The pressure drop in a microchannel Fischer-Tropsch reactor was investigated by means of a fluid dynamics model developed by the authors. The developed model takes into account roughness of the microchannel wall induced by catalyst particle deposition on the surface of the microchannel. The presented simulation procedure takes into account the variation of the synthesis product composition and the variation of thermal properties of the liquid and gas phases along the microchannel length as functions of pressure, temperature, conversion rate and chain growth coefficient. Liquid and gaseous products down flow are modeled in the annular flow approximation. The obtained results are presented for two general types of microchannels, i.e. for rough-walled and for smooth-walled microchannels. It is shown that fluid dynamics in rough-walled and smooth-walled microchannels are dramatically different. It is established that a mean critical diameter can be introduced. The microchannels with diameter below the mean critical value can experience operation difficulty due to by high aerodynamic resistance or can even become completely flooded.
The Fischer-Tropsch synthesis is a key step in the process of synthetic crude production from natural gas or other carbon-containing feedstock. The chemistry of the process includes conversion of syngas (CO and H2 mixture) into hydrocarbons over Co metal or Fe metal catalyst. The molecular mass distribution of produced hydrocarbons obeys Anderson-Schulz-Flory law [
Currently are realized two types of Fischer-Tropsch reactors in industrial practice, namely slurry-bed reactors and fixed-bed reactors. Both types of the reactors possess certain advantages and drawbacks related with thermal stability, diffusion limitations and aerodynamic resistance.
The recent successes in microchannel fabrication technologies have led to very active research and development of microchannel systems for Fischer-Tropsch synthesis as a challenge to conventional slurry-bed and fixed-bed reactors [2-6]. Cobalt catalyst microparticles of 50 - 300 µm size get deposited on the inner surface of a microchannel thus forming a randomly roughed inner surface. Such micro reactors promise easy transport of syngas to catalyst surface and excellent heat removal conditions. The efficiency of a microchannel system increases with decrease in the microchannel diameter. However, the hydrodynamic resistance can ruin the reactor operation even at very good diffusion and hat removal efficiencies. The importance of hydrodynamic resistance cannot be overestimated. It is difficult, however, to estimate this value.
Indeed, experimental measurements must be carried out with the use of gas-liquid mixtures of proper composition, which changes along the channel, otherwise the results cannot be correct. The computational methods available in literature are good for modeling flow in smooth channel, without taking into account evolution of properties and composition along the channel [7-9]. It is necessary to note that hydrodynamics of smooth-walled and rough-walled channels can be dramatically different even for laminar flow. Another important factor is that literature computational methods suggest that gas flow and liquid film flow are considered as independent, which results in substantial error in liquid film thickness value [
This work is devoted to investigation of hydrodynamic resistance in microchannel Fischer-Tropsch reactors and its influence on the reactor operation. Theoretical model was published in our previous work [
It is consider a vertical cylindrical microchannel. Procedure of numerical generation a random surface inside the ceramic microchannel is carried out in accordance with the technical method of coating the channel walls by catalyst microparticles. It is selected the average number of spherical particles per unit length of the microchannel. For cylindrical coordinate system microparticles are modeled as random rings located on the perimeter of the cylinder. The distance between the micro inclusions and the height of the roughness are also independent random variables. The number of particles is recalculated in order to preserve a given volume fraction of particulate catalyst inside the microchannel. Volume fraction of cobalt particles is in the range 20% - 30%.