Advanced Control of the Waste Water Treatment Unit in White Nile Tannery

Transcription

1 International Journal of Engineering, Applied and Management Sciences Paradigms, Vol. 43, Issue Publishing Month: January 27 Advanced Control of the Waste Water Treatment Unit in White Nile Tannery Tahani M. Elnorani, Gurashi A. Gasmelseed 2 and Ibrahim H. Elamin 3 Department of Chemical Engineering, University of Science and Technology, Khartoum (Sudan) Tahani.mamoun@gmail.com 2 Departments of Chemical Engineering, University of Science and Technology, Khartoum (Sudan) gurashigar@hotmail.com 3 Departments of Chemical Engineering, University of Science and Technology, Khartoum (Sudan) Ibrahimelamin@hotmail.com Publishing Date: January, 27 Abstract The aim of this work was the study of tannery wastewater in White Nile tannery. Treatment of tannery waste water is a combination of three methods, biological in an aeration tank, chemical in a mixing tank and physical in a sedimentation tank. A design of tannery equipment was design and controlled. Feedback connection control strategy was developed to control the rate of air in aeration tank, and concentration of chemical in quick mixing tank and control of level in sedimentation tank. The block diagrams of the systems were constructed and the process transfer functions were identified. Then the overall transfer functions, the open and closedloops, and the characteristic equations were determined, and the control systems were tuned to obtain the adjustable parameters using RouthHurwitz, Direct Substitution, Root locus, and Bode methods. The adjustable parameters were inserted into the characteristic equation for the offset investigation, stability analysis and response simulation. It is found that using PID controller for the feedback loop provides the highest gain and lowest overshoots than P and PI controllers. Keywords: Water Treatment Unit, White Nile Tannery, Waste Water. I. Introduction White Nile tannery started production in 975 as public sector tannery, privatized in 992 and was then rehabilitated. Its daily capacity is 6 pieces light skins (sheep or goat) and pieces of heavy hides (cow) []. Basic Steps of Treatment:. Screening: Bar screening, removal of larger solids and Fine screening should drastically reduce the amount of fine suspended solids [2]. 2. Biological oxidation tank: This treatment is called the activated sludge process. This is because air and seed sludge from the plant treatment process are added to the wastewater to break it down further. Air pumped into large aeration tanks mixes the wastewater and sludge that stimulates the growth of oxygenusing bacteria and other tiny organisms that are naturally present in the sewage. [3]. 3. Chemical treatment, mixing tank: Chemicals are added in order to improve and accelerate the settling of suspended solids, especially of fine and colloidal matter in wastewater treatment operations, the processes of coagulation and flocculation are employed to separate suspended solids from water. White Nile tannery effluent treatments are: Alum: industrial aluminum sulphide, and poly aluminum chloride [2] 4. Settling, sedimentation tank: The purpose of sedimentation is to separate the sewage into two main components, sludge and settled sewage, which by being treated separately are normally dealt with more efficiently and economically. Generally up to5 5

2 International Journal of Engineering, Applied and Management Sciences Paradigms, Vol. 43, Issue Publishing Month: January 27 per cent of the total polluting load in the sewage is removed by sedimentation. 5. Sludge drying beds: Sludge drying bed is used as the last step of dewatering system. Sludge of the system is dewatered on these beds by evaporation and drainage. Moreover, cationic polymer can be added to get faster and easier dewatering process (poly aluminum chloride. The polymer accelerates the particle agglomeration, increasing the total amount of water that can be drained and reducing the amount of water that needs to be evaporated. Close sand sludge drying bed is used in this dewatering process. This type of beds includes gravel, sand and sludge layers. Also, at the bottom part, under the gravel layer, there are plastic pipes for the under drainage. The water recycles to aeration tank. [4] X a (s) X in s + X a (s) x r s =.9 ( S+) +.9 ( S+)..2 Quick Mixing Tank: Theoretical models of chemical process are based on conservation laws such as conservation of mass and energy: Rate of mass accumulation= rate of mass in rate of mass out II. Methodology A. steps of models and simulation of the systems:. Aeration tank model and control: Figure 2: Physical diagram for quick mixing tank Modeling assumptions:. Mixing is perfect in each tanks 2. Temperature in each tank is constant 3. Density is constant and negligible Modeling equations for concentration of the chemicals [5]: G p = X (s) + X (s) = X 2 s W s (72.3S+) (72.3S+) 3. Sedimentation tank model and control: Figure : Schematic representation of activated sludge process [4] Modeling assumptions:. An ideal flow in a complete mixreactor 2. Input and output flow rates are equal 3. Density is constant and negligible Concentration along time= inflow outflow+ generation loss (mortality) Modeling Equations: We define the following transfer function [5]: Figure 3: Physical diagram of sedimentation tank Modeling assumptions:. Laminar flow 2. Constant density Modeling equations for level control [5]: H (s) Q (s) = R τs+ = S+ 4 5

