Classification and application range of centrifuges

Introduction

The finer the solid particles in the suspension, the more difficult it is to separate them, and the fine particles carried away in the filtrate or separated liquid will increase. In this case, the centrifuge needs to have a higher separation factor to effectively separate; when the viscosity of the liquid in the suspension is large, the separation speed slows down; the density difference between the components of the suspension or emulsion is large, which is beneficial to centrifugal sedimentation, while the centrifugal filtration of the suspension does not require the density difference between the components.

In ancient China, people tied a pottery jar with one end of a rope, held the other end of the rope, and swung the pottery jar to generate centrifugal force to squeeze out the honey in the pottery jar. This is an early application of the centrifugal separation principle.

Industrial centrifuge

The origin of industrial centrifuge

Industrial centrifuges were born in Europe. For example, in the mid-19th century, three-legged centrifuges for dehydrating textiles and top-suspended centrifuges for separating crystallized sugar in sugar factories appeared successively. These earliest centrifuges were intermittently operated and manually discharged. Due to the improvement of the slag discharge mechanism, continuously operated centrifuges appeared in the 1930s, and intermittently operated centrifuges also developed due to the realization of automatic control.

Industrial centrifuge types

Industrial centrifuges can be divided into three categories according to their structure and separation requirements: filter centrifuges, sedimentation centrifuges and separators.

The selection of a centrifuge requires a comprehensive analysis based on the size and concentration of solid particles in the suspension (or emulsion), the density difference between solids and liquids (or two liquids), the viscosity of the liquid, the characteristics of the filter residue (or sediment), and the requirements for separation. The requirements for the moisture content of the filter residue (sediment) and the clarity of the filtrate (separated liquid) are met, and the type of centrifuge to be used is initially selected. Then, according to the processing volume and the requirements for automation of the operation, the type and specifications of the centrifuge are determined, and finally verified by actual tests.

Working principle

There are dozens (50-80) discs of the same shape and size in the drum of the centrifuge. The discs are stacked at a certain spacing (0.5-1.2mm) to form a disc group. Each disc has several symmetrically distributed circular holes on the circumference of a circle at a certain distance from the axis. When many such discs are stacked, the corresponding circular holes form a vertical channel. After the mixture of two liquids of different gravities enters the centrifugal separator, it enters the gap between the discs through the vertical channel formed by the circular holes on the discs and is carried to rotate at high speed. Due to the difference in the centrifugal sedimentation speed of the two liquids of different gravities, the centrifugal sedimentation speed of the heavy liquid is large, so it moves outward away from the axis, and the centrifugal sedimentation speed of the light liquid is small, so it flows toward the axis. In this way, the two liquids of different gravities are separated in the process of flowing in the gap between the discs. It is used to filter sediment and other solid impurities in water, with an efficiency of more than 90%. The centrifuge has a cylinder that rotates at high speed around its own axis, called a drum, which is usually driven by an electric motor. After the suspension (or emulsion) is added to the drum, it is quickly driven to rotate at the same speed as the drum. Under the action of centrifugal force, the components are separated and discharged separately. Generally, the higher the drum speed, the better the separation effect.

Separator for experimental analysis

A type of separator for experimental analysis, which can be used for liquid clarification and solid particle enrichment, or liquid-liquid separation. This type of separator has different structural types operating under normal pressure, vacuum, and freezing conditions. The working principle of centrifuge is centrifugal filtration and centrifugal sedimentation. Centrifugal filtration is to make the centrifugal pressure generated by the suspension under the centrifugal field act on the filter medium, so that the liquid passes through the filter medium to become filtrate, and the solid particles are trapped on the surface of the filter medium, thereby achieving liquid-solid separation; centrifugal sedimentation is to use the principle that the components of the suspension (or emulsion) with different densities quickly settle and stratify in the centrifugal field to achieve liquid-solid separation or liquid-liquid separation. The important indicator to measure the separation performance of the centrifuge is the separation factor. It indicates the ratio of the centrifugal force to the gravity of the separated material in the drum. The larger the separation factor, the faster the separation and the better the separation effect. The separation factor of industrial centrifuges is generally 100-20000, the separation factor of ultra-speed tubular separators can be as high as 62000, and the separation factor of analytical ultra-speed separators can be as high as 610000. Another factor that determines the processing capacity of the centrifuge is the working area of the drum. The larger the working area, the greater the processing capacity.

