Химия и химические технологии/5 Фундаментальные проблемы создания новых материалов и технологий

Dolidovich A.F., Akhremkova G.S.

A.V.Luikov Heat and Mass Transfer Institute (HMTI)

National Academy of Sciences of the Republic of Belarus

Research studies of transfer processes in dispersed media

relative to gas cleaning systems

The air basin of large cities and industrial centers in many countries is contaminated by different types of such hazardous volatile organic pollutants as alcohols, ketones, ethers, phenols, aromatic hydrocarbons, etc, produced by various technological processes and transport means, which are harmful for peoples and animals, microflora of soil and plants as well as for water sources.

The most known traditional “wet’, “dry” and combined methods of air purification involving scrubbing, condensation, absorption, adsorption, thermal, catalytic, thermocatalytic, and adsorptive-catalytic methods suffer from such drawbacks as the high specific energy and material consumption, utilization of noble and rare earth materials for decontamination purposes, bulky equipment, probability of secondary environmental contamination, high cost of purification, especially relative to the, so called, small-capacity industrial sources containing less than 1,0 - 2,0 g of VOC per 1 m³ of exhaust gases, i.e. the sources which give about 44% of total contamination of the atmosphere with hydrocarbons.

To overcome some of these disadvantages and to reduce their influence on applicability of the traditional environmental protection technologies, a broad spectrum of theoretical and experimental research studies has been carried out at the Luikov Heat and Mass Transfer Institute of the National Academy of Sciences of the Republic of Belarus which has served a basis for development of few novel lower-cost and energy-saving adsorptive, adsorptive-catalytic and thermocatalytic technologies of VOC purification from industrial low-concentration multicomponent exhaust gases.

The present paper is focused on discussion of results of investigations of mechanical, structural, physical and chemical properties, micro-and macrokinetic adsorptive, catalytic and regenerative characteristics of heterogeneous alumina catalyst-adsorbents and other filtering media used in these technologies under conditions of fixed, moving and fluidized beds.

Results of investigations of structural, physical-chemical and strength properties of the tested heterogeneous catalyst-adsorbents and activated carbon carriers described in [1,5] reveal a rather developed specific surface area, high catalytic activity in butane and carbon monoxide oxidation as well as a high mechanical strength of the tested materials. Life tests conducted under severe conditions of a high-temperature fluidized bed in a pilot reactor have demonstrated that they are superior to a great number of well-known metal-oxide heterogeneous catalysts in the thermal and mechanical stability and preserve their adsorptive and catalytic activity for a long time (more than 3.5 thousand of hours) [2,3].

The adsorption properties of the used new catalyst-adsorbents, fresh and after their utilization in a pilot fluidized bed reactor under diesel fuel combustion conditions, as well as of the activated carbons, activated fibrous carbon cloths have been examined by static and dynamic methods with use of individual organic compounds and their multicomponent mixtures, which are close to the characteristic compositions of the exhaust gases in various branches of industry. The main attention in investigations was paid to new developed catalyst-adsorbents because their adsorption properties were completely unknown and because it was suppose that they are best suitable for moving and fluidized bed conditions as regards their mechanical strength and thermal stability. Adsorption isotherms, physical adsorption capacity, total adsorption capacity, and other characteristics of sorbents were determined and demonstrated in [4,5,6,7]. Extensive studies have been made of the kinetics and dynamics adsorption of VOC from dry and moist air flows. Generalization of the obtained experimental data has allowed to derive empirical correlations for calculation of the dynamic adsorption capacity of complete purification with respect to individual VOC and mixtures in dry and moist gas-carrier flows [5].

The catalytic properties of fresh and “aged” catalyst-adsorbents have been investigated with respect to the above mentioned and other individual and mixed organic compounds by the temperature-programmed desorption (TPD) method followed by the chromatographic analysis of oxidation products. The results obtained from the TPD-spectra analysis, and by using other methods and experimental facilities with fixed and fluidized beds of catalyst-adsorbents are in fair agreement and indicate that in a temperature range of 600 to 673 K all the products of thermodesorption of tightly bound modes of VOCs adsorption are being removed from the surface and volume of catalysts and the latter undergo complete regeneration [3].