3 Phase (deg) Magnitude (db) Imaginary Axis International Journal of Engineering, Applied and Management Sciences Paradigms, Vol. 43, Issue Publishing Month: January 27 The steps of control and tuning of the systems: RouthHurwitz Test: Numerical procedures to determine how many roots of a polynomial are in the Right Hand Plane and how many are on the imaginary axis. It doesn't give specific root locations but performing the test is generally far easier than factoring [3].. Direct substituting: Numerical procedure to determine the ultimate gain (Ku) and ultimate period (Pu) 2. Bode Diagram Method: Bode plots are common graphical representation of Amplitude Ratio (AR) and Frequency (θ) functions. A Bode Plot consists of two graphs: Log AR vs. Log ω and θvs. Log ω. To determine the ultimate gain (Ku) and ultimate period (Pu), we have to plot the Open loop Transfer Function first. So, we can get ω co and AR from the plot. Then, by equating the AR with one at the crossover frequency, we can calculate the Ku & Pu. 3. RootLocus Plots Tuning Method: It is graphical representation used to determine the ultimate gain and ultimate period from the root locus [7]. 4. ZieglerNicholas (ZN),Tuning Method: ZieglerNicholas is one of tuning techniques. It goes calculate the parameters according to the following formulas [7] 5. Controller Offset: The Offset of a controller is calculated mathematically by deducting output transfer functions for infinity value from Input transfer function which is ideally to be unity [7]. 6. Unit Step Test: System response is graphical representation of Amplitude Ratio (AR) vs. time functions, after introducing unit step change in the input [7]. 7. Stability in the Zplane: The stability of any system is determined by the location of the roots of its characteristic equation of its transfer function. The characteristic equation of the continuous system is a polynomial in the complex variable S. If all the roots of this polynomial lie in the LHP of the S plane, the system is stable [7]. 8. The stability of a sampleddata System is determined by the location of the roots of a characteristic equation that is polynomial in the complex variable Z [7]. III. Results and discussions i. RouthHurwitz test [6]: For this test characteristic equation of the loop is Characteristic equation = +OLTF = +k c G v G p = 5 k c = k u = ii.direct substituting [6]: k c = k u = W=.5 P u = 2π w = 4.6 iii.root locus criterion [8] Figure 4: Root locus plots for aeration tank iv.bode plots diagram Root Locus Real Axis System: sys Gain: 7.57e+3 Pole: i Damping:.77 Overshoot (%): Frequency (rad/sec):.5 System: sys Gain: 7.57e+3 Pole:.267.5i Damping:.77 Overshoot (%): Frequency (rad/sec):.5 Figure 5: Bode plots for aeration tank [8].5 Bode Diagram Frequency (rad/sec) System: sys Frequency (rad/sec):.5 Magnitude (db): 77.3 System: sys Frequency (rad/sec):.52 Phase (deg): 8 52

