Development of improved pid controller for single effect evaporator

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 05 Issue: 01 | Jan-2016, Available @ http://www.ijret.org 272
DEVELOPMENT OF IMPROVED PID CONTROLLER FOR SINGLE-
EFFECT EVAPORATOR
Aminu Tijjani1
, H. K. Verma2
, Chhaya Sharma3
1
M.Tech Student, Department of Electrical and Electronics Engineering School of Engineering and Technology,
Sharda University, Greater Noida, Uttar Pradesh, India,
mailtoalameen87@gmail.com
2
Distinguished Professor, Department of Electrical and Electronics Engineering School of Engineering and
Technology, Sharda University, Greater Noida, Uttar Pradesh, India,
hk.verma@sharda.ac.in
3
Associate Professor, Department of Paper Technology Campus Indian Institute of Technology, Roorkee Saharanpur
Campus, India,
chhaya1964@rediffmail.com
Abstract
Kraft pulping is the pulping process that is mostly used in paper mills. This is because the chemicals used in cooking are being
recovered, and environmental pollution is minimized, as the chemicals are not discarded into the environment, but rather re-
cycled. These chemicals are recovered by burning the black liquor that leaves the digester in a reactor called a recovery boiler.
Feeding the black liquor directly from the digester to the recovery boiler will cause a water/smelt explosion. This explosion
happens if the percentage of water in the liquor is high, the solid content of the black liquor leaving the digester is usually
between 13-18%. To avoid such disaster the concentration of the black liquor has to be increased, which should in the range of
60-70%, this is done in a single or multiple effect evaporator. In this paper, a single effect evaporator control has been designed
using a Proportional Integral Derivative (PID) controller. During the first design a simple PID controller with a feedback control
was used. Then taking into consideration the load disturbance in the process, a feed forward control has then been introduced
into the controller design. Subsequently the concept of boiling point rise (BPR) to measure concentration has then been used in
the controller design of a single effect evaporator.
Keywords: Boiling Point Rise, Concentration, Evaporators, Feed Forward Control, Paper Mill, PID Controller,
Pulping Process, Single-Effect Evaporator
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1. INTRODUCTION
The paper making process as a whole consist of various sub
sections, wood is a raw material used in paper making.
Trees from the field are cut down into wood logs, then
transporting the wood logs to the paper mill industry. The
wood logs undergo what is known as wood preparation,
upon reaching the paper mill. This wood preparation
includes de-barking, wood chipping and then chip selection.
The selected chip with appropriate size is then sent to a
digester for cooking. The cooking is done under a
predetermined temperature and pressure for a certain period
of time. The cooked pulp then undergoes washing, which
the pulp is then separated from the black liquor. The black
liquor undergoes chemical recovery in order to recover the
used chemicals [4]. The washed pulp then goes to a
bleaching if required. The bleached pulp is then conveyed to
the paper making machine. It is at the paper making
machine were the pulp is transformed into a giant paper
web, and water is squeezed out of it, leaving behind a
delicate paper which need to be dried, and later cut into
required sizes.
The black liquor enters the chemical recovery cycle at the
evaporation section. This is crucial as the black liquor
leaving the digester has low solid content usually 13-18%,
and for the black liquor to be fired into the recovery boiler it
must have at least 58% of solid content [5], otherwise a
water/smelt explosion may occur. For that purpose the
concentration of black liquor has to be increased, and this is
done by evaporation process. The evaporation process is
done in vessels called evaporators.
Evaporators are heat exchangers. This heat exchangers are
bodies in which the evaporation takes place. This
evaporators are also called as effects. The principle
operation of the evaporators is to increase the concentration
of the black liquor in it. Superheated steam or hot air is fed
to the evaporator effect, in which contains the black liquor.
As the steam enters into the wall tubing, it exchanges it is
heat energy with the liquor inside the evaporator. This rises
the temperature of the black liquor to a point whereby it
starts to boil [6]. Upon boiling, the water present in the
liquor is converted to a gaseous form and leave the salt [2].
The steam that gives away it is energy to the liquor changes
to liquid state, and is collected as condensate at one end. In

