Page 343 - Design of Simple and Robust Process Plants

8.4 Control Design 329
and finishing section. So, the front end and the back end of the plant are
stabilized when the reactor is conversion controlled. Such a control design
minimizes swings in recycle flow and puts any disturbance in recycle up-
front to the feed supply system while maintaining the reactor feed constant,
see also below under set recycle streams.
There are many production plants where the feed flow is set (e.g. in olefin plants),
and this is determined by furnace loads. In those situations recycle flows are not
fixed, but some intermediate recycle storage is quite often foreseen to accommodate
a more constant recycle In exceptional cases a downstream plant sets the production
rate of an up-front process; this has been done for gaseous supplies, but it places a
constraint on the supplier plant control design.
± Conversion control of the reaction is essential from control stability perspec-
tive not limited to the reactor section, but for the whole plant. It also sets the
product distribution while its set point represents the economic trade-off
between conversion costs and selectivity. The conversion of a reactor requires
selection of the dominant variable for control; the factors which have to be
considered include temperature, residence time, reactants and catalyst con-
centrations, pressure, and mass and heat transfer rates. The selection of the
dominant variables can be done by evaluating all relevant variables in a static
reactor model. The selection of dominant variables based on a thermody-
namic methodology is discussed later in this section. An accurate, reliable
and fast responding conversion/composition measurement at the outlet of
the reactor is a requirement.
± Control of product quality within safety environmental and operational con-
straints. Quality control is mostly achieved at unit level; however, these can
be divided into separation units and reactor units. An example of product
qualities which are determined at reactor level are hydrogenation reactors ±
either selective hydrogenation such as C2 and C3 hydrogenation to remove
acetylenes and di-olefins; or total hydrogenation such as nitrobenzene, which
is totally converted to aniline. The design of the control structure for quality
is discussed under unit control.
± Set the flows for recycle streams and determine inventory control (pressure
and levels) It is (according to Luyben et al., 1998) fundamental to fix recycle
flow for a process plant. It was said earlier that disturbances, when neither
prevented nor rejected, need to be absorbed. Absorption is done by inventory
control, where flow and composition variations are smoothened or to propa-
gate the disturbance directly outside the process in process or utility streams;
in the latter case it is often also absorbed in an inventory system outside the
process. Recycle loops might be subject to large variations if they are not con-
trolled it is what Luyben et al. called the ªsnowball effectº. The sensitivity of
recycle loop flow rates to small disturbances can cause large swings in these
recycles, which propagates disturbances through recycle flows which are to
be prevented. The solution is to fix recycle flows. This is not conflicting with
reactor control, which is still controlled by its dominant variables, but the