Baku to the distillation column with high pressure. When

Baku Higher Oil School

 

Course: Supervisory Control and Data
Acquisition systems

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Course Project

 

 

 

 

Project name:    Controlling
level of condensate in reflux drum of distillation column

 

 

Student
Name:   Elshan Mikayilov

 

Report submitted on:   
18.01.2018

 

Supervisor: Manafaddin Namazov

 

 

 

 

 

 

 

 

 

 

Contents
Introduction. 1
Background. 2
Objectives. 3
Issues. 4
Piping and
Instrumentation Diagram.. 6
PLC system.. 7
SCADA system.. 7
HMI 7
SCADA components. 7
Conclusion. 7
                                                               

 

 

 

 

 

 

 

 

 

 

 

 

 

Introduction

In
this report I will explain how I designed SCADA system for the level control in
the reflux drum of the crude oil distillation unit. The main purpose is to
control the level of the light oil components in the reflux drum and keeping
the level at the desired value in order to reach the optimal separation and
reflux. Through this report I will firstly give problem statement and include
some details about the distillation process. Consequently, I will show some
possible solutions for the problem in question. Therefore I will show my
solution and how I build the PLC system and made HMI for this system in order
to effectively monitor, gather data about the process and control the
parameters. The whole system is built in the TIA Portal by choosing proper
controller and PC system for the process.

Background

In
this section I will give some brief information about the process itself since
it is crucial to understand the process before designing any process control
system. So, I need firstly determine the inputs and outputs of the system while
considering the conditions and properties that are required to reach the
desired output.

Generally,
distillation is a simple process to separate two or more substances due to the
difference in their boiling points. The same process happens in the crude oil
distillation unit. The crude oil distillation unit (CDU) is the first
processing unit in virtually all petroleum refineries. The CDU distills the
incoming crude oil into various fractions of different boiling ranges, each of
which are then processed further in the other refinery processing units. The
CDU is often referred to as the atmospheric distillation unit because
it operates at slightly above atmospheric pressure.

First
of all, crude oil stream (feed) is heated and pumped to the distillation column
with high pressure. When the crude oil enters the large volume of the
distillation column, the pressure suddenly falls and it makes the components of
the crude oil to separate. This separation happens because of the difference in
the boiling points of the components of crude. The heavy components such as
heavy fuel oils, wax, lubricating oils, asphalt are going to the bottom of the
distillation column while light distillates such as Liquid petroleum gas (LPG),
Gasoline (petrol), Heavy Naphtha are going to the top of the column where they
are condensed. There are also some middle distillates which can be kerosene, automotive
and rail-road diesel fuels, residential heating fuel and other light fuel oils
which leaves the column in various stages. There are sets of trays and packers
mounted inside the column in order to increase the contact between liquid and
vapor to make the separation better. Crude oil distillation column is a
continuous process and the bottom products and top products are not achieved in
one cycle. It means that bottom products are reboiled in the reboiler and sent
back to column while top products are condensed in the condenser and sent back
to the column through reflux drum. Some part of heavy components are taken out
of bottom which are bottom products and remaining are reboiled and sent back to
the column. In the same manner some amount of light components are taken out of
the reflux drum which are called top products and the remaining amount are
refluxed to the column.

My
task is to design control system for reflux drum in order to keep the level of
this tank desired to make the separation better. In this case, I will used PID
control which will efficiently keep the PV closest to the set point.
A proportional–integral–derivative controller (PID controller or three
term controller) is a control
loop feedback mechanism widely used in industrial control systems and a variety of other
applications requiring continuously modulated control. A PID controller
continuously calculates an error value e(t) as the difference between
a desired set point (SP) and a
measured process
variable (PV) and applies a correction based on proportional, integral, and derivative terms which give the
controller its name. The overall control function of PID can be expressed
mathematically as:

Objectives

Before
starting the design process we should consider the objectives that should be
met. These objectives may not lead to the optimal practical integrated
solution, it is just one part of design process and it is not possible to
design the whole control system for distillation process in one month or even
less. So, the current
objectives are:

 

·       
Start and stop
the process by using buttons in the operator room

·       
Pump the feed
through the heater

·       
Heat the feed

·       
Send the feed to
the distillation column

·       
Keep the level
in the reflux drum at the desired value

·       
Use PID for
smooth control

·       
Send some
excessive amount of reflux to the column by controlling valve

To
conclude up, my objective is to design the control system to automatically keep
the level of the light components in the reflux drum at the desired value while
pumping more feed into the column.