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pulp and paper mills, there are different types of evaporators
used, for the purpose of increasing it is concentration, some
of them are. Rising film, falling film, direct contact and
forced circulation [4].
This work focuses on the design of a Proportional Integral
Derivative (PID) Controller for single effect evaporator, for
the purpose of increasing the solid concentration of black
liquor leaving the digester, before it is fed to the recovery
boiler for combustion.
2. CONTROLLER FOR SINGLE EFFECT
EVAPORATOR
2.1 Control Requirements
To achieve the target concentration in the end product, the
rate at which the steam is fed to the heat exchanger has to be
controlled. The rate at which the steam is to be supplied will
be determined based on the solid content concentration of
the black liquor, and to what percentage the concentration is
required to be increased to. For operation and control of the
single effect evaporator, a PID controller is used to control
the degree of opening or closing of a motorized control
valve.
2.2 Simple PID Controller
For a simple PID controller in a single effect evaporator
process, the controller can either be a Micro-process based
[1] or PLC based. The desired set point of the concentration
is inputted into the PID which will be in digital form. A
concentration sensor is placed inside the outlet of the
evaporator so as to measure the concentration of liquor
leaving the evaporator. The sensor measures the
concentration and feds the analog reading to and ADC, the
digital data now is fed to the PID controller. The PID
controller generates an error signal based on the difference
between the measured concentration and set point [3], and
feds it to a DAC. The digital signal is fed to the motorized
control valve, which determines the degree of opening or
closing of the valve based on the error signal fed to it. This
in turns control the amount of steam to be fed to the process.
Fig 1 is a block diagram that illustrates the control strategy
employed here and fig 2 shows the simple PID controller for
a single-effect evaporator.
Fig -1 Block diagram of control strategy of single effect
evaporator
(Csp: Concentration set point, Cm: Measured concentration,
Ca: Actual concentration).
Fig -2: Block diagram of PID controller for a single effect
evaporator
3. PID CONTROLLER WITH FEED FORWARD
CONTROL FOR SINGLE-EFFECT
EVAPORATOR
3.1 Necessity of Feed Forward Control
In the previous control, the controller regulates the amount
of steam flow into the evaporator based on the concentration
measured at the output by the sensor. The rate at which the
process respond is slow, as the rate of steam flow is only
altered if changes has been measured at the output, this does
not take care of disturbance that may occur on the process.
In evaporators there are two major disturbance that can
affect the process, this are, the rate at which the weak liquor
is fed to the process, and the change in concentration of
black liquor entering the process. If the rate at which the
weak liquor is fed to the evaporator is increased, then the
rate of which steam is fed to the process has to be increased,
and vice versa. If the concentration of solid content in the
weak liquor is decreased then more steam will be required to
evaporate. If the concentration of the weak liquor is
increased then less amount of steam will be required inside
the process in order to concentrate it to its required
concentration.
To overcome the effect of this disturbance on the process a
feed forward control is used. This feed forward controller
sends a correcting signal before the disturbance affects the
process [3].
3.2 PID Controller with Feed Forward Control
In this PID controller configuration a feed forward control is
introduced. In the previous control configuration the control
signal sent to the motorized control valve is from an error
generated after measuring the output and comparing with
the preferred set point. This will take a certain time delay
before correcting if there is a disturbance on the process. To
overcome this delay a feed forward controller is used. The
feed forward controller is used to overcome and neutralize
the effect of disturbance on the system, before it even effects
the overall system. Fig 3 shows PID controller scheme.

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Fig-3 Block diagram PID controller with feed forward
control
4. FURTHER IMPROVEMENT IN PID
CONTROLLER
4.1 Boiling Point Rise (BPR) as Measure of
Concentration
Boiling point rise is the difference between the temperature
of a boiling solution and the temperature of boiling water
(pure) at the same pressure. A dhuring chart can be used to
determine the BPR. The BPR is usually expressed in degree
Fahrenheit (o
F) but can also be in degree Celsius (o
C) [8].
Dhuring chart states that, there exist a linear relationship
between the temperature of boiling water and that of a
solution at the same pressure [9]. Since the boiling
temperature of the solution can be measured and the
temperature of steam i.e. the temperature of boiling water.
At the same time measuring the pressure inside the vessel.
Values of these three parameters can be used to compute the
concentration using Eqn 1 or 2.
BPR= (a1+b1Pr)[x/(1-x)] for x<0.65 . . . . . . . . . . . (1)
BPR= [(a2+b2Pr) + (a3+b3Pr)][x/1-x] for x>=0.65. (2)
Where a1, a2, a3, b1, b2 and b3 are experimentally
determined constants and Pr is the pressure in the
evaporator. The concentration is first computed for the first
equation that is x<0.56, if x is found to be grater then 0.65
then the second equation will be used. Therefore if the
temperature of the boiling solution can be measured as well
as the temperature of boiling water and pressure at which
this takes place, then the concentration of that solution can
be calculated. This can be seen in fig 4
Fig -4 Schematic for measuring concentration through BPR
(T1: Temperature sensor that measure the temperature of
steam, T2: Temperature sensor that measures the
temperature of boiling solution, Pr: Pressure sensor that
measure the pressure inside the evaporator).
4.1 PID Controller of a Single Effect Evaporator
Using BPR
In section 3.2 a concentration sensor has been used to
measure the solid content concentration of the liquor. This
concentration sensors are not that efficient in reading the
exact concentration of the liquor, as there are not designed
initially for a particular salt. A Refractometer is used for
measuring concentration, measurement of concentration
using a refractometer is not accurate for higher solid
concentration, but rather works on prediction. Also the cost
of purchasing such sensors is very costly, such as the
specific gravity sensor. In order to overcome such problems
of concentration measurement sensors.
The sensors are replaced with something that will eliminate
those disadvantages. To achieve that, the concept of BPR
fully known as Boiling Point Rise is used. In evaporators, to
measure the boiling temperature of the solution, a
temperature sensor is placed just below the surface of the
boiling solution, another temperature sensor is to be placed
in the vapor region inside the evaporator so as to measure
the boiling point of the steam. A pressure sensor is then
placed in the head of the evaporator so as to measure
accurately the pressure at which the boiling takes place. The
measured parameters are sent to a multiplexer and then to an
ADC so as to convert the analog signal to digital form,
which will be suitable for the computer (controller) to
interpret and manipulate. The ADC output if fed to the
controller that calculates the concentration based on either
equation 1 or 2. Then sends the reading of measured
concentration to the feedback controller. The feedback
controller will generate an error signal based on the
difference between the set point and the measured
concentration. This signal will be sent to DAC, which
converts the digital signal back to an analog signal. The
analog signal is fed to a motorized control valve, this valve
controls the amount of steam to be fed to the evaporator. Fig
5 shows a PID controller block diagram using the concept of
BPR.
Fig-5 Control scheme of single effect evaporator using BPR

