Page 319 - Design of Simple and Robust Process Plants

8.3 Automation of Operation 305
Stoichiometric reactor systems at high conversion without a recycle for precondi-
tioning are more difficult to design for first-pass prime. An example is a furnace,
but this can be made reversible by installing a recycle or dummy load on the process
side. On occasion, the flow of inert material takes the role of conditioning. An exam-
ple is the nitric acid plant, which is shown schematically in Figure 8.10.

Boiler
water Steam
H O Off gas
2
NH 3
Air HNO 3
solution
R R IR IR
IR
IR
Compressor Combustion Reactive-
absorption
Fig. 8.10. Nitric acid process in a block diagram with reversible
compressor and irreversible reactor and reactive absorber.

The nitric acid plant consists of three sections: an air compressor; a NH 3 combustor
where nitrous vapors are formed and the exothermal heat is removed; and finally
absorption in water to form the acid. In such a plant the nitrous vapors are formed
by combustion of NH 3 with air; this reaction is almost instantaneous. An absorber
column where these nitrous vapors react in water requires careful design for its
operation. This process can be conditioned by the air compressor which passes
through the whole process ± in essence it is an open recycle system. To achieve the
required acid concentration the water flow needs to be controlled for steady-state
conditions. This is quite straightforward: during start-up the achievement of first-
pass prime requires careful design of the operation, based on dynamic simulation.
The control of a recycle over part of the column for start-up is a precondition.
Transient operations in continuous processes to achieve different product proper-
ties will always produce intermediate products, as in the case of polymer processing
for polystyrene and polyethylene. One reason for this is that these processing sys-
tems mainly have a large residence time distribution, which results in a large transi-
tion zone that is inherent to the process design, another reason is trajectory control.
Dynamic optimization of the transient operations can reduce the transition zone in
as much as it is impacted by control. The dynamic optimization technique has now
been developed and is approaching commercial status.
For batch processes, the production of first-pass prime products depends both on
accurate metering of the feed streams and temperature control. Currently, the
metering techniques are highly accurate, especially as redundancy is foreseen for
dosing of mass streams by weight cells on process vessels. The argument for this is