4 Amplitude International Journal of Engineering, Applied and Management Sciences Paradigms, Vol. 43, Issue Publishing Month: January 27 Table : Summary for the Comparison between Stability and tuning method Type of k u controll er P.5k u = PI.45k u =3369 P u / PID.6k u =4492 P u / =2.758 τ I (s) τ D (s) = P u /8 = 5.9 = Offset = For PID controller: = Table 3: Offset investigation result offset P PI PID Step Response sys sys2 sys3 Table 2: Using ZieglerNichols table to calculate the controller parameter Method Routh Hurwitz Direct Substitution Root Locus Bode Average of methods v. Offset Investigation: For P controller π f C s = G Y S = output s input = ± π l =.9997 For PI controller: = Offset = For PID controller: = i. Offset investigation: For PI controller: Figure 6: System responses with P controller (sys), PI controller (sys2) and PID controller (sys3) [7]: i. ZTransforms: ZTransforms play the same role for discretetime systems as that played by Laplace transforms for dynamic analysis and design of continuous open or closed loops Systems [8]. HG P G v =.k c ( z ). z[ y z =.k c z [ cz e.36 z ] z ] S S S e. z Stability in the Zplane by root locus: +OLTF=.k c z [ e. 36 z ] + =.k c z [ z. z ] z= = k c =.999 Time (sec) z z +.36 e. z 53

5 International Journal of Engineering, Applied and Management Sciences Paradigms, Vol. 43, Issue Publishing Month: January 27 The system stable of < k c <.999 iv. Bode plot [7] Figure 7: Unit circle in the Zplane 2. Mixing tank Control and tuning: G p = 2, G 72.3S+ v = 2, G.5S+ m = G c = k c.5s+, OLTF =k c G p G v G m i. Routh Hurwitz test [6]: For this test characteristic equation of the loop is: Characteristic equation = +OLTF = k c = ii. Direct substituting [6]: 5.4iw 3 44.w iw k c = W=.55 k c = P u = 2π w =.54 iii. Root locus criterion [7]: Figure 8: Root locus plots for mixing tank Figure 9: Bode plots for mixing tank Table 4: Summary for the Comparison between Stability and tuning method Method k u P u Routh Hurwitz Direct Substitution Root Locus Bode Average of method Table 5: Using ZieglerNichols table to calculate Type of controller P.5k u = PI.45k u = PID.6k u = the controller parameter v. Offset investigation[7]:. For P controller k u τ I (s) τ D (s) P u.2 =.45 P u /2 =.275 P u /8 =

6 Amplitude International Journal of Engineering, Applied and Management Sciences Paradigms, Vol. 43, Issue Publishing Month: January 27 C = 2. For PI controller C = lim S [S S ] = k c + 242k c = = 242 k(+ τ I S )(.5S+) 72.3S+.5S+.5S+ +242k(+ τ I S ) 242k c z z z z z +.39 z 4.296z 3.77z z = k c =.47 The system stable of < k c <.47, k c < z= Offset= = For PID controller G S = 242k(+ τ I S +τ D S)(.5S+) 72.3S+.5S+.5S+ +242k(+ τ I +τ D S) Offset=.554 Table 6: offset investigation result offset P PI PID vi. System response: Figure : Unit Circle in the ZPlane [8] Step Response sys sys2 sys3 G p = S + G v =.5S + G m =.S + G c = k c System: sys3 Time (sec): 6.24 Amplitude: Time (sec) Figure : System response with P controller (sys), PI controller (sys2) and PID controller (sys3) [8] i. RouthHurwitz test [6]: k c = ii. Direct substituting [6]:.2255iw w iw k c = k c = P u = 2π w =.7628 iii. Root locus criterion: vii. ZTransforms HG P G v G m = e TS S. k c S+.5S+ 2.5S+ 242k c z z z z z +.39 z 4.296z 3.77z z Stability in the Zplane by root locus method: +OLTF= Figure 2: Root locus plots for sedimentation tank 55