Structure

The centrifuge consists of a body, a transmission device, a drum, a liquid collection tray, and a liquid inlet bearing seat. The upper part of the drum is a flexible main shaft, and the lower part is a damping floating bearing. The main shaft is connected to the driven wheel by a coupling seat buffer. The motor transmits power to the driven wheel through a conveyor belt and a tensioning wheel, so that the drum rotates at an overspeed around its own axis. A strong centrifugal force field is formed. The material is injected from the bottom liquid inlet, and the centrifugal force forces the liquid to flow upward along the inner wall of the drum, and the liquid is layered due to the density difference of different components.

Product Features

1. Impurities and dirt in the emulsion treated by this machine are easy to clean.

2. It is simple to remove and separate microparticles such as carbon powder, glass powder, aluminum powder, and stainless steel powder.

3. No filter material loss.

4. It will not separate additives in the solution.

5. It is applicable to both oily and water-soluble cutting fluids.

Classification

Centrifugal separation equipment has different models, which can be mainly divided into two categories: laboratory use and factory use.

For laboratory use

1. Ordinary centrifuge

2. High-speed centrifuge

3. Ultra-high-speed centrifuge

4. Refrigerated centrifuge

5. Cream centrifuge Special centrifuge for determining fat in milk (Babcock method).

For factory use

1. Basket centrifuge

Commonly used for centrifugal dehydration in chemical plants and centrifugal separation of slag sweat in food factories

2. Cylinder centrifuge

Commonly used for separation of slag juice in sugar factories

Application

Centrifuges are widely used in chemical, petroleum, food, pharmaceutical, mineral processing, coal, water treatment and shipbuilding sectors.

1. Sugar industry: separation of granulated sugar and honey

2. Salt industry: dehalogenation of crystal salt

3. Starch industry: separation of starch and protein

4. Oil industry: refining of edible oil

5. Beer and beverage liquid: clarification

Function

1. Separation equipment should be able to prevent external microorganisms from mixing into the separated materials.

2. The surface in contact with the separated materials will not produce chemical reactions, adsorption penetration, oxidation and peeling, etc.

3. The internal surface of the centrifuge that contacts the material (centrifugal basket drum, outer shell, etc.) must be polished to the corresponding level, no concave structure is allowed, and all corners require arc transition. The weld must be continuous (i.e., seamless weld), the weld point is smooth, and the weld must be purified after mechanical polishing.

4. The centrifuge should have a cleaning in place (CIP) function (i.e., the equipment can be effectively cleaned without moving or disassembling) to prevent the accumulation of materials and impurities. This is a necessary condition for thorough cleaning of the equipment. In this regard, the fully automated CIP cleaning unit from the German GEAWestfalia separator is highly desirable and has been successfully used for cleaning centrifuges, storage tanks, piping systems and other equipment.

5. In order to ensure that all positions inside the centrifuge can be fully inspected, the centrifuge outer shell cover should have multiple opening methods, namely a large flap, a small flap and a closed cover.

6. The centrifuge should have the function of preventing static electricity and static electricity accumulation, that is, nitrogen should be sealed inside the machine, and the micro-positive pressure inside the centrifuge should be maintained by replenishing and emptying, so as to meet the explosion-proof requirements of the whole machine, and also ensure the quality and safety of special materials that are easy to oxidize.

7. In some cases, the centrifuge should also have the clean function of sterilization in place (SIP). The SIP pressure vessel should be designed with a special spindle seal; as a body designed as a pressure vessel, it should ensure that all equipment surfaces in contact with the material can be fully sterilized under a certain steam pressure.

