Hello students.

Welcome to this course on chemical process control.

The topic for this first week will be introduction to process dynamics and control.

Let us get started with introduction to process control.

Here are the objectives for this particular part of the lecture.

At the end of this lecture, you should be able to articulate what is the need of process

control?

What are the different functions a process control system performs when it is implemented

in a chemical plant?

What are the different elements of a control system?

What are the different types or the control strategies which are possible and what are

the advantages and disadvantages and lastly we will also look at what are the different

types of control problems which exist in a chemical industry.

So let us get started.

Chemical process control is one of the core courses in your chemical engineering curriculum.

You also take different courses such as Heat Transfer, Reaction Engineering and other courses.

In these different courses, you might have come across design of various chemical engineering

equipment.

So just try to look back at what are the different types of assumptions which you make while

designing any equipment for a chemical plant.

Let us take an example from heat transfer.

Let us consider that you are designing a heat exchanger so that you cool a process stream

which is available at 100 degree C and you have to cool it all the way up to 80 degree

C. this can be done by passing a cold fluid from the other side of the heat exchanger.

So let us say you have cooling water which is available at 30 degree Celsius and

so the requirement originally is you have a hot stream which is available at 100 degree

Celsius.

And you want to cool it all the way up to 50 degree Celsius and you have a cooling water

which is available at 30 degree Celsius.

So you can do the calculations for this exchanger and eventually you will find out what is the

UA requirement for this particular exchanger and accordingly you will find out what is

eventually the area of the exchanger and you will buy an exchanger according to that particular

area as well as UA specification.

Now let us think that if you put that exchanger in the real plant, would it always give me

50 degree Celsius as the outlet of the hot stream?

It will give you that provided your hot stream enters at 100 degrees.

Your flow rate of the hot stream remains constant at the value which was designed.

Also your cooling water is available at 30 degree Celsius.

However all these assumptions are not always possible.

It is not always possible to maintain all these assumptions when the plant is actually

operated.

When you design a plant, you always design it at non-fluctuating or constant properties

such as the feed conditions, feed temperature, flow rate, feed composition as well as the

external parameters being constant like the cooling water temperature.

However, when you operate the plant, the plant operation is quite dynamic and it is subjected

to various disturbances.

So let me show you a simple simulation highlighting what happens if you have the cooling water

temperature rather than a constant value of 30 degrees; let us take how a real life cooling

water temperature would vary.

So typically cooling water comes from a cooling water tower and the temperature which you

get is typically dependent on the ambient temperature and you all know that during the

day as well as night time ambient temperature does indeed change and here you can see that

the cooling water temperature cannot be maintained exactly at 30 degrees as which was the assumption

for our design and it does change throughout the day as well as night.

So accordingly if such a cooling water temperature is available for the heat exchanger, the outlet

which comes out from the exchanger the hot side will never be exactly equal to 50 degrees.

However, it will also oscillate like the and it will also change depending on how the cooling

water temperature is changing and what you get at the end is an average performance which

is similar to what we have designed.

However, the local or the instantaneous temperature of the hot stream does not guaranteed to be

at 50 degrees.

So if this particular stream is going to a different unit operation, let us say it is

going to a separation system which requires the inlet temperature of 50 degrees then we

are not able to maintain that particular inlet temperature.

So the take home message is the operation, the actual plant operation is quite dynamic

because it is subjected to various process disturbances, environmental disturbances as

well as there are in some cases, even though you design a system at a particular operating

point, due to different changes in terms of market conditions or different changes in

the topology of the process you want to operate the process at a different operating condition.

So you want the plant to move from one point to the other and thus there are changes in

the desired operating point.

So in such cases you do not go about changing the design every time because it is not always

feasible to change the design of a process every now and then.

So what you end up doing is you implement a system which will ensure that irrespective

of such disturbances as well as fluctuations what you get out of the process is same as

what was the original objective.

And this is the job which a process control system does.

So the aim of the process control system is to maintain the operation of the actual plant

close to whatever is your desired point.

So now control is a very commonly found theme.

So process control deals with control of chemical engineering processes but in general control

is a very common phenomena and let me motivate it through some of the household common control

examples.

So the first example that I am going to take is of taking shower.

So let us say you take shower every day and you typically try to have some feel-good temperature

as well as feel-good flow rate of water when you try to take bath and you are typically

provided with two sorts of nozzles or two sorts of valves so you can change the flow

rate of cold water as well as hot water and accordingly you mix them such that whatever

water you get out of the shower has the required temperature as well as the flow rate.

So here the objective is that you want a certain temperature as well as flow rate of the water

coming out of the shower.

The other example is a pressure cooker.

So when we cook, we typically want to cook it at a higher temperature and thus we want

to maintain a certain pressure inside the pressure cooker.

So how do we maintain that?

