Filtration process

Filtration is known as a mechanical method for separating one or more compounds from fluid based on size differences.By developing the membranes used in this process, this technique is used to isolate dissolving solvents in a solvent or separation of mixtures of gases from each other. It is considered as a two-phase passive and non-passive separator and can be classified according to

Filtration process and examples -

the following characteristics:

1) Nature of the membrane (natural or artificial)

2) Membrane structure (porous or non-porous)

3) Application of membrane (separation of gas, liquid, and solid phases)

4) Membrane activity mechanism (absorption, intrusive, ion exchange)

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Advantages of filtration process:

1. Failure to change the phase or mode during the process.

2. Low energy consumption.

3. Supplying energy only with the use of electricity.

4. No need for phase change equipment such as vaporizers, condensers and …

5-Ability to run at low temperatures.

6. No PH changes and ionic strength during the process

Classification of Filtration Processes:

Types of filtration processes are classified according to the size of the particles or separable molecules, the most important methods of membrane separation are:

Reverse Osmosis, Nano Filtration, Ultrafiltration, Microfiltration, Dialysis, and Electrodialysis

Membrane power table:

Filtration methods:

1. Vertical method

In this method, the feed is fed vertically onto the filter surface and passed through the filter; the particles are deposited on the filter surface; over time, during the process, the thickness of the cake formed on the filter surface increases, which in most cases consists of cake It is compressible and there is no line between the thickness of the cake and the pressure drop. As a result, over time, the filtration rate decreases dramatically, and the emptying of the cake is essential and increases the processing time [2].

2. Tangential Flow

The second method is the tangential flow technique in the feed flow parallel to the filter surface and minimizes the formation of the cake. In this method, the feed is introduced into the filter and the two output streams pass through one filter (another dilute solution) ), And the cake is prevented at the filtration level, which increases mass flux.

The function of filtration devices:

The main filtration equation is obtained using the Darcy law, which links the rate of liquid flow from a solid substance to a flow drop:

Compressive Cake Resistance:

All cakes made from biosolids are compact and, by compressing these cakes, the filtration rate decreases. In cake cakes, cake resistance is a function of pressure drop:

  α is a constant, strongly dependent on the size and shape of the particles forming the cake. S values ​​(cake compression) are between 0 and 1.

0 for tight and incompressible cake and 1 for a very tricky cake

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Types of equipment and systems:

Filtration systems are divided into two general categories of laboratory and industrial systems, and in this process, they use equipment with 5 different designs that are divided as follows:

1-cylindrical filtration systems

2-Hollow fiber filtration systems

3-bed units

4-spiral systems

5. Special systems

Laboratory equipment:

This equipment is used for rapid filtering of low volume specimens, in which the driving force is provided by centrifugal force. An example is placed in the upper part of the cell containing membranes and performed using a centrifuge filtration process. Gets

The equipment is used in clinical and analytical experiments for low volume samples, no control over the polarization phenomenon, and these tests do not filter out the flow of flux. [3]

 The equipment with a higher scale, in this equipment, is the phenomenon of concentration polarization controlled by a stirrer. The magnetic gravity is placed near the membrane surface and the medium is stirred. This device is highly controlled due to the control of the polarization phenomenon of flux.

Industrial equipment:

 1-flat model

2-cylindrical systems

3-Hollow fiber filters

4-spiral systems

5-specific systems

Flat model:

Structurally, they are the simplest, the flat panel or the frame and the page are referred to. These filters have two sides of a rigid flat plate, two layers of membranes, between the rigid plate and the membrane using a spatial material And some of these pages, are placed in a closed space. The flow in these filters is slow. The membrane displacement in this type of filter is usually easy.

In terms of compression density, the energy consumption and intermediate cost of cylindrical and spiral systems are

Cylindrical filters:

These systems are usually produced by shaping polymers in a dominant plastic or paper. More than ceramic membranes are used. The raw material flows from the pipes and the filtered material is removed from the pipe wall. In this type of filter, several filters It can be placed in a single holder. The filter is selected and its size is selected with respect to the minimum energy consumption and construction cost

Hollow fiber filters:

Hollow fiber membranes are in appearance similar to cylindrical membranes, but they have a fundamental difference, this type of membrane is unlike self-supporting cylindrical membranes. In these filters, the feed is flowing from inside the filters and the liquid is flowing in a radial direction from the wall The filter is removed.

