Catalin Florin Petre

Pressure drop through structured packings: Breakdown into the contributing mechanisms by CFD modeling

Determination of dry pressure drops is often the preliminary diagnostic tool for characterizing structured packing-containing columns. One conventional approach that ushered in this area evolves around the use of Ergun expressions along with mandatory experimental pressure drops for the fitting of some empirical constants characterizing a given packing. This method is strictly representational, and incapable of predicting the impact on bed pressure drop of changes in packing geometry, e.g., corrugation angle, channel size, or packing topography. In this work, a combined mesoscale—microscale predictive approach was developed to apprehend the aerodynamic macroscale phenomena in structured packings. The proposed method consists in identifying recurrent mesoscale patterns (the representative elementary units, REU) wherein the constitutive microscale dissipation mechanisms occur. The dissipative phenomena that were identified to be important are: the elbow loss and jet splitting at the packed bed entrance, the elbow loss at the column wall, the elbow loss at the jump from one layer to another, and the collisional losses at the criss-crossing junctions. Each mechanism was simulated over a wide Reynoids range spanning the pure creeping flow to the fully developed turbulent flow using three-dimensional computational fluid dynamics (CFD). Postulating additiveness of dissipation, the overall pressure drop was reconstructed. The approach was validated using experimental dry pressure drop data for five packing types (Flexipac, Gempak, Mellapak, Sulzer BX and Montz-Pak) having different channel sizes, corrugation angles, and surface topography. Our goal was to advocate CFD as a quicker and cheaper means for design and optimization, in terms of energy dissipation, of new structured packing shapes.

Tailoring the pressure drop of structured packings through CFD simulations

A computational fluid dynamic methodology is proposed to breakdown into elementary dissipation mechanisms the overall single-phase gas flow bed pressure drop in towers containing corrugated sheet structured packings. The goal behind was to allow piecewise geometry optimization of such packings in terms of capacity enlargement and efficiency enhancement. The dissipations sorted in order of decreasing importance were the collision losses by jet streams at criss-crossing junctions within corrugated channels, elbow loss by form drag at interlayer transition, elbow loss by jets striking wall and subsequent flow redirection to upper channels, and elbow loss in bed entrance. Replacement of sharp bends at the interlayer junctions by progressive direction change was beneficial for the reduction of the dissipations at the wall and the interlayer junction thus stretching capacity of the structured packing. However, this improvement was not spectacular because the most energy-intensive component (criss-crossing) remained unaffected by such modifications. Computational fluid dynamics is foreseen to be a successful, rapid and economic tool to theoretically explore new geometries coping with this limitation.

Capillary electrophoretic analysis of sulfur and cyanicides speciation during cyanidation of gold complex sulfidic ores

A capillary electrophoretic protocol for the separation and quantification of the most important species potentially liberated during the cyanidation of gold sulfide-rich ores was accomplished in this study. The separation of 11 ions: S2O3(2-), Cu(CN)3(2-), Fe(CN)6(4-), Fe(CN)6(3-), SCN(-), Au(CN)2(-), Ag(CN)2(-), SO4(2-), OCN(-), SO3(2-), and HS(-) was achieved using an indirect UV detection method. The robustness of the analytical protocol was tested by analyzing ions speciation during the cyanidation of two gold sulfide-rich ores. The 1-h cyanidation of the two ores released up to six complexes into solution: S2O3(2-), Cu(CN)3(2-), SCN(-), Fe(CN)6(4-), OCN(-), and SO4(2-). The mineralogy of the ore was found to influence directly the nature and the amount of the dissolved species. Conserving the cyanidation solution for 72 h after sampling resulted in 96% total sulfur recovery. These results allow us to conclude that the analytical protocol developed in this study can become very useful for the optimization of precious-metals cyanidation plants.

Determination of free cyanide and zinc cyanide complex by capillary electrophoresis

A capillary electrophoretic (CE) protocol was developed for the separation and quantification of free cyanide and zinc cyanide complex, two key species in gold cyanidation of zinc-bearing sulfidic ores. Several common carrier electrolytes were implemented in an indirect UV detection method. The effect of electric field strength, injection volume, concentration of electro-osmotic flow (EOF) modifier and UV-absorbing agent in background electrolyte (BGE) was examined while peak height, peak area and noise were considered for optimization. The best results were obtained using a BGE that contained 35 mM sodium chromate, 12 mM free cyanide and 0.45 mM hexamethonium bromide at pH 10.5. Free cyanide concentration was compared to that measured with the conventional silver nitrate titration method in solutions containing free cyanides and weak cyano-complexes. The developed CE protocol proved very robust in capturing the concentration of free cyanides (4% error) unlike the titration method which exhibited substantial sensitivity to the interfering weak cyano-complexes (38% error).

Advances in Chemical Oxidation of Total Reduced Sulfur from Kraft Mills Atmospheric Effluents

Chemical oxidation techniques in use for the reduction of malodorous total reduced sulfur (TRS) emissions in the kraft mills atmospheric effluents were reviewed with an emphasis on recent industrial improvements in chlorine dioxide (ClO2) oxidation of TRS as well as on laboratory developments of an iron-based chemistry process. The ClO2 approach was implemented successfully at the industrial scale in two Québec kraft mills. The approach consisted in mixing the non-condensable gases (NCG) containing the TRS with gaseous chlorine dioxide obtained either as a residue from a bleach plant vent stream or through vaporization of fresh solution. Full-scale tests have shown that the amount of chlorine dioxide injected or mixed in the NCG was sufficient to reduce the TRS load below the 10 ppmv-regulated levels in a cost efficient way as compared with incineration. A prospective approach validated in laboratory conditions and using the iron redox chemistry for alkaline oxidative scrubbing of TRS is being investigated at Laval University to reduce the odor pollution and to convert TRS into valuable sulfur. Two configurations were evaluated, one consisting of homogeneous Fe(III) sequestered in trans-1,2-diaminocyclohexanetetraacetic acid (cdta) chelates and another of heterogeneous Fe(III) as Fe/Ce oxides-hydroxides mixtures. The relative performances, advantages and weaknesses of the various chemical oxidation processes were discussed. In addition, the fundamentals of the alkaline oxidative scrubbing of TRS using the iron-based alkaline approach were summarized in terms of the gas-liquid thermodynamic equilibria and of the homogeneous and heterogeneous iron redox reactions.