Pump shear stress effects were tested by comparing permeability decline data of recycle runs carried out under identical solution and flow conditions but with different exposure times of the antiscalant to pump shear stresses. Induction time data were measured with antiscalants exposed to different extents of pump shear action. Induction times were measured in:
C normal recycle runs characterized by relatively short pump exposure time
C runs in which the pump exposure time was increased by recycling the antiscalant dose in distilled water through the pump for 35–45 h before starting the induction time experiment
C runs in which the pump and membrane exposure times were increased by recycling the antiscalant dose in distilled water through the pump and the membrane for 35–45 h before starting the induction time experiment.
Fig. 15 shows the effect of long exposure of the antiscalant solution in distilled water to pump shear stress on antiscalant performance. Induction times seem to be essentially similar, irrespective of the extent of the antiscalant exposure to the pump shear stress. It may be therefore concluded that antiscalant molecules remain stable and are not degraded by repeated exposure to the shear stress induced by pump impellers.
Fig. 15. Effect of long exposure to pump shear stress on antiscalant performance.
Residence time effects were tested by comparing permeability decline data of once through runs with those of recycle runs performed under identical solution and flow velocity conditions.
Fig. 16 shows permeability results measured under recycle flow and once-through flow conditions respectively. It is seen that induction times in the two systems are substantially similar.
Some antiscalant manufactures consider that the extent of the residence time of a feed solution in an RO system influences the scale suppression effectiveness. The data obtained in this study clearly show that, at least under the experimental conditions covered in this study, there is no significant difference between induction time results of recycle and once-through flow systems. This finding lends further support to the usefulness of the scale propensity techniques developed in our laboratory [1–3].
The main objective of this study was to consolidate the recycle laboratory techniques
Fig. 16. Comparison of recycle flow and once- through flow results
developed in our laboratory for characterizing the scaling propensity of raw feed waters in RO desalination and for a reliable evaluation of the inhibitory effectiveness of antiscalants used to extend water recovery limits. Induction times measurements were carried out in the CaSO4 system, with and without the presence of antiscalants, over a wide range of conditions. The data were successfully correlated by a model based on nucleation theory. Surface energy values for CaSO4 nucleation measured in this study were in excellent agreement with comparable literature results.
Further consolidation of the recycle techniques was obtained by investigating possible fundamental dissimilarities between laboratory test conditions and real field conditions. One dissimilarity is the prolonged exposure of antiscalant molecules to pump shear stress in the laboratory recycle run. Another dissimilarity is the much longer residence time of antiscalants in a laboratory recycle test. The experimental evidence obtained in this study showed that these dissimilarities had no effect on the validity of the laboratory results.
The practical significance of the present investigation is that it lends considerable support to the techniques we have developed for characterizing permissible water recovery limits and evaluating rationally the performance of different antiscalants.
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