optimal periodic operation of reverse osmosis desalination units a. ajbar, k. alhumaizi, e. ali...
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OPTIMAL PERIODIC OPERATION OF REVERSE OSMOSIS DESALINATION UNITS
A. Ajbar, K. AlHumaizi, E. AliChemical Engineering Dept.
King Saud UniversitySaudi Arabia
Introduction
Osmosis/Reverse is widely used as selective separation technique in:
Chemical processes Biological processes Water desalination
RO in Desalination Driven by pressure, RO is a very fine
filtration process using a membrane to filter the salt out from the solution
Membrane RO Performance
Performance is affected by:
Osmotic pressure
Membrane characteristics; permeability and sieving
Problems
Fouling causes irreversible decrease in flux
Caused by accumulating particles on the surface which plugs the pores
fouling is eliminated by shutting the process down and cleaning the membrane
Problems
Concentration polarization results in reversible decline in water flux
Caused by accumulation of rejected salts on the membrane wall, increasing the solute concentration
Can be eliminated by:(i) changes in the characteristics of the
membrane(ii) modification of flow rates and flow regime
Motivation
Periodic input forcing improves the mixing of the solution on the feed side and, hence, reduces the build-up of solute ions near the membrane wall
Forcing Input
Pulse widthInput
Pulse PeriodTime
Pul
se A
mpl
itud
e
Mean value
Effect of pulse width to periodfor feed pressure (Open loop)
1.45
1.5
1.55
1.6
1.65
1.7
1.75
1.8
1.85
1.01
1.012
1.014
1.016
1.018
1.02
1.022
1.024
0 0.2 0.4 0.6 0.8 1
Cw/Cb
q/qss
Effect of Pulse width for both Feed flow and Feed Pressure (Open loop)
1.3
1.35
1.4
1.45
q ave/q
ss
Pulse Width (samples)
0 50 100 150 200 250 3000.8
0.85
0.9
0.95
1
Cp av
e/Cp ss
qave
/qss
Cpave
/Cpss
Feedback Control
Automatically maintain periodic operation
Optimize the cycle parameters, i.e. period, amplitude
Feedback Control
Nonlinear Model Predictive Control:
2
1
M
1=i
2P
1=i1
)())()((min)(),....,(
ikikikMktuktu
tutRty
y: process output; i.e. q, Cp
u: manipulated variable; i.e. uf, Pf
btUA kT )(Subject to:
Input Parameterization
)sin()( 1 mfkf APtPss
)sin()( 2 mfkf Autuss
Setting inputs in periodic fashion:
p
tkπ2
Validated Process Model
iwjipibpiv RTcRppLJ ,2'
,,, )()(
i,wi,v'
j1i,b1ii,bi1i,b c
d
J4)R1(
x
cu
x
cu
dt
dc
21,
72,,
7,
1
,
,272,
7,
11,
105.0105.04
102105.0
iibivipibi
iv
ibiFiibib
i
iib
uJpud
Jx
duxfup
u
up
Closed-loop simulationTs = 1 s, (q/qss)sp = 1.25, (Cp/Cpss)sp = 0.95
0 100 2000
0.5
1
q (
cm3 /s
)
Time (sec)0 100 200
0
200
400
Cp
a (g/
l)
Time (sec)
0 100 2000
50
100
Pf (
bar)
Time (sec)0 100 200
0
50
100
u f (cm
/s)
Time (sec)
0 100 2000.8
1
1.2
1.4
q/q
ss
Time (sec)0 100 200
0.5
1
1.5
Cp
a/Cp
a ss
Time (sec)
Closed-loop simulation Ts = 10 s, (q/qss)sp = 1.25; (Cp/Cpss)sp = 0.9
0 200 4000
0.5
1
q (
cm3 /s
)
Time (sec)0 200 400
0
100
200
Cp
a (g/
l)
Time (sec)
0 200 4000
50
100
Pf (
bar)
Time (sec)0 200 400
36
38
40
u f (cm
/s)
Time (sec)
0 200 4000.5
1
1.5
q/q
ss
Time (sec)0 200 400
0.5
1
1.5
Cp
a/Cp
a ss
Time (sec)
Conclusions
The symmetric cyclic variation of the feed pressure and/or flow showed potential for
an improved performance.
NLMPC generated periodic input functions that managed to enhance the time-averaged value of the permeate production and salt concentration.
Thank You