Analytical and experimental studies have been carried out to explore flow dynamics, heat and mass transfer characteristics in fixed, dense moving and fluidized beds of crushed activated carbons, fibrous activated carbon cloths and catalyst-adsorbents used in cleaning technologies under development [6,8]. During investigations were studied:

· a character of the cold and hot gas flows through the dense fixed beds of dispersed and fibrous activated carbon materials, hydraulic pressure drops of these beds, temperature profiles in the bed with an electric current supplied to it, heat and mass transfer inside the beds with adsorption and desorption of organic substances;

· mechanics, particle velocities and their motion profiles in thin (10 £ l £ 15cm) annular cylindrical and conic moving beds with a gas flow filtration in the transverse direction at different rates, a pressure drops of these beds, characteristics of heat and mass transfer of dense gravity moving beds at adsorption of single-and multicomponent organic substances and with cooling of “hot” particles by a “cold” gas;

· a behavior of the “cold” and “hot” fluidized bed consisting of “active” and “inert” particles with the continuous supply of “active” (in respect to the chemical combustion reaction) particles to the bed, velocities of minimum fluidization and pressure drops of coarse particle beds, a heat and mass transfer rate in “hot” fluidized beds in the case of filtration and catalytic oxidation of hydrocarbon-containing gases as well as with the continuous feed of combustible-adsorbate-ballasted particles to it;

· characteristics of particle discharge from moving and fluidized beds via slots, single or multiple orifices;

· characteristics of plate-type and other feeders as well as some parameters of pneumatic transport of coarse particles in dense and diluted phases.

Correlations were derived and given to determine pressure drops of fixed, gravity-moving and fluidized beds, velocities of minimum fluidization and discharge of granulated catalyst-adsorbents from orifices as well as interphase heat and mass transfer coefficients required for calculation and modeling of the adsorption and catalytic oxidation (combustion) of organic compounds processes in gas purification systems under development which summarize the data of many investigators [5].

The theoretical and experimental research studies performed have allowed to create physical-mathematical models of the processes of adsorptive, adsorptive-catalytic and thermocatalytic purification of the industrial exhaust gases from VOCs and to develop computer programs for their calculation which make it possible to simulate these processes under conditions close to real and to optimize operation and design parameters of gas cleaning systems [9].

Based on these investigations, several modifications of new energy-saving technologies of adsorptive, adsorptive-catalytic and thermocatalytic gas cleaning with a fixed, moving and fluidized beds of adsorbents and catalysts which are intended for decontamination of low-concentrated multicomponent industrial gases from such hazardous VOCs as ethers, alcohols, phenols, aldehydes, ketones, aromatic hydrocarbons, etc. have been developed. These technologies ensure the decreasing in installation dimensions and high value of gas cleaning efficiency from VOCs (up to 98-100%) and thermal energy recuperation (up to 96%).

The schematics, operating principles and some distinctive features of the novel (i) adsorptive (ii) adsorptive-catalytic, (iii) thermocatalytic with a quasistationary fluidized and (iiii) with a fixed catalyst beds and deep intercyclic heat recovery of gas cleaning equipment are described in [9].

References

1. Dolidovich A.F. (1991) Dispersed Systems in the Nature-Protective Technologies, HMTI Press, Academy of Sciences of the BSSR, Minsk, 138p.

2. Dolidovich A.F., G.I.Pal’chonok and Bo Lekkner “Calculation of true heat and mass transfer coefficients between particles and a fluidized bed”, in “Proc. of the Int. Conf. “Fluidization-VII”, Brisbain, Australia, (1992), pp. 550-558.

3. Dolidovich A.F., “Heat and mass transfer phenomena inside capillary-pore bodies as a basis for development of the novel nature-protection technologies”, in “Heat and Mass Transfer-93/94. Col. of science papers”, HMTI Press, Belarus Academy of Sciences, Minsk, (1994), pp.25-38.

4. Dolidovich A. F., Akhremkova G. S. Adsorption of organic substances vapors on activated fibrous and granulated carbon materials. Byelorussian Academy Sciences News Journal, Chemical Sciences Series, 1998, No.2, pp. 38-42.

5. Dolidovich A. F., Akhremkova G. S., Efremtsev V.S. Novel technologies of VOC decontamination in fixed, moving and fluidized catalyst-adsorbent beds. // Canadian J. of Chem. Eng. 1999. Vol. 77, No. 2. Pp. 253 –258.

6. Dolidovich A.F., Akhremkova G. S.(2000) Innovative gas cleaning technologies preventing the environmental contamination by heat and CO₂. Proceeding of 2-nd EUROENVIRONMENT-2000 Conference. Denmark, Aalborg, 18–20 October.

7. Dolidovich A. F., Akhremkova G. S., Lapina V.A., Rubanov A.S. Adsorption of water and organic substances vapors on novel melanin-containing fitoadsorbents. Physic – Chemical Journal (Russia), 2003, V.77, No.1, pp.77-80.

8. Dolidovich A. F., Akhremkova G. S. Theoretical and experimental research studies of pore structure and adsorption properties of carbon fibrous materials. Journal of Eng. Physics and Thermophysics, 2010, v.83, No.5, pp. 861-865.

9. Dolidovich A.F. (2012) Some scientific background for development of advanced energy-saving environment protection equipment with fixed, moving and fluidized beds. Journal of Engineering Physics and Thermophysics, v.85, No.1, pp. 43-58.