The way a pressure cooker works is if the pressure of the steam inside the cooker increases

or reaches the value which we desire then the whistle gets blown up.

So as the whistle opens, the steam goes out and suddenly the pressure starts dropping.

As soon as the pressure reaches the lower value, the whistle again goes back.

So that way by having the manual whistle you are able to maintain pressure inside a pressure

cooker.

The next example is crossing the road.

So when we want to cross the road, we typically have an objective that we want to go from

one end of the road to the other without getting hit by any vehicle obviously.

And what happens is we have control on how fast we can go and when can we cross the road

and the disturbances here are the cars or the other vehicles are coming from both the

sides.

So we have to ensure or we have to predict whether we are able to cross the road without

being hit.

So in that case you try to predict how fast the car is coming, how much time would it

take till it reaches you and accordingly you try to calculate whether you would be able

to cross the road or not safely.

And lastly all of us have air-condition at home and the job of the air conditioner is

to maintain the temperature which we set through its remote.

So if we set a particular feel good temperature for the room then it is the job of the air

conditioner to maintain that temperature irrespective of whatever is the outlet temperature or how

many people are inside the house.

So in a way it tries to maintain that temperature by manipulating its operation.

So in the end through all these four examples you can see that every control system has

an associated objective with it.

If you take the shower example, here we want to maintain the temperature and flow rate.

In the case of cooker example, we want to maintain the pressure.

In the case of crossing the road, we want to cross the road without getting hit or cross

the road safely.

Or in the case of air conditioner we want to maintain a constant temperature.

So similarly when we talk about process control then these objectives are related to the process.

So if we have a chemical plant, then the control system will be used to satisfy some of these

objectives as well as some of the constraints.

So let us again try to pause and think about if you are operating a chemical plant what

are the different types of or what are the different objectives which an operator has

to maintain.

So first and foremost as the process or a chemical plant has been setup to make money

then the primary objective of operating a plant is to make profit.

Now if you have to make profit out of a chemical plant you have to make sure that whatever

the product which you are making meets the specification.

So if your product does not meet specification there is no way you can generate profit out

of that plant.

So another operating objective is to make sure that your product meets the desired production

specs.

The other thing which you might want is you are not going to make the product only once.

You purchase an equipment and then you want that equipment last its lifetime that is 10

years or 15 years.

And that can only happen if you take good care of your equipment.

So while making product out of your plant you also want to make sure that you are protecting

the equipment, you are not running it towards its limit so that you can take your plant

or you can use your machinery for a very long time.

While doing all this, you have to make sure that you are not polluting the environment.

Because that is what eventually even you are going to live in, so while achieving profit

as well as production spec you also have to keep one eye towards meeting environmental

regulations.

And lastly you also have to do all this within a safe environment.

You cannot subject your workers or your labourer or also the surrounding people who are living

around your plant, so their safety is also important.

So when you are operating a chemical plant you have to ensure that these are the different

operational objectives or constraints within which you have to operate.

So if you connect the two slides we should have some sort of control strategies which

will take care of all of these.

So before moving forward let us see whether these objectives which we talked about are

all these objectives taken care of in the same order or there is a special hierarchy

in which these different objectives have to be satisfied.

So as it turns out this is the hierarchy in which these different decisions or constraint

are to be satisfied.

So first and foremost the plant or the operator has to ensure the safety of the personnel

who are working inside the plant as well as those who are around the plant.

So the safety of personnel, equipment, environment takes always a prior, the first seat.

Once you ensure that the plant is safe to operate, you try to make sure that the product

which you are getting out is of required grade.

Then you also try to minimize whatever are the, whatever is the burden on the environmental

system.

So you want to ensure the specs while minimizing the effluents which goes to the effluent treatment.

Then you look at elongating the life of your equipment by trying to ensure that all the

equipment operate within the safe limit.

And once all these things are ensured then try to look at improving the profit.

So in a way even though your primary objective of operating a plant is to make money, it

typically comes as the last layer of your operational constraint.

And there will always be a control system associated with all these different layers.

So let us say if you are a plant operator and you have to ensure all these things then

in order to have, in order to ensure whether the plant is safe or whether your product

specifications are met or not, what you need to know is where are you currently standing.

If you want to ensure safety you want to know that how far I am away from the safety.

Or if you want to ensure that the product specs are met or not, you want to know what

are the current product spec and how far you are away from the boundary.

So all that requires an eye into the system and that is done by doing what is known as

process monitoring.

So you have to have install different instruments inside the plant which will give you a measurement

about how or what is the current state of the process.

Whether it is close to any operating constraint or it is away from the constraint.

And once you know where your system is at then and you know what is your target performance

then you can take some action and then move the plant from your current point to the desired

point.

So that becomes the role of process control system.

