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