Page 131 - Design of Simple and Robust Process Plants

116 Chapter 4 Process Synthesis and Design Optimization
between a supplier plant and a receiver plant ± direct integration between plants.
Examples are a single oxygen plant coupled to ethylene oxide plant, or a hydrogen
plant coupled to a hydrogenation process. In all these case the decision must be
made whether to operate as one unit without intermediate storage, or to install inter-
mediate storage. These decisions can be supported by quantified information about
process reliability and availability, with the cost of interruptions versus inventory cost
(see Chapter 7). Simple and robust plants opt for minimizing inventory by improving
plant robustness, but realistic calculations are needed to guide the decision making.
When a decision has been made to operate as one section, then direct integration
is applied, although adequate provision is required to obtain robust operation and
control. This last situation is often applied between an exothermic reaction section
and finishing section where the heat is applied for separation.
In case the decision is to operate sections independently, then the integration
between sections needs to have some back-up provisions. This might be from an
independent source (as provided by utility systems), from a second source (as in
coupled hydrogen supply systems), or from an inventory system (Figure 4.27).
These decisions might be supported by reliability and vulnerability calculations.
Options for integration at utility levels are illustrated in Figure 4.28. Three alterna-
tives for integration are shown: direct coupled; coupled with internal back-up; and
external utility back-up.

Inventory
Back-up

Section A Section B

Direct coupled
Fig. 4.27. Integration options between processes or sections:
direct coupled, through storage, with back-up.

In order to optimize the design, a good insight into reliability and the availability
of supplies is essential, both internal and external. Bear in mind that over-demand-
ing guarantees in this respect will be a disadvantage for the receiver ± it all boils
down to a problem of optimization. Guarantees have a price ticket that is ultimately
paid by the customer. The reliability and availability of utility and raw-material sup-
plies and site vulnerability will be discussed in Chapter 7.
To summarize, process integration is a development by which the efficiency (raw
material and energy utilization) of an operation may be increased, the options for
such integration being: direct coupled; coupling through inventory; or with a back-
up supply. The constraints that occur are in availability, variability and disturbances
while violation will result in loss of production and/or a poorer quality operation.