SODIUM POLYPHOSPHATE-MODIFIED CLASS C/CLASS F FLY ASH BLEND CEMENTS FOR GEOTHERMAL WELLS.

The authors investigated the usefulness of the coal combustion by-products, Class C fly ash (C) and Class F fly ash (F), in developing cost-effective acid-resistant phosphate-based cements for geothermal wells. In the temperature range of 20-100 C, sodium polyphosphate (NaP) as the acidic cement-forming solution preferentially reacted with calcium sulfate and lime in the C as the base solid reactant through the exothermic acid-base reaction route, rather than with the tricalcium aluminate in C. This reaction led to the formation of hydroxyapatite (HOAp). In contrast, there was no acid-base reaction between the F as the acidic solid reactant and NaP. After autoclaving the cements at 250 C, a well-crystallized HOAp phase was formed in the NaP-modified C cement that was responsible for densifying the cement's structure, thereby conferring low water permeability and good compressive strength on the cement. however, the HOAp was susceptible to hot CO{sub 2}-laden H{sub 2}SO{sub 4} solution (pH 1.1), allowing some acid erosion of the cement. On the other hand, the mullite in F hydrothermally reacted with the Na from NaP to form the analcime phase. Although this phase played a pivotal role in abating acid erosion, its generation created an undesirable porous structure in the cement. They demonstrated that blending fly ash with a C/F ratio of 70/30 resulted in the most suitable properties for acid-resistant phosphate-based cement systems

Thromboses veineuses profondes du membre supérieur

MONTPELLIER-BU Médecine UPM (341722108) / SudocPARIS-BIUM (751062103) / SudocMONTPELLIER-BU Médecine (341722104) / SudocSudocFranceF

Coupling chromatographic separation and crystallisation of enantiomers

Abstrac

Coupling chromatographic separation and crystallisation of enantiomers

Abstrac

Theoretical and experimental investigations of coupling chromatography with preferential crystallization for enantioseparation

In the recent years, the rise of effort for improving of already established separation process has been motivated by the necessity of separation of the racemic mixtures to the enantiomeric pure components. The most commonly used classical resolution methods are chromatographic, crystallization and membrane technologies. Enantioselective preferential crystallization [1] is an attractive, inexpensive method for separation of mixture that crystallizes as a conglomerate i.e., a physical mixture of crystals where each crystal is enantiomerically pure. In solution such systems tend to reach an equilibrium state in which the liquid phase will have racemic composition and the solid phase will consist of a mixture of crystals of both enantiomers. However, before approaching this state, it is possible to preferentially produce just one of the enantiomers after seeding with homochiral crystals. Perfectly mixed batch crystallizers can be described mathematically using a dynamic, one dimensional model which includes experimentally determined kinetic parameters. Such a model was applied in order to describe the process for the D, L-threonine-H2O system [2]. Based on the simplified approach, attractive and more effective operation modes using several crystallizers can be studied e.g., two crystallizers coupled via liquid phase where in each vessel one of both enantiomers is crystallizing simultaneously and an exchange of the liquid phase between the crystallizers (see Fig. 2) leads to an increase of the process productivity [3]. Preferential crystallization processes are sometimes carried out with an initial enantiomeric excess of the preferred enantiomer. Fig. 2 shows that by an increase of the initial enantiomeric excess higher process productivity might be obtainable using a coupled batch operation mode. The intention of this work is the analysis and optimization of the process combination of chromatography and simultaneous preferential crystallization [4]. The chromatography will be used to gain certain amount of pure enantiomer required for a given enantiomeric excess. The optimized process parameters for both separate processes as well as for the hybrid process will be given in this presentation. Parallel to the theoretical analysis, an experimental validation of this process is currently performed and results will be also presented. [1] COLLINS, A.N., SHELDRAKE, G.N., CROSBY, J. (1994): Chirality in Industry: The Commercial Manufacture and Applications of Optically Active Compounds, John Wiley & Sons [2] ELSNER, M.P., FERNÁNDEZ MENÉNDEZ, D., ALONSO MUSLERA, E., SEIDEL-MORGENSTERN, A. (2005): Experimental study and simplified mathematical description of preferential crystallization, Chirality 17 (S1), S183-S195 [3] ELSNER M.P., ZIOMEK, G., SEIDEL-MORGENSTERN, A. (2007): Simultaneous preferential crystallization in a coupled, batch operation mode. Part I: Theoretical analysis and optimization. Submitted to Chemical Engineering Science. [4] KASPEREIT, M. (2006): Separation of Enantiomers by a Process Combination of Chromatography and Crystallisation. PhD thesis, Otto von Guericke University, Magdeburg

Nonisothermal Reactive Chromatography

Selective adsorbents can be used in non-steady state fixed bed reactors to enhance the conversion of reversible reactions through separation of products, which diminishes the undesired backward reactions. This is the main idea of chromatographic reactors that are currently intensively investigated considering various liquid feed stocks (e.g. alcohol/acid mixtures or esters in water) [1,2]. In most of the studies reported up to now, the processes have been considered to proceed under isothermal conditions. Only recently, more attention was paid to the aspect that, in such periodically operated reactors, significant temperature effects can occur [3,4]. Reliable prediction of the performance of chromatographic reactors also at large scale requires that thermal effects due to adsorption, reaction, and also mixing are properly accounted for. In the present work, thermal effects in reactive chromatography are investigated both theoretically and experimentally. The limiting case of simultaneous chemical and phase equilibrium is discussed in the framework of the equilibrium theory. Behaviour of single column and simulated moving bed chromatographic reactors is analysed with numerical simulations. Esterifications of acetic acid with methanol and ethanol to produce methyl acetate and ethyl acetate were used as model reactions. Amberlyst 15 and Finex CS16G ion-exchange resins were chosen as the stationary phases. Adsorption enthalpies were determined from chromatographic data at various temperatures. Reactor experiments were carried out in thermally insulated (adiabatic) columns, fitted with several thermocouples. The self-amplifying nature of thermal waves in the reactor is due to nearly equal propagation velocities of the reaction front and the thermal wave. Experiments under isothermal conditions were carried out as a reference. References: 1. Mazzotti, M., Kruglov, A., Neri, B., Gelosa, D., Morbidelli, M., Chem. Eng. Sci., 51(1996), 1827-1836 2. Vu Dinh, T., Seidel-Morgenstern, A., Grüner, S., Kienle, A., Ind. Eng. Chem. Res. 44(2005), 9565 - 9574 3. Sainio, T., Doctoral dissertation, Acta Universitatis Lappeenrantaensis 218, 2005 4. Migliorini C., M. Wendlinger, M. Mazzotti, M. Morbidelli, Ind. Eng. Chem. Res. 40(2001) 2606-261