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4.3 Improved PID Controller of Single Effect
Evaporator using BPR with Feed Forward
Using the concept of BPR to determine the concentration of
liquor, as well as using a feed forward control to overcome
any effect due to disturbance will be the fore more best
control design for a single effect evaporator. This can be
seen in fig 6.
Fig-6 PID controller using BPR with feed forward control
5 CONCLUSION
In this paper, single effect evaporation process was
discussed with four different control configurations, in order
to achieve the end target of attaining the value of
concentration accurately.
A simple PID controller was first designed with a feedback
control, then taking into consideration of disturbances on the
process, a feed forward control was then employed to
overcome that effect. Later using a concept of BPR to
determine the concentration was used, which is more
accurate than using conventional concentration measuring
sensors and less cost of their purchase. Combining all this
together gives an improved PID controller for single effect
evaporator.
A single effect evaporator is not commonly used anymore
these days since it is not economical, as the output vapor is
not re-used in the process.
For that purpose a multi-effect evaporation process has
replaced it these days. Authors are working on the
development of a PID controller for multi-effect
evaporators.
For that purpose a multi-effect evaporation process has
replaced it these days. Authors are working on the
development of a PID controller for multi-effect
evaporators.
REFERENCES
[1]. Frank Joachim and Mayer ZU. Heringdort, “Flexible
Microprocessor Based Evaporation Controller”, Article,
Journal of Scientific Engineering and Technology, 2012
[2]. Mr Prashant Balpande, G.S. Zamre and A.P Gawande,
“Thermal Integration in Multiple Effect Evaporators” Paper,
International Journal for Engineering Application and
technology, 2013
[3]. Dhara J.Shah and C.G. Bhagchandi, “Design, Modelling
and Simulation of Multiple Effect Evaporators”, Paper,
International Journal of scientific Engineering and
Technology, 2012
[4]. Kaj Henricson, "Chemical Recovery Cycle", Pulping
technology, 2005
[5]. BLRB Advisory Committee "Recommended Good
Practice Safe Firing of Black Liquor in Black Liquor
Recovery Boiler", 2012
[6]. Zhu, M., Xie, H., Zhang, B. and Guan, X, "The
Characteristics of the Evaporator/Evaporator for Direct
Expansion Solar Assisted Heat Pump System", Journal of
Power and Energy Engineering, 2013
[7]. Guillermo H. Crapiste and J.E Lozano, “Effect of
concentration and Pressure on Boiling Point Rise of Apple
Juice and Related Sugar Solutions”, Article, Journal of Food
Science, 2006
[8]. Marta Bialik , Peter Sedin , and Hans Theliander "
Boiling Point Rise Calculations in Sodium Salt Solutions",
Article, Industrial and Engineering Chemistry Research,
2008
[9]. Wikipedia, “Dühring's rule”, at
https://en.wikipedia.org/wiki/D%C3%BChring%27s_rule,
accessed on 20/10/2015
BIOGRAPHIES
Aminu Tijjani graduated in Electrical
Engineering in 2012 from Kano University
of Science and Technology wudil, Kano
state Nigeria in 2012, Then offering his
Master of Technology in Sharda University
in Electrical and Electronics with
specialization in Instrumentation and
Control.
H. K. Verma graduated in Electrical
Engineering in 1967 from the University of
Jodhpur and obtained Master of Engineering
and Ph.D. degrees in 1969 and 1977,
respectively, from the erstwhile University
of Roorkee (now Indian Institute of
Technology Roorkee). Prof. Verma has published over 200
research papers, and guided 15 Ph.D. theses and 130
M.E./M.Tech. dissertations. He was honoured twice, first in
2004 and again in 2009, by the Indian Institute of
Technology Roorkee as a Distinguished Teacher.
Chhaya Sharma (chayafpt@iitr.ernet.in)
received Ph.D. (Pulp and Paper
Engineering) in 1994 from IIT Roorkee
(erstwhile University of Roorkee). She
started her career as Research Fellow (JRF,
SRF and RA) 1988 then joined as faculty in
July 2000 at IIT-Roorkee and continuing
till date. She has published more than 45 research papers

Development of improved pid controller for single effect evaporator

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