Page 399 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

As stated previously and in Chapter 24, this section should be eliminated or minimized through pollution
prevention and green engineering. However, especially if the contaminant has little or no value if
concentrated, there will be relatively dilute waste streams generated and sent to the environmental control
section. Here, they are concentrated (by the separation techniques discussed earlier) and then disposed of
(by incineration, neutralization, oxidation, burial, or other means). The keys are to concentrate the waste
and to make it benign.

12.7 Major Process Control Loops

During the initial synthesis of the PFD, the major control loops are developed. These control loops affect
more than just one unit of a process. For example, the level control in the condensate tank of a distillation
column is necessary for plant operation. On the other hand, a reactor temperature controller that changes
the flowrate of molten salt through the cooling tubes of a reactor is a major loop and should be shown on
the PFD.

It is through the early development of major control loops that significant design improvements can be
made. In the high-temperature exothermic reactor, for example, failure to consider the control loop might
lead one to propose an integrated heat-exchanger network that is difficult or impossible to control.

Beyond the importance of the control loops in maintaining steady-state material balance control,
assurance of product purity, and safety, they provide focal points for the optimization that will follow the
initial PFD synthesis. As described in Chapter 14, the controlled variables are the variables over which
we have a choice. We find the best values of these variables through optimization. These loops also
provide early clues to the flexibility of the process operation. For example, if the feed to the reactor is cut
in half, less heat needs to be removed. Therefore, there must be an increase in the temperature of the
cooling medium, which occurs when the coolant flowrate is reduced. Process control can be both very

difficult and extremely important in biological processes, as demonstrated in Example 12.5.

Example 12.5

In biological waste treatment, microbes that eat the waste and produce benign products are used. In one
class of such processes, called activated sludge, the culture is separated from reactor output and
recycled. What specfic process control issues arise in such a process?

The culture must maintain sufficient activity throughout the recycle. The recycle conditions may become
nutrient poor if nearly all the waste nutrient is consumed, leading to loss of activity. The activity of the
culture might be adversely affected by upsets in the feed conditions to the reactor, such as pH extremes or
high concentrations of compounds toxic to the culture. Therefore, feed-forward control using
measurements of the feed conditions should be considered, as well as control of a reserve of the culture to
be used in case an uncontrolled process upset leads to culture death. Immediate, on-line analysis of
culture activity is difficult, but off-line measurements can be incorporated into the control scheme.

12.8 Flow Summary Table