Elena Razkazova-Velkova

Visible light photocatalytic activity of TiO2 deposited on activated carbon

Four photocatalyst samples, prepared from beech sawdust, were synthesized by an original method, combining pyrolysis and impregnation — two of them: TiO2 + activated carbon and other two — only activated carbon. The pyrolysis process has been carried out at two different temperatures — 680°C and 830°C. The prepared samples were characterized by a series of methods — XRD, BET, SEM and DTA/TG. The most important result was achieving visible light photocatalytic activity with an azo dye pollutant for both materials. The TiO2/AC-680°C sample demonstrated higher activity under visible light illumination than the TiO2/AC-830°C sample. The visible light activity was attributed to the active carbon component in the composite materials, which was evidenced by the photocatalytic tests with bare carbon (without any TiO2) manifesting visible light activity. The AC-680°C carbon was superior to the AC-830°C under visible illumination probably due to its higher specific surface area and porous texture. UV-light testing of the photocatalytic activity revealed that the TiO2/AC-680°C sample was higher than that of the TiO2/AC-830°C under polychromatic UV-A illumination (320–400 nm with a maximum at l = 365 nm). The TiO2/AC-680°C sample was also more efficient with the monochromatic UV-C illumination (l = 254 nm).Graphical abstract

A NEW COLUMN PACKING FOR OPERATION AT EXTREMELY LOW LIQUID LOADS

Abstract It has been demonstrated that the ‘honeycomb packing’ made of sintered PVC sheets is well wetted even at extremely low liquid velocities. For example, at a liquid superficial velocity of 0.00008 m3/(m2 s) the effective surface is between 114 and 178 m2/m3 depending on the particular packing design. Having high effective surface at extremely low superficial velocities, the packing operates at full counter-current flow, while other packings demand a re-circulation of the absorbent. It is shown that compared to some of the highly efficient packings, the use of the new packing leads to considerable pressure drop reduction and to an increase of the volumetric mass-transfer coefficient.

New Packings for Operation at Extremely Low Liquid Superficial Velocity

It is well known that the column apparatuses operating at full countercurrent flow ensure maximum driving force for mass transfer when equilibrium processes are used. It is also known, from the mass balance, that the ratio between gas and liquid superficial velocity, in case of countercurrent flow, is determined from the initial and end concentrations of the absorbed component in both phases. In many cases, especially in purification of waste gases, when the initial gas concentration is very low and its solubility is high, the necessary calculated liquid superficial velocity is extremely low. The lack of packings able to operate effectively at these conditions requires a division of the packing bed into layers with recirculation of the liquid phase in each of them, i.e., a refusal of the principle of complete countercurrent flow takes place. The paper shows the possibility to use this principle even at extremely low liquid superficial velocity.

A Method for the Simultaneous Cleansing of H2S and SO2

A method for the simultaneous electrochemical purification of hydrogen sulfide and sulfur dioxide from sea water or industrial wastes is proposed. Fundamentally the method is based on the electrochemical affinity of the pair H2S and SO2. The reactions (oxidation of H2S and reduction of SO2) proceed on а proper catalyst in a flow reactor, without an external power by electrochemical means. The partial curves of oxidation of H2S and reduction of SO2 have been studied electrochemically on different catalysts. Following the additive principle the rate of the process has been found by intersection of the curves. The overall process rate has been studied in a flow type reactor. Similar values of the process rate have been found and these prove the electrochemical mechanism of the reactions. As a result the electrochemical method at adequate conditions is developed. The process is able to completely convert the initial reagents (concentrations ), which is difficult given the chemical kinetics.