Centrifugal separation technology for tubular separator

The centrifugal separation technology of the tubular separator is a technique for separating substances according to the centrifugal force generated by the rotation of the centrifuge and the factors such as the sedimentation coefficient, mass, density and buoyancy of the particles.
When the material enters the separation barrel of the tubular separator, due to the different density of the materials of different components, the force in the centrifugal force field is different, which directly leads to the difference of the trajectory and the movement rate of the centrifugal force field.

As shown in the figure, the separation tube (drum) of the tubular separator rotates around the central axis at a certain speed. After the mixture enters the separation cylinder, different components form different layers. The liquid phase formed by the liquid component (light liquid) with a small density is closer to the center of rotation, the liquid phase (heavy liquid) with a slightly higher density is farther from the center of rotation, and the insoluble solid phase is concentrated on the inner wall of the cylinder to form Filter cake.

The centrifugal separator has analysis, preparation and analysis-preparation according to the application; according to the structural characteristics, there are various types such as tube type, hanging blue type, and dish type; according to the rotation speed, it can be divided into normal speed (low speed), high speed and Speeding three.

The effect of centrifugal separation is related to many factors. In addition to the centrifuge type, centrifugation method, centrifugation medium, and density gradient described above, the main purpose is to determine the rotational speed and centrifugation time of the tubular centrifuge. Also pay attention to the conditions such as pH and temperature of the centrifuge medium solution.

■ Centrifugal force

The centrifugal force (Fc) of the material particles in the centrifugal field is determined by the mass of the particles (m) and the centrifugal acceleration (ac): Fc = m ac

The magnitude of the centrifugal acceleration depends on the rotational speed of the rotor and the radius of rotation of the particles: ac = ω ² r

Where ω: angular velocity of the rotor (rad/s); r: radius of rotation, ie the distance (cm) of the particle from the center of the axis of rotation.

If the speed is expressed in the usual number of revolutions per minute (r/min), then:

Where n: rotor revolutions per minute (r/min)

In describing the centrifugation conditions, low-speed centrifugation is usually expressed in revolutions per minute of the rotor, such as 4000 rpm; and in high-speed centrifugation, especially in ultracentrifugation, it is often expressed by relative centrifugal force, such as 65000 g.

Relative centrifugal force refers to the ratio of the centrifugal force to the gravity (gravity) of the particles. which is

RCF=Fc/Fg= mw ² r / mg = w ² r / g

Where RCF: relative centrifugal force (g); n: rotor revolutions per minute (rpm); r: radius of rotation (cm); g: gravity acceleration, 9.8 m/s ²

It can be seen that the magnitude of the centrifugal force is proportional to the square of the rotational speed and the radius of rotation. Under the condition of a certain speed, the farther the particle is from the axis, the greater the centrifugal force it receives. During centrifugation, as the particles move in the centrifuge tube, the centrifugal force they undergo also changes. In actual work, the data of centrifugal force refers to its average value. That is, the centrifugal force received by the particles in the centrifugation solution.

■ Centrifugal time

The concept of centrifugation time varies depending on the method of centrifugation. For differential centrifugation, it refers to the time when a certain particle completely settles to the bottom of the centrifuge tube. For iso-density gradient centrifugation, the centrifugation time refers to the equilibrium time at which the particles completely reach the iso-density point; and the time of the density-gradient centrifugation refers to the time at which the well-defined bands are formed.

The zone formation time or equilibrium time required for density gradient centrifugation and iso-density gradient centrifugation is complicated and can be determined experimentally. The settling time required for differential centrifugation can be calculated by calculation.

The settling time of the particles refers to the time required for the particles to completely settle from the liquid level of the centrifuged sample to the bottom of the centrifuge tube, also referred to as the clarification time. The settling time is determined by the particle settling velocity and settling distance.

For particles with known sedimentation coefficients, the settling time can be calculated by the following formula:

Where t: settling time (s); S: sedimentation coefficient of particles (1x10-13S); ω: angular velocity of the rotor (rad/s); r1, r2: the center of the rotating shaft to the liquid level of the sample and the bottom of the centrifuge tube Distance (cm).

The brackets in the above formula are unique constants for a particular rotor, called the rotor efficiency factor or K value. which is

The efficiency factor K of the rotor is related to the radius and speed of the rotor. For particles with a certain sedimentation coefficient S, the smaller the K value, the shorter the settling time and the higher the efficiency of use of the rotor.

For spherical particles that do not know their sedimentation coefficient, press down to estimate the settling time:

Where: T: settling time (s); μ: viscosity of the medium solution (g / (cm.s)); ρ, ρ0: particle and medium solution density (g / cm3); d: average particle diameter (cm ); r1, r2: the distance from the center of the rotating shaft to the bottom of the centrifuge tube and the liquid surface (cm).

■ Temperature and pH

In order to prevent agglomeration, denaturation and deactivation of the substance to be separated, in addition to the attention in the selection of the centrifugal medium, it is necessary to control the centrifugation conditions such as the temperature of the liquid material entering the tube separator and the Ph value of the medium solution. The centrifugation temperature is generally controlled at about 4 ° C. For some enzymes with good thermal stability, centrifugation can also be carried out at room temperature. However, when the high-speed tubular centrifuge is centrifuged, the rotor rotates at a high speed to generate heat, which causes an increase in temperature. Therefore, a refrigeration system must be used to keep the temperature within a certain range. The pH of the centrifuged medium solution should be in the pH range of enzyme stability, and a buffer may be used if necessary. In addition, peracid or overbase may also cause corrosion of the rotor and other parts of the centrifuge and should be avoided.