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The filter usually consists of inlet flow, a left-flow outlet fluid, and two paths for the fluid to be smooth. The flow of the feed and the fluid can be aligned or disjointed. Typically, 3000-50 separate fibers can be placed in the main chamber. Often, the flows are of a quiet type. The pressure is due to the flow rate and the length of the membrane is 20-5 psi. Among all filter models, this type has a high surface-to-volume ratio. Volume holdings are low. In terms of energy consumption, it is very low. Due to the narrow flow path and the high speed of the current, the stress intensity is high. [2]

Spiral systems:

The most compact and cheapest are filtration systems. They are widely used. On each side, there is a flat plate of two flat membranes that are closed from three sides and connected to the central edge by the edge of the edge and the membrane It is wound around the center of the core. The flattened material enters the core after passing through the membrane. The wrapped around the core is placed inside a metal or plastic shell that has a high-pressure bearing. One way to increase the filter surface without The need to increase its length is the use of several membranes with a common core. The thread in these types of filters is turbulent [1] and [2]

Special systems:

Systems that are used in MF and UF should be designed to minimize the phenomenon of polarization and pollution. An approach to reduce the polarization phenomenon and increase the intensity of the flow is the movement of the membrane, which produces different designs in the filters:

1-Circuit systems

2. Vibrating systems

3-Dean Vortices systems

Circulating systems:

In this model, the membrane is located within a cylinder and moves along one axis. The gas passes through the space between the two cylinders and the liquid flows through the membrane. At a specific turning point, the Taylor Vortex phenomenon is created, due to its stress intensity of 100,000 Approximately 10 times more than conventional filters. These equipment are suitable for semi-industrial and smaller scale in the pharmaceutical and biotechnology industries, but due to mechanical problems related to cylinders and the ratio of surface to volume and low density of membranes cannot be increased to industrial size…

The other type of circulation systems is flat plate filter filters suitable for industrial applications. In these types of filters, the membrane is located between two plate rigid discs, which is caused by the circular discs tensing over 400,000 reverse seconds, and the fluid is passed through the membrane he does.

Vibrating Filters:

Another way to move the membrane is to use the V-SEP technique. The model includes a series of flat plate membranes that are separated by a layer 1-2mm in diameter. Industrial examples of these filters can be up to 150 disks with a surface area of ​​30 m. The tension on the membrane surface reaches an inverse of 150,000 s. This tension is high for the high concentration of samples to be used.

Common filtration:

A type of filtration in which a large and large cake is formed usually contains larger particles or microorganisms.

Equipment required for conventional filtration:

1-Filters “Page and Frame” Pressure

Horizontal 2-page filter

3-sheet vertical filter

4-candle filter

5-Vacuum Rotary Filter

 Vertical sheet filtering:

The vertical sheet of the sheet shown in the figure needs a little surface, but it should be placed in a space where space above is empty to remove filter and cake sheets. This filter provides a high filtration rate for the existing volume.

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Candle filter:

Tubes that are mounted on a reservoir filter are operated so that the cake is formed on the outer surface and the filtered material moves upwards inside the tubes. These pipes are distinguished by using the “Wash in the opposite direction”.

Vacuum Rotary Filter:

A rotary vacuum filter is used for large volumes of filtration in biological separation. They are used on a large scale where solids are hardly filtered. Therefore, they are used extensively in commercial operations. From here, These filters operate automatically. There are also low labor costs. The filter consists of a rotating cylinder shown at the bottom. The pressure outside the atomic cylinder is atmospheric, but inside the cylinder is a relative vacuum. A portion of the cylinder is immersed in a solution and during the process, it runs at a slow speed. The solution is sucked into the cylinder and the solids contained in the solvent on the outer surface of the cylinder form a cake. When the cake Outside the solvent container it is washed and dried and then separated from the cylinder. The stage of separating the cake from the cylinder is done using a blade [3]


Microfiltration is one of the important membrane processes that are the basis of its physical separation, the size of solids dissolved, and the turbidity and removed microorganisms are determined by the pore size of the membranes from 0.2 to 20 microns. In this method, separation of bacteria, colloid suspended matter, and polymer materials are removed, and microparticles pass through smaller pores than the membrane.

