M.Sc Thesis

M.Sc StudentSchwartz Rinat
SubjectMagnesium Salt Dissolution in Desalinated Water
DepartmentDepartment of Chemical Engineering
Supervisors PROFESSOR EMERITUS Raphael Semiat
Full Thesis textFull thesis text - English Version


There is current interest in examining the need to add magnesium ions to desalinated water and in developing viable processes for this purpose. The specific objective of this research was to develop a promising simple process for adding magnesium ions to desalinated water. In exploratory work magnesia dissolution in a mixed flow reactor was examined. The main effort of this research was investigation of magnesia dissolution by acidified desalinated water in a bed packed with magnesia pellets. The kinetics of magnesia dissolution was analyzed assuming a mechanism of mass transfer control.

In the exploratory investigation magnesia dissolution by desalinated water acidified with sulfuric acid was performed in a short packed column under recycle conditions.  The dissolution data indicated that the system deviated from ideal mixed flow conditions. A good analysis of the results was obtained by correlating the data according to the dispersion model.

The main investigation was conducted in a column of 98mm diameter containing a 91cm packed length of magnesia spherical pellets of average diameter of 3.5mm. The process of magnesia dissolution by either CO2 or H2SO4 acidified water was monitored by analysis of solutions taken from 7 sampling outlets along the column.  The kinetic model was based on the premise that the dissolution driving force was proportional to the difference between bulk magnesium concentration and equilibrium magnesium concentration on solution-pellet interface. The model was tested in a series of experiments in which the initial acid concentration was varied in the range of 2.5 to 10 mmol/L CO2 and 1 to 5 mmol/L H2SO4, contact times in the range of 2.3 to 6.9 minutes and feed flow rates providing Reynolds numbers in the range of  7.7 to 23.1.  The functional relationships predicted by the model between dissolved Mg concentration and solution pH as a function of contact times was confirmed by the experimental results, yielding mass transfer correlations which are in substantial agreement with literature data. Striking differences were observed between CO2 and H2SO4 dissolutions showing a clear advantage to H2SO4 dissolution. With H2SO4 dissolution, Mg concentration reached rapidly near equilibrium conditions within short contact time of 1.1 to 2.6 minutes while with CO2, in the same acid concentrations and contact times, Mg concentrations were far from equilibrium.