1. OPTIMAL PERIODIC OPERATION OF REVERSE OSMOSIS DESALINATION UNITS A. Ajbar, K. AlHumaizi, E. Ali Chemical Engineering Dept. King Saud University Saudi Arabia

2. Introduction Osmosis/Reverse is widely used as selective separation technique in: Chemical processes Biological processes Water desalination

3. RO in Desalination Driven by pressure, RO is a very fine filtration process using a membrane to filter the salt out from the solution

4. Membrane RO Performance Performance is affected by: Osmotic pressure Membrane characteristics; permeability and sieving

5. 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

6. 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

7. 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

8. Forcing Input

9. Effect of pulse width to period for feed pressure (Open loop)

10. Effect of Pulse width for both Feed flow and Feed Pressure (Open loop)

11. Feedback Control Automatically maintain periodic operation Optimize the cycle parameters, i.e. period, amplitude

12. Feedback Control Nonlinear Model Predictive Control: y: process output; i.e. q, C p u: manipulated variable; i.e. u f, P f Subject to:

13. Input Parameterization Setting inputs in periodic fashion:

14. Validated Process Model

15. Closed-loop simulation T s = 1 s, (q/q ss ) sp = 1.25, (Cp/Cp ss ) sp = 0.95

16. Closed-loop simulation T s = 10 s, (q/q ss ) sp = 1.25; (Cp/Cp ss ) sp = 0.9

17. 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.

18. Thank You