Assessment of hydrocarbon adsorption constants under reaction conditions

ULISES SEDRAN

Cocoyoc, Morelos, México

Workshop; Combinatorial Chemistry & Technologies, Molecular Design and their Industrial Applications; 2006

ICS - UNIDO

The actual adsorption parameters of the products of a catalytic chemical reaction can be assessed in a batch fluidized bed with internal circulation. Two methods (equilibrium or direct, and mass balance error minimization) were used based on the mass balances of the test, the composition of two different samples taken an instant before reactor evacuation and after completing it, and a Henry equilibrium model. The reaction was the cracking of decane over a commercial equilibrium FCC catalyst at 400 and 450 ºC, during 6 to 12 s reaction times, on a CREC Riser Simulator reactor. The direct method can yield information on the individual adsorption parameters of each different species in the reacting mixture. By definition the method does not take into account the effect of diffusion limitations affecting the adsorption parameters, but it is possible to gather important information on hydrocarbon adsorption under reaction conditions. For example, the adsorption constants for branched paraffins were higher than those of the linear compounds with the same number of carbon atoms per molecule, or xylenes showed higher values than toluene among aromatics. Diffusion resistances to mass transfer reflected into some calculated adsorption constants. The mass balance error minimization method uses hydrocarbon lumps and was developed with a model of the evacuation step of the experiments that considers the possible diffusion limitations. The method yielded consistent results for the adsorption constants of the various lumps.  Adsorption issues are particularly significant in residual feedstock processing. A novel methodology was developed in order to evaluate the reactivity of resid feedstocks over FCC catalysts. An atmospheric resid from a naphthenic crude was dissolved into two solvents (toluene and methyl naphthalene) selected due to their low reactivity, and converted over two commercial equilibrium FCC catalysts with different characteristics (conventional and resid catalysts) in the CREC Riser Simulator. Conditions typical of the commercial process were used: temperatures 500 and 550 ºC, cat to oil ratio 5.8 and reaction times up to 25 s. The pure solvents were also converted under the same experimental conditions, so as to generate background information to compare against that from the conversion of the dissolved resid feedstocks. Careful mass balances calculations and the combined analysis of the reaction products of the experiments with the resid dissolved into both solvents, allowed to assess the individual yields of all the compounds produced exclusively by the resid. The results observed, where it is possible to distinguish clearly the effects of the various catalyst properties, grant reliability to the proposed method.