טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentLevy Naomi
SubjectInvestigation of Low-pH Fe(II) Oxidation and Development
of a Method for Fe(III) Regeneration as Part of
a Process Aimed at H2S(g) Removal
DepartmentDepartment of Civil and Environmental Engineering
Supervisor Professor Ori Lahav
Full Thesis textFull thesis text - English Version


Abstract

Hydrogen sulfide is emitted from environmental facilities and industrial processes such as petroleum refineries, anaerobic digestion processes, etc. The removal of H2S from gaseous streams is required for reasons related to general public health, occupational safety and sometimes for process operation purposes.

Liquid Redox Sulfur Recovery (LRSR) is a promising process for H2S(g) removal which is based on reactive H2S(g) absorption and the use of a catalyst redox couple. The most currently used couple is Fe(III)/Fe(II). Fe(III) oxidizes H2S(aq) to S0, and the formed Fe(II) is oxidized back to Fe(III) by O2(aq). Operation at pH7 to pH9 is favorable since the absorption of H2S(g) is efficient and Fe(II) is oxidized spontaneously and rapidly by O2. However, at this pH range it is required to add organic chelates in order to avoid rapid precipitation of Fe(III) species. At pH~2 the solubility of ferric species is high enough to avoid the need for chelating agents, and the low Fe(II) oxidation rate can be accelerated by the use of Fe(II) oxidizing bacteria. However, process dependence upon sensitive autotrophic biomass and the fact that precipitation still occurs, detract from the attractiveness of this approach.

The current work investigated two new approaches for applying the LRSR process at pH1.0. The first method is based on catalytic oxidation of Fe(II) by O2 in the presence of Cu2+ and phosphate. The second is based on electrochemical oxidation of Fe(II), either directly (oxidation of Fe(II) on the anode) or indirectly (oxidation of Fe(II) by chlorine formed on the anode). The very low operational pH (pH1.0) was chosen in order to minimize precipitation of ferric species for long-term LRSR operations. Two other factors to be considered in the process are the efficient reactive-absorption of H2S(g) and rapid oxidation of Fe(II).

The catalytic oxidation method, although proven to accelerate Fe(II) kinetics in lab tests, was found infeasible, since despite of the low pH, H2S introduced to the reactor precipitated with Cu2+ to form CuS, and Fe(III)-phosphate solids also precipitated quite rapidly. The direct electro-oxidation method was also found inapplicable since the efficiency of the reactive-absorption of H2S(g) was very low in the absence of chlorides. In contrast, the indirect electro-oxidation method was found highly feasible: the reactive-absorption efficiency of H2S(g) was high in the presence of chlorides, there was no appreciable amount of precipitates and Fe(II) oxidation rate was very high.