Dieter Bathen

Increasing environmental concerns, stricter legal requirements, and a wide range of industrial applications have led to growing demand for activated carbon worldwide. The energy-intensive production of fresh activated carbon causes significant CO₂ emissions and contributes to global competition for renewable carbon-based raw materials. Although (thermal) reactivation of spent activated carbon can drastically reduce the demand for fresh material, the reactivation process itself is still mostly based on experience and empirical knowledge locked into activated carbon companies. Despite the vast number of papers published in the field, practically relevant, systematic, and quantitative knowledge on the thermal reactivation process is barely available. This paper presents a simple and robust methodology for the development of a predictive model for the production of reactivated carbon with a defined product quality under energetically optimized conditions. An exhausted activated carbon sample was subjected to 26 reactivation experiments in a specially designed laboratory rotary kiln, whereas the experiments were planned and evaluated with statistical design of experiments. The influence of the reactivation conditions (heating rate, heating time, H₂O/N₂ volume ratio, and CO₂/N₂ volume ratio) on the specific surface area, energy consumption, yield, and adsorption capacity for diatrizoic acid were evaluated. The BET surface of the reactivated carbons ranged between 590 m²/g and 769 m²/g, whereas the respective fresh carbon had a BET surface of 843 m²/g. The adsorption capacity for diatrizoic acid measured as the maximum solid phase concentration q_m derived from the Langmuir equation varied between 24.4 g/kg and 69.7 g/kg (fresh carbon: 59.6 g/kg). It was possible to describe the dependency of the quality criteria on different reactivation parameters using mathematical expressions, whereas the response surface methodology with nonlinear regression was applied to build the models. A reactivation experiment under statistically optimized conditions resulted in energy savings up to 65%, whereas the properties of the reactivated sample were close to the predicted values.

In addition to the adsorption mechanism, the heat released during exothermic adsorption influences the chemical reactions that follow during heterogeneous catalysis. Both steps depend on the structure and surface chemistry of the catalyst. An example of a typical catalyst is the faujasite zeolite. For faujasite zeolites, the influence of the Si/Al ratio and the number of Na⁺ and Ca²⁺ cations on the heat of adsorption was therefore investigated in a systematic study. A comparison between a NaX (Sodium type X faujasite) and a NaY (Sodium type Y faujasite) zeolite reveals that a higher Si/Al ratio and therefore a smaller number of the cations in faujasite zeolites leads to lower loadings and heats. The exchange of Na⁺ cations for Ca²⁺ cations also has an influence on the adsorption process. Loadings and heats first decrease slightly at a low degree of exchange and increase significantly with higher calcium contents. If stronger interactions are required for heterogeneous catalysis, then the CaNaX zeolites must have a degree of exchange above 53%. The energetic contributions show that the highest-quality adsorption sites III and III’ make a contribution to the load-dependent heat of adsorption, which is about 1.4 times (site III) and about 1.8 times (site III’) larger than that of adsorption site II.

In this article, for the first time the chemical and mechanical properties of novel adsorbents based on the coating of activated carbons with silicon carbide are reported. The adsorbents are prepared by chemical vapor infiltration (CVI) of activated carbons with tetramethylsilane (TMS) as a precursor. A comparison of two different modified types of activated carbon, C40/4 Extra and A35/4 Extra, each infiltrated with 25%-mass at infiltration temperatures of 973.15 and 1098.15 K, respectively, is presented. Adsorption properties were characterized by measuring nitrogen isotherms and volatile organic compounds (VOC) isotherms in gas phase and excess isotherms in liquid phase. In addition, the physico-chemical properties including the bulk density, ash content, particle hardness, abrasion, conductivity, water-soluble components, and pH value were determined. Furthermore, the first experiments in a fluidized bed adsorber are presented. The results show that the adsorption properties of the modified adsorbents are mainly maintained. The particle hardness and the abrasion resistance increases with increasing infiltration temperature, which leads to an overall increasing of mechanical stability. A modification of the chemical stability as a result of the infiltration experiments is not observed.

Zur simultanen Messung von Adsorptionsenthalpie und Beladung wurde eine neue Messmethodik entwickelt. Ein volumetrisches Adsorptionsmessgerät wird durch eine kalorimetrische Messanordnung erweitert, in der die Druckdifferenz zwischen zwei identischen Sensorgasvolumina gemessen wird, die eine Messzelle und eine Referenzzelle umschließen. Bei der Adsorption tritt infolge der Sorptionswärme ein Druckanstieg im Gasvolumen um die Messzelle auf. Aus dem zeitlichen Verlauf lässt sich nach einer Kalibrierung die Sorptionswärme berechnen. Erste Ergebnisse der Adsorptionsenthalpie von CO₂ an einem 13X‐Zeolithen werden gezeigt.

New periodic mesoporous organosilicas were synthesized using [bis(triethoxysilyl)vinyl]aniline (BTEVA) and [bis(triethoxysilyl)vinyl]benzene (BTEVB) as precursors and by functionalizing the BTEVA PMO with aminopropyl groups. All PMO materials showed a high degree of hexagonal order in the respective P‐XRDs as well as high surface areas with narrow pore size distributions. The PMOs together with activated carbon were investigated with regards to their hexanal adsorption capacity in a gravimetric apparatus with an attached GC‐MS. For the amine PMOs two different regions during adsorption were found. In the first region loadings in the range of 30 wt% were reached by a fast adsorption process. In the second region the loading reached 50 wt% but with slower kinetics. In desorption experiments up to 150 °C the loading decreased from 50 wt% to 30 wt%, where an equilibrium state was observed. With GC‐MS and NMR the first region could be assigned to a chemisorption process, where the hexanal forms an imine group with the amine function in a Schiff base reaction. The subsequent second region can be assigned to weaker and thus reversible physisorption. The total capacity (first and second region) is substantially higher for the amine PMOs than for the activated carbon. Due to the chemical bonding in the chemisorption process, the desorption behavior is completely different: compared to the activated carbon the bonding of hexanal is much stronger for the amine PMOs.

Highly pure organic solvents gain more and more importance in optical, electronical, pharmaceutical and chemical industry. In this context the removal of water in the lower ppm‐ and ppb‐range is a major challenge since water is always present, e. g., air humidity, and leads to catalyst poisoning and undesired side reactions. One technique to remove water is by adsorption on zeolites, silica gels or aluminas. Despite single technical solutions already existing, systematic measurements of influencing parameters are still missing. The research project adresses the adsorption of water from different organic solvents. In the following experiments with linear primary alcohols and acetic acid esters are discussed as these solvents are often used in industry in low water content qualities.