M.Sc Thesis

M.Sc StudentKesler Vladislav
SubjectInjection of a Concentrated Solution into a Flowing Stream
DepartmentDepartment of Chemical Engineering
Supervisors PROFESSOR EMERITUS Raphael Semiat
Full Thesis textFull thesis text - English Version


The motivation for this research arises from a difficulty encountered in desalination practice. It is sometimes necessary to dose into a desalination feed stream a caustic solution in order to achieve a controlled pH increase, as for example in the application of boron reduction measures. In order to restrict the size of the caustic feed system, the NaOH is dosed using a highly concentrated solution.

The problem is that desalination feed waters usually contains ions of sparingly soluble salts such as CaCO3 and Mg(OH)2 which tend to precipitate at high pH levels. The main goal of the present work is to delineate flow and concentration conditions at which dosage of a concentrated NaOH solution to sea water concentrates will not induce significant precipitation of CaCO3 and Mg(OH)2.

The experimental system constructed consisted of a vertical glass tube equipped with a capillary tube located at the center of the glass tube. A concentrated NaOH solution was injected through the capillary tube. A movable tube at the bottom of the pipe was used to sample the flowing sea water solution along the tube axis.

The main results of this study are as follows:

?         The CaCO3 saturation index (SI) profile along the flow axis derived from at pH measurements showed four distinct phases: an initial phase in which there was an abrupt sharp drop in the SI level, a transition phase in which the pH declined very slightly over a substantial distance, a third phase in which the SI declined continuously and a fourth phase in which SI reached a final asymptotic value.

?         The turbidity decline along the axial distance of the tube was continuous. The asymptotic turbidity level was reached before the SI level reached its asymptotic level.

?         A generalized empirical correlation found in the literature provided a rough fit to the turbidity data of the present study. The fit was significantly improved by slight modification of the empirical correlation.

?         The NaOH dosage rate had a significant effect on the extent of the axial dissipation distance. However, the level of the Re number of the flowing solution had a moderate effect on the dissipation results.

The results of this study provide hitherto lacking data that can be used to define more confidently scaling boundary conditions in processes aiming to control the boron content in desalinated sea water.