 

Issues

While
designing a control system there are some vital problems that should be
considered before design process. The first problem is related to the
instrumentation of the system. In my task distillation process happens under
high pressure and temperature conditions which means that proper transmitters,
final control elements and other equipment should be chosen with care to
correspond to explosion zone requirements (Ex0, Ex 1, Ex2) and SIL levels.  Safety integrity
level (SIL) is defined as a relative level of risk-reduction provided by
a safety function, or to specify a
target level of risk reduction.
In simple terms, SIL is a measurement of performance required for a safety instrumented function (SIF). The
requirements for a given SIL are not consistent among all of the functional
safety standards. In the functional safety standards based on the IEC 61508 standard, four SILs are defined, with
SIL 4 the most dependable and SIL 1 the least. A SIL is determined based on a
number of quantitative factors in combination with qualitative factors such as
development process and safety life cycle management.

Hazardous areas
are classified into zones based on an assessment of the frequency of the
occurrence and duration of an explosive gas atmosphere, as follows:

·        
Zone 0: An area in which an explosive gas atmosphere
is present continuously or for long periods;

 

 

·        
Zone 1: An area in which an explosive gas atmosphere
is likely to occur in normal operation;

·        
Zone 2: An area in which an explosive gas atmosphere
is not likely to occur in normal operation and, if it occurs, will only exist
for a short time.

 

Another main problem that
should be considered is selection of control strategy that will safely,
efficiently and fully implement the desires of the customer. There are many
types of control systems that have application in various fields. If your process
is simple, not hazardous and high quality is not required it means that you can
choose basic control algorithm which will be easy to monitor and configure.
Generally, there are two types of control: open loop control and closed loop
control. In open loop control, the controller take actions without having a
sense of output. In closed loop control there is feedback coming from the
output and the controller take proper actions according the error between
process variable and the set point. Of course, closed loop control is more
sophisticated and reliable. So, I will implement close loop control in my
control system.

Selection of proper control
algorithm is also essential which can include on-off control, batch control,
linear control, relay control, proportional control, PI control, PD control,
PID control and so on. For example, on-off control is easy to design and
implement, but there will significant oscillation and overshoot from the set
point.

 

In my example, I will used
PID control which will efficiently keep the PV closest to the set point.
A proportional–integral–derivative controller (PID
controller or three term controller) is a control
loop feedback mechanism widely used in industrial control systems and a variety of other
applications requiring continuously modulated control. A PID controller
continuously calculates an error value e(t) as the difference between
a desired set point (SP) and a
measured process
variable (PV) and applies a correction based on proportional, integral, and derivative terms which give the
controller its name.

 

Piping and
Instrumentation Diagram

A piping
and instrumentation diagram (P&ID) is a detailed diagram in the process industry which shows the piping and vessels in the process flow, together with the instrumentation and control devices. A piping and
instrumentation diagram (P&ID) is defined by the Institute of
Instrumentation and Control as follows:

·       
A diagram which shows the interconnection of
process equipment and the instrumentation used to control the process. In the
process industry, a standard set of symbols is used to prepare drawings of
processes. The instrument symbols used in these drawings are generally based
on International Society of
Automation (ISA) Standard S5.1

·       
The primary schematic drawing used for laying out
a process
control installation.

Below, the P&ID of my task is
shown:

 

Here in this picture you can see
feed in which is pumped by a pump to the distillation tower. After the feed
enters the distillation column light and heavy components will be separated due
to their different boiling points. Heavy components will go bottom and reboiler
will re-boil them in order to feedback some part of this liquid while some
amount will be taken out as bottom product. We can see that flow rate of bottom
product is controlled with LC which will take the measurement from LT level
transmitter. In the re-boiler the heating is supplied by steam.

On the top of the distillation
column light components are taken out. First of all, heated vapor goes through
condenser to condense down and convert to liquid phase. Therefore, this liquid
is directed to a horizontally located tank called reflux drum. In reflux drum
some amount of light components are taken out as top distillate products and
some amount again is sent back to distillation column to separate. Level of the
reflux drum is controlled by LC level controller in order to make the
separation quality better. Another pump will pump the liquid and LV level valve
will control the flow rate according to the set point of the level in the tank.
Overhead gases are also taken from reflux drum when the pressure inside the
tank is so high. Relief valve can be used for this purpose.

SCADA system

Supervisory control and
data acquisition (SCADA) is a control system architecture that uses computers, networked data
communications and graphical user interfaces for
high-level process supervisory management, but uses other peripheral devices
such as programmable logic controllers and discrete PID controllers to interface to the process plant or machinery. The
operator interfaces which enable monitoring and the issuing of process
commands, such as controller set point changes, are handled through the SCADA
supervisory computer system. However, the real-time control logic or controller
calculations are performed by networked modules which connect to the field
sensors and actuators.

