Page 320 - Design of Simple and Robust Process Plants

306 Chapter 8 Instrumentation, Automation of Operation and Control
safety, but mass flows are also verified. The model predictive control techniques for
batch processes are matured to achieve consistent temperature control. The higher
consistency in mass flow and temperature control results in consistent batches. The
objective for a batch plant to produce each batch at specification can be achieved
with current techniques.
The conditioning of a process is not only limited to achieve FPPP but also to
satisfy mechanical constraints. Mechanical systems are constrained by their capabil-
ity of handling temperature transients. These constraints are respected by smooth
start-up of units exposed to temperature transients. Units operated at extreme high
or low temperatures may be fitted with temperature sensors for monitoring heating
and cooling effects.
The smooth start-up of equipment subject to temperature transients is carried out
manually, by slowly opening valves. If it is necessary for this to be automated
through temperature trajectory control, then specific precautions must be taken for
sufficient instrumentation with adequate valve sizing and selection.
The above discussion teaches us that FPPP, in most situations, can be achieved
by careful process and operational designs. Dynamic simulation and optimization
techniques for the start-up of irreversible reactions are efficient tools to design effi-
ciently transient operations.

8.3.2.3 Start-up and shut-down of a process
In the previous section, three basic flowsheets of continuous process plants were
described: a hydration process; a hydro-dealkylation process for benzene production;
and a nitric acid process (Figures 8.8±8.10), all of which have different reversible
and irreversible units. Here, the methodology is presented on how to start a contin-
uous process plant, the three processes presented being those used for verification.
The methodology for start-up of a continuous process plant with a reaction and
finishing section is:

. Take internal utility systems in operation which are in connection with exter-
nal supplies, such as the power, steam grids, air, nitrogen and water systems.
. Take internal utility systems in operation not directly supplied from external,
such as the process cooling water system.
. Fill and bring reversible process and utility units at an initial state called ªpro-
cess waitº, such as filling up refrigeration systems with refrigerant, or filling
process steam systems with boiler water.
. Take reversible utility systems in operation, such as refrigeration. For a refrig-
eration unit, partial heat removal is required, and this can be accomplished
by lining-up a reversible process unit. (This reversible process unit must be
brought to an initial state to remove any heat from the refrigeration unit.)
The pressure and temperature levels of the utility systems are brought to (or
close to) the objected operational conditions. The units will still operate at
reduced loads due to lack of demand.
. Take reversible process systems in the operation, such as distillation, absor-
ber-stripper or extraction-stripper combinations, by circulation.