Page 326 - Design of Simple and Robust Process Plants

312 Chapter 8 Instrumentation, Automation of Operation and Control
The same applies for batch or semi-batch exothermal or gas-releasing reactions,
where a delayed reaction may happen that results in the build up of too-high con-
centrations of reactant. Subsequently, this situation may cause too-high reaction
rates later in the batch, resulting in an uncontrolled run away reaction.
These situations need adequate protection. For the adiabatic plug flow reactor the
system can be safeguarded by running a computer model of the reaction in parallel
to the actual reaction, and using built-in deviation settings for alarming and inter-
locking. An alternative for real-time running of the model is to compare the reaction
with historic reaction profiles of the planned transient operation.
For batch reactions that are normally operated at isothermal conditions, the above
method requires another approach. In that case, a cumulative heat balance can be
measured around the reactor. This is used to calculate the conversion of the reac-
tants, by comparing this with the cumulative reactant feed times the heat of reac-
tion. The amount of unconverted reactant can be obtained by subtraction of the
potential heat generated from the feed of the actual heat generated. The level of
unconverted reactants over time can be compared for each recipe with a standard
profile, and deviation settings for alarming and interlocking implemented. The safe-
guarding can also be provided by running a real-time model in parallel with the
reaction, and comparing the measured and theoretical heat release for deviations.
The models used for alarming can be of a simple form, and in general are of the
Arrhenius type, with the main contributing reactants and catalyst concentrations
applied to the equations. The objective is to measure the conversion to identify
potential runaways, and not for product distribution.

8.3.3.1 Alerting/alarming
The terms alerting and alarming are defined as follows:

. Alerting is intended to warn operators of the start of new transient actions,
and any forthcoming deviation of the process.
. Alarming is intended to warn operators to take action, because the process is
moving to an undesirable situation.
Alerting and alarming is very important in a process plant, and especially so in a
simple and robust designed plant. Such a plant is operated with fewer people, but
still requires a high level of attention. Potential problems may arise when a situation
escalates and alarm ªshowersº take place, as this leads to confusion of operation at a
point in time that rational behavior is urgently needed.
An even more important facet is to keep the operator's attention on the operating
process. Therefore, operation must be kept involved in all main operational actions,
by demanding various manual actions or confirmations. The target is to ªkeep
operation people alertº, and consequently a good alarming strategy is required (Tsia
et al., 2000) which includes the following: