The design of industrial catalysts requires that the diffusivity of the reacting species within the catalyst be accurately known. Nowhere is this more important than in the area of coal liquefaction and upgrading of coal liquids. In this area one is faced with the task of processing a number of heavy oils, containing metals and other contaminants, in a variety of process dependent solvents. It is important, therefore, on the basis of predicting catalyst activity, selectivity, and optimizing reactor performance, that the diffusivities of these oil species be accurately known. Several studies exist [l], emphasizing thk importance of diffusion in processes involving coal and petroleum liquids. Spry and Sawyer [2] measured the demetallization rates of a crude oil using various catalysts and found these rates to depend on average pore size, in agreement with their simple model, based on the theory of Anderson and Quinn [3]. Inoguchi [4] observed that optimum activity for desulfurization of heavy crudes occurs with catalysts with pore sizes of 1008, and for vanadium removal with pore sizes between 120 and 1408,. Eigenson et al. [ti] found increased catalytic activity for Moo3 on Also3, when the pore size increased from 70 to 1508,. Significant intraparticle diffusion effects have been observed during asphaltene cracking and the dekulfurization of asphaltenic and nonasphaltenic fractions of a residuum by Philippopoulos and Papayannakos [SI. Ternan and \ coworkers [7] have reported significant pore size effects during the catalytic processing of Athabasca bitumen. Studies have also been published on the importance of intraparticle diffusion during coal liquefaction and coal liquid upgrading, including those of Polinski and coworkers [8], Scooter and Crynes [9], and Yen et al. [lo]. Workers at Amoco have found [ll] that micropore volume in a critical pore size range, i.e. narrowly distributed about 1208, diameter pores, was very important in coal liquefaction catalyst activity. McCormick et al. [12] have shown during their hydrotreating studies that activities for both hydrogenation and dehydrogenation reactions are highly correlated (> 99% confidence) with the pore volume in the preferred range (60-2008,) in diameter. The importance of catalyst average pore diameter and pore length during the hydrocracking of solvent refined coals and lignites has been discussed by Berg and Kim [13]. Theoretical studies have also been published, which incorporate hindered diffusion in order t o predict optimal pore sizes during petroleum liquid upgrading [14] and catalytic coal liquefaction [15]. The molecules comprising coal liquids can range from less than 10 to several hundred 8, in diameter. Their size is, therefore, comparable t o the average pore size of most hydroprocessing catalysts. Thus, during processing, transport of these molecules into the catalyst occurs mainly by "configurational" or "hindered diffusion," which is the result of two phenomena occurring in 1 the pores; the distribution of solute molecules in the pores is a...
