Page 73 - Design of Simple and Robust Process Plants

3.3 Design Philosophies 57
. Hot process wait condition (hot standby): at this state the inventory and
defined temperatures are established.
. Recycle process wait: at this condition the inventory, temperature, pressure,
specifications and internal recycle streams are established.
The cold process wait is the cold starting condition for an initial processing step.
Hot process wait or hot stand-by is a condition which forms the bases for actual
production at temperature. Examples include a steam turbine which is at slow roll
with a slipstream of steam to enable fast start-up, or a distillation column at total
reflux.
Recycle process wait can be applied to reversible unit operation where all process
conditions, including an internal recycle, are established. Examples include an eva-
porator where the bottom and overhead stream are returned as recycle, a distillation
column with recycle of its top and bottom stream into the feed, or a refrigeration
unit with a minimum cooling duty from a reversible process unit.
The development of the hot stand-by and recycle mode condition as a start for the
production is essential in order to achieve first-pass prime production. Most plants
have a reaction section followed by a finishing section. In such a configuration it is a
standard approach to have the downstream finishing section in such a condition (hot
stand-by or recycle mode) that the process and products are at specification before
any feed is started. For irreversible units, such as a reactor or an extraction, a recycle
mode cannot be established. In these situations the system must be pre-conditioned
to such a level, to produce on-specification products from the start. For a reaction
section, a hot stand-by condition is often established by recycling one of the compo-
nents or a carrier over the system at the required inlet temperature. Such a pre-select-
ed condition in line with a hot stand-by or recycle finishing section will enable
smooth start-up and first-pass prime for production. It should be said that sufficient
effort needs to be spend in analyzing the system dynamically to develop such an
operational procedure. In all cases, a thorough understanding of the process (often
reflected in a dynamic simulation of the process) is required to meet this objective.

Summary
. Just-in-time production (JIP) has as objective to minimize the logistic provi-
sions. It requires production on request and at specification, following the
concept of total quality control (TQC).
. ªPrevent versus cureº is an approach to prevent off-specification production,
resulting in the elimination of intermediate storage and blending facilities
for products out of specification. Implementation is often realized through
feed forward control actions and direct recycle provisions in the process.
. Production of first-pass prime product requires an operational strategy to
avoid plant provisions for recovery of off-specification products.
. The development of operating strategies with intermediate standby condi-
tions leading to the first-pass prime production are essential. The develop-
ment of operation strategy requires detailed process knowledge to be cap-
tured in dynamic models.