So as we have multiple constraints on multiple different layers or hierarchy of objectives

even the control system in a chemical plant also has lot of hierarchies.

So here is a typical hierarchy of process control activities in a chemical plant.

So at the bottom layer is your actual process and the first thing is the measurement and

actuation.

So these are all the equipment or these are all the instruments which are put into the

system either to read value from the process or to take some action on the process.

So these are the hardware elements inside the process and they will operate at a very

fast rate, let us say at a second level or at the time interval of few seconds.

And on top of that, the first and primary layer of control is the safety logic.

So this is the fall back or the primary safety which is inbuilt into the system.

So irrespective of whether you have an additional control system or not, this particular safety

logic will always ensure that your system cannot violate any basic safety boundaries.

So for example in the cooker example, in the example of that steam pressure cooker which

I showed you, the pressure cooker also comes with a safety valve.

So if there is some problem in terms of the whistle and the whistle does not open or close,

then it gets blocked.

Then there is always a rupture disc which is on top of the cooker which will open if

the pressure reaches some unsafe limit.

So as soon as that happens irrespective of whether the whistle is working or not, the

burst disc will burst and then the pressure will be released.

So every chemical system will always be associated with some sort of safety logic.

Which will ensure that even though there is no control system or the control system fails

that particular logic will ensure safety of the plant.

Now on top of that if the first level of control which you ensure which is the regulatory control.

So these are the basic control actions which are taken and those will be taken at the frequency

of few seconds.

And then there are other advance control strategies which are on top of that which will subsequently

ensure additional objective.

So first primary regulatory control will try to ensure the product, the basic objectives

of the control system and then as you go above this particular hierarchy then you will move

towards making more profit out of the plant.

So let me explain all these hierarchies for a simple example of CSTR which is going to

carry out a reaction, which is going to generate some gaseous product along with a liquid product.

So let us say this is the reaction which is getting carried out in this CSTR.

So this is how your feed comes in.

There will be a feed valve.

There will be one product valve for B. And there is also some gas getting generated.

So there will be a gas valve here and this is an elevated temperature reaction, exothermic.

So in order to maintain temperature you would have to have some cooling inside the system.

So this is a system for which we will look at what are the different hierarchies of the

control system.

So this is our process.

So the main process I can simply highlight as this which includes reactor and jacket.

Then we will have measurement and actuation.

So here the measurements would be of pressure inside the reactor.

The measurement can also be temperature inside the reactor.

It may be composition of the product which you are getting out.

In terms of actuation you may have different feed flow valves and you may have some product

valves.

So this is how the system can be actuated.

Now the first and foremost is the safety control logic.

So in this typical example, what you would want as a safety precaution is that the pressure

inside the reactor should not blow up if there is unnecessary production of C.

So in that case, what you want to ensure is, if the pressure inside the vessel goes to

a very high value there should be some safe route and that is typically achieved by having

something known as a pressure relief valve.

So you typically have a pressure relief valve as a safety precaution which will ensure that

if the pressure inside the reactor goes to a very high value, then it will open up and

have a safe release of the gaseous product.

Then we move on to the regulatory layer.

So regulatory layer is the basic control system which has to operate or which has to take

decisions of maintaining the operation at a timeframe of few seconds.

So typically for this particular system it will involve controlling the temperature.

So we typically represent it as TIC which refers to temperature indication and control.

So the regulatory layer will ensure that the temperature inside this reactor is maintained

at a particular level.

This is done by manipulating the cooling water flow rate by using this particular actuation.

And the idea here is, if I maintain a particular temperature in this reactor, then we are also

somehow ensuring that if all the other conditions remain the same then even the conversion or

the product purity remains more or less at the same value or the desired value.

Then the next level of control is the supervisory control.

Now even though we are controlling temperature at the regulatory level, our main objective

out of this reactor is to get a product of required purity.

So what we want is a particular composition to be maintained at a desired value.

So your composition control will come at the supervisory level and it will what it will

do is it will try to dictate how the temperature controller loop change operates so as to ensure

a particular value of the composition.

And then we go on to the higher value to let us say if we talk about the real time optimization,

it will try to find out what is the best value at which this particular composition should

be maintained so that I minimize the cooling water or I maximize profit and then lastly

when we talk about the planning and scheduling level, it actually looks at what are the market

conditions, what is the demand, what are the raw material cost.

And accordingly it tries to predict at what particular time, which particular product

or what particular purity has to be maintained, how much amount of product has to be produced.

All that planning type of decisions are taken at the higher level.

So with this simple example we have what we could see is there are different objectives

in a chemical plant and there is no single control system which maintains all these objectives.

There is always the hierarchy of decision making and hierarchy of control systems and

each control system has an associated hardware with it and an objective associated with it.

So we will take a short break and when we come back we will look at what are the different

functions of a chemical or control system.

Thank you.