Hydrodynamics in a pressurized cocurrent gas-liquid trickle-bed reactor

The influence of gas density on total external liquid hold-up, pressure drop and gas-liquid interfacial area, under trickle-flow conditions, and the transition to pulse flow have been investigated with nitrogen or helium as the gas phase up to 7.5 MPa. It is concluded that the hydrodynamics depends on the gas density and not on the reactor pressure. At higher gas densities, the operating region for trickle flow becomes larger while the gas-liquid interfacial area increases only slightly. Pressure gradient and liquid hold-up are considerably affected by gas density and correlations have been derived for these parameters

Does the fluid elasticity influence the dispersion in packed beds?

Reasons are given why the axial dispersion in a gas flowing through a packed bed may be influenced by the elasticity - or compressibility - of the fluid. To support this hypothesis, experiments have been done in a packed column at pressures from 0.13 to 2.0 MPa. The elasticity E of a gas is proportional to the pressure P and the compressibility to 1/P. The axial dispersion coefficients as determined were found to be a function of the pressure in the packed bed in the turbulent flow region of 3 < Rep < 150 if the Bodenstein number is plotted as a function of the particle Reynolds number. This is shown to be an artifact. The pressure influence is eliminated, if Bom, ax is plotted versus the ratio of the kinetic forces over the elastic forces u2/E. Regrettably, Bom, ax seems to be independent of u2/E. For the moment we only can conclude that Bom, ax in the turbulent region is a unique function of the velocity of the gas which flows through the packed bed. Although the fact that a constant Bo value is obtained when plotted against u2/E, the experimental results are so intriguing we wanted to make them public already now. The experimental work proceeds

Recent Developments in Residence Time Distribution

We use to apply the concepts of Danckwerts to describe residence time distribution (RTD) phenomena, although Danckwerts himself in his original paper warned the reader, that his approach would hardly ever describe the RTD problems adequately. Realistic experiments are known that only may be interpreted with the Danckwerts approach, provided we introduce negative dispersion coefficients. Numerous papers have been written to give a possible physical interpretation of the boundary conditions at the inlet and the outlet of the system; but still they keep puzzling experts. In many experiments never a upstreams dispersion in packed beds could be found despite such predictions by the theory of Danckwerts.We, therefore, derived a new model to interpret RTD phenomena, which still maintains the ease of a one-dimensional approach. We called the model the “wave model “, because of the analogy with the description of wave problems in hydrodynamics. This model only needs boundary conditions at the inlet of the system, adequately describes dispersion in packed beds and asks for realistic dispersion parameters in the case where the Danckwerts model would need negative dispersion coefficients. The wave model will be explained and applied.Chaotic temperature fields in running packed bed reactors have been observed which could not be interpreted with the usual models to describe the behaviour of packed bed plug flow reactors. This led to the investigation of the influence of free convection on the RTD in packed bed reactors. The results of this investigation will be explained and the correlation of the dispersion data will be shown to be quite accurate. Extrapolation of the correlated data to industrial equipment warns for very serious consequences in such equipment, which will be explained. Urgent large scale experiments are required

Neues Verfahren zur Methanol- und Ammoniak-Synthese. Der Gas/Feststoff/Feststoff-Rieselströmungsreaktor - ein neuer Reaktortyp zur Führung chemischer Gleichgewichtsprozesse

New process for the production of methanol and ammonia. The gas/solids/solids thrickle flow reactor - a new kind of reactor for chemical equilibrium processes. A new process for the production of ammonia or methanol has been developed in the high pressure laboratory of Twente Technical University. The reactants can be made to react completely in a single reactor pass, thus avoiding expensive recirculation. Complete reaction without recirculation is accomplished by a combination of two reversible processes: a chemical reaction and a selective adsorption of the reaction product on a solid in constant flow. On introduction of an inert-gas free stoichiometric mixture of reactants into the reactor, the exhaust gas outlet can be closed. If there is an excess of one of the reactants or of inert gas, it can leave from the top of the reactor. Such a process has been accomplished in a new gas/solids/solids trickle flow reactor in which a granulated adsorbent rains through a fixed bed of catalyst. The present article describes the use of this kind of reactor for methanol synthesis. Considerable savings in production costs are expected relative to the modern Lurgi low pressure process. A pilot plant is to be built to characterize and evaluate the new process. Royal Dutch Shell has registered a patent application

Interfacial areas and gas hold-ups in bubble columns and packed bubble columns at elevated pressures

Interfacial areas and gas hold-ups have been determined at pressures up to 1.85 MPa in a bubble column with a diameter of 85.5 mm and for superficial gas velocities between 1 and 10 cm s−1. In some experiments the bubble column was packed with glass cylinders of length 5.0 mm and diameter 4.0 mm. The interfacial areas were determined by the chemical method using the model reaction between CO2 and aqueous diethanolamine (DEA) and hold-ups by observation of height differences.\ud \ud The interfacial areas in the packed bubble column are unaffected by pressure. The gas hold-ups as well as the interfacial areas in the bubble column increase with increasing operating pressure. The magnitude of the pressure influence depends on the superficial gas velocity. The positive influence of pressure on the gas hold-ups and the interfacial in the bubble column originates from the formation of smaller bubbles at the gas distributor

Residence time distribution and hold-up in a cocurrent upflow packed bed reactor at elevated pressure

The residence time distribution in liquid phase was measured in a cocurrent upflow packed bed reactor for the system methanol-hydrogen at low Reynolds numbers and at elevated pressure. The plug flow with axial dispersion model was used to describe mixing in the system. The imperfect pulse method was used to measure the system response to a tracer pulse input. The parameters were calculated using the weighted moments method. The influence of the weighting factor was investigated. The experimental and theoretical outputs, as calculated by convolution, agreed very well. Different types of correlations were used for the Bodenstein number and liquid hold-up. From these correlations, the optimal one was selected for each parameter. A comparison was made between the ordinary moments and the weighted moments methods which led to the conclusion that the latter method is superior with respect to the accuracy of the estimated parameters and therefore strongly recommended