Mixing Suspensions in Tall Vessels with a Draught Tube

A tall vessel with a telescopic draught tube is proposed for mixing suspensions. The paper presents the relations for calculating the agitator power consumption and the speed necessary to keep a particle in suspension.

Mixing Flow Characteristics in a Vessel Agitated by the Screw Impeller With a Draught Tube

The circulating flow in a vessel induced by rotating impellers has drawn a lot of interests in industries for mixing different fluids. It used to rely on experiments to correlate the performance with system parameters because of the theoretical difficulty to analyze such a complex flow. The recent development of computational methods makes it possible to obtain the entire flow field via solving the Navier–Stokes equations. In this study, a computational procedure, based on multiple frames of reference and unstructured grid methodology, was used to investigate the flow in a vessel stirred by a screw impeller rotating in a draught tube. The performance of the mixer was characterized by circulation number, power number, and nondimensionalized mixing energy. The effects on these dimensionless parameters were examined by varying the settings of tank diameter, shaft diameter, screw pitch, and the clearance between the impeller and the draught tube. Also investigated was the flow system without the draught tube. The flow mechanisms to cause these effects were delineated in detail.

A Numerical Analysis on the Flow Characteristics in a Helical Screw Agitator With a Draught Tube

Mechanical agitators are widely used in several industries which can mix one or more reactants, or mix some of the high viscous fluids. In general, simple impeller type is applied to mix low viscous fluids, while a helical screw agitator (HSA) is applied to the high viscous fluids. The study of flow characteristics inside the agitator is important because the mixing efficiency is greatly affected by the existence of draught tube. The HSA with a draught tube is known more efficient than the others. In this study, three-dimensional model consisting of outer vessel, helical screw and draught tube are considered. The draught tube in the vessel disturbs radial flow, hence it makes intermixing between the screw and tube. Also, the pitch of screw is capable of influencing the flow pattern. Consequently, the shapes of vessel, helical screw and draught tube are the main parameters for improving the performance of HSA. Numerical analyses were carried out with different geometrical configurations. Using a commercial CFD code, Fluent, velocity and pressure distributions inside of the HSA are obtained under steady, laminar flow and no-slip condition. Results are graphically depicted with several parameters.

Pumping Capacity of Pitched Blade Impellers in a Tall Vessel with a Draught Tube

A study was made of the pumping capacity of pitched blade impellers (two, three, four, five and six blade pitched blade impellers with pitch angles α = 35° and 45°) coaxially located in a cylindrical pilot plant vessel with cylindrical draught tube provided with a standard dished bottom. The draught tube was equipped with four equally spaced radial baffles above the impeller pumping liquid upwards towards the liquid surface. In all investigated cases the liquid aspect ratio H/T = 1.2 - 1.5, the draught tube / vessel diameter ratios DT /T = 0.2 and 0.4 and the impeller / draught tube diameter ratio D/DT = 0.875. The pumping capacity of the impeller was calculated from the radial profile of the axial component of the mean velocity in the draught tube below the impeller at such an axial distance from the impeller that the rotor does not affect the vorticity of the flow. The mean velocity was measured using a laser Doppler anemometer with forward scatter mode in a transparent draught tube and a transparent vessel of diameter T = 400 mm. Two series of experiments were performed, both of them under a turbulent regime of flow of the agitated liquid. First, the optimum height of the dished bottom was sought, and then the dependences of the dimensionless flow rate criterion and the impeller power number on the number of impeller blades were determined for both pitch angles tested under conditions of optimum ratio HT /DT. It follows from the results of the experiments that the optimum ratio HT /DT = 0.25 when the cross sectional areas of the horizontal flow around the bottom and the vertical inflow to the draught tube are the same. For all the tested pitched blade impellers the impeller power number when α = 45° exceeds the value of this quantity when pitch angle α = 35°, while the flow rate number when α = 35° exceeds this quantity when α = 45°. On the other hand, the absolute values of the impeller power number when the draught tube was introduced correspond fairly well to the dimensionless impeller power input measured in a system without a draught tube. However, the absolute values of the flow rate number found in the former system are significantly lower than the dimensionless impeller pumping capacity determined in the latter system. The hydraulic efficiency of pitched blade impellers N3Qp/Po for the investigated geometry of the agitated systems does not depend on the number of impeller blades, but it is significantly lower than the quantity determined in an agitated system with a dished bottom but without the draught tube.

Airlift-reactor design and test for aerobic environmental bioprocesses with extremely high solid contents at high temperatures

Bioprocesses at high temperatures gained considerably in importance within the last years and several new applications for aerobic, extreme thermophilic environmental bioprocesses are emerging. However, this development is not yet matched by adequate bioreactor designs, especially if it comes to the treatment of solids. In this communication we propose the use of airlift reactors to bridge this gap. The design of an internal draught tube bioreactor (AreaRiser/AreaDowncomer. 1; Height/Diameter . 8) is described in detail. The influence of the temperature on gas hold-up, liquid velocity and mixing characteristics was investigated. It was shown that this reactor could hold up to 1 t quartz sand per m3 in suspension at moderate aeration rates. Despite the decreasing oxygen solubility, the oxygen transfer rate increased with rising temperature due to the improved mass transfer parameters. With rising solid content, the oxygen transfer rate increased and reached a maximum at a solid content of about 140 kg m-3 before it decreased again. However, it is only slightly reduced at the highest solid contents. The results demonstrate that aerobic bioprocesses at high temperatures are not only feasible, but can be very efficient if carried out in proper bioreactors.