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The Introduction of Tangential Flow Filtration——Holves Time:2018.01.29
    Tangential flow filtration is an effective method for concentrating and desalting 10ml to several thousand liters of sample solution. It can be used to isolate large biomolecules from small biomolecules, capture cell suspension and clarify fermentation and cell lysis. TFF can be applied to a range of applications including protein chemistry, molecular biology, immunology, biochemistry and microbiology. In this paper, the principle, main parameters and application of tangential flow filtration technology are introduced.
    TFF is a tangential flow filter, which is a pressure-driven, molecular-sized membrane separation process. With TFF, the sample mixture is not forced through a single channel through a single channel like the dc filter. Instead, the fluid passes through the surface of the membrane through repeated cycles. This "cleaning" behavior caused by pressure reduces the accumulation of initial samples on the membrane surface. The target molecules with larger molecular weight than the membrane were retained, while the small molecules and the buffer passed through the membrane. Tangential flow filtration is an effective method for concentrating and desalting 10ml to several thousand liters of sample solution. It can be used to isolate large biomolecules from small biomolecules, capture cell suspension and clarify fermentation and cell lysis. TFF can be applied to a range of applications including protein chemistry, molecular biology, immunology, biochemistry and microbiology. In this paper, the principle, main parameters and application of tangential flow filtration technology are introduced.
    Conventional filtering is to point to in under the action of pressure, the liquid directly through filtration membrane into the downstream, while the large particles or molecules trapped in the upstream of membrane or internal, small particles or molecules through the membrane into the downstream. In the operation of the body flow direction is perpendicular to the membrane surface into the downstream, so some people call it the "Dead End Filtration" .The application of conventional filtration includes clarifying filtration, sterilization filtering and virus filtering, etc., which is not the focus of this article. And the tangential Flow Filtration is refers to the direction of liquid Flow is parallel to the membrane surface, under the action of pressure, only part of the liquid Filtration membrane into the downstream, the operation mode has also been called "cross-flow Filtration". Because the tangential flow is continuously "scour" on the surface of the film package during the filtration process, it effectively alleviates the accumulation of large particles and molecules on the membrane in this operation mode, which makes this mode of operation have unique advantages in many applications.
In the tangential flow filtration, the pump impels the fluid through the membrane surface to wash away the molecules that are trapped on the membrane, thus minimizing the scaling degree of the membrane surface. At the same time, the tangential fluid also produces pressure perpendicular to the membrane, pushing the solute and small molecules through the membrane. This completes filtering. Using subdivision mesh separate sand and pebbles of simulation test, help to understand the mechanism of tangential flow filtration: mesh eye symbol on the membrane pore, and sand and pebbles symbol to separate molecules, in dc filter, sand - stone mixture forced to screen eye direction, as some of the smaller sand through the screen mesh eye drops, forming a pebble layer in the surface of screen mesh, block at the top of the sand to screen direction and through the screen mesh in the dc filter, increase the pressure, can only to pressure on the mixture, not help promote separation; , by contrast, in tangential flow filtration mode, through the mixture of recirculation prevent the formation of limiting layer, the recirculation is similar to: vibrating screen mesh to remove obstruction of pebbles, made falling is located at the top of the mixture of sand and through the screen mesh. Therefore, it is more efficient to use tangential flow filtration for biological molecular separation, and it is faster to concentrate or infiltrate.
    (A) apply direct pressure to the mixture to cause the bottom sand to drop; A pebble bed is formed on the surface of the screen to prevent the top sand grains from moving towards the screen and through the screen.
    (B) Vibrating screen mesh, which destroys the accumulation of pebbles at the bottom of the mixture to enable complete separation; In the tangential flow filtration, the fluid-flow mechanical action of feed flow is equivalent to the vibration in this case.
    For TFF procedures we often encounter some problems, such as the target protein recovery rate is low, the inlet filter after a period of time is increased pressure, etc., how to effectively solve these problems? We must be clear about the important parameters of tangential flow filtration and their interrelation, and ensure that the experimental conditions suitable for the samples are determined through the exploration and optimization of experimental conditions. These parameters are summed up as two pressure parameters and two velocity parameters.
    Differential pressure is the pressure of the inlet and outlet of the difference, transmembrane pressure (TMP) refers to the average pressure membrane filter upstream and downstream (TMP = + PR (PF) / 2 - Pp), TFF experiment, we focus on the pressure difference across the membrane pressure, tangential velocity, etc., this is closely linked with several parameters, pressure difference decided to tangential velocity and the tangential flow rate, when the differential case, tangential velocity and the tangential flow rate is constant, the transmembrane pressure is to control the speed of liquid filtration so we have to control the inlet pressure and flow pressure, the two decided the TMP and differential pressure, pressure when the differential/tangential velocity under the condition of constant, TMP become important factors in the process of tangential flow, need to be optimized.
    The tangential flow is the velocity of the liquid flow velocity on the membrane surface. It is the deposition of particles to prevent particles from accumulating on the membrane surface. The scour caused by tangential flow takes particles away from the membrane surface. Inlet pressure and tangential velocity (tangential water flux or return velocity), when you increase the tangential flow rate, you increase the pressure difference between the inlet and the reflux ∆p at the same time. If you double the tangential velocity, you end up doubling the ∆p . Partial closure of the backflow valve will increase TMP, which will eventually push the liquid through the membrane. The more the backflow valve closes, the larger the TMP will be, and the more liquid the liquid will increase through. For most ultrafiltration applications, the liquid valve is open. 

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