streams of reaction vessel, it guarantees the quality of the product.
1) REACTOR:
Here, In the above diagram:
a) Three temperature sensing instruments are used one each at the entry and exit of the
reactor, and one for the controlling temperature of the cooling water that exits the reactor.
Two of them are monitored from the control room.
b) One Flow controller is deployed at the entry of cooling water, to control flow rate of cooling
water. Drain is also provided for removal of extra cooling water.
c) One bypass valve is provided at the entry of feed to be used in case of emergency.
d) Motor and normal control valve are provided at each pipe to ensure the flowrate within
specified parameter.
e) Vent is provided for removal of vapor from heat exchanger (reactor).