Search Results (3)

Solid–liquid separation is a fundamental operation in process engineering and thus an important part of many process chains in the preparation of slurries in the chemical industry and other parts of the industrial environment. For the separation of micron-sized particles which, due to their size, do not settle or settle very slowly in the earth’s gravity field, centrifuges are often used. The preferred choice are often decanter centrifuges because they work continuously and stabilize the process against product fluctuations due to their adjustment possibilities. The design of the apparatus is complex: The main components of the apparatus are the cylindrical-conical bowl, which rotates at a high speed, and a screw located inside the bowl, which rotates in the same direction at a low differential speed to transport the separated solids out of the apparatus. Geometrical properties of the apparatus, as well as the adjustable operating parameters, such as rotational speed or differential speed, have a significant influence on the separation. In practice, analytical models and the experience of the manufacturers form the basis for the design. Characteristics of the disperse phase, interactions with the liquid, as well as the influence of the flow on the separation, are not taken into account. As a consequence, the transfer to industrial scale always requires a large number of pilot-scale experiments, which are time-consuming and expensive. Due to the increasing computational power, computational fluid dynamics (CFD) provides one possibility to minimize the experimental effort in centrifuge design. In this work, the open-source software OpenFOAM is used to simulate the multi-phase flow in a laboratory decanter centrifuge. For validation, experiments were carried out on a laboratory scale and the main operating parameters, such as speed, differential speed, and volume flow rate, were varied. The simulation results show a good agreement with the experimental data. Furthermore, the numerical investigations show the influence of the flow on the separation of the particles. To evaluate the transportability of a material, the transport efficiency was introduced as a dimensionless parameter. In addition, the simulation allows the consideration of the individual velocity components, making it possible to generate an impression of the complex three-dimensional flow in the apparatus for the first time. Full article

The separation of immiscible liquids is critical in many industrial processes, such as water treatment, different extraction processes, the petroleum industry, food production, and medicine. This work provides an overview of present research on the separation of liquid mixtures. A brief summary of the thermodynamic basis is provided, covering phase equilibrium, phase diagrams, and thermodynamic properties of phases. Additionally, the fundamentals of dispersion, necessary for discussing liquid–liquid separation, are presented. Subsequently, different liquid–liquid separation methods are discussed, highlighting their advantages and limitations. These methods include decanters, coalescers, centrifugal separators, membranes and electro-coalescers for liquid–liquid separation. Phase properties, dispersion formation, and time and space constraints specify the most efficient separation method. Phase recycling is also briefly discussed as a method to reduce the environmental impact of liquid–liquid extraction with subsequent phase separation. In summary, liquid–liquid separation methods are compared and future perspectives of liquid–liquid separation are discussed. Full article

In static conditions, the only mechanism available for sludge/water separation is sludge sedimentation by gravity. In dynamic conditions an additional mechanism is available: sludge momentum preservation. In order to achieve a better understanding of the operation of a secondary sedimentation tank (SST), the authors analyzed the behavior of the sludge blanket (taking note of the concentration in vertical and horizontal directions) and how it relates to the hydrodynamic fields within the SST. These findings have been interpreted based on hydrodynamic principles: momentum preservation, in case of any energy loss; motion of fluids from an area with higher potential energy to an area with lower potential energy; and the ratio between inertia and gravity forces. The results indicated that the sludge blanket momentum is a parameter of great importance for understanding the behavior of an SST. According to these principles, a longitudinal flow rectangular clarifier has been converted into a transverse flow clarifier, obtaining considerable improvement in operating performance. Moreover, it should be noted that there are already design strategies based on the optimization of water/sludge different momentum as a mechanism to improve the performances of a secondary clarifier. Peripheral feeding in the circular decanter; perforated baffles installed on a rectangular decanter; and the distance to be maintained between the bottom wall of a rectangular SST and the clarified water collection channel are all design strategies explained on the basis of the different sludge/water momentum rather than solid flux theory. Full article