Page 72 - Design of Simple and Robust Process Plants

56 Chapter 3 Design Philosophies
of the potential for fire by installing a sprinkler system on the tank, and the removal
of any adjacent fire source formed part of the solution. Additional solutions included
the elimination of all other potential pressure increases by installing instrumental
protection with appropriate precautions and in-process relief.

Quench

Vent
P

NH3
NH3
WASTE TANK

Pressure relieve with quench Instrumental protection with
water absorber and waste tank sprinkler system no waste

Fig. 3.5. NH 3 storage with recovery system versus a simple
instrumental protection system.

3.3.10.2 Design for first-pass prime production
The design of first-pass prime (Aelion and Powers, 1991; Verwijs et al., 1995) is a
concept that has not yet been extensively explored, though it has the same objective
as ªprevent versus cureº. The concept is to avoid expensive provisions in plants in
order to cope with off-specification products that have originated from the start or
recovery of a facility. A careful design of start-up and standby procedures is required
to achieve the objective of first-pass prime production, see Chapter 8. Therefore, it is
important to distinguish between process units as being reversible or irreversible.
A reversible unit operation is defined (Verwijs et al., 1995) as: ªA process system
that can be operated stand alone without any process stream fed in or out of the
system, due to the presence of inverse operationsº. This yields the possibility of con-
ditioning a process system during an operational state of the process. Irreversible
unit operations can only be operated by supplying an appropriate feed.
Automated plants have defined process states. Next to the actual production state,
transient steps are defined for start-up, shutdown, and product changes. Process
wait states, also called standby, are also introduced, which have defined process con-
ditions, before the process is moved to another operational state. Process wait states
can be differentiated in:

. Cold process wait condition (cold standby): at this state the inventory in the
units is established.