Methane steam reforming is the most common industrial process used for almost the 50% of the world’s hydrogen production. Commonly, this reaction is performed in fixed bed reactors and several stages are needed for separating hydrogen with the desired purity. The membrane reactors represent a valid alternative to the fixed bed reactors, by combining the reforming reaction for producing hydrogen and its separation in only one stage. This article deals with the recent progress on methane steam reforming reaction, giving a short overview on catalysts utilization as well as on the fundamentals of membrane reactors, also summarizing the relevant advancements in this field.

Advances on methane steam reforming to produce hydrogen through membrane reactors technology: A review

The power consumption of a coaxial mixer consisting of a wall-scraping anchor and different dispersion impellers (radial discharge) operating in co- and counterrotating modes has been experimentally characterized in the case of viscous Newtonian and non-Newtonian fluids. It was found that the anchor speed did not affect the power consumption of the dispersion turbines. The power consumption of the anchor was found to increase when the dispersion impellers were used in counterrotating mode and to decrease when they were used in corotating mode. Following the Metzner and Otto approach [AIChE J. 1957, 3 (1), 3−10], new correlations based on the impeller geometry for the generalized Reynolds number and the power number are proposed, and it is shown that a power master curve can be generated for speed ratios larger than 10.

Power Characteristics in Coaxial Mixing: Newtonian and Non-Newtonian Fluids

Determination of mixing time and degree of homogeneity in stirred vessels with large eddy simulation

Hydrodynamic investigation of USP dissolution test apparatus II

The flow pattern and the distribution of energy dissipation rate in a batch rotor–stator mixer have been investigated. Sliding mesh and standard k–ɛ turbulence model were employed to predict velocity and energy dissipation rate distributions verified experimentally by the Laser Doppler Anemometry measurements. The agreement between predicted and measured bulk flow field as well as the flow pattern of jets emerging from the stator holes was very good. Results showed that the periodicity of the jet can be related to the rotor's velocity and number of blades. The energy balance based on measured velocity distribution indicated that about 70% of energy is dissipated in close proximity to the mixing head. Both simulation and measurement showed that the jet velocity and flowrate through the holes were proportional to N while the energy dissipation rate scaled with N3.

Flow pattern, periodicity and energy dissipation in a batch rotor–stator mixer

CFD-based computations of the flow field, power consumption and mixing time are presented for a mechanically stirred eight-blade paddle impeller in an unbaffled vessel over a range of Reynolds numbers covering laminar, transitional and turbulent flow regimes. The flow field calculations were performed using the sliding mesh technique to account for the motion of the impeller, and mixing time studies were done using a simulated tracer injection experiment. The effect of grid density and the choice of the turbulence model were investigated. The results are compared with flow field data from Dong et al . 1 , and power and mixing time correlations from the literature and show satisfactory agreement. It is shown that the product of mixing time and rotational speed remains constant for paddle impellers for laminar flow and that the use of a low Reynolds number turbulence model is necessary for good prediction of mixing time in the transitional flow.

CFD Study of Power and Mixing Time for Paddle Mixing in Unbaffled Vessels

In 1957 Metzner and Otto proposed a simplified way of calculating the power consumption in impeller-driven systems for power-law type of fluids. Assuming a proportionality between the shear rate and rotational speed of the impeller, they reported that the proportionality constant was independent of the geometric and fluid properties of the mixing system. This was later contested by a number of researchers, and different correlations have been given to evaluate the proportionality constant. An in-depth investigation of the Metzner–Otto concept is discussed with the help of computational fluid dynamics (CFD) techniques to calculate the flow field created by an anchor impeller. Fundamentally, both of the assumptions are broadly valid for typical operating parameters used in anchor-driven impeller systems. A new correlation, based on the Metzner–Otto concept and making use of the present CFD simulations, was developed to calculate power consumption in anchor-driven mixing systems.

Mixing of power-law fluids using anchors: Metzner-Otto concept revisited

Fuel cell based modular power generation can be achieved by miniaturization and process intensification of equipments in the process. Fuel cells require hydrogen rich gas which can be generated through reforming and water gas shift reaction. The water gas shift reactor being kinetically limited occupies more volume to achieve the required CO conversion. A membrane reactor integrates the reaction and hydrogen separation stages and hence reduces the volume requirement. Computational Fluid Dynamics offers virtual prototyping of the reactor and thus helps in design, optimization and scale up of reactors. In this study customized User Defined Functions (UDFs) were developed to analyze the performance of low temperature water gas shift membrane reactor. The models were validated using literature data for the parameters – synthesis gas compositions, time factor, sweep flow rate and steam to CO ratio. The effect of all these parameters on the reactor was analyzed for CO conversion, H 2 recovery, DaPe, concentration polarization, concentration profiles and conversion index. The simulations have showed that the UDFs developed were capable of simulating the membrane reactor and this can be used for the design and optimization of the membrane reactor for any process conditions.

CFD analysis of water gas shift membrane reactor

Helical ribbon impellers are widely used in chemical and process industries for the mixing of pseudoplastic fluids of high viscosity. The design of such impellers is based on an assumed linear relation between shear rate and the rotation speed of the impeller. A number of computational fluid dynamics (CFD) simulations of the flow field have been carried to verify this hypothesis. It is shown that while the shear rate varies greatly within the mixing vessel, there does exist a linear relationship between the impeller speed and the local shear rate near the tip of the impeller. The proportionality constant K s associated with this linear relation is found to be dependent on the geometric parameters of the system, but is largely independent of the flow behavior index. Based on these results, a new correlation, applicable to both Newtonian and power-law fluids for power consumption, is proposed.

S. Murthy Shekhar

Papers

CFD Study of Power and Mixing Time for Paddle Mixing in Unbaffled Vessels

Mixing of power-law fluids using anchors: Metzner-Otto concept revisited

CFD analysis of water gas shift membrane reactor

Mixing of pseudoplastic fluids using helical ribbon impellers

Development of novel cabinet solar dryer using UV sheet and its performance evaluation: An experimental study