8. For scraper centrifuges, the selection of filter screens is very important. The double-layer lightweight industrial plastic (PP) filter screen can prevent any fiber or tiny fragments from falling off. Its high temperature resistance and non-deformation characteristics are the prerequisites for SIP high temperature sterilization. The removal of residual filter cakes should also be considered to prevent confusion between batches of products.

9. It should have auxiliary detection functions; the centrifuge is a high-speed rotary machine. Through continuous detection of the vibration amplitude of the equipment, it can pre-alarm when the centrifuge produces harmful vibrations and make corresponding protective actions. In addition, there are oxygen detection devices, etc. Check valves and noise-reducing devices are set in the patented product centrifuge of Glacier Metal Co. Ltd.

10. Use isolation devices to separate the process area of the centrifuge that processes materials from the equipment area including the transmission and hydraulic systems.

Development

Driven by new technologies and the development of related industries, the progress of foreign centrifugal separation technology is mainly reflected in the following aspects:

Strengthen theoretical research and select the best design scheme

Sweden's ALFA-LAVAL company found in the study of disc flow channels that the velocity distribution on the cross section of the disc gap depends on a dimensionless number "λ", and the "λ" of industrial centrifuges is usually between 5 and 28. As the "λ" value increases, the speed of the disc increases and the thin layer decreases, which can increase the Reynolds number and ease the vortex. Through the ingenious design of the disc spacers and distribution holes, the feed rate can be increased by 20%.

In order to select the best solution, the researchers used flow field separation method, finite element simulation method, large gradient density ladder method, inverse modal analysis method, etc. to conduct theoretical research on the working performance and key parts of the centrifuge. For example, B. Sisger of Russia derived a series of formulas for analyzing the fluid dynamics of centrifuges, providing a theoretical basis for designing centrifuges with excellent performance. He also conducted an optimization study on the depth of the weir pool in the horizontal screw centrifuge with internal washing.

Improvement of technical parameters and the advent of new models

In order to improve the purity of products and meet the requirements of energy and environmental protection, high parameters have become the development characteristics of foreign models. Since bioengineering requires the separation of extremely fine particles, such as bacteria, enzymes and insulin, and the smaller the cells, the lower the recovery rate. Westfalia Company of Germany used SA7-47-476 piston disc machine for testing, and found that the cell size of NM (neisseriameningitidis) was 0.8×1.0μm, and the recovery rate was about 80%, while the cell size of TP (treponemaphagedenis) was 0.2×10μm, and the recovery rate was only about 10%. Therefore, the latest disc machine used in bioengineering can handle 0.1μm particles and has a higher rotation speed. For example, the BTAX510 model of ALFA-LAVAL of Sweden and the CSA160 model of Westfalia of Germany have a separation factor of up to 15,000.

As the process requirements increase, new models are constantly coming out. The BH-46 Merco nozzle disc separator used by Dorr-Oliver of the United States for starch industry has an inner diameter of 1.2m, a drum weight of 4.5T and a total machine weight of 21T. It is driven by two motors with a power of 220kW. The maximum production capacity is twice that of the BH-36 model, that is, 450M3/h, and the equivalent sedimentation area has reached 250,000m2, which is the highest among disc separators.

The BTUX510 disc separator used by ALFA-LAVAL of Sweden for biotechnology has an automatically adjustable vortex nozzle. The relationship between the viscosity and concentration of the nozzle feed is used to provide a constant solid phase discharge concentration, which is independent of changes in feed rate and solid content. The horizontal screw centrifuge with a separation factor of 10,000 can make up for the shortcomings of the tubular separator to some extent. The BTNX3560-A model is characterized by advanced rotational dynamic design. The main bearing is replaced with an elastic mount, which can extend the bearing life and greatly reduce the noise during the operation of the machine; the gearbox and the drum are connected with vibration reduction, which can increase the critical speed of the rotor; the conveying pipe is designed with an extra-large diameter.

