Journal of Applied and Industrial Sciences, 2014, 2 (1): 35-45, ISSN: 2328-4595 (PRINT), ISSN: 2328-4609 (ONLINE) Research Article 35 Abstract-Two control strategies were undertaken for the system water-Acetic acid- normal hexane in a rotating disc contactor (R.D.C).The first cascade control system consists of a raffinate composition variable transmitted to the primary controller that transmits it’s output as the set point to the level controller of the secondary loop. The second strategy takes the extract composition transmitted to the primary controller which transmits its output to the secondary level controller. The transfer functions for both primary and secondary loops were identified, the overall transfer functions were calculated for both primary and secondary loops and consequently tuned by Routh, root locus and Bode methods. The adjustable parameters for primary and secondary loops K c’s , τ i’s , τ d’s were determined and optimized by Ziegler-Nichols method. The offsets were determined and it was found that they were reasonable for the secondary loop, but considerably high for the primary loop. Therefore the gain Kc 1 of the primary loop was refined by trial and error to 3.5. This increase in the primary gain reduced the offset to (- 0.239) which is a reasonable value and the primary parameters were adjusted accordingly. The response of each method of tuning was plotted and the characteristics of the close loop responses were compared and found to be in agreement as shown in figures 7,8,9,10,11,12,13,14,15,16,17,18. Index Terms-Cascade control, Tuning, Offset, Responses investigation. I. INTRODUCTION iquid-liquid extraction is a process that separates components based upon solubility in solvents. The liquid-liquid extraction is termed as solvent extraction, extraction, or liquid extraction. The basic principle behind extraction involves the contacting of a solution with another solvent which is immiscible with the original; the solvent is also soluble with specific solute contained in the solution. Two phases are formed after the addition of the solvent, due to the differences in densities. The solvent is chosen, so that the solute in solution has more affinity toward the added solvent, therefore mass transfer of the solute from the solution to the solvent occurs, further separation of the extracted solute and the solvent will be necessary. [1] The control of these units can often be problematic, partially due to their multiphase nature, and partially due to the difficulty of on-line measurements of output variables. Thus, the reliable simulation of the transient behaviour of these columns is extremely valuable. The column hydrodynamics including flooding, entrainment, weeping, and phase inversion will be controlled. [1] Routh Criterion The Routh criterion determines the number of the roots of the characteristic equation that lie on the left-half plane, the right -half plane or on the imaginary axis if the system is stable, unstable or critically stable respectively. The Root Locus Method This method gives an approximate graphical representation of the root as the gain is varied. This is very useful in the design of a system since it gives the position of the poles of the system in the S-plane for all values of the gain [ 2 and 3]. Bode plot Bode plot is the graph on semi log paper of the amplitude ratio and phase angle as the frequency is varied from zero to infinity. From the plot the ultimate gain and period can be determined and used for tuning[2-5]. Control systems with multiple loops The feedback control configuration involves one measurement (output) and one manipulated variable in a single loop. There are other simple control configurations which may be used. If there are more than one measurement and one manipulated variable or one measurement and more than one manipulated variables in such case the control systems with multiple loops may arise, typical example is the cascade control. [6 and 7] Cascade control In a cascade control configuration one manipulated variable and more than one measurement exist. Industrial Controllers These are a combination of two or three modes together. They are usually in actions of P, PI and PID. [1, 2] Tuning of a Cascade Control System in a Rotating Disk Contactor (RDC) H.A. Khalil *1 , G .A. Gasmelseed 2 , A.E Elhassan 3 (1, 2, 3) Department of Chemical Engineering, Faculty of Engineering, University of Science and Technology Email:[email protected]Telephone: +249919634134 Email:[email protected]Telephone: +249999773300 (Received: December 10, 2013; Accepted: February 04, 2014) L *Corresponding author Email:[email protected]Telephone: +249123896466
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Journal of Applied and Industrial Sciences, 2014, 2 (1): 35-45, ISSN: 2328-4595 (PRINT), ISSN: 2328-4609 (ONLINE)
Research Article 35
Abstract-Two control strategies were undertaken for the system
water-Acetic acid- normal hexane in a rotating disc contactor
(R.D.C).The first cascade control system consists of a raffinate
composition variable transmitted to the primary controller that
transmits it’s output as the set point to the level controller of the
secondary loop. The second strategy takes the extract composition
transmitted to the primary controller which transmits its output
to the secondary level controller. The transfer functions for both
primary and secondary loops were identified, the overall transfer
functions were calculated for both primary and secondary loops
and consequently tuned by Routh, root locus and Bode methods.
The adjustable parameters for primary and secondary loops Kc’s ,
τi’s, τd’s were determined and optimized by Ziegler-Nichols
method. The offsets were determined and it was found that they
were reasonable for the secondary loop, but considerably high for
the primary loop. Therefore the gain Kc1 of the primary loop was
refined by trial and error to 3.5. This increase in the primary gain
reduced the offset to (- 0.239) which is a reasonable value and the
primary parameters were adjusted accordingly. The response of
each method of tuning was plotted and the characteristics of the
close loop responses were compared and found to be in agreement
as shown in figures 7,8,9,10,11,12,13,14,15,16,17,18.
Index Terms-Cascade control, Tuning, Offset, Responses
investigation.
I. INTRODUCTION
iquid-liquid extraction is a process that separates
components based upon solubility in solvents. The
liquid-liquid extraction is termed as solvent extraction,
extraction, or liquid extraction. The basic principle behind
extraction involves the contacting of a solution with another
solvent which is immiscible with the original; the solvent is
also soluble with specific solute contained in the solution. Two
phases are formed after the addition of the solvent, due to the
differences in densities. The solvent is chosen, so that the solute
in solution has more affinity toward the added solvent,
therefore mass transfer of the solute from the solution to the
solvent occurs, further separation of the extracted solute and the
solvent will be necessary. [1] The control of these units can
often be problematic, partially due to their multiphase nature,
and partially due to the difficulty of on-line measurements of
output variables. Thus, the reliable simulation of the transient
behaviour of these columns is extremely valuable. The column
hydrodynamics including flooding, entrainment, weeping, and
phase inversion will be controlled. [1]
Routh Criterion
The Routh criterion determines the number of the roots of the
characteristic equation that lie on the left-half plane, the right
-half plane or on the imaginary axis if the system is stable,
unstable or critically stable respectively.
The Root Locus Method
This method gives an approximate graphical representation of
the root as the gain is varied. This is very useful in the design of
a system since it gives the position of the poles of the system in
the S-plane for all values of the gain [ 2 and 3].
Bode plot
Bode plot is the graph on semi log paper of the amplitude ratio
and phase angle as the frequency is varied from zero to infinity.
From the plot the ultimate gain and period can be determined
and used for tuning[2-5].
Control systems with multiple loops
The feedback control configuration involves one measurement
(output) and one manipulated variable in a single loop. There
are other simple control configurations which may be used. If
there are more than one measurement and one manipulated
variable or one measurement and more than one manipulated
variables in such case the control systems with multiple loops
may arise, typical example is the cascade control. [6 and 7]
Cascade control
In a cascade control configuration one manipulated variable
and more than one measurement exist.
Industrial Controllers
These are a combination of two or three modes together. They
are usually in actions of P, PI and PID. [1, 2]
Tuning of a Cascade Control System in a Rotating Disk
Contactor (RDC) H.A. Khalil
*1, G .A. Gasmelseed
2, A.E Elhassan
3
(1, 2, 3)Department of Chemical Engineering, Faculty of Engineering, University of Science and Technology