Page 344 - Design of Simple and Robust Process Plants

330 Chapter 8 Instrumentation, Automation of Operation and Control
feed rate can be set in ratio to the recycle. By fixing the recycle flows the pro-
cess operates very stable. Most hydrogenation reactors, recycle hydrogen over
the reactor system and operate at maximum flow rate of the recycle compres-
sor which is a widely accepted control mode, which a way of setting the
recycle flow.
Inventory control includes pressure, as well as level. The stream which has the larg-
est effect on the inventory should be used for its control ± this is known as the
Richardson rule. A well-known example is that of distillation columns with large
reflux ratios where the level is controlled by the reflux instead of the distillate flow.
For pressure regulation, the cooling duty of a condenser is often selected for manip-
ulation. In case inertia are available, the removal of the inertia has a large effect on
the cooling duty, even when it is only a small stream, and so this is often selected.
± Check component balances: it is important to understand these for control
purposes, particularly at impurity level. They might come from different
sources to be named: introduced with the feed, formed in the reaction, or
introduced with additives which eventually might fall apart. They must be
recognized specifically for situations with recycles, as they will build up due
to their integrated effect, and must exit the system somewhere. Their build
up needs to be quantified, and any effect on reaction or separation under-
stood. The places where these components leave the system must be identi-
fied, and eventually CVs and MVs assigned for their removal. Also, in
straight-through systems it is worthwhile understanding their effect. In such
a situation they either must leave the system with product streams (in which
case there is an effect on the separation and purity), or a separate outlet
(purge stream) must be created, like a vent or side stream. Even when the
impurities leave with the products, build up may occur in the separations
and affect any quality measurement, like a temperature in a separation col-
umn. The concentration levels of impurities might be subject to large swings,
and this can handicap operation and control if the mechanism is insuffi-
ciently understood. The swings might be caused by varying feed and additive
compositions, varying reaction conditions, or by variations caused by control.

Unit control is primarily based on feed-back control, while some cascading and
feed-forward action might also be involved. The plant wide control approach must
be decided before unit control can be designed. It determines the constraints for the
control design next to the operational, safety and environmental constraints. Unit
control still offers many opportunities for pairings CVs and MVs. A methodology
must be followed to achieve a robust basic control layer which is the objective for a
simple and robust process plant. The methodology emphasizes several elements,
which are:
. Selection of dominant variables and its pairing for control.
. Selection of self-optimizing control options.
. Determination of static interaction for selected options.
. Determination of dynamic interaction for selected options