In my case, I need to build a simple
SCADA system to control distillation process and monitor the level of the
reflux drum. SCADA systems have some crucial components and they are discussed
below.

SCADA
components

My SCADA system is hierarchal and
there are process control level and supervisory level. In my project, the
process control level consists of one PLC, two level sensors, two valves with
actuators, pumps, heaters, condensers and so on. These final control elements
and transmitters are connected to PLC and then they get commands from PLC
according to the written program or commands of the operator from the operator
room. The supervisory level consists of PC system which has centralized
computer getting data from PLCs and monitors to show human machine interface,
input devices (such as mouse, keyboards) to enter some command to the SCADA
system.

The general device configuration of
the system is shown below:

Here we can see that PLC is selected
as CPU 315-2-PN/DP which has I/O module inside. This PLC is connected to our PC
system which is SIMATIC PC Station through PN/IE line. The HMI is programmed
with WinCC RT Advanced and IE general is used as communication module.

PLC system

After device selection and
configuration is finished I need to program the PLC in order to make it serve for
our purpose. LAD language will be used to program this CPU. This PLC will have
some input from push buttons (Start and Stop), measurements from sensor coming
to Input modules and some output commands to final control elements. All these
data variables will be handled through PLC tags and proper naming is important
in order to make easy to use and modify. The main OB block of my PLC is shown
below, it contains 7 networks:

 

 

 

 

Network 1 is built to implement
basic start and stop function. Start (M0.0) contact is normally open contact
which will start energizing the Coil (Q0.0) when pressed. Stop (M0.1) is
normally closed contact which will be open when it is pressed and the whole
system will stop running. Auxiliary contact Q0.0 is used here also to keep the
system running even if the Start is de-energized.

 

In network 2, after start is pressed
and coil is energized Pump_feed (Q0.1) will start running and the heater (Q0.2)
will also start heating the feed. At the same time re-boiler and pump at the
bottom of the column will start running.

In network 3, after the start, the
scaling will be done. The feedback coming from the level sensor will be
simulated by a slider in the HMI screen. I considered this input as a voltage
level between 0 V and 10 V. Of course, this voltage level doesn’t show the real
level of the tank. It means we need to scale this input in order to make it
suitable for processing. I have done scaling by using math operators. My sensor
sends signal from 0-10 V and level range of the tank is between 0-150 cm. From
this relation real value of level can be calculated with this equation: real
level=(sensor_reading*150)/10. This equation will make the MD10(level) variable
between 0 and 150 cm. In network 3, you can see that firstly, MUL operator is
used to multiply sensor reading to 150 and DIV operator is used to divide this
value to 10 to get the actual level. However, math operation can be done for
only double integer variables. So, my MD10 is Dint variable. However, PID works
with only real variables. So, I have used CONV block in network 4 to convert
from integer to real. So, MD20 is real actual level in the tank.

In network 5, you can see implementation
of PID algorithm. This is continuous controller and there are many pins. First
of all, manual on pin should be set to false in order to run automatically. So,
I connect M0.5 boolean variable to MAN_ON to make it false. MD20 is the PV (process
variable) which is connected to PV_IN pin and set point is set as 70 cm. Output
of this PID is taken from LMN tag which is connected to MD100 variable.

In network 6, SUB block is used to
make the output of PID suitable for our application. As I explained earlier
there is inverse relationship between level of the tank and the flow rate of
drainage valve. This PID gives manipulated variable for direct control. So, I
need to subtract this MV from 100 in order to make it suitable for our
application. Output of PID (MD100) is subtracted from 100 and MD110 is real
valve opening percentage which will be sent to actuator of valve.

In network 7, comparator is used to
simulate the drainage valve. If the valve opening percentage is greater than 0
it means that some proportion of the valve is open and we will turn the valve to
green in the HMI. If the percentage is 0 the valve will stay red showing that it
is not running.

Human Machine Interface

HMI is a part of SCADA systems and
it is physically screens of PC systems which are available to operators. Operators
can monitor the process via HMI and also they can give control commands to
PLCs. I designed graphical interface in a way that it is easy to understand and
monitor the main points.  The overall design
of the screen before running is like this:

 

 

 

 

 

HMI screen after running:

HMI screen after pressing Start
button:

 

Increasing the level with slider: PV
is below the SP (70 cm)

Tank level above set point: PID is
changing the valve position (59 %)

 

 

Higher tank level:

 

Conclusion