The SZ piston pusher centrifuge newly developed by Krauss-Maffei in Germany is small in size but more effective in solid-liquid separation. There are also double-cone horizontal screw centrifuges used by Flottweg in Germany for handling difficult-to-separate materials.

Strengthen dynamic monitoring and automation

With the development of automatic control and sensor technology, many advanced automatic control methods have been introduced into centrifuges, and all-round measurements are made on various parameters during the operation of the centrifuge, such as temperature, flow, speed, amplitude and noise. That is, sensors of various performances are installed in the centrifuge, and the collected information is input into the computer. After system processing, the changes in various parameters can be understood in time to take corresponding measures. As a result, unmanned disc separators emerged.

Various combination machines and special machines

ALFA-LAVAL combines a screw conveyor on a disc separator, and Krauss-Maffei combines a conical drum on a piston pusher. In addition, in order to improve the separation performance of the centrifuge and seek the best operating conditions, Westfalia's disc separator varieties are the most in the world.

Progress in design

With the development of computer technology, CAD technology and modular design have been widely used. With the increasingly fierce competition in the global market, the manufacturing industry faces the huge challenge of improving customer value. Since the 1990s, "mass customization" has gradually emerged in the manufacturing industry. That is, "producing goods and providing services with an efficiency close to mass production to meet the personalized needs of customers." Due to the importance of design in the product life cycle, design for mass customization (DFMC) has become a new trend in design.

Application of new materials

In order to improve the performance, strength, rigidity, wear resistance and corrosion resistance of separation machinery, a number of new materials have emerged. For example, engineering plastics, cemented carbide and excellent wear-resistant and corrosion-resistant stainless steel materials. France has developed a rotor made of hard ceramic material. The outer layer of the rotor adopts the method of single wire winding to increase the strength of the rotor. The United Kingdom has also developed a continuous fiber composite rotor composed of synthetic resin. However, in disc separators, due to the need for higher mechanical strength and certain corrosion resistance, duplex stainless steel is widely used by manufacturers of separators in Western European countries. Russia has successfully developed a new type of austenite-ferrite duplex steel 04X25H5M2 (04Cr25Ni5Mo2). The forgings of this steel are used in the manufacture of drums and have sufficient mechanical strength and certain plasticity.

In order to make up for the lack of corrosion resistance and strength, some advanced manufacturers generally adopt the self-strengthening technology of drums. In addition, WischnouskiiF et al. in Germany [developed 16] new materials for separation drums. This centrifugally cast duplex stainless steel has the characteristics of high mechanical strength, good plasticity and corrosion resistance.

The future development trend of centrifugal separators will be to strengthen separation performance, develop large centrifugal separators, improve slag unloading mechanisms, increase special and combined drum centrifuges, strengthen separation theory research and research on centrifugal separation process optimization control technology. Improving separation performance includes increasing drum speed; adding new driving force during centrifugal separation; accelerating slag pushing speed; increasing drum length to extend centrifugal sedimentation separation time, etc. The development of large centrifugal separators mainly involves increasing drum diameter and using double-sided drums to improve processing capacity, thereby reducing equipment investment, energy consumption and maintenance costs for processing unit volume of materials.

Theoretical research

Mainly study the flow conditions of fluids in the drum and the formation mechanism of filter residues, and study the calculation method of minimum separation degree and processing capacity.

Centrifugal separation technology

Centrifugal separation is the fastest, most economical and most effective purification method today

Features:

Fast: The centrifugal separation speed is more than 5000-10000 times the separation speed of the natural sedimentation method.

Economic: No need to replace the filter element, low operating cost. Small footprint, compact

Effective: 1. Simultaneous separation of liquid, liquid and solid phases. 2. Strong processing capacity.

Centrifugal separation = clean coolant, cleaning fluid