Hi there, and welcome to this lecture on process flow diagrams: simple control loops.
My name is Marina Miletic.
Now that we've looked at examples of temperature, pressure, level, and flow rate control loops,
let's next look at an example of combining two types on controllers for the same piece
of equipment.
Depending on the process, reactors are frequently the source of many unexpected upsets and problems,
simply because so many things can go wrong, especially when extreme temperature and pressure
conditions are present.
So as much as possible a simple, yet effective, control system should be designed for each
reactor.
R-101 is an isothermal high-pressure reactor.
If either temperature or pressure are outside of their design range, the operation of all
the process units downstream will be affected, and it's very likely that not enough desired
product will be made.
So the purpose of these feedback control loops will involve three factors; controlling the
temperature of the process stream going into, and through, the reactor, and to control the
pressure going in to the reactor.
Let's start with controlling the inlet temperature of the reactor.
Think for a moment where you would measure your temperature signal, and what you would
control to achieve a consistent reactor inlet temperature.
Let's take a closer look at the reactor and its supporting equipment.
It's probably a good idea to measure the temperature here, send the signal to a temperature indicator
controller, which can process it, and send a signal to both valves controlling the high
pressure steam going through E-101 and E-102.
Based on the specific duties of these heat exchangers, these valves are opened or closed
to achieve an even increase or decrease in temperature.
As you might expect, the high pressure steam going to both of these heat exchangers should
be controlled, and not just one.
This is because these streams mix before the reactor and there shouldn't be a large temperature
discrepancy between them.
So this is a suitable control system for the inlet temperature going to R-101.
Now let's examine how we should control the temperature inside the reactor.
Keep in mind, just because the inlet temperature is being controlled, it doesn't mean the reactor
itself is controlled at a specific temperature.
Overall the reactions occurring are exothermic, and the reactor is designed to operate at
a constant temperature.
This means that there should be a jacket, or some kind of cooling element around or
inside the reactor which is absorbing heat as the reaction proceeds.
Think for a moment where you would measure temperature and what you would control to
maintain constant temperature inside the reactor.
The temperature should be read from inside the reactor, sent to a temperature indicator
controller, which then should control the flow rate of cooling utility going through
the jacket to either absorb more or less heat.
This is probably the best way of showing that the utility going through the cooling jacket
is being controlled.
However, if you would like to add a notation that makes it conspicuous, you can state the
utility being used next to the control valve.
Lastly, let's consider the best way to control the pressure of the reactor.
Remember, pressure is best controlled with a unit such as a pump, compressor, or depressurization
valve.
And note that these cannot be inside the reactor, they must be before it.
So we'll need to include compressor C-101 and C-102 in our control loop.
Consider where you would measure the pressure, and what you would control to either increase
or decrease pressure in the reactor.
It's a good idea to measure pressure inside the reactor, or even right before the reactor,
send that pressure to a pressure indicator controller, which then will send an electrical
signal to vary the power sent to each of these compressors.
Depending on the power required for each of these compressors, power can be increased
or decreased to evenly vary the pressure of the process stream.
Just as with the heat exchangers, the pressure should not be controlled by just one compressor.
This is because these pressures should be nearly equivalent when the streams are combined
here before the reactor.
Taken together these three control loops should maintain safe and consistent conditions during
reactor operation.
Let's look at another example, this time with an improperly designed temperature control
loop.
Let's say we'd like to control the exit distillate temperature from T-102.
Here's one example of how we might do this.
What's wrong with this control loop?
You obviously want to avoid a control loop like this where the dead time is too large,
the exit temperature is not directly related to the heat exchanger attempting to control
it, and only one of the parallel heat exchangers is being controlled.
Let's look at another example.
This is another way of trying to control T-102's distillate temperature.
What might be wrong with this loop?
This is also a bad idea since even this heat exchanger is too far away from the final product
stream, there's still at least a 10 minute dead time in this loop.
Also, the distillate temperature is not directly related to this heat exchanger.
Let's look at another example.
Will this control loop adequately control the distillate stream temperature?
Probably not, because you might remember from previous lectures that the temperature of
the distillate and bottoms is determined by the design dimensions of the column, the feed
flow rate and composition, and the condenser and reboiler duties.
All of these factors are responsible for the temperature of the distillate stream.
Distillate temperature is determined by the distillate concentration and condenser duty
more than anything else, and therefore can be difficult to control.
Also, this control loop is receiving its signal from the reboiler, which is excellent for
reboiler control, but it doesn't help control the distillate temperature directly.
So not even this shorter control loop is the best option.
It's best to have a well-designed column with an appropriately sized condenser.
As a last step let's do a review of the top ten most important concepts.
Process control is absolutely instrumental for the safety, efficiency, and effectiveness
of a process.
This is because control is critical to making the right amount of product and the right
concentration continuously.
Not properly controlling a process means time, money, and resources are being wasted.
Be sure to put temperature, pressure, level, and flow rate control loops in potentially
vulnerable areas of a process, which are prone to runaway reactions, unwanted side reactions,
or unexpected concentration changes.
Be sure to pay close attention to reactors and separators since these are the most critical
pieces of equipment.
Simple control loops identify a type of controller, whether it's temperature, pressure, level,
or flow rate, where the controller is receiving its signal, and what the controller is controlling,
whether it's a pump, compressor, fresh feed, or utility flow rate.
Remember that control valves should be placed on streams that can be independently adjusted
such as the flow rate of fresh feed coming from a storage tank, or the flow rate of steam
or cooling water going to a heat exchanger.
Related to this point, avoid putting control valves on process streams unless they're at
the very beginning or end of the process, or unless the control valve is controlling
the liquid level in a flash, distillation column, or vessel.
Recall that a capillary or liquid signal is a solid line.
A pneumatic or air pressure signal is designated as a line with hash marks.
And an electric or voltage signal is designated with a dashed line.
Also remember that process equipment is not one-size-fits-all, flow rate cannot be arbitrarily
increased or decreased in process equipment with the expectation that the same product,
at the same concentration, will come out.
This will not happen if flow rate is modified considerably from its design specification.
Also it's important to design control loops so that they're short, with small dead times,
to make sure that the signal or output variable is directly related to the adjusted variable
and a minimum number of variables are involved in the control loop so that it's not over
designed.
Also, especially in the case of an isothermal reactor, inlet temperature, interior temperature,
and inlet pressure of a reactor should all be controlled, which requires the design of
a few different control loops.
Lastly, distillate temperature is not easily controlled because it's related more to the
concentration coming out of the column.
It's more common to control reboiler heat duty, though this effects the distillate temperature
indirectly.