Microfiltration method is used in the following industries:

– Pre-treatment for nano-filtration and reverse osmosis

– Biologic purification of contaminated waters

– solid separation – liquid for the food and medicine industry

– Sterilization in drinks and medications

– Transparent fruit juices


Ultrafiltration is a membrane separation technology based on the semi-permeable membrane for separation. It is a technology for the removal of fine particles and microbial contamination. The difference in pressure, the force of the flow, to pass through the membrane.


Flux and better resistance to pollution

High density and good flexibility

Good tolerance of acidic and thermal conditions

Better resistance to bacteria

Maintain in dry conditions

Resistant to oxidizing agents

Main Uses

Pre-treatment of RO systems

Purification and removal of bacteria in drinking industries, milk, and mineral water

Urban and Industrial Wastewater Treatment

Refining the oil in the water cycle

Recycling and refrigeration of refrigeration and water treatment in various industries

General application in other fields such as chemical, electricity, food, oil, and textile

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Ultrafiltration is a membrane process. Such a process depends on the permeability of the membrane, which is related to the size of the membrane. Similar processes of this type have been named: ultrafiltration, reverse osmosis, microfiltration, hyperfiltration, and diafiltration. Recent attempts to distinguish between these processes are based on the materials isolated by them, as shown in Figure 5 of this split. According to the shape, there are many similarities in particle size intervals.

Ultrafiltration processes and the like have three specific characteristics: these processes use a high-speed cross-section, are known by the type of membrane and in actual equipment depend on the membrane geometry. Each of these attributes has value for reflection.

Crossover flow

The ultrafiltration process always involves a high-velocity cross-flow on the membrane surface and perpendicular to the flux passing through the membrane. During the action of ultrafiltration, the formation of cake reduces the flow of flux, and the cross flow causes the cake to form on the membrane. The low cake in the ultrafiltration process makes it different from the ordinary filtration process. When a coarse solution of the molecule is subjected to ultrafiltration, the currents flow reduces the accumulation of coarse molecules near the surface of the membrane. This aggregation is called “polarization”, which increases osmotic pressure and thus reduces flow through the membrane.


Ultrafiltration is strongly dependent on the type of membrane, which is different from conventional filtration, in which the filter option usually has less effect on passing through the cake. In ultrafiltration, the membrane plays an important role, given that the cross flow decreases the formation of cake or the concentration of polarization. A thin layer of organic solution is made in water, glass or a neutral base. The porosity membrane is equal to 0.80 and the average porosity size is in the range of 0.1 to 1 μm. Although the porosity at the surface is greater than 1 μm, the vent contraction along the sponge matrix reduces the effective pore size to 0.5 m. Primary membranes of this kind are cellulose esters, but similar structures are made of other polymeric materials such as nylon, polyvinyl chloride, and acrylonitrile. These polymers improve membrane stability over a wide range of pH, temperature and organic solvents. Such membranes are characterized by a molecular weight range (MWCO) and are available, for example, in sizes of 1000, 10,000, 100,000 and 1,000,000 Daltons. 100,000 Daltons membranes are commonly used to separate cells.

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Hot stretch films are made of non-polymeric materials such as polypropylene. The polymeric layer is located under pull and rupture to form smaller pores, and the final building is eventually formed and hardened. These membranes have a porosity of about 35% and a thickness of 0.003 centimeters. Regarding hydrophobicity, these membranes should be prepared by ultrafiltration of aqueous solutions with an alcoholic mixture. These membranes are also the best membrane for use in pulmonary arteries but are not very common for ultrafiltration.

They have the least porosity (about 3%) and the more pores are single-distributive. Since these membranes are usually ten times thinner, they have similar permeability to other types of membranes. These membranes are made of mica or polycarbonate by placing alumina films in a non-toxic form. The exposed parts of the films are affected by hydrofluoric acid and pores are created.


The third characteristic of ultrafiltration is the membrane geometry in real equipment. This geometry binds cross flow and membrane materials on the effect of ultrafiltration.

