13) Good Curie law behavior was observed for most samples down to liquid helium temperature. Although some showed deviations near 10°K, this was related to the prior history of the sample.

No abstract

Although multicomponent reactive transport modeling is gaining wider application in various geoscience fields, it continues to present significant mathematical and computational challenges. There is a need to solve and compare the solutions to complex benchmark problems, using a variety of codes, because such intercomparisons can reveal promising numerical solution approaches and increase confidence in the application of reactive transport codes. In this contribution, the results and performance of five current reactive transport codes are compared for the 1D and 2D subproblems of the so-called easy test case of the MoMaS benchmark (Carrayrou et al., Comput Geosci, 2009, this issue). This benchmark presents a simple fictitious reactive transport problem that highlights the main numerical difficulties encountered in real reactive transport problems. As a group, the codes include iterative and noniterative operator splitting and global implicit solution approaches. The 1D easy advective and 1D easy diffusive scenarios were solved using all codes, and, in general, there was a good agreement, with solution discrepancies limited to regions with rapid concentration changes. Computational demands were typically consistent with what was expected for the various solution approaches. The differences between solutions given by the three codes solving the 2D problem are more important. The very high computing effort required by the 2D problem illustrates the importance of parallel computations. The most important outcome of the benchmark exercise is that all codes are able to generate comparable results for problems of significant complexity and computational difficulty.Keywords MoMaS · Benchmark · Code intercomparison · Numerical methods for reactive transport · Direct substitution approach (DSA) · Differential and algebraic equations (DAE) · Sequential iterative approach (SIA) · Sequential noniterative approach (SNIA) 484 Comput Geosci (2010) 14:483-502

The temperature-programmed desorption (tpd) of the amount of ammonia which is preadsorbed at about 373 K at HZSM-5 zeolites yields a complex desorption curve consisting of two overlapped peaks (fl and ~,lPeak). Parts of the ammonia desorbed can be attributed to SiOHAI groups considering also H-MAS NMR measurements.The course of the desorption of both peaks is describable by a rate equation which considers a dependence of the desorption energy on the degree of coverage or an energy distribution, as could be shown by various methods of evaluation. The calculated dependence of the desorption energy on the ammonia amount adsorbed well agrees with data of literature of adsorption heats determined miero-calorimetrieally.Temperature-programmed desorption (tpd) of bases [3,[4][5][6][7][20][21][22][23][24][25][26][27][28][29][30][31] is, beside of IR spectroscopy [1-9], 1H-MAS-NMR spectroscopy [10][11][12][13][14][15][16] and microcalorimetry [2,8,9,[17][18][19][20] a method frequently used for the characterization of acidic properties of HZSM-5 zeolites.Tpd of ammonia adsorbed at low temperature provides 3 peaks more or less strongly overlapped (a, fl or LT, and 7 or HT peak [7,20]), which are ascribed to acidic centres of different strength. The a peak is ascribed in this case to the physisorption of ammonia. The reason for the ~ peak is the NH3 desorption from strongly acidic OH groups, which can be characterized in the IR spectrum by the vibration band at about 3600 cm -1. The fl peak is explained in different manner by several authors: Tops0e [7] ascribes this peak to desorption from weak acidic centres according to a IR band in the 3720...3740 cm -1 region or to the adsbrption on Na + and Hidalgo [5] to a broad IR band at 3350...3650 cm -1. Lok [3] interpretes the peak by splitting of physisorbed ammonia and Schnabel et al. [6] discuss a desorption from extra-framework aluminium.

In this article, an approach for the efficient numerical solution of multi-species reactive transport problems in porous media is described. The objective of this approach is to reformulate the given system of partial and ordinary differential equations (PDEs, ODEs) and algebraic equations (AEs), describing local equilibrium, in such a way that the couplings and nonlinearities are concentrated in a rather small number of equations, leading to the decoupling of some linear partial differential equations from the nonlinear system. Thus, the system is handled in the spirit of a global implicit approach (one step method) avoiding operator splitting techniques, solved by Newton's method as the basic algorithmic ingredient. The reduction of the problem size helps to limit the large computational costs of numerical simulations of such problems. If the model contains equilibrium precipitation-dissolution reactions of minerals, then these are considered as complementarity conditions and rewritten as semismooth equations, and the whole nonlinear system is solved by the semismooth Newton method.