I. Centrifugal separation is a commonly employed method in the process of concentrating fermentation broths. The selection of appropriate equipment for this task directly impacts the recovery rate of microbial cells, cell viability, and overall production efficiency. Tubular, disc, and horizontal screw centrifuges represent the three most widely used categories of centrifugal equipment; however, due to differences in their structural designs and operating principles, their suitability for specific application scenarios varies significantly. Drawing upon practical experience and relevant literature, this article provides an overview of the principles, structural features, and selection criteria for these three types of centrifuges, serving as a valuable reference for the concentration of fermentation broths.
II. While the core operating principles of these three types of centrifuges are fundamentally identical—all utilizing centrifugal force to replace gravity and facilitate the rapid separation of components with differing densities (e.g., microbial cells and fermentation broth) within a liquid mixture—variations in their structural designs result in distinct strengths regarding separation efficiency, processing capacity, and suitability for specific applications.
1. The tubular centrifuge is characterized primarily by its "high rotational speed and high separation factor." Structurally, it consists mainly of a rotating bowl (rotor), a feed pipe, an overflow pipe, a transmission system, and a casing. The rotating bowl features a slender, tubular shape with a relatively small diameter (typically 50–150 mm) and a considerable length; this design effectively extends the separation path of the liquid mixture, thereby enhancing separation performance. Its operating principle involves feeding the liquid mixture into the bottom inlet of the high-speed rotating bowl. With rotational speeds reaching between 10,000 and 25,000 revolutions per minute (rpm), the bowl generates an extremely powerful centrifugal force, causing the denser microbial cells to rapidly settle against the inner wall of the bowl. Simultaneously, the clarified liquid rises along the inner wall of the bowl and is discharged through the overflow outlet at the top, thereby achieving the separation of the microbial cells from the clarified liquid. Tubular centrifuges feature a compact structure and a small footprint; moreover, their separation process is gentle, exerting minimal impact on microbial cell viability. Consequently, they are well-suited for concentrating fermentation broths with low solid content (≤ 5%) and low viscosity, and are particularly effective for the collection of high-activity microbial cells in pilot-scale and small-scale production settings.
2. Disc centrifuges represent the mainstream equipment for large-scale industrial production. Structurally, their core components consist of a rotating bowl and a stack of discs. The bowl is cylindrical in shape, housing a stack of dozens to hundreds of conical discs that create narrow channels between them; the unit is also equipped with a feed device, a solids discharge mechanism, and a drive system. The operating principle relies on the multi-layered discs stacked within the bowl to significantly increase the separation surface area and reduce the sedimentation distance. As the feed liquid enters the bowl, centrifugal force causes the microbial cells to settle along the surfaces of the discs toward the bottom of the bowl, while the clarified liquid is discharged from the top; this configuration enables continuous feeding and continuous discharge. Compared to tubular centrifuges, disc centrifuges offer higher processing capacities and a greater degree of automation. They feature a moderate separation factor (3,000–10,000 rpm) and exert gentle shear forces, thereby satisfying the demands for continuous separation of large-scale fermentation broths while simultaneously preserving microbial cell viability. Suitable for processing feed liquids with a solid content ranging from 2% to 10%, the disc centrifuge is the preferred equipment for the industrial-scale production of microbial powders.
3. Horizontal screw centrifuges operate based on the principle of continuous sedimentation augmented by a screw conveyor. The feed liquid enters the high-speed rotating bowl via a feed pipe; under the influence of centrifugal force, the microbial cells settle against the inner wall of the bowl. A screw conveyor, rotating at a differential speed relative to the bowl, pushes the settled cells toward the conical end of the bowl for discharge through the solids outlet, while the clarified liquid overflows from the opposite end of the bowl, thereby achieving continuous separation and continuous solids discharge. Structurally, a horizontal screw centrifuge comprises a rotating bowl, a screw conveyor, a feed device, a differential gear unit, and a casing. The bowl is horizontally oriented and cylindrical in shape, featuring a cylindrical section at one end and a conical section at the other; the screw conveyor maintains a specific rotational speed difference relative to the bowl and is responsible for the discharge of settled solids. Its advantages lie in its high throughput and its ability to process feed liquids with high solids content (5%–20%) and significant impurity levels. However, it generates relatively high shear forces, which can have a certain impact on microbial cell viability; consequently, it is primarily utilized for the preliminary separation of fermentation broths or in large-scale production scenarios where strict requirements regarding microbial activity are not a priority.
III. So, how does one go about selecting the appropriate centrifuge from these three types? The selection process must be guided by factors such as production scale, the characteristics of the feed liquid, and the specific requirements for spore preservation, necessitating a comprehensive assessment that weighs the distinct features of each of the three equipment categories. For small-scale laboratory or pilot-scale production—particularly when the objective is to achieve high viability in the final microbial powder or to process feed liquids with low solids content—the tubular centrifuge is the preferred choice; this can be paired with a refrigeration system (set at 4°C) to provide additional protection for the microbial cells. For large-scale industrial production requiring continuous operation and the processing of feed liquids with moderate solids content, the disc centrifuge is the preferred option, as it effectively balances processing efficiency with the preservation of microbial activity. Conversely, if the feed liquid contains high levels of impurities and solids, and if the requirements for the viability of the microbial powder are not particularly stringent, a horizontal screw centrifuge may be selected as a cost-effective solution for processing.
VolgendeAn Analysis Of Bioreactors