This model includes the alternating layers of the membrane, the porous layer of the holder and the distribution areas for the feed and fluid passage. The unit can have a square or oval surface and can be presented as membranes with a surface of up to 1500 square meters. These membranes can be detached from the system to clean or replace faulty membranes. These membranes have the smallest amount of surface area per unit volume of common types, and therefore, the flux per unit volume is lower in the ultrafiltration process.

Another suggestion is the use of sketch and pipe designs. This structure is similar to the heat exchanger of the shell and tube, which has a number of pipes at the ends attached to a common sheet. The flow of the feed enters the duct and the penetrating material passes through the wall, leaving the remaining liquid from the other end of the tube. This geometry has been widely advertised. In our opinion, this model has the disadvantages of other geometries in combination. Cleaning and service This type are harder than the type of plate and frame, and the surface area is less than the volume unit, resulting in fewer fluxes than the torsion and hollow fibers.

The third type is a torsion curtain. This design is similar to a large membrane-shaped envelope with an intermediate layer for feed passage. The food flows around the envelope under high pressure throughout the membrane and is collected inside the envelope. This step is easy, but what complicates it is that the envelope is not flat and round the central channel of the feed is wrapped up.

The fluid passes through the membrane to the inside of the envelope in the direction of the pipe and then moves towards the end of the unit. As a result, the device is similar to the roulette, the layers of the membrane passing through the feed, the membrane and the liquid movement distances, formed around a central welded tube and a compact unit with a membrane surface of up to 2000 square meters.

The ultrafiltration torsion plot has a much larger surface area than a sheet design and can be tubular structures. As a result, the filtration intensity is greater per unit volume. These membranes are much harder to clean, and if a part of the membrane is damaged, the whole membrane should be discarded. As a result, when the feed is relatively pure, we tend to use this type of membrane. For example, super-pure water can be referred to as reverse osmosis.

The final form of ultrafiltration is a hollow fiber design. This shape is similar to tubular membrane systems, which is different in scale. These fibers usually have a diameter of 0.01 cm, while the diameter of the pipes is about 1 cm. Hollow fiber ultrafiltration schemes are in contradiction with the plate and frame forms discussed initially. Because these fibers produce a surface area of ​​very high up to 3000 membranes, they produce the highest amount of flux per unit volume. These devices are easily taken and hard to clean. When some of these fibers fail, the whole structure of the membrane should be discarded and not usable.


Each structure is used for special conditions, but this is not clearly recognizable. We can not always have a reliable proposal. Especially for torsion and hollow fibers, there is a need for pre-filtration before the ultrafiltration stage.

All of these membrane designs require additional equipment

Each process consists of a reservoir, a feed pump, and a membrane module. The pump allows flow through the membrane to be at least 10 times more intense than the usual flux passing through the membrane and the remaining mixture is returned to the system. In sum, the combination of crossover current, membrane type, and its geometry represent the ultrafiltration process and the basis for the analysis to be discussed.

Types of microfiltration and ultrafiltration processes

1. Non-permanent operation

2- One-step process

3. Non-permanent operation with food

4- Multiple-step process with return flow

Uninterrupted operation:

In this method, a pump for feeding to the filter and its circulation is used and the process continues until the final condensation. In this process, a temperature converter can be controlled with a heat exchanger in the circulating solution stream. This system is the easiest method To condense a specific volume of a sample. In this system, the flux is high. The disadvantages of this system can be described as low flexibility.

One-step process:

In this method, the remaining fluid is not returned. In most UF and MF methods, the stream is flattened out of the remaining. The efficiency in this method is very low. In order to improve it, the filters require a very high level. Wastewater treatment, toxic waste removal, etc. The residence time of the sample in this system is at the lowest level.

Continuous Operation with Feeding:

The system continuously enters the feed filtration process while the fluid level is fixed in the main reservoir. This system is used in industrial processes.

The time to carry out the non-continuous process with food is higher due to the low flux ratio than the non-continuous one, so the remaining sample is pumped far more.

Multistage processes:

In this system, only the concentration in the last stage is high and works in the previous stages of low concentration and maximum flask. Increasing the process will increase the cost of control, valves, and plumbing. The residence time and the required tank volume are less than the discontinuous system… These systems run on average 1 to 3 hours every 24 hours. The cleaning process takes place.

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