Process Control Project

Process Control Project
Process Control Project
Student Name
Higher Colleges of Technology Abu Dhabi Men’s College
ECH 3043: Process Control
Lecturer’s Name
16th April, 2019
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Process Control Project
Introduction
Feedback is a control strategy that uses data from measured parameters to manipulate other
system variables so as to get an anticipated outcome. In this meachanism, the measured
variable is the controlled variable. This variable is measured and compared to a targeted value
or set-point. The disparity between the actual measured value and the target value is known as
the error. The feedback system works to reduce the error.
There are two types of feedback control which are the negative and the positive feedback.
The negative feedback tends to cause increase/decrease when the change in the measured
variable has decreased/increased respectively. This results in the convergence towards an
equilibrium. In contrast, the positive feedback causes divergence of the system from the
equilibrium.
The feedback control boast of several advantages due to the acquisition of the controller input
from the process output. First and foremost, modelling the controller system is a very simple
technique as no mathematical modelling of the whole system is needed be. Likewise, the
control can be copied from one system to another. Furthermore, unexpected disturbances that
had been unaccounted for during the design phase can also be accounted for (University of
Michigan Chemical Engineering , 2006).
The fact that the feedback control uses the process output its controller input also adds
various disadvantages to the control mechanism which include:
• Control action is taken only when the process output has digressed from set point.
This also includes the digressions that occur at the very beginning of the process.
Furthermore, the output sensor may fail to note the error and the deviation may persist
leading in controller inefficiency.
• Causes system instability in the closed-loop system as response may be oscillatory.
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Process Control Project
• The feedback controllers do not resolve predictive loads that may cause system
disturbances (University of Michigan Chemical Engineering , 2006).
Finally, the feedback control is a single input single output (SISO) system that can only
obtain signal from a single sensor which is a limiting factor for a system which has multiple
sensors or requires multiple control from a single output. For this reason, more complex
multiple input and multiple output control systems have been developed. Examples are the
split range control and the feedfoward-feedback controls as discussed below.
Split Range Control
The split range control loop is used in situations where there are more than one manipulated
variables (process input) and only one control input.
Industrial Application of Split Range Control
This mechanism is employed in a liquid-vapour hydrocarbon separator. Typical of the split
range controller, the splitter specifies the order in which the valve operate as the output of the
control process (controlled variable) varies.
The figure 1 below is a schematic representation of a hydrocarbon liquid-vapour separator.
The objective of the control mechanism is to maintain the pressure within the vessel within a
specified acceptable range that will allow for the flashing of the hydrocarbon vapours from
the liquid-vapour mixture. The splitter is the pressure indicating controller (PIC) in the
diagram. The controller, PIC, measures and controls the pressure within the separating vessel.
In this case, the controlled variable is the pressure, a primary measurement, within the
separator. The manipulated variables are the flows of the vapour to the flaring section and the
flow of fuel gases into the separator depending on the prevailing state of the controlled
variable. The disturbance in the system is caused by the flow of new feed into the separator
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Process Control Project
which part of it is vaporized as the remaining liquid is collected at the base of the separator.
The pressure is measured directly using pressure sensing devices (EnggCyclopedia, 2019).
Figure 1 Split Range Control on a Hydrocarbon Liquid-Vapour Separator. Adapted from
“Split Range Control Working Principle,” by Inst Tools, 2019, Inst Tools. Retrieved April 18,
2019, from https://instrumentationtools.com/split-range-control-working-principle/
The signal from the controller is split such that it is sent to two pressure control vales
depending on the input signal to the controller. When the pressure in the separator increases
until it is equal or greater than the set point of the controller, the controller sends a signal to
control valve PV-1. The valve opens allowing the gaseous vapours to be let out to flare. This
causes a reduction of pressure in the vessel to the desired pressure range. On the other hand,
when the pressure falls far below the set point pressure, the controller (splitter) sends a signal
to the control valve PV-2 which allows for the flue gases to flow into the separator to
counteract for the pressure deficiency.
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