Kuhs Werner F, Staykova Doroteya K, Salamatin Andrey N
GZG Abt. Kristallographie, Georg-August-Universität Göttingen, Göttingen 37077, Germany.
J Phys Chem B. 2006 Jul 6;110(26):13283-95. doi: 10.1021/jp061060f.
Neutron diffraction runs and gas-consumption experiments based on pressure-volume-temperature measurements are conducted to study the kinetics of methane hydrate formation from hydrogenated and deuterated ice powder samples in the temperature range of 245-270 K up to high degrees of transformation. An improved theory of the hydrate growth in a polydisperse ensemble of randomly packed ice spheres is developed to provide a quantitative interpretation of the data in terms of kinetic model parameters. This paper continues the research line of our earlier study which was limited to the monodisperse case and shorter reaction times (Staykova et al., 2003). As before, we distinguish the process of initial hydrate film spreading over the ice particle surface (stage I) and the subsequent hydrate shell growth (stage II) which includes two steps, i.e., an interfacial clathration reaction and the gas and water transport (diffusion) through the hydrate layer surrounding the shrinking ice cores. Although kinetics of hydrate formation at stage II is clearly dominated by the diffusion mechanism which becomes the limiting step at temperatures above 263 K, both steps are shown to be essential at lower temperatures. The permeation coefficient D is estimated as (1.46 +/- 0.44) x 10(-12) m2/h at 263 K with an activation energy Q(D) approximately 52.1 kJ/mol. This value is close to the energy of breaking hydrogen bonds in ice Ih and suggests that this process is the rate-limiting step in hydrate formation from ice in the slower diffusion-controlled part of the reaction.
进行了基于压力-体积-温度测量的中子衍射实验和气消耗实验，以研究在245 - 270 K温度范围内，氢化和氘化冰粉样品形成甲烷水合物的动力学，直至达到高转化率。建立了一种改进的理论，用于描述随机堆积的多分散冰球集合体中水合物的生长过程，以便根据动力学模型参数对数据进行定量解释。本文延续了我们早期研究的路线，早期研究仅限于单分散情况和较短的反应时间（Staykova等人，2003年）。和以前一样，我们区分了初始水合物膜在冰颗粒表面铺展的过程（阶段I）和随后水合物壳层生长的过程（阶段II），阶段II包括两个步骤，即界面笼形包合反应以及气体和水通过围绕收缩冰核的水合物层的传输（扩散）。尽管在阶段II水合物形成的动力学显然由扩散机制主导，在温度高于263 K时扩散成为限制步骤，但在较低温度下这两个步骤都被证明是必不可少的。在263 K时，渗透系数D估计为(1.46 ± 0.44)×10⁻¹² m²/h，活化能Q(D)约为52.1 kJ/mol。这个值接近冰Ih中氢键断裂的能量，表明在反应较慢的扩散控制部分，这个过程是冰形成水合物的速率限制步骤。