7 Amplitude International Journal of Engineering, Applied and Management Sciences Paradigms, Vol. 43, Issue Publishing Month: January 27 iv. Bode diagram: C =.8333k c = k c Offset =.992 =.8 2. For PI controller C = lim S [S.8333 k(+ τ I S )(.S+) 4.7S+.5S+.S k(+ τ I S ) S ] =.58 Offset= = For PID controller C = lim S [S.8333k( + τ I S + τ DS)(.S + ) S ] 4.7S +.5S +.S k( + τ I S + τ DS) Figure 3: Bode Plots for Sedimentation Tank Table 7: Summary for the Comparison between Stability and tuning method Method k u P u Routh Hurwitz Direct Substitution Root Locus Bode Average of method Table 8: Using ZieglerNichols tables to calculate the Type of controller P.5k u = PI.45k u = PID.6k u = controller parameter k u τ I (s) τ D (s) P u /.2 =.6347 P u /8 =.952 C =.254 Offset= =.254 Table 9: Offset investigation result offset P PI PID vi. System response: Step Response Time (sec) Figure 4: System Responses with P Controller (sys), PI Controller (sys2) and PID Controller (sys3) [8] sys sys2 sys3 v. Offset investigation[7]:. For P controller 56

8 Imaginary Axis International Journal of Engineering, Applied and Management Sciences Paradigms, Vol. 43, Issue Publishing Month: January 27 vii. ZTransforms HG P G v G m = e TS S. k c S+.5S+.S+.8333k c z. 5 z z z z + 2,94 6 z z z z k c z. 5 z z z z + 2,94 6 z z z z = z= k c = /T.9 /T.7 /T.6 /T PoleZero Map.5 /T.4 /T..3 /T /T. /T /T /T.2.9 /T. /T /T.2 /T.8.7 /T.3 /T.6 /T.4 /T.5 /T Real Axis Figure 5: Unit circle in the Zplane [8] IV. Conclusions and Recommendation In the present study, investigation of the tannery wastewater from different tanning processes gave a number of conclusions. Results of the analysis showed that the tannery wastewater from different tanning processes is highly with a disagreeable ph, alkalinity, acidity, total solids total dissolved solids, suspended solid, chemical oxygen demand, Biochemical oxygen demand, chrome, chlorides and sulfides, the recommendations of the study to eliminate the hazards tanneries waste by using treatment unit, tannery used solutions from soaking through retanning processes. There will be saving in water and clean environment. Advance control has a great effect on the response of the treatment unit in (White Nile tannery), which is divided in three tanks in series, aeration tank, quick mixing tank and sedimentation tank respectably. It increases stability and eliminate offset. There are many methods that are used to get the adjustable parameters such as RouthHurwitz, Direct substitution and there are another two graphical method, Bode and root locus. ZieglerNicholas criterion is used to tune the adjustable parameters. PID controller for the three tanks provides the highest gain than P, and PI controllers, P controller eliminates the Offset in the three processes, Z_ transform blot also used to check the stability of the systems. V. Acknowledgements The authors thank to the Faculty of Graduate College and Scientific Research of Karary University and White Nile tannery staff for their help and support, this Paper is generated from a Thesis in partial fulfillment for degree of Ph.D. in Chemical Engineering 57

9 References International Journal of Engineering, Applied and Management Sciences Paradigms, Vol. 43, Issue Publishing Month: January 27 [] PEATE WF: Occupational Skin Disease American Family Physician Am FAM Physician 22, 66: [2] Introduction to treatment of tannery effluents, United Nations Industrial Development Organization Vienna, 2. [3] wsystemprocess.shtml [4] sludgedryingbeds.html [5] John Wiley and sons, process dynamics and control, New York, second edition 24. [6] M E. Abu Goukh, controlling techniques and system stability, department of chemical engineering university of Khartoum press, Khartoum, 23. [7] G A. Gasmelseed, A text book of chemical engineering, process control, GTown book store and press, Khartoum, 2, PP, 346. [8] P. C. Chau, Chemical Process Control: A First Course with MATLAB, 2. 58