Page 341 - Design of Simple and Robust Process Plants

8.4 Control Design 327
. Operational disturbances are caused by operational activities; for example,
when units or components are taken in or out of operation for cleaning,
regeneration, or repair. Even if these activities are automated, they are still to
be recognized as a disturbance.
. Control disturbances which cause propagation or even amplification of a dis-
turbance through the process system, particular so for recycling of distur-
bances. These are negatively influenced by; inappropriate control design, and
wrong tuning of control loops.
The impact of both types of disturbances can be handled in different ways they can
be categorized as:

. Prevention or limitation of a disturbance is a preferred action, examples are:
± Design internal utility grids less sensitive to external (site) variations by oper-
ating the internal system with a pressure controller slightly above or below
the external grid level, depending on an import or export situation.
± Inventory with mixing devices will minimize composition or temperature
variations.
± Smoothly ramping of set point changes particular for flow, pressure and tem-
perature and economically determined set points
± Gradually and smoothly taking units in or out of operation
± Design for back-up supplies like is applicable for heat-integrated systems (see
Figure 4.29 in Chapter 4).
± Correct for any concentration variation of a feed stream. For example hydro-
genation reactors often need to control the mass flow of hydrogen, this
means correction for any impurities is required.
. Rejection of disturbance to outside the process is easily applied for utility sys-
tems and inventory feed systems. Examples are:
± Any deviation in utility consumption such as steam is exported to external
supplies. Site utility systems are normally designed with short response
times to enable fast compensation of demand variations.
± Reactors experience changes in conversion which have an impact on recycle
flows, and as such varies the demand on feed of reactants. These variations
can be absorbed at the supplier, or in the inventory.
. Absorption of disturbances depends on process design and control design.
Examples are:
± Inventory in reactors, column bottoms, reflux drums and surge drums to
smooth variations in flow and/or composition.
± De-coupling of interactions to direct a disturbance outside the process and
avoid recycling or transmitting to other process sections.
These three methods for disturbance handling should be recognized and exploited
in sequential order of the above discussion and in full understanding between pro-
cess and control designer. Design modification might be essential to limit the effect
of disturbances.